1 Introduction

Lithium-ion batteries (LIBs) are currently the most preferred energy storage devices in portable electronic devices due to the high energy density, high operating voltage, and low self-discharge [1, 2]. Electrode materials, electrolytes, and separators, as important components in LIBs, have been intensively developed in recent years [3–6]. However, the binder, as an electrochemically inactive material, which functions to bind together the active material, the conducting agent, and the current collector, has attracted less attention [7–9].

PVDF has been successfully used as a binder for both the positive and negative electrodes in commercial LIBs, but it has some critical drawbacks [10, 11]. PVDF binder is generally dissolved in the volatile, flammable, or explosive N-methyl-2-pyrrolidone (NMP), which poses serious pollution to the atmosphere. In addition, high cost and low flexibility of PVDF also limits its application in large-scale batteries. Therefore, the development of alternative binders with less cost and more environmental friendly feature is essential. Recently, water-soluble polymers, such as sodium carboxymethyl cellulose (CMC) [12–14], polyacrylic acid (PAA) [15, 16], and alginate [17], have been explored as binders, especially for Si negative electrode which has severe volume change of about 400 % during cycling. The hydrogen and/or covalent bonds formed between the carboxylic or hydroxyl groups of these water-soluble polymers and the silicon particles could remarkably accommodate the huge volume change, thus significantly improved the electrochemical properties of the electrodes [12, 17].

Chitosan and its derivatives are a family of natural polymers containing polar groups such as amine, carboxyl, and hydroxyl groups. But chitosan itself is insoluble in water. By milling the mixture of silicon powder, chitosan, and carbon blacks in the water, Larcher and co-workers [18] were able to prepare Si negative electrode, which showed fairly moderate electrochemical performances. Similar to Si, SnS2 as an anode material has high theoretical capacity of 645 mAh g−1, and also suffers from the severe volume changes (up to 300 %) during cycling [19–25].

In this paper, we synthesized a micro/nano-structured SnS2 using biomolecular l-cysteine and SnCl4·5H2O as sulfur source and tin source, respectively. The electrochemical performances of SnS2 electrodes were studied using water soluble binders of C-chitosan and chitosan lactate, compared with the conventional binders of CMC and PVDF.

2 Experimental

2.1 Chemicals and materials

l-cysteine and SnCl4·5H2O were purchased from Guangzhou Chemical Reagent Factory (China). PVDF (Solvay Solef®6020) was from Shenzhen Micro Electron Co., LTD (China), C-chitosan and chitosan lactate (viscosity = 10–80 mpa s) were both purchased from AOKANG Chemical Reagent (China); CMC (viscosity = 800–1200 mpa s) was obtained from Sigma-Aldrich. Chemical structures of the polymers mentioned above were shown in Table 1.

Table 1 The chemical structures of four polymer binders
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2.2 Preparation of micro/nano-structured SnS2

Micro/nano-structured SnS2 was synthesized through a modified procedure reported before [26]. In a typical synthesis, l-cysteine (C3H7NS, 0.25 g, 2.1 mmol) and SnCl4·5H2O (0.35 g, 1 mmol) were dissolved in deionized water (40 mL). After being stirred for 10 min, the mixture was transferred into a Teflon-lined autoclave (50 mL), and incubated in an electric oven at 180 °C for 24 h. The autoclave was then removed from the oven and cooled down to room temperature. The yellow precipitate was collected by centrifugation, washed with deionized water and ethanol, and finally dried at 80 °C for 12 h.

2.3 Physicochemical characterization

The morphologies of the as-prepared SnS2 were observed by a SEM (Hitachi S-4800, Japan) equipped with an energy-dispersive detector (EDS) and TEM (JEOL JEM 2100 F, Japan) images with electron diffraction patterns. The phase identification was performed by X-ray diffraction (XRD, PANALYTICAL Incorporated, the Netherlands) from 10° to 80° with a CuKa source (λ = 0.154178 nm). Raman spectroscopy was carried out to monitor the variations in the sample using a HR800 Confocal Raman system (HORIBA Jobin-Yvon, France) with 532 nm diode Laser excitation on a 300 lines/mm grating at room temperature. The specific surface area was calculated using the Brunauer–Emmet–Teller (BET) method, and the Barrett–Joyner–Halenda (BJH) pore size distribution was determined using the desorption branch of the isotherm (SI-MP-10/PoreMaster 3, Quanatachrome Instruments, America).

