Effect of Radiation on the Morphology of Lithium-Ion Battery Cathodes

Effect of Radiation on the Morphology of Lithium-ion Battery Cathodes

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Dandan He, B.S.

Graduate Program in Nuclear Engineering

The Ohio State University

2014

Thesis Committee:

Lei Cao, Advisor

Tunc Aldemir

Mingzhai Sun

Dandan He

2014

Abstract

The Lithium ion (Li-ion) battery is widely used as power source for consumer electronic devices due to its high energy density, large specific capacity. Recently, application of the Li-ion battery has been extended to aerospace, in which the outer space’s radiation environment has more stringent requirements for the battery performance. This type of battery also provides power for critical modern rescue, sampling equipment in nuclear environment, such as the robots deployed in the aftermath of Fukushima nuclear accident. As one of the most important components of these emergency response robots, the stability of the Li-ion battery under radiation is of crucial importance. The radiation effects on materials are generally categorized into four types: ionization, atomic displacement, impurity production, and energy release. To our knowledge, there has been no definitive study of such effects on

Li-ion batteries and how ionizing radiation affects Li-ion batteries’ structure, strength, deformation, and electrical properties to final failure.

ii In this work, the surface morphology of the Li-ion battery cathode before and after neutron and gamma ray radiation were characterized by atomic force microscopy

(AFM). Distinct particle coarsening (size increase) of the cathode after irradiation was observed, which may primarily came from the crystal boundary migration driven by internal stress due to irradiation. There was a difference in the cathode particle size between thermal plus fast neutrons and fast neutrons alone and charge status of the

Li-ion battery was also found to affect the cathode particle size change under radiations. X-ray diffraction (XRD) patterns of the Li-ion battery cathodes showed that a crystal structure disordering occurred during irradiation process. Electrical test was then carried and a substantial capacity loss of the battery after gamma irradiation was seen based on the discharge curves. The possible corresponding radiation effects on the Li-ion battery were discussed.

iii

Dedication

Dedicated to my family

Acknowledgments

First of all, my deepest appreciation goes to Prof. Cao, my advisor. Without the opportunity and support he offered, this work would be impossible. His expertise and rigorous thinking challenged me through the project and encouraged me to reach my goal in a timely manner. The serious and critical attitude I learned from him will definitely contribute much to my future work.

Also, I would like to express my gratitude to Dr. Sun from the Davis Heart and Lung

Research Institute. He gave me much assistance and support on the Atomic Force

Microscopy. The experiment could not have started without his help. I am truly grateful to Dr. Qiu for his continual assistance. His encouragement gave me much motivation through the work. Since my research required interdisciplinary work, the support from other collaborators helps a lot for the final work. I am thankful to Prof.

Yuan Zheng and Mr. Shimeng Li from Department of Electrical and Computer

Engineering for their help on the battery electrical test. Also I want to thank Prof.

v Jason Hattrick-Simpers, from the University of South Carolina, for the XRD work. I thank to the support staffs at the Ohio State University Research Reactor. I thank to

Prof. Aldemir for serving as my committee member and providing important comments. In addition, I would like to show my greatest appreciation to my family and friends. In my most difficult time, it is their faith and encouragement that gave me the courage to move on. Finally, I want to thank to DTRA the financial support.

Without the funding support, this work can not continue.

Vita

October 13, 1992 ...... Born-Anhui, China June, 2013 ...... B.S. Mechanical Engineering, Huazhong University of Science and Technology 2013-present ...... Graduate Research Associate, The Ohio State University

Fields of Study

Major Field: Nuclear Engineering

vii

Table of Contents

Abstract ...... ii Dedication ...... iv Acknowledgments ...... v Vita ...... vii Lists of Tables ...... x List of Figures ...... xi Chapter 1: Introduction ...... 1 Chapter 2: Overview of radiation effect on materials ...... 4 2.1. Ionization ...... 4 2.2. Atomic displacement ...... 6 2.3. Impurity production ...... 8 2.4. Energy release ...... 9 Chapter 3: Related concepts about the Li-ion battery ...... 11 3.1. Structure and mechanism of the Li-ion battery ...... 11 3.2. Electrical performance characteristics ...... 13 3.3. Degradation of the Li-ion battery ...... 14 3.3.1. Formation of SEI ...... 15 3.3.2. Thermal degradation ...... 17 3.3.3. Mechanical and structure degradation ...... 18 3.3.4. Lithium plating and dendrite formation ...... 19 3.3.5. Chemical decomposition and corrosion ...... 20 3.4. Characterization techniques ...... 21 Chapter 4: Radiation effects on Li-ion battery ...... 28 4.1. Radiation effects on the electrical performance ...... 28 4.2. Radiation effects on the electrodes and electrolyte...... 29 4.3. Neutron radiation effects on the Li-ion battery ...... 31

viii Chapter 5: Experimental Setup for AFM measurement ...... 33 5.1. Samples ...... 33 5.2. Discharging of the Li-ion polymer battery ...... 35 5.3. Neutrons and Gamma Irradiation ...... 36 5.3.1. Neutron irradiation ...... 36 5.3.2. Gamma irradiation ...... 37 5.4. Measurement ...... 38 Chapter 6: AFM characterization of the Li-ion battery cathode ...... 40 6.1. AFM characterization of cathode samples after neutron radiation ...... 40 6.2. AFM characterizations of cathode after gamma radiation ...... 45 6.3. Comparison between charged and uncharged cathodes ...... 46 6.4. Theory and discussion ...... 47 Chapter 7: XRD characterization of the Li-ion battery cathode ...... 52 7.1. Principle of XRD ...... 52 7.2. Experimental setup ...... 53 7.3. Results and discussion ...... 54 Chapter 8: Battery Electrical Performance Test ...... 58 8.1. Samples ...... 58 8.2. Experiment Apparatus ...... 59 8.3. Experiment procedure ...... 60 8.4. Results and discussion ...... 61 Chapter 9: Summary and Outlook ...... 66 Appendix A: PDF Card 70-2685 ...... 69 Reference ...... 71

Lists of Tables

Table 2.1 Radiation effect on materials ...... 10 Table 3.1 Characterization techniques for Li-ion battery ...... 27 Table 5.1 Components of Li-ion polymer battery ...... 34 Table 5.2 Technical specifications of the lithium polymer battery ...... 35 Table 5.3 Neutron irradiation Fluence ...... 36 Table 6.1 Comparison between charged and uncharged cathode parameters ...... 47 Table 7.1 Peak intensity ratio of peak (003) to peak (104) ...... 57 Table 8.1 Technical specifications of the LiFePO4 ...... 59 Table 8.2 Capacity loss after irradiation of the lithium polymer battery ...... 64

