Ni-Mediated Spin Engineering in FeOOH from Spent LiFePO ₄ Cathodes for Enhanced Oxygen Electrocatalysis

The retirement of lithium iron phosphate (LiFePO₄, LFP) batteries presents a resource recovery challenge. Here, we report an effective strategy to transform spent LFP cathodes into efficient oxygen evolution reaction (OER) catalysts through <i>in situ</i> Ni modification and electrochemical activation. This process converts LFP into defect-rich, hollow Ni-FeOOH architectures. Ni incorporation modulates the Fe valence and induces a spin-state transition from high-spin to low-spin. <i>In situ</i> characterization and theoretical calculations reveal that the resulting low-spin Fe^(3-δ)+ species accelerate the formation of the active Ni-FeOOH phase and optimize the adsorption of oxygen intermediates. Consequently, the engineered catalyst exhibits outstanding OER activity, requiring only a 281 mV overpotential to achieve 10 mA cm^-2, significantly outperforming FeOOH derived from pristine LFP. This work highlights spin-state engineering for OER enhancement and provides a sustainable path to valorize retired batteries into high-performance catalysts, promoting resource circularity and advanced energy technologies.