J. Cent. South Univ. (2020) 27: 3239-3248

DOI: https://doi.org/10.1007/s11771-020-4543-3

Process for recycle of spent lithium iron phosphate battery via a selective leaching-precipitation method

LI Hao-yu(李昊昱), YE Hua(叶华), SUN Ming-cang(孙明藏), CHEN Wu-jie(陈武杰)

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Applying spent lithium iron phosphate battery as raw material, valuable metals in spent lithium ion battery were effectively recovered through separation of active material, selective leaching, and stepwise chemical precipitation. Using stoichiometric Na₂S₂O₈ as an oxidant and adding low-concentration H₂SO₄ as a leaching agent was proposed. This route was totally different from the conventional methods of dissolving all of the elements into solution by using excess mineral acid. When experiments were done under optimal conditions (Na₂S₂O₈-to-Li molar ratio 0.45,0.30 mol/L H₂SO₄, 60 °C, 1.5 h), leaching efficiencies of 97.53% for Li⁺, 1.39% for Fe³⁺, and 2.58% for PO₄^3- were recorded. FePO₄ was then recovered by a precipitation method from the leachate while maintaining the pH at 2.0. The mother liquor was concentrated and maintained at a temperature of approximately 100 °C, and then a saturated sodium carbonate solution was added to precipitate Li₂CO₃. The lithium recovery yield was close to 80%.

Key words: lithium iron phosphate batteries; selective leaching; recovery; sodium persulfate; lithium carbonate

Cite this article as: LI Hao-yu, YE Hua, SUN Ming-cang, CHEN Wu-jie. Process for recycle of spent lithium iron phosphate battery via a selective leaching-precipitation method [J]. Journal of Central South University, 2020, 27(11): 3239-3248. DOI: https://doi.org/10.1007/s11771-020-4543-3.

1 Introduction

Lithium-ion batteries (LIBs) are important energy storage devices for lightweight and mobile commerce applications compared to ordinary batteries [1]. Owing to their high energy density, inflated energy capacity, higher battery voltage, wide operating temperature range, and high stability in charge-discharge cycles, LIBs have been widely applied in numerous electronic devices, such as computers, digital camcorders, and mobile telephones, and also as energy-storage systems in power sources for electric cars [2-4] . The growing energy demand for electric vehicles (EVs) and consumer electronics (CE) has boosted the proliferation of discarded batteries and increased environmental pollution. Approximately 13000 t of LiFePO₄ materials were spent worldwide in making LiFePO₄ batteries in 2014. China’s LiFePO₄ exportation was close to 33000 t, accounting for 65% of the global market in 2015. Meanwhile, the LiFePO₄ markets are expected to expand continuously and the average growth rate from 2016 to 2020 is estimated at 20% [5]. LIB manufacturers will be forced to face the difficulties of waste disposal when their service life ends. Therefore, it is imperative for researchers to find an effective method of disposal that avoids environmental pollution and wasting of resources [6]. The recycling of LIBs can improve the efficiency of natural resource utilization and greatly reduce their production costs [7]. The major element components’ content of LIBs is certainly higher than that of ores under certain circumstances [8, 9]. The LIBs for these cathode material mostly consist of 5%-7% Li, 5%-10% Ni, 5%-20% Co, 15% organics, and 7% plastic (this composition may vary slightly owing to different manufacturing processes) [7, 10]. Therefore, the recycling and reuse of waste LiFePO₄ batteries will face unprecedented opportunities and challenges.

