Linden D., Reddy T.B. (eds.) Handbook of batteries

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BASIC CONCEPTS 1.5

1.2.1 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. The

general 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 high-

capacity 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.

1.2.2 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 ‘‘accu-

mulators.’’

The applications of secondary batteries fall into two main categories:

1. 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 sys-

tems, emergency no-fail and standby (UPS) power sources, hybrid electric vehicles and

stationary energy storage (SES) systems for electric utility load leveling.

2. 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 perform-

ance. 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 replace-

ment 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.

1.2.3 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.

1.6 CHAPTER ONE

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 the

cell reaction, but have catalytic properties which enhance the electroreduction or electro-

oxidation of the reactants (the active materials).

The anode active materials used in fuel cells are generally gaseous or liquid (compared

with 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 less

polluting means for converting hydrogen and carbonaceous or fossil fuels to electricity com-

pared 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

1. Direct systems where fuels, such as hydrogen, methanol and hydrazine, can react directly

in the fuel cell

2. 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 of

fuel and oxidant, the type of electrolyte, the temperature of operation, and the application,

etc.

More recently, fuel cell technology has moved towards portable applications, historically

the 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 ‘‘refu-

eled’’ 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 scope

and 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 ap-

plications, 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 ‘‘Bibliography.’’

BASIC CONCEPTS 1.7

1.3 OPERATION OF A CELL

1.3.1 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.

FIGURE 1.1 Electrochemical op-

eration of a cell (discharge).

The discharge reaction can be written, assuming a metal as the anode material and a

cathode material such as chlorine (Cl

), as follows:

Negative electrode: anodic reaction (oxidation, loss of electrons)

⫹

Zn → Zn ⫹ 2e

Positive electrode: cathodic reaction (reduction, gain of electrons)

⫺

Cl ⫹ 2e → 2Cl

Overall reaction (discharge):

⫹⫺

Zn ⫹ Cl → Zn ⫹ 2Cl (ZnCl )

1.8 CHAPTER ONE

1.3.2 Charge

During the recharge of a rechargeable or storage cell, the current ﬂow is reversed and oxi-

dation 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 /Cl

cell, the reaction on charge can be written as follows:

Negative electrode: cathodic reaction (reduction, gain of electrons)

⫹

Zn ⫹ 2e → Zn

Positive electrode: anodic reaction (oxidation, loss of electrons)

⫺

2Cl → Cl ⫹ 2e

Overall reaction (charge):

⫹⫺

Zn ⫹ 2Cl → Zn ⫹ Cl

FIGURE 1.2 Electrochemical op-

eration of a cell (charge).

1.3.3 Speciﬁc Example: Nickel-Cadmium Cell

The processes that produce electricity in a cell are chemical reactions which either release

or 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) ⫹ 2e

At the cathode, nickel oxide (or more accurately nickel oxyhydroxide) is reduced to nickel

hydroxide with the acceptance of an electron,

⫺

NiOOH ⫹ HO⫹ e → OH ⫹ Ni(OH)

BASIC CONCEPTS 1.9

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,

⫹ 2NiOOH ⫹ 2H O → Cd(OH) ⫹ 2Ni(OH)

222

This is the discharge process. If this were a primary non-rechargeable cell, at the end of

discharge, it would be exhausted and discarded. The nickel-cadmium battery system is, how-

ever, a secondary (rechargeable) system, and on recharge the reactions are reversed. At the

negative electrode the reaction is:

⫺

Cd(OH) ⫹ 2e → Cd ⫹ 2OH

At the positive electrode the reaction is:

⫺

Ni(OH) ⫹ OH → NiOOH ⫹ HO⫹ e

After recharge, the secondary battery reverts to its original chemical state and is ready for

further discharge. These are the fundamental principles involved in the charge–discharge

mechanisms of a typical secondary battery.

1.3.4 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

⫹

2H → 4H ⫹ 4e

while the cathodic reaction is

⫹

O ⫹ 4H ⫹ 4e → 2H O

The overall reaction is the oxidation of hydrogen by oxygen, with water as the reaction

product.

