Impact of Structural Transformation on ElectrochemicalPerformances of Li-Rich Cathode Materials: The Case of Li2RuO3

Feng Zheng,† Shiyao Zheng,‡ Peng Zhang,*,§ Xiaofeng Zhang,† Shunqing Wu,*,† Yong Yang,‡

and Zi-zhong Zhu†

†Department of Physics, OSED, Key Laboratory of Low Dimensional Condensed Matter Physics (Department of Education ofFujian Province), Jiujiang Research Institute, and ‡State Key Laboratory for Physical Chemistry of Solid Surfaces, CollaborativeInnovation 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

*S Supporting Information

ABSTRACT: Exploration of Li-rich transition-metal (TM) oxideswith active anionic redox reaction has paved a promising way for thedesign of high-capacity Li-ion battery cathode materials. In thepresent work, we show that our predicted two-step structuraltransformation and resulted structural evolution for Li2RuO3 agreewell with our experiments. The anionic oxidation occurs upondelithiation, and the reduction of structural stability would initiatecation migration from Li−Ru layers to adjacent Li layers, which canin turn restabilize the delithiated structures and suppress the oxygenevolution, providing a good explanation on the observed highreversible capacity. However, the gradual migration of cations willcause a serious voltage decay of Li2RuO3 upon cycling, as well aspoor cycling kinetics because of the fact that migrating cations in Lilayers impede the Li diffusion. Our finding significantly broadens the current understanding on the electrochemistry of Li2RuO3and provides important guidelines for the future design of Li-rich TM oxides as high-capacity cathode materials.

1. INTRODUCTION

Li-ion batteries (LIBs) have contributed greatly to the humansociety over the past 30 years, especially by dominating theportable electronics field.1,2 More recently, new areas ofapplications for LIBs have quickly emerged, such as poweringelectric vehicles and assembling grid storage systems forrenewable energy sources.3 However, the energy density ofcommercial LIBs is still not sufficiently high to meet all theseapplications, which is mainly limited by the capacity of theircathode materials.4 For instance, the two main families ofcathode materials for commercial LIBs include the layeredtransition-metal (TM) oxides, such as LiCoO2,

5 andpolyanionic compounds, such as LiFePO4,

6 which both exhibitlimited capacities to be below 200 mA h/g, severely impedingtheir large-scale applications. To address this issue, over thepast two decades, enormous efforts have been made to developnew cathode materials with higher capacities.7−10 Morerecently, the discovery of anionic redox activity in Li-richTM oxides has been shown to offer a promising way to achievethis goal.11,12 Taking advantage of the cumulative cationic andanionic redox processes, materials such as Li-rich NMC(Li1+xNiyCozMn1−x−y−zO2) can exhibit a capacity of over 250mA h/g.13,14 However, the asset provided by such staggeringcapacities is negated by the capacity fade upon cycling, mainly

because of the irreversible loss of lattice oxygen in thesematerials.15−18

Experimentally, it has been found that unlike 3d Li-rich TMoxides, materials based on the 4d and 5d TMs, such asLi2RuO3, Li2IrO3, and their derivatives, can possess reversiblecapacities as high as ∼260−270 mA h/g, with negligiblecapacity fade and O2 release upon cycling.19−21 This may beascribed to the increasing covalency of the TM−O bonds inthese materials, which suppresses the O2 release,22 whichindicates that the oxygen redox chemistry can be tunedthrough covalency modifications. Actually, besides TM−Ocovalency, structural transformation can also impact theanionic redox chemistry. The evidence has been recentlyfound in 3d TM oxides, such as Li1.17Ni0.21Co0.08Mn0.54O2,exhibiting a high capacity maintained up to 500 cycles, whichwas ascribed to the TM migration from Li−TM to Li layers.23

These observations imply that the structural variation mustalso be considered to understand the cycling performance ofLi-rich TM oxides. However, the underlying mechanismbehind this phenomenon is still far from clear. On the otherhand, it has been demonstrated that the cation migration may

