Rechargeable lithium-ion batteries are ubiquitous in portable electronics today thanks to their high-energy density, high specific power and long cycle-life. These devices consist of two electrodes – an anode and cathode – separated by an electrolyte. When the battery is being charged with electrical energy, lithium ions move from the cathode through the electrolyte to the anode, where they are absorbed into the bulk of the anode material. One of the main problems in these batteries is lithium dendrite growth on the lithiated graphite anodes in lithium-ion batteries. These dendrites short-circuit the battery because they penetrate the device separator and touch the cathode. This can cause the batteries to catch fire.

Magnesium-based rechargeable batteries are promising alternatives thanks to their high-energy density, better safety (the magnesium metal anode is dendrite-free) and lower cost (magnesium is abundant in the Earth’s crust). Most magnesium batteries made to date, however, suffer from slow diffusion of divalent magnesium cations in the cathode and inefficient electrolytes.

New battery cathode

Researchers led by Yan Yao have now designed a new battery cathode made from magnesium chloride cations in expanded titanium disulphide in which the magnesium chloride cations travel fast. Unlike in previous magnesium-based batteries, the magnesium-chloride bond (which is difficult to break) is not broken. Magnesium ions produced by breaking this bond move extremely slowly through the host, which lowers the battery’s overall efficiency.

The new battery stores energy by instead inserting magnesium monochloride into the expanded titanium disulphide host. By retaining the magnesium-chloride bond, the cathode demonstrates much faster diffusion than traditional magnesium versions, and doubles the charge the cathode can store, explains Yao.

Titanium disulphide inter-layers expanded by 300%

The device has a storage capacity of 400 mAh/g, compared to 100 mAh/g for previous magnesium batteries. Traditional lithium ions batteries, for their part, have a capacity of about 200 mAh/g. The key to the new battery is to expand the distance between the layers in the titanium disulphide structure by 300%, or from 0.57 nm to 1.8 nm (using organic “pillars”). This allows the magnesium chloride to be inserted – or intercalated – without having to break the magnesium-chloride bond.

“We hope this is a general strategy,” says Yao. “By inserting various bulky cations in higher voltage hosts, we eventually aim to create high-energy batteries at a lower price, especially for applications like electric vehicles or stationary storage.”

The new magnesium battery is detailed in Nature Communications doi:10.1038/s41467-017-00431-9.