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Enhancing ionic conductivity of solid electrolyte by lithium substitution in halogenated Li-Argyrodite
Journal of Power Sources 450 (2020) 227601
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Enhancing ionic conductivity of solid electrolyte by lithium substitution in halogenated Li-Argyrodite Zhuoran Zhang a, b, 1, Jianxing Zhang a, 1, Huanhuan Jia a, b, Linfeng Peng a, c, Tao An a, b, Jia Xie a, * a
State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China c School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Al3þ partially substitutes Liþ at the 24 g and 48 h sites of Li6PS5Br. � Li5.4Al0.2PS5Br shows a room tempera ture ionic conductivity of 2.4 � 10 3 S cm 1. � The shortened distance of inter-cage jump leads to improved ionic conductivity. � All-solid-state cell TiS2/Li5.4Al0.2PS5Br/ Li exhibits promising performance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid electrolyte Argyrodite Lithium substitution Ionic conductivity
The demand for batteries with high safety and high energy density produces considerable efforts toward the development of all-solid-batteries and solid state electrolytes. Halogen-substituted lithium argyrodites, Li6PS5X (X ¼ Cl, Br), represent a class of promising and suitable sulfide-based solid electrolytes with high conductivity and good processability. To further extend the scope and better understand the structure-property relationship, we systematically explore the effect of partial substitution of Liþ by other cations in Li6PS5X (X ¼ Cl, Br). The Xray diffraction (XRD) analysis shows the possibility of successful substitution at Liþ position by various cations. Among them, substitution with a small amount of Al3þ in Li6PS5Br is confirmed by Rietveld refinements. The ionic conductivity is improved by almost three times to 2.4 � 10 3 S cm 1. This improvement is attributed to the shortened distances of inter-cage jumps. In addition, the substituted sample shows better electrochemical per formance in all-solid-state batteries. This work demonstrates the feasibility of performance improvement by manipulating the position of Liþ in the structure of lithium argyrodites, which provides a versatile approach toward designing better performing solid electrolytes.
* Corresponding author. E-mail address: [email protected] (J. Xie). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227601 Received 9 October 2019; Received in revised form 9 December 2019; Accepted 10 December 2019 Available online 19 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 450 (2020) 227601
1. Introduction
Photoelectric Technology Co., Ltd, 99.99%), germanium sulfide (GeS2, Sigma Aldrich., Ltd, 99.8%), sulfur (S, 99.99%, Sigma Aldrich), boron (B, 99.9%, Sigma Aldrich), lithium chloride (LiCl, 99.9%, Sigma Aldrich), lithium bromide (LiBr, 99.8%, Sigma Aldrich). The precursor materials in stoichiometric ratios were hand-ground in an argon-filled glove box with an agate mortar and then transferred to a ZrO2 pot containing ZrO2 balls (18, diameter 10 mm) for high energy ball milling under at 360 rpm for 18 h. After the grinding, the samples were pelletized at 300 MPa and transferred to quartz tubes. The specimens were sealed in quartz tubes under vacuum (~10 Pa) with a Partulab device (MRVS-1002). Sealed quartz tubes with samples were then annealed at a temperature of 823 K for 10 h and slowly cooled down to room temperature. Sintered samples were manually ground into pow ders for subsequent testing. The stoichiometric amounts of starting precursors were weighed according to the molar ratio of Li6–3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3).
Lithium-ion batteries have already occupied the consumer elec tronics market, become the leading energy-storage technology for new energy vehicles [1–3], and started to participate in grid-scale energy storage [4–6]. Currently, the risk of thermal runaway has become a key issue that hinders the further application and development of current Li-ion batteries [7]. Replacing flammable organic electrolytes with solid-state electrolytes with fast ionic conductions and negligible elec tronic transport can eliminate security risks [8–10]. Moreover, the performance of solid-state electrolytes considerably affects the cycling stability, rate performance, and service life of all-solid-state batteries [11]. Compared to traditional organic electrolytes, solid electrolytes can prevent the formation of lithium dendrites and withstand high voltages above 5 V [12–14]. Thus, various solid electrolytes with high ionic conductivities have been developed, including polymers, oxides, and sulfides [15–20]. Polymer solid electrolytes refer to a salt-in-polymer complex [19]. Oxide solid electrolytes can be classified as an amorphous state that is mainly composed of LiPON (Li3.3PO3.9N0.17) and a crystalline state that is mainly composed of perovskite-type, NASICON-type, and garnet mate rials [18,21–24]. Sulfide solid electrolytes are mainly classified into glass, glass ceramic, and crystalline; these electrolytes exhibit some of the best ionic conductivities [15,25]. Sulfide solid electrolytes are me chanically soft and can be easily processed. But they usually have nar row electrochemical window and the ones with thiophosphate groups are sensitive to moisture. Currently, the research focus is mainly to further improve the conductivities, the interfacial stability as well as the stability against moisture. The main system of glass and glass ceramics is Li2S–P2S5, and the crystalline state is mainly divided into LGPS (Li10GeP2S12), thio-LISICON (thio-lithium superionic conductor), and argyrodite (Li6PS5X) [26–28]. Among them, Li-argyrodites have received considerable attention owing to high ionic conductivity and potentially low cost. Meanwhile, the structure of argyrodites is amenable to a wide range of elemental substitutions. Several reports have been published about anion/cation substitutions in Li-argyrodite-type structures [28–33]. Recent studies have developed another approach in which the substitution of Li with Ge and Fe in Li7PS6 can form the high-temperature phase (Li3.5Ge(Ge0.5P0.5)S6 [Li7Ge3PS12], Li6Fe0.5PS6) [34,35]. Li6.15Al0.15Si1.35S6 has been also tested by researchers, in which Al or Si partially replace Li [36]. The commonality between these three structures is that the high-temperature cubic phase of argyrodite is formed without halogen. At the same time, their ionic conductivities can reach 2.4 � 10 4 S cm 1. Inspired by previous studies, we explore the effect of partial substi tution of Liþ by various cations in halogenated argyrodite Li6PS5X (X ¼ Cl, Br). It is determined that various cations may partially substitute Liþ in halogenated argyrodites. Among them, a rigorous investigation into partial substitution of Liþ by Al3þ in Li6PS5Br has been carried out, which shows the formation of solid solutions and improvement of ionic conductivity by increasing vacancies and shortening the inter-cage jump distance. Furthermore, an all-solid battery with Li5.4Al0.2PS5Br as the electrolyte exhibits good compatibility with metallic lithium. The cor relations between the components, structures, and ion transports found in this work can provide insight into the design strategies for new solid electrolytes in the argyrodite family.
