Basic Concepts of Battery
Contents:
- Components of Cells and Batteries
- Classification of Cells and Batteries
- Operation of a Cell
- Theoretical Cell Voltage, Capacity, and Energy
- Specific Energy and Energy Density of Practical Batteries
- Upper Limits of Specific Energy and Energy Density
ABOUT THE EDITORS
David Linden has been active in battery research, development, and engineering for more than 50 years. He was Director of the Power Sources Division of the U.S. Army Electronics R&D Command. Many of the batteries and power sources currently in use, including lithium batteries and fuel cells, resulted from R&D programs at that Division. Mr. Linden is now a battery consultant working with Duracell, Inc. and other companies on the development and application of newer primary and rechargeable batteries. He is a member of national and international groups establishing standards for these new technologies.
1. COMPONENTS OF CELLS AND BATTERIES
A battery is a device that converts the chemical energy contained in its active materials directly into electric energy by means of an electrochemical oxidation-reduction (redox) reaction. In the case of a rechargeable system, the battery is recharged by a reversal of the process. This type of reaction involves the transfer of electrons from one material to another through an electric circuit. In a nonelectrochemical redox reaction, such as rusting or burning, the transfer of electrons occurs directly and only heat is involved. As the battery electrochemically converts chemical energy into electric energy, it is not subject, as are combustion or heat engines, to the limitations of the Carnot cycle dictated by the second law of thermodynamics. Batteries, therefore, are capable of having higher energy conversion efficiencies.
While the term ‘‘battery’’ is often used, the basic electrochemical unit being referred tois the ‘‘cell.’’ A battery consists of one or more of these cells, connected in series or parallel, or both, depending on the desired output voltage and capacity.*
The cell consists of three major components:
- The anode or negative electrode—the reducing or fuel electrode—which gives up electrons to the external circuit and is oxidized during the electrochemical reaction.
- The cathode or positive electrode—the oxidizing electrode—which accepts electrons from the external circuit and is reduced during the electrochemical reaction.
*Cell vs. Battery: A cell is the basic electrochemical unit providing a source of electrical energy by direct conversion of chemical energy. The cell consists of an assembly of electrodes, separators, electrolyte, container and terminals. A battery consists of one or more electrochemical cells, electrically connected in an appropriate series / parallel arrangement to provide the required operating voltage and current levels, including, if any, monitors, controls and other ancillary components (e.g. fuses, diodes), case, terminals and markings. (Although much less popular, in some publications, the term ‘‘battery’’ is considered to contain two or more cells.)
Popular usage considers the ‘‘battery’’ and not the ‘‘cell’’ to be the product that is soldor provided to the ‘‘user.’’ In this 3rd Edition, the term ‘‘cell’’ will be used when describing the cell component of the battery and its chemistry. The term ‘‘battery’’ will be used when presenting performance characteristics, etc. of the product. Most often, the electrical data is presented on the basis of a single-cell battery. The performance of a multicell battery will usually be different than the performance of the individual cells or a single-cell battery (see Section 3.2.13).
- The electrolyte—the ionic conductor—which provides the medium for transfer of charge, as ions, inside the cell between the anode and cathode. The electrolyte is typically a liquid, such as water or other solvents, with dissolved salts, acids, or alkalis to impart ionic conductivity. Some batteries use solid electrolytes, which are ionic conductors at the operating temperature of the cell.
The most advantageous combinations of anode and cathode materials are those that will be lightest and give a high cell voltage and capacity (see Sec. 1.4). Such combinations may not always be practical, however, due to reactivity with other cell components, polarization, difficulty in handling, high cost, and other deficiencies.
In a practical system, the anode is selected with the following properties in mind: efficiency as a reducing agent, high coulombic output (Ah/g), good conductivity, stability, easeof fabrication, and low cost. Hydrogen is attractive as an anode material, but obviously, must be contained by some means, which effectively reduces its electrochemical equivalence. Practically, metals are mainly used as the anode material. Zinc has been a predominant anode because it has these favorable properties. Lithium, the lightest metal, with a high value of electrochemical equivalence, has become a very attractive anode as suitable and compatible electrolytes and cell designs have been developed to control its activity.
