High-Conductivity Argyrodite Li6PS5Cl Solid Electrolytes Prepared via

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High-Conductivity Argyrodite LiPSCl Solid Electrolytes Prepared via Optimizing Sintering Processes for All-Solid-State Lithium-Sulfur Batteries Shuo Wang, Yibo Zhang, Xue Zhang, Ting Liu, Yuanhua Lin, Yang Shen, Liangliang Li, and Cewen Nan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15121 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

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ACS Applied Materials & Interfaces

High-Conductivity Argyrodite Li6PS5Cl Solid Electrolytes Prepared via Optimizing Sintering Processes for All-Solid-State Lithium-Sulfur Batteries Shuo Wang, Yibo Zhang, Xue Zhang, Ting Liu, Yuan-Hua Lin, Yang Shen, Liangliang Li*, Ce-Wen Nan* State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Keywords: Argyrodite Li6PS5Cl, sulfur solid electrolyte, solid-state sintering, all-solid-state

battery, lithium sulfur battery

Abstract Highly Li-ion conductive Li6PS5Cl solid-state electrolytes (SSEs) were prepared by solid-state sintering method. The influence of sintering temperature and duration on the phase, ionic conductivity, and activation energy of Li6PS5Cl was systematically investigated. The Li6PS5Cl electrolyte with high ionic conductivity of 3.15×10-3 S cm-1 at room temperature (RT) was obtained by sintering at 550 C for just 10 min, which was more efficient taking into account such a short preparation time in comparison with other 1

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reported methods to synthesize Li6PS5Cl SSEs. All-solid-state lithium sulfur batteries (ASSLSBs) based on the Li6PS5Cl SSE were assembled by using the nano-sulfur/multiwall carbon nano-tube composite combined with Li6PS5Cl as the cathode and Li-In alloy as the anode. The cell delivered a high discharge capacity of 1850 mAh g-1 at RT for the first full cycle at 0.176 mA cm-2 (~ 0.1 C). The discharge capacity was 1393 mAh g-1 after 50 cycles. In addition, the Coulombic efficiency remained near 100% during galvanostatic cycling. The experimental data showed that Li6PS5Cl was a good candidate for the SSE used in ASSLSBs.

Introduction Rapid development of electrical vehicles (EVs) and mobile electronic devices motivates the burgeoning demands for batteries with high energy density.1-3 Conventional lithium batteries with oxide cathode materials (e.g. LiCoO2 and LiFePO4) have been widely used in EVs; however, the low theoretical capacity of these oxide cathode materials limits their applications.4, 5 Therefore, it is urgent to develop high-energy-density and low-cost batteries. Sulfur is a promising active cathode material due to its abundance in earth and low cost coupled with a high theoretical capacity of 1675 mAh g-1. Lithium-sulfur (Li-S) batteries due to their high theoretical energy density (~2600 W h kg-1) are recognized as a promising candidate to meet the demands.6-9 Currently, liquid lithium-sulfur batteries face some challenges: (1) the organic liquid electrolytes are flammable, (2) the lithium dendrites 2


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can penetrate the separator resulting in short circuit, and (3) the polysulfide shuttle effects lead to poor Coulombic efficiency.10-12 One of the most promising strategies to solve these issues is to replace the liquid electrolytes with solid-state electrolytes (SSEs).13-18 Among various kinds of inorganic SSEs, sulfide SSEs have drawn a lot of attention because of their high Li-ion conductivities, easy processability, and compatible interface with sulfur-based cathodes.19 In 2011, Li10GeP2S12 was reported to have an ultra-high ion conductivity of 1.2×10-2 S cm-1 at RT.20 But, the starting material GeS2 was expensive. Argyrodites Li6PS5Cl was reported as a promising electrolyte with a high ionic conductivity in the range of 10-3-10-2 S cm-1 at room temperature (RT).21 The typical preparation method for Li6PS5Cl SSEs is ball milling.22-25 Sylvain et al. reported a Li6PS5Cl electrolyte with a high ionic conductivity of 1.33×10-3 S cm-1 synthesized by ball milling for 10 h at 600 rpm.23 To improve the crystallinity and ionic conductivity of the Li6PS5Cl electrolyte, annealing after ball milling is adopted by many groups. Yu et al. prepared a Li6PS5Cl electrolyte with a conductivity of 1.1×10-3 S cm-1 by ball milling at 550 rpm for 10 h followed by annealing at 550 C for 5 h.24 The structure and conductivity of Li6PS5Cl with different milling times were investigated in this work. Recently, Zhang et al. prepared a Li6PS5Cl electrolyte with a high ionic conductivity of 1.29×10-3 S cm-1 by ball milling at 500 rpm for 24 h followed by annealing at 300 C for 10 h.25 These two processes above are complex and time-consuming. Another method to prepare Li6PS5Cl is the liquid phase technique. This method was facile, but the ion conductivity was low and just in the order 3



