Is There A Lead-acid Hybrid BatteryCapacitor In The future?
by Donald Georgi
Lead is dead! (?) Typical comments have been around for the past fifteen years. Such comments are usually followed up by such statement as : “Oh sure, there are still a few million cars driving around with Lead-acid batteries, but lead is poisonous and has low energy density. It is relatively safe if you don’t overcharge and ignite the outgassing hydrogen; it is robust if you swap out the car battery every three to five years.” But the question that should be asked is: “Does this chemistry have a future?
So when the faithful gathered at the BCI (Battery Council International) Convention in Palm Springs this May to sift through the current state of Lead-acid batteries, there were new exciting announcements in Lead-acid relating to the production growth matching that of the economic recovery this past year. Although the presentations were well planned and addressed, not much of the news was technically new until Dr. Andrew Burke spoke of the automotive fuel cell’s effect on the battery market. Although he reminded the audience of the recent growth of Nickel-metal hydride and Lithium-ion batteries in current hybrid research, he planted a unique seed for Lead-acid in a new configuration which can be best described as a hybrid battery/capacitor, hereafter referred to as a BattCap. (BattCap is BD’s coined phrase and has no relation to any scientific or business group.)
When the power in an auto is to be shared between a gasoline engine and a rechargeable battery, the result is a hybrid auto. So, what would be the result be if the energy stored in a battery were shared with an integral double-layer capacitor and a battery electrode? Should the new combination qualify as a hybrid battery-capacitor ( BattCap?)
The new combination is neither a high-tech capacitor soldered in parallel with the Lead-acid battery inside the case, nor is it a hybrid capacitor1 which is a combination of an electolytic and electrochemical capacitor. It is also not a pseudo-capacitor which is a double layer capacitor having the carbon electrode materials replaced with metal oxide at both electrodes.2 The BattCap has a carbon electrochemical capacitor at one terminal and a battery electrode at the other. (See Figure 1.) So, could it have the carbon replaced with metal oxide and be a Pseudo-BattCap? Possibly, but this discussion is limited to a carbon-based construction.
Why is the Lead-acid version of the BattCap hybrid being considered?
Lead-acid batteries are not all things to all people. The battery used to start the auto is optimized to give very high currents for very short periods of time, whereas the telecom backup battery (which must perform for many hours when the power distribution grid goes down) does not require high peak currents but does need maximum energy content over a long discharge period. The instrument or boat battery, which must deliver power almost to its last electron, is maximized to recover from repeated deep discharges.
In the hybrid auto configuration, the need for immediate passing power places the high power requirement on the battery, but concurrently, the more energy available from the battery, the smaller its physical size can be. That equates to needing both more energy density along with greater power density.
High energy density is the realm of Nickel-metal hydride and Lithium-ion batteries, neither of which provides stellar power density, especially when optimized for maximum energy density. While providing superior energy density to Lead-acid batteries, they also carry a premium in price, a factor which has played an important part in the lack of acceptance first of the pure electric cars, and now the gasoline/battery hybrid.
The recipe for success for energy storage is to have high power density, high energy density, rapid charge acceptance for dynamic braking, and while the ideal is being defined, include long cycle and calendar life and low cost. It is difficult to embody all these characteristics in a single chemistry.
Hybrid battery/supercapacitor technology tries to use the best features of both the carbon capacitor and the Lead-acid battery. Papers have been reviewed which have hybrid vehicles perform with an IC engine, an individual supercapacitor and another individual battery.3
For the BattCap, consideration is being given to a new integrated device configured with the carbon-based capacitor as the negative electrode and a lead-oxide (PbO2) positive electrode which are connected by a sulfuric acid electrolyte and separator. (See Figure 1) This is the subject of investigation by the University of California’s Dr. Andrew Burke and associates.4 The concept has been pursued for only a few years, but it has withstood the rigors of theoretical and experimental investigations. Similar work has been done by Dr. John Miller of JME and the American Electric Power Corp. with some Russian scientists.