Received: March 27, 2019Revised: May 13, 2019Published: May 15, 2019

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2019, 123, 13491−13499

© 2019 American Chemical Society 13491 DOI: 10.1021/acs.jpcc.9b02887J. Phys. Chem. C 2019, 123, 13491−13499

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impose negative effects on the cathode performances, such as aserious voltage decay,16,24 which should also be carefullyconcerned with.To address these issues, here we perform a comprehensive

study on the structural transformation and its influence on theelectrochemical performance of Li-rich TM oxides by using thefirst-principles calculations combined with experimentalmeasurements. The well-studied Li2RuO3 was chosen as theprototype material for our investigation, taking advantage ofthe fact that Li2RuO3 has similar structure and electrochemicalperformance to other Li-rich TM oxides but simplestoichiometry and redox chemistry.19,25,26 Our results indicatethat a two-step structural transformation would occur forLi2RuO3 upon cycling: one is related to the rearrangement ofoxygen array when half of the Li ions are extracted and theother to the cation migration upon further delithiation. Thecalculated electronic structures reveal that the removal ofelectrons from the O 2p orbitals of LixRuO3 (x < 1) upondelithiation will lower the stability of its host structure andtrigger the cation migration from Li−Ru layers to adjacent Liones. As a consequence, the significant change of oxygen localenvironments can restabilize the delithiated structures andeffectively suppress the O2 release. This process can be viewedas a structural self-regulation driven simultaneously by theanionic redox reaction in Li-rich oxides, which can explain itshigh reversible capacity observed in experiments.19 However,some drawbacks such as the voltage decay and poor ionickinetics are also found because of the cation migration basedon our theoretical simulations.

2. COMPUTATIONAL DETAILSOur calculations were carried out by using the projector-augmented wave27 representations within density functionaltheory as implemented in the Vienna ab initio simulationpackage.28,29 The exchange and correlation energy was treatedwithin the spin-polarized generalized gradient approximation(GGA) and parameterized by Perdew−Burke−Ernzerhofformula.30 The effects due to the localization of the d electronsof the TM ions were taken into account with the GGA + Uapproach of Dudarev et al.31 An effective parameter U−J wasset to 4.0 eV for Ru, which has been proved to be a goodapproximation in Ru-based compounds.24 Wave functionswere expanded in plane waves up to a kinetic energy cutoff of500 eV. Brillouin-zone integrations were approximated byusing special k-point sampling of Monkhorst−Pack scheme32

with a k-point mesh resolution of 2π × 0.03 Å−1. Latticevectors and atomic coordinates were fully relaxed until theforce 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 units(f.u.). For the NEB calculations, the standard GGA functionalwas used and the lattice constants of a given structure werefixed as their equilibrium values, with all the internal degrees offreedom fully relaxed.The relative stability of LixRuO3 polymorphs at each Li

composition x was evaluated by

E Ex

Ex

E

(Li RuO )2

(Li RuO ) 12

(RuO )

xf 3 2 3

3

Δ = − + −Ä

ÇÅÅÅÅÅÅÅÅ

ikjjj

y{zzz

É

ÖÑÑÑÑÑÑÑÑ (1)

where E is the calculated total energy for a given structure.The average voltage (V) versus Li/Li+ was calculated as

VE E x x E

x x e

(Li host) (Li host) ( ) Li

( )x xtot tot 2 1 tot

2 1

2 1= −− − − [ ]

−(2)

where x2 and x1 are the Li composition before and after thelithium extraction from the host structure, respectively.The reaction enthalpy associated with the formation of

oxygen vacancy in LixRuO3 was calculated by

HE E y E

y

(Li RuO ) (Li RuO ) ( /2)

/2x x ytot 3 tot 3 O2Δ =

− −−

(3)

Here, we first calculated the total energy of a single oxygenmolecule (EO2

) in a 20 × 20 × 20 Å3 periodic box, and the

correction proposed by Ceder et al.34 was then added to EO2.

