Hybrid

The assembled large version of the Lead-acid hybrid supercapacitor/battery is readied for testing at the University of California, Davis. While not as esthetically enclosed as a production battery, the experimental device provides data to determine the characteristics of the design. The lead oxide battery electrode was fabricated using materials from a thin film Inspiron battery. Carbon sheets 350 microns in thickness formed the negative double layer capacitor electrode, giving it a 2000 m2/gm surface area. The electrolyte is sulfuric acid and the separator is paper. +

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.)

Dr. Andrew Burke of the Institute of Transportation Studies at the University of California, Davis, presented a novel combination of a supercapacitor electrode sandwiched with a Lead-acid battery electrode into a hybrid device which would combine the high power of the supercap, the energy of the battery and the low cost of Lead-acid construction. If successful, it could mark a new path for the progression of Lead-acid chemistry. +

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.

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.

The general operating principles of the device are that the electrical charge stored in the device is dependent on the capacitance of the carbon electrode and its rated operating Voltage is dependent on the characteristics of the battery-like electrode. In general, the two electrodes are sized such that the carbon electrode is deeply charged/discharged and the battery-like electrode undergoes only very shallow charge/discharge cycles to promote cycle life. The current collector material is selected to be compatible with the electrolyte and battery-like material. Collector material thickness is selected with trade-offs between energy density and power density. +

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.

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.

Figure 2: The cell Voltage of the hybrid capacitor moves up and down as the carbon electrode is charged and discharged. The rated Voltage of the cell is the sum of the standard potential of the battery-like electrode and Voltage change in the carbon electrode unless the sum exceeds the maximum value for the gassing Voltage of the battery-like electrode. Energy stored is the sum of the energy stored at the negative and positive electrodes.

One of the advantages of the hybrid device is that the Voltage change at the carbon electrode can be 1 Volt compared to 0.5 Volts in a carbon/carbon device. The flatter discharge profile of the battery electrode makes the greater carbon electrode Voltage possible. +

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.

Figure 3: Rangone curves for various high power devices show where the Lead-acid BattCap fits into the performance field of hybrid devices. With 21 cm2 of active area, the device produces 6 Amps. Capacitance is between 38-44 Farads, and the BattCap has an internal resistance ranging from 0.015 to 0.03 Ohms. Providing 80 W/kg of power, the device has 8.7 Wh/kg of energy density.

Competitive hybrid devices such as the Telcordia hybrid with lithium/titanium-oxide have similar performance, but Lead-acid hybrids offer the possibility of lower costs when used in hybrid vehicles. The ideal characteristics of the Lead-acid hybrid BattCap are projected to be 15.7 Wh/kg with 8900 W/kg at 95 % efficiency. Present supercapacitors provide nominal 5 Wh/kg and cost $2.50/Wh. The projected cost for the Lead-acid hybrid BattCap could be in the range of $0.35/Wh. Supercaps cost about $10/kg whereas projected estimates for the BattCap could be $4.50/kg. +

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.