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.