Russian scientists publish new insights into lithium-ion battery structure

There are many reactions in the process of charging and discharging lithium-ion batteries . Although lithium-ion batteries are becoming more and more popular, there are still some unknown mechanisms. To improve the performance of lithium-ion batteries, continuous observation is one of the best ways, but considering the complex structure of lithium-ion batteries and the limitations of microscopy technology, this is not an easy task.

Scientists from the Skoltech Center for Energy Science and Technology in Moscow have developed a new method to take a closer look at some of the processes that make lithium batteries work - the formation of the solid electrolyte interphase (SEI), which the researchers describe as a thin layer of electrolyte reduction products that forms on the surface of a lithium-ion battery anode during the first few cycles.

The researchers say the formation of this film is critical to slowing battery degradation. However, in situ measurements of SEI formation have proven difficult, and replacing commercial battery materials with more uniform alternatives in the lab is the only way to get results.

"Since battery-grade materials are usually powders, it is challenging to visualize dynamic processes on their surfaces by atomic force microscopy, especially in liquid environments," said Skoltech scientist Sergey Luchkin. "Standard battery electrodes are too rough for such measurements, and isolated particles tend to detach from the substrate during scanning. To overcome this problem, we embedded the particles in epoxy resin and performed cross-sectioning in order to firmly fix the particles in the substrate."

In addition to highly oriented pyrolytic graphite, one of the more uniform carbon materials previously used to study the SEI, the Skoltech group applied its cross-section process to electrodes of mesogenic graphite and non-graphitizable amorphous carbon, allowing the researchers to observe the thickness of the formed SEI layer and to assess its electrical and mechanical properties.

Their research shows that the conditions for SEI formation vary greatly depending on the electrode material, and that SEI adhesion is related to the surface roughness of the electrode. Rougher surfaces promote reduced degradation, as the SEI is able to penetrate more porous surfaces and gain better adhesion.

Applying the cross-sectional method to the lithium-manganese-cobalt cathode revealed no signs of SEI layer formation. According to the scientists, this result suggests that future research should acknowledge the fundamental differences in the stabilization mechanisms between anodes and cathodes in lithium-ion batteries.

Their findings were published in Scientific Reports.