10/13/2016 Battery ­ AccessScience from McGraw­Hill Education

(http://www.accessscience.com/)

Battery Article by: Anglin, Donald L. Consultant, Automotive and Technical Writing, Charlottesville, Virginia. Sadoway, Donald R. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097­8542.075200 (http://dx.doi.org/10.1036/1097­8542.075200)

Content

Types Components Size Selection and applications Ratings Life Primary Batteries Zinc­carbon cells Magnesium cells Alkaline­manganese dioxide cells Mercuric oxide cells Silver oxide cells Zinc­air cells Lithium cells Solid­electrolyte cells Reserve batteries Zinc­silver oxide reserve batteries Magnesium water­activated batteries Lithium­anode reserve batteries Thermal batteries Secondary Batteries Lead­acid batteries Nickel­cadmium batteries Nickel­metal hydride batteries Silver­zinc batteries Sodium­sulfur batteries Zinc­air batteries Lithium­ion batteries Lithium­solid polymer electrolyte (SPE) batteries Outlook Bibliography Additional Readings

An electrochemical device that stores chemical energy which can be converted into electrical energy, thereby providing a direct­current voltage source. Although the term “battery” is properly applied to a group of two or more electrochemical cells connected together electrically, both single­cell and multicell devices are called battery. See also: Electrochemistry (/content/electrochemistry/220300); Electromotive force (cells) (/content/electromotive­force­cells/223300)

Types

http://www.accessscience.com/content/battery/075200 1/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education The two general types are the primary battery and the secondary battery. The primary battery delivers current as the result of a chemical reaction that is not efficiently reversible. Practically, this makes the primary battery nonrechargeable. Only one intermittent or continuous discharge can be obtained before the chemicals placed in it during manufacture are consumed. Then the discharged primary battery must be replaced. The secondary or storage battery is rechargeable because it delivers current as the result of a chemical reaction that is easily reversible. When a charging current flows through its terminals in the direction opposite to the current flow during discharge, the active materials in the secondary battery return to approximately their original charged condition.

Components

The cell is the basic electrochemical unit. It has three essential parts: (1) a negative electrode (the anode) and (2) a positive electrode (the cathode) that are in contact with (3) an electrolyte solution. The electrodes are metal rods, sheets, or plates that are used to receive electrical energy (in secondary cells), store electrical energy chemically, and deliver electrical energy as the result of the reactions that occur at the electrode­solution surfaces. Solid polymer or plastic active materials have been developed that can serve as the cathode in rechargeable batteries. The electrolyte is a chemical compound (salt, acid, or base) that when dissolved in a solvent forms a solution that becomes an ionic conductor of electricity, but essentially insulating toward electrons—properties that are prerequisites for any electrolyte. In the cell or battery, this electrolyte solution is the conducting medium in which the flow of electric current between electrodes takes place by the migration of ions. When water is the solvent, an aqueous solution is formed. Some cells have a nonaqueous electrolyte, for example, when alcohol is used as the solvent. Other cells have a solid electrolyte that when used with solid electrodes can form a leak­free solid­state cell or battery.

During charging of a secondary cell, the negative electrode becomes the cathode and the positive electrode becomes the anode. However, electrode designation as positive or negative is unaffected by the operating mode of the cell or battery. Two or more cells internally connected together electrically, in series or parallel, form a battery of a given voltage. Typical are the rectangular 9­V primary battery, which has six flat 1.5­V zinc­carbon or alkaline “dry” cells connected in series, and the 12­V automotive or secondary battery, which has six 2.1­V lead­acid “wet” cells connected in series.

Size

Both primary and secondary cells are manufactured in many sizes, shapes, and terminal arrangements, from the miniature coin­ or button­shaped battery (which has a diameter greater than its height) and the small cylindrical penlight battery to the large submarine battery, where a single rectangular cell has weighed 1 ton (0.9 metric ton). For optimum performance, the battery must be constructed for its particular application consistent with cost, weight, space, and operational requirements. Automotive and aircraft batteries are secondary batteries that have relatively thin positive and negative plates with thin, porous, envelope separators to conserve space and weight and to provide high rates of current discharge at low temperatures. Standby batteries are secondary batteries that use thick plates and thick separators to provide long life. Solid­state batteries can be constructed with unusual features and in irregular sizes and shapes. Size and weight reductions in all types of batteries continue to be made through use of new materials and methods of construction.

Selection and applications

Batteries are probably the most reliable source of power known. Most critical electrical circuits are protected in some manner by battery power. Since a battery has no moving parts, tests, calculations, or comparisons are made to predict the conditions of the cells in some batteries. Growing battery usage reflects the increased demand for portable computers; mobile voice, data, and video communications; and new or redesigned/repowered products in the consumer, industrial, and

http://www.accessscience.com/content/battery/075200 2/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education transportation sectors. Further growth may result from significant increases in dc system operating voltage, such as from 12 V to 24 V or 48 V, which can provide much higher power generally with less weight, greatly broadening the range of potential battery applications.

For most applications, the basic choice in selection is whether to use either a primary (nonrechargeable) or a secondary (rechargeable) cell or battery. Electrical characteristics affecting selection include maximum and minimum voltage, current drain, and pulse current (if any), its duration and frequency of occurrence. Other factors such as performance in the specific application, operating environment, and final packaging of the cell or battery also must be considered.

Primary battery usage

Primary batteries are used as a source of dc power where the following requirements are important:

1. Electrical charging equipment or power is not readily available.

2. Convenience is of major importance, such as in a hand or pocket flashlight.

3. Standby power is desirable without cell deterioration during periods of nonuse for days or years. Reserve­electrolyte designs may be necessary, as in torpedo, guided missile, and some emergency light and power batteries.

4. The cost of a discharge is not of primary importance.

Secondary battery usage

Secondary batteries are used as a source of dc power where the following requirements are important:

1. The battery is the primary source of power and numerous discharge­recharge cycles are required, as in wheelchairs and golf carts, industrial hand and forklift trucks, electric cars and trucks, and boats and submarines.

2. The battery is used to supply large, short­time (or relatively small, longer­time), repetitive power requirements, as in automotive and aircraft batteries which provide power for starting internal combustion engines.

3. Standby power is required and the battery is continuously connected to a voltage­controlled dc circuit. The battery is said to “float” by drawing from the dc circuit only sufficient current to compensate automatically for the battery's own internal self­discharge. Computers and communications networks and emergency light and power batteries are in this category.

4. Long periods of low­current­rate discharge followed subsequently by recharge are required, as in marine buoys and lighthouses, and instrumentation for monitoring conditions such as earthquakes and other seismic disturbances.

5. The very large capacitance is beneficial to the circuit, as in telephone exchanges.

Ratings

Two key ratings of a cell are its voltage and ampere­hour (Ah) capacity. The voltage is determined by the chemical system created by the active materials used for the negative and positive electrodes (anode and cathode). The ampere­hour capacity is determined by the amount of the active materials contained in the cell. The product of these terms is the energy output or watthour capacity of the battery. In actual practice, only one­third to one­half of the theoretical capacity may be available. Battery performance varies with temperature, current drain, cutoff voltage, operating schedule, and storage conditions prior to use, as well as the particular design.

http://www.accessscience.com/content/battery/075200 3/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education Many primary batteries are rated by average service capacity in milliampere­hours (mAh). This is the number of hours of discharge that can be obtained when discharging at a specified temperature through a specified fixed resistance to a specified final or cutoff voltage, which is either at the point of rapid voltage drop or at minimum usable voltage.

