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Article

Spent LiFePO4 to High-Value LiF: Enhanced Mechanical Chlorination Coupled with a Fluorination Reaction Mechanism

by
Chao Liang
1,
Nengwu Zhu
1,2,3,4,*,
Fei Li
1,5,*,
Pengfei Zhang
1,
Pingxiao Wu
1,2 and
Yaxi Hu
1
1
School of Environment and Energy, South China University of Technology, Guangzhou 510006, China
2
The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters Ministry of Education, Guangzhou 510006, China
3
Guangdong Environmental Protection Key Laboratory of Solid Waste Treatment and Recycling, Guangzhou 510006, China
4
Guangdong Provincial Key Laboratory of Solid Wastes Pollution Control and Recycling, Guangzhou 510006, China
5
School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China
*
Authors to whom correspondence should be addressed.
Processes 2025, 13(5), 1478; https://doi.org/10.3390/pr13051478
Submission received: 4 April 2025 / Revised: 30 April 2025 / Accepted: 8 May 2025 / Published: 12 May 2025
(This article belongs to the Section Chemical Processes and Systems)
Browse Figures
Versions Notes

Abstract

:
LiFePO4 (LFP) batteries are among the earliest commercialized and most discarded lithium-ion batteries. Although existing recovery technologies focus on the conversion of LiFePO4 to Li2CO3, challenges associated with achieving near-full recovery and high-value products remain. This study proposes a strategy for the conversion of spent LiFePO4 to LiF by mechanical chlorination coupled with a fluorination reaction. The optimum conditions were determined to be a ball-to-powder ratio (BPR) = 15, NH4Cl:LFP = 3, H2O2 = 2.0 mL, rotation speed = 600 rpm, and grinding time = 12 h. Results showed that 97.14% Li was converted into LiCl by H2O2–NH4Cl mechanical chlorination. When chlorinated intermediates were immersed in water, FePO4 could be harvested, and 96.79% Li could be recovered as LiF with a purity of 99.50% after adding NH4F. When Cl-functionalized renewable resin was used to exchange 99.89% F−, 0.63 g NH4Cl per litre of LiF conversion residual liquid was derived. The favourable results were attributed to the 1O2 generated by H2O2, which had a strong electron affinity to break Li–O bonds and provided superior conditions for the combination of Li and Cl. During fluorination, the formation of LiF reduced the ion concentration, and the entropy decreased, contributing to the spontaneous reaction. Therefore, the proposed method paves the way for near-full recovery and high-value products of spent LiFePO4.

1. Introduction

Driven by the popularity of digital products and electric vehicles, the production of lithium-ion batteries has increased significantly in recent years [1,2]. In particular, the shipment of vehicle power batteries increased from 158.2 GWh in 2020 to 1051.2 GWh in 2024 [3]. LiFePO4 batteries have gained prominence due to their low cost, superior cycle performance, and enhanced safety [4,5]. Considering the average lifespan of 5 years, it is expected that the amount of spent power batteries requiring scrapping and treatment in China will reach 2.373 million tons by 2030, including 1.531 million tons of LFP batteries, accounting for 64.51% [6]. However, there are still obstacles to large-scale recycling due to the economy of existing recycling methods. This implies that it is particularly important to improve the economy of technology for the recovery of Li from LiFePO4 to support the sustainability of the system. Therefore, near-full recovery and high-value products of spent LiFePO4 cathode material are the main topics of the present study.
Generally, the methods for extracting valuable elements from spent LFP batteries are usually categorized as complete leaching and selective leaching methods [7,8]. Complete leaching requires an acidic environment, typically maintained with acids such as H3PO4 or H2SO4, to dissolve all elements in the LFP, which are then separated based on their distinct properties. Under the conditions of 1 mol/L H2SO4, a 50 g/L solid-to-liquid ratio, 80 °C, a 1 h reaction time, and a 300 rpm stirring speed, 99.3% Li and 98.6% Fe were successfully leached from spent LFP cathode materials [9]. Furthermore, 94.29% Li and 97.67% Fe have also been leached by H3PO4, with Li and Fe recovered in the forms of Li3PO4 and FePO4·2H2O, respectively [10]. The advantage of selective leaching is that it can be conducted under weakly acidic or neutral conditions to reduce costs and environmental impact [7]. For example, more than 96% Li were be selectively leached from spent LFP cathode materials under conditions of 30 °C, a 10 mL/g liquid-to-solid ratio, 0.5 mol/L HCOOH, a 1.5% H2O2 concentration, and a 2 h reaction time [11]. However, existing recycling technologies are more focused on the leaching of valuable elements in the preliminary stage, with few reports on how to achieve the goals of near-full recovery and high-value products of spent LFP cathode material. Selective leaching and the high-value recovery of Li represent significant challenges for related enterprises [12].
Recently, mechanochemistry was demonstrated to disrupt the LFP structure, reduce particle size, and enhance leaching efficiency [8,13]. Mechanochemistry has also revealed that chemical reactions can occur with the addition of various assistants. For example, Na3Cit and EDTA–2Na were used as co-grinding agents to release Li from the LFP crystal structure [14,15]. Partial chlorination of Li might be achieved when chlorinating agents are added in the mechanical chlorination process. It was shown that the free radicals generated by H2O2 in the reaction selectively broke the Li–O bonds, thereby increasing the Li chlorination efficiency [16]. Therefore, the coupling of H2O2 and mechanical chlorination might greatly improve the efficiency of bond breaking and the selective chlorination of Li, providing intermediates for the derivation of high-value lithium products.
In this work, a green and sustainable method for the preparation of high-value LiF by H2O2-enhanced mechanical chlorination coupled with a fluorination reaction is proposed. The chlorination behaviour of Li in LFP cathode material is analysed from the perspective of free radicals for the first time, and the fluorination reaction mechanism leading to a high-value product is expounded upon. More importantly, NH4Cl was successfully derived for reuse in mechanical chlorination when the Cl-functionalized renewable resin was used to exchange 99.89% F−. This method is simple to operate and has considerable potential for industrial applications. Moreover, it is able to realize the selective chlorination of Li under conditions of low BPR and a high solid–liquid ratio. The proposed method may be of great significance in mitigating or even avoiding the risk of insufficient resource supply.

