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

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LITHIUM-ION BATTERIES 35.19

FIGURE 35.20 Potential of the carbon negative electrode in a Li-

ion cell on the ﬁrst cycle illustrating the irreversible capacity asso-

ciated with (a) coke or (b) artiﬁcial graphite materials.

On the ﬁrst cycle, passivation layers are formed on the surface of the electrodes. These

layers have been shown to result from the reaction of the electrolyte with the electrode

surface. The passivation layers contain lithium that is no longer electrochemically active,

thus their formation results in irreversible capacity, an undesirable property of all current

materials that occurs largely on the ﬁrst cycle. The capacity difference between the charge

and discharge curves in Fig. 35.20 results from irreversible capacity.

To emphasize the impact of the negative electrode material on cell voltage, Fig. 35.21

shows the discharge voltage of commercial 18650-type C/LiCoO

Li-ion cells with different

electrode materials. As shown, cells with graphite negatives have ﬂatter discharge curves

than cells with coke negative electrodes. Since most commercial products now on the market

have a ﬂat discharge curve and high average voltage, they apparently use a graphitic negative

electrode material.

35.20 CHAPTER THIRTY-FIVE

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Cell Voltage (V)

Discharge Capacity (Ah)

Graphite/LiCoO

Coke/LiCoO

FIGURE 35.21 Effect of the carbon type on the discharge proﬁle of Li-ion cells. (Courtesy of the

University of South Carolina.)

Properties of Carbons. Performance and physical characteristics of various carbons are

included in Table 35.4. An ideal material would offer high speciﬁc capacity without irre-

versible capacity. The carbon used in the cells commercialized by Sony in 1990 was a

petroleum coke. Cokes are compatible with a wide variety of electrolyte solvents, including

PC, but have lower capacity than graphitic materials. MCMB carbons offer good capacity,

⬃300 mAh /g, and low irreversible capacity, ⬃20 mAh/g. Lower cost graphites offer higher

capacity,

⬃350 mAh /g, but higher irreversible capacity, ⬃50 mAh/g, and have higher fade

rates than MCMB carbons, thus not necessarily higher energy density.

This is illustrated

in Fig. 35.22, which compares the reversible and irreversible capacity and energy density of

two MCMB materials and an artiﬁcial graphite. In this case, the graphite offers higher

capacity but also higher irreversible capacity than the MCMB’s, thus intermediate energy

density. In general, irreversible capacity may be correlated to the surface area of a material,

thus the interest in low surface area, spherical materials. The MCMB 25-28 has lower speciﬁc

surface area than MCMB 10-28, thus less irreversible capacity. In practice, a particle size

less than

⬃30

␮

m is required for rate capability to the C rate. MCMB carbon can have a

variety of structures, depending on how the graphite planes are oriented within the sphere.

The performance of MCMB is related to its structure. The laboratory preparation and prop-

erties of a variety of MCMB carbons have been reported.

TABLE 35.4 Properties and Performance of Various Carbons (Experimental Values). (From Ref. 67.)

Carbon Type

Speciﬁc

capacity

(mAh/ g)

Irreversible

capacity

(mAh/ g)

Particle

size

(

␮

BET surface

area (m

/g)

KS6 Synthetic graphite 316 60 6 22

KS15 Synthetic graphite 350 190 15 14

KS44 Synthetic graphite 345 45 44 10

MCMB 25-28 Graphite sphere 305 19 26 0.86

MCMB 10-28 Graphite sphere 290 30 10 2.64

Sterling 2700 Graphitized

Carbon Black

200 152 .075 30

XP30 Petroleum coke 220 55 45 N /A

Repsol LQNC Needle coke 234 104 45 6.7

Grasker Carbon ﬁber 363 35 23 11

Sugar carbon Hard carbon 575 215 N /A 40

LITHIUM-ION BATTERIES 35.21

LK702 MCMB 10-28 MCMB 25-28

100

150

200

250

300

Energy Density (mWh/cm

) and Q

IRREV

(mAh/g)

REV

(mAh/g)

Anode Material

REVERSIBLE

(mAhg) - Cycle 8

AVE

IRREVERSIBLE

(mAh/g) - Cycle 1

AVE

Energy Density (mWh/cm

) - Cycle 8

AVE

FIGURE 35.22 The energy density, reversible and irreversible capacity of carbons commonly used as

negative electrode materials.

