Magnesium is a promising candidate as an energy carrier for next-generation batteries. However, if magnesium batteries are to replace lithium-ion batteries, cycle performance and capacity need to be improved. To this end, a research team focused on a new cathode material with a spinel structure. After extensive characterization and electrochemical performance experiments, they discovered a special composition that could open the door for high-performance magnesium rechargeable batteries.

Lithium-ion batteries have been unmatched in terms of overall performance across a wide range of applications, as evidenced in applications ranging from portable electronics to cellular base stations. However, they have some important drawbacks that are hard to ignore. For one thing, lithium is quite expensive, and the fact that it’s mined at breakneck rates doesn’t help either. Furthermore, the energy density of lithium-ion batteries is insufficient to give autonomy to electric vehicles and heavy machinery. These concerns, combined with the fact that batteries are very unsafe when punctured or exposed to high temperatures, have led scientists to look for alternative technologies.

Magnesium rechargeable batteries show great promise for a green future because of their energy density, safety, and cost. But the lack of high-performance cathode materials has hindered their development. Now, a research team has developed a liquid sulfur/sulfide composite cathode that could enable high-rate magnesium batteries. Magnesium rechargeable batteries (MRBs), using high-capacity magnesium metal as the anode material, are promising candidates for next-generation batteries due to their energy density, safety, and cost. However, the lack of high-performance cathode materials hinders their development. Like their Li-ion counterparts, transition metal oxides are the dominant cathode materials in MRBs. However, the slow diffusion of magnesium ions inside the oxide poses a serious problem. To overcome this problem, some researchers have explored sulfur-based materials. However, sulfur-based cathodes for MRBs have severe limitations: low electronic conductivity, slow diffusion of Mg in solid Mg–S compounds, and solubility of polysulfides in electrolytes, which lead to low rate performance and poor cycle performance.

Among the various elements being tested as efficient energy carriers for rechargeable batteries, magnesium (Mg) is a promising candidate. In addition to safety and abundance, Mg also has the potential to achieve higher battery capacity. However, there are some issues that need to be addressed first. These include the low voltage window provided by magnesium ions, and the unreliable cycle performance observed in magnesium battery materials.

To address these issues, a research team led by Professor Yasushi Idemoto, Vice President of Tokyo University of Science in Japan, has been searching for new cathode materials for magnesium batteries. In particular, they have been looking for ways to improve the performance of cathode materials based on MgV (V:vanadium) systems. Fortunately, they have now found the right path to success, as reported in the latest research, published online December 8, 2022, and published January 1, 2023, in the Journal of Electroanalytical Chemistry, Volume 928.

Focusing on the Mg1.33V1.67O4 system, but substituting a certain amount of vanadium with manganese (Mn), the researchers obtained the material with the molecular formula Mg1.33V1.67−xMnxO4, where x ranges from 0.1 to 0.4. Although the system has a high theoretical capacity, more details about its structure, cyclability, and cathode performance need to be analyzed to understand its practical use. The researchers therefore characterized the synthesized cathode material using a variety of standard techniques.

First, they investigated the composition, crystal structure, electron distribution, and particle morphology of the Mg1.33V1.67−xMnxO4 compound using X-ray diffraction and absorption and transmission electron microscopy. Analysis shows that Mg1.33V1.67−xMnxO4 has a spinel structure with a very uniform composition. Next, the researchers performed a series of electrochemical measurements to evaluate the battery performance of Mg1.33V1.67−xMnxO4, using different electrolytes and testing the resulting charge/discharge performance at different temperatures.

Beyond lithium: a promising cathode material for magnesium rechargeable batteries

The team observed high discharge capacities for these cathode materials—especially Mg 1.33 V 1.57 Mn 0.1 O 4—but it also varied significantly with cycle number. To understand why, they analyzed the local structure near the vanadium atoms in the material. “The seemingly particularly stable crystal structure coupled with the substantial charge compensation of vanadium led us to observe the excellent charge-discharge performance of Mg1.33V1.57Mn0.1O4,” said Prof. Idemoto. “Taken together, our results suggest that Mg 1.33 V 1.57 Mn 0.1 O4 may be a good candidate cathode material for magnesium rechargeable batteries.”

Prof. Idemoto is satisfied with the current research results and is hopeful for the future, concluding, “Through future research and development, magnesium batteries may surpass lithium-ion batteries due to the higher energy density of the former.”

In fact, alternative MgV systems may eventually lead to the long-awaited next-generation batteries. Let’s hope that the much-anticipated lithium replacement that will satisfy our need for rechargeable batteries comes to fruition soon!