Direct relithiation and efficient regeneration of spent LiFePO4 materials through thermochemical healing

The environmentally friendly and low-cost recycling of spent LiFePO4 (LFP) cathode materials has become an urgent problem. This paper aims to employ eutectic Li+ molten-salt-assisted roasting approach to relithiation and regenerating of spent LFP materials under ambient conditions. Via Li+ compensation and structure reshaping, LiFePO4 cathode material with various degradation conditions can be successfully regenerated, which enables the renovation of the electrochemical performance (the capacity, cycling stability, and rate capability) to the levels of the pristine LFP. It opens a door to the prospect of recycling and remanufacturing degraded cathode materials by this new method, having a strong potential for industrial application.


Introduction
Ever since the invention of lithium iron phosphate (LiFePO 4 ) in 1997, the olivine-type LiFePO 4 is one of the most used cathode materials for lithium-ion batteries (LIBs) [1,2].With the explosive growth of electronics and electric vehicles, the yield of lithium-ion power battery (mainly be LiFePO 4 batteries) has dramatically increased, leading to an increase of end-of-life LIBs [3][4][5][6][7].Waste power battery materials is regarded as a valuable urban mine, and the proper handling of spent LIBs has become an important and urgent task, in which the recycling and re-use of cathode materials to reclaim lithium (Li) and highly valuable metals is the major purpose [8].
Traditionally, hydrometallurgical extraction of lithium, used for recovery ternary materials, is widely adopted and extended to recycle waste LFP materials.Via acid leaching and extraction, the valuable lithium is reclaimed in the form of individual products (Li 2 CO 3 , Li 3 PO 4 ), but Fe, P components, and the leaching residues cannot be efficiently re-used and properly handled [3,9,10].Moreover, the new cathode materials with re-configurable architectural and new high-performance are obtained through the hydrometallurgical process, involving leaching, calcination, and resynthesis under mild conditions [11][12][13].For example, Jai et al. retrieved Li 2 CO 3 and FePO 4 from spent LiFePO 4 by organic acids and regenerated LiFePO 4 [12].Although hydrometallurgy can return the high-value lithium back into the LIB production chain, the design process is very tedious and inefficient.Recently, the direct regeneration method is to heal the composition and repair the structural defects of degraded powders by supplementing active lithium and other loss elements, which is inspired by the loss of active lithium in Li-deficient materials and the stability of the crystal structure in the cycle process [3,[14][15][16].The direct regeneration strategy can greatly diminish the production cost of LFP material and reduce environmental pollution.The born-again LiFePO 4 particles are effectively obtained by hydrothermal process or/and heat-treated and exhibited an ideal capacity and long cycling stability in Li-ion batteries, but generally requires severe operating conditions such as high pressure or/and high temperature [14,16,17].Therefore, the simpler, efficient, and environment friendly strategies are needed to be explored for direct regenerating of scrapped LFP cathodes.
Eutectic molten salts, a class of homogenous system, are melts or solidifies at eutectic melting point, which react with the precursors and provide a liquid environment during crystal crystallization and particle growth (known as "dissolution-recrystallization" mechanism) [18][19][20].Meanwhile, the ion pairs of molten salts exhibit the following characteristics: high ionic concentration and diffusion rate [19,20].Many researchers think that the eutectic molten system can be used as reaction medium for "solvent-free" chemical reactions of materials.For example, the high-performance LIB cathodes were synthesized using Li-based eutectic molten salts [20,21].According to the report, the eutectic melting point of LiNO 3 -LiOH (molar ratio of 3:2) is 175 °C.The structure defects, phase, and Li-composition existed in degrade cathodes may be reformed and compensated through the reaction of substance and Li-salt medium, which can address the multifaceted problems of high pressure and high temperature.
The molten salt mixtures have better dissolving capacity, abundant Li + , and strong ion diffusion rate at homogeneous thermal circumstance, which can quickly penetrate the internal Li-deficient material, compensating Li + deficiency and repairing damaged structure through a "dissolution-recrystallization" mechanism.Herein, we confirm the direct recycling of the degraded LFP via a eutectic Li + molten-salt assisted roasting strategy to compensate for the loss of Li + (Fig. 1).The regenerated LiFePO 4 cathodes deliver a comparable discharge capacity of 151.2 mAh g −1 at 0.2 C. For the cycling stability, it displays capacity retention of 97.73% over 150 cycles at 1 C.The proposed eutectic Li + moltensalt-assisted roasting strategy is much benign for regeneration the used LiFePO 4 materials.

