Active sodium loss in practical anode-free sodium batteries: mechanisms, challenges, and strategies

Abstract

Anode-free sodium metal batteries (AFSBs) offer a compelling route toward high-energy and sustainable electrochemical storage by eliminating excess sodium and inactive anode hosts. Yet their practical viability is fundamentally limited by rapid and irreversible active sodium loss. In anode-free architectures, cyclable sodium is the governing “currency” of cell lifetime: any interfacial irreversibility directly translates into catastrophic capacity decay. Unlike lithium systems, sodium depletion in AFSBs is driven by Na-specific physicochemical constraints—including a large ionic radius, low melting point, extreme volumetric expansion (≈260%), and intrinsically fragile solid–electrolyte interphase (SEI) mechanics—giving rise to distinct degradation pathways spanning sparse nucleation, porous growth, dynamic SEI fracture–repassivation, thermally induced morphology collapse, and coupled cathode–anode inventory feedback. This review establishes a multiscale mechanistic framework linking intrinsic sodium properties to cell-level failure, and critically assesses emerging mitigation strategies across sodiophilic current-collector engineering, multifunctional interphase design, sodium supplementation, and operation-protocol optimization. By integrating these approaches within a “source-process-inventory-environment” regulation paradigm, we outline key design rules and future priorities required to suppress sodium depletion and accelerate the translation of AFSBs from laboratory concepts into practical high-energy batteries.