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

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RECHARGEABLE LITHIUM BATTERIES (AMBIENT TEMPERATURE) 34.11

FIGURE 34.5 Reversible discharge capacity of cathode materials for lithium-

ion cells (cells fabricated with graphite anode). (Courtesy Yardney Technical

Products, Inc.)

TABLE 34.7 Diffusion Coefﬁcients of Li-Ion in Lithiated

Transition Metal Oxides

Metal oxides Techniques D

⫹

,cm

/s References

LiCoO

Transient

Impedance

⫻ 10

⫺

5 ⫻ 10

⫺

5 ⫻ 10

⫺

LiNiO

Impedance 2 ⫻ 10

⫺

LiMn

Transient

⫺

K. Mizushima, P. C. Jones, P. J. Wieseman, and J. B. Good-

enough, Solid State Ionics 3/4:171, 1981.

S. Kikkawa, S. Miyazaki, and M. Koizumi, J. Power Sources 14:

231, 1985.

M. G. S. R. Thomas, P. Bruce, and J. Goodenough, Solid State

Ionics 18 / 19:794, 1986.

P. G. Bruce, A. Lisowska-Oleksiak, M. Y. Saidi, and C. A. Vin-

cent, Solid State Ionics 57:353, 1992.

P. G. Dickens and G. F. Reynolds, Solid State Ionics 5:53, 1981.

D. Guyomard and J. M. Tarascon, J. Electrochem. Soc. 139:937,

1992.

Inorganic Electrolytes. Another choice of active materials for the cathode is the inorganic

electrolytes such as liquid SO

-based LiAlCl

or LiGaCl

. Reduction and oxidation occur

during the discharge-charge process on carbon surfaces. The cell reaction depends on the

type of carbon and electrolytes used. On relatively low-surface-area carbon such as acetylene

black, a simple SO

couple is involved in the charge-discharge process,

⫽

discharge

⫽

—— —



2SO ⫹ 2e SO

———



charge

The reduction of SO

occurs at about 2.9 V at 1 mA/cm

34.12 CHAPTER THIRTY-FOUR

When high-surface-area carbons such as Ketjen black and SO

-based LiAlCl

electrolytes

are used,

a carbon-electrolyte complex is believed to take part in the discharge-charge

process.

Solid Cathodes. Solid-state couples can also be used as cathode materials in SO

-based

inorganic electrolytes. The high reduction potential of SO

limits the selection of the positive

electrode materials that can be used in SO

electrolytes to those compounds that reduce at

potential above 2.90 V. Thus CuCl

, which has an open-circuit voltage of 3.43 V and dis-

charges at about 3.35 V at 1 mA /cm

, corresponding to the reduction process of

discharge

⫹

—— —



Cu ⫹ e Cu

———



charge

meets these criteria. The second reduction process (Cu

⫹

⫹ e → Cu) occurs at 2.5 V. In

principle, the discharge of Li/CuCl

cells should not occur below 2.9 V, since the cathode

will be passivated by lithium dithionite (LiS

) produced from the reduction of SO

LiCoO

is being investigated in SO

-based inorganic electrolytes.

12,65,66

This cathode ma-

terial has a higher voltage and the SO

-based LiAlCl

electrolyte is potentially suitable

because of its wide electrochemical voltage window.

34.2.3 Electrolytes

The choice of electrolyte for rechargeable lithium batteries is also critical. The electrolyte

should have the following characteristics:

1. Good ionic conductivity (

⬎10

⫺

S/ cm from ⫺40 to 90⬚C) to minimize internal resistance.

2. Lithium ion transference number approaching unity (to limit concentration polarization)

3. Wide electrochemical voltage window (0 to 5 V)

4. Thermal stability (up to 70

⬚C)

5. Compatibility with other cell components

Aprotic Organic Electrolytes. Aprotic liquid organic electrolyte solvents, such as dioxo-

lane, propylene carbonate, ethylene carbonate, diethyl carbonate and ethylmethyl carbonate

are the most common electrolyte solvents because of their low reactivity with lithium. A list

of the electrolyte solvents used in rechargeable lithium batteries with their major character-

istics is given in Table 34.8.

Choices for the electrolyte solute and their ionic conductivities

in various solvents at different temperatures are listed in Table 34.9. These organic liquid

electrolytes generally have conductivities that are about two orders of magnitude lower than

aqueous electrolytes.

