Concomitant Intercalation Behavior of Li-Mg Dual Cations into Mo6S8
Authors: Li Hongyi , Yagi Shunsuke , Ichitsubo Tetsu 
Published: February 2020

There is a growing demand for high energy-density rechargeable batteries to construct high performance electric vehicles and renewable energy storage systems. As a candidate of post Li-ion batteries (LIBs), Mg rechargeable batteries (MRBs) have a great potential to attain considerable energy density higher than current commercial LIBs. This is mainly because Mg shows a non-dendritic growth behavior during charging. This feature allows Mg metal to be used as an anode material, which is in a sharp contrast to Li metal anode showing the dangerous dendritic growth. However, during developing intercalation-type cathode materials for MRBs, we inevitably face great difficulties as follows. The divalent Mg cations show an inherent large coulombic interaction with host materials; there are only a few materials can allow the intercalation of Mg ions. Although the operating potential is relatively low (about 1.1 V vs. Mg), the Chevrel compound Mo6S8 is the unique material that Mg ions can be intercalated reversibly at room temperature with an excellent cyclability [1]. Even in such excellent MRB cathode materials, Mg ions show a sluggish diffusion behavior and large overpotential inevitably occurs at a certain level of current value.
The sluggish diffusion behavior of Mg cations is seemingly insurmountable unless operating temperature is elevated (~100 ℃). However, while we have been investigating the feasibility of rocking-chair type Li-Mg dual-salt batteries [2], we have found an interesting phenomenon that intercalation of Mg ions into Mo6S8 can be accelerated by concomitant intercalation with Li ions in a Li-Mg dual-salt electrolyte, LiTFSA-Mg(TFSA)2/triglyme(G3), even at room temperature. After fully discharging the Mo6S8 cathode in electrolyte with the same concentration (0.5 mol L-1) of Li and Mg salts, almost the same amount of Li and Mg ions are intercalated, amounting to the full capacity of Mo6S8 (~120 mAh g-1). In addition, Li and Mg ions show a cooperative intercalation behavior in Li-Mg dual-salt electrolyte. During discharge, Li ions are firstly intercalated and occupy the inner sites of Mo6S8 cathode, then subsequently intercalated Mg ions exchange the sites with the pre-intercalated Li ions. Further importantly, the intercalation potential of Mg in the dual-salt electrolyte is found to be higher than that in the single Mg-salt electrolyte. Thus, with the aid of Li ions, the diffusion of Mg ions seems to be accelerated, which would reduce the overpotential significantly. Similar experiments were also carried out in a Cl-contained Li-Mg dual-salt electrolyte, LiHMDS-Mg(HMDS)2-AlCl3/G3. In contrast, the cooperative intercalation behavior is not observed; Li ions were preferentially intercalated into Mo6S8 in the whole discharge process. This result is consistent with the previous studies [3,4] on Daniell-type Li-Mg dual-salt batteries, in which Mo6S8 was used as cathodes and Cl-contained all-phenyl complex (APC) electrolyte was employed as the base of Li-Mg dual-salt electrolytes. The reason why the same phenomenon cannot be observed in these Cl-contained electrolytes would be attributed to the different coordination state of Mg ions in these electrolytes.
Thus, the cooperative intercalation behavior of Li and Mg ions indicates that the inherent sluggish diffusion of Mg ions in solid can be improved by concomitant intercalation with rapid-diffusion Li ions. Furthermore, the coordination state of Mg ions in electrolytes is, of course, suggested to be an important factor for the intercalation of Mg ions.
References:
[1] D. Aurbach et al., Nature, 407, 724–727(2000)
[2] H. Li et al., J. Mater. Chem. A, 5, 3534-3540(2017)
[3] Y. Cheng et al., Chem. Commun., 50, 9644–9646(2014)
[4] J. H. Cho et al., J. Am. Chem. Soc., 136, 16116–16119(2014)