Understanding the Good Kinetics of Mo6S8 As Cathode in Mg Ion Batteries By Key Electronic States

Abstract

Mg rechargeable batteries are being considered as one of the most prospective alternatives for Li rechargeable batteries because of their merits including Mg abundance, absence of Mg dendrite and high theoretical specific capacity attributed to two-electron charge transfer reaction process. [1] However，the achievable performance of Mg rechargeable batteries is still limited regarding their practical application. The key limitation to this lies in cathode. To date, Chevrel phase Mo6(SxSe1-x)8 is the only cathode material in which Mg2+ can be efficiently intercalated/deintercalated. Extensive theoretical and experimental studies have been done to screen out promising cathode materials by constructing the crystal structure similar to Chevrel phase. However, successful research has been rarely reported. It indicates that only taking into account of the crystal structure is not sufficient enough to explore the Mg storage/transport behavior in the electrode. Given the facts that the two-electron charge transfer process was involved in Mg batteries while the great success has been made in the studies on lithium ion batteries through the key electronic states [2-5], the evolution of electronic structure in Chevrel phase might provide more important information to elucidate the physical nature of Mg storage/transport in the electrode. However, the experimental study on the evolution of electronic structure in Chevrel phase cathode is rare. In this report, the study on the unoccupied states of a typical Chevrel phase Mo6S8 including Mo L-edge and S K-edge were conducted by tender X-ray absorption spectroscopy. In contrast with the conventional understanding that only Mo cation is involved in the charge transfer process, the key electronic states at the pre-edge of S spectra were also found to be evolved regularly with different states of charge/discharge. It is assigned to the hybridization between Mo 4d states and S 3p states. The evolution of this state opens a gate to reveal the nature of good kinetics in Chevrel phase. 1. Aurbach, D., et al., Prototype systems for rechargeable magnesium batteries. Nature, 2000. 407(6805): p.724-727.2. Liu, G., et al., Polymers with Tailored Electronic Structure for High Capacity Lithium Battery Electrodes. Advanced Materials, 2011. 23(40): p. 4679-4683.3. Liu, X., et al., Phase Transformation and Lithiation Effect on Electronic Structure of LixFePO4: An In-Depth Study by Soft X-ray and Simulations. Journal of the American Chemical Society, 2012. 134(33): p. 13708-13715.4. Yang, W., et al., Key electronic states in lithium battery materials probed by soft X-ray spectroscopy. Journal of Electron Spectroscopy and Related Phenomena, 2013. 190, Part A(0): p. 64-74.5. Liu, X., W. Yang, and Z. Liu, Recent Progress on Synchrotron-Based In Situ Soft X-ray Spectroscopy for Energy Materials. Advanced Materials, 2014: 7710-7729.