Unravelling the Chemistry and MicrostructureEvolution of a Cathodic Interface in Sulfide-Based All-Solid-State Li-Ion BatteriesXia Li,† Zhouhong Ren,‡ Mohammad Norouzi Banis,† Sixu Deng,† Yang Zhao,† Qian Sun,†

Changhong Wang,† Xiaofei Yang,† Weihan Li,†,∥ Jianwen Liang,† Xiaona Li,† Yipeng Sun,†,∥

Keegan Adair,† Ruying Li,† Yongfeng Hu,§ Tsun-Kong Sham,∥ Huan Huang,⊥ Li Zhang,# Shigang Lu,#

Jun Luo,*,‡ and Xueliang Sun*,†

†Department of Mechanical and Materials Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada‡Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materialsand Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384,China§Canadian Light Source, 44 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2V3, Canada∥Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B9, Canada⊥Glabat Solid-State Battery Inc., 700 Collip Circle, London, Ontario N6G 4X8, Canada#China Automotive Battery Research Institute Co. Ltd., Fifth Floor, No. 43, Mining Building, North Sanhuan Middle Road, Beijing100088, China

*S Supporting Information

ABSTRACT: All-solid-state lithium-ion batteries (SSLIBs) are promis-ing candidates to meet the requirement of electric vehicles due to theintrinsic safety characteristics and high theoretical energy density. Astable cathodic interface is critical for maximizing the performance ofSSLIBs. In this study, operando X-ray absorption near-edge spectros-copy (XANES) combined with transmission electron microscopy (TEM)and electron energy loss spectroscopy (EELS) is employed to investigatethe interfacial behavior between the Ni-rich layered cathodes and sulfidesolid-state electrolyte. The study demonstrates a metastable inter-mediate state of sulfide electrolyte at high voltage and parasitic reactionswith cathodes during the charge/discharge process, which leads to thesurface structural reconstruction of Ni-rich cathodes. Constructing auniform interlayer by atomic layer deposition (ALD) is also employed inthis study to further investigate the cathodic interface stability. Theseresults provide new insight into the cathodic interface reaction mechanism and highlight the importance of advancedoperando characterizations for SSLIBs.

Li-ion batteries (LIBs) have played an essential role inthe portable electronic device market and areconsidered promising candidates as energy storage

devices for electric vehicle applications.1−3 However, thepopular organic liquid electrolytes raise serious concerns dueto their flammable and toxic properties.4,5 Fortunately, thereplacement of liquid with highly conductive solid-stateelectrolytes (SSEs) may address these issues.6−8 Therefore,the development of all-solid-state lithium ion batteries(SSLIBs) has aroused widespread interests due to theirintrinsic safety features that circumvent the use of theflammable and toxic organic liquid electrolytes.9 Furthermore,SSLIBs are proposed to have the potential to compete or even

go beyond the energy density of liquid-based LIBs.10,11 Amongthe developed SSEs, sulfide-based inorganic SSEs, in particular,have demonstrated high ionic conductivity (10−2−10−4 Scm−1), which is almost comparable with that of liquid-basedLi-ion electrolytes.12−15 However, the state-of-the-art SSLIBswith sulfide-based SSEs still suffer from severe issues, such aslimited cycle life and rapid performance degradation, which arelargely related to the unstable interface between electrode

Received: August 4, 2019Accepted: September 16, 2019Published: September 16, 2019

Letterhttp://pubs.acs.org/journal/aelccpCite This: ACS Energy Lett. 2019, 4, 2480−2488

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materials and SSEs that are detrimental to Li-ion transport.16

Some studies have reported interfacial phenomena betweenlithium metal oxide cathode materials and sulfide-basedelectrolytes, such as the space charge effect, the diffusion oftransition metals into SSEs, the decomposition of sulfide SSEsto form the inactive interlayer, etc.17−22 To suppress the sidereactions and stabilize the cathode/SSE interface, variouscoating materials, such as SSE coatings (Li2S−P2S5), inactivecoatings (Al2O3, Li2CO3), and ionic conductive coatings(LiNbO3, Li3PO4, LiTaO3), have been developed for Li-ioncathodes to improve the electrochemical performance ofsulfide-based SSLIBs.23−27

