DF Recycling of lithium-ion batteries Determination of optimal parameters for the application of hy- drogen peroxide as reducing agent in the leaching process Master’s thesis in Innovative and Sustainable Chemical Engineering PIAMCHEEWA BENJAMASUTIN RAKSINA PROMPHAN Department of Chemistry & Chemical Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2020 Master’s thesis 2020 Recycling of lithium-ion batteries Determination of optimal parameters for the application of hydrogen peroxide as reducing agent in the leaching process PIAMCHEEWA BENJAMASUTIN RAKSINA PROMPHAN DF Department of Chemistry and Chemical Engineering Division of Industrial Materials Recycling Chalmers University of Technology Gothenburg, Sweden 2020 Recycling of lithium-ion batteries Determination of optimal parameters for the application of hydrogen peroxide as reducing agent in the leaching process PIAMCHEEWA BENJAMASUTIN and RAKSINA PROMPHAN © PIAMCHEEWA BENJAMASUTIN and RAKSINA PROMPHAN, 2020. Supervisor: Dr. Martina Petranikova, Chalmers Supervisor: Dr. Magnus Paulsson, Nouryon Examiner: Professor Britt-Marie Steenari, Chalmers Master’s Thesis 2020 Department of Chemistry and Chemical Engineering Division of Industrial Materials Recycling Chalmers University of Technology SE-412 96 Gothenburg Telephone +46 31 772 1000 A collaboration with Nouryon Pulp and Performance Chemicals AB Cover: Sulfuric acid leaching solution containing hydrogen peroxide for four cathode materials which are LCO, NMC111, NMC622, and NMC811. iv Recycling of lithium-ion batteries: Determination of optimal parameters for the application of hydrogen peroxide as reducing agent in the leaching process PIAMCHEEWA BENJAMASUTIN and RAKSINA PROMPHAN Department of Chemistry and Chemical Engineering Chalmers University of Technology Abstract The use of secondary Li-ion batteries has grown significantly in recent years because1 of their high energy density and are currently used in a wide range of applications2 such as electronic appliances, energy storage applications, and electric vehicles. Due3 to the limited resources and environmental problems after end-of-use, the recycling4 of valuable metals from spent batteries is substantially essential. In this work,5 sulfuric acid (H2SO4) leaching with the help of a reducing agent (hydrogen peroxide)6 of four different cathode materials was studied. The cathode materials that were7 investigated are LCO, NMC111, NMC622, and NMC811. The aim was to determine8 the optimal leaching conditions including leaching temperature, acid concentration,9 solid-to-liquid ratio, amount (%v/v), and addition strategy of the reducing agent.10 The optimal leaching temperature and acid concentration, without the addition of11 hydrogen peroxide and current collectors, were 50◦C and 2 M H2SO4, respectively.12 A solid-to-liquid ratio of 1:20 g/mL was selected for further leaching experiments13 carried out when hydrogen peroxide was added as a reducing agent. In addition,14 a better mixing was found to promote the leaching performance. Both metals’15 leaching efficiencies for cobalt, lithium, nickel, and manganese and the hydrogen16 peroxide consumption were determined in order to determine the optimal hydrogen17 peroxide concentration in the leaching solution and the best way to add hydrogen18 peroxide. Different amounts of hydrogen peroxide were needed to efficiently leach the19 four different cathode materials studied. Addition of hydrogen peroxide once at the20 beginning of leaching yielded 100% leaching efficiency faster than adding hydrogen21 peroxide at several occasions (same total hydrogen peroxide charge). Moreover, an22 addition of copper and aluminum foils, which represent the current collectors that23 also can act as reducing agents, can improve all metal leaching efficiencies except24 for lithium because lithium doesn’t need to change oxidation state. It was thus25 shown that the proposed leaching conditions can effectively leach valuable metals26 out from pure cathode materials. Crushed spent cathode material ("black mass")27 with the composition of Li1.087Ni0.308Mn0.300Co0.392O2 was then leached with the28 optimum conditions for pure cathode material (NMC111). The outcome was a29 leaching efficiency of almost 100% for cobalt, nickel, and manganese and with low30 amounts of residual hydrogen peroxide in the leachate.31 Keywords: Sulfuric acid leaching, hydrometallurgical recycling, Li-ion batteries,32 LCO, NMC, hydrogen peroxide, leaching efficiency.33 v 34 Acknowledgements35 First and foremost, we would like to appreciatingly express our gratitude toward our36 supervisor, Dr. Martina Petranikova, for giving us an opportunity to do this inter-37 esting thesis on the topic of recycling Li-ion batteries. Also with her kind support,38 patience and immense knowledge helped us all the time doing our master’s thesis39 work: performing experiment and writing this report. We could not imagined to40 have a better supervisor for our research.41 Beside our supervisor, we also would like to extend our gratitude to Dr. Magnus42 Paulsson and Ph.Lic. Pia Hellström from Nouryon Pulp and Performance Chemi-43 cals AB for being our advisors, providing us knowledge about chemistry of hydrogen44 peroxide, kind support of such substance and very useful recommendations.45 We would like to thank Britt-Marie Steenari, our examiner, for letting us to do this46 thesis.47 Our acknowledgement also goes to our laboratory colleagues at the department48 of Nuclear Chemistry/Industrial Material Recycling for giving advice and recom-49 mendations. Also giving thanks to Luis Guillermo Gonzalez Fonseca and Stellan50 Holgersson for the help with ICP-OES and Nathalia Cristine Vieceli for her assist51 on analyzing the component of spent Li-ion batteries.52 Piamcheewa Benjamasutin and Raksina Promphan, Gothenburg, June 202053 vii 54 Contents55 List of Figures xiii56 List of Tables xvii57 1 Introduction 158 2 Theory 359 2.1 Main components in batteries . . . . . . . . . . . . . . . . . . . . . . 360 2.1.1 Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 2.1.2 Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562 2.1.2.1 Lithium Cobalt Oxide (LiCoO2)(LCO) . . . . . . . . 563 2.1.2.2 Lithium Nickel Manganese Cobalt Oxides64 (Li(CoxNiyMnz)O2)(NMC) . . . . . . . . . . . . . . . 665 2.1.3 Current collector and separator . . . . . . . . . . . . . . . . . 666 2.1.4 Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 767 2.2 Processes for recycling Li-ion batteries . . . . . . . . . . . . . . . . . 768 2.2.1 Hydrometallurgical process . . . . . . . . . . . . . . . . . . . . 869 2.2.1.1 Pretreatment steps . . . . . . . . . . . . . . . . . . . 970 2.2.1.2 Leaching . . . . . . . . . . . . . . . . . . . . . . . . . 971 2.2.1.3 Solvent extraction and precipitation . . . . . . . . . 1272 2.3 Aim and objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473 2.4 Scope of work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474 3 Methods 1575 3.1 Materials and reagents . . . . . . . . . . . . . . . . . . . . . . . . . . 1576 3.2 Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1577 3.3 Determination of metal concentration in leaching solution . . . . . . . 1678 3.4 Determination of hydrogen peroxide in leaching solution . . . . . . . 1779 4 Results 1980 4.1 Effect of leaching temperature . . . . . . . . . . . . . . . . . . . . . . 1981 4.1.1 Leaching of LCO . . . . . . . . . . . . . . . . . . . . . . . . . 1982 4.1.2 Leaching of NMC111 . . . . . . . . . . . . . . . . . . . . . . . 2083 4.1.3 Leaching of NMC622 . . . . . . . . . . . . . . . . . . . . . . . 2184 4.1.4 Leaching of NMC811 . . . . . . . . . . . . . . . . . . . . . . . 2285 4.2 Effect of mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2486 4.2.1 Leaching of LCO . . . . . . . . . . . . . . . . . . . . . . . . . 2587 ix Contents 4.2.2 Leaching of NMC111 . . . . . . . . . . . . . . . . . . . . . . . 2588 4.2.3 Leaching of NMC622 . . . . . . . . . . . . . . . . . . . . . . . 2689 4.2.4 Leaching of NMC811 . . . . . . . . . . . . . . . . . . . . . . . 2790 4.3 Effect of acid concentration . . . . . . . . . . . . . . . . . . . . . . . 2891 4.3.1 Effect of solid-to-liquid ratio . . . . . . . . . . . . . . . . . . . 2992 4.3.1.1 Leaching of LCO . . . . . . . . . . . . . . . . . . . . 2993 4.3.1.2 Leaching of NMC111 . . . . . . . . . . . . . . . . . . 3094 4.3.1.3 Leaching of NMC622 . . . . . . . . . . . . . . . . . . 3295 4.3.1.4 Leaching of NMC811 . . . . . . . . . . . . . . . . . . 3396 4.3.2 Effect of current collectors . . . . . . . . . . . . . . . . . . . . 3597 4.3.2.1 Leaching of LCO . . . . . . . . . . . . . . . . . . . . 3598 4.3.2.2 Leaching of NMC111 . . . . . . . . . . . . . . . . . . 3699 4.3.2.3 Leaching of NMC622 . . . . . . . . . . . . . . . . . . 38100 4.3.2.4 Leaching of NMC811 . . . . . . . . . . . . . . . . . . 40101 4.4 Determination of optimal amount and addition strategy for hydrogen102 peroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41103 4.4.1 Pre-determination of the optimal hydrogen peroxide volume104 percentage (%v/v) for different cathode materials . . . . . . . 41105 4.4.2 Determination of the optimal addition strategy for hydrogen106 peroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44107 4.4.2.1 Determination of hydrogen peroxide consumption . . 44108 4.4.2.2 Determination of leaching efficiency . . . . . . . . . . 47109 4.4.2.2.1 Leaching of LCO . . . . . . . . . . . . . . . 47110 4.4.2.2.2 Leaching of NMC111 . . . . . . . . . . . . . 48111 4.4.2.2.3 Leaching of NMC622 . . . . . . . . . . . . . 49112 4.4.2.2.4 Leaching of NMC811 . . . . . . . . . . . . . 50113 4.5 Determination of leaching efficiency and hydrogen peroxide consump-114 tion in the presence of copper and aluminium foils . . . . . . . . . . . 51115 4.5.1 Determination of hydrogen peroxide consumption . . . . . . . 51116 4.5.1.1 Leaching of LCO . . . . . . . . . . . . . . . . . . . . 52117 4.5.1.2 Leaching of NMC111 . . . . . . . . . . . . . . . . . . 53118 4.5.1.3 Leaching of NMC622 . . . . . . . . . . . . . . . . . . 54119 4.5.1.4 Leaching of NMC811 . . . . . . . . . . . . . . . . . . 55120 4.5.2 Determination of leaching efficiency . . . . . . . . . . . . . . . 55121 4.5.2.1 Leaching of LCO . . . . . . . . . . . . . . . . . . . . 55122 4.5.3 Leaching of NMC111 . . . . . . . . . . . . . . . . . . . . . . . 56123 4.5.4 Leaching of NMC622 . . . . . . . . . . . . . . . . . . . . . . . 57124 4.5.5 Leaching of NMC811 . . . . . . . . . . . . . . . . . . . . . . . 57125 4.6 Testing the optimal conditions on the real NMC cathode waste material 58126 5 Conclusion 61127 Bibliography 63128 A Appendix 1 I129 A.1 Calculation to find limiting reagent of the reaction at the condition130 of solid-to-liquid ratio of 1:10 g/ml . . . . . . . . . . . . . . . . . . . I131 x Contents A.1.1 LCO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II132 A.1.2 NMC111 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II133 A.1.3 NMC622 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II134 A.1.4 NMC811 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III135 A.2 Calculation of theoretical hydrogen peroxide needed at the condition136 of solid-to-liquid ratio of 1:20 g/ml . . . . . . . . . . . . . . . . . . . III137 A.2.1 LCO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV138 A.2.2 NMC111 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV139 A.2.3 NMC622 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V140 A.2.4 NMC811 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V141 xi Contents xii List of Figures142 2.1 Schematic illustration of a lithium ion battery showing charge/dis-143 charge processes [15]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4144 2.2 Crystal structure of the three lithium-insertion compounds in which145 the Li+ ions are mobile through the 2-D (layered), 3-D (spinel) and146 1-D (olivine) frameworks [28]. . . . . . . . . . . . . . . . . . . . . . . 6147 2.3 A general flowsheet for the combined recycling process [44]. . . . . . . 