Principles and Applications of Lithium Secondary Batteries E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 504

Veröffentlichungsjahr: 2012

Beschreibung

Lithium secondary batteries have been key to mobile electronics since 1990. Large-format batteries typically for electric vehicles and energy storage systems are attracting much attention due to current energy and environmental issues. Lithium batteries are expected to play a central role in boosting green technologies. Therefore, a large number of scientists and engineers are carrying out research and development on lithium secondary batteries. The book is written in a straightforward fashion suitable for undergraduate and graduate students, as well as scientists, and engineers starting out in the field. The chapters in this book have been thoroughly edited by a collective of experts to achieve a cohesive book with a consistent style, level, and philosophy. They cover a wide range of topics, including principles and technologies of key materials such as the cathode, anode, electrolyte, and separator. Battery technologies such as design, manufacturing processes, and evaluation methods as well as applications are addressed. In addition, analytical methods for determining electrochemical and other properties of batteries are also included. Hence, this book is a must-have for everyone interested in obtaining all the basic information on lithium secondary batteries.

Leseprobe

Contents

Cover

Related Titles

Title Page

Copyright

List of Contributors

Preface

Chapter 1: Introduction

1.1 History of Batteries

1.2 Development of Cell Technology

1.3 Overview of Lithium Secondary Batteries

1.4 Future of Lithium Secondary Batteries

Chapter 2: The Basic of Battery Chemistry

2.1 Components of Batteries

2.2 Voltage and Current of Batteries

2.3 Battery Characteristics

Chapter 3: Materials for Lithium Secondary Batteries

3.1 Cathode Materials

3.2 Anode Materials

3.3 Electrolytes

3.4 Interfacial Reactions and Characteristics

Chapter 4: Electrochemical and Material Property Analysis

4.1 Electrochemical Analysis

4.2 Material Property Analysis

Chapter 5: Battery Design and Manufacturing

5.1 Battery Design

5.2 Battery Manufacturing Process

Chapter 6: Battery Performance Evaluation

6.1 Charge and Discharge Curves of Cells

6.2 Cycle Life of Batteries

6.3 Battery Capacity

6.4 Discharge Characteristics by Discharge Rate

6.5 Temperature Characteristics

6.6 Energy and Power Density (Gravimetric/Volumetric)

6.7 Applications

Index

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Print ISBN: 978-3-527-33151-2

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List of Contributors

Chil-Hoon Doh

Korea Electrotechnology Research Institute (KERI)

Battery Piezoelectric Research Center 12 Bulmosan-ro 10beon-gil, Seongsan-gu Changwon-si, Gyeongsangnam-do, 642-120 Republic of Korea

Kyoo-Seung Han

Chungnam National Univ.

Department of Fine Chemical Engineering and Applied Chemistry 99 Daehak-ro, Yuseong-gu Daejeon, 305-764 Republic of Korea

Young-Sik Hong

Seoul National Univ. of Education

Department of Science Education 96 Seochojungang-ro, Seocho-gu Seoul, 137-742 Republic of Korea

Kisuk Kang

Seoul National Univ.

Department of Materials Science and Eng. 1 Gwanak-ro, Gwanak-gu Seoul, 151-742 Republic of Korea

Dong-Won Kim

Hanyang Univ.

Department of Chemical Engineering 222 Wangsimni-ro, Seongdong-gu Seoul, 133-791 Republic of Korea

Jae Kook Kim

Chonnam National Univ.

Department of Materials Science and Eng. 77 Yongbong-ro, Buk-gu Gwangju, 500-757 Republic of Korea

Sung-Soo Kim

Chungnam National Univ.

Graduate School of Green Energy Tech. 99 Deahak-ro, Yuseong-gu Daejeon, 305-764 Republic of Korea

Chang Woo Lee

Kyunghee Univ.

Department of Chemical Engineering 26 Kyunghee-daero, Dongdaemun-gu Seoul, 130-701 Republic of Korea

Sung-Man Lee

Kangwon National Univ.

Department of Advanced Materials Science and Eng. 1 Kangwondaehak-gil, Chuncheon-si Gangwon-do, 200-701 Republic of Korea

Sang-Young Lee

Kangwon National Univ.