Although some reaction mechanisms are proposed, most ofthe reported studies employed ex situ characterizations toanalyze the cathode/SSE interface before and after theelectrochemical test.17,18,28,29 It should be noted that thesulfide SSEs are highly moisture- and air-sensitive, andtherefore, postcharacterizations have many challenges todirectly reflect the reaction process. Therefore, the underlyingmechanisms of sulfide SSEs, such as when and how the sidereaction occurs, if there are metastable intermediate phasesthat existed, are not fully understood. Furthermore, consider-ing the employed cathode materials in SSLIBs, many reactionmechanisms and the structural evolution, such as how thesurface structure reconstruction and inner cracking evolveduring an electrochemical process and how the developedcoating materials work for cathodes to alleviate the sidereactions and stabilize the cathode/SSEs interface, should alsobe further investigated. These unrevealed fundamental studiesmay impede the design of highly stable and conductive SSE/cathode interfaces and also hinder the development of high-performance SSLIBs.X-ray absorption near-edge spectroscopy (XANES) has been

widely employed in liquid-based LIBs to provide informationon the chemical state and local electronic structure of batterymaterials.30−32 In particular, advanced operando XANESstudy, which is a well-known powerful analytic technique tomonitor the active intermediate products and reveal theelectrochemical reaction mechanism, is used to probe thebattery materials while the cell is undergoing electrochemicalcycling.33,34 In this study, operando XANES combined withscanning transmission electron microscopy (STEM) with high-angle annular dark field (HAADF) and electron energy lossspectroscopy (EELS) is carried out to investigate the interfaceevolution between cathodes and sulfide SSEs in SSLIBs duringthe charge/discharge process. A commercial Ni-rich layeredoxide LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode andLi10GeP2S12 (LGPS) SSE are employed in this study. Toinvestigate the coating strategy for cathodes in SSLIBs, atomiclayer deposited (ALD) lithium niobium oxide (LiNbOx) with acontrollable and ultrathin thickness is applied as the coatinglayer for NMC811, which is proposed to stabilize the interfaceand improve the electrochemical performance of SSLIBs. Thecomparisons of the bare and coated NMC811 cathodes withelectrochemical testing and physicochemical characterizationsreveal the interfacial chemistry and microstructure evolutionbetween cathodes and sulfide SSEs during the electrochemicalreactions. The study illustrates the instability of LGPS as wellas the parasitic reactions with bare NMC811 during thecharging process and byproduct Li2S formation afterward.Furthermore, the surficial structure reconstruction and micro-structure cracks of the Ni-rich layered cathode in sulfide solid-state batteries are also investigated. This work highlights that

the complete operando XANES analysis combined with TEMprovides a detailed scenario of the interface side reactions and,more importantly, presents the experimental value of operandocharacterizations in the study of SSLIBs.Figure 1 illustrates the morphology and fine structure of the

bare and ALD LiNbOx-coated NMC811 (LNO-NMC811). In

this study, we employ an LNO thin film via ALD as a coatingmaterial for Ni-rich cathodes. In our previous study, the ALDLNO thin film demonstrated an acceptable ionic conductivity(6 × 10−8 S cm−1) at room temperature as well as conformalgrowth and controllable thickness during deposition, which ispromising as an interfacial engineering material in solid-statebatteries.35 To investigate the thickness effect, 2, 5, and 10 nmof LNO are deposited on NMC811 cathodes by controllingthe ALD reaction cycles (10, 25, and 50 cycles). As shown inFigure 1a,b, commercial NMC811 cathodes are secondarysphere particles with a size of approximately 5−10 μm. After25 cycles of ALD coating, the surface of the cathode particleappears rougher (Figure 1c), and an amorphous and conformalcoating with the thickness of around 5 nm can be observed atthe surface of NMC811 (Figure 1d). The deposition thicknesscorresponds well with the reported linear growth rate ataround 0.2 nm per cycle of ALD LNO thin films.35 Elementalmapping of scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX) shown in Figure 1e,f demon-strates the uniform distribution of Nb element among theNMC811 particles, further confirming the success of LNOcoating on cathodes. X-ray diffraction (XRD) and XANES areconducted to evaluate the crystal and chemical structure of

Figure 1. Physical characterizations of bare and LNO-coatedNMC811 composites. (a,b) SEM image of bare NMC811. (c) SEMimage, (d) HRTEM image, and (e,f) SEM-EDX element mappingof ALD LNO-coated NMC811.

