Materials with negative thermal expansion as electrodes for lithium-ion batteries
Most solids expand as temperatures increase and shrink
as they cool. Some materials do the opposite, expanding in the cold.
Lithium titanium phosphate is one such substance and could provide a
solution to the problem of steeply declining performance of lithium-ion
batteries in cold environments. In the journal Angewandte Chemie,
a Chinese team has demonstrated its suitability for use in electrodes
for rechargeable batteries.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Lithium-ion batteries and other rechargeable batteries
based on metal ions provide our portable devices with electricity, power
vehicles, and store solar and wind energy. They work well—as long as it
is warm. As temperatures drop, the performance of these batteries can
decrease sharply—a problem for electric cars, aerospace, and military
applications. Countermeasures such as integrated heaters, improved
electrolytes, or electrode coatings increase the cost and complexity of
battery production or reduce performance.
One of the causes of the cold problem is the slowed
diffusion of lithium ions within the electrode material. A team from
Donghua University and Fudan University in Shanghai, as well as Inner
Mongolia University in Hohhot has proposed a new approach to tackling
this issue: electrodes made of electrochemical energy-storage materials
with negative thermal expansion (NTE), such as lithium titanium
phosphate LiTi2(PO4)3 (LTP). Led by
Liming Wu, Chunfu Lin, and Renchao Che, the team used LTP as a model
substance to demonstrate that electrode materials with NTE properties
can provide good performance at low temperatures.
Analysis of the crystal structure revealed a
three-dimensional lattice of TiO6 octahedra and PO4
tetrahedra with an open, flexible structure that contains both
“cavities” and “channels”, where lithium ions can lodge. When cooled,
the structure stretches along one of its crystal axes. By using
spectrometric and electron microscopic analyses in conjunction with
computer modeling, the team determined that the vibrational modes of the
atoms change at low temperature. This increases the occurrence of
special transverse vibrations of certain oxygen atoms, increasing their
distances from each other and widening the cavities in the lattice. This
facilitates storage and transport of the lithium ions. At −10 °C, their
diffusion rate is still at 84% of the value obtained at 25 °C.
Electrochemical tests on carbon-coated LTP at −10 °C also showed good
electrochemical performance with high capacity and a high rate
capability, as well as a high retention of capacity over 1000
charge/discharge cycles.
Materials with negative thermal expansion are thus highly
promising for use as an electrode material in lithium-ion batteries in
cold environments.
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About the Author
Dr Chunfu Lin is an Associate Professor of Physics at
Donghua University, and has been working in the area of energy storage
and conversion for over 20 years.
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