New cathode material for rechargeable magnesium batteries

Magnesium is a promising candidate as an energy carrier for next-generation batteries; however, the cycling performance and capacity of magnesium batteries need to improve if they are to replace lithium-ion batteries. To this end, a research team has analysed a novel cathode material with a spinel structure, Mg_1.33V_1.67−xMn_xO₄. Following extensive characterisation and electrochemical performance experiments, the researchers have found a composition that could open doors to high-performance magnesium rechargeable batteries.

Lithium-ion batteries are unrivalled in terms of overall performance for a range of applications. However, they do have some notable disadvantages, as lithium is expensive and is being mined at an extreme pace. The energy density of lithium-ion batteries is not enough to grant autonomy to electric vehicles and heavy machinery. These concerns, coupled with the fact that the batteries are unsafe when punctured or at high temperatures, have caused scientists to look for alternative technologies.

Among the various elements being tested as efficient energy carriers for rechargeable batteries, magnesium (Mg) is a promising candidate. Apart from its safety and abundance, Mg could help realise higher battery capacities. However, some problems need to be solved first, such as the low voltage window that Mg ions provide, as well as the unreliable cycling performance observed in Mg battery materials.

To solve these issues, a research team led by Vice President and Professor Yasushi Idemoto from Tokyo University of Science, Japan, has been searching for ways to improve the performance of cathode materials for Mg batteries, based on the MgV (V: vanadium) system. As reported in the Journal of Electroanalytical Chemistry, they have now found a solution. The researchers focused on the Mg_1.33V_1.67O₄ system but substituted some amount of vanadium with manganese (Mn), obtaining materials with the formula Mg_1.33V_1.67-xMn_xO₄, where x goes from 0.1 to 0.4. While this system offered high theoretical capacity, more details about its structure, cyclability and cathode performance needed to be analysed to understand its practical utility. The researchers characterised the synthesised cathode materials using a variety of standard techniques.

Electrochemical properties and crystal and electronic structure changes during charge/discharge of spinel type cathode-materials Mg_1.33V_1.67-xMn_xO₄ for magnesium secondary batteries. Image credit: Idemoto et al.

They studied the composition, crystal structure, electron distribution and particle morphologies of Mg_1.33V_1.67-xMn_xO₄ compounds using X-ray diffraction and absorption, as well as transmission electron microscopy. The analyses showed that Mg_1.33V_1.67-xMn_xO₄ has a spinel structure with a uniform composition. Next, the researchers conducted a series of electrochemical measurements to evaluate the battery performance of Mg_1.33V_1.67-xMn_xO₄, using different electrolytes and testing the resulting charge/discharge properties at various temperatures. They found a high discharge capacity for these cathode materials — especially Mg_1.33V_1.67-xMn_xO₄ — but it also varied significantly depending on the cycle number. To understand why, they analysed the local structure near the vanadium atoms in the material.

Idemoto said that the particularly stable crystal structure along with a large amount of charge compensation by vanadium leads to the superior charge-discharge properties the researchers observed for Mg_1.33V_1.57Mn_0.1O₄.

“Taken together, our results indicate that Mg_1.33V_1.57Mn_0.1O₄ could be a good candidate cathode material for magnesium rechargeable batteries,” Idemoto said.

According to Idemoto, magnesium batteries could surpass lithium-ion batteries with more research and development, thanks to the former’s higher energy density.

Top image credit: iStock.com/MF3d