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bare and LNO-coated NMC811 composites. As shown inFigure S1, there are no obvious differences of NMC811 beforeand after LNO coating, indicating that the material propertiesare maintained throughout the ALD process and no newimpurities are introduced.The bare and coated NMC811 composites were then mixed

with LGPS as cathode materials and assembled in SSLIBs.SEM images of the top view of the pressed NMC811-LGPSpellet are shown in Figure S2, and the detailed SSLIB assemblyprocess is listed in the Experimental Section. Electrochemicalimpedance spectroscopy (EIS) plots of LGPS at varioustemperatures are presented in Figure S3. Fitting theconductivities derived from the temperature-dependentimpedance with an Arrhenius law results in ionic conductivityof 2.07 × 10−3 S cm−1 for the commercial LGPS at roomtemperature, which is similar to the reported values.19,36−38

Electrochemical characterizations of assembled SSLIBs areshown in Figures 2 and S4. The thickness effect of the LNOcoating layer for the performance of NMC811 cathodes is first

optimized in Figure S4, and 5 nm of LNO-NMC811 presentsthe best electrochemical performance. Comparing the charge/discharge profiles, the bare NMC811 demonstrates asignificantly irreversible capacity at the first cycle with a lowCoulombic efficiency of 65%. In addition, there is a slight slopecircled in Figure 2a at the initial charging process,corresponding to the space charge effect reported by previousstudies.12,39 Furthermore, the bare NMC811 shows that severevoltage plateaus fade in the following cycles and the capacitydrops rapidly. On the other hand, among the coated cathodes,5 nm of LNO-NMC811 demonstrates significantly improvedelectrochemical performance. The initial charge stage does notexhibit the same sloping voltage profile as the bare NMC811,indicating that the LNO coating effectively prevents theoccurrence of space charge behavior. Furthermore, theirreversible capacity of the first cycle is significantly reduced,and the Coulombic efficiency exceeds 82%. The cyclingperformance of the as-prepared cathodes is shown in Figures2c and S4. Compared with the bare and LNO-coatedNMC811 cathodes with the different thicknesses of LNOcoating layers, 5 nm of LNO-NMC811 demonstratessignificantly improved cycling capacity and stability. After 50cycles, the discharge capacity remains at 99 mAh g−1, while thedischarge capacity of the bare NMC811 is only 20 mAh g−1.The differences of the electrochemical performance areprimarily related to the interface resistance between thecathode and sulfide electrolyte. The charge transfer resistance,including both Li-ion transportation and electron conductionresistance, is one of the factors that affects the electrochemicalperformance of SSLIBs.11 More importantly, the side reactionsbetween sulfide SSEs and cathodes result in the large resistanceat the interface with the decomposition of SSEs duringcycling.12,17 Therefore, constructing the coating layer with anappropriate thickness for cathodes is a necessary step tosuppress the side reactions with sulfide SSEs and achieve thegood electrochemical performance. EIS measurement isconducted to evaluate the resistance of as-assembled batteries,as shown in Figure 2d,e. Before electrochemical cycling, thetwo batteries after assembly demonstrate small surface chargetransfer resistances, indicating good conductivity at thecathode/SSE interface. However, after 10 h of rest, the twobatteries appear to have differences in the EIS plots. Thesemicircle at the high-frequency region of the bare NMC811-LGPS battery becomes significantly larger, indicating anincreased resistance during the resting period. Furthermore,after one charge/discharge cycle and 2 h of rest, the resistanceof the battery with bare NMC811 increases more obviously,which is indicative of the unstable interface between the bareNi-rich cathode and LGPS during cycling. In order toinvestigate the interfacial stability between the In anode andLGPS SSEs, the EIS plots and CV curves of In-LGPS-Inbattery are shown in Figure S5. After several hours of rest, theEIS plots of the In-LGPS-In battery are almost not changed.For the CV test, no obvious anodic peak can be observedduring the scanning process, indicating the stable anodicinterface during electrochemical reactions.The electrochemical characterizations have provided evi-

