Development of the Lithium-Ion

Battery and Recent Technological

Trends

Akira Yoshino

YOSHINO LABORATORY, ASAHI KASEI CORP., 2-1, SAMEJIMA, FUJI-SHI,

SHIZUOKA, JAPAN

CHAPTER OUTLINE

1. Introduction ....................................................................................................................................... 2

2. Development of the Practical LIB .................................................................................................... 3

3. Development of Cathode Materials ................................................................................................ 7

3.1. History of Cathode Material Development............................................................................. 7

3.2. Recent Technological Trends of Cathode Materials............................................................... 7

3.2.1. Three Morphologies of Cathode Materials ........................................................................ 7

3.2.2. Layered Rock Salt Structure Materials (two dimensional)................................................... 8

3.2.3. Spinel Structure Materials (three dimensional)................................................................... 8

3.2.4. Olivine Structure Materials (one dimensional).................................................................... 8

3.3. Latest Research on Cathode Materials .................................................................................... 8

3.3.1. Layered LCO Series (two dimensional) ............................................................................... 8

3.3.2. Layered LiNiO

Series (two dimensional)............................................................................ 9

3.3.3. Layered Mn Compound Series (two dimensional).............................................................. 9

3.3.4. Spinel Structure Cathode Materials (three dimensional) .................................................. 10

3.3.5. Olivine Structure Cathode Materials (one dimensional) ................................................... 10

4. Development of Anode Materials ................................................................................................. 11

4.1. History of Anode Material Development.............................................................................. 11

4.2. Recent Research on Anode Materials .................................................................................... 12

5. Development of Electrolyte Solutions .......................................................................................... 13

5.1. History of Electrolyte Solution Development ....................................................................... 13

5.2. Recent Research on Electrolyte Solutions ............................................................................. 14

6. Separator Technology..................................................................................................................... 15

6.1. Separator Production Methods and Characteristics ............................................................. 15

6.1.1. Dry-process One-component System ............................................................................... 15

6.1.2. Wet-process Two-component System ............................................................................. 16

Lithium-Ion Batteries: Advances and Applications.

http://dx.doi.org/10.1016/B978-0-444-59513-3.00001-7

6.1.3. Wet-process Three-component System ........................................................................... 16

6.1.4. Shutdown Function ......................................................................................................... 16

6.2. Recent Separator Developments............................................................................................ 17

6.2.1. New Materials ................................................................................................................. 17

6.2.2. Inorganic Coating ............................................................................................................ 18

6.2.3. Separators Containing Inorganic Material........................................................................ 18

6.2.4. Nonwoven Separators ..................................................................................................... 18

6.2.5. Laminated Separators ...................................................................................................... 19

7. Conclusion ........................................................................................................................................ 19

References............................................................................................................................................. 19

Introduction

In the 1980s, information technology (IT) advanced significantly with the development of
portable electronic products such as video cameras, mobile phones, and notebook
computers. This technological revolution led to a growing need for rechargeable batteries
with greater capacity or with reduced size and weight for a given capacity. Conventional
rechargeable batteries available or under development at that time such as lead–acid,
nickel–cadmium, and nickel–metal hydride batteries used aqueous electrolytes, which
posed limitations on increasing the energy density and reducing the size and weight.
Thus, there remained an unmet need for a new, small and lightweight rechargeable
battery to be put into practical use. Research on the lithium-ion battery (LIB) started in
the early 1980s, and the first commercialization was achieved in 1991. Since then, LIBs
have grown to become the dominant power storage solution for portable IT devices. The
LIB market has continued to expand rapidly for over 15 years, and its worldwide scale
now exceeds JP Yen 10

millions ($13,000 millions) as shown in

Figure 1.1

200

400

600

800

1000

1200

1992

2000

2010

Year

l sa

s a

ts (

illio

)

FIGURE 1.1 Expansion of worldwide LIB demand.

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

The four major components of the LIB are the cathode, anode, electrolyte, and

separator. LIBs generally produce an average cell voltage of around 3.7 V and operate on
the relatively simple principle of reversible intercalation of Li ions in the cathode and
anode. The most commonly used material for the cathode is lithium cobalt oxide, LiCoO

and some form of carbon is generally used for the anode. In the completely discharged
state, Li atoms are only contained as part of the cathode. On charging, Li ions are released
from the cathode and migrate through the electrolyte into the carbon of the anode. The
reverse reaction occurs during discharging, and electric energy is stored or released by
repeating these reactions reversibly. The rapidly growing LIB market described above
owes to this safe and efficient principle. This chapter focuses on two subjects: the
development of the first practical LIB configuration in the 1980s and the recent tech-
nological trends for LIBs. The principle of the present commercial LIB as described above
was completed in 1985 with a patent filed by the author

[1]

. This invention and other

successive patents illustrate the origins of the key LIB components and configuration, as
well as the technological requirements associated with them. Since many of the
requirements at the time are analogous to the objectives of current research, reviewing
the initial stage of LIB development will provide rich context for and insight into the
overall technology. Following a brief overview of the initial development, sections
devoted to each major LIB component will begin with a description of the early tech-
nological innovations followed by a discussion of recent technology and research.