Preparation and recovery of spent LFP
The spent LFP of LIBs were provided by Shenzhen Qiantai Renewable Energy Technology Co., Ltd, (Shenzhen, China).The waste LFP powder was collected and pre-treated according to the reporting method [19,22].
Cycling LFP (C-LFP) powers and citric acid (CA) were mixed with eutectic lithium salt mixture (LiNO 3 :LiOH = 3:2).The mixture was heated at different temperatures for 4 h for relithiation and repair.Subsequently, the powders were washed with deionized water to remove the excess amount of Li salts; the regenerated LFP was centrifuged and finally dried at 80 ℃ for 12 h.The regenerated LFP samples obtained at given temperatures of 350, 450, and 550 ℃ were defined as R-LFP-350, R-LFP-450, and R-LFP-550, respectively.The R-LFP-no CA material was obtained without CA.

Characterization of materials
Thermogravimetric analyzer (TGA) of the mixture of waste LFP and the eutectic Li salts was performed in the same temperature range using Mettler TGA/DSC3 + .The crystalline structures and chemical state of the elements of all cathodes were characterized by X-ray diffraction (XRD, λ = 0.154 nm, Bruker D8 Phaser, Japan) with Cu Kα radiation and X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC k-Alpha), respectively.The chemical composition of Li and Fe of the samples was analyzed by inductively coupled plasma optical emission spectrometry (ICP-MS, Agilent 7700 s).The TEM and HR-TEM images of the materials Fig. 1 The illustration of the relithiation process for Li composition recovery via homogeneous thermochemical process.The olivine-type structure of C-LFP shows the Fe vacancies (Fe v , as-defined H0), Li vacancies (Li v , H1), and Fe occupation in Li site (Fe Li ).Right side displays R-LFP with all the Fe 3+ being reduced to Fe 2+  were characterized with a transmission electron microscope (FEI Tecnai G2 F20).

Electrochemical characterization
The slurry mixed the active materials, Super P65, and polyvinylidene fluoride binder with a mass ratio of 8:1:1 to synthesis the electrode sheets onto Al foil.The active mass loading was about 2.5 mg cm −2 .The electrochemical performances were tested using 2025 type coin cell with lithium foil as anode and 1 M LiPF 6 in ethylene carbonate and diethyl carbonate (1:1 wt%) as electrolyte.The coin cells were charged and discharged between 2.5 and 4.0 V (versus Li + /Li) by applying a current density of 0.2-5 C (1 C = 170 mA g −1 ).The electrochemical performances of the assembled coin cells were tested at room temperature by Neware Battery Test System (Neware Technology Ltd., China).

