High-Performance Dual-Ion Battery Based on a Layered Tin Disulfide Anode.

Energy issues have attracted great concern worldwide. Developing new energy has been the main choice, and the exploitation of the electrochemical energy storage devices plays an important role. Herein, a high-performance dual-ion battery system is proposed, which consists of a graphite cathode and SnS2 anode, with a high-concentration lithium salt electrolyte (4 M LiTFSI). The benefits from the typical sandwich-like layer structure of SnS2 are as follows: the highest discharge specific capacity of the battery could reach 130.0 mA h g-1 at a current density of 100 mA g-1, and even under an ultra-high current density of 2000 mA g-1, the highest capacity of 66.3 mA h g-1 is still achieved, with an outstanding capacity retention over 100% after 1000 cycles. Inspiringly, this system delivers an excellent low self-discharge of 1.19%/h, surpassing most of the reported dual-ion batteries. In addition, the working mechanism and structural stability are also investigated by X-ray diffraction and Raman spectra, indicating a good reversibility. These results reveal that this graphite/SnS2 dual-ion battery system could provide a promising alternative for a future high-performance energy storage device.

Introduction

At present, the development of energy storage devices with a high energy density has received increasing attention due to constantly rising energy demands.[1,2] Since their creative commercialization in 1991, lithium-ion batteries (LIBs) have changed our way of life and provide energy for various equipment, such as portable electronics and electric vehicles.[3−5] However, the limited lithium resources and security issues have caused serious concern for the large-scale application of LIBs.[6,7] Therefore, great efforts have been made to develop a high-performance and safe next-generation energy storage system. Owing to the unique mechanism based on the simultaneous intercalation of cations and anions into the anode and cathode, respectively, during the charging process, dual-ion batteries (DIBs) could provide a high working voltage and energy density.[8−13] Besides, DIBs show several typical characteristics of low cost and safety and are environmentally friendly and so have been considered promising energy storage devices.[14−16]

In the DIB systems, graphite has been the most commonly used electrode material due to its richness, ease of production, and non-toxicity.[17−20] In addition, the redox amphotericity supports that graphite could serve as a cathode and anode active material, and the first reported DIB configuration is also based on the dual-graphite electrodes. However, graphite shows a relatively low specific capacity (LiC6 372 mA h g–1) and serious exfoliation occurring on the surface of the graphite anode caused by the continuous intercalation of large cations.[21−23] Furthermore, the dual-graphite batteries usually display a severe self-discharge phenomenon, which limits the further commercial application of DIBs. Consequently, searching for a more suitable anode material with considerable capacity and good stability is admirable. Two-dimensional transition-metal dichalcogenide (TMD) materials have received increasing interest due to their excellent electrochemical and mechanical properties and could be promising candidates for electrochemical energy storage.[24−26] WS2 (tungsten disulfide) and MoS2 (molybdenum disulfide) are two typical transition-metal sulfides, which have been proposed as electrode materials for various energy storage systems. Recently, Bellani et al.[27] prepared few-layered WS2 flakes and applied them as an anode material for lithium salt electrolyte-based DIBs. The cutoff operating voltage of the system could reach 4.0 V and shows a considerable specific discharge capacity of ∼83 mA h g–1 at 100 mA g–1. Li et al.[18] designed a novel DIB accompanied by a MoS2 cathode, and the larger layered spacing of MoS2 effectively enhances the anion intercalation kinetics, leading to a high specific capacity (135 mA h g–1) and stable cycling ability. Similar to WS2 and MoS2, SnS2 (one of the post TMD materials) shows a large interlayer spacing and weak van der Waals, which could benefit the intercalation of ions and achieve a high specific capacity.[28−35] Yin et al.[34] synthesized a SnS2 nanoflake (around 100–200 nm) and designed an LIB based on a SnS2 anode. The battery showed a high discharge specific capacity of 1485.8 mA h g–1 and exhibited outstanding rate performance. In addition, the electrochemical intercalation behavior of sodium and potassium ions into the SnS2 crystal have also been investigated, and the results reveal good electron transport kinetics.[31,33] Consequently, SnS2 could be a suitable anode material for a DIB to enhance the electrochemical performances.

