Oxygen Activity in Li-Rich Disordered Rock-Salt Oxide and theInfluence of LiNbO3 Surface Modification on the ElectrochemicalPerformanceMusa Ali Cambaz,† Bhaghavathi P. Vinayan,† Holger Geßwein,‡ Alexander Schiele,§

Angelina Sarapulova,‡ Thomas Diemant,∥ Andrey Mazilkin,§,⊥ Torsten Brezesinski,§

R. Jurgen Behm,†,∥ Helmut Ehrenberg,†,‡ and Maximilian Fichtner*,†,§

†Helmholtz Institute Ulm (HIU) Electrochemical Energy Storage, Helmholtzstrasse 11, 89081 Ulm, Germany‡Institute for Applied Materials, Karlsruhe Institute of Technology (KIT), Herrmann von Helmholtz Platz 1, 76344Eggenstein Leopoldshafen, Germany§Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Herrmann von Helmholtz Platz 1, 76344Eggenstein Leopoldshafen, Germany∥Institute of Surface Chemistry and Catalysis, Ulm University, Albert Einstein Allee 47, 89081 Ulm, Germany⊥Institute of Solid State Physics of the Russian Academy of Sciences, Academician Ossipyan Street 2, 142432 Chernogolovka, Russia

ABSTRACT: Li rich disordered rock salt oxides such asLi1.2Ni1/3Ti1/3Mo2/15O2 are receiving increasing attention ashigh capacity cathodes due to their potential as high energymaterials with variable elemental composition. However, thefirst cycle oxygen release lowers the cycling performance dueto cation densification and structural reconstruction on thesurface region. This work explores the influence of lithiumexcess on the charge compensation mechanism and the effectof surface modification with LiNbO3 on the cyclingperformance. Moving from a stoichiometric LiNi0.5Ti0.5O2composition toward Li rich Li1.2Ni1/3Ti1/3Mo2/15O2, oxygen redox is accompanied by oxygen release. Thereby, cationic chargecompensation is governed by the Ni2+/3+ and Mo3+/6+ redox reaction. Contrary to the bulk oxidation state of Mo6+ in thecharged state, a mixed Mo valence on the surface is found by XPS. Furthermore, it is observed that smaller particle sizes result inhigher specific capacities. Tailoring the surface properties of Li1.2Ni1/3Ti1/3Mo2/15O2 with a solid electrolyte layer of LiNbO3altered the voltage profile, resulting in a higher average discharge voltage as compared to the unmodified material. The resultshint at the interdiffusion of cations from the metal oxide surface coating into the electrode material, leading to bulk compositionchanges (doping) and a segregated Nb rich surface. The main finding of this work is the enhanced cycling stability and lowerimpedance of the surface modified compound. We argue that surface densification is mitigated by the Nb doping/surfacemodification.

■ INTRODUCTION

The development of lithium ion batteries (LIBs) with highenergy density is one of the primary objectives in the energystorage field. The cathode material is considered as thebottleneck for improving the energy density. Conventionallayered oxides like LiCoO2 and LiNi1/3Mn1/3Co1/3O2 exhibitstable specific capacities between 145 and 165 mAh g−1.1−3

Therefore, Li rich layered oxides xLi2MnO3·(1−x)LiMO2 (M= 3d transition metals), which promise higher capacities, arenow pursued as future cathode materials.4,5 These compoundsoffer specific capacities exceeding 200 mAh g−1, which resultsfrom the fact that charge can be stored on both the transitionmetal (Tm) cation and the oxygen anion.6−9 All of thesematerials have ordered structures. Li rich disordered rock saltoxides (DRS) with α LiFeO2 type structure lately have

attracted considerable attention. Even though lithium migration is hindered in cation disordered rock salt oxides, they canin principle function well as high capacity cathode materials.For a threshold ratio lithium:transition metal (Li:Tm) ≥ 1.09,the lithium sites can percolate sufficiently within the structureand form favorable diffusion pathways.10,11 Recently, Yabuuchiet al.12,13 demonstrated that for Li rich cation disorderedLi3NbO4−MO (M = Fe, Ni, Mn), a significant portion of thecharge comes from reversible oxygen redox and is not mainlydue to oxygen release. Oxygen redox can be beneficial, as itcontributes to the capacity in the high voltage range. However,

too much oxidation seems to be detrimental, as oxygen losscan be triggered. This leads to a near surface cationdensification, which can severely impede the Li diffusion.14

The generation of highly reactive oxygen radicals (in the formof O2

−/O− or O22− in the bulk of the material) or even oxygen

vacancies (after oxygen loss) on the particle surface during theelectrochemical process is presumed to result in the formationof a resistive surface layer due to electrolyte decomposition andstructural transformation.15−17 Thus, strategies to stabilize theparticle surface to alleviate or even overcome these problemsare needed to improve the performance of Li rich disorderedrock salt oxides.Li1.2Ni1/3Ti1/3Mo2/15O2 is a promising cathode material that

exhibits high specific capacities but suffers from oxygenrelease.10,18 Here, we propose a surface modification strategyusing LiNbO3 in order to reduce the surface degradation andimprove the electrochemical performance.19,20

■ RESULT AND DISCUSSION

Synthesis and Characterization. We chose to investigatea solid solution between the stoichiometric cation disordered

