Inhibiting degradation of LiCoO2 cathode material by anisotropic strain during delithiation

Lithium cobalt oxides (LiCoO2) possess a high theoretical specific capacity of 274 mAhg−1. However, when LiCoO2 is charged at the voltage higher than 4.2 V, there exist significant structure transition and capacity fade. In this study, we used HRTEM to observe the phase evolution of LiCoO2 cathode material after 100 cycles, and found that LiCoO2 phase would degrade to Co3O4 phase. The phase transition of Co3O4 from LiCoO2 gave rise to lattice expansion, by which the anisotropic strain was proposed by first-principles calculation to inhibit LiCoO2 degradation. Results show that the anisotropic strain via the extension of lattice parameter c and the compression of a enables to simultaneously impede lattice oxygen loss and structure transition of LiCoO2 during delithiation at high voltage. In this case, the elongation of interplanar spacing also increases the diffusivity of Li ions in LiCoO2, contributing to rate performance.


Introduction
Lithium-ion batteries (LIBs) have been increasingly applied in mobile equipments and electric vehicles because of their high energy density [1][2][3]. Compared with anode materials, the performance of cathode materials is vital to the improvement on power density of LIBs [4]. Since LiCoO 2 was discovered by Goodenough's group [3], a series of cathode materials, such as Li-rich [5], Mn-rich [6], Ni-rich [7] transition metal oxides, has been developed, and much progress has been made to further improve their power density and cycling stability. Nevertheless, as an archetypal cathode material, LiCoO 2 has been still the widely employed cathode material [8]. However, until now the commercialized LiCoO 2 only exhibits little more than half of its theoretical capacity. Such a large irreversible capacity is attributed mainly to the presence of an irreversible phase transition during delithiation and/or lithiation processes [9][10][11].
In the process of delithiation, there is an insulator-metal phase transition in the initial low-voltage region for LiCoO 2 [12].While a half of the Li + ions are removed, the material experiences an order-disorder transition [10], which drives the phase transition from hexagonal O3 structure to monoclinic structure, and then back to O3 structure. Furthermore, at the voltage above 4.2 V, delithiated LiCoO 2 tends to experience the O3-(H1-3)-O1 phase transition [10]. Along with the O3-(H1-3)-O1 phase transition, the Fermi level shifts into the valence band, and both Co 3d-t 2g and O 2p states become broader and subsequently hybridized [11]. The Fermi level would reach a critical value when it touches the top of the O 2p bands, indicating that LiCoO 2 is exactly charged, which is the intrinsic voltage limit. With further charging, the Fermi level crosses the O 2p states, accompanying with holes migration into the O 2p states. This results in oxidation of O 2− and oxygen loss from the lattice, hence leading to degradation of LiCoO 2 material [13][14][15]. It is evident that the energy proximity of the Fermi level and the O 2p bands determines the intrinsic voltage limit of LiCoO 2 cathode material. Therefore, in order to increase the capacity of LiCoO 2 , the phase transition during delithiation should be suppressed and the top of O 2pstates in valence band should be lower. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Many efforts have been made to maintain the structural stability and improve the electrochemical performance of LiCoO 2 , in which cations doping have been proved to be efficient, including Mg [16,17], Zr [18], Al [19], Ni [20], Fe [21], Cr [22], Mn [23], and Ti [24], et al Although through doping the specific capacity of LiCoO 2 has been improved, it is still far below the theoretical value of 273 mAh g −1 . As aforementioned, it is phase transition or structure degradation at high charging voltage that decreases the capacity of LiCoO 2 cathode material. In this study, we used HRTEM to observe the microstructure evolution of LiCoO 2 material after charge/discharge cycling. The degradation mechanism of LiCoO 2 was discussed. Moreover, an approach was proposed by anisotropic strain to inhibit LiCoO 2 degradation during delithiation.

