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Drawing a Soft Interface: An Effective Interfacial Modification Strategy for

Garnet-type Solid-state Li Batteries

Article  in  ACS Energy Letters · May 2018

DOI: 10.1021/acsenergylett.8b00453

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Drawing a Soft Interface: An EffectiveInterfacial Modification Strategy for Garnet-Type Solid-State Li BatteriesYuanjun Shao,†,‡,# Hongchun Wang,‡,# Zhengliang Gong,*,‡ Dawei Wang,§ Bizhu Zheng,§

Jianping Zhu,§ Yaxiang Lu,*,† Yong-Sheng Hu,† Xiangxin Guo,∥ Hong Li,† Xuejie Huang,†

Yong Yang,*,‡,§ Ce-Wen Nan,⊥ and Liquan Chen†

†Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, ChineseAcademy of Sciences, School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China‡College of Energy, Xiamen University, Xiamen 361005, China§State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and ChemicalEngineering, Xiamen University, Xiamen 361005, China∥State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, ChineseAcademy of Sciences, Shanghai, 200050, China⊥School of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University, Beijing100084, China

*S Supporting Information

ABSTRACT: Garnet-type solid-state electrolytes (SSEs) are con-sidered to be a good choice for solid-state batteries, yet theinterfacial issues with metallic Li limit their applications. Herein, wepropose an ultrasimple and effective strategy to enhance theinterfacial connection between garnet SSEs and Li metal just bydrawing a graphite-based soft interface with a pencil. Bothexperimental analysis and theoretical calculations confirm that thereaction between the graphite-based interfacial layer and metalliclithium forms a lithiated connection interface with good lithium-ionic and electronic conductivity. Compared to the reportedinterfacial materials, the graphite provides a soft interface withbetter ductility and compressibility. With improvement by this softinterface, the impedance of symmetric Li cells significantlydecreases and the cell cycle is stable for over 1000 h. Moreover, a solid-state battery with Li-metal anode, ternaryNCM523 cathode, and treated-garnet SSEs is fabricated and displays excellent rate capability and long cycling performance.

Although lithium-ion batteries (LIBs) are currentlywidely used in many fields, particularly in the portableelectronics and automotive industries, the flammable

organic electrolytes used in conventional batteries may not onlyincur safety issues but also limit the applications of high-voltagecathodes because of their intrinsic narrow electrochemicalwindow.1,2 Solid-state batteries (SSBs), one of the mostpromising next-generation energy storage devices, have nowbeen extensively developed because of their high safety andlong cycle life.3,4 Furthermore, given the fact that Li-metalanode possesses the highest theoretical capacity (3860 mAhg−1) and the lowest potential (−3.040 V vs Li/Li+), greatimprovement in energy density is expected when paired withsolid-state electrolytes (SSEs).5

The high ionic conductivity and perfect contact interface areboth the most important features of SSEs materials whenemploying for the design of solid-state lithium batteries. A largenumber of SSEs materials have been reported, includingperovskite,6 antiperovskite,7 LISICON,8 thio-LISICON,9 NA-SICON,10 garnet,11 sulfide glass ceramic,12−14 and so on. Theconductivity of most SSEs is close to the level of 1 mS cm−1 atroom temperature and could meet the requirements incommerc ia l ba t te r ies . In par t i cu la r , the su lfideLi9.54Si1.74P1.44S11.7Cl0.3 even exhibits a high conductivity of 25mS cm−1, which exceeds that of most organic liquid

Received: March 21, 2018Accepted: May 1, 2018

LetterCite This: ACS Energy Lett. 2018, 3, 1212−1218

© XXXX American Chemical Society 1212 DOI: 10.1021/acsenergylett.8b00453ACS Energy Lett. 2018, 3, 1212−1218

electrolytes.15 However, most electrolytes, such as Ti-based,Ge-based, and sulfide materials, are thermodynamical unstableagainst metallic Li and have difficulty forming a stable interfacedirectly.16−19 Solving the interfacial problem is increasinglycrucial for the development of high-energy-density solid-statelithium batteries. Garnet-type SSEs Li7La3Zr2O12 with a wideelectrochemical window have attracted extensive attention dueto the chemical and electrochemical stability against metallicLi.11 With aliovalent ion doping at Li, La and Zr sites, such asCa2+, Al3+, Ga3+, Nb5+, Ta5+, Te6+, W6+, and so on, the ionicconductivity of cubic phase could be improved close to 1mS cm−1 at room temperature.20 However, the poor wettingproperties of garnet SSEs with molten metallic Li and theuneven polished surface of sintered ceramic jointly causemicroscopic gaps at the interface and limit the contact area,which leads to a huge interfacial impedance and poorelectrochemical cycling performance.21,22

