Overview of the Oxygen Behavior in the Degradation of Li2MnO3

Article pubs.acs.org/JPCC

Overview of the Oxygen Behavior in the Degradation of Li2MnO3 Cathode Material Eunseog Cho,* Kihong Kim, Changhoon Jung, Seung-Woo Seo, Kyoungmin Min, Hyo Sug Lee, Gyeong-Su Park, and Jaikwang Shin Platform Technology Lab, Samsung Advanced Institute of Technology, 130 Samsung-ro, Suwon, Gyeonggi-do 16678, Republic of Korea S Supporting Information *

ABSTRACT: The Li2MnO3 cathode material is vulnerable to complex degradation behaviors during the operation of battery although it has attracted much attention recently due to its potentially large capacity. In this study, we comprehensively examined the degradation process in Li2MnO3, using theoretical density functional computations as well as experimental techniques (in situ X-ray absorption near edge structure spectroscopy, X-ray diffraction, and Raman spectroscopy). Our study reveals that during the delithiation process, the Li ions mixed in the Mn layer are removed together with those in the Li layer, thereby inducing the release of oxygen atoms. The oxygen loss reaction is energetically favorable at the highly delithiated states, and it can reduce the plateau voltage in the charging curve. Such oxygen loss was observed during or even before the second cycle and furthermore it accelerates the phase transformation of the layered structure to a spinel one. Our results also suggest that oxygen release can be prevented when H ions are exchanged with Li ions during the charging process.

1. INTRODUCTION Li-rich layered oxides have been widely investigated as cathode materials with large capacity and long life cycle for the Li-ion battery.1 Among them, Li2MnO3 is considered as a potential candidate for the commercialization of electric vehicles because it contains a larger number of exploitable Li ions, which leads to higher capacity (theoretical capacity of 458 mAh/g) in comparison to other conventional cathode materials such as LiCoO2.2 In addition, this material also has been proven to be effective for increasing the capacity of conventional cathode when inserted as a composite structure.3,4 For example, the overlithiated layered oxide (OLO), a composite of LiNixCoyMn1−x−yO2 (NCM)5 and Li2MnO3, exhibits large capacity over 200 mAh/g, which is attributed to Li2MnO3.6 Although Li2MnO3 and its composite have a proven large capacity, the capacity fade occurs quickly during the cycling due to degradation, thereby limiting its practical use as a cathode. This degradation has been suggested to originate from complex phenomena such as the oxygen release,11−17 phase transformation,18−20 and Li+/H+ exchange reaction,21−24 but the detailed mechanisms governing these degradation behaviors have not been fully understood yet. Interestingly, the Li2MnO3 was believed to be electrochemically inactive in the past studies.7,8 This belief originates from the oxidation number of Mn and the crystal structure of Li2MnO3. For the cathode to be electrochemically active, the transition metals should lose their electrons to satisfy the charge neutrality when the Li ions are extracted from the cathode during the charging process. For the Li2MnO3, each Mn atom forms a local octahedral structure with six © XXXX American Chemical Society

neighboring oxygen atoms (MnO6) and its oxidation number is +4 in the pristine structure. However, in order to maintain this octahedral structure after the Li extraction, the oxidation number of Mn should not exceed +4.9 Therefore, the octahedral structure of MnO6 within Li2MnO3 could become deformed or collapse if the Li2MnO3 is electrochemically operated because electron loss due to the extraction of Li ions would cause its oxidation number to exceed +4. However, recent researches have proved that Li2MnO3 can be electrochemically active while maintaining its octahedral structure at the high charging voltage of approximately 4.5 V.10,11 In order to discover why Li2MnO3 operates, recent studies have focused on the role of oxygen during the electrochemical process.11−13 They speculate that O rather than Mn loses electrons to operate the cathode during the charging process. However, intuitively, when O loses its electron, O2− residing in the oxide forms neutral oxygen atoms, resulting in the formation of O2 gas, and this is likely to be released from the cathode. In other words, the operation of Li2MnO3 can be closely related to the role of oxygen in terms of degradation. Meanwhile, other studies point out the possibility that the H+ generated from the decomposing electrolyte can enter Li2MnO3 through ion exchange with the Li+.21−24 They suggest that the Li+/H+ exchange reaction might distort the local structure within Li2MnO3, thus preventing the extracted Li+ from returning to the original sites during the discharging Received: May 22, 2017 Revised: September 4, 2017 Published: September 7, 2017 A

