Significantly improved interface between PVDF-based polymer electrolyte and lithium metal via thermal-electrochemical treatment

Polymer-based solid-state electrolytes have attracted much attention for their potential applications in solid-state lithium (Li) metal batteries due to their flexibility, good interfacial contact with electrodes, low cost, and easy scale-up. However, the thermal response of polymer-based electrolytes is still one of the main concerns. Here, we thoroughly investigated the thermal response of the interfacial stability between a poly(vinylidene fluoride) (PVDF)-based polymer electrolyte and Li metal and found that the Li symmetric cells cycled at 60 °C presented a low polarization voltage and long life due to the balance between the interfacial diffusion kinetics and electrochemical reaction rate at a current density of 0.3, 0.5 or 1 mA cm−2. The interface layer between the PVDF-based electrolyte and Li metal formed at 60 °C was uniformly thin and had a smooth surface, whereas the ones formed at 30 or 90 °C were unevenly thick or showed cracks. Based on the findings in the temperature dependency of the interface layer, we proposed a thermal-electrochemical treatment method through which a stable interface was in-situ formed at 60 °C and 0.3 mA cm−2, rendering an ultralong cycle life, such as 2600 h at a current density of 0.3 mA cm−2 and 30 °C, to the Li symmetric cell with the PVDF-based polymer electrolyte. The thermal-electrochemical treatment improved the cycle performance of LiFePO4||PVDF||Li solid-state batteries. This work provides a strategy to effectively enhance the interfacial stability between the PVDF-based electrolyte and Li metal through rational thermal-electrochemical treatment.