Department of Chemical Engineering 1 Kangwondaehak-gil, Chuncheon-si Gangwon-do, 200-701 Republic of Korea

Young-Gi Lee

Electronics and Telecommunications Research Institute (ETRI)

Power Control Device Research Team 218 Gajeong-ro, Yuseong-gu Daejeon, 305-700 Republic of Korea

Yong Min Lee

Hanbat National Univ.

Department of Chemical and Biological Eng. 125 Dongseo-daero, Yuseong-gu Daejeon, 305-719 Republic of Korea

Hong-Kyu Park

LG Chem

Battery Research Institute 104-1 Munji-dong, Yuseong-gu Daejeon, 305-380 Republic of Korea

Jung-Ki Park

Korea Advanced Institute of Science and Technology (KAIST)

Department of Chemical and Biomolecular Eng. 291 Deahak-ro, Yuseong-gu Daejeon, 305-701 Republic of Korea

Seung-Wan Song

Chungnam National Univ.

Department of Fine Chemical Engineering and Applied Chemistry 99 Deahak-ro, Yuseong-gu Daejeon, 305-764 Republic of Korea

Preface

With lithium secondary batteries being considered a key energy storage system, there has been growing expectations for revolutionary technological developments in lithium secondary batteries. However, it is not easy to come across a systematic and logical textbook on the principles and applications of lithium secondary batteries and I felt the pressing need for a comprehensive textbook on lithium secondary batteries for a wide range of readers. I partnered with other experts who shared the same vision and began working on this book. Our writers are researchers of lithium secondary batteries from universities, research centers, and industries.

Before writing, we agreed on a writing philosophy that extends beyond facts to provide straightforward fundamental explanations throughout all the chapters in a single, uniform pattern. In this respect, I myself, as a representing author, reviewed all the content and remodeled all the materials of the writers on the same philosophy. I am afraid our goal has not been fully attained but it is suffice to say that we have worked to the best of our abilities.

Many discussions were held concerning the structure and organization of this book. The introduction is followed by the basics of electrochemical reactions occurring within the battery. Next, we present the structure and properties of key components such as the anode, cathode, and electrolyte, as well as their interfacial reactions. In addition, we cover techniques used in the analysis of electrochemical and physical properties related to batteries. The book also includes sections on battery design, manufacturing, and performance evaluation.

This book was first conceptualized in early 2006 and took almost 4 years to complete. It has been a long journey with tens of meetings between all of our writers. We managed to get through this difficult process through mutual encouragement and in the hope of making significant contributions in relevant fields. We usually ordered packed lunches for these meetings, and I think many of our participating writers will look back with fond memories.

Particularly, I would like to express my appreciation to all who have made this book possible. I would like to thank Professor Yong-Mook Kang who spared no effort in reviewing our sections on cathodes, Professor Hochun Lee for his faithful review of electrochemical analysis techniques, Professor Doo-Kyung Yang for preparing the draft on NMR analysis, and Professor Nam-Soon Choi for her dedication and review of electrolytes and interfacial reactions. I appreciate the support of Miss Myung-su Lee, who was willing to facilitate our countless weekend meetings. Last but not least, it is the readers we are most thankful to.

From my laboratory at KAIST,

Jung-Ki Park, Representing Author

Chapter 1

Introduction

With the proliferation of mobile telecommunication devices arising from remarkable developments in information technology (IT), the twenty-first century is moving toward a ubiquitous society, where high-quality information services are available regardless of time and place. The establishment of a ubiquitous society can be traced back to lithium secondary batteries, which were first commercialized in the early 1990s. Compared to other secondary batteries, lithium secondary batteries not only have higher working voltage and energy density, but also have long service life. Such superior characteristics enable secondary lithium batteries to fulfill complex requirements for diversified growth in devices. Global efforts are underway to further develop the existing technology of lithium secondary batteries and expand their use from eco-friendly transportation to various fields, such as power storage, health care, and defense.

A fundamental and systematic understanding of lithium secondary batteries is essential for the continuous development of related technologies along with technological innovation.

