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

ABSTRACT 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 Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 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 LiFePO 4 ‐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.