List of Figures

Figure 2.1 Atomic displacement by cascade collision ...... 7 Figure 3.1 Structure of the Li-ion battery ...... 12 Figure 3.2 Formation of an SEI on the electrode surface (adapted from [14]) .... 16 Figure 3.3 Li-ion cell operating window (adapted from [20]) ...... 18 Figure 3.4 Formation of heterogeneous lithium metallic plating and dendrite ... 20 Figure 3.5 Contact mode of AFM (adapted from [31]) ...... 23 Figure 3.6 Tapping mode of AFM (adapted from [31]) ...... 23 Figure 4.1 Overview of radiation effect on Li-ion battery ...... 32 Figure 5.1 The Li-ion polymer battery and its components ...... 34 Figure 5.2 Cadmium Button ...... 37 Figure 5.3 Co-60 irradiator ...... 38 Figure 5.4 Fixed cathode sample ...... 39 Figure 5.5 The Bioscope II AFM ...... 39 Figure 6.1 Height images of uncharged cathode under neutron irradiation ...... 42 Figure 6.2 Particle size distributions of uncharged cathode under different neutron dose for (a) fast + thermal neutrons and (b) only fast neutrons. Under each dose, the top point of the upper line is the maximum and the bottom of the lower line is the minimum. The band inside the box refers to the median. The bottom and top box are the first and third quartiles...... 43 Figure 6.3 Roughness of uncharged cathode under neutron irradiation ...... 44 Figure 6.4 Height images of charged cathode under gamma irradiation ...... 45 Figure 6.5 Particle size of charged cathode under gamma irradiation ...... 46 Figure 6.6 Roughness of charged cathode under gamma irradiation ...... 46 Figure 7.1 Bragg reflection on atomic planes ...... 53 Figure 7.2 Miniflex II X-ray diffraction system ...... 54 Figure 7.3 X-ray diffraction patterns for the Li-ion battery cathode ...... 55

xi Figure 7.4 Layered structure of LiCoO2 with R3m symmetry (adapted from [52]) ...... 55 Figure 7.5 Peaks splitting for (a) pristine battery cathodes and (b) irradiated battery cathodes ...... 56 Figure 8.1 The rechargeable LiFePO4 battery ...... 58 Figure 8.2 BK Precision 8500 ...... 60 Figure 8.3 Discharge curve of Lithium polymer battery and LiFePO4 battery. . 63

xii

Chapter 1: Introduction

The concept of the lithium battery was first proposed by M.S. Whitingham in 1970.

Since then, many studies have been carried out, and the performance of the lithium-ion (Li-ion) batteries has improved significantly. However, it is still under intense development. The first commercial Li-ion battery was released by Sony and

Asahi Kasei in 1991. As the development progresses, the Li-ion batteries showed many unique properties, such as high energy density, large specific capacity, lightweight design, no memory effect and long lifespan [1]. These advantages have made the Li-ion batteries one of the popular rechargeable batteries for consumer electronics such as the mobile phone and the notebook computer.

Driven by the advancement of technology, application of the Li-ion batteries has also been extended to many other areas, such as for power tools, electrical vehicles, and military and aerospace application, which have more stringent requirements for the performance of the Li-ion batteries. One concern is the stability of the Li-ion battery

1 under harsh environments (e.g., in outer space). Because of its promising properties, the rechargeable Li-ion battery has been utilized in spacecraft for planetary exploration missions. However, outer space is filled with a large amount of radiation.

To study the behavior of the Li-ion battery in a radiation environment, the U.S.

National Aeronautics and Space Administration (NASA) started a flight battery program [2] and carried out the related research. In addition to the aerospace application, Li-ion batteries are also providing power for those critical modern rescues and sampling equipment in nuclear environment followed perhaps natural disaster. For example, robots were designed and deployed recently to carry out surveillance missions in the nuclear plants after the Fukushima accident [3].

Considering the released radioactive materials in the building, the radiation tolerance of the electronic components of the robots, including the source of power, the Li-ion battery, must be studied to make sure that the robots will operate normally. Therefore, understanding the radiation effect on the performance of the Li-ion battery is necessary for its application in a harsh radiation environment.

In this writing, the general radiation effects on the materials are discussed, followed

2 by a review of the Li-ion battery that concludes a discussion of degradation mechanisms and characterizations techniques. As far as the experimental work is concerned, the atomic force microscopy (AFM) and X-ray diffraction (XRD) was used to characterize the Li-ion battery cathode before and after neutron and gamma irradiation, including the cathode surface morphology and crystal structure. The electrical performance of the battery was also measured.

Chapter 2: Overview of radiation effect on materials

The radiation effect on material and devices depends on many factors, including the type of the radiation, the type of the material, the interaction of the radiation with materials, the radiation energy deposited in the material, the mechanism the device works, and the contribution of the material to the whole device [4]. Generally, the mechanism of radiation effect on materials can be categorized into four types: ionization, atomic displacement, impurity production, and energy release [5].

2.1. Ionization

Ionization is the process by which a neutral atom or molecule becomes charged by gaining or losing electrons. Radiation that is capable of creating reactive ions by ionization is known as ionizing radiation. Both charged and neutral radiation can cause ionization, the difference lies in the specific ionization mechanism. Charged particles such as charged nuclei, alpha particles, electrons and protons, often ionize atom directly through columbic force if they possess enough energy. However, the

4 interaction between electrically neutral radiation (photon radiation and neutron radiation) and matter is not as strong, and most of the ionization effects come from secondary ionization. For example, the photoelectric effect and the Compton effect are two common methods by which photons interact with mater. Either of these interactions can produce an electron (secondary electron), which in turn can ionize more atoms [6].

The effect of this type of radiation is especially evident for those electrical devices because it may modify the electrical properties (e.g., conductivity) of the material.

The subsequent current induced by the radiation may damage the material and cause problems at the device level. High-energy ionizing radiation can also lead to radiolysis, in which one or several chemical bonds will break, and free radicals hence form. Some of these free radicals may react with other radicals or with surrounding materials. Others will diffuse and cause swelling. Also, chemical reactions, such as corrosion and polymerization can be affected by getting activation energy from the ionizing radiation. For metals, ionization can change the crystal structure and make them amorphous, which will eventually influence the mechanical properties.

