Letter pubs.acs.org/JPCL
How Do Li Atoms Pass through the Al2O3 Coating Layer during Lithiation in Li-ion Batteries? Sung Chul Jung and Young-Kyu Han* Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea S Supporting Information *
ABSTRACT: We studied the lithiation of Al2O3 and found the energetically most favorable composition of Li3.4Al2O3 using ab initio molecular dynamics simulations. The calculated Li/Al ratio and corresponding volume expansion ratio agree well with reported experimental observations. The Al atoms accept electrons from the incoming Li atoms during lithiation, leading to the formation of various Al structures, that is, isolated atoms, dimers, trimers, and ring- and chain-type clusters. The Li atoms in the optimal concentration diffuse faster by four (five) orders of magnitude than the Al (O) atoms, and they also diffuse faster by four orders of magnitude than the Li atoms in a dilute Li concentration. We suggest that in Li-ion batteries the lithiation of the Al2O3 coating layer proceeds until a thermodynamically stable phase is reached; then, extra Li atoms overflow into the electrode by passing through the coating layer. SECTION: Molecular Structure, Quantum Chemistry, and General Theory
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at x = 3.4 is in excellent agreement with the experimental ratio of 1.6 reported by Aaltonen et al.,14 who used inductively coupled plasma mass spectroscopy to analyze the composition of amorphous Li2O−Al2O3 films of thicknesses up to ∼110 nm, grown by ALD as Li1.6Al1.0Oz. This good agreement between experiment and theory indicates that a golden ratio of Li/Al, in general, for lithium aluminum oxides is ∼1.6 from the thermodynamic point of view. We present the volume changes of LixAl2O3 as a function of x (0 ≤ x ≤ 4) in Figure 1b. The volume of LixAl2O3 increases almost linearly as the Li concentration (x) increases. The volume expansion at x = 3.4 is V/V0 = 2.1, as indicated by the arrow in Figure 1b. Liu et al.15 used in situ transmission electron microscopy to examine the evolution of the morphology of Al nanowires with naturally oxidized Al2O3 surface layers; the authors observed that the lithiation of Al2O3 coating layer first occurs with the formation of a stable Li−Al− O glass tube and is followed by the lithiation of the inner Al core. On the basis of the reported thickness change of the Al2O3 layer during lithiation,15 the experimentally observed volume expansion of Al2O3 layer is V/V0 = 2.25, which is in good accord with our estimated V/V0 value of 2.1 at x = 3.4. This agreement strongly suggests that during the lithiation process the Al2O3 coating layer first absorbs lithium from the electrolyte, at up to 1.7 Li atoms per Al2O3 unit, to reach the thermodynamically stable composition of Li3.4Al2O3. The lithiated Al2O3 coating layer then delivers extra lithium atoms to the electrode while maintaining this composition.
tabilization of the electrode surfaces is of great importance for improving the performance of lithium-ion batteries. The electrode surfaces are exposed to the electrolyte and can participate in side reactions with electrolyte components, which leads to the formation of a solid electrolyte interphase (SEI) film on both the cathode and anode surfaces. This SEI film, more generally the electrode−electrolyte interface, is a critical part of the Li-ion cell because its instability directly results in an aging of the cell either at rest or on cycling. Surface coating is one controllable approach that can be taken to stabilize the electrode−electrolyte interface and yields an artificial protective layer that serves as a substitute for the SEI film.1,2 In particular, a conformal ultrathin coating created by atomic layer deposition (ALD) has been widely used for both the cathode and anode surfaces; this type of coating can improve the durability, rate capability, and safety of lithium-ion batteries.3−13 Despite an appreciable number of beneficial functions reported for surface coating, very little work has been done to identify the Li transport process that occurs through surface coating layer. One excellent coating material is Al2O3, which has been employed as an ALD coating for LiCoO2, LiMn2O4, Fe3O4, MoO3, Cu−Si, graphite, and Si electrodes.4−13 In the present study, we examined the lithiation of Al2O3 using ab initio molecular dynamics (MD) simulations to understand how Li atoms pass through the Al2O3 coating layer in lithium-ion batteries. We evaluated the formation energies of LixAl2O3, where x = 0.0 (Al40O60), 0.2, 0.4, 1.0, 1.4, 2.0, 2.5, 3.0, 3.5, and 4.0 (Li80Al40O60) and depicted the results as a function of x, as shown in Figure 1a. The x value with the minimum energy is determined to be x = 3.4 by a least-squares fit to the calculated data around x = 3.5. The Li-to-Al atomic ratio of 1.7 © 2013 American Chemical Society
Received: June 14, 2013 Accepted: July 26, 2013 Published: July 26, 2013 2681
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Figure 2. (a) Partial radial distribution functions gαβ(r) for the Li−O, Al−O, and Al−Al pairs. (b) Average partial coordination numbers of O (CNO−Al and CNO−Li). The cutoff bond distances are 2.4 Å for CNO−Al and 2.6 Å for CNO−Li. (c) Total charge of Li and Al cations directly attached to O, defined as Qtotal = (+1) × CNO−Li + (+3) × CNO−Al.