34.13

TABLE 34.8 Characteristics of Organic Solvents*

Characteristic

␥

-BL THF 1,2-DME PC EC DMC DEC DEE Dioxolane (DN)

Structural formula

Boiling point, ⬚C 202–204 65–67 85 240 248 91 126 121 78

Melting point,

⬚C ⫺43 ⫺109 ⫺58 ⫺49 ⫺39–40 4.6 ⫺43 ⫺74 ⫺95

Density, g /cm

1.13 0.887 0.866 1.198 1.322 1.071 0.98 0.842 1.060

Viscosity at 25

⬚C, cP 1.75 0.48 0.455 2.5 1.86

(at 40

⬚C)

0.59 0.75 0.65 0.58

Dielectric constant at

⬚C

39 7.75 7.20 64.4 89.6

(at 40⬚C)

3.12 2.82 5.1 6.79

Molecular weight 86.09 72.10 90.12 102.0 88.1 90.08 118.13 118.18 74.1

Typical H

O content,

ppm

⬍10 ⬍10 ⬍10 ⬍10 ⬍10 ⬍10 ⬍10 ⬍10 ⬍10

Electrolytic

conductivity at 20

⬚C,

1M LiAsF

, mS/cm

10.62 12.87 19.40 5.28 6.97 11.00

(1.9 M)

5.00

(1.5 M)

⬃10.00† ⬃11.20†

␥

-BL ⫽

␥

-butyrolactone; THF ⫽ tetrahydrofuran; 1,2-DME ⫽ 1,2-dimethoxyethane; PC ⫽ propylene carbonate;

⫽ ethylene carbonate; DMC ⫽ dimethyl carbonate; DEC ⫽ diethyl carbonate; DEE ⫽ diethoxyethane.

† Estimation based on Walden’s rule.

Source: From Ref. 8.

34.14

TABLE 34.9 Ionic Conductivity of Some 1 Molar Organic Liquid Electrolytes Used in Secondary Lithium Battery Systems

Salt Solvents

Solvent,

vol %

Conductivities at

⬚C, mS / cm

⫺40 ⫺20 ⫺0 20 40 60 80 References

LiPF

EC/PC

2-MeTHF/ EC / PC

EC/ DMC

EC/ DME

EC/ DEC

50/50

75/ 12.5 / 12.5

33/67

0.23

2.43

—

1.36

4.46

1.2

8.0

2.5

3.45

6.75

5.0

13.6

4.4

6.56

9.24

10.0

18.1

7.0

10.34

11.64

—

25.2

9.7

14.63

14.00

20.0

31.9

12.9

19.35

16.22

—

†

‡

LiASF

EC/ DME

PC/ DME

2-MeTHF/ EC / PC

50/50

75/ 12.5 / 12.5

Freeze

2.54

5.27

4.43

4.67

9.50

8.37

6.91

14.52

13.15

9.90

20.64

18.46

12.76

26.65

23.92

15.52

32.57

28.18

18.18

LiCF

EC/PC

DME/PC

2-MeTHF/ EC / PC

50/50

75/ 12.5 / 12.5

0.02

—

0.50

0.55

2.61

Freeze

0.93

1.24

4.17

5.32

1.34

2.22

5.88

7.41

1.78

3.45

7.46

9.43

2.31

4.88

9.07

11.44

2.81

6.43

10.61

13.20

3.30

LiN(CF

)

EC/PC

EC/ DME

PC/ DME

2-MeTHF/ EC / PC

50/50

75/ 12.5 / 12.5

0.28

—

2.07

1.21

Freeze

3.92

3.40

2.80

7.87

7.19

5.12

12.08

11.23

7.06

7.69

16.58

15.51

8.71

10.70

21.25

19.88

10.41

13.86

25.97

24.30

12.02

LiBF

EC/PC

2-MeTHF/ EC / PC

EC/ DMC

EC/ DEC

EC/ DME

50/50

75/ 12.5 / 12.5

33/67

0.19

—

1.11

0.38

1.3

1.2

6.7

2.41

0.92

3.5

2.0

9.9

4.25

1.64

4.9

3.2

12.7

6.27

2.53

6.4

4.4

15.6

8.51

3.43

7.8

5.5

18.5

10.79

4.29

—

‡

LiClO

EC/ DMC

EC/ DEC

EC/ DME

33/67

—

1.0

1.8

8.4

5.7

3.5

12.3

8.4

5.2

16.5

11.0

7.3

20.3

13.9

9.4

23.9

—

‡

* J. T. Dudley et al., J. Power Sources, 35:59–82, 1991.

