Correlating wavelength dependence in LiMn2O4 cathode photo-accelerated fast charging with deformations in local structure

Electric vehicles are one of the best tools we currently have to combat the effects of climate change. There is one outstanding problem though: it takes a significant amount of time to charge a vehicle. Even fasting-charging units take much longer to “fill up” an EV than do gas pumps to fill a tank. This may seem like a mere annoyance, but for people pressed for time this is no small acceptance hurdle between internal combustion and EVs.

There have been numerous efforts at the material-level to improve charging rates through engineered electrode coatings and nanostructuring of active materials, which limit energy density in the battery pack. Additionally, the study of light interaction with energy storage materials has gained interest for use in photo-rechargeable batteries, and integrated solar energy storage systems, though there have been few studies specifically investigating light interaction with commercially-relevant battery materials. Facilitation of electron movement in a battery material is a key for lowering the resistance to flow of charge. Administration of light to induce light-matter interactions is a possibility to locally alter the electronic nature of the material. In particular, Spinel LiMn2O4 (LMO), a cathode material, was shown to dramatically increase the charging current when exposed to white light during a voltage hold.

Now, a team of researchers led by André D. Taylor, professor of Chemical and Biomolecular Engineering and member of the Sustainable Engineering Initiative, as well as graduated PhD student Jason Lipton and Christopher Johnson of the Argonne National Laboratory, have discovered how different wavelengths of light can change the current in LMO cathodes. The team showed that illuminating with red light results in a higher charging rate compared to both ultraviolet illumination of equal optical power and dark conditions. 

The team analyzed the effect of red light in the context of the electronic structure and possible excitations, and showed that Mn d-d electronic transitions occurring under red light illumination are largely responsible for the increased charging rate. They further demonstrated through X-ray absorption spectroscopy methods that LMO Mn-Mn bond distances shorten after d-electron excitation. The shrinkage in the crystal volume beneficially contributes to delithiation kinetics by lowering the resistance to lithium-ion conduction.

The results provide a roadmap for rapid discovery of candidate materials for photo-accelerated fast charging, through the review of calculated density of states data. Besides LMO, there is a wealth of materials to investigate with strong potential for photo-electrochemically induced activity. Once photo-acceleration is optimized for a given materials system, it is possible to envision using a small amount of the source current from a fast-charging station to power thin, flexible LEDs built into the spiral wound 18650 cells in a battery pack, enhancing the fast-charging capabilities in next-generation EVs while minimizing impact to energy density.