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 to practical high-energy batteries.