2. Materials and Methods

2.1. Materials and Sample Pretreatment

Spent LFP batteries were provided by a new energy environmental protection technology company in Shenzhen. The LFP cathode material was obtained after pretreatment of the batteries. The pretreatment of the batteries mainly included discharge, disassembly, pyrolysis, crushing, and screening (Figure 1). Firstly, the battery was immersed in 5% NaCl solution for 48 h to achieve the discharge of the battery. After the discharge, the battery was taken out and dried naturally overnight. After the voltage was determined to be zero by a digital multimeter, the battery was placed in a glove box in an argon environment for disassembly, and the cathode was taken out. The cathode was pyrolyzed at a temperature of 450 °C, a holding time of 60 min, and a heating rate of 10 °C/min to achieve the separation of the cathode material and the aluminum foil [17]. The particle size of the obtained cathode material was 0.15 mm after crushing and screening. About 300 g of spent LFP cathode materials were recycled. The elemental contents of the spent LFP cathode material are shown in Table 1. There was uncertainty in the measurements of the experimental device.
Ammonium chloride (NH4Cl), hydrogen peroxide (H2O2, 30%), ammonium fluoride (NH4F) and others were purchased from Aladdin Chemical Co. Ltd. (Shanghai, China). The mechanical chlorination reaction was conducted in a planetary ball mill (F-P400, Focucy, Changsha China). The Cl-functionalized resin was purchased from Lanxiao Co., Ltd. (Xi’an, China). Deionized water (resistivity 18.2 MΩ·cm) was used throughout the whole experiment.
The leaching efficiencies LEi of different metals from the cathode material were calculated by Equation (1):
L F i = C i V i m ω % × 100 %
where Ci and Vi are the concentration of the metal (mg/L) and leachate volume (L), respectively; m and ω% are the mass of the LFP cathode material (mg) and the mass fraction of the metals in the LFP cathode materials, respectively.

2.2. Mechanical Chlorination and Leaching Process

Exactly 0.1 g LFP cathode material was ground with NH4Cl and H2O2 in a 100 mL zirconia (ZrO2) ball milling jar with ZrO2 balls. After grinding, the mixture was washed out from the ball milling jar using deionized water for further water leaching experiments. The effects of BPR (5–75), the mass ratio of NH4Cl to LFP (0–5), the volume of H2O2 (0–3.0 mL), the rotation speed (100–700 rpm), and the grinding time (3–15 h) on the chlorination efficiencies of the metals were preliminarily investigated by single-factor experiments [18]. Meanwhile, the effects of leaching temperature (20–80 °C) and leaching time (0.5–4 h) on the leaching efficiencies of metals were studied in the water leaching experiment [19].
After the water leaching process, suction filtration was performed using 0.22 µm filter paper to separate the mixture into filter residue and filtrate. The filter residue was used for characterization experiments after washing, drying, and grinding. The filtrate was homogenised and the chlorination efficiencies of Fe2+/Fe3+ and Li+ were determined by ICP-OES.