DOI: 10.1021/acs.jpcc.7b04937 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) The crystal structure of Li2MnO3; each color is represented as follows: violet (Li), green (Mn), and red (O). The label of oxygen atom (O#) is randomly assigned. (b) The variation of total energy calculated from Li2−xMnO3 structures (a total number of 162) as a function of delithiation; the parentheses indicates the number of possible configurations at each delithiation states. (c) The ratio of Li ions stay in the Li-only (squares) and in the Li−Mn mixed (circles) layers at each delithiation states. (d) The total energies (solid filled black) of Li1.75MnO2.75 when a specific oxygen atom (O#) is removed from the Li1.75MnO3 structure. The atomic charge (empty filled red) of the removed oxygen atom (O#) calculated from Li1.75MnO3. Note that the unit-cell structure contains 12 O atoms. Note 0 eV corresponds to the most stable structure.

situ cells are fabricated and utilized to measure the XANES spectra and capacity-voltage curves. The X-ray diffraction (XRD) patterns, Raman spectra, and inductively coupled plasma−atomic emission spectrometry (ICP-AES) data during cycling are also obtained. The computational results are closely compared with the experimental findings to elucidate the details of degradation mechanism.

process. In addition, some researchers report that the layered structure can undergo phase transformation to the spinel structure during cycling.18−20 For example, Amalraj et al. observed the formation of spinel phase in Li2MnO3 after the third charging to 4.60 V using transmission electron microscopy (TEM).19 However, a previous experiment using the extended X-ray absorption fine structure (EXAFS) comments that the phase transformation could not be observed during the cycling of Li2MnO3.2 Therefore, further comprehensive studies for Li2MnO3 degradation process are required to clarify the origin of these complicated process. In this paper, we investigate the degradation process of Li2MnO3 using theoretical calculations based on density functional theory (DFT), as well as various experimental techniques. While the main focus is on the behaviors of oxygen, the layered-to-spinel phase transformation and the Li+/H+ exchange reactions are also investigated, since the latter two also have been discussed in the literature as the origins of degradation in Li2MnO3. On the theoretical side, we examine the changes in the structural and electronic properties during the delithiation when the oxygen atoms are released. On the contrary to the previous theoretical studies which mainly considered the delithiated structure (Li2−xMnO3) without oxygen defects, we focus on the effect of oxygen released structure (Li2−xMnO3−y) and its correlation with delithiation process.25−28 The energetics of the oxygen loss reaction, the Xray absorption near edge structure (XANES) spectral changes, and the phase diagrams are also calculated. Experimentally, in

2. METHODS 2.1. Computational Details. First-principles calculations were performed by the pseudopotential plane wave method, using the Vienna Ab initio Simulation Package (VASP).29,30 We adopted the generalized gradient approximation (GGA) implemented by Perdew, Burke, and Ernzerhof (PBE)31 for the exchange correlation energy functional with the spin polarization method. The on-site Coulomb interaction (GGA +U approach),32 which has been proved to accurately predict the theoretical voltage,26,28 was used to describe the strongly correlated d electrons of the Mn atoms. The Hubbard U value for Mn was taken as 5.0 eV, which is a theoretically estimated value for Mn4+ in the MnO2 spinel structure.25,33 For the magnetic ordering of Mn, our calculation predicts that the pristine Li2MnO3 with the antiferromagnetic ordering gives 2.84 meV/atom lower in the energy than that with the ferromagnetic configuration. However, the ferromagnetic configuration is only considered here since this energy difference is negligible and the magnetism for the defective structures is not main concern for this study. An 8 × 4 × 8 B


