A Study on the Charge–Discharge Mechanism of Ordered Mesoporous MoS2 Anode Material Via Synchrotron-Based x-Ray Analysis
Authors: Choi Woo Sung , Choi Yun Seok , Jeong Mihee , Park Hyunyoung , Kim JI Man , Yoon Won-Sub 
Published: February 2020

MoS2 has superior electrochemical properties suitable for lithium ion battery, such as high theoretical capacity based on conversion reaction (670 mAhg-1), low volumetric expansion during electrochemical cycling (only ~103%) [1] and the week van der waals forces between layers, which enable facile intercalation / deintercalation of Li-ion in this material. So, MoS2 has been studied as a replacement material for commercialized graphite. The mechanism of capacity expression is known to consist of an irreversible intercalation reaction in the first cycle (MoS2 + xLi+ + xe- → LixMoS2) and a reversible conversion reaction (MoS2 + 4Li+ + 4e- ↔ Mo + 2Li2S) [2, 3]. However, there are some recent studies that contradict with this above mentioned electrochemical reaction mechanism. These studies agree about the irreversible intercalation reactions in the first cycle, but they suggest that the reversible reaction comes from the different reaction path (Li2S ↔ 2Li+ + S-), similar to a lithium-sulfur battery reaction. For example Shyamal K. Das et al [
4
] used Li2S and Mo metal XRD peak data to support this new reaction mechanism. Debin Kong et al [5] observed sulfur redox curve in cyclic voltammetry and In Jaewon Jin et al [6] report the presence of sulfur element using the TEM image after the full charge which again contradicts with the traditional reported mechanism for MoS2. To date, this debate is ongoing, and there is no definite study yet to fully support the either mechanism.
In order to clarify the lithium storage mechanism in MoS2, we first selected the model structure to ascertain the complete mechanism by obtaining the maximum reversible capacity from this material. We synthesized MoS2 ordered mesoporous with 3-D nanostructure by using KIT-6 template. As shown in Figure 1, the model material exhibited a discharge /charge capacity of 1664 mAhg-1 / 1062 mAhg-1 in the first cycle and reversible discharge / charge capacity of 1357 mAhg-1 / 1343 mAhg-1 after 50thcycles.
We utilized synchrotron radiation based characterization techniques to directly observe the local and bulk structural changes in the well-synthesized mesoporous MoS2 model material during the electrochemical cycling. The oxidation state changes of Mo and S were observed during the 1st irreversible and 2nd reversible electrochemical cycle using XAS and XPS. The in situ XRD experiment was used to confirm the formation of Li2S and/or Mo metal during the reversible reaction. In addition, SAXS was also used to observe the pore structure dynamics during the cycling of mesopourous MoS2 electrode.
More detailed discussion will be presented at the 232nd ECS Meeting

Reference

[1] Tyler Stephenson et al., Energy Environ. Sci., 2014, 7, 209–231
[2] Qiang Wang et al., J. Phys. Chem. C, 2007, 111 (4), pp 1675–1682
[3] Kun Chang et al., Chem. Commun., 2011, 47, 4252-4254.
[4] Shyamal K. Das et al., J. Mater. Chem., 2012, 22, 12988
[5] Debin Kong et al., Energy Environ. Sci., 2014, 7, 209-231
[6] Jaewon Jin et al., Nanoscale, 2015, 7, 11280–11285

Figure 1