Inter-diffusion of Cations from Metal Oxide Surface Coatings into

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Interdiffusion of Cations from Metal Oxide Surface Coatings into LiCoO2 During Sintering Yujuan Zhao,†,‡,§,∥ Jing Li,‡ and J. R. Dahn*,‡ †

College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China Department of Physics and Atmosphere Science, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada § Guyue New Material Research Institute, Beijing University of Technology, Beijing 100124, China ∥ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China ‡

S Supporting Information *

ABSTRACT: Surface coatings on positive electrode materials can improve their electrochemical performance in lithium ion batteries, especially at high operating voltages. Cations in the surface coating can diffuse into the positive electrode material during sintering steps after coating. A simple “two-pellet” method was used to measure the concentration profiles of M = Al3+, Mg2+ ions from the pure MOx coating layer to the pure LiCoO2 matrix after heat treatment at various temperatures using energy dispersive spectroscopy. The interdiffusion coefficients between M atoms and Co increase with heating temperature. Accordingly, the diffusion distances of M and Co between the MOx and the LiCoO2 are affected by the sintering temperature and time. The activation barrier for diffusion was determined using the Arrhenius equation to be about 88 kJ/mol for the Al3+/Co3+ couple and about 100 kJ/mol for the Mg2+/ Co3+ couple. The measured and fitting diffusion constants were used to simulate the diffusion Al3+ in a 100 nm Al2O3 coating layer on spherical LiCoO2 particles during heat treatment. These results will be very helpful to guide the design of surfacemodified positive electrode materials.

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INTRODUCTION Recently, more researchers1−4 are focusing on high energy density Li-ion cells with high operating potentials (≥4.2 V). High working potentials cause poor capacity retention due to the instability of delithiated cathode materials in organic electrolyte.5−7 Modification by surface coating8−10 is an important method to improve the electrochemical performance of positive electrode materials in lithium ion batteries. Coating layers can provide a physical protection barrier or a doped surface layer to impede side reactions between cathode materials and electrolytes, thereby improving the reversible capacity, first cycle Coulombic efficiency, cycling behavior, rate capability, and overcharge tolerance. Metal oxide MOx (M = Al, Mg, Ti...)11−13 coatings are often applied on lithiated transition metal oxides. In order to protect electrodes being attacked by the electrolyte and to avoid introducing ohmic resistance, the ideal coating layer would be thin and uniform and a Li+ ion conductor. Various methods, such as sol−gel,14,15 precipitation,16,17 mechanofusion mixing,18,19 chemical vapor deposition,20 and atomic layer deposition,21 have been developed to apply coatings on the surface of active material particles. Figure 1a shows a schematic of a typical coating process. Whatever wet chemical method or solid mechanical mixing was © 2017 American Chemical Society

Figure 1. Schematic diagram of (a) the coating and sintering process for a particle of active materials, and (b) the variation of concentration within the spherical particle after sintering.

used to coat the active material particles, a subsequent heating treatment is needed. During the heating process, a MOx coating Received: March 25, 2017 Revised: May 27, 2017 Published: May 29, 2017 5239

DOI: 10.1021/acs.chemmater.7b01219 Chem. Mater. 2017, 29, 5239−5248

Article

Chemistry of Materials

Figure 2. Procedure for assembling the pellet stacks for SEM and EDS experiments.

constants were used to predict composition versus position in coated LiCoO2 particles sintered at various temperatures for various times. Such simulations demonstrate that the knowledge of the interdiffusion coefficients can help guide the rational synthesis of coated positive electrode materials.

