From Spent Batteries to Green Hydrogen: Catalytic Upcycling of Lithium‐Ion Battery Cathodes for Water Electrolysis

The accelerating global "dual-carbon" transition and the rapid proliferation of electric vehicles are driving an unprecedented surge in spent lithium-ion batteries (LIBs), with the first major retirement peak expected around 2030. Cathode materials form a pivotal bridge between urban mining and green-hydrogen technologies, coupling environmental risks with the strategic importance of critical metals. This review delivers a comprehensive overview of the recycling and upcycling landscape for the three dominant cathode families-LiCoO₂, LiNi_xCo_yMn_1-x-yO₂, and LiFePO₄. We outline the compositional and structural features of these materials, evaluate pretreatment protocols, and critically compare pyrometallurgical, hydrometallurgical, and direct-regeneration strategies. We then highlight how multiscale structure-activity correlations guide the transformation of regenerated cathodes into high-performance electrocatalysts, with emphasis on defect engineering, electronic-structure modulation, interfacial coupling, and the assembly of conductive networks to accelerate both hydrogen- and oxygen-evolution pathways. Finally, we propose a forward-looking design framework that integrates atomic-site dynamics, multimetallic synergy, and process-environment co-optimization, while underscoring emerging opportunities in machine-learning-guided inverse design, operando mechanistic mapping, and device-level implementation. This review provides a conceptual blueprint for integrating battery recycling with green-hydrogen production in a closed-loop materials ecosystem.