Direct Hydrometallurgical Regeneration of Spent Lithium Iron Phosphate Cathodes Using Citric Acid for Closed-Loop Electric Bus Battery Recycling

The rapid electrification of public transit systems has led to a massive surge in retired electric bus batteries, particularly those utilizing lithium iron phosphate (LiFePO₄) chemistry. Conventional recycling processes, such as pyrometallurgy and indirect hydrometallurgy, are highly energy-intensive and economically unsustainable for LiFePO₄ due to the low intrinsic market value of iron and phosphorus. In this study, we present a highly efficient, closed-loop direct hydrometallurgical regeneration process using citric acid as both a green chelating agent and an in-situ carbon source. Spent LiFePO₄ cathodes harvested from retired electric buses were selectively relithiatied and structurally restored. The citric acid-mediated hydrothermal treatment, followed by a short calcination step, successfully repaired the degraded lithium-deficient crystal lattice. The regenerated LiFePO₄ exhibited excellent electrochemical performance, delivering an initial discharge capacity of 151.2 mAh g^-1 at 0.1 C and maintaining 94.3% capacity retention after 500 cycles at 1 C. Life cycle assessment indicated that this direct regeneration pathway reduces greenhouse gas emissions by 62% and energy consumption by 48% compared to virgin cathode production, demonstrating a highly sustainable, low-carbon approach for closing the loop on electric bus transit batteries.