Understanding the chlorine adsorption mechanism on carbon surfaces for Li-Cl battery cathodes

초록

With growing interest in Li-Cl batteries as a next-generation energy storage system, an efficient method for storing Cl at the cathode is required. To elucidate the primary mechanisms of Cl adsorption on carbon surfaces and to propose effective electrode design strategies, a theoretical investigation based on atomistic simulations has been performed. The Cl adsorption is found to be governed by the amount of free Cl generated upon delithiation, the number of surface carbon atoms with dangling bonds that act as adsorption sites, and a minimum separation between adsorbed Cl atoms, which is determined in this study to be similar to 2.8 & Aring;. Physicochemical analysis indicates that this separation originates from electrostatic repulsion between adsorbates, arising from the slight ionic character of the predominantly covalent Cl-C bond, thereby placing the observed distance between the covalent and ionic diameters of Cl. This required minimum distance is identified as the key factor that ultimately determines the Cl adsorption density and the specific capacity of the carbon cathode. Based on this finding, an outlook on cathode energy density is presented, indicating that a carbon specific surface area exceeding 760 m(2)/g for a Li/Cl system and 1500 m(2)/g for a Li/SOCl2 system is necessary for competitiveness. While this assumes efficient utilization of the adsorbent surface, the high migration energy barrier of adsorbed Cl (similar to 1.4 eV) suggests that enabling smooth transport of Cl prior to adsorption is also a critical design consideration.

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