Secondary batteries, such as automotive batteries, have been rated by ampere­hour capacity. Typically, this is the amount of current that the battery can deliver for 20 h without the temperature­corrected cell voltages dropping below 1.75 V per cell. A battery capable of giving 2.5 A for 20 h is rated at 50 amperehours at the 20­h rate. This same battery may provide an engine­cranking current of 150 A for only 8 min at 80°F (27°C) or for 4 min at 0°F (−18°C), giving a service of only 20 and 10 ampere­hours, respectively. By multiplying ampere­hours by average voltage during discharge, a more practical watt­hour rating is obtained. Automotive batteries also are rated by reserve capacity (RC) and cold­cranking amps (CCA). Reserve capacity is the length of time that a fully charged battery at 80°F (27°C) can deliver 25 amperes before the voltage falls to 10.5 V. A typical rating is 125 min, which is the length of time the battery could carry a minimum electrical load after failure of the vehicle's charging system. Cold­cranking amps is a measure of the ability of a battery to crank an engine when the battery is cold. It is measured by the number of amperes that a 12 V battery can deliver for 30 s when it is at 0°F (−18°C) without the battery voltage falling below 7.2 V. A typical CCA rating for a battery with a reserve capacity of 125 min is 430 amperes. The CCA rating for most automobile batteries is between 300 and 600 amperes.

Life

The life expectancy of a cell or battery depends on its design and materials, as well as its application and operating conditions. Life expectancy is measured by shelf life and service life. Shelf life is the expected time that elapses before a stored battery becomes inoperative due to age or deterioration, or unusable due to its own internal self­discharge. Service life is the expected length of time or number of discharge­charge cycles through which a battery remains capable of delivering a specified percentage of its capacity after it has been put into service. This can vary from the one shot or single discharge obtainable from primary cells to 10,000 or more discharge­charge cycles obtainable from some secondary batteries. Automotive batteries may last as little as 18 months in hot climates and 10–12 years in cold climates, but typically they have an average life of 3 years and may last for 6000 cycles. Industrial batteries have a 10–20­year service life. Standby sizes may be expected to float across the dc bus 8– 30 years. Generally the most costly, largest, and heaviest cells have the longest service life.

Many batteries experience an abrupt loss of voltage without warning when the active materials are depleted. However, in some batteries, open­circuit voltage serves as a state­of­charge indicator, decreasing slightly but continuously with loss of capacity. Many electronic devices now include a battery that will last for the life of the product. This eliminates the need for the consumer or user to replace the battery and risk causing damage to the device or the battery by inadvertently shorting the terminals, reversing the polarity, or installing a similar­size battery having the wrong chemistry.

To obtain the maximum life and ensure reliability of batteries, the manufacturer's recommendations for storage and maintenance must be followed. The stated shelf life and temperature of primary cells must not be exceeded. For dry reserve­electrolyte primary cells and secondary cells of the dry construction with charged plates, the cell or battery container must be protected against moisture, and storage must be within specified temperature limits. Wet, charged secondary batteries may require periodic charging and water addition, depending upon the construction.

Primary Batteries

http://www.accessscience.com/content/battery/075200 4/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education A primary cell or battery is not intended to be recharged and is discarded when it has delivered all its electrical energy (Fig. 1). Several kinds of primary cell are widely used, particularly in portable devices and equipment, providing freedom from the dependence on alternating­current line power. They are convenient, lightweight, and usually relatively inexpensive sources of electrical energy that provide high energy density (long service life) at low­to­moderate or intermittent discharge rates, good shelf life, and ease of use while requiring little or no maintenance.

Fig. 1 Diagram of a zinc­alkaline­manganese dioxide cylindrical cell.

Primary cells are classified by their electrolyte, which may be described as aqueous, nonaqueous, aprotic, or solid. In most primary cells the electrolyte is immobilized by a gelling agent or mixed as a paste, with the term “dry cell” commonly applied to the zinc­carbon Leclanche cell and sometimes to other types. An aqueous electrolyte or electrolyte system is used in zinc­carbon, magnesium, alkaline­manganese dioxide, mercuric oxide, silver oxide, and zinc­air cells. Nonaqueous electrolyte systems are used in lithium cells and batteries. See also: Electrolyte (/content/electrolyte/221800)

Typical characteristics of different types of primary batteries and their applications are summarized in Table 1. Performance characteristics of primary batteries at various temperatures are shown in Fig. 2.

http://www.accessscience.com/content/battery/075200 5/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education

Fig. 2 Performance characteristics of primary batteries at various temperatures.

Zinc­carbon cells

The zinc­carbon or LeClanche dry cell was invented in 1866 and continues to be popular. It is made in sizes of varying diameter and height, and batteries are available in voltages ranging from 1.5 to 510 V. Common cell construction uses the cylindrical zinc container or can as the negative electrode and manganese dioxide (mixed with carbon black to increase conductivity and retain moisture) as the positive active material, with the electrical connection made through a center carbon electrode. The slightly acidic electrolyte is an aqueous solution of ammonium chloride and may also contain zinc chloride, immobilized in a paste or the paper separator. The electrochemical reaction between the cathode, anode, and electrolyte in a zinc­carbon LeClanche cell is shown in reaction (1 (075200#075200RX0010)).

http://www.accessscience.com/content/battery/075200 6/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education

(1)

Typical open­circuit voltage of a fresh LeClanche cell is over 1.55 V. The closed­circuit voltage gradually declines as a function of the depth of discharge.

Significant improvements in capacity and shelf life of zinc­carbon batteries have been made through the use of new cell designs, such as the paper­lined cell, and new materials. Upgrading to the use of purer beneficiated manganese dioxide and an electrolyte that consists mainly of zinc chloride and water has created the heavy­duty or zinc chloride cell. Its construction is similar to the zinc­carbon cell, but having an anode of high­purity zinc alloy and typically a higher proportion of carbon to manganese dioxide and a greater volume of slightly more acidic electrolyte. The electrochemical reaction in a zinc chloride cell is shown in reaction (2 (075200#075200RX0020)).

(2)

Open­circuit voltage of a fresh zinc chloride cell typically is over 1.60 V. As in the LeClanche cell, closed­circuit voltage gradually declines as a function of the depth of discharge.

Compared to zinc­carbon batteries, zinc chloride batteries have improved high­rate and low­temperature performance and are available in voltages ranging from 1.5 to 12 V. A common design is the flat cell, used in multicell batteries such as the 9­ V battery, which offers better volume utilization and, in some designs, better high­rate performance. Characteristics of standard zinc­carbon and zinc chloride cells are given in Table 2.

