A frustrated solution

Chem http://doi.org/cknh (2018)

Magnesium-ion batteries have been purported as one of the candidate technologies to surpass lithium-ion batteries. But with safety concerns less pressing and theoretical energy density about five times that of commercial Li-ion batteries, the most demanding problem in Mg-ion batteries research is the shortage of cathode materials that can reversibly intercalate and de-intercalate magnesium ions. Part of the issue resides in the greater charge-to-radius ratio of Mg²⁺ with respect to Li⁺. When Mg²⁺ ions intercalate they tend to polarize the cathode material, generating strong electrostatic interactions that limit Mg²⁺ diffusion. Andrews et al. have now shown that a metastable phase of V₂O₅ (ζ-V₂O₅) can reversibly intercalate and de-intercalate magnesium ions.

The ζ-V₂O₅ phase is a polymorph with an open tunnel framework that allows Mg²⁺ ions to diffuse within the material. Within this open framework there are several sites that magnesium can coordinate to, but the interactions remain weak at all of these sites. The researchers call this situation ‘frustrated coordination’, and it helps Mg²⁺ diffuse through the channels without needing to overcome large energy barriers. It is also worth noting that magnesation is accompanied by only a small volumetric change of <1%. Using this material, Andrews et al. show a Mg-ion battery with reversible capacity of 90 mAh g^–1 over 100 cycles.