Novel efficient regeneration of high-performance Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials from spent LiMn2O4 batteries

Journal of Alloys and Compounds 783 (2019) 357e362

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Novel efficient regeneration of high-performance Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials from spent LiMn2O4 batteries Yannan Zhang 1, Yiyong Zhang 1, Yingjie Zhang, Peng Dong**, Qi Meng*, Mingli Xu National and Local Joint Engineering Laboratory for Lithium-ion Batteries and Materials Preparation Technology, Key Laboratory of Advanced Battery Materials of Yunnan Province, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 October 2018 Received in revised form 20 December 2018 Accepted 30 December 2018 Available online 31 December 2018

The wide usage of LiMn2O4 batteries makes it imperative that they be recovered and recycled. A novel efficient process of recycling was developed to recover spent LiMn2O4 batteries for regeneration of highperformance Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials. The process comprises leaching and coprecipitation. The Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials recovered showed promising electrochemical performance with discharge capacities of 239.4 mAh g
1 at 0.1 C in the voltage range of 2.5 e4.6 V, and displayed perfect cyclic performance with 81.0% capacity retention after 100 cycles at 0.1 C. The results indicate that regeneration of high-performance Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials is a promising strategy for recycling spent LiMn2O4 batteries. © 2018 Elsevier B.V. All rights reserved.

Keywords: Spent lithium-ion batteries Recycling Regeneration Spent LiMn2O4 Li1.2[Mn0.56Ni0.16Co0.08]O2

1. Introduction Lithium-ion batteries (LIBs) have been widely applied in portable electronic devices and electric vehicles because of their high cell voltage, high-energy density, modest size, etc. [1e3]. Due to the worldwide consumption of LIBs, large quantities of spent LIBs are discarded after service lifetimes of 3e5 years [4,5]. The quantity of spent LIBs in the European Union alone is estimated to reach 13,000 tonnes in 2020 [6]. Spent LIBs contain hazardous materials and hence cannot be disposed into landfills [7]. However, the valuable metals present in spent LIBs have been identified as attractive secondary sources [8,9]. Hence, recycling of spent LIBs is beneficial from economic as well as environmental perspectives. Generally, recycling technologies can be categorised into recovery of metals and regeneration of cathode materials. The recovery of metals from spent LIBs has been widely reported, and includes the methods of hydrometallurgy and pyro-metallurgy

* Corresponding author. ** Corresponding author. E-mail address: [email protected] (Q. Meng). 1 Y.N. Zhang and Y.Y. Zhang contributed equally to this work and share first authorship. https://doi.org/10.1016/j.jallcom.2018.12.359 0925-8388/© 2018 Elsevier B.V. All rights reserved.

[10,38]. Hydrometallurgy has become the primary mechanism for recovering metals because of its high recovery rate, low energy consumption, and simple operation [11]. Hydrometallurgy involves the processes of pre-treatment, leaching, and separation [12,39]. During leaching, the spent cathode materials can be leached with inorganic acids (hydrochloric acid [13], sulfuric acid, phosphoric acid [14], etc.) and organic acids (succinic acid, citric acid, aspartic acid, etc. [15e17]) with the aid of reductants (sodium sulphite [18], hydrogen peroxide, glucose [2], etc). The process of separation has also been reported, and mainly involves precipitation and solvent extraction [19]. However, it is difficult to separate valuable metals from the leaching solution because of the similarity of the metals and high process costs [20,21]. Some studies have focused on the regeneration of cathode materials from spent LIBs [22]. The cathode materials regenerated include LiCoO2, LiCoxMnyNizO2, and LiFePO4. Sita et al. regenerated LiCoO2 material by a two-step process of thermal decomposition followed by a solid state reaction [23]. The material obtained displayed good initial charge and discharge capacities (130 mAh g
1 and 125 mAh g
1, respectively), and these values are very close to those presented by fresh LiCoO2 material. Recently, Zhang et al. regenerated LiNi1/3Co1/3Mn1/3O2 by solid state reaction using spent cathode material. The capacity of the material obtained was also