The theoretical speciﬁc capacity of carbon (LiC

) is 372 mAh/ g. Hard carbon materials

offer higher capacity, over 1000 mAh /g, but have not achieved broad acceptance because

they have greater irreversible capacity and higher voltage,

⬃1 V vs. lithium,

than graphitic

materials. Hard carbons are highly disorganized. Mechanisms have been proposed to explain

incorporation of Li above the theoretical capacity of graphite. That proposed by Sato suggests

lithium occupies nearest neighbor sites between pairs of graphene sheets.

A mechanism

proposed by Dahn et al. suggests additional lithium may bind hydrogen containing regions

of carbon,

as supported by theoretical studies which illustrate the importance of hydrogen-

terminated edge regions.

35.2.5 Electrolytes

Four types of electrolytes have been used in Li-ion batteries: liquid electrolytes, gel electro-

lytes, polymer electrolytes and ceramic electrolytes. Liquid electrolytes are solutions of a

lithium salt in organic solvents, typically carbonates. As described in a monograph,

polymer electrolyte is a liquid- and solvent-free material, where an ionically conducting

phase is formed by dissolving a salt in a high molecular weight polymer, whereas a gel

electrolyte is an ionically conductive material wherein a salt and a solvent are dissolved or

mixed with a high molecular weight polymer. Gel electrolytes developed for Li-ion batteries

are typically ﬁlms of PVDF-HFP, LiPF

or LiBF

salt, and carbonate solvent. Fumed silica

may be added to the PVDF-HFP ﬁlm for additional structural integrity. Potential advantages

of polymer electrolytes include improved safety properties resulting from their low volatility

and high viscosity, as they do not contain a volatile, ﬂammable solvent component. A pos-

sible advantage of gel electrolytes is that the liquid phase is absorbed within the polymer,

thus less likely to leak from a battery, however, in a typical Li-ion battery employing a liquid

electrolyte, the electrolyte is almost completely absorbed into the electrode and separator

materials. In the marketplace and the literature, gel electrolytes are often termed gel-polymer

electrolytes, and cells that employ gel (or gel-polymer) electrolytes are termed gel-polymer

or simply polymer cells. Ceramic electrolytes refer to inorganic, solid-state materials that are

ionically conductive.

35.22 CHAPTER THIRTY-FIVE

Most Li-ion electrolytes in current use utilize LiPF

as the salt as its solutions offer high

ionic conductivity,

⬎10

⫺

S/ cm, high lithium ion transference number (⬃0.35), and accept-

able safety properties. As reviewed below, many other salts have attracted industrial interest,

notably LiBF

. Electrolytes in current use are formulated with carbonate solvents. Carbonates

are aprotic, polar, and have a high dielectric, thus can solvate lithium salts to high concen-

tration (

⬎1 M). They also provide compatibility with cell electrode materials over a broad

range of potential. While the industry focused initially on propylene carbonate (PC)-based

solutions, current formulations utilize other carbonates, notably ethylene carbonate (EC),

dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC), as

PC causes degradation in graphite electrodes as it co-intercalates with lithium, resulting in

exfoliation. The choice of solvents for a Li-ion electrolyte is also inﬂuenced by any low

temperature requirements of the application. Low temperature electrolytes utilize low vis-

cosity solutions with low freezing points.

Salts. Salts commonly used in Li-ion cells are listed in Table 35.5. Most cells currently

marketed use LiPF

as its solutions have high conductivity and good safety properties. How-

ever, the salt is costly, hygroscopic, and LiPF

yields hydroﬂuoric acid (HF) upon reaction

with water, thus must be handled in a dry environment. Organic salts have also been devel-

oped. They are more stable to water, thus easier to handle. In particular, BETI (lithium

bisperﬂuoroethanesulfonimide) has received signiﬁcant attention as its solutions offer high

conductivity, it is stable to water, it can be easily dried, and it does not cause aluminum

corrosion—an issue with other organic salts such as lithium triﬂate.

