- Email: [email protected]
Rational coating of Li7P3S11 solid electrolyte on MoS2 electrode for all-solid-state lithium ion batteries
Journal of Power Sources 374 (2018) 107–112
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Rational coating of Li7P3S11 solid electrolyte on MoS2 electrode for all-solidstate lithium ion batteries
T
R.C. Xua, X.L. Wanga,∗, S.Z. Zhanga, Y. Xiaa, X.H. Xiaa,∗∗, J.B. Wub, J.P. Tua,∗∗∗ a
State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China b School of Physics and Electronic Engineering, Taizhou University, Taizhou 318000, China
H I G H L I G H T S MoS /Li P S composite cathode is prepared by a solution method. • The Li P S layer on MoS particles intimates the interfacial contract. • The • The high performance is attributed to the interfacial architecture. 2
7 3 11
7 3 11
2
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid sulfide electrolyte Interface modification All-solid-state battery Molybdenum disulfide
Large interfacial resistance between electrode and electrolyte limits the development of high-performance allsolid-state batteries. Herein we report a uniform coating of Li7P3S11 solid electrolyte on MoS2 to form a MoS2/ Li7P3S11 composite electrode for all-solid-state lithium ion batteries. The as-synthesized Li7P3S11 processes a high ionic of 2.0 mS cm−1 at room temperature. Due to homogeneous union and reduced interfacial resistance, the assembled all-solid-state batteries with the MoS2/Li7P3S11 composite electrode exhibit higher reversible capacity of 547.1 mAh g−1 at 0.1 C and better cycling stability than the counterpart based on untreated MoS2. Our study provides a new reference for design/fabrication of advanced electrode materials for high-performance all-solidstate batteries.
1. Introduction Traditional liquid lithium ion batteries (LIBs) suffer from potential safety problems due to their flammable organic liquid electrolytes [1,2]. This greatly limits their applications and promotes the development of safe LIBs. Over the past decades, great efforts have been focused on developing safe all-solid-state (ASS) LIBs, which are considered as good candidates to replace liquid batteries because of their nonflammable solid electrolytes [3]. In addition, ASS LIBs offer many other advantages over liquid batteries such as high energy densities, excellent thermal/chemical stability and wide electrochemical stability widow [4–7]. Moreover, ASS LIBs are envisioned to be used as main power for portable devices, electronic vehicles and state grids owing to their high safety and high energy density. Solid electrolytes can be divided into three categories: inorganic solid electrolytes, solid polymer electrolyte and composite solid
∗
electrolytes [8,9]. Among them, inorganic solid electrolytes are classified into oxide and sulfide electrolytes according to the types of active materials. Particularly, sulfide solid electrolytes have been widely investigated for ASS LIBs as they possess small grain boundary resistance, high lithium ionic conductivity of 1–10 mS cm−1 at room temperature and good electrochemical stability [10]. In 2005, Tatsumisago et al. [11] reported a fast lithium ionic conductor Li7P3S11 with a high conductivity up to 3.2 mS cm−1 at room temperature. Inspired by his research, a growing number of works on sulfide solid electrolytes spring up. Recently, Kamaya et al. [12] reported a lithium superionic conductor Li10GeP2S12 with an extremely high ionic conductivity of 12 mS cm−1 at room temperature, close to that of conventional liquid electrolytes. Among the explored sulfide electrolytes, Li9.54Si1.74P1.44S11.7Cl0.3 shows an exceptionally high conductivity of 25 mS cm−1 at room temperature [13]. The lithium ionic conductivity of the solid electrolyte is approaching the demand of ASS LIBs.
Corresponding author. Corresponding author. Corresponding author. E-mail addresses: [email protected] (X.L. Wang), [email protected] (X.H. Xia), [email protected], [email protected] (J.P. Tu).
∗∗
∗∗∗
https://doi.org/10.1016/j.jpowsour.2017.10.093 Received 24 August 2017; Received in revised form 30 October 2017; Accepted 31 October 2017 Available online 14 November 2017 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
2.2. Materials characterizations
However, due to the point contacts at the interface of ASS LIBs, the number of electrochemical active sites tends to little accompanied by large interface resistance [14]. And the large interface resistance between electrodes and electrolyte lead to the degradation of the battery performance. To minimize the interfacial resistance between electrodes and electrolyte, a large amount of interfacial processing techniques have been explored and some progresses have been achieved. For example, Tatsumisago et al. [15] developed an interface modification method by depositing gold thin films and the results showed that the gold films decreased the interfacial resistance and improved the chemical stability. Aso et al. [16] reported a highly conductive 80Li2S−20P2S5 solid electrolyte and its coating on NiS particles to form intimate solid−solid contacts between NiS and solid electrolyte. The ASS batteries with the 80Li2S−20P2S5 coated composite exhibited better cycle performance than the cell with the uncoated one. In addition, Yao et al. [17] fabricated a general interfacial architecture that Li7P3S11 electrolyte particles anchored on cobalt sulfide nanosheets by an in-situ liquid-phase approach, which intimated the contact between electrolyte and active materials and improved the electrochemical performance of ASS LIBs. These results indicate that interface modifications are effective in reducing the interfacial resistance and enhancing the performance of ASS LIBs. MoS2 has been widely studied as an active material due to its high capacity (670 mAh g−1), good chemical stability, and low toxicity [18–24]. However, the utilization of active MoS2 materials is limited by the large interfacial resistance between electrode and solid electrolyte. In this present work, we report a MoS2/Li7P3S11 composite electrode synthesized by a solution method. The Li7P3S11 layer is coated on MoS2 particles to intimate the contact between the electrode and solid electrolyte. An ASS battery with the MoS2/Li7P3S11 composite electrode shows better cycling stability and rate performance than the counterpart based on untreated MoS2. The interface modification of electrode for ASS LIBs is a very effective method for enhancing the efficiency of lithium ion transport, leading to excellent electrochemical properties.
The X-ray diffraction (XRD) patterns of the as-synthesized products were recorded on an X'Pert PRO instrument with copper Kα radiation from 10° to 80°. The morphologies and microstructures of the materials were examined using a Hitachi S-4800 field-emission scanning electron microscope (SEM) and a TecnaiG2 F20 field emission transmission instrument (TEM). The ionic conductivity of the solid electrolyte was measured by a PARSTAT MC multi-channel electrochemical workstation. 2.3. Electrochemical evaluation The ionic conductivity of the solid electrolyte was tested through electrochemical impedance spectroscopy (EIS) using a blocking symmetric In/Li7P3S11/In cell. The as-synthesized Li7P3S11 powder was cold-pressed between two indium foils in a die (10 mm in diameter) at a pressure of 380 MPa. The EIS measurements were carried out for the assembled cell at frequencies from 1 MHz to 1 Hz with the amplitude of 10 mV. A non-blocking Li/Li7P3S11/Li cell was assembled to further determine the ionic conductivity of the solid electrolyte through a chronoamperometry test. All-solid-state cells were assembled in a dry argon-filled glovebox by using the MoS2/Li7P3S11 composite as the working electrode, Li7P3S11 as electrolyte and lithium foil as the counter electrode. 80 mg of Li7P3S11 solid electrolyte was placed in a 10 mm die and cold-pressed at a pressure of 380 MPa. Subsequently, 3–5 mg MoS2/Li7P3S11 working electrode was placed on the top of the electrolyte pellet. The formed two-layer pallet was cold-pressed at 380 MPa for 3 min. After that, a lithium foil was placed on the other side of the solid electrolyte as a counter and reference electrode and again cold-pressed at 120 MPa. And two stainless steel disks were attached to the both sides of the cell as current collectors. Electrochemical performances of the assembled all-solid-state cells were investigated using a LAND battery test system. The galvanostatic charge-discharge tests were carried out in the voltage window of 0.1 V–3.0 V at room temperature. Cyclic voltammetry (CV) measurements were conducted on a CHI 600D electrochemistry workstation from 0.1 to 3.0 V (vs. Li/Li+) at 0.1 mV s−1. The EIS of the cells was tested on a PARSTAT MC multi-channel electrochemical workstation.