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theoretical study of the delithiation phenomenon, it is essential to obtain the energetically more favorable delithiated structures. In other words, we should first determine which Li ions should be extracted during the delithiation within the reliable framework. In the case of only one extracted Li atom, simply three crystallographically inequivalent Li sites (2b, 2c, and 4h) need to be considered. However, as delithiation proceeds, the number of possible combinations of Li extraction sites increases rapidly. For example, at 50% delithiation the number of possible configurations in a unit cell (8 Li atoms) is 70, which demands considerable computational resource. Nevertheless, we calculated the total energy for each of the 162 possible delithiated configurations from complete enumeration. Then we compared the obtained energies to choose the most stable structure for each delithiation state. The change of lattice parameters as a function of delithiation (Li2−xMnO3) is shown in Table S1. As shown in Figure 1b, the energy gap between the most and the least stable structures increases as delithiation proceeds. Especially at 50% delithiation, this energy difference reaches 0.042 eV/atom, which corresponds to the value of about 0.21 V in terms of cell voltage. Thus, it is important to search all the delithiated structures to determine the most stable one at specific delithiation state, which allow us to predict the energetically most favored order of Li removal from the cathode. As shown in Figure 1c, in the pristine Li2MnO3 structure, 75% of Li ions occupy the Li-only layers and 25% are in the Li−Mn mixed layers. During the first stage of delithiation (0−12.5%), the Li ions are only removed from the Li-only layer, and in the next stage (12.5−25%) Li ions only in the Li− Mn mixed layer are extracted. Similarly, Li extraction occurs in the Li-only layer during 25−37.5% delithiation, and in the mixed layer during 37.5−50% delithiation. Thus, all Li sites in the Li−Mn mixed layer are vacant at 50% delithiation. Recent calculations based on the nudged elastic band method explain that the interlayer Li diffusion from the Li−Mn mixed layer to the Li-layer shows comparable or smaller activation barrier than that of the intralayer Li diffusion on the Li layer.44,45 This suggests that the Li ions on the mixed layer can also be delithiated through the diffusion to the vacant site on the Li layer. The Li ions in Li2MnO3 also form local octahedral structures (LiO6) with their six neighboring oxygen atoms; therefore, Li extraction obviously affects these oxygen atoms. One expected effect is the release of oxygen, and thus we need to determine the order of oxygen atom removal. These oxygen-loss (O-loss) structures were calculated using steps identical to those for determining Li removal: the total energies for each of O-loss configurations after their geometry optimization are calculated and the most stable one at each of delithiated states is selected as the Li2−xMnO3−y structure. The O-loss process maintains the monoclinic shape of the pristine structure although the lattice deviation from the pristine structure increases as the O-loss proceeds (see Table S1). For example, Figure 1d shows the energies of Li1.75MnO2.75 (8% O-loss) when a specific oxygen atom (O#) is removed from the Li1.75MnO3 structure, whose unit cell contains 12 O atoms. The atomic charge of the respective removed oxygen atom (qO#) calculated from the Li1.75MnO3 structure is also represented in the graph. The atomic charge clearly shows that the six neighboring oxygen atoms (#2, #3, #4, #7, #8, and #12) of the removed Li are more oxidized compared to the other oxygen atoms since when one Li atom is delithiated, the neighboring oxygen atoms are prone to lose their charge to satisfy the charge compensation. The

mesh was used for k-point sampling over the Brillouin zone. The energy cutoff was selected to be 500 eV, and the atomic positions were fully relaxed until the ionic force on each atom was below 0.02 eV/Å. The cell parameters were optimized for the pristine, the delithiated, and the oxygen-loss structures to obtain reliable lattice constants. The atomic charge (q) was calculated by Bader charge analysis35 on the electron densities obtained from DFT calculations. The XANES spectra for the Mn K-edge were calculated using the WIEN2K package,34 based on the optimized structures obtained by the VASP calculation. 2.2. Experimental Details. Li2MnO3 was synthesized from a mixture of Li2CO3 and MnCO3 in a 1:1 molar ratio. The mixture was calcined in Al crucible at 850 °C for 6 h, then quenched to room temperature. The working electrodes were fabricated from a mixture of the prepared Li2MnO3, carbon black, and a polyvinylidenefluoride (PVdF) binder (8:1:1 by weight) on Al foil. Electrochemical measurements were carried out using half-cell type 2032 coin cells, each containing the Li2MnO3 working electrode, Li metal counter electrode, and an electrolyte of 1.3 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 w/w). The cells were tested in the potential range of 2.0−4.7 V at 0.1 C. A schematic view of the cell assembly used for the in situ XANES is shown in Figure S1, Supporting Information (SI). A hole (ϕ = 10 mm) was punched in the 2032 coin cell case. The cathode coated with Al foil (ϕ = 15 mm) was used as the current collector, and epoxy was applied along the edge of the Al foil to prevent electrolyte leakage. The Al foil was spot-welded at two points for electric contact with the stainless steel case. A PVdF separator (ϕ = 19 mm) was placed between the cathode and Li foil. All cells used for in situ XANES were assembled in a dry room. The XANES measurements were performed in fluorescence mode at 7D XAFS beamline at the Pohang Accelerator Laboratory. The cell was placed in plastic holder, which was connected to a potentiostat (Biologics, VSP-300). All the cells were cycled galvanostatically within the voltage range of 2.0−4.7 V at the rate of 0.1 C using the potentiostat. The crystal structures of Li2MnO3 electrodes were investigated by powder XRD (Bruker, D8 Advance) with Cu Kα source (λ = 1.54 Å) within 2θ = 10° ∼ 90°. Raman analysis was conducted at room temperature using a micro-Raman spectrometer (inVia Raman microscope, RENISHAW). The samples were excited by the 514 nm line of an Ar+ laser. In order to prevent thermal damage of the Li2MnO3 samples, the power of the incident laser beam was maintained at below 1 mW.