Fm3m Li2NiTiO4 (LiNi0.5Ti0.5O2) and the lithium enrichedcation ordered P1 Li4MoO5(Li1.6Mo0.4O2), targeting lithiumrich cation disordered Li−Ni−Ti−Mo oxide. For a clearrepresentation of the lithium to transition metal ratio, we usethe notation LiMO2. LiNi0.5Ti0.5O2 is adopting an α LiFeO2

type crystal structure, where Li and the transition metals sharethe same octahedral site and are randomly distributed withoutany long range order. These types of compounds are referredto as cation disordered rock salt oxides in the literature.21

Depending on the degree of the random occupation (frompartial to full occupancy), the extent of cation disorder varies.For the cation ordered triclinic P1 Li4MoO5, Mo6+ ions occupythe octahedral sites in a cubic close packed (ccp) anion latticewith Li+ occupying the remaining octahedral sites. Thestructure is built up of [Mo2O10] clusters, consisting of edgesharing MoO6 octahedra.

22,23 The hypothetical binary mixturebetween “Li4MoO5−Li2NiTiO4” forms a single phase solidsolution with Li1.2Ni1/3Ti1/3Mo2/15O2 stoichiometry andcrystallizes in the α LiFeO2 type structure, as illustrated inFigure 1a.

Figure 1. (a) XRD pattern of LiNi1/2Ti1/2O2 (LNT 0), Li1.2Ni1/3Ti1/3Mo2/15O2 (LNTM 20), LiNbO3 coated Li1.2Ni1/3Ti1/3Mo2/15O2 (Nb LNTM20), and Li4MoO5. (b) STEM image of Nb LNTM 20 and (c) EDX mapping results obtained on Nb LNTM 20 and the line profile for the Nb Ledge.

Li1.2Ni1/3Ti1/3Mo2/15O2 has been synthesized and subsequently coated with 5 wt % LiNbO3 on the pristine powder(see experimental details below). No evidence of the presenceof a LiNbO3 phase was found in the diffraction data of theLiNbO3 coated Li1.2Ni1/3Ti1/3Mo2/15O2. It has been shownthat the metal oxide coating can react with the bulk materialunder high temperature treatment.24,25 This leads to niobiumsubstitution in this specific case. In the following,LiNi0.5Ti0.5O2 is referred to as LNT 0, Li1.2Ni1/3Ti1/3Mo2/15O2

as LNTM 20, and LiNbO3 coated Li1.2Ni1/3Ti1/3Mo2/15O2 asNb LNTM 20. The number at the end of the abbreviationindicates the lithium excess. Rietveld refinement analysis ofLNT 0, LNTM 20, and Nb LNTM 20 is shown in Figure

S1a−c of the Supporting Information (SI). The refined latticeparameter for a single phase fit in Fm3m space group is a =4.141830(21) Å for LNT 0, a = 4.146881(33) Å for LNTM20, and a = 4.146980(24) Å for Nb LNTM 20. Nb LNTM 20shows a slightly larger a lattice parameter, which is consistentwith the larger cation radius of Nb5+, compared to Ni2+ andTi4+. The chemical composition of the compounds has beendetermined by inductively coupled plasma optical emissionspectroscopy (ICP OES) and is given in Table S1 (SI).Transmission electron microscopy (TEM) images are shownin Figure S2 (SI), depicting small spherical crystallites around100 nm in diameter for LNT 0 and LNTM 20. For moreinsight into the coating, dark field scanning transmission

Figure 2. (a) Charge−discharge profiles of LNT 0, LNTM 20, and Nb LNTM 20 with the corresponding differential capacity plots of thedischarge cycle at 20 mA g−1. (b) Rate capability test for all compounds in the range of 4.5−1.5 V vs Li+/Li. Voltage profiles of (c) LNTM 20 and(d) Nb LNTM 20 for different specific currents. (e) Cycling stability of all compounds for various cutoff voltages at 20 mA g−1. (f) Charge−discharge profiles of LNTM 20 (1st cycle) and Nb LNTM 20 (1st and 26th cycles).

electron microscopy energy dispersive X ray spectroscopy(STEM EDX) mapping of Nb LNTM 20 was carried out.The corresponding maps of O, Ni, Ti, Mo, and Nb are shownin Figure 1b,c. The Nb signal was found throughout theparticle, with an increasing intensity from the center towardthe edge. This indicates some Nb segregation to the outersurface. The combined X ray diffraction (XRD) and STEMEDX data suggest partial incorporation of Nb into the Li−Ni−Ti−Mo bulk lattice. A line scan along a crystallite grainconfirms a Nb rich outer surface layer of varying thickness (upto 12 nm). We estimate that ∼47% of the Nb is substitutedinto the phase, leading to a nominal stoichiometry ofLixNi0.328Ti0.328Mo0.131Nb0.016O2. X ray photoelectron spectroscopy (XPS) results in Figure S3 (SI) indicate that Nb is in the+5 oxidation state.Electrochemical Properties. The electrochemical proper

