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1 DOI: /NCHEM.2524 The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials Dong-Hwa Seo 1,2, Jinhyuk Lee 1,2, Alexander Urban 2, Rahul Malik 1, ShinYoung Kang 1, and Gerbrand Ceder 1,2,3 * 1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2 Department of Materials Science and Engineering, UC Berkeley, Berkeley, CA 94720, USA 3 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA These authors contributed equally to this work. *Corresponding author: gceder@berkeley.edu NATURE CHEMISTRY 1

2 Contents Computational details p3 Preparation of the structure models p3 The optimal mixing parameters of Li-excess metal oxides p4 The oxidation state of TM and oxygen ions p7 Impact of the mixing parameter on the competition between TM and oxygen redox p9 Conditions for the O O bond formation p14 Invariant energy level of the Li-O-Li state p18 References p19 NATURE CHEMISTRY 2

3 Computational details All density-functional theory (DFT) calculations employed a plane wave basis set with a energy cutoff of 520 ev. The number of k-point was 2,000 divided by the number of atoms in the unit cell. The lattice parameters and atomic positions of all structures were optimized until the residual forces were smaller than 0.05 ev Å -1. The iso-values for the isosurfaces of the charge densities were 0.02 e Å -3 in Figures 1d and 2b. The iso-values for the isosurfaces of the spin densities were 0.05 e Å -3 for all cases in Figure 3. Preparation of the structure models To investigate oxygen redox activity in Li1.2Ni0.2Mn0.6O2, Li2Ru0.5Sn0.5O3, Li1.2Ni0.33Ti0.33Mo0.13O2, and Li1.3Mn0.4Nb0.3O2, we prepared simplified structural models of Li1.17Ni0.25Mn0.58O2, Li2Ru0.5Sn0.5O3, Li1.17Ni0.33Ti0.42Mo0.08O2, and Li1.25Mn0.5Nb0.25O2. For Li1.17Ni0.25Mn0.58O2 and Li1.17Ni0.33Ti0.42Mo0.08O2, a large number of cation arrangements was created within a supercell containing twelve formula units of the monoclinic LiMO2 (M = Ni, Mn, Ti, Mo) primitive cell (space group: C2/m, ICSD No ) using the enumeration method by Hart et al. 1-3 To model the cation-disordered structures of Li1.17Ni0.33Ti0.42Mo0.08O2, the cation compositions of Li:Ni:Ti:Mo were fixed to 7:2:2:1 and 7:2:3:0 for each layer along the c direction. Stable structures of these materials were determined by ranking fifty cation arrangements by their electrostatic energies and by their DFT energies. 4 The structures of Li2Ru0.5Sn0.5O3 and Li1.25Mn0.5Nb0.25O2 were prepared with the same technique within a supercell of the Li2RuO3 conventional cell (space group: C2/c, ICSD No ) and within the primitive unit cell of Li3NbO4 (space group: I-43m, ICSD No ), NATURE CHEMISTRY 3

4 respectively. The same enumeration technique was used to generate Li/vacancy ordering for delithiated phases of Li1.17-xNi0.25Mn0.58O2, Li2-xRu0.5Sn0.5O3, Li1.17-xNi0.33Ti0.42Mo0.08O2, and Li1.25-xMn0.5Nb0.25O2. The delithiated phases were determined by ranking thirty Li/vacancy arrangements by their electrostatic energies and by their DFT energies at each composition. The DFT energies were evaluated within GGA+U. 5 The U values of 6.0 ev, 3.9 ev, 4.0 ev, 4.4 ev, and 1.5 ev were used for Ni, Mn, Ru, Mo, and Nb, respectively. 6 The optimal mixing parameters of Li-excess metal oxides The redox mechanism of Li-excess (transition) metal oxides is determined by the competition between the TM and oxygen redox. An accurate description of the band structure in which the relative positioning of the oxygen and TM levels is correct is essential to investigate this interplay. Conventional GGA and GGA+U cannot predict the band structures of TM oxides with the required accuracy, as the self-interaction error results in an over-delocalization of the electrons Here, we employed a first-principles methodology that was specifically developed to predict accurate electronic band structures of Li-excess metal oxides and has previously been validated for MO (M = Mn, Co, Ni) and LiMO2 (M = Co, Ni). 11 The method is based on the HSE06 hybrid functional with optimized hybrid mixing parameters. In this study, the hybrid mixing parameter for each system was adjusted to reproduce the band gaps predicted by G0W0 calculations 12 due to the lack of experimental references. 11 We performed non-self-consistent G0W0 calculations based on initial wavefunctions and eigenvalues obtained from GGA+U calculations 13, 14 and the usual random-phase approximation (RPA) 15 with the Vienna ab initio simulation package (VASP). 16 For all GW calculations, a plane-wave energy cutoff NATURE CHEMISTRY 4

