Two-Dimensional Ultrathin Single-Crystalline Sns Nanoflakes As Anode Material for Li-Ion Batteries
Authors: Chen Chun-Yi , Yokoshima Tokihiko , Nara Hiroki , Momma Toshiyuki , Osaka Tetsuya 
Published: February 2020

In recent years, rechargeable Li-ion batteries (LIBs) have attracted much attention because of the wide applications not only for portable electronics but also electric vehicles. However, the current LIBs do not meet the requirements of the market, such as high energy density, long cycle life, and low cost. To solve this problem, great efforts have been made to develop new materials for both anode and cathode. For anode materials, SnS is considered as a potential candidate to replace the conventional graphite anode, because it has much higher theoretical capacity (782 mA h g-1). Herein, we report the fabrication of ultrathin single-crystalline SnS nanoflakes by facile hydrothermal method. The SnS nanoflakes possess two-dimensional layer structures which have large surface area and open-edge morphologies, and are expected to enhance the electrochemical performance when compared with their bulk counterparts.
Sn commercial foils (99.5% purity, Nilaco Corp) were rinsed with ethanol, and then placed into the cysteine-containing solution. The solution was subsequently transferred to a Teflon-line autoclave. The reaction was conducted at 160 ̊C for 24h, and the aotoclave was cooled down to room temperature. After washing and drying, Sn foil with SnS nanoflakes was directly used as anode of the coin cell without any binder, and Li foil was used as counter electrode.
Figure 1 (a) shows surface morphology of the Sn foil. The randomly aligned flake-like structures were observed on the Sn foil. The thickness of the nanoflakes was less than 20 nm, and the width was in the range of micrometer. The HRTEM image and diffraction pattern are given in Figure 1 (b). Clear lattice fringes and the dot diffraction pattern both reveal that the SnS nanoflakes are single crystalline. The lattice fringes with lattice spacing of 3.4 Å and 3.7 Å correspond well to the (012) and (101) planes of SnS, respectively.
The cyclic voltammogram (CV) profiles of the SnS nanoflakes on Sn foil are demonstrated in Figure 1 (c). The potential applied is ranged from 0.5 V to 2.0 V to avoid involvement of Sn foil in the reaction. During the first cycle, anodic peaks were observed at ca. 0.72, 0.80, and 1.85 V. Sharp anodic peaks at 0.72 and 0.80 V can be attributed to the electrochemical de-alloying of Sn–Li alloy. Cathodic peaks at ca. 0.68 and 1.4 V were also observed. The cathodic peaks can be attributed to the decomposition of SnS into Sn and the subsequent reaction between Sn and Li ion. The charge-discharge profiles show the initial reversible capacity was 990 mA h g-1, and much higher than the theoretical reversible capacity. This is attributed to synergistic effect of unique internal layer structure and ultra-thin flake structures.
In summary, the two-dimensional ultrathin single-crystalline SnS nanoflakes are proved as one of the promising candidate as anode material for the new generation of LIBs.

Figure 1. (a) SEM image and (b) HRTEM image and diffraction pattern of SnS nanoflakes. (c) CV profiles of the ultra-thin SnS nanoflakes electrode from 0.5 to 2.0 V with scan rate of 0.1 mVs-1.
REFERENCE

J. Mater. Chem., 2012, 22, 23091–23097.