3O2 positive electrode

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01352 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Applied Materials & Interfaces

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 and 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 KEYWORDS: mechanism, dissolution, Li-ion battery, NMC cathode, recycling.

ABSTRACT: The sustainability through the energy and environmental costs involve the development of new cathode materials in considering the materials 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 acidic solutions use. This study addresses the mechanistic and limiting aspects on the dissolution of layered LiNi1/3Mn1/3Co1/3O2 oxide in acidic solution. The results shows a dissolution of active cathode material in two steps, which leads to form a well-defined core-shell structure inducing an enrichment in manganese on the particles 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 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 side reactions, by the modification of potential, explain the structural and compositional evolutions observed towards a composite material MnO2.LixMO2 (M = Ni, Mn, Co). The study shows a clear way to produce new cathode materials and recover transition metals (TM) from LIBs by hydrometallurgical processes.

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, hydrochloric) and organic acids9-13 (citric, oxalic, malic, ascorbic acids). The general approach consists in determining the operating conditions in acidic solution for the best leaching efficiency. The moderate efficiency to dissolve oxide conducts to add reductive agents as hydrogen peroxide,7, 14 glucose15 or metallic compounds.16 Recently Joulié et al.16 discussed the dissolution of NMC in acid solutions. They suggested a dissolution of LIBs positive 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 surfacecontrolled 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 on the dissolution mechanisms of cathode materials are related to the development of new materials by the solid state chemistry. Extensive research 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 stable LiMn2O4 (spinel structure) and Li3MnO3 (rock salt) phases has provided a first base of understanding.22-23 The ex situ acid delithiation routes were developed in a variety of both acid solutions and cathode materials (H2SO4, HNO3, HCl).22-33

Introduction Rechargeable Li-ion cells are 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 positive electrode material for high capacity batteries (274 mAh g–1). Nevertheless, due to a high cost and possibly a limited availability of Co, as well as safety issues, such oxide cannot be considered as a candidate in large scale batteries for automotive applications.2 Li(Ni,Mn,Co)O2 based electrodes such as LiNi1/3Mn1/3Co1/3O2, referred as NMC, are entering now the automotive application market.3 Since recently, Li-ion batteries (LIBs) are used to power electric vehicles (EVs), gradually replacing nickel metal hydride batteries. The market penetration of EVs is expected to increase as the gasoline prices rise and the pressure increases to reduce carbon and particles emissions from fossil fuel use.4 The treatment of spent LIBs through the recovery of valuable metals (Co, Ni, Mn) is a major concern for their development. For instance The European 2006/66/CE directive drives manufacturers to improve the recycling efficiency of LIBs with a minimum recycling yields of 50 wt.%. Most of the LIB recycling processes developed and reported in academia is hydrometallurgical process because it is less energy consumption. Many research works are focused on the development of cost effective and environmentally acceptable 1

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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 remaining in the spinel framework. Zhecheva et al.27 observed a Li1xHxCoO2 metastable layered phases by acid leaching of LiCoO2 in the H2SO4 and HCl acids. 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 et al.28 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 the 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 dissolutionreprecipitation mechanism instead of solid state transformation by disproportionation. In H2SO4 solution, the oxidation of sulfate to persulfate (E / = 2.01 V vs. NHE ) may initiate the dissolution and then catalyze the Li2MnO3 to αand γ-MnO2 transformation. 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 mechanisms still remain elusive while a better understanding is required to control the structure of new lithium-insertion compounds. We report herein the dissolution mechanisms of LiNi1/3Mn1/3Co1/3O2 material in acid solution, explaining the leaching performances and structural evolutions of lithiated transition metal oxide electrodes. This work included spectroscopic measurements (XPS), morphological and structural analyses (XRD, STEM, HRTEM) in conjunction with electrochemical measurements. The specific nature of our work shows a clear and exciting path to recover transition metals from LIBs and produce new cathode materials. The knowledge of the dissolution mechanisms of LiNi1/3Mn1/3Co1/3O2 material in aqueous solution can also be a source of understanding in the factors influencing the chemical lithium extraction in a non-aqueous medium and understand better the degradation mechanism of positive electrodes (LiMn2O4, LiNi1/3Co1/3Mn1/3O2, LiCoO2 and LiFePO4)36 used in aqueous rechargeable lithium batteries.

