Researchers from  Skoltech, Moscow State University (MSU), Tokyo University of Science and the University of Strasbourg have identified the type of electrochemical reaction associated with charge storage in the anode material for sodium-ion batteries (SIB).

The demand for lithium-ion (LIB) batteries and the average size of storage devices are constantly growing, however this growth trend is encountering multiple barriers, such the high cost of lithium salts, limited global reserves of lithium and uneven distribution of lithium-containing deposits across countries.

To overcome these hurdles, scientists are working on SIB, an alternative technology that may challenge both LIB and lead-acid batteries.

 While there is a broad diversity of compositions and structures that could be suitable for SIB cathodes or electrolytes, the anode still remains a stumbling block.

Graphite, which is successfully used in LIB, does not work for SIB because the sizes of carbon hexagons and sodium cations differ too much to provide intercalation.

Hard carbon seems to be the only material that can actually be used in the anode. Hard carbon formed by an irregular arrangement of distorted graphite-like layers demonstrates sodium-ion storage properties comparable to those of graphite in LIB, however it still remains unclear why and how this happens.

“There are several hypotheses as to how sodium could be introduced into hard carbon. In our study, we validated and slightly expanded one of them,” says researcher Zoya Bobyleva, “we found that hard carbon exhibits intercalation-type behavior to accumulate most of the charge, which is great news. Intercalation is exactly what the battery needs, while the surface processes associated with “pseudocapacitance” are the responsibility of supercapacitors that form a very narrow niche among chemical power sources.”

“This work is remarkable not only in showing how hard carbon works in the sodium-ion system but also in finding a way to produce hard carbon with a capacity of over 300 mAh/g comparable to that of graphite in LIB,” says Evgeny Antipov, a Skoltech professor and head of the Department of Electrochemistry at the MSU Faculty of Chemistry, “creating and optimising a new method takes a lot of painstaking effort that typically remains behind the scenes and is hardly ever reported in scientific papers, so it is important for us to show the ultimate result: we succeeded in making good anode materials for SIB and we know how they work.”