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Innovative Application of Acid Leaching to Regenerate Li(Ni1/3Co1/3Mn1/3)O2 Cathodes from Spent Lithium-Ion Batteries Xiaoxiao Zhang, Yifan Bian, Siwenyu Xu, Ersha Fan, Qing Xue, Yibiao Guan, Feng Wu, Li Li, and Renjie Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04373 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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ACS Sustainable Chemistry & Engineering

Innovative Application of Acid Leaching to Regenerate Li(Ni1/3Co1/3Mn1/3)O2 Cathodes from Spent Lithium-Ion Batteries Xiaoxiao Zhanga, Yifan Biana, Siwenyu Xua, Ersha Fana, Qing Xuea, Yibiao Guanc, Feng Wua,b , Li Lia,b* and Renjie Chena,b* a.

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. b.

c.

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China.

State Key Laboratory of Operation and Control of Renewable Energy & Storage Systems, China Electric Power Research Institute, Beijing 100192, China *

Corresponding author: E-mail address: [email protected]

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS: spent lithium-ion batteries, regeneration, oxalic acid, leaching, cathode material ABSTRACT: Rapid development of energy storage system causes a burst demand of lithium-ion batteries (LIBs), and large number of spent LIBs with high valuable metals are produced. Here we propose a novel application of oxalic acid leaching to regenerate Li(Ni1/3Co1/3Mn1/3)O2 (NCM) cathodes from spent LIBs. With lithium dissolving into the solution, the transition metals transform into oxalate precipitates and deposit on the surface of spent NCM cathodes, separating lithium and transition metals in one simple step. After mixing with certain amount of Li2CO3, the oxalate precipitates together with unreacted NCM are directly calcined into new NCM cathodes. The regenerated NCM after 10 min leaching exhibits the best electrochemical performances, delivering the highest initial specific discharge capacity of 168 mA h g–1 at 0.2C and 153.7 mA h g–1 after 150 cycles with a high capacity retention of 91.5%. The excellent electrochemical performances are attributed to the submicron particles and voids after calcination, as well as the optimal elements proportion. This process can make the most of valuable metals in the spent cathodes, with >98.5% Ni, Co, and Mn recycled. It is simple and effective, and provides a novel perspective of recycling cathodes from spent LIBs.

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INTRODUCTION Over the past decades, lithium-ion batteries (LIBs) have been widely applied as power source in consumer electronics, electric vehicles and stationary energy storage.1-4 In consequence, the amount of spent LIBs has increased greatly worldwide. It is predicted that the number of spent LIBs in 2020 will surpass 25 billion units, which are about 500 thousand tonnes.5 The large amount of LIBs are the secondary source of many elements, especially the strategic cobalt resource in the cathodes.5-7 Meanwhile, the components like organic solvents and heavy metals are severe threats to the environment and human health.6 So how to recycle and dispose the spent LIBs effectively and environmentally is becoming urgent, especially with a massive number of retiring power LIBs from electric vehicles in the near future.4, 8 At present, there are mainly two strategies for the recycling of spent LIBs, pyrometallurgy and hydrometallurgy method, one of which receives more attention due to its milder recycling process and higher purity of products.9-13 Generally, a typical hydrometallurgical recycling process is composed of pre-treatment, acid leaching, separation and purification, and synthesis of new products.14-17 During this process, the spent cathode is subject to a complete change from solid to liquid state and back to solid state again, which makes the whole recycling process complex to some extent. However, a simpler and effective process is more desirable in practice. In light of previous research of recycling and capacity fading mechanism of LIBs, it is well-known that most of the spent cathodes still maintain the original structure with slight amount of impurities after regular cycling.13, 18-22 Based on this feature of spent LIBs, we propose a novel and simple method to regenerate spent cathodes of LIBs. In this study, spent Li(Ni1/3Co1/3Mn1/3)O2 (NCM) cathode with multi-metals and high market share is used as an example to preliminarily investigate the feasibility of the regeneration strategy, which 3