2.3. Fluorination Reaction and Defluoridation Process

The conversion of LiCl to LiF was achieved by using NH4F as a precipitant in the filtrate, without introducing any impurity ions. The LiF was used for characterization experiments after washing, drying, and grinding. Subsequently, excessive F− in the LiF conversion residual liquid needed to be removed. The removal of F− in the LiF conversion residual liquid required a Cl-functionalized ion exchange resin with an Al-loaded functional group. The group could exhibit strong selectivity to F− in the presence of other ions. The NH4Cl in the solution was used for characterization experiments after evaporation and crystallization.

2.4. Characterization Methods

The concentrations of Fe2+/Fe3+ and Li+ were obtained by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Avio 550 Max, Waltham, MA, USA). The concentration of F− in the LiF conversion residual liquid was determined by ion chromatography (IC, Thermo Scientific ICS-1100, Waltham, MA, USA). The characterization was conducted using an X-ray diffractometer (XRD, MalvernPanalytical Empyrean, Overijssel, The Netherlands), with data collected via a step scan method with a scan speed of 10°/min and a scan angle (2θ) of 10°–80°. The functional groups’ information in samples was obtained using Fourier transform infrared (FT-IR, Thermo Fisher Scientific Nicolet iS20, Waltham, MA, USA) spectroscopy. XPS spectra were obtained by an X-ray photoelectron spectrometer (XPS, Thermo Fisher Escalab Xi+, Waltham, MA, USA).

3. Results and Discussion

3.1. Optimization of Mechanical Chlorination of LiFePO4

An appropriate increase of BPR was found to be beneficial for enhancing the chlorination efficiency of Li. More ZrO2 balls can increase the interface area and make the material force uniform. When the BPR was set at 15, the chlorination efficiency of Li was 89.74% (Figure 2a). However, further increases in the BPR led to a decrease in the chlorination efficiency of Li, while the chlorination efficiency of Fe increased. This trend was attributed to the excessive number of balls causing agglomeration, which in turn hindered the reaction [20]. Therefore, a BPR of 15 was selected as the optimal condition. Compared with another study [20], this study was better able to achieve the release of Li under the condition of a low BPR.
The effect of the mass ratio of NH4Cl to LFP on chlorination efficiency is shown in Figure 2b. Within an appropriate range, increasing the ratio enhanced the chlorination efficiency of Li. When the ratio was 1, the chlorination efficiency of Li reached 86.49%, while that of Fe was 0.2%. As the ratio increased to 3, the chlorination efficiency of Li significantly improved to 97.14%, with the Fe chlorination efficiency remaining as low as 0.01%. However, further increasing the ratio led to a slight decrease in the chlorination efficiency of Li. This decline can be attributed to the nature of the reaction being fluid–solid and following the shrinking core model. During mechanical chlorination, a liquid layer formed around solid particles, and the chlorination process of Li was a diffusion-controlled process. Higher concentrations of NH4Cl in the solution reduced the diffusion rate and hindered the chlorination process of Li [21]. Therefore, a mass ratio of 3 for NH4Cl to LFP was determined to be the ideal condition.
The effect of the volume of H2O2 on chlorination efficiency is shown in Figure 2c. The Li chlorination efficiency increased with the addition of H2O2 until 2.0 mL. At this volume, the Li chlorination efficiency reached 97.14%, while that of Fe decreased to 0.01%. However, further increasing the volume of H2O2 led to a decline in the chlorination efficiency of Li. It was observed that an appropriate amount of liquid medium could prevent LFP from sticking to the inner wall of the jar and the surface of balls, thereby promoting the mechanical chlorination reaction. Conversely, an excessive amount of liquid in the system is harmful, as it might reduce the energy transfer during the ball milling impact process [22]. Therefore, the optimal volume of H2O2 added was determined to be 2.0 mL.
The effect of rotation speed on the Li and Fe chlorination efficiencies is reflected in Figure 2d. A higher rotation speed has the ability to disrupt the LFP structure, reduce the particle size, and promote a more complete reaction, thereby enhancing chlorination efficiency. Specifically, the chlorination efficiency of Li reached 97.14% at a rotation speed of 600 rpm. However, further increases in rotation speed led to a decrease in chlorination efficiency. This decline was attributed to the fact that excessively high rotation speeds caused some LFP material to be carried into the blind corner positions of the jar, resulting in incomplete reactions. Consequently, it was determined that 600 rpm was the ideal rotation speed for this process.
The effect of grinding time on the chlorination efficiencies of Li and Fe is illustrated in Figure 2e. When the grinding time was less than 9 h, the chlorination efficiency of Li was approximately 80%. Increasing the grinding time beyond this point led to a significant improvement in efficiency. A critical point was reached after 12 h of grinding, which facilitated the detachment of Li from the framework. At this stage, the chlorination efficiency of Li reached 97.14%, and further increases in grinding time did not enhance this efficiency. The structure was destroyed more thoroughly with increasing grinding time, while Li was much easier to extract, indicating that the grinding time has little impact on the extraction of Li [20]. Considering both time and economic costs, the ideal grinding time was determined to be 12 h.
In conclusion, the optimum conditions for the selective chlorination of Li were determined to be a BPR = 15, NH4Cl:LFP = 3, H2O2 = 2.0 mL, rotation speed = 600 rpm, and grinding time = 12 h. The mechanical process could make more Li convert to soluble LiCl, which provides good conditions for the leaching of Li during the leaching process.

