Factors controlling the performance of lithium-metal solid-state batteries with polyethylene oxide-based composite polymer electrolytes

Abstract

Solid composite polymer electrolytes (CPEs) have emerged as a promising option due to their excellent ionic conductivity, mechanical flexibility, and compatibility with Li metal electrodes. In this study, polyethylene oxide (PEO) was selected as the base polymer, and a composite was formed with LLZTO and oxygen-vacancy LLZTO (OV-LLZTO) as an active ceramic filler. The surface defects in OV-LLZTO enhance its bonding with the PEO chains, leading to improved interfacial resistance, enhanced mechanical stability, prevention of PEO crystallization, mitigation of LLZTO nanoparticle agglomeration, and improved Li⁺ ion conductivity. The removal of oxygen atoms from the LLZTO crystal results in lattice contraction, which strengthens the interaction between the LLZTO and PEO polymer chains, thereby reducing interfacial resistance and improving lithium-ion conductivity. In solid-state battery performance, the ionic conductivity and transference number of the solid electrolyte are crucial, along with thermal, mechanical, and electrochemical stability. While pristine PEO electrolytes exhibit higher conductivity than composites, they have a lower transference number and inferior stability compared to the composite electrolytes. As the temperature increases, the transference number of the polymer electrolyte increases due to increased ion mobility; however, with aging it decreases due to the formation of a passivation layer. A solid-state full cell employing the PEO/OV-LLZTO electrolyte was used to demonstrate high-rate capability (10C rate) and excellent capacity retention at 60 °C with a cathode areal loading of ∼0.2 mAh cm⁻², underscoring its potential for high-performance battery applications.