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reproduced seven times to determine the stoichiometry of the NMC at 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 supplied Li(Ni,Mn,Co)O2 material. Chemical reactants used for experiments are 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). 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 TM 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, X-ray diffraction patterns are obtained by using X-ray diffractometer Bruker D8 Advance with a Θ-2Θ Lynxeyes detector. The patterns were recorded in the 2θ range 8–80°, making 2θ step-scan intervals of 0.05° with 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 taken by a scanning electron microscope LEO 1530 FEG-SEM. Residues are covered by an epoxy resin. Images are recorded by transmission electron microscopy using a FEI TECNAI microscope operated at 200 kV to identify the structure of residues. For studying chemical evolutions, Electron Dispersive X-ray analyses are performed using a FEI OSIRIS microscope operated in STEM mode at 200 kV. X-ray photoelectron spectrometry (XPS) allowed the surface characterization of remaining residual solid particles by using an MXPS Omicron spectrometer. The energy resolution used for high resolution spectra was set to 0.4 mV. Quantitative analysis is achieved, based on peak areas weighted by the Scofield relative sensitivity factors (RSF). The calibration of binding energy scale is performed with the C 1s line (284.8 eV) from the carbon contamination layer. Electrochemical study 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 (PVDF) in N-methyl-2pyrrolidone (NMP) is deposited on glassy carbon of the working electrode and dried at 80 °C as the coin cells 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 vs. the standard hydrogen electrode (SHE). The counter electrode is a platinum wire diving in the electrochemical cell. During the open circuit voltage (OCV) measurements, the NMC electrode is immersed and the acid is added after starting the open circuit voltage acquisition under a constant stirring of 500 rpm.

Materials and methods Materials In the present work, NMC material, Li(Ni1/3Mn1/3Co1/3)O2 supplied by Umicore inc. and referenced as MX6 is investigated. The particles mean size of NMC material powder is measured 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 (BET) with Beck-man Coulter SA3100 surface area analyzer. Elemental titration of NMC material is determined by an Induced Coupled Plasma Optical Emission Spectrometer (Agilent Technologies 700 Series ICP-OES) with a complete dissolution in acid (50 vol.% of HNO3 4 mol.L-1 and 50 vol.% of HCl 4 mol.L-1) assisted by a microwave digestion system (Multiwave 3000, Perkin Elmer, Anton Paar). The titration is 2


LixNi1/3Mn1/3Co1/3O2 phases remain similar with peaks shifts (003), (108), (110) indicating variations of a and c lattice parameters (Table 1). The shift of planes (003) to lower 2θ values for the pattern (b) indicates a lattice expansion in the c direction associated with decreasing lithium content from 1 to 0.48, which is known to be induced by the electrostatic repulsion between oxygen anions.41 The leaching progress from 15 minutes to 24 hours decreases the lithium content from 0.43 to 0.38 in the material, while the (003) planes shifts to higher 2θ values (pattern c). It reflects the contraction of c parameter due to a lower electrostatic repulsion likely due to metal migration to vacancies created by the Li removal. The overlapping of (006)/(102) couple of 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 of oxygen released from the structure, which is observed during the first step of dissolution. The oxygen ions release diminishes the total electrostatic repulsion, therefore the c parameter as well. The separation of (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 of the c-lattice parameter.42 The refined lattice parameters are reported in the 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% due to the opposite evolution both a and c lattice parameters.