2. Experimental 2.1. Materials synthesis Li7P3S11 coated MoS2 electrode was prepared by a solution method. Li2S (99.9%, Alfa Aesar), P2S5 (99.9%, Alfa Aesar) and MoS2 (99%, Alfa Aesar) powders were used as the starting materials. Firstly, 5.0 mmol MoS2 was added in 40 ml acetonitrile (99.8%, Alfa Aesar) solvent and stirred for 2 h at room temperature. To prepare the Li7P3S11 coated MoS2 electrode, 1.4 mmol Li2S and 0.6 mmol P2S5 powders were dissolved in the above solution. The solution was stirred for 24 h at room temperature to ensure sufficient reaction and then dried at 80 °C to remove the organic solvent. The obtained powder was further heated at 250 °C for 2 h to enable the crystallization of the Li7P3S11 glass. Finally, the powder was ground and the MoS2/Li7P3S11 composite was obtained. The Li7P3S11 solid electrolyte is also prepared by the same solution method mentioned above. 11.7 mmol Li2S and 5.0 mmol P2S5 powders were dissolved in 40 mL acetonitrile under stirring at room temperature for 24 h. After that, the solution was dried at 80 °C to evaporate the solvent. The obtained powder was heated at 250 °C in argon atmosphere for 2 h. All the preparation processes performed in a dry glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). The working electrode of solid-state cell was prepared by grinding in an agate mortar under argon atmosphere. It consists of MoS2/ Li7P3S11 composite, Li7P3S11 solid electrolyte and acetylene carbon black (AB) with a weight ratio of 30: 60: 10, namely MoS2/Li7P3S11 composite electrode (the net weight ratio of MoS2 in the composite is 24%). For comparison, the MoS2−Li7P3S11−AB composite was also prepared by the same procedure mentioned above except without the Li7P3S11 coating on MoS2 particles, namely untreated MoS2 electrode.
3. Results and discussion Fig. 1 shows the schematic illustration of the preparation process of MoS2/Li7P3S11 composite electrode. First, MoS2 as the active material was dissolved in the acetonitrile solvent separately. Li2S (70 mol%) and P2S5 (30 mol%) as the raw materials for preparing Li7P3S11 solid electrolyte layer were then added into the solution and stirred for 24 h. During the mixing process, there are no apparent precipitation could be observed. Then, the solution was dried at 80 °C to remove the organic solvent. The solid electrolyte coating on MoS2 is amorphous, which needs to be further heat-treated at 250 °C for 2 h to enable the crystallization. The heating process was designed to crystallize the Li7P3S11 glass for enhancing the ionic conductivity of the solid electrolyte layer. And then the composite of Li7P3S11 glass-ceramic coated MoS2 was obtained. The MoS2/Li7P3S11 composite, Li7P3S11 electrolyte and AB were mixed uniformly to as the working electrode, which processes both high ionic and electronic conductivity. An XRD pattern of the as-synthesized MoS2/Li7P3S11 composite is shown in Fig. 2. The main diffraction peaks in the XRD pattern for the composite (2θ = 14.4°, 39.4°, 44.2° and 49.8°) is assigned to the 2H−MoS2 phase [25,26]. In addition, it can be demonstrated from the magnified pattern from 10° to 35° that the characteristic peaks of Li7P3S11 phase (2θ = 18.0°, 19.7°, 21.8°, 23.7° and 25.9°) [27–29] confirm the successful preparation of MoS2/Li7P3S11 composite. Because the Li7P3S11 coating is thin on the MoS2 particles and the peak 108
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
Fig. 1. Schematic for preparation of MoS2/ Li7P3S11 composite electrode.
intensity of the Li7P3S11 phase is extremely weak, the diffraction peaks of Li7P3S11 is inconspicuous and submerged in the XRD pattern. Apparently, no diffraction peaks corresponding to other phase or impurities can be observed, which indicates the feasibility of the synthesized method. The SEM images of MoS2 and MoS2/Li7P3S11 composite are shown in Fig. 3a and b, respectively. The high magnification SEM images of both materials are also shown in Fig. S1. The MoS2 shows the layered crystal structure and the particles are 1–3 μm in diameters. After treating, it can be clearly seen that there is a coating on the surface of MoS2 particles. The TEM images further confirm that the coating grows on the surface of MoS2 particles for the MoS2/Li7P3S11 composite (Fig. 3c and d). In addition, the TEM image of MoS2/Li7P3S11 shows that the thickness of coating is around 30 nm (Fig. S2). The coating on the MoS2 particles imitates the contract between the electrode and electrolyte and decreases the interfacial resistance, enhancing the high rate and long cycle performance of ASS LIBs. Fig. 3e shows a highresolution TEM image of the MoS2/Li7P3S11 composite. The lattice firings with inter-planar spacing of 0.303 nm and 0.274 nm, which is corresponding to the (2-1-1) lattice plane of Li7P3S11 [17], and (100) lattice plane of 2H−MoS2 [30], respectively. The results demonstrate that the Li7P3S11 electrolyte layer successfully grows on the surface of
Fig. 2. An XRD pattern of the as-synthesized MoS2/Li7P3S11 composite.
Fig. 3. SEM images of (a) MoS2 and (b) MoS2/Li7P3S11 composite. TEM images of (c) MoS2 and (d) MoS2/Li7P3S11 composite. (e) A HRTEM image of MoS2/Li7P3S11 composite. (f) Elemental mapping images of sulfur, phosphorus and molybdenum of the composite.
109
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
The lithium storage capacity of the all-solid-state cells was investigated by galvanostatic charge-discharge test at C/10 rate (corresponding to a current density of 0.102 mA cm−1) in the voltage window of 0.1–3.0 V at room temperature. Fig. 4b shows the charge-discharge curves of the all-solid-state cells with MoS2/Li7P3S11 composite for the 1st, 2nd, 5th, 10th, 30th and 60th cycles. It is shown that two plateaus around 1.1 and 0.6 V are observed in the first discharge process, representing the formation of LixMoS2 and the conversion reaction involving the decomposition of LixMoS2 to Li2S to Mo, respectively. After the first discharge cycle, the MoS2/Li7P3S11 composite displays two short but discernible plateaus at 1.90 and 1.20 V, corresponding to the conversion from sulfur to polysulfides and then to Li2S. During the charge process, the working electrode shows an obvious potential plateau around 2.30 V. Voltage profiles of the MoS2/Li7P3S11 electrode are consistent with CV results mentioned above. The MoS2/ Li7P3S11 electrode shows an initial discharge capacity of 868.4 mAh g−1 and a reversible charge capacity of 669.2 mAh g−1 with a Coulombic efficiency of 77.1%. It is noted that the discharge capacity has exceeded the theoretical capacity (≈670 mAh g−1). The theoretical capacity of MoS2 is based on the conversion reaction (MoS2 + 4 Li+ + 4 e− → 2 Li2S + Mo). Here, the excess lithium storage is ascribed to the contribution of lithium ion intercalation/deintercalation at interfaces [50]. In addition, the formation of SEI layer and the capacity of activated carbon can also contribute to the high practical capacity [47,51]. From the second cycle onwards, the MoS2/Li7P3S11 composite manifests excellent cycling stability and still retains a reversible capacity of 547.1 mAh g−1 after 60 cycles, corresponding to approximately 75% of the discharge capacity of the second cycle (Fig. 4d). The good cycling stability is mainly attributed to the coating on the MoS2, which intimates the contact between the electrode and solid electrolyte and decreases the interface resistance. For comparison, the untreated MoS2 particles were assembled cells by the same method, except the coating of Li7P3S11. The charge-discharge curves of the allsolid-state cells with untreated MoS2 electrode is shown in Fig. 4c. Under the identical test conditions, the untreated MoS2 electrode exhibits an initial discharge capacity of around 740 mAh g−1 and shows a fast capacity fading during cycling. After 60 cycles, a capacity of only around 230 mAh g−1 is retained and the capacity retention is about 40%, much lower than that of the modified one (Fig. 4d). The poor contact between MoS2 and Li7P3S11 solid electrolyte leads to low utilization of active material, resulting the rapid capacity decay. Fig. 4e shows the long cycling performance of the untreated MoS2 and MoS2/ Li7P3S11 composite electrodes at the rate of 1.0 C. The MoS2/Li7P3S11 composite still