⫹ O → 2H O

22 2

1.4 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.)

1.4.1 Free Energy

Whenever a reaction occurs, there is a decrease in the free energy of the system, which is

expressed as

⌬G ⫽⫺nFE

where F

⫽ constant known as Faraday (⬇96,500 C or 26.8 Ah)

⫽ number of electrons involved in stoichiometric reaction

⫽ standard potential, V

1.10 CHAPTER ONE

1.4.2 Theoretical Voltage

The standard potential of the cell is determined by the type of active materials contained in

the 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 1.1. A

more complete list is presented in Appendix B.

The standard potential of a cell can be calculated from the standard electrode potentials

as 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

⫹ Cl

→ ZnCl

, the standard cell potential is:

⫹

Zn → Zn ⫹ 2e ⫺(⫺0.76 V)

⫺

Cl → 2Cl ⫺ 2e 1.36 V

E⬚ ⫽ 2.12 V

The cell voltage is also dependent on other factors, including concentration and temper-

ature, as expressed by the Nernst equation (covered in detail in Chap. 2).

1.4.3 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 mate-

rials. 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 Appen-

dix 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/Cl

cell is 0.394 Ah /g, that is,

—

Zn ⫹ Cl → ZnCl

(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 ap-

propriate data for ampere-hours per cubic centimeter as listed in Table 1.1.

The theoretical voltages and capacities of a number of the major electrochemical systems

are 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.

BASIC CONCEPTS 1.11

TABLE 1.1 Characteristics of Electrode Materials*

Material

Atomic or

molecular

weight, g

Standard

reduction

potential

at 25

⬚C, V

Valence

change

Melting

point, ⬚C

Density,

g/cm

Electrochemical equivalents

Ah/g g/Ah Ah/cm

‡

Anode materials

(Li)C

(1)

(2)

2.01

6.94

23.0

24.3

26.9

40.1

55.8

65.4

112.4

207.2

72.06

116.2

32.04

⫺0.83†

⫺3.01

⫺2.71

⫺2.38

⫺2.69†

⫺1.66

⫺2.84

⫺2.35†

⫺0.44

⫺0.88†

⫺0.76

⫺1.25†

⫺0.40

⫺0.81†

⫺0.13

⬃⫺2.8

⫺0.83†

—

180

650

659

851

1528

419

321

327

—

0.54

0.97

1.74

2.69

1.54

7.85

7.14

8.65

11.34

2.25

—

26.59

3.86

1.16

2.20

2.98

1.34

0.96

0.82

0.48

0.26

0.37

0.45

5.02

0.037

0.259

0.858

0.454

0.335

0.748

1.04

1.22

2.10

3.87

2.68

2.21

0.20

2.06

1.14

3.8

8.1

2.06

7.5

5.8

4.1

2.9

0.84

—

Cathode materials

MnO

NiOOH

CuCl

FeS

AgO

HgO

PbO

CoO

(3)

32.0

71.0

64.0

86.9

91.7

99.0

119.9

123.8

159.8

216.6

231.7

239.2

253.8

1.23

0.40†

1.36

—

1.28‡

0.49†

0.14

—

0.57†

1.07

0.10†

0.35†

1.69

⬃2.7

0.54

0.5

—

5.0

7.4

3.5

—

7.4

—

11.1

7.1

9.4

—

4.94

3.35

0.756

0.419

0.308

0.292

0.270

0.89

0.432

0.335

0.247

0.231

0.224

0.137

0.211

0.30

1.32

2.38

3.24

3.42

3.69

1.12

2.31

2.98

4.05

4.33

4.45

7.29

4.73

1.54

2.16

0.95

4.35

3.20

2.74

1.64

2.11

—

1.04

* See also Appendixes B and C.

† Basic electrolyte: all others, aqueous acid electrolyte.

‡ Based on density values shown.

(1) Calculations based only on weight of carbon.

(2) Based on 1.7% H

storage by weight.