5 2.2. Atomic displacement

Atomic displacement refers to the displacement of atom from its normal positions in the material structure. Usually, it is caused by the collision of passing particles with the material. The atom ejected from its lattice site by irradiation is called as a primary knock-on atom (PKA). A vacancy is left after the atomic displacement and those displaced atoms further interchange in the lattice structure. A displaced atom lodging in a nearby position is known as an interstitial. Such a vacancy-interstitial pair is called a Frenkel pair. After the initial displacement, the primary knock-on atoms can in turn lead to a cascade of knock-on atoms if they have enough energy. For example, under fast neutron irradiation, most of the displaced atoms come from the collision by the primary knock-on atoms as they slow down [7]. Figure 2.1 shows the process of the collision cascade. When the radiation dose is low, some displacements will recover through a combination of the interstitials and the vacancies. Under this condition, the vibration of the atoms in the lattice structure will increase.

Figure 2.1 Atomic displacement by cascade collision

Driven by the energy stored in the crystal lattice (a transfer of the particle kinetic energy), the displaced atom can mitigate to a vacancy site to fix the defect [8].

However, some of the displacement is permanent, and the accumulation of such defect may cause serious material degradation. After the displacement, the ordered structure of the material will be disrupted. Since the microstructure of a material is closely related to its macroscopic properties, the lattice change may change the mechanical properties of the material, such as the hardness and strength of a solid. For insulators and semiconductors, such displacements can works as donors and traps, thus affecting the conductivity of the device.

7 2.3. Impurity production

Radiation can introduce impurities into the material. One primary source for the impurities is nuclear transmutation. By nuclear transmutation, one atom or isotope is converted to another. Either nuclear decay or nuclear reactions can lead to the transmutation. Ions may come directly from an external source or they may be produced by a nuclear reaction or decay process, which will eventually capture electrons, making them neutral after slowing down. For example, protons and alpha particles can become hydrogen and helium, respectively. The impurity may cause the swelling of the material and in turn change its mechanical properties.

In addition, the transmutation may introduce radioactive species into a material’s inner structure. One such typical mechanism is neutron activation, by which the atomic nuclei captures neutrons and becomes excited [9]. The newly formed nuclei may undergo deexcitation by emitting gamma rays or charged particles. Another mechanism is photodisintegration or phototransmutation, during which the atomic nucleus may become excited after absorbing high-energy gamma rays, resulting in the

8 ejection of a single proton, neutron or alpha particle from the nucleus [10]. The end product could alter the electrical and mechanical properties of the material.

2.4. Energy release

Generally, radiation could lead to energy deposition in the material through interactions with the material. The deposited energy will express itself as thermal heating or broken chemical bonds. During the slowing-down process for energetic ions, part of the energy of the moving ions will be transferred to the atoms in the material by collision, and the recoil particle can cause a cascade of collision. A hot region (e.g., thermal spike) will form as a result of the collisions and finally disrupt the structure and form an amorphous material. This effect may be a serious concern for materials used for radiation shielding. The accumulation of large amounts of energy may finally cause the failure of the material.

Based on the above discussion, some common types of radiations and their corresponding effects on materials are listed in the following table.

Radiation effect Radiation Ionization Atomic Impurity Energy displacement production release Charged Alpha Directly Yes Helium Over short particles buildup cause range pressurization Beta Directly Some N/A Localized displacement heat deposition Proton Directly Yes Helium Over short buildup cause range pressurization Neutron Thermal Indirectly Indirectly Directly Indirectly neutron through absorption Fast Indirectly Multiple Indirectly reaction neutron displacement through scattering interactions Photon Gamma/ Indirectly Rare N/A Gamma X ray displacement heating over substantial distance Fission Highly Considerable Considerable Be impurities Over short fragment charged Ionization displacement with respect range ions to the lattice Table 2.1 Radiation effect on materials

Chapter 3: Related concepts about the Li-ion battery

3.1. Structure and mechanism of the Li-ion battery

There are several types of Li-ion batteries based on the different cathode materials, such as lithium cobalt oxide, lithium manganese, lithium iron phosphate and others.

For commercial Li-ion batteries, the typical material used for the cathode is a Li metal oxide; graphite is used as the anode. The electrode is bonded to a current collector by a polymer binder. The compositions of the electrolyte is primarily lithium salt and carbonate solvents. The thin layer of a porous separator facilitates the diffusion of lithium ions and stops the solvent flow. During the discharging process, lithium ions move from the graphite layer to the lithium oxide (intercalation); they move back again (disintercalation) during charging. Take a graphite-LiCoO2 battery as an example, the reactions in each electrode during the charging process are

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11 Figure 3.1 shows the general structure of the Li-ion battery and the charging-discharging process.

Figure 3.1 Structure of the Li-ion battery

When the battery is in discharge mode (used as power supply), the lithium ions mitigate from the anode and intercalate with the cathode inside the battery. For the outer loaded circuit, a flow of positive charge is seen coming from the cathode to the anode (also the current direction), whereas the electrons move in the opposite

12 direction. Under this condition, the cathode works as a positive electrode and the anode works as a positive electrode. The process is reversed when the battery is charged by another source. A current then flows into the cathode and from the anode of the battery. Inside the battery, the lithium ions move from the lithium metal oxide electrode to the graphite electrode, which is consistent with the circuit current direction. The electrons produced from the lithium oxide split at the cathode make the electrode negative. The cathode hence becomes the negative electrode and the anode becomes the positive electrode. Therefore, the reference to an electrode being positive or negative is not absolute; it depends on the specific status of the battery, whether it is supplying power or being charged. From the electrical point of view, whether an electrode is positive or negative is decided by the potential of the electrode and Li metal always has the negative potential, thus being the cathode for any half-cell structure.

3.2. Electrical performance characteristics

The performance of the battery is characterized by kinds of parameters, such as voltage, capacity and cycle life, all of which decide the application of the battery

13 directly and can be affected by different operating conditions. By studying the varying pattern of these parameters, the properties and corresponding mechanisms of the battery can be understood better.

The discharge curve is often used to describe the capacity fade of the battery after multiple cycles. Generally, the voltage of the battery is plotted with respect to the time or capacity. The power delivered by the battery decreases through the discharge process. The actual measured voltage of the battery is determined by many factors, including the internal impedance, load current, state of charge, temperature and other surrounding environmental conditions. The discharge curve is relatively flat for Li-ion batteries since the capacity and internal impedance of the battery is affected by physical and electrochemical reactions inside the battery directly. The discharge curve can be a very helpful indicator to reveal the possible change mechanisms of a battery.