2.4 Electrochemical measurements

SnS2 was used as active material, carbon black (CB) as the conductive agent and C-chitosan (3.5 wt%) and chitosan lactate (3.5 wt%) aqueous solution are used as the binder, respectively. Measured amounts of SnS2, CB, and C-chitosan (chitosan lactate) were mixed according to 75 wt% active material, 15 wt% CB, and 10 wt% binder. The slurry was coated onto a copper foil and dried in a vacuum oven at 110 °C for 12 h to remove the water solvent. To compare the effect of the binders on electrochemical performance, 12 wt% PVDF solution in N-methyl pyrrolidinone (NMP), and 3.5 wt% CMC aqueous solution were used as the binders for SnS2, respectively. The coin cells (CR2025) were assembled to test the electrochemical performance of the as-prepared SnS2, using 1 M LiPF6 EC/DEC (1:1 by volume) as the electrolyte, a microporous membrane (Celguard 2400, USA) as the separator. The cells were charged and discharged galvanostatically in the fixed voltage window from 0 mV to 2.0 V (versus Li+/Li) on a Shenzhen Neware battery tester (China) at 25 °C. Cyclic voltammetry (2.0–0 V, 0.2 mV s−1) was performed by using an electrochemical workstation (Zennium/IM6, Germany). The rate capability was evaluated by varying the rate from 0.5 to 5C. Electrochemical impedance spectroscopy (EIS) was measured with electrochemical workstation (Zennium/IM6, Germany) by applying an alternating voltage of 5 mV over the frequency ranging from 10−2 to 105 Hz.

3 Results and discussion

3.1 Structure and composition characterization

The crystallographic structure and phase purity of the as-prepared samples were examined by XRD (Fig. 1a). All the peaks of the sample in the XRD pattern can be readily indexed to a pure hexagonal phase of SnS2 in good agreement with the reported values (JCPDS card, No. 23-0677) [27]. No peaks of other phases were detected, indicating the high purity of the as-synthesized product. Raman spectrum was used to further confirm the composition of the sample. The Raman spectrum of the prepared SnS2 showed an intense peak at about 311 cm−1 (Fig. 1b), which was attributed to the A1g mode of SnS2 according to the group theory analysis by previous studies [28, 29].

Fig. 1
figure 1

a XRD pattern and b Raman spectrum of the prepared SnS2 sample

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The morphologies and structures of the prepared samples were examined by SEM and TEM (Fig. 2). The low magnification SEM image showed that SnS2 with uniformed spherical morphologies were observed. The magnified SEM image revealed an interesting phenomenon that SnS2 had a uniformed 3D micro/nano structure with an average diameter of 2.0 μm, which was assembled by many interleaving and slightly bending nanoflakes with a thickness less than 30 nm (Fig. 2a, b). TEM images of SnS2 confirmed the highly dispersed micro/nano structure (Fig. 2c). The corresponding electron diffraction pattern (inset in Fig. 2c), taken under the electron beam perpendicular to the surface of a nanosheet, could be indexed as a hexagonal phase SnS2 along the [001] axis. The magnified image showed clearly the lattice stripe of the nanoplate with the interplanar distance of 0.59 nm between the neighboring lattices which is consistent with the (001) planes of a hexagonal phase SnS2 (Fig. 2d).

Fig. 2
figure 2

Morphologies of the as-synthesized SnS2. a, b SEM images of the micro/nano– structured SnS2 prepared using the hydrothermal method; c TEM image of SnS2, inset showing the corresponding electron diffraction pattern; d HRTEM image of the micro/nano-structured SnS2

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3.2 BET surface area

Nitrogen gas adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) characterization were used to measure specific surfaces areas (SSA)and the pore size distribution of the hierarchical flowerlike SnS2 (Fig. 3). The type-IV isotherm with a hysteresis loop in the range of 0.4–1.0 P/P0 was in accordance with the assembled plate-like structure [30, 31]. The quantitative calculation showed that the as-prepared SnS2 sample had a surface area of 78.4 m2 g−1. The pore size distribution of the SnS2 sample was shown in Fig. 3b. Two different mesopore sizes of 4.3 nm and 13.0 nm were observed, which offered a high surface area and thus avoided the permanent trapping of lithium ions that is possible with micropores [32].

Fig. 3
figure 3

a N2 adsorption–desorption isotherm and b pore size distribution curves of SnS2

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3.3 Electrochemical performance

Cyclic voltammetry of SnS2 electrode using C-chitosan binder was performed between 0.0 and 2.0 V at a scan rate of 0.2 mV s−1 (Fig. 4). Two broad peaks at about 1.1 and 0.8 V in the first potential sweeping process disappeared in the second run, due to the transformation of SnS2 nanosheets to metallic tin and Li2S (Eq. 1) [33, 34]. In the more cathodic potential range of 0.1–0.5 V, a broad peak at about 0.1 V in the first cathodic sweep was observed, which originated from the reaction of Li ions and Sn metal (Eq. 2). For the following cycles, the intercalation of lithium ions from Sn occurred in the potential range of 0.0–0.5 V and their deintercalation occurred in the potential range of 0.5–0.7 V. The additional peak at 1.9 V could be attributed to the lithium intercalation of the SnS2 layers without phase decomposition [34, 35].

