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Molybdenum-Sulfur Cathode Shows Promise

Molybdenum-Sulfur Cathode Shows Promise
Developing a new “hybrid” cathode from molybdenum and sulfur may increase both gravimetric and volumetric energy density in future lithium ion batteries.

Lithium ion batteries are made up of three major parts. The cathode is the positive electrode that stores lithium ions during discharge. The anode is the negative electrode where the lithium ions are stored during the charging cycle. The third component is the electrolyte, which allows the lithium ions to travel freely between the two electrodes. In commercial lithium ion battery cells, the anode is typically made from layers of carbon graphite, which can hold lithium ions in a process called intercalation. The electrolyte is typically an organic liquid materials, and the cathode is made up from a transition metal oxide.

All three components of lithium ion batteries are subject to intense research to find ways to increase not only the energy output per pound (gravimetric energy density), but also the energy per liter (volumetric energy density). These two goals are often difficult to attain at the same time.

Molecular diagram shows the structure of molybdenum sulfide, one of the materials used to create the new kind of cathode for lithium-sulfur batteries. (Image source: MIT)

Creating a Hybrid

A recent news release from MIT, describes work conducted by researchers at MIT and in China to develop a new version of the cathode. In the release, the team describes their concept as a “hybrid” cathode. That is because it combines aspects of two different approaches to increase both gravimetric energy density and volumetric energy density. The new cathode incorporates both a type of molybdenum sulfide called Chevrel-phase, and pure sulfur, and the results suggest in both gravimetric and volumetric energy densities can be achieved.

In a typical lithium-ion battery the cathodes is made of a transition metal oxide, such as lithium cobalt oxide, and known as an intercalation type. According to the MIT news release, intercalation cathodes can “… provide a high volumetric energy density — packing 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.”

There is another type of cathode, called the conversion type, using sulfur that gets transformed structurally and that can even temporarily become dissolved in the electrolyte. “Theoretically, these [batteries] have very good gravimetric energy density,” said Ju Li, an MIT professor of nuclear science and engineering, and of materials science and engineering. “But the volumetric density is low,” partly because they tend to require a lot of extra materials, including an excess of electrolyte and carbon, used to provide conductivity,” he added in the news release.

In their new hybrid system, the researchers have managed to combine the two approaches into a new cathode which appears to provide improvements in both energy densities. They used particles of molybdenum sulfide and pure sulfur and compressed them to make the solid cathode. “It is like the primer and TNT in an explosive, one fast-acting, and one with higher energy per weight,” Li says.

Encouraging Results

The electrical conductivity of the combined material is also relatively high, which reduces the need for carbon, usually required to provide conductivity. Typical sulfur cathodes are made up of 20 to 30 percent carbon, Li says, but the new version needs only 10 percent carbon.

The new hybrid cathode provides encouraging results. According to the news release, “Today’s commercial lithium-ion batteries can have energy densities of about 250 watt-hours per kilogram and 700 watt-hours per liter, whereas lithium-sulfur batteries top out at about 400 watt-hours per kilogram but only 400 watt-hours per liter. The new version, in its initial version that has not yet gone through an optimization process, can already reach more than 360 watt-hours per kilogram and 581 watt-hours per liter, Li says.  It can beat both lithium-ion and lithium-sulfur batteries in terms of the combination of these energy densities.”

With further work, Li says, “We think we can get to 400 watt-hours per kilogram and 700 watt-hours per liter,” with that latter figure equaling that of lithium-ion. One reason the results appear promising is the manner in which the research has been undertaken. Instead of testing small coin cells with capacities of only several milliamp-hours, the team have produced three-layer pouch cells with a capacity of more than 1,000 milliamp-hours. By building test cells that are comparable to some commercial batteries such as those used in electric vehicles, they are able to better evaluate the performance of their new hybrid cathode. Further work will include improving the number of charge-discharge cycles the battery can go through before losing too much power. Li says it’s, “…not the cathode’s problem”; it has to do with the overall cell design, and we’re working on that.”

Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.

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