Engineering black phosphorus and its composites for advanced anodes in alkali metal-ion batteries: progress and challenges.

The rapid advancement of energy storage technologies has spurred the search for advanced anode materials for alkali metal-ion batteries (AMIBs). Black phosphorus (BP), a layered phosphorus allotrope whose few-layer form (phosphorene) is a two-dimensional (2D) material, has garnered significant attention due to its high theoretical capacity (∼2596 mAh g-1), favorable ion-transport properties, and tunable electronic structure, making it an attractive anode candidate for AMIBs. However, practical implementation is hindered by air/moisture instability, large alloying-induced volume changes, and unstable solid electrolyte interphase (SEI) formation. This review summarizes recent progress in engineering BP for AMIB anodes. BP's structural and physicochemical properties, representative synthesis/exfoliation routes, and alkali metal-ion storage mechanisms are first outlined. Composite and interfacial engineering strategies, including carbon integration, metallic reinforcement, transition-metal-compound hybrids, polymer encapsulation, metal-organic framework (MOF)/covalent organic framework (COF) frameworks, and few-layer BP-based composites, are then highlighted to regulate charge/ion transport, buffer mechanical strain, and stabilize SEI evolution. These advances are discussed in two parts, focusing on Li-ion systems and then extending to Na/K-ion batteries. Finally, we outline remaining challenges and future opportunities toward scalable, durable, and high-performance BP-based anodes for next-generation AMIBs.