5 of 347 ev was used. A large numbers of bands (over five times the number of occupied states) were included in the calculations to guarantee convergence of the unoccupied states. Supplementary Fig. 1 shows the total density of states (tdos) of Li1.17Ni0.25Mn0.58O2 calculated with HSE06 and various mixing parameters. Supplementary Fig. 2 shows the band gaps of Li1.17Ni0.25Mn0.58O2 calculated with HSE06 as a function of the mixing parameter. The band gap of Li1.17Ni0.25Mn0.58O2 linearly increases with the mixing parameter between 0.10 and The tdos and band gap of Li1.17Ni0.25Mn0.58O2 obtained from GW calculations are also shown in Supplementary Figs. 1 and 2, respectively. The optimal mixing parameter of Li1.17Ni0.25Mn0.58O2 was determined by comparing the GW-band gap with the HSE-band gap for various mixing parameters and was found to be The same approach was followed for Li2MnO3, Li2Ru0.5Sn0.5O3, Li1.17Ni0.33Ti0.42Mo0.08O2, and Li1.25Mn0.5Nb0.25O2, as shown in Supplementary Fig. 3. The optimal mixing parameters for Li2MnO3, Li2Ru0.5Sn0.5O3, Li1.17Ni0.33Ti0.42Mo0.08O2, and Li1.25Mn0.5Nb0.25O2 are 0.17, 0.32, 0.27 and 0.25, respectively. The optimal mixing parameter for layered LiNiO2 (0.18) was used for partially cation-mixed layered LiNiO2, which was previously obtained by adjustment to experimental band gaps. 11 We used these optimal mixing parameters to calculate the projected density of states and the spin densities of these materials reported in this work (Figures 1, 2 and 3). Note that the band gaps of the Li-excess metal oxides tend to increase linearly with the mixing parameter, which is in agreement with previous reports 11, 17 on lithium metal oxides. NATURE CHEMISTRY 5

6 Figure 1. Total density of states (tdos) of Li 1.17Ni 0.25Mn 0.58O 2 calculated with HSE06 and various mixing parameters. The numbers on the left side indicate the mixing parameters. The tdos obtained from a calculation within the GW approximation is also presented for comparison. Figure 2. The band gap of Li 1.17Ni 0.25Mn 0.58O 2 calculated with HSE06 and various mixing parameters. The dashed line indicates the band gap predicted by G 0W 0. The black line indicates the trend of the band gap as a function of the mixing parameter for values between 0 and 0.4. The red line connects the GGA-band gap (α = 0) and the standard HSE06-band gap (α = 0.25). NATURE CHEMISTRY 6

7 Figure 3. The band gap of (a) Li 2MnO 3, (b) Li 2Ru 0.5Sn 0.5O 3, (c) Li 1.17Ni 0.33Ti 0.42Mo 0.08O 2, and (d) Li 1.25Mn 0.5Nb 0.25O 2 calculated with HSE06 and various mixing parameters. The dashed lines indicate the band gaps predicted by G 0W 0. The black lines indicate the trend of the band gaps as a function of the mixing parameter for values between 0 and 0.4. The red lines connect the GGAband gaps (α = 0) and the standard HSE06-band gaps (α = 0.25). The oxidation state of TM and oxygen ions The oxidation states of TM and oxygen ions in the different Li-excess materials (Figure 3) were determined by comparing calculated magnetizations (average net magnetic moments) of TM and oxygen ions with the number of unpaired electrons of the corresponding ions at each oxidation state. For example, Supplementary Fig. 4 shows the magnetizations of Mn ions and oxygen ions in Li1.25-xMn0.5Nb0.25O2 (x = 0, 0.25, 0.50, 0.75, 1.0, and 1.25) predicted by HSE06 with optimal mixing parameter (0.25). The number of unpaired electrons is 4 for Mn 3+, 3 for Mn 4+, 0 for O 2 and 1 for O 1, as shown in Supplementary Fig. 4. At fully lithiated state (x = 0), the magnetizations are around NATURE CHEMISTRY 7