maintains a capacity of 238.1 mAh g−1 at 400 cycles with Coulombic efficiency of almost 100%, and the capacity fading is probably due to the formation of interfacial products such as Li2S and Li3P at Li/electrolyte interface and the bulk MoS2 flakes used as electrode. In contrast, the untreated MoS2 shows a low discharge capacity and dramatically decreases in 260 cycles. It is considered that the composition of MoS2 with Li7P3S11 layer can effectively increase the contract area with solid electrolyte, provide more reaction sites, and benefit the migration of lithium, leading to an excellent cycling performance of all-solid-state batteries. Thus, the advantage of the coating structure of MoS2/Li7P3S11 composite for electrochemical performance is quite apparent. To evaluate the rate capability, the MoS2/Li7P3S11 composite electrode is cycled at different rates from 0.1 C to 1 C over a voltage window of 0.1–3.0 V at room temperature. Fig. 4f shows the rate capability of untreated MoS2 and MoS2/Li7P3S11 composite electrodes. The discharge capacities of the MoS2/Li7P3S11 electrode are 676.5, 565.4, 431.6 and 293.6 mAh g−1 at 0.1 C, 0.2 C, 0.5 C and 1.0 C, respectively. For comparison, the untreated MoS2 electrode is only able to discharge a capacity of 591.8, 454.7, 272.5 and 100.1 mAh g−1 at 0.1 C, 0.2 C, 0.5 C and 1.0 C, respectively. The results demonstrate the excellent rate performance of the MoS2/Li7P3S11 composite electrode, benefiting from the coating of Li7P3S11 electrolyte for the improvement of the ionic
MoS2 particles. To further study the component distribution of the MoS2/Li7P3S11 composite, the energy dispersive spectroscopy (EDS) elemental mapping of the sample is carried out in Fig. 3f, which certifies that the elements of sulfur, phosphorus and molybdenum uniformly distribute in the composite. In addition, the Li7P3S11 glass-ceramic electrolyte for ASS LIBs is also prepared by solution method plus annealing. The XRD of the assynthesized Li7P3S11 solid electrolyte is shown in Fig. S3. The diffraction peaks in the XRD pattern are identified to characteristic peaks of Li7P3S11, which means the Li7P3S11 glass-ceramic is successfully prepared. The Li7P3S11 glass ceramic has the space group of P-1 with P2S7 di-tetrahedral and PS4 tetrahedral units per unit cell [31–33]. Lithium ions surround the P2S7 di-tetrahedral and PS4 tetrahedral and transport through open space among the units. A SEM image shows the sizes of the Li7P3S11 glass-ceramic particles are approximately 200–800 nm (Fig. S4a). The small size of particles increases the contact area between the particles and reduces the grain boundary resistance, resulting in fast lithium ion transport. The surface morphology of the solid electrolyte after cold pressing is observed in Fig. S4b. A few micropores and cracks can be discovered on the surface, which ensures the high ionic conductivity of solid electrolyte. Before assembling ASS LIBs, the ionic conductivity of the Li7P3S11 solid electrolyte is tested using an In/ Li7P3S11 solid electrolyte/In blocking symmetric cell by EIS measurement at room temperature (Fig. S5). Through the Nyquist plot, the impedance of the solid electrolyte is about 64.9 Ω at 298 K and the calculated ionic conductivity is 2.0 mS cm−1, which has reached the requirement of ASS LIBs. To further evaluate the compatibility of solid electrolyte with lithium metal, a Li/Li7P3S11 solid electrolyte/Li nonblocking symmetric cell is fabricated. The DC polarization curve of the cell at a current density of 1.2 mA cm−2 at room temperature is shown in Fig. S6, which indicates that the compatibility with lithium metal of Li7P3S11 is very good although the duration time of stability test is not very long [34,35]. Wenzel et al. [36] found that the solid electrolyte interface (SEI) consisting of the decomposition products Li2S and Li3P grew slowly at the interface between lithium and Li7P3S11. And the SEI stabilized after a formation period and was limited to a few nanometers. The electrochemical stability between the solid electrolyte and lithium metal is a significant factor for high performance ASS LIBs. ASS cells with untreated MoS2 and MoS2/Li7P3S11 composite electrodes are assembled in an argon-filled glovebox to investigate the electrochemical performance. The schematic of an all-solid-state lithium cell is shown in Fig. S7. The Li7P3S11 glass-ceramic is used as the solid electrolyte to separate the working electrode and counter electrode. The untreated MoS2 and MoS2/Li7P3S11 composite mixed with Li7P3S11 and AB additive are used as working electrodes, respectively. In addition, the lithium metal is used as the counter electrode and two stainless steel disks are used as current collectors. Fig. 4a shows the CV curves of the first three cycles of the all-solid-state cell with MoS2/ Li7P3S11 composite electrode at a scanning rate of 0.1 mV s−1 between 0.1 and 3.0 V (vs. Li/Li+) at room temperature. Two peaks at around 0.82 V and 0.37 V in the first cathodic scan can be attributed to the intercalation of Li+ into 2HeMoS2 lattice accompanied with the formation of 1T−LixMoS2 (MoS2 + xLi++xe−→ LixMoS2) [37–40], and the conversion reaction of the lithium intercalates (LixMoS2) to form metallic Mo and Li2S (LiXMoS2 + (4-x) Li+ + (4-x) e− → Mo + 2Li2S) [41,42]. In the subsequent cathodic scans, the reduction peaks at 0.82 V and 0.37 V disappear while two new peaks around 1.85 V and 1.10 V occur. The new cathodic peaks imply the appearance of a multistep Li insertion and the formation of Li2S (nS8 + 16Li++ 16e− → 8Li2Sn and Li2Sn + (2n-2) Li++ (2n-2) e−→ nLi2S) [43–46]. And the peak at about 0.25 V is ascribed to the electrochemical process of the interfacial lithium storage and the formation of a SEI layer on the electrode surface [26,47]. In the anodic scan, a very weak anodic peak at about 1.70 V is assigned to the oxidation of Mo metal and an obvious peak at about 2.40 V is attributed to the oxidation reaction of Li2S into sulfur [41,48,49]. 110
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
Fig. 4. (a) CV curves of MoS2/Li7P3S11 composite electrode at a scanning rate of 0.1 mV s−1 between 0.1 and 3.0 V (Li/Li+). Charge-discharge curves of the all-solid-state cells for (b) MoS2/Li7P3S11 composite and (c) untreated MoS2 electrodes at 0.1 C at room temperature. (d) Cycle performance of untreated MoS2 and MoS2/Li7P3S11 composite electrodes at a rate of 0.1 C at room temperature. (e) Cycle performance of untreated MoS2 and MoS2/Li7P3S11 composite electrodes ata high rate of 1.0 C and the corresponding Coulombic efficiencies of MoS2/Li7P3S11 composite. (f) Rate capability of MoS2/Li7P3S11 composite electrode. (g) Nyquist plots of the all-solid-state cell with untreated MoS2 and MoS2/Li7P3S11 composite electrodes.
111
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
conductivity. In order to further investigate the superior electrochemical performance of MoS2/Li7P3S11 composite, the Nyquist plots of the all-solidstate cell with untreated MoS2 and MoS2/Li7P3S11 composite electrodes are compared in Fig. 4g. The Nyquist plots consist of a single semicircle in the high frequency and an inclined line at low frequency, which represent charge transfer (Rct)/interface resistance (Rif) and diffusion impedance (wo), respectively [12,52]. In addition, the intercept of xaxis in high frequency region represents the bulk resistance of electrolyte (Rbulk) [53]. Obviously, the MoS2/Li7P3S11 composite features a much smaller semicircle diameter than that of untreated MoS2, indicating a small interfacial resistance and a rapid charge-transfer reaction for Li+ insertion and extraction due the combination with the Li7P3S11 electrolyte layer. And the small interfacial resistance leads to an excellent cycle and rate performance of the MoS2/Li7P3S11 electrode. The interfacial architecture of Li7P3S11 electrolyte coating on MoS2 particles enlarges the contact area between the electrolyte and MoS2 particles and offers more active sites for Li+ insertion and extraction, leading to a high utilization of the active materials and excellent electrochemical performance. Moreover, the electrolyte layer can restrain volume charge and guarantee the intimate contract during cycling, resulting in a high cycling stability.
