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

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LITHIUM-ION BATTERIES 35.29

35.2.6 Separator Materials

Li-ion cells use thin (10 to 30

␮

m), microporous ﬁlms to electrically isolate the positive and

negative electrodes. To date, all commercially available liquid electrolyte cells use micro-

porous polyoleﬁn materials as they provide excellent mechanical properties, chemical sta-

bility and acceptable cost. Nonwoven materials have also been developed but have not been

widely accepted, in part due to the difﬁculty in fabricating thin materials with uniform, high

strength.

Requirements for Li-ion separators include:

•

High machine direction strength to permit automated winding

•

Does not yield or shrink in width

•

Resistant to puncture by electrode materials

•

Effective pore size less than 1

␮

•

Easily wetted by electrolyte

•

Compatible and stable in contact with electrolyte and electrode materials

Microporous polyoleﬁn materials in current use are made of polyethylene, polypropylene

or laminates of polyethylene and polypropylene. Also available are surfactant coated mate-

rials, designed to offer improved wetting by the electrolyte. These materials are fabricated

by either a dry, extrusion type process or a wet, solvent based process.

The properties of

commercial materials, including pore dimensions, porosity, and permeability, have been re-

ported.

Commercial materials offer pore size of 0.03

␮

m to 0.1

␮

m, and 30 to 50% porosity,

as illustrated by the SEM micrograph of a commercial material in Fig. 35.28.

FIGURE 35.28 SEM micrograph of Celgard 3501 separator. (Courtesy of Yardney Technical Products,

Inc.)

35.30 CHAPTER THIRTY-FIVE

The low melting point of polyethylene (PE) materials enables their use as a thermal fuse.

As the temperature approaches the melting point of the polymer, 135

⬚C for polyethylene and

165

⬚C for polypropylene (PPE), porosity is lost.

Tri-layer materials (PPE/PE/PPE) have

been developed where a polypropylene layer is designed to maintain the integrity of the

ﬁlm, while the low melting point of polyethylene layers is intended to shutdown the cell if

an over-temperature condition is reached.

35.2.7 Additives

To further improve battery performance, electrolyte additives have been developed. Some,

such as BF

and related complexes, are designed to passivate the surface of electrode

materials thereby reducing their propensity to degrade. Others, such as hexamethyldisilazane

(HMDS), have been used to reduce interfacial resistance,

and react with and immobilize

water and HF, thus improving cell performance:

(CH ) SiN(H)Si(CH ) ⫹ HO→ (CH ) SiOSi(CH ) ⫹ NH

33 33 2 33 33 3

NH ⫹ HF → NH F

To illustrate the utility of one such additive, Fig. 35.29 shows the speciﬁc capacity of a

Li/ LiMn

battery cycled at 55⬚C using an electrolyte with and without HMDS additive.

As shown, the cell with the HMDS additive provided lower capacity fade in this case, as

the additive removed the water and HF impurities that contribute to the degradation of the

electrode materials.

0 1020304050

100

120

140

Specific Capacity

Cycle

With HMDS

Without HMDS

FIGURE 35.29 The speciﬁc capacity of C / LiMn

battery when cycled at 55⬚Cin

an electrolyte with and without HMDS additive. Cycled at C/ 2 rate, 4.2 V to 3.5 V.

(Courtesy of Carus Chemical.)

LITHIUM-ION BATTERIES 35.31

35.3 CONSTRUCTION OF CYLINDRICAL AND PRISMATIC Li-ION

CELLS AND BATTERIES

Cylindrical and prismatic Li-ion batteries have been developed. Wound designs are typical

in small cells (

⬍4 Ah); however in large cell designs, prismatic conﬁgurations with ﬂat plate

construction are more common. For prismatic Li-ion batteries two cell design types are

practiced, ﬂat mandrel wound pseudo-prismatic designs and ﬂat-plate true prismatic designs.

Since Li-ion cells are fabricated in the discharged state, they must be charged before use.

35.3.1 Construction of Wound Li-ion Cells

The construction of a cylindrical wound Li-ion cell is illustrated in Fig. 35.30. The fabrication

of wound prismatic cells is similar to cylindrical versions except that a ﬂat mandrel is used

instead of a cylindrical mandrel. A schematic diagram of a wound prismatic cell is shown

in Fig. 35.31. The construction consists of a positive and negative electrode separated by a