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high, amounting to 201 mAh g
1 on charging and 155 mAh g
1 on discharging [24]. However, the regeneration of LiMn2O4 materials from spent LIBs has rarely been reported. The LiMn2O4-based LIBs have been widely used in hybrid electric vehicles, large-scale energy storage, and electric power tools due to their higher energy density, lower cost, safety, and environment friendliness [25]. The market share of LiMn2O4 materials is increasing annually with the rapid development of new energy vehicles worldwide. Therefore, it is important and essential to find a simple process for the regeneration of LiMn2O4 materials. Lithium-rich Mn-based oxides, with the general notation xLi2MnO3(1ex)LiMO2 (M ¼ Mn, Ni, Co), are growing increasingly appealing owing to their outstanding discharge capacity of over 250 mAh g
1, wide operating voltage range, and low cost, which renders them suitable to meet the expected increase in demand for high-energy density in Li-ion batteries [26]. Hence, a novel process for recycling spent LiMn2O4 was developed using a technique of leaching-associated co-precipitation for the synthesis of Li1.2[Mn0.56Ni0.16Co0.08]O2 (LLMO) cathode materials. The spent LiMn2O4 cathode materials were simply and efficiently reused without the complicated separation process entailed in conventional methods of recovery. The physical characteristics and electrochemical performance of the regenerated cathode materials and spent LiMn2O4 materials were studied in detail. The recovered Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials show promising electrochemical performance. 2. Experimental section 2.1. Spent LiMn2O4 material preparation and regeneration Spent LiMn2O4 was procured from 18650 batteries collected from Dongguan Power Long Battery Technology Co., Ltd. (Dongguan, China). All the chemical reagents used in this study were of analytical grade, and all the solutions were prepared with deionised water. Spent LiMn2O4 powder was collected from the spent batteries by discharging, dismantling, separating, and detaching, according to our previous report [2,27]. Following a typical process of leaching-associated co-precipitation, leaching was performed in a 200 mL beaker equipped with a magnetic stirrer and temperature control. The spent LiMn2O4 material was introduced in stoichiometric amounts into a leaching solution of 2 mol L
1 HNO3 and 5 vol% H2O2 at 75  C at 300 rpm. After leaching, the solution was filtered and washed with deionised water. The Mn and Li contents of the filtrate were characterised using an atomic adsorption spectrophotometer (AAS). The characterisation of the leaching residue (filter residue) was carried out using a scanning electron microscope (SEM). In order to fabricate Li-rich Mn-based Li1.2[Mn0.56Ni0.16Co0.08]O2 powders, the Mn0.7Ni0.2Co0.1CO3 precursor was prepared as follows. Stoichiometric amounts of the as-prepared filtrate were used as Mn source and mixed with Ni(NO3)2$6H2O and Co(NO3)2$6H2O in the molar ratio of Mn: Ni: Co ¼ 7: 2: 1. The resultant solution was mixed with Na2CO3 (2 mol L
1, 99%, Aldrich) in a parallel-flow reaction vessel at a speed of 5 mL min
1. Next, NH3$H2O solution (2 mol L
1) was added into the reaction solution until the pH of the system was up to 8.2 ± 0.2 and stirred thoroughly at a stirring rate of 500 rpm throughout the process of synthesis. The Mn0.7Ni0.2Co0.1CO3 precursor was obtained by the method of precipitation i.e., it was washed with distilled water thrice and then dried at 80  C for 12 h. Next, Li ions from the filtrate were also recovered by the method of precipitation. Firstly, the impurities in the filtrate were thoroughly removed by precipitation at a pH of 14. Then, a Li-enriched liquor was obtained, and its concentration was adjusted to