Experimental
In this study, LiCoO 2 cathode powder was synthesized by solid-state reaction at 1 000 o C [25]. Co 3 O 4 and Li 2 CO 3 were used as starting materials, as the sources of Li and Co, respectively. The mixture of the starting materials with the stoichiometric ratio of LiCoO 2 , where 5% excess lithium carbonate was added to compensate for the volatilization of lithium during calcination. The mixed starting materials were then heat-treated in the air at 1 000 o C for 12 h. To assess the electrochemical performance, the synthesized LiCoO 2 powders was mixed with 10 wt% acetylene black, 10 wt% PVDF binder and N-methyl-2-pyrrolidene (NMP) to prepare a homogenous slurry, which was pasted at Al foil, dried at 120°C and employed as a cathode in 2032-type coincell Li-ion batteries. The coin-cells were assembled by using Cellgrad 2300 as a separator, metallic lithium foil as a counter electrode, and 1 M LiPF 6 dissolved in water as the electrolyte. After 100 charging-discharging cycles under 4.3-2.0 V, the battery was disassembled in an insert Ar atmosphere. The electrode was clean with dimethyl carbonate, and cathode materials were stripped from the electrode of disassembled batteries and were dispersed in ethanol to prepare samples for TEM observation. The microstructure of LiCoO 2 cathode particle was observed by transmission electron microscopy (TEM, JEM 2100 F).
A first-principles calculation was performed by using the Vienna ab initio simulation package (VASP) [26] within the density functional (DFT) theory frame with the supplied PAW pseudopotentials [27,28] and Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) [29]. The cut-off energy for the plane wave basis was set as 570 eV, and the Monkhorst-Pack k grid was set as 6×6×6 for sampling Brillouin zone.

