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J Materiomics 2 (2016) 256e264www.ceramsoc.com/en/ www.journals.elsevier.com/journal-of-materiomics/

Chemical compatibility between garnet-like solid state electrolyteLi6.75La3Zr1.75Ta0.25O12 and major commercial lithium battery cathode

materials

Yaoyu Ren*, Ting Liu, Yang Shen, Yuanhua Lin, Ce-Wen Nan

School of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China

Received 2 January 2016; revised 15 March 2016; accepted 2 April 2016

Available online 12 April 2016

Abstract

High-temperature chemical compatibilities between garnet-like solid state electrolyte of Li6.75La3Zr1.75Ta0.25O12 (LLZTO) and the fourmajor commercial lithium battery cathode materials, i.e., LiCoO2 (LCO), LiMn2O4 (LMO), LiNi0.33Co0.33Mn0.33O2 (NCM) and LiFePO4 (LFP)were investigated by X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), respectively. The samevolume fraction of LLZTO and electrode powders were mixed and heated at 400e900 �C for 1 h. According to the results by XRD, LMO andLFP react with LLZTO at approximately 500 �C. The Raman data reveal that both LCO and NCM can react with LLZTO at approximately 700�C, which lead to generate LaCoO3 and LaCo1�xMnxO3 with a rhombohedral symmetry, respectively.© 2016 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Cathodes; Garnet-type solid electrolyte; Reactivity

1. Introduction

Substituting solid state inorganic electrolytes for liquidorganic electrolytes to manufacture all-solid-state lithium-ionbatteries is one promising way to thoroughly resolve the safetyissues bothering the existing commercial lithium-ion batteries.However, some issues may arise with respect to batteryfabrication. One concern is that the rigidity of the solid elec-trolyte may make it difficult to achieve a good contact betweenthe electrolyte and cathodes. The poor contact would give riseto a high contact resistance, leading to a poor battery perfor-mance. This is especially so for the oxide-type solid

* Corresponding author. Current address: Department of Mechanical Engi-

neering, University of South Carolina, Postal address: 541 Main Street,

Columbia, SC 29208, United States. Fax: þ1 803 777 0137.

E-mail addresses: yroneal@gmail.com (Y. Ren), liuting20002000@163.

com (T. Liu), shyang_mse@mail.tsinghua.edu.cn (Y. Shen), linyh@mail.

tsinghua.edu.cn (Y. Lin), cwnan@mail.tsinghua.edu.cn (C.-W. Nan).

Peer review under responsibility of The Chinese Ceramic Society.

http://dx.doi.org/10.1016/j.jmat.2016.04.003

2352-8478/© 2016 The Chinese Ceramic Society. Production and hosting by Elsevi

creativecommons.org/licenses/by-nc-nd/4.0/).

electrolyte. In contrast to the sulfide counterpart, which isrelatively soft and can be incorporated into the battery by coldcompaction, the oxide electrolyte usually resorts to high-temperature treatment to ensure a better contact. The tem-perature is typically higher than 400 �C, which then posesrequirement for the chemical compatibility between electro-lyte and cathodes.

Garnet-like oxide electrolytes represent a new type of po-tential solid state electrolytes, which exhibit a wide electro-chemical window and a high stability in contact with lithiummetal [1,2]. In 2007, the Li7La3Zr2O12 garnet-like oxide withhigh room-temperature lithium-ionic conductivity of over10�4 S cm�1 was reported by Murugan et al. [3]. With thepartial substitution of tantalum or niobium for zirconium, theconductivity can be further enhanced to around 10�3 S cm�1

without sacrificing other physical properties [4e6]. Thanga-durai et al. investigated the chemical compatibility betweenLi6BaLa2Ta2O12 garnet-like oxide and a series of cathodematerials using X-ray diffraction (XRD) [7]. Their resultsindicated that LiCoO2 had the highest stability against

