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35.1

CHAPTER 35

LITHIUM-ION BATTERIES

Grant M. Ehrlich

35.1 GENERAL CHARACTERISTICS

Lithium-ion (Li-ion) batteries are comprised of cells that employ lithium intercalation com-

pounds as the positive and negative materials. As a battery is cycled, lithium ions (Li

⫹

)

exchange between the positive and negative electrodes. They are also referred to as rocking-

chair batteries as the lithium ions ‘‘rock’’ back and forth between the positive and negative

electrodes as the cell is charged and discharged. The positive electrode material is typically

a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO

), or a material

with a tunneled structure, such as lithium manganese oxide (LiMn

), on a current collector

of aluminum foil. The negative electrode material is typically a graphitic carbon, also a

layered material, on a copper current collector. In the charge/ discharge process, lithium ions

are inserted or extracted from interstitial space between atomic layers within the active

materials.

The ﬁrst batteries to be marketed, and the majority of those currently available, utilize

LiCoO

as the positive electrode material. Lithium cobalt oxide offers good electrical per-

formance, is easily prepared, has good safety properties, and is relatively insensitive to pro-

cess variation and moisture. More recently lower cost or higher performance materials, such

as LiMn

or lithium nickel cobalt oxide (LiNi

⫺

), have been introduced, permitting

development of cells and batteries with improved performance. The batteries that were ﬁrst

commercialized employed cells with coke negative electrode materials. As improved graph-

ites became available, the industry shifted to graphitic carbons as negative electrode materials

as they offer higher speciﬁc capacity with improved cycle life and rate capability.

The Li-ion battery market has grown in a decade from an R&D interest to sales of over

400 million units in 1999. Market value at the OEM level was estimated to be $1.86 billion

in 2000.

By 2005, the market is expected to grow to over 1.1 billion units with value of

over $4 billion (¥455 billion),

while the average unit price is expected to fall 46% from

1999 to 2005. Market interest in this cost-effective, high performance, and safe technology

has driven spectacular growth, as illustrated in Fig. 35.1. This technology has rapidly become

the standard power source in a broad array of markets, and battery performance continues

to improve as Li-ion batteries are applied to an increasingly diverse range of applications.

To meet market demand, an array of designs has been developed, including spiral wound

cylindrical, wound prismatic and ﬂat plate prismatic designs in small (0.1 Ah) to large (160

Ah) sizes. Applications now addressed with Li-ion batteries include consumer electronics,

such as cell phones, laptop computers, and personal data assistants, as well as military

electronics, including radios, mine detectors and thermal weapons sights. Anticipated appli-

cations include aircraft, space craft, satellites, and electric or hybrid electric vehicles.

35.2 CHAPTER THIRTY-FIVE

1998 2000 2002 2004 2006

0.0

5.0x10

1.0x10

1.5x10

2.0x10

Worldwide Demand

Average Price

Year

Li-ion Battery Worldwide Demand (Units)

Average Price (U. S. Dollars)

FIGURE 35.1 Current and anticipated worldwide demand and average price for Li-ion

batteries. (From Ref. 1.)

TABLE 35.1 Advantages and Disadvantages of Li-ion Batteries

Advantages Disadvantages

Sealed cells; no maintenance required Moderate initial cost

Long cycle life Degrades at high temperature

Broad temperature range of operation Need for protective circuitry

Long shelf life

Low self-discharge rate

Rapid charge capability

High rate and high power discharge capability

High coulombic and energy efﬁciency

High speciﬁc energy and energy density

No memory effect

Capacity loss or thermal runaway when over-

charged.

Venting and possible thermal runaway when

crushed

Cylindrical designs typically offer lower power

density than NiCd or NiMH

The major advantages and disadvantages of Li-ion batteries, relative to other types of

batteries, are summarized in Table 35.1. The high speciﬁc energy (

⬃150 Wh/kg) and energy

density (

⬃400 Wh/L) of commercial products makes them attractive for weight or volume

sensitive applications. Li-ion batteries offer a low self-discharge rate (2% to 8% per month),

long cycle life (greater than 1000 cycles) and a broad temperature range of operation (charge

⫺20⬚Cto60⬚C, discharge at ⫺40⬚Cto65⬚C), enabling their use in a wide variety of

applications. A wide array of sizes and shapes is now available from a variety of manufac-

turers. Single cells typically operate in the range of 2.5 to 4.2 V, approximately three times

that of NiCd or NiMH cells, and thus require fewer cells for a battery of a given voltage.

Li-ion batteries can offer high rate capability. Discharge at 5C continuous, or 25C pulse, has

been demonstrated. The combination of these qualities within a cost effective, hermetic

package has enabled diverse application of the technology.

