The development of efficient strategies to recycle lithium-ion battery (LIB) electrode materials is an important yet challenging goal for the sustainable management of battery waste. This work reports a facile and economically efficient method to convert spent cathode material, LiFePO4, into a high-performance NiFe oxy/hydroxide catalyst for the oxygen evolution reaction (OER). Herein, Ni-LiFePO4 is synthesized via the wetness impregnation method and further evolves into defect-rich NiFe oxy/hydroxide nanosheets during the OER. The introduction of the Ni promoter together with in situ evolution strengthens the electronic interactions among the metal sites and creates an abundance of defects. Experimentally, the evolved Ni-LiFePO4 delivers a low overpotential of 285 mV at 10 mA cm(-2) and a small Tafel slope of 45 mV dec(-1), outperforming pristine LiFePO4 and is even superior to the benchmark catalyst RuO2. Density functional theory (DFT) calculations reveal that the introduction of Ni effectively activates Fe sites by optimizing the free energy of the *OOH intermediate and that the abundance of oxygen defects facilitates the oxygen desorption step, synergistically enhancing the OER performance of LiFePO4. As a green and versatile method, this is a new opportunity for the scalable fabrication of excellent electrocatalysts based on spent cathode materials.