er B.V. This is an open access article under the CC BY-NC-ND license (http://

257Y. Ren et al. / J Materiomics 2 (2016) 256e264

chemical reaction with Li6BaLa2Ta2O12 up to 900 �C. Kimet al. investigated the interface between Li7La3Zr2O12 andLiCoO2 after heat treatment at 700 �C, and identified a newphase La2CoO4 via nano-beam electron diffraction [8]. In theirsubsequent work, the impurity phase caused a large cathode/electrolyte interfacial resistance, leading to the degraded bat-tery performance [9]. Ohta et al. found no detectable reactionbetween Li6.75La3Zr1.75Nb0.25O12 and LiCoO2 deposited viapulse laser deposition at 600 �C [10]. To our knowledge, littlework on the chemical compatibility between Ta-dopedLi7La3Zr2O12 (Ta-doped LLZO) and cathode materials havebeen reported. Huang et al. assembled an all-solid-state batteryby coating the surface of Ta-doped LLZO electrolyte withLiCoO2 sol followed by calcination at 700 �C [11]. The bat-tery could be operated under a cyclic voltammetry mode,while no information about the reactivity between electrolyteand cathode was mentioned.

In our previous work, Li6.75La3Zr1.75Ta0.25O12 (i.e.,LLZTO) with a room-temperature conductivity close to10�3 S cm�1 was synthesized [12]. In this paper, the chemicalcompatibility between LLZTO and four major commerciallithium battery cathode materials, i.e., LiCoO2 (LCO), LiM-n2O4(LMO), LiNi0.33Co0.33Mn0.33O2(NCM) and LiFePO4

(LFP) was investigated. This research could be used as ascreening process for choosing the proper cathode materialsfor LLZTO. All these cathode materials exhibit electro-chemical potential higher than 4 V vs. Liþ/Li, thus higherenergy density could be achieved.

2. Experimental

The chemical compatibility between LLZTO and thecathode materials was investigated by firing the powdermixture of the two types of materials at various temperaturesfollowed by phase detection mainly via X-ray diffraction(XRD). Cathode materials were all commercial products.Scanning electronic microscope (SEM) images in Fig. 1demonstrated the particle size and morphology of these cath-ode materials used. LiFePO4 powder was coated with amor-phous carbon.

LLZTO powder was synthesized via solid-state reaction ofthe raw materials as reported previously [12]. No alumina wasintentionally added as a raw material in this work. The rawmaterials were thoroughly mixed in 2-propanol for 8 h anddried. The dry raw materials mixture was firstly calcined at900 �C for 6 h, and then at 1100 �C for another 6 h to ensure acomplete crystallinity of the final product. The calcination wascarried out in alumina crucibles so Al contamination of thefinal product cannot be avoided. In this sense, the actualcomposition of the electrolyte material is essentiallyLieAleLaeZreTaeO. However, the Al contamination,which was not intentionally doped in LLZO like Ta, was notsupposed to affect the final results as indicated in the nextsection. No impurity phases in the as-calcined LLZTO powderwere detected by XRD (see Fig. 2). The powder was sievedthrough a 100-mesh sieve and thoroughly mixed with cathodepowder in a volume ratio of 1:1 using mortar and pestle. The

mixture was then pressed into pellets under 300 MPa to ensurethe good contacts among particles of LLZTO and the cathodematerials. The pellets were fired at 400e900 �C, which waschosen to cover the common temperature range required tosinter cathode materials on solid oxide electrolytes [7,9,13,14].Besides, cathode materials are not stable at > 900 �C. Theduration at target temperatures was set to be 1 h unless thelonger duration is needed to promote possible reactions. Insuch a case, the duration was 10 h. The mixtures ofLLZTO þ LCO, LLZTO þ LMO and LLZTO þ NCM werefired in air, and those of LLZTO þ LFP were fired in a flow of92 vol.% Ar þ 8 vol.% H2 to avoid the oxidation of Fe2þ.

XRD was conducted on Bruker D8 Advance A25 using CuKa radiation. Raman spectroscopy and X-ray photoelectronspectroscopy (XPS) were also utilized to further investigatethe reactivity between LLZTO and LCO, and LLZTO andNCM. Raman spectroscopy was conducted on a LabRAM HREvolution (HORIBA Jobin Yvon). With the aid of an opticalmicroscope, a laser beam (l ¼ 514.5 nm) with the spot size of1 mm was radiated on the interface between a LLZTO particleand a LCO (or NCM) particle in order to detect possible newphases. XPS analysis were performed on a Thermo FisherScientific ESCALAB 250Xi instrument using a mono-chromatic Al KaX-ray source (Energy: 1786.6 eV). Thebinding energies were calibrated with respect to carbon at284.8 eV.

