Despite the superior performance of lithium-ion batteries (LIBs) compared to other rechargeable batteries, these power systems have certain drawbacks. Lithium is a limited resource, and its extraction and use pose significant environmental challenges, as the liquid electrolytes used in LIBs are toxic and flammable. Researchers have increasingly turned to sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) as promising alternatives,

The electrode materials’ capacity in these systems still lags behind that of LIBs, prompting Tokyo University Science researchers in Japan to develop high-capacity electrode materials for NIBs and KIBs. A novel synthesis strategy is described in Advanced Energy Materials for nanostructured hard carbon (HC) electrodes that exhibit unprecedented performance. This amorphous form of carbon lacks a well-defined crystalline structure and is valued for its strength and resistance.

Use of zinc oxide as a template during electrode synthesis improves the capacity to store charge carriers, making it a promising electrode candidate for sodium-ion batteries. Source: Shinichi Komaba/Tokyo University of Science

The researchers previously discovered a method to use magnesium oxide (MgO) as a template during the synthesis of hard carbon electrodes for NIBs. This approach altered the nanostructure of the electrodes, resulting in the creation of nanopores when MgO was removed and significantly increased the electrodes’ capacity to store sodium ions.

Building on this advance, the focus turned toward the utility of compounds composed of zinc and calcium as effective nano-templates for hard carbon electrodes. Different hard carbon samples formed with zinc oxide and calcium carbonate were examined, and a comparative analysis of their performance against those synthesized using magnesium oxide was conducted.

Zinc oxide was demonstrated to offer great potential as the negative electrode material in NIBs. Optimization of the zinc oxide concentration incorporated into the hard carbon matrix during synthesis resulted in a reversible capacity of 464 mAh/g with a high initial coulombic efficiency of 91.7% and a low average potential of 0.18 V versus sodium ions. A NIB fabricated using the optimized zinc oxide-templated hard carbon as the negative electrode exhibited an energy density of 312 Wh/kg, a value equivalent to the energy density of certain types of currently commercialized LIBs. This electrode material also exhibited a significant capacity of 381 mAh/g when incorporated into a KIB, further showcasing its potential.