of 10-5-10-4 S cm-1 at RT.26-28 The solid-state sintering is an easy method suitable for mass production. Deiseroth et al. reported a Li6PS5Cl electrolyte synthesized by sintering at 550 C for 7 days.21 Zhang et al. prepared a Li6PS5Cl electrolyte by heating a mixture of Li2S, P2S5, and LiCl to 600 C at 0.3 C/min, and the ionic conductivity of Li6PS5Cl was 1.810-3 S cm-1 at RT.29 Most recently, Yu et al. prepared a Li6PS5Cl electrolyte with a very high ion conductivity of 4.96×10-3 S cm-1 at 26.2 C by sintering the mixed precursors at 550 C for 10 h.30 Various cathode active materials such as LiCoO2, LiNi1/3Co1/3Mn1/3O2, S, and Li2S have been used in the all-solid-state batteries with Li6PS5Cl as the electrolyte.15, 23-26, 31, 32 Han et al. assembled high-performance all-solid-state lithium sulfur batteries (ASSLSBs) using a mixed conducive Li2S nanocomposite as the cathode and Li6PS5Cl as the electrolyte.15 The cell delivered a large reversible capacity of 830 mAh g-1 at 50 mA g-1 for 60 cycles at RT. Yu et al. combined S with Li6PS5Cl to assemble an all-solid-state battery. This battery displayed a high initial discharge capacity of 1400 mAh g-1, and the capacity decayed rapidly below 400 mAh g-1 after 20 cycles.24 In the previous work, the effects of sintering temperature and duration on the phase and ionic conductivity of Li6PS5Cl have rarely been studied systematically; there is lack of an efficient method to synthesize high-conductivity Li6PS5Cl in a short period of time; in addition, the cycling performance of the ASSLSBs with Li6PS5Cl electrolyte is poor. In this study, we systematically investigated the influence of sintering temperature and 4


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duration on the phase and ion conductivity of Li6PS5Cl and obtained a Li6PS5Cl electrolyte with a high ion conductivity of 3.15×10-3 S cm-1 by sintering at 550 C for just 10 min. The conductivity of Li6PS5Cl was enhanced to 3.3810-3 S cm-1 with process optimization. We fabricated ASSLSBs using the nano-sulfur/multi-wall carbon nano-tube (MWCNT) composite as the cathode, Li6PS5Cl as the electrolyte, and Li-In alloy as the anode. The discharge capacity reached 1850 mAh g-1 at RT for the first cycle at 0.176 mA cm-2 and remained 1393 mAh g-1 after 50 cycles. This excellent performance demonstrated that the Li6PS5Cl electrolyte was a good candidate for the SSE used in ASSLSBs.

Experimental Section Synthesis of Li6PS5Cl Li6PS5Cl SSEs were prepared by solid-state sintering method. Reagent-grade Li2S (Alfa, 99.9%), LiCl (Aladdin, 99.9%), and P2S5 (Aladdin, 99%) powders were used as starting materials. All of the reagents were weighed according to a Li2S/LiCl/P2S5 molar ratio of 5:2:1 and mixed by ball milling using a planetary ball mill apparatus (Fritsch Pulverisette 7) at a rotation speed of 100 rpm for 1 hour. The total weight of the mixture was about 1 g. Subsequently, the mixture was placed in a quartz tube for sintering. All the procedures were conducted under Ar atmosphere in a dry glovebox (O2 < 0.1 ppm, H2O < 0.1 ppm). The mixture was heated at 350, 400, 440, 460, 480, 500, 520, 550, 570, or 620 C for 10 h. The heating rate was 1.5 C/min. The tube was slowly cooled down to room 5