3. Results and discussion

3.1. Analysis by XRD

Figs. 3e6 show the XRD patterns of mixtures of LLZTOand cathode materials fired at 400e900 �C. The results indi-cate that LCO and NCM are more stable against chemicalreaction with LLZTO than the other two cathode materials asdifferent types of impurity phases are largely identified in themixture of LMO þ LLZTO (see Fig. 5a) and LFP þ LLZTO(see Fig. 6a). No discernable impurity phases were observed inthe LCO þ LLZTO mixture even after firing at 900 �C for10 h (see Fig. 3). For the NCM þ LLZTO mixture, a rathersmall amount of impurity was detected in the mixture heatedat 900 �C (marked by arrows in Fig. 4a). After heating for alonger time, i.e., 10 h, a greater amount of the impurity can begenerated as suggested by the increased intensity of the cor-responding XRD peaks (see Fig. 4a). As shown in the enlargedview in Fig. 4b, the impurity peaks locate between the cor-responding peaks for LaMnO3.15 (JC-PDF, card No. 32-0484),LaCoO3 (JC-PDF, card No. 48-0123), and LaNiO3 (JC-PDF,card No. 88-0633), suggesting that the impurity could be Mn-contained perovskite with rhombohedral symmetry. Consid-ering the relative location of the peaks, we find that the im-purity could be either LaCo1�xMnxO3 (x � 0.5) [15,16], orLa(Mn, Ni)O3 solid solution with a composition close toLaNiO3 [17], or a solid solution between these two. Due to thelimit information obtained from XRD results, more precisedetections using Raman and XPS were made to further iden-tify the impurities, and the results will be shown in the next

Fig. 1. SEM images for (a) LCO, (b) NCM, (c) LMO and (d) LFP commercial powder; insets in (b) and (c) show enlarged views of the respect sample.

Fig. 2. XRD pattern for LLZTO powder calcined at 1100 �C.Fig. 3. XRD patterns for LCO þ LLZTO powder mixture calcined in air at

different temperatures.

258 Y. Ren et al. / J Materiomics 2 (2016) 256e264

section. Before that, the reactions in LMO þ LLZTO andLFP þ LLZTO are discussed below in detail.

For the case of LMO þ LLZTO (see Fig. 5a), the mainreflection peaks (at 2q ¼ 18.6�, 36.2�, and 44.0�) ascribed tospinel-like structure LMO shift to higher angles, correspond-ing to Li0.89Mn1.78O4, with the reaction temperature increasingfrom 400 �C to 600 �C (see Fig. 5b). A new phase with a peakat 2q ¼ 44.8� was detected in the reaction product at 500 �C.In the higher temperature reaction products, the intensity ofthis peak increases, and several other peaks appear (indicatedby solid circle in Fig. 5a and b), all of which are ascribed toLi2MnO3 [18]. The main peak of this phase at 2q ¼ 18.8�

overlaps with that of Li0.89Mn1.78O4, both peaks are thusundistinguishable. The concentration of Li2MnO3 in theproduct increases with increasing reaction temperature while

that of Li0.89Mn1.78O4 decreases, as suggested by the increaseand the decrease of the peak intensity of the former and thelater phases, respectively. In the reaction products at600e800 �C, the reflections due to cubic garnet-like LLZTOare absent, indicating the decomposition of LLZTO. Mean-while, new phases were detected with peaks at 29e30� and32e33�, which are temporarily ascribed to LaxZr1�xO2�x/2

solid solution and La1�xMnO3±d, respectively, according tothe PDF cards No. 82-1010 (La0.1Zr0.9O1.95), 88-1007 (ZrO2),83-1344 (La2O3), 78-0390 (Mn2O3), 89-0682 (LaMnO3), 89-0684 (La0.92MnO2.88), and 89-0687 (La0.96MnO3.05). Theexact composition of the new phases, however, cannot bedetermined. In the reaction product at 800 �C, La2Zr2O7 phasewas detected. In order to clarify whether the decomposition ofLMO into other phases is due to heating process, LMO powder

Fig. 4. (a) XRD patterns for NCM þ LLZTO powder mixture calcined in air at different temperatures, and (b) enlarged view of the 2q regions 30e35� and 45e50�

for the mixture fired at 900 �C for different durations.