2.2. X-ray powder diffraction and rietveld analysis X-ray diffraction measurements were carried out at room tempera ture with a SmartLab-SE diffractometer (Rigaku; Cu Kα1 radiation, λ ¼ 1.54059 Å). Measurements were carried out in the 2θ range of 10� and 60� with a step width of 0.015� . Samples were placed in custom glass slides and sealed with polymer films to prevent exposure to moisture. The XRD refinement of samples used the same equipment as before, except that the scan range and step width were changed to 10� to 120� and 0.01� . The refined sample was filtered through a 400 mesh screen to ensure a small and uniform particle radius. The raw data were processed with the CMPR software and Rietveld refinement was carried out using the GSAS software [37]. Fit indicators of Rwp, Rexp and χ2 were used to evaluate the accuracy of the refined result. The following parameters were initially determined through refinement: (1) lattice parameters, (2) crystal structure, (3) structure parameters, and (4) bond distance. 2.3. Electrochemical characterization Characterizations of electrochemical performance are AC impedance spectroscopy, CV curve and charge and discharge performance. The ionic conductivities of the solid electrolyte Li6–3xAlxPS5Br (0 < x < 0.4), Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) were measured using AC impedance spectroscopy with a Solar tron 1260 frequency response analyzer. The pellets pressed by powders with a thickness of 1–2 mm packed into argon filled pouch cells. An amplitude of 20 mV was applied to electrolytes at frequencies ranging from 10 Hz to 10 MHz. The activation energies of Li6–3xAlxPS5Br (0 < x < 0.4) were conducted in the temperature range of 301 K–353 K. The electrochemical window of Li5.4Al0.2PS5Br was evaluated by cyclic voltammetry (CV): a Li/Li5.4Al0.2PS5Br/Au cell was assembled, Li was the counter and reference electrode and Au served as the working electrode. The CV test used a Solartron 1470E CellTest system with a scan rate of 1 mV s 1 between 0.5 V and 5 V. The compatibility of the solid electrolyte with metallic lithium was further proved by symmetric cell cycling data. Two pieces of Li foil were attached to the Li5.4Al0.2PS5Br pellet for a symmetric cell test. Galvanostatic cycling of the Li/Li5.4Al0.2PS5Br/Li cell was tested using a battery test system (Land) at 28 � C. The current density is 0.53 mA cm 1. The TiS2/ Li5.4Al0.2PS5Br/Li all-solid-state battery was assembled and tested for cycling using a battery tester (Land CT2001A). The galvanostatic cycling was performed at a C-rate of 0.1C in room temperature. The anode of complete cell was lithium metal, and the cathode consisted of TiS2 and Li5.4Al0.2PS5Br a mass ratio of 7:3. The mixed cathode was prepared in an agate mortar.
2. Experiment section 2.1. Synthesis of argyrodites Li6–3xAlxPS5Br, Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) were synthesized by a classic solidstate synthesis. The precursor materials are lithium sulfide (Li2S, 99.98%, Sigma Aldrich), phosphorus pentasulfide (P2S5, 99%, Mucklin), aluminum sulfide (Al2S3, 99%, Alfa Aesar), silicon sulfide (SiS2, KaiYada 2
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Journal of Power Sources 450 (2020) 227601
Fig. 1. (a) Schematic diagram of varies element substitution in argyrodite Li7PS6. (b) schematic of cations substituted Li on halogen-containing argyrodites. (The orange solid line is the attempt of this paper.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. (a) X-ray diffraction patterns of Li6PS5Br, Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br). (b) X-ray diffraction patterns of Li6–3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2, 0.25, 0.3) and Li6PS5Br.