Combining with the advantages of LIBs and DIBs, a novel DIB based on a lithium salt electrolyte has been constructed, coupling with a natural graphite cathode and SnS2 anode. In addition, owing to the fact that the cations and anions come from the electrolyte, the electrolyte has also been considered as an active material (like cathode and anode materials) for a DIB. Generally, a highly concentrated electrolyte could enhance the electrochemical performance and achieve a higher energy density. Therefore, 4 M LiTFSI [bis(trifluoromethane)sulfonimide lithium salt] in the hybrid solvent of ethyl-methyl carbonate (EMC) and Pyr14TFSI [1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide] is prepared for the DIB system. The addition of the Pyr14TFSI ionic liquid enhanced the cycling stability and Coulombic efficiency. Furthermore, the electrochemical performance and working mechanism have been comprehensively investigated, and the battery achieves a high discharge specific capacity and excellent rate performance, showing that SnS2 could be a promising anode material for a DIB.

Results and Discussion

The SnS2 sample was synthesized via a simple solvothermal method. The crystal structure and morphology of SnS2 powder have been investigated, and the results are shown in Figure . The XRD (X-ray diffraction) pattern of SnS2 shows several characteristic peaks located at around 14, 28, 32, 41, 50, and 52°, which could be assigned to the crystal planes of (001), (100), (101), (102), (110), and (111), respectively (Figure a).[34] Notably, all diffraction peaks coincide well with the JCPDS card of SnS2 (JCPDS no. 23-0677), indicating the pure-phase 2T-hexagonal of the as-prepared SnS2, which shows a well-defined layered crystal structure with a P3̅m1 space group. To further confirm the crystal structure of SnS2, a Raman spectrum has been recorded. Figure b shows the Raman spectrum of SnS2, and a sharp peak located at 311 appears, which belongs to the out-of-plane vibration mode (A1g) of the SnS2 phase.[28] No other peak is observed, demonstrating that the SnS2 obtained shows high purity, corresponding to the result of the XRD pattern. The chemical status of SnS2 powder was characterized by X-ray photoelectron spectrometry (XPS), and the result is shown in Figure c. The signals which appeared at 159.5, 282.4, 484.5, and 530 eV could be attributed to the S, C, Sn, and O elements, respectively. In addition, the elements of C and O may be caused by the CO2 and O2 from the atmosphere, respectively. Furthermore, the high-resolution spectra of Sn 3d and S 2p are shown in Figure S1. Two signal peaks emerged at around 484.5 and 492.9 eV belonging to Sn 3d5/2 and Sn 3d3/2, respectively, confirming the existence of Sn4+ (Figure S1a).[31]

Figure 1

Structure and morphology of SnS2: (a) XRD pattern, (b) Raman spectrum, (c) XPS spectrum, and (d) SEM image of SnS2.

Besides, Figure S1b shows the high-resolution spectrum of S 2p; two signal peaks at 159.5 and 160.3 eV could be assigned to the S 2p3/2 and S 2p1/2 of S2–, respectively. The surface morphology and structure of SnS2 powder was further characterized by scanning electron microscopy (SEM), and flake-like SnS2 could be clearly observed, which shows a lamellar structure connected by weak van der Waals forces, revealing the potential favorable ion transport path. In addition, the element distribution of the SnS2 powder has been investigated by energy-dispersive X-ray spectroscopy, and the results are shown in Figure S2 and Table S1. The elements of C, O, S, Sn, and Cl have been detected and coincide well with the results of XPS. Besides, the atom percentages of the S and Sn elements are 30.82 and 16.91, respectively (Table S1), in accordance with the stoichiometric ratio of SnS2. The thermostability of SnS2 was also studied by thermogravimetry (Figure S3); the initial increased weight might be caused by the adsorption of N2. The weight loss below 200 °C could be assigned to the moisture, and the weight loss between 200 and 500 °C could be attributed to the impurities and residual raw materials. Furthermore, it could be found that SnS2 began to decompose over 500 °C, indicating good thermostability.

