High Capacity and Superior Cyclic Performances of All-Solid-State

multiple carbons, a kind of monolithic all-solid-state batteries were prepared with ... discharge capacity of 480.3 mAh g-1 at 0.176 mA cm-2 after 60 ...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10029−10035

High Capacity and Superior Cyclic Performances of All-Solid-State Lithium Batteries Enabled by a Glass−Ceramics Solo Yibo Zhang, Rujun Chen, Ting Liu, Bingqing Xu, Xue Zhang, Liangliang Li, Yuanhua Lin, Ce-Wen Nan,* and Yang Shen* State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: By using highly Li-ion conductive 78Li2S−22P2S5 glass− ceramic (7822gc) as both the electrolyte and active material in the composite cathode obtained via ball-milling the 7822gc with multiple carbons, a kind of monolithic all-solid-state batteries were prepared with a lithium−indium foil as the anode. Such 7822gc-based monolithic batteries present stable discharge capacity of 480.3 mA h g−1 at 0.176 mA cm−2 after 60 cycles, which is three times larger than that of the previous work, with the highest capacity obtained so far among all attempts of using sulfide electrolytes as the active materials. High capacity retention of 90.6% and Coulombic efficiency of higher than 99% with high active material loading of 7 mg cm−2 were also obtained. X-ray photoelectron spectroscopy was used to reveal the electrochemical reaction mechanisms in the 7822gc cathode. KEYWORDS: all-solid-state battery, 78Li2S−22P2S5, glass−ceramic, solid electrolyte, lithium ion battery

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Li9.54Si1.74P1.44S11.7Cl0.316 recently reported achieved ultrahigh ionic conductivity of more than 1.0 × 10−2 S cm−1. However, the utilization of the expensive element Ge and the electrochemical instability between electrolytes and low-voltage anode materials such as lithium metals have restricted their applications.17 Meanwhile, the classical Li2S−P2S5 glass− ceramic solid electrolytes are still attracting great attention because of their inexpensive ingredients, high ionic conductivity of about 10−3 S cm−1 at room temperature, and good electrochemical stability. Of particular interest, a high ionic conductivity of 1.7 × 10−2 S cm−1 and a low activation energy of 17 kJ mol−1 were achieved in the 70Li2S−30P2S5 system, even though an all-solid-state battery using this material has not been reported.18 Recently, we have demonstrated the usability of the 70Li2S−30P2S519 and 78Li2S−22P2S520 glass−ceramic (7822gc) electrolytes with a high conductivity of more than 1.0 × 10−3 S cm−1. Especially, the 7822gc prepared via simple processing has achieved the highest conductivity of 1.78 × 10−3 S cm−1 in the 78Li2S−22P2S5 system.21,22 High-performance all-solid-state lithium batteries with FeS2 or Li2S as cathode active materials and the Li−In foil as the anode material have also been prepared to demonstrate the usability of the 7822gc electrolyte. As for the problem of poor rate performance, additive materials used to construct the ionic and electronic conduction paths in the electrode have limited the contents of active

INTRODUCTION The prevalence of the electric vehicles, wearable products, and renewable electricity is greatly promoting wide applications of lithium-ion batteries in the fields of energy storage.1−3 However, the dangerous flammable organic liquid electrolytes used in the commercial lithium-ion batteries have safety concerns with respect to vehicles or electronic products. Thus, all-solid-state batteries with inorganic incombustible electrolytes exhibiting enhanced safety, higher durability, and larger capacity have been put forward to solve these safety issues and are attracting increased attention.4−6 Nevertheless, the commercialization of the all-solid-state lithium-ion batteries is currently hampered because of several problems, for example, interfacial issues and compromised cyclic and rate performances. Roots of the interfacial issues and the poor cyclic performance are the poor interface contacts between solid electrolytes and electrodes, which could result in high interfacial resistance and the low ionic conductivity of solid electrolytes compared to the liquid counterparts.7,8 Therefore, a superior solid electrolyte with high ionic conductivity and low interface resistance to electrode contact is critical to improving battery performance in all-solid-state systems.9−12 Scores of inorganic solid electrolytes with high lithium-ion conductivity have been investigated. Among them, with the advantages of higher ionic conductivity, a more convenient preparation method, and better interface contacting with electrodes than oxides, sulfur electrolytes have been widely studied,13 though their application in the open air is restricted because of their high sensitivity to moisture.14 Among the family of sulfur electrolyt es, Li 1 0 GeP 2 S 1 2 1 5 and © 2018 American Chemical Society

Received: November 30, 2017 Accepted: February 23, 2018 Published: February 23, 2018 10029