Development of the Practical LIB

The author and colleagues focused on creating a practical new nonaqueous electrolyte
rechargeable battery to meet the emerging need for a small and lightweight power source
for portable electronics. Our essential achievements made in the 1980s were as follows:
(1) proposition of fundamental technology for composition of the LIB, in which LiCoO

used as the cathode and a carbonaceous material with a certain crystalline structure is
used as the anode; (2) invention of essential constituent technologies for the electrodes,
electrolyte, and separator; and (3) development of peripheral technology such as safety
device technology, protective circuit technology, and charging and discharging
technology.

The first step to develop the practical LIBs was the adoption of LiCoO

for the cathode.

LiCoO

was first disclosed by Goodenough et al.

[2,3]

and it remains the most commonly

used cathode material at present. One anode material that was gaining attention at the
time was graphite

[4]

, but it was known that propylene carbonate, which was then the

common organic electrolyte solvent, would decompose during charging when graphite
was used. Furthermore, the use of a solid electrolyte resulted in electrical resistance
which was too high to enable practical charging and discharging. We created a working
model LIB using LiCoO

as the cathode and polyacetylene as the anode, but rejected

polyacetylene due to its low density (high bulk) which precluded reduction of cell size.
Studying several carbonaceous materials for their suitability as anode, we found that

Chapter 1 • Development of the Lithium-Ion Battery 3

a carbonaceous material with a certain crystalline structure provided greater capacity
without causing decomposition of the propylene carbonate electrolyte solvent, as
graphite did. The secondary battery which we successfully fabricated based on this new
combination of component materials enabled stable charging and discharging, over
many cycles for a long period

[1]

This combination of electrode materials marked a new concept of a secondary battery

based on the transfer of Li ions. Cell reaction without chemical transformation provided
stable battery characteristics over a long service life, including excellent cycle durability
with little degradation by side reactions, and excellent storage characteristics.
Furthermore, this development also enabled simple and efficient assembly in the dis-
charged state, with no special atmosphere required because LiCoO

is very stable in air,

despite containing Li ions, and the anode is composed of carbonaceous material which is
also stable.

Another key step was the development of essential constituent technologies including

technology for fabricating electrodes and technology for assembling batteries. In the
basic structure of the typical LIB, a multilayer electrode assembly (electrode coil), pre-
pared by winding sheets of cathode and anode with separator membrane in between, is
inserted into a battery can. This is then infused with nonaqueous electrolyte solution
composed of LiPF

or LiBF

dissolved in a mixture of carbonate compounds and sealed.

Both the cathode and the anode are structured with the electrode material coated on both
sides of a current collector. The current collectors conduct electricity from the active
electrode materials to tabs connected to the electrode terminals. Aluminum foil is used
for the cathode current collector and copper foil is used for the anode current collector,
the thickness of each being around 10

mm. Some batteries with this configuration were

subjected to “abuse test” for safety evaluation and proved that the basic LIB cell design
provides the required level of safety, and this cleared the way for the commercialization of
the LIB as we know it today.

The ionic conductivity of nonaqueous electrolyte is lower than that of aqueous elec-

trolyte, so in order to obtain discharge power using the former which is comparable to
that using the latter, lower current density for a given area of electrode surface was
required to prevent the excessive generation of Joule heat. We achieved high current
discharge using nonaqueous electrolyte by devising flat-sheet electrodes wound into a
coil shape. Practical application was achieved with technology for fabricating thin-film
electrodes (100–250

mm) in which a thin metal foil is used as a current collector and

both surfaces of the foil are coated with electrode active material. Our selection of
aluminum as cathode current collector was one of the most important aspects of this
development. Previously, only precious metals such as gold and platinum were consid-
ered to be able to withstand a high voltage of 4 V or more. However, we found that
aluminum foil was suitable for use as a cathode current collector because a passivation
layer forms on the aluminum surface

[5]

Another notable invention relating to constituent technologies essential for achieving

a practical LIB is a highly functional membrane separator for safety. The use of a

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

microporous polyethylene membrane (20–30

mm thick) for use as separator provides a

“shutdown” function in which the material of the separator melts to close the micropores
and shut off battery operation in the case of abnormal heat generation

[6]

In summary, these essential constituent technologies impart the LIB with the

following characteristics: (1) high cell voltage of 4 V or more enabled through the use of
LiCoO

as the cathode and aluminum foil as the cathode current collector; (2) high

current discharge enabled with large-area thin-film electrodes using metal foil as the
current collector with electrode material coated on both sides; (3) efficient, high-speed
electrode production; (4) high-density packaging with the coil-shaped, multilayer thin-
film electrode assembly emplaced in a battery can; and (5) significantly improved bat-
tery safety with a polyethylene microporous membrane having a certain thermal char-
acteristic used as separator.

