Post Li-ion batteries using multivalent ions, such as Mg²⁺, Ca²⁺, Zn²⁺, have attracted much attention in recent years, but developing appropriate intercalation cathode materials for these multivalent cations remains a difficult task due to the sluggish solid-phase diffusion behavior, which is usually caused by the strong coulomb interaction between multivalent cations with the ionic cathode hosts. As an attempt to solve this problem, we proposed a new battery technology, which is referred to as Li-Mg dual-cation batteries.[1-2] The Li-Mg dual-cation batteries possess a so-called “rocking-chair”-type structure, where Li ion and Mg ion are involved in each half-cell reaction simultaneously.

When exploring the characteristics of these batteries, we found an interesting phenomenon that Mg intercalation usually shows higher discharge potential in Li-Mg dual-cation electrolyte than in Mg single-cation electrolyte. The increase of discharge potential (i.e. decrease of the overpotential for Mg intercalation) strongly suggests that the co-existing Li ion is helpful to promote the Mg intercalation with cathode hosts. To elucidate what causes this phenomenon, we performed the first-principles calculations[3-4] with the Nudged Elastic Band (NEB) method[5] to estimate the activation energy of Mg cation migration in cathode hosts that are discharged in Li or Mg single-cation and Li-Mg dual-cation electrolytes and finally revealed a facilitating mechanism for the solid-phase diffusion of Mg cation.[6]

In this presentation, we give a comprehensive explanation on the co-intercalation behavior of Li ion and Mg ion in Li-Mg dual-cation batteries, especially from a thermodynamic perspective. This work advances the fundamental understanding of ionic conduction phenomena of multivalent cations, which is indispensable for developing future energy storage devices.

[1] T. Ichitsubo, S. Okamoto, T. Kawaguchi, Y, Kumagai, F. Oba, S. Yagi, N. Goto, T, Doi, E. Matsubara, J. Mater. Chem. A 3, 10188(2015).

[2] H. Li, T. Ichitsubo, S. Yagi, E. Matsubara, J. Mater. Chem. A 5, 3534(2017).

[3] P. E. Blöchl, Phys. Rev. B 50, 17953(1994).

[4] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77, 3865(1996).

[5] G. Henkelman, B. P. Uberuaga, H. Jónsson, J. Chem. Phys. 113, 9901(2000).

[6] H. Li, N. L. Okamoto, T. Hatakeyama, Y, Kumagai, F. Oba, T. Ichitsubo, Adv. Energy Mater. 8, 1801475(2018).