LITHIUM-ION BATTERIES 35.3

A disadvantage of Li-ion batteries is that they degrade when discharged below 2 V and

may vent when overcharged as they do not have a chemical mechanism to manage over-

charge, unlike aqueous cell chemistries. Li-ion batteries typically employ management cir-

cuitry and mechanical disconnect devices to provide protection from over-discharge, over-

charge or over temperature conditions. Another disadvantage of Li-ion products is that they

permanently lose capacity at elevated temperatures (65

⬚C), albeit at a lower rate than most

NiCd or NiMH products.

35.1.1 Designation and Markings

The International Electrotechnical Commission (IEC) is developing standards for the des-

ignation, marking, electrical testing, and safety testing of Li-ion cells and batteries. A pro-

posed designation and marking system for Li-ion cells utilizes ﬁve ﬁgures in the case of

cylindrical cells and six ﬁgures in the case of prismatic cells. The ﬁrst letter designates the

type of negative electrode employed, I for an intercalation electrode. The second letter des-

ignates the type of positive electrode employed, such as C for a cobalt type, N for Nickel

type, M for Manganese type or V for a Vanadium type. The third letter will designate the

shape of the cell, R for round or P for prismatic. In the case of round cells, the next two

ﬁgures will designate the diameter in millimeters and then the next three ﬁgures the height

of the cell, in tenths of millimeters. For prismatic cells, the next three ﬁgures will designate

the thickness in tenths of millimeters and width and height of the cell, in millimeters. In the

event that any dimension exceeds 100 mm, a solidus (/) is to be placed between the ﬁgures.

If a dimension is less than 1 mm, the dimension is preceded by a t (t7

⫽ 0.7 mm). For

example the common round cells that use the C /LiCoO

cell chemistry are designated

ICR18650 as they are 18 mm in diameter and 65.0 mm in height.

The proposed designation standard for Li-ion batteries will involve a number that will

designate the number of series connected cells, followed by three letters that will designate

the negative electrode system (I for intercalation), the positive electrode system (C for cobalt,

N for nickel, M for manganese), and the battery’s shape (R for round, P for prismatic). The

letters will be followed by two ﬁgures in the case of round batteries, to designate the diameter

and height of the battery, in millimeters and tenths of millimeters or three ﬁgures in the case

of prismatic batteries, to designate the thickness, width and height of the battery, in milli-

meters. If a dimension exceeds 100 mm, a solidus is used between the ﬁgures. If the battery

comprises two or more parallel cells, the number of parallel connected cells is added after

the ﬁgures that designate the dimensions. For example, a battery designated 1ICP206870-2

would be a prismatic Li-ion battery with two C/ LiCoO

cells connected in parallel, with

thickness 20 mm, width 68 mm and height 70 mm.

The latent IEC standards should be obtained for detailed information on nomenclature,

performance and safety guidelines.

ANSI standard C18.2M ‘‘Standard for Portable Rechargeable Cells and Batteries’’ in-

cludes standards for portable Li-ion batteries.

35.4 CHAPTER THIRTY-FIVE

35.2 CHEMISTRY

The electrochemically active electrode materials in Li-ion batteries are a lithium metal oxide

for the positive electrode and lithiated carbon for the negative electrode. These materials are

adhered to a metal foil current collector with a binder, typically polyvinylidene ﬂuoride

(PVDF) or the copolymer polyvinylidene ﬂuoride–hexaﬂuroropropylene (PVDF-HFP), and

a conductive diluent, typically a high-surface-area carbon black or graphite. The positive and

negative electrodes are electrically isolated by a microporous polyethylene or polypropylene

separator ﬁlm in products that employ a liquid electrolyte, a layer of gel-polymer electrolyte

in gel-polymer batteries, or a layer of solid electrolyte in solid-state batteries.

Since the commercialization of Li-ion batteries by Sony in 1990, a broad array of variants

has been introduced. One type, gel-polymer Li-ion batteries, utilizes the same active materials

as products that employ liquid electrolytes, but in a different construction that enables the

fabrication of cells with a thin form factor. Gel-polymer batteries, also referred to as polymer

Li-ion batteries in the marketplace, are products where the microporous separator ﬁlm used

in conventional batteries is substituted by a layer of PVDF-HFP, or other polymer, impreg-

nated with liquid electrolyte and the solid current collector foil is typically substituted with

an open expanded metal current collector grid. In gel-polymer cells, the positive, separator,

and negative layers are bound by the polymer, typically PVDF-HFP, and can be laminated

together to form a monolithic device. Despite these differences, the active cell chemistry

may remain identical to that in cylindrical or prismatic Li-ion batteries.

