Quantifying the Effect of Electronic Conductivity on the Rate Performance of Nanocomposite Battery Electrodes

While it is well-known that the electronic conductivity of electrodes has a critical impact on rate performance in batteries, this relationship has been quantified only by computer simulations. Here we investigate the relationship between electrode electronic conductivity and rate performance in a model cathode system of lithium-nickel-manganese-cobalt-oxide (NMC) filled with various quantities of carbon black, single-walled carbon nanotubes, and graphene. We find extreme conductivity anisotropy and significant differences in the dependence of conductivity on mass fraction among the different fillers. Fitting capacity versus rate curves yielded the characteristic time associated with charge/discharge. This parameter increased linearly with the inverse of the out-of-plane electronic conductivity, with all data points falling on the same master curve. Using a simple mechanistic model for the characteristic time, we develop an equation that matches the experimental data almost perfectly with no adjustable parameters. This implies that increasing the electrode conductivity improves the rate performance by decreasing the RC charging time of the electrode and shows rate performance to be optimized for any electrode once σ_OOP > 1 S/m, a condition achieved by including <1 wt % single-walled carbon nanotubes in the electrode.