8148 2.4 Suitable extractants for extracting nickel, cobalt, and copper at dif-149 ferent pHs [60–65]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13150 4.1 Leaching of LCO: Influence of temperature (reaction conditions: 2 M151 H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). . . . 19152 4.2 Leaching of NMC111: Influence of temperature (reaction conditions:153 2 M H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). 20154 4.3 Leaching of NMC622: Influence of temperature (reaction conditions:155 2 M H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). 21156 4.4 Leaching of NMC811: Influence of temperature (reaction conditions:157 2 M H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). 23158 4.5 Leaching efficiency after 60 minutes leaching of all cathode materials:159 Influence of temperature (reaction conditions: 2 M H2SO4, no H2O2,160 solid-liquid ratio of 1:100 (50 mL solution)). . . . . . . . . . . . . . . 24161 4.6 Leaching of LCO: Influence of mixing (reaction conditions: 2 M162 H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:100 (10 and 50163 mL solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25164 4.7 Leaching of NMC111: Influence of mixing (reaction conditions: 2 M165 H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:100 (10 and 50166 mL solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26167 4.8 Leaching of NMC622: Influence of mixing (reaction conditions: 2 M168 H2SO4, T=50◦C, no H2O2, solid-t0-liquid ratio of 1:100 (10 and 50169 mL solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27170 4.9 Leaching of NMC811: Influence of mixing (reaction conditions: 2 M171 H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:100 (10 and 50172 mL solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28173 4.10 Leaching of LCO: Influence of solid-to-liquid ratio (reaction condi-174 tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:10,175 1:20, and 1:100 (10 mL solution)). . . . . . . . . . . . . . . . . . . . . 30176 xiii List of Figures 4.11 Leaching of NMC111: Influence of solid-to-liquid ratio (reaction con-177 ditions: 2 M H2SO4, T=50◦C, no H2O2, solid-liquid ratio of 1:10,178 1:20, and 1:100 (10 mL solution)). . . . . . . . . . . . . . . . . . . . . 31179 4.12 Leaching of NMC622: Influence of solid-to-liquid ratio (reaction con-180 ditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:10,181 1:20, and 1:100 (10 mL solution)). . . . . . . . . . . . . . . . . . . . . 32182 4.13 Leaching of NMC811: Influence of solid-to-liquid ratio (reaction con-183 ditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:10,184 1:20, and 1:100 (10 mL solution)). . . . . . . . . . . . . . . . . . . . . 34185 4.14 Leaching of all materials: Influence of solid-to-liquid ratio (reaction186 conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of187 1:100, 1:20, and 1:10, 10 mL solution, and 60 minutes leaching time). 35188 4.15 Leaching of LCO: Influence of current collectors (reaction conditions:189 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10 mL190 solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36191 4.16 Leaching of NMC111: Influence of current collectors (reaction condi-192 tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10193 mL solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37194 4.17 Leaching of NMC622: Influence of current collectors (reaction condi-195 tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10196 mL solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39197 4.18 Leaching of NMC811: Influence of current collectors (reaction condi-198 tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10199 mL solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40200 4.19 Leaching solution of all materials with no addition of current collec-201 tors (reaction conditions: 2 M H2SO4, T=50◦C, with H2O2, solid-to-202 liquid ratio of 1:20 (10 mL solution)). . . . . . . . . . . . . . . . . . . 43203 4.20 Leaching solution of all materials with an addition of current collec-204 tors (reaction conditions: 2 M H2SO4, T=50◦C, with H2O2, solid-to-205 liquid ratio of 1:20 (10 mL solution)). . . . . . . . . . . . . . . . . . . 43206 4.21 Leaching of LCO: Influence of H2O2 addition strategy on residual207 H2O2 concentration (reaction conditions: 2 M H2SO4, T=50◦C, 7%v/v208 H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . . . . . . 45209 4.22 Leaching of NMC111: Influence of addition strategy on residual H2O2210 concentration (reaction conditions: 2 M H2SO4, T=50◦C, 3%v/v211 H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . . . . . . 45212 4.23 Leaching of NMC622: Influence of addition strategy on residual H2O2213 concentration (reaction conditions: 2 M H2SO4, T=50◦C, 4%v/v214 H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . . . . . . 46215 4.24 Leaching of NMC811: Influence of addition strategy on residual H2O2216 concentration (reaction conditions: 2 M H2SO4, T=50◦C, 3%v/v217 H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . . . . . . 47218 xiv List of Figures 4.25 Leaching of LCO: Influence of H2O2 addition strategy on leaching219 efficiency (reaction conditions: 2 M H2SO4, T=50◦C, 7%v/v H2O2,220 solid-to-liquid ratio of 1:20 (40 mL solution)). Thick lines represent221 the case when all H2O2 was added at the beginning and thin lines222 represent the case when H2O2 was added at multiple steps. . . . . . . 48223 4.26 Leaching of NMC111: Influence of H2O2 addition strategy on leaching224 efficiency (reaction conditions: 2 M H2SO4, T=50◦C, 3%v/v H2O2,225 solid-to-liquid ratio of 1:20 (40 mL solution)). Thick lines represent226 the case when all H2O2 was added at the beginning and thin lines227 represent the case when H2O2 was added at multiple steps. . . . . . . 49228 4.27 Leaching of NMC622: Influence of H2O2 addition strategy on leaching229 efficiency (reaction conditions: 2 M H2SO4 T=50◦C, 4%v/v H2O2,230 solid-to-liquid ratio of 1:20 (40 mL solution)). Thick lines represent231 the case when all H2O2 was added at the beginning and thin lines232 represent the case when H2O2 was added at multiple steps. . . . . . . 50233 4.28 Leaching of NMC811: Influence of H2O2 addition strategy on leaching234 efficiency (reaction conditions: 2 M H2SO4, T=50◦C, 3%v/v H2O2235 solid-to-liquid ratio of 1:20 (40 mL solution)). Thick lines represent236 the case when all H2O2 was added at the beginning and thin lines237 represent the case when H2O2 was added at multiple steps. . . . . . . 50238 4.29 Leaching of LCO: Influence of the addition of current collectors on239 residual H2O2 concentration (reaction conditions: 2 M H2SO4, T=50◦C,240 7%v/v (no current collectors added) and 8%v/v (current collectors241 added) H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . 52242 4.30 Leaching of NMC111: Influence of the addition of current collectors243 on residual H2O2 concentration (reaction conditions: 2 M H2SO4,244 T=50◦C, 3%v/v H2O2 (both with and without current collectors),245 solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . . . . . . . . . 53246 4.31 Leaching of NMC622: Influence of the addition of current collectors247 on residual H2O2 concentration (reaction conditions: 2 M H2SO4,248 T=50◦C, 4%v/v (no current collectors added) and 6%v/v (current249 collectors added) H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). 54250 4.32 Leaching of NMC811: Influence of the addition of current collectors251 on residual H2O2 concentration (reaction conditions: 2 M H2SO4,252 T=50◦C, 3%v/v (no current collectors added) and 3.5%v/v (current253 collectors added) H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). 55254 4.33 Leaching of LCO: Influence of the addition of current collectors on255 leaching efficiency (reaction conditions: 2 M H2SO4, T=50◦C, 8%v/v256 H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . . . . . . 56257 4.34 Leaching of NMC111: Influence of the addition of current collectors258 on leaching efficiency (reaction conditions: 2 M H2SO4, T=50◦C,259 3%v/v H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . 56260 4.35 Leaching of NMC622: Influence of the addition of current collectors261 on leaching efficiency (reaction conditions: 2 M H2SO4, T=50◦C,262 6%v/v H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . . 57263 xv List of Figures 4.36 Leaching of NMC811: Influence of the addition of current collectors264 on leaching efficiency (reaction conditions: 2 M H2SO4, T=50◦C,265 3.5%v/v H2O2, solid-to-liquid ratio of 1:20 (40 mL solution)). . . . . 58266 4.37 Leaching of black mass: Leaching efficiency (reaction conditions: 2267 M H2SO4, T=50◦C, 3%v/v H2O2, solid-to-liquid ratio of 1:20 (40 mL268 solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59269 4.38 Leaching of black mass: Residual H2O2 concentration (reaction con-270 ditions: 2 M H2SO4, 3%v/v H2O2, solid-to-liquid ratio of 1:20 (40 mL271 solution)). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60272 xvi List of Tables273 2.1 Specific energy (energy density) of commercialized cathode materials274 [21–23]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5275 2.2 Material energy density (mAh/g) of LCO and different NMC cathode276 materials [31]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6277 2.3 Summary of related literature about leaching process. . . . . . . . . . 10278 2.4 Summary of related literature about sulfuric acid leaching process279 cooperated with hydrogen peroxide. . . . . . . . . . . . . . . . . . . . 12280 3.1 Selected wavelengths in ICP-OES analysis . . . . . . . . . . . . . . . 16281 4.1 Theoretical of 2 M H2SO4 needed per gram of each cathode material282 and the amount of H2SO4 added at different solid-to-liquid ratios. . . 29283 4.2 Copper and aluminum concentration in leachate and percent recovery284 after 45 minutes leaching. . . . . . . . . . . . . . . . . . . . . . . . . 41285 4.3 The theoretical volume and concentration of H2O2 (59 wt%) needed. 42286 4.4 The volume percentage of H2O2 needed to fully dissolve the cathode287 materials and addition time of H2O2 (59% of H2O2 was used). . . . . 42288 4.5 The residual amount of H2O2 after leaching for 60 minutes with and289 without current collectors. . . . . . . . . . . . . . . . . . . . . . . . . 44290 4.6 The amount of H2O2 used in the leaching trials (in %v/v and g/L). . 51291 4.7 Black mass composition . . . . . . . . . . . . . . . . . . . . . . . . . 59292 xvii List of Tables xviii 1293 Introduction294 Nowadays, transportation around the world predominantly relies on fossil-based295 fuel which is the main source of CO2 emissions in the recent decades. The more296 environmentally friendly technologies are emerging from concern of environmental297 issues from the emission of conventional vehicles. The lithium-ion secondary bat-298 teries are crucial for many electrical devices including electric vehicles (EVs) due299 to their compactness and lightweight. Lithium ions have a small size that can pro-300 mote the ability to intercalate in both electrodes. So, lithium-ion batteries have301 higher energy density compared to other types of a battery such as Nickel Cad-302 mium (Ni-Cd) and Nickel-metal hydride (Ni-MH) [1]. An increase in demand for303 Li-ion batteries (LiBs) is reflected as a Compound Annual Growth Rate (CAGR).304 The battery’s market is expected to grow approximately at a CAGR of 12.31%305 during 2019-2024 [2]. The mechanism and structure of a lithium-ion battery are306 fairly simple. There are electrochemical cells connected in series or parallel and307 each cell has a negative and a positive electrode which are divided by an electrolytic308 solution and a porous separator [3]. In the battery compartment, the electrical en-309 ergy will be generated by the conversion of chemical energy via redox reactions at310 the cathode and anode. The working principle has two modes which are charging311 and discharging. There are many possible metal composition types of cathode ma-312 terials for lithium ion batteries: Lithium Cobalt Oxide (LiCoO2, LCO), Lithium313 Manganese Oxide (LiMn2O4)/Li2MnO3/LiMnO2/Li2MnO2, LMO), Lithium Iron314 Phosphate (LiFePO4, LFP), Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2,315 NCA), and Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2, NMC) [4]. The316 strategic metal lithium is becoming an essential material for greener technology in317 the future. Due to the high demand of lithium for the lithium-ion batteries man-318 ufacturing, the worldwide mining production of lithium increased 13% in 2017 [5].319 Although lithium is a strategic metal, the valuable metals contained in the cathode320 are cobalt, nickel, and to some extent manganese. In addition to lithium, cobalt is321 one of the main components in spent LiBs (5-20 wt.