Results and discussion
It is commonly recognized that Li vacancy defects (Li v ) and Fe occupation of Li site (Fe Li ) are one of the major issues that are responsible for the performance degradation of LFP cathode.Due to the formation of Li v defects, the iron at the iron-rich grain boundaries of degraded LFP is partly oxidized into Fe 3+ ; meanwhile, the partial cations (e.g., Fe 2+ ) start to migrate into lithium layer and occupy lithium site (Fig. 1), forming the as-defined ''anti-site'' defects, which ultimately obstructs the Li + diffusions [1,23,24].As it turns out, there are two different phases: olivine-type FePO 4 (FP) and LiFePO 4 .Inspired by the Li v and Fe Li defects in cycling LFP material, we should precisely resolve the Li v and antisite defects, thereby resume their electrochemical activity of the C-LFP.It requires high activation energy to complete the migration process of Fe ions returning to the original position (H0).In our design, a reaction environment involving ambient-pressure and low-temperature provides relithiationreduction system, which can be beneficial to re-dose lithium ions and the migration and reduction of Fe ions.The eutectic Li-molten salt solution is considered to be a naturally homogeneous ionic conductor, facilitating the relithiation-related Li + diffusion into the Li-deficient particles during the reaction process.The citric acid, an electrical conductor, donates electrons in the reaction system, reducing the Fe 3+ and subsequently dropping the migration barrier to shift Fe 2+ from the H1 to H0 site (Fig. 1).With that in mind, the relithiation mechanism reactions are presented below [23,25]: To determine the appropriate reaction condition and explain the regeneration process, thermal analysis of C-LFP powders and the mixture (C-LFP and eutectic Li salts) were conducted (Fig. 2a).Two stages of the mass loss were observed in the TG curve of the mixture: the first mass loss of 19% above 100 °C corresponds to the loss of crystal water from LiNO 3 and LiOH; the second predominant mass loss of about 15% in the range of 150-350 ℃ was associated with the melting of the eutectic Li salts and the gas evolution, because the O 2 , water (vapor), and NO 2 were generated through the lithiation [3,26].A mass loss slightly of C-LFP above 550 °C was observed, which was attributed to the oxidation of PVDF and conductive carbon in the sample to form carbon dioxide.Therefore, a series of temperature ranges of 350-550 ℃ were used for the relithiation experiment in this work.
To promote the relithiation and crystallization of the spent particles, the C-LFP powders were treated by the molten salt thermochemical process at different annealed temperatures.The chemical compositions of the C-LFP and R-LFP materials were monitored and compared (Fig. 2b).The Li/Fe molar ratio of degraded LFP was only 0.45, which should be largely to blame for the capacity loss in spent LIBs after a long time.After regeneration (1) The crystalline structures of the spent LFP and R-LFP samples with different reaction temperature were analyzed by XRD, as shown in Fig. 3a.The peak annotated with diamonds were typically attributed to the olivine FP (JCPDS 37-0478), while the other diffraction peaks were consistent with the olivine LFP crystal system (JCPDS 40-1499) [27,28].The existence of the FePO 4 phase was a result of lithium loss after long-term cycling [16,23,29].After relithiation, the XRD patterns of all R-LFP powers showed good accordance to the olivine LFP (JCPDS 40-1499) [27,28].It was directly demonstrated that the Li + from eutectic Li-molten salt lithium was effectively supplemented into the Li-poor materials through homogeneous thermochemical process, and the delithiated crystal structure of FP was successfully converted to LFP phase.From the quality point of view, it provided a high-purity LFP phase without the Fe 2 O 3 phase in the regenerated product.
Figure 3b shows the XPS spectra of Fe 2p for the C-LFP, R-LFP, and pristine LFP materials.The binding energies at 711.96 (Fe 2p 3/2 ) and 725.62 eV (Fe 2p 1/2 ) of C-LFP samples indicated the Fe 3+ valence state, further confirming the presence of the FePO 4 in the spent materials.The XPS spectra of R-LFP and pristine LFP displayed obvious shifts in two main peaks: one was the shift of Fe 2p 3/2 from 711.96 eV (Fe 3+ ) to 710.68 eV (Fe 2+ ), and the other was the shift of Fe 2p 1/2 from 725.62 eV (Fe 3+ ) to723.92 eV (Fe 2+ ).In addition, Fig. 3c shows the survey XPS spectra of C-LFP, R-LFP, and pristine LFP.Interestingly, a similar peak shift has been reported in the literature [28,29].The XRD result indicated the conversion of the FePO 4 to the LiFePO 4 phase through thermochemical healing [30,31].
The detailed structural evolution of C-LFP and R-LFP samples were carefully examined by high-resolution TEM (HR-TEM).The TEM images (Fig. 4a and c) showed that spent LiFePO 4 and R-LFP particles were irregular shaped with an average size of nearly 200 nm.HR-TEM of the C-LFP sample has a d-spacing of 0.344 nm, confirming the formation of a FePO 4 phase after repeated long-term cycling of the battery.The appearance of FePO 4 in C-LFP sample proved that the loss of lithium was the chief factor affecting behind the failure of LFP batteries.Different interplanar spacings of 0.392 nm (Fig. 4d), 0.246 nm (Fig. 4f), and 0.347 nm (Fig. 4g) were observed from lattice fringes of the R-LFP sample, which can be indexed to the (210), (121), and (111) planes of LFP, respectively.Furthermore, the SEM images of three sets of samples were shown in Fig. 5.It can be seen that the particles of R-LFP-550 were relatively uniform and there were no impurities around them, implying the impurities in the spent material have been removed completely and the spent LFP has been successfully regenerated.However, R-LFP-450 and R-LFP-350 had obvious agglomeration phenomenon, with particles of different sizes.Moreover, the particles of R-LFP-350 were adhered with some impurities.It can be concluded that R-LFP-450 and R-LFP-350 had not been completely repaired.
To investigate the efficiency of regeneration, the electrochemical properties of spent LFP and all R-LFP were evaluated under a voltage range of 2.5-4.0V. Figure 6 1).In addition, by comparing the capacity retention, capacity under high current density, and preparation methods, it can be seen that our material has achieved a good performance at lower reaction temperatures.
The rate performance of spent LFP and all R-LFP electrodes was also evaluated at various current densities (Fig. 7a).The spent LFP showed specific capacities of 71.69, 62.4, 49.1, 35.5, and 18.9 mAh g −1 at 0.2, 0.5, 1, 2, and 5 C, respectively.The R-LFP-550-no CA electrode showed inferior rate capability; the capacities were about 140.6, 89.5, and 65.1 mAh g −1 at 0.  conducive to electron transfer and used as a buffer layer for volume effect of Li + in the process of charge and discharge.As for the cycling performance, the R-LFP-550 cathode displayed a specific capacity of 128.0 mAh g −1 in the first cycle at 1 C and 125.1 mAh g −1 after 150 cycles with a capacity retention of 97.73%.The capacity retention of R-LFP-no-CA only reached 74.59% after 150 cycles.All R-LFP with CA have excellent cycling performances, comparing to that of R-LFP-no-CA.In addition, all materials exhibit high coulombic efficiency, except for R-LFP-550-no CA, which has poor performance in the early stages of cycling.

