3O2 Positive Electrode

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 16424−16435

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Dissolution Mechanisms of LiNi1/3Mn1/3Co1/3O2 Positive Electrode Material from Lithium-Ion Batteries in Acid Solution Emmanuel Billy,*,†,‡ Marion Joulié,†,‡ Richard Laucournet,†,‡ Adrien Boulineau,†,‡ Eric De Vito,†,‡ and Daniel Meyer§ †

Université Grenoble Alpes, F-38000 Grenoble, France CEA-LITEN, F-38054 Grenoble, France § Institut de Chimie Séparative de Marcoule (ICSM), UMR 5257 CEACNRSUMENSCM, Centre de Marcoule, BP 17171, 30207 Bagnols-sur-Cèze Cedex, France ‡

S Supporting Information *

ABSTRACT: The sustainability through the energy and environmental costs involve the development of new cathode materials, considering the material abundance, the toxicity, and the end of life. Currently, some synthesis methods of new cathode materials and a large majority of recycling processes are based on the use of acidic solutions. This study addresses the mechanistic and limiting aspects on the dissolution of the layered LiNi1/3Mn1/3Co1/3O2 oxide in acidic solution. The results show a dissolution of the active cathode material in two steps, which leads to the formation of a well-defined core− shell structure inducing an enrichment in manganese on the particle surface. The crucial role of lithium extraction is discussed and considered as the source of a “self-regulating” dissolution process. The delithiation involves a cumulative charge compensation by the cationic and anionic redox reactions. The electrons generated from the compensation of charge conduct to the dissolution by the protons. The delithiation and its implications on the side reactions, by the modification of the potential, explain the structural and compositional evolutions observed toward a composite material MnO2·LixMO2 (M = Ni, Mn, and Co). The study shows a clear way to produce new cathode materials and recover transition metals from Li-ion batteries by hydrometallurgical processes. KEYWORDS: mechanism, dissolution, Li-ion battery, NMC cathode, recycling

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INTRODUCTION

directive drives the manufacturers to improve the recycling efficiency of LIBs with a minimum recycling yield of 50 wt %. Most of the LIB recycling processes developed and reported in academia are hydrometallurgical because they are less energy-consuming. Many research works are focused on the development of cost-effective and environmentally acceptable solutions to recover the valuable metals contained in the cathode material. The reducing leaching of cathodic powders is performed in both inorganic5−8 (sulfuric, nitric, and hydrochloric) and organic9−13 (citric, oxalic, malic, and ascorbic) acids. The general approach consists of determining the operating conditions in the acidic solutions for the best leaching efficiency. The moderate leaching efficiency for cathode materials conducts to add reductive agents such as the hydrogen peroxide,7,14 the glucose15 or a metal. Recently, Joulié et al.16 discussed the dissolution of NMC in acid solutions. They suggested the dissolution of LIB-positive

Rechargeable Li-ion cells are the key components of portable devices, entertainment, computing, and telecommunication equipment required by our current mobile society.1 For portable energy storage, the layered oxide LiCoO2 is mainly used as a positive electrode material for high-capacity batteries (274 mA h g−1). Nevertheless, because of the high cost and possibly a limited availability of Co, as well as safety issues, such an oxide cannot be considered as an ideal candidate in largescale batteries for automotive applications.2 Li(Ni,Mn,Co)O2based electrodes such as LiNi1/3Mn1/3Co1/3O2, referred as NMC, are now entering the automotive application market.3 Since recently, Li-ion batteries (LIBs) are used to power electric vehicles (EVs), gradually replacing the nickel metal hydride batteries. The market penetration of EVs is expected to increase as the gasoline prices rise and the pressure increases to reduce the carbon and particle emissions from fossil fuel use.4 The treatment of spent LIBs through the recovery of valuable metals (Co, Ni, and Mn) is a major concern for their development. For instance, the European 2006/66/CE © 2018 American Chemical Society

Received: January 27, 2018 Accepted: April 17, 2018 Published: April 17, 2018 16424

DOI: 10.1021/acsami.8b01352 ACS Appl. Mater. Interfaces 2018, 10, 16424−16435

Research Article

ACS Applied Materials & Interfaces electrodes in two steps: a fast first step controlled by the pH of the solution and a slower second step subjected to the electrochemical processes involving a surface-controlled dissolution. Although great efforts were made to develop an efficient leaching solution, the mechanisms of cathode material dissolution are not well understood. To date, the main advances in the dissolution mechanisms of cathode materials are related to the development of new materials by the solid-state chemistry. Extensive research studies have been focusing on the preparation of stable solid oxide solutions with layered manganese oxides.17−21 The synthesis of new metastable MnO2 compounds by acid delithiation of the stable LiMn2O4 (spinel structure) and Li3MnO3 (rock salt) phases has provided the first base for understanding.22,23 The ex situ acid delithiation routes were developed in a variety of both acid solutions and cathode materials (H2SO4, HNO3, and HCl).22−33 The first acid delithiation mechanism of LiMn2O4 was proposed by Hunter22 and extended by Thackeray et al.23 The proposed acid delithiation of LiMn2O4 occurs by solid-state diffusion. The described mechanism involves an acid-assisted dissolution of lithium oxide (Li2O) and a disproportionation of Mn3+ in sulfuric acid. This disproportionation reaction produces Mn2+ ions in the acid solution and Mn4+ ions remain in the spinel framework. Zhecheva and Stayanova27 observed the metastable layered phases of Li1−xHxCoO2 by acid leaching of LiCoO2 in H2SO4 and HCl. They concluded that the acid leaching leads simultaneously to lithium extraction and proton exchange in the framework of the parent-layered structure. It is commonly accepted that the delithiation of layered oxides in acid solution starts with an exchange of Li+ ions by H+ ions in the lattice. Gupta and Manthiram28 studied the chemical extraction of lithium from LiCoO2 with various oxidizing agents in dilute sulfuric acid. They reported a disproportionation of Co3+ to Co2+ and Co4+, as Mn from the spinel LiMn2O4 material, reporting a small degree of ion exchange of Li+ by H+. Later, Shao-Horn et al.34 suggested a dissolution−reprecipitation mechanism instead of the solid-state transformation by disproportionation. In H2SO4 solution, the oxidation of sulfate to persulfate (ES0 O 2− /SO 2− = 2.01 V vs NHE ) may initiate the 2 8

influencing the chemical lithium extraction in a nonaqueous medium and understanding better the degradation mechanism of the positive electrodes (LiMn2O4, LiNi1/3Co1/3Mn1/3O2, LiCoO2, and LiFePO4)36 used in aqueous rechargeable lithium batteries.