3.3. Degradation of the Li-ion battery

The degradation of the Li-ion battery is a very complex process, and it is the main cause for the failure of the battery except for those direct damages. From the electrical point of view，the capacity fade and power fade of the battery reflects this kind of

14 degradation. The capacity fade is the loss of discharge capacity of the battery over time [11], whereas the power fade (voltage of the battery) refers to the deterioration in the battery’s ability to deliver power, which is closely associated with the impedance of the battery [12]. There are several known primary mechanisms resulting in the degradation, such as the formation of a solid electrolyte interphase (SEI), thermal degradation, mechanical and structural degradation, lithium plating [13-14], chemical decompositions and corrosion.

3.3.1. Formation of SEI

The SEI is a self-forming passivation layer on the surface of the anode during charging [15]. It is caused by electrolyte instability and chemical reactions between the electrolyte and the electrodes. To some degree, the layer can provide protection for the electrode from further reactions with the electrolyte while still allowing the transportation of the lithium ions [16]. Figure 3.2 shows the formation process of the

SEI, which is made up of the decomposition products of the electrolyte.

Figure 3.2 Formation of an SEI on the electrode surface (adapted from [14])

As the decomposition of the electrolyte continues, the thickness of the SEI increases and more lithium ions are consumed as a result of a series of chemical reactions.

However, the SEI layer is not stable and may crack because of the volume change of the graphite lattice during the charging and discharging process [17]. This crack formation can cause the loss of the active material and result in greater loss of lithium ions. The decrease in the number of the lithium ions is believed to be the direct reason for the capacity fade. Furthermore, over time, the penetration of the SEI into the electrodes may reduce the electrode active surface area and block the pores in the electrode. This would limit the transport of the lithium ions, leading to transfer resistance and increase of impedance [18], finally causing the power fade of the battery.

16 3.3.2. Thermal degradation

The Li-ion battery is sensitive to temperature and thus is limited to a narrow temperature range for application. By affecting the compositions and chemical reactions, the thermal condition of the battery is directly related to the performance and degradation of the battery. With increasing temperature, the battery reaction rate also increases. When the temperature reaches a certain point, thermal runaway, a self-sustaining increase in cell heat as components react exothermically, may happen for the Li-ion battery [19].

Figure 3.3 shows the Li-ion cell operating window, which is related to the temperature and the voltage. It is clear that the thermal effect on the performance of the Li-ion battery is significant, and damage will occur once the temperature goes beyond the limited range (green box).

Figure 3.3 Li-ion cell operating window (adapted from [20])

3.3.3. Mechanical and structure degradation

For the Li-ion battery, the insertion and extraction of lithium ions can cause the volume change of the electrode. As a result, mechanical stress within the electrodes is generated, which may induce the disordering of the particles. Moreover, excessive stress may lead to electrode fracture and fatigue [21-22]. This type of fracture may cause the degradation of the battery in several ways. One possible mechanism is that a fragment of the fractured particles may deviate from the original conducting matrix

[23] position and cause a loss of capacity [24]. Another way that the fractured particles may result in reduced capacity is by decreasing the amount of active material

18 and surface area. Also, the ions movement rates may be affected by stress and fracture.

A study on the intercalation-induced stress and heat generation in a Li-ion battery showed that both the parameters increase with an increasing in the number of equivalent particles and that spherical particles seem more resistant to those failures than ellipsoidal electrode particles with bigger aspect ratios [25]. Therefore, during the charging and discharging process, the structure and electrochemical characteristics change as a result of the intercalation of the lithium ions.

3.3.4. Lithium plating and dendrite formation

The lithium plating refers to the deposition of metallic lithium on the surface of the anode when the lithium ions cannot insert into the layers of the anode in time, which may cause the formation of dendrite [26]. The consequence of the lithium plating is the decrease in the number of the lithium ions, resulting in capacity loss. The lithium metal can break away from the electrode surface and accumulate in the separator.

With an increase of lithium metal in the separator, a conductive path forms and produces a short circuit [27]. Also, as the lithium plating grows, the metallic protrusions can damage the separator and lead to thermal runaway [28]. Based on the

19 distribution of the metallic plating, the lithium plating is classified into two types, homogeneous and heterogeneous (or localized) lithium plating. When the charge current is very big, homogeneous lithium plating occurs and covers a large area of the anode surface. Heterogeneous lithium plating is usually seen at the edges or corners of the anode. Figure 3.4 below shows heterogeneous lithium plating and dendrite formation condition on the surface of the anode.

Figure 3.4 Formation of heterogeneous lithium metallic plating and dendrite

3.3.5. Chemical decomposition and corrosion

Decomposition reactions inside a Li-ion battery play an important role in the degradation of the battery. Electrolyte decomposition is the direct cause for the SEI formation at the surface of the electrode，and it can cause both capacity and power

20 fade of the battery at the end. The binder, which is a critical component in supporting the composite electrode material and its interaction with the electrolyte, may decompose, resulting in loss of particles contact and volume changes (structural degradation), thereby affect the potential of the battery.

In addition, the corrosion of the collectors could not be neglected, especially the aluminum collector. Localized corrosion has been found on the surface of the aluminum during the electrical cycling and many “pits” (mounds) are observed based on SEM photograph [29]. As a result, a power fade occurs and the degradation becomes worse due to the inhomogeneous distribution of the potential [14]. As compared to the corrosion of aluminum, copper corrosion has not been evident and not much damage has been found.

3.4. Characterization techniques

Characterization techniques are of great importance to the study of batteries, including the electrical performance, the microstructure, chemical compositions, and interphase phenomenon of the electrodes and electrolyte [30]. Driven by this requirement, many advanced characterization methods for the Li-ion battery have

21 been proposed in addition to conventional measurements and analysis tools. Several common characterization methods for the Li-ion battery are presented here.

Atomic force microscopy (AFM) is one of the most widely used techniques to study the surface characteristics of battery electrodes. There are two operating modes for

AFM, the contact mode and the tapping mode (see Figure 3.5 and 3.6), both of which use a sharp tip at the end of the cantilever to scan the surface. A piezoelectric element is utilized to turn the cantilever into a strain gauge. A laser beam emitted from a laser on the surface of the cantilever is reflected into the split photodiode detector and then transferred to a deflection signal.

Figure 3.5 Contact mode of AFM (adapted from [31])

Figure 3.6 Tapping mode of AFM (adapted from [31])

23 Even with the same basic principles, the two operating modes are used based on different sample surface characteristics and experimental requirements. With the contact mode, the tip on the cantilever keeps in contact with the surface of the sample during the scanning process, which may result in damage to the sample surface.

However, in tapping mode, the tip taps the sample surface lightly. One problem of this mode is the loss of some surface information.

