Impact of Structural Transformation on Electrochemical Performances

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C: Energy Conversion and Storage; Energy and Charge Transport

Impact of Structural Transformation on Electrochemical Performances of Li-Rich Cathode Materials: The Case of LiRuO 2

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Feng Zheng, Shiyao Zheng, Peng Zhang, Xiaofeng Zhang, Shunqing Wu, Yong Yang, and Zi-Zhong Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02887 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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The Journal of Physical Chemistry

Impact of Structural Transformation on Electrochemical Performances of Li-Rich Cathode Materials: The Case of Li2RuO3 Feng Zheng, † Shiyao, Zheng, ‡ Peng Zhang, *,§ Xiaofeng Zhang, † Shunqing Wu, *,† Yong Yang, ‡ Zi-zhong Zhu† †Department

of Physics, OSED, Key Laboratory of Low Dimensional Condensed Matter Physics (Department of Education of Fujian Province), Jiujiang Research Institute, Xiamen University, Xiamen 361005, China. ‡State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, and College of Energy, Xiamen University, Xiamen 361005, China. §College of Electronic Science and Technology, Shenzhen University, Shenzhen, Guangdong 518060, China. *email: [email protected] *email: [email protected]

Abstract Exploration of Li-rich transition-metal oxides with active anionic redox reaction has paved a promising way for the design of high-capacity Li-ion battery cathode materials. In the present work, we show that our predicted two-step structural transformation and resulted structural evolution for Li2RuO3 agrees well with our experiments. The anionic oxidation occurs upon delithiation, the reduction of structural stability would initiate cation migration from Li-Ru layers to adjacent Li layers, which can in turn re-stabilize the delithiated structures and suppress the oxygen evolution, providing a good explanation on the observed high reversible capacity. However, the gradual migration of cation will cause a serious voltage decay of Li2RuO3 upon cycling, as well as poor cycling kinetics, due to the fact that migrating cations in Li layers impede the Li diffusion. Our finding significantly broaden the current understanding on the electrochemistry of the Li2RuO3 and provide important guideline for the future design of Li-rich transition-metal oxides as high-capacity cathode materials.

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1. Introduction Li-ion batteries (LIBs) have contributed greatly to the human society over the past 30 years, especially by dominating the portable electronics field.1,2 More recently, new areas of applications for LIBs have quickly emerged, such as powering electric vehicles and assembling grid storage systems for renewable energy sources.3 However, the energy density of commercial LIBs is still not sufficiently high to meet all these applications, which is mainly limited by the capacity of their cathode materials.4 For instance, the two main families of cathode materials for commercial LIBs include the layered transition metal (TM) oxides, such as LiCoO2,5 and polyanionic compounds, such as LiFePO4,6 which both exhibit limited capacities to be below 200 mAh/g, severely impeding their large-scale applications. To address this issue, over the past two decades, enormous efforts have been devoted to developing new cathode materials with higher capacities.7-10 More recently, the discovery of anionic redox activity in Lirich TM oxides has been shown to offer a promising way to achieve this goal.11,12 Taking advantage of the cumulative cationic and anionic redox processes, materials such as Li-rich NMC (Li1+xNiyCozMn1-x-y-zO2) can exhibit a capacity of over 250 mAh/g.13,14 However, the asset provided by such staggering capacities is negated by the capacity fade upon cycling, mainly because of the irreversible loss of lattice oxygen in these materials.15-18 Experimentally, it has been found that unlike 3d Li-rich TM oxides, materials based on the 4d and 5d TMs, such as Li2RuO3, Li2IrO3 and their derivatives, can possess reversible capacities as high as ~260-270 mAh/g, with negligible capacity fade and O2 release upon cycling.19-21 This may be ascribed to the increasing covalency of the TMO bonds in these materials that suppresses the O2 release,22 which indicates that the oxygen redox chemistry can be tuned through covalency modifications. Actually, besides TM-O covalency, structural transformation can also impact the anionic redox chemistry. The evidence has been recently found in 3d TM oxides, such as Li1.17Ni0.21Co0.08Mn0.54O2, exhibiting a high capacity maintained up to 500 cycles, which was ascribed to the TM migration from Li-TM to Li layers.23 These observations 2