DOI: 10.1021/acsami.7b18211 ACS Appl. Mater. Interfaces 2018, 10, 10029−10035

Research Article

ACS Applied Materials & Interfaces materials in the composite cathode.23−25 Thus, the capacities per gram of total composite cathode for the all-solid-state batteries are sacrificed to obtain a better power density. The Li2S−P2S5 electrolytes contain an Li2S component, which is a kind of cathode material with high theoretical capacity. Thus, the electrolyte itself could be used as an active material as well. The 7822gc electrolyte might have residual Li2S crystalline phase and high conductive phase in the glass−ceramic matrix which may also have electrochemical activity. Thus, both high power density and high energy density could be expected in the all-solid-state batteries with the 7822gc as both electrolyte and active cathode material. Here, we report a kind of monolithic all-solid-state battery with the 7822gc as both electrolyte and cathode active material and the lithium−indium (Li−In) foil as the anode. The composite cathodes are prepared through ball-milling 7822gc and multiple carbons (MC) without any other normal active materials. Such 7822gc-based all-solid-state batteries present stable discharge capacity of 480.3 mA h g−1 at 0.176 mA cm−2 after 60 cycles, which is three times larger than that of the reported work, with the highest capacity obtained so far of all attempts of using sulfide electrolytes as the active materials.23,26 High capacity retention of 90.6% and excellent Coulombic efficiency higher than 99% are also obtained with high loading of 7822gc. All above effectively demonstrate the usability of this monolithic all-solid-state battery simply based on the 7822gc.

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Figure 1. Schematic configuration of the monolithic cell 7822gc−MC| 7822gc|Li−In. Yellow polyhedrons denote the 7822gc particles, and dark dots and rods denote the MC particles. weight of active materials (i.e., the 7822gc in the composite electrodes). 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 AC impedance measurements were performed for the cells before and after cycles using an impedance analyzer (ZAHNER-elektrik IM6) in the frequency range of 1 Hz to 8 MHz with an AC amplitude of 50 mV. X-ray diffraction patterns (XRD, Rigaku D/max-2500 diffraction meter with a Cu Kα radiation source) were obtained for the phase identification of the composite cathode samples in air at room temperature. To prevent from air exposure, the samples were sealed in an air-tight container covered with a polyimide thin film and mounted on the X-ray diffractometer. Scanning election microscopy (SEM, JEOLJSM-7001F) was used to observe the cross-section microstructure of the cells before and after the cycles coupled with an energy dispersive spectroscopy (EDS). The surface chemistry of the 7822gc− MC cathodes was examined by X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250Xi spectrometer. To prepare the sample for XPS test, the electrodes were charged or discharged to a certain voltage and held at that voltage for 12 h. Then, the whole cells were transferred into the XPS chamber under inert conditions in a nitrogenfilled glove bag. Ar+ sputtering was performed for 180 s on the surface of the discharged and charged cathodes to remove the oxidized surface layer. XPS data were collected using an Al Kα X-ray source (1486.6 eV). The working pressure of the chamber was lower than 7.0 × 10−6 mbar. All reported binding energy values are calibrated to the C 1s peak at 284.8 eV.

EXPERIMENTAL SECTION

To prepare the 7822gc electrolytes, reagent-grade chemicals P2S5 (Aldrich, 99%) and Li2S (Alfa, 99.9%) were mixed thoroughly in appropriate molar ratios. Then, the mechanical milling using a highenergy planetary ball mill (FRITSCH PULVERISETTE 7) was performed for 25 h. All the processing was carried out in a dry Ar-filled glovebox with [H2O], [O2] < 1 ppm. The obtained 78Li2S−22P2S5 glass electrolyte was ground into powder by a motor in the glovebox; then the glass powder was heat-treated at 170 °C for 0.5 h and then 240 °C for 3 h in the glovebox to obtain the glass−ceramic electrolytes. The performance of 7822gc electrolytes can be found in our previous report.20 MC used as the conductive additive was prepared by ball-milling Super P carbon (TIMCAL) (66.7 wt %) and VGCF (vapor grown carbon fibers, Showa Denko) (33.3 wt %) together. Hong et al. have demonstrated that zero-dimensional Super P could form good electronic conduction paths in short-range regions and VGCF could provide long-range conduction paths. Thus, Super P and VGCF together could form a homogeneous and synergistic hierarchical conductive network.27 The composite cathodes were obtained by ballmilling the mixture with 70−90 wt % 7822gc and 30−10% wt % MC powders, and the loading of the active material 7822gc is about 7 mg cm−2. All-solid-state cells of 7822gc−MC|7822gc|Li−In (see Figure 1) were fabricated with the obtained 7822gc−MC as a working composite electrode and the 7822gc as the solid electrolyte. The electrochemical performance of all-solid-state cells were tested in a Swagelok cell, wherein the solid electrolyte and electrodes were coldpressed inside an insulating poly(ether−ether−ketone) PEEK tube (diameter = 12 mm). First, the electrolyte layer was obtained by compressing 7822gc powders at 100 MPa. Then, the composite cathodes were pressed on the one side of the electrolyte pellet at 150 MPa, and the Li−In foil as the anode was attached at 100 MPa on the other side. Finally, the three-layered cell pellets were pressed into a mode (3ESTC15, Kejing Star, China) at 100 MPa with two stainless steel disks as current collectors on both sides. The total mass of the composite cathodes is about 10 mg. The cells were cycled using a charge−discharge measurement device (C2001A, LAND, China) at room temperature. The cut-off voltage was 0.0−3.0 V (vs Li−In), which is equivalent to 0.6−3.6 V (vs Li). The capacity is based on the