temperature after sintering. The samples sintered at different temperature were named as 350-10h, 400-10h, 440-10h, 460-10h, 480-10h, 500-10h, 520-10h, 550-10h, 570-10h, and 620-10h, respectively. After the optimal reaction temperature was found, the reaction duration was changed to optimize the process. For example, the ball-milled mixture was heated at 550 C for 10 min, 1 h, 2 h, 3 h, 5 h, 10 h, 15h or 20h. These samples were designated as 550-10min, 550-1h, 550-2h, 550-3h, 550-5h, 550-10h, 550-15h, and 55020h, respectively. Materials characterization X-ray diffractometry was carried out using Rigaku D/max-2500 diffraction meter with a Cu Kα radiation source to identify the phases of the samples at RT. The samples were sealed in an airtight container covered with a polyimide thin film to avoid reaction with moisture and oxygen and mounted on the X-ray diffractometer. Scanning election microscopy (SEM, Zeiss Merlin field-emission) was used to observe the microstructure of the samples. Raman spectra were obtained using a Raman spectrometer (LabRAM HR Evolution) at an excitation of 532 nm. All samples were sealed in a chamber with a glass window in the glovebox. Electrochemical Performance Measurements The powder was pressed into pellets with a diameter of 12 mm under 150 MPa for ionic conductivity measurement. Two stainless steel disks were attached to the pellets as current collectors. The AC impedance measurement was conducted using an impedance 6


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analyzer (ZAHNER-elektrik IM6) in the frequency range of 1 Hz to 1 MHz with an applied voltage of 50 mV. The ionic conductivity σ was calculated by the equation σ = L/RS, where R was the total resistance of the solid electrolyte, L was the sample thickness, and S was the area of the solid electrolyte. The 𝐸𝑎 was determined from the slope of the Arrhenius plot. Impedance spectra were recorded at 5 C intervals during heating from 35 to 60 C. To evaluate the electrochemical stability of the Li6PS5Cl electrolyte, lithium metal||Li6PS5Cl electrolyte||stainless steel cell was conducted by cyclic voltammetry (CV) using a potentiostat (Biologic VMP3) at a scan rate of 0.2 mV s-1 at voltages ranging from -0.5 V to 5 V at RT. The blocking stainless steel||Li6PS5Cl||stainless steel cell was used for the direct-current (DC) polarization test with a constant voltage of 2 V to determine the electronic conductivity. ASSLSBs were fabricated by employing nano-sulfur/MWCNT composites combined with Li6PS5Cl as the cathode, Li6PS5Cl as the solid electrolyte, and the Li-In foil as the anode (See Figure 1). For the composite cathode, the nano-sulfur/MWCNT composite was prepared according to the method reported by Chen et al..33 The weight ratio of nano-sulfur to MWCNT was around 6:4. Then, the as-synthesized nano-sulfur/MWCNT powder was mixed with the Li6PS5Cl SSE with a weight ratio of 4:6 by ball-milling at 300 rpm for 1 hour. The all-solid-state cells were fabricated as follows. Firstly, 130 mg of Li6PS5Cl powder was pressed under 150 MPa to form a solid electrolyte pellet. Then, the cathode composite powder (about 5 mg) was pressed on the top of the pellet at 150 MPa. Next, the 7



Li-In foil was attached on the other side of the pellet as the anode at 100 MPa. Finally, the formed three-layered pellet was cold-pressed under 150 MPa. All the processes were conducted in a poly(ether-ether-ketone) (PEEK) mold (diameter: 12 mm) with stainless steel rods as current collectors. The charge-discharge behavior of the cells was tested using a battery test system (LAND, C2001A) under a cut-off voltage of 0.0 - 3.0 V (vs. Li-In) at RT. The current density was set as 0.044 mA cm−2 for the first two cycles and 0.176 mA cm−2 for the following cycles. The CV measurements of the solid-state batteries were performed under the voltage of 0.0 - 3.0 V with a sweep speed of 0.5 mV/s.

Figure 1. Schematic diagram of a nano-sulfur/MWCNT composite cathode||Li6PS5Cl||LiIn ASSLSB. Yellow spheres, red spheres, and dark short rods denote the Li6PS5Cl particles, sulfur particles, and MWCNTs, respectively.