In addition to the surface morphology, AFM can also be used to measure electrical properties of the sample surface. The conductivity of the surface can be obtained by allowing the current to pass through the tips. Also, a scanning spreading resistance microscopy (SSRM) module in AFM has been developed to measure the surface resistance characteristics of the battery material [32].

Scanning electron microscopy (SEM) is also an effective technique to characterize the surface morphology of the sample with high-energy scattered electron beam. The electrons interact with the sample and produce an image of the sample. Also, information on the composition can be obtained. For the Li-ion battery, SEM is often used to identify the particle morphology and nanostructure of the active material [33].

24 A similar electron microscopy technique is transmission electron microscopy (TEM).

TEM uses transmitted electrons to provide more detailed information inside the sample’s surface, such as the crystallization, phase changes, and stress. Chen et al.

[2006] used TEM to obtain the crystal image of LiFePO4 [34].

XRD is a primary tool for probing crystal structure of crystalline materials. The X-ray applied on a sample can be diffracted into many different directions by the atoms in the material. For the Li-ion battery, phase analysis of the electrode material based on

XRD can provide information on the content of the active material (electrochemically

essential materials to support normal operations of the battery, like LiCoO2), which is important for studying the change mechanism of the electrical properties. In the study carried out by Li et al. [2003], the diffraction pattern on an operational Li-ion battery obtained by synchrotron XRD reflected the structural and phase changes that occurred during the charging and discharging process [35].

Raman spectroscopy (RS) can also be helpful to study the structural and chemical changes of the Li-ion battery by identifying the material intensity in the electrodes

[36]. The RS is a nondestructive or noninvasive technique, which makes it a good

25 candidate for the measurement of the Li-ion battery owing to the highly reactive materials in the battery [37]. In addition, there are no demanding requirements for the sample preparation.

Neutron depth profiling (NDP) is an effective technique for obtaining some special element (6Li, 10B, etc.) concentration at the near-surface region of the host materials as a function of depth based on nuclear reactions. Neutrons pass through the material and react with the particular isotope of the element, producing charged particles after neutron absorption. By measuring the energy spectrum of those emitted charged particles, the corresponding penetration depth and elemental concentration profile is obtained. The aging mechanism of the Li-ion battery is studied by the NDP technique

[38]. Some common characterization techniques for the Li-ion battery are listed in

Table 3.1. The surface morphology mentioned in the table includes the particle size, shape and distribution.

Technique Abbr. Application Atomic Force Microscopy AFM Surface morphology Scanning Electron Microscopy SEM Surface morphology Compositional difference (backscatter mode) Transmission Electron TEM Surface morphology Microscopy Atomic structure Crystallization Phase change X-ray Diffraction XRD Phase identification Crystal structure Raman Spectroscopy RS Chemical composition Carbon identification Neutron Depth Profiling NDP Lithium distribution X-ray Photoelectron XPS Chemical composition state Spectroscopy Electrochemical Impedance EIS Electrochemical properties Spectroscopy Inner structure Nuclear Magnetic Resonance NMR Electrochemical structure difference Electron Energy Loss EELS Lithium distribution Spectroscopy Chemical state of transition metal Glow Discharge Mass GDMS Impurity mass analysis Spectrometry Trace element Fourier Transform Infrated FTIR Chemical composition analysis Spectroscopy Neutron Reflectometry NR Depth profile of chemical element Table 3.1 Characterization techniques for Li-ion battery

Chapter 4: Radiation effects on Li-ion battery

My research about radiation effects on Li-ion battery is mainly focused on the following aspects: 1) characterizing the interaction of the radiation with specific components of the battery based on the observation of the material structure with

AFM and XRD, 2) measuring the electrical properties of the Li-ion battery prior to and after irradiation, and 3) proposing a mechanism for the radiation effects based on the obtained experimental data. An overview about some related research is made in this chapter.

4.1. Radiation effects on the electrical performance

An electrical testing on Li-ion prototype cells used in aerospace applications has been carried by NASA. The result showed that there was only a small decrease in the capacity of the cell (~10%) upon irradiation (Co-60) up to 25 Mrad and that the radiation dose rate had a minimal effect on the performance of the cell [39]. From these results, the tolerance of the Li-ion battery to high gamma radiation seems very

28 robust. However, as pointed out in the research article by Ding et al. (2006), there was a substantial capacity loss on the LiCoO2 half-cell (20%) and the LiCoO2/Li full-cell

(50%) after irradiation by Co-60 at a lower radiation level (14.4 Mrad) [40]. This large difference may be caused by the differences in the batteries, i.e., the improved stability of the Li-ion battery used in the aerospace application.

4.2. Radiation effects on the electrodes and electrolyte

Since the cathode of the Li-ion battery determines the number of the active lithium ions, which has a direct relationship with the electrical properties of the battery, changes to the cathode when exposed to radiation provide important informations when considering the radiation effects. Changes to the crystal structure of a LiCoO2 cathode has been observed after gamma irradiation based on Raman spectroscopy and

XRD analysis [41]. The structural changes were related to the crystal displacement caused by the intensified wagging vibration of the lithium ions. As a result, the electrochemical properties of the Li-ion battery were affected.

The interaction between the radiation and the electrolyte should also be considered because some electrolyte compositions are potentially susceptible to degradation

29 under radiation. Evident color changes of the electrolyte solution after gamma irradiation have been observed [40]. The Fourier transform infrared spectroscopy

(FTIR) spectra of the electrolyte solution demonstrated the production of hydroxyl group and an increase in the number of carboxyl groups, which resulted in the production of carboxyl acid and ethane and changed the solution color [40]. The electrochemical reaction between the increased carboxyl groups and the lithium was proved to affect the Li-ion cell electrical performance [42]. Magaly et al. (2010) carried out further investigation about the gamma ray effect on the electrolyte with different solvents and compositions. An electrolyte with a vinylene carbonate (VC) additive and LiPF6 as a lithium salt showed a significant difference in its spectrum after exposure to 2 Mrad gamma dose as compared with the other combinations [43].

Hence, an appropriate choice of electrolyte compositions can make the Li-ion battery more stable under radiation exposure.

As for the anode, very few studies could be found on how the anode is affected by radiation. One possible consideration may be that the structure of the anode material, graphite, is stable. However, during the manufacturing of a Li-ion battery, a polymer

30 coating that can improve the electrochemical performance by providing better conductivity and decrease the contact of graphite with electrolyte [44] is often applied to the surface of the anode. Similar to the aforementioned degradation of the electrolyte, this type of polymer is susceptible to degradation.