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without LLZTO was heated at 600e800 �C. No new phaseswere detected by XRD (see Fig. 5c), which consist with theprevious results [19]. Therefore, it can be concluded that thedecomposition of LMO was induced by LLZTO. LLZTOreacting with LMO to form La1�xMnO3±d is the main causefor the decomposition of LLZTO and LMO. A further inves-tigation via other techniques is needed to clarify the detailedmechanism of this reaction. Since the reaction products, i.e.,LaxZr1�xO2�x/2, La2Zr2O7 and La1�xMnO3±d, are not elec-trochemically active for Liþ ion intercalation/deintercalation,the cathode performance of LMO is expected to degrade afterreaction with LLZTO.

For the case of LFP þ LLZTO, the reaction betweenLLZTO and LFP can be distinguished by the apparently dis-appearing peaks, which are ascribed to LLZTO and LFP in thereaction products at 500e800 �C (see Fig. 6a). Some phases,i.e., Li3PO4 and Fe, are identified with the peak intensitiesincreasing with increasing temperature. In the reaction productat 800 �C, La2Zr2O7 phase was detected. The reaction is notsupposed to be induced by the reducing atmosphere since nodecomposition of LFP occurred in the same atmosphere in theinvestigated temperature range (see Fig. 6b). LLZTO itself issupposed to decompose neither since none of its elements arereducible under the same condition. We suggest that the re-action was induced by the incorporation of Li from LLZTOinto LFP, resulting in the formation of Li3PO4 and iron oxide,which was successively reduced to Fe by H2.

3.2. Observations by optical microscope

Although the XRD results indicate no reactions betweenLLZTO and LCO and minor reaction between LLZTO andNCM, we clearly observe the color change of the reactionmixture from black to green when firing temperature increases(see the inset in Fig. 7). The major color change occurs onLLZTO powder (see 1 in Fig. 7), from white to green for both

cases. Kim et al. found the similar phenomenon in the powdermixture of LCO and LLZO after firing at 700 �C [8]. Byinvestigating the interface between a LLZO pellet and a pulselaser deposited LCO thin film, they found a new phase cor-responding to La2CoO4, as suggested by the nano-beamelectron diffraction result. Considering the limit detectioncapability of XRD, we used Raman and XPS to more clearlyinvestigate the reactivity between LLZTO and LCO and be-tween LLZTO and NCM particles next.

3.3. Raman spectroscopic studies

Fig. 8 shows the Raman spectra of LCO þ LLZTO andNCM þ LLZTO as well as pure LLZTO, LCO, and NCM. Thespectra of all the pure compounds are consistent with theprevious results [20e22] even after heat treatment at 900 �Cfor LCO (see Fig. 8a) and NCM (see Fig. 8b). After reaction, aphase with the peak locating at 685 cm�1 is clearly identifiedfor the LCO þ LLZTO mixture (see Fig. 8a). The peak can bediscernable for the samples heated at 700e900 �C. No such apeak appears for pure LLZTO and pure LCO even heated at900 �C, indicating that the new phase was formed due to theheat-induced reaction between the two oxides. The increasingintensity of the peak with increasing heat-treatment tempera-ture indicates an increasing amount of the new phase, whichcan be correlated to the color change in the appearance of thesamples (see Fig. 7a). The similar results are obtained forNCM þ LLZTO mixture; and a peak at 691 cm�1 is wellresolved for the reaction products at 800e900 �C (see Fig. 8b).

The Raman peak at 685e691 cm�1 can be assigned to thesymmetric OeMeO (M ¼ Ni, Co, Mn) stretching mode vi-bration of [MO6] in many compounds [23e29]. ForNCM þ LLZTO, the mode can be ascribed to [MO6] inrhombohedral LaMO3 [26,30], in accordance with the XRDresult (see Fig. 4b). The Raman peak at 685 cm�1 for theLCO þ LLZTO mixture can be attributed to LaCoO3

Fig. 5. XRD patterns for (a) LMO þ LLZTO powder mixture, (b) enlarged

view of the 2q regions 18e20� and 35e46�, and (c) LMO power calcined in

air at different temperatures.