The cathode must be an efficient oxidizing agent, be stable when in contact with theelectrolyte, and have a useful working voltage. Oxygen can be used directly from ambient air being drawn into the cell, as in the zinc/air battery. However, most of the common cathode materials are metallic oxides. Other cathode materials, such as the halogens and the oxyhalides, sulfur and its oxides, are used for special battery systems.
The electrolyte must have good ionic conductivity but not be electronically conductive, as this would cause internal short-circuiting. Other important characteristics are nonreactivity with the electrode materials, little change in properties with change in temperature, safety in handling, and low cost. Most electrolytes are aqueous solutions, but there are important exceptions as, for example, in thermal and lithium anode batteries, where molten salt and other nonaqueous electrolytes are used to avoid the reaction of the anode with the electrolyte.
Physically the anode and cathode electrodes are electronically isolated in the cell to prevent internal short-circuiting, but are surrounded by the electrolyte. In practical cell designs a separator material is used to separate the anode and cathode electrodes mechanically. The separator, however, is permeable to the electrolyte in order to maintain the desired ionic conductivity. In some cases the electrolyte is immobilized for a nonspill design. Electrically conducting grid structures or materials may also be added to the electrodes to reduce internal resistance.
The cell itself can be built in many shapes and configurations—cylindrical, button, flat, and prismatic—and the cell components are designed to accommodate the particular cell shape. The cells are sealed in a variety of ways to prevent leakage and dry-out. Some cells are provided with venting devices or other means to allow accumulated gases to escape. Suitable cases or containers, means for terminal connection and labeling are added to complete the fabrication of the cell and battery.
2. CLASSIFICATION OF CELLS AND BATTERIES
Electrochemical cells and batteries are identified as primary (nonrechargeable) or secondary (rechargeable), depending on their capability of being electrically recharged. Within this classification, other classifications are used to identify particular structures or designs. The classification used in this handbook for the different types of electrochemical cells and batteries is described in this section.
2.1 Primary Cells or Batteries
These batteries are not capable of being easily or effectively recharged electrically and, hence, are discharged once and discarded. Many primary cells in which the electrolyte is contained by an absorbent or separator material (there is no free or liquid electrolyte) are termed ‘‘dry cells.’’
The primary battery is a convenient, usually inexpensive, lightweight source of packaged power for portable electronic and electric devices, lighting, photographic equipment, toys, memory backup, and a host of other applications, giving freedom from utility power. Thegeneral advantages of primary batteries are good shelf life, high energy density at low to moderate discharge rates, little, if any, maintenance, and ease of use. Although large highcapacity primary batteries are used in military applications, signaling, standby power, and so on, the vast majority of primary batteries are the familiar single cell cylindrical and flat button batteries or multicell batteries using these component cells.
2.2 Secondary or Rechargeable Cells or Batteries
These batteries can be recharged electrically, after discharge, to their original condition by passing current through them in the opposite direction to that of the discharge current. They are storage devices for electric energy and are known also as ‘‘storage batteries’’ or ‘‘accumulators.’’
The applications of secondary batteries fall into two main categories:
- Those applications in which the secondary battery is used as an energy-storage device, generally being electrically connected to and charged by a prime energy source and delivering its energy to the load on demand. Examples are automotive and aircraft systems, emergency no-fail and standby (UPS) power sources, hybrid electric vehicles and stationary energy storage (SES) systems for electric utility load leveling.
- Those applications in which the secondary battery is used or discharged essentially as a primary battery, but recharged after use rather than being discarded. Secondary batteries are used in this manner as, for example, in portable consumer electronics, power tools, electric vehicles, etc., for cost savings (as they can be recharged rather than replaced), and in applications requiring power drains beyond the capability of primary batteries.
Secondary batteries are characterized (in addition to their ability to be recharged) by high power density, high discharge rate, flat discharge curves, and good low-temperature performance. Their energy densities are generally lower than those of primary batteries. Their charge retention also is poorer than that of most primary batteries, although the capacity of the secondary battery that is lost on standing can be restored by recharging.