It is extensively reported that introducing a buffer layerbetween garnet SSEs and metallic lithium is an effective way toimprove their contact and decrease the interfacial impedance.Recently, a series of interfacial layer materials have beenreported by Hu et al., including Al2O3

23, Al24, Si25, Ge26,ZnO27, and so on.28,29 For these or similar interfacial layerswhich were prepared by the techniques such as atomic layerdeposition (ALD), plasma-enhanced chemical vapor deposition(PECVD), physical vapor deposition (PVD), and so on, are allproved to be effective and have obtained the expected results.However, these strategies are somewhat complicated and costly,which may limit the practical applications of the garnet solid-state lithium batteries in the future. In addition, the reportedinterface layer are all inorganic oxide or metal materials, and thepoor ductility and compressibility may not endure for the longcycle life and the large amount of Li plating−stripping found insolid-state lithium batteries. Moreover, Jiang’s work proposedthat soft substrates could release the compressive stress duringlithium electroplating process and impede the lithium dendritegrowth.30 Therefore, it is very necessary to develop a simple,convenient, and efficient strategy to prepare a soft interfacelayer on garnet SSEs to mitigate Li dendrite growth andenhance the cells’ performance.In this work, an utterly simple and easily operated strategy is

demonstrated to address this interfacial issue by simply using apencil to paint a graphite-based layer on the top of the garnet-type Li5.9Al0.2La3Zr1.75W0.25O12 (LALZWO) ceramic andgenerate a soft interface between SSEs and Li metal. The

wettability and interfacial contact between garnet-type SSEsand Li metal have been evidently improved, which leads to asignificant decrease in interfacial impedance. Besides, it alsodemonstrates that the graphite-based interface layer could belithiated to form the LiC6; thus, the lithium ion and electroncould diffuse in the in situ formed lithiated graphite layerspontaneously at room temperature, which may ensure theuniform distribution of lithium ions in the interfacial layer.Moreover, combined with the good ductility and compressi-bility of graphite materials, this soft interfacial layer provides aclose connection at interface and exhibits excellent stability,resulting in a good electrochemical cycling performance insymmetric cells. Finally, on the basis of this novel interfaceengineering strategy and the previously developed toothpaste-like cathode fabrication method, a solid-state lithium batterywith excellent performance is successfully designed with Li-metal anode, garnet electrolyte, and ternary NCM523 cathode.We believe that our strategy is simple, convenient, and efficient;it could be extended to solve interfacial contact problemsbetween other electrolytes and Li metal, and will become apromising strategy for practical SSB applications.Tungsten (W)-doped garnet Li5.9Al0.2La3Zr1.75W0.25O12

(LALZWO) ceramic pellets were prepared by followingprevious work and used as the base electrolyte to investigatethe interfacial properties.31,32 As shown in Figure S1, the X-raydiffraction (XRD) pattern of the sintered LALZWO ceramicpellet matches well with that of cubic garnet-phaseLi7La3Zr2O12 (ICSD 063-0174), indicating that the sinteredLALZWO is pure cubic phase, which has a higher Li-ionconductivity compared with that of tetragonal phase. The Li(24d) and Zr (16a) sites were doped by Al and W atoms,respectively, as shown in Figure 1a, and the Li-ion transportroute was 24d-96h-48g-96h-24d, which had been proven byNMR technology in previous work.32 In comparison, tungsten-free samples were also synthesized. The ionic conductivity ofthe pure LALZO and LALZWO were evaluated by EIS coupledwith gold electrodes at temperatures ranging from 15 to 85 °C,and the resulting Arrhenius behaviors are summarized in Figure1b. The Al and W codoping significantly improves the phasestability and ionic conductivity of samples, leading to an ionicconductivity of 5.2 × 10−4 S cm−1 at room temperature and anactivation energy of 0.36 eV, respectively. Figure 1c displays anatomic force microscopy (AFM) image of the polished ceramicpellet, indicating the morphology of the local environment anda surface roughness of about 500 nm.

Figure 1. Characterization of the as-prepared LALZWO garnet electrolyte. (a) Crystal model of garnet-structured LALZWO. (b) Arrheniusplots of the LLZO and LALZWO to present the ionic conductivity. (c) AFM image of top view of the LALZWO ceramic pellet. Color bar is0−650 nm.