260 Y. Ren et al. / J Materiomics 2 (2016) 256e264

perovskite [27]. Unlike the LCO þ LLZTO mixture, however,the intensity of the peak at 691 cm�1 for the NCM þ LLZTOsample heated at 900 �C (see Fig. 8b) is relatively low,compared to the counterpart for the LCO þ LLZTO. Sinceonly the impurity in NCM þ LLZTO can be detected by XRD(see Fig. 4b), showing that the amount of impurity inNCM þ LLZTO is greater than that in LCO þ LLZTO, therelatively low intensity of the Raman peak could be explained

by the possible loss of degeneracy upon lowering of thesymmetry of the [MO6] octahedron and the increase of the unitcell to more than one LaMO3 unit [31,32], which may result inbroad bands with low intensities. Besides, a small shoulder,with its intensity increasing with increasing heating tempera-ture, appears at 577 cm�1 (see Fig. 8b), showing the formationof another new phase. Li(Ni, Co, Mn)O2 with a layeredstructure has a Raman band corresponding to the symmetricOeMeO (M ¼ Ni, Co, Mn) stretching mode vibration of[MO6] at this frequency [20]. The peak of such a mode, i.e.,A1g mode, for LCO and NCM is shown in Fig. 8a and b,respectively. In general, the peak frequency of this mode shiftsto a greater value in the order of Ni < Co < Mn [20,33,34].The peak frequency is distinctly lower than that of NCM, buthigher than that of LiNiO2(545 cm�1 [35]). Accordingly, thenew phase could be minor amount of Li(Ni, Co, Mn)O2 solidsolution but with a composition with higher Ni content thanNCM [36,37]. This solid solution would be expected to beanother product from the reaction between LLZTO and NCMif we consider that the impurity in NCM þ LLZTO detectedby XRD is LaCo1�xMnxO3 (x � 0.5).

Note that a peak at 685e691 cm�1 can be also attributed toother compounds such as Co3O4 or CoO [28], or La2CoO4

[29,38]. Clearly, some of these compounds may cause changesof valence state of transition metals, which may be detectableby XPS.

3.4. Analysis by XPS

Fig. 9 compares the XPS results of the transition metal 2porbitals for LCO and NCM in the pristine state, after heated at900 �C alone, and with LLZTO. No satellite peaks at785e788 eV indicative of Co2þ were detected for all thesamples (see Fig. 9a and b), indicating that the cobalt is pre-sent mainly as a trivalent cation [39]. In Fig. 9a, the mainCo2p3/2 peaks at 779.8 eV obtained from a pristine LCOpowder correspond well to the trivalent state of cobalt ions[39,40]. No peak shift was observed for the pure LCO afterheated at 900 �C. After reaction with LLZTO at 900 �C, a shiftof the Co 2p peaks for LCO to lower binding energy wasobserved, indicating an increasing concentration of Co3þ [41].This phenomenon is consistent with the case of assigning thenew phase detected in the Raman spectra (see Fig. 8a) toLaCoO3 [42], while excludes the possibility of assigning it toother compounds with a great concentration of Co2þ, e.g.,CoO and La2CoO4 [43e46].

For the case of NCM þ LLZTO, the main peak bindingenergies of Co2p (Fig. 9b), Mn2p (see Fig. 9c) and Ni2p (seeFig. 9d) in the XPS spectra for pristine NCM are 780, 642.3and 854.5 eV, respectively, suggesting that the correspondingelements are mainly in the valence state of Mn4þ, Ni2þ andCo3þ [47,48]. No obvious peak shift was observed for all theelements in the pure NCM after heated at 900 �C. After re-action with LLZTO, a shift of the Co2p peaks for NCM tolower binding energy was observed in the XPS (see Fig. 9b).Besides, the Mn2p peaks also shift to a lower binding energy(see Fig. 9c). No change of the Ni2p peak position was

Fig. 6. XRD patterns for (a) LFP þ LLZTO powder mixture, and (b) LFP powder calcined in a flow of 92 vol.% Ar þ 8 vol.% H2 at different temperatures.