Figure 1. (a) Formation energy of LixAl2O3, defined as Ef(x) = Etot(LixAl2O3) − xEtot(Li) − Etot(Al2O3), where Etot(LixAl2O3) is the total energy per LixAl2O3 unit, Etot(Li) is the total energy per atom of bcc Li bulk, and Etot(Al2O3) is the total energy per Al2O3 unit. The inset shows the atomic structure of Li3.5Al2O3 in which green, white, and red balls represent the Li, Al, and O atoms, respectively. (b) Volume expansion of LixAl2O3 (0 ≤ x ≤ 4). The cross indicates the concentration of Li at the lowest energy (x = 3.4), and the arrow indicates the corresponding volume expansion (V/V0 = 2.1).
number of O over the whole region of 0 ≤ x ≤ 4 (Figure 2b) and (ii) the total charge of cations attached directly to O, in contrast, gradually decreases in the region of x ≤ 2 and remains virtually unchanged in the high Li-content region of 2 ≤ x ≤ 4 (Figure 2c). When the Al(III) around O is replaced by Li(I) during lithiation, at x = 2, one Al atom is detached from O and two Li atoms are attached to O, as shown in Figure 2b. The total coordination number of O increases by 1.0, and the total cation charge around O decreases by 1.2. This drastic replacement of Al by incoming Li no longer occurs in the range of x ≥ 2 because further decreases in the total countercation charge would be energetically unfavorable from an electrostatic point of view. We performed Bader charge analyses to quantify the charge distribution (Q) associated with the Li, Al, and O atoms during lithiation. Figure 3a shows the average Bader populations of the Li, Al, and O atoms. The Q(Al) and Q(O) values are +2.44 and −1.63, respectively, for the pristine Al2O3. During lithiation, Al undergoes an appreciable change from +2.44 (x = 0) to +0.87 (x = 4), that is, the reduction of Al from Al(III) to Al(I) while the charge states of Li and O exhibit only a slight variation over the entire range of x. We can conclude that the Al(III) atoms are reduced during lithiation and that the O atoms act as bystanders. Note that this electron redistribution reflects an average picture for the Li, Al, and O elements. An intriguing result hidden in Figure 3a is that individual Al atoms have a wide variety of charge states depending on the local structures of Al. Note also that Al atoms with different charge states from +2.4 to −1.5 emerge during lithiation, as shown in Figure 3b.