35.2.1 Intercalation Processes

The active materials in Li-ion cells operate by reversibly incorporating lithium in an inter-

calation process, a topotactic reaction where lithium ions are reversibly removed or inserted

into a host without a signiﬁcant structural change to the host. The positive material in a Li-

ion cell is a metal oxide, with either a layered or tunneled structure. The graphitic carbon

negative materials have a layered structure similar to graphite. Thus the metal oxide, graphite,

and other materials act as hosts, incorporating lithium ions, guests, reversibly to form ‘‘sand-

wich’’ like structures.

Intercalation materials, originally discovered by the Chinese 2700 years ago,

have been

the subject of contemporary chemical research for only the last half-century. Today, inter-

calation compounds form the basis for a host of technologies ranging from superconductors

to catalysis. Intercalation materials in common use include graphite,

5,6

layered silicates such

as talc (Mg

(OH)

(Si

)),

clays, and layered transition metal dichalcogenides, such as

TiS

The intercalation of a variety of electron donors, including lithium, and electron ac-

ceptors, such as halogens, into graphite has been studied.

6,7

The ﬁeld of graphite intercalation

compounds is especially rich,

both in the diversity of the chemistry and the depth of study.

Of particular interest to the ﬁeld of Li-ion batteries is the work on alkali metal intercalation

of graphite and related carbons, in particular Li

(0 ⬍ x ⬍ 1).

When a Li-ion cell is

charged, the positive material is oxidized and the negative material is reduced. In this process,

lithium ions are de-intercalated from the positive material and intercalated into the negative

material, as illustrated in Fig. 35.2. In this scheme, LiMO

represents the metal oxide positive

material, such as LiCoO

, and C the carbonaceous negative material, such as graphite. The

reverse happens on discharge. As metallic lithium is not present in the cell, Li-ion batteries

are chemically less reactive, safer, and offer longer cycle life than possible with rechargeable

lithium batteries that employ lithium metal as the negative electrode material. The charge-

discharge process in a Li-ion cell is further illustrated graphically in Fig. 35.3. In the ﬁgure,

the layered active materials are shown on metallic current collectors.

LITHIUM-ION BATTERIES 35.5

1-x

+ xLi

+ xe

LiMO

charge

discharge

Positive:

Negative:

Overall:

C + xLi

+ xe

charge

discharge

charge

discharge

LiMO

+ C Li

C + Li

1-x

FIGURE 35.2 Electrode and cell reactions in a Li-ion cell.

on Charge

on Discharge

1-x

on Charge

on Discharge

Intercalation

FIGURE 35.3 Schematic of the electrochemical process in a Li-ion cell.

35.6 CHAPTER THIRTY-FIVE

35.2.2 Positive Electrode Materials

Positive electrode materials in commercially available Li-ion batteries utilize a lithiated metal

oxide as the active material. The ﬁrst Li-ion products marketed by Sony used LiCoO

Goodenough and Mizushima developed this material, as described in a series of patents.

Recently, cells have been developed that utilize less costly materials, such as LiMn

(spi-

nel), or materials with higher coulombic capacity, such as LiNi

⫺

. Commercial interest

in LiNiO

has waned as its instability, driven by the energetic formation of NiO and oxygen,

has been shown to contribute to safety issues.

Viable electrode materials must satisfy a number of requirements, as summarized in Table

35.2. These factors guide the selection and development of positive electrode materials. To

enable high capacity, materials must incorporate a large amount of lithium. Further, the

materials must reversibly exchange that lithium with little structural change to permit long

cycle life, high coulombic efﬁciency, and high energy efﬁciency. To achieve high cell voltage

and high energy density, the lithium exchange reaction must occur at a high potential relative

to lithium. When a cell is charged or discharged, an electron is removed or returned to the

positive material. So that this process can occur at a high rate, the electronic conductivity

and Li

⫹

mobility in the material must be high. Also, the material must be compatible with

the other materials in the cell; in particular it must not be soluble in the electrolyte. Finally,

the material must be of acceptable cost. To minimize cost, preparation from inexpensive

materials in a low-cost process is preferred.