␮

mto25

␮

m thick microporous polyethylene or polypropylene separator. Positive elec-

trodes consist of 10

␮

mto25

␮

m Al foil coated with the active material to a total thickness

typically

⬃180

␮

m. Negative electrodes are typically 10

␮

mto20

␮

m Cu foil coated with

a carbonaceous active material to a total thickness

⬃200

␮

m. The thin coatings and separator

are required because of the low conductivity of non-aqueous electrolytes,

⬃10 mS /cm,

and

slow Li

⫹

diffusion in the positive and negative electrode materials, about ⬃10

⫺

Typically a single tab at the end of the wind is used to connect the current collectors to their

respective terminals. The case, commonly used as the negative terminal, is typically Ni-

plated steel. When used as the positive terminal, the case is typically aluminum. Most com-

mercially available cells utilize a header that incorporates disconnect devices, activated by

pressure or temperature, such as a PTC device, and a safety vent. One design is illustrated

in Fig. 35.32. These devices can limit cell performance at high rates, i.e. in a typical 18650

cell, 12 A discharge results in disconnect after 20 seconds once the temperature of the

disconnect device reaches 70

⬚C due to resistive heating. The header-can seal is typically

formed through a crimp.

In a typical 18650 cell, the positive current collector is coated on both sides with 12 g

of LiCoO

resulting in an electrode thickness of ⬃7.0 mil. Typically 6.5 g of carbon is used

on the negative electrode and the positive to negative ratio is such that the carbon negative

is utilized at a maximum of 270 mAh /g, 10% less than the capacity typical for MCMB

carbon and 100 mAh/g less than the theoretical capacity of carbon, (372 mAh/g).

The

mass distribution for the components in two 18650 products is illustrated in Fig. 35.33.

For specialized applications, such as for satellites, larger cylindrical cells have been de-

veloped. The ‘‘25 Ah’’ cells developed by Blue Star Advanced Technology, are depicted in

Fig. 35.34. These products utilize a LiCoO

based positive and a graphite negative. As shown,

the header incorporates two glass-to-metal seal terminals, a rupture disk and a ﬁll port. The

mass of the major components of one cell is described in Table 35.10. As indicated, the cell

container accounts for 14% of the mass while the electrodes and the electrolyte account for

81% of the cell mass. This cell delivers 121 Wh /kg and 280 Wh /L, slightly less than the

lower aspect ‘‘25 Ah Design II’’ cell which offers 125 Wh/ Kg and 265 Wh/L.

35.32 CHAPTER THIRTY-FIVE

FIGURE 35.30 Cross-sectional view of a cylindrical Li-ion cell. (Courtesy of

the University of South Carolina. Reproduced with permission from the Journal

of Power Sources.)

FIGURE 35.31 Schematic drawing of a wound prismatic cell. (Cour-

tesy of Japan Storage Battery Co., Ltd.)

LITHIUM-ION BATTERIES 35.33

FIGURE 35.32 Detail of the construction of a cell header with a breaker and vent mechanism for an

abnormal rise of internal pressure, (1) Aluminum burst disk, (2) Aluminum lead. (Courtesy of Sony

Corp.)

Sanyo

Separator

5.0%

Can

18.1%

Header

2.9%

Negative

Electrode

29.5%

Positive

Electrode

34.7%

Electrolyte

9.8%

Sony

Separator

4.4%

Can

24.2%

Header

3.4%

Negative

Electrode

23.0%

Positive

Electrode

39.4%

Electrolyte

5.5%

FIGURE 35.33 Mass distribution in 18650 Li-ion products. The batteries

weighed 39.4 g and 37.7 g respectively. (Courtesy of the University of South

Carolina.)

29Ah

121 Wh/Kg

280 Wh/L

890g

4.8 cm

20.9 cm

6.7 cm

11.5 cm

28.5 Ah

125 Wh/Kg

265 Wh/L

848.0g

8.9 cm

6.35 cm

24.0 Ah

110 Wh/Kg

219 Wh/L

784.3g

FIGURE 35.34 25 Ah cylindrical Li-ion cells. (Courtesy Blue Star Advanced Technologies. Re-

produced with permission from SAE paper 01-1390, 1999.)

35.34 CHAPTER THIRTY-FIVE

TABLE 35.10 Mass Analysis of a 29 Ah

Cylindrical Cell. (Courtesy of Blue Star Advanced

Technologies. Reproduced with permission from

SAE paper 01-2640, 1999.)

Component Mass (g) % of total

Case 108.5 12.2

Cap assembly 15.5 1.8

Electrolyte 217.9 24.5

Positive electrode 339.2 38.1

Negative electrode 165.0 18.5

Miscellaneous 43.4 4.9

Total 889.5 100

Fill Port

Rupture Disk

Positive Terminal

Cell Cover

Cell Case

Negative Terminal

Fill Tube

Rupture Disk

FIGURE 35.35 Schematic view showing the header and electrodes of 7 Ah (case negative) and 40

Ah (case neutral) ﬂat plate prismatic Li-ion cells. (Courtesy of Yardney Technical Products, Inc.)

