Mechanism Study of MoS₂ Electrode Using the Synchrotron-Based X-Ray Analysis

MoS2 has superior electrochemical properties suitable for lithium-ion battery, such as high theoretical capacity based on conversion reaction (670 mAh g-1), low volumetric expansion during electrochemical cycling (only ~103%) and week van der waals forces between layers, which enable facile intercalation/deintercalation.[1] Besides, MoS2 at a much lower price than other anode materials (e.g., sub-micron MoS2 powder retails for dollars per kilogram) has been studied as a substitute for commercialized graphite. Based on these advantages, there were various researches to enhance Li storage performance by morphology control, and as another strategy of mixing with carbon to improve electron/Li-ion conductivity. However, despite the continuous developments in aspects of lithium-ion battery performance, it is noteworthy that the specific mechanism of MoS2 has not yet been demonstrated. After confirmation of 2H to 1T phase transition & lithium intercalation at the beginning part of 1st discharge process and the following conversion reaction at 0.5 V, many researchers have believed that subsequent reactions will follow this reversible intercalation and conversion. However, some recent studies were contradicting the abovementioned electrochemical reaction mechanism. They agreed about the initial intercalation and following conversion reactions in the 1st cycle, but they suggested that the Li+ storage capacity derived from different reaction pathway similar to lithium-sulfur battery reaction (Li2S ↔ 2Li+ + S2-). For example, Fang et al.[2] presented S (222) and Mo (110) metal XRD peak after recharged to 3.0 V as evidence for this newly proposed mechanism. Stephenson et al.[1] observed the coupled sulfur redox reaction in cyclic voltammogram, and Jin et al.[3] adduced the presence of sulfur element at the fully charged state through TEM, which again disproves the conventionally accepted mechanism for MoS2. Also, Sen et al.[4] proceeded XANES and EXAFS study with Ab-initio density functional (DFT) calculations to consider and explain the rationale for Li-S reaction. Despite the various evidences of Li-S like reactions, the majority of researches still believe that reversible conversion between MoS2 is possible. Jiao et al.[5] certified the repeated conversion reaction by comparing the CV curves of metallic and 2H MoS2. Su et al.[6] confirmed the reversible formation of lamellar MoS2 nanograins during charge-discharge processes using in situ TEM analysis. Moreover, there was another suggestion about the existence of Mo and MoS3 upon cycling.[7] However, these previous studies were carried out with synthesized nanostructures or mixed with carbon composites to enhance the electrochemical performance. These manners could alter the inherent nature of MoS2 and hinder the understanding of the intrinsic reaction mechanism. Therefore we performed the mechanism study using commercial MoS2 to eliminate the effect of other morphological or composite effects and by utilizing the synchrotron X-ray analysis, we will discuss the 1st irreversible reaction in brief and be more focused on the reaction mechanism after 2nd cycle which is directly related to the reversible capacity systematically. Reference [1] T. Stephenson et al., Energy Environ. Sci., vol. 7, no. 1, pp. 209–231, 2014. [2] X. Fang et al., Chem. - An Asian J., vol. 7, no. 5, pp. 1013–1017, 2012. [3] J. Jin et al., Nanoscale, vol. 7, no. 26, pp. 11280–11285, 2015. [4] U. K. Sen et al., Nanoscale, vol. 6, no. 17, pp. 10243–10254, 2014. [5] Y. Jiao et al., Adv. Energy Mater., vol. 8, no. 15, pp. 1–9, 2018. [6] Q. Su et al., Sci. Rep., vol. 7, no. 1, pp. 1–10, 2017. [7] F. Zhou et al., Angew. Chemie - Int. Ed., vol. 53, no. 43, pp. 11552–11556, 2014.