Researchers at MIT and in China are proposing a new class of dense intercalation-conversion hybrid cathodes by combining intercalation-type Mo₆S₈ with conversion-type sulfur (HMSC) to realize a Li–S full cell.

In a paper in Nature Energy, the researchers report that the mechanically hard Mo₆S₈ with fast Li-ion transport ability, high electronic conductivity, active capacity contribution and high affinity for lithium polysulfides is an ideal backbone to immobilize the sulfur species and unlock their high gravimetric capacity.

A pouch cell assembled based on the hybrid cathode and a 2× excess Li metal anode is able to simultaneously deliver a gravimetric energy density of 366 Wh kg⁻¹ and a volumetric energy density of 581 Wh l⁻¹.

Design strategy for jointly high gravimetric–volumetric energy density. The relationship between cathode porosity and predicted cathode-specific volumetric energy density e_v (a) and gravimetric energy density e_g (b) of the hybrid Mo₆S₈/S₈ cathode with all carbon included (HMSC) and C/S₈ cathode with different C/S₈ and Mo₆S₈/S₈ ratios. Xue et al.

Anion-redox lithium–sulfur (Li–S) is one of the most promising conversion battery chemistries with high theoretical cathode energy density of 2,600 Wh kg-1 based on the weight of Li₂S, S₈ + 16^e− + 16Li⁺ = 8Li₂S, several times higher than conventional lithium-ion battery (LIB) cathodes based on transition metal cation–redox intercalation reactions.

Unfortunately, the actual full-cell energy densities are a far cry from the theoretical values resulting from the excessive use of inactive components, such as electrolyte and conductive carbon. The electronic insulating nature of the S₈ and Li₂S phases (as compared to, say, Li_xCoO₂, with its high Co³⁺ ↔ Co⁴⁺ polaron mobility) means that for the anion-redox reaction S^α−↔S^β−+(α−β)e− to proceed (where 0≤α,β≤2 is the average sulfur valence reflecting a mix of ionic and covalent bonding, often in the physical form of S_n²⁻, where α=2/n), the S₈ must physically dissolve into liquid electrolytes as Sn2− (electrolyte), transform into S_m²⁻ (electrolyte) and eventually redeposit somewhere else as solid phases.

… to reach high full-cell E_g and E_v [gravimetric and volumetric energy density], which is much more meaningful for pushing Li–S batteries into commercial development, Li–S batteries must work under rigorous conditions including high active material loading, lean electrolyte and low cathode porosity. Until now, however, few reports have been devoted to improving the joint E_g–E_v energy densities by decreasing the inactive components in sulfur cathodes.

… our strategy is to design an intercalation-conversion hybrid cathode material by introducing electrochemically active Chevrel-phase Mo₆S₈ with fast lithium intercalation reactions and high tap density to hybridize with S₈.

—Xue et al.

Intercalation type cathodes, which use compounds such as lithium cobalt oxide, provide a high volumetric energy density. These cathodes can maintain their structure and dimensions while incorporating lithium atoms into their crystalline structure.

Conversion type cathodes use sulfur that gets transformed structurally and is even temporarily dissolved in the electrolyte.

Theoretically, these Li-S 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, says MIT Professor Ju Li, co-corresponding author of the paper.

In the new 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, thus reducing the need for carbon and lowering the overall volume, Li says. Typical sulfur cathodes are made up of 20 to 30 percent carbon, he says, but the new version needs only 10 percent carbon.

The net effect of using the new material is substantial. 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—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.

Already, the team has gone a step further than many laboratory experiments aimed at developing a large-scale battery prototype: Instead of testing small coin cells with capacities of only several milliamp-hours, they 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.

So far, the new cell can’t 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,” Li says.

Even in its present early form, he says, this may be useful for some niche applications, such as a drone with long range, where both weight and volume matter more than longevity.

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.

Resources

• Weijiang Xue, Zhe Shi, Liumin Suo, Chao Wang, Ziqiang Wang, Haozhe Wang, Kang Pyo So, Andrea Maurano, Daiwei Yu, Yuming Chen, Long Qie, Zhi Zhu, Guiyin Xu, Jing Kong & Ju Li (2019) “Intercalation-conversion hybrid cathodes enabling Li–S full-cell architectures with jointly superior gravimetric and volumetric energy densities” Nature Energy doi: 10.1038/s41560-019-0351-0