The electrochemical performances of the graphite/SnS2 DIB system were studied in CR 2025 button cells, and the electrolyte used was 4 M LiTFSI. Figure a shows the influence of the Pyr14TFSI ionic liquid in the electrolyte solvent system. In the EMC system, the battery delivers a high initial discharge specific capacity of 124.3 mA h g–1. Unfortunately, the capacity decays very quickly, and only 21.7 mA h g–1 remained after 10 cycles, which could be attributed to the collapse of the graphite crystal structure caused by the solvent co-intercalation and the decomposition of EMC.[36−38] Interestingly, the battery based on the EMC–Pyr14TFSI (volume ratio: 1:1) system shows better electrochemical performance. The initial discharge specific capacity of the battery reaches 114.3 mA h g–1 and then increases to 119.1 mA h g–1, far beyond that of the EMC system. In addition, the cycling performances of the DIB in different electrolytes are displayed in Figure S4. Obviously, the DIBs in the organic solvent electrolyte system could not achieve a stable cycle and show a low Coulombic efficiency, which could be attributed to the electrolyte decomposition and co-intercalation of the solvent (Figure S4a,b). Due to the addition of Pyr14TFSI, the batteries show a higher Coulombic efficiency (Figure S4c,d), indicating that the Pyr14TFSI ionic liquid might prevent the co-intercalation of EMC and enhances the stability. Compared with that of other electrolyte systems, the battery based on the 4 M LiTFSI (EMC–Pyr14TFSI) system exhibits higher discharge capacity, higher middle discharge, and better cycling performance (Figures S4 and S5). Therefore, 4 M LiTFSI (EMC–Pyr14TFSI) electrolyte was chosen in the subsequent tests. To investigate the electrochemical intercalation behaviors of ions, cyclic voltammetry (CV) tests were conducted in the two-electrode system in a negative scan direction. The working electrode is the SnS2 anode, coupled with the graphite counter electrode in the window of −3.6 to −1.0 V, and the result is shown in Figure b. Two pairs of redox peaks could be seen in the curves, and with the increase in scan cycles, the reduction peaks shift from −2.67 and −3.10 to −3.22 and −3.43 V, respectively, revealing the intercalation of Li ions and TFSI– cations during the charging process.[39−41] Two oxidation peaks appear at −3.02 and −2.27 V, and the curves overlap well, confirming the de-intercalation of Li ions and TFSI– cations. Besides, Figure S6 shows the CV curves of the different low cutoff voltages (−3.7 and −3.8 V). Compared with that of the cutoff voltage of −3.6 V, the first curves of the lower cutoff voltages exhibit well-defined oxidation peaks and also show greater electrochemical polarization.

Figure 2

(a) Electrochemical performances of the battery in different solvents; (b) CV curves at the cutoff voltage of −3.6 V; (c) charge–discharge curves of the system under various upper cutoff voltages; (d) charge–discharge curves of the system under an upper cutoff voltage of 3.6 V at a current density of 100 mA g–1.

Furthermore, to find a suitable upper cutoff voltage for galvanostatic charge–discharge, the electrochemical performances of the system under the various upper cutoff voltages (3.5, 3.6, 3.7, and 3.8 V) were analyzed. As shown in Figure c, all the charge curves show two plateaus of 2.2 to 2.4 and 3.5 to 3.6 V, corresponding to the two reduction peaks of the first CV curve. Similarly, the discharge curves possess two slopes around 3.3 to 2.8 and 2.7 to 1.9 V, coinciding with the two oxidation peaks. The battery based on a cutoff voltage of 3.8 V shows the highest initial discharge specific capacity of 161.1 mA h g–1, and the capacities of other cutoff voltages are 147.9 mA h g–1 (3.7 V), 114.3 mA h g–1 (3.6 V), and 71.2 mA h g–1 (3.5 V). In addition, the cycle performances are further demonstrated by Figure S7. It could be seen that the higher cutoff voltage contributes to a higher discharge specific capacity, and the higher cutoff voltage could also lead to the sacrifice of Coulombic efficiency, which could be ascribed to the occurrence of a side reaction and electrolyte decomposition.[42,43] The battery based on a cutoff voltage of 3.6 V shows the best cycling performance and maintains a high capacity retention of 83.5% after 100 cycles, far better than that of 3.5 V (43.5%), 3.7 V (40.0%), and 3.8 V (43.0%). Consequently, owing to the considerable capacity, the high Coulombic efficiency, and good capacity retention, 3.6 V could be a suitable cutoff voltage for the system. Figure d shows the charge–discharge curves of different cycles under a current density of 100 mA g–1 in the working window of 1.0 to 3.6 V. With the increase in charge–discharge cycles, the discharge specific capacity increases from 114.3 mA h g–1 to the highest capacity of 130.3 mA g–1 after 21 cycles, and the Coulombic efficiency also increases to 96.2%, demonstrating the presence of an electrochemical activation process.[44,45] After 30 cycles, the discharge specific capacity maintains at 127.2 mA h g–1, keeping an excellent capacity retention of 111.3%. Furthermore, the discharge curves are well overlapped, indicating the low electrochemical polarization and good cycling stability.

