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Phase Separation of Li 2 S/S at Nanoscale during Electrochemical Lithiation of the Solid-State Lithium–Sulfur Battery Using In Situ TEM

Zhenzhong Yang , Zhiyong Zhu , Jie Ma , Dongdong Xiao , Xian Kui , Yuan Yao , Richeng Yu , Xiao Wei , * Lin Gu , * Yong-Sheng Hu , * Hong Li , and Xixiang Zhang *

Dr. Z. Yang, Dr. J. Ma, Dr. D. Xiao, Dr. Y. Yao, Prof. R. Yu, Prof. L. Gu, Prof. Y.-S. Hu, Prof. H. Li Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences Beijing 100190 , China E-mail: l.gu@iphy.ac.cn; yshu@aphy.iphy.ac.cn Dr. Z. Zhu, Prof. X. Zhang Division of Physical Science and Engineering King Abdullah University of Science and Technology (KAUST) Thuwal 23955-6900 , Saudi Arabia E-mail: xixiang.zhang@kaust.edu.sa X. Kui, Prof. L. Gu School of Physical Sciences University of Chinese Academy of Sciences Beijing 100049 , China Dr. X. Wei Department of Material Science and Engineering Zhejiang University Hangzhou 310027 , China E-mail: mseweixiao@zju.edu.cn Prof. L. Gu Collaborative Innovation Center of Quantum Matter Beijing 100190 , China

DOI: 10.1002/aenm.201600806

conductive multiporous hosts [ 6 ] and use of the electrolyte addi-tives or hybrid anode structure. [ 7 ] However, the replacement of liquid electrolyte with solid electrolyte initially eliminates the polysulfi de shuttle effect and dendrite formation. On the other hand, solid-state electrolytes with moderate ionic conductivities induce lower power densities [ 8 ] and substitution with fast ionic conductors is reported to improve the power densities. [ 9 ]

In order to improve the performance of Li–S batteries, it is crucial to gain an in-depth understanding of the microscopic mechanism of the electrochemical reaction of the S cathode during the charge/discharge processes. A number of studies dealing with the mechanism of Li–S batteries were performed using in situ X-ray diffraction. [ 10 ] However, these studies could not provide information at high enough spatial resolutions, and thus, the detailed mechanism of the process at the nanoscale is still not well understood. [ 10 ]

Although transmission electron microscopy (TEM) is a powerful tool for investigating microstructural changes, [ 11 ] imaging S with TEM is still very challenging because S has a relatively low vapor pressure and it is sensitive to the high energy electron beam. [ 12 ] In this study, a solid-state Li–S nano-battery is designed for in situ TEM observation and studies were performed to gain a better understanding of the electro-chemical lithiation process of carbon-coated sulfur materials.

The pristine S powder used here is bought from Alfa Aesar, and its X-ray diffraction pattern (XRD) is shown in Figure S1, Supporting Information, which is consistent with the stand powder diffraction of S 8 (JCPDS card No. 01-078-1888). The S samples were fi rst dispersed on a TEM grid and then encap-sulated with carbon coatings using a sputter with coating preci-sion of 1 Å to yield coating thicknesses of ≈5 nm. A typical TEM image and annular dark-fi eld (ADF)-scanning TEM (STEM) image of the sample are shown, respectively, in Figure 1 a,b. It can be seen that the S sample exhibits a reticular structure. The element distribution of the selected area in Figure 1 b (red box) is depicted in Figure 1 c,d. It is clear that the carbon coating is uniformly distributed on the S particles, which confers to them a particular stability that makes it possible to perform in situ TEM experiments. Because of the poor electronic conductivity of S, carbon becomes the most common additive material employed in Li–S batteries to enhance the charge transport due to its substantial conductivity. Accordingly, carbon is used in this study to enhance the conductivity of the S samples, without effecting the investigation of the S reaction mechanisms.