1.1 History of Batteries

A battery can be defined as a system that uses electrochemical reaction to directly convert the chemical energy of an electrode material into electric energy. The battery was first described in an 1800 study by Volta, an Italian professor at the University of Pavia, and published by the Royal Society of London. In 1786, Galvani of Italy discovered that touching a frog's leg with a metal object caused muscular convulsions. He claimed that “animal electricity” was generated from within the frog and transported through its muscles. Volta, who doubted the credibility of animal electricity, confirmed that the animal's body fluid merely served as an electrolyte between two different metals. In 1800, Volta invented the voltaic pile, in which an electric current is produced by connecting the two ends of a stack of two metal disks separated by cloth soaked in an alkaline solution. This was the first form of the battery as we know it today [1].

A 2000-year-old clay jar, believed to be the earliest specimen of a battery, was discovered at a historic site near Baghdad in 1932 (Figure 1.1b). This clay jar had a height of 15 cm and contained a copper cylinder that was held in place by copper and iron rods. The rods had been corroded by acid. Although the artifact is accepted by some scholars as a primitive cell, it is uncertain whether it was indeed used for such a purpose.

Figure 1.1 The voltaic pile (a) and the Baghdad battery (b).

Batteries can be classified into primary batteries, which are used once and disposed, and secondary or rechargeable batteries, which can be recharged and used multiple times. Since the invention of the voltaic pile, various batteries have been developed and commercialized.

The first widely used primary battery was the Leclanché (or manganese) cell invented in 1865 by Leclanché, a French engineer. The Leclanché cell, containing a zinc anode, a manganese dioxide (MnO2) cathode, and an acidic aqueous electrolyte of ammonium chloride (NH4Cl) and zinc chloride (ZnCl2), had a wide range of applications with an electromotive force of 1.5 V. Later, the aqueous electrolyte in the Leclanché cell was replaced with an alkaline electrolyte of potassium hydroxide (KOH). This became the alkaline battery, which enhanced capacity and discharge, with the same voltage. New types of primary batteries later emerged, such as zinc–air batteries (1.4 V) and silver oxide batteries (1.5 V). The performance of primary batteries was greatly improved in the 1970s when 3 V lithium primary batteries with lithium as an anode became commercialized.

The oldest type of secondary batteries is lead–acid battery, invented by French physicist Planté in 1859. Lead–acid batteries have a lead peroxide anode, a lead cathode, and weak sulfuric acid as an electrolyte. With an electromotive force of 2 V per cell, they are commonly used as storage batteries in motor vehicles. When NiCd (1.2 V) batteries became widespread in 1984, they began to replace primary batteries in small electric appliances [2]. However, owing to the harmful environmental effects of cadmium, NiCd batteries are not as widely used today.

In the early 1990s, NiMH (1.2 V) cells were favored over NiCd batteries for their eco-friendliness and enhanced performance. This was followed by the emergence of 3 V lithium secondary batteries with greatly improved energy density. Compact and lightweight lithium secondary batteries soon dominated the market for portable devices, including cell phones, laptops, and camcorders [3].

1.2 Development of Cell Technology

After the invention of the voltaic pile in 1800, two significant milestones were reached in the 200-year history of cell technology. One was the development of primary batteries into secondary batteries and the other was the advancement to a working voltage of 3 V. Lithium secondary batteries, which use lithium ions as the main charge carrier, can maintain a high average discharge voltage of 3.7 V despite being lightweight. With the highest energy density among all currently available batteries, lithium secondary batteries have led the revolution of cell technology.

Looking at the changes in energy density with developments in secondary cell technology, lead–acid batteries have a specific energy of 30 Wh/kg and energy density of 100 Wh/l, whereas the energy density of lithium secondary batteries has shown an annual increase of 10%. At present, cylindrical lithium secondary batteries have a specific energy of 200 Wh/kg and energy density of 600 Wh/l (Figure 1.2). The specific energy of lithium secondary batteries is five times that of lead–acid batteries and three times higher than that of NiMH cells [4].

Figure 1.2 Changes in energy density with developments in cell technology.

NiMH cells, a type of secondary batteries, have limited working voltage and energy density, but are attractive in terms of application to hybrid electric vehicles (HEVs) owing to their high stability. Recently, the advent of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) has brought greater attention to lithium secondary batteries, which have higher energy and output compared to NiMH cells. Because secondary batteries in electric vehicles should offer fast charging time, lightweight, and high performance, future technology developments surrounding lithium secondary batteries are likely to be highly competitive. We can expect revolutionary and continuous technological progress that will overcome current limitations.

