Effects of Flexible Group Length of Phosphonate Monomers on the Performance of Gel Polymer Electrolytes for Sodium-Ion Batteries with Ultralong Cycling Life

Using gel polymer electrolytes instead of liquid electrolytes is one of the methods to avoid safety hazards such as leakage, decomposition, and dendrite growth caused by conventional electrolytes. In this work, a series of functional monomers of methyl phosphonate with different flexible-group chain lengths are devised and synthesized. On the basis of these phosphonates, the gel copolymer electrolytes are obtained by in situ thermal copolymerization with trifluoroethyl methacrylate and methyl methacrylate. All gel copolymer electrolytes have excellent thermal stabilities, wide electrochemical stable windows (≥5.0 V vs Na/Na + ), and adequate ionic conductivities (>1.4 × 10 –3 S cm –1 ), and the sodium ion conduction mechanism is discussed and supplemented by a theoretical calculation. The assembled Na 3 V 2 (PO 4 ) 3 /Na batteries with gel copolymer electrolytes all have better ultralong cycling stabilities than liquid cells with higher capacity retentions above 76% after 5000 cycles. Furthermore, the Na 3 V 2 (PO 4 ) 3 / GPE1–GPE3 /Na batteries with shorter lengths of flexible-group chains have much better cycling stabilities and much higher capacity retentions up to 87% after 5000 cycles, and the Na 3 V 2 (PO 4 ) 3 / GPE1–GPE2 /Na batteries with shorter lengths of flexible-group chains have better rate performance than GPE4 -based batteries with longer chain lengths. In addition, the polarization properties, the interfacial compositions obtained by XPS, and the morphology characterizations of separators after cycling show that the gel polymer electrolytes with short-chain monomers have better interfacial stabilities and better capabilities of inhibiting decomposition of liquid electrolytes. In summary, the influence of multifunctional phosphonate monomers with different chain lengths on battery performance is studied in detail, and the structure–activity relationships of phosphonate-based gel polymer electrolytes are discussed. That provides a reference for the design and preparation of gel polymer electrolytes with better performance.