8 3.75 for all of the Mn ions, indicating that they are in the 3+ oxidation state. After extracting two out of ten lithium ions (x = 0.25), the magnetizations of two Mn ions become around 3.0 and those of the other two Mn ions remain around 3.75, indicating that half of the Mn ions become oxidized to 4+. Since the magnetizations of all of the Mn ions become around 3.0 at x = 0.5 in Li1.25-xMn0.5Nb0.25O2, we conclude that all of the Mn ions become fully oxidized to 4+ at this composition. The magnetizations of the oxygen ions, on the other hand, are close to 0 for compositions with 0 x 0.50, thus oxygen ions remain O 2. After extracting six out of ten lithium ions (x = 0.75), the magnetizations of two oxygen ions become around 0.75, indicating that oxygen ions start to get oxidized. More oxygen ions become oxidized upon further delithiation as shown in Supplementary Fig. 4b. The number of oxygen ions that becomes oxidized to O 1 is equal to the number of extracted lithium ions after all Mn ions have been fully oxidized to Mn 4+, as shown in Supplementary Fig. 4, confirming that the holes are localized on individual oxygen ions. Note that the magnetizations of Nb ions remain 0 even after complete delithiation, implying that Nb ions are not redox active in this material. Figure 4. The magnetizations of (a) Mn ions and (b) oxygen ions in Li 1.25-xMn 0.5Nb 0.25O 2 (x = 0, 0.25, 0.50, 0.75, 1.0, and 1.25) predicted by HSE06 with optimal mixing parameter (0.25). The electronic configurations of Mn 3+, Mn 4+, O 2 and O 1 are presented. NATURE CHEMISTRY 8

9 Impact of the mixing parameter on the competition between TM and oxygen redox The relative position of the TM redox level to the oxygen redox level sensitively depends on the mixing parameter, 11 thus the competition between TM and oxygen redox also changes with the value of the mixing parameter. To investigate the effect of the mixing parameter on the competition between TM and oxygen redox, the oxidation states of TM and oxygen ions in Li1.25-xMn0.5Nb0.25O2 and Li1.17-xNi0.33Ti0.42Mo0.08O2 were calculated with various mixing parameters. Supplementary Fig. 5 shows the average magnetizations of Mn and oxygen ions in Li1.25-xMn0.5Nb0.25O2 as a function of the lithium content with various mixing parameters. For mixing parameters of 0.25 and 0.35, the average magnetization of Mn ions decreases from around 3.75 to around 3.0 until extraction of 0.5 Li from Li1.25Mn0.5Nb0.25O2 and then remains around 3.0 until complete delithiation, indicating that Mn ions are first oxidized from 3+ to 4+ and remain in the 4+ oxidation state. For the mixing parameter of 0.15, the average magnetizations of Mn ions continuously decrease below 3.0 after extracting more than 0.5 Li. The average magnetizations of oxygen ions for the mixing parameter of 0.15 are slightly lower than those for the mixing parameters of 0.25 and This indicates that the HSE06 with lower mixing parameter predicts more Mn oxidation (to slightly above 4+) and less oxygen oxidation due to the lower relative energy level of the orphaned O 2p state. Therefore, the mixing parameter has to be properly adjusted to quantitatively predict the competition between TM and oxygen redox in Li-excess materials. However, even for HSE06 calculations with mixing parameters between 0.15 and 0.35, no overlap between the Mn 3+ /Mn 4+ band and the orphaned O 2p state in Li1.25-xMn0.5Nb0.25O2 is observed. The same analysis was performed for Li1.17-xNi0.33Ti0.42Mo0.08O2, as shown in NATURE CHEMISTRY 9