LIBs usually include four main parts: the anode, cathode, electrolyte, and membrane. The anode is a typical Cu sheet wrapped by a compound of graphite, conductor, binder polyvinylidene fluoride (PVDF), and additional reagents, such as LiPF₆ [11]. Similarly, the cathode is an Al foil enclosed by an admixture of active cathode materials, PVDF adhesive, electric conductor, and additional reagents. So far, researchers have published data on the recovery of LiFePO₄ cathode materials from spent LIBs, and they have mainly focused on existing hydrometallurgical methods [5, 12, 13] and direct regeneration methods [14-16]. Recycling of waste LiFePO₄ via the above- mentioned methods has been widely reported.Table 1 shows the different methods of recycling LiFePO₄ cathode materials. In the leaching processes of recycling and regeneration of LiFePO₄ powders, inorganic acids (such as HCl, H₃PO₄, and H₂SO₄) usually combined with hydrogen peroxide are used as leaching agents to dissolve all elements into the solution. Chemical precipitation methods are further adopted to solve the problems incurred in the separation procedure by using NaOH or Na₃PO₄ [5, 12, 13]. However, the solution after acid leaching treatment is not purified, which will result in a higher content of manganese, calcium, etc., all of which will affect the quality of the final product. The direct regeneration methods refer to recycling waste LIBs via some simple steps and regenerating them directly in the battery manufacturing process. In recycling waste LiFePO₄ powders, those materials comprising the cathodes could be separated effectively and later reused following high-temperature heating in organic solvents [14-16]. However, recovered cathode materials are vulnerable to impurities; in addition, a cathode’s structure will also become drastically changed after multiple charge-discharge cycles, leading to low electrochemical performances in the process.

In general, if a viable method of recycling waste cathode materials is industrialized, the chemical industry technique needs to be simpler, more efficient, more economical, and more environmentally friendly. Under these circumstances, it is undoubtedly necessary to explore a green, environmentally friendly process route for recycling spent LiFePO₄. In this paper, we present an overall method of recovering LiFePO₄ from spent LIBs, mainly including the recovery of Al foil, Fe salt, and Li salt. The route of selective leaching and recovery of Li, Fe, and P from spent LIBs was designed by using stoichiometric Na₂S₂O₈ as an oxidant and adding low-concentration H₂SO₄ as a leaching agent. It is totally different from the conventional methods of dissolving all of the elements into solution by using excess inorganic acid. The optimal method presented in this study provides a scalable, environmentally friendly, and simple route for selective leaching and recycling of valuable metals from spent LIBs.

2 Experimental

2.1 Materials

The chemical reagents used in this work include sodium persulfate, sulfuric acid solution, sodium hydroxide, and sodium carbonate (supplied by Shenghua Science Research Institution, Changsha, China), and all were of analytical grade. Before the leaching experiment, LiFePO₄ powders were obtained from spent batteries (provided by Jinlu New Energy Co., Ltd., Yichun, Jiangxi,China) by adopting the ball-milling method, and its main elemental contents (wt.%) were 35.44% Fe, 4.28% Li, and 19.22% P.

Table 1 Various methods of recycling LiFePO₄ cathode materials

2.2 Experimental procedures

Prior to beginning the recycling projects, the waste batteries were completely discharged and then disassembled, after which the cylindrical materials were removed from the waste LIBs. The experiments consisted of the following main procedures.

2.2.1 Separation of LiFePO₄ from spent LIBs

The chosen separation method, ball-milling, was used to recover the LiFePO₄ powders and Al foils, with the LiFePO₄ powders effectively separated from the Al collectors by oscillating the sieve. During this experiment, the separation results obtained at different rotational speeds and different duration times were studied to determine the best conditions (300 r/min, 60 min); the best screen mesh (0.85 mm) was determined through comparison with results using different screen apertures.However, the residual aluminum slag (about 1.44%) after ball milling was mixed into LiFePO₄ powder. Finally, the LiFePO₄ powders were pretreated with NaOH solution (conditions: [OH^-]=2.0 mol/L, solid/liquid (S/L) ratio=1 g:3 mL) to eliminate Al interference. Table 2 shows the effect of alkaline washing on the separation of LiFePO₄ powder.

This procedure resulted in 97.63% of LiFePO₄ and 98.56% of Al foils being separated effectively from the materials.

2.2.2 Selective leaching of cathode materials (LiFePO₄)

In the leaching process, the theory of oxidation reduction was applied to investigating selective separation of Li⁺ and FePO₄ from the cathode materials. In the recovery of waste LIBs such as LiCoO₂ and LiNi_xCo_yMn_1-x-yO₂ [17-19], it has been proved that hydrogen peroxide has strong oxidizing property in an acidic environment instead of exhibiting reducing property, and hydrogen peroxide can effectively promote the dissolution of Li, Co, Ni, etc. LI et al [5] pointed out that hydrogen peroxide can have better selectivity for lithium iron phosphate leaching at lower H⁺ concentration, and that the Li⁺ was expected to separate from LiFePO₄ powders and dissolve in solution in which H₂O₂ acted as an oxidation during the reaction (see Eq. (1)). However, to achieve the best reaction conditions, the H₂O₂/Li molar ratio was about 4 times its theoretical value, which excessively increases the amount of hydrogen peroxide used.