%) and as high as 25% of the322 cobalt produced globally is found in LiBs [6]. The EU has identified that cobalt is323 a critical raw material due to limited reserve and many strategic and irreplaceable324 industrial uses [7]. Cobalt is the most expensive metal among others as the price is325 30,000 USD/MT [8]. Therefore, recovering of cobalt can definitely return benefits in326 terms of material depletion and economics. There are some issues with lithium-ion327 batteries. Since the average life cycle of the battery is relatively short, only around328 10 years, several hundred thousand tons of batteries are disposed annually within329 EU [9, 10]. In 2017, only 46% of the batteries sold in the EU were collected for330 recycling and the rest undergoes inadequate disposal that can lead to environmen-331 1 1. Introduction tal problems [11]. The rapid growth of battery demand also affects future battery332 production since lithium can be considered as a scarce natural resource and it is333 expected to be totally mined out by 2050. Therefore, in the near future valuable/s-334 carce materials such as lithium, cobalt, nickel and manganese should be recycled in335 order to reduce the impact of raw material depletion [9].336 2 2337 Theory338 2.1 Main components in batteries339 The battery is an electrochemical cell that can be connected in parallel or series.340 Each cell contains four main components which are electrode, electrolyte, separa-341 tor, and current collector. The positive and negative electrode are separated by342 an electrolyte that allows the transfer of positive ions from one electrode to an-343 other. According to Figure 2.1, there are two working modes that are charging344 and discharging. During the charging process, lithium ions will be released from the345 cathode to the anode via the electrolyte, free electrons will form and flow through an346 external circuit to a negative collector at the anode. During the discharging process,347 the flow of lithium ions will occur in the opposite direction. The performance of the348 battery depends on the battery chemistry and material of each component in the349 compartment. According to European Portable Battery Association (EPBA), spent350 LiBs (LCO chemistry) are composed of Aluminium —15-25%, Carbon, amorphous,351 powder —0.1-1%, Copper foil —5-15%, Diethyl Carbonate (DEC) —1-10%, Ethy-352 lene Carbonate (EC) —1-10%, Methyl Ethyl Carbonate (MEC) —1-10%, Lithium353 Hexafluorophosphate (LiPF6) —1-5%, Graphite, powder —10-30%, Lithium Cobalt354 Oxide (LCO) —25-45%, Poly (vinylidene fluoride) (PVDF) —0.5-2%, steel, nickel355 and inert polymer [3, 12–14].356 3 2. Theory Figure 2.1: Schematic illustration of a lithium ion battery showing charge/dis- charge processes [15]. 2.1.1 Anode357 The anode is a negative electrode that tends to lose electrons and forms positive358 ions. The generated electrons will flow through an external circuit. There are many359 types of anodes that are used commercially and the anode can be composed either360 of carbon/graphite or non-carbon materials such as transition metal oxides [16–18].361 The anode has a significant impact on improving the energy density of a lithium-ion362 cell, therefore the anode should fulfill the following requirements [16].363 • Long cycle life.364 • High rate capability and low potential against cathode material.365 • High reversible gravimetric and volumetric capacity.366 • The material must be low cost and environmentally friendly.367 Commonly used anode materials are carbon-based graphite which has high order and368 micro-structure texture. Moreover, the carbonaceous material has low cost and low369 operational voltage which are very important factors for batteries. Those properties370 allow lithium to form the intercalated compound as shown in Equation 2.1 [19].371 xM(s) + Li+ + e- LiMx(s) (2.1) 372 373 The graphite structure can store up to one Li+ for every six carbon atoms between374 each graphene layer. The theoretical specific capacity of graphite is 372 mAhg-1 375 [20]. The ability of graphite to intercalate anions promotes the use of graphite376 for rechargeable batteries and graphite has excellent properties compared to other377 anodes.378 4 2. Theory 2.1.2 Cathode379 The cathode is a positive electrode comprised of active materials with different380 natures and is usually composed of lithium-containing materials. There are many381 lithium-based cathode materials that are commercialized as shown in Table 2.1, each382 cathode material has a different specific energy. NMC, NCA, and LFP cathode383 chemistries are produced in higher amounts compared to LCO, but LCO is the384 most common type of battery that is used in various applications, however it also385 has drawbacks such as a high environmental risk. The effective way to improve the386 performance of the battery is to make the cathode fulfill the following requirements.387 • The material must contain a readily reducible/oxidizable ion.388 • The material must react with lithium very rapidly on both lithium insertion389 and removal to give high power.390 • The material must be low cost and environmentally friendly.391 • The material should be a good electronic conductor.392 Table 2.1: Specific energy (energy density) of commercialized cathode materials [21–23]. Cathode Specific Energy (Wh/kg) Li(NiCoAl)O2 (NCA) 230 Li(CoxNiyMnz)O2 (NMC) 200 LiCoO2 (LCO) 180 LiMn2O4 (LMO) 120 LiFePO4 (LFP) 110 Li2TiO3 (LTO) 65 The two cathode materials that this study focused on are LCO (typically used in393 portable electronics) and NMC (used in electrical vehicles).394 2.1.2.1 Lithium Cobalt Oxide (LiCoO2)(LCO)395 LCO was the first cathode that was introduced since 1980 by Oxford University and396 Tokyo University’s Koichi Mizushima and commercialized by Sony Corporation in397 1991 [24]. There are two types of LCO which are low temperature (LT-LCO) rep-398 resenting in cubic form and high temperature (HT-LCO) representing in hexagonal399 form as shown in Figure 2.2. The structure can also be described as transition metal400 oxide layers separated by layers of Li+ ions, represented by green dots in Figure 2.2.401 The crystallinity of structure is the important feature in achieving high-performance402 rechargeable batteries, that is as high specific capacity, low self-discharge, and ex-403 cellent cycle life as possible [25–27]. The important role of cobalt is to stabilize the404 cathode structure but it is costly and less available compared to other transition405 metals like nickel and manganese. Therefore, the further development of the cath-406 ode will focus on cheaper material such as using nickel and manganese instead of407 cobalt in order to reduce the cost and also environmental impact.408 5 2. Theory Figure 2.2: Crystal structure of the three lithium-insertion compounds in which the Li+ ions are mobile through the 2-D (layered), 3-D (spinel) and 1-D (olivine) frameworks [28]. 2.1.2.2 Lithium Nickel Manganese Cobalt Oxides409 (Li(CoxNiyMnz)O2)(NMC)410 NMC is the nickel rich cathode where x+y+z is equal to 1. There are many possi-411 ble ratios of cobalt, nickel and manganese such as LiNi1/3Mn1/3Co1/3O2 (NMC111),412 LiNi0.6Mn0.2Co0.2O2 (NMC622), and LiNi0.8Mn0.1Co0.1O2 (NMC811). According to413 Table 2.2, the capacity has been improved by the increase of nickel content. The ca-414 pacity of NMC811 is increased by almost 31% compared to NMC111. The mixtures415 of cobalt, nickel, and manganese are designed to combine the specialty properties of416 each material together with minimizing the drawbacks. A nickel rich composition417 can improve the energy density of lithium-ion batteries significantly [29]. Moreover,418 as stated in Ozhuku et al. [30], manganese can improve thermal stability, also the419 addition of some aluminum such as in the NCA cathode material is expected to420 exhibit more thermal stability and longer cycling life.421 Table 2.2: Material energy density (mAh/g) of LCO and different NMC cathode materials [31]. Cathode material Capacity (mAh/g) LCO 199.3 NMC111 154.8 NMC622 175.8 NMC811 203.4 2.1.3 Current collector and separator422 The current collectors allow electrons to transport to (or from) the electrodes and423 they are located on the external surface of the electrodes. During the discharge424 process, it will collect charges that are generated during charging and the current425 collectors will permit the connection to an external circuit source [32]. The impor-426 tant role of current collectors is to enhance electron transfer. The current collectors427 are usually made from inexpensive metals and mostly in the form of a thin foil to428 improve the adhesive property. Materials used for positive and negative current429 6 2. Theory collector will be different due to corrosive property. For that reason, copper foil is430 used for the anode and aluminum foil for the cathode.431 The separator is one of the important components of a LiB cell that is not432 involved in the electrochemical reactions (i.e., it is an isolator with no electrical433 conductivity). The main function is to physically separate positive and negative434 electrodes from each other while still allow the transport of ions [33]. Moreover, the435 separator prevents electrical short circuit in the cell [34]. In theory, the separator436 should have zero ionic resistance, but in practical, low ionic resistance is acceptable437 [35]. Separator can be classified into three types; porous membrane, composite sepa-438 rator, and non-woven mat. Nowadays, the most common separator for non-aqueous439 electrolytes is a porous membrane made of polyethylene (PE) and/or polypropylene440 (PP) [36].441 2.1.4 Electrolyte442 The electrolyte is one important key that determines the function of a battery. The443 electrolyte is a medium that allows the transport of ions in order to convert chemical444 energy to electrical energy, that is a high ionic conductivity is important [37]. The445 electrolyte is usually a lithium containing material in order to facilitate the transfer446 of lithium ions. The typical non-aqueous electrolyte for commercial batteries is a447 solution of LiPF6 salt in ethylene carbonate solvent together with additives with no448 more than 5% of the composition [38–40]. The electrolyte additives can improve449 the performance by providing higher conductivity. Moreover, the electrolyte can450 influence the recycling process since it spreads throughout the pores of electrodes451 and separators. Therefore, the types of electrolyte is crucial since the electrolyte is452 very sensitive to other components such as electrodes. During the battery operation,453 a solid electrolyte interphase (SEI) will be formed on the graphite surface that454 compete with the reversible lithium intercalation and this determines the long-term455 performance of LiBs such as safety, power capability, shelf life, and cycle life [41,456 42]. SEI is formed due to electrolyte decomposition after the first cycle [43]. The457 formation of SEI consumes electrolytes and also reduces the battery capacity. The458 formed SEI should prevent electrolyte decomposition and should act as a good ionic459 conductor to help facilitate the transport of lithium ions. Therefore, the electrolyte460 must be designed to contain at least one material that reacts with lithium under the461 formation of an insoluble electrolyte interphase.462 2.2 Processes for recycling Li-ion batteries463 Generally, there are four methods to recycle spent Li-ion