Magnesium cells

http://www.accessscience.com/content/battery/075200 7/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education The magnesium­carbon primary cell is basically a cylindrical zinc­carbon cell in which the can or container is made of magnesium or its alloy instead of zinc. Magnesium has a greater electrochemical potential than zinc, providing an open­ circuit voltage of about 2.0 V. Developed for military use in radios and other equipment, the magnesium cell has twice the capacity or service life of a zinc­carbon cell of equivalent size, and longer shelf life, particularly at elevated temperatures. However, when the load is applied to a magnesium cell, a characteristic voltage drop occurs briefly before the cell recovers to its usable voltage. This may make the magnesium cell unacceptable for use in some applications. The magnesium cell has a vent for the escape of hydrogen gas, which forms during discharge.

Alkaline­manganese dioxide cells

These cells became the most popular battery during the 1980s. Available in voltages ranging from 1.5 to 12 V and in a variety of sizes and shapes, they have higher energy density, better low­temperature and high­rate performance, and longer shelf life than a zinc­carbon battery. Alkaline and zinc­carbon cells are chemically similar, both using zinc for the anode and manganese dioxide for the cathode active materials. However, the cells differ significantly in construction, electrolyte, and formulation of the active materials.

The container for the cylindrical alkaline cell is a steel can that does not participate in the electrochemical reaction. A gelled mixture of zinc powder and electrolyte is used for the anode. The highly alkaline electrolyte is an aqueous solution of potassium hydroxide, which is more conductive than the salt of the zinc­carbon cell. The cathode is a highly compacted, conductive mixture of high­purity manganese dioxide and graphite or carbon. To prevent contact between the anode and cathode, which would result in a very active chemical reaction, a barrier or separator is placed between the two. A typical separator is paper or fabric soaked with electrolyte. The separator prevents solid­particle migration, while the electrolyte promotes ionic conductivity. The electrochemical reaction in an alkaline­manganese dioxide cell is shown in reaction (3 (075200#075200RX0030)).

(3) Open­circuit voltage of a fresh cylindrical alkaline cell typically is 1.58 V. Closed­circuit voltage gradually declines as a function of the depth of discharge.

Modifying the internal design of zinc­alkaline cells has resulted in cells that have an average of 20–50% higher amperehour capacity or service life. This newer cell design provides a significantly greater volume for the active materials, the factor most responsible for improved performance. The modified cell is manufactured with a nickel­plated steel can and an outside poly(vinyl chloride) jacket (Fig. 1). A diaphragm vent is incorporated into the assembly in case of pressure buildup. Alkaline­manganese dioxide cells also are made in miniature sizes with flat, circular pellet­type cathodes and homogeneous gelled anodes.

Mercuric oxide cells

The zinc­mercuric oxide cell (Fig. 3) has high capacity per unit charge, relatively constant output voltage during discharge, and good storage qualities. It is constructed in a sealed but vented structure, with the active materials balanced to prevent the formation of hydrogen when discharged. Three basic structures are used: the wound anode, the flat pressed­powder anode, and the cylindrical pressed­powder electrode. Typically, the anode is zinc, the cathode is mercuric oxide, and the electrolyte is alkaline. The electrochemical reaction in a mercuric oxide cell is shown in reaction (4 (075200#075200RX0040)).

(4)

http://www.accessscience.com/content/battery/075200 8/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education Open­circuit voltage of a mercuric oxide cell is 1.35 V, with some batteries delivering up to 97.2 V.

Fig. 3 Diagram showing the design of a mercury button cell.

Mercuric oxide batteries have been made in various sizes and configurations, from the miniature button 16 mAh to the large 14 Ah cylindrical cell, and are suited for use at both normal and moderately high temperatures. However, the low­ temperature performance is poor, particularly under heavy discharge loads. Use at temperatures below 32°F (0°C) is generally not recommended. Mercuric oxide batteries have been used for hearing aids, cameras, watches, calculators, implanted heart pacemakers and other medical applications, emergency radios, and military equipment. Today concerns about health hazards and environmental regulations related to disposal generally prevent manufacture and sale of mercuric oxide batteries. They are replaced with newer, less expensive designs such as zinc­air that do not contain mercury or its compounds.

The cadmium­mercuric oxide battery is similar to the zinc­mercuric oxide battery. The substitution of cadmium for the zinc anode lowers the cell voltage but offers a very stable system, with a shelf life of up to 10 years, and improved performance at low temperatures. Its watthour capacity, because of the lower voltage, is about 60% of the zinc­mercuric oxide battery. However, health concerns and environmental regulations related to disposal also limit the availability and usage of batteries containing cadmium.

Silver oxide cells

The zinc­silver oxide primary cell is similar to the zinc­mercuric oxide cell, but uses silver oxide in place of mercuric oxide (Fig. 3). This results in a higher cell voltage and energy. In the small button­cell configuration, this is a significant advantage for use in hearing aids, photographic applications, watches, and calculators. The silver oxide battery has a higher voltage than a mercury battery; a flatter discharge curve than an alkaline battery; good resistance to acceleration, shock, and vibration; an essentially constant and low internal resistance; and good performance characteristics at temperature extremes.

http://www.accessscience.com/content/battery/075200 9/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education Silver oxide batteries generally have a flat circular cathode and a homogeneous gelled anode. The cathode is a mixture of silver oxide with a low percentage of manganese dioxide and graphite, and the anode is a mixture of amalgamated zinc powder. A highly alkaline electrolyte of either sodium hydroxide or potassium hydroxide is used, although potassium hydroxide makes the battery more difficult to seal. The separator is a material that prevents migration of any solid particles in the battery.

The chemical reaction in a silver oxide cell is shown in reaction (5 (075200#075200RX0050)).

(5) Open­circuit voltage of a silver oxide cell is 1.6 V, with batteries available having a nominal voltage of 6 V.

Zinc­air cells

Zinc­air cells (Fig. 4) have the highest energy density of the commercially available primary cells. They use atmospheric oxygen for the active cathode material, so there is no need to include the cathode material in the cell. This allows the cathode electrode to be very thin. The remaining space can be used for increasing the amount of zinc, which is the active anode material, resulting in a higher cell capacity (Fig. 4a).

Fig. 4 Zinc­air cell. (a) Cross section compared with metal oxide cell. (b) Major components.

The zinc­air cell is usually constructed in a button configuration (Fig. 4b). The cell consists of two cans, isolated from each other by an annular insulator. One can contains the zinc anode and the other the air or oxygen cathode. The anode can is fabricated from a triclad metal; the external material is nickel for good electrical conductivity, the middle layer is stainless steel to provide strength, and the inner surface is copper, which is compatible with the cell components. The cathode can is fabricated of nickel­plated steel and contains holes that allow air to enter the cell. A layer of permeable polytetrafluoroethylene (Teflon) serves as a separator to assure the proper distribution of air and limit the entrance or exit of moisture. The anode is an amalgamated gelled zinc powder and electrolyte. Although the cathode uses atmospheric oxygen as the active material, a mixture of carbon, polytetrafluoroethylene, and manganese dioxide is impressed on a nickel­plated screen. The carbon and manganese dioxide serve as catalysts for the oxygen reaction. The electrolyte is an aqueous solution of potassium hydroxide with a small amount of zinc oxide.

http://www.accessscience.com/content/battery/075200 10/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education The air holes of the zinc­air cell are sealed until the cell is used, to inhibit the entrance of air. In this condition, the cells can retain more than 95% of their rated capacity after 1 year of storage at room temperature. The cells are activated by removing the seal which permits the flow of air into the cell. The chemical reaction in a zinc­air cell is shown in reaction (6 (075200#075200RX0060)).