3.2. Water Leaching Separation Effect of Li and Fe

The effect of leaching temperature is shown in Figure 3a. When the leaching temperature was raised to 60 °C, the Li leaching efficiency was increased to 97.14%, while that of Fe reached 0.01%. However, further increasing the temperature led to a decrease in Li leaching efficiency, while the Fe leaching efficiency increased. This phenomenon might be attributed to the accelerated phase transformation or structural rearrangement of the cathode material under high-temperature conditions, which in turn inhibits the diffusion of Li [23].
Regarding leaching time (Figure 3b), the Li leaching efficiency at 30 min initially reached 87.6%, indicating a fast reaction rate. The Li leaching efficiency could continue to increase to 97.14% at 180 min, while the Fe leaching efficiency decreased with increasing leaching time. It was worth noting that the leaching experiments consistently showed that the Fe leaching efficiency remained below 1%. The decrease in Li leaching efficiency might be due to the redeposition of Li on the surface of the material or the inner wall of the equipment, resulting from excessive leaching.
As a result, the suitable leaching conditions were as follows: leaching temperature = 60 °C and leaching time = 180 min.

3.3. Mechanism

3.3.1. Mechanism of H2O2-Enhanced Mechanical Chlorination of LFP

The breakage of Li–O bonds of LFP in the mechanochemical process has always been a great challenge. Only a small portion of Li–O bonds can be broken under natural ball milling, which is far from meeting the needs of chlorination and the resource conversion of Li. The addition of H2O2 may reduce the energy barrier of breaking Li–O bonds, but has little effect on Fe–O bonds. This shows that free radicals generated by H2O2 during mechanical chlorination might play an important role in the breaking of bonds in LFP. H2O2 activation can generate various reactive oxygen species, such as hydroperoxyl radical (HO2●), superoxide radical (O2−●), hydroxyl radical (●OH), and singlet oxygen radical (1O2) [24,25,26,27]. The reaction was as follows:
Fe2+ + H2O2→Fe3+ + OH− + ●OH
H2O2→2H2O + O2
Fe2+ + ●OH→Fe3+ + OH−
●OH + H2O2→2H2O + HO2●
Fe3+ + HO2●→Fe2+ + H+ + O2
Fe2+ + HO2● + H+→Fe3+ + H2O2
HO2●→O2−● + H+
HO2 + ●OH→H2O +1O2
Fe2+ + O2→Fe3+ + O2−●
2Fe2+ + H2O2→2Fe3+ + 2OH−
H2O2→●OH→HO2●→O2−●→1O2
However, the strong reactivity of these free radicals means that they have a short lifetime, making it difficult to directly capture and quantify. This challenges the understanding of the formation and reaction mechanisms of free radicals in the process of H2O2-enhanced mechanical chlorination of LFP [28]. Currently, quenchers are added to the system to solve this problem. By comparing the chlorination efficiencies of Li and Fe with the addition of quenchers, the existence and formation mechanism of free radicals can be confirmed and the reaction pathway between free radicals and LFP can be inferred [25]. There are many kinds of ·OH quenchers, such as benzoic acid [29], phenol [30], and alcohols [31,32], among which alcohols are the most widely used. The most commonly used quencher for HO2● and O2−● is p-benzoquinone (BQ) [33]. The most common quencher of 1O2 is furfuryl alcohol (FFA) [34]. To minimize any impact on the experiment, methanol (MeOH), n-butanol (NBA), and tert-butanol (TBA) were selected as the quenching agents of ·OH, BQ was selected as the quenching agent of HO2● and O2−●, and FFA was selected as the quenching agent of 1O2. Under the above optimal experimental conditions, an appropriate number of different types of quenchers were added, and the chlorination efficiencies of Li and Fe under different quenchers were tested, as shown in Figure 4.
