Increasing Volumetric Capacity Diminishes Rate Performance in Li-Ion Battery Electrodes

Combining high-capacity electrodes with good rate performance is essential for maximising both energy and power density in Li-ion batteries. While much effort has been dedicated to increasing both capacity and rate performance, little consideration has been made as to how an increase in specific or volumetric capacity might directly affect rate performance. Here, we quantitatively examine the relationship between the volumetric capacity of Li-storing electrodes and their rate performance using graphite/boron-nitride composite electrodes with a range of compositions as a model system. The rate performance of these cells is evaluated by fitting capacity vs rate curves to a semi-empirical equation and extracting a characteristic charge/discharge time. As graphite content and so electrode capacity are increased, we observe a linear, threefold increase of this characteristic time, representing a significant degradation in rate performance. This shows that capacity and rate performance are anti-correlated, an observation is consistent with the predictions of a simple physical model which shows this effect to be associated with the RC charging time of the electrode. Using no adjustable parameters, we find excellent agreement between the model and our experimental data.