dence of the side reactions between bare NMC811 and LGPSand demonstrated the different interfacial behavior betweenthe bare and coated NMC811 with LGPS. To further revealthe interface chemistry, ex situ and operando XANES studieswere conducted. Figure S6a,b illustrates the ex situ XANES ofcathodes and SSEs before battery assembly. Interestingly, just

Figure 2. Electrochemical characterizations of bare and LNO-coated NMC811 in SSLIBs. (a,b) Charge/discharge profiles, (c)cycling performance, and (d,e) EIS plots of batteries at a variousresting times and after the charge/discharge test.

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after the bare NMC811 and LGPS composites mixed andrested over 72 h, the feature at 2470.7 eV of the S K-edgepresented a shift in the edge jump from 2469.9 to 2470.2 eV,while the feature at 8353 eV of the Ni K-edge shifted to lowerenergy (8352.2 eV), which is indicative of some interactionbetween LGPS and NMC811. When the sulfide SSEs andcathodes contact each other, the Li elements from sulfide SSEsprefer to be attracted by lithium metal oxide cathodes, leadingto reduced Li concentration in the SSEs and new interlayerformation. This phenomenon is called space charge effect,corresponding to the slope at the initial charging proc-ess.12,39,40 Furthermore, the interdiffusion of transition metalsfrom cathodes and sulfur from SSEs leads to the decom-position of battery materials and formation of insulatedinterphases.29,41,42 All of these side reactions result in thechanges of the chemical state and local environments of LGPSand NMC811, corresponding to the energy shift of the featuresin the S and Ni K-edge spectra. On the other hand, with theprotection of the coating layer, the NMC811 and LGPSmixture at the same conditions is very stable without obvious

changes from the XANES results, indicating that the coatinglayer enhances the chemical stability of the interface betweenthe cathode and sulfide SSEs. After the charge/dischargecycling test, as shown in Figure S6c,d, the feature at 2470.7 eVin the S K-edge spectrum of bare the NMC-LGPS systembecomes very broad and shifts to higher energy, indicating thedecomposition of LGPS with bare NMC811 during cycling.Furthermore, the Ni K-edge spectra of both bare and coatedNMC811 are shifted to higher energy after a long time cycling,indicating the irreversible chemical state change of the Nielement of NMC811 in SSLIBs. However, due to the inherentfeature of ex situ characterization, the detailed reactionmanner, such as when, where, and how the interfacial sidereactions proceed, is still unknown.To overcome the limitation of ex situ characterizations,

operando XANES was conducted with real-time measurementsof the batteries during electrochemical cycling. A modified coincell configuration was utilized as the operando cell with anopened window that allowed the incident X-rays to penetratethrough, as shown in Figure 3a. It should be noticed that the

Figure 3. Operando XANES study of SSLIBs with bare and LNO-coated NMC811 cathodes during cycling: (a) schematic figure of theoperando cell and obtained Bruker spectrum in a large photon energy region. Operando S K-edge spectra with first derivative mapping, NiK-edge spectra, and charge/discharge profiles of (b−d) bare NMC811-LGPS and (e−g) LNO NMC811-LGPS SSLIBs.

