A research group from Hokkaido University, Tohoku University, and Nagoya Institute of Technology has successfully developed a low-cost, high-capacity, long-life cathode material for lithium-ion batteries that is free of rare metals.
Lithium-ion batteries, which have now become indispensable in daily life, currently use rare metals such as cobalt and nickel as positive electrode materials. As demand for lithium-ion batteries increases, supply chain risks such as resource depletion and price hikes are becoming serious issues. Therefore, this research group has been developing a lithium-ion battery cathode material whose main component is iron, which is free of rare metals.
Lithium iron oxide (Li5FeO4), which the research group developed in previous research, has been shown to have more than twice the capacity of lithium iron phosphate (LiFePO4), which has already been commercialized. However, Li2FeO5 has the disadvantage of poor cycle life due to the release of oxidized oxygen molecules during charging. In this study, in order to improve the cycle characteristics of Li4FeO5, we searched for a method to suppress the oxygen desorption reaction that occurs during charging.
As a result, they found that oxygen desorption reactions can be suppressed by introducing p-block elements (elements from groups 5 to 4 of the periodic table) such as silicon and phosphorus into Li13FeO18. Among the p-block elements, elements in groups 13 to 16 in particular are known to form strong covalent bonds with oxygen, which contributes to suppressing oxygen desorption and improving cyclability.
When evaluating the capacity retention rate of the developed material after 10 cycles of charging and discharging, we found a significant improvement from 50% to a maximum of 90%. The material with silicon introduced had the highest performance, but in terms of the overall energy density of the positive electrode, including the redox reaction of iron, the material with phosphorus and germanium had the highest performance. In this way, by stabilizing the unstable redox reaction of oxygen and utilizing the two redox reactions of iron and oxygen, the overall energy capacity of the positive electrode can be increased.
The results of this research are considered to be useful as design guidelines for high-performance rare metal-free cathode materials, and the progress of this research is expected to contribute to a low-carbon society and global warming countermeasures.