Theoretical Study Of The Effects Of Boron-doped Graphite Cathode On Absorptions Of Alkali Metals

Recently, the battery industry, as an important section of energy industry, is paid more attention and developed rapidly owing to the energy crisis and environmental pollution. Lithium ion battery is the focus of worldwide R&D because of its good safety, the flexibility of shape designing, high capacity, and good cycle life. Improving and enhancing the electrochemical performance of batteries mainly depends on the change-discharge property of negative electrode materials.Boron-substituted carbons materials have been examined from experimental and theoretical viewpoints and achieved much progress.In order to clarify the reason why boron-substituted disordered carbons can store more Li atoms than pristine carbons, we employed several polycyclic hydrocarbons as model clusters for disordered carbons and investigated the effect of boron substitutions on the properties of model clusters by using the ONIOM method. It is found that boron substitution created an electron acceptor level in a lower energy region than that for the pristine carbon. As a result, the Li absorption energy for the boron–substituted cluster was larger than that for the pristine carbon. And with the increase of the concentration of boron, voltage of battery considerably increases in all cases.Secondly, density functional theory is used to study the adsorption of alkali metals on boron–doped graphite models. Five types of adsorption site on single layer boron–substituted cluster are investigated: (1) the center of a ring, (2) the middle of c-c bond, (3) the middle of c-b bond, (4) the on-top of a carbon atom, and (5) the on-top of a boron atom.K and Na adsorptions will be mainly based upon on the center of a ring, the most stable position, and Li adsorptions will be mainly based upon on the"on–top site"directly above a boron atom, the most stable position.In the following discussion, the comparison of K and Na adsorptions will be mainly based upon on the center of a ring, the most stable position for K and Na. The binding energies show that Na is stronger than K in adsorption.The reason is that an alkali metal is normally adsorbed on the basal plane of graphite through the interaction of the SOMO of the metal and the HOMO or LUMO of the boron-substituted carbons materials. One can see that the energy difference (SOMONa- LUMO) is smaller than (SOMOK- LUMO), which may be the key reason why Na has a stronger adsorption than K, although the smaller size of Na atom could also be another reason.