Recycling of spent LiFePO4 batteries with an acid-free process to high-rate Na2FeP2O7@C cathodes for sodium-ion storage

Abstract

The escalating retirement of lithium iron phosphate (LiFePO₄, LFP) batteries necessitates the development of sustainable recycling strategies that avoid traditional acid/alkali consumption and secondary pollution. This study pioneers an efficient and green approach for directly converting spent LiFePO₄ (S-LFP) cathodes into high-performance sodium-ion battery (SIB) cathodes. By employing sodium dihydrogen phosphate (NaH₂PO₄)-assisted roasting under an oxygen atmosphere, lithium is selectively extracted in the form of water-soluble complex phosphates (Li₂NaPO₄ and LiNa₅(PO₄)₂) via in situ solid-phase reactions, while iron is stabilized as insoluble NaFeP₂O₇, thereby eliminating the use of strong acids/alkalis and the generation of high-salinity wastewater. Water leaching achieved exceptional Li/Fe separation efficiency, with 99.31% lithium recovery and only 0.24% iron leaching. The recovered lithium phosphate (Li₃PO₄) is used to regenerate high-performance LFP cathodes. Simultaneously, the iron-rich residue is directly converted into a carbon-coated sodium iron pyrophosphate cathode (Na₂FeP₂O₇@C, denoted as NFPO) through carbothermal reduction. The NFPO cathode exhibits outstanding SIB performance, including an ultrahigh rate capability (46.6 mAh g⁻¹ at 30 C) and exceptional cycling stability (98.6% capacity retention after 1500 cycles at 5 C). Full-cell validation (NFPO∥HC) confirms practical viability with 91.8% capacity retention after 100 cycles at 1 C. This article provides a new paradigm for the iterative transformation of used lithium-ion batteries into sodium-ion batteries.