The schematic diagram of the in situ device based on Nano-factory Instruments AB TEM holder is depicted in Figure 2 a. [ 13 ]

Although lithium ion batteries (LIBs) have higher energy den-sities among rechargeable batteries, they still cannot meet the increasing demands for modern electric vehicles or smart grids. [ 1 ] Developing electrode materials with substantial capacity is highly desirable to improve the energy density of batteries. Theoretically, sulfur (S) as a cathode material has a capacity of ≈1673 mA h g −1 and specifi c energy of ≈2600 Wh kg −1 . [ 2 ] These values are fi vefolds higher than those of common materials currently used in cathodes (e.g., LiCoO 2 ). Furthermore, S is nontoxic, low-cost, and naturally abundant. As a result, S has been identifi ed as a promising cathode material for the next-generation rechargeable lithium batteries. [ 3 ] Intense studies have led to improvements in the specifi c capacities of the lithium–sulfur (Li–S) batteries, which today approach their theoretical capacity values. Furthermore, the cycle life of some selected cells has surpassed 1000 cycles. [ 4 ] However, many draw-backs still need to be solved, including the low electric conduc-tivity of S, the large volume expansion of S during lithiation, the safety problem caused by the formation of lithium dendrites, and the shuttle effect due to the solubility of lithium polysulfi de (Li 2 S x , 4 ≤ x ≤ 8). [ 2,5 ] Research efforts have been devoted to over-coming these problems, including encapsulation of S in the

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It consists of a half TEM grid with the S samples sealed by carbon coating and a tungsten tip with metallic lithium. A Keithley power supply is used to provide a constant voltage to drive the lithiation reaction of S. During the transient transfer of the Li–S battery into the TEM, the surface layer of the metallic lithium deposited on tungsten tip is oxidized to Li 2 O, which is then used as a solid electrolyte. Typical ADF-STEM images of the S samples at the pristine state and after lithiation are, respectively, shown in Figure 2 b,c.

TEM images captured during lithiation process of the S par-ticles taken from the video (Movie S1, Supporting Information, the frame rate is accelerated 15 times) are illustrated in Figure 3 . The lithium source (Li/Li 2 O) on the tungsten tip is located on the left side of the sample. Rather than directly touching the S, the lithium source comes into contact with the supported carbon fi lm of the TEM grid. Here, the supported carbon fi lm and carbon coating are used as the conducting matrix, which prevents any infl uence of mechanical forces on morphological changes to the S particles or to the carbon coating during the lithiation process. In this prototype solid-state Li–S nanobattery, the contact resistance between W tip and the Li/Li 2 O layer, Li/Li 2 O, and supporting carbon fi lm of TEM grid is high. To initiate lithiation process of S, we applied a driving voltage of ≈10 V using a Keithley power supply. As shown in Movie S1 of Supporting Information, the lithiation pro-cess is initially very slow, probably due to the poor contact between the tungsten tip and the supporting carbon fi lm. It should be noted that the image contrast of S particles become weaker as the lithiation reaction proceeds. This indicates the transformation of the

S particles into lithium sulfi de (Li x S). As the lithiation process continues, dark spots appear in increased numbers (Figure 3 c). A typical high magnifi cation TEM image of a lithiated S sample is depicted in Figure S2 of the Supporting Information, where dark spots could clearly be seen. The selected area electron dif-fraction (SAED) patterns captured during the lithiation process (Figure 3 d–f) illustrates that the crystal S turns into polycrystal-line S and Li 2 S (Figure 3 e), then eventually to pure Li 2 S at the end of the lithiation process (Figure 3 f). Figure 3 f suggests that the dark spots in the lithiated S sample seen in Figure 3 c are crystalline Li 2 S. [ 12 ]

To further understand the electrochemical lithiation process of the S cathode, in situ STEM studies are performed and the results are shown in Movie S2 of the Supporting Information, (the frame rate is accelerated 10 times). ADF-STEM images of Figure 4 a–c captured from Movie S2 of the Supporting Infor-mation correspond to images of S at different stages: before, during, and after lithiation. The video displays a number of bright dots formed during the initial stage of the lithiation pro-cess, where some vanish as the lithiation process progresses as shown in Figure 4 b (the typical bright dots are marked by red arrows). Taking into consideration the SAED patterns of Figure 3 e, the bright dots should possibly be remnant S that gradually transform into lithiated products (Li x S). As the lithia-tion process continues, new bright dots appear and grow bigger that might be formed crystalline Li x S (Figure 4 c). These data suggest the clear coexistence of the nanoscale S and Li x S phase during the lithiation process, where the resulting composite facilitate both the electron conductivity and Li + diffusion.

