Quantifying the effect of electrical conductivity on the rate-performance of nanocomposite battery electrodes

While it is well-known that electrode conductivity has a critical impact on rate-performance in battery electrodes, this relationship has been quantified only by computer simulations. Here we investigate the relationship between electrode conductivity and rate-performance in Lithium-Nickel-Manganese-Cobalt-Oxide (NMC) cathodes filled with various quantities of carbon black, single-walled carbon nanotubes and graphene. The electrode conductivity is always extremely anisotropic with the out-of-plane conductivity, which is most relevant to rate-performance, roughly 1000 smaller than the in-plane conductivity. For all fillers the conductivity increases with filler loading although the nanotube-filled electrodes show by far the most rapid increase. 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 conductivity, with all data points falling on the same master curve. Using a simple mechanistic model for the characteristic time, we develop an equation which 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. This model shows the effect of electrode resistance on the rate-performance to become negligible in almost all cases once the out-of-plane conductivity of the electrode exceeds 1 S/m. Our results show that this can be achieved by including <1wt% single-walled carbon nanotubes in the electrode.