Experimental and Molecular Simulation Studies on the Hygroscopic Behavior and Mechanism of Lifsi

Abstract

Lithium bis(fluorosulfonyl)imide (LiFSI) is a new type of lithium salt, which has unique effects in improving the cycling ability and energy capacity of lithium batteries. It is also highly safe and environmentally friendly, and currently has the basic conditions for industrialization. The commercially available LiFSI particles have a small particle size and are highly susceptible to water absorption during storage or use. However, there is currently a lack of basic research on its moisture absorption mechanism, thermodynamic properties, and crystallization process, resulting in a lack of theoretical basis for how to optimize its crystallization process to slow down moisture absorption. Based on this, this article systematically studied the moisture absorption process of LiFSI using a combination of experiments and molecular simulations. The results showed that the moisture absorption rate of LiFSI increased with increasing ambient humidity. Under the same humidity, the initial moisture absorption rate of large-grained crystals is 44% lower than that of small-grained crystals. Through molecular simulation, the mechanism of Li+ and FSI- in the moisture absorption process was clarified: the exposed Li+ on the surface of LiFSI crystals easily combines with the O atom of H2O molecules to form a Li+…O coordination structure, while the H atom of H2O molecules forms an O-H…O hydrogen bond with the O atom of FSI- anion, which is the main reason for LiFSI's strong moisture absorption. The average adsorption energy of water molecules on the {1 -1 1} crystal plane is the largest (-101.39 kJ·mol-1), while the {2 -1 0} crystal plane has stronger adsorption sites (-174.94 kJ·mol-1). By adjusting the morphology or particle size of the crystal, the exposure of its high-energy moisture-absorbing surface can be reduced, which can reduce its moisture absorption rate. The above research results provide theoretical guidance for the preparation of large-sized low-hygroscopic lithium salts.

Keywords: Electrolyte, LiFSI, Molecular simulation, Moisture absorption