Impact of Cell Components on Rate and Power Capability of Li-Ion Cells

Many applications require Li-ion cells that can support high rates and/or high-power pulses. In order to effectively design such cells, it is important to understand the factors that control the rate and/or power capability of Li-ion cells.

Work done at TIAX has shown a surprising impact of the separator employed in the cell on the rate/power capability of Li-ion cells. Figure 1 shows the rate capability of a high-nickel cathode material as a function of discharge rate for three different electrode loadings. The rate capability appears independent of loading for rates less than 1 C. However, at rates higher than 2C, the rate capability appears to depend strongly on the loading. From these data it is tempting to conclude that this result is a consequence of increased ion diffusion limitation in the thicker electrode – essentially the thickness of the electrode at the loading of 27.1mg/cm² is almost twice the thickness of the electrode at a loading of 15.6mg/cm². However, a different conclusion emerges if these same data are plotted as a function of areal current density instead of C-rate (Figure 2). The discharge capacities appear independent of the electrode loading, but depend strongly on the areal current density. Essentially, the ionic limitations are not in the electrode, but elsewhere in the cell. 

Studies at TIAX have shown that the primary ionic limitation under these conditions occurs in the separator. Data presented in Figure 3 demonstrate this effect. Essentially, by changing only the separator the discharge capacity at a current density of 16 mA/cm²(corresponding to 5 C) can be increased by a factor of four.

These combined results discussed above show that the ionic transport limitations in the separator play a major role in determining the rate capability of Li-ion cells. In this paper, we will present rate data using separators with a wide range of physical properties, and demonstrate a correlation between the rate capability and separator properties. Using data comparing the rate capability of different cathode active materials, we will discuss the implications of our findings for research aimed at understanding the power and rate capability of active materials. Finally, we will provide coin cell design guidelines for measuring the rate capabilities of active materials.