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.