layer is formed from the coating precursor and interdiffusion between M and the metals in the active material particles occurs, as shown in Figure 1b. To obtain the desired particles, cation diffusion between the coating layer and the active particles should be controlled by adjusting the heating temperature and time. Generally, with cations diffusing from the coating layer to the active particles, the surface of the active particles can become covered with a layer of doped ionic conducting material, such as Li[Co1−zAlz]O2; it was reported that solid solution LiCo1−yAlyO2 was formed on the surface of LiCoO2 by reacting with the coating material to improve the structural stability of LiCoO2 during cycling.22 Dogan’s group23 has studied the evolution of a surface alumina coating on several positive electrode materials during heat treatment. LiAlO2 was observed on the surface of NCM523 after heating to 600 or 800 °C, due to lithium diffusion from the bulk and reaction with alumina at high temperature. Therefore, Al diffusion into the bulk is not preferred in NCM532. However, when the Al2O3 coating layer was applied to LiCoO2, the Al diffused into the bulk and formed a Li[Co1−zAlz]O2 solid solution. Moreover, Thackeray et al.24 used XAS to elucidate the chemical and structural details of a LiNiPO4 coating on Li1.2Mn0.4Co0.4O2 to help understand the structure of the surface-modified materials at the atomic scale. To further understand the kinetic behavior of the cation diffusion between a MOx coating layer and the active particles during the heating process, Li et al. used a novel, simple “twopellet” experiment25 to measure the interdiffusion coefficients of transition metal cations in lithium transition metal oxides at elevated temperature. The distinct interface between pressed flat pellets makes composition analysis of the interface region simple, since composition only varies in one dimension perpendicular to the interface between the pellets. In the work here, Al2O3 and MgO were selected as oxide coating layers. These have been shown to be effective coating layers for cathode materials, especially at high voltage and temperature.26,27 Moreover, the diffusion of Al3+ from different compositions of coating layers such as Al2O3, LiAlO2, and LiCo0.5Al0.5O2 into LiCoO2 was measured. The structures of Al2O3, LiAlO2, and LiCo0.5Al0.5O2 are shown in Figure S1. In this work, a series of experiments to measure the interdiffusion coefficients of the metal ions (Al3+, and Mg2+) in the coating layer and Co3+ in lithium transition metal oxides were designed and conducted at various temperatures. Pellet stacks of the pure LiCoO2 and the pure coating phase were pressed in contact and heated at high temperature. On the basis of the measured composition versus position profiles, interdiffusion constants at various temperatures were determined using best fits to Fick’s second law. Activation energy barriers for each diffusion couple were then determined using the Arrhenius equation. The measured and extrapolated (to lower temperature) interdiffusion

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EXPERIMENTS

Materials. For powder A, LiCoO2 active material was synthesized by mixing Li2CO3 (99%, Alfa Aesar) and CoCO3 (99%, Alfa Aesar) powders with a Li/M ratio of 1.08:1. The mixture was then heated at 1000 °C for 10 h in air. The XRD pattern of the obtained LiCoO2 is shown in Figure S2a. The particle size of the LiCoO2 obtained was about 30−50 μm. Large particles were desired so that smooth interfaces between a large particle and the coating phase could be found at the pellet/pellet interface. For powder B, aluminum oxide (α-Al2O3, activated, Sigma-Aldrich) has a high specific surface area of about 149 m2/g. Magnesium oxide (MgO, −325 mesh, Sigma-Aldrich) has a specific surface area of about 125 m2/g. Lithium aluminate (γ-LiAlO2, Sigma-Aldrich) powder has a particle size of about 2−3 μm. LiCo0.5Al0.5O2 was synthesized via mixing LiOH·H2O (98+%, Sigma-Aldrich), Al(OH)3 (Sigma-Aldrich), and Co(OH)2 (made by Umicore) with Li/M ratio 1.05:1 and sintering the mixture at 900 °C for 6 h. The XRD pattern of the obtained LiCo0.5Al0.5O2 is shown in Figure S1b. Almost all peaks can be indexed on the basis of the hexagonal α-NaFeO2 structure (R3m ̅ ). There are some small additional peaks of impurities, like Li5AlO4, indicated by stars in Figure S2b. Pellets. Figure 2 shows the procedure for assembling the pellet stacks. For each pellet, powder A was first compressed to 15 MPa for 5 min, and then powder B was added to the pellet mold on top of pellet A. The powders were compressed again to 70 MPa for 20 min to ensure good contact at the interface. The pellet stacks were heated at various temperatures and times. The heated pellet stacks were purposely fractured into smaller pieces, which were subsequently embedded into Crystal Bond (CrystalBond 555, SPI Supplies/Structure Probe Inc.) and polished to a mirror surface finish for scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) experiments. The SEM studies were made on the cross-sections of the diffusion couple pellet stacks. SEM and EDS Line Scans. SEM evaluation was conducted using a Hitachi S-4700 SEM with a cold field emission source, equipped with an 80 mm2 silicon drift EDS detector (Oxford Instruments). The beam diameter was less than 10 nm. EDS line scans were collected with an accelerating voltage of 15 kV, a beam current of 10 μA, and an acquisition time of 20 s for each data point. The penetration depth of the electron beam was ∼800 nm, while the X-rays originate primarily from a radial distance of less than 80 nm from the electron beam, according to our previous report of Monte Carlo simulations of electron trajectories in various materials using the CASINO software.25 The total number of points measured in the line scans depended on the diffusion couple. Between 20 and 30 points were collected for each diffusion couple, separated by a distance of ∼0.5 to ∼1.0 μm, depending on the sample. Generally, two different line scans were measured for each position of the sample, and the atomic concentration profiles versus position with 5240