4.3. Neutron radiation effects on the Li-ion battery

Considering that the focus of most research about the radiation effect on the Li-ion battery is the aerospace application of the battery, gamma rays, which are the primary radiation source in outer space, attracts more attention. However, in a nuclear environment, such as in a nuclear reactor, neutron radiation can also be part of a mixed radiation environment, in addition to gamma rays. Thus the neutron radiation effect must also be taken into account. In a study by Qiu et al. (2013), a decrease in the current of a Li-ion cell after exposure to thermal neutron was observed [45], which showed that the neutron radiation does affect the performance of the Li-ion battery. One of the known interactions is the thermal neutron capture, which is defined as

! ! ! !�� + � → !�� + !�

31 As the above equation shows, upon neutron radiation, lithium splits into an alpha and a triton particle. Because of the large thermal neutron absorption cross section of lithium (940 barns) [46], the consumption of lithium by the neutron absorption may, in theory, affect the capacity and power of the Li-ion battery. Nevertheless, the neutron absorption reaction would not play a big role as it is limited by the neutron flux and the lithium density in the battery. An overview about changes to a Li-ion battery under radiation is summarized in Figure 4.1.

Figure 4.1 Overview of radiation effect on Li-ion battery

Chapter 5: Experimental Setup for AFM measurement

5.1. Samples

The Li-ion battery used in this study was a lithium polymer battery known as pouch format cell. The main difference between this battery and more common Li-ion batteries lies in the formation of the lithium salt electrolyte, which is held in a solid polymer composite rather than an organic solvent. The advantages of the lithium polymer battery are its potentially low cost and flexible packaging shape. For electrical devices where space is at a premium, the flexibility characteristic of the lithium polymer battery becomes attractive. The battery sample used in the current experiment was from Superior Lithium Polymer Battery (SLPB) series from Kokam company, and it was used for the monitor robots in the aftermath of the Fukushima nuclear accident. A photograph of the battery and its components is shown in Figure

5.1. The cathode mainly consists of LiCoO2, with aluminum as the current collector and graphite covering on copper foil as the anode. Table 5.1 shows the detailed

33 compositions information of the battery.

Figure 5.1 The Li-ion polymer battery and its components

Component Chemical composition Mass content (%) Cathode Lithium Cobalt Dioxide (LiCoO2) 20–50

Anode Carbon (Graphite) 15–35 Binder PVDF (Polyvinylidene Fluoride) <8 (-(C2H2F2)n-) Electrolyte Ethylene Carbonate (EC)(C3H4O3) 10–20 Ethylmethyl Carbonate (EMC)(C4H8O3) Lithium Hexafluorophosphate (LiPF6) Collector Aluminum foil 3–12 Copper foil 3–12 Cover Aluminum film <5 Table 5.1 Components of Li-ion polymer battery

Some technical specifications of the battery are listed in Table 5.2.

Item Value Rated capacity Typ. 25 mAh Nominal voltage 3.7 V Lower limit voltage 2.7 V Upper limit voltage 4.2±0.03 V Maximum charge current 50 mA Maximum discharge current 50 mA Cycle More than 1000 cycles Table 5.2 Technical specifications of the lithium polymer battery

5.2. Discharging of the Li-ion polymer battery

Prior to irradiation, the pristine Li-ion polymer battery was discharged by connecting it to a 1000Ω resistor. The voltage of the pristine battery was measured to be approximately 3.7V (not fully charged) and the capacity of the battery was 25 mAh.

The discharging process lasted approximately 7 h until the measured voltage was reduced to approximately zero.

C C 0.025 ∗ 1000 T = = = = 6.76 hr I V 3.7 R

35 5.3. Neutrons and Gamma Irradiation

5.3.1. Neutron irradiation

The cathode samples taken from the pristine charged and fully discharged batteries were irradiated through the “rabbit” facility at the Ohio State University Research

Reactor (OSURR). Fast plus thermal neutrons (fast + thermal) and fast neutrons alone

(fast) were applied to the two kinds of cathode samples respectively. Each type of neutron radiation had four different neutron fluences (1E12, 1E13, 1E14 and 1E15 neutrons/cm2). The detailed irradiation parameters are listed in Table 5.3.

Reactor Thermal neutron Total neutron Fluence Neutron Power (kw) flux (n/cm2/s) flux (n/cm2/s) (n/cm2) type 5 5.1E+09 2.5E+10 1E12 fast+thermal 5 5.1E+09 2.5E+10 1E12 fast 5 5.1E+09 2.5E+10 1E13 fast+thermal 100 1.0E+11 5.0E+11 1E13 fast 100 1.0E+11 5.0E+11 1E14 fast+thermal 450 4.6E+11 2.3E+12 1E14 fast 450 4.6E+11 2.3E+12 1E15 fast+thermal 450 4.6E+11 2.3E+12 1E15 fast Table 5.3 Neutron irradiation Fluence

Owing to the mixed radiation environment in the reactor, which includes fast neutrons

36 and thermal neutrons, a cadmium button (Figure 5.2) was used to filter the thermal neutron out in order to produce only fast neutron radiation on the samples. Cadmium is widely used as neutron absorber because of the relative highly thermal neutron capture cross-section value.

Figure 5.2 Cadmium Button

5.3.2. Gamma irradiation

A Co-60 irradiator (Figure 5.3) was used to irradiate the pristine battery (packed). At a dose rate of 39.2 krad/hr, the samples were exposed to the gamma rays for 2.5, 18 and 70h. Based on the dose rate, the yields for the three different dose amounts were

98, 706, and 2744 krad, respectively.

Figure 5.3 Co-60 irradiator

5.4. Measurement

AFM was used to observe the surface characteristics of the cathode samples prior to and after irradiation. Considering the relatively hard surface of the samples, contact mode was selected. The cathode sample was fixed on the center of the glass slide and

AFM images were obtained with a 1 × 1 µm scan size. The minor axis of the particles located within the 1 µm2 scan area were measured and compared.

Figure 5.4 Fixed cathode sample

Figure 5.5 The Bioscope II AFM

Chapter 6: AFM characterization of the Li-ion battery cathode

The cathode surface was scanned under the contact mode of AFM. Height images

(also called the topography image) for a 1 × 1 µm scan area were obtained. The height image shows the surface characteristics by providing height information.

6.1. AFM characterization of cathode samples after neutron radiation

After neutron irradiation, the cathode samples were measured by AFM. The corresponding height images of the uncharged cathodes under different neutron doses are shown in Figure 6.1. A number of elliptical particles can be seen in each image.

