Factors Influencing Li-Ion Energy and Power Capabilities and Implications for Active Material Research
Authors: Ofer David , Rempel Jane , Pullen Adrian , Dalton-Castor Sharon , Barnett Brian , Sriramulu Suresh 
Published: February 2020

Li-ion technology enabled a revolution in consumer electronics by offering the highest energy density among rechargeable battery chemistries resulting in dramatically improved runtimes in a variety of devices. Given its unique combination of energy, power capability and life, lithium-ion technology is poised to revolutionize several emerging applications such as automotive, electric bikes, robotics, and air-borne drones for consumer and business applications. Cells used for each of these applications, which have markedly discharge rates and duty cycles, and must be optimized for energy density and specific energy. For example, the typical consumer electronics application has discharge rates of C/10, battery electric vehicles (BEVs) C/3, plug-in hybrid electric vehicles (PHEVs) 4C, hybrid electric vehicles (HEVs) 10 C, and drones 5C to 10 C.  Designing a Li-ion cell with optimum energy density for each of these duty cycles requires not only selection of suitable active materials, but also suitable electrolytes and separators, and electrode designs that can support the specific duty cycles.

 At CAMX Power, we have been designing Li-ion cells for a wide range of applications including vehicles, electronics applications, lead-acid replacement batteries, and drone applications. Our work has revealed surprising impact of inactive materials on the energy and power capability of Li-ion cells. An important finding has been the role of the separator in determining the rate/power capability, and even energy density of Li-ion cells. Figure 1 shows the discharge rate performance 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 ion-diffusion limitations in the thicker electrode – essentially the thickness of the electrode at the loading of 27.1 mg/cm2 is almost twice the thickness of the electrode at a loading of 15.6 mg/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. These studies 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.

 We have used this improved understanding of the factors that control the rate and power capability of Li-ion cells to design and fabricate high energy Li-ion cells that can be discharged at very high rates. Figure 4 shows measured performance data of a 1.1 Ah CAM-7/silicon-anode based cell that can support > 270 Wh/kg at a 5C rate. A photograph of the cell is shown in Figure 5.

In this presentation, we will further discuss the role of inactive components and cell designs on the rate and power capability of Li-ion cells. 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. We will show how judicious selection of test conditions can prevent erroneous conclusions of active material rate capability. We will provide coin cell design guidelines for measuring the rate capabilities of active materials. Finally, we will describe the key design details of the CAM-7/silicon cells that enable achieving this very high specific energy at 5C discharge rate.

Figure 1