Stable Cycling LithiumâSulfur Solid Batteries with Enhanced Li

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Stable Cycling Lithium-Sulfur Solid Battery with Enhanced Li/Li10GeP2S12 Solid Electrolyte Interface Stability Ediga Umeshbabu, Bizhu Zheng, Jianping Zhu, Hongchun Wang, Yixiao Li, and Yong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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ACS Applied Materials & Interfaces

Stable Cycling Lithium-Sulfur Solid Battery with Enhanced Li/Li10GeP2S12 Solid Electrolyte Interface Stability Ediga Umeshbabu,§ Bizhu Zheng,§ Jianping Zhu,§ Hongchun Wang,‡ Yixiao Li§ and Yong Yang*§, ‡ §Collaborative

Innovation Center of Chemistry for Energy Materials, State Key Laboratory for

Physical Chemistry of Solid Surface, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡College

of Energy, Xiamen University, Xiamen 361005, China

Abstract We herein explore a facile and straightforward approach to enhance the interface stability between the lithium superionic conducting Li10GeP2S12 (LGPS) solid electrolyte and Li metal by employing ionic liquid such as 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/Nmethyl-N-Propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR13TFSI) as the interface modifier. The results demonstrated that do the presence of 1 M LiTFSI/PYR13TFSI ionic liquid; the interface stability at the electrode/solid electrolyte (i.e., Li/LGPS) was improved remarkably by forming an in-situ solid electrolyte interphase (SEI) layer. As a result, an effectively reduced interfacial resistance, from 2021  cm2 to 142  cm2 and stable Li stripping/plating performance (over 1200 h at 0.038 mA cm-2 and 1000 h at 0.1 mA cm-2) was achieved in the Li/LGPS/Li symmetric cells. On this basis, the Li-S solid-state batteries were further architecture with one of the S@C composite (where the C is ketjen black carbon (KBC) or PBX 51-type activated carbon (PBX51C) or multi-walled carbon nanotubes (MCNTs)) cathode and the LGPS solid electrolyte. The batteries with S@KBC electrodes delivered an excellent discharge/charge performance with a high initial discharge capacity of 1017 mAh g-1 and better stability than those of the batteries with the S@PBX51C and S@MCNTs electrodes. Featuring with high surface area, unique 1 ACS Paragon Plus Environment

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beneficial pore-structure and better particle dispersion of sulfur in S@KBC composite facilitate high sulfur utilization and also increases the intimate contact between electrode and LGPS solid electrolyte interface during discharge/charge process. KEYWORDS: Lithium-sulfur batteries, Li10GeP2S12 solid electrolyte, carbon materials, ionic liquid, electrode/electrolyte interface stability

1. Introduction Replacing the flammable liquid electrolytes with incombustible inorganic solid electrolytes in Li-S batteries have attracted widespread attention owing to its myriad advantages, such as safety, reliability, high energy density, long life-span, inhibition of polysulfide dissolution and the ability to prevent Li dendrite growth.1-7 Moreover, the sulfur working cathode is nontoxic, earth-abundant, low-cost and has a high theoretical capacity of 1675 mAh g-1.8 Nevertheless, the solid-state Li-S batteries (SSLSBs) possess several bottlenecks that mainly concentrate at the electrode/electrolyte interface region.2, 9-13 The bottlenecks mainly arise from highly insulating nature of sulfur (  510-30 S/cm at 25 C) limits the active material utilization, which impedes the transfer of Li ions and electrons. And also arise from poor compatibility and stability of the SEs against the electrodes (including both anode and cathode), which deteriorate the electrode/SEs interface and further increases the interfacial resistance for mass and charge transfer. As a result of these obstacles, the battery cells become inefficient and often lead to performance failure during the repeated electrochemical discharge/charge cycles. Therefore, the development of favorable electrode-electrolyte interface region, including the cathode/solid electrolyte and anode/solid electrolyte in solid-state Li-S batteries is a key challenge to achieve better performance and long endurance.

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A number of breakthroughs have been reported to increase conductivity, sulfur utilization as well as overcome the stress/strain barriers in SSLSBs.14-19 One prevailing approach to surmount those difficulties is unifying the sulfur with electrically conducting material (e.g., Cu metal or nanocarbon) and ionically conducting SE (e.g., Li3PS4) under ball-milling and/or liquid phase approach not only increasing the electron-conducting path but also enhancing the Li+ conduction path into the sulfur materials.6,20,21 For instance, Nagao et al.7 synthesized composite working electrodes comprising of sulfur, Li2S.P2S5 SE and acetylene black (AB), by using hand ground with mortar and high-energy ball-milling approaches. In analogy with physical mixing, the ballmilled composite as the working electrode exhibited an excellent discharge/charge performance and favorable intimate electrode-electrolyte contacts in SSLSBs. Yao et al.6 fabricated reduced graphene oxide (rGO) with a nanosized (2 nm) sulfur coating to render the interfacial resistance and stress/strain of the sulfur positive electrodes. Solid-state Li-S batteries by this cathode (0.40.5 mg cm2 loadings) exhibit an excellent performance (capacity retention of 830 mAh g-1 after 750 cycles at 1C and 60 C) and a small interface resistance at the electrode/electrolyte interface. More recently, Janek and coworkers17 developed a novel and new hot-press setup to minimize the interfacial resistances between the sulfur electrode and the Li7P3S11 SE in SSLSBs. By introducing this methodology, the grain boundary and interfacial resistance could minimize greatly and empower a fast ion transport through the interface and bulk. In consequence, high sulfur utilization of 82% (corresponding capacity is 1370 mAh g-1), promising cycling- and rate capability is achieved in solid-state Li-In/Li7P3S11/S-composite batteries. To achieve solid-state Li-S batteries with high energy density compared to the contemporary conventional Li-S batteries, utilization of Li metal as anode is prerequisite as it improves the cell voltage and specific capacity.22,23 Unfortunately, most of SEs such as Li10GeP2S12, Li10SnP2S12