Some batteries, known as ‘‘mechanically rechargeable types,’’ are ‘‘recharged’’ by replacement of the discharged or depleted electrode, usually the metal anode, with a fresh one. Some of the metal/ air batteries (Chap. 38) are representative of this type of battery.
2.3 Reserve Batteries
In these primary types, a key component is separated from the rest of the battery prior to activation. In this condition, chemical deterioration or self-discharge is essentially eliminated, and the battery is capable of long-term storage. Usually the electrolyte is the component that is isolated. In other systems, such as the thermal battery, the battery is inactive until it is heated, melting a solid electrolyte, which then becomes conductive.
The reserve battery design is used to meet extremely long or environmentally severe storage requirements that cannot be met with an ‘‘active’’ battery designed for the same performance characteristics. These batteries are used, for example, to deliver high power for relatively short periods of time, in missiles, torpedoes, and other weapon systems.
1.2.4 Fuel Cells
Fuel cells, like batteries, are electrochemical galvanic cells that convert chemical energy directly into electrical energy and are not subject to the Carnot cycle limitations of heat engines. Fuel cells are similar to batteries except that the active materials are not an integral part of the device (as in a battery), but are fed into the fuel cell from an external source when power is desired. The fuel cell differs from a battery in that it has the capability of producing electrical energy as long as the active materials are fed to the electrodes (assuming the electrodes do not fail). The battery will cease to produce electrical energy when the limiting reactant stored within the battery is consumed.
The electrode materials of the fuel cell are inert in that they are not consumed during thecell reaction, but have catalytic properties which enhance the electroreduction or electrooxidation of the reactants (the active materials).
The anode active materials used in fuel cells are generally gaseous or liquid (comparedwith the metal anodes generally used in most batteries) and are fed into the anode side of the fuel cell. As these materials are more like the conventional fuels used in heat engines, the term ‘‘fuel cell’’ has become popular to describe these devices. Oxygen or air is the predominant oxidant and is fed into the cathode side of the fuel cell.
Fuel cells have been of interest for over 150 years as a potentially more efficient and lesspolluting means for converting hydrogen and carbonaceous or fossil fuels to electricity compared to conventional engines. A well known application of the fuel cell has been the use of the hydrogen/oxygen fuel cell, using cryogenic fuels, in space vehicles for over 40 years. Use of the fuel cell in terrestrial applications has been developing slowly, but recent advances has revitalized interest in air-breathing systems for a variety of applications, including utility power, load leveling, dispersed or on-site electric generators and electric vehicles.
Fuel cell technology can be classified into two categories
- Direct systems where fuels, such as hydrogen, methanol and hydrazine, can react directly in the fuel cell
- Indirect systems in which the fuel, such as natural gas or other fossil fuel, is first converted by reforming to a hydrogen-rich gas which is then fed into the fuel cell
Fuel cell systems can take a number of configurations depending on the combinations offuel and oxidant, the type of electrolyte, the temperature of operation, and the application, etc.
More recently, fuel cell technology has moved towards portable applications, historicallythe domain of batteries, with power levels from less than 1 to about 100 watts, blurring the distinction between batteries and fuel cells. Metal/ air batteries (see Chap. 38), particularly those in which the metal is periodically replaced, can be considered a ‘‘fuel cell’’ with the metal being the fuel. Similarly, small fuel cells, now under development, which are ‘‘refueled’’ by replacing an ampule of fuel can be considered a ‘‘battery.’’
Fuel cells were not included in the 2nd Edition of this Handbook as the technical scopeand applications at that time differed from that of the battery. Now that small to medium size fuel cells may become competitive with batteries for portable electronic and other applications, these portable devices are covered in Chap. 42. Information on the larger fuel cells for electric vehicles, utility power, etc can be obtained from the references listed in Appendix F ‘‘
3. OPERATION OF A CELL
3.1 Discharge
The operation of a cell during discharge is also shown schematically in Fig. 1.1. When the cell is connected to an external load, electrons flow from the anode, which is oxidized, through the external load to the cathode, where the electrons are accepted and the cathode material is reduced. The electric circuit is completed in the electrolyte by the flow of anions (negative ions) and cations (positive ions) to the anode and cathode, respectively.
