Researchers at the University of Eastern Finland have combined mesoporous silicon microparticles, made from plant waste, with carbon nanotubes to improve the performance of silicon anodes in Li-ion batteries.

“In the future, silicon will gradually replace carbon as the anode material in Li-ion batteries,” according to the university. “The capacity of silicon is ten times higher than the capacity of graphite, which is nowadays used as the anode material. Using silicon in the anode makes it possible to even double the capacity of the total battery cell.”

However, silicon anodes generally swell and shrink so much as lithium ions enter and leave that the anode material rapidly crumbles, rendering the electrode useless.

According to the researchers, mesoporous silicon grainss – particles around 21µm in diameter with ~5nm pores in this case – are less susceptible to this damage. They point out that porous materials have been previously been made for anode use, but using processes that are unlikely to scale from the laboratory.

In this case the grains are made from the ash of barley husks, which contains natural amorphous porous silica structures, that can be chemically converted to porous silicon structures.

Carbon nanotubes are introduced to increase the conductivity of the otherwise semiconducting material.

Again, this has been done before, according to to the researchers, but their twist is that they have found a way to pre-treat the silicon particles and the nanotubes in ways that allows them to bond chemically – conjugate – after mixing: “With the right type of conjugation, also the electrical conductivity and mechanical durability of the material was improved”.

The result is a silicon composite with an average discharge capacity of 2Ah/g at a rate of 0.1C (472mAh/g at 1C).

In one set of tests, after initial charge-discarge cycles ‘formed’ the anode, a stable capacity of ~1.15Ah/g was seen up to 110 cycles – after which capacity declined steadily.

Next, the researchers are aiming to produce a complete silicon anode with a solid electrolyte.

“The progress of the Li-ion battery research is very exciting,” said University of Eastern Finland professor Vesa-Pekka Lehto, “and we want to contribute to the field with our know-how related to mesoporous structures of silicon.”

He added: “Hopefully, the EU will invest more in the basic research of batteries to pave the wave for high performance batteries and to support the competitiveness of Europe in this field. The Battery 2030+ roadmap would be essential in supporting this progress.”

This article very much skims the surface of the research. For further information:

‘Conjugation with carbon nanotubes improves the performance of mesoporous silicon as Li-ion battery anode‘ describes the hybrid material and its performance in Nature Scientiic Reports.

While ‘Cascading use of barley husk ash to produce silicon for composite anodes of Li-ion batteries‘ describes making the porous silicon particles.

Image – Mesoporous silicon micro-particles are bound together with carbon nanotubes at multiple points. By Timo Ikonen.