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

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 x 10-4 S cm-1 at room temperature, which makes the symmetric Li||Li batteries show an ultra-long cycle life (approximate to 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 LiNi0.6Co0.2Mn0.2O2 (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. We incorporate HFA into PVDF-based polymer-in-salt solid electrolyte to form n-AGGs with continuous Li-ion transport path and a LiF-rich SEI, enhancing both ionic conductivity and stability. NCM622||Li cells stably operate over 600 cycles at 0.2C.