1.3 Overview of Lithium Secondary Batteries

For a cell to be characterized as a secondary battery, the anode and cathode have to repeat charging and discharging. The electrode structure should be kept stable during the insertion and extraction of ions within electrodes, while an electrolyte acts as an ion transfer medium. The charging of a lithium secondary battery is illustrated in Figure 1.3.

Figure 1.3 Movement of Li+ in an electrolyte and insertion/extraction of Li+ within electrodes in a lithium secondary battery.

Charge neutrality occurs when ions flowing into electrodes collide with electrons entering through a conductor, thus forming a medium to store electric energy in the electrodes. Furthermore, the rate of reactions is increased as ions from the electrolyte are drawn to the electrodes. In other words, the overall reaction time of a cell heavily depends on the movement of ions between electrolyte and electrodes. The amount of ions inserted into electrodes for charge neutrality determines the electrical storage capacity. Ultimately, the types of ions and materials are main factors that influence the amount of electric energy to be stored. Cells based on lithium ions (Li+) are known as lithium secondary batteries.

Lithium, which is the lightest of all metals and has the lowest standard reduction potential, is able to generate a working voltage greater than 3 V. With a high specific energy and energy density, it is suitable for use as an anode material. Since the working voltage of lithium secondary batteries is greater than the decomposition voltage for water, organic electrolytes should be used instead of aqueous solutions. Materials that facilitate the insertion and extraction of Li+ ions are appropriate as electrodes.

Lithium secondary batteries use a transition metal oxide as an cathode and carbon as a anode. The electrolyte of lithium ion batteries (LIBs) is held in an organic solvent, while that of lithium ion polymer batteries (LIPBs) is a solid polymer composite.

As shown in Figure 1.4, commercialized lithium secondary batteries can be classified according to cell shape and component materials. The various forms of batteries include cylindrical laptop batteries, prismatic cells for portable devices, single-cell coin-shaped batteries, and pouch-shaped cells cased in aluminum plastic composites [4].

Figure 1.4 Different shapes of lithium secondary batteries: (a) cylindrical, (b) coin, (c) prismatic, (d) pouch [4]. Reprinted by Permission from Macmillan Publishers Ltd: [4], copyright 2001.

Table 1.1 shows the key components of a lithium secondary battery. Its materials can be described as follows. Because lithium is removed from the lattice structure and released as ions, stable transition metal oxides are used as cathodes. Anode materials should have a standard reduction potential similar to that of lithium so as to stabilize the released ions and provide a large electromotive force. The electrolyte consists of a lithium salt in an organic solvent, thus maintaining electrochemical and thermal stability within the range of the working voltage. In addition, separators made of polymers or ceramics have a high-temperature melt integrity, which prevents short circuits caused by electrical contact between the cathode and the anode.

Table 1.1 Characteristics and examples of key components in a lithium secondary battery.

1.4 Future of Lithium Secondary Batteries

To date, the development of lithium secondary batteries has focused on small electric appliances and portable IT devices. Lithium secondary batteries are expected to build upon these achievements and create new applications described by buzzwords, such as “green energy,” “wireless charging,” “self-development,” “recycle,” “from portable to wearable,” and “flexible.” It is an important task to make advance preparations for future batteries by considering the functions of these applications.

Among the future applications of lithium secondary batteries, medium- and large-sized cells show great promise. Energy storage systems, in the form of batteries for electric vehicles and robots, or high-performance lithium secondary batteries capable of storing alternative energy such as solar, wind, and marine energy, are viewed as a key component of next-generation smart grid technology.

Other types of future lithium secondary batteries are microcells and flexible batteries. Microcells can be applied to RFID/USN, MEMS/NEMS, and embedded medical devices, while flexible batteries are mainly used in wearable computers and flexible displays. The structural control and manufacturing process of these batteries will be very different from today's methods.

The development of all-solid-state lithium secondary batteries is highly anticipated as well. With massive recalls caused by frequent battery explosions, an important challenge that lies ahead is to resolve the instability problem of existing liquid electrolytes through application of electrolytes consisting of polymers or organic/inorganic composites, along with the development of suitable electrode materials and processes.

References

1. Vincent, C.A. and Scrosati, B. (1997) Modern Batteries: An Introduction to Electrochemical Power Sources, 2nd edn, Arnold, London.

