Towards cheaper and better EV batteries: Sulphide or Magnesium?

Latest research in alternatives to a conventional lithium-ion battery focuses on components made from magnesium and sulphide. Laboratory results indicate that the abundance and chemical properties of these materials could help bring a cheaper battery with more than 300% higher energy density to market in the not so distant future.

The cost and performance of electric car batteries are considered by many as the major barrier to faster uptake of EVs. Intensive research in the field of battery chemistry points to some promising alternative solutions. Although the commercialisation of such novel batteries is years away, laboratory tests indicate that both cost reduction and power density enhancement are in the horizon.

More than 3 times higher energy storage capacity with lithium-sulphide

Although with promisingly high theoretical energy storage capacity, a battery that uses lithium metal in one electrode and sulphur in the other has one major issue: high reactivity of lithium metal with water causes short circuits and fires. Latest research focuses on a similar battery that doesn't require lithium metal. The work could lead to commercial batteries that store more than three times as much energy as the lithium-ion batteries currently used in electric vehicles.

In the new type of battery, the sulphur electrode is replaced with a lithium-sulphide material. This becomes the source of the lithium, so the lithium metal is no longer required and can be replaced with graphite. The problem is that use of lithium sulphide slows down the charging and reduces energy storage.

But two recent papers introduce ways to make lithium-sulphide batteries more practical.

Research of Yi Cui, a materials science professor at Stanford, focused on charging the battery at a higher voltage than usual for its first charge. This changes the chemistry of the electrode and addresses the conductivity problem of lithium-sulphide batteries. Even then, the lithium sulphide had to be mixed with carbon to improve its conductivity.

Yuegang Zhang, a staff scientist at Lawrence Berkeley National Laboratory, demonstrated a new way to mix the carbon with the lithium sulphide that greatly reduces the amount of carbon needed in the cathode. This improvement could nearly double the overall battery storage capacity, from 350 to 610 Wh/kg. (Lithium-ion batteries in electric vehicles now typically store less than 200 Wh/kg).

Toyota developing magnesium-ion batteries

In November 2012, researchers at the Toyota Research Institute of North America (TRINA) in Michigan published a paper in the journal Chemical Communications that describes experiments involving a magnesium-ion battery with a new kind of anode, made of tin, and the same type of electrolytes used in lithium-ion batteries. According to lead author of the paper, Nikhilendra Singh, the latest tests showed most promising performance and open the path for further research.

In an article published by MIT, Yuyan Shao, a senior scientist at the Pacific Northwest National Laboratory explains two primary areas of research. One is focused on making batteries with magnesium metal anode. This type of anode transfers charge efficiently but is incompatible with conventional electrolytes.

Another potential solution is to use a different anode material that works with conventional electrolytes. Toyota’s paper demonstrates that that it is worth further research, specifically aimed at finding a high-capacity, high-voltage cathode.

Magnesium is an abundant material and its ions have a positive charge of two. Although the chemistry involved in making a magnesium-ion battery work efficiently has yet to be perfected, use of magnesium instead of lithium could someday offer a cheaper and more energy-dense alternative.