The free energy of the reaction was finally evaluated by ΔG =ΔH − TΔS, with TΔS = 0.63 eV for the O2 gas under standardconditions obtained from JANAF thermochemical table.35

3. RESULTS AND DISCUSSION3.1. Structural Evolution and Voltage Profile. We start

with the discussion of the structural transformation of LixRuO3(0 ≤ x ≤ 2) during the electrochemical cycling. Four differentLixRuO3 polymorphs, including two pristine structures (theC2/c and R3 polymorphs) and two cycled structures (the C2/c-cycl and R3-cycl polymorphs), were considered (the detailedstructural information is given in the Supporting Information).Figure 1 plots the calculated formation energies of these

LixRuO3 polymorphs as a function of Li content (x). It is clearto see that the polymorph with space group C2/c (denoted asC2/c LixRuO3) is most stable at x = 2, which is consistent withthe experimental lines of evidence that the synthesizedLi2RuO3 always crystallized in a monoclinic C2/c structure.19

As Li ions are extracted, we found a stable intermediate phasebetween x = 2 and x = 1 (at x = 1.25), which indicates that thefirst Li extraction should correspond to two two-phasereactions. When more Li ions are extracted, that is, 0 ≤ x ≤1, there are several intermediate phases on the convex hull,indicating a possible solid-solution reaction in this region.

Figure 1. Calculated formation energy of LixRuO3 at different Liconcentrations. The olive line indicates the convex hull based on theC2/c polymorph.

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However, we note that the calculations show anotherpolymorph with space group R3 that has a lower formationenergy than its C2/c counterpart at x = 1, as illustrated inFigure 1, which suggests that a structural transformation fromthe C2/c to R3 polymorph would occur at this composition.Previous experimental results showed that in Ru-based Li-richoxides the Ru ions tend to migrate from Li−Ru layers toadjacent Li layers at high charge states, causing a seriousvoltage fade of these materials.24 Here, our calculationsindicate that the Ru migration would not occur in LixRuO3when x > 1 because of the relatively high formation energy ofthe resultant structures (i.e., C2/c-cycl and R3-cycl structures),but it is energetically favorable 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-basedLi-rich oxides is an irreversible process, with the cationaccumulation increasing gradually upon charge−dischargecycles.24 This irreversibility may be attributed to the hysteresisin the migration pathways of TM ions during the charge−discharge process.23 According to the experiments, for the firstdischarge and subsequent cycles, we assume that the Rumigration back to Li−Ru layers cannot occur, thus thestructural evolution would depend only on the relative stabilitybetween the C2/c-cycl and R3-cycl LixRuO3 polymorphs. Asshown in Figure 1, our calculations show that the R3-cyclpolymorph is more stable than the C2/c-cycl polymorph for x =0, 1, and 1.25, whereas the C2/c-cycl polymorph becomesstable at x = 2, which reveals that a structural transformationfrom the R3-cycl to C2/c-cycl polymorphs would occur whenwe reinsert all the Li ions back.In order to test this theoretical prediction of the structural

evolution of Li2RuO3, the experimental measurements havealso been carried out (the method is given in the SupportingInformation). Figure 2a shows the voltage profiles of Li2RuO3for the first five cycles between 2.0 and 4.6 V, together with thecalculated voltage plateaus for the first charge process. In thefirst charge process, the experimental measurements showthree voltage plateaus, which are consistent with thecalculations, as shown in Figure 2a. The first two plateauscorrespond to the first Li extraction from Li2RuO3, suggestingthat two stable intermediate phases must exist, which are foundto be at the compositions of x ≈ 1.4 and 1, respectively, whichis also in agreement with the theoretical prediction. For thesecond Li extraction, only one plateau is found, indicating thatLi2RuO3 would experience a two-phase reaction upon furtherdelithiation. In the first discharge and subsequent cycles, onthe other hand, only S-shape voltage profiles are observed,indicating that a significant structural change must occur at theend of the first charge process.The ex situ X-ray diffraction (XRD) patterns of Li2RuO3 at

