A team of researchers at the Indian Institute of Technology (IIT) Madras has been able to enhance the capacity retention of anode material in lithium ion batteries four-fold compared with commercially available ones.

The team was able to develop a novel composite which can deliver specific capacity retention of 1,120 mAh/g after 10,000 cycles, and work at high current density (ability to draw more current from a cell within a short time) with long cycle life by combining two types of lithium ion storage mechanisms. The results were published in the Journal of Materials Chemistry A.

Compared with graphite, carbon nanotube (CNT) has several advantages as an anode material. However, the efficiency (or irreversible capacity) of carbon nanotube anode is an issue. Also, the lithium ions that get inserted into the carbon nanotube during charging do not fully come out during discharge. So not all lithium ions inserted into carbon nanotube comes out to contribute to the useful capacity.

“We overcame this by using carbon nanotubes with a few layers unzipped, which can be called as partially exfoliated carbon nanotubes,” says Sripada Raghu from the Department of Physics, IIT Madras and one of the authors of the paper. The unzipping does not affect the core of the CNT structure, which renders very good electrical conductivity, while the partially exfoliated outer layers have very good ability to store lithium ions. “The outer layers of partially exfoliated carbon nanotubes are quite similar to a few-layered graphene,” Mr. Raghu says.

To enhance the capacity even further, the researchers incorporated sulphur in the exfoliated layers. The theoretical capacity of sulphur is very high (1,675 mAh/g). So the team wanted to take advantage of this property of sulphur.

Besides getting inserted into the layers of exfoliated carbon nanotube, the lithium ions interact with sulphur in a chain of reactions leading to the formation of lithium polysulphides. Higher-order polysulphides are initially formed and later the stable lower-order polysulphides are formed. “The lower-order polysulphides are desirable,” says Ananya Gangadharan from the Department of Physics, IIT Madras and the first author of the paper.

“We have clubbed the two storage mechanisms — lithium ion insertion reaction from the lithium ion battery and sulphur redox reaction from the lithium sulphur battery — in one battery. That’s why we are able to achieve high capacity retention even after 10,000 cycles at high current density,” says Ms. Gangadharan.

“We have already patented our anode material. We are now trying to combine the anode with suitable cathodes and test the enhancement in efficiency and capacity retention so we can replace the commercial anodes with ours,” says Prof. Sundara Ramaprabhu from the Department of Physics, IIT Madras and the corresponding author of the paper. “Work is currently on to further enhance the capacity and use the material as an electrode in both lithium ion and lithium sulphur batteries.”

Published in The Hindu on January 7, 2017