Electrolyte-Dependent Sodium Plating for Anode-Free Na-Ion Batteries Studied by Operando Optical Microscopy

Anode-free sodium ion batteries (SIBs) promise higher energy density and lower costs, by eliminating the need for an anode host material; however, achieving efficient Na plating/stripping remains a major challenge. Here, three electrolyte classes - carbonate-based, glyme-based, and a localized high-concentration electrolyte-are evaluated for Na plating/stripping on a commercial carbon-coated aluminium current collector. Measurements across a broad temperature and current range (-30°C-+60°C, 0.25-14 mA cm^-2-) and studies on the Na growth modes by operando optical microscopy reveal the superior behavior of the glyme-based electrolyte, including a uniform crystalline metal deposition. In anode-free full cells with Na₄Fe₃(PO₄)₂P₂O₇ as cathode, this electrolyte enables superior cycling with 75.3% capacity retention over 400 cycles at areal loadings above 3 mAh cm^-2-. Projected energy densities of 290 Wh/kg and 751 Wh/l are calculated at the cell-stack level, exceeding current LiFePO₄-based Li-ion batteries. The excellent Na plating/stripping behavior is evidenced by a particularly low initial areal capacity loss (IACL, mAh cm^-2). The IACL parameter represents the first cycle Na inventory loss that must be compensated by the cathode. Unlike for conventional Na-ion cells with traditional anodes, the IACL is a constant for anode-free cells. For the given cell, the IACL amounts to only 0.14-0.16 mAh cm^-2 (~5% of the 3 mAh cm^-2 cathode areal capacity). This highlights the potential of anode-free SIBs using commercially available components.