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Adding porosity to battery electrodes is believed to be universally usefulfor adding space to accommodate volumetric expansion, electrolyte access to allactive materials, helping to mitigate poor C-rate performance for thickerelectrodes and for allowing infilling with other materials. Ordered porouselectrode, such as inverse opals that have macroporosity, have been a modelsystem: binder and conductive additive free, interconnected electrically,defined porosity and pore size with thickness, good electrolyte wettability andsurprisingly good electrode performance in half cells and Li-battery cells atnormal rates. We show that the intrinsic electronic conductivity is important,and at fast rates the intrinsic conductivity ultimately suppresses any chargestorage in electrode materials. Using a model system of inverse opal V2O5in aflooded Li battery three-electrode cell, whose Li electrochemistry is very wellunderstood, 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 ismodified under lithiation using X-ray diffraction, Raman scattering andelectron microscopy, and that little or no reaction occurs to the material athigher rates. We also use chronoamperometry to examine rate behaviour and linkthe limitations in high rate performance, and complete capacity suppression, tothe intrinsic out-of-plane conductivity of the IO network. The data show thateven idealized electrodes with nanoscale dimensions, functional porosity andfull material interconnectivity, are fundamentally limited for high rateperformance when they are less conductive even when fully soaked withelectrolyte. While adding so-called functional size reduction, porosity etc.can be useful for some materials, these potential benefits are clearly notuniversally useful for high rate electrodes in Li-ion batteries.

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