Toward Anode-Free Sodium Metal Batteries: Engineering Metal Utilization from Excess to Zero.

Sodium metal batteries (SMBs) with limited metal content or anode‑free configurations hold significant potential to mitigate safety risks associated with active sodium (Na) metal, while delivering high energy density and reducing environmental impact. Nevertheless, the absence of a sustained Na supply readily leads to rapid battery performance degradation due to the irreversible consumption of Na source during repeated charge/discharge cycles. Consequently, there is an urgent need for a comprehensive and detailed review on enhancing metal utilization in SMBs, particularly addressing failure mechanisms and strategies to extend battery lifespan. This review provides in-depth discussions on the relationship between metal utilization and the depth of discharge, and the bridge transitioning from uncontrolled sodium content to anode-free configuration. Based on this, the proposed review will focus on the variations in metal utilization, elucidating the distinct characteristics and design strategies of sodium metal batteries with varying metal content scenarios (limited metal content and anode‑free configurations). This review will systematically overview their configuration, working mechanisms, inherent advantages, and associated challenges. It will also highlight the latest advancements in performance enhancement and conclude with a brief discussion on the future research directions for SMBs.