Crystal defect with potential: positrons analyse battery materials

In ‘defect engineering’, defects are purposely created in the atomic lattice to change material properties. An interdisciplinary research team has detected crystal defects at FRM II’s positron source NEPOMUC that make batteries more efficient.

Dr Thomas Gigl (left) and Dr Stefan Seidlmayer at the NEPOMUC positron source. © Wenzel Schürmann / TUM

Dr Thomas Gigl (left) and Dr Stefan Seidlmayer at the NEPOMUC positron source. © Wenzel Schürmann / TUM

Great potential with some challenges

Lithium titanate (Li4Ti5O12, LTO) is a promising anode material for batteries. It offers a long service life with a low self-discharge rate and outstanding thermal stability. Thermal stability is important because the performance of a battery can decrease at too high temperatures and the battery can also catch fire.

Due to the rapid degradation of LTO, there is a swift decline in capacity and voltage. The aim of the research team around Dr Yu-Te Chan and Dr Cristina Grosu from the Technical University of Munich (TUM) was therefore to gain a better understanding of LTO – more specifically through a gas treatment that alters the material’s composition. This process removes oxygen atoms, creating atomic vacancies. The treated LTO exhibits increased electrical conductivity, but: “The problem is that until now, nobody really understood why this happens at the atomic level,” reveals Dr Yu-Te Chan, Postdoctoral Fellow at the Chair of Theoretical Chemistry at TUM.

Battery cells are to become even more durable and quicker to charge. © Andreas Heddergott / TUM

Battery cells are to become even more durable and quicker to charge. © Andreas Heddergott / TUM

Positrons show defects

Using positron annihilation lifetime spectroscopy (PALS) and the Coincidence Doppler-broadening spectrometer (CDBS) at FRM II, the researchers were able to identify defects in the LTO crystal. Specifically, this is achieved through positrons that annihilate with electrons in the vicinity of oxygen vacancies in the crystal. “The duration of the positron lifetime is key; this varies depending on the vacancy,” says Yu-Te Chan. The deeper the oxygen vacancies lie within the material, the further the positron beam must penetrate. “From this, we can precisely deduce which defects the material has and at what depth they are located,” explains Prof. Dr Christoph Hugenschmidt, NEPOMUC’s instrument manager. With the help of the two researchers, Dr Thomas Gigl and Dr Stefan Seidlmayer, the team carried out the PALS and the measurements at the CDBS.

Prof. Dr Christoph Hugenschmidt is the instrument scientist responsible for the instrument NEPOMUC. © Wenzel Schuermann / TUM

Prof. Dr Christoph Hugenschmidt is the instrument scientist responsible for the instrument NEPOMUC. © Wenzel Schuermann / TUM

Discoveries about surface structure

“For me, the most surprising thing was what we found deep inside the material,” says Stefan Seidlmayer. The expectation had been that the oxygen vacancies would increase conductivity in the crystal. On the surface, this is indeed the case. However, the researchers discovered that in the bulk the cause lies in lithium ions bouncing around inside the crystal. Another important discovery is the ‘facet effect’: certain crystal orientations have better conductivity than others. “This is very useful because it explains why LTO nanoparticles with certain shapes work better,” reveals Cristina Grosu, team leader of the Electric Vehicle Lab at the Chair of Automotive Engineering at TUM.

The new insights into the methodology and structure of LTO give the team confidence: “Now other researchers can use the same approach,” says Yu-Te Chan, “and that could help develop the next generation of safe batteries.”