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imply that the structural variation must be also considered to understand the cycling performance of Li-rich TM oxides. However, the underlying mechanism behind this phenomenon is still far from clear. On the other hand, it has been demonstrated that the cation migration may impose negative effects on the cathode performances, such as a serious voltage decay,16,24 which should also be carefully concerned with. To address these issues, here we perform a comprehensive study on the structural transformation and its influence on the electrochemical performance of Li-rich TM oxides by using the first-principles calculations combined with experimental measurements. The well-studied Li2RuO3 was chosen as the prototype material for our investigation, taking advantage of the fact that Li2RuO3 has similar structure and electrochemical performance to other Li-rich TM oxides, but simple stoichiometry and redox chemistry.19,25,26 Our results indicate that a two-step structural transformation would occur for Li2RuO3 upon cycling; one is related to the rearrangement of oxygen array when half of the Li ions are extracted, and the other to the cation migration upon further delithiation. The calculated electronic structures reveal that the removal of electrons from the O 2p orbitals of LixRuO3 (x < 1) upon delithiation will lower the stability of its host structure, and trigger the cation migration from Li-Ru layers to adjacent Li ones. As a consequence, the significant change of oxygen local environments can re-stabilize the delithiated structures and effectively suppress the O2 release. This process can be viewed as a structural self-regulation driven simultaneously by the anionic redox reaction in Li-rich oxides, which can explain its high reversible capacity observed in experiments.19 However, some drawbacks like the voltage decay and poor ionic kinetics are also found due to the cation migration based on our theoretical simulations.

2. Computational details Our calculations were carried out by using the projector-augmented wave (PAW)27 representations within density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP).28,29 The exchange and correlation energy was 3



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treated within the spin-polarized generalized gradient approximation (GGA) and parameterized by Perdew-Burke-Ernzerhof formula (PBE).30 The effects due to the localization of the d electrons of the transition metal ions were taken into account with the GGA+U approach of Dudarev et al.31 An effective parameter U-J was set to 4.0 eV for Ru, which has been proved to be a good approximation in Ru-based compounds.24 Wave functions were expanded in plane waves up to a kinetic energy cut-off of 500 eV. Brillouin-zone integrations were approximated by using special k-point sampling of Monkhorst-Pack scheme32 with a k-point mesh resolution of 2π × 0.03 Å ―1. Lattice vectors and atomic coordinates were fully relaxed until the force on each atom was less than 0.01 eV ∙ Å ―1. The activation barriers for the Li migration in Li2RuO3 were calculated with the climbing nudged elastic band (NEB) method33 in a 2 × 1 × 1 supercell containing 16 formula unites (f.u.). For the NEB calculations, the standard GGA functional was used and the lattice constants of a given structure were fixed as their equilibrium values, with all the internal degrees of freedom fully relaxed. The relative stability of LixRuO3 polymorphs at each Li composition x was evaluated by 𝑥

𝑥

∆𝐸𝑓 = 𝐸(𝐿𝑖𝑥𝑅𝑢𝑂3) ―[2𝐸(𝐿𝑖2𝑅𝑢𝑂3) +(1 ― 2)𝐸(𝑅𝑢𝑂3)],

(1)

where E is the calculated total energy for a given structure. The average voltage (V) vs. Li/Li+ was calculated as Etot(Li𝑥2Host) - Etot(Li𝑥1Host) - (𝑥2 - 𝑥1)Etot[Li] , 𝑉= (𝑥 - 𝑥 )𝑒 2

1

(2)

where x2 and x1 are the Li composition before and after the lithium extraction from the host structure, respectively. The reaction enthalpy associated with the formation of oxygen vacancy in LixRuO3 was calculated by ∆H =

𝐸𝑡𝑜𝑡 (𝐿𝑖𝑥𝑅𝑢𝑂3) ― 𝐸𝑡𝑜𝑡(𝐿𝑖𝑥𝑅𝑢𝑂3 ― 𝑦) ― (𝑦/2)𝐸𝑂2 𝑦/2

.