(6)

\noindent The open­circuit voltage of a zinc­air cell is about 1.4 V, providing a flat discharge with the operating voltage between 1.35 and 1.1 V, depending on the discharge conditions. The cell is capable of low to moderately high discharge rates, and is used in applications requiring a relatively short operating time before replacement.

Zinc­air cells are manufactured in sizes from 50 to 6500 mAh. Multicell batteries are available in a wide range of voltage and capacity ratings, including a nominal 8.4­V zinc­air battery that is interchangeable in some applications with the zinc chloride and alkaline 9­V battery.

Lithium cells

Lithium is a silvery­white element that is the lightest metal, having a density only about half that of water, with which the alkaline lithium is highly reactive. Lithium has good conductivity, and its standard potential and electrochemical equivalence are higher than those of any other metal. It is widely used in various forms as battery anode material (negative electrode), especially in coin­size primary cells. There are several different types of lithium cells, primary as well as reserve and secondary, similar to the variety of cells using a zinc anode. Each type of lithium cell differs in the cathode material for the positive electrode, electrolyte, and cell chemistry as well as in physical, mechanical, and electrical features (Table 3). Performance advantages of lithium cells over other primary cells include high voltage (may be above 3 V), high energy density, operation over a wide temperature range, and long shelf life. Lithium cells are manufactured in different sizes and configurations, ranging in capacity from milliamperehours to over 20,000 Ah, with characteristics matched to the general application. See also: Lithium (/content/lithium/387000)

Table 3 ­ Classification of lithium primary cells*

Cell Typical Power Operating Shelf Typical Nominal cell Key classification electrolyte capability Size, Ah range, °F (°C) life, cathodes voltage, V characteristics years

Soluble Organic or Moderate 0.5−20,000 −67 to 158 5−10 Sulfur dioxide 3.0 High energy cathode to output;

(liquid or inorganic high (−55 to 70) Thionylchloride 3.6 high power gas) power, output;

(w/solute) W Sulfuryl 3.9 low chloride temperature

operation; long shelf life

Solid cathode Organic Low to 0.03−5 −40 to 122 5−8 Silver chromate 3.1 High energy output for

(w/solute) (−40 to 50) Manganese 3.0 moderate moderate dioxide power

power, Carbon 2.6 requirements; mW monofluoride

http://www.accessscience.com/content/battery/075200 11/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education Table 3 ­ Classification of lithium primary cells

Copper(I) sulfide 1.7 nonpressurized cells Iron disulfide 1.6 Iron sulfide 1.5

Copper(II) 1.5 oxide

Solid Solid state Very low 0.003−0.5 32 to 212 10−25 I2poly(2­ 2.8 Excellent shelf electrolyte vinylpyridine) life;

power, (0 to 100) solid­state— μW no

leakage; long­term microampere discharge *From D. Linden (ed.), Handbook of Batteries and Fuel Cells, McGraw­Hill, 2d ed., 1995.

In a lithium cell, nonaqueous solvents must be used for the electrolyte because of the solubility and reactivity of lithium in aqueous solutions. Organic solvents, such as acetonitrile and propylene carbonate, and inorganic solvents, such as thionyl chloride, are typical. A compatible solute is added to provide the necessary electrolyte conductivity. Solid­cathode cells are generally manufactured as cylindrical cells in sizes up to about 30 Ah in both low­ and high­rate constructions. The low­rate, or high­energy, designs use a bobbin­type construction to maximize the volume for active materials (Fig. 5a). These cells have the highest energy density of cells of similar size. The high­rate cells use a jelly­roll (spiral­wound) construction, which provides the large electrode surface required for the high discharge rates (Fig. 5b). These cells deliver the highest rates of all primary batteries and are widely used in military applications. Early ambient­temperature lithium systems used lithium­ sulfur dioxide chemistry, and found applications in medical implants, weapons systems, and spacecraft. These cells provide 3 V and have seven times the energy of a typical alkaline battery. However, other lithium chemistry has been developed. Four types of cells are lithium­carbon monofluoride, lithium­manganese dioxide, and lithium­iron disulfide, all of which have solid cathodes, and lithium­thionyl chloride, which has a liquid cathode.

Fig. 5 Solid cathode cells. (a) Lithium­manganese dioxide bobbin cell. (b) Lithium­manganese dioxide spiral­wound cell. (Duracell, Inc.)

http://www.accessscience.com/content/battery/075200 12/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education Lithium­carbon monofluoride batteries have a solid carbon monofluoride cathode and characteristics that allow the battery to be used for the life of the product in which the battery is installed, such as computer real­time clock and memory backup applications, instead of having scheduled replacement intervals. The cell has a storage and operational temperature range of −40 to +85°C, and a long life made possible by a self­discharge rate that may be less than 0.2% of capacity loss per year. As a lithium­carbon monofluoride cell discharges while powering a specified load, internal resistance generally remains low and level while the closed­circuit voltage profile remains high, flat, and stable until the depth of discharge exceeds approximately 85%. When completely discharged, this type of lithium cell may be disposed of as nonhazardous waste. Batteries containing unreacted lithium metal are considered reactive hazardous waste and must be disposed of following the applicable local, state, and federal regulations.

Lithium­manganese dioxide cells have a solid manganese dioxide cathode and characteristics that make the preferred use of the cell to be where periodic replacement is routinely performed. The cell can supply both pulse loads and very small current drains, typical needs for microprocessor applications. As the cell discharges, its internal resistance increases because of the manganese dioxide, causing a tapered discharge profile and a declining closed circuit voltage. The manganese dioxide also limits the maximum temperature rating of the cell to 140°F (60°C), a temperature at which self­ discharge may increase to a rate of over 8% per year. However, the cell costs less than a lithium­carbon monofluoride cell, and is widely used especially for applications requiring intermittent pulsing such as remote keyless entry systems. When completely discharged, this type of lithium cell may be disposed of as non­hazardous waste.

Lithium­iron disulfide cells have an anode of lithium foil in contact with a stainless­steel can and a cathode­material mixture of graphite and iron disulfide. These cells have an open­circuit voltage of 1.8 V, a nominal voltage of 1.5 V, and can be used in any application that uses other AA­size 1.5­V batteries. Each battery contains two safety devices: a thermal switch which acts as a current limiter if the battery begins to overheat, and a pressure­relief vent that opens at a predetermined temperature. This type of lithium cell may be disposed of using the same procedures approved for other cells.