2LiFePO₄+H₂SO₄+H₂O₂→2FePO₄↓+Li₂SO₄+2H₂O         (1)

Therefore, the key to recycling LIBs is to choose a more effective oxidant. The persulfate anion (S₂O₈^2-) is a strong two-electron oxidizing agent with a redox potential of 2.08 V and higher oxidizing power than hydrogen peroxide (E^Θ=1.7 V) [20]. The following experiment was therefore conducted using sodium persulfate instead of hydrogen peroxide. During the leaching experiment, Li⁺ was selectively dissolved into the leaching solution. Meanwhile, Fe²⁺ in LiFePO₄ powders is easily oxidized to Fe³⁺, which will combine with PO₄^3- and remain in the leaching residue as FePO₄. The following reactions occurred in the selective leaching process [21-23]:

             (2)

in which Fe²⁺ activates S₂O₈^2- decomposition and then promotes the formation ofat a temperature of approximately 20 °C [24]. In the next reaction, excess Fe²⁺ reacts with to transform Fe²⁺ into Fe³⁺, according to:

                   (3)

The total reaction in the leaching process could be concluded to be the one for the recovery of FePO₄ with Na₂S₂O₈; thus, the reaction equation is as follows:

                    (4)

LiFePO₄ materials are leached selectively by using a low concentration of H₂SO₄ and Na₂S₂O₈/Li molar ratio. A low concentration of H₂SO₄ plays an important role in two aspects: improving the leaching rate of Li⁺ and reducing experimental costs. The agitation speed was set to 250 r/min during all of the leaching. To determine the technical feasibility of Li⁺ and FePO₄ separation during leaching, the following operational variables were investigated: Na₂S₂O₈/Li molar ratio (0–0.6), H₂SO₄ concentration (0–0.6 mol/L), temperature (30- 80 °C), duration (0.5–2.5 h), and H₂SO₄/Li molar ratio (0.39-0.72). The resulting solution was separated by filtration, and then the concentrations of Li, Fe, and P were determined with inductively coupled plasma–optical emission spectrometry (ICP-OES) (using a PerkinElmer Optima 5300DV ICP-OES system) after leaching.

Table 2 Main components of each element in separated LiFePO₄ powder

2.2.3 Effect of pH values on Fe/P molar ratio of recovered FePO₄

After selective leaching experiments, as shown in Figure 1, Li⁺ in solution would be expectedly separated with FePO₄, but 1.39% of Fe³⁺ and 2.58% of PO₄^3- were dissolved simultaneously into solution to avoid the interference of lithium phosphate on the final product, lithium carbonate. Therefore, the Fe³⁺/PO₄^3- molar ratio in the solution was adjusted to 1.1:1 by the addition of Fe₂(SO₄)₃ to precipitate and recover FePO₄ [27]. This reaction can be expressed as:

                     (5)

Finally, pH value are an important factor in controlling the precipitation of FePO₄, and they were studied in the next process.

2.2.4 Recovery of Li₂CO₃

Recovery of Li₂CO₃ mainly includes the following processes: The filtrate was concentrated and boiled, then a saturated sodium carbonate solution was added and filtered, and the filter cake was washed [25]. Finally, the filter cake was dried to achieve a constant mass at 110 °C, and Li₂CO₃ was then obtained.

