Co-Precipitation Synthesis of Sns-C Composite Used As Stable Anode in Li-Ion Battery
Authors: Yu Denis Yau Wai , Li Yingshun , Kang Wenpei , He Jieqing , Zhou Hui 
Published: February 2020

Graphite is the anode material commonly used in commercial lithium-ion batteries (LIB). The practical capacity of graphite is already close to its theoretical limit, so alternative anode materials are needed to further increase the energy density of LIB. For this purpose, high-capacity active materials with stable charge-discharge performance are needed. Among the different materials, metal-alloy, metal-oxides and sulphides are studied because they undergo alloying and/or conversion reaction to give capacities larger than that of graphite (> 370mAh/g).1,2,3. However, these materials exhibit large volume change during lithium insertion and extraction, which typically leads to fast capacity fading with cycling.4,5

In this study, we focus on high-capacity SnS material for lithium-ion applications. Both Sn and S can react with Li, resulting in a theoretical capacity of 1109 mAh/g (6.25 e-per SnS). To buffer the large volume change during charge and discharge, SnS-C composite is prepared. The carbon composite can also increase the electrical conductivity of the material by forming interconnected conductive network.
A one-pot co-precipitation method is developed to synthesize the SnS-C composite from SnCl2, Na2S and organic precursors. The precipitates were then heat-treated in N2atmosphere to carbonize the organic materials. X-ray diffraction (XRD) shows that the materials are phase-pure with an orthorhombic crystal structure (JCPDS #65-3812). The materials were made into electrodes and tested with lithium metal counter electrode comparing with that of SnS (purchased from Sigma-Aldrich). Figure 1 (a) and (b) show the charge-discharge behavior of SnS and SnS-C composite. Initial capacities of SnS and SnS-C are similar (about 900 mAh/g at 100mA/g). The charge-discharge curves show two regions of reaction: <1V (which corresponds to Sn/Li alloying) and 1.5-2.5V (which corresponds to Li/S reaction). With cycling, SnS shows significant loss in capacity, whereas SnS-C composite can maintain stable cycling performance with about 700 mAh/g at 250 mA/g after 100 cycles (Fig. 1(c)). Cycle efficiency of SnS-C is also significantly better than SnS (Fig. 1(d)).
The carbon composite introduced during the preparation process significantly improved the cycle performance. After 55 cycles, only the Sn/Li reaction below 1V is observed for the SnS sample (Fig. 1(a)). On the other hand, SnS-C composite still show both Sn/Li and Li/S reactions (Fig. 1(b)). This suggests that in addition to providing electrical conductivity and buffering for volume expansion, the carbon composite is able to facilitate recombination of SnS after charging.
Further work on the effect of cycle performance on carbon content, as well as the mechanical properties of the materials are underway and will be presented at the meeting.

Acknowledgement

This research is sponsored by the Innovation Technology Funding (ITS/363/13) managed by Innovation and Technology Commission from the Government of the Hong Kong SAR.
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Figure 1.Voltage vs. capacity profiles of (A) SnS and (B) SnS-C composite cell cycling between 0.012 and 2.5 V vs. Li at 22 °C. Lines of solid 1, long dash 2, long dash dot 5 were cycled at 100 mA/g. Lines of dash 6, dash dot 15, dot 55 were cycled at 250 mA/g. Black vertical dash line in (A) and (B) separates the conversion (left) and alloy (right) reaction in the material. (C) Capacity and (D) coulombic efficiency plotted as a function of cycle number for SnS (cross) and SnS-C composite (square) cells. Capacity was evaluated based on weight of active material.

Figure 1