Some holes in S closed during the lithiation, indicating the expansion of the sample during the process. However, the expansion ratio is much smaller than the theoretical value and should expand by ≈80% during the transformation from S to Li 2 S. [ 3c ] The latter might be due to: (i) the pristine S is not as dense as the crystalline S, which would cause some lithiation products to fi ll in available vacancies during the process; (ii) the carbon coating limits the expansion during the lithiation pro-cess; (iii) sulfur is liable to evaporate under high vacuum and

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Figure 1. a) Typical TEM image of the carbon coated S particles, where the S sample supported by carbon fi lm has a netted structure. b) ADF-STEM image of the S sample and element maps of C and S corresponding to the area outlined in red in (b).

Figure 2. a) Schematic diagram of the electrochemical device set for in situ TEM observation of the solid-state Li–S nanobattery. Typical ADF-STEM images of S sample b) before and c) after lithiation.

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irradiation of high energy electron beam, sulfur loss might occurred during the lithiation process because the coated carbon layer may crack because of the large expansion during the transformation from S to Li 2 S.

Considering that S is sensitive to high vacuum and irradia-tion of electron beam, the comparison experiment of carbon coated S without the high driving voltage was carried out, as shown in Movies S3 and S4 of the Supporting Information,

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Figure 3. Selected images of sample evolution during the lithiation process of S. a–c) TEM images captured from the video of the lithiation process and d–f) represent their corresponding SAED patterns. (a) and (c) correspond to the images of the pristine sample and lithiated products, respectively.

Figure 4. ADF-STEM images and EELS spectrum of S and lithiated S samples. Typical ADF-STEM images of S samples a) before, b) during, and c) after lithiation. d) Low energy EELS. e) S-L 2,3 edge of S, Li 2 S, and lithiated S in the in situ experiment, respectively. f) Low-energy STEM-EELS spectra of the bright dots corresponding to the red dashed circle area in the inset.

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the frame rate is accelerated 20 times. It can be seen that there is no obvious change of S sample under vacuum and electron beam. The experiment results indicate that the phe-nomenon we observed during the in situ STEM experiment is mainly caused by elec-trochemical lithiation.

To clarify the reaction mechanism of the lithiation process, electron energy loss spectra (EELS) were performed and the results are shown in Figure 4 d,f. The low-loss EELS of pristine S, lithiation product (Li x S), and commercially available Li 2 S powders are illustrated in Figure 4 d. The Li–K edge could be distinguished from the low-loss EELS of Li x S, further confi rming the occurrence of the lithiation process. Figure 4 d,e clearly depicts the signifi cant difference in both the low-loss and core-loss EELS between the S and Li x S. Furthermore, plasma spectrum of the lithiated product Li x S agrees well with that of Li 2 S, where two peaks at 14.7 and 18.5 eV are observed. This is consistent with the calculated EELS data of S and Li 2 S shown in Figure S3 of the Supporting Informa-tion. Based on the core-loss EELS data, the lithiation product should be a composite of Li 2 S and S (Figure 4 e and Figure S3, Sup-porting Information), suggesting a direct transformation of S to Li 2 S without passing through interme-diates of lithium polysulfi de. The STEM-EELS data also sup-port the hypothesis that the bright spots are due to S and Li 2 S (Figure 4 f), corroborating the SAED results. Because the lithi-ation process in our experiment was stopped when the SAED and EELS data were collected, these results refl ect a more static state than a dynamic process. Thus, the possibility that the lithium polysulfi de ((Li 2 S x , 2 ≤ x ≤ 8)) is formed and quickly transformed into Li 2 S cannot be excluded.