(3)

Here, we first calculated the total energy of a single oxygen molecule (𝐸𝑂2) in a 20 × 20 × 20 Å3 periodic box and the correction proposed by Ceder et al.34 was then 4


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added to

𝐸𝑂2. The free energy of the reaction was finally evaluated by

∆G = ∆H ― T∆S, with T∆S = 0.63 eV for the O2 gas under standard conditions obtained from JANAF thermochemical Table.35

3. Results and discussion 3.1. Structural evolution and voltage profile We start with the discussion of the structural transformation of LixRuO3 (0≤x≤2) during the electrochemical cycling. Four different LixRuO3 polymorphs, including two pristine structures (the C2/c and 𝑅3 polymorphs) and two cycled structures (the C2/ccycl and 𝑅3-cycl polymorphs), were considered (the detailed structural information are given in Supporting Information). Figure 1 plots the calculated formation energies of these LixRuO3 polymorphs, as a function of Li content (x). It is clear to see that the polymorph with space group C2/c (denoted as C2/c LixRuO3) is most stable at x = 2, which is consistent with the experimental evidences that the synthesized Li2RuO3 always crystallized in a monoclinic C2/c structure.19 As Li ions are extracted, we found a stable intermediate phase between x = 2 and x = 1 (at x = 1.25), which indicates that the first Li extraction should correspond to two two-phase reactions. When more Li ions are extracted, i.e. 0≤x≤1, there are several intermediate phases on the convex hull, indicating a possible solid-solution reaction in this region. However, we note that the calculations show another polymorph with space group 𝑅3 has a lower formation energy than its C2/c counterpart at 𝑥 = 1, as illustrated in Figure 1, which suggests that a structural transformation from the C2/c to 𝑅3 polymorph would occur at this composition. Previous experimental results showed that in Ru-based Li-rich oxides the Ru ions tend to migrate from Li-Ru layers to adjacent Li layers at high charge states, causing a serious voltage fade of these materials.24 Here, our calculations indicate that the Ru migration would not occur in LixRuO3 when x > 1, due to the relatively high formation energy of the resultant structures (i.e. C2/c-cycl and 𝑅3-cycl structures), but 5



it is energetically favourable at high charge states (e.g. x = 0), as illustrated in Figure 1, in agreement with experiments. Moreover, it has also shown that the Ru migration in Ru-based Li-rich oxides is an irreversible process, with the cation accumulation increases gradually upon charge-discharge cycles.24 This irreversibility may be attributed to the hysteresis in the migration pathways of TM ions during the chargedischarge process.23 According to the experiments, for the first discharge and subsequent cycles, we assume that the Ru migration back to Li-Ru layers cannot occur, thus the structural evolution would depend only on the relative stability between the C2/c-cycl and 𝑅3-cycl LixRuO3 polymorphs. As shown in Figure 1, our calculations show that the 𝑅3-cycl polymorph is more stable than the C2/c-cycl polymorph for x = 0, 1 and 1.25, while the C2/c-cycl polymorph becomes stable at x = 2, which reveals that a structural transformation from the 𝑅3-cycl to C2/c-cycl polymorphs would occur when we reinsert all the Li ions back.

Figure 1. Calculated formation energy of LixRuO3 at different Li concentrations. The olive line indicates the convex hull based on the C2/c polymorph.

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Figure 2. (a) Experimentally measured charge-discharge voltage profiles of Li2RuO3 at a current density of 10 mA/g between 2.0 and 4.6 V combined with the calculated voltage profile for the first charge cycle. (b) Ex situ XRD patterns of Li2RuO3 at different preset voltages during the charge-discharge process (traces i-vii in (a)). The peaks with asterisk represent current collector Al, which have some overlap with that of sample i and ii. (c) Calculated XRD patterns for different LixRuO3 polymorphs. 7