TABLE 35.5 Salts Used in Electrolytes for Li-ion Cells

Common name Formula

Mol. wt.

(g/ mol)

Typical

impurities Comments

Lithium

hexaﬂuorophosphate

LiPF

151.9 H

O (15ppm)

HF (100ppm)

Most commonly used

Lithium

tetraﬂuoroborate

LiBF

93.74 H

O (15ppm)

HF (75ppm)

Less hygroscopic than

LiPF

Lithium perchlorate LiClO

106.39 H

O (15ppm)

HF (75ppm)

When dry, less stable than

alternatives

Lithium

hexaﬂuoroarsenate

LiAsF

195.85 H

O (75ppm)

HF (15ppm)

Contains arsenic

Lithium triﬂate LiSO

156.01 H

(100ppm)

Al corrosion above 2.8 V,

stable to water

Lithium

bisperﬂuoroethane-

sulfonimide (BETI)

LiN(SO

)

387 N / A No Al corrosion below

4.4 V, stable to water

Solvents. A wide variety of solvents, including carbonates, ethers and acetates, has been

evaluated for non-aqueous electrolytes. The industry has now focused on the carbonates as

they offer excellent stability, good safety properties and compatibility with electrode mate-

rials. Neat carbonate solvents typically have intrinsic solution conductivity less than 10

⫺

S/ cm, dielectric constant ⬎3, and solvate lithium salts to high concentration. Table 35.6

presents the properties of some commonly used solvents. The properties of these and other

materials are included in a recent review.

35.23

TABLE 35.6 Characteristics of Organic Solvents

Characteristic EC PC DMC EMC DEC 1,2-DME AN THF

␥

-BL

Structure

CCH

BP (⬚C) 248 242 90 109 126 84 81 66 206

MP (

⬚C) 39 ⫺48 4 ⫺55 ⫺43 ⫺58 ⫺46 ⫺108 ⫺43

Density (g / ml) 1.41 1.21 1.07 1.0 0.97 0.87 0.78 0.89 1.13

Viscosity (cP) 1.86 (40

⬚C) 2.5 0.59 0.65 0.75 0.455 0.34 0.48 1.75

Dielectric

constant

89.6 (40

⬚C) 64.4 3.12 2.9 2.82 7.2 38.8 7.75 39

Donor

number

16.4 15 8.7

6.5

— 14 –—

Mol. wt. 88.1 102.1 90.1 104.1 118.1 90.1 41.0 72.1 86.1

*EC ⫽ ethylene carbonate, PC ⫽ propylene carbonate, DMC ⫽ dimethyl carbonate, EMC ⫽ ethyl methyl carbonate,

DEC

⫽ diethyl carbonate, DME ⫽ dimethylether, AN⫽acetonitrile, THF⫽tetrahydrofuran,

␥

BL ⫽

␥

-butyrolactone.

(From Refs. 69, 70, 71 and 72.)

35.24 CHAPTER THIRTY-FIVE

TABLE 35.7 Conductivity, in mS/ cm, of 1M LiPF

Solutions in Various Solvents.

(Courtesy of Merck KGaA, Darmstadt, Germany.)

Solvent ⫺40.0⬚C ⫺20.0⬚C 0.0⬚C 20.0⬚C 40.0⬚C 60.0⬚C 80.0⬚C

DEC — 1.4 2.1 2.9 3.6 4.3 4.9

EMC 1.1 2.2 3.2 4.3 5.2 6.2 7.1

PC 0.2 1.1 2.8 5.2 8.4 12.2 16.3

DMC — 1.4 4.7 6.5 7.9 9.1 10.0

EC ———6.9 10.6 15.5 20.6

MA 8.3 12.0 14.9 17.1 18.7 20.0 —

MF 15.8 20.8 25.0 28.3 ———

* DEC ⫽ diethyl carbonate, EMC ⫽ ethyl methyl carbonate, PC ⫽ propylene carbonate, DMC ⫽

dimethyl carbonate, EC⫽ethylene carbonate, MA⫽methyl acetate, MF ⫽ methyl formate.