The final part of our development was the peripheral technology which was instru-

mental in the development of a practical LIB, including safety device technology, pro-
tective circuit technology, and charging and discharging technology. One key example is a
positive temperature coefficient device which is sensitive to both electric current and
temperature. Incorporation of this device in the LIB results in greatly improved safety,
particularly in terms of protection against overcharging

[7]

These are the key technological developments that made the current LIB possible.

Further developments accelerated quickly with commercialization in the early 1990s, and
the pace is picking up again as new applications outside portable electronics come to the
fore. Further technological development today is generally focused on novel composi-
tions of the major LIB components, the cathode, anode, electrolyte, and separator.
Characteristics of these components are improved either individually or together to
enhance the battery performance. For example, the energy density of the LIB has risen
from 200 Wh/l in the early 1990s to the current 600 Wh/l, as shown in

Figure 1.2

. This

steady increase in energy density has enabled the power module of portable IT devices to
significantly decrease in size and weight, and the major contributor to this development
has been the development of high-capacity anode materials. Capacity improvement of
carbon-based anodes is, however, thought to be approaching a ceiling lately, and
therefore an increasing amount of research is focused on the development of new ma-
terials capable of reversibly intercalating Li ions with a higher capacity than carbon.

A tripling of energy density would, in principle, result in the bare cell price per watt-

hour dropping by one-third. Further price reduction is possible by improving material
characteristics and cutting production costs. In fact, LIBs have historically achieved an
astonishing level of price reduction, far exceeding the rate of increase in capacity. This is
illustrated in

Figure 1.3

, which shows the price trend for bare cylindrical 18,650 cells, the

general-purpose LIB configuration. When high-volume production began in earnest
around 1994, the bare cell price was

U300; this dropped to U22.5 in the early 2000s. The

achievement of such a low price per watt-hour for portable applications is furthermore
significant in that it begins to make the LIB affordable for medium- and large-scale
applications such as electric vehicles and storage batteries for residential use.

Chapter 1 • Development of the Lithium-Ion Battery 5

Such dramatic price reduction together with further technological developments are

leading the LIB’s rapid advancement into medium- and large-scale applications, most
notably hybrid electric vehicle (HEV), plug-in hybrid electric vehicle, battery electric
vehicle (BEV), and energy storage system for buildings. The following sections of this
chapter review the technological improvements which took place from the start of
commercialization to the present and describe recent trends of development for each
major LIB component.

100

200

300

400

500

600

700

800

1992

2000

2010

Energy density of LIB (Wh/l)

Year

FIGURE 1.2 Capacity increase of LIB.

100

200

300

400

500

600

700

1992

2000

2010

Price of LIB 18650 (JP ¥/Wh)

Year

FIGURE 1.3 Trend of LIB bare cell price.

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

Development of Cathode Materials

3.1.

History of Cathode Material Development

In 1995 when mass production of the LIB was just underway, LiCoO

(LCO) was by far the

dominant cathode material, with the spinel LiMn

(LMO) occupying only a small part

of the 650-t total market as shown in

Figure 1.4

Although LCO remains the most common cathode material, its share of the total

market has been gradually eroded by other materials due to considerations of cost and
resource availability. By 2010, the market share for LCO had declined to 40%, and the use
of LiNi

1/3

(NMC), a ternary system with nickel, manganese, and cobalt, had

increased

dramatically.

Although

their

market

are

relatively

small,

LiNi

0.8

0.15

0.05

(NCA) and LMO are preferred for certain applications. Although only

finding limited use so far, phosphates with an olivine structure are a promising new class
of cathode materials with LiFePO

(LFP) being the most common. As shown in

Figure 1.4

total shipments of LIB cathode materials reached 45,000 t in 2010, and this is poised to
further expand as other new cathode materials are developed especially for medium- and
large-scale applications.

3.2.

Recent Technological Trends of Cathode Materials

3.2.1.

Three Morphologies of Cathode Materials

The LIB cathode materials are transition metal oxides containing lithium, and they are a
type of functional ceramics. For such a material to be used as LIB cathode, the Li ions
must be able to diffuse freely through the crystal structure. The morphology of the
crystal structure, being one-, two-, or three-dimensional, determines the number of
dimensions in which Li ions are able to move. Cathode materials currently in use or
under development are described below in accordance with the following three
morphologies.

FIGURE 1.4 Major materials in the LIB cathode market. LCO, LiCoO

; LMO, LiMn

; NMC, LiNi

1/3

; NCA,

LiNi

0.8

0.15

0.05

; LFP, LiFePO

Chapter 1 • Development of the Lithium-Ion Battery 7

3.2.2.

