The impact of residual solvent on catholyte performance in solid-state batteries

Abstract

All-solid-state batteries (ASSBs) are attractive due to their safety, use of the Li metal anode, high energy density, and innovative processing routes. However, high interfacial resistance, especially on the cathode side, is one of the major challenges for commercialization of ASSBs. Catholyte, either a liquid or solid, is added to lower the cathode/electrolyte interfacial resistance. In this study, we find that residual N-methylpyrrolidone solvent remaining in a PVDF/LiTFSI solid polymer catholyte after incomplete drying can dramatically lower the interfacial resistance between the Li_6.25Al_0.25La₃Zr₂O₁₂ electrolyte and LiNi_1/3Mn_1/3Co_1/3O₂ cathode-active material. Cells with varying amounts of residual solvent are compared to optimize the residual solvent loading. With moderate residual solvent, the discharge capacity reaches 142 mA h g⁻¹ when cycled at 25 °C and 0.5 C and the capacity retention is 60.5% after 125 cycles. Discharge capacity retention is improved at −10 °C. The conductivity of the free-standing PVDF/LiTFSI film, mimicking the PVDF/LiTFSI catholyte, verifies the role of residual NMP in the cathode. This study demonstrates the possibility of widely differing results for ASSBs when the cathode is not completely dried. Also, it provides a hint for a potential method to lower the cathode/electrolyte interfacial resistance.