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MATERIALS AND METHODS

Materials. In the present work, the NMC material, Li(Ni1/3Mn1/3Co1/3)O2, supplied by Umicore Inc. and referenced as MX6 is investigated. The particle mean size of the NMC material powder is measured as 7.684 μm by laser grading with Malvern Mastersizer 2000. The isotherms are used to calculate the specific surface area at 0.40 m2·g−1, using the Brunauer−Emmett−Teller theory with a Beckman Coulter SA3100 surface area analyzer. Elemental titration of the NMC material is determined by an inductively coupled plasma optical emission spectrometer (Agilent Technologies 700 Series ICP−OES), with a complete dissolution in acid (50 vol % of 4 mol·L−1 HNO3 and 50 vol % of 4 mol·L−1 HCl), assisted by a microwave digestion system (Multiwave 3000, PerkinElmer, Anton Paar). The titration is reproduced seven times to determine the stoichiometry of the NMC material, Li1.043Ni0.333Mn0.296Co0.328O2−δ (8.1 wt % Li, 22 wt % Ni, 18.3 wt % Mn, and 21.8 wt % Co), with a standard deviation of ±0.001. The composition results for each metal element is close to the expected stoichiometry 3/3/3 of the supplied Li(Ni,Mn,Co)O2 material. The chemical reactants used for the experiments are of reagent grade, supplied by Sigma-Aldrich. Experimental Methods. Leaching. The positive electrode material is leached in a 0.1 L one-neck glass reactor placed into an oil bath to control the temperature. The solution is stirred with a magnetic bar at 500 rpm. Before complete filtration, 1 mL is sampled and filtered for analysis through 0.2 μm syringe filters in Teflon. The unleached material is filtered with a paper filter (VWR folded qualitative filter paper, 313, particle retention 5−8 μm). The concentrations of Li, Ni, Mn, Co, Al, and Cu in the leaching liquor are measured by ICP−OES. The dissolved oxygen measurement is performed with the SG6SevenGo pro from Mettler Toledo with an accuracy of ±0.1 mg L−1. Solid Residue Analyses. The recovered residual solids are analyzed by different techniques. First, the XRD patterns are obtained by using an X-ray diffractometer Bruker D8 ADVANCE with a θ−2θ LYNXEYE detector. The patterns were recorded in a 2θ range of 8°− 80°, with 2θ step-scan intervals of 0.05° at a constant counting time of 10 s. The patterns were analyzed by the EVA program, which is a part of the Bruker software package for structural analysis. Then, high-resolution images are obtained by a scanning electron microscope LEO 1530 FEG-SEM. The residues are covered by an epoxy resin. The images are recorded by transmission electron microscopy using an FEI Tecnai microscope operated at 200 kV to identify the structure of the residues. For studying chemical evolutions, electron-dispersive X-ray analyses are performed using an FEI Osiris microscope operated in STEM mode at 200 kV. XPS allowed the surface characterization of the remaining residual solid particles by using an MXPS Omicron spectrometer. The energy resolution used for the high-resolution spectra was set to 0.4 mV. A quantitative analysis is achieved, based on the peak areas weighted by Scofield’s relative sensitivity factors. The calibration of the binding energy scale is performed with the C 1s line (284.8 eV) from the carbon contamination layer. Electrochemical Study. The electrochemical measurements are performed using a potentiostat galvanostat EIS analyzer PARSTAT 4000. An ink composed by 92 wt % of NMC, 4 wt % of carbon Super P, and 4 wt % of polyvinylidene fluoride in N-methyl-2-pyrrolidone is deposited on to a glassy carbon of the working electrode and dried at 80 °C as the coin cell formulations.37 All measurements are carried out in a cell using a double junction, thermally controlled. A Luggin capillary is used to control the placement of the Ag/AgCl (3 mol·L−1 KCl) reference electrode and prevent any contamination. For convenience, all potentials will be reported versus the standard

4

dissolution and then catalyze Li2MnO3 to α- and γ-MnO2 transformations. Recently, Knight et al.35 confirmed a disproportionation mechanism with Mn3+ for spinel oxides LiMn2−xNixO4 (0 ≤ x ≤ 0.5) and LiMn2−xMxO4 (0 ≤ x ≤ 0.5) (M = Cr, Fe, and Co). The delithiation was largely discussed by the mechanisms that still remain elusive, whereas a better understanding is required to control the structure of the new lithium insertion compounds. We report herein the dissolution mechanisms of the LiNi1/3Mn1/3Co1/3O2 material in acid solution, explaining the leaching performances and structural evolutions of the lithiated transition metal (TM) oxide electrodes. This work included spectroscopic measurements [X-ray photoelectron spectrometry (XPS)], morphological, and structural analyses [X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM)] in conjunction with the electrochemical measurements. The specific nature of our work shows a clear and an exciting path to recover TMs from LIBs and produce new cathode materials. The knowledge of the dissolution mechanisms of the LiNi1/3Mn1/3Co1/3O2 material in an aqueous solution can also be a source of understanding the factors 16425


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ACS Applied Materials & Interfaces hydrogen electrode (SHE). The counter electrode is a platinum wire in the electrochemical cell. During the open-circuit voltage (OCV) measurements, the NMC electrode is immersed, and the acid is added after starting the OCV acquisition under a constant stirring of 500 rpm.