Results and discussion

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Figure 2. XRD patterns (a) and Raman spectra (b) of the Li6PS5Cl samples synthesized by solid-state sintering at different temperatures for 10 h.

Figure 2a shows the XRD patterns of the Li6PS5Cl samples synthesized by solid-state sintering at 350, 400, 440, 460, 480, 550, and 620 C for 10 h. The diffraction pattern of the mixture without heat treatment is corresponding to the starting materials, Li2S, LiCl and P2S5. Regarding the 350-10h sample, the main diffraction peaks are indexed as crystalline Li6PS5Cl, and the minor peaks of Li2S, LiCl, and Li4P2S6 impurities are also detected. The intensity of the diffraction peaks of the impurities gradually decreases with increasing sintering temperature. It can be observed that the Li4P2S6 impurity phase is transformed to Li6PS5Cl phase when the sintering temperature increases. The major peaks of the 460-10h sample can be indexed to the Li6PS5Cl crystal structure, except a weak peak for Li2S at 27° that disappears when the sintering temperature is 480 C. As for the 48010h and 550-10h samples, a pure Li6PS5Cl crystalline phase is obtained. When the sintering 9



temperature increases to 620 C, weak peaks for LiCl, Li2S, and unknown phase appear; therefore, a temperature of higher than 620 C is not suitable for obtaining pure Li6PS5Cl electrolytes. Thus, 550 C is the optimal sintering temperature to obtain Li6PS5Cl electrolytes with a pure Li6PS5Cl phase. The Raman spectra of Li6PS5Cl samples synthesized by solid-state sintering at different temperatures for 10 h are shown in Figure 2b. A main peak at 484 cm-1 and a small peak at 382 cm-1 are observed in the 350-10h sample. The peak at 484 cm-1 is attributed to the stretching of PS43-, which indicates the presence of the high ionic conductivity phase Li6PS5Cl.31, 34-35 This result is consistent with the previous reports that argyrodite-type solid electrolyte Li6PS5Cl was constructed of PS43- units.21 In addition, the peak at 382 cm-1 is for P2S64-, which belongs to the Li4P2S6 phase.34, 35 With increasing the sintering temperature, the intensity of the P2S64- peak gradually decreases and disappears at 460 C. From 460 to 620 C, only the PS43- peak shows up. The Raman results are in accordance with the XRD data above.

Figure 3. Ion conductivities of Li6PS5Cl samples at room temperature as a function of 10


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sintering temperature (a) and sintering duration (b), respectively.

The Nyquist spectra of Li6PS5Cl SSEs measured at 25 C are presented in Figure S1a. The resistance of the Li6PS5Cl electrolyte decreases as the sintering temperature increases. When the temperature increases to 620 C, the resistance increases. The ionic conductivities of the Li6PS5Cl SSEs sintered at different temperatures were calculated and shown in Figure 3a. The ionic conductivity of the 350-10h sample is 5.42×10-4 S cm-1. With the increase of the sintering temperature, the ionic conductivity of the Li6PS5Cl electrolyte increases rapidly until 460 C, then rises slowly, reaches the maximum at 550 C, and then decreases. The ionic conductivity of the 550-10h sample is as high as 3.34×103

S cm-1 at RT. From 350 to 460 C, the rapid increase of the ionic conductivity might be

due to the reduction of the LiCl, Li2S, and Li4P2S6 impurities with poor ionic conductivity.19, 36

When the temperature continues to increase, these impurities disappear and the

crystallinity of Li6PS5Cl SSEs increases, leading to the slow increase of the ionic conductivity. As for the 620-10h sample, its conductivity decreases due to the formation of LiCl and Li2S impurities. Thus, the optimized sintering temperature is 550 C.

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Figure 4. (a) Arrhenius plots of the Li6PS5Cl samples synthesized by solid-state sintering at different temperatures for 10 h, and (b) the dependence of the activation energy of Li6PS5Cl samples on sintering temperature.