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RESULTS AND DISCUSSIONS The cycling performances of the 7822gc−MC|7822gc|Li−In all-solid-state cells cycled between 0.0 and 3.0 V (vs Li−In) at relatively high current density (0.176 mA cm−2) are shown in Figure 2. Initial discharge capacity of the cells with 70 wt % 7822gc−30 wt % MC as the cathode is 655 mA h g−1 at 0.044 mA cm−2. The cell retains a stable discharge capacity of 480.3 mA h g−1 at 0.176 mA cm−2 after 60 cycles. High capacity retention of 90.6% during the last 50 cycles is obtained. The cells with 80 wt % 7822gc−20 wt % MC as the composite cathode possess an initial discharge capacity of 419 mA h g−1 at 0.044 mA cm−2. Then, during the high-rate cycling, the capacity slowly increases with fluctuations to 328.9 mA h g−1 at 0.044 mA cm−2 after 60 cycles. As for the cells with 90 wt % 7822gc− 10 wt % MC as the composite cathodes, the initial discharge capacity is only 9.2 mA h g−1 at 0.044 mA cm−2. Then, the capacity keeps as low as 2.6 mA h g−1 after 60 cycles at 0.176 mA cm−2. Except for the cell with 90 wt % 7822gc−10 wt % MC as the composite cathodes, the others deliver a high Coulombic efficiency larger than 99% during the last 50 cycles. For the first several high-rate cycles, the relatively low Coulombic efficiency and increasing capacity demonstrate that the composite cathodes need to be activated to achieve 10030

DOI: 10.1021/acsami.7b18211 ACS Appl. Mater. Interfaces 2018, 10, 10029−10035

Research Article

ACS Applied Materials & Interfaces

Figure 2. Cycling performance of x wt % 7822gc−(100 − x) wt % MC|7822gc|Li−In all-solid-state cells. (a) Charge, discharge capacity, and Coulombic efficiency as a function of the cycle number at the current of 0.176 mA cm−2. (b) The corresponding discharge−charge voltage profiles of the cells using 70 wt % 7822gc−30 wt % MC as the electrodes at 2nd, 10th, and 60th cycles.

Figure 3. Rate performance of x wt % 7822gc−(100 − x) wt % MC| 7822gc|Li−In all-solid-state cells. (a) Charge and discharge capacity under different cyclic currents (i.e., 0.044, 0.088, 0.176, 0.352, and 0.044 mA cm−2) as a function of the cycle number. (b) The corresponding discharge−charge voltage profiles at different currents of the cells using 70 wt % 7822gc−30 wt % MC as the electrodes.

the tested current decreases back to 0.044 mA cm−2, the discharge capacity obtains a high capacity retention of 94.5% (592 mA h g−1). However, for the cell with 80 wt % 7822gc−20 wt % MC as the cathodes, the capacity is 426 mA h g−1 at 0.044 mA cm−2. In addition, the cells retain 59.8% of their capacity at 0.352 mA cm−2 (i.e., 235 mA h g−1). When the current goes back to 0.044 mA cm−2, the capacity retention is 89.5% (351 mA h g−1). The discharge capacity of the cell with 90 wt % 7822gc−10 wt % MC as the cathodes is rather low, only 6.7 mA h g−1 for the first full cycle at 0.044 mA cm−2. The capacity retention of this cell is only 26.9% at 0.352 mA cm−2 (i.e., 1.8 mA h g−1). When the current decreases back to 0.044 mA cm−2, the discharge capacity slowly increases to 8.0 mA h g−1 at 0.044 mA cm−2. Among all these composite cathodes, the 70 wt % 7822gc−30 wt % MC electrodes exhibit the best performance, which possess the highest capacity and the best capacity retention after high-rate cycling. The comparison illustrates that sufficient electric conductive paths are important for satisfactory performance of the 7822gc−MC composite cathodes because the 7822gc owns poor electronic conductivity and good ionic conductivity. Good electronic conductive paths are rather essential to inspire the capacity potential of the 7822gc. The discharge−charge voltage profiles of the 7822gc−MC| 7822gc|Li−In cells with 70 wt % 7822gc−30 wt % MC as the cathodes are presented in Figure 3b. Two plateaus at about 1.8 and 2.3 V during the charge process and another two at about 1.3 and 0.6 V during the discharge process are observed. Among them the plateaus at 1.8 and 1.3 V have also been observed by Hakari et al., which mainly attribute to the

