Enhancing Electrochemical Interfacial Stability in Solid-State Lithium Metal Batteries through MgF2–Filled Composite Polymer Electrolyte Design

Poly(vinylidene fluoride) (PVDF)-based solid electrolytes have been extensively studied for lithium-ion batteries due to their high ionic conductivity, which is achieved by accommodating excess lithium salts to form a “salt-in-polymer” structure. However, when prepared using N,N-dimethylformamide (DMF) as a solvent, the residual DMF causes severe interfacial side reactions with the lithium metal anode, significantly hindering Li+ conduction and promoting lithium dendrite growth. This study addresses these challenges by incorporating magnesium fluoride (MgF2) particles as inert fillers in PVDF-based solid electrolytes. The MgF2 effectively restricts DMF movement, thereby increasing the electrochemical window to 4.72 V. The competitive effect of MgF2 with Li+ restricts TFSI– movement, releasing more free Li+, and results in an ionic conductivity of 0.88 mS cm–1 and a lithium-ion transference number of 0.55. Additionally, an in situ formed Li3N-LiF interfacial layer between the lithium metal and electrolyte during cycling effectively inhibits lithium dendrite growth. At a current density of 0.1 mA cm–2, the lithium symmetric cell operates stably for over 1000 h. Coin cells with LiFePO4 and NCM811 cathodes achieve discharge specific capacities of 146.5 mAh g–1 at 30 °C and 0.5 C (with no capacity decay after 75 cycles) and 104.8 mAh g–1 at 30 °C and 1 C (with 64.85% capacity retention after 400 cycles), respectively. These results demonstrate that MgF2 enhances the electrochemical stability and performance of PVDF-based solid electrolytes, offering a promising approach to improving lithium-ion battery safety and efficiency.