Results and discussion Leaching kinetic of LiNi1/3Mn1/3Co1/3O2 material in sulfuric acid The Figure 1 shows the NMC dissolution kinetic in sulfuric acid at 1 mol.L-1 and 30 °C with 4% as weight solid/liquid ratio. The results indicate a dissolution kinetic divided in two steps. The first step (Step I) is a fast material dissolution from 0 to 14 minutes accompanied of few gas bubbles emission in solution. The leaching efficiency is 71%, 33%, 34%, and 40% respectively for lithium, nickel, cobalt and manganese. The first dissolution step is limited as observed by others studies for LiNi1/3Mn1/3Co1/3O2,16 LiCoO2,7, 13, 38 LiNi0.8Co0.15Al0.05O2,8 LiMnyCo1-yO224 compounds. Even if the leaching efficiencies are not comparable due to both changes in operating conditions and cathode materials, they reveal some similarities. The leaching operated in the nitric and hydrochloric acids reveal also a fast material dissolution (Supplementary Information, SI-1). The second step (step II) is a 43 days slow stage, with 100%, 57% and 56% respectively for 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 in two periods, the step IIa shows a decrease of manganese in solution without evolution of Co2+, Ni2+ and Li+ ions (Figure 1), while the step IIb indicates a concomitant dissolution.

(A)

∆

•

Pristine

Ramsdellite-MnO2

(B)

∆ Mn0.33Co0.33Ni0.33O(OH) ∇ Li0.48Mn0.33Co0.33Ni0.33O2 ♦ Li0.38Mn0.33Co0.33Ni0.33O2 # HMn0.33Co0.33Ni0.33O2 Li4Mn14O27.xH2O (birnessite) # ∗ Nsutite:Mn1.82Mn0.12O3.50(OH)0.5 # #∗ ∆ # ∆• • # ∆ # •∗ •∗ ∆ •∆• •∗ •∆ •∗ ∆• (d) ∆ ∗•

Li4Mn14O27.xH2O

Step ΙΙ ♦

15

20

25

30

0.01

0.02

∇

∇

∇

5

10 15 20 25 30 35 40 45

♦

♦

(c) ♦♦

∇

(b) ∇ ∇∇

40

45

(c)

*

(104)

∇

50

55

60

65

70

(a)

75

80

(108)

(b)

CuKα 2θ (Deg.)

0 0.00

35

♦

(105)

(006) (102)

(101)

(003)

10

30 min

♦ ♦

(201) (116)

*

∇

20

♦ ∇

∇

*

Li Ni Mn Co

40

♦ ♦

∇

(108) (110) (113)

*

Intensity (a.u.)

60

(d)

♦

♦

Intensity (a.u)

ΙΙb

(009) (107)

ΙΙa

(110)

Step Ι

(003)

80

*

*

100

Leaching efficiency (%)

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


*

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(a) 12 14 16 18 20 62

64

66

CuKα 2θ (Deg.)

Figure 2: X-ray diffraction patterns of the pristine NMC powder (a) after leaching in H2SO4 1 mol.L-1 during 15 minutes (b), 24 hours (c) and 43 days (d) - Note that the “d” pattern have not the same intensity scale. The graph (B) is an enlargements in the 12 – 21° (2θ) and 61 – 67.5° (2θ) ranges.

Time (days)

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

Structural evolution during leaching The Figure 2A shows the X-ray diffraction (XRD) patterns of residual solids after the leaching of the LiNi1/3Mn1/3Co1/3O2 powder. The diffraction lines of pristine material are well indexed with Miller indices for each peak in the rhombohedral system with 3 space group and a single-phase α-NaFeO2type structure without any impurity phase. As seen in diffractogram (a), both the (006)/(102) and (108)/(110) doublets are well separated, which indicate a good hexagonal ordering of layered NMC materials.39 The patterns (b) and (c) indicate deintercalated phases in lithium and the formation of a neoformed phase. The diffraction peaks at 2θ = 12.6° and 25.5° are assigned to (001) and (002) planes of a birnessitetype layered structure.40 The deintercalated

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 Mn O 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 birnessitelike materials.43-44 The 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 gammatype-manganese oxide (nsutite and Ramsdellite structure), as reported in the literature.45 3