TABLE 35.2 Requirements for Li-ion Positive Electrode

Materials

High free energy of reaction with lithium

Can incorporate large quantities of lithium

Reversibly incorporates lithium without structural change

High lithium ion diffusivity

Good electronic conductivity

Insoluble in the electrolyte

Prepared from inexpensive reagents

Low cost synthesis

Characteristics of Positive Electrode Materials. A variety of positive electrode materials

has been developed and many of these are commercially available. All commercially avail-

able materials have one of two structure types. LiCoO

, LiNiO

, and related materials such

as LiNi

⫺

, have layered structures whereas LiMn

or ‘‘spinel’’ materials have a

three-dimensional ‘‘framework’’ structure. The term spinel formally refers to the mineral

(MgAl

), although the term is used for materials with equivalent structure. The ideal

layered structure of MnO

is shown in Fig. 35.4. In the case of LiCoO

or LiNi

1-x

, the

cobalt or nickel atoms would reside within oxygen octahedra, and the lithium atoms would

reside in the space between the oxygen layers. The LiMn

(spinel) materials, however,

have a three-dimensional framework or tunneled structure based on

␭

-MnO

, as illustrated

in Fig. 35.5. In spinel, lithium ﬁlls one-eighth of the tetrahedral sites within the

␭

-MnO

structure as Mn-centered oxygen octahedra ﬁll one-half of the octahedral sites.

Commercially, the structural difference between these materials has had unusually high

impact because the original patent by Goodenough and Mizuchima,

granted to the United

Kingdom Atomic Energy Agency (AEA) in 1981, referred to ‘‘mixed oxides having the

layered structure of

␣

-NaCrO

,’’ thus cells using spinel did not fall within the scope of

Goodenough and Mizuchima’s patents.

LITHIUM-ION BATTERIES 35.7

FIGURE 35.4 The idealized structure of layered-

MnO

.(Courtesy of CISR.)

-MnO

LiMn

FIGURE 35.5 The idealized structure of

␭

-MnO

and LiMn

spinel. The model on the left shows the

manganese centered oxygen octahedra of

␭

-MnO

. In the model on the right of LiMn

, oxygen is grey and

lithium is black. (Courtesy of CISR and Michael Tucker.)

The voltage and capacity characteristics of common positive electrode materials are sum-

marized in Table 35.3. The most commonly used positive electrode material, LiCoO

(Fig.

35.6), offers good capacity, 155 mAh /g, and high voltage, 3.9 V vs. Li. The LiNi

⫺

materials offer higher capacity, up to 220 mAh/ g, albeit at ⬃0.2 V lower than LiCoO

LiMn

Spinel, LiMn

, is also of commercial interest, particularly for applications that are cost

sensitive or require exceptional stability upon abuse. It has lower capacity, 120 mAh/g,

slightly higher voltage, 4.0 V vs. Li, but has higher capacity loss on storage, especially at

elevated temperature, relative to cells that use LiCoO

or LiNi

⫺

, described below.

Properties of LiNiO

-Based Positive Electrode Materials. Despite its high capacity and

low cost, LiNiO

is not widely used commercially because of the energy evolved upon

decomposition, the relatively low temperature at which self-heating ensues, and the difﬁculty

of preparing the material consistently in quantity.

The synthesis, electrochemistry, and

structural properties of LiNiO

have been described,

as have the morphological changes

that can occur when the material is charged to 4.2 V vs. lithium.

35.8 CHAPTER THIRTY-FIVE

TABLE 35.3 Characterisitcs of Positive Electrode Materials*

Material

Speciﬁc

Capacity

(mAh/ g)

Midpoint

V vs. Li

(at 0.05C) Advantages or disadvantages

LiCoO

155 3.88 Most common commercially, Co is expensive

LiNi

0.7

0.3

190 3.70 Intermediate cost

LiNi

0.8

0.2

205 3.73 Intermediate cost

LiNi

0.9

0.1

220 3.76 Highest speciﬁc capacity

LiNiO

200 3.55 Most exothermic decomposition

LiMn

120 4.00 Mn is inexpensive, low toxicity, least exothermic decomposition

* Experimental values.

FIGURE 35.6 The idealized layered structure of

LiCoO

, Li is speckled, O white and Co black.

The energy evolved on decomposition is illustrated in Fig. 35.7, which shows Differential

Scanning Calorimetry (DSC) data from electrodes that were charged at 4.5 V for 40 hours

prior to heating in a DSC. As shown, LiNiO

evolves the most energy and has a low onset

temperature. The decomposition of LiNiO

has been shown to follow two paths, both leading

to NiO;

low temperature (LT)-LiNiO

has been shown to be less stable than high temper-

ature (HT)-LiNiO

, and the stability is dependent on the particle size. To mitigate the insta-

bility of LiNiO

, cobalt substituted materials are now commonplace. As shown in Fig. 35.7,

LiCoO

has a higher onset temperature and evolves less energy than LiNiO

. As expected,

the intermediate phase LiNi

0.8

0.2

has an onset temperature and energy evolution between

that of LiNiO

and LiCoO