Engineering a well-connected ion-conduction network and interface chemistry for high-performance PVDF-based polymer-in-salt electrolytes

Abstract

The challenges posed by the low ionic conductivity at room temperature and the poor compatibility of the Li metal/electrolyte interface restrict the practical application of solid-state polymer electrolytes. Herein, a bifunctional heptafluorobutyric anhydride (HFA) additive has been proposed to overcome the abovementioned problems of the polyvinylidene fluoride (PVDF)-based polymer-in-salt solid electrolyte. The experimental data, density functional theory (DFT) calculations and the molecular dynamics (MD) simulations demonstrate that the HFA additive can promote the formation of nanometric aggregates (n-AGGs), which can construct a continuous and fast Li⁺ ion transport network in the solid electrolyte. Meanwhile, the HFA additive exhibits a lower LUMO energy level, and then it can contribute a LiF-rich SEI at the Li metal/electrolyte interface. As a result, the fabricated solid electrolytes with HFA additive can deliver an improved ion conductivity of 2.41 × 10⁻⁴ S cm⁻¹ at room temperature, which makes the symmetric Li‖Li batteries show an ultra-long cycle life (≈1700 h at 0.1 mA h cm⁻²). Besides, the HFA-containing solid electrolytes show greatly enhanced cycle stability compared to the pristine electrolyte in the full cells when paired with a LiFePO₄ cathode or high-voltage LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) cathode. This work provides a critical insight into the mechanism of a new additive to enhance the electrochemical performances of PVDF-based polymer-in-salt solid-state electrolytes.