The performances of the system at a variety of current densities are shown in Figure . Figure a exhibits the first cycle charge–discharge curves under the current densities of 100 to 2000 mA g–1. With the increase in current density, the initial discharge capacity decreases from 114.3 to 27.7 mA h g–1, and the gaps between the charging plateaus and discharging plateaus also grow, which could be caused by the electrochemical polarization.[46] Owing to the electrochemical activation process, all the capacities of the batteries increase in the subsequent cycles, and the highest discharge specific capacities reach 130.3, 121.9, 96.4, 99.3, 45.2, 34.8—, and 66.3 mA g h–1 at the current densities of 100 mA g–1, 200 mA g–1, 300 mA g h–1, 400 mA g h–1, 500 mA g h–1, 1000 mA g h–1, and 2000 mA g h–1, respectively (Figure S8).

Figure 3

(a) Charge–discharge curves at various current densities; (b) rate capacity of the system.

Furthermore, the rate performance of the system is shown in Figure b. The battery was operated at the current densities of 100 to 400 mA g–1 in the first 50 cycles, and then the current density returns to 100 mA g–1 in the subsequent cycles. With the increase in current density, the discharge specific capacity reduces from 103.7 to 41.4 mA g–1. Inspiringly, the battery achieves a high capacity of 122.0 mA g–1, and after 50 cycles, the capacity still remains at 100 mA h g–1, indicating an excellent rate performance. Furthermore, the long-term cycle performance of the system at the high current density of 2000 mA g–1 is shown in Figure . It can obviously be seen that the battery experienced a unique activation process as long as hundreds of cycles, which could benefit from the sandwich-like layer structure and weak van der Waals forces between the spaces of the SnS2 crystal. Besides, the discharge specific capacity still remains at a high value of 40.5 mA h g–1, which is 1.46 times the initial discharge specific capacity, indicating that the system could be a promising fast charge device.

Figure 4

Long-term cycling performance of the system under a high current density of 2000 mA g–1.

Self-discharge is an important factor for DIB systems, and the serious self-discharge phenomenon will limit further practical applications.[47] Especially in the dual-graphite or dual-carbon systems, the batteries usually deliver a high value of self-discharge. Therefore, the self-discharge performances of the system were also studied. Figure a,b shows the first cycle voltage–time curve and voltage–capacity curve of the battery without resting, respectively, which was operated at the current density of 100 mA g–1, and a discharge specific capacity of 114.3 mA g–1 is achieved. The contrastive battery was rested for 5 h after being charged to the upper cutoff voltage of 3.6 V and then discharged directly.

Figure 5

Self-discharge performance of the battery: (a) voltage–time curve of the unrested battery; (b) voltage–capacity curve of the unrested battery; (c) voltage–time curve of the rested battery; (d) voltage–capacity curve of the rested battery.

It could be found that the voltage reduces from 3.6 to 3.1 V after resting for 5 h (Figure c), and the remaining discharge specific capacity reaches 107.5 mA h g–1 (Figure d). Consequently, according to previous work, the self-discharge rate of the system is calculated as low as 1.19%/h, which is superior to the reported DIBs system (Table S2).[48−50] Furthermore, the self-discharge of the system after cycling is demonstrated in Figure S9, which was performed based on the sixth cycle. Figures S6a and 6b show the charge–discharge curves of the former six cycles and the voltage–capacity curve of the sixth cycle, respectively, and the discharge specific capacity of the sixth cycle is 115.6 mA h g–1. The controlled battery was rested for 5 h after the sixth charge, and the voltage decreased to 3.11 V, virtually no change compared with that tested in the first cycle. After the full discharge, the capacity still remains at 99.0 mA h g–1, resulting in a low self-discharge of 2.86%/h, indicating that this DIB system shows excellent stability and great potential for further practical applications.

Figure 6

XRD patterns of the electrode in different states: (a) graphite cathode and (b) SnS2 anode.