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Consistent with the results of previous reports, the molten Limetal cannot wet the bare garnet appropriately, as shown inFigures S2 and S3, where the molten Li reunites to form a ball,resulting in poor contact and large interfacial resistance.Normally, to decrease the interfacial impedance, it is extremelynecessary to build an interfacial layer between the LALZWOceramic pellet and metallic lithium. On the basis of recentworks,23−27 this interfacial material must have three importantproperties: (a) ability to form a close bond with garnet pellet,(b) good reaction characteristic with lithium to form a mediumlayer/phase which has Li-ion conduction capability, and (c)good compatibility with Li metal. Given the fact that graphite isa widely used anode material in lithium ion battery and couldbe easily lithiated to form LiC6,

33−35 it can be an idealinterfacial layer material. Furthermore, compared to thereported inorganic oxide or metal materials, as shown inTable 1,34 the graphite displays lower Young’s modulus and

better ductility and compressibility, which means that thegraphite could establish a soft interlayer and thus provide theextra benefit to the cycling stability and enhance the interfaceconnection between garnet electrolyte and Li metal.An interface is realized in this work by simply drawing a

graphite-based layer with a pencil on the surface of garnet

ceramic pellet, as shown in Figure 2a. The image in Figure 2bdemonstrates the great improvement of the wetting propertiesof molten Li metal with graphite layer on the surface of garnetSSEs. The XRD pattern and Raman spectroscopy revealed thegraphite properties of the drawn interlayer, as shown in Figures2c and S4. The (002), (101), and (004) characteristic peaks ofgraphite are observed in the XRD pattern of the interlayer-modified LALZWO ceramic pellets, which is also proved by thehigher intensity of G peak relative to that of the D peak inRaman spectroscopy. The relationship between the mass ofgraphite-coated garnet and the painting number (the number ofstrokes with the pencil) is shown in Figure 2d. The loadingmass of graphite layer tends to reach a maximum with theincrease of the painting number, which implies that theinterfacial layer can be coated only to the surface of garnetceramic rather than the as-painted graphite layer repeatedly.The SEM images confirm the close connection between thegraphite-based interfacial layer and LALZWO ceramic pellets,and the maximum thickness of the graphite-based interfaciallayer is about 1 μm, as shown in Figures 2e, S5, and S6, whichreflects that the coated graphite layer and lithiated graphitelayer both form a close connect with garnet and modify theuneven surface of the LALZWO ceramic pellet effectively. TheSEM images in Figures 2f and S7 demonstrate a close interfacialconnection between modified the LALZWO ceramic pellet andmetallic lithium, further revealing that the graphite interfaciallayer effectively improved the wetting properties of garnet SSEswith molten Li metal and could eliminate microscopic gaps atthe interface.

Table 1. Young’s Modulus of the Reported Interfacial LayerMaterial and Graphite34

material Al Ge Si ZnO Al2O3 graphite

Young’s modulus (GPa) 69 79.9 107 135 390 27

Figure 2. Preparation and characterizations of graphite-based interface layer. (a) Schematic of the preparation of graphite-based interfacelayer simply by drawing with a pencil. (b) Comparison of the wetting behaviors of molten metal lithium on LALZWO ceramics with andwithout graphite-based interface layer. (c) XRD pattern of the graphite interface coated on sintered LALZWO ceramic electrolyte. (d) Plot ofthe total mass of graphite-coated garnet vs the painting number. (e) SEM image of the graphite-coated LALZWO ceramics. (f) Interface SEMimage of the graphite-coated LALZWO ceramic with metallic Li

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To understand the effectiveness of this interface wettingmethod, experiment and theoretical calculations were con-ducted to explore the formation mechanism of the graphite-based interface layer. First, a graphite-based layer was coated onhalf of one side of the ceramic pellet and made to contact withmolten metallic lithium (conducted at a heating platform, 210°C). As shown in Figure 3a (region 1) and in Figure S8 (wholesurface region), this interfacial layer showed a rapid and clearcolor change from black to gold, indicating the occurrence of areaction between graphite and lithium metal and the formation

of LiC6.33,36,37 After cooling the half area lithiated pellet (region

1 in Figure 3a, which has golden color) to room temperature,we draw the graphite layer on one-quarter of the ceramic pellet(region 2 in Figure 3b) once again and placed this pellet in aglovebox for aging. Panels b, c, and d of Figure 3 record theaging photographs of 0, 3, and 8 h, respectively, which showedthat the gold color spread into the newly painted graphite layergradually and completely covered this painted area (see colorchange in region 2), indicating that the lithium ion and electroncan diffuse from lithium metal to graphite through the newly

Figure 3. Mechanism of graphite-based interface layer reacted with Li metal. (a−d) Experimental evidence of the reactions occurring betweengraphite and Li metal (a) demonstrating the resulting gold color by contacting the melted Li metal with drawn graphite interface on half of thegarnet solid-state electrolyte and displaying the color change by drawing graphite on another quarter of pure ceramic and setting aside atroom temperature for (b) 0 h, (c) 3 h, and (d) 8 h. (e) Schematic of the mechanism of graphite reacted with Li metal via the generated LiC6.