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Figure 3. SEM images of the cross-sections of pellet stacks (a1) LiCoO2/Al2O3, (b1) LiCoO2/MgO, and (c1) LiCoO2/LiCo0.5Al0.5O2. Parts a2, b2, and c2 show the cross-sections of the corresponding pellet stacks at a higher magnification. The red dashed lines are the approximate positions of the interface. Similarly, eq 5 can be discretized in time and space, and the concentrations at time t + Δt can be determined from the concentration at time t with

an average diffusion constant of two EDS lines are presented in Figures 4−7. Each pellet stack collected several position data, and the average diffusion constants of different positions are listed in the Figures 8 and 9 with error bar. Interdiffusion Model in Binary Systems. a. Linear Flow for Laminar Pellets. For the pellet stacks, the one-dimensional diffusion equation (linear flow model) was defined by Fick’s second law:28

⎡ Ci + 1, t − 2Ci , t + Ci − 1, t 1 Ci + 1, t − Ci − 1, t ⎤ Ci , t +Δt = D̃ ⎢ + · ⎥Δt 2 ⎣ ⎦ r Δr Δr 2 + Ci , t

∂C ∂ 2C = D̃ 2 (1) ∂t ∂x In eq 1, D̃ is the interdiffusivity, which was assumed to be independent of concentration. C is concentration at time, t, and position, x, of one of the metal elements (since CA + CB = 1), where t is the time since the beginning of heating and x is the direction perpendicular to the interface between the two pellets. The concentration profile and heating time can be discretized by equidistant segments of size Δx and Δt, respectively. The concentration gradient at point xi can be determined from the concentrations of the neighboring points xi−1 and xi+1 as

C − Ci − 1 ⎛ ∂C ⎞ ⎜ ⎟ = i+1 ⎝ ∂x ⎠x = xi 2Δx

Symmetric boundary conditions in the radial flow model were applied. At r = 0

⎡ Ci , t − 2C0, t + C −1, t 1 Ci , t − C0, t ⎤ C0, t +Δt = D̃ ⎢ + · ⎥Δt + Ci , t ⎣ Δr Δr 2 Δr 2 ⎦ (7) and at r = rn ⎡ Cn − 1, t − 2Cn , t + Cn + 1, t 1 Cn , t − Cn − 1, t ⎤ Cn , t +Δt = D̃ ⎢ + · ⎥Δt 2 rn Δr Δr 2 ⎣ ⎦ + Ci , t

(2)

Δt

= D̃

Ci + 1, t − 2Ci , t + Ci − 1, t Δx 2

(3)

Concentrations at the next time step can be determined from the initial concentrations with Euler forward integration:

