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Achieving efficient and stable interface between metallic lithium and garnet-type solid electrolyte through a thin indium tin oxide interlayer
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Achieving efficient and stable interface between metallic lithium and garnet-type solid electrolyte through a thin indium tin oxide interlayer Jiatao Lou a, Guoguang Wang b, Yang Xia a, Chu Liang a, Hui Huang a, Yongping Gan a, Xinyong Tao a, Jun Zhang a, **, Wenkui Zhang a, * a b
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, PR China Hengdian Group DMEGC Magnetics Co., Ltd, Dongyang, 322118, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� ITO interlayer achieves efficient and stable Li/garnet interface. � Li/garnet interfacial resistance is reduced from 1192 to 32 Ω cm2. � Li/ITO-garnet-ITO/Li symmetric cell cycles over 800 h at 0.2 mA cm 2. � Critical current density of Li/ITOgarnet-ITO/Li cell reaches 1.05 mA cm 2. � Li/ITO-garnet/LiFePO4 hybrid cell shows stable electrochemical performance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Li7La3Zr2O12 Solid-state electrolytes Solid-state lithium batteries Indium tin oxide Lithium/electrolyte interface
Garnet-type Li7La3Zr2O12 (LLZO) ceramic electrolytes are promising solid electrolytes in solid-state lithium-ion batteries due to their relatively high ionic conductivity and high stability against lithium metal. However, the poor contact between garnet electrolytes and Li metal causes high interfacial resistance. In this work, we report a strategy to tackle this problem by modifying the surface of the Li6.4La3Zr1.4Ta0.6O12 (LLZTO) pellets with indium tin oxide (ITO), which is widely used as transparent conductive films in electronic devices. Lithium is tightly soldered on the garnet pellets through rapid reaction with the ITO interlayer. Thus the interfacial resistance of Li/LLZTO dramatically decreases from 1192 Ω cm2 to 32 Ω cm2. Due to the superior ability to homogenize current distribution, the lithiated ITO layer can protect the LLZTO pellets with low relative density (~92%) surviving at a critical current density up to 1.05 mA cm 2. Moreover, the Li/ITO-LLZTO-ITO/Li symmetric cell shows stable lithium plating/stripping over 800 h at 0.2 mA cm 2 without polarization increasing. Hybrid solidstate Li/ITO-LLZTO/LiFePO4 cell with good cycle stability and rate performance is also achieved. This work demonstrates that the ITO thin film, through conversion and alloying reactions, can effectively solder lithium anode on garnet-type solid electrolyte for high-performance lithium batteries.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (W. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227440 Received 1 September 2019; Received in revised form 23 October 2019; Accepted 9 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jiatao Lou, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227440
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1. Introduction
SnO2 as the interlayer, which react with lithium to form nanosized LixSn homogenously dispersed in Li2O matrix [38]. The LixSn alloy enhanced the transport of lithium ions and electrons at the interface, and Li2O can effectively restrict volume change of the alloy. This implies that the composite interlayer is much more stable than the simple alloy. As known as a typical kind of N-type oxide semiconductor, indium tin oxides (ITO) is widely used in light-emitting diodes (LEDs) and solar cells as transparent conductive films. Due to the superior electrical conductivity of ITO, Nan et al. [39] sintered the cathode materials on the garnet pellet by using ITO particle as the conductive agent. Herein, we propose In2(1 x)Sn2xO3 (ITO: 90 wt% In2O3, 10 wt% SnO2) as an artifi cial intermediate layer between Li anode and Li6.4La3Zr1.4Ta0.6O12. By the conversion reaction between Li and ITO at 200 � C, an interface composed of Li oxide, LixIn, and LixSn is formed. The reaction can firmly solder metallic Li on LLZTO pellet. More importantly, the unique structure formed by the reaction can maintain interface integrity during cycling. The area specific resistance between metallic Li and LLZTO is drastically reduced from 1192 Ω cm2 to 32 Ω cm2. After more than 800 h lithium plating/stripping cycles, the interfacial impedance keeps stable with no obvious polarization voltage increase. Moreover, the critical current density (CCD) of the less dense pellets prepared with conven tional sintering process can reach 1.05 mA cm 2. Owning to the opti mized interface, a Li/ITO-LLZTO/LiFePO4 full cell exhibits good cycle and rate performance. It is believed that this wok will provide inspira tion for solving Li/garnet interface problems and promoting the devel opment of solid-state batteries.
Li-ion batteries (LIBs) have been widely used in portable electronics and electric vehicles owing to their high working voltage, long cycle life, no memory effect and environmental friendliness [1,2]. However, traditional organic liquid electrolytes used in LIBs cannot match high-voltage cathodes due to their narrow electrochemical window, thus hampering the energy density. Even worse, their flammability proper ties could incur safety concerns when Li dendrites propagating and finally penetrating the separator [3]. Recently, solid-state electrolytes (SSE) have been regarded as an ideal alternative due to their wide electrochemical window and inflammability [4–6]. During the past few decades, various inorganic SSEs have been studied, such as lithium phosphorus oxynitride (LiPON) [7], perovskite and anti-perovskite types [8,9], hydrides [10,11], NASICON-type [12], sulfide-type [13,14], garnet-type [15–17], etc. Among the aforemen tioned SSEs, garnet-type has superior overall performance including high ionic conductivity at room temperature, wide electrochemical window and maintaining stable interface against Li. However, there are two notable problems stem from the preparation process of garnet. One unavoidable problem is that the density of the pellet affects its electro chemical performance to a great extent. The relative density of the bulk garnet can exceed ~98% by hot press sintering [18], field assisted sin tering technology (FAST) [19], and spark plasma sintering (SPS) [20]. However, the density of bulk garnet prepared by powder compression can hardly reach ~94%. The density certainly affects the grain boundary resistance and Liþ conductivity. Moreover, dense garnet pellets with low number of grain boundaries can inhibit lithium dendrites effectively. It has been reported that metallic lithium grew along the inter-particle pores of garnet, followed by the generation and expansion of cracks [21]. Therefore, the garnet pellet with low relative density is more easily penetrated by metallic lithium. Many works based on powder-compression garnet, even modifying the particles already, can just reach the critical current density (CCD) as 0.8 mA cm 2 [22,23]. This result is far from reaching the galvanostatic cycling current of lithium metal battery based on liquid electrolyte. Another challenge for garnet type solid electrolytes is their poor contact to metallic Li, which is mainly caused by the contamination of the surface. It has been found that the garnet-type electrolytes will be easily contaminated by Li2CO3 in moist air because of Hþ/Liþ exchange [24,25]. The as-formed Li2CO3 which known as a poor conductor of lithium ions will be adhere to the grain boundaries and surface of the garnet. It seriously hinders the transport of lithium ions and results in high interfacial impedance. To remove of Li2CO3 contamination, different methods have been investigated, so that molten Li can easily wet the intrinsic-lithiophilic garnet surface [26,27]. However, even if annealing or polishing the garnet pellet can remove Li2CO3 completely, it is energy wasting and time consuming. Moreover, hydrocarbon and carbonate would contaminate surface again even the treated pellet stored in inert atmosphere. Alternatively, introducing artificial inter layer between metallic Li and garnet pellet has been demonstrated as an effective way to reduce interfacial impedance [28]. Based on alloy or conversion reaction with lithium, the interlayer can facilitate transport of lithium ions and electrons, reducing the interfacial impedance greatly. For examples, Chen et al. modified garnet SSEs with graphite interlayer by simply painting the pellet surface with a pencil [29]. Hu et al. engineered the Li/garnet interface by forming a Li–Al alloy [30]. Other interlayers have been proposed by means of magnetron sputtering such as Si [31], Ge [32], Sn [33], Mg [34], Au [35] and Ag [36]. They are based on alloy reactions between modification layers with molten Li. The drawback of these strategies is uncontrollable volume change dur ing alloy reactions. Meanwhile, continuous lithium ions flowing through the interface would inevitably cause co-diffusion of elements. Thus, the modified layers will be out of function due to volume-change-caused pulverization [37]. Metal oxides are considered to be a more desirable interlayer due to their conversion reaction with Li. Guo et al. applied
2. Experimental Preparation of garnet LLZTO electrolytes. Cubic Garnet LLZTO powder was prepared by traditional solid-state reactions and subsequent sintering. Briefly, stoichiometric amounts of La(OH)3 (Aladdin Reagent, 99.9%), ZrO2 (Aladdin Reagent, 99.99%), Ta2O5 (Aladdin Reagent, 99.99%), and 15 wt% excess of LiOH (Aladdin Reagent, 98%) for additional lithium volatilization during the sintering process were ball milled at 300 rpm for 12 h. After the ball milling process, the precursor powders were sintered in air at 950 � C for 12 h. After that, 1.2 wt% Al2O3 (Aladdin Reagent, 99.99%) was added to the sintered powders in order to stabilize cubic phase by ball-milling for another 12 h at 300 rpm. Finally, the powder was pressed into pellets under the pressure of 30 MPa and then sintered at 1140 � C for 12 h in a muffle furnace. The pellets were covered by the mother powder to reduce lithium loss during sintering. The sintered pellets were well polished and stored in an argonfilled glove box for further use. Preparation of ITO Artificial interlayer. ITO thin films were deposited on the polished LLZTO pellets by magnetron sputtering. ITO (90 wt% In2O3, 10 wt% SnO2) was used as target. The sputtering process was carried out in Ar atmosphere with the mode of direct-current at room temperature. The working pressure of the sputtering chamber was 0.7 Pa. The deposition rate was approximately calculated as 20 nm/min, as shown in Fig. S1. The thickness of the film was optimized to 40 nm in experiment by fixing deposition time at 2 min. Assembly of Symmetric Cells and Hybrid Solid-State Full Cells. To prepare the Li/ITO-LLZTO-ITO/Li symmetric cell, two Li disks (~0.1 mm thick and ~10.0 mm in diameter) were gently attached to both side of the ITO-modified LLZTO pellets (~0.4 mm thick and ~11.0 mm in diameter). The sandwich structure was heated at 200 � C for 15 min. After cooling down naturally, the symmetric cell was assembled in a swagelok cell. For comparison, a Li/LLZTO/Li symmetric cell was also assembled without ITO modification. To prepare the Li/ ITO-LLZTO/LFP hybrid solid-state full cells, LiFePO4 (LFP) electrode was firstly prepared by mixing carbon coated LiFePO4 powder, carbon black and polyvinylidene fluoride in N-methyl-2-pyrrolidone with a mass ratio of 8 : 1: 1 and then casted on carbon coated Al foil by doctor blade. After thoroughly evaporating the solvent, the cathode strip was punched into disks with active materials mass loading of ~2 mg cm 2. 2
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Fig. 1. Characterization of the LLZTO Electrolyte. (a) XRD pattern of the as-prepared LLZTO matches well with cubic structure Li5La3Nb2O12. (b) Cross-sectional SEM image of the LLZTO pellet. (c) EIS profiles of the LLZTO electrolyte at different temperatures from 22 � C–60 � C. (d) Arrhenius plots of the LLZTO ionic conductivity.
Finally, a membrane (Celgard 2400) absorbed tiny amount of liquid electrolyte (1.0 mol L 1 LiPF6 in ethylene carbonate and diethyl car bonate (EC/DEC (volume ratio 1:1)) was placed between the cathode and the garnet pellet. The other side of LLZTO pellet was modified by ITO and soldered on Li disk. The full cell was sealed in 2032 coin cell. All the battery assembly process was carried out in an Ar-filled glovebox. Characterizations. The crystal structures of samples were examined by X-ray diffraction (XRD, Rigaku Ultima IV), using Cu Kα radiation (λ ¼ 0.15418 nm). The morphology of samples was observed by scan ning electron microscopy (SEM, Hitachi S4700) equipped with energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) results were obtained using an Axis Ultra DLD system (Kratos), using a monochromatic Al Kα (1486.6 eV) X-ray source. Electrochemical impedance spectroscopy (EIS) measurements were measured by Zen nium electrochemical workstation (ZAHNER, Germany), working fre quency range from 4 MHz to 10 Hz with an amplitude of 10 mV. The ionic conductivity of LLZTO pellet was tested after sputtering Ag layers on both sides of the pellet as blocking electrode. Linear cyclic voltam mogram (LSV) measurements were recorded on a CHI3232B electro chemical workstation (Chenhua, Shanghai, China) at a scanning rate of 0.1 mV s 1 in the voltage range of 0–6.0 V. Galvanostatical chargedischarge tests of full batteries were performed at various current den sities (e.g. 1C ¼ 170 mA g 1) in a voltage range of 4.0–2.5 V on Neware battery testers. The symmetric cells were tested by the Neware at 30 � C. The cycled Li/ITO-LLZTO-ITO/Li symmetric cell was disassembled in an Ar-filled glovebox, cut by a very sharp blade to exposure the interface, and then transferred in Ar atmosphere for cross-sectional SEM observation.
