ARUMUGAMMANTHIRAM The University of Texas at Austin Austin, TX
INTRODUCTION
Batteries are the major power sources for portable elec- tronic devices and toys. They are also used in automo- biles for starting, lighting, and ignition (SLI batteries). At present, the worldwide battery market exceeds $30 billion per year. Rapid technological advances and miniaturiza- tion in electronics have created an ever-increasing demand for compact, lightweight batteries. For example, popular portable electronic devices such as cellular phones, lap- top computers, and camcorders require batteries of high energy density. Additionally, a need for more efficient use of available energy resources as well as air-quality con- trol have created enormous interest in electric vehicles.
For example, the major automobile manufacturers around the globe are engaged in developing advanced batteries for electric vehicles in response to increased environmen- tal regulations and legislative mandates. The advanced and high energy density batteries have become possible due to the discovery and development of smart materials and processes. This article, after providing a brief introduc- tion to the basic electrochemical concepts and the princi- ples involved in batteries, presents the materials and elec- trochemical aspects of high energy density (lithium-ion) batteries.
ELECTROCHEMICAL CONCEPTS
A battery is an electrochemical cell that converts the chem- ical energy of a reaction directly into electrical energy. This section covers briefly the fundamental principles of electro- chemical cells. For more detailed information, readers are referred to several excellent texts available in the litera- ture (1– 4).
Electrochemical Cells
Figure 1 shows a schematic of an electrochemical cell that consists of three components: an anode or negative elec- trode, a cathode or positive electrode, and an electrolyte or ionic conductor. During the electrochemical reaction, the anode M is oxidized and it gives up electrons to the exter- nal circuit:
M→Mn++ne−, (1)
and the cathode X accepts the electrons from the external circuit and is reduced:
X+ne−→Xn−. (2)
The electrolyte, on the other hand, acts as a medium for charge transfer between the anode and cathode as ions inside the cell. The overall cell reaction is given by adding the two half-cell reactions (1) and (2):
M+X→Mn++Xn−. (3) The amount of electricity that passes through an elec- trochemical cell is related by the Faraday law to the masses of reactants involved and products formed. If a current of Iamperes flows in the circuit for a time oftseconds, then the amount of charge Qtransferred across any interface in the cell is equal toItcoulombs. Now, in accordance with the Faraday law, the number of molesNmof the reactants M or X [see Eqs. (1) and (2)] consumed by the passage ofIt coulombs is given by
Nm= It
nNAe, (4)
wheren,NA, andeare, respectively, the number of electrons given up or accepted by each M or X, Avogadro’s number, and the charge on an electron. The product NAeis called the Faraday constantF, which is equal to 96,487 C mol−1, and Eq. (4) can be reduced to
Nm= It
nF. (5)
Consequently, the theoretical capacityQof the electrode is given by
Q=It=nF Nm. (6) One gram-equivalent weight of an electrode, for example, theoretically has a capacity of 96,487 C or 26.8 Ah. Gram- equivalent weight is defined as the atomic or molecular weight in grams divided by the number of electronsnin- volved in the reaction.
Thermodynamics of Electrochemical Cells
The driving force for an electrochemical cell to deliver elec- trical energy to an external circuit is the decrease in the standard free energyGoof the cell reaction [Eq. (3)]. The free energyGois related to the standard cell potentialEo by
Go= −nFEo, (7) wherenand Fare, respectively, the number of electrons involved in the reaction and the Faraday constant. The cell potentialEois the difference between the electrode poten- tials of the cathode and anode. The values ofEofor various electrochemical couples are given in terms of standard re- duction/oxidation potentials in textbooks and handbooks 68
BATTERY APPLICATIONS 69
Anode M
Load
− +
e e
Cathode X
Separator
Electrolyte Electrolyte
Figure 1. Schematic of an electrochemical cell.
(5,6). A positive value ofEomeans that the cell reaction oc- curs spontaneously. The standard potentialEois the equi- librium potential when all of the cell components are in their standard states. For example, the solution species have unit molar activities, the gases have pressures of 1 atmosphere, and the solid phases are in their most stable form in their standard states. For conditions other than the standard state, the cell potentialEis given by the Nernst equation,
E=Eo− RT
nF lnaMn+aXn−
aMaX
, (8)
whereRis the gas constant,Tis the absolute temperature, andaMn+,aXn−,aM, andaXare the activities of the products and reactants involved in cell reaction (3). At room tem- peratureT=298 K, the Nernst equation can be simplified to
E= Eo− 0.0591
n logaMn+aXn−
aMaX
. (9)
The cell potential also depends on the temperature and pressure. The dependences are related to the thermody- namic quantities by
∂E
∂T
P
= S
nF (10)
and
∂E
∂P
T
= −V
nF, (11)
where Sis the entropy change and V is the volume change. Thus, the measurement of the cell potential can be used to determine thermodynamic quantities such as G,S, enthalpy changeH, and equilibrium constants.