1 mol L
1 through evaporation. Finally, Na2CO3 (0.011 mol) was added to the Li-enriched liquor (10 mL) at 80  C. The precipitated Li2CO3 was washed with boiling ultrapure water and dried in a vacuum oven for 12 h. Meanwhile, the dry carbonate precursors Mn0.7Ni0.2Co0.1CO3 obtained were mixed thoroughly with LiOH,H2O in a molar ratio of Li/M (MnþNiþCo) ¼ 1.5 by ball milling. Finally, the Li1.2Mn0.56Ni0.16Co0.08O2 active materials recovered were calcined at 800  C for 10 h in air atmosphere and cooled to room temperature (the heating rate and furnace cooling rate were controlled to 3  C min
1). 2.2. Analytical methods The elemental contents of the solution were analysed using an AAS (Thermo iCE 3000, USA). The morphologies of the samples were examined with a SEM (TESCAN VEGA3, CZE). The X-ray diffraction (XRD, Rigaku, Japan) was performed with Cu Ka radiation (1.5418 Å) at 40 kV, 150 mA, with a slow scanning speed of 2 min
1. The elemental composition of the final product is determined by using an inductively coupled plasma atomic emission spectrometer (ICP-AES, Thermo-6000, USA). The chemical changes in the elements during synthesis were detected by X-ray photoelectron spectroscopy (XPS, with a PHI5000 Versa probe-II spectrometer). A transition electron microscope (TEM, Tecnai G2 F20 STwin) was employed for microstructure and morphology analyses. The coin cell preparation and procedures of electrochemical analysis were based on our previous reports [28]. The cathode material consisted of 80 wt% of Li1.2[Mn0.56Ni0.16Co0.08]O2, 10 wt% of the conductive carbon black (Super P), and 10 wt% of polyvinylidene fluoride, which were thoroughly mixed and bonded with Nmethyl-2-pyrrolidinone solvent. The average mass loading of LLMO materials on electrode was 1.51 mg cm2. Lithium metal sheet and Celgard 2300 film were selected as anode and separator, respectively. The electrolyte solution was composed of 1 M LiPF6 in ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 1:1:1. To evaluate the electrochemical performance, charge-discharge tests were carried out at different current densities in a voltage range of 2.5e4.6 V with a LANDCT2001A test system, and cyclic voltammetry (CV) was recorded at a scanning rate of 0.1 mV s
1 between 2.0 and 4.8 V using an electrochemical workstation (CHI660D, Shanghai, China). 3. Results and discussion 3.1. Characterisation of spent LiMn2O4 materials and leaching mechanism The XRD profiles of the spent cathode materials are shown in Fig. 1. The diffraction peaks of spinel-structured LiMn2O4 match well with PDF#88-0598, belonging to the Fd-3m space group. Meanwhile, the Mn and Li contents of the sample were 59.46% and 3.00%, respectively. The Li in cathode materials may be lost during the processes of charge-discharge and dismantling preparation. Therefore, the main constituent of the spent cathode powder obtained is spent LiMn2O4 materials. The spent LiMn2O4 materials were leached using HNO3 and H2O2. During leaching, HNO3 can release Hþ, and this dissociation reaction can be expressed as follows:

HNO3 ¼ Hþ þ NO
3

(1)

The leaching of valuable metals from spent LiMn2O4 materials involves the conversion of high-valence Mn (Mn3þ and Mn4þ) in the solid phase to Mn2þ in the aqueous phase. Therefore, H2O2 was

Y. Zhang et al. / Journal of Alloys and Compounds 783 (2019) 357e362

Fig. 3. Rietveld refinement results of the XRD patterns of Li1.2[Mn0.56Ni0.16Co0.08]O2 materials.