Ci , t +Δt = D̃

Ci + 1, t − 2Ci , t + Ci − 1, t Δx 2

Δt + Ci , t

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RESULTS AND DISCUSSION Figure 3 presents SEM images of the cross-sections of pellet stacks. The pellets were prepared with diffusion couples of Mn+/ Co3+ (Mn+ = Al3+, Mg2+) under various conditions. The LiCoO2 pellets are shown on the left side for all images in Figure 3, showing a clear smooth cross-section of each particle. The right sides of the SEM images in Figure 3a1,b1,c1 are Al2O3, MgO, and LiCo0.5Al0.5O2 pellets, respectively. A close contact between the two pellets of the pellet stacks can be observed, and the boundaries of pellets in Figure 3a1,b1 are very distinct. In Figure 3c, the boundary between LiCoO2 and LiCo0.5Al0.5O2 is not so clear, and the particles at the interface seem to merge together, due to the similar sintering properties of both pellets. Figure

(4)

Additionally, symmetric boundary conditions were assumed with the introduction of hypothetical points x−1 and xn+1 at both ends of the sample, which were set to have the same values as the points at positions x2 and xn−1, respectively. If boundary and initial conditions are formulated, one can get the solutions of this equation via least-squares fitting to the measured concentration profile. b. Radial Flow for Spherical Coated Particles. A radial flow model was used for the simulation of interdiffusion in the spherical coated active material particles. Fick’s second law can be written as29

⎛ ∂ 2C ⎞ 1 ∂C ∂C = D̃ ⎜ 2 ⎟ + · ∂t r ∂r ⎝ ∂r ⎠

(8)

c. Least-Squares Fitting. Least-squares fitting was performed using the linear flow model for the data collected in the pellet interdiffusion experiments. The step size, Δx, was set to be that of the experimental concentration profile, while the sinter time was set to match that of the experiments. The interdiffusivity, the initial position of the interface, and the initial concentrations at the left and right side were refined during the fitting.

Equation 1 can then be discretized in time and space as

Ci , t +Δt − Ci , t

(6)

(5) 5241


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Figure 4. Al and Co atomic concentration profiles versus position, measured using EDS line scans, for the Al2O3/LiCoO2 pellet stacks sintered at different temperatures: (a) 850 °C for 10 h, (b) 900 °C for 10 h, (c) 950 °C for 10 h, (d) 1000 °C for 5 h, (d) 1100 °C for 5 h. The symbols show the original data points. The dashed lines show the initial concentration profiles, and the solid lines show the calculated profiles from Fick’s law.

smooth and compact cross-sections on both sides, the concentration variation of Al and Co through the interface is clear and smooth. The fitted curves calculated by Fick’s second law match the measured concentration profile very well and yield interdiffusion coefficients for the Al3+/Co3+ couple at 850, 900, 950, 1000, and 1100 °C of 0.039 × 10−16, 0.056 × 10−16, 0.064 × 10−16, 0.12 × 10−16, and 0.24 × 10−16 m2/s, respectively. The interdiffusion coefficients increase with sintering temperature. Accordingly, the diffusion distances through the interface between the Al2O3 section and the LiCoO2 particle are affected by the sintering temperatures and times. As shown in Figure 4a, from the right section of Co3+ to the left section of Al3+, the atomic concentration of Co decreases and that of Al increases through the interface. The diffusion distance is measured between the two intersections of the initial Co concentration line and fitted Co concentration line from the left side to the interface, which is about 1.5 μm after sintering at 850 °C for 10 h. The diffusion distance of the pellet stack sintered at 950 °C for 10 h is about 1.8 μm, which is similar to that of a pellet stack sintered at 1000 °C for 5 h. The pellet stack sintered at 1100 °C for 5 h shows the largest diffusion distance, about 2.3 μm. Figure 5 shows Al and Co atomic concentration profiles plotted versus position for LiAlO2/LiCoO2 pellet stacks. Similarly, the initial atomic concentration of Co was ∼100% on the left, and the initial atomic concentration of Al was ∼100% on the right. The fitted curves calculated by Fick’s law match the measured concentration profile very well. The fits to the measured profiles using Fick’s Law yielded interdiffusion coefficients for the Al3+/Co3+ couple of 0.042 × 10−16, 0.070 × 10−16, 0.114 × 10−16, and 0.25 × 10−16 m2/s at 800, 900, 950, and