Figure 4. Characterizations of graphite-based interface layer in symmetric Li cells. (a) Comparison of EIS profiles of the symmetric Li garnetcells with and without graphite-based interface. The inset shows the enlarged impedance curve of the former. (b) Comparison of galvanostaticcycling performance of symmetric cells including Li/bare-garnet/Li and Li/modified-garnet/Li at a current density of 50 μA cm−2. (c)Galvanostatic cycling of Li/graphite-interface-treated garnet/Li cell at a current density of 300 uA cm−2 and insets showing the magnifiedcurve from 0 to 10 h, 495 to 505 h, and 993 to 1003 h.

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generated LiC6 spontaneously at room temperature. The resultsrevealed the generated LiC6 is a good lithium-ionic andelectronic conductor at room temperature. These experimentsprovide strong evidence and assist to propose a viable reactionmechanism to illustrate the phenomenon between metalliclithium and the graphite layer, as shown in Figure 3e. Thetheoretical calculations based on density functional theory(DFT) were used for interpreting the Gibbs free energy of thereaction. Based on the structures of graphite and LiC6 (FigureS9), the calculated ΔG of the reaction Li + C6 → LiC6 is−10.59 kJ mol−1, indicating that the reaction is a spontaneousprocess on thermodynamics and it is in line with theexperimental results. Furthermore, the spontaneous diffusionmechanism might ensure the Li ion distributed uniformly in theinterface layer; thus, an even current distribution could beachieved while cycling, which should have a positive effect inpreventing lithium dendrite growth. Therefore, the graphitecoating could react with metallic Li rapidly and form a highly

conductive interlayer between the garnet electrolyte and Limetal anode, which will improve the wetting behavior greatly.To verify the effect of graphite modification on the

improvement of the garnet/Li interface, symmetric Li/graph-ite-garnet-graphite/Li cells and Li/bare-garnet/Li cells wereprepared and evaluated. EIS was used to quantify the changesof interfacial resistance. A significant arc decrease from 1350 to105 Ω cm2 was observed in Figure 4a. Both arc impedancescould be attributed to the combination of LALZWO grainboundary impedance and the interfacial impedance betweenLALZWO and metallic Li,23 and this improvement in thecomplex impedance could be attributed to the betterconnection between the LALZWO ceramic pellet and metallicLi and the modification of the coated soft graphite-based layeron the interface. Galvanostatic cycling experiments were carriedout to evaluate the stability and Li-ion transport capabilityacross the interface. As shown in Figure 4b, with this graphiteinterface layer, the plating−stripping of the symmetric cell (50uA cm−2, 50 uAh) is more stable with a very small overpotential

Table 2. Comparison of Cycling Performance for Interfacial Layer Materials in Symmetric Li Cell

material Al Ge Si ZnO Al2O3 Gel graphite graphite(80 °C)

current density (mA·cm−2) 0.2 0.1 0.2 0.1 0.2 0.125 0.3 0.5Li electrodeposition (mAh) 0.017 0.008 0.033 0.017 0.1 0.125 0.3 3ref 24 26 25 27 23 28 this work this work

Figure 5. Fabrication schematic and room-temperature performance of solid-state battery based on the pencil-drawing graphite interfacemethod. (a) Schematic of solid-state battery with Li metal anode, graphite coated LALZWO solid-state electrolyte, and ternary NCM523cathode. (b) Charge and discharge profiles of the battery at different rates. (c) Cycling performance of the battery at different rates. (d)Cycling performance of the battery at a rate of 0.5 C with first 3 cycles tested at 0.1C.

