Study of composite polymer electrolytes incorporating LLZO particles in a PEO matrix in high voltage all solid-state lithium batteries

Abstract

The impact of different Li₇La₃Zr₂O₁₂ (LLZO) fillers irrespective of dopants on the electrochemical performance of solid polymer electrolytes (SPEs) is systematically investigated. LLZO particles doped with Al and Ta/Nb were synthesized via electrospinning (Al-LLZO) and the solid-state reaction (bulk Al-LLZO and Ta/Nb-LLZO), alongside a commercial LLZO Ref. Three SPE composites S1 (electrospun Al-LLZO), S2 (bulk Al-LLZO), and S3 (Ta/Nb-LLZO) were fabricated by dispersing the fillers into a PEO–LiTFSI matrix. Among them, S2 exhibited the highest ionic conductivity (10⁻³ S cm⁻¹ at 60 °C), outperforming S1 and S3 (10⁻⁴ S cm⁻¹). All SPEs demonstrated a stable electrochemical window of 2.5–4.2 V, confirmed via cyclic voltammetry. Symmetric cell testing revealed that the Ref sample, with smaller and more uniformly distributed LLZO particles, achieved the lowest overpotential. Full-cell cycling with an NMC811‖SPE‖Li–metal at 60 °C yielded discharge capacities of 80–115 mAh g⁻¹ for S2, S3, and the Ref, whereas S1 underperformed. Despite these variations, the solid polymer electrolytes demonstrate promising stability when in contact with both the Li metal anode and the NMC811 cathode, highlighting its potential for use in high voltage solid-state batteries at elevated temperatures.