3 Results and discussion

3.1 Effect of different H₂SO₄ concentrations

Figure 1(a) shows the effect of different H₂SO₄ concentrations on leaching efficiency. Experiments were conducted at a Na₂S₂O₈/Li molar ratio (no H₂SO₄ added, theoretical value 0.5) of 0.5, a liquor-to-solid (L/S) ratio of 10 mL/g, a temperature of 50 °C, and a duration of 2.0 h. When the H₂SO₄ concentration is 0, at this point the solution pH=7.5, the leaching efficiencies are 87.03% for Li⁺, 0.00% for Fe³⁺, and 0.02% for PO₄^3-. These results clearly reveal that sodium persulfate has a strong oxidizability and a good selective leaching effect without sulfuric acid assistance. A reasonable explanation for this phenomenon is that lithium iron phosphate has a FeO₆ octahedron and a PO₄ tetrahedron as a skeleton structure [26], and it is difficult to decompose it under the action of lower H⁺ concentration, but Li⁺ easily escapes from the olivine-type LiFePO₄ material through a two- dimensional channel. With H₂SO₄ concentrations increasing from 0.00 to 0.30 mol/L, the leaching ratio of Li⁺ significantly increased, from 87.03% to 96.62%, while Fe³⁺ and PO₄^3- still remained at a low leaching rate (Fe³⁺ and PO₄^3- less than 1.32% and 1.98%, respectively). However, when the concentration of H₂SO₄ was in the range 0.30-0.60 mol/L (pH values simultaneously changed from 1.8 to 0.4), the leaching ratio of Li⁺ hardly increases with the further increase of H₂SO₄ concentration, but it leads to the continuous increase of Fe³⁺ concentration (from 1.32% to 45.12%) and PO₄^3- concentration (from 1.98% to 48.24%). To achieve the maximum efficiency of Li and lower Fe and P leaching efficiency, the optimal H₂SO₄ concentration was determined to be 0.30 mol/L by experiments.

3.2 Effect of Na₂S₂O₈-to-Li molar ratio

When the operational variables were controlled at a H₂SO₄ concentration of 0.30 mol/L, a temperature of 50 °C, an L/S ratio of 10 mL/g, and a duration of 2.0 h, experiments with a different Na₂S₂O₈/Li molar ratios were conducted. When the Na₂S₂O₈/Li molar ratio changed from 0 (without addition of Na₂S₂O₈) to 0.45, the Li leaching ratio increased by approximately 77% (from 18.32% to 95.83%), and the Fe and P leaching rates were reduced from 15.34% to 1.45% and from 17.87% to 1.34%, respectively. The results (Figure 1(b)) show that when combined with Na₂S₂O₈, the leaching rate of Li is remarkably increased and that the leaching rates of Fe and P are thus suppressed. A reasonable explanation for this is that Na₂S₂O₈ acts as an oxidant for transferring Fe from the 2+ state to the 3+ state, which is beneficial to the formation of FePO₄ precipitate, thus making the structure of LiFePO₄ easy to decompose in H₂SO₄ solution. Moreover, sulfuric acid also improves the leaching efficiency of Li and achieves the purpose of reducing the amount of sodium persulfate. When the Na₂S₂O₈/Li molar ratio was further increased to above 0.6, a negligible effect on leaching efficiency was obvious. Therefore, the Na₂S₂O₈-to-Li molar ratio should be set at 0.45, which also greatly reduces the leaching efficiency of Fe and P.

Figure 1 Effects of various conditions on leaching rates of Li, Fe, and P:

3.3 Effect of reaction temperature and time

Figure 1(c) shows the effect of reaction temperature on leaching efficiency. The operational variables are as follows: Na₂S₂O₈/Li molar ratio 0.45, H₂SO₄ concentration 0.30 mol/L, L/S ratio 10 mL/g, and duration 2.0 h. With reaction temperature increasing from 30 °C to 60 °C, the leaching rates of all of the elements increase slightly. When the reaction temperature is above 60 °C, the leaching rate of Li⁺ hardly changes as temperature increases. Therefore, the optimal leaching temperature should be set at 60 °C to obtain the highest leaching efficiency of Li (up to 97.87%). However, temperatures below 60 °C are also feasible due to the small effect on leaching, especially when considering cost issues in industrial applications.

Figure 1(d) shows the effect of reaction time on leaching efficiency with Na₂S₂O₈/Li molar ratio 0.38, H₂SO₄ concentration 0.30 mol/L, L/S ratio 10 mL/g, and temperature 60 °C. As above, a duration of 1.5 h was concluded to be optimal leaching time. Simultaneously, it was seen from the relationship diagram of reaction time that the leaching efficiency of P was always higher than Fe under different conditions. The main reason for this phenomenon was that the whole reaction was carried out in a weakly acidic environment, and Fe³⁺ in the solution was easily precipitated with Fe(OH)₃ in the process.