3. RESULTS 3.1. Electronic Characteristics of Oxygen-Loss Structures. Figure 1a describes the crystal structure of the monoclinic Li2MnO3. Of the total amount of Li atoms in this simulation cell, 25% of them replace 33% of the total Mn sites (Wyckoff position of 2b site) to form a Li−Mn mixed layer, while the remaining 75% of Li atoms occupy 2c and 4h sites in the Li-only layer between the two Li−Mn mixed layers. The Li2MnO3 crystal is composed of alternatively stacked mixed Li−Mn and Li-only layers, forming an ordered C2/m structure. Figure 1b shows the total energies of 162 optimized configurations that we calculated for the Li2−xMnO3 (x = 0.25, 0.5, 0.75, and 1.0) compositions. We consider stoichiometries up to x = 1.0 (50% delithiation) since this approximately corresponds to the limit of experimental Li extraction that avoids abrupt instability during cycling.23 For C


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Figure 2. (a) Changes in DOS of Li2−xMnO3 and (b) of Li2−xMnO2.75 as the delithiation proceeds; black is total DOS, blue and red lines indicate the partial DOSs of Mn and O atoms, respectively. Percentage value in the left indicates the degree of delithiation. Vertical line represents the Fermi energy.

level, and the density of hole states start to increase. The increase of the hole states indicates the oxygen atom loses its electron and becomes oxidized during the delithiation. On the other hand, the Mn atom hardly loses its electron during the delithiation, since Mn contributes little to the VBs of total DOS. The DOS results suggest that oxygen, not manganese, is responsible for charge compensation in the Li2−xMnO3 structures, which is also presented in the previous DOS studies.25,26 Figure 2b shows the variation of partial DOS of the oxygen defective structures (Li 2−xMnO 2.75) during the delithiation process. The defective states due to the O-loss emerge just below the Fermi level, and the states pass the Fermi level as delithiation proceeds. Considering the fact that the defective states are composed of O and Mn atoms, both can participate in the charge compensation process in the O-loss structures. This compensation mechanism is also clearly shown by the calculated atomic charges of Mn and O atoms (qMn and qO, respectively) as functions of O-loss. In Figure 3, qMn does not show any variation during the delithiation of Li2−xMnO3, but qO continues to increase as the Li ions are extracted. Such oxidation (increase of charge) of the O atom further supports that oxygen, not Mn, plays the main role in the charge compensation process, which was discussed earlier in the context of Figure 2a. Moreover, the oxidation in oxygen atom as a result of the charge compensation could induce the release of oxysimigen during delithiation process. Once the oxygen is released from the cathode, the oxidation of Mn can then occur during delithiation. The values of qMn for both Li2−xMnO2.75 (8% O-loss) and Li2−xMnO2.5 (17% O-loss) structures increase during the delithiation, and the rate of increase is higher with more oxygen removal. Therefore, not only O but also Mn can participate in the charge compensation process, analogously to the results of the DOS of the O-loss structures (Figure 2b). However, it should be noted that even in the O-loss structures, the values of qMn at 50% delithiation are similar at all levels of O-loss. Therefore, although the O-loss initiates the oxidation of Mn, the final charge of Mn does not exceed its value in the pristine Li2MnO3 structure. In the pristine and two O-loss structures, qO increases with delithiation, and at any given delithiation stage, it decreases with increasing O-loss. This indicates that although delithiation can induce oxygen release,

role of oxygen in the charge compensation process is discussed in Figures 2 and 3. The energies and the atomic charges follow

Figure 3. Computed atomic charge variations of Mn and O atoms in Li2−xMnO3 (no O-loss; black square), Li2−xMnO2.75 (8% O-loss; red circle), and Li2−xMnO2.5 (17% O-loss; blue triangle) during delithiation.

similar trends in the plot: the energy of the O-loss structure decreases (i.e., becomes more stable) when the less negatively charged oxygen (more oxidized oxygen) is removed. The same trend is also observed at other stages of delithiation (Li2−xMnO2.75 with x = 0, 0.5, 0.75, and 1.0, shown in Figure S2, SI). This indicates that the more oxidized oxygen atom in Li2−xMnO3 is preferentially released from the cathode in terms of energetics. For the electrochemical activity of the cathode material, both Li ions and electrons should be simultaneously removed from the cathode during the delithiation for charge balance. Therefore, the transition metal and/or oxygen atoms should lose electrons. To understand this charge compensation process, the change of partial density of states (DOS) of Li2−xMnO3 during the delithiation was calculated and shown in Figure 2a. In all delithiated structures, it is clear that the valence bands (VBs) of total DOSs are mainly occupied by the oxygen states, and most of the Mn states contribute to the conduction bands of total DOSs. As delithiation proceeds, the VB maximum of oxygen atom continues to move over the Fermi D