Lithium polysulfi des at various compositions of Li 2 S 2 , Li 2 S 4 , Li 2 S 6 , and Li 2 S 8 were prepared by dissolving S and Li 2 S in dimethyl sulfoxide (DMSO) at a concentration of 0.1 mol L −1 . The low-loss and core-loss EELS spectra of these composites are shown in Figure S4 of the Supporting Information. No signifi -cant difference between the spectra of Li 2 S 2 , Li 2 S 4 , Li 2 S 6 , and Li 2 S 8 is observed, except a slight energy shift in the spectrum of the plasma. The EELS spectra of lithium polysulfi de are vastly different from the experimental results of Li x S, suggesting that lithium polysulfi de is likely not present during the lithiation process.

Given the results collect here, we summarize the lithia-tion process of S as follows: S particles partially fi rst turn into nanocrystalline S and Li 2 S and the remnant S transforms into Li 2 S. The Li 2 S nanocrystals gradually grow larger as the lithi-ation process progresses. The reaction mechanism diagram is schematically drawn in Figure 5 . During the lithiation process, an S/Li 2 S phase separation at the nanoscale happens. This phase separation does not only reduces the diffusion distance but also provides the S/Li 2 S interfaces network with a better electron and Li + ion transportation. [ 12,14 ] This is consistent with

previously published results showing one plateau in the voltage profi le of the solid-state Li–S battery. [ 8 ] It is worth noting that the core–shell reaction is not observed as we predicted. The reason might be due to the fact that both the S and Li 2 S are insulated and the electrons can be transferred through the inter-face between the S and Li 2 S nanocrystals. In addition, the for-mation of the nanocrystals provides a shorter pathway for both the electron and ions, which favors the electrochemical diffu-sion processes. As the S phase reduces, the Li 2 S increases as the lithiation process continues. Finally, pure Li 2 S is formed at the end of the lithiation process.

As mentioned above, we observed a novel lithiation process of solid-state Li–S battery in this in situ TEM experiment, which is different from the lithiation reaction in traditional liquid Li–S battery. It should be mentioned that though the carbon coated S cathode is stable under high vacuum and high energy electron beam, the electrochemical environment is still dif-ferent from that in practical cell, the electron beam and vacuum environment might additionally change the phase transforma-tion kinetics and result in some loss of S and sulfi des. How-ever, our fi nding could provide valuable insights and give useful suggestion to the novel design of solid-state Li–S battery.

In summary, a solid-state Li–S nanobattery is designed inside a transmission electron microscope and electrochemical lithiation of the S cathode was successfully observed using in situ (S)TEM. The carbon coating we deposited on the S parti-cles can effi ciently protect the S from irradiation damage and high vacuum circumstance. S/Li 2 S phase separation phenom-enon is observed for the fi rst time in the solid-state Li–S battery. This nanophase separation not only reduce the diffusion

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Figure 5. The schematic diagram of the electrochemical mechanism of a solid-state Li–S bat-tery. a) Pristine S sample with the carbon coating. b) The phase separation of S and Li 2 S during the early stage of the lithiation process. c) The phase of S reduces and that of Li 2 S increases as the lithiation process progresses. d) The pure Li 2 S phase formed at the end of lithiation process.

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distance but also provides the S/Li 2 S interfaces network, which is favorable for the Li + and electron diffusion during the lithi-ation process. As a result, the S particles at sub-micrometer scale can be fully used regardless the lack of direct contact with the carbon conductive additives. Both the electron diffrac-tion and EELS data suggest the direct transformation of S into Li 2 S without a detectable intermediate product of lithium poly-sulfi de (Li 2 S x , 4 ≤ x ≤ 8). This is different from the liquid Li–S batteries, where a two-step reaction takes place at the S cathode during the electrochemical lithiation process. These fi ndings clearly demonstrate that in situ TEM is a very powerful tool for exploring the mechanism of the initial reaction in solid-state Li–S batteries. In-depth understanding of the mechanism may provide new insights into how to improve the power density, cycle life, and commercialization of Li–S batteries.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Program on Key Basic Research Project ( 2014CB921002 ), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB07030200 ), and National Natural Science Foundation of China ( 51522212 , 51421002 , and 51102208 ), and the Fundamental Research Funds for the Central Universities and by King Abdullah University of Science and Technology . Zhenzhong Yang acknowledges support from KAUST during his stay as an exchange student.

Received: April 17, 2016 Revised: June 11, 2016

Published online: July 26, 2016

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