The desire to protect the environment is driving consumer demands for electric vehicles that use non-polluting propulsion systems. Nickel-metal hydride and several versions of lithium-ion (Li-ion) batteries have been used in the power management of electric vehicles with mixed results.
Some power-management systems also have used ultracapacitors to augment the performance of electric vehicles. And though it has been around for about 10 years, the lithium-iron-phosphate (LiFePO4) battery is getting the attention of electric vehicle manufacturers (Fig. 1).
Lithium-based batteries offer one of the best energy-toweight ratios and a short life cycle. Also, they don’t produce any memory effect. Yet some Li-ion batteries can be dangerous if they’re mistreated, and a lack of proper care can reduce their lifespan.
Most lithium batteries previously used in electric vehicles employed a lithium-cobalt-oxide (LiCoO2) cathode material. Other lithium battery cathodes used either lithiummanganese oxide (LiMn2O4) or lithium-nickel oxide (LiNiO2). Anodes were always made of carbon. But these lithium batteries have two key disadvantages—slow charge and discharge rates.
The Pros and Cons of LiFePO4
The LiFePO4 cathode battery improves the charge/discharge rate as well as the ability to store energy. And because it is derived from Li-ion technology, the LiFePO4 chemistry shares many of the advantages and disadvantages of Li-ion chemistry. For example, LiFePO4 cells can supply a higher discharge current.
Also, LiFePO4 is an intrinsically safer cathode material than LiCoO2. The Fe-P-O bond is stronger than the Co-O bond, so when the battery is abused (short-circuited, overheated, etc.), the oxygen atoms are much harder to remove.
Breakdown can occur under extreme heating, generally over 800°C. Yet LiFePO4 batteries don’t have the thermal runaway that LiCoO2 batteries may exhibit. LiFePO4 batteries also have the best safety characteristics, accommodating up to 2000 charge/discharge cycles. But LiFePO4 technology has a negative side, compared to other Li-ion technologies.
The minimum cell discharge voltage is 2.8 V; its working voltage is 3.0 to 3.3 V; and the maximum charge voltage is 3.6 V. A conventional Li-ion battery charge voltage is 4.2 V. Also, the LiFePO4 capacity/size ratio is lower than the Li- CoO2 battery, so it requires further development to improve this characteristic.
Still, LiFePO4 batteries have a potentially lower cost than their lithium-based counterparts, particularly when they are more widely used. They suit electric bikes, scooters, and cars, as well as power tools, UPS, and solar energy systems. Battery manufacturers around the world are currently working to find a way to get the maximum storage performance out of smaller and lighter LiFePO4 batteries.
Electric vehicle batteries have a limited ability to capture and regenerate energy, or provide bursts of high power during short duration events, such as acceleration and braking. This high-power limitation reduces the efficiency of the electric drive system. Because most vehicles are in a constant brake/acceleration state, the ability to capture and regenerate braking energy can be important (Fig. 2).
Vehicle manufacturers require an electric power and storage system that overcomes the limitations of the batteries as well as those of the vehicles themselves. Therefore, ultracapacitors provide a solution by using the regenerative braking to store energy that could be applied for further acceleration or for the basic energy needs of supplementary electrical systems. The associated power-management system controls ultracap operation.
Ultracaps offer performance usable down to –40°C, while most batteries don’t operate reliably below 0°C without heating. Also, ultracaps have a long life cycle and usually run for the duration of the lifetime of the machine where they are installed, resulting in cost savings.
They have an efficiency of 85% to 95%, compared with an average of 70% or lower for most batteries. Environmentally friendly, they are 70% recyclable and do not include heavy metals. And, they can provide more than 10 times the power of batteries.
Ultracaps are electrochemical capacitors with an unusually high energy density compared with common capacitors—on the order of thousands of times greater than a highcapacity electrolytic capacitor. A typical D-cell sized electrolytic capacitor may have a value measured in microfarads, whereas the same size ultracap could store several farads for an improvement of about 10,000 times. Larger commercial ultracaps have capacities as high as 3000 farads.
A conventional capacitor stores energy by removing charge carriers, typically electrons, from one metal plate and depositing them on another. The total energy that’s stored in this fashion is a combination of the number of charges stored and the potential between the plates.
The number of charges is essentially a function of size and the material properties of the plates, whereas the potential between the plates is limited by the dielectric breakdown between the plates. Various materials can be inserted between the plates to allow higher voltages to be stored, leading to higher energy densities for a given size.
In contrast with traditional capacitors, ultracaps do not have a conventional dielectric. Their structure contains an electrical double layer. In a double layer, the effective thickness of the dielectric is exceedingly thin—on the order of nanometers. Combined with the very large surface area, that thinness is responsible for their very high capacitances in practical sizes.
In an electrical double layer, each layer by itself is quite conductive. But the physics at the interface where the layers are effectively in contact means that no significant current can flow between the layers.
However, the double layer can withstand only a low voltage. That means that ultracaps that are rated for higher voltages must be made of matched series-connected individual ultracaps, much like series-connected cells in higher-voltage batteries.
Ultracaps improve storage density through the use of a nanoporous material in place of the conventional insulating barrier, typically activated charcoal. Activated charcoal is a powder made up of extremely small and rough particles. In bulk, they form a low-density volume of particles with holes between them that resembles a sponge.
The overall surface area of even a thin layer of such a material is many times greater than a traditional material like aluminum, allowing many more electrons to be stored in any given volume. The downside is that the charcoal replaces improved insulators used in conventional devices. So in general, ultracaps operate at potentials of about 2 to 3 V.