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structure of the operando cell is different from that of themodel cell, which is adopted for electrochemical tests (FigureS7). The operando XANES proceeded in a full and half cycle(charge/discharge/charge) of batteries during electrochemicalcycling. The obtained results of the bare NMC811-LGPSbattery are listed in Figure 3b−d, consisting of the S K-edgespectra with first derivative mapping (Figure 3b), Ni K-edgespectra (Figure 3c), and charge/discharge/charge profiles(Figure 3d). The corresponding results of LNO-NMC811-LGPS SSLIB are listed in Figure 3e−g. First, Ni K-edge spectraof both bare and coated cathodes demonstrate similarevolution during the cycling process. As shown in Figure3c,g, the spectrum presents a white line at 8353 eV,corresponding to the 1s to 4p transition.43,44 During thecharge/discharge process, the feature at 8353 eV initially shiftsto higher energy and then returns back, indicating the Ni2+/Ni3+ and Ni3+/Ni4+ electrochemical redox reaction in thedelithiation/lithiation process of NMC811.1,3,45 The corre-sponding Ni K-edge first derivation spectra in Figure S8 furthersubstantiate the evolution. Second, the interpretation of the SK-edge spectral features in this system is much morecomplicated. The S K-edge white line of LGPS with bareNMC811 is at 2470.7 eV (Figure 3b). During the first chargingprocess, this feature gradually shifts to lower energy. Whencharging at the high-voltage stage (>3.5 V), the feature shiftsfrom 2470.7 to 2470.4 eV, indicating the instability of LGPS.

Furthermore, during the subsequent discharge process, theshifted feature reverses back to 2470.7 eV gradually, while anew feature at 2472.5 eV becomes more prominent. Comparedto the standard reference samples (Figure S9), the appearanceof the new peak can be assigned to the S 1s to Li2S σ*transition,46 indicating that LGPS is first decomposed to Li2Srather than metal sulfides or polysulfide. The whole process isrepeatable that the feature at 2470.7 eV in the second chargingprocess shifts to lower energy again. The first derivativemapping of the S K-edge spectra gives more direct observationof the evolution, in which the bright red and blue regionspresent a consistent shift and skew in the image. On the otherhand, the S K-edge spectra of LGPS with LNO-NMC811 arevery stable during the whole electrochemical reaction and donot show the obvious shift in features during the electro-chemical reaction processes. When comparing the firstderivative mappings of the two S K-edge spectra, the brightred and blue regions of the LNO-NMC811-LGPS battery arevery straight, which is indicative of the stabilized interfacebetween LGPS and the protected cathode during the firstcharge/discharge/charge process. It should be noted that thedeveloped coin-cell configuration with an open windowinduces potential polarization of SSLIBs. The discharge voltageplateaus measured in operando study are lower than regularcharge/discharge characterizations, which needs furtheradvancements in solid-state cell configuration for operando

Figure 4. Operando XANES spectra of transition metals of bare and LNO-coated NMC811 cathodes. (a,c) Mn and Co K-edge spectra of bareNMC; (b,d) Mn and Co K-edge spectra of LNO-NMC811.

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XANES studies. Apart from the S and Ni K-edge spectra,studies of transition metal Mn and Co K-edge spectra are alsoconducted in the operando XANES characterization, as shownin Figure 4. The Mn K-edge spectrum exhibits the pre-edge ataround 6541 eV and the white line at 6559 eV. During thecharge/discharge process, the peaks at 6559 eV of both bareand coated NMC (Figure 4a,b) slightly shift to higherenergy.2,43 These shifts are indicative of local environmentchanges of Mn during the delithiation/lithiation process andrelated to the structural degradation of the cathode duringcycling with LGPS SSE.1−3,43,47 Compared to the Mn K-edgespectra, the spectra of Co K-edge in both bare and coatedNMC811 (Figure 4c,d) do not display obvious shifts,indicating that the Co element from NMC is relatively stableduring electrochemical reaction in SSLIBs.To further understand the microstructure and chemical