comprises of mild oxalic acid leaching and following calcination. Oxalic acid is chosen as a green leaching agent, which is natural with low toxicity and can be decomposed with little environmental pollution. It is a metabolite of organisms and widely distributed in vegetables, animals and fungi, and it has been proved feasible in the leaching of LiCoO2 (LCO) in the reported literatures.23-24 Different from the previous reports, the leaching extent is controlled in this work to regenerate spent cathode materials. Unlike Li+ dissolving in the oxalic acid solution, Ni, Co, and Mn elements form oxalate precipitates due to their low solubility and the reducibility of oxalic acid.25-26 The spent NCM cathodes can be regenerated to different degrees by controlling the leaching time, the effect of which on the electrochemical performances is investigated. The regenerated NCM cathode is obtained after calcination with small amount of Li2CO3 supplement. The lithium in the leaching solution is concentrated and recovered as Li2CO3 using saturated Na2CO3 solution at 80–90 °C, as reported in the literatures.10, 12 The whole regeneration process is simple and effective for the spent NCM cathode, and has great potential application in regenerating different kinds of spent cathode materials in the future. EXPERIMENTAL SECTION Regeneration process. This study aims to investigate the feasibility of oxalic acid leaching reaction preliminarily, so we minimized the effect of other subordinate factors such as the impurities and elements ratio. 18650 cells with NCM cathodes were purchased and cycled to ~80% of its original capacity. The impurities were minimized through a designed pretreatment that we applied in our previous research.27-28 Specifically, after discharging, the cathode and anode were separated manually instead of crushing and sieving to avoid Fe, Cu, Al, etc. impurities. The electrodes were then immersed in dimethyl carbonate (DMC) to remove and recycle the electrolyte residues. N-methyl-2-pyrrolidone (NMP) was used to separate the spent cathode materials from the aluminum foil, which was recycled in 4


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metallic solid state to avoid Al impurity. Meanwhile, NMP was recycled and reused for several times. In the following, the cathode was calcined at 700 °C to eliminate organic carbon and binder impurities. The gas generated during calcination was collected and purified to reduce environmental pollution. Then the regeneration process with oxalic acid leaching and calcination was employed. Specifically, 0.6 M oxalic acid and a measured amount of spent NCM cathode powder were placed into a 100 mL three-necked and round-bottomed reactor with an impeller stirrer and a condenser. The whole reactor was equipped in a water bath to maintain the temperature at 70 °C. The solid to liquid ratio during leaching was set at 20 g L–1. After leaching, the precipitates with different leaching time (10, 30, 60, and 120 min) were filtered and dried, labelled as OA-10-P, OA-30-P, OA-60-P, and OA-120-P, respectively. Finally, the precipitates were mixed with a certain amount of Li2CO3, and calcined at 900 °C for 14 h to obtain the regenerated NCM cathode materials, labelled as OA-10, OA-30, OA-60, and OA-120, respectively. For the recovery of lithium in the leaching solution, NaOH was added to adjust the pH as 11 to remove residual Ni/Co/Mn. Then, the leaching solution was condensed to reach a relatively high concentration of Li (>10g/L), which is essential to the precipitation of Li2CO3. Then, saturated Na2CO3 was added into the solution under 90 °C. The obtained Li2CO3 precipitation was washed with hot water, filtrated, and dried for 24 h. The flowsheet of the regeneration process is shown in Fig. 1. For comparison, certain amounts of NiSO4·6H2O, CoSO4·7H2O, MnSO4·H2O (corresponding to the amount of transition metal contents in the spent sample during leaching experiment) and 0.6 M oxalic acid were mixed in the reactor and reacted at 70 °C for 2 h. The precipitate and following calcined sample were labelled as OA-CS-P and OA-CS, respectively. Materials characterization. Concentration of Li, Co, Ni and Mn in the leachate and regenerated materials was tested by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 5



8300). Crystal structure of the materials was characterized using X-ray diffractometer (XRD; Rigaku Ultima IV-185) with a Cu Kα radiation source. The source tension and current are 40 kV and 40 mA, respectively. Data were acquired with a speed of 8° min−1 over a 2θ range of 10°−80°. Morphologies of the samples were characterized by a field emission scanning electron microscope (FESEM, FEI, Quanta 200f). X-ray photoelectron spectroscopy (XPS, PHI Quantera) was used to detect the surface elements of the materials. Thermal gravimetric analysis (TG) and differential scanning calorimetry (DSC) were conducted using a simultaneous thermal analyzer (NETZSCH STA 449F3) at a scan rate of 5 °C minute–1 from room temperature to 800 °C under air atmosphere.