34, and 40%, respectively, for lithium, nickel, cobalt, and manganese. The first dissolution step is limited as observed in other studies for the LiNi1/3Mn1/3Co1/3O2,16 LiCoO2,7,13,38 LiNi0.8Co0.15Al0.05O2,8 and LiMnyCo1−yO224 compounds. Even if the leaching efficiencies are not comparable because of both changes in the operating conditions and cathode materials, they reveal some similarities. The leaching operated in the nitric and hydrochloric acids also reveal a fast material dissolution (Supporting Information, SI-1). The second step (step II) is a 43-day slow stage, with 100, 57, and 56%, respectively, of lithium, nickel, and cobalt leached. The second step is distinguished by a steady decrease of manganese up to 0%. The second step can be decomposed into two periods: step IIa shows a decrease of manganese in solution without the evolution of Co2+, Ni2+, and Li+ ions (Figure 1), whereas step IIb indicates a concomitant dissolution. Structural Evolution during Leaching. Figure 2A shows the XRD patterns of residual solids after the leaching of the LiNi1/3Mn1/3Co1/3O2 powder. The diffraction lines of the pristine material are well-indexed with the Miller indices for each peak in the rhombohedral system with an R3̅m space group and a single-phase α-NaFeO2-type structure without any impurity phase. As seen in diffractogram (a), both the (006)/ (102) and the (108)/(110) doublets are well-separated, which indicates a good hexagonal ordering of the layered NMC materials.39 The patterns (b) and (c) indicate the deintercalated phases in lithium and the formation of a neoformed phase. The diffraction peaks at 2θ = 12.6 and 25.5° are assigned to the (001) and (002) planes of a birnessite-type layered structure.40 The deintercalated LixNi1/3Mn1/3Co1/3O2 phases remain similar with the peak shifts (003), (108), and (110), indicating the variations of the lattice parameters a and c (Table 1). The shift of the plane (003) to lower 2θ values for the pattern (b) indicates a lattice expansion in the c direction associated with a decreasing lithium content from 1 to 0.48, which is known to

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RESULTS AND DISCUSSION Leaching Kinetics of the LiNi1/3Mn1/3Co1/3O2 Material in Sulfuric Acid. Figure 1 shows the NMC dissolution kinetics

Figure 1. Dissolution kinetics of LiNi1/3Mn1/3Co1/3O2 particles in 1 mol·L−1 H2SO4 at 30 °C with the S/L ratio fixed at 4%.

in sulfuric acid at 1 mol·L−1 and 30 °C, with 4% as the weighted solid/liquid ratio. The results indicate the dissolution kinetics divided in two steps. The first step (step I) is a fast material dissolution from 0 to 14 min accompanied by an emission of a few gas bubbles in solution. The leaching efficiency is 71, 33,

Figure 2. (A) XRD patterns of the pristine NMC powder (a) after leaching in 1 mol·L−1 H2SO4 during 15 min (b), 24 h (c), and 43 days (d)note that the “d” pattern does not have the same intensity scale. The graph (B) is an enlargement in the 12°−21° (2θ) and 61°−67.5° (2θ) ranges. 16426


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ACS Applied Materials & Interfaces

Table 1. Variation of the Cell Parameters of the LiNi1/3Mn1/3Co1/3O2 Powder (a) and after Leaching in 1 mol·L−1 H2SO4 at 30 °C during 15 min (b), 24 h (c), and 43 days (d), and the Evolution of the Lithium Content of Particles lattice parameters of the NMC powder

a

nominal lithium content

sample

leaching time

a (Å)

b (Å)

c (Å)

Va(Å3)

ΔVb(%)

XRD

ICP

a b c d

pristine material 15 min 24 min 43 days

2.85730 2.81730 2.82700 22.87450

2.85730 2.81730 2.82700 2.87450

14.2250 14.5340 14.3450 13.9930

100.5760 99.9038 99.2848 100.1300

0 −0.67% −1.28% −0.44%

1 0.48 0.38 0

1.04 0.47 0.35 0

Unit cell volume. bΔV = volume variation with the pristine material.

be induced by the electrostatic repulsion between the oxygen anions.41 The leaching progress from 15 min to 24 h decreases the lithium content from 0.43 to 0.38 in the material, while the (003) plane shifts to higher 2θ values (pattern (c)). It reflects the contraction of the parameter c because of a lower electrostatic repulsion likely due to the metal migration to the vacancies created by the Li removal. The overlapping of the (006)/(102) couple of the diffraction line and the strong broadening of peaks suggest a disordered lamellar phase, with a mixing of lithium and metal ions between the slab and the interslab space. The contraction could result in an oxygen release from the structure, which is observed during the first step of dissolution. The oxygen ion release diminishes the total electrostatic repulsion, thereby the c parameter as well. The separation of the (108)/(110) peaks increases and decreases, which indicates that the a-lattice parameter first decreases and then increases as the lithium content decreases in opposition to the c-lattice parameter.42 The refined lattice parameters are reported in Table 1 together with the cell volume as a function of the lithium content determined by ICP−OES and XRD. The unit cell volume changes by only about 1% because of the opposite evolution of both a- and c-lattice parameters. The patterns (b) and (c) reveal the formation of a new Lirich birnessite-type phase with a progressive crystallinity according to the leaching time. The neoformed phase is a monoclinic structure crystal, with the Mn xz+Oy -layered intercalated lithium and water molecules corresponding to a Li4Mn14O27·xH2O structure. The corresponding calculated average oxidation state (z+) is 3.6, as reported for the birnessite-like materials.43,44 Manganese is oxidized with different oxidation states, ranging from +2 to +4, into a highly hydrated layered structure. After 43 days of leaching (pattern (d)), the birnessite structure disappears and is replaced by a γtype manganese oxide (nsutite and Ramsdellite structure), as reported in the literature.45 The diffraction pattern (d) shows a poor crystallinity, but the Bragg peaks can also be indexed on the protonated and hydrolyzed NMC phase. After 43 days of aging in the acidic solution, the structural integrity of the NMC phase subsists and coexists with the neoformed phases of the manganese oxide type. Bulk and Surface Composition Investigations. Figure 3 shows the HRTEM observations and Figure 4 depicts the concentration profiles obtained by STEM−energy-dispersive Xray spectrometry (EDXS) experiments from the particles presented in the Supporting Information, SI-2. The pristine material presents a lamellar structure with an interplanar distance of 4.74 Å and without an amorphous surface. The composition profiles presented in Figure 4a indicate a homogeneous elemental distribution in accordance with the preliminary chemical analyses.

