Energy Density vs Power Density
As discussed, in electrical energy storage devices, there exists a trade-off between energy storage capacity (i.e. energy density or specific energy) and power delivery capability (i.e. power density or specific power). Figure 1 is a Ragone diagram that shows the relationship between power and energy in various technologies. It is therefore critical to identify the application requirements in terms of energy and power for an energy storage device. There is not a one-size-fits- all solution. For this reason, Saft offers four levels of power and energy products:
• E series: high energy Li-ion
• P and M series: medium to high power
• V series: very high power; and
• U series: ultra-high power.
The power and energy capacities of these different battery technologies are shown in Figure 1.
For modern high-power electric weapon systems such as direct energy weapons (lasers), high-power microwave and kinetic energy weapons (electromagnetic launchers), the power requirement is very high with a very short duration (i.e. a pulsed power requirement). Traditionally, electrostatic capacitors were the preferred power sources; however, a capacitor’s power delivery is inversely exponential with respect to time. For a majority of such applications, some sort of pulse forming/shaping circuits would be required.
Recently, Saft has developed a new line of ultra-high-power batteries that have demonstrated pulsed power capability equal to or higher than that of the supercapacitor or Li-ion capacitor at a power level of tens of kW/kg. This new technology widens the choice of power sources for the electric fire community. The new pulsed power sources based on Li-ion technology have two distinct product groups: Li-ion battery and Li-ion capacitor. The Li-ion capacitor has also been termed Li-ion hybrid capacitor, because the Li-ion capacitor is composed of one Li-ion battery electrode that carries out an electrochemical reaction and one capacitor electrode that stores charges electrostatically. Figure 2 is a summarized overview of the performance specifications of each technology.
As can be seen from the specifications, no one technology can surpass the rest in all performance categories. For example, the super-capacitor can achieve one million cycles but has the lowest power and energy density. The Li-ion battery can deliver the most current, voltage, power, and energy, but its cycle life is limited. This is due to the fact that energy charge and discharge involve Li-ion extraction and insertion into electrode materials. This action causes physical dimension change and material fatigue over cycling. Since the Li-ion capacitor is half battery, its cycle life is also limited relative to the super capacitor.
By innovatively enhancing the cell’s mechanical design while optimizing the electrochemical portion of the cell, Saft SDD has successfully developed a Li-ion battery that can simultaneously supply power and energy density that is high enough to satisfy the requirements of a range of high-power defense and space applications. And while its cycle life is comparably limited, the 40,000 cycles achieved by the Saft VL5U (shown here) provides such an application with a cycle life ample enough to outlast the weapon itself.
This article was written by Yvonne Chen, Sr. Research Scientist, Saft SDD (Bagnolet, France). For more information, visit http://info.hotims.com/40440-501.