ties were analyzed by galvanostatic charge−discharge measurements. Figure 2a shows the charge−discharge profiles forLNT 0, LNTM 20, and Nb LNTM 20 half cells. The sampleswere cycled in the voltage range of 4.5−1.5 V vs Li+/Li using aconstant specific current of 20 mA g−1. The specific dischargecapacity increased significantly from 100 to 220 mAh g−1 uponchanging from stoichiometric LNT 0 to Li rich LNTM 20.This increase in accessible capacity agrees with expectationsbased on percolation theory.10,11,26 The latter predicts that the0 TM site can percolate and form extended lithium networkswith 9% excess lithium, thus allowing more lithium to beintercalated/deintercalated. For Nb LNTM 20, the voltageprofile changed compared to that of the pristine sample. This isobvious from the corresponding differential capacity curves.Remarkably, Nb LNTM 20 showed a longer and more definedupper voltage plateau and higher capacity contribution in thehigh voltage regime >2.5 V than LNTM 20. LNTM 20showed an overall higher initial specific discharge capacity.However, a significantly higher fraction of the capacity isgained in the low voltage regime (<2.5 V), accompanied by alow voltage plateau around 2.2 V. In order to assess theinfluence of particle size on the accessible specific capacity, thesintering time was varied. The average particle sizes of LNTM20 sintered for 2 and 4 h and Nb LNTM 20 sintered for 4 hwere 96, 220, and 176 nm, respectively, as shown in FigureS4a−c. LNTM 20 with an average particle size of 220 nmdisplayed a much lower specific discharge capacity of 170 mAhg−1 as compared to LNTM 20 with an average size of 96 nm(Figure S5, SI), indicating a particle size dependence of theaccessible capacity. This points to diffusion limitations. Figure2b illustrates the rate performance of all compounds, whichinvolved five consecutive cycles at a constant specific current.At higher currents, Nb LNTM 20 retained more of thecapacity than both LNT 0 and LNTM 20. For specific currentsof 20, 50, 100, 200, and 400 mA g−1, Nb LNTM 20 exhibitedspecific capacities of 200, 174, 157, 135, and 108 mAh g−1,whereas for LNTM 20 the corresponding values were 220,164, 137, 115, and 95 mAh g−1. For specific currents >50 mAg−1, Nb LNTM 20 showed higher discharge capacities. Therespective first cycle charge−discharge profiles for each currentstep are shown for both LNTM 20 and Nb LNTM 20 inFigure 2c,d. For higher currents, we observed an increase inpolarization and a decrease in the specific capacity. Figure 2eshows the cycling stability of the investigated electrodematerials. Nb LNTM 20 demonstrated better capacity retention compared with LNTM 20. The better rate capability, inconjunction with the improved cycling stability, further

highlights the importance of surface modification. Lowcapacity retention for Li rich rock salt oxides is a widelyobserved phenomenon.8,27,28 It is believed to originate fromthe participation of anions in the charge compensation process(oxygen redox/release), as discussed below. Note that thecharge−discharge profiles for Nb LNTM 20 in the later cyclesreveal the appearance of a low voltage plateau and the voltageprofile resembles the behavior of LNTM 20, as shown inFigure 2f. With prolonged cycling, the voltage hysteresisincreased along with an increased capacity contribution fromthe low voltage region <2.5 V. Furthermore, the cyclingstability of LNTM 20 appears to depend on the charge cutoffvoltage, showing severe degradation when charged to highpotentials. Being charged to high potentials, the Li rich DRScan exhibit irreversible phenomena like oxygen evolution fromthe surface and side reactions.27 The first cycle oxygen releasefor LNTM 20 is believed to cause cation densification andstructural reconstruction of the surface phase, therebyimpeding Li conductivity and leading to impedance buildup.The observed plateau around 2.2 V has been related to surfacedensification after oxygen loss.18,29 For Nb LNTM 20, thisplateau is greatly reduced. Thus, surface modification seems tobe beneficial in protecting the reactive surface and impedingsurface densification.30 For a relatively narrow voltage windowof 4.1−2.0 V, oxygen evolution or oxygen participation can besuppressed, and better cycling stability is achieved. This will bediscussed in the following sections. Cycling LNTM 20 in asmaller voltage window leads to more symmetric profiles withlower voltage hysteresis, as shown in Figure S6 (SI). NbLNTM 20 was subjected to milling in organic solvent andsintering as additional processing steps, which may affect itselectrochemistry. In order to elucidate the influence of suchadditional steps on the electrochemistry, LNTM 20 wasprocessed in the same way, without the addition of lithiumand niobium precursors. The resulting charge−dischargeprofile is presented in Figure S7a and the rate capability testshown in Figure S7b of the SI. The processed LNTM 20showed a slightly lower specific capacity and a larger hysteresis,possibly due to lithium leaching during the solvent treatment.As mentioned above, we hypothesize that the surface phase

evolution is indicated by the appearance of the low voltageplateau ∼2.2 V. For the modified compound, the first dischargedoes not reveal any visible low voltage plateau, which onlyappears with higher cycle numbers, as shown in Figure 2f. Onthe contrary, the unmodified sample shows already in the firstdischarge a visible low voltage plateau from which we inferredthat the surface phase evolution is mitigated for Nb LNTM 20.Electrochemical impedance spectra were measured to gain abetter understanding of the improved cycling performance.Figure 3 shows the ac impedance spectra (Nyquist plots) ofpristine LNTM 20 and of surface modified Nb LNTM 20,measured after 2 and 40 cycles for the discharged state. Theequivalent circuit31 is shown in the inset, and the fitting resultsare listed in Table S2 (SI). The spectra exhibit two semicirclesat high and low frequencies and a Warburg tail at very lowfrequencies. The first, potential independent semicircle isattributed to the surface processes, e.g., the resistance of Li+

migration through electrode surface films (surface filmresistance R1). The second, potential dependent semicirclereflects the charge transfer (interfacial) resistance R2.