3a2,b2,c2 shows the cross-section of the pellet stacks at a higher magnification. Figure 3a2 shows the smoothest surface of both LiCoO2 and Al2O3 pellets. Some small pores were observed in the MgO pellet in Figure 3b2 due to incomplete densification. Because Al2O3 and MgO powders had very large specific surface areas of about 125−150 m2/g, the pellet stacks of Al2O3 and MgO were easy to densify during sintering and could be polished smoothly. In Figure 3c2, it is apparent that the surface of the LiCo0.5Al0.5O2 pellet was quite rough, even after sintering at 1050 °C. LiCo0.5Al0.5O2 particles were synthesized with a solid state method at 900 °C leading to large particles (about 3−6 μm), which agglomerate during sintering in a noncompact way. EDS line scans were performed on the samples shown in Figure 3 in a direction perpendicular to the interface between a selected large LiCoO2 particle with smooth cross-section and the opposite Mn+ pellet. Figures 4−6 show the Al and Co atomic concentration profiles plotted versus position, measured using EDS line scans for the Al2O3/LiCoO2, LiAlO2/LiCoO2, and LiAl0.5Co0.5O2/LiCoO2 pellet stacks sintered at various temperature from 800 to 1100 °C, respectively. Figure 7 shows the Mg and Co profiles for the MgO/LiCoO2 pellets sintered at 800, 900, 950, and 1000 °C, respectively. The symbols in Figures 4−7 show the original data points. The dashed lines indicate the initial concentration profiles before sintering, and the solid lines show the results of fitting Fick’s law to the data. Figure 4 shows that the initial atomic concentration of Co is ∼100% on the left and the initial atomic concentration of Al is ∼100% on the right, corresponding to the initial individual compositions. Because the Al2O3/LiCoO2 pellet stacks have 5242


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Figure 5. Al and Co atomic concentration profiles versus position for the γ-LiAlO2/LiCoO2 pellets sintered at different temperatures: (a) 800 °C for 20 h, (b) 900 °C for 10 h, (c) 950 °C for 10 h, and (d) 1050 °C for 5 h. The symbols show the original data points. The dashed lines show the initial concentration profiles, and the solid lines show the calculated profiles from Fick’s law.

Mg2+/Co3+ couple of 0.034 × 10−16, 0.080 × 10−16, 0.087 × 10−16, and 0.21 × 10−16 m2/s at 800, 900, 950, and 1000 °C, respectively. Figure 8a shows all of the interdiffusion constants of the Al3+/ Co3+ couples in the various pellet stacks as a function of temperature. In Figure 8a, the red circles show the interdiffusion constants of Al3+/Co3+ in composite pellets of Al2O3/LiCoO2; the blue triangles are data points for pellet stacks of LiAlO2/ LiCoO2, and the black diamonds are those for pellet stacks of LiCo0.5Al0.5O2/LiCoO2, respectively. The red line in Figure 8a shows the fit of the Arrhenius equation to all of the Al3+/Co3+ data points. The fits of Arrhenius’ law to the data from the individual Al3+/Co3+ pellet stacks are shown in Supporting Information Figure S3. The error in the measured values in Figure 8 makes it very hard to distinguish the activation energies and frequency factors for the individual fits with any confidence. Therefore, only the parameters from the fit to all of the data, as shown in Figure 8a, are given in Table 1. Interdiffusion constants of the Mg2+/Co3+ couple as a function of temperature are shown in Figure 8b. The fit of Arrhenius’ law to the data is also shown in Figure 8b, and the fitted parameters are given in Table 1. Figure 9a,b shows Arrhenius plots of the merged interdiffusion constants of Al3+/Co3+ and Mg2+ /Co3+couples with −ln(D) on the vertical axis and 1/T on the horizontal axis. The slope of the red lines in Figure 9a,b, multiplied by the molar gas constant, R (units are J/(mol K)), gives the activation energy barrier for diffusion of each of the diffusion couples. Table 1 lists the activation energies and frequency factors determined from the data presented in Figures 8 and 9. The Al3+/Co3+ couple shows a

