Towards low-temperature structural battery electrolytes for sustainable multifunctional energy storage

Abstract

The electrification of the transport sector has increased the demand for efficient and lightweight energy systems. One promising solution is structural batteries which are multifunctional materials that store energy and carry mechanical load, simultaneously. This multifunctionality is enabled partly by the structural battery electrolyte, a semi-solid electrolyte consisting of a liquid electrolyte for ionic transport, and a porous polymer providing mechanical stability. However, at low temperatures (< -30 °C), these systems exhibit a significant loss of capacity due to increased viscosity or freezing of the electrolyte, limiting ion transport. This presents a critical challenge for applications in Nordic climates, defence, and aerospace systems, where operation is in extreme cold. In this thesis, six different techniques were used to analyse the electrochemical and mechanical performance of five novel electrolyte systems. The results found that the diglyme-based electrolytes showed improved electrochemical performance compared to the reference electrolyte, with the 70:30 DG:FEC composition exhibiting the highest ionic conductivity in the range of 10−4 S/cm at 25 °C and 10−6 S/cm at -40 °C. This system also showed a specific capacity of 430 mAh/g at 25 °C and maintained over 100 mAh/g at -15 °C. Mechanical testing confirmed that the structural integrity of the electrolyte was preserved across the temperature range and morphology analysis revealed a stable electrolyte-electrode interface after cycling, with preserved porosity. Overall, the results demonstrate that diglyme-based structural electrolytes enable improved low-temperature performance while maintaining mechanical functionality, highlighting their potential for cold-climate energy storage applications.