(LSPS), Li7P3S11, Li1.5Al0.5Ge1.5(PO4)3 (LAGP), etc., are not compatible with Li metal anode, which is a focal impediment for the battery performance.16-18, 24-27 Moreover, it has been reported that these SEs will undergo reduction reactions by consuming lithium ions and electrons from the lithium metal anode and form interface layer which is so-called the solid electrolyte interphase between SE and Li metal electrode.28 If the formed SEI layer has high lithium ionic conductivity and minimal electronic conductivity, the SEI layer acts as a protective layer to impede further reactions between the Li metal and the SEs. Such kind of behavior observed usually in the LiPON, Li7P3S11 and Li6PS5Cl SEs system.29-31 However, if the SEI layer is a mixed electronic and Li ion conductor it will continue to grow as long as contact is perpetuated between SEs and Li metal. This result in the continuous degradation of SEs and the growth of interfacial layers causing capacity fading, increased resistance and short-circuiting of the cell. The SEs of LAGP, LGPS, and LSPS are the classical examples for this category.24,32,33 The interfacial barriers such as large interfacial resistance and incompatibility between Li metal and SEs have been tentatively approached in several ways by many research groups.13, 34-37 For instance, the surface amendments of SEs and/or the passivation lithium metal electrodes.34-43 Generally, elements/compounds such as Si, Ge, Au, LiH2PO4, etc. have been adopted to improve the interface between SE and Li metal.34,37,41 Another approach is alloying of thin-film/bulk-type Li metal.13,15 For instance, a Li-In metal alloy with a voltage of 0.62 V versus Li/Li has been considered as an anode in Li-S solid-state batteries to suppress the reduction interface reactions of SEs.16,17 The third approach is an accommodation of dual layer solid electrolyte configuration such as Li10GeP2S12@75%Li2S-24%P2S5-1%P2O5 where 75%Li2S-24%P2S5-1%P2O5 performs as stable interface layer against Li anode in order to avoid the reduction of Li metal with LGPS SE.4,19 However, the aforementioned approaches are expensive, quite complicated and also time-


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consuming process. In fact, some of the approaches, particularly utilization of Li-M alloy as anode mitigate the cell voltage and thus sacrifice the energy density. Recently, incorporation of ionic liquids (ILs) into SEs has become a promising approach to improve the interface compatibility across electrode and solid electrolyte, owing to their superior safety properties such as low vapor pressure, non-flammability, less volatility, high Li ionic conductivity, high voltage stability windows, and excellent thermal stability compared to organic liquid electrolytes.12,

44-46

Among the existing ionic liquids, the pyrrolidinium-based ILs and

which consisting of the bis(trifluoromethanesulfonyl) imide (TFSI) anion demonstrates its applications in high performance solid-state Li batteries.12,46 For example, Oh et al. reported incorporation of a small quantity of [Li(triglyme)]+[TFSI] solvate IL to the composite cathode could effectively transform the contact mode from solid-solid to solid-liquid and afford an excellent Li conducting network so that it could ensure intimate contact across electroactive material particles and the Li3PS4 SE.45 We have also recently demonstrated that addition of ionic liquid such as LiTFSI/PYR13TFSI on Li10SnP2S12 (LSPS) SE surface in order to improve the Li/LSPS interface stability.12 The formation of an SEI layer through decomposition of IL is imperative in enhanced electrode-solid electrolyte interface. As a result, a small interfacial resistance and an excellent stripping/plating stability are achieved in Li-metal solid-state batteries. It is further interesting and challenging that along with enhancing Li/SEs interface, the improvement of cathode/SEs interface is also an imperative in order to improve the overall cell energy density, rate capability and cycling stability. Moreover, the effects of different hierarchical carbon additives on sulfur cathode performance in SSLSBs have not been reported yet.



In this work, enhancement of the compatibility of lithium superionic conducting Li10GeP2S12 solid electrolyte with Li metal is eventually achieved by promoting 1M LiTFSI/PYR13TFSI ionic liquid as the LGPS SE surface modifier. Meanwhile, the effects of carbon additives on sulfur composite cathodes are also studied in Li-S solid batteries. The systematic and comprehensive experiment results demonstrate that when the addition of a small amount ionic liquid, remarkably stabilized the LGPS SE interface with Li metal anode by forming in-situ SEI layer on Li metal electrode. In consequence, a small interfacial impedance and stable Li strip/plat cycling life are achieved in Li/LGPS/Li symmetric cells. Moreover, such a high interface stabilized LGPS SE combined with the S@KBC working electrode in Li-S solid-state batteries displayed a high capacity and exhibited better cycle performance than the cells with S@PBX51C and S@MCNTs electrodes.