32 TheWarburg tail is ascribed to the diffusion of Li ions into theelectrode material (Warburg element). Both materials (pristineand Nb coated) show a similar starting behavior and display

growth in impedance with electrochemical cycling. The chargetransfer resistances “R2” of the pristine and surface modifiedcathode after the 2nd and 40th cycles have been compared. R2is 20 and 67 Ω for the pristine cathode, and 8 and 23 Ω for thesurface modified compound, respectively. During prolongedcycling, the charge transfer resistance significantly increases forthe pristine cathode (by 44 vs 16 Ω for the surface modified

cathode). The increase in charge transfer resistance istentatively attributed to structural changes on the surface.These results demonstrate that the surface modified compound shows a relatively low impedance after oxygen loss ascompared to pristine material. However, the increase inimpedance with cycling is significant, in accordance with thecontinuing capacity fading seen for Nb LNTM 20.The electrode kinetics depend on both the ion transport and

electronic conductivity and can limit the overall performance.The total conductivity of LNT 0 and LNTM 20 was studiedby ac impedance spectroscopy at various temperatures, asshown in Figures S8 and S9 of the SI. The activation energy(Ea) was calculated via the Arrhenius equation, from the slopeof ln σ vs T−1, where σ is the conductivity and T represents theabsolute temperature. LNTM 20 shows a higher roomtemperature conductivity of ∼3.0 × 10−6 S cm−1 comparedto LNT 0 (∼7.1 × 10−8 S cm−1). The donation of d electronsfrom Mo to the conduction band can explain the increase inconductivity for LNTM 20. The activation energy (Ea) forLNT 0 (319 ± 19 meV) is higher than for LNTM 20 (249 ±16 meV). Overall, the conductivity is sufficiently high so thatno electronic limitations are to be expected. Important tomention is that, with the state of charge and the number ofcycles, the conductivity can vary by several orders ofmagnitude.33

For lithium rich materials, both oxygen redox and latticeoxygen loss upon cycling are well known phenomena.34,35 Inorder to find out whether oxygen release takes place in the caseof overlithiated LNTM 20, in situ differential electrochemicalmass spectrometry (DEMS) measurements were conductedand the results compared to LNT 0, which served as referencematerial. To this end, half cells were charged at C/10 to 4.5 V(1C = 210 mA g−1 for LNTM 20 and 100 mA g−1 for LNT 0)while monitoring gas evolution. As depicted in Figures 4 andS10 (SI), the LNTM 20 and LNT 0 half cells showed firstcycle charge capacities of 157 and 88 mAh g−1. The specificcapacities were lower for the thicker electrodes used in theDEMS measurements. CO2 evolution was observed, startingaround 3.9 V for LNTM 20 and 4.1 V for LNT 0. Thequantities detected for LNTM 20 were below 2 ppm up to 4.1V (50 mAh g−1), thus making a precise determination of theonset potential difficult. As expected, significant oxygen releasewas only observed for the overlithiated material, starting at 4.4V. This is indicative of lattice changes during cycling operation,eventually leading to an oxygen deficient (surface) structure.

Figure 3. Impedance spectra obtained for the discharged state (1.5 V)after the 2nd and 40th cycles, respectively, on (a, top) LNTM 20 and(b, bottom) Nb LNTM 20. The corresponding equivalent circuit isshown in the inset.

Figure 4. DEMS measurements of the first charge cycle at C/10 to 4.5 V for LNTM 20 half cells. The cell voltage (black) is shown together withthe O2 (blue) and CO2 (red) evolution.

Chemical reactions of released oxygen with the electrolyte arenot the only source of CO2 evolution, as both materials showsimilar behavior. Electrochemical electrolyte oxidation36,37 anddecomposition of residual carbonate38,39 species from thesynthesis, for instance, may also account for the CO2 evolution.Structural Changes upon Cycling. In situ XRD

measurements were carried out to investigate the structuralchanges of LNTM 20 during two consecutive cycles between4.5 and 1.5 V at C/30. The voltage profile and the changes inthe a lattice parameter from single phase XRD refinement areshown in Figure 5, with an enlarged section displaying the

pattern around the 002 reflection. During the first charge, threeregimes can be seen. The first charge regime until 4 V reveals asloped voltage profile, where the a lattice parameter iscontinuously decreasing, thus reflecting lattice contraction.This finding can be associated with a Ni2+/3+ oxidation process[Ni2+ (r = 0.69 Å) and Ni3+ (r = 0.56 Å)]. The second regimeis along the 4.3 V plateau, where the lattice parameter changesonly slightly, and the third regime occurs toward the end of thecharge, where the lattice parameter is continuously decreasing.This result can be attributed to oxygen oxidation and thenrelease. In the midregime, the lattice parameter is onlychanging in the beginning and at the end of charge. Weargue that oxygen (O2−) oxidation is associated with ashrinkage of the oxygen framework because of the smallersize of the oxygen anion or due to the formation of peroxo likespecies with shorter oxygen−oxygen bonds.40 During the firstdischarge, the lattice parameter continuously increases (latticeexpansion) until 2.2 V. Further discharge affects the latticeonly slightly. During the second cycle upon discharge, a morepronounced plateau is observed for voltages <2.2 V, with onlyslight changes in the a lattice parameter.Redox Mechanism. Further information on the redox

mechanism of LNTM 20 and LNT 0 was obtained from X rayabsorption near edge structure (XANES) measurements.