Figure 3. HRTEM images of an initial NMC particle (a) and those after leaching in H2SO4 during (b) 15 min, (c) 24 h, and (d) 43 days, and the insets are the magnified regions.

After 15 min of leaching, large microcracks appear (inset of Figure 3b), increasing the specific surface area from 0.4 to 6.2 m2·g−1 (cf. Supporting Information, SI-3). The dissolution causes exfoliation at the surface of the particles because of solvent intercalation and/or gas evolution inside the layered structure. The average size distribution of the particles is reduced from 7.684 to 4.095 μm because of a significant leaching of Mn, Co, and Ni (40%). After 24 h, the leaching efficiency as well as the particle damage increases (Figure 3c). The specific surface area increases up to 45 m2 g−1, and the average size distribution decreases down to 2.412 μm likely because of the mechanical fragmentation of the particles. Their profile of the elemental composition in Figure 4c shows a manganese accumulation on the outer layer of the particle, with a distribution gradient of TMs. The near-surface region is enriched in manganese partially crystallized under a birnessite phase. The bulk composition corresponds to a delithiated material with the Mn/Ni/Co atomic ratios of 1:1:1 similar to the pristine compound. After 43 days, the submicronic scale (inset of Figure 3d) shows small fibrous needles intergrown in the particle. The needlelike morphology enhances the specific surface area at 68 m2·g−1. Figure 4d indicates the ongoing surface enrichment in manganese to form a well-defined core−shell structure. The 16427


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Figure 4. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis corresponding to the EDXS elemental maps with the cross-sectional composition line profile of an initial NMC particle (a) and that after leaching in 1 mol·L−1 H2SO4 during (b) 15 min, (c) 24 h, and (d) 43 days.

NMC phase, wherein is inserted a compositional gradient of TMs including a shell of manganese, describing a composite material MnO2·LixMO2 (M = Ni, Mn, and Co). Figure 5 indicates the online electrode potentials, dissolution measurements, and the relationships between the OCV profile

chemical distribution inside the particle is uniformly distributed in Mn/Ni/Co. The composition of the particle evolves gradually from the core to reach a Mn-rich surface describing a Mn-shell growing inward of the particle. The compositional results obtained from the EDXS mapping indicate a monotonous variation in both the Mn enrichment and the depletion of Co and Ni, with a constant Co/Ni ratio of 1:1. The surface composition analyses of the residual particles by XPS are summarized in Table 2. It is well-known that such a LiTable 2. Surface Composition (at. %) of Residual Particles Measured by XPS after 5 min, 15 min, and 18 h of Leaching in 1 mol·L−1 H2SO4 at 30 °C leaching time

O (%)

Li (%)

Ni (%)

Mn (%)

Co (%)

pristine material 5 min 15 min 18 h

54.2 60.6 60.9 69.5

35.4 16.3 14.6 2.5

3.0 7.4 7.5 7.0

4.5 9.9 10.5 14.3

2.9 5.9 6.5 6.6

containing material is extremely sensitive to oxygen and is prone to surface enrichment in lithium oxides: this explains the higher concentration of Li and O for the pristine material in Table 2. From the beginning of leaching, the surface of the particles is steadily depleted in lithium. The surface concentration of Mn, Co, and Ni is stable up to 15 min, but the Mn concentration rises after 18 h. The surface enrichment in manganese is consecutive to the first step of leaching. The structural analysis shows three main structural and chemical evolutions of particles. The first evolution leads to a single delithiated NMC phase, which exhibits a rhombohedral symmetry. The second is the formation of a thin crystallized film of birnessite, forming a shell at the surface of the delithiated particles. The third is the growth of Mn needles on the surface, with the transformation of the metastable birnessite phase into a γ-type manganese oxide. The particle core is an

Figure 5. OCV measurement of LiNi1/3Mn1/3Co1/3O2 particles and lithium content determined from the leaching efficiency in 1 mol·L−1 H2SO4 at 30 °C with the S/L ratio fixed at 4%.

and delithiation of the NMC particles. It shows the OCV evolution with the composition of the LiNi1/3Mn1/3Co1/3O2 particles immersed in sulfuric acid. The OCV curve defines the operating potentials, which are assimilated to the corrosion potentials of the first phase of dissolution. The OCV profile stands out by an immediate sharp rise of the potential from 0.6 to 1 V (vs SHE) followed by a gradual weakening before reaching an equilibrium around 1.3 V. The rise of the potential is attributed to the progressive delithiation of the material. The lithium depletion in the particles is caused by an excess of 16428


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protons in solution promote the reduction reaction (eq 7). The process can be described as a chemical reaction coupled to an electron transfer mechanism. The surface protonation initiates the extraction of lithium by an acid−base reaction, and the deintercalation generates the electrons for the redox dissolution.