different preset voltages (traces i−vii in Figure 2a) during thecharge−discharge cycles are given in Figure 2b. The XRD offully lithiated Li2RuO3 (trace “i” in Figure 2a) is indexed to alayered monoclinic C2/c structure, consistent with theprevious report.19 When charged to 3.6 V (upon 0.6 Li+

extraction), the diffraction peaks of the lithiated Li2RuO3structure are still visible. Thus, the structure of Li1.4RuO3can also be indexed to the same C2/c structure, although withslightly different lattice parameters.36,37 This result agreesqualitatively with our calculations which predict that a stableLi1.25RuO3 phase should exist and crystallize into the C2/cstructure, as shown in Figure 1. The calculated XRD pattern ofLi1.25RuO3 is given in Figure 2c (and Figure S1), which is also

comparable to that of Li1.4RuO3. The slight difference betweenthe XRD patterns of these two structures should be ascribed totheir different Li contents and lattice parameters. Whencharged to 4.0 V (about half of the Li ions are extracted), anew set of diffraction peaks can be observed at ∼36°, ∼41°,and ∼53°, indicating a clear structural change. Comparing theexperimental and calculated XRD patterns, we see that theLixRuO3 structure may transform from the C2/c to R3polymorph at x = 1, as depicted in Figure 2c. This structuraltransformation agrees with our calculations that the R3structure has a lower energy than the C2/c structure forLiRuO3. Upon further delithiation, the calculations reveal thatthe cycled phase (R3-cycl RuO3) will be more stable than thepristine one (R3 RuO3), as shown in Figure 1. The calculatedXRD pattern of R3-cycl RuO3 is slightly different from that ofR3 LiRuO3, with only a shift of the ∼19° peak (see Figure 2c).This result coincides with the experimental observation thatthe first diffraction peak shifts to a higher angle of ∼19°, whenfurther charged to high voltage of 4.6 V, which suggests a

Figure 2. (a) Experimentally measured charge−discharge voltageprofiles of Li2RuO3 at a current density of 10 mA/g between 2.0 and4.6 V combined with the calculated voltage profile for the first chargecycle. (b) Ex situ XRD patterns of Li2RuO3 at different preset voltagesduring the charge−discharge process [traces i−vii in (a)]. The peakswith asterisk represent current collector Al, which have some overlapwith that of samples (i) and (ii). (c) Calculated XRD patterns fordifferent LixRuO3 polymorphs.

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reduction of lattice parameter c because of cation migration asdiscussed below.When discharged to 2.0 V in the first cycle, the change of

XRD patterns indicates that Li2RuO3 would undergo anotherstructural change, as shown in Figure 2b. The experimentalXRD 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 thatthis structural transformation is reversible (Figures S2 and S3),which explains why the S-shape profiles can be uniquelyobserved as depicted in Figure 2a.It has been reported previously that this topical variation of

voltage profiles in Li-rich TM oxides is strongly correlated tothe TM migration, which would occur only at high chargestates.24 Our calculations also reveal that the Ru migration inLixRuO3 would only occur when x < 1. To further demonstratethis result, we set the cutoff voltage to be <3.8 V, which willpresumably not trigger the Ru migration. The measuredvoltage profiles show that the staircase-like curves aremaintained for five charge−discharge cycles, demonstratingthe absence of significant structural transformation (FigureS4). Furthermore, the previous reports suggested that for Li-rich TM oxides, a voltage decay would be associated with thegradual accumulation of TM ions in Li layers because of TMmigration,24,38 which can also be found in our experiments(see Figure 2a). However, as the voltage cutoff is set to bebelow 3.8 V to decrease the Ru migration during cycles, thevoltage decay is found to be reduced (Figure S5). This findingis further reinforced by our theoretical calculations. We havecompared the amount of voltage fade in Li2RuO3 with different