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When there is no O-loss, the calculated voltage during the whole delithiation process is almost constant at 4.6 V, whose value agrees with the experimental plateau voltage of the first charging curve. When 8% oxygen is removed from the oxide, the calculated voltage starts at around 3.09 V and continues to increase to 4.42 V at 50% delithiation. These trends are similar to those observed experimentally from the second cycle. Thus, from the comparison of calculation and experiment the charging curve at the first cycle corresponds to the no O-loss structure, and those at the second and subsequent cycles denote the O-loss structures. This implies that oxygen atoms can be released from Li2MnO3 as early as at the second cycle. In the case of 17% O-loss, the voltage also continues to increase as delithiation proceeds, but its value (2.48∼2.85 V) is much lower than that observed from the experiment. Hence, 17% of O-loss is considered unlikely to occur for 50 charge−discharge cycles. In order to investigate whether oxygen gas evolution is energetically possible, we calculated the free-energy change (ΔG) of the O-loss reaction at 300 K and plotted it in Figure 5. We selected 8% and 17% O-loss structures, and the overall reaction is adopted as

additional oxygen release from the oxygen deficient structure becomes more difficult. 3.2. Energetics and Verification of Oxygen-loss. Figure 4a shows the capacity−voltage curves measured for 50 charge−

Li 2 − xMnO3 → Li 2 − xMnO3 − y +

y 2O2

(2)

Figure 4. (a) Capacity−voltage curves measured for 50 cycles of charge−discharge. (b) Calculated voltage changes in Li2−xMnO3 (no O-loss; black square), Li2−xMnO2.75 (8% O-loss; red circle) and Li2−xMnO2.5 (17% O-loss; blue triangle) during delithiation. Figure 5. Free-energy changes (ΔG) of the O-loss reactions at 300 K in Li2−xMnO2.75 (8% O-loss; red circle) and Li2−xMnO2.5 (17% O-loss; blue triangle) according to the delithiation. Horizontal dashed line indicates when ΔG = 0.

discharge cycles. At the first cycle, the charging curve quickly reaches the plateau voltage of around 4.6 V and maintains this value during further charging. At the second cycle, the plateau is lowered to around 3.0 V but the final voltage is the same as observed in the first charging process. As the cycling proceeds, the capacity continues to decrease at around 4.6 V, but all the charging curves resemble that of the second cycle. To understand the origin of the dramatic difference between the first and the second charging observed in the experiments, we calculated the voltage changes according to the levels of Li extraction (Figure 4b). To include the effect from the oxygen defects, the 0%, 8%, and 17% O-loss structures were modeled. The voltage (V) is calculated as

The free-energy changes are calculated using the following equation ΔG = ΔH − T ΔS ≈ E o(Li 2 − xMnO3 − y ) +

y − E o(Li 2 − xMnO3) − T ΔS(O2 ) 2E*(O2)

(3)

where ΔH and ΔS are the changes of enthalpy and entropy, respectively; and T is the temperature. Eo is the total energy of the system obtained from DFT calculation at 0 K, and E*(O2) is the corrected energy of the oxygen molecule. The binding energy of the oxygen molecule is well-known to be overestimated in the framework of GGA calculations. Such an error is associated with adding electrons to the oxygen p orbital to form anions from the O2 molecule, and it also affects the energies of oxide materials.36,37 Wang et al. calculated this constant error as 1.36 eV per O2 molecule in the oxidation reaction within the GGA+U framework, by fitting the formation enthalpy of transition metal oxides.38 Their correction factor produced oxidation energies in good agree-

V (x1 ≤ x ≤ x 2) ⎡ E[Li x MnO3 − y ] − E[Li x MnO3 − y ] − (x 2 − x1)μ[Li] ⎤ 2 1 ⎥ = −⎢ ⎢⎣ ⎥⎦ (x 2 − x1)e (1)

where E[Lix2MnO3−y] and E[Lix1MnO3−y] represent the total energies of the system at Li concentrations of x2 and x1, respectively, and μ[Li] represents the chemical potential of Li metal. E