lithium leached per formula unit (empty symbol in Figure 5), which leads to a positive shift of the particle potential induced by the charge compensation of the TMs. In the NMC voltage window of 2.5−4.7 V, the charge compensation results in two steps at different charge states. The Ni 2+ /Ni 4+ couple compensates the charge up to the deintercalation of 2/3 Li per formula unit (∼3.8 V vs Li), and the Co3+/Co4+ couple is involved in the range 2/3 ≤ x ≤ 1 (∼4.6 V vs Li). The Mn4+ is considered as electrochemically inactive and stabilizes the crystal structure.46,47 In a LIB, the average equilibrium voltage, V(x), is related to the difference in the Gibbs free energy, ΔG(x), between the delithiated phase (charged state) and the lithiated phase (discharged state) by the general equation

V (x ) =

−ΔG(x) zF

3(Li3Ni IIMn IV CoIIIO6 )b + (Li3Ni IIMn IV CoIIIO6 )s Ox ⎯⎯⎯→ (Ni IVMn IV CoIV O6 )s + 3(Li 2Ni IIIMn IV CoIIIO6 )b + 6Li+ + 6e− (Ni IVMn IV CoIV O6 )s + 12H+ + 6e− Red ⎯⎯⎯⎯→ Ni 2 + + Mn 2 + + Co2 + + 6H 2O

where F is the Faraday constant, z is the number of electrons involved, and x is the lithium content.48 Thus, the delithiation phenomenon changes the internal energy of the reaction and increases the operating potential. Dissolution Mechanism during the First Step of Leaching. A lithium depletion occurs during the first leaching step and induces a charge compensation by the TM which generates one electron per lithium deintercalated. In the absence of an external circuit, the electrons can recombine inside the particle to reduce TM and promote the oxide dissolution during the removal of the lithium ion. Such a process thus requires a spontaneous extraction of lithium. The deintercalation of lithium can be promoted by an excess of positive charge at the interface, as suggested in Thackeray’s mechanism.23 The surface charge of the electrode is determined by the relative position of the oxide to the potential of zero charge (pzc). The standard description of the chemical phenomena involved in acid−base equilibrium is given by49 (2)

M−OH + H+ ↔ M−OH 2+

(3)

3(Li3Ni IIMn IV CoIIIO6 )b + (Li3Ni IIMn IV CoIIIO6 )s + 12H+ → Ni 2 + + Mn 2 + + Co2 + + 6Li+ + 6H 2O + 3(Li 2Ni IIIMn IVCoIIIO6 )b

LiNi II1/3Mn IV1/3CoIII1/3−O2 2 − + 2H+ (4)

LiNi II1/3Mn IV1/3CoIII1/3−(OH)2 + 2H+ ↔ LiNi II1/3Mn IV1/3CoIII1/3−(OH 2)2+

(8)

The reaction of dissolution (eq 8) is in agreement with the measurements of the potential (Figure 5) and of the surface composition (Table 2). The bulk of the particles is oxidized and the surface is depleted in lithium. During the dissolution, the surface valence distribution (formal valences) is unknown, but it can be apprehended in considering the stoichiometry of the reaction. In eq 8, the atomic ratio of the dissolved Li+/(Ni2+ + Mn2+ + Co2+) ions is 2. The ratio calculated from the leaching is steady around 2.1, assuming a compensation by Ni2+→4+ and Co3+→4+ (cf. Supporting Information, SI-4). Thus, the formal valences of TMs at the interface should be at the four valence states, as described in the equations. A large variation is observed at the beginning of the leaching with a ratio around 2.5. The over-stoichiometry in lithium (electrons per metal dissolved) can be explained by the recombination inside the material and/or by the side reactions. During chemical delithiation, an exchange reaction of Li+ by H+ occurs in the oxide cathodes.50 Consequently, the number of Li+ available and the number of electrons actually generated are overestimated because of the exchange reaction of Li+ by H+. Additionally, a secondary reaction is clearly identified with a few bubbles of gas accompanying the first phase of dissolution. The online O2 measurements in solution reveal the release of dioxygen (cf. Supporting Information, SI-5). The XPS results in Figure 6 show significant changes with the leaching time. For reasons of clarity, the evolution of Mn 2p, Li 1s, and Co 2p core peaks are described separately in the Supporting Information, SI-6. Regarding the O 1s spectrum of the initial material, a welldefined profile peak is observed at 529.7 eV, characteristic of O2− anions of the crystalline network (denoted O2− lattice), whereas at ∼532 and ∼533 eV, they are clearly reminiscent peaks of weakly adsorbed oxygen surface species.51 The peak at ∼531 eV corresponds to the more oxidized oxygen state, that is, peroxide-like O− ions.51−55 The O 1s spectrum remarkably changes when the sample is leached for 5 min, and the peak intensity at 531 eV increases. The phenomenon is amplified with the leaching time and with an increase in the O−/O2− ratio (Supporting Information, SI-7). The results argued that the oxygen ions participate in the redox

The symbol  represents the set of bonds linking the surface ions to the solid framework. The surface charge density is a function of pH. The pHpzc is defined as the pH at which the surface charge (σ0) is equal to zero. Below the pHpzc of the solid, the surface is positively charged, indicating an excess of surface protons, whereas above the pHpzc the surface is negatively charged, indicating a proton deficiency. If the leaching process occurs below the pzc, the surface of the oxide can behave as a Lewis base (electron pair donor) and conduct a protonation reaction at the interface.

