A single-ion-conducting polymer and high-entropy Li-garnet composite electrolyte with simultaneous enhancement in ion transport and mechanical properties

Abstract

Enabling the lithium metal anode has been the holy grail for improving the energy density for the next generation advanced batteries. Developing electrolytes that will suppress Li dendrite growth and provide sufficient ionic conductivity remains a major challenge in this field. In this study, we develop a polymer–ceramic composite electrolyte for lithium metal batteries. The polymer matrix is a vinyl ethylene carbonate (VEC) based single-ion-conducting polymer electrolyte. The ceramic filler is a Li₇La₃Zr_0.5Nb_0.5Ta_0.5Hf_0.5O₁₂ high-entropy Li-garnet (HE Li-garnet) ceramic, which is less prone to surface Li₂CO₃ formation compared to Al-doped Li garnets. The addition of HE Li-garnet leads to a 7-fold increase in the ionic conductivity (8.6 × 10⁻⁵ S cm⁻¹ at 30 °C) compared to the pure polymer, while maintaining a high Li⁺ transference of 0.73. Proton nuclear magnetic resonance and thermogravimetric analysis results suggest that the addition of HE Li-garnet results in a lower degree of polymerization of VEC, leaving more unpolymerized VEC monomers in the matrix, serving as the governing mechanism for conductivity enhancement. The favorable interactions between HE Li-garnet particles and the polymer matrix lead to a stable and well-mixed composite with 2-fold enhancement of storage modulus at 40 °C. The simultaneous ion transport and mechanical property enhancement significantly improves the composite electrolyte's dendrite resistance and cycle life in Li symmetric cells. This work highlights the positive role HE Li-garnet can play in improving polymer electrolytes to enable lithium metal anodes.