3. Results and discussions Cubic garnet phase Li6.4La3Zr1.4Ta0.6O12 was prepared by conven tional solid-state method including mechanical milling and subsequent sintering. According to previous work, Al2O3 was introduced to stable cubic garnet phase at room temperature [40]. XRD pattern (Fig. 1a) shows that all the diffraction peaks can be indexed to the standard pattern of Li5La3Nb2O12 (PDF#45-0109), indicating that the synthesized LLZTO is in pure garnet phase. The relative density of sintered pellet is measured to be ~92% by the Archimedes method in ethanol. The cross-sectional SEM image of the sintered pellet is shown in Fig. 1b. It is found that most grains are tightly combined. In some area, the grain boundaries disappear due to grain amalgamation, while pores can also be seen owing to the low relative density. As seen from EIS spectra (Fig. S2), the semicircle of high frequency part comes from the contri bution of the total impedance contained bulk and grain boundary resistance. The tail at low-frequency is related to Warburg impedance attributed from capacitive behavior of Ag blocking electrodes. The Li-ion conductivity of the LLZTO pellet derived from the low-frequency intercept is calculated to be 3.22 � 10 4 S cm 1 at 22 � C. Fig. 1c dem onstrates the Nyquist plots of the LLZTO pellet range from 22 to 60 � C. Obviously, the Li-ion conduction of the LLZTO pellet is elevated during the increasing temperature. The activation energy of Li-ion conduction is 0.37 eV calculated by the Arrhenius equation, obtained from the Arrhenius plots in Fig. 1d. The XPS spectra of bare LLZTO pellet are shown in Fig. S3, exhibiting peaks at 55.0 eV for Li 1s, 290.0 eV for C 1s and 531.5 eV for O 1s, which match very well with Li2CO3 reported by previous works [27,41]. This result confirms that Li2CO3 is the main contaminant on the surface of the sintered pellet, as many groups have reported [42,43]. Fig. S4a shows 3
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Fig. 2. Cross-sectional SEM images of the Li/LLZTO interface (a, b) without and (c, d) with an ITO interlayer. Much improved interfacial contact is achieved with the ITO modification layer.
Fig. 3. O 1s, In 3d, Sn 3d and Li 1S XPS spectra of ITO modification layer on LLZTO before (named as ITO-LLZTO) and after (named as Li/ITO-LLZTO) lithiation.
the rough surface of the polished pellet due to inevitable lithium vola tilization during sintering. Thus, the bare LLZTO pellet shows poor wetting properties with molten lithium, as can be seen in Fig. 2a and b that obvious gaps exist at the Li/LLZTO interface. The poor contact between Li and LLZTO will cause uneven distribution of current at interface, which will accelerate lithium dendrite growth and penetration through the bulk LLZTO pellet. Despite the existence of Li2CO3, the surface of pellet can be turned from lithiophobic to lithiophilic by introducing an ITO film. Based on the conversion reactions between ITO film and Li, the Li foil has been tightly soldered on LLZTO pellet. As
shown in Fig. 2c, no gaps can be observed. Moreover, the LLZTO grains combine tightly with Li (Fig. 2d), indicating that the ITO film effectively enhances contact at interface. After introducing an ITO thin film, the color of bare LLZTO turns from white to light yellowish (Fig. S5a). The surface morphologies before and after ITO coating were observed by SEM. As shown in Fig. S4b, the ITO thin film exhibits a continuously distributed island structure, which is very different from the morphology of bare LLZTO. To solder Li and ITO-LLZTO together, a metallic lithium disk was placed on the ITO-LLZTO pellet and heated to 200 � C by a hot plate. The light 4
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Fig. 4. (a) Comparison of EIS profiles of the symmetric cells using LLZTO with and without ITO modification. (b) Nyquist plots and fitting result of the symmetric cell based on the ITO-LLZTO-ITO. Inset image shows the equivalent circuit of the symmetric cell. (c) Comparison of galvanostatic cycling performance of Li/LLZTO/Li symmetric cells with and without ITO modification at a current density of 0.1 mA cm 2. (d) Critical current density (CCD) test of the Li/ITO-LLZTO-ITO/Li symmetry cell. (e) Galvanostatic cycling of the Li/ITO-LLZTO-ITO/Li symmetry cell at a current density of 0.2 mA cm 2.
yellowish film rapidly changes to black in less than 1 min, as shown in Fig. S5b, indicating the reaction between the ITO nanolayer and molten Li. The surface of ITO-LLZTO was observed by SEM before and after lithiation, as shown in Fig. S6. The corresponding EDS mapping images for O, In, Sn and La elements are also presented, and the elemental percentages are listed in Table S1. It is found that the signals of O, Sn and In are homogeneous, while the signal of La is very weak, indicating that the ITO film covers uniformly on the LLZTO pellet. The EDS-derived percentages of La and Zr elementals are very low, confirming that the surface of LLZTO is uniformly covered by the ITO thin film. After con tacting with lithium at 200 � C, it is found that the island-like morphology of the surface of ITO-LLZTO changes to a rougher surface with spherical particles within 100 nm in size. Obviously, the signal of La became a little stronger in the EDS mapping image. This phenomenon
indicates that the ITO layer is etched by lithium and leave more exposed area allowing La and Zr to be detectable, which is in consistent with the surface morphology change. Fig. 3 shows the XPS spectra of ITO film on LLZTO before and after lithiation. In brief, the peak of In 3d and Sn 3d shifts from higher energy to lower energy because of the reduction by metallic lithium. Due to excessive lithium supply, the metallic In and Sn reduced from ITO film instantly forms LixIn and LixSn alloy. It is well known that SnO2, as a high capacity anode material for lithium batte ries, can react with lithium to form Li2O and LixSn [44]. In2O3 and ITO has also been studied as a lithium storage material [45,46]. The intensity of In 3d is much stronger than Sn 3d, which corresponds to original ratio in ITO target. During the reaction, Li oxide simultaneously generates according to the XPS result of O 1s and Li 1s. In order to evaluate the effect of ITO film on improving contact at 5
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Fig. 5. (a) Schematic configuration of the Li/ITO-LLZTO/LFP cell. (b) Cycling performance of the cell at 0.2 C-rate. (c) Selected voltage profiles of the Li/ITO-LLZTO/ LFP cell at 0.2C (1st, 50th, and 100th cycle). (d) Rate performance of the Li/ITO-LLZTO/LFP cell.