Fig. 1 Flowsheet of the regeneration processes of NCM cathodes from spent LIBs. Electrochemical measurements. For the fabrication of the cathode, the regenerated material was mixed with acetylene black and PVDF (8:1:1 by weight) in NMP. The slurry was coated onto Al foil and dried overnight at 80 °C in a vacuum oven. The obtained electrode was roll-pressed and punched into small pieces with a diameter of 1.1 cm. Then 2025 coin-type half cells were assembled in a glove box filled with high-purity argon using Li metal as anode and Celgard 2400 membrane as separator. 6


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The electrolyte solution was 1 M LiPF6 in ethyl carbonate (EC) and DMC (1:1 by volume). Loading mass of the regenerated NCM powders in the electrode are about 3.5~4 mg cm–2. Electrochemical measurements (galvanostatic charge/discharge cycling and rate capability tests) were performed using a Land battery test system (Land CT2001A, Wuhan, China) in the voltage range of 2.8−4.3 V at different rates from 0.2 to 5C (1C=150 mA g–1). Cyclic voltammograms measurement was performed on a CHI660D electrochemical workstation at the potential window of 2.5–4.4 V at a scan rate of 0.1 mV s–1. Electrochemical impedance spectroscopy (EIS) analysis was carried out from 105 Hz to 0.01 Hz using an IM6 electrochemical impedance analyzer with an AC perturbation signal of 5 mV. Potentials throughout the paper are referenced to the Li/Li+ couple. RESULTS AND DISCUSSION Pre-treatment characterization. Fig. 2 shows the XRD patterns and SEM images of the spent NCM cathodes before and after calcination treatment. It can be seen that the spent NCM cathodes mainly maintain its original hexagonal layered structure and contain some carbon additives. After calcination, a little amount of impurities of Co3O4 or NiCo2O4 are found and the peak of carbon additives disappears. The layered structure still retains, although the split of (006)/(102) and (108)/(110) peaks is not as clear as those before calcination. The SEM images show that the large aggregates become loose after removal of PVDF binder in calcination, and the particle size of spent spherical NCM is about 7~10 µm. Table S1 shows the mass percent of metal elements in the spent NCM cathodes before acid leaching. It can be seen that the contents of main metal elements are slight lower than the theoretical values, indicating a small loss of metals in the spent LIBs. Meanwhile, impurities like Fe and Cu are not detected in the spent cathodes, and only a slight amount of Al is found which comes from the Al2O3 coating or Al doping in the commercial NCM cathodes. 7



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Fig. 2 XRD patterns and SEM images of spent NCM cathodes before and after calcination. Structure and morphology characterization. Oxalic acid is a commonly used dicarboxylic acid with strong acidity and reducibility. It coordinates a metal ion such as Ni2+, Co2+, and Mn2+ to form oxalate complex of a five-membered ring structure. These complexes usually precipitate as solids due to their low solubility, as shown in Table 1.24-25 So, it can be expected that the spent NCM cathodes can react with oxalic acid in the following path: (1) Li+ will dissolve into oxalic acid solution, and transition metals with higher oxidation states (Co3+ and Mn4+) will be reduced to +2 valence; (2) Ni2+, Co2+, and Mn2+ ions will combine with oxalate ions to form the corresponding oxalate precipitates. The reaction can be presented as follows: 4H2C2O4 + 2LiNi1/3Co1/3Mn1/3O2 = Li2C2O4 + 2(Ni1/3Co1/3Mn1/3)C2O4 + 4H2O + 2CO2

(1)

Table 1 Equilibrium reactions and constants of oxalic acid and transition metal oxalates. Equation