degrees of Ru migration, where the voltage fade is found toincrease 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 ofthe corresponding phases listed in Table 1. The structure ofC2/c Li2RuO3 can be viewed as a derivation from that oflayered oxides (LiMO2) by substituting excess Li ions for Ruions in RuO2 layers. Upon delithiation, Li ions in Li layers willbe extracted first and vacancies left are then simultaneouslycompensated by the diffusion of Li ions from Li−Ru layers tothe tetrahedral sites (TdLi sites) in Li layers. This delithiationprocess is similar with the other Li-rich TM oxides.39 Whenhalf of the Li ion are extracted, LixRuO3 will transform fromthe C2/c to R3 polymorphs, which is attributed to therearrangement of oxygen array from cubic to hexagonal closepacking.36 This phase transition will lead to the variation ofstructural symmetry. Therefore, the obvious lattice parameterschange can be observed between C2/c Li1.25RuO3 and R3LiRuO3, as shown in Table 1, which is due to the differentchoice of unit cells in the two different structural symmetry.However, the change of volume is small (<7%) during thecharge−discharge cycle as seen in Table 1. This suggests thatthe effect of stress and strain in this material during thelithiation−delithiation cycle would be small. When delithiatedto RuO3, the R3 structure will transform to the R3-cyclstructure with a reduced lattice parameter c, as shown in Table1 (14.43 Å of R3 LiRuO3 vs 13.90 Å of R3-cycl RuO3). Thisvariation of lattice parameter c is quite different from otherlayered oxides, for which the lattice parameter c is usuallyincreased upon delithiation, because of the expansion of theinterlayer space, caused by the enhanced repulsion between thetwo neighboring oxygen layers. However, if the cationmigration is taken into account, this abnormal lattice variation

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

Table 1. Calculated Lattice Parameters and Volume Change of Different LixRuO3 Polymorphs as Given in Figure 3

space group (a, b, c) (Å) (α, β, γ) (deg) volume (Å3/f.u.) volume change (%)

Li2RuO3 C2/c 5.17, 9.00, 9.90 90.0, 100.2, 90.0 56.68 0C2/c-cycl 10.41, 9.00, 9.95 90.0, 100.3, 89.8 57.26 1.02

Li1.25RuO3 C2/c 5.23, 8.93, 9.84 89.8, 97.5, 90.1 56.77 0.16LiRuO3 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 −5.56RuO3 R3-cycl 10.45, 10.22, 13.90 88.4, 91.1, 121.1 52.97 −6.55

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can be understood. As shown in Figure 3, for R3-cycl RuO3, Ruions migrate to Li layers, and therefore, the contraction of thelattice parameter c can be attributed to the shorter Ru−Obonds in R3-cycl RuO3, compared to the Li−O bonds in R3LiRuO3 (Figure S7). In subsequent cycles, LixRuO3 willundergo the reversible structural transformation from the R3-cycl to C2/c-cycl polymorphs, as denoted by the red arrows inFigure 3.3.2. Effect of Cation Migration on the Stability of

Oxygen Redox Couple. It has been reported that the cationmigration would have a significant impact on the anionic redoxchemistry in Li-rich TM oxides.23 To understand thisphenomenon, we next turn to examine the effect of cationmigration 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

because the fully lithiated phase Li2RuO3 is well-known to bestable against O2 release. For R3 LiRuO3, a positive formationenergy G(VO) = 1.40 eV is found, implying that VO cannotform easily, so the host structure of this compound is stable,whereas for R3 RuO3, we found that G(VO) = −4.06 eVbecomes negative, suggesting a spontaneous formation of VO.