CCD of the cell can be confirmed as 1.05 mA cm 2, which is close to the value of cells assembled with hot pressed LLZTO pellets [38]. Moreover, the symmetric Li/ITO-LLZTO-ITO/Li cell shows stable galvanostatic cycling over 800 h with a smooth plateau of 36 mV under 0.2 mA cm 2 at 30 � C, as shown in Fig. 4e. Each plating/stripping cycle is periodically continued for 1 h, thus area-specific capacity is 0.2 mA h. After 800 h cycling, almost unchanged polarization can be observed. By using Ohm’s law, the total resistance of the symmetrical cell is calcu lated as 180 Ω cm2. This value is close to the EIS-obtained total resis tance of ~185 Ω cm2. The deviation derives from the temperature at which EIS test performed (tested at room temperature). The curve of galvanostatic cycling is generally stable and flat, slight fluctuations in the middle part can be ascribed to temperature perturbation during the test. After 800 h galvanostatic cycling, the symmetric Li/ITO-LLZTO-ITO/Li cell was disassembled. Then the Li/LLZTO inter face was exposed by cutting the whole sandwich structure into half. As shown in Fig. S9, not only no lithium dendrite penetrates into the bulk LLZTO, but also no obvious gaps appears at the interface. During long time Li plating/stripping, continuous Liþ flux could easily take away the highly reactive metal elements, which would lead to failure of the modification layer. However, the interface between LLZTO and metallic Li remains stable in this work. Such superior interface stability can be attributed to the formation of lithium oxide, which may be regarded as an artificial SEI layer. The specific mechanism requires further research. These encouraging results imply the ITO modification layer can effec tively homogenize current distribution at interface. Thus the ITO modified LLZTO pellets, even with low relative density, are able to prevent lithium dendrites penetrating the bulk LLZTO at high current density. In order to further demonstrate the interface stability and to extend application of ITO modified LLZTO in lithium-ion battery, Li/ITOLLZTO/LFP hybrid solid state full cells were assembled and tested. The structure of the full cell is shown in Fig. 5a. A piece of commer cialized separator wetted with a tiny quantity of the liquid electrolyte
interface and cycle stability, the symmetric Li/ITO-LLZTO-ITO/Li cells and Li/LLZTO/Li cells were assembled. For a fair comparison, both symmetric cells were assembled following the same procedure asdescribed in the experimental section. As shown in Fig. 4a, the Nyquist plot for the symmetric cell of non-modified LLZTO exhibits one semicircle. This semicircle is assigned to the interfacial resistance. Considering the symmetry of the Li/LLZTO interfaces, the interfacial resistance of a single Li/LLZTO interface is 1192 Ω cm2. Compared with the non-modified LLZTO, the symmetric cell of Li/ITO-LLZTO-ITO/Li exhibits two semicircles, as shown in Fig. 4b. These two semicircles can be ascribed to the contributions of the interface between ITO and LLZTO, and the interface between ITO and Li, respectively [38]. Therefore, the total resistance of a single Li/ITO/LLZTO interface is calculated to be 32 Ω cm2. By introducing ITO modification film, the interface impedance between LLZTO and lithium can be drastically reduced from 1192 Ω cm2 to 32 Ω cm2 (Fig. S7). The electrochemical window of garnet is at least up to 6.0 V, as reported by previous works [47,48]. Herein, the influence of the ITO modification layer to the electrochemical window of garnet is evaluated. Fig. S8 displays the LSV curve of the Li/ITO-LLZTO/Pt cell in the voltage range of 0–6.0 V at 60 � C. No obvious oxidation or reduction peak can be observed in the whole cycle, indicating that the ITO modification layer does not affect the electrochemical window of LLZTO versus lithium. Critical current density (CCD) is a key indicator of electrolyte/electrode stability at an extremely high current density. As shown in Fig. 4c, the Li/LLZTO/Li cell displays large over-potential and reached short circuit in semi-circle under 0.1 mA cm 2 at 30 � C. The voltage drops sharply to 1.6 mV after two cycles, indicating occurrence of lithium penetration. Due to the poor contact between LLZTO and Li, uneven local current distribution drives uneven Li plating and stripping which cause irreversible short circuit. For comparison, the Li/ITO-LLZTO-ITO/Li cell shows stable cycling performance at the same current density. The Li/ITO-LLZTO-ITO/Li cell was tested by galvanostatic cycling from 0.05 mA cm 2 with stepped increasing current at 0.05 mA cm 2 at 30 � C. As shown in Fig. 4d, the 6
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was introduced between LiFePO4 cathode and LLZTO electrolyte. The Nyquist plots of the hybrid solid-state Li/ITO-LLZTO/LFP full cell before cycling was recorded. As shown in Fig. S10, the total area specific resistance of the full cell is no more than 600 Ω cm2. The galvanostatic charge/discharge performance of Li/ITO-LLZTO/LFP full cell at a cur rent density of 0.2C is shown in Fig. 5b. The initial charge and discharge capacities are 153.5 and 150.4 mA h g 1, respectively, corresponding to a coulombic efficiency of 97.9%. After operating 100 cycles, the discharge capacity remains stable without any fading, and the coulombic efficiency maintains as high as ~99%. Flat voltage plateaus of different cycles with 0.16 V polarization could be seen in Fig. 5c. As shown in Fig. 5d, the discharge capacities are 160.3, 150.9, 128.6 and 117.7 mA h g 1 at 0.1, 0.2, 0.5 and 1C, respectively. After high-rate cycling, the cell recovers a discharge capacity of 160.3 mA h g 1 at 0.1C. These results indicates that the ITO modification layer supplies a stable interface for superior performance of the full cells.
