Researchers in the U.S. and in China say they have created a new version of a key component for lithium-ion batteries, the cathode.
The team said their concept combines aspects of two different approaches that have been used before, one to increase the energy output per pound (gravimetric energy density), the other for the energy per liter (volumetric energy density).
The work is described in the journal Nature Energy, in a paper by Ju Li, an MIT professor of nuclear science and engineering and of materials science and engineering; Weijiang Xue, an MIT postdoc; and 13 others.
Lithium-ion batteries tend to use cathodes (one of the two electrodes in a battery) made of a transition metal oxide. But batteries with cathodes made of sulfur are considered a promising alternative to reduce weight.
The cathodes are usually made in one of two ways, known as intercalation types or conversion types. Intercalation types, which use compounds such as lithium cobalt oxide, provide a high volumetric energy density. That means they pack a lot of punch per volume because of their high densities. These cathodes can maintain their structure and dimensions while incorporating lithium atoms into their crystalline structure.
The other cathode approach, called the conversion type, uses sulfur that is transformed structurally and is even temporarily dissolved in the electrolyte. Theoretically, these [batteries] have excellent gravimetric energy density, the researchers said. But the volumetric density is low, in part because they tend to require extra materials, including an excess of electrolyte and carbon, used to provide conductivity.
In their hybrid system, the researchers combine the two approaches into a new cathode that incorporates both a type of molybdenum sulfide called Chevrel-phase and pure sulfur, which together appear to provide the best aspects of both. They used particles of the two materials and compressed them to make the solid cathode.
Among other advantages, the electrical conductivity of the combined material is relatively high. The researchers said that this reduces the need for carbon and lowers the overall volume. Typical sulfur cathodes are made up of 20-30% carbon, but the new version needs around 10% carbon.
The net effect of using the new material may be substantial, the researchers said. Today's commercial lithium-ion batteries can have energy densities of about 250 watt-hours per kilogram and 700 watt-hours per liter. Lithium-sulfur batteries top out at about 400 watt-hours per kilogram and 400 watt-hours per liter.
A promising design
The new version (in its initial version that has not yet gone through an optimization process) can reach more than 360 watt-hours per kilogram and 581 watt-hours per liter. It can beat both lithium-ion and lithium-sulfur batteries in terms of the combination of these energy densities.
With further work, the researchers said they think they can get to 400 watt-hours per kilogram and 700 watt-hours per liter, with the latter figure roughly equaling that of lithium-ion. The team has produced a three-layer pouch cell (a standard subunit in batteries for products such as electric vehicles) with a capacity of more than 1,000 milliamp-hours. This is comparable to some commercial batteries, indicating that the new device likely does match its predicted characteristics.
So far, the new cell can't quite live up to the longevity of lithium-ion batteries in terms of the number of charge-discharge cycles it can go through before losing too much power to be useful. But that limitation is "not the cathode's problem"; it has to do with the overall cell design, and "we're working on that," one researcher said.
The work was supported by the Samsung Advanced Institute of Technology, the National Key Technologies R&D Program of China, the National Science Foundation of China and MIT's Department of Materials Science and Engineering. The team also included professor Jing Kong and others at MIT, as well as researchers at the Chinese Academy of Sciences in Beijing, the Songshan Lake Materials Laboratory in Guangdong, China, the Samsung Advanced Institute of Technology America in Burlington, Massachusetts, and Tongji University in Shanghai.
