A metalophilic, anion-trapped composite gel electrolyte enables highly stable electrode/electrolyte interfaces in sodium metal batteries

Next-generation rechargeable sodium batteries consisted of layered oxide cathode and sodium metal anode, are desired to meet future energy-storage needs with high energy density and intrinsic low cost. But the undesirable dissolution of transition-metal from layered oxide cathode and severe growth of Na dendrites in the liquid electrolyte block their practical application. Herein, to address these issues, we report a metalophilic, anion-trapped composite gel electrolyte (MACGE) that is capable of simultaneously stabilizing Na metal anode and layered oxide cathode to form stable electrode/electrolyte interfaces. The MACGE is achieved by grafting sodium alginate on one side of boron-contained polymer in-situ modified glass fiber, leading to excellent mechanical properties and high electrolyte wettability/uptake capability. The resulting MACGE shows high ionic conductivity (4.12 mS cm−1), considerable Na+ transference number (0.76), and superior interfacial compatibility. Benefiting from the unique structure and composition, the MACGE can not only selectively transfer Na+ through anion trapping effect of boron moiety, reducing interfacial polarization and suppressing dendrite growth, but also act as cathode protecting layer to effectively suppress continuous dissolution of transition-metal from the cathode through the chemical chelating interaction. As a result, the sodium metal batteries using MACGE exhibit remarkably enhanced cell performances, including excellent cycling stability with 91.5% capacity retention at 0.2 C after 200 cycles, and superior rate capability with 85.8% capacity retention at 3 C after 800 cycles.