The Arrhenius plots of the Li6PS5Cl SSEs sintered at different sintering temperatures are shown in Figure 4a. Figure 4b shows the activation energy 𝐸𝑎 calculated from the slope of the linear Arrhenius plot according to the Arrhenius equation σ(𝑇) = 𝐴exp( ― 𝐸𝑎 /𝑘𝑇), where T is the absolute temperature, A is a pre-exponential factor, and k is the Boltzmann constant. For the 350-10h sample, the 𝐸𝑎 value is 0.34 eV. As the sintering temperature increases, the 𝐸𝑎 values of the 400-10h and 440-10h samples are 0.32 eV and 0.29 eV, respectively, which are lower than that of the 350-10h sample. The reduction of LiCl, Li2S, and Li4P2S6 impurities results in the decrease of the 𝐸𝑎. When the temperature is more than 440 C, 𝐸𝑎 remains constant at 0.29 eV, which is low in comparison with other reported SSEs.23, 29, 37 Thus, a sintering temperature of 550 C offers both higher ionic conductivity and lower 𝐸𝑎.

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Figure 5. XRD patterns (a) and Raman spectra (b) of the Li6PS5Cl samples synthesized by solid-state sintering at 550 C for different sintering durations.

Figure 5a shows the XRD patterns of the Li6PS5Cl samples synthesized by solid-state sintering at 550 C for 10 min, 1 h, 2 h, 3 h, 5 h, 10 h, 15 h, and 20 h. As for the 550-10min sample, a weak peak attributed to Li2S impurity is detected besides the main diffraction peaks for Li6PS5Cl. The Li2S peak intensity decreases when the sintering time increases. After a heat treatment duration of 3 h or more, the Li2S peak disappears and a pure Li6PS5Cl phase is obtained. Figure 5b shows the Raman spectra of Li6PS5Cl samples synthesized by solid-state sintering at 550 C for different sintering durations. The Raman bands around 424 cm-1 associated to the PS43- group are observed in all samples, indicating a Li6PS5Cl phase. Figure S1b shows the RT Nyquist spectra of Li6PS5Cl SSEs synthesized at 550 C for various durations. The resistance of the Li6PS5Cl electrolyte decreases slowly to the lowest 13



value and then increases with the increasing sintering duration. The ionic conductivities for these samples were calculated and shown in Figure 3b. With the duration increases, the ionic conductivity of Li6PS5Cl increases slowly, reaches the maximum at 15 h, and then decreases. The ionic conductivity of the 550-10min sample with minor Li2S impurity is 3.15×10-3 S cm-1. When the duration increases to 3 h, the ionic conductivity of Li6PS5Cl increases to 3.20×10-3 S cm-1 with pure Li6PS5Cl. The ionic conductivity reaches at the maximum of 3.38×10-3 S cm-1 at 15 h and then decreases at a longer duration. The decrease of the ionic conductivity may be due to the evaporation of lithium when the sintering duration is more than 15 h.38 In general, the ionic conductivity is not sensitive to the sintering duration. A duration of 10 min is sufficient to obtain a high ionic conductivity over 3×10-3 S cm-1. The ionic conductivity data of Li6PS5Cl SSEs prepared by various methods that have been recently reported are compared and summarized in Table 1. The ionic conductivity of our 550-10min sample with minor Li2S impurity is 3.15×10-3 S cm-1. The electronic conductivity of this sample is 3.40×10-9 S cm-1 (Figure S2). The Li+ transference number was very close to 1 according to Figure S3, which is considerably higher than those of liquid electrolytes39, 40 and polymer electrolytes41. As for the 550-15h sample with pure Li6PS5Cl, the ionic conductivity is 3.38×10-3 S cm-1. Clearly, the conductivity of the 55015h sample is one of the highest values compared with others obtained by liquid phase chemical synthesis, ball milling followed by annealing, and solid-state sintering.22-30 14


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Taking into account both the ion conductivity of Li6PS5Cl and preparation time, a high ionic conductivity over 310-3 S cm-1 can be obtained by sintering at 550 C for just 10 min. Furthermore, a duration time of 10 min is much shorter in comparison compared with others works. Such a short duration time can save energy and increase production efficiency.

Table 1. Summary of the conductivities of Li6PS5Cl SSEs synthesized by different preparation methods. Ionic Reference

Conductivity (mS

Yubuchi et.al, 201526

Nataly et al., 201827

Nataly et al., 201828 Yu et al., 201624 Zhang et al., 201825 Boulineau et al., 201223 Zhang et al., 201829 Rao et al., 201122

Temperature

Preparation Processes

cm-1) Ball milling at 600 rpm for 45 h to obtain Li6PS5Cl precursor.

