Study of Stripping and Deposition of Magnesium in Mg Batteries
Despite the results that Mg is not forming dendrites, we have noticed non homogenous stripping and deposition of Mg in the different electrolyte solutions. Such non homogenous deposition can overgrow the separator, leading to formation of short circuits. To better understand reasons and to control the parameters that are influencing different morphologies of Mg deposition, we have decided to perform a detailed and systematic study how different electrolyte solutions and impurities (particularly H2O) influence process of Mg stripping and deposition.
All the electrochemical measurements were performed in a two electrode beaker cell inside the glove-box (Ar atmosphere). Morphology of Mg foil and nano Mg powder was checked by scanning electron microscope. Solvents with different quantity of water were used for preparation of the electrolyte solutions. We performed two different sets of the electrochemical measurements; cyclic voltammetry and potentiometric cycling. While cyclic voltammetry showed a reversible stripping and deposition process, the potentiometric cycling revealed more information about the efficiency and kinetics of the process. Figure 1a presents Mg stripping and deposition process at a constant current in the first few cycles, showing that the electrodeposition process is taking part at two different potentials. Those two steps can be only observed when the source of Mg is not infinitive (as a positive electrode we typically used copper foil).
To better understand the electrochemical processes occurring during the stripping and the deposition, we performed a controlled experiment where Mg was galvanostatically deposited on the Cu (Cu1) positive electrode (charge used for stripping was 0.5 Ah). Then Mg negative electrode was replaced by a fresh Cu electrode (Cu2) and Mg, deposited on Cu1 electrode, was galvanostatically cycled between two Cu electrodes (Cu1 and Cu2), which were examined under SEM at various stages of Mg deposition. Mg deposits appear at two different particle sizes (Figure 1b) and we found that smaller particles are consumed at the beginning of galvanostatic cycling; for the stripping of larger particles a certain overpotenital is required, which is then observed as a second potential. Working with different current densities we additionally found out that the size of particles depends on the current density, used for galvanostatic experiment. Low current densities are leading to large spherical particles, while with high current densities we obtained numerous small spherical particles. We need to point out that non homogenous electrodeposition of Mg can lead to disintegration of the bigger Mg particles from Mg foil surface and consequently to lower Coulombic efficiency as it was observed from our experiments.
Careful analyses of Mg foil reveal a pitting corrosion on the surface. The reason for pitting corrosion formation has been studied. Our focus was on the water content in the solvent, which was found to have a large effect on the surface density and size of corroded spots. Besides lower Coulombic efficiency, problems related with the corrosion can importantly influence Mg battery behavior.
Although the Mg does not form dendrites, the use of the Mg foil has several restrictions, among them two are exposed in this work. The solution to overcome those can be use of an electrode, prepared from Mg nano powder. Nano Mg powder deposition and stripping follows similar mechanisms as Mg foil, but powder exhibits some improved kinetic properties, which are quite beneficial for overall battery performance and surface deposition seems more uniform than in the case of the foil. Mg foil and nano powder were also tested in the complete Mg battery and we can confirm superiority of Mg nano powder over Mg foil.