Taking the Ru migration into account, however, a drasticincrease of G(VO) to 0.18 eV for R3-cycl RuO3 was found,indicating a significant stabilization effect of cation migrationagainst O2 release, which coincides with the experimentalobservations.23 To reveal the underlying mechanism, wesystematically investigate the electronic structural evolutionof LixRuO3 (x = 1, 0). Figure 4 plots the calculated projecteddensity of states (PDOS) for LixRuO3. The results show thatR3 LiRuO3 is a semiconductor with a band gap of ∼1.1 eV.The valence band maximum is dominated by O 2p states,mixed with some Ru 4d states. That is to say, upon furtherdelithiation, holes will be introduced greatly into the O 2pband. This can be clearly seen in the right panel of Figure 4b,which illustrates distinct unoccupied O 2p states above theFermi level. As predicted by Zhang and Wei,40 these emptystates can accommodate electrons dropping from the defectlevel induced by the formation of VO and thus gain energy,which would balance the energy cost for breaking the Ru−Obonds and therefore facilitate the formation of VO. Never-theless, this unstable R3 RuO3 will trigger the cation migrationto reconstruct its electronic structure. R3-cycl RuO3 changesback to a semiconductor, as shown in the right panel of Figure

Figure 4. Crystal structure and calculated projected 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 Oions to be one, two, and three, respectively.

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4c. As a result, no energy gain would be expected as VO forms,which leads to a positive G(VO). We find that this electronicstructure variation from R3 RuO3 to R3-cycl RuO3 is relatedclosely to the change of oxygen local environment. For R3RuO3, all the oxygen atoms are bonded to two Ru atoms andthe RuO6 octahedra all connect to each other to form ahoneycomb-like ordering, as shown in the left panel of Figure4b. The Ru−O bonds of R3 RuO3 are in the range of 1.928−1.930 Å (Figure S7). However, the cation migration will breakthe honeycomb structure and change the oxygen localenvironment. As illustrated in the left panel of Figure 4c, thecoordination number of oxygen can be 1, 2, and 3 in R3-cyclRuO3. Moreover, the increased Ru−O bonds (1.679−2.280 Å,as shown in Figure S7) can also be found. Consequently, thevariation of bonding property and the associated largerdistortion of the RuO6 octahedra cause a significant

reconstruction of the electronic structure and lower the energyof R3-cycl RuO3, which essentially suppresses the formation ofVO.

3.3. Effect of Cation Migration on the Li Ion Diffusionin Li2RuO3. Besides the anionic redox stability, the poorkinetics shed another limit to the practical applications of Li-rich TM oxides, which was proposed to originate from theinsulating nature of these materials.41−43 However, aside fromthe electronic conductivity, the ionic conductivity should alsoplay an important role in determining the kinetics, which isactually the dominant factor in most of the cathodematerials.9,41,44 To estimate the impact of cation migrationon the kinetics of Li-rich TM oxides, the climbing NEBcalculations were used to investigate the Li diffusion in C2/cLi2RuO3 and its cycled structure, as shown in Figure 5. ForC2/c Li2RuO3, four most probable Li diffusion paths were

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

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considered, as shown in Figure 5a: path 1from Li−Ru to Lilayers (A−B); path 2a straight trajectory along the b-axis inLi layers (B−C−D); path 3a zigzag trajectory along the a-axis in Li layers (E−B−F), which needs to pass through oneRu−Ru dumbbell in each hop; path 4another zigzagtrajectory along the a-axis in Li planes (G−C−H), whichneeds to pass through two Ru−Ru dumbbells in each hop. Thecalculated activation barriers (Ea) along these paths in C2/cLi2RuO3 are given in Figure 5c−f. We found that the Ea for theLi diffusion from Li−Ru to Li layers is 0.43 eV that is smallerthan that in C2/m Li2MnO3.

41 In Li layers, the Ea arecalculated to be 0.65, 0.38, and 0.59 eV for path 2, path 3, andpath 4, respectively. It is interesting to see that the smallest Eafound here is comparable to that of LiCoO2

45 and LiFePO4,46

which should guarantee good ionic conductivity in thismaterial. However, as the cation migration occurs, the casechanges dramatically. Here, we considered a cycled structurewith a single Ru atom placed into Li layers within a 2 ×1 × 1supercell as shown in Figure 5b. Although our model is by nomeans exhaustive, the influence on the Li diffusion can still berationalized. The results show that Ea increases for all the fourdiffusion paths because of the large electrostatic repulsionbetween Li+ and Ru4+, especially for path 3, where Ea isincreased to 0.71 eV, as shown in Figure 5c−f. The activationbarriers and diffusion coefficients of these two structures aresummarized 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 Ruions migrate into Li layers, the ionic conductivity of Li2RuO3will drop markedly, leading to a poor overall kinetics of thismaterial. Because the cation migration was found commonly inLi-rich TM oxides, we suspect that the ionic diffusion, ratherthan the electronic conductivity, may act as the major limit totheir kinetic performances.