[11] F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, Adv. Mater. 17 (2005) 918–921. [12] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, Nat. Mater. 10 (2011) 682–686. [13] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nat. Energy 1 (2016) 16030. [14] A.C. Luntz, J. Voss, K. Reuter, J. Phys. Chem. Lett. 6 (2015) 4599–4604. [15] A. Kato, A. Hayashi, M. Tatsumisago, J. Power Sources 309 (2016) 27–32. [16] K. Aso, A. Sakuda, A. Hayashi, M. Tatsumisago, ACS Appl. Mater. Interfaces 5 (2013) 686–690. [17] X.Y. Yao, D. Liu, C.S. Wang, P. Long, G. Peng, Y.S. Hu, H. Li, L.Q. Chen, X.X. Xu, Nano Lett. 16 (2016) 7148–7154. [18] C.F. Zhang, Z.Y. Wang, Z.P. Guo, X.W. Lou, ACS Appl. Mater. Interfaces 4 (2012) 3765–3768. [19] C.B. Zhu, X.K. Mu, P.A. van Aken, Y. Yu, J. Maier, Angew. Chem. Int. Ed. 53 (2014) 2152–2156. [20] X.H. Cao, Y.M. Shi, W.H. Shi, X.H. Rui, Q.Y. Yan, J. Kong, H. Zhang, Small 9 (2013) 3433–3438. [21] D. Xie, W.J. Tang, Y.D. Wang, X.H. Xia, Y. Zhong, D. Zhou, D.H. Wang, X.L. Wang, J.P. Tu, Nano Res. 9 (2016) 1618–1629. [22] Y.L. Jia, H.Q. Wan, L. Chen, H.D. Zhou, J.M. Chen, J. Power Sources 354 (2017) 1–9. [23] D.H. Youn, C. Jo, J.Y. Kim, J. Lee, J.S. Lee, J. Power Sources 295 (2015) 228–234. [24] M. Yang, S. Ko, J.S. Im, B.G. Choi, J. Power Sources 288 (2015) 76–81. [25] K. Bindumadhavan, S.K. Srivastava, S. Mahanty, Chem. Commun. 49 (2013) 1823–1825. [26] J.B. Ding, Y. Zhou, Y.G. Li, S.J. Guo, X.Q. Huang, Chem. Mater. 28 (2016) 2074–2080. [27] H. Yamane, M. Shibata, Y. Shimane, T. Junke, Y. Seino, S. Adams, K. Minami, A. Hayashi, M. Tatsumisago, Solid State Ionics 178 (2007) 1163–1167. [28] M.R. Busche, D.A. Weber, Y. Schneider, C. Dietrich, S. Wenzel, T. Leichtweiss, D. Schröder, W. Zhang, H. Weigand, D. Walter, S.J. Sedlmaier, D. Houtarde, L.F. Nazar, J. Janek, Chem. Mater. 28 (2016) 6152–6165. [29] R.C. Xu, X.H. Xia, Z.J. Yao, X.L. Wang, C.D. Gu, J.P. Tu, Electrochim. Acta 219 (2016) 235–240. [30] L.F. Fei, S.J. Lei, W.B. Zhang, W. Lu, Z.Y. Lin, C.H. Lam, Y. Chai, Y. Wang, Nat. Commun. 7 (2016) 12206. [31] I.H. Chu, H. Nguyen, S. Hy, Y.C. Lin, Z. Wang, Z. Xu, Z. Deng, Y.S. Meng, S.P. Ong, ACS Appl. Mater. Interfaces 8 (2016) 7843–7853. [32] R.C. Xu, X.H. Xia, X.L. Wang, Y. Xia, J.P. Tu, J. Mater. Chem. A 5 (2017) 2829–2834. [33] K. Mori, K. Enjuji, S. Murata, K. Shibata, Y. Kawakita, M. Yonemura, Y. Onodera, T. Fukunaga, Phys. Rev. Appl. 4 (2015) 054008. [34] E. Rangasamy, Z.C. Liu, M. Gobet, K. Pilar, G. Sahu, W. Zhou, H. Wu, S. Greenbaum, C.D. Liang, J. Am. Chem. Soc. 137 (2015) 1384–1387. [35] Z.C. Liu, W.J. Fu, E.A. Payzant, X. Yu, Z.L. Wu, N.J. Dudney, J. Kiggans, K.L. Hong, A.J. Rondinone, C.D. Liang, J. Am. Chem. Soc. 135 (2013) 975–978. [36] S. Wenzel, D.A. Weber, T. Leichtweiss, M.R. Busche, J. Sann, J. Janek, Solid State Ionics 286 (2016) 24–33. [37] H.L. Li, K. Yu, H. Fu, B.J. Guo, X. Lei, Z.Q. Zhu, J. Phys. Chem. C 119 (2015) 7959–7968. [38] J. Shao, Q.T. Qu, Z.M. Wan, T. Gao, Z.C. Zuo, H.H. Zheng, ACS Appl. Mater. Interfaces 7 (2015) 22927–22934. [39] J.Y. Wan, W.Z. Bao, Y. Liu, J.Q. Dai, F. Shen, L.H. Zhou, X.H. Cai, D. Urban, Y.Y. Li, K. Jungjohann, M.S. Fuhrer, L. Hu, Adv. Energy Mater. 5 (2015) 1401742. [40] D. Xie, D.H. Wang, W.J. Tang, X.H. Xia, Y.J. Zhang, X.L. Wang, C.D. Gu, J.P. Tu, J. Power Sources 307 (2016) 510–518. [41] C.Y. Zhao, X. Wang, J.H. Kong, J.M. Ang, P.S. Lee, Z.L. Liu, X.H. Lu, ACS Appl. Mater. Interfaces 8 (2016) 2372–2379. [42] C.Y. Zhao, J.H. Kong, X.Y. Yao, X.S. Tang, Y.L. Dong, S.L. Phua, X.H. Lu, ACS Appl. Mater. Interfaces 6 (2014) 6392–6398. [43] Z.M. Wan, J. Shao, J.J. Yun, H.Y. Zheng, T. Gao, M. Shen, Q.T. Qu, H.H. Zheng, Small 10 (2014) 4975–4981. [44] D. Xie, X.H. Xia, Y. Zhong, Y.D. Wang, D.H. Wang, X.L. Wang, J.P. Tu, Adv. Energy Mater. 7 (2017) 1601804. [45] L.C. Yang, S.N. Wang, J.J. Mao, J.W. Deng, Q.S. Gao, Y. Tang, O.G. Schmidt, Adv. Mater. 25 (2013) 1180–1184. [46] D. Xie, X.H. Xia, W.J. Tang, Y. Zhong, Y.D. Wang, D.H. Wang, X.L. Wang, J.P. Tu, J. Mater. Chem. A 5 (2017) 7578–7585. [47] M.H. Chen, X.S. Yin, M.V. Reddy, S. Adams, J. Mater. Chem. A 3 (2015) 10698–10702. [48] K. Chang, W.X. Chen, J. Mater. Chem. 21 (2011) 17175. [49] D. Xie, W.J. Tang, X.H. Xia, D.H. Wang, D. Zhou, F. Shi, X.L. Wang, C.D. Gu, J.P. Tu, J. Power Sources 296 (2015) 392–399. [50] T. Stephenson, Z. Li, B. Olsen, D. Mitlin, Energy Environ. Sci. 7 (2014) 209–231. [51] D. Xie, X.H. Xia, Y.D. Wang, D.H. Wang, Y. Zhong, W.J. Tang, X.L. Wang, J.P. Tu, Chem. Eur. J. 22 (2016) 11617–11623. [52] B.R. Shin, Y.J. Nam, D.Y. Oh, D.H. Kim, J.W. Kim, Y.S. Jung, Electrochim. Acta 146 (2014) 395–402. [53] T. Ohtomo, A. Hayashi, M. Tatsumisago, Y. Tsuchida, S. Hama, K. Kawamoto, J. Power Sources 233 (2013) 231–235.
4. Conclusions In summary, a MoS2/Li7P3S11 composite electrode is successfully prepared by a solution method for all-solid-state batteries. Intimate combination between MoS2 and Li7P3S11 with high ionic conductivity, the Li7P3S11 coating dramatically decreases the interfacial resistance between the electrode and electrolyte, which enables a marked improvement on the electrochemical properties of the ASS electrode. The ASS batteries based on the MoS2/Li7P3S11 composite electrode exhibits a high initial discharge capacity of 868.4 mAh g−1 at 0.1 C with a high Coulombic efficiency and a high reversible capacity of 547.1 mAh g−1 is retained after 60 cycles. Moreover, the obtained all-solid-state battery possesses excellent rate performance and cycling stability. Our developed design strategy for ASS electrode provides a new way for synthesis of high-performance all-solid-state batteries. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant. Nos. 51571180, 51502263), Qianjiang Talents Plan D (Grant. No. QJD1602029), Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), Startup Foundation for Hundred-Talent Program of Zhejiang University. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2017.10.093. References [1] J.W. Wen, Y. Yu, C.H. Chen, Mater. Express 2 (2012) 197–212. [2] Z.J. Yao, X.H. Xia, Y. Zhong, Y.D. Wang, B.W. Zhang, D. Xie, X.L. Wang, J.P. Tu, Y.Z. Huang, J. Mater. Chem. A 5 (2017) 8916–8921. [3] J.W. Fergus, J. Power Sources 195 (2010) 4554–4569. [4] R.J. Chen, W.J. Qu, X. Guo, L. Li, F. Wu, Mater. Horiz. 3 (2016) 487–516. [5] Y.S. Jung, D.Y. Oh, Y.J. Nam, K.H. Park, Isr. J. Chem. 55 (2015) 472–485. [6] K. Takada, Acta Mater. 61 (2013) 759–770. [7] Z. Lin, C.D. Liang, J. Mater. Chem. A 3 (2015) 936–958. [8] A. Manthiram, X.W. Yu, S.F. Wang, Nat. Rev. Mater. 2 (2017) 16103. [9] R.C. Xu, X.H. Xia, S.H. Li, S.Z. Zhang, X.L. Wang, J.P. Tu, J. Mater. Chem. A 5 (2017) 6310–6317. [10] A. Hayashi, M. Tatsumisago, Electron. Mater. Lett. 8 (2012) 199–207.