Fig. 4
figure 4

Cyclic voltammetry for micro/nano-structured SnS2 using C-chitosan binder for the first three cycles from 2.0 to 0 V versus Li/Li+

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$${\text{SnS}}_{2} + 4{\text{Li}} \to {\text{Sn}} + 2{\text{Li}}_{ 2} {\text{S}}$$
(1)
$${\text{Sn}} + 4.4{\text{Li}} \leftrightarrow {\text{Li}}_{4.4} {\text{Sn}}$$
(2)

Micro/nano-structured SnS2 using C-chitosan, chitosan lactate or CMC binder exhibit a higher specific capacity than that of PVDF. The first charge–discharge profiles of SnS2 electrodes cycled between 2.0 V and 0.0 v at 0.5C showed that SnS2 electrodes using C-chitosan, chitosan lactate, CMC, and PVDF binder delivered a first discharge capacity of 899, 806, and 937 and 661 mAh g−1, respectively (Fig. 5a). After 50 cycles, SnS2 electrode maintained a discharge specific capacity of 479 mAh g−1 (C-chitosan), 488 mAh g−1 (Chitosan lactate), 544 mAh g−1(CMC), and 265 mAh g−1 (PVDF), respectively (Fig. 5b). High coulombic efficiency (CE) is critical for practical applications and is challenging to achieve for Sn-based anodes with huge volume change due to the need to maintain a stable SEI layer [24]. SnS2 electrode using C-chitosan, chitosan lactate, CMC binder showed higher initial coulombic efficiency (ICE) than PVDF; 47.5, 62.1, 61.0, and 60.8 % for PVDF, CMC, C-chitosan, and Chitosan lactate, respectively. The rate capability (from 0.1 to 5C) of micro/nano-structured SnS2 using different binders was also studied (Fig. 5c). SnS2 electrode using water soluble binders (e.g., C-chitosan, chitosan lactate, or CMC) displayed better rate capability than PVDF. Even at higher rates, such as 5C, SnS2 electrode using C-chitosan, chitosan lactate, CMC, and PVDF binder, delivered a higher specific capacity of 458, 432, and 442 mAh g−1, respectively, as compared with 175 mAh g−1 for PVDF.

Fig. 5
figure 5

The electrochemcial properties of micro/nano-structured SnS2 using different binders. a The first charge–discharge curves of SnS2 between 2 and 0 V; b the cycling performance at 0.5C rate; c the rate capability of SnS2 from 0.1 to 5C; d impedance spectra of SnS2 electrode after two cycles

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We analyzed the impact of the chemical structures of four binders on electrochemical performances (Table 1). Apparently, the water-soluble binders contain various kinds of polar groups such as carboxy, hydroxyl, or amine group in their chemical structures when compared with PVDF. These polar groups can ensure better interfacial interaction through hydrogen and/or covalent bonds with SnS2 [12, 17], and the amine groups from chitosan derivatives, could possibly act as an acid scavenger (HF generated from the decomposition reaction of electrolytic salt) so as to improve the cell cyclability [36].

The electrochemical impedance spectroscopy (EIS) was used to characterize the impedance properties of SnS2 electrode using different binders after 2 cycles (Fig. 5d). All the Nyquist plots included a semicircle and a linear part, indicating that the electrode reactions were controlled by a mixture of charge transfer and diffusion steps. The semicircle part at high frequencies reflected the charge transfer process and the sloping straight line at low frequencies corresponds to the Li-ion diffusion in the bulk. The sum of the charge transfer resistance (R ct) and surface film resistance (R sf) of SnS2 electrode using C-chitosan, Chitosan lactate, or CMC binder was lower than that of PVDF. The enhanced rate capability could be attributed to an efficient hydrogen bond network formed by the polar groups of the water-soluble polymers and the active particles which facilitated the fast transfer of Li+ ions, and the formation of a stable SEI layer at the surface of SnS2 electrode [37, 38].

4 Conclusions

Micro/nano-structured SnS2 with high yield was successfully synthesized through a simple one-pot hydrothermal route. The electrochemical performance of micro/nano-structured SnS2 using the different binders (C-chitosan, Chitosan lactate, CMC, or PVDF) was investigated. Micro/nano-structured SnS2 using water-soluble binders (C-chitosan, Chitosan lactate, CMC) showed higher initial coulombic efficiency, reversible capacity and rate capability, compared with that of PVDF. After 50 cycles at 0.5C rate, the SnS2 electrode using C-chitosan, Chitosan lactate, CMC, and PVDF binder, delivered a discharge specific capacity of 479, 488, 544, and 265 mAh g−1, respectively. At a much higher rate of 5C, SnS2 electrode using C-chitosan, Chitosan lactate, CMC, and PVDF binder, exhibited a specific capacity of 458, 432, 442, and 175 mAh g−1, respectively. This work shows that chitosan derivatives can serve as a new family of water-soluble binders and potentially replace non-aqueous PVDF binder for LIBs.