† D. Guyomard and J. M. Tarascon, J. Electrochem. Soc. 140:3071–3081, 1993.

‡ S. Sosnowski and S. Hossain, unpublished results, Yardney Technical Products, Inc.

RECHARGEABLE LITHIUM BATTERIES (AMBIENT TEMPERATURE) 34.15

FIGURE 34.6 Variation of conductivity with temperature of dif-

ferent classes of polymer electrolytes. PEO ⫽ polyethylene oxide.

CPE ⫽ cross-linked polyvinylether. MEEP ⫽ poly[bis(methoxy eth-

oxy ethoxide)]. PAN ⫽ polyacrylonitrile.

Polymer Electrolytes. An alternative to the liquid electrolytes is a solid polymer electrolyte

(SPE) formed by incorporating lithium salts into polymer matrices and casting into thin

ﬁlms. These can be used as both the electrolyte and separator. These electrolytes have lower

ionic conductivities and low lithium-ion transport numbers compared to the liquid electro-

lytes, but they are less reactive with lithium, which should enhance the safety of the battery.

The use of thin polymer ﬁlms or operation at higher temperatures (60–100⬚C) compensate

in part for the lower conductivity of the polymer ﬁlm. The solid polymers also offer the

advantages of a ‘‘nonliquid’’ battery and the ﬂexibility of designing thin batteries in a variety

of conﬁgurations.

Initially, high-molecular-weight polymers such as polyethylene oxide (PEO) and lithium

salts such as LiClO

and LiN(CF

)

were used.

These PEO-lithium salt electrolytes

have good mechanical properties but low conductivities, which are on the order of 10

⫺

S/cm at 20⬚C. A signiﬁcant improvement in conductivity to approximately 10

⫺

S/cm has

been achieved with the combination of modiﬁed comb-shaped PEO structures with lithium

salts,

but these types of solid polymer electrolytes have poor mechanical properties and

their conductivity is still two orders of magnitude lower than that of most organic liquid

electrolytes. Further improvement in conductivity was obtained with the addition of liquid

plasticizers such as polypropylene carbonate.

15,16

The amount of plasticizer may be as high

as 70%, resulting in limited chemical and mechanical stability. Another class of polymer

electrolytes called ‘‘gelled’’ electrolytes has been developed by trapping liquid solutions of

lithium salts in aprotic organic solvents [for example, LiClO

in propylene carbonate-ethylene

carbonate (PC/EC) solvent] into a solid polymer matrix such as polyacrylonitrile (PAN).

17,18

The ‘‘gel’’ electrolytes are made by inserting liquid electrolyte solutions into polymer cages

with an immobilization procedure such as cross-linking, gelliﬁcation, and casting. Cross-

linking may be carried out by ultraviolet, electron-beam, or gamma-ray irradiation. Conduc-

tivities as high as 10

⫺

S/cm at 20⬚C and transference number around 0.6 have been obtained.

However, these plasticized and gelled electrolytes are more reactive with lithium than true

solid polymers. The conductivites of the various classes of polymer electrolytes and their

variations with temperature are presented in Fig. 34.6.

34.16 CHAPTER THIRTY-FOUR

FIGURE 34.7 (a) Conductivity of LiAlCl

/SO

electrolytes at various

temperatures. (b) Vapor pressure of SO

and SO

-based electrolytes.

Solid-State Electrolytes. True all solid-state ionic electrolytes such as lithium phosphorus

oxynitride (LiPON) provide adequate conductivity for use in thin ﬁlm solid-state batteries

(see Sec. 35.8.)

Inorganic Electrolytes. The SO

-based inorganic electrolytes are another alternative for

use in rechargeable lithium batteries. These electrolytes are attractive because they offer the

highest ionic conductivity of any electrolyte used in rechargeable lithium batteries. Figure

34.7a shows the ionic conductivity of SO

-based LiAlCl

electrolytes at various tempera-

tures.