3. Result and discussion
(M ¼ Al, B; X ¼ Cl, Br), and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) are shown in Fig. 2a. Li6PS5Cl-substituted samples show more impurities. Though both Ge and Si can be successfully substituted at the P position in Li7PS6 to form a solid solution [31], the impurity phases of Li5.2Si0.2PS5Cl, Li5.2Si0.2PS5Br, and Li5.2Ge0.2PS5Br indicate that Ge and Si cannot easily substitute Li in Li6PS5X (X ¼ Cl, Br). Thus, we conducted an in-depth study of Li6 3xAlxPS5Br owing to the higher purity and high ionic conductivity of Li5.4Al0.2PS5Br. The X-ray diffraction patterns of Li6 3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2, 0.25, and 0.3) and high temperature phase Li6PS5Br are shown in Fig. 2b. It is observed that Li6 3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2, 0.25, and 0.3) are consistent with the XRD pattern of Li6PS5Br, which indicates that the novel solid solutions exhibit a cubic argyrodite-type structure. As the content of Al increases (x > 0.2 in Li6 3xAlxPS5Br), the crystal linity of samples deteriorates, and the impurity phase of Al2S3 can be observed. These phenomena indicate that the upper limit of the solid solubility of Al in Li6PS5Br is probably reached. The XRD patterns of Li6 3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2) are almost free of impurities, espe cially the Li5.4Al0.2PS5Br sample. It is suggested that Al3þ may substitute Liþ in the crystal lattice to form a solid solution. Furthermore, it is also
3.1. XRD characterization Previous examples of various elemental substitutions in argyrodite Li7PS6 are summarized in Fig. 1a. It is observed that the four different sites in Li7PS6 can be substituted by different cations, but the specific replacement scheme is still determined according to the specific situa tion [26,28,32]. To explore the effect of Liþ substitution in halogen-containing argyrodites, we evaluated the substitution of Li6PS5X (X ¼ Cl, Br) by several cations according to the ionic radius (Fig. 1a). Li6PS5I does not have high conductivity. It is generally believed that the substitution of the P site will increase the volume of the (P/M)S4 tetrahedron to obtain high conductivity. Thus, no attempt is made to replace Liþ in Li6PS5I. Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) were synthesized, as shown in Fig. 1b, in which orange lines represent the attempts of this study. The principle of the experimental design is to pair halogen ions with larger radii with cations with larger radii. Representative X-ray diffraction patterns of Li6PS5Br, Li5.4M0.2PS5X 3
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Journal of Power Sources 450 (2020) 227601
argyrodite in the F 43m space group via the Rietveld refinement of the X-ray diffraction pattern (Fig. 3). The specific parameters are shown in Table 1. Therein, the Al3þ portion occupies the 24 g Wyckoff position of Liþ at 48 h and 24 g sites. Owing to the larger radius of Liþ (68 pm) than that of Al3þ (53 pm), the lattice parameter of the crystal is reduced to 9.978 Å, which is smaller than that of Li6PS5Br (10.181 Å). The reduced lattice parameter indicated the successful incorporation of Al3þ in the Liþ site. In addition, the refinement results indicate that Al3þ substitu tion does not considerably change the site disorder between Br and S2–. It is generally believed that Li ions migrate in three possible jump processes, i.e., inter-cage jump, intra-cage jump, and doublet jump. The material can have high ionic conductivity only when these three jump processes are active at the same time. Owing to the influence of sub stitution, the three path lengths of lithium-ion jump have changed. The paths of lithium-ion jump in a crystal were compared in Li6PS5Br and Li5.4Al0.2PS5Br, as shown in Fig. 4. The longer doublet jump distance 48 h 24 g 48 h is attributed to larger Columbic repulsion provided by Al3þ. With an increase in the distance of the double jump, the distance of intra-cage and inter-cage jumps is shortened. In general, the ratedetermining step of the ion migration is inter-cage jump [27], and its shortening may be the reason for the improvement of the ionic conductivity.
Fig. 3. Rietveld refinement pattern of Li5.4Al0.2PS5Br at 298 K. The black points are experimentally measured data; the red part is the calculated intensities; the blue line is difference curve. The structure shown in the inset is crystal structure of the synthesized Li5.4Al0.2PS5Br. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Crystallographic data for Li5.4Al0.2PS5Br from X-ray diffraction. Refined pa rameters are shown with uncertainty in brackets.
3.3. Electrochemical properties
space group F43m a ¼ 9.97179 (1) � A
Table S1 shows the room temperature ionic conductivities of powder samples Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br). The ionic conductivities of Li5.2Si0.2PS5Br and Li5.4B0.2PS5Br increase to approximately 1.50 mS cm 1 compared to 0.9 mS cm 1 for pristine Li6PS5Br. Fig. 5aFig. S1 and S2 show the ionic conductivities of cold-pressed pellets of Li6 3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3) as well as the extracted activation energies obtained via the impedance method. The specific values are shown in Tables S2 and S3. Temperature-dependent impedance spectroscopy was performed on Li6 3xAlxPS5Br. The corresponding impedance responses are shown in Fig. 5b. The Arrhenius data also provides information on the activation barrier of migration. As shown in Fig. 5a, with an increased fraction of Al3þ, the conductivity reaches the maximum value of 2.4 � 10 3 S cm 1 for Li5.4Al0.2PS5Br. The ionic conductivities of the samples show the tendency of the W-shaped trend in the interval. The x ¼ 0.15 and x ¼ 0.25 samples sintered at different temperatures are shown in Fig. S3 and Table S4. When sintered at 450 � C, the crystallinity of Li5.25Al0.25PS5Br became significantly better, and the ionic conductivity of Li5.25Al0.25PS5Br also reached 1.41 � 10 3 S cm 1. The trend of W-type indicates that the preparation conditions of some samples are not optimal, which requires extensive study in the future. Activation en ergies show the same tendency, and the specific values of the activation energy are also added to Table S3. In contrast, the ionic conductivity and
Fit residuals (Rwp, Rexp, χ 2): 5.73%, 4.37%, 1.985 Atom
Site
x
y
z
Occ.
