Lithium-superrich iron oxides for cost-effective, high-capacity, and cyclable cathodes. (Illustration: Science Graphics. Co., Ltd. with modification)

The energy capacity and charge-recharge cycling — cyclability — of lithium-iron-oxide, a cost-effective cathode material for rechargeable Li-ion batteries, is improved by adding small amounts of abundant elements. The development, achieved by researchers at Hokkaido University, Tohoku University, and Nagoya Institute of Technology, is reported in the journal ACS Materials Letters.

Li-ion batteries have become indispensable in modern life, used in a multitude of applications including mobile phones, electric vehicles (EVs), and large power-storage systems. A constant research effort is underway to increase their capacity, efficiency, and sustainability. A major challenge is to reduce the reliance on rare and expensive resources. One approach is to use more efficient and sustainable materials for the battery cathodes, where key electron exchange processes occur.

The researchers worked to improve the performance of cathodes based on a particular lithium-iron-oxide compound. In 2023, they reported a promising cathode material, Li5FeO4, that exhibits a high capacity using iron and oxygen redox reactions. However, its development encountered problems associated with the production of oxygen during charging-recharging cycling.

Capacity retention of lithium-iron-oxide cathode is improved from 50 percent to 90 percent when doped with abundantly available elements such as aluminum, silicon, phosphorus, and sulfur. (Illustration: Hiroaki Kobayashi)

“We have now found that the cyclability could be significantly enhanced by doping small amounts of abundantly available elements such as aluminum, silicon, phosphorus, and sulfur into the cathode’s crystal structure,” said Hokkaido University Associate Professor Hiroaki Kobayashi.

“Suppressing an oxygen evolution, a decomposition reaction in batteries,” Kobayashi told Tech Briefs when asked about the biggest technical challenge of the work.

A crucial chemical aspect of the enhancement proved to be the formation of strong covalent bonds between the dopant and oxygen atoms within the structure. These bonds hold atoms together when electrons are shared between the atoms, rather than the ionic interaction between positive and negatively charged ions.

“The covalent bonding between the dopant and oxygen atoms makes the problematic release of oxygen less energetically favorable, and therefore less likely to occur,” said Kobayashi.

He said in the aforementioned Tech Briefs interview that developing “a high-capacity cathode material using only cost-effective elements, such as iron” was the catalyst for the work.

The researchers used X-ray absorption analysis and theoretical calculations to explore the fine details of changes in the structure of the cathode material caused by introducing different dopant elements. This allowed them to propose theoretical explanations for the improvements they observed.

They also used electrochemical analysis to quantify the improvements in the cathode’s energy capacity, stability, and the cycling between charging and discharging phases, showing an increase in capacity retention from 50 percent to 90 percent.

“Our developed lithium iron oxide can operate as a high-capacity cathode material,” he told Tech Briefs.

The next phase of the research will include exploring the challenges and possibilities in scaling up the methods into technology ready for commercialization.

Kobayashi said that the team’s work is “ongoing” and that “improving the capacity retention up to 99 percent” is the group’s ultimate goal.