The energy densities of lithium-sulfur batteries are at least twice as much as that of their rival Li-ion batteries of the same weight. Therefore, scientists are highly interested in developing rechargeable lithium-sulfur batteries.

There is a common by-product of papermaking industry: lignosulfonate. Researchers from Rensselaer Polytechnic Institute (RPI) use lignosulfonate to make low-cost electrode materials for lithium-sulfur batteries. The research team invented a prototype of lithium-sulfur battery for a watch. They will scale up the prototype in the next stage.

With risen temperatures, when sulfur is combined with carbon, sulfur is highly conductive. Thus, it is highly anticipated in new battery technologies.

A rechargeable battery mainly consists of two electrodes, a liquid electrolyte that is placed in between the two electrodes, and a separator. A lithium-sulfur battery's cathode is made of sulfur-carbon matrix, and its anode is composed of a lithium metal oxide.

Different forms of carbon (for instance, nanotubes and complex carbon foams) have been used by scientists to stabilize sulfur in a fixed place. However, the result was not satisfying.

However, there is a huge difficulty. Sulfur might easily dissolve into a battery's electrolyte. Accordingly, the two electrodes worsen after merely a few cycles.

The team from Rensselaer Polytechnic Institute (RPI) has found a simple way to make the best sulfur-based cathode from a single raw material.

First, the researchers would dry the main paper byproducts, lignosulfonate. Then, they would put it into a quartz tube furnace, which would be heated to about 700 degrees Celsius. The extreme heat will remove most of the sulfur gas while keeping some polysulfides (chains of sulfur atoms) because their purpose is to deeply trap the poly-sulfides in an activated carbon matrix.

Scientists would repeat the heating processes until proper amounts of sulfur is embedded in the carbon matrix. The carbon matrix prevents the embedded sulfur from dissolving into the battery's electrolyte. The team will then ground up the material and mix it with an inert polymer binder, so as to coat the cathode on aluminum foil.

The current prototype of a lithium–sulfur battery is made for a watch. It can last around 200 cycles.

The research team plans to scale up the prototype, in order to enhance the discharge rate and the battery's cycle life.

This new battery might power larger data centers, and offer a lower-cost option for micro-grids and the traditional electric grids.

(Photo credit: Rensselaer Polytechnic Institute)