  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.