Table 1: Variation of cell parameters of LiNi1/3Mn1/3Co1/3O2 powder (a), and after leaching in H2SO4 1 mol.L-1 at 30°C during 15 minutes (b), 24 hours (c) and 43 days (d) and evolution of lithium content of particles. Sample

Leaching time

Nominal lithium content

Lattice parameters of the NMC powder a (Å)

b (Å)

c (Å)

(1)

(2)

V (Å3)

∆V (%)

XRD

ICP

a

Pristine material

2.85730

2.85730

14.2250

100.5760

0

1

1.04

b

15 min

2.81730

2.81730

14.5340

99.9038

-0.67%

0.48

0.47

c

24 min

2.82700

2.82700

14.3450

99.2848

-1.28%

0.38

0.35

d

43 days

22.87450

2.87450

13.9930

100.1300

-0.44%

0

0

Unit cell volume ; (2)∆V = Volume variation with the pristine material

The diffraction pattern (d) shows a poor crystallinity but the Bragg peaks can be also indexed on NMC phase’s protonated and hydrolyzed. After 43 days of ageing in acidic solution, the structural integrity of the NMC phase subsists and coexists with neoformed phases of manganese oxide-type.

After 15 minutes of leaching, large micro-cracks appear (inset Figure 3b), increasing the specific surface area from 0.4 to 6.2 m2.g-1 (cf. Supporting Information, SI-3). The dissolution causes the exfoliation at the surface of particles due to solvent intercalation and/or gas evolution inside the layered structure. The average size distribution of particles is reduced from 7.684 to 4.095 µm due to a significant leaching of Mn, Co and Ni (40%).

Bulk and surface compositions investigations The Figure 3 shows HRTEM observations and the Figure 4 depicts concentration profiles obtained by STEM-EDX experiments from particles presented in Supporting Information, SI2.

100

100

(a) 80

Relative composition (at.%)

(b)

Co Mn Ni

60

40

20

Co Mn Ni

80


(1)

0

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100

(c)

(d)

Co Mn Ni

60

40

20

0

Co Mn Ni

80


80


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

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60

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Position (µm)

1.0

1.2

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0.9

1.2

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1.8

Position (µm)

Figure 4 : HAADF-STEM analysis corresponding EDS elemental maps with the cross-section composition line profile of initial NMC particle (a) and after leaching in H2SO4 1 mol.L-1 during (b) 15 minutes, (c) 24 hours and (d) 43 days.

After 24 hours, the leaching efficiency increases and the particles damage as well (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 due to particles mechanical fragmentation. Their profile of elemental composition in Figure 4c shows a manganese accumulation on the outer layer of the particle, with a gradient distribution of transition metals. The near-surface region is enriched in manganese partially crystallized under a birnessite phase. The bulk composition corresponds to a delithiated material with Mn/Ni/Co atomic ratios of 1:1:1 similar to pristine compound. After 43 days, the submicronic scale (inset Figure 3d) shows small fibrous needles intergrown in the particle. The needlelike morphology enhances the specific surface area at 68 m2.g-

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

The pristine material presents a lamellar structure with 4.74 Å as interplanar distance and without amorphous surface. The composition profiles presented in Figure 4a indicates a homogeneous elemental distribution in accordance with the preliminar chemical analyses.

4


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1

. Figure 4d indicates the surface enrichment in manganese ongoing to form a well-defined core-shell structure. The chemical distribution inside the particle is uniformly distributed in Mn/Ni/Co. The composition of particle evolves gradually from the core to reach a Mn-rich surface describing a Mnshell growing inwards of the particle. The compositional results obtained from the EDX mapping indicate a monotonous variation 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 residual particles by Xray photoelectron spectrometry (XPS) are summarized in Table 2. It is well known that such Li-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 particles is steadily depleted in lithium. The surface concentration in Mn, Co and Ni is stable up to 15 minutes before rising for manganese after 18 hours. The surface enrichment in manganese is consecutive to the first step of leaching. Table 2 : Surface composition (at.%) of residual particles measured by XPS after 5 minutes, 15 minutes and 18 hours of leaching in H2SO4 1 mol.L-1 at 30 °C. Leaching time