4. CONCLUSIONSWe have carried out the first-principles calculations, combinedwith the experimental measurements, to study the structuraltransformation and its effect on the electrochemical perform-ance of Li2RuO3, a prototypical family member of Li-rich TMoxides. We identify a complex two-step structural trans-formation for the first charge process in LixRuO3, that is, thefirst is related to the rearrangement of oxygen array occurringat x = 1 (upon half delithiation), followed by the Ru migrationfrom Li−Ru to Li layers upon further delithiation (x < 1). Thecalculated voltage profiles and XRD patterns show goodagreement with our experimental measurements. For thesubsequent cycling processes, our results indicate that Li2RuO3will undergo a reversible structural transition between the R3-cycl and C2/c-cycl polymorphs, with the migrating Ru graduallyaccumulating in Li layers. This cation migration and

accumulation in Li layers are found to be caused by theanionic redox reaction of Li2RuO3. As more than one Li ionsare extracted from Li2RuO3, holes are introduced into the O 2pbands, which tend to destabilize the host structure of thematerial. As a response, we found that the Ru migration can betriggered to restabilize the anionic redox chemistry againstoxygen evolution. This is mainly due to the significant changeof oxygen local environments and the resultant large distortionof the RuO6 octahedron after cation migration, which canreconstruct the electronic structure and thus lower the energyof system. Therefore, the Ru migration can be viewed as astructural self-regulation triggered by the anionic redoxreaction, which can explain a high capacity maintained uponprolonged cycling in Li2RuO3.

19 However, on the other hand,our calculations also indicate that the gradual accumulation ofRu ions in Li layers will lead to a serious voltage fade uponcycling, which lowers the energy output of the materials, and toa poor Li diffusion. Therefore, further extensive studies may bestill needed to achieve the trade-off between the reversiblecapacity and other electrochemical properties of Li-rich TMoxides.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.9b02887.

Details of structural information and experimentalsection, comparisons of experimental and calculatedXRD, charge−discharge voltage profiles of Li2RuO3 forthe first five cycles between 2.0 and 3.8 V, averagevoltage of Li2RuO3 for the first 20 cycles, calculatedaverage voltages of Li2RuO3 in different percentages ofRu migration, and average Li−O and Ru−O bondlengths in LixRuO3 (x = 1, 0) (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: pengzhang@szu.edu.cn (P.Z.).*E-mail: wsq@xmu.edu.cn (S.W.).ORCIDShiyao Zheng: 0000-0001-5002-5204Peng Zhang: 0000-0001-8515-8875Shunqing Wu: 0000-0002-2545-0054Zi-zhong Zhu: 0000-0001-5353-4418NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the National Natural ScienceFoundation of China (grant nos. 11874307 and21761132030), the National Key R&D Program of China(nos. 2016YFB0901502 and 2016YFA0202601), the Funda-mental Research Funds for the Central Universities (grant no.20720180020), and the Supercomputing Center of the USTC.

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Table 2. Calculated Activation Barriers (Ea) and EstimatedDiffusion Coefficients (D300K) for Various MigrationPathways in C2/c Li2RuO3 and Its Cycled Structure fromClimbing NEB Methodsa

structure Ea (eV) D300K (cm2/s)

C2/c Li2RuO3 0.43/0.65/0.38/0/59 10−10/10−13/10−8/10−12

cycled Li2RuO3 0.45/0.70/0.71/0.72 10−10/10−14/10−14/10−14

aD = d2ν·exp(Ea/kBT), where ν = 1013 Hz is assumed for estimationand d is the hopping distance.

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