In Situ Polymerized Gel Electrolytes on Na-Metal/Electrodes for High-Performance Sodium-Ion Batteries

A solid sodium-ion conducting gel polymer electrolyte (GPE) is presented, directly synthesized on the sodium-metal/electrodes, as an alternative solid-like electrolyte to conventional liquid and ceramic electrolytes for high-performance sodium-ion batteries (SIBs). The GPE is obtained via an efficient synthesis protocol, wherein a mixture of liquid electrolyte (LE) and polymer precursors (PP) is in situ polymerized inside a prefabricated porous electrospun polyacrylonitrile (PAN) membrane, stabilized on either Na-metal or Na₃V₂(PO₄) (NVP) electrode. The membrane's high porosity allows substantially high LE (≈60 vol%) confinement within its pores, which, following the polymerization, leads to a densely packed, mechanically and thermally stable amorphous gel. The GPE exhibited high sodium-ion conductivity (≈1 mS cm^-1 ≈ LE ion conductivity) and transference number (0.63), wide electrochemical stability window (5.2 V), and excellent interfacial stability. The chemically stable GPE mitigated dendritic growth, leading to successive sodium-stripping/plating over 100s of cycles with low overpotentials at varying current densities. While the solid-electrolyte interphase (SEI) growth on the Na-electrode in the case of LE, studied using electrochemical impedance spectroscopy, reveals an unstable self-similar variation of growth with time (≈1 month), the GPE exhibits a stable SEI growth over longer durations of time (≈2 months). In (Na||Na₃V₂(PO₄)₃) configuration, i.e. sodium-metal batteries (SMBs) and symmetric Na₃V₂(PO₄)₃||Na₃V₂(PO₄)₃ batteries, the GPE shows remarkable cyclability, delivering consistent performance at 1C over 500-1000 cycles at room temperature. The diversity of GPE is also demonstrated via its remarkably stable cyclability over 100s of cycles with novel Sn-based alloy anodes, an upcoming alternative anode to hard carbons.