Progress and Perspectives on Confinement Engineering to Revive Alloying-Type Anode Materials for Sodium-Ion Batteries

With the rise of sodium-ion batteries (SIBs), alloying-type anodes offer high specific capacities and suitable operating potentials through multi-electron reactions, positioning them as key to enhancing the energy density of SIBs. However, their serious volume expansion leads to electrode pulverization and repeated rupture of the solid electrolyte interphase (SEI) film, severely compromising the cycle life. Confinement engineering has recently emerged as a powerful strategy for addressing the intrinsic limitations of alloying-type anodes. Nevertheless, systematic classification and in-depth mechanism clarification of diverse confinement strategies remain lacking. To fill this gap, this review systematically summarizes the fundamental principles, structural motifs, and design strategies of confinement engineering. Different from previous material-oriented reviews, this work categorizes confinement engineering into self-confinement and extrinsic confinement, and profoundly clarifies the structure-regulation mechanism at the micro/nano scale, emphasizing the critical role of confinement engineering in mitigating volume expansion, stabilizing SEI films, and enhancing reaction kinetics. Key advances in alloying-type anode development are highlighted, alongside a critical assessment of the persisting challenges. Finally, future directions are outlined to provide theoretical guidance and innovative strategies for accelerating the commercialization of alloying-type anode materials for high-energy-density and long-cycle-life SIBs.