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Hard carbon for a high energy sodium battery

Scientists in Japan demonstrated a hard-carbon electrode that can greatly increase the capacity of a sodium-ion battery. With further work on the long-term performance, the discovery could make sodium-ion batteries better able to compete on energy density with their lithium-ion counterparts.

Sodium-ion batteries are a promising energy storage technology, one that has already seen limited commercialization in the stationary storage segment. And sodium-ion has attracted plenty of attention from researchers, since it offers an alternative to lithium-ion batteries that relies on much cheaper, more abundant materials.

In terms of energy density, sodium-ion technology is a little way behind lithium. This means it is widely seen as impractical for applications such as electric vehicles or consumer electronics, where the size and weight of the battery are a primary concern. A new discovery from scientists at the Tokyo University of Science (TOS), however, could be set to turn this assumption on its head.

A group at the university looked to carbon electrode materials to boost sodium-ion battery capacity, and developed a technique to fabricate a porous, hard carbon anode. The technique is described in the paper New hard-carbon anode material for sodium-ion batteries will solve the lithium conundrum, published in Angewandte Chemie, International Edition.

Magnesium oxide template

Key to the process is the use of magnesium oxide (MgO) as a ‘template’ for the size and structure of the pores. Particles of MgO are formed into a carbon matrix, and pretreated at 600 degrees Celsius, before acid leaching and carbonization at 1500 degrees Celsius completes the process. After a series of experiments to optimize the MgO template and calculate ideal fabrication conditions, the group was able to fabricate hard carbon with a capacity of 478 millamp-hours per gram, and 88% Couloumbic efficiency (charge transfer efficiency) at the first cycle.

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Shinichi Komaba, Professor at TOS notes that previously the highest value reported for this material was 438 mAh/g, and this was achieved with processing at even higher temperatures. Calculations presented by TOS show that a sodium-ion battery utilizing this anode would operate at a slightly lower voltage difference than today’s standard lithium-ion batteries, but would still achieve approximately a 19% increase in energy density (1600 watt-hours per kg, versus 1430.)

“Our study proves that it is possible to realize high-energy sodium-ion batteries, overturning the common belief that lithium-ion batteries have a higher energy density,” says Komaba. “The hard carbon with extremely high capacity that we developed has opened a door towards the design of new sodium-storing materials.”

Other battery concepts under investigation promise energy densities far beyond what TOS has achieved here, and it is not clear how much more performance could be squeezed out of this new concept. The work, however, could force researchers to think again about what is possible with sodium-ion batteries. The next step will be to evaluate the practicality of the approach, and whether the materials can maintain stability over many cycles, and reach a lifetime that’s at least comparable to today’s lithium-ion technology.

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Source: pv magazine