O

Li

Ni

Mn

Co

Pristine material

54,2%

35,4%

3,0%

4,5%

2,9%

5 min

60,6%

16,3%

7,4%

9,9%

5,9%

15 min

60,9%

14,6%

7,5%

10,5%

6,5%

18 h

69,5%

2,5%

7,0%

14,3%

6,6%

4,6

E vs. Li (V)

4,2 4,0 3,8 3,6 3,4

OCV vs. SHE (V)

4,4

1,4

1,0

1,2

0,8

1,0

0,6

0,8

0,4

0,6

0,2

0,4

0,0 0

5

10

15

20

Lithium content (x)

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

25

Time (min)

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

In the NMC voltage window 2.5–4.7 V, the charge compensation results in two steps at different charge states. The Ni2+/Ni4+ couple compensates the charge up to 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, ∆G(x), between the delithiated phase (charged state) and lithiated phase (discharged state) by the general equation !"#$ =

%∆' "($ )*

(1)

where F is the Faraday constant, z the number of electrons involved and x the lithium content.48 Thus, the delithiation phenomenon changes the internal energy of the reaction and increases the operating potential.

The structural analysis shows three main structural and chemical evolutions of particles. The first evolution leads to a single NMC phase delithiated where rhombohedral symmetry is kept. The second is the formation of a thin crystallized film of birnessite forming a shell at the surface of delithiated particles. The third, is the growth of Mn needles in surface with transformation of the metastable birnessite phase into a gamma-type-manganese oxide. The particles core is a NMC phase, wherein is inserted a compositional gradient of transition metals including a shell of manganese, describing a composite material MnO2.LixMO2 (M = Ni, Mn, Co). The Figure 5 indicates the on-line electrode potentials and dissolution measurements relationships between open circuit voltage (OCV) profile and delithiation of NMC particles. It shows the OCV evolution with the composition of LiNi1/3Mn1/3Co1/3O2 particles immersed into the sulfuric acid. The OCV curve defines the operating potentials, which are assimilated to corrosion potentials of the first phase of dissolution. The OCV profile is stands out by an immediate sharp rise of potential from 0.6 V to 1 V (vs. SHE) followed by a gradual weakening before reaching an equilibrium around 1.3 V. The rise of potential is attributed to the progressive delithiation of material. The lithium depletion of particles is caused by the excess of lithium leached per formula unit (empty symbol in Figure 5), which leads to a positive shift of particles potential induced by the charge-compensation reactions of TM.

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 absence of external circuit, the electrons can recombine inside the particle to reduce TM and promote the oxide dissolution during Li-ion removal. Such process requires thus a spontaneous extraction of lithium. The deintercalation of lithium can be promote by an excess of positive charge at the interface as suggested in the 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 ≡ , − .% + 0 ↔ ≡ , − .0 (2) ≡ , − .0 + 0 ↔ ≡ , − .02 (3) 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) equals zero. Below the pHpzc of solid, the surface is positively charged, indicating an excess of surface 5



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 to a protonation reaction at the interface.

For reasons of clarity, the evolution of Mn 2p, Li 1s and Co 2p core peaks are described separately in the supplementary information, SI-6. O1s

Lattice Oxygen (O-/O2) 2

≡ 3454688 ,968: ; $? + "34= 54 88 ,98: ;< 888 .> $@

(c)

(b)

A(

BC "54 8: ,98: ;< 8: .> $@ + 3"342 54 888 ,98: ;< 888 .> $? + 6 34 + 6E % (6)