Polarization Losses in Electrochemical Cells
The amount of electrical energy that an electrochemical cell can deliver is related to the free energy change of the cell reaction [Eq. (7)]. However, when a currentIis passed through the cell, part of the energy is lost as waste heat due to polarization losses in the cell. The polarization loss can be classified into three types: activation polarization, concentration polarization, and ohmic polarization. Acti- vation polarization is related to the kinetics of electrode reactions. Concentration polarization is related to the con- centration differences of the reactants and products at the electrode surfaces and in the bulk as a result of mass trans- fer. Ohmic polarization, usually referred to as internalIR drop, is related to the internal impedance of the cell, which is a sum of the ionic resistance of the electrolyte and the electronic resistance of the electrodes.
The different polarization losses are indicated schemat- ically in Fig. 2 as a function of operating current (2). The operating (measured) cell voltageEopis given by
Eop=Eoc−η, (12) where Eoc is the open-circuit voltage andηis the overvol- tage from polarization. The overvoltageηis a measure of the deviation of the cell voltage Eopfrom the equilibrium open-circuit voltageEoc. The overvoltageηfrom the three different polarizations is given by
η=ηa+ηc+I Ri, (13) where ηa is the activation polarization at the anode and cathode,ηc is the concentration polarization at the anode and cathode,Iis the load or operating current, andRiis the internal resistance of the cell. The degree of polarization in- creases and the measured cell voltageEopdecreases as cur- rent increases. Therefore, the cell will operate close to the open-circuit voltage Eoc and deliver most of the expected energy only at very low operating currents. Obviously, the
Cell voltage (V)
Eop Eoc
(a) (b) (c)
Current (A)
Figure 2. Variation of cell voltage with operating current illus- trating polarization losses: (a) ohmic polarization, (b) activation polarization, and (c) concentration polarization.
70 BATTERY APPLICATIONS
intrinsic properties of the electrodes and electrolytes as well as the engineering design of the cell will influence the polarization losses and hence the performance and ef- ficiency of electrochemical cells.
BATTERIES
Performance Parameters
As mentioned in the previous section, a battery is an elec- trochemical device that converts stored chemical energy directly into electrical energy. The performance character- istics of a battery are assessed in terms of several param- eters discussed later (1–3). The cell voltageEopis the dif- ference between the electrode potentials of the cathodeEc
and anode Ea:
Eop=Ec−Ea. (14) Although the theoretical capacity Q of a cell or half-cell is given by Eq. (6), it is often convenient to calculate the specific capacityQspfor purposes of comparison. The spe- cific capacityQspis obtained by dividing the capacityQof the cell or half-cell by the massmor volumeVof the cell or half-cell and is usually expressed in terms of Ah/kg or Ah/L:
Qsp= Q
m, (15)
or
Qsp= Q
V. (16)
The available energyεof a cell is given by the product of the cell capacityQand the average operating voltageEop
and again is usually given in terms of either gravimetric energy density (specific energyεsp) in Wh/kg or volumetric energy density in Wh/L:
εsp=QspEop. (17)
The powerPdelivered by the cell is given by the product of the currentIflowing and the associated cell voltageEopand is generally given in terms of gravimetric power density (specific power Psp) in W/ kg or volumetric power density in W/ L:
Psp=IspEop (18) whereIspis the current density (current per unit weight or volume).
The discharge characteristic of a battery is another im- portant parameter, which is given in terms of a plot of cell voltage versus capacity. The discharge profile and the fi- nal capacity obtainable depend on the current densityIsp
used. Figure 3 compares the discharge profiles for various current densities. A useful way of defining the influence of current density on discharge curves is in terms ofC
Cell voltage (V) 10 C
4 C Ideal battery
C/2
0 20 40 60 80
Discharge capacity (%)
100
Figure 3. Discharge profiles at variousCrates that illustrate the influence of current density.
rates:
C= Id
Qn
, (19)
where Idand Qn are, respectively, discharge current and nominal capacity. For example, aCrate ofτ implies that the nominal capacity of the cell is delivered in 1/τ hours under the specified current density. In an ideal battery, the discharge voltage drops sharply to zero when the chemi- cal reaction reaches completion and the stored energy is fully consumed (Fig. 3). The discharge curves deviate from the ideal curve as the discharge rate (or current density) increases due to the polarization losses discussed in the previous section.