RECHARGEABLE LITHIUM BATTERIES (AMBIENT TEMPERATURE) 34.17

The SO

-based electrolytes, however, present potential safety problems. One disadvantage,

the high vapor pressure of SO

, can be reduced by adding electrolyte salts such as LiAlCl

and LiGaCl

. These form complexes exothermically with the SO

and reduce the vapor

pressure. For example, as shown in Fig. 34.7b, the vapor pressure of LiAlCl

䡠 3SO

is below

atmospheric pressure at 20

⬚C.

34.3 CHARACTERISTICS OF LITHIUM RECHARGEABLE BATTERIES

A number of different battery systems have been investigated for the development of lithium

rechargeable batteries in order to achieve the high speciﬁc energy and charge retention that

lithium batteries can offer without sacriﬁcing other important characteristics, such as speciﬁc

power and cycle life, while maintaining safe and reliable operation. These approaches are

summarized in Fig. 34.1.

The rechargeable lithium batteries are generally characterized by a high cell voltage, good

charge retention, higher speciﬁc energy but poorer high-rate performance, and poorer cycle

life than conventional aqueous rechargeable batteries.

Rechargeable lithium metal batteries, because of their many potential advantages, have

been considered for use in a wide variety of applications. Because of the reactivity of lithium

and the possibility of safety problems, emphasis also has been placed on achieving safe

operation under normal and abusive conditions.

For these reasons, too, commercialization of rechargeable lithium batteries has been lim-

ited. They have been introduced into the market only on a limited scale and in small cell

sizes. Coin-type batteries have been commercially available for use in low-power portable

applications and as memory backup. Small cylindrical cells, using a lithium metal anode,

have been marketed brieﬂy for consumer electronics applications but were withdrawn when

safety problems arose. Rechargeable lithium metal batteries (including ambient-temperature

as well as high-temperature types), because of their high energy density, have been investi-

gated for applications requiring larger size cells and batteries as, for example, electric ve-

hicles. More recently, the lithium-ion type battery was introduced into the consumer market,

again in small cylindrical and prismatic sizes for camcorders, cell phones and other portable

electronics. The lithium ion battery became the dominant rechargeable lithium system during

the 1990s.

34.3.1 Electrochemical Systems

The different types of ambient-temperature lithium metal rechargeable batteries have been

classiﬁed into four design categories. These are identiﬁed in Table 34.10, which lists rep-

resentative chemical systems and the key advantages and disadvantages of each class. The

components and chemical reactions and the performance characteristics of typical examples

of the ﬁve types are summarize and compared in Sec. 34.3.2.

Liquid Organic Electrolyte Batteries. These cells use lithium metal for the negative elec-

trode, a liquid organic aprotic solution for the electrolyte, and transition metal compounds

(oxides, sulﬁdes, and selenides) for the positive electrode. These transition metal compounds

are insertion or intercalation compounds and possess a structure into which lithium ions can

be inserted or from which they can be removed during discharge and charge, respectively,

discharge

—— —



xLi ⫹ MB LiMB

———



xzy

charge

where M

is the transition metal compound.

34.18 CHAPTER THIRTY-FOUR

TABLE 34.10 Classiﬁcation of Rechargeable Lithium Metal Batteries (Ambient Temperature)

1. Liquid organic electrolyte cells (solid cathode cells using intercalation compounds for cathode, a

liquid organic electrolyte, and a metallic lithium anode):

High speciﬁc energy

Moderate-rate capability

Potential safety problems

Low cycle life

Low self-discharge rate

Examples: Li/ MoS

, Li/ MnO

, Li/TiS

, Li/ NbSe

, Li/V

, Li/ LiCoO

, Li/ LiNiO

2. Polymer electrolyte cells (cells using polymer electrolyte, intercalation compounds for cathode,

and lithium metal for anode):

High energy density

Safer design

Low electrolyte conductivity (poor high-rate capability)

Poor low-temperature performance

Low self-discharge rate

Examples: Li/ PEO-LiClO

/VO

and others

3. Inorganic electrolyte cells (liquid cathode cells using inorganic cathode materials which also

function as electrolyte solvent):

High energy density

High-rate capability

Excellent shelf life

Ability to sustain overcharge

Safety problems, toxicity

Capacity fade on cycling

Examples: Li/ SO

, Li/ CuCl

4. Lithium alloy cells (cells with lithium-alloy anodes, liquid organic electrolytes, and a variety of

cathodes):

