Lightweight Polyanion Frameworks for Li- and Mg-Ion Batteries

Abstract

Mg-ion batteries are a promising “beyond Li-ion” technology as they offer high theoretical capacities, use cheap and abundant materials, and Mg metal can be used as the anode.1 However, the search for high voltage cathodes has been hindered by electrolyte incompatibility and sluggish kinetics which can impede intercalation and charging rates.2 MgMB2O5 (M = Mn, Fe, Co) are shown to be high voltage, high capacity cathode materials for both Li- and Mg-ion batteries. The open structure facilitates ion diffusion while the light weight polyanion framework increases capacity and voltage. The materials are produced from metal oxalate and boric acid starting materials via a solid state synthesis process. The Mg:M ratio can be altered to manipulate capacity and ordering of the cations.Mg can be fully removed from MgMnB2O5 giving a capacity of up to 300 mAhg-1 while maintaining the structure. Removal of Mg is confirmed by refinement of powder neutron diffraction data. In a full Mg-ion cell a reversible discharge capacity of ~100 mAhg-1 has been achieved. However, due to the incompatibility of the cell with available electrolytes this is accompanied by irreversible side reactions on charge. Some promising resolutions to the electrolyte problem are discussed. Mg can also be removed from the structure when M = Co or Fe, with the change in transition metal leading to a change in operating voltage. The results presented show that this class of material is a promising candidate for a high capacity, high voltage Mg-ion cathode material.In a Li-ion battery 1.5 Li can be intercalated into the manganese based material after Mg has been removed, giving a specific capacity of up to 250 mAhg-1 above 1.5 V. The capacity is retained over hundreds of cycles with little drop off observed even at rates of 5C. Ex-situ XRD shows a doubling of the unit cell when the material is cycled, suggesting charge ordering is occurring. Neutron diffraction refinements show a change in volume of ~2%, associated with insertion or removal of Li ions.