To further investigate the structural changes of the graphite cathode and SnS2 anode during the charge–discharge process, the XRD patterns were analyzed to characterize the cycled electrodes. Figure a shows the XRD patterns of the graphite cathode at the various states, and the pristine graphite cathode exhibits a strong typical diffraction peak located at around 26.53°, corresponding to the (002) crystal plane of graphite. With the increase of charging voltage, the (002) peak shows reduced intensity and splits into two new peaks at nearly 25 and 28° after the battery is fully charged (Figure S10), which could be assigned to the graphite intercalation compounds of (00n) and (00n + 1), respectively, demonstrating the intercalation of TFSI– anions.[41,51−53] On the other hand, the (002) peak shifts return to 26.44° and show increased intensity along with the de-intercalation of TFSI– anions during the discharging process (Figure S11). In addition, owing to the partial irreversibility, the two new peaks located at 25 and 28° still could be seen, coinciding with the results of galvanostatic charge–discharge (a low initial Coulombic efficiency). The pristine SnS2 anode maintains the several typical diffraction peaks of the SnS2 crystal, especially the (001) peak (Figure b). The (001) peak shifts to a high value during the charging process and disappears at the fully discharged state, which is caused by the intercalation of Li+ cations. In addition, simultaneously, several weak peaks emerge at around 20°, which could be ascribed to the existence of the Li–Sn alloy and Li–SnS2 compounds.[30] Conversely, the (001) peak reappears due to the de-intercalation of Li+ cations during the discharging process, indicating that the diffusion path of Li+ cations could be along with the (001) plane in the SnS2 crystal structure. Furthermore, Figure S11 also demonstrates the structural changes of the SnS2 anode during the charge–discharge process. Several new peaks emerge in the fully charged curve, which could be attributed to the formation of the Li–Sn alloy and Li–SnS2 compounds. Besides, the peak of A1g also shows a right shift and decreased intensity, indicating the intercalation of Li+ cations, corresponding to the results of XRD patterns. Notably, the fully discharged curve only shows the sharp (A1g) peak of SnS2, demonstrating an excellent reversibility of the charge–discharge process.

Conclusions

In summary, SnS2 was successfully obtained via a simple solvothermal method, and a novel DIB system has been proposed based on the graphite cathode and SnS2 anode, coupled with a high-concentration lithium salt electrolyte. Owing to the addition of the Pyr14TFSI ionic liquid, the stability of the system has been greatly improved. The initial capacity of the system could reach 114.3 mA h g–1 under the current density of 100 mA h g–1, and inspiringly, the capacity increases to the highest value of 130.0 mA h g–1 in the subsequent cycle. The rate capacity and cycling performance are also outstanding, and even at the high current density of 2000 mA g–1, the highest discharge specific capacity of 66.3 mA g h–1 is achieved, and the capacity retention exceeds 100% after 1000 cycles. In addition, the system shows a very weak self-discharge of 1.19%/h and still maintains a low level at 2.86%/h after being cycled, which is superior to the other DIB systems. Besides, the working mechanism of the system and the structural stability of the electrodes have been investigated by XRD and Raman spectra. Consequently, this DIB system could be a promising energy storage device for future large-scale applications.

Experimental Detail

Material Preparation

The SnS2 powder was prepared by the solvothermal method. 1.40 g of stannic chloride pentahydrate (SnCl4·5H2O) was mixed with 0.60 g of thioacetamide in 100 mL of ethyl alcohol, and the mixture was stirred for 1 h. Then, the solution was transferred to a Teflon-lined stainless-steel autoclave and reacted in an air-dry oven for 10 h under 180 °C. After centrifugation and washing by deionized water and ethyl alcohol, a golden powder was successfully obtained.

Electrode Preparation

The SnS2 anode was prepared by mixing 70 wt % SnS2 powder, 20 wt % acetylene black, and 10 wt % sodium carboxymethylcellulose in a solvent of deionized water. The slurry was coated on the Al foil, and after drying in a vacuum oven under 80 °C, the film was cut into wafers with a diameter of 14 mm. The average mass loading of the SnS2 anode is 1.23 mg cm–1. Similarly, the graphite cathode consists of 90 wt % graphite, 2 wt % acetylene black, and 8 wt % poly(vinylidene fluoride), and the average mass loading of the graphite cathode is 3.44 mg cm–1.

Electrochemical Measurement

The performances of the batteries were analyzed on CR2025 coin-type cells, with 100 μL of 4 M LiTFSI electrolyte in a solvent of EMC–Pyr14TFSI (volume ratio: 1:1), and the cells were assembled in Mikrouna glovebox ([O2] < 0.01 ppm, [H2O] < 0.01 ppm) under an argon atmosphere. The galvanostatic charge–discharge tests were performed on a Neware CT-4008 battery test system (Shenzhen China) at room temperature. CV tests were characterized on a CHI660E (Shanghai China).

Characterization of the Material and Electrode

The XRD patterns were obtained using X’Pert Powder (PANalytical, Netherlands), and the Raman spectra were recorded on an HJY LabRAM Aramis (Horiba Jobin Yvon). The morphology and valence state were characterized using a Merlin field emission electron microscope (Zeiss, Germany) and an Axis Ultra DLD (Kratos, United Kingdom).