In order to test this theoretical prediction of the structural evolution of Li2RuO3, the experimental measurements have also been carried out (the method is given in Supporting Information). Figure 2a shows the voltage profiles of Li2RuO3 for the first five cycles between 2.0 V and 4.6 V, together with the calculated voltage plateaus for the first charge process. In the first charge process, the experimental measurements show three voltage plateaus, which is consistent with the calculations, as shown in Figure 2a. The first two plateaus correspond to the first Li extraction from Li2RuO3, suggesting that two stable intermediate phases must exist, which are found to be at the compositions of 𝑥 ≈ 1.4 and 1, respectively, which is also in agreement with the theoretical prediction. For the second Li extraction, only one plateau is found, indicating that Li2RuO3 would experience a two-phase reaction upon further delithiation. In the first discharge and subsequent cycles, on the other hand, only Sshape voltage profiles is observed, indicating that a significant structural change must occur at the end of the first charge process. The ex-situ XRD patterns of Li2RuO3 at different preset voltages (traces i-vii in Figure 2a) during the charge-discharge cycles are given in Figure 2b. The XRD of fully lithiated Li2RuO3 (trace “i” in Figure 2a) is indexed to a layered monoclinic C2/c structure, consistent with the previous report.19 When charged to 3.6 V (upon 0.6 Li+ extraction), the diffraction peaks of the lithiated Li2RuO3 structure are still visible. Thus, the structure of Li1.4RuO3 can also be indexed to the same C2/c structure, although with slightly different lattice parameters.36,37 This result agrees qualitatively with our calculations which predict that a stable Li1.25RuO3 phase should exist and crystalize into the C2/c structure, as shown in Figure 1. The calculated XRD pattern of Li1.25RuO3 is given in Figure 2c (and Figure S1), which is also comparable to that of Li1.4RuO3. The slight difference between the XRD patterns of these two structures should be ascribed to their different Li contents and lattice parameters. When charged to 4.0 V (about half of the Li ions are extracted), a new set of diffraction peaks can be observed at ~ 36°, ~ 41° and ~ 53°, indicating a clear structural change. Comparing the experimental and 8


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calculated XRD patterns, we see that the LixRuO3 structure may transform from the C2/c to 𝑅3 polymorph at 𝑥 = 1, as depicted in Figure 2c. This structural transformation agrees with our calculations that the 𝑅3 structure has a lower energy than the C2/c structure for LiRuO3. Upon further delithiation, the calculations reveal that the cycled phase (𝑅3-cycl RuO3) will be more stable than the pristine one (𝑅3 RuO3), as shown in Figure 1. The calculated XRD pattern of 𝑅3-cycl RuO3 is slightly different from that of 𝑅3 LiRuO3, with only a shift of the ~19° peak (see Figure 2c). This result coincides with the experimental observation that the first diffraction peak shifts to a higher angle of ~19°, when further charged to high voltage of 4.6 V, which suggests a reduction of lattice parameter c due to cation migration as discussed below. When discharged to 2.0 V in the first cycle, the change of XRD patterns indicates that Li2RuO3 would undergo another structural change, as shown in Figure 2b. The experimental XRD pattern agrees with the calculated XRD pattern for C2/c-cycl Li2RuO3, as shown in Figure 2c (also see Figure S2), indicating a structural transformation from the 𝑅3-cycl to C2/c-cycl polymorphs, consistent with the theoretical prediction. Moreover, in the subsequent cycles, the XRD data show that this structural transformation is reversible (Figure S2 and Figure S3), which explains why the S-shape profiles can be uniquely observed as depicted in Figure 2a. It has been reported previously that this topical variation of voltage profiles in Lirich TM oxides is strongly correlated to the TM migration, which would occur only at high charge states.24 Our calculations also reveal that the Ru migration in LixRuO3 would only occur when x < 1. To further demonstrate this result, we set the cutoff voltage to be < 3.8 V, which will presumably not trigger the Ru migration. The measured voltage profiles show that the staircase-like curves are maintained for five charge-discharge cycles, demonstrating the absence of significant structural transformation (Figure S4). Furthermore, the previous reports suggested that, for Lirich TM oxides, a voltage decay would be associated with the gradual accumulation of TM ions in Li layers due to TM migration ,24,38 which can be also found in our experiments (see Figure 2a). However, as the voltage cutoff is set to be below 3.8 V to 9



