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

Abstract

The impact of different Li₇La₃Zr₂O₁₂ (LLZO) fillers 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 nanofibers) and 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 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 composite electrolyte demonstrates promising stability in contact with both Li metal anode and NMC811 cathode, highlighting its potential for use in high voltage solid-state batteries at the elevated temperatures.Broader contextAdvancing solid-state batteries is essential for meeting growing demands for safer and higher-energy storage in electric vehicles and renewable-energy systems. Solid polymer electrolytes (SPEs) offer good processability but require enhanced ionic conductivity and interfacial stability to compete with liquid electrolytes. Incorporating ceramic fillers such as garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a promising strategy, yet the influence of filler composition, morphology, and synthesis route on SPE performance remains insufficiently understood.This work provides a systematic comparison of SPEs containing different LLZO filler nanofibrous Al-LLZO, bulk Al-LLZO, and Ta/Nb-LLZO within a PEO-LiTFSI matrix. By correlating filler characteristics with ionic conductivity, overpotential behaviour, and full cell cycling, the study identifies which LLZO attributes most effectively enhance composite electrolyte performance. These insights support the rational design of polymer ceramic electrolytes for high voltage solid-state batteries operating at elevated temperatures, contributing to safer and more sustainable energy storage technologies.