The discharge reaction can be written, assuming a metal as the anode material and a cathode material such as chlorine (Cl2), as follows:
Negative electrode: anodic reaction (oxidation, loss of electrons)
Zn → Zn2+ + 2e
Positive electrode: cathodic reaction (reduction, gain of electrons)
Cl2+ 2e → 2Cl-
Overall reaction (discharge):
Zn + Cl2→ Zn2++ 2Cl- (ZnCl2 )
3.2 Charge
During the recharge of a rechargeable or storage cell, the current flow is reversed and oxidation takes place at the positive electrode and reduction at the negative electrode, as shown in Fig. 1.2. As the anode is, by definition, the electrode at which oxidation occurs and the cathode the one where reduction takes place, the positive electrode is now the anode and the negative the cathode.
In the example of the Zn/Cl2 cell, the reaction on charge can be written as follows:
Negative electrode: cathodic reaction (reduction, gain of electrons)
Zn2+ +2e → Zn
Positive electrode: anodic reaction (oxidation, loss of electrons)
2Cl-→ Cl2 + 2e
Overall reaction (charge):
Zn2+ + 2Cl-→ Zn + Cl2
3.3 Specific Example: Nickel-Cadmium Cell
The processes that produce electricity in a cell are chemical reactions which either releaseor consume electrons as the electrode reaction proceeds to completion. This can be illustrated with the specific example of the reactions of the nickel-cadmium cell. At the anode (negative electrode), the discharge reaction is the oxidation of cadmium metal to cadmium hydroxide with the release of two electrons,
Cd + 2OH-→ Cd(OH)2+2e
At the cathode, nickel oxide (or more accurately nickel oxyhydroxide) is reduced to nickel hydroxide with the acceptance of an electron,
NiOOH + H2O +e → OH- + Ni(OH)2
When these two ‘‘half-cell’’ reactions occur (by connection of the electrodes to an external discharge circuit), the overall cell reaction converts cadmium to cadmium hydroxide at the anode and nickel oxyhydroxide to nickel hydroxide at the cathode,
Cd + 2NiOOH + 2H2O → Cd(OH)2+2Ni(OH)2
This is the discharge process. If this were a primary non-rechargeable cell, at the end ofdischarge, it would be exhausted and discarded. The nickel-cadmium battery system is, however, a secondary (rechargeable) system, and on recharge the reactions are reversed. At the negative electrode the reaction is:
Cd(OH)2+ 2e → Cd + 2OH-
At the positive electrode the reaction is:
Ni(OH)2+ OH-→ NiOOH +H2O +e
After recharge, the secondary battery reverts to its original chemical state and is ready forfurther discharge. These are the fundamental principles involved in the charge–discharge mechanisms of a typical secondary battery.
3.4 Fuel Cell
A typical fuel cell reaction is illustrated by the hydrogen/oxygen fuel cell. In this device, hydrogen is oxidized at the anode, electrocatalyzed by platinum or platinum alloys, while at the cathode oxygen is reduced, again with platinum or platinum alloys as electrocatalysts. The simplified anodic reaction is
2H2→ 4H++ 4e
while the cathodic reaction is
O2+ 4H+ + 4e → 2H2O
The overall reaction is the oxidation of hydrogen by oxygen, with water as the reaction product.
2H2+ O2→ 2H2O
4. THEORETICAL CELL VOLTAGE, CAPACITY, AND ENERGY
The theoretical voltage and capacity of a cell are a function of the anode and cathode materials. (See Chap. 2 for detailed electrochemical theory.)
4.1 Free Energy
Whenever a reaction occurs, there is a decrease in the free energy of the system, which is expressed as
Delta G0=-nFE0
where F = constant known as Faraday (~96,500 C or 26.8 Ah)
n = number of electrons involved in stoichiometric reaction
E0= standard potential, V
1.4.2 Theoretical Voltage
The standard potential of the cell is determined by the type of active materials contained inthe cell. It can be calculated from free-energy data or obtained experimentally. A listing of electrode potentials (reduction potentials) under standard conditions is given in Table 1.1A more complete list is presented in Appendix B.