the subsequent excellent cyclic performance. The continuous rising capacity of the cell using 80 wt % 7822gc−20 wt % MC as the electrodes may represent a prolonged activated process. Among all attempts of using general sulfide electrolytes as the active materials so far, the obtained high capacity is three times larger than that of the reported work, exhibiting the highest capacity.23,26 The excellent performance is also comparable with those for all-solid-state cells with metal sulfides or Li2S as the cathodes,28−30 and our monolithic all-solid-state cells even have higher loading of active materials (about 7 mg cm−2) and better capacity retention. For further illustration, Figure 2b shows the charge− discharge curves of 7822gc−MC|7822gc|Li−In cells with 70 wt % 7822gc−30 wt % MC as the cathodes. It should be noted that the current density of the 2nd cycle is 0.044 mA cm−2 and 0.176 mA cm−2 for the others. Overlapping of the 10th and the 60thdischarge curves demonstrates the good cycling performance with little attenuation during the galvanostatic cycling. High capacity and good retention without rapid degradation clearly indicate that the charge−discharge reaction of the 78Li2S−22P2S5 system active material is reversible. The rate capabilities of the 7822gc−MC|7822gc|Li−In allsolid-state cells with three types of composite cathodes cycled between 0.0 and 3.0 V (vs Li−In) at room temperature are compared in Figure 3a. The discharge capacity of the cell with 70 wt % 7822gc−30 wt % MC as the cathodes is the highest, which achieves 646 mA h g−1 for the first full cycle at 0.044 mA cm−2. The capacity retention of this cell is 64.9% at 0.352 mA cm−2 (407 mA h g−1) with respect to the initial capacity. When 10031

DOI: 10.1021/acsami.7b18211 ACS Appl. Mater. Interfaces 2018, 10, 10029−10035

Research Article

ACS Applied Materials & Interfaces electrochemical reaction of the glass matrix itself as the active material.23 The other two plateaus may relate to the Li2S crystal phase or some other materials generated during the discharge process. Both of these two reactions contribute to the high capacity of 646.1 mA h g−1 at 0.044 mA cm−2. We separate the discharge process into two parts. The regions with voltage higher than 0.7 V are called region I and those with voltage lower than 0.7 V are called region II. By comparing the cell cycling on different rates, we can see that the speed of the capacity attenuation contributed by regions I and II is different. As the current density increases from 0.044 to 0.352 mA cm−2, the discharge capacity above 0.7 V decreases from 360 to 310 mA h g−1. High capacity retention of 86.1% is achieved. While the capacity below 0.7 V rapidly decreases from 290 to 110 mA h g−1 and only 37.9% of the initial capacity is retained. When the cells cycle at high current density (0.352 mA cm−2), about 73.8% of the cycling capacity comes from region I. However, when the current density is as low as 0.044 mA cm−2, about 44.6% of the cycling capacity comes from region II. Thus, at low current density, the capacity contributed by region II is comparable to that of region I. When operated at higher current density, region II would provide little recyclable energy and even lose the electrochemical activity. To balance both high capacity and high rate, the current is set at 0.176 mA cm−2 for the galvanostatic cycling test mentioned above. EIS measurements are performed to further understand the good electrochemical performances of the 7822gc−MC composite cathodes. The Nyquist plots of the all-solid-state cells with 70 wt % 7822gc−30 wt % MC as the composite cathodes at room temperature are shown in Figure 4. The total

Figure 5. XRD pattern of the 7822gc electrolyte material and the composite cathodes before and after 60 cycles. The phases in the samples can be indexed to Li2S, Li7PS6 and the high conductivity phase.

