PMMA In Situ Combination and FEC Interface Modification co‐Enhancing the Stability of SN ‐Based Electrolyte for Solid‐State Lithium Battery

Succinonitrile (SN) electrolyte is emerging as an ideal candidate for solid‐state electrolytes due to its high ionic conductivity, wide electrochemical stability window, and low cost. However, severe side reactions with lithium metal and poor mechanical strength have significantly hindered the rapid development of SN‐based electrolytes. In this study, an in situ thermal polymerization technique was employed to integrate SN with cross‐linked poly(methyl methacrylate) (PMMA), yielding a composite solid electrolyte that combines high ionic conductivity with excellent mechanical properties. The optimized SN‐PMMA50 electrolyte achieves a room‐temperature ionic conductivity of 2.95 × 10 −4 S cm −1 , an electrochemical stability window of 4.75 V, a lithium‐ion transference number of 0.68, and a tensile strength of 165 MPa. Furthermore, the introduction of fluoroethylene carbonate (FEC) as a solid electrolyte interphase (SEI)‐forming additive significantly suppresses the side reactions between SN and the lithium anode. The optimized SN‐PMMA50‐FEC30 composite electrolyte exhibits excellent electrochemical performance and cycling stability. The assembled Li/LiFePO 4 solid‐state battery retains 96% capacity after 200 cycles at room temperature. The incorporation of PMMA and FEC markedly enhances the mechanical properties of the electrolyte and the stability of the electrode/electrolyte interface, which provides a highly promising strategy for the application of SN‐based polymer electrolytes in lithium metal batteries.