0.014

RT

Then, the precursor is dissolved into EtOH solution followed by stirring for 24 h and drying at 80 C for 3 h. Ball milling at 600 rpm for 40 h to obtain Li6PS5Cl precursor.

0.06

RT

Then, the precursor is dissolved into a mixture solution followed by stirring and drying at 150 C. Ball milling at 600 rpm for 40 h to obtain Li6PS5Cl precursor.

0.6

25 °C

Then, the precursor is dissolved into a mixture solution followed by stirring and drying at 180 C.

1.1

RT

1.29

RT

1.33

25 °C

1.8

RT

1.9

RT

High-energy ball milling at 550 rpm for 10 h followed by annealing at 550 C for 5 h. Ball milling at 500 rpm for 24 h followed by annealing at 300 C for 10 h. Ball milling at 600 rpm for 10 h with Li2S impurity. A mixture of Li2S, P2S5 and LiCl was heated to 600 C at 0.3 C/min and cooled down. Ball milling for 20 h followed by annealing at 550 C for 5 h

15



Yu et al.,

4.96

26.2 °C

Our work

3.15

25 °C

Our work

3.38

25 °C

201830

A mixture of Li2S, P2S5 and LiCl was heated at a reaction temperature of 550 C for 10 h and cooled down A mixture of Li2S, P2S5 and LiCl was heated at a reaction temperature of 550 C for 10 min and cooled down A mixture of Li2S, P2S5 and LiCl was heated at a reaction temperature of 550 C for 15 h and cooled down

SEM images of the 550-15h sample are shown in Figure S4. The morphology of Li6PS5Cl powder (Figure S4a) shows that the size of the particles is inhomogeneous. Most particles have a size of 1-3 μm, and a few particles have a size of around 10 μm. The average size of the particles is small, which is beneficial to increase the contact area between the active material and the electrolyte in the composite cathode. It can be seen that a good contact between particles is achieved in the Li6PS5Cl pellet prepared by cold pressing without sintering (Figure S4b).

Figure 6. The CV curve of the Stainless steel||Li6PS5Cl||Li cell.

The electrochemical window is important to evaluate the stability of the Li6PS5Cl 16


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electrolyte against lithium metal. The CV curve of the Stainless steel||Li6PS5Cl||Li cell is shown in Figure 6. Cathodic and anodic currents that correspond to lithium deposition (Li++ e- → Li) and dissolution (Li → Li+ + e-), respectively, are observed around 0 V vs Li/Li+ in the voltammogram. Meanwhile, there is no significant current owing to electrolyte decomposition in the potential range from -0.5 to 5 V vs Li/Li+. Therefore, the Li6PS5Cl electrolyte exhibits high stability against Li metal with an electrochemical window of more than 5 V. CV was used to reveal the electrochemical reaction mechanism of the nanosulfur/MWCNT cathode, as shown in Figure 7a. In the first cycle, a remarkable cathode peak can be observed at 0.93 V (vs Li-In) due to the reduction of S8 molecules to Li2S. Meanwhile, an anode peak is detected at 2.12 V (vs Li-In) representing the oxidation of Li2S back to S8.16, 42 The CV curves after the first cycle are almost identical to each other, demonstrating a good electrochemical reversibility of the nano-sulfur/MWCNT composite cathode.

17



Figure

7.

(a)

Cycling

performance

of

nano-sulfur/MWCNT

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composite

cathode||Li6PS5Cl||Li-In all-solid-state cells. (b) Charge, discharge capacity, and Coulombic efficiency as a function of the cycle number at 0.176 mA cm-2. (c) The corresponding discharge–charge voltage profiles of the cells at 2nd, 3rd, 20th and 50th cycles.