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4. Conclusion In summary, we report an effective way to improve the wettability of the solid-state electrolyte LLZTO to metallic lithium by an ITO inter mediate layer. The lithiated ITO can ensure tight contact of the Li/ LLZTO interface. The effectiveness of this strategy has been proved by reducing interfacial resistance of Li/ITO-LLZTO-ITO/Li symmetric cell from 1192 to 32 Ω cm2. The CCD of the symmetric cell can reach 1.05 mA cm 2 for LLZTO pellets with low relative density prepared by conventional powder compressing and sintering. Uniform local current distribution achieved by interface modification enables the LLZTO pel lets to defense lithium dendrite penetration, not relying on high density pellets sintered by hot pressing or SPS technique. The stable interface enables a symmetric cell being cycled for more than 800 h under a current density of 0.2 mA cm 2 at 30 � C. Based on the modified LLZTO electrolyte, hybrid solid-state battery coupled with LiFePO4 cathode shows a good cycling performance and enhanced rate property. Considering the already industrialized ITO film preparation process, this work shows its practicality to solve the Li/garnet interface problem. Acknowledgements The authors acknowledge the support by the National Natural Sci ence Foundation of China (NSFC) under grant No. 21972127, 51677170, 51777194, and 51722210, and Zhejiang Provincial Natural Science Foundation of China under grant No. LR20E020002, LY17E020010, LY18B030028 and D18E020007. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227440. References [1] [2] [3] [4] [5] [6] [7]
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7
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Achieving efficient and stable interface between metallic lithium and garnet-type solid electrolyte through a thin indium tin oxide interlayer Jiatao Lou a, Guoguang Wang b, Yang Xia a, Chu Liang a, Hui Huang a, Yongping Gan a, Xinyong Tao a, Jun Zhang a, **, Wenkui Zhang a, * a b
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, PR China Hengdian Group DMEGC Magnetics Co., Ltd, Dongyang, 322118, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� ITO interlayer achieves efficient and stable Li/garnet interface. � Li/garnet interfacial resistance is reduced from 1192 to 32 Ω cm2. � Li/ITO-garnet-ITO/Li symmetric cell cycles over 800 h at 0.2 mA cm 2. � Critical current density of Li/ITOgarnet-ITO/Li cell reaches 1.05 mA cm 2. � Li/ITO-garnet/LiFePO4 hybrid cell shows stable electrochemical performance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Li7La3Zr2O12 Solid-state electrolytes Solid-state lithium batteries Indium tin oxide Lithium/electrolyte interface
Garnet-type Li7La3Zr2O12 (LLZO) ceramic electrolytes are promising solid electrolytes in solid-state lithium-ion batteries due to their relatively high ionic conductivity and high stability against lithium metal. However, the poor contact between garnet electrolytes and Li metal causes high interfacial resistance. In this work, we report a strategy to tackle this problem by modifying the surface of the Li6.4La3Zr1.4Ta0.6O12 (LLZTO) pellets with indium tin oxide (ITO), which is widely used as transparent conductive films in electronic devices. Lithium is tightly soldered on the garnet pellets through rapid reaction with the ITO interlayer. Thus the interfacial resistance of Li/LLZTO dramatically decreases from 1192 Ω cm2 to 32 Ω cm2. Due to the superior ability to homogenize current distribution, the lithiated ITO layer can protect the LLZTO pellets with low relative density (~92%) surviving at a critical current density up to 1.05 mA cm 2. Moreover, the Li/ITO-LLZTO-ITO/Li symmetric cell shows stable lithium plating/stripping over 800 h at 0.2 mA cm 2 without polarization increasing. Hybrid solidstate Li/ITO-LLZTO/LiFePO4 cell with good cycle stability and rate performance is also achieved. This work demonstrates that the ITO thin film, through conversion and alloying reactions, can effectively solder lithium anode on garnet-type solid electrolyte for high-performance lithium batteries.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (W. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227440 Received 1 September 2019; Received in revised form 23 October 2019; Accepted 9 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jiatao Lou, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227440
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1. Introduction
SnO2 as the interlayer, which react with lithium to form nanosized LixSn homogenously dispersed in Li2O matrix [38]. The LixSn alloy enhanced the transport of lithium ions and electrons at the interface, and Li2O can effectively restrict volume change of the alloy. This implies that the composite interlayer is much more stable than the simple alloy. As known as a typical kind of N-type oxide semiconductor, indium tin oxides (ITO) is widely used in light-emitting diodes (LEDs) and solar cells as transparent conductive films. Due to the superior electrical conductivity of ITO, Nan et al. [39] sintered the cathode materials on the garnet pellet by using ITO particle as the conductive agent. Herein, we propose In2(1 x)Sn2xO3 (ITO: 90 wt% In2O3, 10 wt% SnO2) as an artifi cial intermediate layer between Li anode and Li6.4La3Zr1.4Ta0.6O12. By the conversion reaction between Li and ITO at 200 � C, an interface composed of Li oxide, LixIn, and LixSn is formed. The reaction can firmly solder metallic Li on LLZTO pellet. More importantly, the unique structure formed by the reaction can maintain interface integrity during cycling. The area specific resistance between metallic Li and LLZTO is drastically reduced from 1192 Ω cm2 to 32 Ω cm2. After more than 800 h lithium plating/stripping cycles, the interfacial impedance keeps stable with no obvious polarization voltage increase. Moreover, the critical current density (CCD) of the less dense pellets prepared with conven tional sintering process can reach 1.05 mA cm 2. Owning to the opti mized interface, a Li/ITO-LLZTO/LiFePO4 full cell exhibits good cycle and rate performance. It is believed that this wok will provide inspira tion for solving Li/garnet interface problems and promoting the devel opment of solid-state batteries.
Li-ion batteries (LIBs) have been widely used in portable electronics and electric vehicles owing to their high working voltage, long cycle life, no memory effect and environmental friendliness [1,2]. However, traditional organic liquid electrolytes used in LIBs cannot match high-voltage cathodes due to their narrow electrochemical window, thus hampering the energy density. Even worse, their flammability proper ties could incur safety concerns when Li dendrites propagating and finally penetrating the separator [3]. Recently, solid-state electrolytes (SSE) have been regarded as an ideal alternative due to their wide electrochemical window and inflammability [4–6]. During the past few decades, various inorganic SSEs have been studied, such as lithium phosphorus oxynitride (LiPON) [7], perovskite and anti-perovskite types [8,9], hydrides [10,11], NASICON-type [12], sulfide-type [13,14], garnet-type [15–17], etc. Among the aforemen tioned SSEs, garnet-type has superior overall performance including high ionic conductivity at room temperature, wide electrochemical window and maintaining stable interface against Li. However, there are two notable problems stem from the preparation process of garnet. One unavoidable problem is that the density of the pellet affects its electro chemical performance to a great extent. The relative density of the bulk garnet can exceed ~98% by hot press sintering [18], field assisted sin tering technology (FAST) [19], and spark plasma sintering (SPS) [20]. However, the density of bulk garnet prepared by powder compression can hardly reach ~94%. The density certainly affects the grain boundary resistance and Liþ conductivity. Moreover, dense garnet pellets with low number of grain boundaries can inhibit lithium dendrites effectively. It has been reported that metallic lithium grew along the inter-particle pores of garnet, followed by the generation and expansion of cracks [21]. Therefore, the garnet pellet with low relative density is more easily penetrated by metallic lithium. Many works based on powder-compression garnet, even modifying the particles already, can just reach the critical current density (CCD) as 0.8 mA cm 2 [22,23]. This result is far from reaching the galvanostatic cycling current of lithium metal battery based on liquid electrolyte. Another challenge for garnet type solid electrolytes is their poor contact to metallic Li, which is mainly caused by the contamination of the surface. It has been found that the garnet-type electrolytes will be easily contaminated by Li2CO3 in moist air because of Hþ/Liþ exchange [24,25]. The as-formed Li2CO3 which known as a poor conductor of lithium ions will be adhere to the grain boundaries and surface of the garnet. It seriously hinders the transport of lithium ions and results in high interfacial impedance. To remove of Li2CO3 contamination, different methods have been investigated, so that molten Li can easily wet the intrinsic-lithiophilic garnet surface [26,27]. However, even if annealing or polishing the garnet pellet can remove Li2CO3 completely, it is energy wasting and time consuming. Moreover, hydrocarbon and carbonate would contaminate surface again even the treated pellet stored in inert atmosphere. Alternatively, introducing artificial inter layer between metallic Li and garnet pellet has been demonstrated as an effective way to reduce interfacial impedance [28]. Based on alloy or conversion reaction with lithium, the interlayer can facilitate transport of lithium ions and electrons, reducing the interfacial impedance greatly. For examples, Chen et al. modified garnet SSEs with graphite interlayer by simply painting the pellet surface with a pencil [29]. Hu et al. engineered the Li/garnet interface by forming a Li–Al alloy [30]. Other interlayers have been proposed by means of magnetron sputtering such as Si [31], Ge [32], Sn [33], Mg [34], Au [35] and Ag [36]. They are based on alloy reactions between modification layers with molten Li. The drawback of these strategies is uncontrollable volume change dur ing alloy reactions. Meanwhile, continuous lithium ions flowing through the interface would inevitably cause co-diffusion of elements. Thus, the modified layers will be out of function due to volume-change-caused pulverization [37]. Metal oxides are considered to be a more desirable interlayer due to their conversion reaction with Li. Guo et al. applied
2. Experimental Preparation of garnet LLZTO electrolytes. Cubic Garnet LLZTO powder was prepared by traditional solid-state reactions and subsequent sintering. Briefly, stoichiometric amounts of La(OH)3 (Aladdin Reagent, 99.9%), ZrO2 (Aladdin Reagent, 99.99%), Ta2O5 (Aladdin Reagent, 99.99%), and 15 wt% excess of LiOH (Aladdin Reagent, 98%) for additional lithium volatilization during the sintering process were ball milled at 300 rpm for 12 h. After the ball milling process, the precursor powders were sintered in air at 950 � C for 12 h. After that, 1.2 wt% Al2O3 (Aladdin Reagent, 99.99%) was added to the sintered powders in order to stabilize cubic phase by ball-milling for another 12 h at 300 rpm. Finally, the powder was pressed into pellets under the pressure of 30 MPa and then sintered at 1140 � C for 12 h in a muffle furnace. The pellets were covered by the mother powder to reduce lithium loss during sintering. The sintered pellets were well polished and stored in an argonfilled glove box for further use. Preparation of ITO Artificial interlayer. ITO thin films were deposited on the polished LLZTO pellets by magnetron sputtering. ITO (90 wt% In2O3, 10 wt% SnO2) was used as target. The sputtering process was carried out in Ar atmosphere with the mode of direct-current at room temperature. The working pressure of the sputtering chamber was 0.7 Pa. The deposition rate was approximately calculated as 20 nm/min, as shown in Fig. S1. The thickness of the film was optimized to 40 nm in experiment by fixing deposition time at 2 min. Assembly of Symmetric Cells and Hybrid Solid-State Full Cells. To prepare the Li/ITO-LLZTO-ITO/Li symmetric cell, two Li disks (~0.1 mm thick and ~10.0 mm in diameter) were gently attached to both side of the ITO-modified LLZTO pellets (~0.4 mm thick and ~11.0 mm in diameter). The sandwich structure was heated at 200 � C for 15 min. After cooling down naturally, the symmetric cell was assembled in a swagelok cell. For comparison, a Li/LLZTO/Li symmetric cell was also assembled without ITO modification. To prepare the Li/ ITO-LLZTO/LFP hybrid solid-state full cells, LiFePO4 (LFP) electrode was firstly prepared by mixing carbon coated LiFePO4 powder, carbon black and polyvinylidene fluoride in N-methyl-2-pyrrolidone with a mass ratio of 8 : 1: 1 and then casted on carbon coated Al foil by doctor blade. After thoroughly evaporating the solvent, the cathode strip was punched into disks with active materials mass loading of ~2 mg cm 2. 2
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Fig. 1. Characterization of the LLZTO Electrolyte. (a) XRD pattern of the as-prepared LLZTO matches well with cubic structure Li5La3Nb2O12. (b) Cross-sectional SEM image of the LLZTO pellet. (c) EIS profiles of the LLZTO electrolyte at different temperatures from 22 � C–60 � C. (d) Arrhenius plots of the LLZTO ionic conductivity.
Finally, a membrane (Celgard 2400) absorbed tiny amount of liquid electrolyte (1.0 mol L 1 LiPF6 in ethylene carbonate and diethyl car bonate (EC/DEC (volume ratio 1:1)) was placed between the cathode and the garnet pellet. The other side of LLZTO pellet was modified by ITO and soldered on Li disk. The full cell was sealed in 2032 coin cell. All the battery assembly process was carried out in an Ar-filled glovebox. Characterizations. The crystal structures of samples were examined by X-ray diffraction (XRD, Rigaku Ultima IV), using Cu Kα radiation (λ ¼ 0.15418 nm). The morphology of samples was observed by scan ning electron microscopy (SEM, Hitachi S4700) equipped with energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) results were obtained using an Axis Ultra DLD system (Kratos), using a monochromatic Al Kα (1486.6 eV) X-ray source. Electrochemical impedance spectroscopy (EIS) measurements were measured by Zen nium electrochemical workstation (ZAHNER, Germany), working fre quency range from 4 MHz to 10 Hz with an amplitude of 10 mV. The ionic conductivity of LLZTO pellet was tested after sputtering Ag layers on both sides of the pellet as blocking electrode. Linear cyclic voltam mogram (LSV) measurements were recorded on a CHI3232B electro chemical workstation (Chenhua, Shanghai, China) at a scanning rate of 0.1 mV s 1 in the voltage range of 0–6.0 V. Galvanostatical chargedischarge tests of full batteries were performed at various current den sities (e.g. 1C ¼ 170 mA g 1) in a voltage range of 4.0–2.5 V on Neware battery testers. The symmetric cells were tested by the Neware at 30 � C. The cycled Li/ITO-LLZTO-ITO/Li symmetric cell was disassembled in an Ar-filled glovebox, cut by a very sharp blade to exposure the interface, and then transferred in Ar atmosphere for cross-sectional SEM observation.
