Enabling High‐Voltage Polymer‐Based Solid‐State Batteries Through Reinforcements with LiAlO₂ Fillers

Poor ionic conductivity, low Li ⁺ transference number, and limited electrochemical stability plague all‐solid‐state Li‐metal batteries based on solid polymer electrolytes (SPEs). One strategy to overcome these hurdles is the insertion of ceramic fillers to generate composite polymer electrolytes (CPEs). These are based either on active (ion‐conductive) fillers like Li ₇ La ₃ Zr ₂ O ₁₂ or passive (non‐conductive) fillers like Al ₂ O _3. In this work, the effect of passive Li‐containing fillers is showcased, exemplified by a CPE platform of poly(trimethylene carbonate) (PTMC:LiTFSI) with LiAlO ₂ particles. The inclusion of such fillers shows a strikingly positive effect. The ionic conductivity is greatly improved by one order of magnitude at 20 wt% of LiAlO ₂ compared to the pristine PTMC SPE. Moreover, the Li ⁺ transference number is significantly boosted and reaches values close to unity ( T ₊ = 0.97 at 20 wt% of LiAlO ₂ ), effectively rendering the material a single‐ion conductor. The CPEs show outstanding cycling stability vs Li‐metal, and electrochemical stability of up to 5 V vs Li ⁺ /Li. When implemented in a solid‐state battery cell with LiNi _0.33 Mn _0.33 Co _0.33 O ₂ (NMC111) and Li‐metal, a stable cycling performance for over 100 cycles is observed. This demonstrates the potential of using microsized and cost‐effective LiAlO ₂ fillers in CPEs for applications in all‐solid‐state Li‐metal batteries.