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Cycling performance of a nano-sulfur/MWCNT composite cathode||Li6PS5Cl||Li-In ASSLSB between 0.0 V and 3.0 V (vs. Li-In) is shown in Figure 7b. The current density is 0.044 mA cm−2 (~ 0.02 C) for the first two cycles and 0.176 mA cm−2 (~ 0.1 C) for the following cycles. The initial discharge capacity of the cell is 1850 mAh g-1 at RT for the first full cycle at 0.176 mA cm-2. The discharge capacity increases to 1866 mAh g-1 at the 5th cycle and declines to 1393 mAh g-1 after 50 cycles, corresponding to a capacity retention of 75%. This is the highest capacity achieved in all-solid-state cells using Li6PS5Cl material as solid electrolytes.15, 23-26, 29, 32, 43-44 The increase of the capacity during the initial several cycles is due to the activation process for the sulfur cathode, which has been reported for the Li-S batteries.15, 45 Note that the reversible capacity of the all-solidstate cell in the first 35 cycles is even higher than the theoretical capacity of sulfur. The extra capacity of the battery could be attributed to the Li6PS5Cl electrolyte in the composite cathodes that may act as active cathode material.14, 16 It has been reported that Li6PS5Cl electrolyte shows some reversible electrochemical activity, which could contribute to the reversible capacity of the battery.43 The cell delivers a high coulombic efficiency of nearly 100% after three cycles, demonstrating a high reversibility. All these results show that Li6PS5Cl electrolyte is applicable as an electrolyte for ASSLSBs. Figure 7c shows the charge-discharge curves of the nano-sulfur/MWCNT composite cathode||Li6PS5Cl||Li-In cell. The current density of the 2nd cycle is 0.044 mA cm−2 and that of the other cycles is 0.176 mA cm−2. Only one discharge plateau at 1.5 V (vs. Li-In) 19



is detected due to the transformation from S8 to S2-. Meanwhile, only one charge plateau at 1.8 V (vs. Li-In) is observed corresponding to the transformation from Li2S to S8. These results indicate a direct reaction between sulfur and Li2S during the discharge/charge process in the ASSLSB, which is in accordance with the CV scan in Figure 5a. Compared to the Li-S batteries with an organic liquid electrolyte, the sulfur redox chemistry is distinguished in ASSLSB and there is no any other charge or discharge plateau related to the formation of polysulfides in the ASSLSBs, which is in accordance with the reported works.14-16,

18, 36

Due to the elimination of polysulfide shuttle effect, the nano-

sulfur/MWCNT composite cathode||Li6PS5Cl||Li-In cell exhibits a high reversibility. Overlapping of the 3rd and the 20th discharge curves indicates good cycling performance with little attenuation during initial galvanostatic cycling. Although the discharge capacity rapidly decreases after 20 cycles, the discharge capacity of the 50th cycle is still as high as 1393 mAh g-1.

Conclusions Li6PS5Cl SSEs with high ionic conductivities were synthesized by solid-state sintering. The influence of sintering temperature and duration on the phase, ionic conductivity, and activation energy of the Li6PS5Cl was investigated. It was found that a duration of 10 min at 550 C was enough to obtain the Li6PS5Cl with minor Li2S impurity and a high ionic conductivity of 3.15×10-3 S cm-1. Such a short preparation time can increase the production 20


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efficiency and reduce energy consumption. ASSLSBs based on Li6PS5Cl SSEs were assembled. An ultra-high discharge capacity of 1850 mAh g-1 was achieved at RT for the first full cycle at 0.176 mA cm-2. After 50 cycles, the discharge capacity was 1393 mAh g-1. The Coulombic efficiency remained near 100% during the charge-discharge cycles. These results show that Li6PS5Cl SSEs possess good potential for the applications in ASSLSBs.

Acknowledgments This work was supported by Basic Science Center Program of National Natural Science Foundation of China (NSFC) under Grant No. 51788104 and NSFC projects under Grant Nos. 51572149, 51532002, 51572141, and 51625202.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06658. Nyquist spectra measured at 25 C for Li6PS5Cl SSEs synthesized at various sintering temperatures for 10 h and at 550 C for different durations; direct-current (DC) polarization of a blocking stainless steel||Li6PS5Cl||stainless steel cell; current-time curves of the 550-10min sample; SEM images of the 550-15h Li6PS5Cl powder and the surface of a Li6PS5Cl pellet.

21



Corresponding Author *Corresponding author e-mail: [email protected] Phone: +86-10-62797162 Fax: +86-10-62771160 Postal address: State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China *Corresponding author e-mail: [email protected] Phone: +86-10-62773587 Fax: +86-10-62771160 Postal address: State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

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High-Conductivity Argyrodite Li6PS5Cl Solid Electrolytes Prepared via

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