↔ LiNi II1/3Mn IV1/3CoIII1/3−(OH)2

(7)

where the subscripts s and b refer to the surface and bulk of particles, respectively, and the net reaction is

(1)

M−O− + H+ ↔ M−OH

(6)

(5)

As a result of the protolytic reactions, the positive species accumulated at the interface lead to a positive surface charge. The difference in the charge is balanced by the reactions of dissociation, adsorption, and likely by the lithium extraction. The lithium extraction involves the charge compensation of TM and releases electrons (eq 6). Mobile electrons and 16429


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reaction of charge compensation. The cation−anion dual charge compensation mechanism occurs in the first step of dissolution. The chemical instability of the layered Li1−xNi1−y−zMnyCozO2 cathodes was reported on lithium-ion batteries in the literature.56 Numerous studies54,57 exemplify the contributions of the bulk and surface oxygen to high charge−discharge capacity in Li-rich layered oxides. During a deep charge, the oxidation of oxygen ions (O2−/O− or O2n−) leads to an oxygen loss from the lattice.52,58 The relative chemical stability can be understood by considering the band diagram and the electronic structure during the charge compensation. The anionic redox is activated as soon as the bottom portion of the O-band merges with the Fermi level.59 An overlap of the M3+/4+:3d band with the top of the O2−:2p band, showing an oxidation of O2− ions and an ultimate oxygen loss from the lattice, is shown in Figure 7.56 In the leaching process, the O2 evolution is attributed to the instability of peroxide-like O− ions. Unlike in conventional stoichiometric-layered cathode materials, the local environment at the surface is largely perturbed by the intensive dissolution. The surface reorganization has to conduct to a chemical stabilization reaction, which can be described as an O2 release according to eq 9. (2MIVO2 3 −)s → (MIVO2 )s + O2

(9)

(with M = Co, Ni, and Mn). The surface oxygen is oxidized in O2 gas, which is lost irreversibly from the structure, leaving some oxygen vacancies at the surface or in the subsurface layers. The O2 evolution explains the large microcracks observed on the particles (Figure 3b). Kim et al.21 described a similar process with a Li2MnO3 material during a battery cycling at a high potential. The intermediate superoxide species at the electrode surface are

Figure 6. O 1s photoelectron spectra of (a) the LiNi1/3Mn1/3Co1/3O2 initial material, after leaching in 1 mol·L−1 H2SO4 during (b) 5 min, (c) 15 min, and (d) 18 h. The binding energies were calibrated against the C 1s line from the carbon contamination layer (284.8 eV).

Figure 7. Process of O2 release during the first phase of dissolution. (a) Macroscopic representation of LixNi1/3Mn1/3Co1/3O2 particle delithiated. (b) Crystal structure and the delithiation between the core and the surface of particles. (c−e) Schematic representation of the energy-level vs density of states, showing the respective motion of Fermi level with respect to the lithium content, leading to anionic redox processes and then O2 release. 16430


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extraction of lithium and a discontinuous pathway for fast Liion transportation increase the energy barrier for Li+ migration and impact the reaction kinetics. Figure 9 represents the dissolution kinetics of lithium (a) and the lithium content (b) in the NMC particles for leaching, with

immediately stabilized by the chemical release of oxygen at the surface. Limitation of the First Step of Dissolution. The mechanism of dissolution can be considered as “self-regulating” as long as the lithium deintercalation reaction occurs and provides electrons at the solid/liquid interface. Although the Li+ deintercalation reaction is incomplete, the leaching is hardly stopped after few minutes. The formation of an interfacial blocking layer in manganese that impedes delithiation is not detected. The cross-sectional composition profile indicates a homogeneous elemental distribution (Figure 4b), and the surface composition in TM is conformed to the initial material slightly depleted in lithium (Table 2). The limitation of the first phase of dissolution is rather slowed down than blocked, likely because of the structural and chemical evolutions. Figure 8 is a simplified E−pH diagram reporting the operating potential values from the OCV curves (inset) after

Figure 9. Dissolution kinetics of lithium in (a) and lithium content in (b) from the NMC particles in 1 mol·L−1 sulfuric acid, at 30 °C with an S/L ratio fixed at 4% at pH values 0, 1, 2, 3, and 4, and (c) is a schematic of Li+ depletion in the NMC particles with the pH decrease in solution.

Figure 8. E−pH diagram at 25 °C of the equilibrium lines O2/H2O (dash), MnO2/Mn2+ (full), and the OCV measurements at 30 min of the NMC material in H2SO4, according to the pH of the solution. The inset is the OCV measurement of NMC during 30 min at pH = 0.16 (a); pH = 1 (b); pH = 2 (c); pH = 3 (d); pH = 4 (e); and pH = 5 (f) in H2SO4 solution.

the pH set at 0, 1, 2, 3, and 4. The lithium content is calculated considering the leaching efficiency of TM and Li+ (Supporting Information, SI-8). The lithium dissolution decreases from 70% (pH = 0) to 8% (pH = 4), whereas the lithium content in the particles increases respectively from 0.45 to 0.95. When the dissolution pH rises, the surface layer is proportionally increased in lithium. The transport properties should be facilitated because of a shorter pathway for the Li+ migration, denoted δLi+depleted in Figure 9c. The results do not show a correlation between the dissolution kinetics and the depletion in lithium, assuming the control of the reaction by the potential rather than the mass transfer of species. It supposes a larger kinetic limitation by the free energy of reaction (ΔrG(x)). The driving force is reduced and the dissolution is certainly limited by the oxide reduction reaction by protons, involving a surface-controlled dissolution. The overall results point out a dissolution mechanism governed by the delithiation process which controls the driving force and the transport properties. Consequently, the amount of lithium per unit of material is fundamental. It imposes the potential and the electron number generated, and therefore the leaching efficiency for the first step of dissolution. Second Step of Dissolution. The formation of a Li-rich birnessite-type phase and a transformation in the γ-type manganese oxide with a concomitant decrease of Mn2+ concentration in solution (Figure 10a) are highlighted during the second step of dissolution. The structural evolution is consecutive to the first step of dissolution from 30 min to 24 h