2. Besenhard, J. (1998) Handbook of Battery Materials, Wiley-VCH Verlag GmbH.

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

4. Tarascon, J.M. and Armand, M. (2001) Nature, 414 (15), 359.

Chapter 2

The Basic of Battery Chemistry

Electrochemistry is the study of electron transfer caused by redox reactions at the interface of an electron conductor, such as a metal or a semiconductor, and an ionic conductor, such as an electrolyte. Technologies based on electrochemistry include batteries, semiconductors, etching, electrolysis, and plating. In this book, electrochemistry refers to the conversion of chemical energy into electric energy in various systems such as primary batteries, secondary batteries, and fuel cells. In particular, this chapter describes the electrochemical aspects of secondary batteries.

2.1 Components of Batteries

2.1.1 Electrochemical Cells and Batteries

An electrochemical cell is the smallest unit of a device that converts chemical energy to electric energy, or vice versa. In general, a battery has multiple electrochemical cells, but it may be used to refer to a single cell. An electrochemical cell consists of two different electrodes and an electrolyte. The two electrodes of different electric potential create a potential difference when immersed in the electrolyte. This potential difference is also known as electromotive force.

Electric potential, denoted by V, is the potential energy of a unit charge within an electric field, and electromotive force drives current in an electric circuit. Redox reactions occur at each electrode due to this force and the generated electrons pass through the external circuit. To maintain charge neutrality of the electrolyte, redox reactions continue at the electrodes until the cell reaches electrochemical equilibrium.

2.1.2 Battery Components and Electrodes

As explained above, a battery (or an electrochemical cell) is a device that converts chemical energy to electric energy, or vice versa, using redox reactions. Figure 2.1 shows the components of a cell, including the cathode, anode, electrolyte, and a separator to prevent short circuits between the electrodes.

Figure 2.1 A battery or electrochemical cell (discharge).

When electrochemical redox reactions occur at the electrodes, ions are shuttled between the anode and the cathode through the electrolyte. At the same time, electron transfer takes place between the two electrodes. These electrons travel through the external wire connecting the two electrodes, thus forming a closed circuit.

In a discharging battery, electrochemical oxidation (oxidation, A → A+ + e−) of the electrode proceeds at the negative terminal, which is termed an anode. Discharging is the process of converting chemical energy carried by the battery into electric energy. Electrons transferred from the negative terminal through the external circuit engage in reduction (reduction, B+ + e− → B) at the positive terminal, which is known as a cathode. The electrolyte serves as an ionic conductor between the two electrodes and should be distinguished from an electron conductor.

While redox reactions at the electrodes are irreversible for primary batteries, they are reversible and repeatable in secondary batteries. Here, “reversible” means that redox reactions are repeated within the same electrode. One advantage of secondary batteries over primary batteries is that they can be recharged repeatedly. In the case of secondary batteries, both oxidation and reduction reactions can occur at the same electrode. It means that a cathode during discharging can be an anode during charging. However, in the conventional point of view, the terms remain the same for both charging and discharging, with the oxidative electrode as the anode and the reductive electrode as the cathode during discharging, spontaneous electrochemical reaction.

2.1.3 Full Cells and Half Cells

Electrochemical cells, in the form of a full cell or a half cell, are often used to analyze the electrochemical properties of batteries. A full cell takes on the complete form of a battery, where electrochemical reactions occur at both the cathode and the anode, and allows direct measurement of battery characteristics and performance. With additional use of a reference electrode, the full cell is able to obtain the electric potential difference between the two electrodes through individual measurements at the cathode and anode. On the other hand, the counter electrode of a half cell is used as the reference electrode to facilitate measurement and analysis of activities at the working voltage. This is useful in understanding the basic properties of each electrode material. Experiments may be performed using a full cell or a half cell, depending on one's purpose.

2.1.4 Electrochemical Reaction and Electric Potential

The electrochemical reaction occurring during discharge is related to the amount of electric energy that the battery can deliver. Consider the following electrochemical reaction at a given electrode:

(2.1)

Here, p, q, r, and s are stoichiometric coefficients of A, B, C, and D, which are different chemical species. Gibbs free energy for the above equation is given by Eq. (2.2), where a is the activity.

