Origin of storage capacity enhancement by replacing univalent ion with multivalent ion for energy storage

The storage capacity or capacitance of a material could be enhanced significantly by replacing univalent ion with multivalent ion in the energy storage field. However, the mechanism of the enhancement is unknown. Here, we dedicate to understand the origin of the enhancement on the storage capacity of multivalent ions over univalent ions. The experimental results show that the specific capacitance and charge-discharge rate of α-MnO₂ are doubled by using Ca²⁺ cation to replace Na⁺ cation in the electrolyte as the energy storage medium. The First-principles calculations are used for a further understanding for the enhancement on the capacity, charge rate and the insertion mechanism. The given number of cations (two Na⁺ or Ca²⁺ ions) can be preferably stabled in one α-MnO₂ unit cell to decrease the irreversible tetragonal-orthorhombic deformation caused by John-Teller effect. Because the insertion of Ca²⁺ triggers double electron transfer than Na⁺, the capacity and charge-discharge rate of α-MnO₂ using Ca²⁺ cation as storage medium are doubled. The result pave a path to understand the enhancement on the storage capacity by replacing the univalent ions (such as Li⁺, Na⁺, K⁺, etc.) with multivalent ions (such as Ca²⁺, Mg²⁺, Zn²⁺, Al³⁺, etc.).