Conclusions
In summary, a eutectic Li + molten-salt-assisted roasting strategy was proposed to regenerate spent LFP cathode material without much complicated processing or hightemperature treatment.The capacity degradation mechanism of the spent LFP cathode was revealed through combining XRD, ICP, and TEM characterization.The regenerated LiFePO 4 cathode material delivered a discharge capacity of 151.2 mAh g −1 at 0.2 C. For the cycling stability, it

Fig. 2 a
Fig. 2 a TG curves of C-LFP powders and the mixture of C-LFP and eutectic Li salts (inset of the magnification of C-LFP at 400-700 °C).b Li/Fe molar ratio of regenerated LFP materials at different temperatures

Fig. 3 a
Fig. 3 a XRD patterns of C-LFP and regenerated materials with different reaction temperature b and c High-resolution Fe 2p XPS spectra and survey spectrum of the C-LFP, R-LFP, and pristine LFP materials 2, 2, and 5 C.Moreover, the R-LFP-550 delivered a capacity of 150.2, 120.4,and 101.5 mAh g −1 at 0.2, 2, and 5 C, which was superior to R-LFPno-CA, R-LFP-450, and R-LFP-350, especially at high rates.The specific capacities of R-LFP were quickly retained after cycling at 0.2 C. The carbon layer in R-LFP materials was

Fig. 4 Fig. 6
Fig. 4 High-resolution TEM images and lattice fringe spacing of a-b the spent LFP and c-g R-LFP nanoparticle

Fig. 7 a
Fig. 7 a Rate performance and b cycling performance of the C-LFP, R-LFP, and R-LFP-no CA cathodes

Table 1
The comparisons of this work and other regeneration strategy in Ar for 4 h 151.2 (0.2 C) 101.5 (5 C) 97.73% (150st, 1 C) Our work