1050 °C, respectively. However, the experimental data points are a little scattered, because the LiAlO2 particles are somewhat loose after sintering; therefore, the LiAlO2/LiCoO2 composite pellet could not be polished very smoothly. At sintering temperatures of 900 °C (10 h), 950 °C (10 h), and 1050 °C (5 h), the diffusion distances increased from 1.9 μm to 2.4 μm and to 2.5 μm, respectively, distances which are slightly larger than those of the Al2O3/LiCoO2 pellet stacks. Figure 6 shows the Al and Co atomic concentration profiles plotted versus position for LiCo0.5Al0.5O2/LiCoO2 pellet stacks. The initial atomic concentration of Co and Al is ∼50% on the left, while that of Co is ∼100% and that of Al is ∼0% on the right, as expected on the basis of the initial compositions of the individual pellets. The interdiffusion coefficients for the Al3+/ Co3+ couple were 0.063 × 10−16, 0.070 × 10−16, 0.28 × 10−16, and 0.30 × 10−16 m2/s at 900, 950, 1000, and 1050 °C, respectively, for the experiments in Figure 6. Figure 7 shows Mg and Co atomic concentration profiles plotted versus position for MgO/LiCoO2 pellet stacks. Luo et al.30 showed that Lix[Co1−yMgy]O2 solid solution samples only exist in the range 0 < y < 0.1. Possible evidence for this limited solid solution range can be seen in Figure 7a,b,d by the “plateau” in Mg content at about 10−15% and the corresponding “plateau” in Co content at about 85−90% just to the left of the initial interface between the MgO and LiCoO2. In spite of this limited solid solution range, the concentration profiles were still fitted with Fick’s law assuming that Mg and Co were fully miscible. Figure 7 shows that the fitted curves calculated with Fick’s law match the measured concentration profiles quite well. The fits to the measured profiles yielded interdiffusion coefficients for the 5243


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Figure 6. Al and Co atomic concentration profiles versus position for the LiCo0.5Al0.5O2/LiCoO2 composite pellets sintered at different temperatures: (a) 900 °C for 10 h, (b) 950 °C for 10 h, (c) 1000 °C for 5 h, and (d) 1050 °C for 5 h. The symbols show the original data points. The dashed lines show the initial concentration profiles, and the solid lines show the calculated profiles from Fick’s law.

slightly lower activation barrier of 88 ± 4 kJ/mol, while the Mg2+/Co3+ couple shows an activation barrier of 100 ± 4 kJ/mol, as listed in Table 1. On the basis of the measured diffusion constants for various diffusion couples, interdiffusion during sintering of Mn+/Co3+ (Mn+ = Al3+, Mg2+) couples found in bulk LiCoO2 particles and MOx coating layers can be simulated. The effect of sintering on the diffusion distance and concentration variation of Mn+/Co3+ can be simulated by substituting the measured diffusion constants into the radial flow model. Figure 10 shows the simulation results for the atomic concentrations in spherical LiCoO2 particles coated with Al2O3 sintered at 500 and 800 °C for 2, 5, and 10 h, respectively. From the parameters listed in Table 2, the interdiffusion constants (0.0035 × 10−16 m2/s at 500 °C and 0.030 × 10−16 m2/s at 800 °C) were calculated using the Arrhenius equation. It is clear that the 500 °C estimate is subject to a large error as it involves an extrapolation of 300 °C from the closest data point. The active LiCoO2 was assumed to be a 10 μm diameter spherical particle having an initial Al2O3 coating layer thickness of 100 nm. The very fine red and blue dashed lines represent the initial concentrations of Co3+ and Al3+, respectively. The colored solid and dashed lines show the concentration variation of Co3+ and Al3+ composition after different heating times in the simulation results. Figure 10a shows the results after sintering at only 500 °C for different times. After heating at 500 °C for 2 h, the Al3+/Co3+ diffusion distance is about 0.2 μm, shown in Figure 10a1. Because

