Bilayer electrolyte design toward high-voltage durable solid-state lithium metal batteries

Abstract

To obtain higher energy density, the choice of LiCoO₂ (LCO) with extraordinary theoretical volumetric energy density as the cathode has absolute advantages, and more discharge capacity can be obtained by increasing the cut-off voltage. However, several challenges, including surface degradation, irreversible structural transformation, cobalt dissolution, and detrimental side reactions with electrolyte, arise when charging to a high cut-off voltage (>4.2 V vs. Li/Li⁺), which lead to rapid capacity decay. Herein, we constructed poly(vinyl carbonate) (PVC)/lithium difluoro(oxalato) borate (LiDFOB) interface layer on the surface of LCO by artificial design, aiming at the interface between Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO)-based composite electrolyte and LCO. The electrode plate is composed of 1 M LiDFOB/VC and LCO cathode (named as 1 M D-LCO). A B–F/LiF-rich cathode electrolyte interphase (CEI) layer is formed during electrochemical cycling, which effectively suppresses the side reactions between LCO and electrolyte under high voltage and improves the interfacial stability. As a result, the 1 M D-LCO|trimethylolpropane trimethacrylate (TMPTMA)-1,6-hexanediol diacrylate (HDDA)/LLZTO|Li cell delivers an initial discharge capacity of 141.2 mA h g⁻¹ at 2C (4.4 V, 83.57% after 500 cycles) and superior high voltage stability (4.6 V, 75.12% after 150 cycles), which is one of the best results for 1 M D-LCO|TMPTMA–HDDA/LLZTO|Li reported to date. This work provides an effective method for the compatibility of cathode and solid-state electrolytes.