30 min in acidic solution. The lithium deintercalation rises the potential for all pH values and reduces considerably the driving force magnitude for reducing the oxides. The E−pH evolution is described by a blue line with a slope of 133 mV per decade close to the equilibrium line MnO2/Mn2+. The slope value is only indicative due to a semi-equilibrium of the NMC surface, which causes some deviations with a straight line. However, a slope around 120 mV per decade shows the valence state 4+, in coherence with eq 7 (MO2/M2+). The E−pH evolution indicates the reduction in the operating potential with the acid, describing the correlation between the leaching limitation and the potential. The kinetic is also affected by structural considerations. The lithium deintercalation decreases the lattice volume and induces vacancies and defects in the particles. The overlapping of the doublet (006/102) on the diffractogram in Figure 2b exhibits a disorder degree of cations in the lamellar phase.60 The presence of the TM ions in the lithium planes can interfere with the lithium-ion diffusion path because of the electrostatic repulsion between Li+ and Mz+.61,62 The lithium extraction rate is also influenced by the decrease in the interslab distance of the lithium layers (Table 1). Additionally, the lithium-depleted layers enhance deeper the Li+ migration pathways. The deep 16431


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Figure 10. Dissolution kinetics of the LiNi1/3Mn1/3Co1/3O2 particles in 1 mol·L−1 H2SO4 at 30 °C with the S/L ratio fixed at 4% in the range 0−20 days (a), and (b) is a focus in the range 0−24 h. HRTEM images evidencing the layered structure after 30 min of dissolution, (c) after 24 h of dissolution, and (d) the phase transition from the layered structure in the subsurface to the spinel structure on the surface (delimited by the red line).

dissolution could result from the H+−Li+ ion−exchange reaction according to the redox reaction.

(Figure 10c,d). In solution, the average leaching efficiency does not evolve significantly (Figure 10b); the leaching of lithium increases progressively from 76 to 83%, whereas the leaching efficiency of Mn2+, Co2+, and Ni2+ ions fluctuates in the 37− 41% range. The structural and chemical modifications show the changes at the particle surface and explain the reorganization toward a core−shell structure. The structural rearrangement is driven by the surface vacancies resulting both the extensive delithiation and the oxygen release. Additionally, the high potential value of particles, around 1.2 V versus SHE (after 30 min of dissolution), makes possible the side reactions such as the disproportionation of manganese, initially described by Hunter,22 and/or the oxidation of the divalent manganese ions. The lithium-depleted surface, that is, concentrated in vacancies, gives preferential diffusion channels for the acidic solution, assuring the transport of species from the surface to the bulk. Consequently, the composition of the particle core evolves gradually to reach a Mn-rich surface describing a Mnlayer growing inward of the particles. It explains the monotonous variation with the Mn enrichment and the depletion of Co and Ni with a constant Co/Ni ratio of 1:1. The surface transformation is possible because of the structural compatibility of the rhombohedral and monoclinic symmetry, respectively, of the layered and birnessite phase. However, the birnessite phase is metastable and tends to turn into a spinellike phase. Figure 10d indicates the spinel domains on the fringes, not observed by XRD, because of a few extended domains. After an extended period of leaching (Figure 10a; step IIa), the manganese concentration in solution decreases by 25%, whereas the concentrations of Co, Ni, and Li are stable. The decrease in the manganese concentration is associated to the redox reaction occurring between the divalent manganese ions in solution and the surface. The highly oxidized core of the particles, consecutive to the first dissolution phase, is reduced to a valence state where the element is insoluble. The reduction of TM might induce a charge compensation in the solid framework and the co-intercalation of protons. It was demonstrated that a deep lithium extraction of LixNi1/3Mn1/3Co1/3O2 shows a significant concentration of protons in the lattice.50 The proton co-intercalation compensates the reduction of TM (M = Co, Ni, and Mn) and the subsequent exchange reaction with lithium. The second step of

LixMz +Oy ·γ H 2O + w Mnaq 2 + H 2O; H+ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ LixMz − 1Oy ·γ H 2O + HzMn wOy ·α H 2O (10)

LixMz − 1Oy ·γ H 2O + HzMn wOy ·α H 2O ↔ Lix − 1HMz − 1Oy ·γ H 2O + LiHz − 1Mn wOy ·α H 2O (11)

The co-intercalation of protons and water molecules are essential for the dissolution and the birnessite-phase formation. They ensure the permeation of particles and organize the formation of a birnessite phase and the transformation in a γtype manganese oxide. Many studies demonstrated the importance of alkali insertion and post TMs to create a tunnel structure of the birnessite crystal.43,45,63 The presence of the tunnel makes possible the transportation of species and the progression of the dissolution. Beyond 5 days, the leaching of cobalt, nickel, and lithium resumes (Figure 10a; step IIb). The progress of the dissolution reduces the TMs in a soluble form. The dissolution is stopped when the Mn2+ ions are completely consumed. Figure 11 indicates the kinetics of leaching of the NMC particles immersed into sulfuric acid at 70 °C. Once the equilibrium is reached after 24 h, Mn2+ ions are added in the solution (24 h). The leaching curve is similar to that in Figure 1, except that the thermal activation rises the dissolution, whereas the addition of Mn2+ reactivates the reaction of dissolution. Thus, the second phase of dissolution is governed by the presence of divalent manganese in solution.

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CONCLUSION This study addresses the mechanistic and limitation of the layered LiNi1/3Mn1/3Co1/3O2 oxide leaching used in lithium batteries. It aims at highlighting the fundamental dissolution mechanisms in acidic solution. The main results indicate the dissolution of the active cathode material in two steps. The first step is “self-regulating” by the lithium deintercalation and the charge compensation of TMs and partially by the oxygen reaction. The participation of the oxygen reaction in the 16432



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Emmanuel Billy: 0000-0002-0405-1118 Richard Laucournet: 0000-0002-9766-2806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Materials Transversal Program of the French Atomic and Alternative Energies Commission (CEA). XPS, XRD, and TEM measurements were performed by the Nanocharacterization Platform (PFNC) at Minatec center (Grenoble, France).