the coating layer is very thin (only 100 nm), the composition of the surface coating layer changed markedly from initial pure Al2O3 to solid solution Li[Co1−zAlz]O2 having about 20% of Co3+ and 80% of Al3+. In Figure 10a3, after heating for 10 h, the diffusion distance is about 0.4 μm, and the composition of the outer surface is about 55% Co3+ and 45% Al3+. Figure 10b shows the concentration profiles after heating at 800 °C for various times. Figure 10 b1 suggests that all of the 100 nm Al2O3 surface layer would be converted to Li[Co1−zAlz]O2 with a maximum value of z at the surface of z = 0.35 for the 2 h heating. After heating for 10 h, the values of z near the surface would be 0.16, shown in Figure 10b3. The diffusion distances increase from 0.5 μm to about 1.2 μm as the heating time increases from 2 to 10 h leading to regions where z in Li[Co1−zAlz]O2 varies with position. Figure 11 shows the simulation results of spherical LiCoO2 particles coated with an Al2O3 coating layer thickness of 500 nm sintered at 500 °C (in Figure 11 a1,a2,a3) and at 800 °C (in Figure 11 b1,b2,b3) for 2, 5, and 10 h, respectively. This simulation was performed to show (Figure 11a) that only a tiny fraction of the Al2O3 in the thick layer (or in larger Al2O3 particles contacting the LiCoO2 surface) would be converted to Li[Co1−zAlz]O2 at 500 °C. By contrast, Figure 11b3 shows that after heating for 10 h at 800 °C the surface concentration is about 70% Al, suggesting that substantial conversion to Li[Co1−zAlz]O2 with z near 0.70 would occur. The lengths of the diffusion zones in Figure 11b and in Figure 10b are the same. This is to be expected because the diffusion constants and temperatures 5244


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Figure 7. Mg and Co atomic concentration profiles versus position for the MgO/LiCoO2 pellets sintered at different temperatures: (a) 800 °C for 20 h, (b) 900 °C for 10 h, (c) 950 °C for 10 h, and (d) 1000 °C for 5 h. The symbols show the original data points. The dashed lines show the initial concentration profiles, and the solid lines show the calculated profiles from Fick’s law.

Figure 8. (a) All of the interdiffusion constants of the Al3+/Co3+ couples in various pellet stacks as a function of temperature and the fit of the Arrhenius law to all of the Al3+/Co3+ data points (red circle ●, Al2O3/LiCoO2; blue triangle ▲, LiAlO2/LiCoO2; black diamond ◆, LiCo0.5Al0.5O2/LiCoO2). (b) Interdiffusion constants of the Mg2+/Co3+ couple as a function of temperature and the fit of the Arrhenius law to the data.

considered in these figures are the same. A comparison between Figures 10b and 11b really shows how the initial thickness of the coating layer controls the maximum Al content at the surface after the 800 °C heating step. An additional simulation for a 250 nm coating layer is shown in Figure S4. The results of this work allow one to predict the range of interdiffusion of the cations when coated positive electrode materials are heat treated. The interdiffusion constants were directly measured and fit using the “two-pellet” approach and Fick’s law. The results listed in Table 2 can be used by others to simulate how Al and Mg in coating layers diffuse into LiCoO2 during heat treatment.

Table 1. Summary of the Activation Energies and Frequency Factors for Diffusion Measured in This Worka activation energy diffusion couple 3+

3+

Al /Co Mg2+/Co3−

kJ/mol (±4)

eV/atom (±0.04)

D0 (m2/s) (±0.3) × 10−13

88 100

0.91 1.03

0.56 2.08

a

According to the Arrhenius equation (−ln D) = Ea/RT + (−ln D0) derived from D = D0 exp(−Ea/RT), one can calculate D at any temperature, as is done in Table 2.