112
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Rational coating of Li7P3S11 solid electrolyte on MoS2 electrode for all-solidstate lithium ion batteries
T
R.C. Xua, X.L. Wanga,∗, S.Z. Zhanga, Y. Xiaa, X.H. Xiaa,∗∗, J.B. Wub, J.P. Tua,∗∗∗ a
State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China b School of Physics and Electronic Engineering, Taizhou University, Taizhou 318000, China
H I G H L I G H T S MoS /Li P S composite cathode is prepared by a solution method. • The Li P S layer on MoS particles intimates the interfacial contract. • The • The high performance is attributed to the interfacial architecture. 2
7 3 11
7 3 11
2
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid sulfide electrolyte Interface modification All-solid-state battery Molybdenum disulfide
Large interfacial resistance between electrode and electrolyte limits the development of high-performance allsolid-state batteries. Herein we report a uniform coating of Li7P3S11 solid electrolyte on MoS2 to form a MoS2/ Li7P3S11 composite electrode for all-solid-state lithium ion batteries. The as-synthesized Li7P3S11 processes a high ionic of 2.0 mS cm−1 at room temperature. Due to homogeneous union and reduced interfacial resistance, the assembled all-solid-state batteries with the MoS2/Li7P3S11 composite electrode exhibit higher reversible capacity of 547.1 mAh g−1 at 0.1 C and better cycling stability than the counterpart based on untreated MoS2. Our study provides a new reference for design/fabrication of advanced electrode materials for high-performance all-solidstate batteries.
1. Introduction Traditional liquid lithium ion batteries (LIBs) suffer from potential safety problems due to their flammable organic liquid electrolytes [1,2]. This greatly limits their applications and promotes the development of safe LIBs. Over the past decades, great efforts have been focused on developing safe all-solid-state (ASS) LIBs, which are considered as good candidates to replace liquid batteries because of their nonflammable solid electrolytes [3]. In addition, ASS LIBs offer many other advantages over liquid batteries such as high energy densities, excellent thermal/chemical stability and wide electrochemical stability widow [4–7]. Moreover, ASS LIBs are envisioned to be used as main power for portable devices, electronic vehicles and state grids owing to their high safety and high energy density. Solid electrolytes can be divided into three categories: inorganic solid electrolytes, solid polymer electrolyte and composite solid
∗
electrolytes [8,9]. Among them, inorganic solid electrolytes are classified into oxide and sulfide electrolytes according to the types of active materials. Particularly, sulfide solid electrolytes have been widely investigated for ASS LIBs as they possess small grain boundary resistance, high lithium ionic conductivity of 1–10 mS cm−1 at room temperature and good electrochemical stability [10]. In 2005, Tatsumisago et al. [11] reported a fast lithium ionic conductor Li7P3S11 with a high conductivity up to 3.2 mS cm−1 at room temperature. Inspired by his research, a growing number of works on sulfide solid electrolytes spring up. Recently, Kamaya et al. [12] reported a lithium superionic conductor Li10GeP2S12 with an extremely high ionic conductivity of 12 mS cm−1 at room temperature, close to that of conventional liquid electrolytes. Among the explored sulfide electrolytes, Li9.54Si1.74P1.44S11.7Cl0.3 shows an exceptionally high conductivity of 25 mS cm−1 at room temperature [13]. The lithium ionic conductivity of the solid electrolyte is approaching the demand of ASS LIBs.
Corresponding author. Corresponding author. Corresponding author. E-mail addresses: [email protected] (X.L. Wang), [email protected] (X.H. Xia), [email protected], [email protected] (J.P. Tu).
∗∗
∗∗∗
https://doi.org/10.1016/j.jpowsour.2017.10.093 Received 24 August 2017; Received in revised form 30 October 2017; Accepted 31 October 2017 Available online 14 November 2017 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
2.2. Materials characterizations
However, due to the point contacts at the interface of ASS LIBs, the number of electrochemical active sites tends to little accompanied by large interface resistance [14]. And the large interface resistance between electrodes and electrolyte lead to the degradation of the battery performance. To minimize the interfacial resistance between electrodes and electrolyte, a large amount of interfacial processing techniques have been explored and some progresses have been achieved. For example, Tatsumisago et al. [15] developed an interface modification method by depositing gold thin films and the results showed that the gold films decreased the interfacial resistance and improved the chemical stability. Aso et al. [16] reported a highly conductive 80Li2S−20P2S5 solid electrolyte and its coating on NiS particles to form intimate solid−solid contacts between NiS and solid electrolyte. The ASS batteries with the 80Li2S−20P2S5 coated composite exhibited better cycle performance than the cell with the uncoated one. In addition, Yao et al. [17] fabricated a general interfacial architecture that Li7P3S11 electrolyte particles anchored on cobalt sulfide nanosheets by an in-situ liquid-phase approach, which intimated the contact between electrolyte and active materials and improved the electrochemical performance of ASS LIBs. These results indicate that interface modifications are effective in reducing the interfacial resistance and enhancing the performance of ASS LIBs. MoS2 has been widely studied as an active material due to its high capacity (670 mAh g−1), good chemical stability, and low toxicity [18–24]. However, the utilization of active MoS2 materials is limited by the large interfacial resistance between electrode and solid electrolyte. In this present work, we report a MoS2/Li7P3S11 composite electrode synthesized by a solution method. The Li7P3S11 layer is coated on MoS2 particles to intimate the contact between the electrode and solid electrolyte. An ASS battery with the MoS2/Li7P3S11 composite electrode shows better cycling stability and rate performance than the counterpart based on untreated MoS2. The interface modification of electrode for ASS LIBs is a very effective method for enhancing the efficiency of lithium ion transport, leading to excellent electrochemical properties.
The X-ray diffraction (XRD) patterns of the as-synthesized products were recorded on an X'Pert PRO instrument with copper Kα radiation from 10° to 80°. The morphologies and microstructures of the materials were examined using a Hitachi S-4800 field-emission scanning electron microscope (SEM) and a TecnaiG2 F20 field emission transmission instrument (TEM). The ionic conductivity of the solid electrolyte was measured by a PARSTAT MC multi-channel electrochemical workstation. 2.3. Electrochemical evaluation The ionic conductivity of the solid electrolyte was tested through electrochemical impedance spectroscopy (EIS) using a blocking symmetric In/Li7P3S11/In cell. The as-synthesized Li7P3S11 powder was cold-pressed between two indium foils in a die (10 mm in diameter) at a pressure of 380 MPa. The EIS measurements were carried out for the assembled cell at frequencies from 1 MHz to 1 Hz with the amplitude of 10 mV. A non-blocking Li/Li7P3S11/Li cell was assembled to further determine the ionic conductivity of the solid electrolyte through a chronoamperometry test. All-solid-state cells were assembled in a dry argon-filled glovebox by using the MoS2/Li7P3S11 composite as the working electrode, Li7P3S11 as electrolyte and lithium foil as the counter electrode. 80 mg of Li7P3S11 solid electrolyte was placed in a 10 mm die and cold-pressed at a pressure of 380 MPa. Subsequently, 3–5 mg MoS2/Li7P3S11 working electrode was placed on the top of the electrolyte pellet. The formed two-layer pallet was cold-pressed at 380 MPa for 3 min. After that, a lithium foil was placed on the other side of the solid electrolyte as a counter and reference electrode and again cold-pressed at 120 MPa. And two stainless steel disks were attached to the both sides of the cell as current collectors. Electrochemical performances of the assembled all-solid-state cells were investigated using a LAND battery test system. The galvanostatic charge-discharge tests were carried out in the voltage window of 0.1 V–3.0 V at room temperature. Cyclic voltammetry (CV) measurements were conducted on a CHI 600D electrochemistry workstation from 0.1 to 3.0 V (vs. Li/Li+) at 0.1 mV s−1. The EIS of the cells was tested on a PARSTAT MC multi-channel electrochemical workstation.