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of 6 mV at room temperature. In contrast, the Li/bare-garnet/Li shows a large polarization with a fluctuated potential growthand drop at the same condition, reaching 150 mV at the firstplating cycle, which is regarded as a typical phenomenon of Liinfiltration into the SSEs because the subsequent cycles showan uneven lithium deposition and dissolution.38 To verify theadvantage of the better ductility and compressibility from thegraphite-based soft interfacial layer, Figure 4c displays thecycling performance of a symmetric Li cell tested at a currentdensity of 300 μA cm−2 with 1 h per direction at roomtemperature (the highest current density and largest amount ofLi plating−stripping reported so far, as shown in Table 2),demonstrating that the cell cycles stably over 1000 h with noobvious polarization or Li infiltration. There is a modest changein EIS and SEM images of the Li/garnet interface in thesymmetric Li cell after cycling for 1000 h, as shown in FiguresS10 and S11, indicating the excellent stability of the softinterfacial layer during long cycling. Simultaneously, thesymmetric Li cell was also tested at a higher current densitywith a greater amount of Li plating−stripping (500 μA cm−2, 3mAh) at 80 °C; both the potential curves and the SEM image(as shown in Figure S12) further verify the advantage of thegraphite-based soft interfacial layer on the stability. All theresults confirm that the easily achieved graphite interfacial layerfacilitates the lithium ion diffusion, enhances the interfacialconnection, and improves the interfacial stability effectively,which confirm the previously illustrated mechanism and thegood ductility and compressibility of the graphite materials.In addition to the application in the symmetric cell, this

novel interface engineering strategy can be potentially used insolid-state lithium batteries. The schematic of the designedsolid-state battery is shown in Figure 5a. The batteries wereprepared based on the Li/graphite-painted garnet/toothpaste-like cathode prototype. The toothpaste-like cathode wasprepared with ionic liquid as the wetting agent and carbonblack as the conductive agent, which has been proven feasibleand achieved the ideal cycle performance in a solid-statesodium battery.39 Considering the energy density and powerdensity, the ternary LiNi0.5Co0.2Mn0.3O2 (NCM523) waschosen as the active material. The charge−discharge curvesand rate performance are shown in panels b and c of Figure 5,respectively. The cathode material achieved a reversible specificcapacity of 175 mAh g−1 at the current density of 0.1 C and 141mAh g−1 at the current density of 3 C, which was consistentwith the performance of the materials in a lithium-ion batterywith organic liquid electrolyte. With the Coulombic efficiencyover 99%, as shown in Figure 5d, there is no obvious capacitydecay at 0.5 C and more than 80% capacity retention after 500cycles. The batteries were also tested at a high current densityof 2 C at high temperature of 60 °C, as shown in Figure S16,and also display excellent electrochemical performance. Theresults indicate that the easily prepared graphite interfacial layercan be used in the solid-state battery to achieve the desiredelectrochemical performance, which shows the great potentialfor widespread use in the future.In summary, a graphite-based soft interfacial layer has been

proposed and achieved on a garnet-type LALZWO ceramicpellet via an utterly simple strategy of directly drawing with apencil. The interfacial layer significantly improves the wettingproperties of garnet SSEs with molten metallic Li and reducesthe interface impedance in symmetric Li cells effectively. Bothexperimental analysis and theoretical calculations have con-firmed that the graphite interlayer is lithiated to form the

golden LiC6 and the Li ion and electron could transform frommetallic Li to graphite through the generated LiC6 sponta-neously at room temperature, which ensures the evendistribution of Li ions at the interface layer. Benefiting fromthis mechanism and the good ductility and compressibility ofthe graphite materials, the Li plating−stripping process showsmuch better cycling stability in the graphite-based softinterfacial layer coated symmetric cell than in the bare garnetsymmetric cell, implying the great improvement of theinterfacial stability. Combined with the prepared toothpaste-like ternary NCM523 cathode and metallic Li anode, thefabricated solid-state batteries achieve excellent performance.Compared with the reported interfacial layer preparationmethods, the graphite-based soft interface proposed in thiswork is much more simple and easily handled with lower cost,and it can effectively improve the interfacial contact andstability. These results shed light on the research andapplications of garnet SSEs in solid-state lithium batteries.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.8b00453.

Experimental details; XRD patterns, SEM and AFMimages, and Raman spectra characterizations of theLALZWO material and graphite-based interfacial layer;and additional EIS and electrochemical performance ofthe symmetric Li cells and the solid-state NCM523/modified-garnet/Li batteries (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: yyang@xmu.edu.cn.*E-mail: zlgong@xmu.edu.cn.*E-mail: yxlu@iphy.ac.cn.ORCIDYuanjun Shao: 0000-0002-4932-057XYong-Sheng Hu: 0000-0002-8430-6474Hong Li: 0000-0002-8659-086XXuejie Huang: 0000-0001-5900-678XAuthor Contributions#Y.S. and H.W. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is financially supported by National Natural ScienceFoundation of China (NSSFC, Grant Nos. 21233004,51725206, 21473148, 21428303, 21303147, and 51421002)and National Key Research and Development Program ofChina (Grant No. 2016YFB0901500)

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ACS Energy Letters Letter

DOI: 10.1021/acsenergylett.8b00453ACS Energy Lett. 2018, 3, 1212−1218

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