Lithium­thionyl chloride cells were designed for the military and have been used for nuclear surety applications, which require long­life power sources for command and control systems. These cells have a voltage of about 3.6 V, making possible the use of fewer cells for the same application. They are classed as high­energy, high­current­drain­rate cells, with twice the energy density of lithium­sulfur dioxide cells, and will last twice as long. Lithium­thionyl chloride cells have a soluble cathode which is liquid, toxic, and corrosive. Each cell is hermetically sealed, with a designed­in fuse to avoid rupture or explosion if the battery is inadvertently charged or shorted. A lithium­thionyl chloride cell should be replaced only by a trained technician. Because thionyl chloride is highly toxic and corrosive, this battery is an environmental hazard which requires special handling and disposal.

Solid­electrolyte cells

These may be classified as (1) cells using a solid crystalline salt (such as lithium iodide) as the electrolyte and (2) cells using a solid­polymer electrolyte. With either type of electrolyte, the conductivity must be nearly 100% ionic as any electronic conductivity causes a discharge of the cell, which limits shelf life. Solid­polymer electrolytes are based on the characteristic that solutions of alkali metal salts dissolved in certain polymers, such as poly(ethylene oxide), form solid polymers that have reasonable ionic conductivity. The solid polymers can be fabricated in thin cross sections, are more stable than liquid electrolytes, and can serve as the separators. Most development work on thin­film polymer electrolyte batteries has been for rechargeable batteries, using a lithium anode and solid cathode materials. Energy densities of 5–10 times that of lead­acid cells have been calculated. The technology can also be applied to the design of primary cells and batteries. See also: Electrolytic conductance (/content/electrolytic­conductance/221900); Ionic crystals (/content/ionic­crystals/352100)

http://www.accessscience.com/content/battery/075200 13/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education The most widely used solid­electrolyte cells have lithium iodide as the solid electrolyte. These batteries have a solid lithium foil anode and a cathode that is largely iodine. The iodine is made conductive by the addition of an organic compound, poly(2­vinylpyridine). This iodine­polyvinylpyridine cathode has a tarlike consistency in the fresh battery, then solidifies gradually as the battery is discharged. Lithium and iodine are consumed during the discharge, and the reaction product, lithium iodide, forms in place in the region between the two reactants where it serves as the cell separator. Because the electrolyte continues to form as the battery discharges, the overall resistance of the cell continually increases with discharge. This results in a drop in cell voltage for a given current drain. The nominal cell voltage is around 2.8 V. These batteries have a very low rate of self­discharge, a long storage life, and high reliability, but can be discharged only at low rates because of the low conductivity of the electrolyte. Applications for these batteries include memory backup and implanted cardiac pacemakers.

A typical implantable battery has a volume of 0.4 in.3 (6 cm3), weighs 0.8 oz (23 g), and has a 2 Ah capacity. The lifetime of this battery is 5–10 years since pacemakers typically draw only 15­30 microamperes. The battery is considered to be discharged when the output voltage drops to 1.8 V. This may be detected by monitoring the patient's pulse rate. See also: Medical control systems (/content/medical­control­systems/412800)

Cells that use only solid components, including an ion­conducting solid electrolyte, are called solid­state cells or batteries. Some of these have electrodes made of fast­ion conductors, materials that are good conductors for both ions and electrons. Examples include the layered­structure disulfides of titanium and vanadium, TiS2 and VS2, which can be used as the cathode or sink for lithium, and aluminum­lithium alloys, which can be used as the anode or source of lithium in lithium batteries. These materials can sustain high reaction rates because of their unique crystalline structures which allow the incorporation of ions into their crystalline lattices without destruction of those lattices. See also: Energy storage (/content/energy­storage/233100); Intercalation compounds (/content/intercalation­compounds/348250); Solid­ state chemistry (/content/solid­state­chemistry/634800)

In a solid­state cell, the polycrystalline pressed electrolyte is interspaced between a metallic anode and the solid cathode material. The electrodes are applied to the electrolyte by mechanically pressing the materials together, or in some cases the electrolyte is formed in place by reaction between the two electrodes. These cells are then stacked together to form a battery of the required voltage. A carbon current collector is often used on the cathode side, and this is frequently admixed with the cathode material. If the cathode material is sufficiently conductive, for example titanium disulfide, no carbon conductor is needed.

Reserve batteries

Most primary batteries are ready for use at the time of manufacture. However, high­energy primary batteries are often limited in their application by poor charge retention resulting from self­discharge between the electrolyte and the active electrode materials. The use of an automatically activated battery, or reserve battery, can overcome this problem, especially when the battery is required to produce high currents for relatively short periods of time (seconds or minutes), typical of military applications. A reserve battery is manufactured as a complete but inert battery that is stored in an inactive condition by keeping one of the critical cell components, such as the electrolyte, separated from the rest of the battery. The battery is activated just prior to use by adding this component manually or automatically. An important design consideration is to ensure that the electrolyte or other withheld component is delivered as quickly as possible at the time of activation while avoiding chemical short­circuiting of the cells.

Zinc­silver oxide reserve batteries

http://www.accessscience.com/content/battery/075200 14/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education Automatically activated batteries have been used in missile and spacecraft applications, where the battery has to be activated from a remote source. The package contains a mechanism that drives the electrolyte out of the reservoir and into the cells of the battery. Most of these automatically activated batteries use zinc­silver oxide chemistry with a solution of potassium hydroxide for the electrolyte in order to achieve the required high discharge rates.

Magnesium water­activated batteries

Several types of water­activated (dunk type) batteries use magnesium anodes and silver chloride or cuprous chloride cathodes. The batteries are assembled dry, with the active elements separated by an absorbent. Activation occurs by pouring water into the battery container or by immersing the battery in water or seawater. Magnesium­silver chloride batteries have many marine applications, including buoys, beacons, flares, and safety lights. They also power the pingers which are attached to and help locate the cockpit voice recorder and flight data recorder in downed aircraft, and emergency lighting for submarine escape hatches. Magnesium­copper iodide and magnesium­cuprous chloride batteries have similar capabilities.

Lithium­anode reserve batteries

Several types of batteries having a lithium anode are available as reserve batteries. These include batteries using lithium­ sulfur dioxide, lithium­thionyl chloride, and lithium­vanadium pentoxide chemistry. Lithium­sulfur dioxide and lithium­thionyl chloride reserve batteries may be gas­activated, with activation triggered by electrical, mechanical, or pressure signals. The lithium­vanadium pentoxide battery is an electrolyte­activated battery. The electrolyte is contained in a glass ampule or vial, which is broken by mechanical shock, allowing the electrolyte to disperse within the cell.

Thermal batteries

A thermal battery, or fused­electrolyte battery, is a type of reserve battery that is activated by the application of heat. At room temperature, the electrolyte is a solid and has a very low conductivity, rendering the battery essentially inert. When the temperature is raised above the melting point of the electrolyte, the molten electrolyte becomes ionically conductive, and the battery is then capable of delivering electrical energy. Thermal batteries were introduced in 1955 to solve the wet­ stand time limitation of aqueous systems, and subsequently were used as the main power source for nuclear weapon systems.