(a) 536 535 534 533 532 531 530 529 528 527 526

FGH

"54 8: ,98: ;< 8: .> $@ + 120 + 6E % BIC 54 2 + ,92 + ;< 2 + 6 02 . (7) Where the subscripts s and b refer to surface and bulk of particles, the net reaction is: 3"34= 54 88 ,98: ;< 888 .> $? + "34= 54 88 ,98: ;< 888 .> $@ + 120 → 54 2 + ,92 + ;< 2 + 6 34 + 6 02 . + 3"342 54 888 ,98: ;< 888 .> $? (8) The reaction of dissolution (equation 8) is in agreement with the measurements of potential (Figure 5) and surface compositions (Table 2). The bulk of particles is oxidized and the surface is depleted in lithium ongoing of reaction. During the dissolution, the surface valence distribution (formal valences) is unknown, but it can be apprehended in considering the stoichiometry of the reaction. In equation 8, the Li+/(Ni2+ + Mn2+ + Co2+) atomic ratio of ions dissolved is two. 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 transition metals at the interface should be at the four valence state 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 recombination inside the material and/or by side reactions. During the chemical delithiation, an exchange reaction of Li+ by H+ occurs into oxide cathodes.50 Consequently the number of Li+ available and the number of electrons actually generated is overestimated due to the exchange reaction of Li+ by H+. Additionally, a secondary reaction is clearly identified with few bubbles of gas accompanying the first phase of dissolution. The on-line 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.

Binding energy (eV)

Figure 6 : O 1s photoelectron spectra of (a) LiNi1/3Mn1/3Co1/3O2 initial material, after leaching in H2SO4 1 mol.L-1 during (b) 5 minutes, (c) 15 minutes and (d) 18 hours. The binding energies were calibrated against the C1s line from the carbon contamination layer (284.8 eV).

Regarding to the O 1s spectrum of initial material, a welldefined profile peak is observed at 529.7 eV characteristic of O2− anions of the crystalline network (denoted O2- lattice), while at ∼532 eV 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, i.e., peroxide-like O- ions.51-55 The O 1s spectrum remarkably changes when the sample is leached 5 minutes, the peak intensity at 531 eV increases. The phenomenon is amplified with the leaching time and the increase of O-/O2- ratio (Supplementary Information, SI-7). The results argued that oxygen ions participate in the redox reaction of charge compensation. The cation–anion dual charge compensation mechanism occurs in the first step of dissolution. The chemical instability of layered Li1−xNi1−y−zMnyCozO2 cathodes was reported on lithium ion batteries in literature.56 Numerous studies54, 57 exemplify the contributions of bulk and surface oxygen on high charge– discharge capacity in Li-rich layered oxides. During a deep charge, an oxidation of oxygen ions (O2-/O- or O2n-) lead 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 The Figure 7 overlap of the M3+/4+:3d band with the top of the O2−:2p band shows an 6


Page 7 of 13 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


oxidation of O2− ions and an ultimate oxygen loss from the lattice.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 a O2 release according to equation 9. "2,8: .2=% $@ → ",8: .2 $@ + .2 (9) (with M= Co, Ni. 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 sub-surface layers. The O2 evolution explains large micro-cracks observed on the particles (Figure 3b). Kim et al.21 described a similar process with a Li2MnO3 material during a battery cycling at high potential. The intermediate superoxide species at the electrode surface are immediately stabilized by the chemical release of oxygen at the surface.

phase of dissolution is rather slowed down than blocked, likely due to structural and chemical evolutions. Figure 8 is a simplified E-pH diagram reporting the operating potential values from OCV curves (inset) after 30 minutes 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 the equation 7 (MO2/M2+). The E-pH evolution indicates the reduction of operating potential with the acidity, 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 doublet (006/102) on the diffractogram in Figure 2b exhibits a disorder degree of cations in the lamellar phase.60 The presence of transition metal ions in the lithium planes can interfere with the lithium ion diffusion path due to the electrostatic repulsion between Li+ and Mz+.61-62 The lithium extraction rate is also influenced by the decrease of the inter-slab distance of lithium layers (Table 1). Additionally, the lithium-depleted layers enhance deeper the Li+ migration pathways. The deep extraction of lithium and a discontinuous pathway for fast Li ion transportation increases the energy barrier for Li+ migration and impacts the reaction kinetic.