-40-200 20406080

0.1

0.2

0.4

0.6

0.8

Conductivity (mS/cm)

Temperature (°C)

DEC

EMC

DMC

FIGURE 35.23 Conductivity, in mS /cm, of 1M LiPF

solutions in various solvents.

(Courtesy of Merck KGaA, Darmstadt, Germany.)

Conductivity of Electrolytes. Electrolyte formulations in current Li-ion cells typically util-

ize two to four solvents. Formulations with multiple solvents can provide better cell per-

formance, higher conductivity, and a broader temperature range than possible with a single

solvent electrolyte. For example, ethylene carbonate (EC) is associated with low irreversible

capacity and low capacity fade when used in conjunction with graphitic negative electrodes.

Ethylene carbonate is found in many commercial electrolyte formulations, but is a solid at

room temperature. Multiple solvent formulations often include EC, thereby incorporating its

desirable properties, while using other solvents to lower the freezing point and viscosity of

the mixture.

The conductivity of 1 M LiPF

solutions in solvents commonly used in Li-ion electrolytes

is given in Table 35.7 and plotted in Fig. 35.23 for temperatures between

⫺40⬚C and 80⬚C.

In general these solutions offer high conductivity, 10

⫺

S/ cm, and a few solvents, such as

PC and EMC, offer good conductivity at low temperature and a high boiling point. MA and

MF offer high conductivity, but cell performance is poor if either is used at levels greater

than 25% by weight.

LITHIUM-ION BATTERIES 35.25

TABLE 35.8 Conductivity, in mS/ cm, of LiPF

Solutions in Binary Mixtures, 1:1 by Weight. C ⫽ partially

crystallized, S

⫽ saturated. (Courtesy of Merck KGaA, Darmstadt, Germany.)

Solvents Concentration ⫺40⬚C ⫺20⬚C0⬚C20⬚C40⬚C60⬚C80⬚C

EC:DEC

0.25 M

0.50 M

1.00 M

1.25 M

1.50 M

1.75 M

—

0.7

0.4

—

2.5 (C)

2.2

1.7

—

1.7 (C)

3.0

4.2

3.6

—

4.2

6.4

7.0

6.4

5.6

4.8 (S)

5.8

8.7

10.3

9.7

—

7.3

11.1

13.9

13.5

—

8.8

13.6

17.5

17.4

—

0.25 M ——4.2 5.8 7.8 9.7 11.5

0.50 M ——6.5 9.3 12.8 16.0 19.1

0.75 M — 3.8 6.9 10.3 14.0 17.9 21.6

1.00 M — 3.7 7.0 15.0 19.5 24.0

EC:DMC 1.25 M 0.7 2.7 5.6 9.3 13.7 18.4 23.3

1.50 M — 2.2 5.4 9.3 14.1 19.2 24.7

1.75 M —— —7.5 ———

2.00 M —— —6.7 ———

2.25 M —— —0.9 (S) ———

EC:EMC

0.25 M

0.50 M

1.00 M

1.25 M

3.50 M

—

0.9

0.6

—

3.0

2.7

2.3

—

3.7

5.1

5.3

4.7

—

5.3

7.5

8.5

8.0

0.9 (S)

7.2

10.2

12.2

12.0

—

9.1

12.8

16.3

16.2

—

10.9

15.4

20.3

20.6

—

EC:MA

0.25 M

0.50 M

1.00 M

1.25 M

3.0 M

3.5 M

2.4 (C)

3.1 (C)

3.8

—

4.6

6.7

7.8

7.1

0.5

—

6.3

9.8

12.2

11.8

2.1

—

8.3

13.1

17.1

17.2

5.2

3.4 (S)

10.4

16.0

22.3

22.7

—

12.4

19.3

27.3

28.4

15.4

—

21.8

—

EC:MPC 1.00 M C 1.5 3.6 6.3 9.5 12.9 16.8

* DEC ⫽ diethyl carbonate, EMC ⫽ ethyl methyl carbonate, DMC ⫽ dimethyl carbonate, EC ⫽ ethylene carbonate, MA ⫽ methyl

acetate.