Equilibrium constants k

H2C2O4 ↔ H+ + HC2O4−

5.9×10–2

HC2O4− ↔ H+ + C2O42−

6.4×10–5

NiC2O4 ↔ Ni2+ + C2O42−

7.8×10–10

CoC2O4 ↔ Co2+ + C2O42−

2.7×10–9

MnC2O4 ↔ Mn2+ + C2O42−

1.7×10–7

Fig. 3a shows the XRD patterns of the leaching residues at different time and OA-CS-P oxalate precipitate sample. It can be seen that the peaks of OA-CS-P are well indexed to the monoclinic α-oxalate structure (space group 15, C2/c), indicating that stable MC2O4·2H2O (M=Ni/Co/Mn) precipitates are obtained, which is consistent with previous reports.25-26, 29 Meanwhile, all the other 8


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samples exhibit patterns indexed to a hexagonal layered α-NaFeO2 type structure (space group, R3m) and the same monoclinic α-oxalate structure to OA-CS-P, as depicted in dashed and solid boxes, respectively in Fig. 3a. This implies that oxalic acid can indeed react with spent NCM cathodes to form the corresponding MC2O4·2H2O (M=Ni/Co/Mn) precipitates. Even after 10 min leaching time, peaks of oxalates can be seen clearly. As the leaching time extends, the peak intensities of oxalates increase gradually relative to those of layered structure, indicating the transformation of layered NCM to monoclinic oxalates. The layered structure still exists even after a long leaching time of 2 h, which is different from the oxalic acid leaching with LCO in previous literatures.23-24 We speculate that the larger secondary NCM particles than LCO is the main reason for the different leaching behaviour. The newly formed oxalates can readily deposit on the surface of unreacted NCM cathodes and result in a large mass transfer resistance for the continuous reaction. Fig. 3b shows the XRD patterns of the regenerated NCM cathodes after calcination. It can be observed that the peaks of all the samples are well indexed to a hexagonal layered α-NaFeO2 type structure, and no impurities are found. The OA-10, OA-30, OA-60, and OA-120 samples all show a clearer split of (006)/(102) and (108)/(110) peaks than OA-CS, implying a better layered structure.

Fig. 3 XRD patterns of the leaching residues and OA-CS-P (a), and regenerated NCM cathodes (b). The morphology images of all the samples are depicted in Fig. 4. For the oxalate precipitates (Fig. 4A-4E), the OA-CS-P particles resemble a typical morphology of α-oxalate crystal,25-26, 29 with a block 9



or plate-like shape and particle size of 5~7 µm. After 10 min leaching, OA-10-P exhibits a morphology of loose particles with a size of 1~3 µm, much smaller than the original particle size of spent NCM cathodes (7~10 µm in Fig. 2). This indicates that the large secondary particles are scattered to small lumps when reacting with oxalic acid at first. As the leaching time increases, the particle size increases as well, and the block morphology becomes more and more obvious. When the leaching time extends to 2 h, large secondary blocks (~5 µm) with angular lumps take shape, similar to the morphology of the OA-CS-P sample.

Fig. 4 SEM images of oxalate precipitates (A-E), and regenerated NCM cathodes (a-e). (Inset A, B, and b is the image with larger magnification.)

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SEM images of the regenerated samples are shown in Fig. 4a-4e. It can be seen that the particle size of all the samples decreases to submicron range. In order to obtain a more accurate analysis of the size of regenerated sample, the particle size distribution of OA-10 was measured and the result is shown in Fig. 5. Most of the particle size is in the range of 0.5~5 µm, with a D50 value of 1.5 µm. The submicron particles are loosely connected with each other, leaving a lot of voids which came from the gas release during the calcination. Due to a shorter Li+ diffusion path and better contact with electrolyte, these voids along with the submicron particles are expected to benefit the electrochemical performances. The surfaces of all the samples subject to different leaching time are smooth. By comparison, rough surface and some small residue particles are observed for OA-CS, which is speculated to come from its poor contact and incomplete calcination with Li2CO3 particles.

Fig. 5 Particle size distribution of the OA-10 sample.

