Direct regeneration mechanisms of spent lithium iron phosphate cathode materials for sustainable lithium ion battery recycling

Abstract The rapid adoption of lithium iron phosphate (LiFePO 4 , LFP) batteries in electric vehicles has accelerated end-of-life battery accumulation. Conventional pyrometallurgical and hydrometallurgical routes decompose the cathode structure. High energy consumption, significant greenhouse gas (GHG) emissions, and economic losses for LFP feedstocks are the primary drawbacks of these routes. There is an urgent need for a more sustainable recycling strategy. Direct regeneration offers a lower-emission alternative with demonstrated economic potential under optimized conditions. This review examines the current state of direct regeneration of spent LFP (S-LFP) cathode materials. Active lithium (Li) loss, Li-Fe antisite defect formation, carbon coating degradation, and particle cracking are analyzed as the primary failure mechanisms. Five direct regeneration strategies are systematically evaluated: solid-state sintering, hydrothermal relithiation, electrochemical relithiation, organic lithium salt-assisted relithiation, and molten salt relithiation. The strategies are compared on a common trade-off between processing strength, output rate, and deployment. Feedstock impurity rather than peak recovered capacity is identified as the decisive variable for industrial scale-up. Structural, chemical, and electrochemical characterization methods used to validate regeneration quality are also reviewed. Direct recycling generates GHG emissions of 0.32 kg CO 2 -eq kg −1 to 0.59 kg CO 2 -eq kg −1 and produces consistent economic returns of + $885 t −1 to + $2,556 t −1 from LFP feedstocks. Within the system boundaries and assumptions compiled in this review, direct regeneration emerges as the most favorable of the four recycling approaches evaluated for LFP feedstocks. This ranking is sensitive to feedstock quality and the scope of the underlying studies. Commercial deployment is currently concentrated in China. Adoption in Western markets remains constrained by feedstock variability and capital requirements. Key research gaps include the lack of industrial-scale techno-economic assessments and closed-loop integration with battery manufacturing. Machine learning-assisted process optimization, continuous-flow hydrothermal reactor engineering, and co-located manufacturing-regeneration facilities are proposed as future directions. This work contributes to SDG 7 (Affordable and Clean Energy), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action) by advancing a low-emission and economically viable route for closing the material loop in electric vehicle battery systems.