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The Journal of Physical Chemistry C ment with experimental data and has been used to calculate oxidation energies in other studies.39,40 Therefore, we used the same approach here to correct the enthalpy change by shifting the oxygen energy. Meanwhile, the entropy of solid materials such as oxides does not show any meaningful change with regard to temperature, hence only the entropy change of gaseous O2, ΔS(O2), at 300 K is included, which is 2.13 meV/K from the JANAF thermochemical table.41 As seen in Figure 5, the O-loss reactions are energetically unfavorable (ΔG > 0) at the fully lithiated state, but they become favorable (ΔG < 0) in the highly delithiated regions. Specifically, both 8% and 17% O-loss structures are energetically unfavorable before 30% of delithiation. Very recently, Shin et al. suggested that the O-loss on the surfaces are unfavorable before about 15% of delithiation based on the DFT calculations,47 which supports our result considering that the O-loss from the bulk structure can occur at further delithiated status than that in the surface regions. Our calculation suggests that approximately 30% of Li ions (Li0.6) in the pristine Li content (Li2.0) cannot be returned to the oxide based on energetics after the oxygen atoms are lost. This suggestion is confirmed by the change of Li/Mn ratio in the electrode for 50 cycles, which was measured by ICP-AES and tabulated in Table 1. Before cycling, the ratio of Li/Mn was 2.04, which agrees Table 1. Ratio of Li/Mn Concentration for 50 Cycles Obtained from ICP-AES Measurements sample

ratio (Li/Mn)

0 cycle 1st cycle (charging) 1st cycle (discharging) 10th cycle 30th cycle

2.04 0.98 1.73 1.60 1.46

Figure 6. (a) In-situ XANES spectra of Mn K-edge measured for two charge−discharge cycles. (b) XANES spectral changes of Mn K-edge calculated for Li2−xMnO3 (upper panel) and Li2−xMnO2.75 (lower panel) according to the delithiation. Arrows indicate the direction of spectral changes as the delithiation proceeds.

such spectral change, we also calculated the change of XANES spectra for Mn K-edge within the framework of DFT. We modeled two different structures: no O-loss (Li2−xMnO3) and 8% of O-loss (Li2−xMnO2.75) and calculated the spectral changes as the delithiation proceeds. As shown in Figure 6b, when we remove only the Li ions (i.e., no O-loss), only peak intensities decrease, similar to the first charging measurement in Figure 6a. However, when Li ions are removed from the 8% Oloss structure, the peak is shifted to the right and its intensity is reduced, and the intensities of shoulder peaks at around 9 eV over the Fermi energy (Ef) also displays a substantial reduction. Such spectral change is similar to that observed during the second charging process in experiments. Hence, this analysis demonstrates that only Li ions are removed during the first charging process, and in the second charging process the oxygen atoms are released together with the Li ions. The conclusion of the XANES study, that oxygen can be released at the early stage of cycling, is also consistent with above discussions from the voltage-capacity changes, ICP measurement, and free-energy calculations. 3.3. Layered-to-Spinel Transformation. Next, we investigated the possibility of the phase transformation from the layered structure to cubic spinel phase during cycling. To study the formation of the cubic spinel phase, the XRD patterns and Raman spectra were measured over 50 cycles. Figure 7a shows the change of the XRD pattern from pristine, 10- to 50cycled Li2MnO3. The initial XRD data clearly show that the synthesized Li2MnO3 has the monoclinic, low symmetric structure. As the electrode is cycled, several peaks from Li2MnO3 are reduced, and the structure transforms to a phase with higher symmetry. The XRD pattern for 50-cycled sample provides the clear proof of the phase transformation since

with the stoichiometric value in the pristine Li2MnO3. During the first charging process, half of the Li ions in the oxide were extracted, the same as in our model with 50% delithiation. However, some of the extracted Li ions were not returned after the first discharging (about 13%, assuming that the Mn content did not change). As the cycles continued, more Li ions were lost: after the 10th and 30th cycles, 20% and 27% Li ions were irreversibly lost, respectively. Therefore, the comparison between free-energy calculation and ICP analysis results clearly indicates that oxygen atoms are released from the cathode during cycling. Furthermore, the initial O-loss is suspected to happen before the second cycle, since 13% of Li ions are not returned after the first discharging process. Meanwhile, the 17% O-loss reaction (Figure 5) appears much more favorable in terms of free-energy than the 8% O-loss one at the highly delithiated states. Although we could not experimentally observe the 17% O-loss state in the voltage-capacity study for 50 cycles (Figure 4), longer cycling might induce such O-loss if the layered structure is still maintained. Figure 6a shows the in situ XANES spectra measured for Mn K-edge for two charge−discharge cycles. In the first charging process, the peak intensity at 6561 eV diminishes slightly. In the second charging process, the peak shifts to the higher energy and its intensity is reduced. In addition, the shoulder peak at around 6550 eV becomes substantially weaker. The peak maximum shifts in the bottom right direction for Li2MnO3 was also reported by Yu et al. in an ex-situ experiment,23 but the origin has not been clearly described. To reveal the origin of F


