Polyester-Polycarbonate Polymer Electrolytes Beyond LiFePO4: Influence of Lithium Salt and Applied Potential Range

Rechargeable polymer-based solid-state batteries with metallic lithium anodes and LiNi_xMn_yCo_1−x−yO₂ (NMC)-based cathodes promise safer high-energy-density storage solutions than existing lithium-ion batteries, but have shown challenging to realize. The failure mechanisms that have been suggested for these battery cells have mostly been related to the use of a metallic lithium anode and formation of dendrites during cycling. Here, we approach the issue of using solid polymer electrolytes (SPEs) vs. NMC cathodes by employing a range of materials based on poly(ϵ-caprolactone-co-trimethylene carbonate) (PCL-PTMC) with different salts under various cycling conditions. It is seen that although the ionic conductivity of the electrolyte can be improved by exchanging the lithium salt, it does not immediately correlate to better cycling performance. However, increasing the temperature during battery cycling to improve the ion transport kinetics lowers the polarization of the battery cell and full capacity can be achieved at an upper voltage cut-off that is appropriate for the polymer electrolyte. For these electrolytes, the limit is demonstrated to be 4.4 V vs. Li⁺/Li, and cycling with NMC-111 cathodes is thereby possible provided that the upper cut-off is limited to below this limit.