Coulometric and energy efficiencies and cycle life of sec- ondary (rechargeable) batteries are some additional impor- tant parameters. Coulometric efficiencyqcis defined as
qC= Qd
Qc
, (20)
where Qd and Qc are, respectively, the discharge and charge capacities. The reciprocal of the coulometric effi- ciency is the charging factor f:
f = 1
qC. (21)
Energy efficiencyqEis defined as qE=qc
Ed
Ec
, (22)
where Ed and Ec are, respectively, the average discharge and charge voltages. AqC<1 implies the occurrence of un- wanted side reactions that produce heat during the charg- ing process. Intrinsic cell materials characteristics, cell en- gineering, and cell operating conditions such as current density and temperature can all influenceqC. A qE<qC
implies a deviation of the discharge and charge curves
BATTERY APPLICATIONS 71
Electrolyte/separator HOMO
LUMO
Cathode Anode
Eg
EF
eEoc EF
E
φc
φa
Figure 4. Schematic energy diagram of a cell that has an open circuit.
from the open-circuit voltage profile. Again, polarization losses arising from materials characteristics, cell engineer- ing, and operating conditions can influenceqE.
The cycle life of a battery is the number of times it can be charged and discharged repeatedly before the cell capacity falls below a limiting value. Generally, the limiting value is set around 70 to 80% of the nominal capacity. The cycle life depends on the reversible characteristics (structural and chemical stability) of the electrode materials, cell en- gineering, and operating conditions such as temperature, current density, and depth of discharge.
Design Considerations
The equilibrium cell voltage Eoc and the capacity Q of a battery are determined by the intrinsic properties of the electrode materials. The cell voltage can be maximized by choosing anode materials that have a smaller work func- tionφaand cathode materials that have a larger work func- tionφc. In other words, the anode should be a good reducing agent that has a large negative reduction potential, and the cathode should be a good oxidizing agent that has a large positive reduction potential. A schematic energy diagram of an open circuit is shown in Fig. 4. The open-circuit volt- ageEocof the cell is given by
Eoc= φc−φa
e (23)
Table 1. Major Primary Battery Systems
Cell Voltage Capacity
Battery Anode Cathode Cell Reaction (V) (Ah / kg)a
Leclanche Zn MnO2 Zn+2MnO2→ZnO·Mn2O3 1.6 224
Magnesium Mg MnO2 Mg+2MnO2+H2O→Mn2O3+Mg(OH)2 2.8 271
Alkaline MnO2 Zn MnO2 Zn+2MnO2→ZnO+Mn2O3 1.5 224
Mercury Zn HgO Zn+HgO→ZnO+Hg 1.34 190
Zinc–air Zn O2 Zn+0.5O2→ZnO 1.65 658
Li–SO2 Li SO2 2Li+2SO2→Li2S2O4 3.1 379
Li–MnO2 Li MnO2 Li+MnO2→LiMnO2 3.1 286
aBased only on active cathode and anode materials.
or by the difference between the electrode potentials of the cathode and anode [Eq. (14)]. Thermodynamic stability considerations also require that the Fermi energies EFof the cathode and anode lie within the band gap Eg of the electrolyte, as shown in Fig. 4, so that no unwanted reduc- tion or oxidation of the electrolyte occurs. This implies a limitation of
φc−φa<Eg. (24) Alkali and alkaline-earth metals that have a smaller φa
or a larger negative reduction potential are attractive an- odes, and higher valent transition-metal compounds that have a largerφcor larger positive reduction potentials are attractive cathodes to maximize the cell voltage. The cell capacity, on the other hand, is determined by the atomic or molecular weight of the elements or compounds used as electrodes and the degree of reaction (number of elec- trons involved) per mole of the electrode material [Eq. (6)].
Lightweight elements such as hydrogen, lithium, or oxy- gen and low molecular weight compounds are preferred as electrodes to maximize cell capacity.
In addition to high cell voltage and capacity, several other criteria are important in designing a battery to achieve high efficiency and minimal energy loss. The elec- trolyte should have good ionic conductivity, but should be an electronic insulator to avoid internal short-circuiting.