Layered Rock Salt Structure Materials (two dimensional)

This is a group of compounds with a two-dimensional crystal morphology. While the
most common example is LCO, LiNiO

and LiMnO

are also well known. Although the

latter two compounds have proved to be unsuitable for use as LIB cathode in their simple
form, performance improvements have been achieved by combining them with other
elements to form complex oxides such as NMC, LiNi

0.8

0.2

, and LiNi

0.5

. Some

of these new materials have already been commercialized. Furthermore, solid solution
materials indicated by the general formula Li

MnO

–LiMO

(M being a transition metal

such as Ni or Fe) are recently being researched, notably Li

1.2

0.4

3.2.3.

Spinel Structure Materials (three dimensional)

LMO is the most significant compound in this category which enables Li ions to diffuse in
all three dimensions. Although spinels provide lower discharge capacity than layered rock
salt materials, they do enjoy advantages in terms of lower cost and high stability and are
therefore gaining attention in emerging medium- and large-scale LIB applications.

3.2.4.

Olivine Structure Materials (one dimensional)

LFP is the most well known of the olivines, which restrict Li ion diffusion to a single linear
dimension. Although low ion mobility poses an inherent performance disadvantage, this
has been minimized through the development of nanoparticles and other techniques (see

Section 3.3.5

). Discharge voltage for olivines is relatively low at around 3.5 V, and there is

little scope of increasing the energy density. Nevertheless, there has been some
commercialization of olivine cathode materials due to the outstanding stability they offer.

3.3.

Latest Research on Cathode Materials

3.3.1.

Layered LCO Series (two dimensional)

Although LCO has been studied for many years, there is still scope to improve its per-
formance as LIB cathode by employing new synthesis techniques. In one notable
example, Yamaki et al. report the synthesis of nanoparticles of overlithiated LCO which
provide significantly improved electrochemical properties

[8]

. They employed a proce-

dure in which lithium acetate and cobalt acetate are mixed in solution, dried, and then
calcined at 600

C for 6 h, in contrast to the general method of obtaining bulk LCO

particles by calcining a combination of Co

and Li

at around 900

C, which pro-

duces material with a primary particle diameter of several micrometers. The new method
produced spherical nanoparticles of LCO with a primary particle diameter of 5–25 nm,
and the material contained 9–21 times more Li than conventional bulk LCO particles.
Notably, spherical nanoparticles with a primary particle diameter of about 25 nm were
obtained when the Li content was increased by 8–12 times, while rod-shaped particles of
5 nm diameter and 60 nm length were obtained when the Li content was increased by
21 times. Cathode using the rod-shaped particles provided the greatest improvement in

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

maintaining discharge capacity under high rate conditions and is considered to be
particularly suitable for HEV applications.

3.3.2.

Layered LiNiO

Series (two dimensional)

LiNiO

has been a candidate cathode material for some time. Its low cost and high

discharge capacity of 200 mAh/g or more, which is 40% higher than that of LCO, make it
attractive. However, it is associated with a number of shortcomings, such as outgassing
when stored at high temperature and decreased thermal stability in the charged state. A
significant advancement has been reported by researchers at Panasonic, who developed a
LiNiO

-based material which appears to be practical for use as LIB cathode

[9]

. Although

the technical details are not disclosed, it is presumed that they stabilized the material by
adding cobalt or aluminum. In addition, they report coating the cathode surface with a
“heat-resistance layer” to improve thermal stability. Using their LiNiO

-based cathode

material, they achieved 3.1 Ah capacity in a cylindrical 18650 cell, with energy density of
660 Wh/l and 248 Wh/kg. This outstanding energy density would make such a material
suitable for BEV applications.

3.3.3.

Layered Mn Compound Series (two dimensional)

Layered LiMnO

shows poor discharge performance and therefore has not been a prac-

tical cathode material. However, many researchers have found that performance can be
improved by adding other elements to form more complex compounds, with
LiNi

0.5

being a notable candidate. This material, however, still has poor discharge

performance due to low conductivity, so research has broadly turned to the ternary NMC
system. Research by Ohzuku et al.

[10,11]

revealed that NMC provides relatively high

discharge capacity of 150 mAh/g together with well-balanced battery characteristics,
giving it great potential as an increasingly important cathode material. However, it is to be
noted that the characteristics of this material are highly dependent on the method of
synthesis.

Conventional cathode materials such as LCO with a single transition metal element

are prepared by a simple solid-phase method, in which cobalt acetate and lithium
carbonate are calcined at around 900

C, and consistent product is easily obtained. For

materials with multiple transition metal elements like LMO, on the other hand, it is
crucial to obtain product composition that is stoichiometrically precise and to obtain the
intended crystal structure. Even slight variations can result in widely divergent charac-
teristics when used as cathode material. Strict control of preparation conditions is thus
essential for this class of materials.