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Fig. 6 Thermal analysis curves (TG/DSC) of different oxalate precipitates. Table 2 Thermal analysis results of the oxalate precipitates. Temperature / °C 150–210 290–320

Thermal decomposition

Weight loss rate / % Theoretical OA-CS OA-10 OA-60

(Ni1/3Co1/3Mn1/3)C2O4·2H2O =(Ni1/3Co1/3Mn1/3)C2O4+ 2H2O 3(Ni1/3Co1/3Mn1/3)C2O4+2O2 =(Ni1/3Co1/3Mn1/3)3O4+ 6CO2

OA-120

19.83

18.82

11.58

13.47

13.73

35.3

36.64

21.86

27.17

27.44

Thermal property characterization. Thermal analysis of the oxalate precipitates was conducted, and the TG/DSC results are shown in Fig. 6 and Table 2. Each profile exhibits two obvious weight loss stages, in which the first stage takes place at about 150~210 °C, corresponding to the release of structural H2O from the oxalate precipitates. The second one occurs at 290~320 °C, which is attributed to the decomposition of oxalates to metal oxides.25-26, 29 The weight loss of OA-CS-P in two stages is close to the theoretical values, and the little mismatch is maybe ascribed to the different solubility of metal oxalates to form nonstoichiometric oxalates precipitates. In particular, manganese oxalate has higher solubility, indicating that a small quantity of Mn2+ will dissolve into the solution. In contrast, due to the remaining layered structure with almost no mass loss reaction in this temperature range, the actual weight loss of all the oxalate precipitates is less than the theoretical values. With the increasing leaching time, the proportion of weight loss is rising, indicating the increasing oxalates proportion in 12


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the precipitates. This is consistent with our previous analysis of XRD and SEM results. Meanwhile, the similar weight loss values of OA-60-P and OA-120-P implies that the oxalic acid is difficult to react with spent NCM cathodes completely after 60 min or 120 min, which is attributed to the mass transfer resistance of oxalates on the surface of unreacted NCM cathodes. Contents of metal ions in the oxalate precipitates and regenerated materials were tested and shown in Table 3. It can be seen that the Li contents in the precipitates gradually decrease with the increasing leaching time, in accordance with the increasing Li content in the leaching solution (Fig. S1). Meanwhile, the leaching efficiencies of Ni, Co and Mn in the leachate were tested and all lower than 1.5% (Fig. S1), indicating that more than 98.5% transition metals stay in the precipitates and are regenerated directly. Effects of other parameters (temperature, S/L, and oxalic acid concentration) on the leaching of Li have been tested as well. It can be seen that when the temperature exceeds 70 °C, the leaching efficiency changes little. The same trend is observed in the effect of S/L ratio. For the effect of acid concentration, there is an obvious decrease of leaching efficiency when the concentration is higher than 0.6 M. This is attributed to the block of lithium diffusion to the solution. The molar ratio of Ni, Co and Mn in the regenerated precipitates are close to 1: 1: 1, with a slight loss of Mn. This is consistent with the relatively large solubility of the manganese oxalate. Except a slight amount of Al in the regenerated OA-10 sample (0.008%), no other impurity ions were tested. During the leaching reaction with oxalic acid, the unreacted NCM cathode maintains the original ratio, leading to a proportion closer to the set value. The OA-10 sample has the closest element ratio to the theoretical value, indicating that the shorter reaction time, the better proportion of metal ions in the precipitates and calcined materials. In contrast, the proportion of transition metal ions in OA-CS-P is more different from the set value. After the calcination process with Li2CO3, the OA-CS sample shows a little lithium deficiency, which 13



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may be ascribed to the insufficient contact with Li2CO3 and thus more lithium loss in the high temperature calcination.