2. Experimental section 2.1 Chemicals and materials In our experiments, we used all the chemicals were of analytical grade and employed as received without further purification. Li10GeP2S12 SE (Hefei Kejing Materials Technology Co., Ltd, China), elemental sulfur (Sinopharm Chemical Reagent Co., Ltd, China), Ketjen Black EC600JD Carbon (Lion Corporation), PBX 51-type activated carbon (Cabot Corporation) and multi-walled carbon nanotubes (Graphistrength® C100, ARKEMA), PYR13TFSI ionic liquid (Shanghai CHENGJIE Corporation Co., Ltd., China), and LiTFSI (Zhuhai Smoothway Electronic Materials, China). 2.2 Preparation of sulfur@carbon composites


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Sulfur/ketjen carbon (S@KBC) composite was fabricated by ball-milling followed by a thermal annealing process. Typically, the elemental sulfur and ketjen black carbon (1:1 in wt.) were mixed thoroughly by ball milling for 0.5 h at 300 rpm and for 5 h at 500 rpm. Then, the mixture was transferred to the ampoule, sealed in a stainless steel autoclave and heated at 155 C for 12 h under an argon atmosphere to achieve S@KBC composite. Similarly, the sulfur@carbon composites such as S@PBX51C and S@MCNTs composites with 1:1 in wt. were obtained by a similar method. 2.3 Preparation of 1 M LiFSI/PYR13TFSI Ionic Liquid The preparation of 1 M ionic liquid such as LiTFSI salt dissolved in PYR13TFSI ionic liquid was detailed in our previous study.12 The room temperature ionic conductivity of 0.639 mS cm-1 for freshly prepared 1 M LiTFSI/PYR13TFSI ionic liquid was measured with a DDS-307A conductivity meter. 2.4 Materials characterization The samples X-ray diffractometer (XRD) patterns were collected with X-ray diffractometer (Rigaku Ultima IV) employing Cu-Kα (40 kV, 30 mA) as radiation source. The solid electrolyte and cathode composite samples were covered with a Mylar film to circumvent detrimental side reactions with moist air. The phase purity and crystal lattice constants of the LGPS SE were identified by means of Rietveld refinement analysis using General Structure Analysis System (GSAS) program. Morphology, microstructure and elemental distribution of the samples were performed by Hitachi S-4800 field emission scanning electron microscopy (SEM) equipped with an EDS instrument. For SEM of LGPS and cathode composite samples, an airtight specimen holder was utilized to fully circumvent moisture and air adulteration during sample transfer. Transmission electron microscopy (TEM) pictures were collected on a JEOL JEM-2100 7 ACS Paragon Plus Environment


machine. Sulfur content in the different S@C composites was analyzed by thermal gravimetric analysis (TGA) using a STA 409 PC thermal analyzer (Netzsch, Germany). An Xplora Raman spectrometer (Horiba JY) with an excitation laser line of 638 nm was employed to collect Raman spectra of the samples at room temperature (RT). The nitrogen adsorption/desorption isotherms was performed on a Micromeritics ASAP 2020 surface analyzer to compute Brunauer-EmmettTeller (BET) surface area of the samples. The pore-size distribution plots were acquired by Barrett-Joyner-Halenda (BJH) method. XPS measurements were obtained by PHI 5000 Versa Probe III spectrometer (ULVAC-PHI, Japan) using Al K as the X-ray source. The observed binding energies were calibrated based on the C1s peak (284.8 eV). 2.5 Battery assembly 2.5.1 Symmetric C/LGPS/C cells To quantify Li ionic conductivity of Li10GeP2S12 solid electrolyte, Li-ion blocking symmetric cells, C/LGPS/C was fabricated. For this, first 150 mg of the LGPS solid electrolyte powder was cold-pressed into a dense pellet at a pressure of 360 MPa. Pressing was performed using a test cell, with a polytetrafluoroethylene (PTFE) cylinder body having an inner diameter of 10 mm. The carbon coated Al foils as blocking electrodes were then laid on both sides of the LGPS pellet by pressing at a pressure of 360 MPa. 2.5.2 Symmetric Li/LGPS/Li cells The symmetric Li/LGPS/Li cells were fabricated as follows. First, 75 mg of the LGPS solid electrolyte powder was put in a 10 mm PTFE tube and cold pressed at a pressure of 360 MPa for 2 min. Subsequently, one drop (about 10 L) of 1 M LiTFSI/PYR13TFSI ionic liquid was evenly spread on each side of LGPS pellet surface and two Li metal foils with 2 mm thick were then tightly pressed by hands onto the two surfaces of LGPS pellet to make intimate contact between 8 ACS Paragon Plus Environment

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them. Similarly, the pristine Li/LGPS/Li cells were also assembled by a similar procedure, but no spreading of LiTFSI/PYR13TFSI ionic liquid on the LGPS pellet surface. 2.5.3 Quasi-solid-state S@C/LGPS/Li cells Quasi-solid-state S@C/LGPS/IL/Li batteries were architectured in a custom-made Swagelok cell. Prior to cells assembly, the electrode composite consisting of 40 wt.% of S@C (C  KBC or PBX51 C or MCNTs) composite, 50 wt.% Li10GeP2S12 SE and 10 wt.% acetylene black (AB) were mixed with hand ground by mortar and pestle for 2 h, in order to achieve a homogeneous mixture. For the cell assembly, 75 mg of the LGPS electrolyte powder was placed into 10 mm diameter mold and cold pressed at a 360 MPa. One side of the produced SE pellet was uniformly covered with 5 mg of the composite cathode, followed by pressing at a 360 MPa. Subsequently, one drop of 1 M ionic liquid was spread onto the anode side of LGPS SE pellet surface and a Li metal foil was then attached by hand pressing. For comparison, bare S@C/LGPS/Li solid-state cells were also prepared by a similar process without the addition of 1 M LiTFSI/IL. The sulfur loading in each of the cell is 1.28 mg cm-2. Typical testing systems for batteries (both symmetrical and Li-S solid-state batteries) rely on a custom-made Swagelok-type cell system and the cell assembly processes were conducted under an argon atmosphere in a dry glove box (H2O and O2 1 ppm). 2.6 Electrochemical measurements The EIS measurements of the cells were performed on a Versa STAT MV Multichannel potentiostat/galvanostat instrument with frequency range from 1 Hz to 1 MHz. The galvanostatic charge-discharge (GCD) measurements were performed on a multichannel battery test system (LAND CT-2001A Wuhan, China). The galvanostatic lithium plating/stripping cycling were conducted on Li/LGPS/Li cells at fixed biased current densities of 0.038 and 0.1 mA cm-2 by 9 ACS Paragon Plus Environment


