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X-ray shows how batteries can perform at high voltages

24 Jun 2015  | Paul Buckley

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"It's important to see the defects and know where they are in order to understand how they might change the properties of the material," said Ulvestad. "We are the first to do this imaging in a working battery."

The team used the defects to probe the mechanical properties of the LNMO spinel. The researchers report that the movement of the defects while the battery is charging causes changes in the strain fields, which are described by mathematical equations. Using these equations, the team calculated a material property called the Poisson's ratio and discovered that this value is negative when the material is charged to higher voltages.

The Poisson's ratio expresses how a material behaves under an applied strain. Most materials have a positive Poisson's ratio. This means when a material is enlarged in one dimension, it shrinks in the other two dimensions, and vice versa. For example, stretching a rubber band in one dimension makes it thinner in the other dimensions, while pressing down on a ball in the vertical dimension makes it expand in the horizontal dimensions.

Materials with a negative Poisson's ratio behave the opposite under an applied strain. If a rubber band had a negative Poisson's ratio, stretching it in one dimension would make it fatter (instead of thinner) in the other dimensions. Materials that behave this way are rare.

A negative Poisson's ratio has important applications for lithium-ion battery materials and enables materials to maintain their shape regardless of what type of strain is applied, making them more tolerant to strain.

"The LNMO spinel is incredibly strong. You can expand it or shrink it, and it doesn't crack. No one has ever reported a negative Poisson's ratio in battery materials.

"We hypothesise that this property helps make this cathode material more resistant to strain than other materials at high voltages," said Meng.


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