(3) Based on x

⫽ 0.5; higher valves may be obtained in practice.

1.12

TABLE 1.2 Voltage, Capacity and Speciﬁc Energy of Major Battery Systems—Theoretical and Practical Values

Battery type Anode Cathode Reaction mechanism

Theoretical values†

V g/Ah Ah/kg

Speciﬁc

energy

Wh / kg

Practical battery‡

Nominal

voltage

Speciﬁc

energy

Wh/kg

Energy

density

Wh / L

Primary batteries

Leclanche´ Zn MnO

Zn ⫹ 2MnO

→ ZnO  Mn

1.6 4.46 224 358 1.5 85

(4)

165

(4)

Magnesium Mg MnO

Mg ⫹ 2MnO

⫹ H

O → Mn

⫹ Mg(OH)

2.8 3.69 271 759 1.7 100

(4)

195

(4)

Alkaline MnO

Zn MnO

Zn ⫹ 2MnO

→ ZnO ⫹ Mn

1.5 4.46 224 358 1.5 145

(4)

400

(4)

Mercury Zn HgO Zn ⫹ HgO → ZnO ⫹ Hg 1.34 5.27 190 255 1.35 100

(6)

470

(6)

Mercad Cd HgO Cd ⫹ HgO ⫹ H

O → Cd(OH)

⫹ Hg 0.91 6.15 163 148 0.9 55

(6)

230

(6)

Silver oxide Zn Ag

OZn⫹ Ag

O ⫹ H

O → Zn(OH)

⫹ 2Ag 1.6 5.55 180 288 1.6 135

(6)

525

(6)

Zinc / O

Zn O

Zn ⫹

⁄

→ ZnO 1.65 1.52 658 1085 — — —

Zinc / air Zn Ambient air Zn

⫹ (

⁄2O

) → ZnO 1.65 1.22 820 1353 1.5 370

(6)

1300

(6)

Li / SOCl

Li SOCl

4Li ⫹ 2SOCl

→ 4LiCl ⫹ S ⫹ SO

3.65 3.25 403 1471 3.6 590

(4)

1100

(4)

Li/SO

Li SO

2Li ⫹ 2SO

→ Li

3.1 2.64 379 1175 3.0 260

(5)

415

(5)

LiMnO

Li MnO

Li ⫹ Mn

→ Mn

(Li

⫹

) 3.5 3.50 286 1001 3.0 230

(5)

535

(5)

Li / FeS

Li FeS

4Li ⫹ FeS

→ 2Li

S ⫹ Fe 1.8 1.38 726 1307 1.5 260

(5)

500

(5)

Li / (CF)

Li (CF)

nLi ⫹ (CF)

→ nLiF ⫹ nC 3.1 1.42 706 2189 3.0 250

(5)

635

(5)

Li/I

(3)

Li I

(P2VP) Li ⫹

⁄2I

→ LiI 2.8 4.99 200 560 2.8 245 900

Reserve batteries

Cuprous chloride Mg CuCl Mg ⫹ Cu

→ MgCl

⫹ 2Cu 1.6 4.14 241 386 1.3 60

(7)

Zinc / silver oxide Zn AgO Zn ⫹ AgO ⫹ H

O → Zn(OH)

⫹ Hg 1.81 3.53 283 512 1.5 30

(8)

Thermal Li FeS

See Section 21.3.1 2.1–1.6 1.38 726 1307 2.1–1.6 40

(9)

100

(9)

1.13

Secondary batteries

Lead-acid Pb PbO

Pb ⫹ PbO

⫹ 2H

→ 2PbSO

⫹ 2H

O 2.1 8.32 120 252 2.0 35 70

(10)

Edison Fe Ni oxide Fe ⫹ 2NiOOH ⫹ 2H

O → 2Ni(OH)

⫹ Fe(OH)

1.4 4.46 224 314 1.2 30 55

(10)

Nickel-cadmium Cd Ni oxide Cd ⫹ 2NiOOH ⫹ 2H

O → 2Ni(OH)

⫹ Cd(OH)

