Multilayer PEO/LLZTO composite electrolyte enables high-performance solid-state Li-ion batteries

All-solid-state Li-ion batteries (ASSLBs) are promising systems to power electronic devices and electric vehicles for their high-energy density and safety. PEO-based composite electrolytes containing lithium salts and various fillers have been regarded as the most attractive solid electrolytes for ASSLBs due to their high interfacial compatibility with electrodes, high mechanical flexibility, and easy fabrication. For PEO/lithium salt/ceramic composite electrolytes, both “ceramic-in-polymer” and “polymer in ceramic” can be applied to solid-state batteries. “Ceramic-in-polymer” can provide high ionic conductivity but narrow electrochemical window; “polymer in ceramic” can provide wide electrochemical window but low ionic conductivity. In order to combine the high conductivity of “ceramic in polymer” with the wide electrochemical window of “polymer in ceramic,” we designed multilayer PEO/ Li6.4La3Zr1.4Ta0.6O12(LLZTO) composite electrolytes with low content of LLZTO (10wt %) in the inside layer and high content of LLZTO (40wt %) in the surface layer (PCEs-40–10-40) by a simple solution casting method. The PCEs-40–10-40 showed high ionic conductivity(4.61 × 10–4 $${\mathrm{S}\cdot \mathrm{cm}}^{-1}$$ ) and a wide electrochemical window(> 5.1 V) at 60 ℃. Compared with monolayer composite electrolytes, solid-state LiFePO4|Li batteries with multilayer composite electrolytes showed much better cycling stability (capacity retention of 98.8% after 200 cycles) at 0.5C and 60 ℃. This work shall give practical insights for the applications of PEO-based composite electrolytes.