Most rechargeable batteries are lithium-ion batteries made from relatively scarce elements including precious metals like cobalt. Worldwide lithium reserves themselves are also limited and subject to geopolitical considerations. Alternative chemistries that offer less volatility and more abundance of raw materials are gaining traction, including vanadium, sodium and zinc.
In a new study, scientists from Tokyo University of Science discovered an energy-efficient method to fabricate a hard carbon electrode with enormously high sodium storage capacity. This could pave the way for next-generation sodium-ion batteries made with inexpensive and abundant materials, and having a higher energy density than lithium-ion batteries.
So far, rechargeable lithium-ion batteries occupy the lion’s share of the market due to across-the-board performance in capacity, stability, price, and charging time. As increasing demand for lithium-ion batteries surges with the development of the EV market and other applications, Tokyo University of Science Professor Shinichi Komaba and colleagues have been seeking a solution through alternative and more available materials.
In a recent study published in Angewandte Chemie International Edition, the team found an energy efficient method to produce a novel carbon-based material for sodium-ion batteries. The study focused on the synthesis of hard carbon, a highly porous material that serves as the negative electrode of rechargeable batteries, through the use of magnesium oxide (MgO) as an inorganic template of nano-sized pores inside hard carbon.
The researchers explored a different technique for mixing the ingredients of the MgO template so as to precisely tune the nanostructure of the resulting hard carbon electrode. After multiple experimental and theoretical analyses, they determined the optimal fabrication conditions and ingredients to produce hard carbon with a capacity of 478 mAh/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 to 350 mAh/g,” said Komaba. “Though values near 438 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.” Of course, 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 surpasses that of graphite (372 mAh/g), which is currently used as the negative electrode material in lithium-ion batteries. Moreover, even though a sodium-ion battery with this hard carbon negative electrode 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 (1600 Wh/kg versus 1430 Wh/kg), resulting in +19% increase of energy density.
Komaba added that their study proves that high-energy sodium-ion batteries are indeed possible, countering the current market assumption that lithium-ion batteries have a higher energy density. The hard carbon with extremely high capacity opens the door to the design of new sodium-storing materials.”
Further studies will be required to verify that the proposed material actually offers superior lifetime, input-output characteristics, and low temperature operation in actual sodium-ion batteries.