Magnesium-based batteries that are safer and more powerful could be the next generation energy-storage device. One hurdle to overcome is “speeding up” the rate that Mg ions move in solids. Here, scientists have developed design rules that allow Mg ions to move faster.
Magnesium ions are doubly charged compared to singly charged lithium ions, so these batteries have the potential for storing more energy. In this research, scientists discovered and demonstrated design rules for identifying crystalline solids that magnesium ions can move through quickly—a critical property for faster-charging batteries. The researchers predicted which pathway magnesium ions take in moving through solids. Also, they predicted how to minimize the energy barrier for that movement.
Magnesium is an earth-abundant element, unlike lithium. Also, magnesium-ion batteries could have significantly higher energy density than lithium-ion batteries. However, the poor mobility of magnesium ions was seen as an insurmountable hurdle to creating a commercial magnesium-ion battery. These results show that high magnesium-ion mobility in solids is possible. The study opens the door to a new form of energy storage.
The remarkable mobility of lithium ions in solids has been a key factor in the development of commercial lithium-ion batteries.
A generally perceived obstacle to the development of magnesium-ion batteries is the low mobility observed for the larger, more highly charged magnesium ion. The magnesium ions have two positive charges rather than a single positive charge on the smaller lithium ions. A wide range of solid electrode materials is available for lithium-ion technology, but not magnesium-ion batteries. There have only been a few demonstrations of fully operational magnesium-ion batteries.
A team of researchers from the Joint Center for Energy Storage Research (JCESR) has developed a new set of design rules for better magnesium-ion battery materials. They judiciously tuned the chemistry and crystal structure to enable high magnesium mobility. The team, working at Lawrence Berkeley National Laboratory, calculated the specific local environments that a magnesium ion would have to pass through as it moves within a crystal. Much like a human hiker might choose a pathway through mountainous terrain with a minimal elevation change, the JCESR team focused on materials where there was a pathway for magnesium ions with a minimal change in energy for each step along the pathway.
Flattening the energy profile along the migration pathway in the solid should translate to fast ion diffusion, even for the relatively large, doubly charged magnesium ion. The team applied this approach to the computational screening of a large number of candidate materials for use as a solid electrolyte in magnesium-ion batteries. This design principle guided the team to investigate specific solids, including MgSc2Se4. Synthesis and characterization of this compound resulted in the first demonstration of high magnesium ionic conductivity in a solid at room temperature.
This success in designing a solid magnesium-ion conductor represents an encouraging step towards finding more solids with fast magnesium mobility that can function as electrodes or electrolytes in magnesium-ion batteries.