MIT develops ‘hybrid’ cathode for smaller, lighter batteries

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From pv magazine global

A team of researchers at the Massachusetts Institute of Technology (MIT) has created a new cathode for lithium battery cells, which could allow for smaller and lighter lithium batteries for electric vehicles and consumer electronics.

Lithium-sulfur batteries are widely seen as a promising alternative to current lithium-ion batteries relying on transition metals such as cobalt, since sulfur is both lighter and more abundant than these materials. But issues with conductivity and longevity have so far held them back from commercial production.

The battery developed by MIT and partners including the Samsung Advanced Institute of Technology, the National Key Technologies R&D Program of China and the National Science Foundation of China is described in the paper Intercalation-conversion hybrid cathodes enabling Li-S full-cell architectures with jointly superior gravimetric and volumetric energy densities, published in the journal Nature Energy.

The researchers combined two approaches to cathode design. Intercalation types, which are more common commercially, are based on transition metals and work by incorporating lithium atoms into their crystalline structure. They also looked at conversion types, in which sulfur can transform and partially dissolve during operation of the battery.

Using a molybdenum sulfide known as Chevrel Phase, and pure sulfur, the team says its was able to design a battery cell with the best qualities of both approaches – the molybdenum sulfide providing a “backbone” of fast li-ion transport and high conductivity, unlocking the pure sulfur’s high energy storage capacity. “It is like the primer and TNT in an explosive,” explains lead author and MIT professor Ju Li. “One fast acting, and one with higher energy per weight.” Li goes on to point out that the cell has high electrical conductivity compared to other lithium-sulfur battery concepts, and only requires carbon content of around 10%.

MIT says that an initial version of the battery, without optimization, achieved gravimetric energy density of more than 360 Wh/kg, and volumetric energy density of 581 Wh/liter. Li says that with further work and optimization, he believes that the battery could reach 400 Wh/kg and 700 Wh/liter, which is comparable to commercial lithium-ion batteries.

He also notes that the device his team developed is a full three-layer pouch cell, rather than much smaller “coin cell” devices typically used in research applications. And while the battery did not achieve the longevity or number of charge-discharge cycles that would make it commercially viable, Li says that this not down to issues with the cathode, but the overall cell design. “We’re working on that,” he concludes.