Idemoto et al.

[12]

investigated the effect of LMO preparation method on cathode

performance. They prepared material using both the solid-phase method and the solution
method. In the solution method, Ni, Mn, Co, and Li salts are mixed and dried, and then the
mixture is calcined. The researchers found that material prepared with the solution
method showed stable characteristics, while that prepared with the solid-phase method

Chapter 1 • Development of the Lithium-Ion Battery 9

was affected by cooling conditions after calcination. Since the solid-phase method is
generally used for commercial production, careful control of process conditions is
essential for the stable supply of LMO as a practical cathode material.

Another area of research for cathode materials in the Mn group are solid-solution

materials described by the general formula Li

MnO

–LiMO

(see

Section 3.2.2

Several such materials have been reported, including Li(Li

x/3

< x < 1)

by Numata et al.

[13]

and Li[Cr

(1/3

x/3)

(2/3 2

x/3)

< x < 1) by Lu and Dahn

[14,15]

. With very high discharge capacity, they have great potential for use in

future LIBs.

The mechanism by which these materials achieve such high discharge capacity is not

yet completely clear, and many aspects of these materials are the subject of active
research. There are notable reports focusing on their first-charge electrochemical
behavior

[16]

, mechanisms of deterioration

[17]

, mechanisms of high durability

[18]

mechanisms of high capacity

[19]

, mechanisms of ion transport

[20]

, and the relation-

ships between performance and the crystal structure

[21]

3.3.4.

Spinel Structure Cathode Materials (three dimensional)

LMO is the most well-established cathode material of this group. The spinel structure is
generalized as AB

, with MgAl

being a typical example. In the case of LMO, there are

Li ion tunnels intersecting three-dimensionally through manganese oxide skeleton. The
pioneering research on this material was performed by Thackeray et al.

[22,23]

. LMO is

known to have some shortcomings, including relatively low discharge capacity and the
elution of manganese during charging and discharging as well as during storage at high
temperature. It has been shown, however, that manganese elution can be suppressed by
doping at the Mn site with elements such as Al, Cr, Ti, and Ni, or by increasing the ratio of
Li to Mn

[24]

. Although its discharge capacity is somewhat lower than layered rock salt

cathodes, reasonable cost and high safety characteristics make LMO an attractive cathode
material for medium- and large-scale LIB applications.

3.3.5.

Olivine Structure Cathode Materials (one dimensional)

First reported by a team led by Goodenough

[25]

, LFP is the most well-known olivine

cathode material. Because of the one-dimensional crystal morphology, mobility of Li
ions is limited. The resulting low ion diffusion rate and low ionic conductivity have
made this material difficult to commercialize as cathode. In the early 2000s, A123
Systems, Inc., succeeded in overcoming these hurdles by producing the material in
nanoparticle form, coating the cathode surface with a carbon layer, and doping the
material with a different element such as niobium, and the material has been used in
LIBs for power tools and electric vehicles (EVs). However, the low cell voltage with LFP
prevents improvement in energy density, which limits its appeal in medium- and large-
scale applications. Other olivines such as LiMnPO

and LiCoPO

, that provide 4.1 V and

4.8 V, respectively, are therefore gaining attention.

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

Development of Anode Materials

4.1.

History of Anode Material Development

In 1995, the major anode materials were graphite and hard carbon. About half of the 450 t
of anode material shipped at the time was graphite, either mesophase graphite or syn-
thetic graphite. The former was more popular despite its high price because it more easily
enabled stable battery performance characteristics to be obtained. By 2010, shipments of
anode material grew to 27,000 t, nearly all being some form of graphite, as shown in

Figure 1.5

. The reason graphite gained such overwhelming dominance is the superior

discharge profile of graphite compared to hard carbon.

Figure 1.6

shows the discharge profile for LIB anode made of graphite, and the curve is

characterized by a very broad, flat range. For hard carbon, on the other hand, the
discharge profile is characterized by a steadily declining curve across most of the charge
range, as shown in

Figure 1.7

The rapid spread of mobile phones was the main driver of LIB demand during this

period, and a flat discharge profile is preferable for mobile phone applications. Graphite
thus became the dominant anode material, with many variations developed to achieve
lower cost and increased capacity. Among the various types of graphite, modified natural
graphite has become the most common.

Natural graphite is the most inexpensive graphite material available, but its high

reactivity to electrolyte prevents its use as anode without modification. Technology to
coat the graphite surface with thin carbon layer has become widely used, enabling
modified natural graphite to replace mesophase graphite as the leading anode material. A
more recent development in the anode market is the resurgence of hard carbon. Having
once been all but eliminated as an anode material, hard carbon is now making a
comeback as its discharge profile has been found to be suitable for HEV applications.