1.35 5.52 181 244 1.2 35 100

(5)

Nickel-zinc Zn Ni oxide Zn ⫹ 2NiOOH ⫹ 2H

O → 2Ni(OH)

⫹ Zn(OH)

1.73 4.64 215 372 1.6 60 120

Nickel-hydrogen H

Ni oxide H

⫹ 2NiOOH → 2Ni(OH)

1.5 3.46 289 434 1.2 55 60

Nickel-metal hydride MH

(1)

Ni oxide MH ⫹ NiOOH → M ⫹ Ni(OH)

1.35 5.63 178 240 1.2 75 240

(5)

Silver-zinc Zn AgO Zn ⫹ AgO ⫹ H

O → Zn(OH)

⫹ Ag 1.85 3.53 283 524 1.5 105 180

(10)

Silver-cadmium Cd AgO Cd ⫹ AgO ⫹ H

O → Cd(OH)

⫹ Ag 1.4 4.41 227 318 1.1 70 120

(10)

Zinc / chlorine Zn Cl

Zn ⫹ Cl

→ ZnCl

2.12 2.54 394 835 — — —

Zinc / bromine Zn Br

Zn ⫹ Br

→ ZnBr

1.85 4.17 309 572 1.6 70 60

Lithium-ion Li

(i–x)

CoO

⫹ Li

(i–x)

CoO

→ LiCoO

⫹ C

4.1 9.98 100 410 4.1 150 400

(5)

Lithium / manganese dioxide Li MnO

Li ⫹ Mn

→ Mn

(Li

⫹

) 3.5 3.50 286 1001 3.0 120 265

Lithium / iron disulﬁde

(2)

Li(Al) FeS

2Li(Al) ⫹ FeS

→ Li

FeS

⫹ 2Al 1.73 3.50 285 493 1.7 180

(11)

350

(11)

Lithium / iron monosulﬁde

(2)

Li(Al) FeS 2Li(Al) ⫹ FeS → Li

S ⫹ Fe ⫹ 2Al 1.33 2.90 345 459 1.3 130

(11)

220

(11)

Sodium / sulfur

(2)

Na S 2Na ⫹ 3S → Na

2.1 2.65 377 792 2.0 170

(11)

345

(11)

Sodium / nickel chloride

(2)

Na NiCl

2Na ⫹ NiCl

→ 2NaCl ⫹ Ni 2.58 3.28 305 787 2.6 115

(11)

190

(11)

Fuel cells

/ air

Ambient air

⫹

⁄

→ H

⫹ (

⁄

) → H

1.23

0.336

0.037

2975

26587

3660

32702

Methanol / O

OH O

OH ⫹

⁄2O

→ CO

⫹ 2H

O 1.24 0.50 2000 2480 — — —

Methanol / air CH

OH Ambient air CH

OH ⫹ (

⁄2O

) → CO

⫹ 2H

O 1.24 0.20 5020 6225 — — —

† Based on active anode and cathode materials only, including O

but not air (electrolyte not included).

* These values are for single cell batteries based on identiﬁed design and at discharge rates optimized for energy

density, using midpoint voltage. More speciﬁc values are given in chapters on each battery system.

(1) MH

⫽ metal hydride, data based on 1.7% hydrogen storage (by weight).

(2) High temperature batteries.

(3) Solid electrolyte battery (Li/I

(P2VP)).

(4) Cylindrical bobbin-type batteries.

(5) Cylindrical spiral-wound batteries.

(6) Button type batteries.

(7) Water-activated.

(8) Automatically activated 2- to 10-min rate.

(9) With lithium anodes.

(10) Prismatic batteries.

(11) Value based on cell performance, see appropriate chapter for details.

1.14 CHAPTER ONE

FIGURE 1.3 Components of a cell.

1.4.4 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/Cl

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.

1.5 SPECIFIC ENERGY AND ENERGY DENSITY OF PRACTICAL

BATTERIES

The theoretical electrical properties of cells and batteries are discussed in Sec. 1.4. In sum-

mary, 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.