Limitations of Ordered Macroporous Battery Electrode Materials at High Charge and Discharge Rates

Abstract

Adding porosity to battery electrodes is believed to be universally useful for adding space to accommodate volumetric expansion, electrolyte access to all active materials, helping to mitigate poor C-rate performance for thicker electrodes and for allowing infilling with other materials. Ordered porous electrode, such as inverse opals that have macroporosity, have been a model system: binder and conductive additive free, interconnected electrically, defined porosity and pore size with thickness, good electrolyte wettability and surprisingly good electrode performance in half cells and Li-battery cells at normal rates. We show that the intrinsic electronic conductivity is important, and at fast rates the intrinsic conductivity ultimately suppresses any charge storage in electrode materials. Using a model system of inverse opal V2O5in a flooded Li battery three-electrode cell, whose Li electrochemistry is very well understood, we show that beyond 10 C, electrodes can store almost no charge, but completely recover once reduced to < 1C. We show how the IO material is modified under lithiation using X-ray diffraction, Raman scattering and electron microscopy, and that little or no reaction occurs to the material at higher rates. We also use chronoamperometry to examine rate behaviour and link the limitations in high rate performance, and complete capacity suppression, to the intrinsic out-of-plane conductivity of the IO network. The data show that even idealized electrodes with nanoscale dimensions, functional porosity and full material interconnectivity, are fundamentally limited for high rate performance when they are less conductive even when fully soaked with electrolyte. While adding so-called functional size reduction, porosity etc. can be useful for some materials, these potential benefits are clearly not universally useful for high rate electrodes in Li-ion batteries.