Electro-optical measurements of lithium intercalation/de-intercalation at graphite anode surfaces.

Abstract

Graphite anode behaviour is of great interest for the optimization of Lithium-ion batteries. The improvement of battery performance depends on an understanding of lithium intercalation/de-intercalation in graphite anodes, especially in regards to energy and power density.In this study we present a new approach for investigating graphite anode behaviour under equilbirum and nonequlibirum conditions. It is based on reflectance change measurements from a graphite anode surface, relative to state of charge (SOC). We have introduced an innovative optical test cell and used a photodiode.A reflectance change hysteresis occurs between lithium intercalation and de-intercalation, under equilbrium conditions. This was ascribed to specific lithium-carbon bonds at the graphite particle edge region. Charging and discharging have a unique reflectance change characteristic. This was assigned to SOC nonequilibrium in the anode and serves as clear evidence that limited lithium mobility in the porous microstructure is a signficant loss factor. We then performed a more detailed analysis of the anode loss processes by correlating the anode reflectance change with dynamic electrical excitation. This method, Optical Impedance Spectroscopy (OIS), provided detailed information about the frequency range of the loss processes. It also confirmed our assigning the low frequency range (1mHz-3Hz) to solid state diffusion and lithium transport in the electrolyte filled pores. The results are in excellent agreement with previous EIS studies. We conclude that OIS is applicable for validating physicaly-based graphite anode models and determining model parameters, in combination with EIS.