10 Supplementary Fig. 6. For all of the calculations with mixing parameters 0.15, 0.27 and 0.35, the average magnetizations of Ni ions decrease to +1 and those of the oxygen ions remain close to 0 until 0.33 Li ions are extracted. After extracting more than 0.33 Li, the average magnetizations of oxygen ions increase to around 0.25, whereas those of the Ni ions change by less than 0.5. This shows that the Ni ions first become oxidized to Ni 3+ and then oxygen ions start to get oxidized. Lower mixing parameters (0.15) than the optimal value (0.27) predict less overlap between Ni redox and oxygen redox. However, HSE06 calculations with mixing parameters between 0.15 and 0.35 still predict the overlap between Ni 2+ /Ni 4+ redox and oxygen redox. Therefore, our predictions about the competition between TM and oxygen redox in Li1.25-xMn0.5Nb0.25O2 and Li1.17-xNi0.33Ti0.42Mo0.08O2 are robust with respect to the value of the mixing parameter between 0.15 and Figure 5. The average magnetizations of (a) Mn ions and (b) oxygen ions in Li 1.25-xMn 0.5Nb 0.25O 2 (x = 0, 0.25, 0.50, 0.75, 1.0, and 1.25) with various mixing parameters (α = 0.15, 0.25, 0.35). NATURE CHEMISTRY 10

11 Figure 6. The average magnetizations of (a) Ni ions and (b) oxygen ions in Li1.17-xNi0.33Ti0.42Mo0.08O2 (x = 0, 0.17, 0.33, 0.66, and 1.0) with various mixing parameters (α = 0.15, 0.27, 0.35). The electronic configurations of Ni 2+, Ni 4+, O 2 and O 1 are presented. Figure 7. (a) Probability (%) of finding a Li-O-Li configuration for an oxygen ion as a function of the Li-excess level (x in Li 1+xM 1-xO 2) in the layered and cation-disordered rocksalt structures (b) Average number of Li-O-Li configurations per oxygen as a function of the Li-excess level (x in Li 1+xM 1-xO 2): for the calculations, we assumed a random distribution of cations within the metal layer for the layered structure and for all the cation sites in the cation-disordered structure. NATURE CHEMISTRY 11

12 Figure 8. Snapshots of two neighboring oxygen ions in Li 2-xRu 0.5Sn 0.5O 3 (a) before [x = 0] and (b) after delithiation [x = 2]: The spin densities around oxygen ions are drawn in yellow. The isovalues for the isosurfaces of the spin densities were 0.05 e Å -3. The blue dashed lines show the direction of the Li-O-Li configuration for the two oxygen ions in Li 2Ru 0.5Sn 0.5O 3. The red dashed lines show the direction of O 2p shaped holes in Ru 0.5Sn 0.5O 3. (c, d) The O O and Sn O COOPs before [black] and after delithiation [red] are drawn in (c) and (d), respectively. Insets in figure c are the partial charge densities of two oxygen ions after delithiation for each energy range [red arrows]. Inset in figure d is the partial charge density between Sn and O within 6 ev to 2 ev. (e) A simple schematic of the O O bond formation in Li 2-xRu 0.5Sn 0.5O 3. NATURE CHEMISTRY 12

13 Figure 9. Snapshots of two neighboring oxygen ions in Li 1.17-xNi 0.25Mn 0.58O 2 (a) before [x = 0] and (b) after delithiation [x = 1.17]: The spin densities around oxygen ions are drawn in yellow. The iso-values for the isosurfaces of the spin densities were 0.05 e Å -3. The spin densities around Mn ions are not drawn. The blue dashed lines show the direction of the Li-O-Li configuration for the two oxygen ions in Li 1.17Ni 0.25Mn 0.58O 2. The red dashed lines show the direction of O 2p shaped holes in Ni 0.25Mn 0.58O 2. (c, d) The O O and Mn O COOPs before [black] and after delithiation [red] are drawn in (c) and (d), respectively. (e, f) various local arrangements of (oxidized) Li-O-Li states in Li 1.25-xMn 0.5Nb 0.25O 2 (x = 1.25): the spin densities around O ions are drawn in yellow. NATURE CHEMISTRY 13