Figure S11a (SI) shows the Ni K edge spectra of pristineLNT 0 after charging to 4.5 V and then discharging to 1.5 V.Comparison of LNT 0 with a NiO reference compound revealsthat the edge energy is similar, indicating that the Ni oxidationstate is +2. Figure 6a shows similar results for LNTM 20.

During charging, the Ni K edge shifted significantly to higherenergies, thereby reflecting an increase in oxidation state. Onthe basis of Faraday’s law, the theoretical capacities fordifferent redox processes have been calculated and are listed inTable 1. The oxidation state of Ti was determined by XPS tobe +4, as shown in Figure S12 (SI). Ti4+ was fully oxidized andtherefore assumed to be electrochemically inactive.41,42 Notethat neither XANES nor XPS can rule out the formation ofsmall amounts of Ti3+, in the atom percent range. The

Figure 5. (left) In situ XRD pattern of LNTM 20 for two consecutivecycles in the range of 4.5−1.5 V at C/30. (right) Correspondingvoltage profile and changes in the a lattice parameter from Rietveldrefinement analysis. The red curves are the voltage profile and the reddots the measurement points, where the XRD patterns were collected.

Figure 6. (a) XANES Ni K edge spectrum for LNTM 20 and (b) MoK edge spectrum for LNTM 20. (c) FT EXAFS of Ni.

Table 1. Theoretical Specific Capacity for Different ValenceChanges and the Specific Lithium Capacity of LNT 0 andLNTM 20

theoretical capacity/mAh g−1

compound Ni2+/3+ Ni2+/4+ Mon+1 Mon+2Li specificcapacity

LiNi1/2Ti1/2O2 (LNT-0) 144 287 287Li1.2Ni1/3Ti1/3Mo2/15O2(LNTM-20)

101 202 40 80 364

measured specific capacity for each state of charge can be usedto estimate the average valence change.For LNT 0, the specific capacities for charging to 4.5 V and

subsequently discharging to 1.5 V were 145 and 106 mAh g−1,respectively. This corresponds to 0.5 Li extraction and 0.37 Liintercalation per formula unit (fu), which can be attributed tothe Ni2+/Ni3+ redox couple. Interestingly, LNTM 20 delivered262 mAh g−1 during charge, corresponding to 0.86 Li per fu,which is beyond the theoretical capacity for the Ni2+/4+ redoxcouple with 202 mAh g−1. This suggests that more redox activespecies are involved in the reaction, although electrolyte/electrode side reactions can also be expected. As confirmed byDEMS, O2 evolution takes place during charge, and therefore,oxidation of O2− must be considered as an additionalmechanism for charge compensation. For the consecutivedischarge to 1.5 V, LNTM 20 exhibited a specific dischargecapacity of 212 mAh g−1, corresponding to 0.73 Li per fu.Simultaneously, the Ni K edge shifts back to lower energies,pointing to nickel reduction. In Figure S11b (SI), thecomparison between the Ni K edge of LNT 0 and LNTM20 in the charged state shows similar edge energies, suggestingthat Ni is in the oxidation state +3 in both cases. In conclusion,oxidation to Ni4+ can be excluded for LNTM 20, limiting thenickel redox contribution to Ni2+/3+. Interesting to note is thatthe pre edge peak intensity of Mo increased significantly afterthe first charge and remained at a higher value after thedischarge, which is shown in Figure 6b. This finding indicatesan increased distortion of the MoO6 octahedra compared tothe pristine state. The intensities for these transitions are verysensitive to the coordination environment and the symmetry ofthe metal center. For centrosymmetric symmetry, the pre edgeis weak, as it is limited by a weak electric quadrupole intensitymechanism.43 This pre edge structure is typically observed forα MoO3 with Mo in the oxidation state +6. The increase inpre edge peak, with the shift of the white line to higherenergies, suggests oxidation of Mo upon charging.44 On thebasis of the results of a linear combination fit, the initial bulkoxidation state of Mo is estimated to be +4.7 and increases toaround +6 upon charging. During discharge, it decreases to+3.2.XANES in fluorescence mode is a method to probe the bulk

and investigate the average oxidation state; however, it is notsensitive to changes in the surface or near surface range. Asevidenced by DEMS, O2 release takes place. Therefore, XPSwas used to characterize the surface oxidation state for thecharged material, as shown in Figure S12 (SI). Note that amixed oxidation state with mainly +6 and a minority at +4 isfound for Mo. The partial reduction of Mo at the surface ispossibly promoted after the oxygen loss.45,46 We did notobserve any changes in the oxidation state of Ti. By chargebalancing, we can roughly estimate the oxygen redoxcontribution to the total (reversible) capacity. On the basisof the derived oxidation states, Mo contributes with 52 mAhg−1 to the total specific charge capacity, without anycontribution of the fully oxidized Ti4+. Assuming that duringcharging Ni is oxidized to +3, the Ni2+/3+ redox reaction canaccount for 101 mAh g−1. The remaining charge capacity mustbe compensated by oxygen anions, with an estimatedcontribution of 109 mAh g−1 during the first charge cycle.The local environment of the Ni ions in LNT 0 and LNTM