35.3.2 Construction of Flate-plate Prismatic Li-ion Batteries

The construction of ﬂat-plate prismatic cells is illustrated in Fig. 35.35. As in a wound cell,

a microporous polyethylene or polypropylene separator separates the positive and negative

electrodes. Typically each plate in the cell has a tab, the tabs are bundled and welded to

their respective terminals or to the cell case. Cell cases of either nickel-plated steel or 304L

stainless steel have been used. As shown, the cover typically incorporates one or two ter-

minals, a ﬁll port and a rupture disk. The terminal may be a glass-to-metal seal, for low cost

applications compression type seals have been used, or the terminal may incorporate devices

similar to those found in the header of cylindrical products to provide pressure, temperature

and over current interrupt in one component. The case to cover seal is typically formed either

by TIG or laser welding.

LITHIUM-ION BATTERIES 35.35

35.4 Li-ION BATTERY PERFORMANCE

The general performance characteristics of Li-ion batteries are outlined in Table 35.11. As

indicated in the table, Li-ion batteries have a high voltage, typically operating in the range

of 2.5 to 4.2 V, approximately three times that of NiCd or NiMH. As such, fewer cells are

required for a battery of a given voltage. Li-ion batteries offer high speciﬁc energy and

energy density, batteries with speciﬁc energy over 150 Wh /Kg and energy density over 400

Wh/ L are commercially available. Multiple-tabbed Li-ion batteries also offer high rate ca-

pability, up to 5C continuous or 25C pulse, thus high power density, and low self-discharge

rate, years of calendar life, no memory effect, and a broad temperature range of operation.

Li-ion batteries can be charged from

⫺20⬚Cto60⬚C and discharged from -40⬚Cto65⬚C.

The combination of these qualities within a cost effective, hermetic package has enabled the

diverse applicability of the technology.

TABLE 35.11 General Performance Characteristics of

Li-ion Batteries

Characteristic Performance range

Operational cell voltage 4.2 to 2.5 V

Speciﬁc energy 100 to 158 Wh / kg

Energy density 245 to 430 Wh /L

Continuous rate capability Typical: 1C

High rate: 5C

Pulse rate capability Up to 25C

Cycle life at 100% DOD Typically 3000

Cycle life at 20 to 40% DOD Over 20000

Calendar life Over 5 years

Self discharge rate 2 to 10% /month

Operable temperature range

⫺40⬚Cto65⬚C

Memory effect None

Power density 2000 to 3000 W / L

Speciﬁc power 700 to 1300 W / Kg

35.4.1 Characteristics and Sizes of Lithium Ion Batteries

As illustrated in Table 35.12 and Table 35.13, Li-ion batteries are available in a wide range

of sizes from 0.6 to 160 Ah in both cylindrical and prismatic designs with a range of aspect

ratios. Li-ion battery performance has steadily improved, in the period 1996 to 1999, the

speciﬁc energy of 18650-type cells increased 8% per year while energy density increased

14% per year, on average.

TABLE 35.12 Electrical and Physical Characteristics of Typical Cylindrical C/ LiCoO

or C /LiNi

⫺

Li-ion

Batteries

Type 14500 14650 17500 17670 18500 18650 26650 33600

Height, mm 50.0 65 50 67 50 65 65 60

Diameter, mm 14 14 17 17 18 18 26 33

Volume, ml. 7.7 10 11.3 15.2 12.7 16.5 34.5 51.4

Mass, g. 19 26 25 35 31 42 93 125

Capacity (Ah) .65 .90 0.83 1.25 1.1 1.8 3.2 5.0

Speciﬁc energy (Wh/ kg) 126 128 123 132 131 155 354 150.4

Energy density (Wh/ L) 312 333 273 306 320 410 131 366

35.36

TABLE 35.13 Electrical and Physical Characteristics of Typical Prismatic C/ LiCoO

or C / LiNi

⫺

Li-ion Batteries

Cell type 61/ 19/ 48 46 / 30 / 48 55 /30 /48 81 / 31 / 48 65/ 35/ 67 103 /34 /50 160 /61 /78 280 / 95/ 151 460/ 89/ 128 500 / 130 /208

Shape Prismatic (thickness (0.1 mm)/ width (mm)/ height (mm)

Height, mm 48 48 48 48 67.3 50 78 151 128 208

Width, mm 19.5 30 30 30.5 35.1 34.1 61 95 89 130

Thickness, mm 6.1 4.6 5.5 8.1 6.5 10.3 16 28 46 50

Volume, ml. 5.70 6.6 7.9 11.9 15.3 18.4 76 136 465 1352

Mass, g. 11.5 14 16 24 33 38 185 870 1108 3650

Capacity (Ah) 0.42 .52 0.6 0.9 .96 1.5 7.0 35 40 160

Speciﬁc energy (Wh/ kg) 135 137 138 139 104 146 145 145 156 160

Energy density (Wh/ L) 272 291 281 280 226 301 345 344 372 430

LITHIUM-ION BATTERIES 35.37

0.0 0.5 1.0 1.5

2.5

3.0

3.5

4.0

Voltage (V)

Discharge Capacity (Ah)

0.33 A (1.68 Ah)

0.82 A (1.67 Ah)

1.65 A (1.60 Ah)

2.00 A (1.53 Ah)

2.40 A (1.43 Ah)

3.60 A (1.37 Ah)

2.5

3.0

3.5

4.0

2.5

3.0

3.5

4.0

FIGURE 35.36 Discharge capability of 18650 type C/ LiCoO

batteries at constant current

at 21⬚Cto25⬚C. (Courtesy of the University of South Carolina and NEC Moli Energy.)