Li1 Li2 Al1 Al2 S1 S2 S3 Br1
48 h 24 g 48 h 24 g 16e 4c 4a 4c
0.324 (2) 0.2500 x (Li1) x (Li2) 0.1188 (2) 0.750 0 x (S2)
0.023 (3) 0.017 (8) y (Li1) y (Li2) 0.8812 (2) x (S2) x (S3) x (S2)
0.676 (2) 0.7500 z (Li1) z (Li2) 0.6188 (2) x (S2) x (S3) x (S2)
Br2
4a
x (S1)
x (S1)
x (S1)
P
4b
0.5000
x (P)
x (P)
0.39 (3) 0.12 (1) 0.01 0.014 1 0.80 (2) 0.20 (1) 1-Occ (S2) 1-Occ (S3) 1
2
Beq/� A
9 (4) 0 (3) B (Li1) B (Li2) 3.36 (10) 1.44 (5) 1.0 (5) 2.45 (13) 2.0 (8) 1.0 (4)
possible that the addition of halogen Br (196 pm), which is larger than S2–(184 pm), increases the solid solution range of Al3þ to Liþ substitution. 3.2. Structural analysis The crystal structure of Li5.4Al0.2PS5Br is indexed to cubic lithium
Fig. 4. Jumps distance between the lithium positions (48 h 24 g 48 h, doublet jump), intra-cage jumps (48 h 48 h), and inter-cage jumps in Li5.4Al0.2PS5Br. 4
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Journal of Power Sources 450 (2020) 227601
Fig. 5. (a) Ionic conductivities at room temperature and the activation energy for the series samples of Li6–3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3). (b) Arrhenius plot of the conductivities of Li6–3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3) obtained from the measurements on cold pressed pellets.
Fig. 6. (a) Cyclic voltammetry curve of Au/Li5.4Al0.2PS5Br/Li cell with a scan rate of 1 mV s 1 in the voltage window of 0.5 to 5 V. (b) Li/Li5.4Al0.2PS5Br/Li symmetric cell showing lithium cyclability at a current density of 0.53 mA cm 2, where Li and Au serve as the reference/counter and working electrode, respectively. The step time is 1 h. (c) Representative charge and discharge curves of TiS2/Li5.4Al0.2PS5Br/Li all-solid-state battery at a rate of 0.1C in ambient conditions. (d) Nyquist plots and equivalent circuit for TiS2/Li5.4Al0.2PS5Br/Li all-solid-state cell before and after cycling.
activation energy of Li6PS5Br are 0.9 � 10 3 S cm 1 and 0.20 eV [27]. Fig. S1 shows the Nyquist plots of cold-pressed Li5.4Al0.2PS5Br powders measured between 301 K and 343 K. The change in conductivity and activation energy may be caused by the increased Li vacancies and the shortened inter-cage jump, as previously discussed. Fig. 6a shows the cyclic voltammetry curve of the Au/ Li5.4Al0.2PS5Br/Li cell. Au is the working electrode, and Li is the counter and pseudo reference electrode. The cathodic current at approximately 0 V refers to lithium striping. The cyclic voltammetry (CV) performed in the Au/Li5.4Al0.2PS5Br/Li cell indicated that there were no serious side reactions. However, the CV curve of the Au/Li5.4Al0.2PS5Br/Li cell
operating at 2–4.5 V shown in Fig. S4 also indicates that the stability between the electrolyte and metallic Li is only relative. The compati bility of Li5.4Al0.2PS5Br as a solid electrolyte is further confirmed by the cycling data of the symmetric cell. Fig. 6b shows that the symmetric Li/ Li5.4Al0.2PS5Br/Li cell was fabricated and cycled at the current density of 0.53 mA cm 2 to investigate the compatibility of Li5.4Al0.2PS5Br with metallic Li. The cell voltage stays stable at approximately 0.04 V, which indicates the presence of minimal interfacial reactions between Li5.4Al0.2PS5Br and metallic Li. An all-solid-state battery was assembled with TiS2 as the cathode, metallic lithium as the anode, and Li5.4Al0.2PS5Br as the solid electrolyte. With a charging rate of 0.1C at 5
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room temperature, the initial discharging capacity was 235 mAh g 1, which stabilized at 210 mAh g 1 in subsequent cycles (89% of the initial discharge capacity) (Fig. 6c). Fig. S5 also shows that the rate perfor mance of the all-solid-state battery is limited, which suggests that the battery process needs to be improved. The side reaction distorts the charging curve and increases capacity. In addition, the interface of allsolid-state battery needs to be further optimized. Nyquist plots and the equivalent circuit for TiS2/Li5.4Al0.2PS5Br/Li all-solid-state cell before and after cycling are shown in Fig. 6dand Table S5, which shows the deterioration of the interfacial contact in the cell and to some extent explains the attenuation of capacity.
[12] [13] [14] [15]
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4. Conclusions
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In summary, we used various cations to partially substitute Liþ in halogenated argyrodites. Among them, Li5.4Al0.2PS5Br exhibited the highest ionic conductivity of 2.4 � 10 3 S cm 1 which was three times higher than that of Li6PS5Br. Compared to other studies that replaced the Li site with other cations, Al was first used to substitute halogenated Liargyrodite and enhance conductivity. The conductivity was relatively high in this study on Li-argyrodites (Table S6). The Rietveld refinement results suggest that the change in vacancy and the shortening of intercage jump may be the main cause for the improvement of ionic con ductivity. CV and galvanostatic cycling tests indicate that the material is relatively stable to metallic lithium, and the all-solid-state battery shows better cycling performance. This work presents a new strategy for developing well-performing argyrodite-type solid electrolytes.
[18] [19] [20]
[21] [22]
Declaration of competing interest
[23]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement. This work was supported by the National Basic Research Program of China (973 Program, 2015CB258400) and the Certificate of China Postdoctoral Science Foundation Grant (2017M622422). The authors gratefully acknowledge the Analytical and Testing Center of HUST for allowing us to use its facilities.
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227601.