20 was investigated by extended X ray absorption finestructure (EXAFS) measurements. Figures 6c and S11c (SI)depict the Fourier transformed (FT) spectra of the Ni K edge

for different states of charge. The first coordination shell (Ni−O) reflects the contribution of oxygen, while the second one isrelated to backscattering from Ni ions (Ni−Ni). The mostnotable change takes place in the first coordination shell duringcharge, indicating that the charge compensation results inlower Ni−O peak amplitude. Ni3+ in the low spin state isexpected to result in a Jahn−Teller distortion, which isincreased by oxidation. This leads to a decrease in theamplitude of the Ni−O peak.47 The shape of the firstcoordination shell reveals the existence of two Ni−O peaks,suggesting distorted octahedral coordination. In Figure 6c, theamplitude R decreases for the first charge due to the change inthe coordination environment and completely recovers afterthe first discharge. In the case of LNT 0, in Figure S11c (SI),the amplitude R for the first discharge increases compared tothat of the pristine state, suggesting that the pristine materialpartially contains Ni3+, which is being reduced during the firstdischarge.

■ DISCUSSION AND CONCLUSIONSLi rich cation disordered rock salt oxides have promisingproperties, such as high capacity. Furthermore, they allow fora large number of element combinations, which can befavorable in terms of sustainability and cost. However,challenges have to be overcome in order to achieve viablesystems. So far, extended cycling stability with little capacityfading has not been demonstrated yet. In addition, problemshindering practical applications like voltage hysteresis and poorrate capability still need to be addressed and resolved. Withincreasing lithium excess, new redox processeswhich includeoxygen activitycan be expected. Disadvantages of thisprocess are the associated oxygen release, leading to surfacedegradation, side reactions with the electrolyte, cationdensification, and overall performance degradation. Inparticular, higher resistances lead to lower rate capability.DEMS measurements provided clear evidence that O2 isreleased for Li rich LNTM 20, but not for stoichiometricLNT 0.In order to alleviate the above mentioned drawbacks,

approaches comprising both structural and surface/interfacemodifications have been intensively studied.30,31 Recently, Leeet al.29 demonstrated that substitution of oxygen by fluorinecould mitigate oxygen loss and improve the cycling stability ofthe Li rich Li−Ni−Ti−Mo−(O/F) system. The fluorinesubstitution is an effective way to enable an introduction oflithium excess in low valent systems without the use of heavyhigh valent charge compensators, which can lower the specificcapacity. Practically, this approach aims to increase the cationicredox capacity. Fluorine doping has been reported to mitigateoxygen loss, improve capacity retention, and increase theaverage voltage in some examples.18,48 However, strongerinteractions between lithium and fluorine and lithiummigration into tetrahedral sites for a high states of chargeimpose constraints to the accessible capacity.49 Anotherinteresting approach shown by Shin et al.50 could experimentally demonstrate that oxygen loss can be mitigated bylow level doping of heavy elements such as Ta, resulting insurface modification. The improvement has been rationalizedby the enhanced oxygen retention of Ta and the tendency ofheavy elements to segregate at the surface due to their largerionic radii.Our approach targets the surface modification and physical

protection through a ceramic coating in order to mitigate the

surface phase evolution, which leads to performancedegradation.Here, we propose an approach comprising a surface

treatment with LiNbO3 in order to improve the electrochemical performance of the material. UnmodifiedLi1.2Ni0.333Ti0.333Mo0.133O2 showed a high specific capacitybut displayed a significant polarization for high degrees ofd e l i t h i a t i on (∼4 . 3 V) . The L iNbO3 mod ifi edLi1.2Ni0.333Ti0.333Mo0.133O2, on the other hand, showed lesspolarization, with a higher average discharge voltage and asignificantly higher capacity contribution in the high voltageregime. We argue that the change of the voltage profile can beattributed to the formation of a Li−Ni−Ti−Mo−Nb solidsolution due to partial interdiffusion of cations from the surfacecoating into the active material during sintering. Thisobservation is reflected in the lower impedance growth ofNb LNTM 20 during cycling. We suggest that the lowercharge transfer resistance and impedance increase for thesurface modified compound can be attributed to the mitigatedsurface densification. For further optimization, a uniformcoating is required. In addition, the influence of sinteringtemperature on the interdiffusion mentioned above needs tobe better understood. Alternatively, LiTaO3 is a possiblecandidate compound that could be used for the surfacemodification, since Ta doping in a disordered rock salt hasbeen shown to affect the surface chemistry due to segregationof the heavy element dopant on the surface, thereby mitigatingoxygen loss.50

In summary, we have demonstrated that high delithiation ofLi1.2Ni0.333Ti0.333Mo0.133O2 triggers oxygen release, resulting insubstantial performance decay. The changes in oxidation state,bond distance, and coordination environment for differentstates of charge are illustrated. XANES spectroscopy revealedthat Ni is oxidized from +2 to +3. At the same time, Mochanges its oxidation state from the pristine state with +4.7 toaround +6 in the charged state; it is reduced to +3.2 in theconsecutive discharge cycle.Furthermore, an irreversible structural distortion of the Mo

environment was observed upon cycling. XPS revealed that inthe charged state, Mo has a mixed oxidation state on thesurface, with predominantly +6 and +4. Thus, in the fullycharged state, we estimate that 109 mAh g−1 (around 0.36 Liper formula unit) are charge compensated by oxygen anionoxidation, triggering O2 evolution, accompanied by partialreduction of Mo to +4 near the surface.Moreover, a strong particle size dependence of the accessible

discharge capacity indicates the necessity of using small sizematerial for achieving high specific capacities. To the best ofour knowledge, this is the first time a surface modificationusing the solid state electrolyte LiNbO3 has been tested forcation disordered rock salt oxides. The voltage hysteresis wassignificantly reduced for Nb LNTM 20 over LNTM 20. Thisobservation was reflected by the lower impedance and also therelatively lower impedance buildup as compared to the pristinematerial. Overall, we conclude that surface modifications holdpromise to further improve the rate capability and cyclingperformance of Li rich cation disordered rock salt oxides.