evolution of cathode materials after cycling, XRD and electronmicroscopy techniques are utilized to investigate the NMC811cathodes after charge/discharge cycling. The XRD patterns ofbare NMC811 before and after cycling are shown in FigureS10. The evolution of peaks after cycling indicates thestructural change of MNC811 during the charge/dischargeprocess. Figures 5 shows the SEM images and correspondingEDX elemental mapping of bare and coated NMC811 after 40cycles. The cross-sectional SEM specimens were prepared via afocused ion beam (FIB) technique. Interestingly, the intensiveintergranular cracks can be detected at the surface of the bareNMC811 spherical secondary particle, as shown in Figure 5a,b.With the continued FIB thinning process, the distinct crackscan be observed at the surface and extend to the interior of theparticle, as shown in Figure S11a, suggesting that the cracksoriginate from the surface and then propagate to the bulkstructure of NMC811 during the cycling. SEM-EDX mappingand linear scan spectra in Figures 5c and S12a demonstratethat the sulfur element shows enrichment within the bareNMC811 particle, indicating that the sulfur element is veryeasy to diffuse into the NMC811 particles and forms anunstable interface during battery operation, inducing thestructural evolution and cracks of NMC811 cathodes. Onthe contrary, the phenomena of both surficial cracks and sulfurelement diffusion significantly decline in the LNO-coatedNMC811 particle, indicating that the coating layer effectivelyconstructs a protective interface between the cathode and SSEsto maintain the integrity of the cathode particle and suppressthe occurrence of side reactions. Figures 6 and S13 are theatomic-level structural images and EELS of cycled bare andLNO-coated NMC811 cathodes, respectively. As shown inFigure 6a, the bare NMC811 after cycling presents obviousdifferences in the lattice patterns between the surface and bulkregion. As confirmed by the inset fast Fourier transformation(FFT) in Figure 6a, the lattice pattern at the near-surfacewithin 12 nm can be indexed as the rock-salt phase with aspace group of Fm3m, while the inner region still maintains thelayered structure phase with a space group of R3m, which isclear evidence that the surface structure of bare NMC811 hasbeen changed and reconstructed during cycling due to the sidereactions with sulfide SSEs. EELS spectra further substantiatethe significant features of the structural lattice reconstruction.As shown in Figure 6b,c, the O K-edge spectra presentsuppression of the pre-edge feature from the bulk to surface.The oxygen pre-edge is associated with the transition ofelectrons from the 1s core state to unoccupied 2p stateshybridized with 3d states in transition metals.48,49 Therefore, a

gradual suppression of the oxygen pre-edge indicates theformation of an oxygen vacancy and the reduction of transitionmetals.49 Furthermore, the Mn L-edge spectra show that theratio of the Mn L3/L2 feature is increased from the bulk tosurface, indicating that Mn reduction is more significant in therock-salt region.48,50 The chemical shift of the Ni L-edge is alsonotable, which indicates the reduction of Ni at the surfaceregion. The surface lattice evolution of NMC cathodesassociated with transition metal reduction is widely observedin the liquid-based LIBs, which is mainly derived from the sidereactions with electrolyte during the cycling.51 The atomic-level structural images and EELS spectra in this study confirmthat the severe side reaction-induced surface lattice evolutionof NMC cathodes also can be observed in the sulfide-basedSSBs. Compared to the bare NMC811, the surface phasechange of LNO-coated NMC811 is not obvious, as shown inFigure S13. The lattice pattern of the bulk region (black)presents a clear layered structure feature, and the surfaceregion (green) is slightly different from the bulk region,

Figure 5. Microstructure evolution of NMC811 cathodes aftercycling in SSLIBs identified by SEM images and EDX mapping.(a,b) Cross-sectional FIB-SEM images and (c,d) corresponding S(yellow) and Ni (cyan) maps of the red region in (a) of a bareNMC811 cathode particle. (e,f) Cross-sectional FIB-SEM imagesand (g,h) corresponding S (yellow) and Ni (cyan) maps of the redregion in (e) of an LNO-coated NMC811 cathode particle. Scalebars in the figures are (a,e) 2 μm and (b−d,f−h) 1 μm.

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illustrating the change of the layered structure to a disorderedstructure at the surface region of NMC811 after cycling. TheMn L-edge and O K-edge also present a similar tendency thattransition metals are slightly reduced at the surface, whichindicates that coating can slow down but still cannot stop theoccurrence of side reactions. Therefore, combined with SEM,STEM, and EELS results, it strongly indicated the structuraldegradation of bare NMC811 cathodes after cycling. Theresults demonstrate that the severe side reactions with sulfideSSEs not only induce the diffusion of sulfur into the NMC811particles but also accelerate the formation of microstructuralcracks on the surface of NMC811 cathodes. As a result, thetransition metal reduction and surface structure reconstructionof NMC811 are undeniable in sulfide-based SSLIBs, leading topoor electrochemical performance of batteries.Combing operando XANES, STEM, and EELS results, we