FIGURE 1.5 Major materials in the LIB anode market.

Chapter 1 • Development of the Lithium-Ion Battery 11

4.2.

Recent Research on Anode Materials

As there is little scope to further increase the capacity of graphite anode, research has
turned to other new materials including metal oxides such as Co

, CoO, CuO, and FeO,

and Li metal alloys such as Cu-Sn-Li, Cu-Sb-Li, In-Sn-Li, Si-Li, and Si-C-Li. Li metal alloys
provide much higher capacity than graphite as shown in

Figure 1.8

, but a serious

drawback is the large expansion and contraction of volume which occurs during the
charge–discharge process, as shown in

Figure 1.9

. This problem can be diminished by

forming the material in nanoparticles or using it as a composite with carbon, and some
such materials have begun to be adopted in practical LIBs.

2.5

3.5

4.5

100

Discharge (V)

Depth of discharge (%)

FIGURE 1.6 Discharge curve of the graphite anode.

2.5

3.5

4.5

100

Discharge (V)

Depth of discharge (%)

FIGURE 1.7 Discharge curve of the hard carbon anode.

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

Development of Electrolyte Solutions

5.1.

History of Electrolyte Solution Development

The electrolyte for LIBs is a mixture of organic solvents and an electrolyte salt compound.
The common solvents are a mixture of cyclic carbonate esters, such as ethylene carbonate
and propylene carbonate, and linear carbonate esters, such as dimethyl carbonate and
diethyl carbonate. The solution is completed with the addition of a salt compound such
as LiPF

or LiBF

. Electrolyte solutions must enable the Li ions to transport freely, which

requires both high dielectric constant and low viscosity. Cyclic carbonate esters have a
high dielectric constant but high viscosity, while linear carbonate esters have low vis-
cosity but low dielectric constant. Suitable electrolyte solutions are therefore obtained by
mixing the two.

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

500

1000

1500

2000

2500

3000

3500

4000

Capacity (mAh/g) (including Li)

Capacity (mAh/cc)

(

inc

luding

Metal

LiAl

Graphite

FIGURE 1.8 Theoretical discharge capacity of metal alloy anodes.

100

150

200

250

C6Li

LiAl

Li21Si5

Li3Sb

Li3As

Li21Sn5 Li22Pb5

Volume expansion–contraction /%

LiAl

P b

FIGURE 1.9 Volume expansion

–contraction of metal alloy anodes at charge and discharge.

Chapter 1 • Development of the Lithium-Ion Battery 13

For electrolyte salt, both LiPF

and LiBF

were widely used in 1995, but LiPF

has come

to dominate the market as it expanded from 300 to 3700 t, as shown in

Figure 1.10

5.2.

Recent Research on Electrolyte Solutions

Research on the electrolyte solution is generally focused on one of three areas: functional
electrolyte additives, flame-resistant or nonflammable electrolyte solutions, and new
electrolyte salts.

Functional electrolyte additives are included in the electrolyte solution to improve

battery performance. This concept has been around for some time, and the basic tech-
nology is well established. One early example is the addition of propane sultone to the
nonaqueous electrolyte solution of a rechargeable battery using a metallic lithium anode.
Although this technology was initially developed for metallic lithium batteries, the use of
such additives for LIBs began around 1994. Since then a wide range of additives have been
developed. So many different compounds have been used as additives that they are too
numerous to mention, but notable examples include vinylene carbonate, propane
sultone, phenylcyclohexane, and fluoroethylene carbonate. The selection of additives and
determination of their appropriate formulations have become a key aspect of the pro-
prietary know-how of each battery manufacturer, and the search for new additives
continues apace.

The second area of research is flame-resistant or nonflammable electrolyte solutions.

The main approach is to employ phosphate compounds in some way, either by using
cyclic phosphoric acid ester as solvent or by adding a phosphazene compound as flame
retardant. The next most common approach is to use halogen compounds, especially
fluorine compounds such as fluorocarbon ester and fluorinated ether, as solvent. There is
also the concept of a new safety mechanism whereby a flame retardant is encased in
microcapsules to be released in case of battery malfunction.

The third area of research is new electrolyte salts to replace LiPF

, but there remain

many challenges in terms of performance and cost.

Figure 1.11

shows some of the newly

developed salts which are strong candidates for use in next-generation LIBs. Notable
examples include sulfonyl amides such as lithium bis(trifluoromethylsulfonyl) amide

LiPF

LiBF

LiPF

Others

3700

2010

300

1995

FIGURE 1.10 Major materials in the LIB electrolyte salt market.