High ionic conductivity in the electrolyte is essential to minimize theIRdrop or ohmic polarization. Using a given electrolyte, the IRdrop due to electrolyte resistance can be reduced, and the rate capability can be improved by a higher electrode interfacial area and thin separators. The electrode should have a high electronic conductivity and diffusion rate for the ions to minimize the IR drop. The electronic conductivity of the electrodes can be improved by adding electrically conducting additives such as car- bon. The electrode reaction rates at the cathode and anode should be high enough to minimize activation polariza- tion. This is commonly achieved by using a porous elec- trode design, which can reduce the local current density by providing high surface area. Adequate flow or passage of electrolytes is essential to facilitate mass transfer and min- imize concentration polarization. Electrode porosity and pore size, optimum separator thickness and structure, and concentration of the reactants in the electrolytes are im- portant factors in minimizing concentration polarization.
In addition to these points, the electrolyte should have
72 BATTERY APPLICATIONS
Table 2. Major Secondary Battery Systems
Cell Voltage Capacity
Battery Anode Cathode Cell Reaction (V) (Ah / kg)a
Lead–acid Pb PbO2 Pb+PbO2+2H2SO4→2PbSO4+2H2O 2.1 120
Nickel–cadmium Cd NiOOH Cd+2NiOOH+2H2O→2Ni(OH)2+Cd(OH)2 1.35 181
Nickel–hydrogen H2 NiOOH H2+2NiOOH→2Ni(OH)2 1.5 289
Nickel–metal hydride MH NiOOH MH+NiOOH→M+Ni(OH)2 1.35 206
Lithium-ion Li Li0.5CoO2 0.5Li+Li0.5CoO2→LiCoO2 3.7 137
aBased only on active cathode and anode materials.
good chemical stability and should not undergo any di- rect reaction with the electrodes. In rechargeable batter- ies, chemical reversibility on the electrodes is crucial to maintaining good capacity retention. Raw materials and fabrication costs, cell safety, and environmental factors are additional considerations.
Types of Batteries
Batteries can be classified into two types: primary (non- rechargeable) and secondary (rechargeable) batteries.
Electrode materials undergo irreversible chemical reac- tions in primary batteries, but they exhibit reversible chemical reactions in secondary batteries. Some major pri- mary and secondary battery systems are given in Tables 1 and 2 (2). The tables give the cell reactions, voltage, and ca- pacity for each system. Most of the primary and secondary systems are based on aqueous electrolytes; the lithium- based primary systems in Table 1 and the lithium-ion system in Table 2 are based on nonaqueous electrolytes.
The aqueous systems are limited in cell voltage (≤2.1 V) due to a smaller separationEgbetween the highest occu- pied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of water (Fig. 4) and the con- sequent vulnerability of water to reduction/oxidation reac- tions at higher cell voltages. The use of nonaqueous elec- trolytes that have a largerEg, on the other hand, permits higher cell voltages in lithium-based systems.
SMART BATTERIES
The discovery of smart materials and the development of new processes have revolutionized the electronics indus- try over the years. The continued reduction in the sizes and weights of popular portable electronic devices such as cellular phones and laptop computers has driven the par- allel, development of smart batteries to power them. In this regard, lithium-ion batteries have become appealing because they offer higher energy density (volumetric and gravimetric) compared to other rechargeable systems (Fig.
5) such as lead–acid, nickel–cadmium, and nickel–metal hydride batteries (7). Lithium-ion batteries are smaller and lighter compared to other systems. Lithium-ion bat- teries have become a commercial reality since Sony Cor- poration introduced them in 1990 as a result of the dis- covery of new materials over the years. The history, prin- ciples, current status, and future challenges of lithium-ion technology are briefly discussed in the following sections.
For more detailed information, the readers are referred to several references in the literature (1–3, 8–13).
Lithium-Ion Batteries
Maximizing the energy density of a battery requires using electrode materials that offer both high capacity and high cell voltage Eoc. High cell voltage can be achieved by us- ing an anode material that has a smaller work functionφa
and a cathode material that has a larger work functionφc, as shown in [Eq. (23)]. Lithium metal—the lightest solid in the periodic table—has a high specific capacity and a high standard oxidation potential (smallφa) and is an at- tractive anode for achieving high energy density. Because of this objective, batteries that consist of metallic lithium as an anode and a lithium insertion compound LixMyXz
(M=transition metal and X=nonmetal) as a cathode be- came appealing during the 1970s; a lithium insertion com- pound is a host matrix into/from which the guest species Li+ can be reversibly inserted/extracted. This concept of a secondary lithium battery was initially demonstrated using a layered metal sulfide TiS2 as the cathode and a nonaqueous electrolyte consisting of a lithium salt such as LiClO4dissolved in an organic solvent such as propylene carbonate. In this cell, the Li+ions produced at the anode by oxidation of the metallic lithium during discharge mi- grate through the electrolyte and are inserted into the van der Waals gap between the sulfide layers of TiS2, and the electrons flow through the external circuit from the anode to the cathode to give LixTiS2. During the charging process, the Li+ions are extracted from LixTiS2and the electrons
Smaller
Lithium-ion
Lead-acid
Ni / Cd Ni / MH 250
200 150
100 Lighter
Gravimetric energy density (Wh/kg)
50 0
0 100 200
Volumetric energy density (Wh/L)
300 400
Figure 5. Comparison of the gravimetric and volumetric energy densities of various rechargeable battery systems.