The conductivity of binary 1:1 mixtures of EC with common Li-ion electrolyte solvents

over a range of salt concentrations and temperatures is given in Table 35.8. As indicated,

for many solvent pairs, conductivity is highest with 1 M LiPF

, and these formulations are

liquid from

⫺40⬚Cto80⬚ C. The conductivity of 1 M LiPF

binary solutions with EC are

plotted in Fig. 35.24. As shown, the EC:MA solution offers the highest conductivity although

such levels of MA are associated with high capacity fade.

Other mixtures, including EC:

DEC, EC:DMC, and EC:EMC offer good conductivity and low capacity fade. In particular,

EC:EMC mixtures offer 0.9 mS/cm conductivity at

⫺40⬚C, and low capacity fade.

The conductivity of PC:DME 1:1 solutions of LiPF

, and a variety of organic salts, is

illustrated in Fig. 35.25, which shows the conductivity of the solutions from 0.25 M to 2 M.

As shown, LiPF

offers the highest conductivity, 13 mS/ cm at 1.2 M, although solutions

with the organic salts offer comparable conductivity, up to 11 mS/ cm.

35.26 CHAPTER THIRTY-FIVE

-40-20 0 20406080

0.2

0.4

0.6

0.8

Conductivity (mS/cm)

Temperature (°C)

EC:DEC

EC:DMC

EC:EMC

EC:MA

EC:PC

FIGURE 35.24 Conductivity of 1M LiPF

solutions in various binary mixtures, 1:1 by weight. (Courtesy

of Merck KGaA, Darmstadt, Germany.)

0.0 0.5 1.0 1.5 2.0

Conductivity (mS/cm)

Salt Concentration (M)

LiPF

LiN(SO

)

LiN(SO

)

LiSO

FIGURE 35.25 Conductivity of electrolytes at 20⬚C prepared with PC:DME (1:1, V /V) and various salts.

(Courtesy of 3M, St. Paul, MN.)

LITHIUM-ION BATTERIES 35.27

TABLE 35.9 Conductivity, in mS/ cm, of M LiPF

Solutions in Various Ternary Solvent Mixtures. (Courtesy of Merck

KGaA, Darmstadt, Germany.)

Solvent Wt. ratio ⫺40⬚C ⫺20⬚C0⬚C20⬚C40⬚C60⬚C80⬚C

EC:PC:DMC 20:20:60 ——6.9 10.6 14.5 18.4 22.2

EC:PC:EA 15:25:60 3 6.2 9.8 13.7 17.8 21.6 25.1

EC:PC:EMC 15:25:60 1 2.8 5.3 8.1 11.5 14.6 17.8

EC:PC:MA 15:25:60 4.1 8.1 12.9 17.8 22.8 27.6 boils

EC:PC:MPC 15:25:60 0.5 1.4 3.3 5.6 8.2 10.9 13.9

EC:DMC:EMC 15:25:60 1.4 3.2 5.3 7.6 10 12.1 14.1

EC:DMC:MPC 15:25:60 0.7 1.8 3.4 5.3 7.2 9 10.9

* DEC ⫽ diethyl carbonate, EMC ⫽ ethyl methyl carbonate, PC ⫽ propylene carbonate, DMC ⫽ dimethyl carbonate, EC ⫽ ethylene

carbonate, MA

⫽ methyl acetate, MPC ⫽ methyl propyl carbonate, EA ⫽ ethyl acetate.