Fig. 1. XRD profiles of the spent LiMn2O4 cathode materials.

used as a typical redundant, because of its strong reducibility in acid solution, to facilitate the leaching of spent LiMn2O4 material (Eq. (2)) [29]. Therefore, the leaching reaction can be represented as follows (Eq. (3)):

O2 þ 2Hþ þ 2e
¼ H2 O2

E02 ¼ 0:68V

2LiMn2O4 þ 6Hþ þ 3H2O2 ¼ 4Mn2þ þ 2Liþ þ 6H2O

359

(2) (3)

The SEM images of the spent LiMn2O4 materials and leaching residues at leaching time of 40 min are shown in Fig. 2(a) and (b), respectively. Fig. 2(a) shows the SEM image of the spent LiMn2O4 materials powder; they exhibited irregular morphology with particle size in the range of 1e10 mm. The residual particles become smaller, about 1e7 mm in size, after leaching for 40 min. Severe structural corrosion holes and evidence of gullies can be detected obviously, indicating that the edge of the spent LiMn2O4 material particle is easily destroyed during leaching. 3.2. Characterisation of regenerated Li1.2[Mn0.56Ni0.16Co0.08]O2 materials Fig. 3 exhibits the XRD pattern of the regenerated Li1.2[Mn0.56Ni0.16Co0.08]O2. Most diffraction peaks can be assigned to a welldefined layered a-NaFeO2 structure with R-3m space group. Meanwhile, a weak superlattice diffraction peak appears in the range of 18e22 and can be indexed in the single-layer Li2MnO3 phase structure, which is affected by the rearrangement of transition metal ions in the Li-rich cathode materials [30,31]. The

Fig. 2. SEM images of the spent LiMn2O4 materials (a), and leaching residues at 40 min (b).

Rietveld refinement is applied to qualitatively evaluate the variations in the lattice parameters, and the results are displayed in Table 1. The lattice constant ratio c/a of Li1.2[Mn0.56Ni0.16Co0.08]O2 is 4.983 and higher than 4.899 (the reference value for the ideal hexagonal laminated dense stacked lattice) [32]. The high value of c/a is good for the stability of the lamellar structure. All these results indicate that the as-prepared Li1.2[Mn0.56Ni0.16Co0.08]O2 material is mainly an integrated a-NaFeO2 composite with a singlelayer Li2MnO3 superstructure. The chemical composition of the as-regenerated LLMO sample, as determined by ICP-AES analysis, is presented in Table 2. The final product can be described using the notation Li1.51Mn0.69Ni0.20Co0.11O2.5, which is approximately equivalent to Li1.2Mn0.56Ni0.16Co0.08O2 after the conventional normalisation to two O atoms. The SEM images of the regenerated Li1.2[Mn0.56Ni0.16Co0.08]O2 is shown in Fig. 4(a) and (b). It is found that the Li1.2[Mn0.56Ni0.16Co0.08]O2 regenerated from spent LiMn2O4 materials displays a spherical structure with a secondary particle size of 2e3 mm. The EDX mapping images confirm the coexistence of Ni, Mn, and Co, which are uniformly embedded in the spherical particles. Fig. 4(b) presents the magnified image of the selected region of Li1.2[Mn0.56Ni0.16Co0.08]O2. It can be clearly seen that the primary nano-particles of Li1.2[Mn0.56Ni0.16Co0.08]O2 display smooth edges and a lamellar structure. The TEM detection is performed for observing the microstructure of the prepared Li1.2[Mn0.56Ni0.16Co0.08]O2. As shown in Fig. 4(c), the bulk particles of Li1.2[Mn0.56Ni0.16Co0.08]O2 exhibits clear surface profiles. The high-resolution TEM (HRTEM) image (Fig. 4(d)) of the Li1.2[Mn0.56Ni0.16Co0.08]O2 sample reveals clear lattice fringes from the selected bulk region with a d spacing of 0.47 nm, corresponding to the monoclinic (003) and hexagonal (001) planes. The HRTEM analysis demonstrates that the Li1.2[Mn0.56Ni0.16Co0.08]O2 nano-particles have a layered structure that is highly ordered, which coincides well with the results of XRD refinement. In addition to the XRD and morphology results, XPS is employed to analyse the surface compositions of the as-prepared samples. It can be clearly seen from the full-scan spectrum (Fig. 5 (a)) that all the peaks are characteristic peaks corresponding to C 1s, O 1s, Mn 2p, Co 2p, and Ni 2p, and no other distinct elementary signal is detected. In the Mn 2p spectrum in Fig. 5(b), the binding energies of Mn 2p3/2 and Mn 2p1/2 are located at 642.53 eV and 654.22 eV, respectively, confirming the coexistence of Mn4þ and Mn3þ on the surface of Li1.2[Mn0.56Ni0.16Co0.08]O2. Meanwhile, two main peaks at around 780.07 eV and 795.57 eV corresponding to Co 2p3/2 and Co