decrease the Ru migration during cycles, the voltage decay is found to be reduced (Figure S5). This finding is further reinforced by our theoretical calculations. We have compared the amount of voltage fade in Li2RuO3 with different degrees of Ru migration, where the voltage fade is found to increase as more Ru ions migrate into Li layers (Figure S6). Figure 3 plots a schematic diagram of the whole structural evolution process for LixRuO3, with the lattice parameters of the corresponding phases listed in Table 1. The structure of C2/c Li2RuO3 can be viewed as a derivation from that of layered oxides (LiMO2) by substituting excess Li ions for Ru ions in RuO2 layers. Upon delithiation, Li ions in Li layers will be extracted firstly and vacancies left are then simultaneously compensated by the diffusion of Li ions from Li-Ru layers to the tetrahedral sites (TdLi sites) in Li layers. This delithiation process is similar with the other Li-rich TM oxides.39 When half of the Li ion are extracted, LixRuO3 will transform from the C2/c to 𝑅3 polymorphs, which is attributed to the rearrangement of oxygen array from cubic to hexagonal close packing.36 This phase transition will lead to the variation of structural symmetry. Therefore, the obvious lattice parameters change can be observed between the C2/c Li1.25RuO3 and 𝑅3 LiRuO3, as shown in Table 1, which is due to the different choice of unit cell in the two different structural symmetry. However, the change of volume is small (< 7%) during the charge-discharge cycle as seen in Table 1. This suggests that the effect of stress and strain in this material during the lithiationdelithiation cycle would be small. When delithiated to RuO3, the 𝑅3 structure will transform to the 𝑅3-cycl structure with a reduced lattice parameter c, as shown in Table 1 (14.43 Å of 𝑅3 LiRuO3 vs 13.90 Å of 𝑅3-cycl RuO3). This variation of lattice parameter c is quite different from other layered oxides, for which the lattice parameter c is usually increased upon delithiation, due to the expansion of the interlayer space, caused by the enhanced repulsion between the two neighboring oxygen layers. However, if the cation migration is taken into account, this abnormal lattice variation can be understood. As shown in Figure 3, for the 𝑅3-cycl RuO3, Ru ions migrate to Li layers and therefore the contraction of the lattice parameter c can be attributed to the 10


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shorter Ru-O bonds in 𝑅3-cycl RuO3, compared to the Li-O bonds in 𝑅3 LiRuO3 (Figure S7). In subsequent cycles, LixRuO3 will undergo the reversible structural transformation from the 𝑅3-cycl to C2/c-cycl polymorphs, as denoted by the red arrows in Figure 3.

Figure 3. Schematic illustration of the charge-discharge process for LixRuO3. Black and red arrows represent the initial charge process and subsequent cycles, respectively.

Table 1. Calculated lattice parameters and volume change of different LixRuO3 polymorphs as given in Figure 3.

Li2RuO3

Li1.25RuO3

LiRuO3

Space

(a, b, c)

(α, β, γ)

Volume

Volume

group

(Å)

(deg)

(Å3/f.u.)

change (%)

C2/c

5.17, 9.00, 9.90

90.0, 100.2, 90.0

56.68

0

C2/c-cycl

10.41, 9.00, 9.95

90.0, 100.3, 89.8

57.26

1.02

C2/c

5.23, 8.93, 9.84

89.8, 97.5, 90.1

56.77

0.16

𝑅3

10.28, 10.28, 14.43

90.0, 90.0, 120.0

55.03

-2.92

𝑅3-cycl

10.24, 10.24, 14.17

89.6, 90.5, 120.1

53.53

-5.56

11



RuO3

𝑅3-cycl

10.45, 10.22, 13.90

88.4, 91.1, 121.1

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52.97

-6.55

3.2. Effect of cation migration on the stability of oxygen redox couple It has been reported that the cation migration would have a significant impact on the anionic redox chemistry in Li-rich TM oxides.23 To understand this phenomenon, we next turn to examine the effect of cation migration on the stability of oxygen redox in LixRuO3, especially with respect to the formation of O vacancy (VO). We will focus on the delithiated phases LiRuO3 and RuO3, since the fully lithiated phase Li2RuO3 is well known to be stable against O2 release. For 𝑅3 LiRuO3, a positive formation energy G(VO) = 1.40 eV is found, implying that the VO cannot form easily, so the host structure of this compound is stable. While for 𝑅3 RuO3, we found that G(VO) = -4.06 eV becomes negative, suggesting a spontaneous formation of VO. Taking the Ru migration into account, however, a drastic increase of G(VO) to 0.18 eV for 𝑅3-cycl RuO3 was found, indicating a significant stabilization effect of cation migration against O2 release, which coincides with the experimental observations.23 To reveal the underlying mechanism, we systematically investigate the electronic structural evolution of LixRuO3 (x = 1, 0). Figure 4 plots the calculated projected density of states (PDOS) for LixRuO3. The results show that 𝑅3 LiRuO3 is a semiconductor with a band gap of ~1.1 eV. The valence band maximum (VBM) is dominated by O 2p states, mixed with some Ru 4d states. That is to say, upon further delithiation, holes will be introduced greatly into the O 2p band. This can be clearly seen in the right panel of Figure 4b, which illustrates distinct unoccupied O 2p states above the Fermi level. As predicted by Zhang et al.,40 these empty states can accommodate electrons dropping from the defect level induced by the formation of VO and thus gain energy, which would balance the energy cost for breaking the Ru-O bonds and therefore facilitate the formation of VO. Nevertheless, this unstable 𝑅3 RuO3 will trigger the cation migration to reconstruct its electronic structure. The 𝑅3-cycl RuO3 changes back to a semiconductor, as shown in the right panel of Figure 4c. As a result, no energy gain would be expected 12