(2.2)

The electric work (Wrev) at the equilibrium state is the maximum possible electric energy (Wmax). When the battery is undergoing a chemical reaction, this can be expressed using ΔG, the change in Gibbs free energy.

(2.3)

(2.4)

Meanwhile, electric energy is associated with the charge Q (unit coulomb, C) and electric potential (E) as follows:

(2.5)

Q can be represented as a product of the number of electrons within the cell and the elementary charge. The number of electrons, ne, is the number of moles multiplied by the Avogadro constant (NA, 6.023 × 1023). Q, in terms of moles and elementary charge, is written as follows:

(2.6)

(2.7)

Q can also be described by the following equation.

(2.8)

Here, F is the Faraday constant, which is the elementary charge per mole of electrons (96 485 C/mol). The movement of n moles of electrons due to the potential difference between the two electrodes results in the following expression:

(2.9)

(2.10)

This shows the relationship between the change in Gibbs free energy during equilibrium and electromotive force of the cell.

When all reactants and products are at a standard state, the standard potential is denoted by E°.

(2.11)

Equations (2.2) and (2.11) lead to the following Nernst equation, where the difference in electric potential is affected by the concentration of components involved in the electrochemical reaction:

(2.12)

2.2 Voltage and Current of Batteries

2.2.1 Voltage

Voltage is the electrical driving force and is equal to the electric potential difference between two points in an electric circuit. It is also known as electromotive force and measured in volts (V). Since the actual voltage of a cell is subject to various conditions such as temperature and pressure, a reference point is needed. This corresponds to the standard state (1 bar, 25 °C, and 1 mol/dm3) of an electrode. The standard electric potential, which is the measure of electric potential under equilibrium conditions, sets the basis for the electric potential at each electrode. The actual difference in electric potential between two electrodes can be expressed as follows:

(2.13)

Erxn is the potential difference arising from chemical reactions, while Eright and Eleft correspond to the electric potential at each electrode. For a galvanic cell, where redox reactions occur spontaneously, Erxn takes a positive value. For nonspontaneous redox reactions in an electrolytic cell, the value of Erxn is negative.

When the battery is in an equilibrium state with little or no current flow, it can provide electric energy equal to the amount of ΔG. Since current continues to flow in the battery during discharge, it is considered to be in a nonequilibrium state according to thermodynamics. The maximum possible energy cannot be used, because the voltage at this time is always less than the open circuit voltage (OCV). The open circuit voltage is the difference in the electric potential between two terminals of a device without an external load. The lower value of operating voltage compared to the open circuit voltage can be explained by ohmic polarization and by similar polarization effects from the movement of electric charge at the interface of the electrode/electrolyte. On the other hand, the voltage for the reverse charging reaction is higher than the open circuit voltage. This is due to internal resistance, overcharging caused by activation polarization, ionic conductivity lower than electron conductivity, impurities in electrode materials, and concentration polarization from varying diffusion speeds of lithium ions on the surface and inside the electrodes.

Figure 2.2 shows the voltage induced by periodic current pulses during the actual charging/discharging process. The voltage of the charging/discharging curve was measured by applying current slowly at the standard state. The open circuit voltage profile was recorded in the equilibrium state without permitting any additional flow of current. As demonstrated above, the difference in voltage between actual measurements during charging/discharging and the open circuit voltage can be interpreted as a result of polarization.

Figure 2.2 Charging/discharging voltage of a battery and OCV profile.

2.2.2 Current

Current, which is the rate of flow of electric charge, is closely related to the rate of electrochemical reactions at the electrodes. The rate of electrode reactions is determined by the transfer of electrons from the electrolyte to the electrodes and at the surface of the electrode active material.

At the electrodes, the reactants O and the product R undergo a reversible electrochemical reaction, as shown in Eq. (2.14). The relationship between current and the rate of reactions is given by the Nernst equations in Eqs. (2.15) and (2.16).

(2.14)

(2.15)

(2.16)

Here, υf and υb represent the speed of forward and backward reactions, respectively, while kf and kb are the respective rate constants. Co and CR are the concentration of oxidation and reduction substances, respectively, and Co(x, t) is the concentration as a function of time t and distance x from the electrode surface. Cathodic and anodic currents are denoted by ic and ia, respectively. The number of moles, Faraday constant, and electrode surface area are given as n, F, and A, respectively.