When ·OH was quenched, the Li chlorination efficiency decreased to varying degrees, while the Fe chlorination efficiency did not change significantly. This was due to the limitation of the jar capacity, resulting in the inability to add sufficient quenchers to prevent the chain reaction of free radicals. When BQ was used as the quencher, the production of HO2●, O2−●, and 1O2● was found to be limited. Concurrently, the Li chlorination efficiency was reduced, while the Fe chlorination efficiency was significantly increased by the destruction of the Fe–O bonds by the strongly oxidising ●OH. The chlorination efficiencies of Li and Fe with FFA as a quencher were similar to that of BQ, and the chlorination of Fe was more efficient. This meant that the coexisting ●OH, HO2●, and O2−● were mainly responsible for the breakage of Fe–O bonds, while 1O2 broke Li–O bonds. The 1O2 with a strong electron affinity could polarize Li–O bonds and weaken the strength of the bonds, eventually leading to the breakage of Li–O bonds. The stability of Fe–O was due to the fact that PO43− retained its structure and captured the surrounding Fe3+ to form FePO4, achieving the targeted enrichment of Fe and P.
The XRD pattern of the spent LFP sample is shown in Figure 5 (line (a)). The XRD pattern of the filter residue sample in a H2O–NH4Cl mechanical chlorination system is shown in Figure 5 (line (b)). It was observed that the main phase was LiFePO4, which was consistent with the result that only a small part of Li was chlorinated and leached. The XRD pattern of the LFP sample in the H2O2–NH4Cl mechanical chlorination system exhibited the presence of NH4Cl and FePO4 as the main phases, accompanied by a LiCl phase (Figure 5 (line (c))). The main phase of the filter residue sample in the H2O2–NH4Cl mechanical chlorination system was FePO4 (Figure 5 (line (d))). The XRD pattern confirmed the oxidative transformation of LiFePO4 to FePO4, indicating the deintercalation of Li.
The FT-IR spectra of the samples before and after mechanical chlorination under different conditions is shown in Figure 6. The peak position did not change significantly from Figure 6 (line (a)) to Figure 6 (line (b)). The peaks at 469.02 cm−1 and 502.05 cm−1 were related to the movement of Li+, and their intensity decreased with the decrease of x in LixFePO4 (Figure 6 (line (a)), Figure 6 (line (c))). The stretching vibration of octahedral FeO6 at 636.56 cm−1 in LiFePO4 was transferred to 655.18 cm−1 in the LFP sample of the H2O2–NH4Cl mechanical chlorination system. The PO43− symmetric stretching mode band at 969.01 cm−1 in LiFePO4 gradually shifted to 957.12 cm−1 in FePO4. The peak at 1401.41 cm−1 was related to the antisymmetric stretching of PO43−, indicating the deintercalation of Li and the formation of FePO4 [35]. The peak at 3170.36 cm−1 was due to the vibration of the N–H bonds in the excess NH4Cl. The peak at 3403.52 cm−1 might correspond to the symmetrical stretching vibration of the O–H bonds, indicating that the sample might contain water or hydroxyl groups. The change was observed from Figure 6 (line (c)) to Figure 6 (line (d)). After the NH4Cl component was removed by filtration, the peak corresponding to 3170.36 cm−1 disappeared.
To further confirm the main reaction pathways during the mechanical chlorination process, samples before and after H2O2–NH4Cl mechanical chlorination were analysed by XPS (Figure 7). It could be observed from the spectrum of Li 1s that the characteristic peak of Li shifted after mechanical chlorination, and the characteristic peak of Li 1s moved from a high binding energy (56.20 eV) to a low binding energy (56.10 eV), indicating that Li was removed from the LiFePO4 [36]. After mechanical chlorination, the main peaks of Fe 2p3/2 and Fe 2p1/2 in the LFP cathode material were obviously moved to the high binding energy direction from 711.61 to 712.58 eV and from 724.54 to 726.53 eV. This suggested that the Fe(II) present in LFP had been oxidized to Fe(III) as a result of the mechanical chlorination process [4]. However, from the P2p profile, it could be seen that the peak and the front positions were almost unchanged, indicating that the structure was highly similar to that before mechanical chlorination. It was further confirmed that the conversion of LiFePO4 to FePO4 was accompanied by the deintercalation of Li.