batteries (LiBs): mechani-464 cal treatment, hydrometallurgical treatment, a combination of thermal pretreatment465 and hydrometallurgical treatment, or pyrometallurgical treatment. In the pyromet-466 allurgical process, a high temperatures (above 900◦C) is used and organic compounds467 and graphite are burned, but the process can handle a large volume of waste with-468 out any requirement of mechanical pretreatment. However, the disadvantage is that469 the metals cannot be fully recovered, some of them are usually left in the slag.470 To recover these metals, an additional hydrometallurgical process is needed. On471 7 2. Theory the other hand, hydrometallurgical processes are able to recover valuable metals at472 high purity and high recycle rate and the hydrometallurgical process is more en-473 vironmentally friendly than the pyrometallurgical processes due to applying lower474 temperatures, low energy demand, and less emission of hazardous gases. Due to475 its great performances, hydrometallurgical processes are therefore of interest when476 developing recycling schemes for spent LiBs and this study will focus on dissolution477 of LCO and NMC cathode materials in the presence of the reducing agent hydrogen478 peroxide.479 2.2.1 Hydrometallurgical process480 Hydrometallurgical processes are used to recover valuable metals, such as cobalt and481 lithium, from spent LiBs. There are some pretreatment methods before a hydromet-482 allurgical separation processes can be applied and these are mechanical and thermal483 pretreatments. Thermal pretreatment is also implemented in order to remove or-484 ganic compounds such as binders which can cause problems in further separation485 steps. The process that combines both thermal pretreatment and hydrometallurgi-486 cal processing is known as the combined process. It starts with discharging the spent487 batteries, followed by dismantling, mechanical pretreatment, thermal pretreatment,488 and separation stages followed by the hydrometallurgical process. Hydrometallurgi-489 cal separation processes are for example leaching, solvent extraction, precipitation,490 ion exchange, etc. A general flowsheet of the combined process is shown in Figure491 2.3. However, difference recycling schemes can be seen at different companies.492 Figure 2.3: A general flowsheet for the combined recycling process [44]. 8 2. Theory 2.2.1.1 Pretreatment steps493 As mentioned above, the composition of LiBs is very complex and LiBs cannot494 be recycled directly. Therefore, several pretreatments are necessary to separate all495 particular components and proper management for each component can possibly496 be applied. The pretreatments consist of discharging, dismantling, mechanical, and497 thermal treatments.498 Discharging499 The aim of discharge is to discard the remaining capacity (remove the stored energy)500 in spent LiBs in order to prevent short-circuits and self-ignition. There are different501 discharging methods. The most common is to immerse spent LiBs in a 5 wt% NaCl502 solution to let them completely discharged [45, 46].503 Dismantling and mechanical pretreatment504 These processes are essential for large batteries such as car batteries. Dismantling505 is performed after discharging by removing battery cells and other components.506 Other components, which are cables, printed circuit boards, and casing, will be sent507 to recycling facilities for reprocessing in a proper way. Manual dismantling can508 be applied to large battery size and required several tools such as pincers, knives,509 and saws. Manual dismantling is not feasible for small LiBs where a mechanical510 pretreatment could be applied instead [47]. Methods of mechanical pretreatment511 involve crushing, sieving, magnetic separation, and classification which is to obtain512 a fraction enriched in the cathode active material (black mass). After crushing and513 sieving, the separator, aluminum foils, copper foils, and plastics are mainly in the514 coarse particles while the electrode materials, for instance LCO, and graphite, end515 up in fine fraction [48]. Magnetic separation is used to separate steel out based on516 their different magnetic properties.517 At the same time, the black mass will be sent to battery recycling facili-518 ties in order to separate cathode materials from current collectors (Cu/Al foils)519 by dissolving the organic binders which are between them. The organic binders520 are made of polyvinylidene difluoride (PVDF) and organic solvents such as N,N-521 dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), N-methylpyrrolidone522 (NMP), and dimethylsulfoxide (DMSO) that are used to dissolve PVDF [44] because523 they are all polar and can easily dissolve together. However, the dissolution process524 by organic solvents cannot remove all impurities. The process is also costly because525 of expensive solvents and not suitable for large scale. A calcination process which526 is a thermal process may be needed to remove (decompose) residues.527 Thermal pretreatment528 Additionally, thermal pretreatment is another method that could also be performed529 to remove organic compounds (e.g., PVDF). It is done by burning organic com-530 pounds at high temperature (typically in the range of 500-1150◦C) in a furnace [49].531 This process is simple and suitable for a high loaded sample but it emits toxic gases532 and smoke during the process which are needed to be controlled.533 2.2.1.2 Leaching534 After the black mass is obtained from the previous pretreatment steps, hydrometal-535 lurgical processes can be applied. The leaching process is the first step in hydromet-536 9 2. Theory allurgical processing. It is a process that extracts a certain soluble material from a537 solid by using solvent [50], in this case, the valuable metals presented in the black538 mass will be recovered as metal ions in the leachate. There are different types of539 leaching processes proposed for battery recycling, for example, leaching with inor-540 ganic acids, organic acids, bioleaching, and so on. In this work, the leaching using541 inorganic acids will be explored. The most common inorganic acids used are sulfuric542 acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3). HCl gives higher543 leaching efficiency over the others [51, 52]. Nevertheless, once HCl is used, Cl2 gas544 will be produced as can be seen in Equation 2.2 (example LCO).545 2LiCoO2 + 8HCl 2CoCl2 + Cl2 + 2LiCl + 4H2O (2.2) Chlorine gas poses an environmental problem and requires high corrosion re-546 sistance equipment which leads to higher recycling costs. The use of H2SO4 as an547 inorganic acid will be studied in this work. In addition, the leaching efficiency has548 also been shown to increase with the use of a reducing agent like hydrogen peroxide549 (H2O2). The main purpose with addition of a reducing agent is to change the valence550 state of the metals used in the cathode active material into a more soluble state (e.g.551 Co3+ to Co2+) and by that increases the leaching efficiency. Oxygen is generated552 as a consequence of the reaction between hydrogen peroxide and the black mass.553 In the presence of H2O2 in H2SO4 leaching, the cobalt leaching efficiency can be554 increased by 13% [53]. Incorporation of H2SO4 and H2O2 yields leaching efficiency555 of valuable metals as high as when using HCl [54, 55]. Table 2.3 summarizes the556 related literature about leaching processes, optimum leaching conditions, and metal557 recovery yields.558 Table 2.3: Summary of related literature about leaching process. Leaching media S:L (g/L) Temper- ature (°C) Leaching time (hr) Co leaching efficiency (%) Li leaching efficiency (%) Ref- er- ence 2M H2SO4, 5vol% H2O2 50 80 1 99 99 [54] 4M HCl 20 80 1 99.5 99.9 [55] 1M H2SO4 50 95 4 66.2 93.4 [53] 1M H2SO4, 5vol% H2O2 50 95 4 79.2 94 [53] The proposed sulfuric acid leaching reactions for the different cathode materials559 studied, without any reducing agent present, are shown as Equations 2.3, 2.4, 2.5,560 and 2.6 which represent LCO, NMC111, NMC622 and NMC811, respectively.561 4LiCoO2(s)+6H2SO4(aq) 4CoSO4(aq)+2Li2SO4(aq)+6H2O(l)+O2(g) (2.3) 10 2. Theory 12LiNi0.33Mn0.33Co0.33O2(s) + 18H2SO4(aq) 4NiSO4(aq) + 4CoSO4(aq) +4MnSO4 + 6Li2SO4(aq) +9H2O(l) + 3O2(g) (2.4) 20LiNi0.6Mn0.2Co0.2O2(s) + 30H2SO4(aq) 12NiSO4(aq) + 4CoSO4(aq) +4MnSO4 + 10Li2SO4(aq) +30H2O(l) + 5O2(g) (2.5) 20LiNi0.8Mn0.1Co0.1O2(s) + 30H2SO4(aq) 16NiSO4(aq) + 2CoSO4(aq) +2MnSO4 + 10Li2SO4(aq) +30H2O(l) + 5O2(g) (2.6) Moreover, the corresponding leaching reactions using sulfuric acid as leaching562 agent and the addition of H2O2 as reducing agent are shown below.563 2LiCoO2 + 3H2SO4 +H2O2 2CoSO4 + Li2SO4 + 4H2O +O2 (2.7) 6LiNi0.33Mn0.33Co0.33O2(s) + 9H2SO4(aq) 2NiSO4(aq) + 2CoSO4(aq) +H2O2 +2MnSO4 + 3Li2SO4(aq) +10H2O(l) + 2O2(g) (2.8) 10LiNi0.6Mn0.2Co0.2O2(s) + 15H2SO4(aq) 6NiSO4(aq) + 2CoSO4(aq) +H2O2 +2MnSO4 + 5Li2SO4(aq) +16H2O(l) + 3O2(g) (2.9) 40LiNi0.8Mn0.1Co0.1O2(s) + 60H2SO4(aq) 32NiSO4(aq) + 4CoSO4(aq) +2H2O2 +4MnSO4 + 20Li2SO4(aq) +62H2O(l) + 11O2(g) (2.10) As mentioned before, hydrogen peroxide can be an effective reducing agent to564 leach valuable metals from spent batteries. Table 2.4 summarizes some literature565 that describes the effect of hydrogen peroxide on the leaching with hydrogen perox-566 ide.567 11 2. Theory Table 2.4: Summary of related literature about sulfuric acid leaching process co- operated with hydrogen peroxide. Ma- te- rial Leaching condition Optimal amount of H2O2 H2O2 adding pattern Co leaching efficiency (%) Li leaching efficiency (%) Ref- er- ence Spent LiBs 4 M H2SO4, S:L=1:10, T=85°C 10 vol% Initial adding 95 96 [56] Spent LiBs 2 M H2SO4, S:L=1:20, T=75°C 10 vol% Initial adding 80 99 [57] Spent LiBs 2 M H2SO4, S:L=1:10, T=75°C 5 vol% Initial adding 70 99.1 [58] As a whole, most of the literature describes the sulfuric acid leaching with568 addition of H2O2 as a reducing agent where the proposed amount of H2O2 was added569 at the beginning of the leaching process. However, no one has studied the remaining570 amount of hydrogen peroxide that might be still left after leaching and different571 hydrogen peroxide adding strategies. The novelty of this study is to investigate the572 residual amount of hydrogen peroxide as well as assess different hydrogen peroxide573 addition strategies.574 2.2.1.3 Solvent extraction and precipitation575 After the metals such as Li, Co, Ni, Mn, Cu, Al, Fe are leached from the cathode576 active material, they still need to be recovered from the leachate. Other hydromet-577 allurgical methods, such as solvent extraction, precipitation, and electrochemical578 deposition, are needed in order to separate and recover these metals. Compared to579 precipitation, solvent extraction has a better separation effect due to its selectivity580 of extractants. Since the leaching solution is complex, more than one technique581 could be used to separate pure metals effectively [59].582 Solvent extraction583 Basically, the process consists of two immiscible liquid phases: organic and aqueous584 phases. It involves two operations: Extraction and stripping. Extraction refers to585 when the metals in the aqueous phase are transferred to the organic phase where586 the metals are more soluble into. Extraction is followed by stripping where the587 extracted metals are recovered from the organic phase to another strip solution [50].588 There are several extractants that could be used depending on the selectiv-589 ity of the desired metal and operating pH. Examples of extractants are di-(2-590 ethylhexyl) phosphoric acid (D2EHPA), diethylhexyl phosphoric acid (DEHPA),591 bis-(2,4,4-tri-methyl-pentyl) phosphinic acid (Cyanex 272), trioctylamine (TOA),592 and 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A) [47]. Figure593 2.4 summarizes suitable extractants and the operating pH to extract a specific metal594 ion. Some of the extractants can recover more than one metal ion by using several595 12 2. Theory stages in a series with different extractants. According to [60], D2EHPA was good596 at extracting copper and