periodically charging for 1 h and discharging for 1 h. Discharge and charge measurements of the S@C cathodes in Li-S quasi-solid-state batteries were recorded at different current densities (1C  1670 mA g-1) and the cut-off voltages for discharge and charge were 1.5 and 3.0 V (vs. Li/Li). The applied current density was computed based on sulfur content in the composite cathode. The electrochemical measurements of the as-assembled cells were performed at room temperature.

3. Results and discussion 3.1 Characterization of LGPS and S@C composites The SEM image of Li10GeP2S12 (LGPS) solid electrolyte powder unveils non-uniform surface morphology with a clear aggregation of the particles in the range of micron-scale (Figure 1A). The Raman spectrum of the LGPS SE is shown in Figure 1B. The peaks at around 175 and 278 cm-1 are attributed to deformation of PS43− unit.47 The peak seemed at 415 cm-1, mainly comes from the Ge-S-Ge stretching, and the other peak appeared at 420 cm-1 is the stretching of PS43unit. The peak marked at 384 cm-1 is ascribed to the (GeS0.5S3)3- unit with non-bridging sulfur.48 According to Martin's report, LiS- stretching vibration is found at 363 cm−1.49 In addition, the representative peaks found at 550 and 573 cm-1, due to asymmetric vibrations of the PS43− and P2S74− units.48 The crystal structure and phase purity of the Li10GeP2S12 were further determined from powder XRD pattern and Rietveld refinement, as shown in Figure 1C. The as-obtained results demonstrate that the Li10GeP2S12 SE consists of tetragonal crystal structure with a space group of P42/nmc (137) and the corresponding lattice constants are a  b  8.696 Å and c  12.607 Å, which are in good agreement with the earlier published literature.26,50 To determine the Li ionic conductivity (Li) of the LGPS electrolyte, AC impedance analysis was performed on C/LGPS/C symmetric cells at room temperature within the frequency range from 1 Hz to 1 MHz, as shown in Figure 1D. It is obvious that Nyquist plot consists of a clear 10 ACS Paragon Plus Environment

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semicircle at high frequency range and a vertical capacitive tail at low-frequency is attributed to contribution of the interface between the Li-ionic conductor and blocking carbon electrodes.12,51 The intercept at real axis (Z) in high frequency region can be accredited to the bulk resistance (Rbulk) of the LGPS SE whereas the depressed arc corresponding to the grain boundary response. The total resistance, Rtotal (bulk and grain boundary contributions) of the LGPS SE is 62.3 , estimated from the low-frequency intercept. The Li ionic conductivity (Li) of the LGPS SE can be determined from the total resistance by the equation,  Li  l /( Rtotal  a ) where l and a represent thickness and area of the LGPS SE pellet, respectively. The computed total Li ionic conductivity of the LGPS SE is 2.0410-3 S cm-1, which is comparably lower than the literature reported value of 1210-3 S cm-1.26 In general, the ionic conductivity of SEs greatly depends on synthesis conditions (which can largely influence the amount of impurities) and hot/cold pressing conditions of the solid electrolyte pellets, thus, varying the ionic conductivity of our LGPS solid electrolyte. The powder XRD patterns of sulfur, carbon blacks and sulfur@carbon composites are given in Figure 2A and S1. As expected, the broad diffraction peaks centered at about 24.3 and 43.2, which correspond to the (002) and (100) facets of amorphous carbon.52 However, in all the S@C composites, no pronounced diffraction peaks of sulfur as compared to standard crystalline sulfur, implying that sulfur is in a highly dispersed state inside the pores of carbon. At 155 C, the sulfur melts and is fully confined into porous of the carbon frameworks.53 The salient feature to note is that high-energy ball-milling caused a high degree of dispersion and exfoliation of carbon, as reflected by the pronounced decrease in graphitic peak intensity in all three S@C composites compared to the pristine carbons (Figure S1).52 The comparison Raman spectra of the different S@C composites in Figure 2B exhibit two prominent peaks at around 1315 (D-band) and 1590 11 ACS Paragon Plus Environment