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Mn−O (Figure 8) was constructed using the Thermo-Calc software46 with the formation energies obtained by DFT

Figure 8. (a) Phase diagram composed of Li−Mn−O three elements. (b) Delithiated and O-loss structures are represented as open squares (Li2−xMnO3; x = 0.125, 0.25, 0.375, and 0.5), circles (Li2−xMnO2.75; x = 0, 0.125, 0.25, 0.375, and 0.5), and triangles (Li2−xMnO2.5; x = 0, 0.125, 0.25, 0.375, and 0.5) in the blue dashed square region. Red arrow gives the Li-extraction direction.

calculation. The phase transformation is speculated to be closely related to both the oxygen release and delithiation. Thus, the formation energies of the O-loss structures Li1.75MnO2.75 (8%) and Li1.75MnO2.5 (17%) as well as the delithiated structures are considered. The formation energies of all structures used for the phase diagram are included in Table S2 in SI. The monoclinic Li2MnO3 and cubic spinel LiMn2O4 structures appear in the diagram, which means both are thermodynamically stable compared with other Li−Mn−O structures such as LiMnO 2 and various Li 2−x MnO 3−y compounds. In Figure 8(b, none of the defective Li2−xMnO3‑y structures (0 < x < 0.5, y = 0, 0.25, and 0.5) are more stable than the neighboring intact phases. It is noted that the open squares (Li2−xMnO3), open circles (Li2−xMnO2.75), and open triangles (Li2−xMnO2.5) represent phases that do not appear in the phase diagram; the defective phases move along the direction of the arrow as Li is extracted. This phase diagram indicates that when Li is extracted as well as the oxygen is released from the oxide, the defective Li2−xMnO3−y phases decompose into O2 gas, monoclinic Li2MnO3, and spinel Li2MnO4 phases. A recent atomistic study elucidates that the monoclinic structure is prone to transform to the spinel-like when the Li is excessively delithiated (x > 1.5) for Li2−xMnO3 (no O-loss structure).20 This is because it severely lowers the migration barrier of the Mn atoms allowing the formation of an intermediate state with a tetrahedral coordination. Our phase diagram suggests that the decomposition to the spinel Li2MnO4 phase can be accelerated for thermodynamic reasons as more oxygen atoms are released in the highly delithiated states. 3.4. Li+/H+ Exchange Reaction. Finally, we investigated the Li+/H+ exchange reaction. Figure 9a represents the effect of Li+/H+ exchange on the oxygen loss reaction with the assumption that all extracted Li + are replaced by H + (HxLi2−xMnO3). The ΔG of 8% O-loss reaction at 300 K is calculated as

Figure 7. (a) Changes of XRD patterns for 50 cycles. The stick spectra at bottom (initial) and top (50th cycle) are the theoretical peaks positions of layered Li2MnO3 (black) and spinel LiMn2O4 (blue) structures. Asterisk (*) indicates the peaks of Al foil. (b) Raman spectral changes measured for 50 cycles.

several peaks representing the monoclinic phase are disappeared and new peaks corresponding to the spinel phase are developed. Specifically, the peak at around 21° from the initial phase representing the Li2MnO3 phase19,42 disappears but the first peak at 19° still exists and new doublet peaks at around 36° and 38° arise. The strong peak at 19° and the doublet ones correspond to (111), (311), and (222) peaks of the LiMn2O4 phase,43 respectively and also match to the theoretical peak positions of the spinel structure. This phase transformation is also observed in the measured Raman spectra in Figure 7b. The Raman spectra of the initial Li2MnO3 structure show peaks at 248, 311, 323, 370, 413, 433, 491, and 610 cm−1, all of which are the same as the Raman peaks of Li2MnO3 suggested by other studies.7,42 Especially, the high frequency peak at 610 cm−1 is attributed to the symmetric Mn−O stretching vibration (Ag mode, monoclinic structure) of the octahedral MnO6 local structure. Generally, weaker Raman activity is observed in the spinel LiMn2O4 structure due to its polaronic character with a considerably broad band at approximately 625 cm−1 (A1g mode, cubic structure) and a shoulder peak at 583 cm−1.29 In our spectra, as the cycles proceed, the intensity of Mn−O stretching mode (610 cm−1) decreases, and a neighboring broad peak within the range of 615−650 cm−1 arises. The spectral characteristic after 50 cycles is quite similar to that of the spinel LiMn2O4. Therefore, the above changes in the XRD pattern and Raman spectra strongly indicate that the cycling of Li2MnO3 creates a new phase with higher symmetry, which has many characters of the cubic spinel phase. For a theoretical understanding of the possible phases, the phase diagram of Li−

ΔG = E o(HxLi 2 − xMnO2.75) + 0.125E*(O2 ) − E o(HxLi 2 − xMnO3) − T ΔS(O2 )

(4)

As shown in Figure 5, the 8% O-loss reaction becomes energetically favorable as delithiation proceeds. However, when G


Article


(Figure 6a). Therefore, we speculate that this ion exchange is not a dominant reaction that can be observed at ambient temperature.

