Analogue Molecular Doping Engineering Enables High Ionic Conductivity of Polyvinylidene Fluoride-Based Polymer Electrolytes

Solid polymer electrolytes (SPEs) based on polyvinylidene fluoride (PVDF) are promising candidates due to their outstanding mechanical properties and intrinsic safety features. Unfortunately, the crystalline α phase of PVDF limits the mobility of lithium ions, thus leading to low lithium ion conductivity. Herein, a molecular doping strategy is proposed to achieve high lithium ion conductivity of the PVDF-based electrolyte (<i>md-</i>PVDF) via introducing polyvinylidene dichloride (PVDC) to reduce the generation of the harmful α phase of PVDF. As the molecular analog of PVDF, PVDC is homogeneously dispersed in PVDF at arbitrary concentrations, and it disrupts the crystallization of the PVDF matrix. Moreover, the chlorine functional group in doping molecular PVDC not only enhances the dissociation of Li salt but also reduces the energy barrier of lithium-ion migration. Consequently, the resulting <i>md-</i>PVDF electrolytes show significantly high ionic conductivity (1.4 × 10^-3 S cm^-1 at room temperature). The lithium symmetric batteries with <i>md-</i>PVDF electrolytes cycle stably for over 2000 h at 0.1 mA cm^-2, and the Li||LFP batteries display excellent cycling stability over 500 cycles at a high rate of 5 C. In addition, the <i>md-</i>PVDF electrolytes exhibit outstanding low-temperature performance, achieving an ionic conductivity of 3.0 × 10^-4 S cm^-1 at -5 °C. This work demonstrates a strategy to improve the ionic conductivity of SPEs and to realize fast charging of solid-state lithium.