cm-1 (G-band), which are accredited to the defects/disordered carbon and ordered sp2 graphitized carbon, respectively.4,54 In all samples, the intensity of the D-band is greater than that of the Gband owing to existence of the defects induced by sulfur infiltration into porous carbon matrix. Moreover, the basic features of the sulfur exhibit well-defined Raman bands at 153, 218, 244, 435 and 471 cm-1(Figure 2B).55 In contrast with sulfur, the S@C composites do not exhibit those Raman bands because the sulfur is in a highly dispersed state in the composite.54 Further, TGA profiles for all three S@C composite samples confirmed that the sulfur content is almost 50 wt% (Figure S2). The morphology and microstructure of the prepared S@C composite samples were examined by SEM (Figure 3). The representative images of S@KBC in Figure 3A, B display homogenous mixing of sulfur and ketjen black carbon with an abundant mesopores structure. The pictures of S@PBX51C (Figure 3C, D) and S@MCNTs (Figure 3E, F) also signify good mixing of sulfur and carbon components, but no significant porous structure. To further better understand the microstructure and interaction between sulfur and carbon particles in S@KBC composite, TEM was performed. The combined results of TEM and EDS mapping images for S@KBC sample are shown in Figure 4A-D. As shown in Figure 4A, B, the sulfur particles with the size range of 1020 nm sizes are well adhered on surface of ketjen carbon and uniformly distributed throughout carbon skeleton in the composite. The EDS mapping of carbon and sulfur further confirms the healthy distribution of sulfur in mesoporous carbon framework (Figure 4C, D). The XPS analysis was performed on S@KBC composite to determine the surface chemical bonding state of sulfur. The high-resolution S 2p spectrum can be deconvoluted into two peaks at about 164.3 eV (2p3/2) and 165.4 eV (2p1/2) are ascribed to the characteristic peaks of elemental sulfur (Figure 4E).56 The high-resolution C 1s spectrum shows peaks at about 284.8, 285.4, and 286.3 eV which can


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be attributed to the CC/C-C, C-OH and CO functional groups, respectively, for the ketjen black carbon (Figure 4F).56 The surface textural characteristics of the S@C composite powders were determined by N2 adsorption-desorption isotherms. The isotherms of the different carbon materials and their sulfur composites are shown in Figure 5 and pore-size distribution plots of the corresponding samples are depicted in the insets. The computed BET specific surface areas for the pristine carbons of KBC, PBX51C and MCNTs are 1390, 1124 and 198 m2 g-1, respectively. However, after infiltration of sulfur into those carbons significantly reduces the surface areas to 115, 65 and 19 m2 g-1 for S@KBC, S@PBX51C and S@MCNTs samples. The large attenuation in surface areas of the composites is found due to the blocking of carbon pores by large-sized sulfur particles. Moreover, it can be seen from the BJH plots (insets of Figure 5) that the pristine carbon materials exhibit broad pore-size distribution while after mixing the sulfur in carbon materials reduces the pore-size distribution range, suggesting pores of the carbon materials are filled by the sulfur. Nonetheless, the large specific surface area and favorable mesoporous structure of the S@KBC composite is advantageous in that it presumably facilitates the increase of interface at the electrode/SE that allows the smooth charge transfer with the LGPS solid electrolyte. The positive electrode is a composite, residing a mixture of one of the prepared S@C (where C is KBC or PBX51C or MCNTs) composites, acetylene black (electronic conduction additive) and LGPS SE (Li conduction additive), obtained in two steps. First, the S@C composite was obtained through the ball-milling and subsequent heating. The as-obtained S@C composite was then mixed with LGPS and AB by hand ground with mortar and pestle for at least 2 h, so as to obtain evenly mixed S@C-LGPS-AB composite. Figure S3 signifies the powder XRD patterns of S@KBC-LGPS-AB composite cathode at first and after three days of preparation. There is no



obvious side reaction in the prepared composite even after 3 days, implying good stability of the composite. The SEM picture of the composite cathode is shown in Figure S4A. It can be seen that sulfur, carbon and LGPS components are evenly dispersed and contact firmly, which is highly beneficial and endows balanced prodigious electronic and ionic conductivity of the composite. Moreover, the size reduction and homogeneous dispersion of sulfur and carbon in the composite could increases sulfur utilization during discharge/charge process, leading to a better electrochemical performance with robust rate and long endurance. The EDS images were carried out in order to have a deeper look at the composite cathode (Figure S4). The results confirm the presence of C, S, Ge and P in the composite, which further suggests uniform mixing of sulfur, carbon, and SE in the composite cathode powder. 3.2 Electrochemical studies 3.2.1 Li/LGPS/Li symmetric cells To quantify the effect of ionic liquid such as LiTFSI/PYR13TFSI on Li/LGPS interface and lithium plating/stripping cycling stability, symmetric Li/LGPS/Li cells were assembled with or without addition of ionic liquid. Figures 6A, B represent photographs of the pelletized LGPS SE surface without and with ionic liquid. As seen in Figure 6C, the Nyquist plots of the Li/LGPS/Li symmetric cells, assembled without and with the addition of 1 M ionic liquid. The Nyquist profiles of two different Li/LGPS/Li symmetric cells show a clear difference in their semicircles, representing different resistance of the cells. Usually, the first intercept at real part (Z) of the data in the high frequency is attributable to the bulk-resistance, Rbulk.57 The semicircle at medium frequency region is regarded to the interfacial resistance, Rintf which essentially poised of chargetransfer resistance, Rct and passivation layer resistance, Rsei.58 The resulting impedance data of the cells were simulated with the modal circuit shown in the inset of Figure 6C. It can be noticed 14 ACS Paragon Plus Environment