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Table 1 Results of the crystallographic parameters obtained from X-ray Rietveld refinement of the Li1.2[Mn0.56Ni0.16Co0.08]O2 materials. Sample

Crystallographic Parameters

Li1.2Mn0.56Ni0.16Co0.08O2

a (Å)

c (Å)

c/a

Rp (%)

Rwp (%)

Rexp (%)

2.8603(2)

14.2516(3)

4.9825

8.35

11.63

4.66

Table 2 Stoichiometric molar compositions of the as-regenerated LLMO sample as determined using ICP-AES. Elements

Li

Mn

Ni

Co

LLMO

1.51

0.69

0.20

0.11

2p1/2 are observed in Fig. 5(c), and are consistent with findings in the literature on LiCoO2 and can be attributed to the tervalent Co in the final product [33]. Moreover, as shown in Fig. 5(d), the fitting results of the two main peaks of Ni 2p3/2 and Ni 2p1/2 are located at 854.71 eV and 872.27 eV, respectively, with a binding energy difference of 17.56 eV, which can be assigned to the existence of divalent Ni. The other two main peaks at 861.27 eV and 871.27 eV are the characteristic satellite peaks of Ni2þ and can be ascribed to the multiple splitting of the nickel oxide energy. The XPS results provide strong evidence of the coexistence of Mn4þ, Mn3þ, Co3þ, and Ni2þ in the regenerated Li1.2[Mn0.56Ni0.16Co0.08]O2 sample, and this has also been observed in previously reported works on Li-rich layered oxide materials [34]. Fig. 6(a) displays the charge-discharge curves of the prepared Li1.2[Mn0.56Ni0.16Co0.08]O2 sample at 0.1 C between 2.5 V and 4.6 V. It is clearly observed from the first charge curve that the Li1.2[Mn0.56Ni0.16Co0.08]O2 sample typically displays a smoothly sloping voltage profile at 4.45 V, followed by a long plateau. This can be explained by the decrease in the irreversible capacity fading as a result of the activation of the Li2MO3 phase. Meanwhile, the voltage in the discharge profile gradually declines with increase in the cycle number, which refers to the transformation from the

layered to spinel phase [35]. The corresponding CV profiles of the Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode after the 1st, 3rd, and 5th cycles are shown in Fig. 6(b). Typically, there are two anodic peaks at 4.29 V and 4.68 V in the initial charge process; the former corresponds to the oxidation of the transition ions of Ni2þand Co3þ, and the latter can be ascribed to the simultaneous removal of O2
and Liþ [36]. The cathodic peak at 3.82 V during discharge can be attributed to the reduction of Ni4þ, Co4þ, and Mn4þ [37]. The CV profiles from the initial cycle to the 5th cycle show that both the cathodic and anodic peaks shift slightly to lower voltages and exhibit higher intensity. The reproducibility of the peaks illustrates the favourable reversibility of the reactions of Li extraction from or insertion into the Li1.2[Mn0.56Ni0.16Co0.08]O2 sample. The cyclic performances of the Li1.2[Mn0.56Ni0.16Co0.08]O2 sample at a current density of 0.1 C in the voltage range of 2.5e4.6 V are presented in Fig. 6(c). The Li1.2[Mn0.56Ni0.16Co0.08]O2 sample exhibited an initial discharge capacity of 239.4 mAh g
1 and capacity retention of 81.0% at the end of the 100th cycle. Furthermore, the Coulombic efficiency remained nearly 100%. The rate capabilities of the regenerated Li1.2[Mn0.56Ni0.16Co0.08]O2 electrodes with charge/discharge rates in the range of 0.1e5 C are displayed in Fig. 6(d). Notably, the specific discharge capacity of the Li1.2[Mn0.56Ni0.16Co0.08]O2 sample remained up to 105.1 mAh g
1 at high current densities (5 C). These results indicate that the recycled Li1.2[Mn0.56Ni0.16Co0.08]O2 sample displays a favourable cyclic life and stable rate performance. It also reveals that regeneration of high-performance Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials is a promising strategy for recycling spent LiMn2O4 batteries.

