Electrical energy has been stored in electrochemical batteries, capacitors, and double layer or ultra capacitors. Other mechanisms like pumped hydro, thermal systems, and even flywheels store energy that can be used to generate electricity. Surface-mediated cells (SMC) offer us the advantage of increasing both power and energy densities.
Power Density is a measure of how quickly a charge can be transferred. It is typically measured in units of power (watts, KW, joules/second) over weight. Power is what we need in an electric car to start the vehicle moving, to accelerate, and for regenerative braking. Some devices are known for their high power ratings. Capacitors, ultra caps (often measured in mF and Farads) and flywheels are particularly good with power density because the electricity is held as a static charge on the surface of plates or in the kinetic movement of flywheels. There is no intermediate chemical step as there is with batteries. Devices with a high power density will charge and discharge quickly. Hybrids that have an additional energy source particularly need energy storage that has an emphasis on power density.
Energy Density will tell us how long the electricity can continue to flow. It is measured in units of power over time (watt-hours, KW-hr, joules and often for batteries AH) over weight. Within the chemical bonds of batteries a more electrical energy can be stored than is generally available with capacitors or flywheels. Energy density helps to give us the range in an electrical vehicle. Battery electric vehicles (laptops, and cell phones) that don’t have an additional energy source especially need energy storage that emphasizes energy density. This is particularly true in vehicles as the weight of the battery must be carried using the energy stored in the device.
Other important considerations may be the power and energy density over volume, the battery efficiency, the battery cost (often expressed as a fraction with power density or energy density as in cost/KW-hr), the cost of recycling/disposal and life cycles before failure. Battery manufacturers want to increase the energy density but may wish to increase charge/discharge times, which will also increase power density. Capacitor manufacturers want to increase the energy density of their products. There has been an assumption that overall battery qualities could not be improved without some compromise. SMC technology challenges this assumption in this latest of a series of announced improvements with their technology.
The approach taken by Dr. Bor Jang Co-Founder and Chief Executive Officer of Angstron Materials Inc. (privately held; Dayton, Ohio) uses the “exchange of lithium ionsintercalation or deintercalation.” The operation is more like a capacitor than a battery and the power and energy densities reflect this. The device is presently measured at an energy density of 160 Wh/kg/cell and a power density of 100 kW/kg/cell. This might compare with a Tadiran lithium thionyl chloride battery, which offers an energy density of up to 710 Wh/Kg. However, these are primary energy cells and not secondary, rechargable cells. Atomic batteries can have energy densities many times that of chemical batteries.
The highest power densities are not as frequently discussed. The SMC technology claims to achieve 10 times the power density of the highest ultracapacitors. The highest power densities may be useful for military rail guns where a “power source must provide roughly 6.5 million Amps.”
Angstron Materials Inc. is a spinoff from Nanotek Instruments (privately held, you will need Chinese packets to read the website completely). Nanotek Instruments holds several patents, including one for Nanoscale Graphene Plates (NGPs), a less costly alternative to carbon nanotubes. Earlier this year, it received several patents for “Graphite-carbon composite electrode for supercapacitors.” Other than magnitude, it is not clear if the SMC technology is different than a similar technology announced by MIT last year.
Primary source and photocredit on SMC: Nano Letters