manganese ions at a pH range of 2.6-2.7 and PC-88A was597 then used to recover cobalt and nickel ions at pH 4.5.598 Figure 2.4: Suitable extractants for extracting nickel, cobalt, and copper at differ- ent pHs [60–65]. Precipitation599 Precipitation is an alternative to recover metals. Precipitants containing anions such600 as OH−, C2O4 2− and CO3 2− are added into a leaching solution and then anions will601 attract to metal cations and form insoluble precipitates which can easily be extracted602 out. Sometimes, one metal ion is hard to be precipitated when other metal also has603 the same valence state, such as Co2+ and Ni2+, and the coprecipitation of both604 metals can occur. As a result, [51] showed that Co2+ in the leaching liquor was605 oxidized to Co3+ by adding sodium hypochlorite (NaClO) to recover Co2O3· 3H2O606 by selective precipitation and nickel hydroxide is further precipitated by addition of607 a base. Moreover, precipitation could usually be used before solvent extraction in608 order to remove impurities, such as aluminum, copper, and iron, that will hinder609 the separation of cobalt in the solvent extraction process [44].610 According to all mentioned recycling processes, the final recovered metals can611 possibly be reused in batteries or in other applications to move toward the circular612 economy promoted by restrictive environmental regulations and limited natural re-613 sources. In 2018, the recycling of spent NMC523 and LFP can be profitable based614 on China’s background with the profits of 2256 Euro/ton and 436 Euro/ton, respec-615 tively [66]. However, today’s recycling methods need to be improved to save energy,616 chemicals and time required as much as possible in order to be more cost-effective617 and more accessible.618 13 2. Theory 2.3 Aim and objective619 Due to a rapid increase in the demand of Li-ion batteries, stockpiles of spent bat-620 teries have been produced globally. The hydrometallurgical process is an interesting621 recycling method that is environmentally-friendly and effectively recovers valuable622 metals but it is not cost-effective. In order to make it economically feasible, the623 process parameters should be optimized. The aim and objective of this study is to:624 • Determine the effect of adding a reducing agent, which is hydrogen peroxide,625 on different types of battery active materials (LCO, NMC111, NMC622, and626 NMC811).627 • Optimize the operating parameters in the leaching process namely leaching628 temperature, acid concentration, solid-to-liquid ratio, and addition strategy629 and amount of hydrogen peroxide.630 • Examine whether hydrogen peroxide can be used efficiently to recover valu-631 able metals from industrially mechanically pre-treated spent Li-ion batteries.632 A deeper understanding on how H2O2 influence on leaching process with differ-633 ent surrounding conditions were examined in order to maximize the leaching634 efficiency along with the reduction of time, solvent and energy used.635 2.4 Scope of work636 Four different types of cathode material were studied: LCO, NMC111, NMC622,637 and NMC811. Lithium ion batteries of LCO type is common in portable electronics638 and NMC-type batteries in electrical vehicles. These cathode materials are now639 being used nowadays. This work will study the leaching process, which is a part640 of the hydrometallurgical process to recycle spent batteries, using sulfuric acid as a641 leaching agent with the help of hydrogen peroxide as a reducing agent.642 14 3643 Methods644 3.1 Materials and reagents645 The cathode materials used in this study are LCO and mixed NMC including646 NMC111, NMC622, and NMC811. LCO used was a pure metal oxides from Sigma-647 Aldrich which is in black powder form. Whereas the mixed NMCs were provided648 by Uppsala university and are also in pure form. The sulfuric acid is an essential649 leaching agent in the leaching process of active cathode materials of LiBs. The dif-650 ferent concentrations of sulfuric acid were prepared from concentrated (95% - 97%)651 solution that was supplied by Sigma-Aldrich. The Hydrogen peroxide that was used652 as a reducing agent was kindly supported from Nouryon Functional Chemicals AB,653 the solution has 59-59.5 wt.% (EKA HP C59). Moreover, the aluminum foil and654 copper metal powder were used as current collectors that can present in spent Li-ion655 batteries. The real NMC cathode waste material was provided by Volvo Cars and656 mechanically treated at Akkuser in Finland. The material was dissolved in aqua657 regia for 5 hours at 80◦C and then analyzed by ICP-OES.658 3.2 Leaching659 The leaching processes were done by using sulphuric acid as leaching reagent. The660 desired concentration of sulphuric acid was prepared by diluting high concentrated661 sulphuric acid with Milli-Q water. The process was carried out in either 100 mL662 plastic beaker or 20 mL glass bottle depending on the desired amount of liquid and663 it was immersed in the glass water bath for temperature control. The container was664 covered with the lid to reduce the loss of water from evaporation. It was agitated by665 a magnetic stirrer at 300 rpm in order to improve mixing efficiency, and regulated to666 the desired temperature before introducing cathode material powder. The leaching667 time was started recording after the black mass powder was added. All experiments668 were done in triplicates. The sampling times were at 1, 5, 10, 15, 30, and 60-669 minute when only sulfuric acid was added. When hydrogen peroxide was added, the670 sampling times were changed to 1, 2, 3, 15, 30, and 60-minute. Each sampling time,671 more than 200 µL of leaching sample was withdrawn from the beaker to get enough672 volume for the analysis and immediately filtered by a syringe filter with pore size673 of 0.45 µm and 25mm in diameter. Only 100 µL of leaching sample was actually674 taken out and the rest of the solution and 100 µL of sulfuric acid will be returned to675 the leaching solution to minimize the change of solid-to-liquid ratio. The obtained676 100 µL samples were first diluted by addition of 9.9 mL of 0.5 M nitric acid. The677 15 3. Methods second dilution is necessary before the ICP-OES analysis. The sample was diluted678 as a factor of 1000. Each 1 mL of diluted sample was diluted again with 9 mL of679 0.5 M nitric acid.680 3.3 Determination of metal concentration in leach-681 ing solution682 Metal concentrations in each leaching solution were determined by using ICP-OES.683 The calibration curve was prepared by using the metal solution with the concentra-684 tion of 0 ppm, 5 ppm, 10 ppm and 20 ppm. The new set of standard solutions was685 prepared every time when using ICP-OES measurement since the variation of con-686 centration can be occurred due to temperature change. Firstly, standard solution of687 20 ppm was prepared by using 1000 ppm metal concentration that is provided by688 SPEX CertiPrep (SPEX CertiPrep Group, Metuchen, US). In this case, there are689 4 main metals that are focused on so each 1 mL of lithium, cobalt, manganese and690 nickel is diluted with 0.5 M nitric acid until the volume reaches 50 mL in order to691 make 20 ppm. Then 10 ppm and 5 ppm standard solution were prepared by dilution692 from 20 ppm.693 The suitable wavelength was selected for each metals that needed to analyze.694 The multiple wavelength can be selected however it should exhibit suitable inten-695 sities and also free from spectral interferences. In this experiment the following696 wavelengths that show in Table 3.1 were used for the analysis:697 Table 3.1: Selected wavelengths in ICP-OES analysis Metal element Selected Wavelength (nm) Li 670.784 Co 228.616 Ni 221.648 Mn 257.61 Al 396.153 Cu 327.393 To calculate leaching efficiency, the equation 3.1 and 3.2 are used.698 Leaching efficiency [%] = [(C/1000) · V mmetal ] · 100 (3.1) C is specific metal concentration obtained from ICP-OES analysis [ppm].699 V is the volume of leaching solution [mL].700 mmetal is the amount of certain metal in the cathode material used [g] which is701 calculated from Equation 3.2.702 mmetal = ( MWmetal ·Molar ratio MWcathode ) ·mcathode (3.2) MWmetal is the molecular weight of specific metal [g/mol].703 16 3. Methods Molar ratio is the molar ratio of specific metal in cathode material chemical for-704 mula, for example, the molar ratio of Co is 0.33 in LiNi0.33Mn0.33Co0.33O2 (NMC111)705 [-].706 MWcathode is the molecular weight of specific cathode material [g/mol].707 mcathode is the weight of cathode material [g].708 3.4 Determination of hydrogen peroxide in leach-709 ing solution710 The hydrogen peroxide that was used in all experiments was provided by Nouryon711 under brand name Eka HP C59 which has H2O2 content around 59 - 59.5 wt.%.712 The residual concentration in solution was determined using iodometric method.713 All chemicals used in this method are listed as followings:714 • 2 M Sulfuric acid (H2SO4)715 • 1 M Potassium Iodide solution (KI)716 • Ammonium molybdate ((NH4)6Mo7O24·4H2O) 15% solution717 • 0.05 M Sodium thiosulfate solution (Na2S2O3)718 • Iodine indicator719 For the determination, the sample should be filtered first. For 1-10 vol% H2O2, a720 sample volume of 200 µL is suitable for the titration. The sample should be adjusted721 with deionized water to a volume of 50 mL then followed by the addition of 5 mL722 sulfuric acid and 10 mL of potassium iodide. A few drops of ammonium molybdate723 were added and the titration was done immediately with sodium thiosulfate solution724 to a light-yellow color. A few drops of iodine indicator were added and continue to725 titrate until the solution was colorless. To prevent the decomposition of hydrogen726 peroxide, the titration was performed immediately after each sampling time. The727 results were not corrected for interfering substances present in the leaching solution728 and a minor discrepancy may therefore occur.729 The following equation was used to calculate the remaining concentration of730 hydrogen peroxide.731 Residual hydrogen peroxide [g/L] = [ VNa2S2O3 · CNa2S2O3 ·MW n · Vprov ] (3.3) VNa2S2O3 is the volume of sodium thiosulfate solution used in titration (mL).732 MW is the molecular weight for hydrogen peroxide which is 34 g/mol.733 n is the equimolar factor which is 2.734 Vprov is the volume of sample (mL)735 17 3. Methods 18 4736 Results737 4.1 Effect of leaching temperature738 The leaching process was performed at two different temperatures which are 50◦C739 and 60◦C in order to determine the most suitable temperature in terms of perfor-740 mance, energy demand and economics. The overall leaching time was 60 minutes.741 The solid-to-liquid ratio of 1:100 g/mL was fixed and 50 mL of sulfuric acid was742 used. Each experiment was done in triplicates. In every figures, the y-axis repre-743 sents the metal leaching efficiency and the x-axis represents leaching time from 0 to744 60 minutes. Standard deviation was also calculated and presented in all figures as745 vertical error bars.746 4.1.1 Leaching of LCO747 Figures 4.1a and 4.1b represent the leaching efficiencies of lithium and cobalt re-748 spectively for LCO. All points show the average value from three replicates. Only749 lithium and cobalt leaching were in focus for this cathode material.750 (a) Li leaching efficiency (b) Co leaching efficiency Figure 4.1: Leaching of LCO: Influence of temperature (reaction conditions: 2 M H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). For lithium leaching in Figure 4.1a, both temperatures illustrated the same751 trend. In the first 5 minutes of leaching time, nothing was dissolved. There was no752 lithium represented in the leaching solution so that the leaching efficiency become753 zero for both temperatures. Leaching efficiency started to increase after 10 min-754 utes. The highest standard deviation was observed at 30 minutes. At 60 minutes,755 19 4. Results the highest efficiency was obtained for both 50◦C and 60◦C; the highest leaching756 efficiency was 34.5% at 60◦C which is only 4% higher than at 50◦C.757 In Figure 4.1b, the