Early thermal batteries used calcium­calcium chromate chemistry. This was replaced in 1980 by thermal batteries that used a lithium anode, an iron sulfide cathode, and a lithium chloride­potassium chloride electrolyte. Figure 6 shows a typical multicell lithium­iron disulfide thermal battery, which is activated by an electrical pulse to an integral electric match (squib). Other thermal batteries are activated mechanically using a percussion­type primer. The principal means for heating are pyrotechnic heat sources such as zirconium barium chromate heat paper or a heat pellet containing fine iron powder and potassium perchlorate. When the temperature rises to about 750°F (400°C), the electrolyte becomes molten and conductive. For other sources of electric energy known as batteries or cells See also: Fuel cell (/content/fuel­ cell/274100); Nuclear battery (/content/nuclear­battery/457900); Solar cell (/content/solar­cell/633000)

http://www.accessscience.com/content/battery/075200 15/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education

Fig. 6 Lithium/iron disulfide thermal battery. (Sandia National Laboratories)

Donald L. Anglin

Secondary Batteries

Secondary batteries (also known as accumulators) are rechargeable. This means that the electrochemical reactions in the cell must be reversible so that if the load in the external circuit is replaced by a power supply, the reactions in the cell can be forced to run in reverse, thereby restoring the driving force for reaction and hence recharging the cell. In contrast, primary batteries cannot be recharged because the reactions that produce current cannot be made to run in reverse; instead, totally different reactions occur when the cell is forcibly charged, and in some instances the reaction products are dangerous, that is, explosive or toxic.

The paradigm of battery design is to identify a chemical reaction with a strong driving force and then to fashion a cell that requires the reaction to proceed by a mechanism involving electron transfer, thereby making electrons available to a load in the external circuit. The magnitude of the driving force will determine cell voltage; the kinetics of reaction will determine cell current.

http://www.accessscience.com/content/battery/075200 16/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education There are many chemistries that will serve as the basis for secondary batteries. Distinctions can be made on the basis of the following metrics: voltage, current (maximum, steady­state, and peak), energy density (Wh/kg and Wh/L), power density (W/kg and W/L), service life (cycles to failure), and cost ($/kWh). Table 4 summarizes performance characteristics of some of the secondary battery types. Such comparisons are made difficult by the fact that many performance characteristics are functions of battery size and service conditions. For example, service life is greatly affected by depth of discharge and discharge current. However, both of these operating parameters can be very different for two different battery types that were designed for very different applications.

Table 4 ­ Comparison of performance characteristics of secondary batteries

Characteristics Pb­acid Ni­Cd Ni­MH Zn­Ag Na­S Zn­air* Li­ion Li­SPE†

Nominal voltage, V 2 1.2 1.2 1.5 1.9 1.5 3.6 3.6

Specific energy, Wh/kg 35 40 90 110 80 280 125 500

Specific energy, kJ/kg 126 144 324 396 288 1008 450 1800 Volumetric energy, Wh/L 70 100 245 220 385 440 900 Volumetric energy, kJ/L 252 360 882 792 1386 1584 3240

*Based upon commercial prototype notebook computer battery. †Projections based upon thin­film microbattery test results in the laboratory.

Another classification scheme considers the state of aggregation of the principal cell components, that is, whether the electrolyte is a solid or liquid, and whether the electrode active material is a solid, liquid, or gas. Most batteries have solid electrodes and a liquid electrolyte. However, there are examples of batteries in which the anode and cathode are both liquid, and the electrolyte is solid.

What follows is a series of descriptions of the significant secondary battery chemistries.

Lead­acid batteries

The lead­acid battery is the dominant secondary battery, used in a wide variety of applications, including automotive SLI (starting, lighting, ignition), traction for industrial trucks, emergency power, and UPS (uninterruptible power supplies). The attributes of lead­acid batteries include low cost, high discharge rate, and good performance at subambient temperatures. The first practical lead­acid cell was made by Planté in France in 1860.

The anode is metallic lead. The cathode active material is lead dioxide, which is incorporated into a composite electrode also containing lead sulfate and metallic lead. The electrolyte is an aqueous solution of sulfuric acid, 37% by weight when the battery is fully charged.

The half­cell reactions at each electrode are shown in reactions (7 (075200#075200RX0070)) and (8 (075200#075200RX0080)). The overall cell reaction is the formation of lead sulfate and water, shown in reaction (9 (075200#075200RX0090)). When the battery is discharging,

(7)

(8)

(9) the reactions proceed from left to right; upon charging, the reactions proceed from right to left. The elementary electrochemical reactions are shown in reactions (10 (075200#075200RX0100)) and (11 (075200#075200RX0110)). On http://www.accessscience.com/content/battery/075200 17/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education discharge, electrons are

(10)

(11)

\noindent produced at the anode as metallic lead is oxidized to Pb(II) present in lead sulfate; the complementary reaction at the cathode is the reduction of Pb(IV) present in lead dioxide to Pb(II) present in lead sulfate with the attendant consumption of electrons.

Sulfuric acid functions not only as the electrolyte but also as one of the reactants. Consequently, the acid concentration is a function of the instant state of charge and attains maximum value when the battery is fully charged.

Nominal voltage is 2 V, which decreases as the battery is discharged. The lower limit is governed by the depth of discharge. The theoretical specific energy is 170 Wh/kg. Commercially available lead­acid batteries deliver about 35 Wh/kg, which is about 20\% of theoretical. However, lead­acid batteries are capable of high specific power, have relatively flat discharge profiles, are comparatively insensitive to voltage changes with temperature, can be made for calendar lives exceeding 20 years, and are the least costly of all rechargeable technologies. Thus, when mobility is not a consideration and hence there is no penalty for the high density of lead, lead­acid technology can be found. The disadvantages of lead­acid technology include low specific energy and possibility of hydrogen evolution.

Nickel­cadmium batteries

The nickel­cadmium battery is the dominant alkaline secondary battery and is used in many of the same heavy industrial applications as are lead­acid batteries. At the same time, nickel­cadmium batteries are found in applications requiring portable power such as electronics and power tools. The first nickel­cadmium battery was made by Jungner in Sweden in 1900.

The anode is metallic cadmium. The cathode­active material is nickel oxide present as nickel oxyhydroxide (NiOOH). The active materials of both electrode are incorporated into porous sintered nickel plaques. The electrolyte is an aqueous solution of potassium hydroxide, 31% by weight when the battery is fully charged.

The half­cell reactions at each electrode are shown in reactions (12 (075200#075200RX0120)) and (13 (075200#075200RX0130)). The overall cell reaction is the formation of nickel hydroxide and cadmium hydroxide, shown in reaction (14 (075200#075200RX0140)). The

(12)

(13)

(14) elementary electrochemical reactions are shown in reactions (15 (075200#075200RX0150)) and (16 (075200#075200RX0160)). On discharge, electrons are produced

(15)

(16)

http://www.accessscience.com/content/battery/075200 18/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education \noindent at the anode as metallic cadmium is oxidized to Cd(II) present in cadmium hydroxide; the complementary reaction at the cathode is the reduction of Ni(III) present in nickel oxyhydroxide to Ni(II) present in nickel hydroxide with the attendant consumption of electrons.