14 Conditions for the O O bond formation To study if the hole states from different oxidized oxygen ions overlap and form O O bonds in various Li-excess materials, we computed the crystal orbital overlap population (COOP) between neighboring oxygen ions in Li2-xRu0.5Sn0.5O3 (x = 0 and 2) and Li1.17-xNi0.25Mn0.58O2 (x = 0 and 1.17). The COOP is a bond indicator that shows bonding/antibonding states in a crystal and can be easily constructed by generating an overlap population-weighted density-of-states. The COOPs were computed with the DFT/HSE calculations by the Lobster program In COOPs, positive/negative values indicate bonding/anti-bonding, respectively. Weak O O bonding has been observed in Li2-xRu0.5Sn0.5O3 after very high delithiation. 21, 22 In our calculations, we find that oxygen oxidation first occurs in this compound with the extraction of labile oxygen electrons from the Li-O-Li states (Fig. 3b). Then at very high delithiation, some neighboring oxidized Li-O-Li states (unhybridized O 2p orbitals) hybridize to form weak O O bonding. Specifically, the O O bond forms between two neighboring oxygen ions that have one Li-O-Li and two Li-O- Sn configurations each but no Ru connected to them. Supplementary figures 8a and 8b show snapshots of the two neighboring oxygen ions in Li2-xRu0.5Sn0.5O3 before (x = 0) and after delithiation (x = 2). From the figures, we can see that the two O 2p orbitals that were along the Li-O-Li directions slightly rotate towards each other after delithiation so that -overlap can be formed. From the O O COOP, we find that this σ-overlap after complete delithiation leads to σ b (bonding) states at about 1.0 ev to 0 ev and σ * (antibonding) states at about 2.3 ev to 4.2 ev, which can also be observed in the partial charge densities of the two oxygen ions within each energy range (Insets in red boxes, NATURE CHEMISTRY 14

15 Supplementary Fig. 8c). Such formation of an O O bond after complete delithiation can also be inferred from the shortened O O distance (2.27 Å) as compared to the O O distance (2.71 Å) toward other oxygen ions in the first-coordination sphere. After delithiation the bonding signals below the Fermi level remain in the Sn O COOP, indicating that the non-oxidized O 2p orbitals that are along the Li-O-Sn axes maintain their Sn-O hybridization after the σ-like bonding of the Li-O-Li states (Supplementary Fig. 8d). This can also be captured by the partial charge density between Sn and O within 6 ev to 2 ev which is shifted toward Sn even after delithiation (Inset in Supplementary Fig. 8d). After the σ-bonding of Li-O-Li states, the shortened O O distance also leads to partial π-like overlap between the remaining O 2p orbitals along the Li-O-Sn axes, resulting in bonding and anti-bonding states between 5.3 ev to 1.0 ev as well (Supplementary Fig. 8c). However, as both bonding and antibonding states are electron-filled for those hybridized states, such partial hybridization between the nonoxidized O 2p orbitals along the Li-O-Sn configuration does not stabilize the O O bonding, but is rather a byproduct of the σ-bonding of the Li-O-Li states. This understanding is drawn as a schematic in Supplementary figure 8e. We performed the same analysis for the layered Li1.17-xNi0.25Mn0.58O2 to investigate whether O O bond formation occurs in this material after complete delithiation (Supplementary Figs. 9a 9d). In this material, oxygen oxidation occurs on the oxygen ions with one Li-O-Li and two Li-O-Mn configurations (Fig. 3a). But in contrast to the Li2-xRu0.5Sn0.5O3, we find no clear change in both O O or Mn O COOP after complete delithiation (Supplementary Fig. 9c, 9d). These results indicate that there is no O O bond formation in the material, consistent with the fact that the oxidized Li-O- NATURE CHEMISTRY 15