3.4 Effect of H₂SO₄-to-Li molar ratio

To clearly demonstrate the influence of the amounts of H₂SO₄ and LiFePO₄, different H₂SO₄/Li molar ratios instead of L/S ratios were studied under the following conditions: Na₂S₂O₈/Li molar ratio 0.45, H₂SO₄ concentration 0.30 mol/L, temperature 60 °C, and duration 1.5 h. Since H₂SO₄ concentration and LiFePO₄ quality used in each experiment were both fixed, the higher H₂SO₄/Li molar ratio means that the more H₂SO₄ is added, the larger the solution volume. The results (Figure 1(e)) clearly show the effect of H₂SO₄/Li molar ratio on the leaching efficiency of Li. As the molar ratio increases throughout the selected range, the Li leaching rate rises from 61.23% to 97.53%. However, when the H₂SO₄/Li molar ratio ranges from 0.39 to 0.56, there is no significant change in the leaching rate of Fe and P. As the molar ratio continues to increase, more Fe and P are dissolved in the solution. To reduce the additional amount of sulfuric acid to decrease the amount of alkali added and the volume of the leachate in the next impurity removal step to be as low as possible, a H₂SO₄/Li molar ratio of 0.56 (L/S ratio 11.1 mL/g) can obtain the best leaching results.

3.5 Removal of Fe³⁺ and PO₄^3- from leaching solution

After leaching under the optimized conditions, 1.39% of Fe³⁺ and 2.58% of PO₄^3- were dissolved simultaneously into solution. The Fe³⁺/PO₄^3- molar ratio in the solution was adjusted to 1.1:1 by the addition of Fe₂(SO₄)₃ at this point, with a system pH of 0.45. As shown in Figures 2 and 3, when the pH was greater than 1.0, precipitate began to appear in solution. Figure 2 shows that the Fe³⁺/PO₄^3- molar ratio in the precipitation gradually increases with pH value. When pH values were adjusted to between 1.9 and 2.1, the Fe³⁺/PO₄^3- molar ratio was analyzed as 0.97–1.01, so the precipitate was substantially in the form of FePO₄, and when the pH value was 2.0, the removal efficiencies of PO₄^3- and Fe³⁺ were 99.3% and 90.3%, respectively. With pH value increasing from 2.0 to 3.5, traces of PO₄^3- and excess Fe³⁺ in solution can be further removed. When the final pH value was 3.5, the removal rate of PO₄^3- reached 99.95% and the removal rate of Fe³⁺ was greater than 99.99%. Figure 4 shows the TG-DSC curve of the recovered FePO₄/C during selective leaching. From the TG curve, it can be seen that there are continuous quality changes in the temperature range between 37 °C and 550 °C, which reveals a total weight loss of approximately 4.0%. In addition, the DSC curve shows that there are two upward exothermic peaks of different intensities at 430.0 °C and 550.0 °C. This result is due to the combustion of acetylene black and the decomposition of the binder residue (PVDF) in the FePO₄/C materials. With temperature continuously increasing, a downward endothermic peak is shown at 658 °C and no loss of mass occurs, indicating that FePO₄ began to undergo a phase transition at this temperature (i.e., from amorphous to an alpha quartz structure).

Figure 2 Effect of pH values on Fe/P molar ratio of FePO₄

Figure 3 Effect of pH values on recovery of FePO₄

Figure 4 XRD patterns of recovered FePO₄ (a) and TG-DSC curves (b) of recycled FePO₄/C

To further analyze the crystal structure of the recovered FePO₄ materials, X-ray-diffraction (XRD) patterns of FePO₄ precipitation and FePO₄ leaching residue by burning at 580 °C for 3 h were studied. As shown in Figure 4, two XRD patterns are well consistent with the standard pattern (PDF No. 00-29-0715). This result clearly demonstrates that Fe³⁺ in solution is precipitated in the form of FePO₄ and that FePO₄ remains in the leaching residue during the selective leaching experiment.

