Doubling up on rechargeable ions

Rechargeable lithium-ion batteries are widely used in portable electronic gadgets but they are limited in terms of energy density and safety for automotive applications. The multivalent magnesium ion holds promise in this context because of its double charge but developers have always assumed that this greater charge would stymie its use as an alternative to lithium ions because it would have greater attraction for other ions in the electrolyte. Now, researchers at Berkeley Lab Molecular Foundry, David Prendergast and Liwen Wan, have carried out computer simulations of magnesium ion batteries, which they say, dispel this long-held misconception.

"The catch for multivalent ions is that their increased charge draws more attention to them they become surrounded in the battery's electrolyte by other oppositely charged ions and solvent molecules which can slow down their motion and create energetic penalties to exiting the electrolyte for the electrodes," explains Prendergast. "However, we found the problem may be less dire than is widely believed." [Wan and Prendergast, J Am Chem Soc, 2014, 136, 14456-14464 DOI: 10.1021/ja505967u]

The team used first-principles molecular dynamics simulations to show that the magnesium(II) ions coordinate to only four nearest neighbors in a dichloro-complex electrolytes using tetrahydrofuran solvents rather than six as was previously assumed. The simulations are supported by data from X-ray absorption experiments on magnesium chloride and other magnesium salts. A lower degree of coordination means that the magnesium ion should be able to move more freely through the battery's liquid electrolyte than a six-coordinate species.

"This is good news for magnesium-based batteries, since it means that there are less species to carry around and shed as the battery undergoes discharging or charging," Prendergast explains. "Our findings also suggest that the performance bottlenecks experienced with magnesium-ion batteries to date may not be so much related to the electrolyte itself, but to what happens at the interface between the electrolyte and electrodes as the magnesium ions shed their coordination spheres."

The team's observations suggest that avoiding high concentrations might preclude precipitation of magnesium chloride salt within a battery especially in the proximity of the electrodes. This might be taken into account in Mg-ion battery design.

"Our next step is to look at what happens at the electrolyte-electrode interfaces during charging and discharging cycles, in particular, how efficient is the magnesium desolvation process as it approaches the interface and will there be any precipitation occurring at the interface," Wan told Materials Today.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".