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that the Li/LGPS/Li cells modified with 1 M ionic liquid deliver remarkably a small interfacial resistance of 142  cm2 than that of the unmodified Li/LGPS/Li cells (2021  cm2). It is worth noting that the interfacial resistance of ionic liquid modified LGPS SE is well comparable to or better than the reported sulfide and oxide based SEs.12,36, 59-63 To determine the dynamic interface stability between the ionic liquid modified LGPS SE and Li metal, AC impedance test was carried out on Li/LGPS/Li symmetric cell stored for 15 days at RT. Figure 6D illustrates the variation of the interfacial impedance of 1 M LiTFSI/IL modified Li/LGPS/Li symmetric cells with time evolution under open circuit voltage (OCV). It can be evident that with the increase of time, the bulk-resistance, Rbulk value is almost constant demonstrating that the ionic conductivity of the solid electrolyte does not change with increased storage time. However, there is a marked change in interface charge-transfer resistance, Rintf, first a sharp jump with time in first three days and could keep constant eventually over 15 days, implying a formation of a stable interface across LGPS SE and Li metal electrode as a result of the addition of ionic liquid. The initial increase in resistance is due to the growth of passivation layer which becomes stable with increase in time.64 We further carried out “Li strip/plate test” to evaluate the dynamical interface stability and lithium ion transport capability across the LGPS SE and Li metal interface. Figure 7A, B show the time dependent plating/striping voltage profiles of the Li/LGPS/Li cells at 0.038 and 0.1 mA cm-2. It can be seen from that the Li/LGPS/Li symmetric cells modified with 1 M IL show a flat and highly stable stripping/plating profiles with a small overpotential of 37.5 mV at 0.038 mA cm-2 (Figure 7A). Even after prolonged 1200 h, the cell unveils a stable and flat voltage profiles, signifying a remarkable improvement in cycling stability. As the current density increases to 0.1 mA cm-2 (Figure 7B), the overpotential of the Li stripping/plating remains smaller and delivers



exceptionally stable cycle performance for at least 1000 h. In stark contrast, a spike in overpotential with prolonged time is observed in unmodified Li/LGPS/Li cells, due to unstable interface where the interfacial resistance (Rintf) rises quickly with time (Figure 7A). We further performed SEM images on the LGPS SE pellet surface after long-term stripping/plating cycles (Figure 7C, D). It can be clearly observed that 1 M ionic liquid can efficiently protect from the unfavorable side reactions between LGPS SE and Li metal during stripping and plating cycles, thus, the smooth surface morphology with no significant chemical reactions has been observed between them (Figure 7C). While in the absence of ionic liquid (Figure 7D), the surface of LGPS SE has significant voids and moreover, the SE surface is highly reacted with Li metal during charge/discharge cycling leading to a black in color. The improved interface stability of the Li/LGPS with reduced impedance and excellent cycle stability in ionic liquid modified Li/LGPS/Li cells is attributed to the following aspects.12,65 In solid-liquid hybrid electrolyte system, i.e. ionic liquid modified LGPS SE symmetric cells, (i) IL facilitates a good ionic conducting network and also change the contact mode from solid-solid to solid-liquid which empowers intimate interface contact between LGPS SE and Li metal.65 (ii) Ionic liquids such as LiTFSI/PYR13TFSI increase the Li/LGPS interface stability by the in-situ formation of interface layer on Li metal surface through the putrefaction of ionic liquids.12,66 The as-formed a dense and stable SEI layer further inhibits the chemical reactions that occur at the LGPS SE contacting Li metal and thus helps to increase the related properties of the cells. While, in the absence of ionic liquid in Li/LGPS/Li symmetric cells, the Li/LGPS interface is intrinsically unstable and attributing to the gradual decomposition of the LGPS SE leading to the formation of interface layer consisting low ionic conductivity materials (such as Li2S and Li3P,


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etc.) and a mixed ion-electron conductive Li-Ge layer.32 Thus increasing the interfacial resistance and reduced electrochemical stability. 3.2.2 Quasi-solid-state Li-S cells The effect of the addition of ionic liquid in improving solid-state Li-S battery performance was probed at ambient temperature by using galvanostatic charge/discharge and electrochemical impedance spectroscopy techniques.46 Figure 8A reveals a schematic representation of a quasisolid-state Li-S battery fabricated with sulfur composite consists of S@C, LGPS SE, and AB as the positive electrode, LGPS as SE and Li metal as anode, respectively. At anode side of LGPS SE surface was uniformly covered with a drop of 1 M LiTFSI/PYR13TFSI ionic liquid, so as to improve the interface stability and suppress adverse side reactions between lithium metal and Li10GeP2S12 SE. Figure 8B represents first charge-discharge curves for different S@C composite cathodes in Li-S solid batteries at 83.5 mA g-1. The first discharge capacities for the cells with S@KBC, S@PBX51C and S@MCNTs electrodes are 1068, 783 and 677 mAh g-1, respectively. And their retained reversible capacities after 25th cycles are 868, 375 and 218 mAh g-1 (Figure 8C). Among these composite cathodes, the S@KBC shows enhanced discharge specific capacity and high cycle stability during the charge/discharge process. In order to gain a better insight of the effects of carbon additives on the electrochemical properties of the S@C composites, EIS was carried out in the frequency range of 1 Hz to 1 MHz and the results are displayed in Figure 8D. The solid symbols denote the experiment data and the thick line represents fitting data with the equivalent circuit shown in the inset. The fitting results of three different composite electrodes are shown in Table 1. All the Nyquist profiles comprise a clear semicircle at high frequency region and a near vertical line along the Z in low-frequency region. The intercept at x-axis (at Z) in the high frequency region signifies the bulk-resistance 17 ACS Paragon Plus Environment


