Suppressing storage-induced degradation of Li7La3Zr2O12 via encapsulation with hydrophobicity-tailored polymer nanolayer

Abstract

Solid-state batteries have been proposed as an alternative to conventional lithium-ion batteries to resolve safety issues. Biphasic solid electrolytes (BSEs) based on Li7La3Zr2O12 (LLZO) and a polymer phase have been widely studied because LLZO has high Li+ conductivity and chemical/electrochemical compatibility with Li metal. However, LLZO reacts with H2O and CO2 during storage in air, forming lithium carbonate (Li2CO3) layers on the surface. The extremely low Li+ conductivity of Li2CO3 degrades the Li+-conduction properties of LLZO-based BSEs. Herein, we propose an effective approach to improve the air-stability of LLZO via encapsulation with a hydrophobicity-tailored, Li+-conducting polymer nanolayer. Polyurethane-based polymers are designed to have high hydrophobicity by tuning soft segments and chain extenders and successfully encapsulate the LLZO surface with a thickness of ∼10 nm (P-LLZO). Accelerated durability tests (ADTs) under controlled concentrations of O2, H2O, and CO2 indicate that LLZO encapsulation with hydrophobic polymer effectively mitigates storage-induced degradation by preventing direct contact between LLZO and H2O/CO2. ADT-tested P-LLZO BSE exhibits higher ionic conductivity (σ = 1.3 × 10−4 S cm−1 at 60 °C) compared with that of ADT-tested LLZO BSE (σ = 3.6 × 10−5 S cm−1). A solid-state Li battery with ADT-tested P-LLZO BSE shows enhanced cycling stability than that with ADT-tested LLZO BSE, proving the efficacy of polymer encapsulation. The findings are essential for understanding the role of interfacial engineering in mitigating the degradation of Li+-conduction properties and developing highly conductive LLZO-based BSEs.