■ EXPERIMENTAL SECTIONSynthesis. The procedure to synthesize LiNi0.5Ti0.5O2 and

Li1.2Ni1/3Ti1/3Mo2/15O2 was adopted from Lee et al.29 Li4MoO5 wassynthesized using modified conditions. Li2CO3 (Alfa Aesar, >99%),NiCO3 (Alfa Aesar, 99%), TiO2 nanopowder (<50 nm particle size,

Alfa Aesar, 99.7%), and MoO2 (Alfa Aesar, 99%) were employed asthe precursors. Other than for LiNi0.5Ti0.5O2, stoichiometric amountsof precursors were used. For Li1.2Ni1/3Ti1/3Mo2/15O2 and Li4MoO5,5% excess of Li was used. The precursors were ball milled for 24 hwith 200 rpm using a Fritsch P6 planetary ball mill with a 80 mLsilicon nitride vial and silicon nitride balls, with a ball to powderweight ratio of 20:1. The mixture of the precursors was pelletized andthen sintered at 750 °C for 2 h in air, followed by furnace cooling to200 °C and immediate transfer inside an argon filled glovebox.Li4MoO5 was sintered at 900 °C for 2 h. After sintering, the pelletswere manually ground into a fine powder.

For coating, we prepared a solution between solid lithium ethoxide(Sigma Aldrich, 95%) and liquid niobium isopropoxide solution (AlfaAesar, 99%, 10 wt % niobium isopropoxide in 50:50 (v:v) 2propanol/hexane) with a nominal stoichiometry of 1.05:1.0 of Li toNb. Li1.2Ni1/3Ti1/3Mo2/15O2 was ball milled with the prepared coatingsolution using a Fritsch P9 with a 15 mL vial volume for 12 h at 200rpm (ball to powder ratio of 15:1). Subsequently, the mixture wasdried overnight in an oven in an argon filled glovebox and thenpelletized and sintered at 400 °C for 2 h in air, followed by furnacecooling to 200 °C and immediate transfer into an argon filledglovebox. After the sintering, the pellets were manually ground into afine powder.

Electrochemical Measurements. Electrochemical tests werecarried out in a Swagelok type cell using Li foil as the counterelectrode. Electrode slurries were made of 90 wt % composite and 10wt % polyvinylidene difluoride (PVDF) binder with N methyl 2pyrrolidone (NMP) as the solvent. The composite consists of activematerial and Super C65 carbon black in a weight ratio of 80:20. Themixed slurry was coated on an aluminum foil using the doctor bladetechnique and dried at 120 °C for 12 h under vacuum. Each workingelectrode (12 mm diameter) contained about 3 mg of active material.LP30 from BASF (ethylene carbonate/dimethyl carbonate, 1:1 weightratio with 1 M LiPF6) was used as the electrolyte. Galvanostaticcharge−discharge experiments were conducted at 25 °C in a climatechamber using an Arbin electrochemical workstation.

Impedance Spectroscopy. For a better understanding of thecycling performance of surface modified Nb LNTM 20 relative topristine LNTM 20, electrochemical impedance spectra were measured at 25 °C after the 2nd and 40th cycles. Electrochemicalimpedance spectroscopy (EIS) was performed using a three electrodePAT Cell (EL CELL) with a Li ring as the reference electrode and Limetal (18 mm) as the counter electrode. The working electrode sizewas 18 mm, and aluminum served as the current collector. Theexperiments were conducted using a Bio Logic electrochemicalworkstation with an applied sinusoidal excitation voltage of 10 mVin the frequency range between 200 kHz and 0.1 Hz.

Differential Electrochemical Mass Spectrometry. In situ gasanalysis was performed by use of differential electrochemical massspectrometry (DEMS). The setup has been described elsewhere.37,51

Custom cells with gas inlets and outlets were assembled in an argonfilled glovebox. The cathodes used (40 mm diameter with a 4 mmhole for proper gas extraction) had areal loadings of about 1 mgLNT‑0/cm2 and 1.6 mgLNTM‑20/cm

2. GF/A (42 mm diameter, GE HealthcareLife Sciences, Whatman) was used as the separator, 600 μL of LP47(1 M LiPF6 in ethylene carbonate/diethyl carbonate, 3:7 by weight,BASF) as the electrolyte, and 600 μm thick Li metal foil (AlbemarleGermany GmbH) with a diameter of 40 mm as the counter electrode.A constant carrier gas flow (2.5 mLHe/min, purity 6.0) was appliedduring DEMS measurements for gas extraction. The gas was analyzedvia mass spectrometry (GSD 320, OmniStar Gas Analysis System,Pfeiffer Vacuum GmbH). After each run, a calibration gas of knowncomposition was introduced to quantify the measured ion currents.

