PVDF-based Composite Solid Polymer Electrolyte Incorporated with Cubic-ZrO2-x for Long-cycle Lithium Metal Batteries

Abstract

Solid-state lithium metal batteries (SSLMBs) have great potential as high-energy-density, high-power, and safe energy storage devices. Solid polymer electrolytes based on polyvinylidene fluoride-poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-(PVDF-HFP)) have attracted significant attention due to their high ionic conductivity at room temperature, good flexibility, and unique ion transport properties, making them promising candidates for commercial applications. However, PVDF-based composite solid polymer electrolytes (CSPEs) exhibit poor lithium-ion transport properties and are prone to chemical reactions with the lithium metal surface, posing the risk of forming lithium dendrites. This degradation results in reduced battery performance and cycle life. This work investigates the effects of Co-doped cubic-ZrO_2-x (C-ZrO_2-x), a filler with abundant oxygen vacancies, on the electrochemical properties of PVDF-based CSPE. Zirconium in zirconia can provide Lewis acid sites, while oxygen vacancies can adsorb TFSI⁻ and exhibit a pronounced ability to dissociate lithium salt and accumulate anchored anions. These effects are further amplified after phase transition, from unstable monoclinic phase with a small amount of oxygen vacancies to cubic phase with abundant oxygen vacancies, leading to a higher proportion of free Li⁺ in PVDF-based CSPE. Additionally, the uniform distribution of solid electrolyte interphase with a single LiF component, indicates that C-ZrO_2-x increases the interface stability between electrolyte and lithium metal. The corresponding Li||Li symmetrical cells show a stable cycle lifetime of 730 h at 0.1 mA cm⁻². The specific discharge capacity of Li|CSPE|LiFePO₄ reaches 106.09 mAh g⁻¹ at 1 C. Even after 550 cycles, the capacity retention rate is still 92.13%. These findings ideally contribute to an effective way to achieve high performance and long cycle life of solid-state lithium metal batteries.