Mechanistically Engineered Heterojunction From Spent LFP for Efficient Oxygen Evolution Electrocatalysis

The large-scale retirement of LiFePO₄ (LFP) batteries demands sustainable strategies for material recovery and functional repurposing. However, the inert micrometer-scale morphology and electronically stable lattice of spent LFP hinder its direct catalytic reuse. Herein, we present a structure-guided strategy that upcycles spent LFP into a high-performance oxygen evolution reaction (OER) electrocatalyst. Mild air oxidation transforms LFP into a phosphate-rich Li₃Fe₂(PO₄)₃ (LFP(III)) framework decorated with coronal α-Fe₂O₃ nanodots. This reconfiguration preserves the bulk structure while achieving nanoscale surface activation. Subsequent spatially selective growth of NiO on α-Fe₂O₃ yields a well-defined p-NiO/n-Fe₂O₃ heterojunction, driving interfacial charge redistribution and promoting formation of catalytically active NiOOH species under operational conditions. In situ Raman and XPS analyses reveal that the heterostructure facilitates lattice oxygen participation via an accelerated lattice oxygen mechanism, while the phosphate-rich LFP(III) matrix imparts strong electrostatic repulsion toward Cl^-, effectively suppressing halide-induced corrosion. The resulting LFP(III)/NiO catalyst achieves low overpotentials of 268 and 292 mV at 10 and 100 mA cm^-2, respectively, in chloride-containing electrolytes, along with excellent durability. Techno-economic analysis indicates a six-fold improvement in cost-efficiency over conventional recycling. This work establishes a mechanistically informed and energy-efficient upcycling strategy that bridges battery waste management with functional catalyst design, advancing both sustainable materials chemistry and water-splitting technologies.