The images for both types of neutron irradiation, fast plus thermal and fast alone, show that there is no significant change in the particles when the neutron dose is relatively low (less than 1E13 n/cm2). However, the condition changes as the neutron dose increases, especially when the neutron dose reaches 1E15 n/cm2. At this dosage, the particle size shows a significant increase and the number of particles in each image decreases. To quantifying this change, the conjugate diameter of each complete

40 particle in every image was measured in pixels and the real length in nanometers was calculated using the pixel-to-nanometer ratio (The side length of each image measured 360 pixels and the real scan size is 1 µm. Therefore, the pixel-to-nanometer ration was 0.36 pixel/nm).

The particle size (conjugate diameter of the particles in this study) distributions under different neutron dose are plotted in Figure 6.2. The tendency of the particle size to increase with increasing neutron dose is more obvious with the boxplot than with the direct observation of the images. The particle size varied from approximately 60nm to

125nm. Although there is overlap between the different neutron doses, from the non-irradiated samples to the maximum neutron dose-irradiated samples, the particle size still shows a 20 nm increase for fast plus thermal neutrons and an approximately

30 nm increase for fast neutrons alone.

Unirradiated

Neutron-irradiate Fast plus thermal neutron Only fast neutron d 1E12 n/cm2

1E13 n/cm2

1E14 n/cm2

1E15 n/cm2

Figure 6.1 Height images of uncharged cathode under neutron irradiation

Particle size for fast plus thermal neutron

130 120

110 100 (nm)

90 size 80 75% 70 25%

Particle 60 50 40 0 1.00E+12 1.00E+13 1.00E+14 1.00E+15 Neutrons luence (n/cm2)

(a)

Particle size for fast neutron

130 120

110

(nm) 100

90 size 80 75% 70 25%

Particle 60 50 40 0 1.00E+12 1.00E+13 1.00E+14 1.00E+15 Neutrons luence (n/cm2)

(b) Figure 6.2 Particle size distributions of uncharged cathode under different neutron dose for (a) fast + thermal neutrons and (b) only fast neutrons. Under each dose, the top point of the upper line is the maximum and the bottom of the lower line is the minimum. The band inside the box refers to the median. The bottom and top box are the first and third quartiles.

43 The roughness data collected during the AFM scanning process further demonstrates

this change. In Figure 6.3, the root mean square roughness (Rq) is plotted. As the

neutron dose on the cathode samples increased, the value of Rq also increased. Rq is calculated by taking the square root of the sum of the square of the height and depth from mean value [47]. Since the real particle size change reflected at different

dimensions should be similar, the change of Rq and measured particle size based on the two-dimensional (2-D) images should be consistent.

Figure 6.3 Roughness of uncharged cathode under neutron irradiation

44 6.2. AFM characterizations of cathode after gamma radiation

The height images of the cathode samples after gamma irradiation are shown in

Figure 6.4. Similarly, the particle size was measured and plotted in Figure 6.5.

Compared to the non-irradiated samples, the particle size of the cathode after gamma radiation is bigger, and the difference becomes evident when the gamma dose is much larger. When the gamma dose reaches 2.744 Mrad, a nearly 30nm increase of the particle size was found. In addition, the roughness values (Figure 6.6) show the same trend in terms of the measured particle size.

(a) 0 h (b) 2.5 h (c) 18 h (d) 70 h Figure 6.4 Height images of charged cathode under gamma irradiation

Particle size for gamma irradiation

120

100

nm) 80 (

size

60 75% 40 25% Particle 20

0 0 0.098 0.706 2.744 Gamma dose (Mrad)

Figure 6.5 Particle size of charged cathode under gamma irradiation

Figure 6.6 Roughness of charged cathode under gamma irradiation

6.3. Comparison between charged and uncharged cathodes

A difference in the particle sizes between the charged and uncharged cathode samples

46 was found. The roughness and measured average particle size are listed in Table 6.1.

From the table, it is easy to see that the uncharged cathode shows a bigger particle size and roughness for all the irradiation environments.

Parameter Average particle size (nm) Roughness

(Rq/nm) Irradiation charged uncharged charged uncharged

Unirradiated 67.6 73.5 25.3 30.8 Fast plus thermal neutron 92.0 100.4 49.4 52 1E15 n/cm2 Only fast neutron 95.6 102.1 56.9 60 1E15 n/cm2 Table 6.1 Comparison between charged and uncharged cathode parameters

6.4. Theory and discussion

The increase of the particle size reflects the crystal grain growth, which is the result of the grain boundary migration. The crystal boundary, interface between two adjacent grains or crystallites, will move in the presence of the driving force and lead to the coarsening. Conceptually, the driving force refers to the free energy difference between the two sides of the grain boundary. There are various types of driving force,

47 such as stored strain energy (sometimes called stored deformation energy), surface energy, gradient of temperature, and magnetic filed (material that has anisotropic interactions with the applied filed) [48-49].

The internal strain energy is usually stored in the material in the form of dislocations

(a crystallographic defect). Driven by this force, the grain boundary migration will occur to reduce the dislocation and release the internal strain energy. The atoms located at the boundary of high-energy crystal (high internal strain energy is stored) can move to a crystal lattice with a relative lower energy, thereby removing some dislocations. For the internal strain energy, the driving force is calculated by the following equation [49]:

1 � = ��! 2 where the � is the dislocation density, � is the shear modulus, b represents the

Burgers vector, and ! ��! is the energy per unit length of a dislocation. !

Different from the internal strain energy, the chemical potential gradient is determined by the surface energy, which is directly related to the grain boundary curvature [49-50]. By adjusting the curvature and angles of the boundary, the driving

48 force of the surface energy is achieved and the grain energy is decreased. Therefore, the grain boundary migration is a recovery mechanism that occurs during or after the deformation, achieving new equilibrium of the system by reducing the total free energy. The grain boundary mobility is determined by the following expression [49]:

� � = � exp − ! ��

�! � = �!� � ! �� !" !

where the Q is the activation energy, T is temperature, k is Boltzmann constant, �!

is the initial mobility, d is the migration distance, �!" is the vacancy concentration,

and �! is the Debye frequency.

Under neutron irradiation, the passing neutrons will collide with the cathode material and produce atomic displacement. A number of vacancies will appear during the process and cause a change in the crystal structure of the cathode. Based on literature review, the formation of dislocation loops by collapse of vacancy cluster has been proved [51]. The driving force equation shows that the driving force of the internal strain energy will increase when the number of dislocation increases, which facilities the boundary migration. The grain boundary migration due to vacancy can also be

49 known directly from the grain boundary mobility equation. As a consequence, coarsening of the cathode particles will occur, as seen in the AFM images when exposed to the neutron irradiation.