propose the following interfacial reaction mechanism: thecontact of bare NMC811 and LGPS induces parasitic reactionsat their interface, which leads to chemical state changes of thecathode and sulfide. During the initial charging process,especially at high voltage, LGPS presents a metastableintermediate state. During the following discharge process,the intermediate state cannot fully reverse back. As a result,some of the LGPSs are first decomposed to form a Li2Simpurity phase. At the same time, the parasitic reactionsaccelerate the microstructure degradation of NMC811 fromthe surface to bulk during cycling, leading to the micro-structural cracks at the surface of cathode particles, surficialtransition metal reduction of cathodes, and phase evolutionfrom the layered to rock-salt structure. The parasitic reactionsthat occurred at both sulfide SSE and cathode sides areaccumulated at the interface gradually and cut off the ionicconductivity, resulting in a large irreversible capacity and fastperformance degradation. Coating, on the contrary, helps toprotect the integrity of the NMC811 cathode and stabilize theinterface between the cathode and sulfide SSEs, leading topromising cycling performance of the cathode in SSLIBs.

In summary, we investigated the chemistry and micro-structure evolution at the interface between layered NMC811cathodes and LGPS sulfide SSEs in SSLIBs via operandoXANES and electron microscopy studies. Our results expandthe understanding of the interface mechanism of inorganic-based SSLIBs and demonstrate the potential of operandocharacterization studies to probe the highly reactiveintermediate states and unravel the complex reactionmechanisms of energy materials.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.9b01676.

Detailed experimental procedures; XRD patterns,XANES spectra, and SEM images of as-prepared cathodecomposites; additional electrochemical characteriza-tions; schematic figure of the model cell; ex situ andin situ XANES spectra of various cathodes before andafter cycling; and SEM images, STEM-HAADF imagesand corresponding EELS spectra of bare and modifiedcathodes (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: xsun@eng.uwo.ca (X.S.).*E-mail: jluo@tjut.edu.cn (J.L.).ORCIDMohammad Norouzi Banis: 0000-0002-6144-6837Jun Luo: 0000-0001-5084-2087Xueliang Sun: 0000-0003-2881-8237Author ContributionsXia Li, Zhouhong Ren, and Mohammad Norouzi Baniscontributed equally to this work. Prof. Xueliang Sun supervisedthe overall project. Xia Li conceived this idea, synthesized the

Figure 6. Microstructure evolution of NMC811 cathodes after cycling in SSLIBs identified by STEM-HAADF images and correspondingEELS spectra. (a) Atomic-level STEM-HAADF image of a bare NMC811 cathode after cycling with corresponding FFT patterns inset. (b,c)STEM-HAADF image and corresponding EELS spectra acquired from the regions. Scale bars in the figures are (a,b) 5 nm.

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electrodes, characterized the samples, tested the electro-chemical performances, and wrote this manuscript. ZhouhongRen and Jun Luo performed the TEM characterization atTianjin University of Technology. Xia Li, Mohammad NorouziBanis, Qian Sun, Weihan Li, Jianwen Liang, Yongfeng Hu, andTsun-Kong Sham carried out the synchrotron radiationcharacterization at CLS. Sixu Deng, Changhong Wang, XiaofeiYang, and Xiaona Li performed electrochemical character-ization. Yang Zhao and Yipeng Sun performed the ALDprocess. Keegan Adair polished the language. Ruying Li carriedout SEM characterization and purchased all of the chemicals.All of the authors discussed the results and commented on themanuscript. All authors have given approval to the final versionof the manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was supported by the Natural Sciences andEngineering Research Council of Canada (NSERC), CanadaResearch Chair Program (CRC), Canada Foundation forInnovation (CFI), Ontario Research Fund (ORF), the CanadaLight Source at University of Saskatchewan (CLS), Inter-disciplinary Development Initiatives (IDI) by WesternUniversity, and University of Western Ontario. Xia Liacknowledges support of the Mitacs Elevate PostdoctoralFellowship.

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