leaching of cobalt is represented. The leaching efficiency758 of cobalt in LCO was slightly higher compared to lithium leaching. The leaching759 performance increased from the beginning and increased only a few percentages after760 30 minutes. For both temperatures, the highest leaching efficiency was obtained after761 60 minutes and was around 36%. There was no significant difference between the762 two temperatures, i.e. 50◦C was preferable.763 4.1.2 Leaching of NMC111764 In this section, Figures 4.2a, 4.2b, 4.2c, and 4.2d show the kinetic leaching curve765 of Li, Co, Ni, and Mn which are the main four elements in the mixed cathode766 material. The leaching process was operated in both 50◦C and 60◦C to find the767 optimal temperature.768 (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.2: Leaching of NMC111: Influence of temperature (reaction conditions: 2 M H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). In Figure 4.2a, the leaching efficiency of lithium increased considerably and769 increased gently after 15 minutes for both temperatures. The leaching process at770 60◦C showed slightly higher performance during the overall leaching time. The771 highest leaching efficiency was reached after 60 minutes for both temperatures. The772 maximum leaching efficiency was at 60◦C and was 50.3%.773 20 4. Results In Figure 4.2b, cobalt leaching is illustrated. The leaching efficiency increased774 slowly and leveled off after 15 minutes. Both temperatures showed almost identical775 performance but all values at 60◦C always showed superior result. During the last776 30 minutes of leaching process at 60◦C equilibrium was reached, and the efficiency777 increased less than 1%. The maximum leaching efficiency was 32.4%.778 In Figure 4.2c, the nickel leaching performance at 60◦C was slightly higher than779 at 50◦C during the first 5 minutes of leaching. After that, the leaching efficiency780 was about the same for both temperatures and was about 37% at 60 minutes.781 In the case of Mn in Figure 4.2d, the trend of kinetic curves were almost identical782 for both temperatures. Both leaching efficiency increased slowly during the first783 15 minutes. After 15 minutes, the leaching performance was almost stable. The784 maximum efficiency was obtained at 50◦C/60 minutes and was about 28.8%; a better785 performance compare with 60◦C. Therefore, the more suitable temperature was 50◦C786 since the performance at 60◦C was not significantly better.787 4.1.3 Leaching of NMC622788 The figures in this section show the leaching efficiency for NMC622. Four valuable789 metals including Li, Co, Ni and Mn, which are the main component in this mixed790 NMC cathode material, are also of an interest and measured in concentration for791 calculating the leaching efficiency and comparing the results.792 (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.3: Leaching of NMC622: Influence of temperature (reaction conditions: 2 M H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). 21 4. Results Figure 4.3a represents the leaching efficiency of lithium from NMC622. The793 curve clearly showed that the lithium leaching efficiency increased when time passed794 for both temperatures. At 60◦C, the leaching efficiencies are slightly higher than795 operating at 50◦C for all sampling points. The reason behind this is that an increase796 in temperature can accelerate molecules to move faster and increase energy of parti-797 cles [67]. The maximum Li leaching efficiencies after 60 minutes leaching time were798 approximately 39% and 44% for 50◦C and 60◦C, respectively.799 Figure 4.3b represents the leaching kinetic of cobalt from NMC622. The trend800 of the curves was the same as for lithium (Figure 4.3a). Leaching at 60◦C yields801 marginally higher cobalt leaching efficiency than at 50◦C for all plots and shows802 better leaching performance as seen as higher efficiency (almost 20%) was reached803 at the beginning. After 15 minutes, the curves tended to reach constant efficiencies.804 The maximum cobalt leaching efficiencies of 34% and 36% obtained after 60 minutes805 leaching at 50◦C and 60◦C, respectively.806 Figure 4.3c shows the leaching curve of nickel from NMC622. The nickel leach-807 ing efficiency increases in the first 10–15 minutes. A small increase in efficiency in808 the first 15 minutes was obtained when 60◦C was used. After that, about the same809 values of efficiency were measured for both temperatures. After 30 minutes, the810 curves for both 50◦C and 60◦C also reached the equilibrium and approached about811 33% and 35% efficiency, respectively.812 Figure 4.3d represents the graph plotted between manganese leaching efficiency813 and time for the leaching of NMC622. At 50◦C, the efficiency slightly increased in814 the first 30 minutes and remained constant afterward reaching a leaching efficiency815 of about 16.6%. The kinetic curve for 60◦C went up and reached its peak after 15816 minutes. Then the curve went down gradually and stayed below that of 50◦C. To817 conclude, for leaching of manganese, 50◦C tended to be more effective than 60◦C.818 Therefore, it was shown that there was no significant improvement on efficiency819 when a higher temperature was used. 50◦C could be considered to be the optimal820 temperature to leach NMC622 in order to avoid an unnecessary high energy demand.821 4.1.4 Leaching of NMC811822 The figures shown in this section present the graphs plotted between leaching ef-823 ficiency of a specified metal and leaching time for the leaching of mixed NMC811.824 Two temperatures which are 50◦C and 60◦C were studied.825 22 4. Results (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.4: Leaching of NMC811: Influence of temperature (reaction conditions: 2 M H2SO4, no H2O2, solid-to-liquid ratio of 1:100 (50 mL solution)). Figure 4.4a shows the leaching curve for lithium from NMC811. At the be-826 ginning, the efficiencies measured were rather low. The reason behind this may827 be because the NMC811 was hydrophobic that made it hard to be dissolved when828 initially added into the leaching solution. Beyond this point, the efficiency contin-829 uously increased when 50◦C was used. On the other hand, for 60◦C, the curve was830 getting higher to its peak after 15 minutes leaching and dropped down after that.831 The highest lithium leaching efficiencies that could be reached were 47% and 25%832 for 50◦C and 60◦C, respectively. Therefore, 50◦C yielded better lithium leaching833 performance for NMC811.834 Figure 4.4b displays the curves plotted between cobalt leaching efficiency and835 leaching time. It can be seen that at 50◦C the efficiency increased significantly in836 the first 15 minutes and then increased to some extent afterwards, while at 60◦C,837 the efficiency also increased in the first 15 minutes until it reached the maximum838 value and then declined to lower values. Almost 40% efficiency was reached for 50◦C839 after 60 minutes leaching, whereas 26% for 60◦C (at 15 minutes). Therefore, 50◦C840 is more effective to leach cobalt from NMC811.841 Figure 4.4c presents the nickel leaching efficiency for leaching of NMC811. As842 a result, the same pattern as before was observed. At 50◦C, the efficiency of 40%843 was reached after 60 minutes leaching, when at 60◦C the highest efficiency was844 measured as 30% after 15 minutes. Correspondingly, the lower temperature gives a845 better nickel leaching from NMC811.846 23 4. Results The manganese leaching performance can be seen in Figure 4.4d. No leached847 manganese concentration was detected by ICP-OES regardless of leaching temper-848 ature. This was probably because of too low manganese concentration detected in849 the leachate.850 In respect to the results, 50◦C was more suitable for being used to leach valuable851 metals from NMC811 because such temperature yielded better leaching performance852 than 60◦C and showed more clear pattern of leaching curves. To compare the results,853 a bar graph showing the leaching efficiency at the end of leaching (60 minutes) for854 both temperatures (50, 60°C) can be seen below. Blue bars represent 50◦C and855 orange bars represent 60◦C, vice versa.856 Figure 4.5: Leaching efficiency after 60 minutes leaching of all cathode materials: Influence of temperature (reaction conditions: 2 M H2SO4, no H2O2, solid-liquid ratio of 1:100 (50 mL solution)). Based on the above results, Figure 4.5, the higher temperature (60◦C) did857 not improve the leaching performance significantly. In some cases, especially the858 manganese leaching of mixed NMC cathode material, 60◦C always gave a worse859 efficiency compared to 50◦C. Moreover, the NMC811 leaching efficiency was lower860 when 60◦C leaching temperature was used for all leached metals. Therefore, 50◦C861 was chosen for the further experiments.862 4.2 Effect of mixing863 The effect of mixing was studied by using a solid-to-liquid ratio of 1:100. Two sets of864 experiments conducted by varying the leaching volume (10 and 50 mL). The different865 container was used: 20 mL smaller glass vial and 100 mL bigger and wider-in-width866 plastic container. An equal size of magnet was used for stirring which is more fit867 with small glass vial and it is expected to have a better mixing. The following figures868 show the leaching efficiency of all cathode materials in different leaching scales. The869 24 4. Results thick line represents the leaching in 50 mL and thin line represents the leaching in870 10 mL solution.871 4.2.1 Leaching of LCO872 In this section, the leaching of LCO is presented. The leaching volume was altered873 between 10 mL and 50 mL. The comparison of the leaching efficiency between the874 two different volumes is shown in Figure 4.6.875 Figure 4.6: Leaching of LCO: Influence of mixing (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:100 (10 and 50 mL solution)). According to Figure 4.6, the leaching process in 10 mL solution showed higher876 leaching performance throughout the whole leaching time. Moreover, the highest877 leaching efficiency that could be achieved was much higher compared to when leach-878 ing in 50 ml scale which were 91.5% and 30.7%, respectively. In addition, the cobalt879 leaching efficiency was also higher but not as much as when compared to lithium880 leaching since lithium is monovalent and can be leached more easily especially with a881 perfect mixing. On the other hand, cobalt leaching strongly depends on the change882 of oxidation state, a better mixing alone was probably not enough to overcome its883 limitation and a reducing agent might be needed in order to enhance leaching per-884 formance. Therefore, it was clear that decreasing the leaching volume (i.e. better885 mixing) can improve the leaching efficiency.886 4.2.2 Leaching of NMC111887 Leaching of NMC111 was performed using two different leaching containers with888 different volumes. The result of leaching efficiency is shown below.889 25 4. Results Figure 4.7: Leaching of NMC111: Influence of mixing (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:100 (10 and 50 mL solution)). According to Figure 4.7, the NMC111 kinetic leaching curve showed that a890 higher efficiency was reached when scaling down, and the improvement was obtained891 already in the initial phase of leaching. It was also obvious that the lithium leaching892 performance was noticeably improved more than for the other metals. The lower893 leachability of other metals are, as for LCO, related to the need to change into the894 lower oxidation state while lithium is not involved with this leaching principle. The895 highest leaching efficiency obtained was 96% for lithium.896 4.2.3 Leaching of NMC622897 NMC622 leaching was focused on in this part. Figure 4.8 shows the leaching per-898 formance for the two different scales.899 26 4. Results Figure 4.8: Leaching of NMC622: Influence of mixing (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-t0-liquid ratio of 1:100 (10 and 50 mL solution)). From Figure 4.8, lithium was always the most easily leached element in all cath-900 ode materials. The lithium leaching