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as VO forms, which leads to a positive G(VO). We find that this electronic structure variation from 𝑅3 RuO3 to 𝑅3-cycl RuO3 is relate closely to the change of oxygen local environment. For 𝑅3 RuO3, all the oxygen atoms are bonded to two Ru atoms and the RuO6 octahedra all connect to each other to form a honeycomb-like ordering, as shown in the left panel of Figure 4b. The Ru-O bonds of 𝑅3 RuO3 are in the range of 1.928~1.930 Å (Figure S7). However, the cation migration will break honeycomb structure and change oxygen local environment. As illustrated in the left panel of Figure 4c, the coordination number of oxygen can be 1, 2 and 3 in 𝑅3-cycl RuO3. Moreover, the increased Ru-O bonds (1.679~2.280 Å, as shown in Figure S7) can also be found. Consequently, the variation of bonding property and the associated larger distortion of RuO6 octahedra cause a significant reconstruction of the electronic structure and lower the energy of 𝑅3-cycl RuO3, which essentially suppresses the formation of VO.

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Figure 4. Crystal structure and calculated partial density of states (PDOS) of (a) 𝑅3 LiRuO3, (b) 𝑅3 RuO3 and (c) 𝑅3-cycl RuO3. The Li, Ru and O ions are represented by green, gray and red balls, respectively. The green, blue and black cycles represent the coordination number of O ions to be one, two and three, respectively.

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Figure 5. The calculated kinetic properties of Li2RuO3. Schematic diagrams for four typical paths of (a) pristine C2/c structure and (b) its cycled structure. Li atoms are drawn by small green balls and Ru atoms are colored in grey, to simplify the presentation, oxygen and Li atoms of Li-Ru layers in top view are not drawn. The path 4 of cycled structure is along the C-H-I due to G site occupied by Ru atom. Calculated activation barrier along four major paths: (c) path 1, (d) path 2, (e) path 3 and (f) path 4. 15



3.3. Effect of cation migration on the Li ion diffusion in Li2RuO3 Beside the anionic redox stability, the poor kinetics shed another limit to the practical applications of Li-rich TM oxides, which was proposed to originate from the insulating nature of these materials.41-43 However, aside from the electronic conductivity, the ionic conductivity should also play an important role in determining the kinetics, which is actually the dominate factor in most of the cathode materials.9,41,44 To estimate the impact of cation migration on the kinetics of Li-rich TM oxides, the climbing NEB calculations were used to investigate the Li diffusion in C2/c Li2RuO3 and its cycled structure, as shown in Figure 5. For C2/c Li2RuO3, four most probable Li diffusion paths were considered, as shown in Figure 5a: path 1 – from Li-Ru to Li layers (A-B); path 2 – a straight trajectory along b-axis in Li layers (B-C-D); path 3 – a zigzag trajectory along the a-axis in Li layers (E-B-F), which needs to pass through one Ru-Ru dumbbell in each hop; path 4 – another zigzag trajectory along a-axis in Li planes (G-C-H), which needs to pass through two Ru-Ru dumbbells in each hop. The calculated activation barriers (Ea) along these paths in C2/c Li2RuO3 are given in Figure 5c-f. We found that the Ea for the Li diffusion from Li-Ru to Li layers is 0.43 eV that is smaller than that in C2/m Li2MnO3.41 In Li layers, the Ea are calculated to be 0.65, 0.38 and 0.59 eV for the path 2, path 3 and path 4, respectively. It is interesting to see that the smallest Ea found here is comparable to that of LiCoO245 and LiFePO4,46 which should guarantee good ionic conductivity in this material. However, as the cation migration occurs the case changes dramatically. Here, we considered a cycled structure with a single Ru atom placed into Li layers within a 2 × 1 × 1 supercell as shown in Figure 5b. Although our model is by no means exhaustive, the influence on the Li diffusion can still be rationalized. The results show that Ea increases for all the four diffusion paths because of the large electrostatic repulsion between Li+ and Ru4+, especially for path 3, where Ea is increased to 0.71 eV, as shown in Figure 5c-f. The activation barriers and diffusion coefficients of these two structures are summarized in Table 2. The cycled structure has a relatively smaller diffusion coefficient, indicating a poor Li diffusion. From these results, one can naturally expect that as more Ru ions 16