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Enhancing ionic conductivity of solid electrolyte by lithium substitution in halogenated Li-Argyrodite Zhuoran Zhang a, b, 1, Jianxing Zhang a, 1, Huanhuan Jia a, b, Linfeng Peng a, c, Tao An a, b, Jia Xie a, * a
State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China c School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Al3þ partially substitutes Liþ at the 24 g and 48 h sites of Li6PS5Br. � Li5.4Al0.2PS5Br shows a room tempera ture ionic conductivity of 2.4 � 10 3 S cm 1. � The shortened distance of inter-cage jump leads to improved ionic conductivity. � All-solid-state cell TiS2/Li5.4Al0.2PS5Br/ Li exhibits promising performance.
A R T I C L E I N F O
A B S T R A C T
Keywords: Solid electrolyte Argyrodite Lithium substitution Ionic conductivity
The demand for batteries with high safety and high energy density produces considerable efforts toward the development of all-solid-batteries and solid state electrolytes. Halogen-substituted lithium argyrodites, Li6PS5X (X ¼ Cl, Br), represent a class of promising and suitable sulfide-based solid electrolytes with high conductivity and good processability. To further extend the scope and better understand the structure-property relationship, we systematically explore the effect of partial substitution of Liþ by other cations in Li6PS5X (X ¼ Cl, Br). The Xray diffraction (XRD) analysis shows the possibility of successful substitution at Liþ position by various cations. Among them, substitution with a small amount of Al3þ in Li6PS5Br is confirmed by Rietveld refinements. The ionic conductivity is improved by almost three times to 2.4 � 10 3 S cm 1. This improvement is attributed to the shortened distances of inter-cage jumps. In addition, the substituted sample shows better electrochemical per formance in all-solid-state batteries. This work demonstrates the feasibility of performance improvement by manipulating the position of Liþ in the structure of lithium argyrodites, which provides a versatile approach toward designing better performing solid electrolytes.
* Corresponding author. E-mail address: [email protected] (J. Xie). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227601 Received 9 October 2019; Received in revised form 9 December 2019; Accepted 10 December 2019 Available online 19 December 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
Photoelectric Technology Co., Ltd, 99.99%), germanium sulfide (GeS2, Sigma Aldrich., Ltd, 99.8%), sulfur (S, 99.99%, Sigma Aldrich), boron (B, 99.9%, Sigma Aldrich), lithium chloride (LiCl, 99.9%, Sigma Aldrich), lithium bromide (LiBr, 99.8%, Sigma Aldrich). The precursor materials in stoichiometric ratios were hand-ground in an argon-filled glove box with an agate mortar and then transferred to a ZrO2 pot containing ZrO2 balls (18, diameter 10 mm) for high energy ball milling under at 360 rpm for 18 h. After the grinding, the samples were pelletized at 300 MPa and transferred to quartz tubes. The specimens were sealed in quartz tubes under vacuum (~10 Pa) with a Partulab device (MRVS-1002). Sealed quartz tubes with samples were then annealed at a temperature of 823 K for 10 h and slowly cooled down to room temperature. Sintered samples were manually ground into pow ders for subsequent testing. The stoichiometric amounts of starting precursors were weighed according to the molar ratio of Li6–3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3).
Lithium-ion batteries have already occupied the consumer elec tronics market, become the leading energy-storage technology for new energy vehicles [1–3], and started to participate in grid-scale energy storage [4–6]. Currently, the risk of thermal runaway has become a key issue that hinders the further application and development of current Li-ion batteries [7]. Replacing flammable organic electrolytes with solid-state electrolytes with fast ionic conductions and negligible elec tronic transport can eliminate security risks [8–10]. Moreover, the performance of solid-state electrolytes considerably affects the cycling stability, rate performance, and service life of all-solid-state batteries [11]. Compared to traditional organic electrolytes, solid electrolytes can prevent the formation of lithium dendrites and withstand high voltages above 5 V [12–14]. Thus, various solid electrolytes with high ionic conductivities have been developed, including polymers, oxides, and sulfides [15–20]. Polymer solid electrolytes refer to a salt-in-polymer complex [19]. Oxide solid electrolytes can be classified as an amorphous state that is mainly composed of LiPON (Li3.3PO3.9N0.17) and a crystalline state that is mainly composed of perovskite-type, NASICON-type, and garnet mate rials [18,21–24]. Sulfide solid electrolytes are mainly classified into glass, glass ceramic, and crystalline; these electrolytes exhibit some of the best ionic conductivities [15,25]. Sulfide solid electrolytes are me chanically soft and can be easily processed. But they usually have nar row electrochemical window and the ones with thiophosphate groups are sensitive to moisture. Currently, the research focus is mainly to further improve the conductivities, the interfacial stability as well as the stability against moisture. The main system of glass and glass ceramics is Li2S–P2S5, and the crystalline state is mainly divided into LGPS (Li10GeP2S12), thio-LISICON (thio-lithium superionic conductor), and argyrodite (Li6PS5X) [26–28]. Among them, Li-argyrodites have received considerable attention owing to high ionic conductivity and potentially low cost. Meanwhile, the structure of argyrodites is amenable to a wide range of elemental substitutions. Several reports have been published about anion/cation substitutions in Li-argyrodite-type structures [28–33]. Recent studies have developed another approach in which the substitution of Li with Ge and Fe in Li7PS6 can form the high-temperature phase (Li3.5Ge(Ge0.5P0.5)S6 [Li7Ge3PS12], Li6Fe0.5PS6) [34,35]. Li6.15Al0.15Si1.35S6 has been also tested by researchers, in which Al or Si partially replace Li [36]. The commonality between these three structures is that the high-temperature cubic phase of argyrodite is formed without halogen. At the same time, their ionic conductivities can reach 2.4 � 10 4 S cm 1. Inspired by previous studies, we explore the effect of partial substi tution of Liþ by various cations in halogenated argyrodite Li6PS5X (X ¼ Cl, Br). It is determined that various cations may partially substitute Liþ in halogenated argyrodites. Among them, a rigorous investigation into partial substitution of Liþ by Al3þ in Li6PS5Br has been carried out, which shows the formation of solid solutions and improvement of ionic conductivity by increasing vacancies and shortening the inter-cage jump distance. Furthermore, an all-solid battery with Li5.4Al0.2PS5Br as the electrolyte exhibits good compatibility with metallic lithium. The cor relations between the components, structures, and ion transports found in this work can provide insight into the design strategies for new solid electrolytes in the argyrodite family.