almost reached 100% without any additional901 reducing agents while the leaching efficiency for the other elements was below 50%.902 A higher efficiency was achieved when the small volume was used.903 4.2.4 Leaching of NMC811904 NMC811 leaching was of interest since this cathode chemistry represents the latest905 development. The figure below reviews the influence of mixing in term of metals’906 leaching efficiency.907 27 4. Results Figure 4.9: Leaching of NMC811: Influence of mixing (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:100 (10 and 50 mL solution)). In the case of NMC811 that is illustrated in Figure 4.9, the lithium leaching908 performance reached 100% after only 15 minutes when leaching using 10 mL. Fur-909 thermore, the maximum manganese leaching efficiency was 31.2% when leaching910 using the smaller volume (10 mL) whereas nothing could be leached when using 50911 ml. The leaching performance leveled off after 10-15 minutes for all materials and912 all metals.913 Therefore, the leaching process should be performed using efficient mixing con-914 dition in order to maximize the leaching performance. However, the leaching was915 done in a less efficient mixing, 100 mL container, for the further experiments for916 the reason that a high volume of solution is required in order to avoid the changing917 the leaching volume when taking samples for analysis. The results from this section918 therefore cannot be compared with other experiments using different leaching vol-919 ume because of the different mixing. In addition, the particle size and distribution920 of the cathode materials could also affect the leaching efficiency more or less. Apart921 from the mixing, those factors can also be interesting to study.922 4.3 Effect of acid concentration923 In this section, the solid-to-liquid ratio was increased in order to see whether the924 desired concentration of acid, which is 2 M of H2SO4, is sufficient. The solid-to-925 liquid ratio was varied as 1:100, 1:20, and 1:10 g/mL. By increasing the solid-to-926 liquid ratio, there will be less acid to leach the desired metals and amount of acid927 can be a limiting factor, i.e. a higher acid concentration might be needed. The928 calculations to find the limiting agent in the reaction when 2 M H2SO4 and a solid-929 to-liquid ratio of 1:10 is used, which is the worst case, is presented in Appendix930 A.1. Regarding the calculations, all cathode materials are limiting agents that is,931 at these conditions, the concentration of acid is sufficient theoretically. Moreover,932 28 4. Results the optimal solid-to-liquid ratio will be selected for further experiments, i.e. when933 current collectors are present during leaching (Chapter 4.3.2), when H2O2 is present934 (Chapters 4.4 and 4.5), and when black mass is leached (Chapter 4.6). Moreover,935 with the optimal solid-to-liquid ratio, aluminium foil and copper powder would also936 be added because of the ability to consume acid.937 4.3.1 Effect of solid-to-liquid ratio938 The solid-to-liquid ratio was varied as 1:100, 1:20, and 1:10 g/mL and the same939 amount of liquid was used (10 mL of 2 M H2SO4) to have an efficient mixing. The940 kinetic curves are plotted between leaching efficiency and leaching time for different941 desired solid-to-liquid ratios. Table 4.1 shows the theoretical volume of 2 M H2SO4942 needed to leach different cathode materials based on stoichiometric ratios in the943 reaction equations and the actual volume of H2SO4 used in the leaching for different944 studied solid-to-liquid ratios. To compare, the numbers are divided by each cathode945 material weight.946 Table 4.1: Theoretical of 2 M H2SO4 needed per gram of each cathode material and the amount of H2SO4 added at different solid-to-liquid ratios. Cathode material Theoretical volume needed of H2SO4 per cathode material weight (mL/g) Added volume of H2SO4 per cathode material weight (ml/g) 1:100 1:20 1:10 LCO 7.65 100 20 10NMC111 7.80 NMC622 7.75 NMC811 7.70 According to Table 4.1, the volume of H2SO4 added is higher than the theoreti-947 cal volume needed. It is expected that all leaching with these desired solid-to-liquid948 ratios is not limited by the leaching solution.949 4.3.1.1 Leaching of LCO950 In this section, the leaching of LCO was studied. Figures 4.10a and 4.10b show the951 leaching performances of lithium and cobalt between different solid-to-liquid ratios.952 Blue lines represent the kinetic curve when a solid-to-liquid of 1:100 g/mL was used.953 Orange and gray lines corresponds to 1:20 and 1:10 g/mL, respectively.954 29 4. Results (a) Li leaching efficiency (b) Co leaching efficiency Figure 4.10: Leaching of LCO: Influence of solid-to-liquid ratio (reaction condi- tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:10, 1:20, and 1:100 (10 mL solution)). It can be seen from Figure 4.10a that the inclination of all three kinetic curves955 were the same. The leaching process occurred rapidly during the first 15 minutes956 and then slowed down. For all solid-to-liquid ratios, the lithium leaching efficiencies957 were not reaching certain values within 60 minutes leaching. It was expected that958 the ratio of 1:100 g/mL yielded the highest efficiency because there would be more959 free acid available for leaching. At solid-to-liquid ratios of 1:20 and 1:10, there was960 almost no difference in lithium leaching efficiency. The highest lithium leaching961 efficiencies were 91.5%, 66.7%, and 62.9% for solid-to-liquid ratios of 1:100, 1:20,962 and 1:10 g/mL, correspondingly.963 In regard to Figure 4.10b, the same progression was also observed for all con-964 ditions that the cobalt leaching efficiency increased considerably during the first 15965 minutes and then raised slightly. It can also be seen that the cobalt leaching effi-966 ciency at the ratio of 1:10 g/mL was similar to 1:20 g/mL and that these two ratios967 were inferior to that of 1:100 g/mL.968 4.3.1.2 Leaching of NMC111969 Figures 4.11a, 4.11b, 4.11c, and 4.11d review the influence of the solid-to-liquid ratio970 between 1:100, 1:20, and 1:10 on the leaching of NMC111.971 30 4. Results (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.11: Leaching of NMC111: Influence of solid-to-liquid ratio (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-liquid ratio of 1:10, 1:20, and 1:100 (10 mL solution)). The lithium leaching efficiency of the NMC111 is shown in Figure 4.11a. There972 was a very fast leaching initially especially for 1:100 and 1:20 (the increase after973 the initial 5 minutes of leaching was slower). Li was almost totally leached after 60974 minutes leaching when using the solid-to-liquid ratio of 1:100. The difference in the975 maximum Li leaching efficiency between 1:10 and 1:20 was small.976 According to Figure 4.11b, cobalt leaching efficiency in comparison of the three977 solid-to-liquid ratios is shown. The leaching trends were the same for all solid-to-978 liquid ratios. The curves raised throughout the investigated leaching time interval.979 It was clear that the ratio of 1:100 gave the highest cobalt leaching efficiency while980 the two other ratios gave pretty close values.981 The nickel leaching efficiency when leaching NMC111 is illustrated in Figure982 4.11c. It can be seen that faster initial leaching was obtained when the solid-to-liquid983 ratios of 1:100 and 1:20 were used. The curves were close to each other, i.e. small984 differences between the different ratios. The ratio of 1:10 gave the worst leaching985 efficiency; the maximum nickel leaching efficiency obtained after 60 minutes were986 44.3%, 37.7%, and 33.9% from low to high solid-to-liquid ratio.987 Figure 4.11d shows a plot of the manganese leaching efficiency. There was a988 faster leaching at the beginning and nothing much happened after 10–15 minutes for989 the solid-to-liquid ratios of 1:20 and 1:100. For the solid-to-liquid ratio of 1:100, the990 leaching curve overlapped with that of 1:20 after 10 minutes and small differences991 31 4. Results between the solid-to-liquid ratios of 1:20 and 1:10 after 10-15 minutes were observed.992 The total manganese leaching efficiency after 60 minutes was still low (below 40%),993 therefore a reducing agent might be needed in order to obtain higher efficiency.994 As a result, the kinetic curves of all solid-to-liquid ratios were parallel for all995 metals. The difference in the leaching efficiency between the three solid-to-liquid996 ratios was highest for lithium. The leaching efficiency was high for lithium (79-96%)997 and much lower for the other metals (<50%). However, the 1:100 ratio always gave998 the highest leaching efficiency while the solid-to-liquid ratios of 1:20 and 1:10 gave999 almost identical results.1000 4.3.1.3 Leaching of NMC6221001 Figures 4.12a, 4.12b, 4.12c, and 4.12d show the effect of the solid-to-liquid ratios1002 1:100, 1:20, and 1:10 g/mL on the leaching efficiency of NMC622.1003 (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.12: Leaching of NMC622: Influence of solid-to-liquid ratio (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:10, 1:20, and 1:100 (10 mL solution)). Figure 4.12a illustrates the leaching kinetic curve of lithium over the time.1004 Lithium leaching efficiencies increased the first 30 minutes and then reached an1005 equilibrium. When the solid-liquid ratio was decreased, the lithium leaching effi-1006 ciency was improved to some extent. Lithium was easily leached and the lithium1007 32 4. Results leaching efficiency was high for all solid-to-liquid ratios. Above 90% was achieved1008 at the ratios of 1:100 and 1:20 g/mL and about 85% was reached at 1:10 g/mL.1009 In Figure 4.12b, the cobalt leaching efficiency is shown. The same trend was1010 observed for all ratios and the value at every single point was close to each other.1011 The cobalt leaching was only to a minor extent affected by the solid-to-liquid ratio;1012 the maximum cobalt leaching efficiencies were 43.1%, 42.8%, and 39.6% at the ratio1013 of 1:100, 1:20, and 1:10 g/mL, accordingly.1014 According to Figure 4.12c, at any solid-to-liquid ratios, the nickel was leached1015 out continuously until it reached equilibrium after 30 minutes. The leaching perfor-1016 mance was slightly better at lower solid-to-liquid ratio. The highest nickel leaching1017 efficiency that could be reached were 41.1%, 39.5%, 36.2% when solid-to-liquid ratios1018 of 1:100, 1:20, and 1:10 were used, respectively.1019 Figure 4.12d shows the manganese leaching efficiency of the NMC622 cathode1020 material for the three desired solid-to-liquid ratios. It can be seen that a solid-to-1021 liquid ratio of 1:100 yielded the highest manganese leaching performance while 1:201022 and 1:10 gave about the same result. The maximum efficiencies were 38.3%, 32.8%,1023 and 30.1% for 1:100, 1:20, and 1:10 g/mL, respectively.1024 4.3.1.4 Leaching of NMC8111025 The effect of varying solid-to-liquid ratio in term of metals’ leaching efficiency is1026 illustrated in Figures 4.13a, 4.13b, 4.13c, and 4.13d.1027 33 4. Results (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.13: Leaching of NMC811: Influence of solid-to-liquid ratio (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:10, 1:20, and 1:100 (10 mL solution)). Figure 4.13a is a plot of lithium leaching efficiencies in a comparison between1028 three investigated solid-to-liquid ratios which are 1:100, 1:20, and 1:10. For NMC811,1029 it was surprising that 100% of the Li was leached after 15 minutes, when using a1030 solid-to-liquid ratio of 1:100, in the absence of a reducing agent. Moreover, the Li1031 leaching efficiency for a ratio of 1:20 ratio was almost as good as for 1:100. For the1032 highest solid-to-liquid ratio (1:10), the maximum leaching efficiency was obtained1033 after 30 minutes.1034 The cobalt leaching performance, as seen in Figure 4.13b, increased considerably1035 during the first 15 minutes and reached the maximum efficiency after 30 minutes1036 and slightly dropped afterwards for the cases of 1:10 and 1:20 but for 1:100, it was1037 stable. The cobalt leaching efficiency