(Rbulk) of the LGPS SE.5 This value is almost same for all three electrodes owing to presence of same kind of LGPS SE. The clear semicircle in the middle frequency region reflects the Rct and Rintf between the electrode and the electrolyte.67 The linear part in the lower frequency region reflects diffusion impedance, Wo.55 In comparison, among all three electrodes, the S@KBC had a smaller diameter of the semicircle indicating that low charge-transfer resistance and interfacial resistance than those of the S@PBX51C and S@MCNTs. Therefore, the batteries with S@KBC as the cathode exhibited an excellent discharge/charge performance and better cycling stability. The large surface area, indispensable pore-structure and good sulfur particles dispersion in S@KBC composite is highly responsible for in achieving high sulfur utilization, and favorable intimate interface contact leading to fast-charge transfer rate at the electrode/electrolyte interface. Further, we have chosen the sulfur/ketjen black carbon (S@KBC) composite to in-depth analysis in Li-S solid batteries. Figure 9A illustrates initial five discharge/charge profiles of the S@KBC/LGPS/IL/Li solid-state battery under a fixed current density of 83.5 mA g-1. The cell delivered very high discharge capacities of 1017 and 912 mAh g-1, respectively at 1st and 5th cycles. The discharge capacity of the S@KBC composite is much better than the reported several S@C composite electrodes in SSLSBs (Table S1).14,15, 68-71 It is notable that the discharge and charge curves of the solid-state cell exhibits a single voltage plateau in their potential profiles, as it undergoes a solid-phase Li-S redox reaction (S8  16Li  8Li2S  16e-),46 suggesting that no polysulfide dissolution.16,55 In the literature, a plenty of reports showed similar discharge/charge profiles of SSLSBs with sulfide SEs such as glass/glass-ceramic Li2S-P2S5, Li10GeP2S12, and Li6PS5Cl.4,9,46,55,72 As seen from derivative plots of the corresponding cell in Figure 9B, the redox peaks at about 2.30 and 2.07 V are observed during the charge and discharge process, which are corresponding to lithiation and delithiation process of sulfur in the electrode.16,71 The other peak


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located at around 2.18 V during discharge can be related to the solid-electrolyte contribution. Kanno et al. observed a similar kind of redox behavior in charge/discharge curves of SSLSBs with utilization of thio-LISICON Li3.25Ge0.25P0.75S4 SE.16 An additional broad reduction peak looked at 1.85 V during first discharge cycle, attributed to some irreversible reactions occurring during initial cycle, which might be related to the decomposition of LGPS.73,74 Figure 9C shows 2nd cycle lithiation/delithiation profiles of the S@KBC cathode at vacillate current densities, ranging from 50 to 200 mA g-1. It can be seen that the discharge capacities declines gradually with the increase in current density. The cell can be delivered a discharge specific capacity as high as of 1068 mAh g-1 at 50 mA g-1 and remained about 652 mAh g-1 (i.e., 61% with respect to the initial capacity) even at high applied current of 200 mA g-1, indicates good rate capability of the cell. It is further mentioning that as the current increases to higher values of 167 and 200 mA g-1, the voltage gap between the discharge and charge plateau are increased as a result of polarization associated with higher current density. Further, long-term galvanostatic cycling test for the S@KBC composite electrode was performed over 50 cycles, at a constant current of 83.5 mA g-1 (Figure 9D). The capacity of the cell gradually diminishes with increase in cycle number, and the capacity retentions for 25, 35 and 50th cycles are 95.2, 90.6 and 82.6%, respectively. These values are calculated after the 5th cycle onwards. Even though the capacity diminishes with cycling is more pronounced in the last cycles (mainly 25-50 cycles) but we noticed the capacity fade even within 25 cycles. This can be attributed to the formation of polysulfide species which result in large volume changes of the electrode.64,72 Nevertheless, the cell maintained a reversible discharge capacity as high as of 726 mAh g-1 at the end of 50th cycle. Moreover, the Coulombic efficiency of the corresponding cell increases in first few cycles



and could keep constant eventually; however, it did not exceed 97%, implying some irreversibility of the battery reaction. Further, we have fabricated solid-state batteries without the addition of 1 M ionic liquid of LiTFSI/PYR13TFSI to confirm whether IL can suppress adverse side reactions between LGPS SE and Li metal or not and we measured galvanostatic charge/discharge profiles at a constant current of 83.5 mA g-1 (Figure 10A). The initial discharge capacity of the cell is merely 405 mAh g-1, while the reversible specific capacity is found as low as 50 mAh g-1, which can be attributed to the unstable interface between LGPS SE and Li metal anode due to favoring the side reactions between them during the discharge/charge process, as result broad plateaus are found within the applied potential window.4,75 Further, we performed EIS spectra of the corresponding cells in order to assess the decrease in resistance with the addition of 1M LiTFSI/PYR13TFSI IL (Figure 10B). A comparatively large interface impedance of 69  cm2 can be found in pristine S@KBC/LGPS/Li cell (without the addition of IL) than that of the cells modified with ionic liquid which is about 29.8  cm2 i.e., the resistance decreases by 57 %. The unstable Li/LGPS interfacial is ascribed to the gradual spontaneous reaction between LGPS SE and Li metal, leads to the formation of interphases consisting of Li2S, Li3P and Li-Ge alloy, as a result, high interfacial resistance is achieved in unmodified solid electrolyte based cells.32 Therefore, the addition of a small amount of 1 M LiTFSI/PYR13TFSI IL at anode side of LGPS pellet surface can remarkably increase the battery performance in terms of specific capacity, cyclability and interfacial resistance of S@C/LGPS/Li solid cell due to the enriched interface stability at the electrode/electrolyte (Li/LGPS) interface.