2. Experimental 2.1. Materials synthesis Li7P3S11 coated MoS2 electrode was prepared by a solution method. Li2S (99.9%, Alfa Aesar), P2S5 (99.9%, Alfa Aesar) and MoS2 (99%, Alfa Aesar) powders were used as the starting materials. Firstly, 5.0 mmol MoS2 was added in 40 ml acetonitrile (99.8%, Alfa Aesar) solvent and stirred for 2 h at room temperature. To prepare the Li7P3S11 coated MoS2 electrode, 1.4 mmol Li2S and 0.6 mmol P2S5 powders were dissolved in the above solution. The solution was stirred for 24 h at room temperature to ensure sufficient reaction and then dried at 80 °C to remove the organic solvent. The obtained powder was further heated at 250 °C for 2 h to enable the crystallization of the Li7P3S11 glass. Finally, the powder was ground and the MoS2/Li7P3S11 composite was obtained. The Li7P3S11 solid electrolyte is also prepared by the same solution method mentioned above. 11.7 mmol Li2S and 5.0 mmol P2S5 powders were dissolved in 40 mL acetonitrile under stirring at room temperature for 24 h. After that, the solution was dried at 80 °C to evaporate the solvent. The obtained powder was heated at 250 °C in argon atmosphere for 2 h. All the preparation processes performed in a dry glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). The working electrode of solid-state cell was prepared by grinding in an agate mortar under argon atmosphere. It consists of MoS2/ Li7P3S11 composite, Li7P3S11 solid electrolyte and acetylene carbon black (AB) with a weight ratio of 30: 60: 10, namely MoS2/Li7P3S11 composite electrode (the net weight ratio of MoS2 in the composite is 24%). For comparison, the MoS2−Li7P3S11−AB composite was also prepared by the same procedure mentioned above except without the Li7P3S11 coating on MoS2 particles, namely untreated MoS2 electrode.
3. Results and discussion Fig. 1 shows the schematic illustration of the preparation process of MoS2/Li7P3S11 composite electrode. First, MoS2 as the active material was dissolved in the acetonitrile solvent separately. Li2S (70 mol%) and P2S5 (30 mol%) as the raw materials for preparing Li7P3S11 solid electrolyte layer were then added into the solution and stirred for 24 h. During the mixing process, there are no apparent precipitation could be observed. Then, the solution was dried at 80 °C to remove the organic solvent. The solid electrolyte coating on MoS2 is amorphous, which needs to be further heat-treated at 250 °C for 2 h to enable the crystallization. The heating process was designed to crystallize the Li7P3S11 glass for enhancing the ionic conductivity of the solid electrolyte layer. And then the composite of Li7P3S11 glass-ceramic coated MoS2 was obtained. The MoS2/Li7P3S11 composite, Li7P3S11 electrolyte and AB were mixed uniformly to as the working electrode, which processes both high ionic and electronic conductivity. An XRD pattern of the as-synthesized MoS2/Li7P3S11 composite is shown in Fig. 2. The main diffraction peaks in the XRD pattern for the composite (2θ = 14.4°, 39.4°, 44.2° and 49.8°) is assigned to the 2H−MoS2 phase [25,26]. In addition, it can be demonstrated from the magnified pattern from 10° to 35° that the characteristic peaks of Li7P3S11 phase (2θ = 18.0°, 19.7°, 21.8°, 23.7° and 25.9°) [27–29] confirm the successful preparation of MoS2/Li7P3S11 composite. Because the Li7P3S11 coating is thin on the MoS2 particles and the peak 108
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
Fig. 1. Schematic for preparation of MoS2/ Li7P3S11 composite electrode.
intensity of the Li7P3S11 phase is extremely weak, the diffraction peaks of Li7P3S11 is inconspicuous and submerged in the XRD pattern. Apparently, no diffraction peaks corresponding to other phase or impurities can be observed, which indicates the feasibility of the synthesized method. The SEM images of MoS2 and MoS2/Li7P3S11 composite are shown in Fig. 3a and b, respectively. The high magnification SEM images of both materials are also shown in Fig. S1. The MoS2 shows the layered crystal structure and the particles are 1–3 μm in diameters. After treating, it can be clearly seen that there is a coating on the surface of MoS2 particles. The TEM images further confirm that the coating grows on the surface of MoS2 particles for the MoS2/Li7P3S11 composite (Fig. 3c and d). In addition, the TEM image of MoS2/Li7P3S11 shows that the thickness of coating is around 30 nm (Fig. S2). The coating on the MoS2 particles imitates the contract between the electrode and electrolyte and decreases the interfacial resistance, enhancing the high rate and long cycle performance of ASS LIBs. Fig. 3e shows a highresolution TEM image of the MoS2/Li7P3S11 composite. The lattice firings with inter-planar spacing of 0.303 nm and 0.274 nm, which is corresponding to the (2-1-1) lattice plane of Li7P3S11 [17], and (100) lattice plane of 2H−MoS2 [30], respectively. The results demonstrate that the Li7P3S11 electrolyte layer successfully grows on the surface of
Fig. 2. An XRD pattern of the as-synthesized MoS2/Li7P3S11 composite.
Fig. 3. SEM images of (a) MoS2 and (b) MoS2/Li7P3S11 composite. TEM images of (c) MoS2 and (d) MoS2/Li7P3S11 composite. (e) A HRTEM image of MoS2/Li7P3S11 composite. (f) Elemental mapping images of sulfur, phosphorus and molybdenum of the composite.
109
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
The lithium storage capacity of the all-solid-state cells was investigated by galvanostatic charge-discharge test at C/10 rate (corresponding to a current density of 0.102 mA cm−1) in the voltage window of 0.1–3.0 V at room temperature. Fig. 4b shows the charge-discharge curves of the all-solid-state cells with MoS2/Li7P3S11 composite for the 1st, 2nd, 5th, 10th, 30th and 60th cycles. It is shown that two plateaus around 1.1 and 0.6 V are observed in the first discharge process, representing the formation of LixMoS2 and the conversion reaction involving the decomposition of LixMoS2 to Li2S to Mo, respectively. After the first discharge cycle, the MoS2/Li7P3S11 composite displays two short but discernible plateaus at 1.90 and 1.20 V, corresponding to the conversion from sulfur to polysulfides and then to Li2S. During the charge process, the working electrode shows an obvious potential plateau around 2.30 V. Voltage profiles of the MoS2/Li7P3S11 electrode are consistent with CV results mentioned above. The MoS2/ Li7P3S11 electrode shows an initial discharge capacity of 868.4 mAh g−1 and a reversible charge capacity of 669.2 mAh g−1 with a Coulombic efficiency of 77.1%. It is noted that the discharge capacity has exceeded the theoretical capacity (≈670 mAh g−1). The theoretical capacity of MoS2 is based on the conversion reaction (MoS2 + 4 Li+ + 4 e− → 2 Li2S + Mo). Here, the excess lithium storage is ascribed to the contribution of lithium ion intercalation/deintercalation at interfaces [50]. In addition, the formation of SEI layer and the capacity of activated carbon can also contribute to the high practical capacity [47,51]. From the second cycle onwards, the MoS2/Li7P3S11 composite manifests excellent cycling stability and still retains a reversible capacity of 547.1 mAh g−1 after 60 cycles, corresponding to approximately 75% of the discharge capacity of the second cycle (Fig. 4d). The good cycling stability is mainly attributed to the coating on the MoS2, which intimates the contact between the electrode and solid electrolyte and decreases the interface resistance. For comparison, the untreated MoS2 particles were assembled cells by the same method, except the coating of Li7P3S11. The charge-discharge curves of the allsolid-state cells with untreated MoS2 electrode is shown in Fig. 4c. Under the identical test conditions, the untreated MoS2 electrode exhibits an initial discharge capacity of around 740 mAh g−1 and shows a fast capacity fading during cycling. After 60 cycles, a capacity of only around 230 mAh g−1 is retained and the capacity retention is about 40%, much lower than that of the modified one (Fig. 4d). The poor contact between MoS2 and Li7P3S11 solid electrolyte leads to low utilization of active material, resulting the rapid capacity decay. Fig. 4e shows the long cycling performance of the untreated MoS2 and MoS2/ Li7P3S11 composite electrodes at the rate of 1.0 C. The MoS2/Li7P3S11 composite still