4. CONCLUSION We investigated the degradation process of Li2MnO3 using theoretical calculations within the density functional framework, as well as various experimental measurements. It was found that the Li ions mixed in the Mn layer can be removed together with those in the Li layer during the delithiation, and this Li removal induces the release of oxygen atoms. The oxygen atoms in the cathode are mainly responsible for the charge compensation process. However, after oxygen is released, Mn can also compensate the excess charge. The more oxidized (i.e., less negatively charged) oxygen atom should be preferentially released from the cathode. The oxygenloss reaction is thermodynamically favorable at the highly delithiated states and can decrease the plateau voltage in the charging curve from 4.6 to 3.0 V at the second cycle. XANES study and ICP-AES measurements also proved that oxygen can be released from the cathode during or even before the second cycle. XRD and Raman spectroscopy analyses demonstrated the formation of spinel-like phase from the monoclinic structure, and the phase diagram calculation provides complementary information that oxygen loss can accelerate this phase transformation. Finally, the calculated energetics, atomic charges, and XANES spectra predict that the Li+/H+ exchange reaction can prevent the oxygen release even at the highly delithiated states. Although it is not considered as a major reaction at room temperature, this result points a possible direction for the reduction of oxygen loss.

Figure 9. (a) Free-energy changes (ΔG) of 8% O-loss reaction from HxLi2−xMnO3 at 300 K according to the delithiation; horizontal dashed line indicates the ΔG = 0. (b) XANES spectral changes of Mn K-edge calculated for HxLi2−xMnO3 according to the delithiation.

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the Li+/H+ exchange occurs, oxygen cannot be released regardless of the status of delithiation. This implies that Li+/ H+ exchange can prevent the oxygen release, which supports the previous experimental finding from infrared spectroscopy and 1H NMR analysis that released oxygen was not observed when Li+/H+ exchange occurred.11 We also calculated the changes of atomic charges of both Mn and O in HxLi2−xMnO3 structures as the Li+/H+ exchange reaction proceeds (Figure S3 in SI). The atomic charge of Mn in HxLi2−xMnO3 is not affected by delithiation, the same as qMn in the Li2−xMnO3 structure. However, qO is also almost constant (−1.24e ∼ −1.26e) during delithiation, which contrasts with the case of Li2−xMnO3 where it keeps increasing (Figure 3). These results also indicate that the oxygen release hardly occurs in the presence of the Li+/H+ exchange reaction. Moreover, the exchanged H+ ion moves to the nearby oxygen atom during the geometry optimization and forms the OH bond inside the cathode (Figure S4). We also calculated the voltage of delithiated state when the inserted H+ forms the OH bond (H0.25Li1.75−xMnO3) and compared that with the delithiation voltage of the structure without H+ ion (Li2−xMnO3). Interestingly, the voltage of H0.25Li1.75−xMnO3 gives constant voltage of 4.52∼4.57 V (Table S3), which is almost identical to the voltage of Li2−xMnO3 (∼4.6 V). Figure 9b shows the calculated XANES spectra of Mn K-edge in the HxLi2−xMnO3 structures. As delithiation proceeds, the maximum peak decreases without shifting to higher energy. Moreover, the intensity of the spectrum in the range of 10−16 eV continues to increase. These peak shifts and intensity changes of the HxLi2−xMnO3 structures are different from the experimental XANES spectral changes measured at ambient temperature

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04937. Schematic view of the fabricated in situ cell for the XANES, energy and charge variation of O-loss structures, atomic charge variations in H xLi2‑xMnO3 , crystal structure of H0.25Li1.75MnO3, change of lattice parameters, total energies for the construction of Li−Mn−O phase diagram, and calculated voltage changes in H0.25Li1.75‑xMnO3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. ORCID

Eunseog Cho: 0000-0001-5308-8278 Kyoungmin Min: 0000-0002-1041-6005 Notes

The authors declare no competing financial interest.

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REFERENCES

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Overview of the Oxygen Behavior in the Degradation of Li2MnO3

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