Enhancing the cyclability of lithium-ion battery cathodes

Researchers at Hokkaido University, Tohoku University and the Nagoya Institute of Technology have found that the energy capacity and charge-recharge cycling (cyclability) of lithium-iron-oxide — a cathode material for rechargeable lithium-ion batteries — is improved by adding small amounts of abundant elements.

Lithium-ion batteries are ubiquitous and researchers are seeking ways to increase the capacity, efficiency and sustainability of Li-ion batteries. 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.

Associate Professor Hiroaki Kobayashi from Hokkaido University said the cyclability can be enhanced by doping small amounts of abundantly available elements such as aluminium, silicon, phosphorus and sulfur into the cathode’s crystal structure. A crucial aspect of the enhancement is 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 favourable, and therefore less likely to occur,” Kobayashi said.

The researchers used X-ray absorption analysis and theoretical calculations to explore the 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. The researchers also used electrochemical analysis to quantify the improvements in the cathode’s energy capacity, stability and the cycling between charging and discharging phases, with an increase in capacity retention from 50% to 90%.

The researchers aim to explore the challenges and possibilities in scaling up the methods into technology that is ready for commercialisation. “We will continue to develop these new insights, hoping to make a significant contribution to the advances in battery technology that will be crucial if electric power is to widely replace fossil fuel use, as required by global efforts to combat climate change,” Kobayashi said.

The research findings were published in the journal ACS Materials Letters.

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