Alkali Cation Additives Assisting Magnesium Cation Intercalation in Hollandite-Type Manganese Dioxide Cathodes

In rechargeable magnesium batteries, Mg(2+ )usually exhibits sluggish kinetics and large overpotentials during intercalation into cathode materials due to the strong Coulomb interaction with the host structures. In this work, we show that introducing alkali ions (Li+, Na+, or K+) to construct dual-cation electrolytes significantly reduces the discharge overpotential and increase the discharge capacity of a hollandite-type alpha-MnO(2 )cathode. Two necessary conditions are found to activate the concerted effects: (i) Mg2+ intercalation is energetically preferred by the cathode materials compared to the alkali cation additives. The topotactic Mg2+ intercalation therefore maintains the main cathode reaction and keeps the battery as a "rocking-chair-type" structure. (ii) The Mg2+ and alkali cations coexist in the host structures without phase separation. The mixing of the dual cations along the diffusion path may allow concerted motion, reducing the energy barrier in solid-state diffusion.