Fig. 4. SEM-EDX images of regenerated LLMO (a), magnified image of the selected region (b), HRTEM images of regenerated LLMO (c), and magnified image of the selected region (d).

Y. Zhang et al. / Journal of Alloys and Compounds 783 (2019) 357e362

361

Fig. 5. XPS spectra of Li1.2[Mn0.56Ni0.16Co0.08]O2 sample: Full-scan spectrum (a), Mn 2p (b), Co 2p (c), and Ni 2p (d).

Fig. 6. Electrochemical characterisation of the regenerated LLMO materials: charge and discharge curves at 0.1 C (a), CV curves in the potential range of 2.0e4.8 V at 0.1 mV s
1 (b), cyclic performance at 0.1 C in the potential range of 2.5e4.6 V (c), and rate capability tests at several currents (d).

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4. Conclusion In summary, we developed a novel efficient technique of leaching-associated co-precipitation for the regeneration of Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials from spent LiMn2O4 cathodes, which were simply and efficiently reused without the complicated process of separation that conventional recovery entails. The recovered Li1.2[Mn0.56Ni0.16Co0.08]O2 cathode materials show promising electrochemical performance with a discharge capacity of 239.4 mAh g
1 at a rate of 0.1 C in the voltage range of 2.5e4.6 V and display perfect cyclic performance. This is a promising strategy for recycling LiFePO4, LiCoO2, and Ni/Co/Mn-based spent LIBs for commercial use. Acknowledgements We gratefully acknowledge the National Natural Science Foundation of China (No. 51764029, No. 51764030), and the Applied Basic Research Plan of Yunnan Province (No. 2018FB087). References [1] L. Yao, Y. Xi, G. Xi, Y. Feng, J. Alloys Compd. 680 (2016) 73e79. [2] Q. Meng, Y. Zhang, P. Dong, Waste Manag. 64 (2017) 214e218. [3] L. Zeng, C. Zheng, L. Xia, Y. Wang, M. Wei, J. Mater. Chem. A 1 (2013) 4293e4299. [4] S. Natarajan, A.B. Boricha, H.C. Bajaj, Waste Manag. 77 (2018) 455e465. [5] H. Ebrahimzade, G.R. Khayati, M. Schaffie, J. Environ. Chem. Eng. 6 (2018) 3999e4007. [6] H. Mahandra, R. Singh, B. Gupta, J. Clean. Prod. 172 (2018) 133e142. [7] P. Xiong, L. Zeng, H. Li, C. Zheng, M. Wei, RSC Adv. 5 (2015) 57127e57132. [8] L.P. He, S.Y. Sun, X.F. Song, J.G. Yu, Waste Manag. 64 (2017) 171e181. [9] E. Fan, L. Li, X. Zhang, Y. Bian, Q. Xue, J. Wu, F. Wu, R. Chen, ACS Sustain. Chem. Eng. 6 (2018) 11029e11035. [10] H.I. Kim, G. Moon, I. Choi, J.Y. Lee, R.K. Jyothi, J. Clean. Prod. 187 (2018) 449e458. [11] F. Pagnanelli, E. Moscardini, P. Altimari, T. Abo Atia, L. Toro, Waste Manag. 60 (2017) 706e715. €m, A. Santasalo-Aarnio, R. Serna-Guerrero, Waste [12] S. Ojanen, M. Lundstro Manag. 76 (2018) 242e249.

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