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

(LiTFSA) and lithium bis(pentafluoroethylsulfonyl) amide (LiBETA), lithium fluoroalkyl-
fluorophosphate, lithium bis(oxalate) borate, and lithium perfluorinated boric acid salt
cluster. These compounds are currently under evaluation for commercialization. Lithium
bis(oxalate) borate in particular is considered promising as it is a low-cost material con-
taining no fluorine and made from abundantly available boric acid and oxalic acid.

Separator Technology

6.1.

Separator Production Methods and Characteristics

The LIB separator is a thin microporous membrane made of polyolefin. It is placed
between the anode and cathode to prevent contact between them while enabling Li ions
to pass through. There are three basic categories of separator based on their production
methods, and each displays different morphologies and characteristics which are suitable
for different battery applications. Scanning electron micrographs of typical separators
produced by each of the three methods are shown in

Figure 1.12

6.1.1.

Dry-process One-component System

With this system, the separator is produced extruding molten polymer as a thin film and
then forming pores around spherulites by stretching the film while it cools. It is a “dry”
process in that no solvent is used, and it is a “one-component” system in that only the
polymer material of the membrane itself is used. This system enables separator to be
produced at relatively low cost because of its simplicity, requiring no additional pro-
cessing. However, it permits only limited control of pore size and pore structure, and

(

Lithiu

( 4

(CF

)

um bis(triflu

Lithium flu

ithium perflu

12, Z=H

–

uoromethylsu

uoroalkylfluo

uorinated bo

12–x

H, Cl, Br )

ulfonyl) ami

orophosphat

oric acid salt

de Lithi

t cluster

(CF

ium bis(pent

Lithium

)

tafluoroethyl

m bis(oxalate

–

lsulfonyl) am

e) borate

mide

FIGURE 1.11 New electrolyte salt materials under development.

Chapter 1 • Development of the Lithium-Ion Battery 15

therefore control of physical characteristics of the separator is limited. Notably, it is
difficult to endow the separator with the safety shutdown function when this production
system is used.

6.1.2.

Wet-process Two-component System

With this system, plasticizer is mixed into the polymer before extrusion. Phase separation
between the plasticizer and the polymer occurs in microscopic regions within the molten
bulk during cooling after extrusion, and the pores are formed by removing the plasticizer.
This process is called “wet” because solvent is used to remove the plasticizer, and the
term “two-component” indicates that the polymer and plasticizer are present in the
extruded mass. This system enables control of pore size and pore structure by selecting
different polymer and plasticizer materials, which in turn allows the production of sep-
arators with a wide range of physical characteristics.

6.1.3.

Wet-process Three-component System

This is similar to the system described above except that particles of inorganic filler are
also mixed into the polymer before extrusion. The particles of filler are removed together
with the plasticizer. The advantage of this system is that it is able to form larger pores than
the other two systems, providing greater ion mobility.

6.1.4.

Shutdown Function

Many separators are designed to have the physical characteristics that endow them with a
shutdown function, a safety feature in which the polymer melts to close the micropores
and prevent ion transport between the electrodes in the case of abnormal heat generation
caused by short circuit or other reasons. This function prevents battery overheating and
therefore greatly improves battery safety.

The temperature at which the shutdown function engages is determined by the

melting point of the polymer of which the separator is composed.

Figure 1.13

shows the

(a)

(b)

(c)

(a)

(b)

(c)

FIGURE 1.12 Pore characteristics of microporous separator membrane. (a) Dry-process one-component system,
(b) wet-process two-component system and (c) wet-process three-component system.

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

degree of impedance to ion transport at different temperatures for polyethylene and
polypropylene separators. A rise in impedance at high temperature illustrates that the
pores close upon melting and the shutdown function is engaging. The temperature at
which the shutdown function engages thus depends on the melting point of the polymer.
For polyethylene separators, the impedance rises sharply between 130

C and 140

which means that the battery temperature will not exceed this range even in the case of a
short circuit. For the polypropylene separator, the melting point is 170

C, so the shut-

down function does not engage until the battery temperature approaches this
temperature.

It is worth noting that in this graph you can see that one of the polyethylene separators

maintains high impedance even when temperature continues to increase, while the
impedance for the other one falls off quickly after an initial spike, indicating a rupture of
the membrane. The difference between these two different behavior characteristics de-
pends on a complex combination of factors such as pore size, pore structure, and mo-
lecular weight of the polymer.

6.2.

Recent Separator Developments

6.2.1.

New Materials

All commercial separators so far have been made of polyolefins, but they provide only
limited heat resistance. Research is now focusing on separators made of different ma-
terials which would offer superior heat resistance. These include heat-resistant rubber
such as silicone rubber and fluororubber, aromatic polyamide resin, liquid crystalline
polyester resin, heat-resistant resin containing polyoxyalkylene, and resin with cross-
linked groups. Separators made of such materials are expected to demonstrate not only
high temperature stability and safety but also superior ion transportation for better rate
capability at high current discharge.

