To grasp the wide variety of electrochemical solutions available to the applications engineer, an understanding of three main dimensions of energy measurement and discharge rate capability is an important first step. These three dimensions include: specific energy (Wh/kg) or energy density (Wh/l), specific power (W/kg) or power density (W/l) and discharge rate. By understanding their limitations and optimum use, an applications engineer will be better able to choose the right power solution for his or her application.
The electrochemical universe can generally be divided into four main groups: capacitors, supercapacitors, batteries and fuel cells - based on their unique materials, construction and mode of operation (see graph below).
Capacitors have low energy density, very high power density, and deliver charges very rapidly independent of temperature. Batteries have medium energy and power density, and deliver their energy from one to several decade hours. Fuel cells have very high energy densities, moderately low power densities, and deliver their energy over a time frame of several days.
Where these technology capabilities touch is the domain of hybrid systems where one takes advantage of unique contributions of each technology in unique applications or circumstances.
What types of variables contribute to energy and power densities and discharge capabilities of batteries? For one, increasing the amount of energy liberated by the active materials and packing a large amount of the active materials into a small volume will increase the energy density and specific energy of a battery.
Increasing the power density of a battery system is done by increasing mass transport capability, allowing the rapid movement of ions in and out of electrochemically active interfaces or surfaces, and is achieved by increasing the porosity of the electrodes within the battery.
Optimizing for both energy density and power density is a constant battle, since each are at odds with the other. Increasing energy density, through high packing densities of active materials, lowers the porosity and power density. Likewise, factors that increase power density tend to lower energy density. To cross this chasm, battery engineers employ additives in electrolytes to enhance the conductivity of surface films.
In the next article we will focus on what types of batteries are best suited for specific applications.
By Ralph Wise
Director of Technology
Ultralife
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