Nominal voltage is 1.2 V. The theoretical specific energy is 220 Wh/kg. Commercially available nickel­cadmium batteries deliver about 40 Wh/kg, although cells with plastic­bonded plates can achieve 55 Wh/kg. Nickel­cadmium batteries are capable of withstanding considerable mechanical abuse. They are also chemically quite stable owing to the fact that none of the cell components is attacked by the electrolyte. The disadvantages of nickel­cadmium technology include low specific energy, and the so­called memory effect in which the cell loses capacity as it seemingly conforms to a duty cycle that does not involve full discharge.

Nickel­metal hydride batteries

The nickel­metal hydride battery, commonly designated NiMH, is a comparatively new technology that has found uses in laptop computers, cellular telephones, and other portable devices where high specific energy is sought. Larger cells have been built for use in electric vehicles.

The electrochemistry shares some features with the nickel­cadmium battery. For example, the cathode active material is nickel oxyhydroxide. In contrast, the anode active material is hydrogen, which is present as the hydride of a proprietary metal alloy. The electrolyte is an aqueous solution of potassium hydroxide, roughly 30% by weight.

The half­cell reactions at each electrode are shown in reactions (17 (075200#075200RX0170)) and (18 (075200#075200RX0180)). The overall cell reaction is the formation of nickel hydroxide and extraction of the hydrogen storage alloy, shown in reaction (19 (075200#075200RX0190)).

(17)

(18)

(19)

Here M denotes the hydrogen storage alloy and MH is the hydride compound of same. The elementary electrochemical reactions are shown in reactions (20 (075200#075200RX0200)) and (21 (075200#075200RX0210)). On discharge, electrons are produced

(20)

(21) at the anode as hydrogen stored in the alloy is oxidized to protons, which combine with hydroxyl ions in the electrolyte to form water; the complementary reaction at the cathode is the reduction of Ni(III) present in nickel oxyhydroxide to Ni(II) present in nickel hydroxide with the attendant consumption of electrons.

Nominal voltage is 1.2 V. The theoretical specific energy is 490 Wh/kg. Commercially available nickel­cadmium batteries deliver about 90 Wh/kg, which is substantially higher than nickel­cadmium or lead­acid batteries. However, materials costs associated with the hydrogen storage alloy are high, and thus the final cost per unit charge ($/kWh) is higher for NiMH than for the other two cited technologies.

Silver­zinc batteries

http://www.accessscience.com/content/battery/075200 19/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education The silver­zinc battery boasts the highest specific energy and specific power of any secondary battery containing an aqueous electrolyte. However, the high cost of silver has restricted the battery's use to military and aerospace applications primarily.

The anode is metallic zinc. The cathode active material is silver oxide. The electrolyte is an aqueous solution of potassium hydroxide, 45% by weight.

The discharge reaction occurs in two steps, shown in reactions (22 (075200#075200RX0220)) and (23 (075200#075200RX0230)). The overall cell reaction

(22)

(23) is the production of silver by metallothermic reduction of silver oxide by zinc. The elementary electrochemical reactions are shown in reactions (24 (075200#075200RX0240)), (25 (075200#075200RX0250)), and (26 (075200#075200RX0260)). On discharge, electrons are

(24)

(25)

(26) produced at the anode as zinc is oxidized to Zn(II) present as zinc oxide; the complementary reactions at the cathode are the reduction of Ag(II) present in silver oxide first to Ag(I) present in Ag2O, followed by the reduction of Ag(I) to metallic silver, each step accompanied by the attendant consumption of electrons.

Nominal voltage is 1.5 V. The theoretical specific energy is 590 Wh/kg. Commercially available nickel­cadmium batteries deliver about 120 Wh/kg. However, silver­zinc suffers from the high costs associated with silver, loss of cathode active material through dissolution of silver oxide in the electrolyte, and cell shorting by zinc dendrites on charging, all of which lead to poor cycle life. However, when huge currents are needed (and cost is no object), silver­zinc is the battery of choice.

Sodium­sulfur batteries

The sodium­sulfur battery is unique in several ways. Most batteries have solid electrodes and a liquid electrolyte; the sodium­sulfur battery has molten electrodes separated by a solid membrane serving as the electrolyte. In order to keep the electrodes molten and the electrolyte sufficiently conductive, superambient temperatures are required; the sodium­sulfur battery operates at 300°C (572°F). Sodium­sulfur batteries were once prime candidates for electric vehicle power sources and electric utility energy storage for load leveling. However, in recent years, commercial production has ceased.

The electrolyte is a U­shaped tube made of the ceramic material aluminum oxide, specifically β″­Al2O3 which is a sodium­ ion conductor. The tube is filled with molten metallic sodium which is the anode active material. A metal rod immersed in the sodium serves as current collector. The anode/electrolyte assembly in positioned inside a larger metal container. The space between the metal container and the ceramic electrolyte tube is occupied by carbon felt filled with elemental sulfur which is the cathode active material. The cathode current collector is the carbon felt which is in electrical contact with the outer metal container.

http://www.accessscience.com/content/battery/075200 20/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education The half­cell reactions at each electrode are shown in reactions (27 (075200#075200RX0270)) and (28 (075200#075200RX0280)). The overall cell reaction is the formation of sodium sulfide, shown in reaction (29 (075200#075200RX0290)).

(27)

(28)

(29)

The elementary electrochemical reactions are identical to the above. The key to the cell's operation is the fact that β″­Al2O3 acts as a separator to prevent direct contact of molten sodium and sulfur while allowing only Na+ to pass from the anode chamber to the cathode chamber.

On discharge, electrons are produced at the anode as metallic sodium is oxidized to Na(I) present in the β″­Al2O3 electrolyte as free sodium ions; the complementary reaction at the cathode is the reduction of elemental sulfur to S(II) present as polysulfide ion which reacts immediately to form sodium sulfide. The sodium ions are delivered to the cathode chamber by the solid electrolyte. Thus, as the cell discharges, the sulfur content of the cathode chamber decreases and the sodium sulfide concentration rises.

Nominal voltage is 1.9 V. The theoretical specific energy is 750 Wh/kg. The last commercially available sodium­sulfur batteries delivered about 80 Wh/kg. While sodium­sulfur batteries have performance characteristics comparable to the best batteries with aqueous electrolytes, thermal management requirements (keeping the cell at temperature) and the indeterminacy of the mode of failure, which in turn lead to safety concerns, have limited the successful deployment of this battery technology.

Zinc­air batteries

The combination of a metal anode and an air electrode results in a battery with an inexhaustible supply of cathode reactant. Recharging involves restoring the anode either electrochemically or mechanically, that is, by direct replacement. In principle, extremely high specific energies should be attainable as the theoretical energy density is 1310 Wh/kg.

The anode is metallic zinc. The cathode active material is oxygen, which is present as a component of air. The current collector is high­surface­area carbon. The electrolyte is an aqueous solution of potassium hydroxide 45% by weight, which may be gelled to prevent leakage.

