Ni-Mediated Spin Engineering in FeOOH from Spent LiFePO₄ 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 in situ 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. In situ 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.