The net reaction rate is the difference in forward and backward reaction rates.

(2.17)

In other words, the current generated at the electrodes is highly dependent on the net reaction rate. When the forward and backward reactions proceed at the same rate during equilibrium, the net reaction rate υnet and the net current flow become 0.

2.2.3 Polarization

Polarization is a lack or excess of electrode potential at equilibrium. Since each battery component undergoes charge transfer at different rates, the slowest transfer becomes the rate-limiting process. When current flows between the two terminals of the battery, the actual potential E is always larger (charging) or smaller (discharging) than the equilibrium potential Eeq. Overpotential refers to this potential difference between the actual potential and the equilibrium potential. It is used as a measure of the extent of polarization. The relationship between actual potential (E), equilibrium potential (Eeq), and overpotential (η) can be expressed as follows:

(2.18)

As shown in Figure 2.3, polarization is classified into ohmic polarization (iR drop), activation polarization, and concentration polarization.

Figure 2.3 Effect of current density on the polarization.

Here, iR drop is associated with the electrolyte instead of resistance from electrode reactions. Considering that the iR drop increases proportionally to current density, a drastic decrease in working voltage at high current density conditions can be prevented by minimizing internal resistance.

On the other hand, activation polarization is closely related to electrode characteristics. As an inherent property of active materials, it is strongly influenced by temperature. Concentration polarization results from the concentration gradient of reactants at the surface of active materials. However, these different types of polarization are difficult to distinguish in an actual battery.

2.3 Battery Characteristics

2.3.1 Capacity

The capacity of a battery is the product of the total amount of charge, when completely discharged under given conditions, and time. The theoretical capacity CT is determined by the amount of active materials and is calculated as follows:

(2.19)

Here, F is the Faraday constant and x is the number of moles of electrons produced from the discharging process. The practical capacity, Cp, is smaller than the theoretical capacity because reactants are not 100% utilized in discharge. As the rate of charging/discharging increases, the practical capacity is further reduced due to the iR drop.

In general, the rate of charging/discharging is denoted by Crate. The battery capacity and current drawn from charging/discharging are related by the following equation:

(2.20)

Here, h is the time (in hours) taken to completely discharge (or charge) a battery, i is the current drawn (A), and Cp is the battery capacity (Ah). The reciprocal of h is given by Crate. In other words, as Crate increases, the battery requires less time to be charged or discharged. Battery capacity can be measured using gravimetric specific capacity (Ah/kg, mAh/g) or volumetric specific capacity (Ah/l, mAh/cm3).

2.3.2 Energy Density

Energy density, an important factor in determining battery performance, is the amount of energy stored per unit mass or volume. The maximum energy that can be obtained from 1 mol of reactant is given as follows:

(2.21)

Here, E is the electromotive force of the battery and εT is the theoretical energy (unit Wh, 1 Wh = 3600 J) for the cell reaction of 1 mol.

The actual energy εp, which varies according to the discharging method, from 1 mol of reactant is derived as follows:

(2.22)

As the rate of discharge or discharge current per unit time increases, the electric potential of the battery departs further from the equilibrium potential. Similarly to battery capacity, energy density is measured in Wh/kg, mWh/g or Wh/l, and mWh/cm3.

2.3.3 Power

The power of a battery refers to the energy that can be derived per unit time. The power P is the product of current i and electric potential E.

(2.23)

Electric power is a measure of the amount of current flowing at a given electric potential. When the current increases, the power rises to a peak and declines. The battery voltage drops when the current goes beyond a certain limit, thus leading to a decrease in power. This phenomenon of polarization is related to the diffusion of lithium ions and the internal resistance of the battery. To improve power, it is necessary to enhance the diffusion rate of lithium ions and the electrical conductivity. Similarly to battery capacity and energy, power per unit mass or volume is described as power density.

2.3.4 Cycle Life

Cycle life is the number of charge and discharge cycles that a battery can achieve before its capacity is depleted. A high-performance battery should be able to maintain its capacity even after numerous charge and discharge cycles. The cycle life of lithium secondary batteries strongly depends on the structural stability of electrode active materials during the charging/discharging process. Irreversible capacity, which is the amount of charge lost, is usually observed after the first charge/discharge cycle and results from the formation of a new layer at the interface of electrodes and electrolyte.