We examined the structural changes of LixAl2O3 in the range of x = 0 to 4 by analyzing the radial distribution function (RDF). Figure 2a shows the partial RDFs gαβ(r) for the Li−O, Al−O, and Al−Al pairs obtained by carrying out ab initio MD simulations for 3 ps at T = 300 K. The Li atoms in Al2O3 are strongly bound to the O atoms, with the formation of Li−O bonds of 2.0 Å, which agrees with the experimental data for Li2O2.16 The Al−O bond length of 1.8 Å remains unchanged during lithiation, but the Al−Al bond length undergoes distinct changes: the Al−Al peak at r = 3.2 Å diminishes, while a new Al−Al peak grows at r = 2.8 Å after x = 1 (LiAl2O3). The Al−Al bond length of 2.8 Å is very close to 2.86 Å, the bond length in fcc Al bulk, indicating that the self-cohesion of Al atoms begins at the Li/Al ratio of 0.5 during lithiation. Figure 2b shows the average partial coordination numbers (CNO−Li and CNO−Al) calculated for the O atoms, which clearly shows that the inserted Li atoms gradually substitute for the Al atoms around O during lithiation. Note that at x = 2 (Li2Al2O3) the dominant elements coordinated with the O atoms are changed from Al to Li. The crossing at the Li/Al ratio of one is very interesting because Li and Al have very different formal charge values of +1 and +3, respectively. During the Li insertion into Al2O3, we observed two characteristics concerning the Li− O and Al−O coordinations: (i) the electronegative O atoms tend to stabilize both the electropositive Al and incoming Li atoms, which leads to a steady increase in the total coordination 2682
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The diffusivity of Li atoms in Al2O3 is a crucial factor that influences battery performance. Hao and Wolverton17 used kinetic Monte Carlo simulations and reported that the Li diffusivity (D) in a dilute Li concentration (x = 0.00625) of LixAl2O3 is as low as D = 2.7 × 10−14 m2 s−1 at 600 K. We calculated the self-diffusion coefficient D of Li, Al, and O atoms in LixAl2O3 at T = 300 K. Table 1 compares these calculated D Table 1. Diffusion Properties of LixAl2O3a x
atom
ED
0.2
Li Al O Li Al O
0.65 0.64 0.69 0.38 0.64 0.71
3.5
D0 1.1 2.8 5.2 1.5 2.2 2.7
× × × × × ×
10−3 10−4 10−4 10−3 10−3 10−3
D 1.1 4.4 1.4 7.1 4.6 2.9
× × × × × ×
10−14 10−15 10−15 10−10 10−14 10−15
a
ED (eV) is the activation energy for diffusion, D0 (cm2/s) is the preexponential factor, and D (cm2/s) is the self-diffusion coefficient at T = 300 K.
values for Li, Al, and O atoms between x = 0.2 and 3.5. The diffusivities at x = 0.2 are 1.1 × 10−14, 4.4 × 10−15, and 1.4 × 10−15 cm2/s for Li, Al, and O, respectively, indicating that a small number of Li atoms diffuse faster, by at most one order of magnitude, than the Al and O atoms. As the lithiation proceeds, the diffusivities of Al and O atoms do not change significantly, whereas the diffusivity of Li atoms increases markedly. The diffusivity of Li elements at x = 3.5 is 7.1 × 10−10 cm2/s, which is faster by at least four orders of magnitude than the values of 4.6 × 10−14 and 2.9 × 10−15 cm2/s for Al and O, respectively. The great enhancement of Li diffusivity can be attributed to: (i) the decrease in the number of attractive O atoms around Li (see Figure S1 in the Supporting Information) and (ii) the increase in the number of repulsive Li atoms around Li. We speculate, based on these diffusivity calculations, that while a small number of Li atoms in the Al2O3 coating layer are too slow to escape from the coating layer, a large number of Li atoms can rapidly pass through the coating layer, supporting the fact that the lithiation of the Al2O3-coated electrode proceeded after the formation of a Li−Al−O glass layer surrounding the electrode, as observed experimentally in ref 15. Our calculation results for LixAl2O3 remind us of the lithiation of a crystalline Si (c-Si) electrode in which the c-Si phase first undergoes a transition into an amorphous LixSi (aLixSi) phase during Li insertion.18−20 The composition of the amorphous phase in the two-phase lithiation was reported to be Li3.4Si.21,22 Next, if excess Li atoms flow into the electrode, then the a-Li3.4Si phase maintains a Li/Si ratio of 3.4 and instead extends its domain with a movement of the sharp boundary between the a-Li3.4Si and c-Si phases.23,24 We suggest that the lithiation of Al2O3 proceeds until it reaches the thermodynamically stable phase (Li3.4Al2O3); then, extra Li ions overflow into the electrode by passing through the LixAl2O3 coating layer, similarly to the lithiation process described for Si. Our calculation results do not necessarily mean that the diffusion of Li atoms into the electrode starts only at x = 3.4 (i.e., we do not entirely exclude the possibility that some of the Li atoms at x ≤ 3.4 can diffuse into the electrode). It should be noted that our suggestion was based on both the LixAl2O3 calculation results and their agreement with the experimental results,14,15 not the calculation results alone.