Limitation of the first step of dissolution The mechanism of dissolution can be considered as “selfregulating” as long as lithium deintercalation reaction occurs and provides electrons at the solid/liquid interface. Although the Li+ deintercalation is incomplete, the leaching is hardly stopped after few minutes. The formation of an interfacial blocking layer in manganese that impedes the delithiation is not detected. The cross-section composition profile indicates a homogeneous elemental distribution (Figure 4b) and the surface composition in TM is conformed to initial material slightly depleted in lithium (Table 2). The limitation of the first

Figure 7 : Process of O2 release during the first phase of dissolution. (a) is a macroscopic representation of LixNi1/3Mn1/3Co1/3O2 particle delithiated, (b) the crystal structure and the delithiation between the core and the surface of particles. (c, d and e) are a schematic representation of the energy level versus 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.

7



E vs. SHE (V)

1.6

E vs. SHE (V)

1.4

O2/H2O

1.2

When the dissolution pH rise, the surface layer is proportionally increased in lithium. The transport properties should be facilitated due to a shorter pathways for the Li+ migration, denoted δLi+depleted in Figure 9c. The results doesn’t show a correlation between the dissolution kinetic and the depletion in lithium, assuming a 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 (∆rG(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 number of lithium per unit of material is fundamental. It imposes the potential and the electrons number generated, and therefore the leaching efficiency for the first step of dissolution.

1.2

E30min

1.0

a

0.8

b

0.6

c d e

0.4

f 0

5

10

15

20

25

30

Time (min)

1.0

MnO2/Mn2+

0.8 0.6 0.4 0

1

2

3

4

5

6

pH

Second step of dissolution The formation of a Li-rich birnessite-type phase and a transformation in a gamma-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 minutes to 24 hours (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%, while the leaching efficiency of Mn2+, Co2+ and Ni2+ ions fluctuates in the 37-41% range.

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

The Figure 9 represents the dissolution kinetic of lithium (a) and the lithium content (b) in the NMC particles for a leaching with a pH set at 0, 1, 2, 3 and 4. The lithium content is calculated in considering the leaching efficiency of TM and Li+ (Supporting Information, SI-8). The lithium dissolution decreases from 70% (pH=0) to 8% (pH=4), while the lithium content in the particles increases respectively from 0.45 to 0.95.

100

(a) 80


Leaching efficiency of Lithium (%)

80

(a)

Step ΙΙb

Step ΙΙa

60

40

80 78 76

5 nm 44

(d) 42 40

20

60

38

pH=4

36 0 0.0

40

2.5

5.0

10

Time (days)

pH=3

20

0

(b)

1,0

Lithium content (x)

(c)

Pristine particule

(c)

82

100

pH=0 pH=1 pH=2 pH=3 pH=4

(b)

84


1.8

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

Page 8 of 13

pH=2

pH=1 δLi+depleted 0,6

5

10

15

20

25

30

0

4

8

12

16

20

24

Time (hours)

The structural and chemical modifications show the changes at the particles 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 vs. SHE (after 30 minutes of dissolution), makes possible side reactions as the disproportionation of manganese, initially described by Hunter,22 and/or the oxidation of divalent manganese ions. The lithium depleted surface ie., concentrated in vacancies, gives preferential diffusion channels for the acidic solution, assuring the transport of species from surface to bulk. Consequently, the composition of particle core evolves gradually to reach a Mn-rich surface describing a Mn-layer growing inwards of particles. It ex-

pH=0

0

20

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

0,8

0,4

15

r1

r0

X=0

X=1

X=0

LixNi1/3Mn1/3Co1/3O2

Time (min)

Figure 9: Dissolution kinetic of lithium in (a) and lithium content in (b) from NMC particles in sulfuric acid 1 mol.L-1, at 30 °C with a 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.

8


Page 9 of 13

plains 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 due to a 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. The Figure 10d indicates the spinel domains on the fringes, not observed by XRD due to few extended domain. After an extended period of leaching (Figure 10a; Step IIa), the manganese concentration in solution decreases by 25%, while the concentrations of Co, Ni and Li are stable. The decrease of manganese concentration is associated to the redox reaction occuring between the divalent manganese ions in solution and the surface. The highly oxidized core of 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 protons co-intercalation compensates the reduction of TM (M= Co, Ni. Mn) and the subsequent exchange reaction with lithium. The second step of dissolution could result from the H+– Li+ ion-exchange reaction according to the redox reaction.