-40°C -20°C0°C20°C40°C60°C80°C

0.4

0.6

0.8

Conductivity (mS/cm)

Temperature (°C)

EC:PC:DMC, 20:20:60

EC:PC:EA, 15:25:60

EC:PC:EMC, 15:25:60

EC:PC:MA, 15:25:60

EC:PC:MPC, 15:25:60

EC:DMC:EMC, 15:25:60

EC:DMC:MPC, 15:25:60

FIGURE 35.26 Conductivity of 1 M LiPF

solutions in ternary solvent mixtures. (Courtesy of Merck KGaA,

Darmstadt, Germany.)

The conductivity of selected ternary 1 M LiPF

solutions is given in Table 35.9 and

plotted in Fig. 35.26. These mixtures contain 33% EC, as is typical in current Li-ion elec-

trolytes, but still offer high conductivity and a broad temperature range, illustrating the utility

of multiple component mixtures. For example, four of these mixtures offer at least 1 mS /

cm at

⫺40⬚C, of which three are liquid at 80⬚C. Quaternary mixtures have also been devel-

oped to provide electrolytes with better low temperature performance. The conductivity of

1.0 M LiPF

solutions in various quaternary solvent mixtures is shown in Fig. 35.27. As

illustrated, these solutions offer over 1 mS/ cm at

⫺40⬚C and up to 0.6 mS/cm at ⫺60⬚C.

A survey of the conductivity and solvent properties of electrolytes utilizing either LiAsF

LiPF

, LiSO

, or LiN(SO

)

and a wide variety of solvents, including EC, DME,

ethyldiglyme, triglyme, tetraglyme, sulfolane, Freon, and methylene chloride has been pub-

lished.

35.28 CHAPTER THIRTY-FIVE

-60 -40 -20 0 20 40

0.01

0.1

Conductivity (mS/cm)

Temperature (°C)

EC:DEC:DMC (1:1:1)

EC:DEC:DMC:MA (1:1:1:1)

EC:DEC:DMC:GBL (1:1:1:1)

EC:DEC:DMC:EA (1:1:1:1)

EC:DEC:DMC:MF (1:1:1:1)

EC:DEC:DMC:EB (1:1:1:1)

FIGURE 35.27 The conductivity of 1.0 M LiPF

solutions in quaternary solvent mixtures. (Courtesy of

JPL.)

Electrolyte Formulation, Irreversible Capacity and the SEI. Various electrolytes have

been used in Li-ion batteries. The solvents used must be stable at both the anodic and

cathodic potentials found in Li-ion cells, 0 V to 4.2 V vs. lithium. No practical solvents are

thermodynamically stable with lithium or Li

near 0 V vs. Li, but many solvents undergo

a limited reaction to form a passivation ﬁlm on the electrode surface. This ﬁlm spatially

separates the solvent from the electrode, yet is ionically conductive, and thus allows passage

of lithium ions. The passivation ﬁlm, termed the solid electrolyte interphase (SEI), therefore

imparts extrinsic stability to the system allowing the fabrication of cells that are stable for

years without signiﬁcant degradation.

When the SEI is formed, lithium is incorporated into the passivation ﬁlm. This process

is irreversible and is thus observed as a loss of capacity, primarily on a cell’s ﬁrst cycle. The

amount of irreversible capacity is dependent on the electrolyte formulation and the electrode

materials, in particular the type of carbon used in the negative electrode. As the reaction

occurs at the particle surface, materials with low speciﬁc surface area typically offer lower

irreversible capacity.

Cells with electrolyte formulations that contain alkyl carbonates, in particular EC, have

been shown to offer low capacity fade, low irreversible capacity and high capacity.

In EC

containing electrolytes, the passivation ﬁlm formed on the surface of Li-ion electrodes is

formed with a minimum amount of lithium. This SEI has been shown to consist primarily

of Li

(OCO

(CH

)

OCO

)

and related reaction products, including Li

and LiOCH

of the electrolyte solvent with either lithium or a lithiated species such as Li

. While

solvents other than EC, typically esters or alkyl carbonates such as EMC or MPC,

also

form stable passivation ﬁlms, most solvents do not. If an ester or alkyl carbonate is not used,

graphite can be cycled in a solvent that does not form a stable passivation ﬁlm if an additive,

such as a crown ether

or CO

76,80

is added to the electrolyte.