maintains a capacity of 238.1 mAh g−1 at 400 cycles with Coulombic efficiency of almost 100%, and the capacity fading is probably due to the formation of interfacial products such as Li2S and Li3P at Li/electrolyte interface and the bulk MoS2 flakes used as electrode. In contrast, the untreated MoS2 shows a low discharge capacity and dramatically decreases in 260 cycles. It is considered that the composition of MoS2 with Li7P3S11 layer can effectively increase the contract area with solid electrolyte, provide more reaction sites, and benefit the migration of lithium, leading to an excellent cycling performance of all-solid-state batteries. Thus, the advantage of the coating structure of MoS2/Li7P3S11 composite for electrochemical performance is quite apparent. To evaluate the rate capability, the MoS2/Li7P3S11 composite electrode is cycled at different rates from 0.1 C to 1 C over a voltage window of 0.1–3.0 V at room temperature. Fig. 4f shows the rate capability of untreated MoS2 and MoS2/Li7P3S11 composite electrodes. The discharge capacities of the MoS2/Li7P3S11 electrode are 676.5, 565.4, 431.6 and 293.6 mAh g−1 at 0.1 C, 0.2 C, 0.5 C and 1.0 C, respectively. For comparison, the untreated MoS2 electrode is only able to discharge a capacity of 591.8, 454.7, 272.5 and 100.1 mAh g−1 at 0.1 C, 0.2 C, 0.5 C and 1.0 C, respectively. The results demonstrate the excellent rate performance of the MoS2/Li7P3S11 composite electrode, benefiting from the coating of Li7P3S11 electrolyte for the improvement of the ionic
MoS2 particles. To further study the component distribution of the MoS2/Li7P3S11 composite, the energy dispersive spectroscopy (EDS) elemental mapping of the sample is carried out in Fig. 3f, which certifies that the elements of sulfur, phosphorus and molybdenum uniformly distribute in the composite. In addition, the Li7P3S11 glass-ceramic electrolyte for ASS LIBs is also prepared by solution method plus annealing. The XRD of the assynthesized Li7P3S11 solid electrolyte is shown in Fig. S3. The diffraction peaks in the XRD pattern are identified to characteristic peaks of Li7P3S11, which means the Li7P3S11 glass-ceramic is successfully prepared. The Li7P3S11 glass ceramic has the space group of P-1 with P2S7 di-tetrahedral and PS4 tetrahedral units per unit cell [31–33]. Lithium ions surround the P2S7 di-tetrahedral and PS4 tetrahedral and transport through open space among the units. A SEM image shows the sizes of the Li7P3S11 glass-ceramic particles are approximately 200–800 nm (Fig. S4a). The small size of particles increases the contact area between the particles and reduces the grain boundary resistance, resulting in fast lithium ion transport. The surface morphology of the solid electrolyte after cold pressing is observed in Fig. S4b. A few micropores and cracks can be discovered on the surface, which ensures the high ionic conductivity of solid electrolyte. Before assembling ASS LIBs, the ionic conductivity of the Li7P3S11 solid electrolyte is tested using an In/ Li7P3S11 solid electrolyte/In blocking symmetric cell by EIS measurement at room temperature (Fig. S5). Through the Nyquist plot, the impedance of the solid electrolyte is about 64.9 Ω at 298 K and the calculated ionic conductivity is 2.0 mS cm−1, which has reached the requirement of ASS LIBs. To further evaluate the compatibility of solid electrolyte with lithium metal, a Li/Li7P3S11 solid electrolyte/Li nonblocking symmetric cell is fabricated. The DC polarization curve of the cell at a current density of 1.2 mA cm−2 at room temperature is shown in Fig. S6, which indicates that the compatibility with lithium metal of Li7P3S11 is very good although the duration time of stability test is not very long [34,35]. Wenzel et al. [36] found that the solid electrolyte interface (SEI) consisting of the decomposition products Li2S and Li3P grew slowly at the interface between lithium and Li7P3S11. And the SEI stabilized after a formation period and was limited to a few nanometers. The electrochemical stability between the solid electrolyte and lithium metal is a significant factor for high performance ASS LIBs. ASS cells with untreated MoS2 and MoS2/Li7P3S11 composite electrodes are assembled in an argon-filled glovebox to investigate the electrochemical performance. The schematic of an all-solid-state lithium cell is shown in Fig. S7. The Li7P3S11 glass-ceramic is used as the solid electrolyte to separate the working electrode and counter electrode. The untreated MoS2 and MoS2/Li7P3S11 composite mixed with Li7P3S11 and AB additive are used as working electrodes, respectively. In addition, the lithium metal is used as the counter electrode and two stainless steel disks are used as current collectors. Fig. 4a shows the CV curves of the first three cycles of the all-solid-state cell with MoS2/ Li7P3S11 composite electrode at a scanning rate of 0.1 mV s−1 between 0.1 and 3.0 V (vs. Li/Li+) at room temperature. Two peaks at around 0.82 V and 0.37 V in the first cathodic scan can be attributed to the intercalation of Li+ into 2HeMoS2 lattice accompanied with the formation of 1T−LixMoS2 (MoS2 + xLi++xe−→ LixMoS2) [37–40], and the conversion reaction of the lithium intercalates (LixMoS2) to form metallic Mo and Li2S (LiXMoS2 + (4-x) Li+ + (4-x) e− → Mo + 2Li2S) [41,42]. In the subsequent cathodic scans, the reduction peaks at 0.82 V and 0.37 V disappear while two new peaks around 1.85 V and 1.10 V occur. The new cathodic peaks imply the appearance of a multistep Li insertion and the formation of Li2S (nS8 + 16Li++ 16e− → 8Li2Sn and Li2Sn + (2n-2) Li++ (2n-2) e−→ nLi2S) [43–46]. And the peak at about 0.25 V is ascribed to the electrochemical process of the interfacial lithium storage and the formation of a SEI layer on the electrode surface [26,47]. In the anodic scan, a very weak anodic peak at about 1.70 V is assigned to the oxidation of Mo metal and an obvious peak at about 2.40 V is attributed to the oxidation reaction of Li2S into sulfur [41,48,49]. 110
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
Fig. 4. (a) CV curves of MoS2/Li7P3S11 composite electrode at a scanning rate of 0.1 mV s−1 between 0.1 and 3.0 V (Li/Li+). Charge-discharge curves of the all-solid-state cells for (b) MoS2/Li7P3S11 composite and (c) untreated MoS2 electrodes at 0.1 C at room temperature. (d) Cycle performance of untreated MoS2 and MoS2/Li7P3S11 composite electrodes at a rate of 0.1 C at room temperature. (e) Cycle performance of untreated MoS2 and MoS2/Li7P3S11 composite electrodes ata high rate of 1.0 C and the corresponding Coulombic efficiencies of MoS2/Li7P3S11 composite. (f) Rate capability of MoS2/Li7P3S11 composite electrode. (g) Nyquist plots of the all-solid-state cell with untreated MoS2 and MoS2/Li7P3S11 composite electrodes.
111
Journal of Power Sources 374 (2018) 107–112
R.C. Xu et al.
conductivity. In order to further investigate the superior electrochemical performance of MoS2/Li7P3S11 composite, the Nyquist plots of the all-solidstate cell with untreated MoS2 and MoS2/Li7P3S11 composite electrodes are compared in Fig. 4g. The Nyquist plots consist of a single semicircle in the high frequency and an inclined line at low frequency, which represent charge transfer (Rct)/interface resistance (Rif) and diffusion impedance (wo), respectively [12,52]. In addition, the intercept of xaxis in high frequency region represents the bulk resistance of electrolyte (Rbulk) [53]. Obviously, the MoS2/Li7P3S11 composite features a much smaller semicircle diameter than that of untreated MoS2, indicating a small interfacial resistance and a rapid charge-transfer reaction for Li+ insertion and extraction due the combination with the Li7P3S11 electrolyte layer. And the small interfacial resistance leads to an excellent cycle and rate performance of the MoS2/Li7P3S11 electrode. The interfacial architecture of Li7P3S11 electrolyte coating on MoS2 particles enlarges the contact area between the electrolyte and MoS2 particles and offers more active sites for Li+ insertion and extraction, leading to a high utilization of the active materials and excellent electrochemical performance. Moreover, the electrolyte layer can restrain volume charge and guarantee the intimate contract during cycling, resulting in a high cycling stability.
