Mesoscale polymer regulation for fast-charging solid-state lithium metal batteries

Abstract

Developing solid-state polymer electrolytes with both high voltage and ionic conductivity is essential for practical solid-state batteries. Poly(vinylidene fluoride) (PVDF)-based solid-state electrolytes are attractive for solid-state lithium metal batteries (LMBs). However, their high mesoscale heterogeneity induced by phase separation during electrolyte preparation leads to the formation of large PVDF spherulites. Herein, we demonstrate that the mesoscale heterogeneity causes the accumulation of Li⁺ on the surfaces of oversized PVDF spherulites. The large spherulites with low interface-bulk ratios greatly impede the efficient long-range Li⁺ conduction within the electrolyte. We propose an efficient strategy to regulate the geometric structure of PVDF spherulites by introducing the polyvinyl alcohol (PVA) coated dielectric SrTiO₃ (PVA@STO) as a nucleating agent in a PVDF-based electrolyte. The excellent interfacial compatibility of the PVA coating layer with PVDF facilitates the uniform dispersion of PVA@STO nano-fillers and regulates the nucleation to form abundant finer PVDF spherulites with more interfaces, thereby providing abundant Li⁺ transport pathways. Furthermore, the well-dispersed PVA@STO nano-fillers can effectively dissociate lithium salt to generate more mobile Li⁺ in the PVDF–PVA@STO electrolyte that exhibits a high ionic conductivity of 8.6 × 10⁻⁴ S cm⁻¹. The Li|PVDF–PVA@STO|NCM811 batteries demonstrate excellent high rate cycling performance, retaining 80.8% and 70.5% capacity after 2000 cycles at 5C and 10C, respectively. This work clearly demonstrates the significant effect of mesoscale structure regulation on a polymer structure for high-performance fast-charging solid-state lithium metal batteries.