After N charge/discharge cycles, the capacity retention is given by CN/C1 (%) and the relative capacity decrease is (C1 − CN)/C1. The remaining capacity after N charge/discharge cycles and 1 cycle is CN and C1, respectively. The cycle life is generally affected by the depth of discharge, which varies according to battery type. Lithium secondary batteries tend to display a longer cycle life when charging is repeated at a low depth of discharge, such that capacity is not fully depleted.

2.3.5 Discharge Curves

Repeated cycles of charging and discharging affect the discharge characteristics of a battery, allowing the discharge curve to take various forms depending on discharge conditions, electrical properties, and other measurement variables. Constant current, constant power, and constant external resistance are normal discharge conditions. Electrical properties to be measured include the battery voltage, current, and power, while measurement variables are discharge time, capacity, and lithium ion occupancy. With the same battery consisting the same materials and cell design, various discharge curves can be produced according to measurement conditions. It is essential to compare these discharge curves to obtain a more accurate understanding of battery properties. An actual battery gives a wide range of discharge curves according to battery components. The typical discharge curves of a battery are shown in Figure 2.4.

Figure 2.4 Discharge curves obtained from electrochemical reactions.

Figure 2.4 is a plot of changes in battery voltage as a function of capacity when the battery is discharged under constant current. Since capacity is directly proportional to the time during which current is applied, Figure 2.4 implies the change in voltage with elapsed time. Furthermore, the battery voltage represents the open circuit voltage when the battery is not connected to an external load, or the working voltage when the circuit is closed. The battery voltage at the point at which discharge is complete is known as the cutoff voltage.

In the case of curve I in Figure 2.4, the battery voltage is hardly influenced by reactions occurring within the battery during discharge. Curve II shows two flat regions due to a change in the reaction mechanism. In curve III, the reactants, products, and internal resistance of the battery are continuously changing throughout the discharge process.

For lithium secondary batteries, the change in battery voltage after charging/discharging is given by the Armand equation.

(2.24)

In Eq. (2.24), y is the lithium ion occupancy and ky is the effect of interactions between intercalated lithium ions on the battery voltage. The change in the gradient of the battery voltage based on capacity is determined by direct factors such as the rate of lithium ion diffusion, phase transition, change in lattice structure, and dissolution, as well as additional factors such as the particle size of electrode active materials, temperature, electrolyte characteristics, and porosity of the separator. These factors may change the values of y and k in the Armand reaction.

Under low current density conditions, both voltage and discharge capacity of the working battery come close to the theoretical equilibrium. However, from the trends of in Figure 2.5, we can see that the battery voltage decreases during discharge since the iR drop and overvoltage from polarization increase with discharge current. The battery capacity also decreases when the battery is discharged beyond its cutoff voltage, owing to the rising gradient of discharge curves. These characteristics of the discharge voltage vary greatly with temperature.

Figure 2.5 Effect of current density on the battery voltage.

As shown in Figure 2.6, when the battery is discharged at a low temperature, the reduced chemical activity of the reactants leads to less internal resistance. This gives rise to a sharp decline in the battery voltage along with a decrease in capacity. At higher temperatures, there is a capacity increase caused by lower internal resistance and higher discharge voltage. However, if the temperature is too high, increased chemical activity may result in self-discharge and other unwanted chemical reactions.

Figure 2.6 Effect of temperature on the battery capacity.

Chapter 3

Materials for Lithium Secondary Batteries

3.1

Cathode Materials

3.1.1 Development History of Cathode Materials

Pioneering work on lithium batteries began in the 1910s under G. N. Lewis, and the first Li/(CF)n primary batteries were sold in the 1970s. The cathode material (CF) is an intercalation compound of fluorine, which has the highest electrical conductivity of all elements, and carbon. Attempts were made in the United States to develop Li/MnO batteries, but failed due to problems with moisture control, cell structure, and assembly technology. In 1973, Japan became the first nation to commercialize Li/MnO primary batteries, which gained recognition for their high working voltage of 3 V, obtained through the use of an organic solvent instead of an aqueous solution. These developments laid the foundation for the commercialization of lithium secondary batteries in Japan.