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Figure 9. Arrhenius plots of (a) the merged interdiffusion constants of the Al3+/Co3+ couple and (b) the Mg2+ /Co3+couple. The graphs have −ln(D) on the vertical axis and 1/T on the horizontal axis.

Figure 10. Simulation results for the atomic concentrations versus position in spherical LiCoO2 particles coated with Al2O3 sintered at (a) 500 °C and (b) 800 °C for (1) 2 h, (2) 5 h, and (3) 10 h, respectively, assuming a particle diameter of 10 μm and a coating layer thickness of 100 nm.

Table 2. Interdiffusion Constants of Al3+/Co3+ and Mg2+/Co3+ Diffusion Couples at Various Temperatures from the Fitting and Extrapolated Results interdiffusion constants D (×10−16 m2/s) diffusion couple

500 °C

550 °C

600 °C

650 °C

700 °C

750 °C

800 °C

850 °C

900 °C

950 °C

Al3+/Co3− Mg2+/Co3+

0.0035 0.0021

0.0050 0.0032

0.0072 0.0050

0.010 0.0079

0.015 0.012

0.021 0.019

0.030 0.030

0.044 0.047

0.064 0.073

0.092 0.114

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CONCLUSIONS

determined, respectively. Although interdiffusion coefficients were only measured between 800 and 1100 °C, the Arrhenius equation was extrapolated to predict interdiffusion coefficients every 50 °C from 500 to 950 °C. These are listed in Table 2. On the basis of the measured diffusion constants for various diffusion couples, the effect of sintering on the Mn+/Co3+ concentration profiles was simulated using Fick’s law. An

3+

In this work, the interdiffusion of Al (in different forms: Al2O3, LiAlO2, and LiCo0.5Al0.5O2) and Mg2+ (from MgO) into LiCoO2 was measured at various temperatures using a “two-pellet” method. The interdiffusion coefficients increased with sintering temperature as expected, and activation energies for interdiffusion of 0.91 and 1.03 eV for the Al/Co and Mg/Co couples were 5246


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Figure 11. Simulation results for the atomic concentrations in spherical LiCoO2 particles coated with Al2O3 sintered at (a) 500 °C and (b) 800 °C for (1) 2 h, (2) 5 h, and (3) 10 h, respectively, assuming a particle diameter of 10 μm and a coating layer thickness of 500 nm.

Notes

Al2O3 coating layer of 100 nm thickness is partly converted to Li[Co1−zAlz]O2, with z = 0.45 at the surface during heating at 500 °C for 10 h. However, a 500 nm thick coating layer remains predominantly as Al2O3 after heating at 500 °C. At 800 °C, interdiffusion is much more rapid, and a 100 nm thick Al2O3 coating layer is entirely converted to Li[Co1−zAlz]O2 with z = 0.16 at the surface after heating for 10 h. The range of diffusion is about 500 nm during this 2 h period. The list of interdiffusion constants in Tables 1 and 2 can be used, along with Fick’s law, by others to rationally design coated positive electrode materials.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank NSERC, 3M Canada, and Tesla Motors for the funding of this work under the auspices of the Industrial Chairs program. Other partners, including the Canada Foundation for Innovation and the Nova Scotia Research and Innovation Trust, who helped fund the Facilities for Materials Characterisation (SEM/EDS), managed by the Dalhousie University Institute for Materials Research, are also gratefully acknowledged. Y.Z. acknowledges the generous support of the China Scholarship Council.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01219. Crystal structures of typical layered LiCoO2 and other metal oxides, interdiffusion constants of different Al3+/ Co3+ couples as a function of temperature, and simulation results for the atomic concentrations in spherical LiCoO2 particles coated with Al2O3 assuming a coating layer thickness of 250 nm (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: jeff[email protected]. Phone: 001-902-494-2991. Fax: 001902-494-5191. ORCID

Jing Li: 0000-0003-3698-7102 J. R. Dahn: 0000-0002-6997-2436 5247


Article

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Inter-diffusion of Cations from Metal Oxide Surface Coatings into

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