Fig. 7. Optical micrographs and photos (inset) of (a) LCO þ LLZTO and (b) NCM þ LLZTO powder mixture pellets calcined at different temperatures. LLZTO,

LCO, and NCM particles are marked with number 1, 2, and 3, respectively.

261Y. Ren et al. / J Materiomics 2 (2016) 256e264

observed (see Fig. 9d). These results clearly demonstrate thatthe reactions between NCM and LLZTO involves in thechange of valence state of Mn ions from þ4 to þ3 and Co ionsfrom þ2 to þ3, which is consistent with the possible forma-tion of LaCo1�xMnxO3 (x < 0.4) as the reaction product[49,50]. The unchanged Ni2p peak position upon possiblevariation of Ni concentration in NCM due to the reaction couldbe due to a tendency toward a “NiO”-like surface for nickelcations in lithium nickelate based compound [39].

The above investigations further prove that besides NCM,LCO can also react with LLZTO although the reactivity be-tween LCO and LLZTO is weaker as suggested by XRD.However, whether the reactions have a significant influence onthe performance of solid-state lithium-ion batteries needs to beclarified in a real battery system. Iriyama et al. [9] demon-strated a poor performance of a battery with LCO thin film

cathode deposited on LLZO pellet at 600 �C. In this battery,La2CoO4, the reaction product between LCO and LLZO, wasattributed to be the main obstacle that limits the performance.

3.5. General discussion

One factor affecting the reactivity between LLZTO andcathode materials is the particle size of the reactants. Thenanoparticles with the size of <100 nm have a higher reac-tivity due to the greater specific surface area. In this work, thesize of LLZTO is ~2.5 mm [12]. The size of primary LMO andLFP particles is about 200e400 nm (see Fig. 1c) and200e500 nm (see Fig. 1d), respectively, which are slightlysmaller than that of LCO (i.e., 1e5 mm, see Fig. 1a) and NCM(i.e., ~1 mm, see Fig. 1b). All these particles are too large totrigger any essential enhancement of the reactivity, and thus

Fig. 8. Raman spectra of (a) LCO þ LLZTO and (b) NCM þ LLZTO powder mixture calcined at different temperatures.

Fig. 9. XPS spectra of (a) Co2p for LCO, and (b) Co2p, (c) Mn2p, and (d) Ni2p for NCM.

262 Y. Ren et al. / J Materiomics 2 (2016) 256e264

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size effect on the reactivity between LLZTO and cathodematerials should be minimal. For LMO þ LLZTO andLFP þ LLZTO, the disappearance of both original phasesafter high-temperature calcinations further indicates a ther-modynamically controlled bulk reaction. In contrast, forLCO þ LLZTO and NCM þ LLZTO, the lack of or limitdetectable reaction products even after longer term calcinationmay be due to either dynamically limited reaction or reactionconfined to surface impurities. The latter could be true, byconsidering that LLZTO is not totally stable in air and mayform surface impurities, e.g., Li2CO3, by reacting with hu-midity and CO2 [51].

4. Conclusions

Among the four major commercial lithium battery cathodematerials, LCO and NCM were more stable against reactionwith LLZTO than LMO and LFP. The latter two can react withLLZTO at temperature as low as 500 �C as revealed by XRD,while the former two can react with LLZTO at approximately700 �C. It was indicated that one of the reaction productsbetween LCO and LLZTO and between NCM and LLZTOwas LaCoO3 and LaCo1�xMnxO3 with rhombohedral sym-metry, respectively.

Acknowledgments

This work was supported by China Postdoctoral ScienceFoundation funded project (grant No. 043202211) and theNational Natural Science Foundation of China (grant No.51221291).

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18294e300.

Dr. Yaoyu Ren is currently a Postdoctor fellow in

University of South Carolina. He received a bachelor

degree of Chemical Engineering and Technology

from South China University of Technology and a

doctor degree of Materials Science and Engineering

from Tsinghua University. He was a Postdoctor in

Professor Ce-Wen Nan's group in Tsinghua Univer-

sity from 2013 to 2015. His research interests focus

on designing and optimization of energy-related

ceramic-based devices such as all-solid-state

lithium-based batteries and solid oxide fuel cells.