composite cathodes. It suggests that no new crystal phase is formed after ball-milling treatment. However, the peak intensity of the high conductivity phase in the composite cathodes is greatly reduced owing to the existence of a large volume fraction of MC, which only shows the halo patterns in the XRD testing. The weak peaks assigned to Li7PS6 in the 7822gc were even hard to distinguish in the composite cathodes. Similar XRD patterns were observed for the composite cathodes before and after 60 cycles, which indicates that no new crystal phase is generated during the cycle. Meanwhile, when we carefully examine the XRD result near 28°, as shown in Figure S1, we can find that the peak assigned to Li2S at 26.8° disappeared in the sample before cycle. However, the same peak appeared in the composite cathode after the 60th discharge process. We believe that the remained Li2S phase in the 7822gc finally become part of the glass matrix during the ball-milling mixing process. As a result, no Li2S phase could be found in the composite cathode before the cycle. However, new Li2S phase is generated during the discharge process, leading to the reappearance of the Li2S peak in the sample after 60 cycles. The XRD result coincides with the discharge−charge voltage profiles, which demonstrates that the materials generated during the discharge process lower than 0.7 V is the Li2S crystal phase. Figure 6 shows the cross-sectional microstructure of the cells before and after 60 cycles, which concentrates on the interface

Figure 4. Nyquist plots for the cells with 70 wt % 7822gc−30 wt % MC as the electrodes before cycle and after 10th and 60th cycles.

impedance of the all-solid-state battery before the cycle is 46 Ω, which is similar to that for the 7822gc electrolyte layer itself. The initial interface impedance of the cells is rather low. However, after the 10th cycle, the total impedance of the battery increases to about 130 Ω. The variation of the impedance related to the composite cathode/electrolyte interface is much larger than the one for the electrolyte/Li− In interface.20,31 After subsequent 50 more cycles, the total impedance of the all-solid-state battery merely increases about 20 Ω. As the rate of the deterioration of the interfacial impedance keeps slow, the cells could successfully maintain a high capacity and high Coulombic efficiency after 60 cycles. Figure 5 shows the XRD of the 70 wt % 7822gc−30 wt % MC composite cathodes before and after 60 cycles. Compared with the 7822gc electrolyte, no new peaks are observed in the

Figure 6. (a) Cross-sectional SEM image of the cells, and EDS mappings of (b) carbon and (c) sulfur in the same area and before cycles; (d−f) SEM image and EDS mappings of carbon and sulfur after 60 cycles. 10032

DOI: 10.1021/acsami.7b18211 ACS Appl. Mater. Interfaces 2018, 10, 10029−10035

Research Article

ACS Applied Materials & Interfaces between the composite cathode and the electrolyte. Figure 6a shows the sample before the cycle, and Figure 6b,c shows the EDS mappings of carbon and sulfur. The interface between the composite cathodes and the electrolytes is difficult to distinguish in Figure 6a. Through observing the element mappings, the area enriched with carbon or sulfur can be identified as the composite cathode or the electrolyte layer with ease. Both of these two layers are compressed tightly. The intimate physical contact of the electrode and the electrolyte guarantees the high initial cycling capacity of the cells. Figure 6d shows the cell after 60 cycles, and Figure 6e,f shows the EDS mappings of carbon and sulfur. High structural integrity between the electrode and the electrolyte can also be observed in Figure 6d. Without the help of element mapping, the interface is also very hard to be discerned. Even after 60 cycles, both of the composite cathode and the electrolyte layer are still dense. With the help of element mappings, the area of the composite cathode and the electrolyte can also be distinguished by carbon or sulfur enrichment. The interface contact maintains the sound condition as that before the cycle, which guarantees the high cycling capacity and great capacity retention. Although good physical contact of the interface between the composite cathode and electrolyte after cycle is observed, possible poor chemical contact or space charge layer may have an impact, which results in the increase of the interface impedance and the attenuation of the cycling capacity. To reveal the electrochemical reaction mechanisms of the 7822gc−MC electrode in the all-solid-state cells, XPS was used to characterize the electrodes at various charged or discharged states. Figure 7a shows the discharge−charge voltage profiles of the cells using 70 wt % 7822gc−30 wt % MC as the cathodes for the first two cycles. The electrodes at the fresh state, first fully charged (to 3.0 V), first discharged to 0.64 V, first fully discharged (to 0.0 V), second charged to 1.85 V, and second fully charged (to 3.0 V) are respectively named states 1 to 6, as shown in Figure 7a. Figure S2 shows the XPS spectra of S 2p for the samples. The spectra for the states 1, 3, and 5 are similar, which represents the initiate state of the composite cathodes. Similar XPS spectra of S 2p are also observed for the states 2 and 6, which represents the charge state. The XPS spectra of S 2p for the state 4 shows the discharge state. Figure 7b−d compares the XPS spectra of S 2p for the states 1, 2, and 4, which represents the composite cathodes at fresh, fully charged, and fully discharged states. The corresponding spectra are fitted with spin−orbit coupled 2p3/2 and 2p1/2 doublets, constrained at 1.18 eV separation with a characteristic 2:1 area ratio and equal full width at half-maximum. The results indicate that the cathodes at different states exhibit a main peak around ≈163 eV. The detailed fitting for the main peak of interest results in two peaks at ≈163.3 and 162.1 eV. However, considering the difficulty of distinguishing either of them for the complicated chemical environment of S in the 7822gc samples (e.g. peaks at 164 eV (S8 ring), 163.5 eV (bridging sulfur: P−S−P) and 162.2 eV (non-bridging sulfur: PS)32 in the glass−ceramic matrix, high conductive phase, and lithium sulfides), the peaks are denoted as a high binding energy (B.E.) peak (at ≈163.3 eV) and a low B.E. peak (at ≈162.1 eV).23,33 Nevertheless, another new B.E. peak (at ≈160.2 eV) is observed in the sample of fully discharged states. It represents an electron-rich sulfur species whose B.E. is similar to that of S2− in bulk Li2S (at ≈159.8 eV).34 Therefore, it is denoted as S2−. The electronegativity difference of cations that are bonded with S could tell a general trend of their B.E. positions (S−S >

