Rechargeable Magnesium-ion Batteries (RMBs) must achieve competitive energy densities to be viable alternatives to lithium-ion batteries in commercial applications. The enhancement of RMB energy densities can be achieved by developing novel cathode materials with higher gravimetric capacities and/or operating voltages. Recent literature has identified borates as potential novel cathode materials due to their ability to deliver high operating voltages, attributed to the polyanion inductive effect.

In this comprehensive study, a wide range of different polyanion materials were synthesised and studied as Mg cathode candidates for RMBs. Three different polyanion groups M₃(BO₃)₂, M₂B₂O₅, M₃BO₅, were synthesised with a variety of different transition metals(TM) namely Ni, Fe, Mn.

A key analysis of the effects of ball milling on the first-charge capacity of the polyanions was undertaken. Polyanion cathodes are known to be more ionically insulating than layered metal oxides, with much improvement of extractable capacity being demonstrated in the literature by ball milling. Reducing cathode particle size through ball milling typically enhances reaction kinetics by increasing the surface area, exposing more electrochemically active sites. Additionally, the shorter diffusion path for ions during deintercalation reduces the likelihood of ions becoming trapped within defects.

The ball milling combined with high-temperature cycling was found to produce significant first-charge capacities in all the studied polyanion systems. This promising capacity, alongside the extended voltage plateaus, suggested the potential viability of these candidate cathode materials for RMBs. This promising electrochemical performance was investigated using a wide range of in-depth post-cycling analyses, such as cutting-edge operando XANES. Comprehensive analysis revealed that demagnesiation from the cathode structure accompanied by TM oxidation was not the source of the observed first charge capacity in the polyanions investigated.

A further effort was put into identifying the origin of the promising first charge capacity common to the borate polyanion systems studied in this work. Through HR-TEM and XPS, FTIR we propose a mechanism associated with a non-faradaic and irreversible reaction of an amorphous surface of the cathode particles. Such a surface composition could arise from the synthesis of these materials. The effect of ball-milling activates and increases the surface area and amorphous nature of these surface coatings, which when exposed to the electrolyte at high temperatures yield the significant capacities observed in this work.

We believe this work will be of interest to the wider battery community, in demonstrating a potential surface activation and capacity-generating effect that may arise when ball milling polyanion cathode materials. Further it is important in the Magnesium battery community to highlight that care should be taken in processing cathode materials before testing them as Mg (de)intercalation compounds. We stress the complexity of cathode material behaviour in RMBs and emphasize the necessity of thorough post-cycling characterization to fully understand these complexities.