3. Results and discussions Cubic garnet phase Li6.4La3Zr1.4Ta0.6O12 was prepared by conven tional solid-state method including mechanical milling and subsequent sintering. According to previous work, Al2O3 was introduced to stable cubic garnet phase at room temperature [40]. XRD pattern (Fig. 1a) shows that all the diffraction peaks can be indexed to the standard pattern of Li5La3Nb2O12 (PDF#45-0109), indicating that the synthesized LLZTO is in pure garnet phase. The relative density of sintered pellet is measured to be ~92% by the Archimedes method in ethanol. The cross-sectional SEM image of the sintered pellet is shown in Fig. 1b. It is found that most grains are tightly combined. In some area, the grain boundaries disappear due to grain amalgamation, while pores can also be seen owing to the low relative density. As seen from EIS spectra (Fig. S2), the semicircle of high frequency part comes from the contri bution of the total impedance contained bulk and grain boundary resistance. The tail at low-frequency is related to Warburg impedance attributed from capacitive behavior of Ag blocking electrodes. The Li-ion conductivity of the LLZTO pellet derived from the low-frequency intercept is calculated to be 3.22 � 10 4 S cm 1 at 22 � C. Fig. 1c dem onstrates the Nyquist plots of the LLZTO pellet range from 22 to 60 � C. Obviously, the Li-ion conduction of the LLZTO pellet is elevated during the increasing temperature. The activation energy of Li-ion conduction is 0.37 eV calculated by the Arrhenius equation, obtained from the Arrhenius plots in Fig. 1d. The XPS spectra of bare LLZTO pellet are shown in Fig. S3, exhibiting peaks at 55.0 eV for Li 1s, 290.0 eV for C 1s and 531.5 eV for O 1s, which match very well with Li2CO3 reported by previous works [27,41]. This result confirms that Li2CO3 is the main contaminant on the surface of the sintered pellet, as many groups have reported [42,43]. Fig. S4a shows 3
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Fig. 2. Cross-sectional SEM images of the Li/LLZTO interface (a, b) without and (c, d) with an ITO interlayer. Much improved interfacial contact is achieved with the ITO modification layer.
Fig. 3. O 1s, In 3d, Sn 3d and Li 1S XPS spectra of ITO modification layer on LLZTO before (named as ITO-LLZTO) and after (named as Li/ITO-LLZTO) lithiation.
the rough surface of the polished pellet due to inevitable lithium vola tilization during sintering. Thus, the bare LLZTO pellet shows poor wetting properties with molten lithium, as can be seen in Fig. 2a and b that obvious gaps exist at the Li/LLZTO interface. The poor contact between Li and LLZTO will cause uneven distribution of current at interface, which will accelerate lithium dendrite growth and penetration through the bulk LLZTO pellet. Despite the existence of Li2CO3, the surface of pellet can be turned from lithiophobic to lithiophilic by introducing an ITO film. Based on the conversion reactions between ITO film and Li, the Li foil has been tightly soldered on LLZTO pellet. As
shown in Fig. 2c, no gaps can be observed. Moreover, the LLZTO grains combine tightly with Li (Fig. 2d), indicating that the ITO film effectively enhances contact at interface. After introducing an ITO thin film, the color of bare LLZTO turns from white to light yellowish (Fig. S5a). The surface morphologies before and after ITO coating were observed by SEM. As shown in Fig. S4b, the ITO thin film exhibits a continuously distributed island structure, which is very different from the morphology of bare LLZTO. To solder Li and ITO-LLZTO together, a metallic lithium disk was placed on the ITO-LLZTO pellet and heated to 200 � C by a hot plate. The light 4
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Fig. 4. (a) Comparison of EIS profiles of the symmetric cells using LLZTO with and without ITO modification. (b) Nyquist plots and fitting result of the symmetric cell based on the ITO-LLZTO-ITO. Inset image shows the equivalent circuit of the symmetric cell. (c) Comparison of galvanostatic cycling performance of Li/LLZTO/Li symmetric cells with and without ITO modification at a current density of 0.1 mA cm 2. (d) Critical current density (CCD) test of the Li/ITO-LLZTO-ITO/Li symmetry cell. (e) Galvanostatic cycling of the Li/ITO-LLZTO-ITO/Li symmetry cell at a current density of 0.2 mA cm 2.
yellowish film rapidly changes to black in less than 1 min, as shown in Fig. S5b, indicating the reaction between the ITO nanolayer and molten Li. The surface of ITO-LLZTO was observed by SEM before and after lithiation, as shown in Fig. S6. The corresponding EDS mapping images for O, In, Sn and La elements are also presented, and the elemental percentages are listed in Table S1. It is found that the signals of O, Sn and In are homogeneous, while the signal of La is very weak, indicating that the ITO film covers uniformly on the LLZTO pellet. The EDS-derived percentages of La and Zr elementals are very low, confirming that the surface of LLZTO is uniformly covered by the ITO thin film. After con tacting with lithium at 200 � C, it is found that the island-like morphology of the surface of ITO-LLZTO changes to a rougher surface with spherical particles within 100 nm in size. Obviously, the signal of La became a little stronger in the EDS mapping image. This phenomenon
indicates that the ITO layer is etched by lithium and leave more exposed area allowing La and Zr to be detectable, which is in consistent with the surface morphology change. Fig. 3 shows the XPS spectra of ITO film on LLZTO before and after lithiation. In brief, the peak of In 3d and Sn 3d shifts from higher energy to lower energy because of the reduction by metallic lithium. Due to excessive lithium supply, the metallic In and Sn reduced from ITO film instantly forms LixIn and LixSn alloy. It is well known that SnO2, as a high capacity anode material for lithium batte ries, can react with lithium to form Li2O and LixSn [44]. In2O3 and ITO has also been studied as a lithium storage material [45,46]. The intensity of In 3d is much stronger than Sn 3d, which corresponds to original ratio in ITO target. During the reaction, Li oxide simultaneously generates according to the XPS result of O 1s and Li 1s. In order to evaluate the effect of ITO film on improving contact at 5
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Fig. 5. (a) Schematic configuration of the Li/ITO-LLZTO/LFP cell. (b) Cycling performance of the cell at 0.2 C-rate. (c) Selected voltage profiles of the Li/ITO-LLZTO/ LFP cell at 0.2C (1st, 50th, and 100th cycle). (d) Rate performance of the Li/ITO-LLZTO/LFP cell.