The standard potential of a cell can be calculated from the standard electrode potentialsas follows (the oxidation potential is the negative value of the reduction potential):
Anode (oxidation potential) + cathode (reduction potential) + standard cell potential.
For example, in the reaction Zn + Cl2→ ZnCl2, the standard cell potential is:
Zn → Zn2+ + 2e
Cl2 → 2Cl-- 2e
E0=-(-0.76 V)/1.36V=2.12V
The cell voltage is also dependent on other factors, including concentration and temperature, as expressed by the Nernst equation.
4.3 Theoretical Capacity (Coulombic)
The theoretical capacity of a cell is determined by the amount of active materials in the cell.It is expressed as the total quantity of electricity involved in the electrochemical reaction and is defined in terms of coulombs or ampere-hours. The ‘‘ampere-hour capacity’’ of a battery is directly associated with the quantity of electricity obtained from the active materials. Theoretically 1 gram-equivalent weight of material will deliver 96,487 C or 26.8 Ah. (A gram-equivalent weight is the atomic or molecular weight of the active material in grams divided by the number of electrons involved in the reaction.)
The electrochemical equivalence of typical materials is listed in Table 1.1 and Appendix C.
The theoretical capacity of an electrochemical cell, based only on the active materials participating in the electrochemical reaction, is calculated from the equivalent weight of the reactants. Hence, the theoretical capacity of the Zn/Cl2 cell is 0.394 Ah/g, that is,
Zn + Cl2 → ZnCl2
(0.82 Ah/g) (0.76 Ah/g)
1.22 g/Ah + 1.32 g/Ah = 2.54 g/Ah or 0.394 Ah/g
Similarly, the ampere-hour capacity on a volume basis can be calculated using the appropriatedata for ampere-hours per cubic centimeter as listed in Table 1.1.
The theoretical voltages and capacities of a number of the major electrochemical systemsare given in Table 1.2. These theoretical values are based on the active anode and cathode materials only. Water, electrolyte, or any other materials that may be involved in the cell reaction are not included in the calculation.
4.4 Theoretical Energy*
The capacity of a cell can also considered on an energy (watthour) basis by taking both the voltage and the quantity of electricity into consideration. This theoretical energy value is the maximum value that can be delivered by a specific electrochemical system:
Watthour (Wh) = voltage (V) × ampere-hour (Ah)
In the Zn/Cl2 cell example, if the standard potential is taken as 2.12 V, the theoretical watthour capacity per gram of active material (theoretical gravimetric specific energy or theoretical gravimetric energy density) is:
Specific Energy (Watthours/gram) = 2.12 V × 0.394 Ah/g = 0.835 Wh/g or 835 Wh/kg
Table 1.2 also lists the theoretical specific energy of the various electrochemical systems.
5. SPECIFIC ENERGY AND ENERGY DENSITY OF PRACTICAL BATTERIES
The theoretical electrical properties of cells and batteries are discussed in Sec. 1.4. In summary, the maximum energy that can be delivered by an electrochemical system is based on the types of active materials that are used (this determines the voltage) and on the amount of the active materials that are used (this determines ampere-hour capacity). In practice, only a fraction of the theoretical energy of the battery is realized. This is due to the need for electrolyte and nonreactive components (containers, separators, electrodes) that add to the weight and volume of the battery, as illustrated in Fig. 1.3. Another contributing factor is that the battery does not discharge at the theoretical voltage (thus lowering the average
*The energy output of a cell or battery is often expressed as a ratio of its weight or size.The preferred terminology for this ratio on a weight basis, e.g. Watthours /kilogram (Wh/ kg), is ‘‘specific energy’’; on a volume basis, e.g. Watthours / liter (Wh/ L), it is ‘‘energy density.’’ Quite commonly, however, the term ‘‘energy density’’ is used to refer to either ratio.
voltage), nor is it discharged completely to zero volts (thus reducing the delivered amperehours) (also see Sec. 3.2.1). Further, the active materials in a practical battery are usually not stoichiometrically balanced. This reduces the specific energy because an excess amount of one of the active materials is used.