[11] F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, Adv. Mater. 17 (2005) 918–921. [12] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, Nat. Mater. 10 (2011) 682–686. [13] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nat. Energy 1 (2016) 16030. [14] A.C. Luntz, J. Voss, K. Reuter, J. Phys. Chem. Lett. 6 (2015) 4599–4604. [15] A. Kato, A. Hayashi, M. Tatsumisago, J. Power Sources 309 (2016) 27–32. [16] K. Aso, A. Sakuda, A. Hayashi, M. Tatsumisago, ACS Appl. Mater. Interfaces 5 (2013) 686–690. [17] X.Y. Yao, D. Liu, C.S. Wang, P. Long, G. Peng, Y.S. Hu, H. Li, L.Q. Chen, X.X. Xu, Nano Lett. 16 (2016) 7148–7154. [18] C.F. Zhang, Z.Y. Wang, Z.P. Guo, X.W. Lou, ACS Appl. Mater. Interfaces 4 (2012) 3765–3768. [19] C.B. Zhu, X.K. Mu, P.A. van Aken, Y. Yu, J. Maier, Angew. Chem. Int. Ed. 53 (2014) 2152–2156. [20] X.H. Cao, Y.M. Shi, W.H. Shi, X.H. Rui, Q.Y. Yan, J. Kong, H. Zhang, Small 9 (2013) 3433–3438. [21] D. Xie, W.J. Tang, Y.D. Wang, X.H. Xia, Y. Zhong, D. Zhou, D.H. Wang, X.L. Wang, J.P. Tu, Nano Res. 9 (2016) 1618–1629. [22] Y.L. Jia, H.Q. Wan, L. Chen, H.D. Zhou, J.M. Chen, J. Power Sources 354 (2017) 1–9. [23] D.H. Youn, C. Jo, J.Y. Kim, J. Lee, J.S. Lee, J. Power Sources 295 (2015) 228–234. [24] M. Yang, S. Ko, J.S. Im, B.G. Choi, J. Power Sources 288 (2015) 76–81. [25] K. Bindumadhavan, S.K. Srivastava, S. Mahanty, Chem. Commun. 49 (2013) 1823–1825. [26] J.B. Ding, Y. Zhou, Y.G. Li, S.J. Guo, X.Q. Huang, Chem. Mater. 28 (2016) 2074–2080. [27] H. Yamane, M. Shibata, Y. Shimane, T. Junke, Y. Seino, S. Adams, K. Minami, A. Hayashi, M. Tatsumisago, Solid State Ionics 178 (2007) 1163–1167. [28] M.R. Busche, D.A. Weber, Y. Schneider, C. Dietrich, S. Wenzel, T. Leichtweiss, D. Schröder, W. Zhang, H. Weigand, D. Walter, S.J. Sedlmaier, D. Houtarde, L.F. Nazar, J. Janek, Chem. Mater. 28 (2016) 6152–6165. [29] R.C. Xu, X.H. Xia, Z.J. Yao, X.L. Wang, C.D. Gu, J.P. Tu, Electrochim. Acta 219 (2016) 235–240. [30] L.F. Fei, S.J. Lei, W.B. Zhang, W. Lu, Z.Y. Lin, C.H. Lam, Y. Chai, Y. Wang, Nat. Commun. 7 (2016) 12206. [31] I.H. Chu, H. Nguyen, S. Hy, Y.C. Lin, Z. Wang, Z. Xu, Z. Deng, Y.S. Meng, S.P. Ong, ACS Appl. Mater. Interfaces 8 (2016) 7843–7853. [32] R.C. Xu, X.H. Xia, X.L. Wang, Y. Xia, J.P. Tu, J. Mater. Chem. A 5 (2017) 2829–2834. [33] K. Mori, K. Enjuji, S. Murata, K. Shibata, Y. Kawakita, M. Yonemura, Y. Onodera, T. Fukunaga, Phys. Rev. Appl. 4 (2015) 054008. [34] E. Rangasamy, Z.C. Liu, M. Gobet, K. Pilar, G. Sahu, W. Zhou, H. Wu, S. Greenbaum, C.D. Liang, J. Am. Chem. Soc. 137 (2015) 1384–1387. [35] Z.C. Liu, W.J. Fu, E.A. Payzant, X. Yu, Z.L. Wu, N.J. Dudney, J. Kiggans, K.L. Hong, A.J. Rondinone, C.D. Liang, J. Am. Chem. Soc. 135 (2013) 975–978. [36] S. Wenzel, D.A. Weber, T. Leichtweiss, M.R. Busche, J. Sann, J. Janek, Solid State Ionics 286 (2016) 24–33. [37] H.L. Li, K. Yu, H. Fu, B.J. Guo, X. Lei, Z.Q. Zhu, J. Phys. Chem. C 119 (2015) 7959–7968. [38] J. Shao, Q.T. Qu, Z.M. Wan, T. Gao, Z.C. Zuo, H.H. Zheng, ACS Appl. Mater. Interfaces 7 (2015) 22927–22934. [39] J.Y. Wan, W.Z. Bao, Y. Liu, J.Q. Dai, F. Shen, L.H. Zhou, X.H. Cai, D. Urban, Y.Y. Li, K. Jungjohann, M.S. Fuhrer, L. Hu, Adv. Energy Mater. 5 (2015) 1401742. [40] D. Xie, D.H. Wang, W.J. Tang, X.H. Xia, Y.J. Zhang, X.L. Wang, C.D. Gu, J.P. Tu, J. Power Sources 307 (2016) 510–518. [41] C.Y. Zhao, X. Wang, J.H. Kong, J.M. Ang, P.S. Lee, Z.L. Liu, X.H. Lu, ACS Appl. Mater. Interfaces 8 (2016) 2372–2379. [42] C.Y. Zhao, J.H. Kong, X.Y. Yao, X.S. Tang, Y.L. Dong, S.L. Phua, X.H. Lu, ACS Appl. Mater. Interfaces 6 (2014) 6392–6398. [43] Z.M. Wan, J. Shao, J.J. Yun, H.Y. Zheng, T. Gao, M. Shen, Q.T. Qu, H.H. Zheng, Small 10 (2014) 4975–4981. [44] D. Xie, X.H. Xia, Y. Zhong, Y.D. Wang, D.H. Wang, X.L. Wang, J.P. Tu, Adv. Energy Mater. 7 (2017) 1601804. [45] L.C. Yang, S.N. Wang, J.J. Mao, J.W. Deng, Q.S. Gao, Y. Tang, O.G. Schmidt, Adv. Mater. 25 (2013) 1180–1184. [46] D. Xie, X.H. Xia, W.J. Tang, Y. Zhong, Y.D. Wang, D.H. Wang, X.L. Wang, J.P. Tu, J. Mater. Chem. A 5 (2017) 7578–7585. [47] M.H. Chen, X.S. Yin, M.V. Reddy, S. Adams, J. Mater. Chem. A 3 (2015) 10698–10702. [48] K. Chang, W.X. Chen, J. Mater. Chem. 21 (2011) 17175. [49] D. Xie, W.J. Tang, X.H. Xia, D.H. Wang, D. Zhou, F. Shi, X.L. Wang, C.D. Gu, J.P. Tu, J. Power Sources 296 (2015) 392–399. [50] T. Stephenson, Z. Li, B. Olsen, D. Mitlin, Energy Environ. Sci. 7 (2014) 209–231. [51] D. Xie, X.H. Xia, Y.D. Wang, D.H. Wang, Y. Zhong, W.J. Tang, X.L. Wang, J.P. Tu, Chem. Eur. J. 22 (2016) 11617–11623. [52] B.R. Shin, Y.J. Nam, D.Y. Oh, D.H. Kim, J.W. Kim, Y.S. Jung, Electrochim. Acta 146 (2014) 395–402. [53] T. Ohtomo, A. Hayashi, M. Tatsumisago, Y. Tsuchida, S. Hama, K. Kawamoto, J. Power Sources 233 (2013) 231–235.
4. Conclusions In summary, a MoS2/Li7P3S11 composite electrode is successfully prepared by a solution method for all-solid-state batteries. Intimate combination between MoS2 and Li7P3S11 with high ionic conductivity, the Li7P3S11 coating dramatically decreases the interfacial resistance between the electrode and electrolyte, which enables a marked improvement on the electrochemical properties of the ASS electrode. The ASS batteries based on the MoS2/Li7P3S11 composite electrode exhibits a high initial discharge capacity of 868.4 mAh g−1 at 0.1 C with a high Coulombic efficiency and a high reversible capacity of 547.1 mAh g−1 is retained after 60 cycles. Moreover, the obtained all-solid-state battery possesses excellent rate performance and cycling stability. Our developed design strategy for ASS electrode provides a new way for synthesis of high-performance all-solid-state batteries. Acknowledgement This work is supported by National Natural Science Foundation of China (Grant. Nos. 51571180, 51502263), Qianjiang Talents Plan D (Grant. No. QJD1602029), Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), Startup Foundation for Hundred-Talent Program of Zhejiang University. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2017.10.093. References [1] J.W. Wen, Y. Yu, C.H. Chen, Mater. Express 2 (2012) 197–212. [2] Z.J. Yao, X.H. Xia, Y. Zhong, Y.D. Wang, B.W. Zhang, D. Xie, X.L. Wang, J.P. Tu, Y.Z. Huang, J. Mater. Chem. A 5 (2017) 8916–8921. [3] J.W. Fergus, J. Power Sources 195 (2010) 4554–4569. [4] R.J. Chen, W.J. Qu, X. Guo, L. Li, F. Wu, Mater. Horiz. 3 (2016) 487–516. [5] Y.S. Jung, D.Y. Oh, Y.J. Nam, K.H. Park, Isr. J. Chem. 55 (2015) 472–485. [6] K. Takada, Acta Mater. 61 (2013) 759–770. [7] Z. Lin, C.D. Liang, J. Mater. Chem. A 3 (2015) 936–958. [8] A. Manthiram, X.W. Yu, S.F. Wang, Nat. Rev. Mater. 2 (2017) 16103. [9] R.C. Xu, X.H. Xia, S.H. Li, S.Z. Zhang, X.L. Wang, J.P. Tu, J. Mater. Chem. A 5 (2017) 6310–6317. [10] A. Hayashi, M. Tatsumisago, Electron. Mater. Lett. 8 (2012) 199–207.
112