The half­cell reactions at each electrode are shown in reactions (30 (075200#075200RX0300)) and (31 (075200#075200RX0310)). The overall cell reaction is the oxidation of zinc to form tetrahydroxyzincate or zinc oxide, depending upon the instant composition of the electrolyte, shown in reaction (32 (075200#075200RX0320)). When the concentration

(30)

(31)

(32)

http://www.accessscience.com/content/battery/0752002− 21/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education 2− of Zn(OH)4 exceeds the solubility limit, precipitation of zinc oxide occurs according to reaction (33 (075200#075200RX0330)). Under

(33) these conditions the overall reaction is effectively the oxidation of zinc according to reaction (34 (075200#075200RX0340)). The

(34) elementary electrochemical reactions are shown in reactions (35 (075200#075200RX0350)) and (36 (075200#075200RX0360)). On discharge,

(35)

(36) electrons are produced at the anode as zinc is oxidized to Zn(II) present as or zinc oxide; the complementary reaction at the cathode is the reduction of oxygen present in air to O(II) present in hydroxide.

Nominal voltage is 1.5 V. For applications such as notebook computers demonstration cells have been built that deliver 280 Wh/kg. Other attributes include a relatively flat discharge curve, long shelf life thanks to water activation (battery is stored “dry”), and visual inspection of anode condition to determine state of charge. Among the disadvantages is the complexity of a system that comprises a solid anode, liquid electrolyte, and a gaseous cathode. Because the oxygen electrode does not stand up well to operation as an anode (which must happen on charging), a third electrode must be incorporated into the cell. Alternatively, provision must be made for easy disassembly for replacement of a fresh zinc anode and subsequent flawless reassembly of the “recharged” battery.

Lithium­ion batteries

The lithium­ion battery is fast becoming the dominant battery in applications requiring portable power such as electronics and power tools. As such, it is displacing nickel­cadmium and nickel­metal hydride from some of their traditional uses.

The term “lithium­ion” derives from the fact that there is no elemental lithium in the battery; instead, lithium shuttles between hosts, a behavior that has earned this battery the nickname “rocking chair.” In the anode, the chemical potential of lithium is high, that is, desirably near that of pure lithium; in the cathode, the chemical potential of lithium is low. The anode is a carbonaceous material (graphite or coke) chosen for its electronic conductivity and its ability to intercalate lithium at potentials near that of pure lithium. An example is LiC6. In commercial cells the cathode active material is a lithiated transition­metal oxide such as lithium cobalt oxide. Because lithium is more electropositive than hydrogen, the electrolyte must be nonaqueous and aprotic. A representative formulation is a solution (1:1 by volume) of ethylene carbonate and propylene carbonate containing a suitable lithium salt (at a concentration of about 1 M) such as lithium hexafluorophosphate, LiPF6, which has been introduced in order to raise the conductivity of the electrolyte. For safety, a separator made of a polyolefin such as microporous polypropylene is placed between the electrodes. If the electrolyte temperature exceeds a certain value, the separator melts and current flow ceases.

The half­cell reactions at each electrode are shown in reactions (37 (075200#075200RX0370)) and (38 (075200#075200RX0380)).

(37)

http://www.accessscience.com/content/battery/075200 22/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education The overall cell reaction is the formation of lithium cobalt oxide, shown in reaction (39 (075200#075200RX0390)). The (38) elementary electrochemical reactions are shown in reactions (40 (075200#075200RX0400)) and (41 (075200#075200RX0410)).

(39)

(40)

(41) On discharge, electrons are produced at the anode as elemental lithium is oxidized to Li(I) present in the electrolyte as free lithium ions; the complementary reaction at the cathode is the reduction of Co(IV) to Co(III) with the attendant consumption 0 of electrons. Note that lithium intercalates into carbon as Li , a neutral species, whereas lithium intercalates into LiCoO2 as Li+, a charged species. However, the presence of Li+ in the cathode triggers a valence shift in the host itself: the compensating electron does not neutralize Li+ but is localized on Co4+, converting it to Co3+.

Nominal voltage is 3.6 V. The theoretical specific energy of this cell fitted with a LiCoO2 as cathode active material is 770 Wh/kg. Commercially available lithium­ion batteries deliver about 125 Wh/kg.

Lithium­solid polymer electrolyte (SPE) batteries

Since the 1970s it has been known that, upon addition of appropriate salts, polymers can be rendered lithium­ion conductors. Such materials can serve as electrolytes in lithium batteries. When full measure is given to the capacity for miniaturization of a fully solid­state battery, it becomes evident that, in principle, this has the potential to attain the highest specific energy and specific power of any rechargeable technology. In addition, a lithium­SPE battery offers other advantages: ease of manufacture, immunity from leakage, suppression of lithium dendrite formation, elimination of volatile organic liquids, and mechanical flexibility. It is this last attribute that makes lithium­SPE batteries most intriguing: a multilayer laminate of thin films of metal, polymer, and ceramic measuring some tens of micrometers in thickness, totally flexible, with a specific energy in excess of 500 Wh/kg and capable of delivering power at rates in excess of 1000 W/kg at room temperature, operable over a temperature range from −30°C to +120°C, and costing less than $500/kWh. Realization awaits advances in research in materials science and battery engineering.

Outlook

What is in store for secondary power sources, remains speculative. Perhaps higher­capacity batteries for automobile traction will be developed. There may be a push for thin­film, all­solid­state microbatteries, which will enable distributed power sources in combination with their own charger and electrical appliance or load. An example of such integration might be an electrical device powered by a secondary battery connected to a photovoltaic charger.

Donald R. Sadoway

Bibliography

Claus Daniel and J. O. Besenhard (eds.), Handbook of Battery Materials, Wiley­VCH, 2d ed., 2011

T. R. Crompton, Battery Reference Book, 3d ed., Newnes, 2000

R. A. Huggins, Advanced Batteries: Materials Science Aspects, Springer, 2008

Kirk­Othmer Encyclopedia of Chemical Technology, 5th ed., Wiley, 2007

T. Reddy, Linden's Handbook of Batteries, 4th ed., McGraw­Hill, 2010

http://www.accessscience.com/content/battery/075200 23/24 10/13/2016 Battery ­ AccessScience from McGraw­Hill Education Additional Readings

D. Andrea, Battery Management Systems for Large Lithium­Ion Battery Packs, Artech House, Norwood, MA, 2010

N. J. Giordano, Reasoning and Relationships, Brooks/Cole, Belmont, CA, 2010

J. W. Moore, C. L. Stanitski, and P. C. Jurs, Principles of Chemistry: The Molecular Science, Brooks/Cole, Belmont, CA, 2010

C. D. Rahn and C. Wang, Battery Systems Engineering, John Wiley & Sons, Chichester, West Sussex, UK, 2013

S. Seki et al., Imidazolium­based room­temperature ionic liquid for lithium secondary batteries: Relationships between lithium salt concentration and battery performance characteristics, ECS Electrochem. Lett., 1(6):A77–A79, 2012 DOI: 10.1149/2.003206eel (http://dx.doi.org/10.1149/2.003206eel)

http://www.accessscience.com/content/battery/075200 24/24