Figure 7. (a) The discharge−charge voltage profiles of the cells using 70 wt % 7822gc−30 wt % MC as the electrodes for the first two cycles. The deconvoluted S 2p core XPS spectra of the composite cathodes at the states: (b) 1 at the fresh state, (c) 2 first fully charged (to 3.0 V), and (d) 4 first fully discharged (to 0.0 V).

P−S−P > PS > Li−S). Additionally, the peak would shift to higher binding energy as the oxidation state of S increases. Li2S with a sulfur oxidation state of −2 is always located in the lowest B.E. peak. These results allow us to use the area ratio of the low B.E. peak or the S2− peak to the high B.E. peak to speculate the reaction mechanism in the 7822gc cathodes (Table 1). The area ratio of the low to high B.E. peak decreases Table 1. XPS Binding Energies of High B.E. Peak (Ehigh), Low B.E. Peak (Elow) and S2− Peak (E S2−) of S 2p Spectra and the Area Ratio of Elow or E S2− Peak to Ehigh Peak in 70 wt % 7822gc−30 wt % MC Cathodes samples fresh charged discharged

E S2− [eV]

Elow S 2p3/2 [eV]

Ehigh S 2p3/2 [eV]

area ratio

160.23

162.23 162.03 161.6

163.46 163.35 162.9

3.31:1 1.65:1 7.21:1, 5.56:1

from 3.31:1 for the fresh 7822gc electrode to 1.65:1 after being charged to 3.0 V, which indicates that the oxidation of sulfur (from PS to P−S−P and S−S) occurs during the charge process. However, after the 7822gc is discharged to 0.0 V, the ratio increases back to as high as 7.21:1, which means the reduction of S. Meanwhile, the S2− peak appears during the discharge process. The area ratio of the S2− to the high B.E. peak increases from zero to 5.56:1 during discharging. This demonstrates that the sulfide ion further reduces during the discharge process and transforms from a low or high B.E. state 10033

DOI: 10.1021/acsami.7b18211 ACS Appl. Mater. Interfaces 2018, 10, 10029−10035

Research Article

ACS Applied Materials & Interfaces to S2− state. The generation of S2− state mainly provides the extra discharge capacity of the cell between the voltages ranging from 0.7 to 0.0 V (region II), which has also been confirmed by the XRD test and the discharge−charge voltage profiles. The reduction from P−S−P or S−S to PS in the glass matrix contributes to the discharge capacity of the cell with voltage higher than 0.7 V (region I). These results confirm that the electrode performance of the 7822gc could be attributed to both the oxidation and reduction of the PS, P−S−P and S−S state, as well as the formation and decomposition of the S2− state within the 7822gc material. Both the glass matrix and the new generated Li2S give rise to the high discharge capacity of the 7822gc and the good cycling performance of the all-solidstate batteries enabled by a glass−ceramics solo.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (grant no. 51532002).