3.3.2. Mechanism of Conversion of LiF and Regeneration of NH4Cl

After filtering out FePO4, the concentrated filtrate was converted from LiCl to high-value product LiF by adding NH4F. NH4F and LiCl were dissolved in water and dissociated into NH4+, F−, and Li+, Cl−, respectively (Equations (13) and (14)). When the two substances were mixed in water, the metathesis reaction occurred. NH4+ combined with Cl− to form NH4Cl, and Li+ combined with F− to form LiF (Equations (15) and (16)).
LiCl→Li+ + Cl−
NH4F→NH4+ + F−
Li+ + F−→LiF
NH4+ + Cl−→NH4Cl
From the perspective of lattice energy, a strong point attraction generated by Li+ with a small ionic radius and F− with a high charge density is the main reason for the high lattice energy of LiF [37,38,39]. From the perspective of Gibbs free energy (Figure 8a), the formation process of LiF precipitation reduces the number of ions in the solution, resulting in a decrease in the entropy of the system. The energy released during the process compensates for the heat absorbed during the dissolution process and promotes the forward reaction. From the perspective of solubility differences, LiCl, NH4F, and NH4Cl are easily soluble in water, and the solubility of LiF is sharply lower than that of the other three salts, resulting in its preferential precipitation and promoting the forward reaction.
Moreover, the concentration of Li in a saturated Li2CO3 solution is significantly higher than in a saturated LiF solution under the same conditions (Text S1). This means that the efficiency of Li recovery in the form of LiF was much higher than that of the traditional lithium-ion battery recovery product Li2CO3 (Text S2). LiF is widely used in flux, glaze, dosimetry, soldering, and aluminum melting processes. More importantly, LiF plays a key role in the field of solid-state batteries by modifying the interface, improving ionic conductivity, and enhancing electrochemical stability [40,41,42,43,44]. The XRD pattern in Figure 8b showed that the LiF was well-crystallized.
During ion exchange, F− was adsorbed on the resin to remove F− in the LiF conversion residual liquid. When the defluorinated resin reaches saturation, an aluminum salt solution (such as AlCl3) is usually used as a regenerant for regeneration to restore its performance. The Al3+ in the regeneration solution desorbs F− from the resin to form Al–F complexes [45,46], thereby realizing the regeneration and recycling of the resin. The defluoridation efficiency of resin under different cycles is shown in Figure S4. The main solute of the LiF conversion solution was converted to NH4Cl. The excess NH4Cl in the mechanical chlorination process could be reused in the initial mechanical chlorination stage, together with the NH4Cl generated in the LiF conversion process and derived from the defluoridation process, reducing the consumption of material and thus lowering the cost (Figure S5). The XRD pattern showed that the reused NH4Cl was well-crystallized (Figure 8c).
Therefore, the reaction mechanism of spent LiFePO4 to high-value LiF by H2O2-enhanced mechanical chlorination coupled with a fluorination reaction is shown in Figure 9. Under mechanical forces, the 1O2 generated by H2O2 had a strong electron affinity, which could break the Li–O bonds in LiFePO4 while having a minimal impact on the Fe–O bonds. This selective bond cleavage releases Li+, providing favorable conditions for the combination of Li and Cl. Similar to the charging process of LFP battery, LiFePO4 was oxidized to FePO4, and the crystal structure remained unchanged. The conversion of LiCl to LiF was due to the combination of Li+ with a small ionic radius and F− with a high charge density after adding NH4F. In addition, the formation of a low-solubility LiF precipitate would increase the entropy of the system, thus promoting the forward decomposition reaction. In the process of defluoridation, Al-loaded functional groups in the renewable resin showed a strong selective adsorption of F−, and the same amount of Cl− was released during the ion exchange process to combine with NH 4 + to form NH4Cl. However, the formation of Al–F complexes during resin regeneration requires careful attention.