2.2. X-ray powder diffraction and rietveld analysis X-ray diffraction measurements were carried out at room tempera ture with a SmartLab-SE diffractometer (Rigaku; Cu Kα1 radiation, λ ¼ 1.54059 Å). Measurements were carried out in the 2θ range of 10� and 60� with a step width of 0.015� . Samples were placed in custom glass slides and sealed with polymer films to prevent exposure to moisture. The XRD refinement of samples used the same equipment as before, except that the scan range and step width were changed to 10� to 120� and 0.01� . The refined sample was filtered through a 400 mesh screen to ensure a small and uniform particle radius. The raw data were processed with the CMPR software and Rietveld refinement was carried out using the GSAS software [37]. Fit indicators of Rwp, Rexp and χ2 were used to evaluate the accuracy of the refined result. The following parameters were initially determined through refinement: (1) lattice parameters, (2) crystal structure, (3) structure parameters, and (4) bond distance. 2.3. Electrochemical characterization Characterizations of electrochemical performance are AC impedance spectroscopy, CV curve and charge and discharge performance. The ionic conductivities of the solid electrolyte Li6–3xAlxPS5Br (0 < x < 0.4), Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) were measured using AC impedance spectroscopy with a Solar tron 1260 frequency response analyzer. The pellets pressed by powders with a thickness of 1–2 mm packed into argon filled pouch cells. An amplitude of 20 mV was applied to electrolytes at frequencies ranging from 10 Hz to 10 MHz. The activation energies of Li6–3xAlxPS5Br (0 < x < 0.4) were conducted in the temperature range of 301 K–353 K. The electrochemical window of Li5.4Al0.2PS5Br was evaluated by cyclic voltammetry (CV): a Li/Li5.4Al0.2PS5Br/Au cell was assembled, Li was the counter and reference electrode and Au served as the working electrode. The CV test used a Solartron 1470E CellTest system with a scan rate of 1 mV s 1 between 0.5 V and 5 V. The compatibility of the solid electrolyte with metallic lithium was further proved by symmetric cell cycling data. Two pieces of Li foil were attached to the Li5.4Al0.2PS5Br pellet for a symmetric cell test. Galvanostatic cycling of the Li/Li5.4Al0.2PS5Br/Li cell was tested using a battery test system (Land) at 28 � C. The current density is 0.53 mA cm 1. The TiS2/ Li5.4Al0.2PS5Br/Li all-solid-state battery was assembled and tested for cycling using a battery tester (Land CT2001A). The galvanostatic cycling was performed at a C-rate of 0.1C in room temperature. The anode of complete cell was lithium metal, and the cathode consisted of TiS2 and Li5.4Al0.2PS5Br a mass ratio of 7:3. The mixed cathode was prepared in an agate mortar.
2. Experiment section 2.1. Synthesis of argyrodites Li6–3xAlxPS5Br, Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) were synthesized by a classic solidstate synthesis. The precursor materials are lithium sulfide (Li2S, 99.98%, Sigma Aldrich), phosphorus pentasulfide (P2S5, 99%, Mucklin), aluminum sulfide (Al2S3, 99%, Alfa Aesar), silicon sulfide (SiS2, KaiYada 2
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Fig. 1. (a) Schematic diagram of varies element substitution in argyrodite Li7PS6. (b) schematic of cations substituted Li on halogen-containing argyrodites. (The orange solid line is the attempt of this paper.). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. (a) X-ray diffraction patterns of Li6PS5Br, Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br). (b) X-ray diffraction patterns of Li6–3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2, 0.25, 0.3) and Li6PS5Br.