The most signiﬁcant challenges to the broader application of Li-ion technologies are

related to either stability at high temperature or safety. While batteries may be exposed to

temperatures as high as 70

⬚C for short periods, the rate of degradation of current Li-ion

batteries is signiﬁcant above 60

⬚C. Li-ion batteries are generally safe, although venting can

occur if they are overcharged or crushed. To vent from overcharge, batteries must typically

be charged to greater than 200% of their rated capacity. Protective devices are employed to

prevent ventings under abusive conditions.

In current Li-ion batteries, the overcharge, overdischarge and over temperature issues have

been largely addressed by the incorporation of management circuits into batteries to provide

protection from over-charge, over-discharge or over-temperature. In addition, the circuits may

also perform fuel gauge functions and can record battery history. The controller in the man-

agement circuit typically monitors the voltage of each cell or string of cells in a battery. In

addition, circuits typically include a thermistor or other thermostat for restorable over-

temperature control and a thermal fuse for non-restorable over-temperature control. Addi-

tional information on battery management circuits is found in Sec. 5.6.

35.4.2 Performance of Commerical Cylindrical Batteries

Discharge Rate Capability. The rate capability and capacity of Li-ion batteries are depen-

dent on their design, and varies considerably between manufacturers. Discharge curves for

18650-type C /LiCoO

batteries discharged at rates from 0.33 A to 3.6 A at 21⬚C are shown

in Fig. 35.36. At low rates (C/2), the battery provided 1.67 Ah, and at 1.65 A (the 1 C rate)

1.60 Ah while at the C rate the average voltage was 3.5 V.

Batteries utilizing the C/ LiMn

cell chemistry have also been commercialized. The

spinel cathode materials, as described in the section on cathode materials, are regarded as

providing lower cost and more benign safety properties relative to LiCoO

, although they

possess lower speciﬁc capacity. Shown in Fig. 35.37 is the rate capability and capacity of

18650-type C /LiMn

batteries. The products provide 1.44 Ah at the C /5 rate (0.28 A)

and 1.34 Ah at the C rate (1.4 A), comparable to, but less than the capacity of C/LiCoO

batteries. The discharge curves have a ﬂat voltage proﬁle with average voltage at the C rate

of 3.65 V, 0.15 V higher than C /LiCoO

cells at the C rate.

35.38 CHAPTER THIRTY-FIVE

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

Voltage (V)

Discharge Capacity (Ah)

0.28 A (1.44 Ah)

0.70 A (1.43 Ah)

1.40 A (1.34 Ah)

2.00 A (1.25 Ah)

FIGURE 35.37 Rate capability of an 18650-type C / LiMn

battery at constant current. The

battery was charged in a CCCV (constant current-constant voltage) regime at 1.4 A to 4.2 V for

2.5 hours, then discharged at 21⬚C. (Courtesy of NEC Moli Energy.)

01234567

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

Voltage (V)

Discharge Energy (Wh)

0.5W (1.69 Ah, 6.4 Wh)

1.0W (1.68 Ah, 6.3 Wh)

2.0W (1.67 Ah, 6.2 Wh)

5.0W (1.60 Ah, 5.7 Wh)

FIGURE 35.38 Constant power discharge of an 18650-type C / LiCoO

battery. The battery was charged

in a CCCV regime at 1.65 A to 4.2 V for 2.5 hours at 21⬚C. (Courtesy of NEC Moli Energy.)

The performance of Li-ion batteries under constant power discharge is illustrated in Fig.

35.38 for 18650-type C/LiCoO

batteries, and in Fig. 35.39 for an 18650-type C /LiMn

battery. At 1 W the C /LiCoO

battery provided 6.3 Wh, or 145 Wh /kg and 380 Wh/L. At

1 W the C/LiMn

battery provided 5.5 Wh, or 130 Wh/kg and 332 Wh/L.

The rate capability of several 18650-type batteries is compared in Fig. 35.40 which il-

lustrates the higher capacity of newer products and the 3C rate capability of the A&T

LSR18650 product. The average discharge voltage is lowered signiﬁcantly at the higher rates.