0.5

0.4

-1

Concentration (mol L )

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


0.2

0.1

0.0 0

8

16

24

32

40

48

Time (hours)

Figure 11: NMC dissolution kinetic in H2SO4 1 mol.L-1 at 70 °C with sulphate manganese addition at the end of the second dissolution phase (t=24 h)

Conclusion This study addresses the mechanistic and limitation of layered LiNi1/3Mn1/3Co1/3O2 oxide leaching used in lithium battery. It aims at highlighting the fundamental dissolution mechanisms in acidic solution. The main results indicate a dissolution of the active cathode material in two steps. The first step is “self-regulating” by the lithium deintercalation and the charge compensation of transition metals and partially by the oxygen reaction. The participation of the oxygen reaction in the material, induced the release of O2 gas and the defects formation. The process is related to the protons concentration and certainly initiated by an excess of positive charge at the solid/liquid interface of 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 a second step, the concentration of surface vacancies associated to the high potential of particles leads to a surface reorganization from layered structure to a metastable birnessite phase and a transformation in a gamma-type-manganese oxide. This second step results firstly of the disproportionation reaction of manganese and/or the redox reaction between Mn2+ ions and the particles surface. It induces an enrichment in manganese at the particles surface to form a well-defined coreshell structure. Secondly, the second step of dissolution is controlled by the presence of divalent manganese in solution. This study should promote further work on the generalization of the dissolution mechanism on other lithiated transitionmetal 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 per formula unit and the leaching efficiency has to be extended to other cathode materials (LCO, LMO, NCA, and LFP).

P A ; P R

2 34( , ) .K . L02 . + M,9NO BIIIIC 34( , )%S .K . L02 . + 0) ,9T .K . U02 . (10)

34( , )%S .K . L02 . + 0) ,9T .K . U02 . ↔ 34(%S 0, )%S .K . L02 . + 340)%S ,9T .K . U02 .

Li Ni Mn Co

0.3

(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 gamma-type-manganese oxide. Many studies demonstrated the importance of alkali insertion, and post transition metals to create a tunnel structure of the birnessite crystal structure.43, 45, 63 The presence of tunnel makes possible the transportation of species and the progression of the dissolution. Beyond 5.6 days, the leaching of cobalt, nickel and lithium resumes (Figure 10a; Step IIb). The progress of the dissolution reduces the transition metals in a soluble form. The dissolution is stopped when the Mn2+ ions are completely consumed. Figure 11 indicates the kinetic of leaching of NMC particles immersed into sulfuric acid at 70 °C. Once the equilibrium reached after 24 hours, Mn2+ ions are added in the solution (24h). The leaching curve is similar to the Figure 1 excepted that the thermal activation rises the dissolution, while 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.

ASSOCIATED CONTENT Supporting Information. The dissolution kinetic in hydrochloric and nitric acids, the STEM EDS elemental mappings of parti-

9



cles with the leaching time, the evolution of particle size and active area of NMC particles, the evolution of molar ratio in metals dissolved, the in-situ O2 concentration dissolved with the leaching time, the photoelectron spectra for cobalt, nickel and manganese with the leaching time, the evolution of oxygen anions concentration with leaching time, and the dissolution kinetic according the pH of dissolution

Page 10 of 13

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AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT 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).

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(62) Rougier, A.; Gravereau, P.; Delmas, C. Optimization of the Composition of the Li1-zNi1+zO2 Electrode Materials: Structural, Magnetic, and Electrochemical Studies. Journal of the Electrochemical Society 1996, 143 (4), 1168-1175. (63) Feng, Q.; Kanoh, H.; Ooi, K. Manganese Oxide Porous Crystals. Journal of Materials Chemistry 1999, 9 (2), 319-333.

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3O2 positive electrode

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