dropped when increasing solid-to-liquid ratio1038 but there was no outstanding difference between 1:20 and 1:10.1039 For the nickel leaching efficiency as illustrated in Figure 4.13c, the leaching1040 curve was the same as observed for cobalt leaching. The maximum nickel leaching1041 efficiencies were 51.2%, 41.1%, and 36.5% for 1:100, 1:20, 1:10 g/mL, consequently.1042 In Figure 4.13d, the manganese leaching performance is shown, and an inter-1043 esting trend was observed. The highest performance was measured after 15 minutes1044 and the efficiency gradually dropped after that.1045 Therefore, it can be concluded that the effect of solid-to-liquid ratios on the1046 leaching performance is the same for all cathode materials. Figure 4.14 shows a1047 34 4. Results comparison of the leaching efficiency for different cathode materials and solid-to-1048 liquid ratios. The comparison is done at a leaching time of 60 minutes.1049 Figure 4.14: Leaching of all materials: Influence of solid-to-liquid ratio (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:100, 1:20, and 1:10, 10 mL solution, and 60 minutes leaching time). The decreasing in solid-to-liquid ratio affected the leaching efficiency in all1050 cathode material. Li was especially affected by the solid-to-liquid ratios for LCO1051 whereas for the NMC cathode materials the difference was smaller and varying. It1052 showed gradual improvement on other metals’ leaching efficiencies for all cathode1053 materials. Therefore, it can be concluded that the lowest solid-to-liquid ratio gives1054 the highest leaching efficiency and that the difference between 1:20 and 1:10 was1055 rather small but 1:20 was slightly better. Please note that this evaluation is done1056 without a reducing agent present.1057 A solid-to-liquid ratio of 1:20 was selected for further experiments, when H2O21058 was evaluated as reducing agent, to avoid a too concentrated leachate which is not1059 suitable for the following solvent extraction process [68].1060 4.3.2 Effect of current collectors1061 With the selected solid-to-liquid ratio which was 1:20, aluminium foil and copper1062 powder, which represent the current collectors, were added with an amount of 10%1063 of the cathode’s weight. The addition of Al and Cu foils is necessary since they can1064 probably be presented in the black mass and also affect the leaching process. The1065 leaching time in the following experiments was set to 45 minutes.1066 4.3.2.1 Leaching of LCO1067 Leaching of LCO with an addition of Cu and Al foils was studied in this section.1068 The leaching efficiency when leaching with current collectors present was compared1069 to the one without current collectors. Lithium and cobalt leaching efficiencies are1070 35 4. Results of an interest in the leaching of LCO and the results can be found in Figures 4.15a1071 and 4.15b, respectively.1072 (a) Li leaching efficiency (b) Co leaching efficiency Figure 4.15: Leaching of LCO: Influence of current collectors (reaction conditions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10 mL solution)). According to Figure 4.15a, the blue line represents the kinetic curve of the1073 leaching without current collectors and the orange line represents the kinetic curve1074 of the leaching with current collectors. The maximum leaching performance of1075 lithium was 73.8% after 45 minutes and a higher efficiency might be achieved since1076 equilibrium was not reached. However, lithium is always the easiest element to be1077 leached out whatever treatment condition due to the position of lithium; lithium1078 loosely lies between molecular octahedral formed by cobalt and oxygen atoms in the1079 LCO layer structure [69].1080 The leaching efficiency of cobalt is shown in Figure 4.15b. The maximum cobalt1081 leaching efficiency was 49.9%. Cobalt leaching efficiency was much better when1082 adding the current collectors.Cobalt leaching was more affected by the presence of1083 Al and Cu foils than lithium leaching. The electrochemical potential is involved1084 and affects the leaching reaction. Al and Cu have lower electrochemical potentials1085 compared to cobalt. The standard electrode potentials are -1.662 V for Al, 0.34 V1086 for Cu, and 1.82 V for Co. Due to their low values of electrochemical potential, Al1087 and Cu could act as reducing agents in the leaching system. The leaching process is1088 driven by galvanic interactions between current collectors (Al and Cu) and transition1089 metal oxides that leads to a better dissolution of cobalt in the presence of current1090 collectors. It reduces the oxidation state of Co and promote formation of CoSO4.1091 4.3.2.2 Leaching of NMC1111092 The kinetic leaching curves of Li, Co, Ni, and Mn are illustrated in Figures 4.16a,1093 4.16b, 4.16c, and 4.16d, respectively.1094 36 4. Results (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.16: Leaching of NMC111: Influence of current collectors (reaction condi- tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10 mL solution)). As illustrated in Figure 4.16a, the leaching without Cu and Al foils could reach1095 the maximum Li leaching efficiency faster than the leaching with Cu and Al foils.1096 However, the leaching efficiency was almost the same, about 83-84%, at the end of1097 leaching.1098 In the case of cobalt in Figure 4.16b, without an addition of Cu and Al foils, a1099 slightly higher efficiency could be observed at the beginning and reached a constant1100 value after 10 minutes. On the other hand, with an addition of current collectors,1101 the efficiency was gradually increased and yielded better result. Roughly, the im-1102 provement was 35% when adding current collectors. This was due to the effect of1103 changing to the preferred state of cobalt (Co2+).1104 The same trend was also observed for Ni leaching as shown in Figure 4.16c.1105 With the addition of current collectors, the maximum Ni leaching efficiency was1106 52.3% while the corresponding value without current collectors was 37.7%, i.e. an1107 improvement with 39%.1108 Regarding Figure 4.16d, the trends of the Mn leaching kinetic curve looked the1109 same as those of Co and Ni. With the addition of current collectors, the maximum1110 Mn leaching efficiency was 44.0% whereas without current collectors it was 26.1%1111 (an improvement of 69%). Therefore, the leaching of Mn was strongly affected by1112 the presence of current collectors.1113 From all results, the addition of current collectors to the leaching of NMC1111114 could definitely promote the leaching efficiency except for lithium leaching. How-1115 37 4. Results ever, addition of current collectors affected mostly the cobalt, nickel, and manganese1116 dissolution. The improvement was 35%, 39% ,and 69% for Co, Ni, and Mn, respec-1117 tively. The Li was hardly affected by the reduction process unlike the Co, Ni, and1118 Mn. For Li leaching, the influence from solid-to-liquid ratio, which was mentioned1119 previously, was more important. The effect of current collectors on the lithium1120 leaching is negligible since there is no change in oxidation state. However, a better1121 dissolution of Co can promote the dissolution of Li. The leaching of nickel was also1122 improved by about 39% because of liberation of other metals from the NMC1111123 structure. When the current collectors and cathode material are present together1124 in the leaching solution, the metals from current collectors can function as reducing1125 agents and promote the reduction of the metals as shown in the following equations.1126 Mn4+ + 2e- →Mn2+ (4.1) Co3+ + e- → Co2+ (4.2) The valence state 2+ is the stable state for Mn and Co in an aqueous solution.1127 Their divalent forms are dissolved readily in H2SO4. Therefore, it is necessary to1128 have reducing agents in the system to reduce the transition metal element from high1129 valence states to low valence states to reach high leaching efficiencies.1130 4.3.2.3 Leaching of NMC6221131 Figures 4.17a, 4.17b, 4.17c, and 4.17d present the kinetic curves of NMC622 leaching1132 for all leached metals, including Li, Co, Ni, and Mn, with an influence of an addition1133 of current collectors.1134 38 4. Results (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.17: Leaching of NMC622: Influence of current collectors (reaction condi- tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10 mL solution)). Figure 4.17a shows the lithium leaching efficiency curves of NMC622 for both1135 with and without an addition of current collectors. It can be seen that both curves1136 have the same trend. Therefore, there was no clear improvement after an addition1137 of Cu and Al foils on Li leaching efficiency.1138 According to Figure 4.17b, the same trend was observed for the cobalt leaching1139 efficiency. When the current collectors were added, the cobalt leaching performance1140 was slightly better compared to the leaching without current collectors but the differ-1141 ence was not large. The improvement was 11% when introducing current collectors.1142 In case of Ni, Figure 4.17c, it was clear that when introducing Cu and Al, the1143 nickel leaching performance was better in every sampling. The maximum efficien-1144 cies were 48.4% and 39.5% with and without current collectors, respectively. The1145 improvement was 22.6% when the current collectors were present.1146 As can be seen in Figures 4.17a-4.17d, the improvement when adding of Cu1147 and Al was most pronounced for Mn compared to the other metals when leaching1148 NMC622. The maximum Mn leaching efficiency increased with as much as 38%.1149 To sum up, the addition of Cu and Al foils led to higher leaching efficiency for1150 Co, Ni, and Mn in NMC622. As mentioned above, due to the low electrochemical1151 potentials, Cu and Al could function as reducing agents and by that promote metal1152 dissolution as discussed above.1153 39 4. Results 4.3.2.4 Leaching of NMC8111154 Figures 4.18a, 4.18b, 4.18c, and 4.18d present the leaching performance of NMC8111155 for all leached metals with and without the presence of current collectors.1156 (a) Li leaching efficiency (b) Co leaching efficiency (c) Ni leaching efficiency (d) Mn leaching efficiency Figure 4.18: Leaching of NMC811: Influence of current collectors (reaction condi- tions: 2 M H2SO4, T=50◦C, no H2O2, solid-to-liquid ratio of 1:20 (10 mL solution)). As illustrated in Figure 4.18a, without an addition of Cu and Al, the maximum1157 leaching was obtained after 30 minutes leaching and that the difference in leaching1158 efficiency with or without current collectors was minor. The maximum lithium effi-1159 ciencies for both conditions were 97.8% and 98.8% for without and with an addition1160 of current collectors.1161 Figure 4.18b refers to cobalt leached performance of NMC811 leaching. It was1162 shown that the leaching with the addition of current collectors improved the leaching1163 efficiency after 30 minutes of leaching. As high as 55.4% of cobalt leaching efficiency1164 could be achieved when Cu and Al were added into the solution while 49.4% was1165 obtained when there was no Cu and Al present, i.e. an improvement of 12%.1166 Conforming to Figure 4.18c, nickel leaching efficiency was plotted against time.1167 With an addition of Cu and Al foils, the efficiency was higher after 10 minutes. The1168 improvement was 24% for nickel when introducing current collectors.1169 The largest improvement was clearly observed on manganese leaching that is1170 presented in Figure 4.18d. As can be seen, an addition of Cu and Al foils made the1171 leaching efficiency not to drop down like it was observed when the current collectors1172 40 4. Results were not added. The highest manganese leaching efficiencies were 22% and 32.1%1173 for without and with an addition, consequently, i.e. an improvement of 46%. The1174 concentration of Al and Cu in the final solution and the corresponding percent1175 recovery are shown in Table 4.2 (where their initial concentrations were 500 ppm1176 (10w/w% of each).1177 Table 4.2: Copper and aluminum concentration in leachate and perc