Tag Archives: Energy Density

Rechargeable Magnesium Batteries – Safer And Cheaper Than Li-ion Batteries

Researchers at the University of Houston reported in the journal Nature Communications the discovery of a new design that significantly improves the development of a battery based on magnesium. Magnesium batteries are considered as safe resources of power supply – unlike traditional lithium-ion batteries. They are not flammable or subject to exploding – but their ability to store energy is very limited. But the latest discovery of the new design for the battery cathode drastically increases the storage capacity.

Energy diagrams for the intercalation and diffusion of Mg2+ and MgCl+
Energy diagrams for the intercalation and diffusion of Mg2+ and MgCl+ in magnesium batteries

In order to make magnesium batteries, the magnesium-chloride bond must be broken before inserting magnesium into the host, and this is very hard to do. Hyun Deog Yoo, the first author of the paper, said,

First of all, it is very difficult to break magnesium-chloride bonds. More than that, magnesium ions produced in that way move extremely slowly in the host. That altogether lowers the battery’s efficiency.

The new battery technology stores energy by inserting magnesium monochloride into titanium disulfide, which acts as a host. By keeping the magnesium-chloride bond intact, the cathode showed much faster diffusion than traditional magnesium batteries.

The researchers managed to achieve a storage capacity density of 400 mAh/g – a quadruple increase compared with 100 mAh/g for earlier magnesium batteries. This achievement even overpowered the 200 mAh/g cathode capacity of commercially available lithium-ion batteries. Yoo, who is also the head investigator with the Texas Center for Superconductivity at UH, confirmed this fact.

The cell voltage of a magnesium cell is only 1V which is significantly less than a lithium-ion battery which has 3.7V cell voltage. Higher cell voltage and high cathode capacity made Li-ion batteries the standard. Li-ion batteries suffer from an internal structural breach, known as dendrite growth what makes them catch fire. Being an earth-abundant material, magnesium is less expensive than lithium and is not prone to dendrite growth.

The magnesium monochloride molecules are too large to be inserted into the titanium disulfide using conventional methods. The trick they developed is to expand the titanium disulfide to allow magnesium chloride to be inserted rather than breaking the magnesium-chloride bonds and inserting the magnesium alone. Retaining the magnesium-chloride bond doubled the charge the cathode could store. Yoo said,

We hope this is a general strategy. Inserting various polyatomic ions in higher voltage hosts, we eventually aim to create higher-energy batteries at a lower price, especially for electric vehicles.

New Batteries with 3 & 15 Times Energy Density

With the rapid growth of battery-based devices and tools, efficient energy storage systems are becoming more and more important. Of course there are many researches running around the world working on novel battery technologies. Two new cell technologies are working to deliver energy density of 3 and 15 times of conventional lithium cells.

The first is a group of scientists from Rice University, they solved the dendrite problem of commercial lithium-ion batteries providing a three times capacity rechargeable lithium metal battery. Dendrites are whiskers of lithium that grow inside batteries, and they can cause fires like those in the Samsung Galaxy Note 7. They are considered a major issue for next-generation lithium batteries.

Lithium metal coats the hybrid graphene and carbon nanotube anode in a battery created at Rice University. The lithium metal coats the three-dimensional structure of the anode and avoids forming dendrites. Courtesy of the Tour Group.
Lithium metal coats the hybrid graphene and carbon nanotube anode in a battery created at Rice University. The lithium metal coats the three-dimensional structure of the anode and avoids forming dendrites. Courtesy of the Tour Group.

The main idea of the research is to coat high conductive hybrid graphene and carbon nanotubes with metallic lithium. These low density and high surface area nanotubes have space for lithium particles to slip in and out as the battery charges and discharges.

“Lithium-ion batteries have changed the world, no doubt, but they’re about as good as they’re going to get. Your cellphone’s battery won’t last any longer until new technology comes along.”  – James Tour, leader of the research team

A prototype of a battery with 3.351 Ah/g capacity, retains 80% of the original capacity after 500 charge cycles.

In Japan, and especially at the NIMS (National Institute for Materials Science), another research is working to create a Lithium-air battery that has the highest theoretical energy density because it uses oxygen in air. Its capacity reaches 15 times of the conventional Lithium batteries.

The electrode material has an enormous surface area thanks to ​​carbon nanotubes. Researchers achieved 30 mAh/cm² capacity in the lab, which will be amazing if realized in a commercial product. Work is ongoing to produce real practical samples with high energy density and a system to filter impurities from the air.

Source: Elektor