Today's lithium-ion batteries use cathodes (one of the two electrodes in a battery) made of a transition metal oxide. Batteries with cathodes made of sulfur are considered a promising alternative to reduce weight. Designers of lithium-sulfur batteries face a tradeoff. The cathodes of such batteries are usually made in one of two ways: intercalation or conversion. Intercalation types — which use compounds such as lithium cobalt oxide — 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.
The conversion type uses sulfur that gets transformed structurally and is even temporarily dissolved in the electrolyte. Theoretically, these batteries have very good gravimetric energy density 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.
A hybrid cathode combines aspects of two different approaches that have been used before: one to increase the energy output per pound (gravimetric energy density) and the other for the energy per liter (volumetric energy density). The synergistic combination produces a version that provides the benefits of both.
In the new hybrid system, the two approaches are combined 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. Particles of the two materials were compressed to make the solid cathode.
The electrical conductivity of the combined material is relatively high, reducing the need for carbon and lowering the overall volume. Typical sulfur cathodes are made up of 20 to 30 percent carbon; the new version needs only 10 percent carbon.
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 can reach more than 360 watt-hours per kilogram and 581 watt-hours per liter. Researchers have 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 does match its predicted characteristics.