1.E − 01

1.E + 00

1.E + 01

1.E + 02

1.E + 03

1.E + 04

1.E + 05

100

110

120

130

140

150

160

170

180

Temperature (°C)

Impedance of separator

(Ohm cm

)

(b)

(a)

(c)

FIGURE 1.13 Shutdown temperature for polyethylene separators. Impedance (1 kHz AC) change of electrolyte-
penetrated separators at elevated temperature. (a, b) Polyethylene and (c) polypropylene.

Chapter 1 • Development of the Lithium-Ion Battery 17

6.2.2.

Inorganic Coating

One drawback of polyolefin separators is the possibility of membrane rupture when
battery temperature continues to rise after the shutdown function is engaged (see

Section 6.1.4

). One approach to prevent this is to coat the membrane surface with a

heat-resistant inorganic layer. In addition to inorganic materials such as alumina, silica,
titania, and magnesia, candidates include vitreous materials, antioxidant ceramic
particles, clay minerals, metal salt compounds, and tabular fillers. This technique uses a
heat-resistant resin as a binder to hold the layer onto the surface of the separator.
Aromatic polyamide resin, polyimide resin, liquid crystalline polyester, and aromatic
polyether are used as binder. In addition to providing enhanced safety by preventing
rupture of the separator at high temperature and during overcharging, the addition of an
inorganic layer with antioxidant properties has been found to improve stability on the
side that contacts the cathode. Such coated separators have begun to find limited
commercial use in high-power LIBs employing advanced cathode materials, and they
are expected to become more prominent as the development of such LIBs continues,
but the inevitable added cost entailed by the coating process will pose challenges to
widespread adoption.

6.2.3.

Separators Containing Inorganic Material

Another approach to obtaining greater heat resistance is mixing inorganic material
into the bulk of the separator. This has the additional advantage of increasing ion
permeability. Inorganic materials for this purpose need to have antioxidant character-
istics and resistance to the electrolyte solution. Alumina, silica, and titania are the
leading candidates, although inorganic materials that absorb heat through dehydration
reaction are also attracting attention. This technology is applicable not only for poly-
olefin separators but also for those made from heat-resistant resin as described in

Section 6.2.1

6.2.4.

Nonwoven Separators

Nonwoven fabrics have been considered as an alternative type of separator due to
their low cost and high ion permeability. Nonwovens under consideration include
those made from liquid crystalline polyester, aromatic polyamide, and cellulose due
to their heat-resistance characteristics. However, it is not yet possible to obtain
nonwovens that are thin enough, and the pore sizes are too large for sufficient
electrical insulation. One approach to reduce pore size is to add a porous inorganic
layer to close the larger gaps in the fabric. Materials studied for this purpose
include alumina, silica, and titania, and it is possible to improve insulation charac-
teristics in this way. Evonik Degussa is the pioneer of this technique. Another way to
reduce pore size while also reducing thickness is to form the nonwoven with ultrafine
fibers and special spinning technologies such as flash spinning and electrospinning.

LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

6.2.5.

Laminated Separators

By laminating conventional polyethylene and polypropylene microporous membranes
together, it is possible to obtain a separator with the desired shutdown function together
with protection from rupture. Microporous membrane of liquid crystalline polyester,
polyphenylene ether, aromatic polyamide, polyimide, polyamide imide resin, acrylic
resin, and cross-linked polymer are now being studied as candidates for lamination with
polyethylene in order to gain even greater heat resistance.

Conclusion

Since its commercialization, the LIB has facilitated a remarkable advance in portable
electronics and broadened the accessibility to IT throughout society. The LIB is now
used in practically every field of consumer electronics. In accordance with market
needs, the development of each of the four major LIB components has historically
emphasized characteristics that were suitable to portable electronics applications. As
medium- and large-scale LIB applications begin to emerge, both for electric vehicles
and for stationary power storage, the required characteristics are evolving. The di-
rection of development of each LIB component is now turning toward the new re-
quirements. Emerging new configurations of the LIB are now expected to facilitate a
revolution in these medium- and large-scale applications, just as in portable
electronics.

References

[1] A. Yoshino, K. Sanechika, T. Nakajima, U.S. Patent 4,668,595 (1985).

[2] J.B. Goodenough, K. Mizushima, U.S. Patents 4,302,518 and 4,357,215 (1979).

[3]

K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res. Bull. 15 (1980) 783

[4]

R. Yazami, P. Touzain, J. Power Sources 9 (1983) 365

[5] A. Yoshino, K. Sanechika, Japanese Patent 2128922 (1984).

[6] A. Yoshino, K. Nakanishi, A. Ono, Japanese Patent 2642206 (1989).

[7] A. Yoshino, K. Inoue, Japanese Patent 3035677 (1991).

[8]

J. Yamaki, T. Doi, S. Okada, Funsai 52 (2009) 13