Pseudocapacitive Behavior and Ultrafast Kinetics from Solvated Ion Cointercalation into MoS₂ for Its Alkali Ion Storage

The popularization of electric vehicles and the increasing use of electronic devices highlight the importance of fast charging technology. The charging process of lithium secondary battery is basically limited by a series of processes on the anode side, which include desolvation of lithium ions as well as lithium diffusion through SEI and the anode material. These series of reactions are kinetically sluggish, leading to insufficient power density. Therefore, to unravel this problem, we need to either accelerate each step or skip over some of the steps to make the whole charging process shorter. A solvated ion cointercalation into graphite has turned out to successfully exclude both desolvation of lithium ions and SEI film formation to achieve high kinetics with graphite. Herein, the solvated ion cointercalation into MoS2 demonstrated that it can help to remove desolvation of alkali ions as well as SEI formation, and thereby ultrahigh kinetics and long-term cyclability are attained by the characteristic pseudocapacitive behavior irrespective of the charge/discharge mechanism of anode materials. This phenomenon occurred between 1 and 3 V with MoS2 anode in a novel electrolyte (i.e., 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane/tetraglyme (DME/TGM, v/v = 3:1 by volume)). In detail, its capacity retentions slightly decreased from 95.9% to 91.8%, 89.7%, 87.7%, 84.8%, 77.0%, 67.9%, and 55.1% as current densities increased from 0.1 to 0.2, 0.5, 1, 2, 5, 10, and 20 A g–1, respectively. Meanwhile, it delivered a capacity retention of 90.6% even after 2000 cycles at 1 A g–1. Interestingly, MoS2 with solvated ion cointercalation can display much higher capacity (273.5 mAh g–1) than that of graphite (<120 mAh g–1). As a result of the investigation on its reaction mechanism, the solvated ion cointercalation was validated to occur during the discharge process in MoS2 anode and thus induce pseudocapactive sodiation as well as exclude SEI film formation. This research may help to suggest an alternative way to enhance the kinetics of anode materials for high-power alkali ion batteries.