Scientists show that the loss of oxygen can lead to high-capacity positive electrodes
Khuram Umar Ashraf
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Density states of lithium titanium oxide. The green spheres are lithium and the reds are oxygen.

From our laptop computers and mobile phones to our electric vehicles, rechargeable lithium-ion batteries play an important role in our everyday lives. These batteries have one of the best energy-to-weight ratios, but they lose their capacity to hold energy over time. These batteries could be even more efficient, thanks to the Understanding Charge Separation and Transfer at Interfaces in Energy Materials and Devices Center, or CST.

Depending on the choice of material for the positive and negative electrode as well as the electrolyte, the voltage capacity, life and safety of a lithium-ion battery can change dramatically. The key is oxygen.

A collaborative effort between investigators showed that when manganese in the lithium-ion battery was replaced with another metal, the new metal either promotes or inhibits the loss of oxygen. The oxygen-binding energy is a direct indicator of oxygen stability; the stronger the bond of the oxygen-metal, the harder it is for the oxygen to leave the material and the harder it is to maintain the energy capacity after discharging and charging. This phenomenon was investigated at the CST, where they used computer modeling software and calculations to test and explain the experimental results.

The release of oxygen allowed researchers at the CST to envisage a lithium manganese oxide based lithium-rich layered positive electrode that could increase the capacity of lithium-ion batteries. After creating these lithium-rich layered positive electrodes by replacing manganese with other metals, they sought to explain why these metals would either reduce or increase the loss of oxygen. They discovered that the energy gap of the positive electrode determines the energy required for oxygen to bind. This is because the electrons on the oxygen atom are forced from their stable states below the gap into unoccupied states above the gap. Additionally, they found that oxygen binds more tightly to titanium than to cobalt because titanium increases the energy gap, whereas cobalt lowers the gap. So metals that decrease the energy gap facilitate oxygen loss, whereas metals that increase the energy gap suppress oxygen loss.

In addition to uses for consumer electronics, lithium-ion batteries are used in defense, automotive and aerospace applications, with increasing regularity because of their high energy density. These findings pave the way to increase the capacity of lithium ion batteries even further.

More Information

Xiao P, Z Q Deng, A Manthiram and G Henkelman. 2012. "Calculations of Oxygen Stability in Lithium-Rich Layered Cathodes." The Journal of Physical Chemistry 116(44): 23201-23204. DOI: 10.1021/jp3058788

Acknowledgements

This work was sponsored by Understanding Charge Separation and Transfer at Interfaces in Energy Materials, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences.

About the author(s):

Khuram U. Ashraf is a Ph.D. student working for theĀ Photosynthetic Antenna Research Center in Richard Cogdell's group at the University of Glasgow. His research is focused on the structure and function of the Chlorobaculum tepidum reaction center, in the pursuit of enhancing the absorption, using surface plasmons as a means of devising an artificial photosynthetic device.

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