Figure 3. (a) Average Bader populations of Li, Al, and O. (b) Distribution histograms of the Bader populations of Al. (c) Snapshot of only the Al atoms in Li3.5Al2O3.
First, the main charge state of Al across the whole range of 0 ≤ x ≤ 4 is +2.4, which represents Al atoms enclosed by four to six O atoms and also isolated from the other Al atoms. Next, the charge states from +2.4 to 0.0 appear in the range of x ≥ 0.2 and represent Al atoms with one to three neighboring O atoms. These Al atoms mainly constitute the dimer or trimer. Lastly, the charge states from 0.0 to −1.5 appear in the range of x ≥ 1.4 and represent Al atoms without neighboring O atoms. The Al atoms with negative charge states constitute the chain- and ring-type clusters, as shown in Figure 3c, where the negative charges are attributed to the electron transfers from neighboring Li atoms. Actually, at the most stable composition, various types of Al atoms coexist as isolated atoms, dimers, trimers, and polyatom clusters. 2683
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Xiao et al.12 reported that the Al2O3 coating layer on the Si electrode becomes increasingly more ion-conductive during the first few cycles and ascribed this increasing conductivity to some structural changes occurring in the Al2O3 layer. Our calculation results lead us to believe that energy-driven lithiation in the Al2O3 layer proceeded over the first few cycles and that the Li diffusivity increased with increasing Li concentration during the cycles. In summary, Li3.4Al2O3 is the energetically most favorable composition of lithiated alumina. During lithiation, while the charge states of the O atoms remain unchanged, the Al atoms accept electrons from the incoming Li atoms, leading to the formation of various Al structures, ranging from isolated atoms to dimers, trimers, and ring- and chain-type clusters. The Li atoms in the optimal concentration diffuse faster by four and five orders of magnitude than the Al and O atoms, respectively, and also diffuse faster by four orders of magnitude than the Li atoms in a dilute concentration. This article clarifies the importance of understanding the fully lithiated coating layer, which is the real key component for stable cycling of highperformance electrodes in the ever-expanding lithium-ion battery field.
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COMPUTATIONAL DETAILS Kohn−Sham density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP).25 We employed the Perdew−Burke−Ernzerhof (PBE) exchange and correlation functionals26 and the projectoraugmented wave (PAW) method.27 We investigated the behavior of Li atoms inside the Al2O3 coating layer by simulating amorphous LixAl2O3 bulk structures as a periodic cubic supercell containing 20 × x Li atoms, 40 Al atoms, and 60 O atoms. We performed ab initio MD simulations in the course of preparing the amorphous structures.24,28 The self-diffusion coefficients D of Li, Al, and O atoms in LixAl2O3 at T = 300 K were obtained based on the Einstein relation ⟨r2(t)⟩ = 6Dt, where ⟨r2⟩ is the mean-square displacement. More details of calculation schemes are described in the Supporting Information.
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ASSOCIATED CONTENT
S Supporting Information *
Coordination numbers, mean-square displacements, temperature dependence of diffusivities, formation energy per Li atom, radial distribution functions, density of states, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support by the NRF of Korea Grant (MEST, NRF-2010-C1AAA001-0029018).
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REFERENCES
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