Magnesium batteries are an important energy storage technology being investigated to improve upon, and move beyond, current lithium-ion (Li-ion) battery systems. Some advantages of magnesium-based batteries include magnesium’s abundance, increased volumetric capacity, and lower reactivity compared to Li. However, Mg batteries have many challenging problems hindering their implementation. One major issue is the slow intercalation kinetics of the Mg²⁺ ion in many metal oxide cathodes conventionally used in Li-ion batteries. A potential solution that has been demonstrated is adding water molecules into organic electrolyte; this method has shown increased Mg insertion and performance of cathode materials such as vanadium oxide (V₂O₅) and manganese oxide (MnO₂). MnO₂ is the cathode of interest for this work, and an in-depth investigation is necessary to fully understand the role of the water molecules in the Mg intercalation into this material.

In this work, the Mg insertion mechanism is being investigated in a system of water-in-organic electrolyte, with electrolytes containing an H₂O to Mg ratio of 6:1. The cathode is an amorphous, electrodeposited MnO₂ thin film. Thin film MnO₂ electrodes were prepared so that they could be electrochemically cycled in magnesium perchlorate/propylene carbonate electrolyte, and subsequently examined via ex-situ XPS at different stages of charge and discharge to monitor the surface chemical speciation and any changes. Herein, we will elucidate upon the XPS results to help explain the mechanism of Mg insertion in this mixed electrolyte. Results demonstrate that an Mg(OH)₂ layer forms upon discharge, and this hydroxide layer is reversibly formed and removed during charge and discharge. Additionally, after first cycling electrodes in wet electrolyte and transferring them to dry organic electrolyte and cycling further, the Mg(OH)₂ layer is still observed although in a diminished capacity.

We propose that in the mixed water-in-electrolyte system, there is a combined insertion/conversion reaction mechanism whereby an Mg(OH)₂ layer is formed on the surface. We therefore believe water inserted into the MnO₂ structure upon cycling in wet electrolyte may be responsible for the Mg(OH)₂ formation in dry electrolyte and continual increased Mg-ion capacity. A more complete understanding of the mechanism for this phenomenon may help in further developing strategies to improve metal oxide cathode performance. For instance, one possibility may be trying to reproduce the positive effect of water with another additive that does not react with the Mg anode as water does.