Indium Oxychloride as Current Collector Modifier for Limited‐Sodium Metal Batteries

ABSTRACT Alkali metals attract considerable interest as anodes for high‐energy‐density batteries due to their high theoretical capacity and low electrochemical potential. However, their practical implementation remains limited by the uncontrolled growth of metallic dendrites and the formation of unstable solid electrolyte interphase (SEI) layers during cycling, resulting in poor Coulombic efficiency and short lifespan. Herein, we address these issues in sodium metal batteries through a simple yet effective surface modification using an indium oxychloride coating on the current collector. Our experimental results demonstrate the formation of an artificial SEI hybrid layer composed of NaIn and sodium oxychloride (Na 3 OCl). This hybrid layer combines enhanced ion diffusion, improved interfacial stability, and space charge regulation to enable uniform dendrite‐free sodium deposition. Density functional theory (DFT) calculations further support the superior role of oxychloride chemistry over conventional alloy‐based modification approaches like NaIn, particularly in promoting horizontal plating and suppressing dendrite formation. We further provide clear evidence for our hypothesis using in situ optical microscopy studies, fabricating custom‐designed cells to directly monitor sodium deposition and interfacial evolution during cycling. The modified limited‐sodium anode exhibits ultra‐low polarization (25 mV) and achieves highly reversible plating/stripping for 2100 h at 1 mAh cm −2 in an ether‐based electrolyte, significantly outperforming anode with sodium plated on a pristine Cu current collector, which cycles only for 50 h. The surface‐modified anode shows high reversibility even at high current densities of 12 mA cm −2 . In full‐cell configuration with a Na 3 V 2 (PO 4 ) 3 (NVP) cathode, the system delivers capacities of 107 mAh g −1 at 0.1 C and 90 mAh g −1 at 5 C, while retaining 90.2% of its capacity over 900 cycles at 1 C rate, offering 99.1% Coulombic efficiency. This work showcases the potential of oxychloride‐based interface engineering in enabling safe and high‐performance limited‐sodium metal batteries, paving the way for next‐generation energy storage technologies.