CCD of the cell can be confirmed as 1.05 mA cm 2, which is close to the value of cells assembled with hot pressed LLZTO pellets [38]. Moreover, the symmetric Li/ITO-LLZTO-ITO/Li cell shows stable galvanostatic cycling over 800 h with a smooth plateau of 36 mV under 0.2 mA cm 2 at 30 � C, as shown in Fig. 4e. Each plating/stripping cycle is periodically continued for 1 h, thus area-specific capacity is 0.2 mA h. After 800 h cycling, almost unchanged polarization can be observed. By using Ohm’s law, the total resistance of the symmetrical cell is calcu lated as 180 Ω cm2. This value is close to the EIS-obtained total resis tance of ~185 Ω cm2. The deviation derives from the temperature at which EIS test performed (tested at room temperature). The curve of galvanostatic cycling is generally stable and flat, slight fluctuations in the middle part can be ascribed to temperature perturbation during the test. After 800 h galvanostatic cycling, the symmetric Li/ITO-LLZTO-ITO/Li cell was disassembled. Then the Li/LLZTO inter face was exposed by cutting the whole sandwich structure into half. As shown in Fig. S9, not only no lithium dendrite penetrates into the bulk LLZTO, but also no obvious gaps appears at the interface. During long time Li plating/stripping, continuous Liþ flux could easily take away the highly reactive metal elements, which would lead to failure of the modification layer. However, the interface between LLZTO and metallic Li remains stable in this work. Such superior interface stability can be attributed to the formation of lithium oxide, which may be regarded as an artificial SEI layer. The specific mechanism requires further research. These encouraging results imply the ITO modification layer can effec tively homogenize current distribution at interface. Thus the ITO modified LLZTO pellets, even with low relative density, are able to prevent lithium dendrites penetrating the bulk LLZTO at high current density. In order to further demonstrate the interface stability and to extend application of ITO modified LLZTO in lithium-ion battery, Li/ITOLLZTO/LFP hybrid solid state full cells were assembled and tested. The structure of the full cell is shown in Fig. 5a. A piece of commer cialized separator wetted with a tiny quantity of the liquid electrolyte
interface and cycle stability, the symmetric Li/ITO-LLZTO-ITO/Li cells and Li/LLZTO/Li cells were assembled. For a fair comparison, both symmetric cells were assembled following the same procedure asdescribed in the experimental section. As shown in Fig. 4a, the Nyquist plot for the symmetric cell of non-modified LLZTO exhibits one semicircle. This semicircle is assigned to the interfacial resistance. Considering the symmetry of the Li/LLZTO interfaces, the interfacial resistance of a single Li/LLZTO interface is 1192 Ω cm2. Compared with the non-modified LLZTO, the symmetric cell of Li/ITO-LLZTO-ITO/Li exhibits two semicircles, as shown in Fig. 4b. These two semicircles can be ascribed to the contributions of the interface between ITO and LLZTO, and the interface between ITO and Li, respectively [38]. Therefore, the total resistance of a single Li/ITO/LLZTO interface is calculated to be 32 Ω cm2. By introducing ITO modification film, the interface impedance between LLZTO and lithium can be drastically reduced from 1192 Ω cm2 to 32 Ω cm2 (Fig. S7). The electrochemical window of garnet is at least up to 6.0 V, as reported by previous works [47,48]. Herein, the influence of the ITO modification layer to the electrochemical window of garnet is evaluated. Fig. S8 displays the LSV curve of the Li/ITO-LLZTO/Pt cell in the voltage range of 0–6.0 V at 60 � C. No obvious oxidation or reduction peak can be observed in the whole cycle, indicating that the ITO modification layer does not affect the electrochemical window of LLZTO versus lithium. Critical current density (CCD) is a key indicator of electrolyte/electrode stability at an extremely high current density. As shown in Fig. 4c, the Li/LLZTO/Li cell displays large over-potential and reached short circuit in semi-circle under 0.1 mA cm 2 at 30 � C. The voltage drops sharply to 1.6 mV after two cycles, indicating occurrence of lithium penetration. Due to the poor contact between LLZTO and Li, uneven local current distribution drives uneven Li plating and stripping which cause irreversible short circuit. For comparison, the Li/ITO-LLZTO-ITO/Li cell shows stable cycling performance at the same current density. The Li/ITO-LLZTO-ITO/Li cell was tested by galvanostatic cycling from 0.05 mA cm 2 with stepped increasing current at 0.05 mA cm 2 at 30 � C. As shown in Fig. 4d, the 6
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was introduced between LiFePO4 cathode and LLZTO electrolyte. The Nyquist plots of the hybrid solid-state Li/ITO-LLZTO/LFP full cell before cycling was recorded. As shown in Fig. S10, the total area specific resistance of the full cell is no more than 600 Ω cm2. The galvanostatic charge/discharge performance of Li/ITO-LLZTO/LFP full cell at a cur rent density of 0.2C is shown in Fig. 5b. The initial charge and discharge capacities are 153.5 and 150.4 mA h g 1, respectively, corresponding to a coulombic efficiency of 97.9%. After operating 100 cycles, the discharge capacity remains stable without any fading, and the coulombic efficiency maintains as high as ~99%. Flat voltage plateaus of different cycles with 0.16 V polarization could be seen in Fig. 5c. As shown in Fig. 5d, the discharge capacities are 160.3, 150.9, 128.6 and 117.7 mA h g 1 at 0.1, 0.2, 0.5 and 1C, respectively. After high-rate cycling, the cell recovers a discharge capacity of 160.3 mA h g 1 at 0.1C. These results indicates that the ITO modification layer supplies a stable interface for superior performance of the full cells.
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4. Conclusion In summary, we report an effective way to improve the wettability of the solid-state electrolyte LLZTO to metallic lithium by an ITO inter mediate layer. The lithiated ITO can ensure tight contact of the Li/ LLZTO interface. The effectiveness of this strategy has been proved by reducing interfacial resistance of Li/ITO-LLZTO-ITO/Li symmetric cell from 1192 to 32 Ω cm2. The CCD of the symmetric cell can reach 1.05 mA cm 2 for LLZTO pellets with low relative density prepared by conventional powder compressing and sintering. Uniform local current distribution achieved by interface modification enables the LLZTO pel lets to defense lithium dendrite penetration, not relying on high density pellets sintered by hot pressing or SPS technique. The stable interface enables a symmetric cell being cycled for more than 800 h under a current density of 0.2 mA cm 2 at 30 � C. Based on the modified LLZTO electrolyte, hybrid solid-state battery coupled with LiFePO4 cathode shows a good cycling performance and enhanced rate property. Considering the already industrialized ITO film preparation process, this work shows its practicality to solve the Li/garnet interface problem. Acknowledgements The authors acknowledge the support by the National Natural Sci ence Foundation of China (NSFC) under grant No. 21972127, 51677170, 51777194, and 51722210, and Zhejiang Provincial Natural Science Foundation of China under grant No. LR20E020002, LY17E020010, LY18B030028 and D18E020007. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227440. References [1] [2] [3] [4] [5] [6] [7]
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