3. Result and discussion
(M ¼ Al, B; X ¼ Cl, Br), and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) are shown in Fig. 2a. Li6PS5Cl-substituted samples show more impurities. Though both Ge and Si can be successfully substituted at the P position in Li7PS6 to form a solid solution [31], the impurity phases of Li5.2Si0.2PS5Cl, Li5.2Si0.2PS5Br, and Li5.2Ge0.2PS5Br indicate that Ge and Si cannot easily substitute Li in Li6PS5X (X ¼ Cl, Br). Thus, we conducted an in-depth study of Li6 3xAlxPS5Br owing to the higher purity and high ionic conductivity of Li5.4Al0.2PS5Br. The X-ray diffraction patterns of Li6 3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2, 0.25, and 0.3) and high temperature phase Li6PS5Br are shown in Fig. 2b. It is observed that Li6 3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2, 0.25, and 0.3) are consistent with the XRD pattern of Li6PS5Br, which indicates that the novel solid solutions exhibit a cubic argyrodite-type structure. As the content of Al increases (x > 0.2 in Li6 3xAlxPS5Br), the crystal linity of samples deteriorates, and the impurity phase of Al2S3 can be observed. These phenomena indicate that the upper limit of the solid solubility of Al in Li6PS5Br is probably reached. The XRD patterns of Li6 3xAlxPS5Br (x ¼ 0.1, 0.15, 0.2) are almost free of impurities, espe cially the Li5.4Al0.2PS5Br sample. It is suggested that Al3þ may substitute Liþ in the crystal lattice to form a solid solution. Furthermore, it is also
3.1. XRD characterization Previous examples of various elemental substitutions in argyrodite Li7PS6 are summarized in Fig. 1a. It is observed that the four different sites in Li7PS6 can be substituted by different cations, but the specific replacement scheme is still determined according to the specific situa tion [26,28,32]. To explore the effect of Liþ substitution in halogen-containing argyrodites, we evaluated the substitution of Li6PS5X (X ¼ Cl, Br) by several cations according to the ionic radius (Fig. 1a). Li6PS5I does not have high conductivity. It is generally believed that the substitution of the P site will increase the volume of the (P/M)S4 tetrahedron to obtain high conductivity. Thus, no attempt is made to replace Liþ in Li6PS5I. Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br) were synthesized, as shown in Fig. 1b, in which orange lines represent the attempts of this study. The principle of the experimental design is to pair halogen ions with larger radii with cations with larger radii. Representative X-ray diffraction patterns of Li6PS5Br, Li5.4M0.2PS5X 3
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argyrodite in the F 43m space group via the Rietveld refinement of the X-ray diffraction pattern (Fig. 3). The specific parameters are shown in Table 1. Therein, the Al3þ portion occupies the 24 g Wyckoff position of Liþ at 48 h and 24 g sites. Owing to the larger radius of Liþ (68 pm) than that of Al3þ (53 pm), the lattice parameter of the crystal is reduced to 9.978 Å, which is smaller than that of Li6PS5Br (10.181 Å). The reduced lattice parameter indicated the successful incorporation of Al3þ in the Liþ site. In addition, the refinement results indicate that Al3þ substitu tion does not considerably change the site disorder between Br and S2–. It is generally believed that Li ions migrate in three possible jump processes, i.e., inter-cage jump, intra-cage jump, and doublet jump. The material can have high ionic conductivity only when these three jump processes are active at the same time. Owing to the influence of sub stitution, the three path lengths of lithium-ion jump have changed. The paths of lithium-ion jump in a crystal were compared in Li6PS5Br and Li5.4Al0.2PS5Br, as shown in Fig. 4. The longer doublet jump distance 48 h 24 g 48 h is attributed to larger Columbic repulsion provided by Al3þ. With an increase in the distance of the double jump, the distance of intra-cage and inter-cage jumps is shortened. In general, the ratedetermining step of the ion migration is inter-cage jump [27], and its shortening may be the reason for the improvement of the ionic conductivity.
Fig. 3. Rietveld refinement pattern of Li5.4Al0.2PS5Br at 298 K. The black points are experimentally measured data; the red part is the calculated intensities; the blue line is difference curve. The structure shown in the inset is crystal structure of the synthesized Li5.4Al0.2PS5Br. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 1 Crystallographic data for Li5.4Al0.2PS5Br from X-ray diffraction. Refined pa rameters are shown with uncertainty in brackets.
3.3. Electrochemical properties
space group F43m a ¼ 9.97179 (1) � A
Table S1 shows the room temperature ionic conductivities of powder samples Li5.4M0.2PS5X (M ¼ Al, B; X ¼ Cl, Br) and Li5.2N0.2PS5X (N ¼ Si, Ge; X ¼ Cl, Br). The ionic conductivities of Li5.2Si0.2PS5Br and Li5.4B0.2PS5Br increase to approximately 1.50 mS cm 1 compared to 0.9 mS cm 1 for pristine Li6PS5Br. Fig. 5aFig. S1 and S2 show the ionic conductivities of cold-pressed pellets of Li6 3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3) as well as the extracted activation energies obtained via the impedance method. The specific values are shown in Tables S2 and S3. Temperature-dependent impedance spectroscopy was performed on Li6 3xAlxPS5Br. The corresponding impedance responses are shown in Fig. 5b. The Arrhenius data also provides information on the activation barrier of migration. As shown in Fig. 5a, with an increased fraction of Al3þ, the conductivity reaches the maximum value of 2.4 � 10 3 S cm 1 for Li5.4Al0.2PS5Br. The ionic conductivities of the samples show the tendency of the W-shaped trend in the interval. The x ¼ 0.15 and x ¼ 0.25 samples sintered at different temperatures are shown in Fig. S3 and Table S4. When sintered at 450 � C, the crystallinity of Li5.25Al0.25PS5Br became significantly better, and the ionic conductivity of Li5.25Al0.25PS5Br also reached 1.41 � 10 3 S cm 1. The trend of W-type indicates that the preparation conditions of some samples are not optimal, which requires extensive study in the future. Activation en ergies show the same tendency, and the specific values of the activation energy are also added to Table S3. In contrast, the ionic conductivity and
Fit residuals (Rwp, Rexp, χ 2): 5.73%, 4.37%, 1.985 Atom
Site
x
y
z
Occ.