Coin cell conﬁguration

Lithium alloy considered safer than metallic lithium

Low energy density

Poor cycle life (except on shallow depth of discharge)

Examples: LiAl/ MnO

, LiAl/ V

, LiAl/C, LiC/V

, LiAl/ polymer

The overall discharge process is shown graphically in Fig. 34.8. Lithium ions formed at

the negative electrode during discharge migrate through the electrolyte, and are inserted into

the crystal structure of the host compound at the positive electrode. The electrons travel in

the external circuit to enter into the electronic band structure of the host compound. The

process is reversed during charge. The electrode reactions that occur during this process are

covered in Sec. 34.2.2.

These batteries have the highest energy density of any of the rechargeable lithium bat-

teries, the highest power density (with the exception of the SO

-based batteries), and a low

self-discharge rate, typically less than 1% per month. Safety, however, is a major concern

with this type of lithium battery. Most designs use a three- to ﬁve-fold excess of lithium in

order to obtain a reasonable cycle life to make up for the fact that the active lithium is lost

as it is stripped and replated during cycling. Further, the freshly formed porous high-surface-

area lithium that is formed during recharging is highly reactive and susceptible to forming

dendrites, which could cause internal short circuits.

RECHARGEABLE LITHIUM BATTERIES (AMBIENT TEMPERATURE) 34.19

FIGURE 34.8 Scheme of the discharge process of a

liquid organic electrolyte Li / M

cell. 䡩 ⫽ B; ● ⫽ M.

(From Ref. 8.)

The liquid electrolyte batteries were among the ﬁrst lithium rechargeable batteries to be

investigated and the ﬁrst to be introduced commercially. Small coin-type batteries were

marketed for use in small electronic devices and for memory backup. These batteries, how-

ever, use lithium alloys, which are less reactive than metallic lithium, for the negative elec-

trode. The small cylindrical batteries that were introduced brieﬂy in the 1980s, on a limited

basis, were withdrawn as safety problems arose. In the 1990s, cylindrical AA batteries were

reintroduced, but have recently also been withdrawn from the market. Applications will

probably be limited to coin-type and other small-size batteries until a technological break-

through occurs.

Polymer Electrolyte Batteries. The rechargeable lithium batteries which use solid polymer

electrolytes (SPE) are considered to have a safety advantage over the organic liquid electro-

lytes because of their lower reactivity with lithium and the absence of a volatile, ﬂammable

organic solvent. In their most common form, as shown schematically in Fig. 34.9. these cells

use a lithium-ion conducting polymer membrane which acts both as the electrolyte and as

the separator, a thin lithium metal foil as the negative or anode material and a transition

metal oxide or chalcogenide, such as V

,TiS

,orVO

, blended with carbon and the

polymer electrolyte and backed by a metal-foil current collector as the positive electrode.

The basic structure can be represented as

Li/ PEO-LiX/ M B , PEO-LiX, C /M*

where M

is the intercalation compound, PEO-LiX the polymer electrolyte, and M* the

current collector.

The cell reaction is similar to that in the liquid organic electrolyte cell—intercalation of

lithium into the structure of the cathode during discharge and deintercalation of lithium from

the charged cathode and deposition on the anode during charge,

discharge

—— —



xLi ⫹ MB Li M B

———



xzy

charge

This process is shown schematically in Fig. 34.10.

34.20 CHAPTER THIRTY-FOUR

FIGURE 34.9 Schematic cross section of a solid polymer electrolyte

(SPE) cell. (Courtesy Valence Technology, Inc.)

FIGURE 34.10 Schematic of discharge process of a Li/ SPE / M

cell.

The ionic conductivity of most solid polymer electrolytes is signiﬁcantly lower than that

of the liquid electrolytes. Cells must be designed with thin electrodes and cell components

to minimize the internal cell resistance. The total thickness of a cell assembly is as low as

200

␮

m or thinner. An alternative is to operate at higher temperatures where the conductivity

is higher. While this may be acceptable for electric-vehicle and stand-by batteries, it will not

be acceptable for many portable consumer applications. Newer polymer electrolytes are being

developed using plasticizers or gel-type polymers. These methods increase the conductivity

of the polymers, but since they contain organic solvents, they will be more reactive with the

lithium anode.