Construction, benefits and future direction
The construction consists of a half capacitor made up of a carbon negative electrode, storing energy in the electric double layer; a sulfuric acid electrolyte, and a lead oxide positive electrode which stores energy by electrochemical (Faradic) reaction. To obtain long cycle life, the battery electrode is oversized by about ten times and only a small fraction of its capacity is used for a complete discharge of the carbon electrode. Such a marriage provides relatively high energy density (10-15 Wh/kg), the high current/power for vehicle start/acceleration/braking demands, low cost of Lead-acid batteries, and hopefully, needed cycle life.
One benefit of using the combination of the carbon and battery material is the larger Voltage range for the carbon electrode of 1 Volt instead of the usual 0.5 Volts because the battery electrode operates at a near constant Voltage . Since capacitor energy is proportional, the change in Voltage squared (the energy stored in the supercap carbon electrode) is four times greater. Since charge must be conserved, this allows for a greater energy storage in the battery electrode. In other words, this is the fundamental reason for creating the hybrid and its higher energy density. (See Figure 2) Selecting Lead-acid construction provides for a greater cell Voltage of 2.2 Volts compared to the nickel hydroxide hybrid which only has a rated cell Voltage of 1.5. Then too, there is the projection that a Lead-acid hybrid should be buildable for less money than other hybrids because of the inherent lower cost of the lead materials and their well-developed processing methods. Lead-acid batteries have traditionally had cycle life limitations, but the use of the oversized lead oxide electrode will permit it to cycle through very shallow discharges and thus greatly extend its cycle life.
A number of combinations of the carbon supercapacitor and battery materials are presently being investigated by other researchers. (See figure 3.) Carbon and nickel hydroxide are being pursued at Florida Atlantic University; lithium/ titanium - oxide in the negative electrode with carbon in the positive is being studied by Telecordia Technologies, and carbon in the negative and lithium intercalation materials in the positive by the Ness Corp. in Korea. Each of these devices has advantageous performance in one area or another, but they must all meet all the performance and cost requirements of the demanding transportation applications. Cycle and calendar life of the battery electrodes are the major concerns of the hybrid capacitors. The cycle and calendar life of the carbon electrode is very good. The integral device must retain this capability. Since cost has been a major barrier in the use of supercapacitors, the final proof of suitability for the BattCap requires real production costs with long cycle and calendar life devices.
Dr. Burke is planning a next configuration which will closely resemble a prototype device which would be representative of production units. But high tech solutions usually involve high tech development and manufacturing costs. This may be an advantage for the Lead-acid BattCap in that it uses the familiar and less costly Lead-acid battery based construction. Prior work has been performed on thin film Lead-acid batteries during the era of electric vehicle development in the 1990’s by Boulder Technologies and later by Johnson Controls which acquired the Boulder technology. Dr. Burke hopes to benefit from those experiences with the thin-film lead electrodes on foils. Forming the carbon electrode on thin foils is routine in the carbon/carbon ultrapcap industry, so the major problem in assembling the carbon/lead oxide device is forming the thin lead electrode. Dr. Burke is presently seeking a Lead-acid battery manufacturer as a partner in the next phase of his research.
The Lead-acid BattCap is not a ‘done deal,’ but the rigorous methodology followed by Dr. Burke has brought the concept to a point where there is a possibility for it to make a cost effective contribution in improved energy storage for ICE/hybrid and fuel cell/hybrid vehicles.
3. Batteries Digest Newsletter, Issue 96, p. 8.
4. Development of Advanced Electrochemical Capacitors using Carbon and Lead-oxide Electrodes for Hybrid Vehicle Applications. UCD-ITS-RR-03-2. Prepared for Calstart on Contract CS-AVP99-03 by University of California-Davis.
5. Batteries Digest Newsletter, Issue 36, p 6.