Li1 Li2 Al1 Al2 S1 S2 S3 Br1
48 h 24 g 48 h 24 g 16e 4c 4a 4c
0.324 (2) 0.2500 x (Li1) x (Li2) 0.1188 (2) 0.750 0 x (S2)
0.023 (3) 0.017 (8) y (Li1) y (Li2) 0.8812 (2) x (S2) x (S3) x (S2)
0.676 (2) 0.7500 z (Li1) z (Li2) 0.6188 (2) x (S2) x (S3) x (S2)
Br2
4a
x (S1)
x (S1)
x (S1)
P
4b
0.5000
x (P)
x (P)
0.39 (3) 0.12 (1) 0.01 0.014 1 0.80 (2) 0.20 (1) 1-Occ (S2) 1-Occ (S3) 1
2
Beq/� A
9 (4) 0 (3) B (Li1) B (Li2) 3.36 (10) 1.44 (5) 1.0 (5) 2.45 (13) 2.0 (8) 1.0 (4)
possible that the addition of halogen Br (196 pm), which is larger than S2–(184 pm), increases the solid solution range of Al3þ to Liþ substitution. 3.2. Structural analysis The crystal structure of Li5.4Al0.2PS5Br is indexed to cubic lithium
Fig. 4. Jumps distance between the lithium positions (48 h 24 g 48 h, doublet jump), intra-cage jumps (48 h 48 h), and inter-cage jumps in Li5.4Al0.2PS5Br. 4
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Fig. 5. (a) Ionic conductivities at room temperature and the activation energy for the series samples of Li6–3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3). (b) Arrhenius plot of the conductivities of Li6–3xAlxPS5Br (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3) obtained from the measurements on cold pressed pellets.
Fig. 6. (a) Cyclic voltammetry curve of Au/Li5.4Al0.2PS5Br/Li cell with a scan rate of 1 mV s 1 in the voltage window of 0.5 to 5 V. (b) Li/Li5.4Al0.2PS5Br/Li symmetric cell showing lithium cyclability at a current density of 0.53 mA cm 2, where Li and Au serve as the reference/counter and working electrode, respectively. The step time is 1 h. (c) Representative charge and discharge curves of TiS2/Li5.4Al0.2PS5Br/Li all-solid-state battery at a rate of 0.1C in ambient conditions. (d) Nyquist plots and equivalent circuit for TiS2/Li5.4Al0.2PS5Br/Li all-solid-state cell before and after cycling.
activation energy of Li6PS5Br are 0.9 � 10 3 S cm 1 and 0.20 eV [27]. Fig. S1 shows the Nyquist plots of cold-pressed Li5.4Al0.2PS5Br powders measured between 301 K and 343 K. The change in conductivity and activation energy may be caused by the increased Li vacancies and the shortened inter-cage jump, as previously discussed. Fig. 6a shows the cyclic voltammetry curve of the Au/ Li5.4Al0.2PS5Br/Li cell. Au is the working electrode, and Li is the counter and pseudo reference electrode. The cathodic current at approximately 0 V refers to lithium striping. The cyclic voltammetry (CV) performed in the Au/Li5.4Al0.2PS5Br/Li cell indicated that there were no serious side reactions. However, the CV curve of the Au/Li5.4Al0.2PS5Br/Li cell
operating at 2–4.5 V shown in Fig. S4 also indicates that the stability between the electrolyte and metallic Li is only relative. The compati bility of Li5.4Al0.2PS5Br as a solid electrolyte is further confirmed by the cycling data of the symmetric cell. Fig. 6b shows that the symmetric Li/ Li5.4Al0.2PS5Br/Li cell was fabricated and cycled at the current density of 0.53 mA cm 2 to investigate the compatibility of Li5.4Al0.2PS5Br with metallic Li. The cell voltage stays stable at approximately 0.04 V, which indicates the presence of minimal interfacial reactions between Li5.4Al0.2PS5Br and metallic Li. An all-solid-state battery was assembled with TiS2 as the cathode, metallic lithium as the anode, and Li5.4Al0.2PS5Br as the solid electrolyte. With a charging rate of 0.1C at 5
Journal of Power Sources 450 (2020) 227601
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room temperature, the initial discharging capacity was 235 mAh g 1, which stabilized at 210 mAh g 1 in subsequent cycles (89% of the initial discharge capacity) (Fig. 6c). Fig. S5 also shows that the rate perfor mance of the all-solid-state battery is limited, which suggests that the battery process needs to be improved. The side reaction distorts the charging curve and increases capacity. In addition, the interface of allsolid-state battery needs to be further optimized. Nyquist plots and the equivalent circuit for TiS2/Li5.4Al0.2PS5Br/Li all-solid-state cell before and after cycling are shown in Fig. 6dand Table S5, which shows the deterioration of the interfacial contact in the cell and to some extent explains the attenuation of capacity.
[12] [13] [14] [15]
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4. Conclusions
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In summary, we used various cations to partially substitute Liþ in halogenated argyrodites. Among them, Li5.4Al0.2PS5Br exhibited the highest ionic conductivity of 2.4 � 10 3 S cm 1 which was three times higher than that of Li6PS5Br. Compared to other studies that replaced the Li site with other cations, Al was first used to substitute halogenated Liargyrodite and enhance conductivity. The conductivity was relatively high in this study on Li-argyrodites (Table S6). The Rietveld refinement results suggest that the change in vacancy and the shortening of intercage jump may be the main cause for the improvement of ionic con ductivity. CV and galvanostatic cycling tests indicate that the material is relatively stable to metallic lithium, and the all-solid-state battery shows better cycling performance. This work presents a new strategy for developing well-performing argyrodite-type solid electrolytes.
[18] [19] [20]
[21] [22]
Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement. This work was supported by the National Basic Research Program of China (973 Program, 2015CB258400) and the Certificate of China Postdoctoral Science Foundation Grant (2017M622422). The authors gratefully acknowledge the Analytical and Testing Center of HUST for allowing us to use its facilities.
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[25] [26] [27]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227601.
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