Conductivity. To measure the conductivity, powder materialswere pressed into pellets with a diameter of 8 mm and coated with agold layer on both sides by sputtering. The ac impedance data werecollected in the frequency range from 0.1 Hz to 1 MHz (10 mVrmsamplitude) using a frequency response analyzer (ZAHNER ElektrikGmbH) at various temperatures (25−80 °C). After each temperature,the samples were allowed to equilibrate for a substantial amount of

time. The impedance data were fitted and analyzed using the ZMAN2 software.X-ray Absorption Spectroscopy. The P65 XAS beamline

(PETRA III, Hamburg) provides a relatively large beam (0.5 × 1mm2) and a moderate photon flux density. The 11 period miniundulator delivers a monochromatic photon flux of about 1011 ph/s.Two plane mirrors with variable angle of incidence and three differentsurface coatings were installed in front of the water cooled doublecrystal monochromator. Si(111) and Si(311) crystals were used forthe energy ranges of 4−22 and 7−44 keV, respectively. The P65beamline is also equipped with a 7 pixel HPGe energy dispersivedetector and Si PIPS diode. X ray absorption spectroscopy measurements were performed in both transmission and fluorescence modes.For ex situ measurements, the step scan mode with a duration ofabout 10−11 min for each spectrum was used. The spectra wereprocessed using the Demeter software package based on IFEFFIT andFEFF.52

X-ray Diffraction. In situ X ray diffraction (XRD) patterns werecollected using modified coin cells on a parallel beam laboratorydiffractometer with a microfocus rotating anode (Mo Kα radiation) intransmission geometry and a Pilatus 300 K W area detector. The coincells comprise an LNTM 20 based cathode, a Li metal anode, and aWhatman glass fiber separator. The electrolyte was 1 M LiPF6 inethylene carbonate:dimethyl carbonate 1:1 (w:w) from BASF.Galvanostatic charge−discharge cycling was performed with anIvium potentiostat between 1.5 and 4.5 V at 10 mA g−1. Diffractionpatterns were collected every 300 s. Two consecutive patterns wereadded up to eliminate cosmic spikes on the detector. The diffractionimages were integrated using the pyFAI software53 and analyzed withthe Rietveld method using TOPAS V6.X-ray Photoelectron Spectroscopy. The elemental composi

tion of the surface region was analyzed by X ray photoelectronspectroscopy (XPS) using a Physical Electronics PHI 5800 ESCAsystem. The measurements were carried out with monochromatic AlKα radiation (250 W, 13 kV) at a detection angle of 45° and with passenergies of 93.9 and 29.35 eV for survey and detail measurements,respectively. An electron flood gun was used for sample neutralization.The main C 1s peak was set to 284.8 eV for binding energycalibration. Some samples were subject to Ar+ sputtering (sputter rate∼1 nm min−1, 1 mA, 5 kV) to remove the topmost surface layer.Transmission electron microscopy. Transmission electron

microscopy (TEM) investigations were performed on a TecnaiF20ST transmission electron microscope operated at 200 kV. Samplesfor the TEM study were prepared using an FEI STRATA dual beamfocused ion beam (FIB)/scanning electron microscope. The FIB liftout samples were milled using a Ga ion beam at 30 kV.

■ AUTHOR INFORMATIONCorresponding Author*E mail: m.fichtner@kit.edu.

Musa Ali Cambaz: 0000 0002 4249 3486Bhaghavathi P. Vinayan: 0000 0001 6491 5160Torsten Brezesinski: 0000 0002 4336 263XR. Jurgen Behm: 0000 0002 7565 0628Helmut Ehrenberg: 0000 0002 5134 7130NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support from the FET OPEN project “LiRichFCC”of the European Commission (grant agreement #711792) isgratefully acknowledged. This work contributes to the researchperformed at CELEST (Center for Electrochemical EnergyStorage Ulm Karlsruhe). We want to thank Dr. EdmundWelter from Experiments Division XAS of PETRA III,Hamburg, Germany. This work is partially carried out withthe support of the Karlsruhe Nano Micro Facility.

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Repository KITopen Dies ist ein Postprint/begutachtetes Manuskript.

Empfohlene Zitierung: Cambaz, M. A.; Vinayan, B. P.; Geßwein, H.; Schiele, A.; Sarapuolva, A.; Diemant, T.; Mazilkin, A.; Brezesinski, T.; Behm, R. J.; Ehrenberg, H.; Fichtner, M. Oxygen Activity in Li-Rich Disordered Rock-Salt Oxide and the Influence of LiNbO3 Surface Modification on the Electrochemical Performance. 2019. Chemistry of materials, 31. doi: 10.5445/IR/1000096239/post Zitierung der Originalveröffentlichung: Cambaz, M. A.; Vinayan, B. P.; Geßwein, H.; Schiele, A.; Sarapuolva, A.; Diemant, T.; Mazilkin, A.; Brezesinski, T.; Behm, R. J.; Ehrenberg, H.; Fichtner, M. Oxygen Activity in Li-Rich Disordered Rock-Salt Oxide and the Influence of LiNbO3 Surface Modification on the Electrochemical Performance. 2019. Chemistry of materials, 31 (12), 4330–4340. doi:10.1021/acs.chemmater.8b04504 Lizenzinformationen: KITopen-Lizenz