Direct Repair of the Crystal Structure and Coating Surface of Spent LiFePO4 Materials Enables Superfast Li-Ion Migration

Abstract

The rapid accumulation of spent LiFePO4 (LFP) cathodes from retired lithium-ion batteries necessitates the development of effective and environmental-friendly recycling strategies. In this context, direct regeneration has emerged as a promising approach for reclaiming LFP cathode materials, offering a streamlined pathway to restore their electrochemical functionality. We report an integrated regeneration protocol that simultaneously repairs the degraded crystal structure and reconstructs the damaged carbon coating in spent LFP. The regenerated cathode material had superfast lithium-ion diffusion kinetics and a stable cathode–electrolyte interface, giving a remarkable rate capability with specific capacities of 122 mAh g−1 at 5C and 106 mAh g−1 at 10C (1C = 170 mA g−1). It also maintained capacities of 110.7 mAh g−1 (5C) and 84.1 mAh g−1 (10C) after 400 cycles. It could be used in harsh environments and could be stably cycled at subzero temperatures (−10 and −20 °C) and in solid-state electrolyte batteries. Life cycle assessment combined with economic evaluation using the EverBatt model reveals that this direct regeneration approach has high economic and environmental benefits.

1. Introduction

Lithium-ion batteries (LIBs) have become dominant power source for electric vehicles (EVs) and grid-scale energy storage systems [1–4], especially those using LiFePO4 (LFP) as cathodes [5–7]. With first-generation EV batteries approaching their end of life, there is a critical need to develop sustainable solutions for LFP recycling [8–11]. Conventional pyrometallurgical and hydrometallurgical recycling processes are not cost-effective because of their large energy demands and high reagent consumption [12]. Furthermore, the inherent volatility in the price of the lithium salt increases the financial risk associated with these conventional methodologies that mainly focus on the recovery of lithium compounds [13]. As a result, direct regeneration strategies that restore the structural integrity of degraded cathode materials using targeted crystal repair rather than complete material decomposition, have received increasing interest.

The key failure mechanisms of LFP cathode materials are generally regarded as a damaged crystal structure, including Li loss, Li-Fe anti-site defects (FeLi), irreversible phase transitions and a damaged surface coating layer [14–16]. Recent advances in direct regeneration have demonstrated that the structural recovery of spent LFP (s-LFP) has three essential requirements: (i) a reductive environment to reverse Fe oxidation states, typically achieved through organic reductants like ethanol [17], glycerol [18], lithium triethyl borohydride [19], polycyclic aromatic hydrocarbons [20] or citric acid [21], (ii) lithium supplementation using Li salts such as LiNO3 [22], Li2CO3 [16, 23] or LiOH [21, 24–26] and (iii) coating regeneration to form a uniform carbon layer using glucose, polyvinylidene fluoride, etc. [27, 28]. While pioneering work by Ji et al. used 3,4-dihydroxybenzonitrile dilithium as a multifunctional regeneration agent achieving 88% capacity retention after 400 cycles at 5C [29], its commercial viability is limited by toxicity concerns and prohibitive costs. Meanwhile, most hydrothermal-related reports use multiple reagents and over 140 °C reactions [30, 31]. This highlights the urgent need for developing benign, cost-effective regeneration protocols that remove both structural defects and surface damage.

We report an integrated regeneration strategy that combines low-temperature hydrothermal relithiation with surface engineering. Our approach uses lithium oxalate (Li2C2O4) to simultaneously provide lithium replenishment and produce reductive conditions.