In Fig. 1.4, the following values for some major batteries are plotted:
- The theoretical specific energy (based on the active anode and cathode materials only)
- The theoretical specific energy of a practical battery (accounting for the electrolyte and non-reactive components)
- The actual specific energy of these batteries when discharged at 20_C under optimal discharge conditions
These data show:
- That the weight of the materials of construction reduces the theoretical energy density or of the battery by almost 50 percent, and
- That the actual energy delivered by a practical battery, even when discharged under conditions close to optimum, may only be 50 to 75 percent of that lowered value
Thus, the actual energy that is available from a battery under practical, but close to optimum, discharge conditions is only about 25 to 35 percent of the theoretical energy of the active materials. Chapter 3 covers the performance of batteries when used under more stringent conditions.
These data are shown again in Table 1.2 which, in addition to the theoretical values, liststhe characteristics of each of these batteries based on the actual performance of a practical battery. Again, these values are based on discharge conditions close to optimum for that battery.
The specific energy (Wh/kg) and energy density (Wh/L) delivered by the major batterysystems are also plotted in Fig. 1.5(a) for primary batteries and 1.5(b) for rechargeable batteries. In these figures, the energy storage capability is shown as a field, rather than as a
single optimum value, to illustrate the spread in performance of that battery system under different conditions of use.
In practice, as discussed in detail in Chap. 3, the electrical output of a battery may bereduced even further when it is used under more stringent conditions.
6. UPPER LIMITS OF SPECIFIC ENERGY AND ENERGY DENSITY
Many advances have been made in battery technology in recent years as illustrated in Fig. 1.6, both through continued improvement of a specific electrochemical system and through the development and introduction of new battery chemistries. But batteries are not keeping pace with developments in electronics technology, where performance doubles every 18 months, a phenomenon known as Moore’s Law. Batteries, unlike electronic devices, consume materials when delivering electrical energy and, as discussed in Secs. 1.4 and 1.5, there are theoretical limits to the amount of electrical energy that can be delivered electrochemically by the available materials. The upper limit is now being reached as most of the materials that are practical for use as active materials in batteries have already been investigated and the list of unexplored materials is being depleted.
As shown in Table 1.2, and the other such tables in the Handbook, except for some ofthe ambient air-breathing systems and the hydrogen/oxygen fuel cell, where the weight of the cathode active material is not included in the calculation, the values for the theoretical energy density do not exceed 1500 Wh/ kg. Most of the values are, in fact, lower. Even the values for the hydrogen/ air and the liquid fuel cells have to be lowered to include, at least, the weight and volume of suitable containers for these fuels.
The data in Table 1.2 also show that the specific energy delivered by these batteries, basedon the actual performance when discharged under optimum conditions, does not exceed 450 Wh/ kg, even including the air-breathing systems. Similarly, the energy density values do not exceed 1000 Wh/L. It is also noteworthy that the values for the rechargeable systems are lower than those of the primary batteries due, in part, to a more limited selection of materials that can be recharged practically and the need for designs to facilitate recharging and cycle life.
Recognizing these limitations, while new battery systems will be explored, it will be more difficult to develop a new battery system which will have a significantly higher energy output and still meet the requirements of a successful commercial product, including availability of materials, acceptable cost, safety and environmental acceptability.
Battery research and development will focus on reducing the ratio of inactive to activecomponents to improve energy density, increasing conversion efficiency and rechargability, maximizing performance under the more stringent operating and enhancing safety and environment. The fuel cell is offering opportunities for powering electric vehicles, as a replacement for combustion engines, for use in utility power and possibly for the larger portable applications (see Chap. 42). However, the development of a fuel cell for a small portable applications that will be competitive with batteries presents a formidable challenge.
REFERENCES
- Ralph J. Broad, ‘‘Recent Developments in Batteries for Portable Consumer Electronics Applications,’’ Interface 8:3, Fall 1999, Electrochemical Society, Pennington, NJ.
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