Hard-carbon anode supports high energy density in sodium-ion battery

Cost-effective rechargeable batteries are at the heart of virtually all portable electronic devices, which have become ubiquitous in modern daily life. Moreover, rechargeable batteries are essential components in many environmentally friendly technologies, such as electric cars and systems that harvest renewable energy. So, it shouldn't come as a surprise that a lot of effort is currently being spent on developing better and cheaper rechargeable batteries.

So far, rechargeable lithium-ion batteries hold the number-one spot thanks to their great performance across the board in terms of capacity, stability, price and charging time. However, lithium, and other minor and costly metals like cobalt and copper, are not the most abundant materials in the Earth's crust, and the ever-increasing demand for them will soon lead to supply problems around the world. At the Tokyo University of Science in Japan, Shinichi Komaba and colleagues have been striving to find a solution to this worsening conundrum by developing rechargeable batteries using alternative, more abundant materials.

In a paper in Angewandte Chemie International Edition, Komaba and his colleagues, including researchers from the National Institute for Materials Science and Okayama University, both in Japan, now report an energy-efficient method for producing a novel carbon-based material for sodium-ion batteries. They focused on synthesizing hard carbon, a highly porous material that serves as the negative electrode, or anode, of rechargeable batteries, by using magnesium oxide (MgO) as an inorganic template for producing nano-sized pores inside the material.

The researchers explored a novel technique for mixing the ingredients of the MgO template, so they could precisely tune the nanostructure of the resulting hard-carbon electrode. After multiple experimental and theoretical analyses, they elucidated the optimal fabrication conditions and ingredients to produce hard carbon with a capacity of 478mAh/g, the highest ever reported in this type of material.

"Until now, the capacity of carbon-based negative electrode materials for sodium-ion batteries was mostly around 300–350mAh/g," said Komaba. "Though values near438 mAh/g have been reported, those materials require heat treatment at extremely high temperatures above 1900°C. In contrast, we employed heat treatment at only 1500°C, a relatively low temperature." With lower temperature comes lower energy expenditure, which also means lower cost and less environmental impact.

The capacity of this newly developed hard carbon electrode material is certainly remarkable, and greatly surpasses that of graphite (372mAh/g), which is the standard anode material in lithium-ion batteries. Moreover, even though a sodium-ion battery with this hard carbon anode would in theory operate at a 0.3-volt lower voltage difference than a standard lithium-ion battery, the higher capacity of the former would lead to a much greater energy density by weight (1600Wh/kg versus 1430Wh/kg), producing a 19% increase in energy density.

"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," said Komaba. "The hard carbon with extremely high capacity that we developed has opened a door towards the design of new sodium-storing materials."

Further studies will now be required to verify that the proposed material actually offers superior lifetime and input-output characteristics and low temperature operation in actual sodium-ion batteries.

This story is adapted from material from the Tokyo University of Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.