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(1) Scrosati, B. Battery technologychallenge of portable power. Nature 1995, 373, 557−558. (2) Aricò, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; van Schalkwijk, W. Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater. 2005, 4, 366−377. (3) Tarascon, J.-M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359−367. (4) Ren, Y.; Chen, K.; Chen, R.; Liu, T.; Zhang, Y.; Nan, C.-W. Oxide electrolytes for lithium batteries. J. Am. Ceram. Soc. 2015, 98, 3603− 3623. (5) Chen, R.; Liu, Z.; Li, L.; Wu, F. Electrolyte materials for high energy density lithium-sulfur secondary battery. Chin. Sci. Bull. 2013, 58, 3301. (6) Fergus, J. W. Ceramic and polymeric solid electrolytes for lithium-ion batteries. J. Power Sources 2010, 195, 4554−4569. (7) Tatsumisago, M. Glassy materials based on Li2S for all-solid-state lithium secondary batteries. Solid State Ionics 2004, 175, 13−18. (8) Manthiram, A.; Yu, X.; Wang, S. Lithium battery chemistries enabled by solid-state electrolytes. Nat. Rev. Mater. 2017, 2, 16103. (9) Chen, R.; Liang, W.; Zhang, H.; Wu, F.; Li, L. Preparation and performance of novel LLTO thin film electrolytes for thin film lithium batteries. Chin. Sci. Bull. 2012, 57, 4199−4204. (10) Zhu, Y.; He, X.; Mo, Y. Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first-principles calculations. ACS Appl. Mater. Interfaces 2015, 7, 23685−23693. (11) Minami, K.; Hayashi, A.; Ujiie, S.; Tatsumisago, M. Structure and properties of Li2S-P2S5-P2S3, glass and glass-ceramic electrolytes. J. Power Sources 2009, 189, 651−654. (12) Fu, K.; Gong, Y.; Hitz, G. T.; McOwen, D. W.; Li, Y.; Xu, S.; Wen, Y.; Zhang, L.; Wang, C.; Pastel, G.; Dai, J.; Liu, B.; Xie, H.; Yao, Y.; Wachsman, E. D.; Hu, L. Three-Dimensional Bilayer Garnet Solid Electrolyte Based High Energy Density Lithium Metal-Sulfur Batteries. Energy Environ. Sci. 2017, 10, 1568−1575. (13) Tatsumisago, M.; Nagao, M.; Hayashi, A. Recent development of sulfide solid electrolytes and interfacial modification for all-solidstate rechargeable lithium batteries. J. Asian Ceram. Soc. 2013, 1, 17− 25. (14) Hayashi, A.; Muramatsu, H.; Ohtomo, T.; Hama, S.; Tatsumisago, M. Improvement of chemical stability of Li3PS4 glass electrolytes by adding MxOy (M = Fe, Zn, and Bi) nanoparticles. J. Mater. Chem. A 2013, 1, 6320−6326. (15) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A lithium superionic conductor. Nat. Mater. 2011, 10, 682− 686. (16) Kato, Y.; Hori, S.; Saito, T.; Suzuki, K.; Hirayama, M.; Mitsui, A.; Yonemura, M.; Iba, H.; Kanno, R. High-power all-solid-state batteries using sulfide superionic conductors. Nat. Energy 2016, 1, 16030. (17) Han, F.; Zhu, Y.; He, X.; Mo, Y.; Wang, C. Electrochemical stability of Li10GeP2S12 and Li7La3Zr2O12 solid electrolytes. Adv. Energy Mater. 2016, 6, 1501590. (18) Seino, Y.; Ota, T.; Takada, K.; Hayashi, A.; Tatsumisago, M. A sulphide lithium super ion conductor is superior to liquid ion

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CONCLUSIONS High-performance monolithic all-solid-state cells using the 7822gc as both electrolyte and active material of the composite cathode have been prepared. The cathode is prepared via ballmilling the 7822gc with MC which constructs good electron conduction paths so as to inspire the capacity potential of the 7822gc. The monolithic 7822gc−MC|7822gc|Li−In cells exhibit an initial discharge capacity of 654.5 mA h g−1 at 0.044 mA cm−2 and retain a stable discharge capacity of 480.3 mA h g−1 at 0.176 mA cm−2 after 60 cycles, which is three times larger than the reported work with the highest capacity of all attempts of using sulfide electrolytes as the active materials as we know. The impedance of the cells increases rather slowly with good overlapping of the discharge curves during the cycles. High capacity retention of 90.6% and Coulombic efficiency of higher than 99% with the ultra-high active material loading of 7 mg cm−2 are obtained. Our results show that providing enough electron conduction paths, the use of the 7822gc as both an active material and ionic conduction path in the composite electrode is effective for increasing the capacity of the cells. The XPS analysis of the electrochemical reaction mechanisms of the cells demonstrates that the electrode performance of the 7822gc could be attributed to both the oxidation and reduction of the PS, P−S−P, and S−S state in the glass matrix, as well as the formation and decomposition of the S2− within the 7822gc.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18211.