3.4. Implications

The growing quantity of spent LFP batteries has received extensive attention in recent years. To solve the problem of low resource utilization rate of spent LFP battery, numerous studies on the recovery of Li from spent LFP cathode material has reported the significant leaching efficiency of all valuable metals by strong acids (HCl, H2SO4, etc.), but the separation of Li and Fe usually requires additional steps [47,48]. Unlike previous studies on the recovery of LFP cathode material [18,49,50], this paper proposes a strategy for the conversion of spent LiFePO4 to LiF by H2O2-enhanced mechanical chlorination coupled with a fluorination reaction. The Li chlorination efficiency of the enhanced process was increased from 35.01% to 97.14% compared with the control of only NH4Cl, and the high- value product LiF with 99.50% purity was produced at the conversion rate of 96.79% on this basis (Table 2). Moreover, NH4Cl was successfully derived for reuse in mechanical chlorination when a Cl-functionalized renewable resin was used to exchange 99.89% F−. Therefore, this strategy is promising, and can serve as a potential and effective new choice for the conversion of LiFePO4 to high-value LiF.
However, the purity of the FePO4 product prepared by this method needs to be further improved. The treatment of Al–F complexes produced during resin regeneration should be taken seriously [45,46]. For example, aluminum sulfate and polyaluminum (PACl) have been used as options for the treatment of Al–F complexes. In particular, PACl works better because its main flocculation mechanism is particle bridging, which works under a low turbidity [51]. In addition, adsorption [52] and membrane separation [53] were used to treat Al–F complexes. From the perspective of resin functionalization, a defluorinated resin that could be regenerated with NH4Cl might better replace the resin used in this study to overcome the reuse problem of NH4F. Finally, on the basis of solving the above problems, scale amplification and application verification could be considered.

4. Conclusions

Inspired by the charging process of LFP batteries, this study proposed a strategy for the conversion of spent LiFePO4 to high-value LiF by H2O2-enhanced mechanical chlorination coupled with a fluorination reaction. Under the conditions of a BPR = 15, NH4Cl:LFP = 3, H2O2 = 2.0 mL, rotation speed = 600 rpm, and grinding time = 12 h, the highly selective chlorination of valuable Li was achieved. The chlorination reaction mechanism of the selective breaking of Li–O bonds by 1O2 in the process of mechanical chlorination and the fluorination reaction mechanism to realize the high value of the product were clarified. Up to 96.79% Li was successful converted into high-value LiF, with a purity of 99.50%, and 0.63 g NH4Cl per litre of LiF conversion residual liquid was successfully derived for reuse in mechanical chlorination when the Cl-functionalized renewable resin was used to exchange 99.89% F−. Through H2O2-enhanced mechanical chlorination coupled with a fluorination reaction, near-full recovery and high-value products of spent LiFePO4 were achieved, as well as being simpler and more environmentally friendly. Future work should focus on purifying the by-products of LiFePO4 for subsequent recovery of Fe and phosphate, which will enhance scalability and economic feasibility for industrial adoption. Moreover, the core technology route can also be applied to other spent lithium-ion batteries, such as lithium cobaltate (LiCoO2) and lithium manganate (LiMn2O4).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13051478/s1, Text S1: Calculation of Li+ content in saturated solution; Text S2: Comparison of unit product value calculation; Figure S1: Defluoridation mechanism of Cl-functional resin; Figure S2: Regeneration mechanism of Cl-functional resin; Figure S3: XPS spectra of LFP samples before and after H2O2–NH4Cl mechanical chlorination; Figure S4: Defluoridation efficiency of resin under regeneration times; Figure S5: Chlorination efficiency and fluorination efficiency of Li under number of cycles of reused NH4Cl.