Figure 11. NMC dissolution kinetics in 1 mol·L−1 H2SO4 at 70 °C with the sulfate manganese addition at the end of the second dissolution phase (t = 24 h).

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(1) Tarascon, J.-M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (2) Koga, H.; Croguennec, L.; Mannessiez, P.; Ménétrier, M.; Weill, F.; Bourgeois, L.; Duttine, M.; Suard, E.; Delmas, C. Li1.20Mn0.54Co0.13Ni0.13O2 with Different Particle Sizes as Attractive Positive Electrode Materials for Lithium-Ion Batteries: Insights into Their Structure. J. Phys. Chem. C 2012, 116, 13497−13506. (3) Ohzuku, T.; Makimura, Y. Layered Lithium Insertion Material of LiCo1/3Ni1/3Mn1/3O2 for Lithium-Ion Batteries. Chem. Lett. 2001, 30, 642−643. (4) Notter, D. A.; Gauch, M.; Widmer, R.; Wäger, P.; Stamp, A.; Zah, R.; Althaus, H.-J. Contribution of Li-Ion Batteries to the Environmental Impact of Electric Vehicles. Environ. Sci. Technol. 2010, 44, 6550−6556. (5) Castillo, S.; Ansart, F.; Laberty-Robert, C.; Portal, J. Advances in the Recovering of Spent Lithium Battery Compounds. J. Power Sources 2002, 112, 247−254. (6) Sun, L.; Qiu, K. Vacuum Pyrolysis and Hydrometallurgical Process for the Recovery of Valuable Metals from Spent Lithium-ion Batteries. J. Hazard. Mater. 2011, 194, 378−384. (7) Lee, C. K.; Rhee, K.-I. Reductive Leaching of Cathodic Active Materials from Lithium Ion Battery Wastes. Hydrometallurgy 2003, 68, 5−10. (8) Joulié, M.; Laucournet, R.; Billy, E. Hydrometallurgical Process for the Recovery of High Value Metals from Spent Lithium Nickel Cobalt Aluminum Oxide Based Lithium-ion Batteries. J. Power Sources 2014, 247, 551−555. (9) Li, L.; Ge, J.; Wu, F.; Chen, R.; Chen, S.; Wu, B. Recovery of Cobalt and Lithium from Spent Lithium Ion Batteries Using Organic Citric Acid as Leachant. J. Hazard. Mater. 2010, 176, 288−293. (10) Sun, L.; Qiu, K. Organic Oxalate as Leachant and Precipitant for the Recovery of Valuable Metals from Spent Lithium-ion Batteries. Waste Manag. 2012, 32, 1575−1582. (11) Li, L.; Ge, J.; Chen, R.; Wu, F.; Chen, S.; Zhang, X. Environmental Friendly Leaching Reagent for Cobalt and Lithium Recovery from Spent Lithium-ion Batteries. Waste Manag. 2010, 30, 2615−2621. (12) Granata, G.; Moscardini, E.; Pagnanelli, F.; Trabucco, F.; Toro, L. Product Recovery from Li-ion Battery Wastes Coming from an Industrial Pre-treatment Plant: Lab Scale Tests and Process Simulations. J. Power Sources 2012, 206, 393−401. (13) Li, L.; Lu, J.; Ren, Y.; Zhang, X. X.; Chen, R. J.; Wu, F.; Amine, K. Ascorbic-acid-Assisted Recovery of Cobalt and Lithium from Spent Li-ion Batteries. J. Power Sources 2012, 218, 21−27. (14) Chagnes, A.; Pospiech, B. A Brief Review on Hydrometallurgical Technologies for Recycling Spent Lithium-ion Batteries. J. Chem. Technol. Biotechnol. 2013, 88, 1191−1199. (15) Pagnanelli, F.; Moscardini, E.; Granata, G.; Cerbelli, S.; Agosta, L.; Fieramosca, A.; Toro, L. Acid Reducing Leaching of Cathodic

material induced the release of O2 gas and the defects formation. The process is related to the proton concentration and certainly initiated by an excess of positive charge at the solid/liquid interface of the particles. The electrochemical measurements show a dependence with the pH and the degree of delithiation in the NMC particles. The delithiation process leads to a positive potential shift of particles, which reduces the leaching driving force and limits the dissolution. In the second step, the concentration of the surface vacancies associated to the high potential of particles leads to a surface reorganization from the layered structure to a metastable birnessite phase and a transformation in the γ-type manganese oxide. This second step results first from the disproportionation reaction of manganese and/or the redox reaction between the Mn2+ ions and the particle surface. It induces an enrichment in manganese at the particle surface to form a well-defined core− shell structure. Second, the second step of dissolution is controlled by the presence of a divalent manganese in solution. This study should promote further work on the generalization of the dissolution mechanism on other lithiated transition metal oxide cathode materials produced by ex situ acid delithiation routes. The mechanistic limitation by the lithium deintercalation has to be confirmed. The relationship between the number of lithium ions per formula unit and the leaching efficiency has to be extended to other cathode materials such as lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt aluminium oxide (NCA), and lithium iron phosphate (LFP).

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01352. dissolution kinetics in hydrochloric and nitric acids; STEM EDXS elemental mappings of particles with the leaching time; evolution of particle size and active area of the NMC particles; evolution of molar ratio in the metals dissolved; in situ O2 concentration dissolved with the leaching time; photoelectron spectra for cobalt, nickel, and manganese with the leaching time; evolution of oxygen anions concentration with leaching time; and dissolution kinetics according to the pH of dissolution (PDF) 16433


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3O2 Positive Electrode

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