Table 3 Contents of metal ions in the oxalate precipitates and regenerated samples. Molar ratio Sample Li

Ni

Co

Mn

OA-CS-P

--

0.333

0.358

0.271

OA-10-P

0.377

0.333

0.329

0.303

OA-60-P

0.318

0.333

0.323

0.297

OA-120-P

0.282

0.333

0.326

0.297

OA-CS

0.820

0.333

0.342

0.273

OA-10

1.027

0.333

0.334

0.305

OA-60

0.994

0.333

0.330

0.299

OA-120

1.033

0.333

0.326

0.293

XPS analysis. In order to investigate the valence state of metals at the surface of the regenerated samples, XPS analysis was conducted as shown in Fig. 7. The Ni 2p photoelectron spectra in Fig. 7a show a mixed valence state in the materials, with Ni2+ located at 854.3 eV and Ni3+ at 855.6 eV.30-31 Meanwhile, the molar ratio of Ni2+/Ni3+ has been estimated based on the area ratio of corresponding peaks. In the OA-10 sample, the molar ratio of Ni2+/Ni3+ was 9:5, and the ratio in OA-60 sample was 4:5. The results indicate that the relative content of Ni3+ increases with the increasing leaching time. Because of a small amount of Mn loss with the increasing leaching time, the content of Ni3+ increases to maintain the neutral state of materials. In the Co 2p spectra of all the samples, the peaks at 779.9 eV with satellite peaks at 795.2 eV are assigned to standard Co3+ in LiCoO2.30, 32 In Fig. 7c of Mn 2p spectra, the binding energy values of 642.4 eV agree well with Mn4+ in all the samples.30, 33

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Fig. 7 Ni 2p (a), Co 2p (b), and Mn 2p (c) XPS data of the OA-10, OA-60, and OA-CS samples. Recovery of Li. The dissolved lithium in the solution was concentrated to a relatively high concentration of ~10 g L–1 and then recovered as Li2CO3 using saturated Na2CO3 solution at 90 °C. The recovery efficiency and purity of Li2CO3 was tested to be 81% and 97%, respectively. The XRD of recovered Li2CO3 was shown in Fig. S2. Concluded from above, it can be inferred that the spent NCM cathodes can be regenerated using oxalic acid to form oxalate precipitates MC2O4·2H2O (M=Ni/Co/Mn), and the extent of reaction can be controlled by the leaching time. Long time leaching will lead to a slight loss of Mn into the solution and a short leaching time of 10 min was verified to get the optimal elements proportion. After calcination with certain amount of Li2CO3, the regenerated NCM cathode with layered structure can be

15



obtained. Illustration of this regeneration process is shown in Fig. 8.

Fig. 8 Illustration of the regeneration process of spent NCM materials with oxalic acid leaching and calcination. Electrochemical performances. Galvanostatic charge/discharge cycling tests of all the samples were performed between 2.8 and 4.3 V at 0.2 and 1C rates, and the results are shown in Fig. 9 and Table 4. It can be clearly seen that electrodes regenerated with shorter leaching time exhibit better performances. When cycling at a low rate of 0.2C, OA-10 exhibits the best initial discharge capacity of 168 mA h g–1, and maintains at 153.7 mA h g–1 after 150 cycles with the highest capacity retention of 91.5%. As the rate increases to 1C, OA-10 also delivers a high reversible discharge capacity of 137.3 mA h g–1 after 300 cycles with a high capacity retention of 86.7%. The overlap of charge/discharge curves of OA-10 (inset of Fig. 9a) indicates good reversibility. The discharge voltage peak locates at as high as 3.77 V, which can also be inferred from its CV result in Fig. S3. The OA-30 sample exhibits a similar excellent cycling performance to that of OA-10, while the OA-60 and OA-120 samples deliver a lower discharge capacity in the cycling test. By comparison, OA-CS only attains 101 mA h g–1 at 0.2C after 150 cycles, and 96 mA h g−1 at 1C after 300 cycles. Rate discharge performances from 0.2 to 5C of all the samples were tested as shown in Fig. 9b, and 0.2C rate was applied during charge. OA-10 delivers the highest initial specific discharge capacities of 16