Author Contributions

C.L.: conceptualization, methodology, data curation, writing—original draft. N.Z.: conceptualization, investigation, resources, writing—review and editing. F.L.: conceptualization, data curation, formal analysis, validation. P.Z.: formal analysis, writing—review and editing. P.W.: validation, writing-review and editing. Y.H.: writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the National Natural Science Foundation of China (22276062), the Guangzhou Science and Technology Program (No. 2023B04J0029, No. 2023B04J0022 and No. 2025B04J0010), and the National Key Research and Development Project (2024YFC3908700).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Processes 13 01478 g001
Figure 1. Flow chat of pretreatment of spent LFP batteries.
Figure 1. Flow chat of pretreatment of spent LFP batteries.
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Figure 2. Effects of BPR (a), NH4Cl to LFP mass ratio (b), volume of H2O2 (c), rotation speed (d), and grinding time (e) on the chlorination efficiencies of Li and Fe in mechanical chlorination.
Figure 2. Effects of BPR (a), NH4Cl to LFP mass ratio (b), volume of H2O2 (c), rotation speed (d), and grinding time (e) on the chlorination efficiencies of Li and Fe in mechanical chlorination.
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Figure 3. Effects of leaching temperature (a) and leaching time (b) on the leaching efficiencies of Li and Fe in the leaching.
Figure 3. Effects of leaching temperature (a) and leaching time (b) on the leaching efficiencies of Li and Fe in the leaching.
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Figure 4. Chlorination efficiencies diagram of Li and Fe using different quenchers.
Figure 4. Chlorination efficiencies diagram of Li and Fe using different quenchers.
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Figure 5. XRD patterns of the spent LFP sample (a), the filter residue sample in the H2O–NH4Cl mechanical chlorination system (b), the LFP sample in the H2O2–NH4Cl mechanical chlorination system (c), and the filter residue sample in the H2O2–NH4Cl mechanical chlorination system (d).
Figure 5. XRD patterns of the spent LFP sample (a), the filter residue sample in the H2O–NH4Cl mechanical chlorination system (b), the LFP sample in the H2O2–NH4Cl mechanical chlorination system (c), and the filter residue sample in the H2O2–NH4Cl mechanical chlorination system (d).
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Figure 6. FT-IR spectra of the spent LFP sample (a), the filter residue sample in the H2O–NH4Cl mechanical chlorination system (b), the LFP sample in the H2O2-NH4Cl mechanical chlorination system (c), and the filter residue sample in the H2O2-NH4Cl mechanical chlorination system (d).
Figure 6. FT-IR spectra of the spent LFP sample (a), the filter residue sample in the H2O–NH4Cl mechanical chlorination system (b), the LFP sample in the H2O2-NH4Cl mechanical chlorination system (c), and the filter residue sample in the H2O2-NH4Cl mechanical chlorination system (d).
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Figure 7. XPS spectra of LFP samples before and after H2O2–NH4Cl mechanical chlorination.
Figure 7. XPS spectra of LFP samples before and after H2O2–NH4Cl mechanical chlorination.
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Figure 8. Relationship between Gibbs free energy change and temperature in Li reaction processes (a), XRD patterns of LiF (b), reused NH4Cl (c), and Li2CO3 (d).
Figure 8. Relationship between Gibbs free energy change and temperature in Li reaction processes (a), XRD patterns of LiF (b), reused NH4Cl (c), and Li2CO3 (d).
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Figure 9. Mechanism of enhanced mechanical chlorination coupled with a fluorination reaction.
Figure 9. Mechanism of enhanced mechanical chlorination coupled with a fluorination reaction.
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Table 1. The main elemental contents of the spent LFP materials.
Table 1. The main elemental contents of the spent LFP materials.
ElementsLiFePAl
content (wt%)4.1233.8917.480.19
Table 2. The main element contents of LiF and Li2CO3 products.
Table 2. The main element contents of LiF and Li2CO3 products.
ElementsLiFFeAlMgCu
LiF26.6272.880.050.030.020.01
Li2CO318.660.000.030.020.010.01
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Liang, C.; Zhu, N.; Li, F.; Zhang, P.; Wu, P.; Hu, Y. Spent LiFePO4 to High-Value LiF: Enhanced Mechanical Chlorination Coupled with a Fluorination Reaction Mechanism. Processes 2025, 13, 1478. https://doi.org/10.3390/pr13051478

AMA Style

Liang C, Zhu N, Li F, Zhang P, Wu P, Hu Y. Spent LiFePO4 to High-Value LiF: Enhanced Mechanical Chlorination Coupled with a Fluorination Reaction Mechanism. Processes. 2025; 13(5):1478. https://doi.org/10.3390/pr13051478

Chicago/Turabian Style

Liang, Chao, Nengwu Zhu, Fei Li, Pengfei Zhang, Pingxiao Wu, and Yaxi Hu. 2025. "Spent LiFePO4 to High-Value LiF: Enhanced Mechanical Chlorination Coupled with a Fluorination Reaction Mechanism" Processes 13, no. 5: 1478. https://doi.org/10.3390/pr13051478

APA Style

Liang, C., Zhu, N., Li, F., Zhang, P., Wu, P., & Hu, Y. (2025). Spent LiFePO4 to High-Value LiF: Enhanced Mechanical Chlorination Coupled with a Fluorination Reaction Mechanism. Processes, 13(5), 1478. https://doi.org/10.3390/pr13051478

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