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4. Conclusion In summary, with the purpose of addressing the problems of interface stability across the Liconducting Li10GeP2S12 SE and Li metal anode, an exceptionally small amount of ionic liquid modified LGPS SE based Li/Li symmetric cells were assembled and studied their performance in Li metal batteries. Meanwhile, the additive effects of carbon materials on the performance of LiS cell chemistry are also investigated thoroughly. With enabling 1 M LiTFSI/PYR13TFSI ionic liquid, exceptionally improved the interface stability at the Li/LGPS SE by forming an in-situ SEI layer. As consequence, remarkably improved interface stability of the LGPS SE with Li metal anode in Li/LGPS/Li symmetric cells. The interfacial resistance has been achieved as low as of 142  cm2 and stable Li stripping/plating performance over 1200 h at 0.038 mA cm-2 and 1000 h at 0.1 mA cm-2. Moreover, Li-S solid-state batteries architecture with S@KBC positive electrode and surface stabilized LGPS SE, delivered excellent charge-discharge performance and a small interfacial resistance compared to that of S@PBX51C and S@MCNTs electrodes. For instance, the S@KBC electrode exhibits initial discharge capacity as high as of 1017 mAh g-1 at 83.5 mA cm-2 and retains greater than 750 mAh g-1 even after 50 cycles. Featuring beneficial properties such as high specific surface area, indispensable pore-structure and uneven sulfur particle dispersion in S@KBC composite are substantial in achieving high sulfur utilization and favorable intimate interface contact between active electrode material and LGPS SE. The results herein demonstrates a promising method in improving the anodic interface problems and Li-S solid battery performance using ionic liquid modified sulfide-type Li10GeP2S12 solid electrolytes.



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Supporting Information Powder XRD patterns and TGA profiles of the different S@C composites; XRD, SEM and EDS mapping of S@KBC-LGPS-AB composite are prasent in the supporting information.

Acknowledgements This work was financially supported by National Key Research andDevelopment Program of China

(grant

no.

2016YFB0901502

and

2018YFB0905400)

and

National

Natural

ScienceFoundation of China (grant no. 21761132030, 21621091 and 21473148).

Author Information *Corresponding author, E-mail: [email protected] (Yong Yang) Tel.:/Fax: + 86 592 2185753

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Figure 1. (A) SEM image, (B) Raman Spectrum, (C) X-ray Rietveld refinement patterns, and (D) Nyquist plot of the Li10GeP2S12 (LGPS) solid electrolyte.


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Figure 2. (A) Comparison powder XRD patterns and (B) Raman spectra of sulfur and different sulfur@carbon composites. 31 ACS Paragon Plus Environment


Figure 3. SEM images of (A, B) S@KBC, (C, D) S@PBX51C, and (E, F) S@MCNTs composite samples at different magnifications.


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Figure 4. (A, B) TEM and (C, D) EDS mapping images of S@KBC composite; Core-level XPS spectra of (E) S 2p and (F) C 1s of S@KBC composite sample.



6

Figure 5. N2 adsorption-desorption isotherms of pristine carbons of (A) KBC, (C) PBX51C and (E) MCNTs, and their composites with sulfur: (B) S@KBC, (D) S@PBX51C, and (F) S@MCNTs. In sets show corresponding BJH pore-size distribution plots. 34 ACS Paragon Plus Environment

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Figure 6. Photographs (top views) of the LGPS solid electrolyte pellet surface (A) without and (B) with 1 M LiTFSI/PYR13TFSI (1 M LiTFSI/IL) ionic liquid. (C) Nyquist profiles for the Li/LGPS/Li symmetric cells with and without 1 M LiTFSI/PYR13TFSI ionic liquid and (D) time evolution of impedance response of the Li/LGPS/Li cell with 1 M LiTFSI/IL at various storage times.



Figure 7. (A) Galvanostatic cycling curves of Li/LGPS/Li cells with and without 1 M LiTFSI/IL at a current density of 0.038 mA cm-2, and (B) the Li/LGPS/Li cell with 1 M LiTFSI/IL at 0.1 mA cm-2. SEM images obtained after long-term Li stripping/platting cycles for the Li/LGPS/Li symmetric cells (C) with and (D) without 1 M LiTFSI/IL ionic liquid at a current density of 0.038 mA cm-2.


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Figure 8. (A) A schematic diagram of quasi-solid-state lithium-sulfur battery. Comparison of the (B) galvanostatic discharge/charge profiles, (C) cycling performances, and (D) Nyquist plots for the different sulfur/carbon composite electrodes in quasi-solid-state Li-S batteries. Galvanostatic discharge/charge profiles and cycling stability measurements are obtained at a current density of 83.5 mA g-1.



Figure 9. (A) Galvanostatic discharge/charge profiles and (B) corresponding derivative curves for the S@KBC composite in solid-state Li-S batteries under a current density of 83.5 mA g-1; (C) the 2nd cycle discharge/charge profiles at different current densities and (D) long-term cycling performance of solid-state Li-S battery with S@KBC composite electrode.


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Figure 10. (A) Galvanostatic discharge-charge curves of the S@KBC composite as the cathode in Li-S state-state batteries without the addition of 1 M LiTFSI/PYR13TFSI ionic liquid. (B) Comparison Nyquist plots of the S@KBC electrode in Li-S solid-state batteries with and without 1 M LiTFSI/PYR13TFSI ionic liquid; Inset shows enlarged high-frequency view.



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Table 1. The fitting results of the different sulfur@carbon composite cathodes in quasi-solidstate Li-S batteries. Electrode material

Solid electrolyte resistance, RSE ( cm2)

Interfacial resistance, Rif ( cm2)

S@KBC

11.15

29.86

S@PBX51C

11.81

56.81

S@MCNTs

11.48

64.55


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Graphical abstract

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Stable Cycling LithiumâSulfur Solid Batteries with Enhanced Li

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Stable Cycling LithiumâSulfur Solid Batteries with Enhanced Li