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migrating into Li layers, the ionic conductivity of Li2RuO3 will drop markedly, leading to a poor overall kinetics of this material. Since the cation migration was found commonly in Li-rich TM oxides, we suspect that the ionic diffusion, rather than the electronic conductivity, may act as the major limit to their kinetic performances. Table 2. Calculated activation barriers (Ea) and estimated diffusion coefficients (D300k) for various migration pathway in C2/c Li2RuO3 and its cycled structure from climbing NEB methods. (D = 𝑑2𝜈 ∙ exp

( ), where 𝜈 = 10 𝐸𝑎

13

𝑘𝐵𝑇

Hz is assumed for estimation, and

d is the hopping distance.) Structure

Ea (eV)

D300K (cm2/s)

C2/c Li2RuO3 Cycled Li2RuO3

0.43/0.65/0.38/0/59 0.45/0.70/0.71/0.72

10-10/10-13/10-8/10-12 10-10/10-14/10-14/10-14

4. Conclusions We have carried out the first-principles calculations, combined with the experimental measurements, to study the structural transformation and its effect on electrochemical performance of Li2RuO3, a prototypical family member of Li-rich TM oxides. We identify a complex two-step structural transformation for the first charge process in LixRuO3, i.e., the first is related to the rearrangement of oxygen array occurring at x = 1 (upon half delithiation), following by the Ru migration from Li-Ru to Li layers upon further delithiation (x < 1). The calculated voltage profiles and XRD patterns show good agreement with our experimental measurements. For the subsequent cycling processes, our results indicate that Li2RuO3 will undergo a reversible structural transition between the 𝑅3-cycl and C2/c-cycl polymorphs, with the migrating Ru gradually accumulating in Li layers. This cation migration and accumulation in Li layers is found to be caused by the anionic redox reaction of Li2RuO3. As more than one Li ions are extracted from Li2RuO3, holes are introduced into the O 2p bands, which tends to destabilize the host structure of the material. As a response, we found that the Ru migration can be triggered to re-stabilize the anionic redox chemistry against oxygen evolution. This is mainly because of the significant 17



change of oxygen local environments and the resultant large distortion of RuO6 octahedron after cation migration, which can reconstruct the electronic structure and thus lower the energy of system. Therefore, the Ru migration can be viewed as a structural self-regulation triggered by the anionic redox reaction, which can explain a high capacity maintained upon prolonged cycling in Li2RuO3.19 However, on the other hand, our calculations also indicate that the gradual accumulation of Ru ions in Li layers will lead to a serious voltage fade upon cycling, which lowers the energy output of the materials, and to a poor Li diffusion. Therefore, further extensive studies may be still needed to achieve the trade-off between the reversible capacity and other electrochemical properties of Li-rich TM oxides.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx. Details of structural information and experimental section, comparisons of experimental and calculated XRD, charge-discharge voltage profiles of Li2RuO3 for the first five cycles between 2.0 and 3.8 V, average voltage of Li2RuO3 for the first 20 cycles, calculated average voltages of Li2RuO3 in different percentage of Ru migration, and average Li-O and Ru-O bond lengths in LixRuO3 (x = 1, 0).

Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 11874307, 21761132030), the National Key R&D Program of China (No. 2016YFB0901502, 2016YFA0202601), the Fundamental Research Funds for the Central Universities (Grant No. 20720180020), and the Supercomputing Center of the USTC.

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Impact of Structural Transformation on Electrochemical Performances

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