Results and discussion
Figure 1(a) shows the HRTEM image of degraded LiCoO 2 particles. As can be seen, there have two regions with different phases, respectively. Comparing to the HRTEM images of LiCoO 2 before assembled into battery as shown in figure S1 is available online at stacks.iop.org/MRX/7/025501/mmedia, which demonstrated uniform LiCoO 2 phase in the particle, degraded LiCoO 2 particles consist of regions with different phases. Figure S2   square in figure S2a. The diffraction pattern belongs to two different phases. After carefully indexing the fast Fourier transformation (FFT) of the area marked by the square in figure 1(a), this region was a phase of Co 3 O 4 with Fd-3m symmetry. Figure 1(b) is the FFT result of the marked the square in figure 1(a). The angle between the (003) orientation of LiCoO 2 and the (0-22) orientation of Co 3 O 4 is approximately 37°, which is approximately equal to the angle between the c-axis of the traditional cell of LiCoO 2 and the face diagonal of the rebuild cell LiCoO 2 ,as shown in figure 2. Therefore, TEM analysis demonstrates that the degradation of LiCoO 2 cathode material is attributed to the phase transition from LiCoO 2 to Co 3 O 4 after delithiation.
To further discern the phase transition pathway from LiCoO 2 to Co 3 O 4 , the crystal cell of LiCoO 2 was rebuilt to a rhombohedral presentation for comparing with the conventional cell of Co 3 O 4 , as seen in figure 2. Figure 2(a) shows the conventional cell of LiCoO 2 and the rebuilt cell. The rebuilt cell has the lattice parameters a=b=c=8.01 Å and α=β=γ=89°. Figure 2(b) shows the conventional cell of Co 3 O 4 with lattice parameters a=b=c=8.12 Å and α=β=γ=90°.Through comparing the crystal structures and lattice parameters of LiCoO 2 with Co 3 O 4 , one can see that lattice expansion and oxygen loss occurs during the phase transition from LiCoO 2 to Co 3 O 4 . Thus, lattice expansion and oxygen loss should be inhibited during delithiation to improve the capacity and stability of LiCoO 2 cathode materials.
One effective method to suppress the lattice expansion is compression, and this is equivalent to doping by elements with smaller radii. Meanwhile, lattice compression would reduce interlayer space of crystal, which leads to the decrease in Li + diffusivity. In addition, to avoid the oxidation of O 2− caused by the overlap of Fermi level and the O 2p states, the strain from lattice compression/expansion should change the band structure and make the O 2p states lower. Therefore, we calculated the band structure of LiCoO 2 under different strain conditions using first-principles calculations. Figure 3 shows the band structures of LiCoO 2 crystal under different strain conditions. Significant splitting of the Co 3d states is observed and covalent interaction results in hybridization of Co 3dand O 2p [30]. According to the band structure, an intrinsic voltage limit regarding the decomposition potentials of cathode materials was proposed [14]. At the intrinsic voltage, Co 3d states cross the top of O 2p states. For further charging, the Fermi level falls below the top of O 2p states and holes form in the bonded O 2p states, resulting in oxidation of O 2− . Thus, for inhibiting oxygen loss, the cross point of Co 3d states and O 2p states should move far away from the Fermi level. As shown in figure 3, the cross point of Co 3d states and O 2p states was marked in dash. As can be seen, four different strain types are applied to LiCoO 2 via changing the lattice parameters. The calculated partial density of states indicates that the strain conditions of (a) and (c) would give rise to the decline in the cross point of Co 3d states and O 2p states. This inhibits the oxidation of O 2− when LiCoO 2 cathode is charged to higher voltage. Contrarily, the strain conditions of (b) and (d) increase the cross point of Co 3d states and O 2p states, which would make the oxidation of O 2− easier.
The structure transition should be forbidden, while the oxidation of O 2− is suppressed during delithiation for LiCoO 2 . It is known that structural transition at the high charging voltage is driven by electrochemical potential, so the transition could be impeded as the transition potential barrier increases. Furthermore, for practical application of LiCoO 2 , the diffusivity of Li ions significantly affects the performance of LIBs. LiCoO 2 is a typical layered structural cathode material, and the reduced lattice parameter cmakes the interplanar spacing smaller, resulting in the decrease in Li + diffusivity. Hence, the strain condition (a) enables to inhibit oxygen loss during delithiation, but the shrinkage of lattice parameter ccauses less diffusivity of Li ions. By contrast, it is desired for the strain condition (c) to achieve the intension inhibiting oxygen loss and structural transition.
During delithiation, Li x CoO2 (x<0.5) lost Li + and preferably transformed to Co 3 O 4 because the free energy of Co 3 O 4 was lower than that of the Li x CoO2 (x<0.5) [15]. figure 4 shows the change in the free energy along the proposed transition pathway for the LiCoO 2 with and without strain. As shown in figure 4, Li ions deinsertion from LiCoO 2 is described by transition steps 1-2-3, and transition steps 3-4-5 demonstrate the lattice expansion from a=b=c=8.01 Å to a=b=c=8.12 Å, and transition steps 5-6-7 indicate the oxygen loss. The free energy for each transition step was calculated by using a first-principles calculation method. For LiCoO 2 cathode material, although the system free energy increases due to the delithiation, the potential barrier is surmounted by an applied electric filed. The oxygen loss was energetically preferred after lattice expansion. The potential barriers of lattice expansion for strain condition (c) and without strain were calculated. Clearly, potential barrier is 5.57 eV under the strain condition (c), higher than the unstrain condition. Thus, the anisotropic strain i.e. lattice parameter c extension and acompression, enables to simultaneously inhibit oxygen loss and structural transition during delithiation at high voltage. Also, the increased in interplanar spacing enhances the diffusivity of Li ions, which is available on the rate performance of Li x CoO 2 .
As above theoretical analysis, the structural stability of LiCoO 2 could be improved by lattice strain. The lattice strain could be achieved by doping ions with different radius with Li + or Co 3+ . For example, doping ions with smaller radius such as Al, Ga would compress the lattice, conversely, doping ions with larger radius such as In, Sc would stretch the lattice. However, the lattice strain arising from doping ions with different radius would not be isotropic. Hence, co doping would realize the anisotropic strain.

Conclusion
In conclusion, we used HRTEM to observe the structure evolvement of LiCoO 2 material after 100 chargingdischarging cycles, and found that the degradation compound of LiCoO 2 is Co 3 O 4. The conventional cell of LiCoO 2 was rebuilt to exhibit what is the atoms correspondence between LiCoO 2 and Co 3 O 4 . Accordingly, a structural transition pathway was proposed, containing delithiation of Li ions, lattice expansion, and oxidation of O 2− . Hence, to improve the capacity and charging voltage of LiCoO 2 , the lattice expansion and oxidation of O 2− should be inhibited. The first-principles calculation was employed to obtain the band structure of LiCoO 2 crystal under different strain conditions. Results show that anisotropic strain, i.e. lattice parameter c extension and acompression, inhibits oxygen loss and structural transition without reducing Li + diffusivity during delithiation.