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162.9 mA h g−1, 159.8 mA h g−1, 153.8 mA h g−1, and 146.1 mA h g−1 at 0.5, 1, 2, and 5C rate, respectively. When the discharge rate restores to 0.2C, the discharge capacity of OA-10 can be recovered to 163.6 mA h g−1, indicating that the structure is not destroyed during high rate cycling. EIS plots for the samples of OA-CS and OA-10 after 1st and 300th cycles at 1C are shown in Fig. S4. It can be seen that the RSEI (solid electrolyte interface resistance) and Rct (charge transfer resistance) values of OA-10 after 1st and 300th cycles are lower than those of OA-CS sample. Meanwhile, the Rct resistance of OA-CS increases remarkably from 1st to 300th cycles, implying a poor Li+ transfer at the interface of electrode and thus rate performance. The regenerated NCM cathodes using this novel method exhibit satisfactory or even better electrochemical performances compared with those recycled NCM cathodes reported in the literatures (Table. S2).17, 34-37 The excellent specific discharge capacity and cycling performance are attributed to the submicron particles and voids, and better element proportion in the regenerated materials. It is worth noting that the lithium in the unreacted materials is all reused and more than 98.5% Ni, Co and Mn are recycled during this process.

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Fig. 9 Electrochemical performances of regenerated NCM cathodes: (a) cycling performances at 0.2C (inset is the charge/discharge curves of OA-10 at different cycles), (b) rate performances at different currents (inset is the charge/discharge curves of OA-10 at different rates), (c) cycling performances at 1C (inset is the capacity retention of OA-10 during cycling).

Table 4 Discharge capacities (mA h g–1) and capacity retentions (%) of all the regenerated samples. 0.2C

1C

1st

150th

Capacity

1st

300th

Capacity

discharge

discharge

retention

discharge

discharge

retention

OA-10

168

153.7

91.5

OA-30

164.6

150.4

OA-60

154.3

136.8

OA-120

156

OA-CS

141.1

Sample

158.4

137.3

86.7

91.4

157

135.3

86.2

88.6

148.8

123.8

83.2

141

90.3

150.7

112

74.3

101

71.6

137.3

96

70

CONCLUSION In summary, the spent NCM cathodes of LIBs are regenerated by a simple and novel method, which includes oxalic acid leaching and calcination. A comprehensive investigation was conducted to 18


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preliminarily understand the regeneration process. Different from the traditional acid leaching, the transition metals in NCM cathodes form precipitates during oxalic acid leaching and the transformation extent can be controlled by the leaching time. After calcination with Li2CO3, the layer-structured NCM cathodes can be regenerated, and those with short leaching time of 10~30 min exhibit better electrochemical performances. Specifically, the regenerated NCM cathode with 10 min leaching time delivers the highest initial specific discharge capacity of 168 mA h g–1 at 0.2C, and maintained at 153.7 mA h g–1 after 150 cycles with a high capacity retention of 91.5%. When cycling at 1C, a high reversible discharge capacity of 137.3 mA h g–1 can be obtained after 300 cycles with capacity retention of 86.7%. The excellent electrochemical performances are attributed to the created submicron particles and voids, and the best elements proportion retained during the regeneration process. This novel regeneration method has been successfully applied to recycle spent NCM cathodes and it is believed that it has great potential to regenerate other similar spent cathodes as well in the future. The simple and novel feature provides a new perspective of recycling spent cathodes of LIBs.

ASSOCIATED CONTENT Supporting Information. Effect of various parameters (time, temperature, acid concentration, and solid to liquid ratio) on the leaching efficiencies of metal ions; XRD pattern of recovered Li2CO3; CV curves of the OA-10 sample between 2.5~4.4 V at a scan rate of 0.1 mV s–1. EIS plots for the samples of OA-CS and OA-10 after 1st and 300th cycles at 1C. Mass percent of metal elements in the spent NCM materials before leaching; Comparison of the electrochemical performances of OA-10 with reported results.

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AUTHOR INFORMATION Corresponding Author * Renjie Chen, E-mail address: [email protected] * Li Li, E-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The experimental work of this study was supported by National Key R&D Program of China (2017YFB0102104), the Joint Funds of the National Natural Science Foundation of China (U1564206), the National Natural Science Foundation of China (51772030) and the Major Achievements Transformation Project for Central University in Beijing.

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Table of Content

A novel leaching and regeneration method using environment-friendly oxalic acid has been successfully applied to recycle spent NCM cathodes.

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