3.6 Recovery of Li from leaching solution

After the recovery of PO₄^3- and Fe³⁺ by controlling pH values, the mother liquor concentrated and maintained at a temperature of approximately 100 °C, and then a saturated sodium carbonate solution was added to precipitate Li₂CO₃. Since lithium carbonate solubility is inversely proportional to temperature in an aqueous solution, e.g.,at 0 °C and 0.71 (g/100 g H₂O) at 100 °C [25], Li₂CO₃ products can be effectively recovered after filtration and washing with hot water to eliminate residual mother liquor.

Finally, the result shows that approximately 80% of the Li is recovered as Li₂CO₃ precipitate. Figure 5 clearly reveals that the products of recovered Li₂CO₃ are pure and highly crystalline, and well matched compared to the pattern to Li₂CO₃ (PDF No. 22-1141). In addition, the diffraction peaks from the tests are highly coincident and sharp.

Figure 5 XRD patterns of synthesized Li₂CO₃

4 Conclusions

In this paper, we introduced a relatively simple recovery method of FePO₄ and Li₂CO₃ from spent LIBs. Use of a selective leaching-chemical precipitation method was proposed for the orderly recovery of valuable metals in waste LiFePO₄ batteries. As shown in Figure 6, an overall process flowchart mainly includes the following processes: the amount of acid and alkali required greatly reduced during leaching and precipitation compared with conventional hydrometallurgy method; the impurities in the leachate were low in content and easy to purify; the obtained products achieved effective recovery of elements, such as Li, Fe, P, and reduced environmental problems, such as phosphorus pollution. The conclusions are mainly as follows:

1) For the following operational variables, Na₂S₂O₈/Li molar ratio 0.45, H₂SO₄ concentration 0.30 mol/L, L/S ratio 11.1 mL/g, temperature 60 °C, and duration 1.5 h, results indicate that 97.55% of Li⁺, 1.39% of Fe³⁺, and 2.58% of PO₄^3- are leached at different levels of leaching efficiency.

2) The Fe³⁺/PO₄^3- molar ratio in the solution is adjusted to 1.1:1 by the addition of Fe₂(SO₄)₃ and the pH value increased to 2.0 to achieve precipitation of FePO₄. Then, with the pH value increasing from 2.0 to 3.5, traces of PO₄^3- and excess Fe³⁺ in the solution can be further removed.

Figure 6 Flowchart for leaching and recovery process

3) The mother liquor is concentrated and maintained at a temperature of approximately 100 °C, and then a saturated sodium carbonate solution was added to precipitate the Li₂CO₃.

Contributors

YE Hua provided the idea of the study and developed the research goal. LI Hao-yu led the research activity planning and execution. SUN Ming-cang made great contribution to the improvement of manuscript after the initial edition. CHEN Wu-jie offered some valuable suggestions for analyzing the test data. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

LI Hao-yu, YE Hua, SUN Ming-cang and CHEN Wu-jie declare that they have no conflict of interest.

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(Edited by YANG Hua)

中文导读

选择性浸出-沉淀法回收废磷酸铁锂电池

摘要：以废磷酸铁锂电池为原料，通过活性物质分离、选择性浸出、逐步化学沉淀等工序回收废旧锂离子电池中的有价金属。提出了使用化学计量比的Na₂S₂O₈作为氧化剂并添加低浓度H₂SO₄作为浸出剂进行实验。该方法与传统上使用过量无机酸将所有元素溶解到溶液中的常规方法完全不同。在最佳条件下进行实验下(Na₂S₂O₈和Li的摩尔比0.45；0.30 mol /L H₂SO₄；60 °C；1.5 h)，记录Li⁺的浸出效率为97.53%，Fe³⁺的浸出效率为1.39%，PO₄^3-的浸出效率为2.58%。然后通过控制pH为2.0使用沉淀法从浸出液中回收FePO₄。将母液浓缩并保持在约100 °C的温度，然后加入饱和碳酸钠溶液以沉淀Li₂CO₃，此时锂回收率接近80%。

关键词：磷酸铁锂电池；选择性浸出；回收；过硫酸钠；碳酸锂

Foundation item: Project(Z20160605230001) supported by Hunan Province Non-ferrous Fund Project, China

Received date: 2019-01-22; Accepted date: 2020-08-10

Corresponding author: YE Hua, PhD, Professor; Tel: +86-18974801607; E-mail: 451250423@qq.com; ORCID: https://orcid.org/0000- 0002-8345-6913