Correlating Wavelength Dependence in LiMn₂O₄ Cathode Photo-Accelerated Fast Charging with Deformations in Local Structure

Lithium-ion batteries have revolutionized the possibilities for electrified transportation, as recognized by the 2019 Nobel Prize in chemistry. However, lithium-ion batteries suffer from a critical slow charging limitation, contributing to range anxiety amongst consumers and inhibiting adoption of electric vehicles. Recently, it has been demonstrated that under white light illumination LiMn₂O₄ cathodes undergo photo-accelerated fast charging, essentially improving the kinetics of delithiation without use of low electrode loadings or nanostructured active materials. In this work, using LEDs to select discrete wavelengths of light, it is shown that d-d transitions in Mn are largely responsible for the increased charging rate. This excitation is possible by absorption of red light; however, in UV light the absorption process is dominated by excitation of electrons from O-2p into available Mn-t_2gstates, which is ineffective in promoting enhanced electrochemistry. It is further demonstrated through X-ray absorption spectroscopy methods that LiMn₂O₄ undergoes a reduction in Mn-Mn distance as a result of d-electron excitation which leads to improved kinetics of delithiation. The results presented here set forth a road map for facile identification of future materials with potential for photo-accelerated fast charging.