BATTERY APPLICATIONS 73
Discharge Load
e
Charge e
Discharge
Charge
Electrolyte Li+ Li+
Li+ Li+
Cathode Anode
LixC6 Li1−xCoO2
Figure 6.Schematic of the charging/discharging process in a lithium-ion cell.
flow from the cathode to the anode through the external circuit.
In this cell, however, the chemical reactivity of metallic lithium with the nonaqueous electrolyte results in forming a passivating film on the anode. Although the passivating film prevents further corrosion, it leads to nonuniform plat- ing of lithium during charging, which results in total cell failure due to dendritic short-circuiting and also in serious safety problems due to local overheating. These difficul- ties of the metallic lithium anode forced the use of lithium insertion compounds as both anodes and cathodes. These cells are called lithium-ion cells or rocking-chair cells be- cause the lithium ion shuttles or rocks between the cathode and anode hosts during the charging/discharging process (Fig. 6). This strategy, however, requires careful selection of cathode and anode pairs to maintain high cell voltage (>3 V) and to minimize the added weight of the insertion compound anode.
Although the concept of secondary lithium batteries was initially demonstrated by using a sulfide cathode, it was recognized during the 1980s that it is difficult to achieve high cell voltage using sulfide cathodes because an over- lap of the higher valent Mn+:d energies and the top of the S:3p energy and the formation of S22−ions lead to an inac- cessibility of higher oxidation states for Mn+ in a sulfide LixMySz; the stabilization of the higher oxidation state is essential to maximize the work function φc and thereby the cell voltage Eoc [Eq. (23)]. On the other hand, the lo- cation of O:2p energy much below the S:3p energy and a larger increase of the Mn+:d energies in an oxide compared to those in a sulfide, due to a larger Madelung energy, make the higher valent states accessible in oxides. Accordingly, transition-metal oxide hosts were pursued as cathodes during the 1980s (14–16).
Figure 7 compares the electrochemical potential ranges of some lithium insertion compounds versus metallic
LixV6O13
Li4+xTi5O12
Li Metal
Insertion compound
Voltage (V)
LixCoO2 LixNiO2 LixFePO4
LixTiS2
Lix Coke Lix Graphite
LiAl
LixFe2(SO4)3 Li1−xMn2O4 Li1−xNi0.5Mn1.5O4 LixMnO2
Li1−xCoMnO4 5
4 3 2 1 0
Figure 7. Electrochemical potential ranges of some lithium in- sertion compounds with reference to metallic lithium.
lithium. Among them, LiCoO2, LiNiO2, and LiMn2O4
oxides that have a higher electrode potential of 4 V ver- sus metallic lithium have become attractive cathodes for lithium-ion cells. Graphite and coke that have lower elec- trode potentials <1 V versus metallic lithium and are lightweight have become attractive anodes. In a lithium- ion cell made from, for example, a LiCoO2 cathode and a carbon anode (Fig. 6), the lithium ions migrate from the LiCoO2cathode to the LixC6anode through the electrolyte, and the electrons flow through the external circuit from the cathode to the anode during the charging process. Ex- actly the reverse reaction occurs during the discharging process.
A lithium insertion compound should have several fea- tures to be a successful electrode (cathode or anode) in lithium-ion cells:
rThe cathode should have a high lithium chemical potential (µLi(c)), and the anode should have a low lithium chemical potential (µLi(a)) to maximize the cell voltage:
Eoc= µLi(c)−µLi(a)
F . (25)
The voltage is determined by the energies involved in both electron transfer and Li+ transfer. The energy involved in electron transfer is related to the work functions of the cathode (φc) and anode (φa) as shown in Eq. (23), whereas that involved in Li+transfer is determined by the crystal structure and the coordi- nation geometry of the site into/from which Li+ ions are inserted /extracted (17). If we consider only elec- tron transfer, thenEoccan be given by Eq. (23). This implies that the Mn+ ion in the insertion compound LixMyOzshould have a high oxidation state to be used as a cathode and a low oxidation state to be used as an anode.