16 Li states did not rotate towards each other, and that the O O distance (2.58 Å) remains essentially unchanged from the O-O distance toward the other oxygen ions in the first coordination shell (Supplementary Fig. 9b). These different results for the O O bond formation in Li2-xRu0.5Sn0.5O3 and Li1.17-xNi0.25Mn0.58O2 can be explained by the different metal species in the compounds as the metal to a large extent controls the position and flexibility of M-O bonding. In order for the oxidized Li-O-Li states (orbitals) to form a σ-bond, the oxidized oxygen ions must be displaced to some degree because the oxygen orbitals are at 90 degrees to each other in the ideal rocksalt structure. The ease by which such rotation happens depends on the nature of the M-O bond. One would expect M-3d to O-2p overlap to be strongly directional for transition metals with partially filled d states, 23 making it difficult for the oxygen-p orbitals to rotate. Indeed, in Li2-xRu0.5Sn0.5O3, we find that O O bond formation occurs between oxidized oxygen ions that are coordinated with one Li-O-Li and two Li- O-Sn configurations. The displacement of these oxygens is easier because the completely filled d-shells of Sn (non-transition metal) lead to a less directional Sn-O bond formed by the Sn 5s/p O 2p hybridization. In contrast, the oxidized oxygen ions in Li1.17-xNi0.25Mn0.58O2 are coordinated with one Li-O-Li and two Li-O-Mn configurations. The Mn 3d O 2p hybridization, which stabilizes the Mn-O bond in addition to Mn 4s/p O 2p hybridization, leads to strong and more directional M-O bond. Hence, the displacement becomes more difficult for the oxygens, preventing the oxygen displacement necessary to form an O O bond. Not only the strength of M-O hybridization, but also the geometric arrangement of the Li-O-Li states (orbitals) can affect whether an O O bond forms or not. In cation- NATURE CHEMISTRY 16

17 disordered Li1.25-xMn0.5Nb0.25O2 and Li1.17-xNi0.33Ti0.42Mo0.08O2, we find that many of the oxidized Li-O-Li states are spatially arranged in a way that they cannot hybridize with neighboring Li-O-Li states unless they completely destroy their existing M-O hybridization along the Li-O-M configurations (Supplementary Figs. 9e, 9f and Figs. 3c, 3d). Therefore, we find many of the oxidized Li-O-Li states in the materials remain unhybridized and do not form O O bonds. In short, there are two conditions for the formation of O O bonds after high delithiation: (1) Neighboring oxidized Li-O-Li states (unhybridized O 2p orbitals) that are (2) bonded in such a way that oxygen displacement can lead to the σ-hybridization of the Li-O-Li states without a substantial loss of the M-O hybridization. Our calculations indicate that such O O formation is easier when the oxygens are bonded to non-transition metals. Therefore, O O peroxo-like bond formation depends not only on the Li-excess level, but also on the delithiation level, cation distribution, and the metal species present in the compound. NATURE CHEMISTRY 17

18 Invariant energy level of the Li-O-Li state Figure 10. Projected density of states (PDOS) of (a) an oxygen ion with one Li-O-Li and two Li- O-Ni configurations in partially cation-mixed layered LiNiO 2, (b) an oxygen ion with one Li-O- Li and two Li-O-Mn configurations in layered Li 2MnO 3, and (c) an oxygen ion with one Na-O- Na and two Na-O-Ni configurations in partially cation-mixed layered NaNiO 2. Here, the data from NaNiO 2 is shown to demonstrate that the O 2p orbital along any non-hybridizing metal species (e.g. Li, Na) leads to same labile oxygen states as the Li-O-Li state. To compare the energy levels of the Li-O-Li (or Na-O-Na) states, the energies of the O 2s state in Li 2MnO 3 and NaNiO 2 were aligned with that of the O 2s state in LiNiO 2. The insets show the partial charge density around oxygen within 1.75 ev to 1.1 ev (blue box), all of which show that labile oxygen electrons within the range are from the O 2p orbitals along non-hybridizing species (Li- O-Li, Na-O-Na). From the results, we find that the energy level of the orphaned Li-O-Li states is invariant with respect to the metal species along the Li-O-M (M = Mn, Ni) axes, and is at the same level of the Na-O-Na states. This is because the Li-O-Li or Na-O-Na state is essentially the unhybridized O 2p state. NATURE CHEMISTRY 18

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