Theoretical Analysis on De-Solvation of Lithium, Sodium, and Magnesium Cations to Organic Electrolyte Solvents

De-solvation of a Li ion at an electrode/electrolyte interface can be the rate-determining step of the reaction in lithium-ion secondary batteries. The present study theoretically evaluates the de-solvation energies of Li, Na, and Mg ions to organic electrolyte solvents. The Na-ion complexes revealed commonly smaller de-solvation energies compared to the Li-ion complexes due to the weaker Lewis acidity, while the solvation structures were similar to each other. The Mg-ion complexes showed remarkably larger de-solvation energies because of the double positive charge. The increase of coordination number, which was associated with the change in the solvation structure, was observed for the Mg-ion complexes. Detailed analysis revealed good correlations between the de-solvation Lithium-ion secondary batteries (LIBs) have been widely used as the power sources for portable electronic devices due to their high energy density and long cycle life. Recently, LIBs have also attracted much attention for the use in large electronic devices, such as electric vehicles. Large-scale LIBs in such applications need not only high energy density but also high power density (i.e., rapid charge and discharge). Toward rapid charge/discharge battery reactions, the kinetics of ion transfer is important, namely, diffusivity of Li ion in the ac- tive materials, ion transport in the electrodes/electrolytes, and ion transfer at the electrode/electrolyte interface. In the case of the bat- teries, in which thin film-type electrodes or fine powder-type active materials and thin electrolyte phase with sufficiently high ionic con- ductivity are employed, Li-ion transfer at the electrode/electrolyte interface can be the rate-determining step. Ogumi, Abe, and their coworkers 1-5 experimentally investigated the kinetics of Li-ion trans- fer at a solid/liquid interface using various organic solvents such as ethylene carbonate (EC), diethyl carbonate (DEC), propylene car- bonate (PC), and dimethyl sulfoxide (DMSO). The experimental activation barriers for the interfacial Li-ion transfer showed good correlation with de-solvation energies of Li ion from the last sol- vent molecule evaluated by the density functional theory (DFT) calculations, indicating that de-solvation of Li ion is the rate- determining step of interfacial Li-ion transfer between electrode and electrolyte. To the best of our knowledge, however, systematic investigations on the de-solvation energies of Li ion with a wide variety of solvent specieshavenotbeenreportedsofar.Furthermore,exploringtheissue in Na and Mg ions is of great interest with an eye to more advanced future battery systems. Na-ion secondary batteries (SIBs) attract great interest as an alternative to LIBs, since LIBs are facing the problems of dwindling resources and rising cost. Mg-ion batteries (MIBs) are also under development by many researchers because of the potential of much higher energy density based on a bivalent charge-discharge reaction. The present theoretical study evaluated the de-solvation energies of Li, Na, and Mg ions to 27 organic electrolyte solvents. The depen- dences of de-solvation energies on chemical species of solvents were analyzed with respect to electronic states, geometric structures, and chemical indexes.