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REFERENCES

XRD pattern of the composite cathodes near 28° and XPS spectra of S 2p for the composite cathodes at different states (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected]. cn. Phone: +86-10-62794855. Fax: +86-10-62772507 (C.W.N.). *E-mail: [email protected] (Y.S.). ORCID

Yang Shen: 0000-0002-1421-0629 10034

DOI: 10.1021/acsami.7b18211 ACS Appl. Mater. Interfaces 2018, 10, 10029−10035

Research Article

ACS Applied Materials & Interfaces conductors for use in rechargeable batteries. Energy Environ. Sci. 2014, 7, 627−631. (19) Zhang, Y.; Chen, K.; Shen, Y.; Lin, Y.; Nan, C.-W. Synergistic effect of processing and composition x on conductivity of xLi2S-(100x)P2S5 electrolytes. Solid State Ionics 2017, 305, 1−6. (20) Zhang, Y.; Chen, R.; Liu, T.; Shen, Y.; Lin, Y.; Nan, C.-W. High Capacity, Superior Cyclic Performances in All-Solid-State Lithium-Ion Batteries Based on 78Li2S-22P2S5 Glass-Ceramic Electrolytes Prepared via Simple Heat Treatment. ACS Appl. Mater. Interfaces 2017, 9, 28542−28548. (21) Kim, J.; Eom, M.; Noh, S.; Shin, D. Performance optimization of all-solid-state lithium ion batteries using a Li2S-P2S5 solid electrolyte and LiCoO2 cathode. Electron. Mater. Lett. 2012, 8, 209−213. (22) Trevey, J.; Jang, J. S.; Jung, Y. S.; Stoldt, C. R.; Lee, S.-H. Glass− ceramic LiS−PS electrolytes prepared by a single step ball billing process and their application for all-solid-state lithium-ion batteries. Electrochem. Commun. 2009, 11, 1830−1833. (23) Hakari, T.; Nagao, M.; Hayashi, A.; Tatsumisago, M. All-solidstate lithium batteries with Li3PS4 glass as active material. J. Power Sources 2015, 293, 721−725. (24) Suo, L.; Hu, Y.; Li, H.; Wang, Z.; Chen, L.; Huang, X. Progress on high-energy density lithium-sulfur batteries. Chin. Sci. Bull. 2013, 31, 3172. (25) Xin, S.; Guo, Y.; Wan, L. Electrode materials for lithium secondary batteries with high energy densities. Sci. China: Chem. 2011, 41, 1229. (26) Han, F.; Gao, T.; Zhu, Y.; Gaskell, K. J.; Wang, C. A battery made from a single material. Adv. Mater. 2015, 27, 3473−3483. (27) Hong, S.; Kim, J.; Kim, M.; Meng, X.; Lee, G.; Shin, D. Effect of hybrid conductive additives on all-solid-state lithium batteries using Li2S-P2S5 glass-ceramics. Ceram. Int. 2015, 41, 5066−5071. (28) Chen, M.; Rao, R. P.; Adams, S. The unusual role of Li6PS5Br in all-solid-state CuS/Li6PS5Br/In-Li batteries. Solid State Ionics 2014, 268, 300−304. (29) Wan, H.; Peng, G.; Yao, X.; Yang, J.; Cui, P.; Xu, X. Cu2ZnSnS4/ graphene nanocomposites for ultrafast, long life all-solid-state lithium batteries using lithium metal anode. Energy Storage Mater. 2016, 4, 59−65. (30) Yu, P.; Wang, L.; Wang, J.; Zhao, D.; Tian, C.; Zhao, L.; Yu, H. Graphene-like nanocomposites anchored by Ni3S2 slices for Li-ion storage. RSC Adv. 2016, 6, 48083−48088. (31) Hayashi, A.; Sakuda, A.; Tatsumisago, M. Development of Sulfide Solid Electrolytes and Interface Formation Processes for BulkType All-Solid-State Li and Na Batteries. Front. Energy Res. 2016, 4, 25. (32) Tanibata, N.; Tsukasaki, H.; Deguchi, M.; Mori, S.; Hayashi, A.; Tatsumisago, M. A novel discharge−charge mechanism of a S-P2S5 composite electrode without electrolytes in all-solid-state Li/S batteries. J. Mater. Chem. A 2017, 5, 11224−11228. (33) Xu, Y.; Wen, Y.; Zhu, Y.; Gaskell, K.; Cychosz, K. A.; Eichhorn, B.; Xu, K.; Wang, C. Confined Sulfur in Microporous Carbon Renders Superior Cycling Stability in Li/S Batteries. Adv. Funct. Mater. 2015, 25, 4312−4320. (34) Koerver, R.; Walther, F.; Aygün, I.; Sann, J.; Dietrich, C.; Zeier, W. G.; Janek, J. Redox-active cathode interphases in solid-state batteries. J. Mater. Chem. A 2017, 5, 22750−22760.

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DOI: 10.1021/acsami.7b18211 ACS Appl. Mater. Interfaces 2018, 10, 10029−10035