With higher energy (an average energy of 1.98 MeV), the fast neutron can cause more atomic displacements and vacancies by scattering interaction compared to the thermal neutron (energy less than 0.4 eV). As for thermal neutron capture reactions occurred during the irradiation process, the estimation for the Li consumption rate is about 1010 atoms per second based on the neutron flux, which is a small rate value considering the atom density of Li (1023). For products of the thermal capture, alpha and triton particles, SRIM simulation showed that approximately 99.9% of their initial energy goes to ionization in LiCoO2. Hence, the fast neutrons should have a larger effect on the grain boundary migration than fast plus thermal neutrons with the same fluence, leading to greater changes in the particle size in the end.

However, with gamma rays, the atomic displacement is not evident. Gamma heating over a substantial distance plays a more important role instead. After absorbing the energy from the gamma ray, the vibration of the lithium atoms will be intensified and

50 causes the crystal structure disorder, which will produces internal strain. The stored internal strain energy then drives the grain boundary migration and results in coarsening.

In terms of the particle size difference between a charged and uncharged cathode, one possible mechanism is the insertion of lithium ions into the cathode during the discharging process. After the lithium ion insertion, more lithium oxide forms and exits in the cathode. The agglomeration of more lithium oxides makes the visible particles size increase. Since the uncharged samples were obtained by discharging the pristine charged sample in this experiment, some of the common aging mechanisms

(which occur after many battery cycles) such as SEI were not considered.

Chapter 7: XRD characterization of the Li-ion battery cathode

To further study the crystal structure of the Li-ion battery cathode, XRD was used to characterize the same cathode samples after the AFM measurements. Crystal atoms in the electrode caused the incident X-rays to diffract into different directions, and cathode structure information then was obtained by measuring the angles and intensities of those diffracted beams.

7.1. Principle of XRD

Crystals consist of ordered arrangement of atoms. For incoming X-ray, the beam strikes the atoms’ electrons (also called scatterers) and produces secondary spherical waves (see Figure 7.1). For those scatterers arranged regularly, the corresponding spherical waves will be regular, and follow the Bragg’s law

2�� = �� where d is the space between diffracting planes, θ is the angle between incident ray and scattering planes, λ is the wavelength of incident wave, n is an integer

Figure 7.1 Bragg reflection on atomic planes

With known wavelength of incident wave and measured scatter angles, for specific lattice plane, the spacing value of the reflecting planes is determined.

Then crystal structure information can be obtained with the XRD patterns.

7.2. Experimental setup

In this experiment, Miniflex II X-ray diffraction system (see Figure 7.2) from Rigaku was used to characterize the Li-ion battery cathode. A Cu X-ray tube worked as the

X-ray source and was operated at 30 kV and 15 m. With a 2 degree/min scanning speed and a 0.02 degree scanning step size, XRD patterns were recorded in the 2θ range of 10 degree to 90 degree. Diffracted beams were collected by a scintillation counter detector (NaI).

Figure 7.2 Miniflex II X-ray diffraction system

7.3. Results and discussion

The XRD patterns of the Li-ion battery cathodes under different irradiation environment are shown in Figure 7.3. The phase of the cathode coated layer fits well to LiCoO2 (PDF# 70-2685, detailed card information can be seen in Appendix A).

Clear peaks, such as (003), (006), (104), and (018), can be seen for all the patterns before and after irradiation, proving that no evident amorphization occur during irradiation process. Based on observations of the peaks’ characteristics from the

figure, the cathode material ( single-phase LiCoO2) can be indexed toα-NaFeO2 structure with �3� symmetry. Figure 7.4 shows the layered structure of LiCoO2

54 and its lattice plane.

Figure 7.3 X-ray diffraction patterns for the Li-ion battery cathode

Figure 7.4 Layered structure of LiCoO2 with �3� symmetry (adapted from [52])

Figure 7.5 shows the enlarged patterns for (006) peak. For the pristine battery

55 cathodes, (006) peak and (012) peak splits with each other and can be distinguished

clearly, indicating the ordered layered-structure of LiCoO2 [53]. However, this peak splitting diminishes after irradiation and only (006) peak can be seen in the figure.

This difference may be caused by the disordering of cathode crystal structure due to the neutron irradiation.

(a)

(b) Figure 7.5 Peaks splitting for (a) pristine battery cathodes and (b) irradiated battery cathodes

The intensity ratio of peak (003) to peak (104) is an important value for evaluating the cation mixing of the layered structure, which may affect the capacity and thus directly influence the electrochemical properties [54-55]. Generally, cation disordering occurs during the exchange process of Li and Co ions and lead to the decrease of the

intensity raio (I003/I104). The peak intensity for both peaks is listed in Table 7.1 and its

corresponding ratio is calculated. From the table, a smaller I003/I104 value after irradiation is found, showing that a disordering of the LiCoO2 structure occurs after neutron irradiation.

Neutrons(n/cm2) Unchar Charged 1012 1013 1014 1015 ged

I003 15787 19090 Fast 16483 14422 11775 18190

I104 3112 4898 5895 8215 9948 12280

I003/I104 5.07 3.90 2.80 1.76 1.18 1.48

I003 / / Fast 19778 20805 27781 26871

I104 / / plus 14208 15946 18375 20061

I003/I104 / / thermal 1.39 1.30 1.51 1.34 Table 7.1 Peak intensity ratio of peak (003) to peak (104)

Chapter 8: Battery Electrical Performance Test

8.1. Samples

Besides the lithium polymer battery, another type of Li-ion battery was used for the battery electrical performance test，the lithium iron phosphate (LiFePO4) battery.

With a high discharge current and long cycle life, the LiFePO4 battery is popular for high-power applications, such as power tools and electrical vehicles. The LiFePO4 battery used for the current experiment is shown in Figure 8.1. Some specific parameters for the battery are listed in Table 8.1.

Figure 8.1 The rechargeable LiFePO4 battery

Items Value Nominal capacity 450 mAh Maximum capacity 750 mAh Working voltage 3.2 V Charging cut-off voltage 3.6 V Discharge cut-off voltage 2.2 V

Maximum discharge/charge current 550 mA

Cycle More than 2000 cycles Table 8.1 Technical specifications of the LiFePO4

8.2. Experiment Apparatus

The BK Precision 8500 (Figure 8.2) was selected to carry out the test. As a programmable DC electronic load, the BK Precision Model 8500 is designed for testing and evaluating many kinds of DC power sources [56]. Because of it many advantages, such as a large measuring range, high accuracy, various operating modes and excellent display resolution, the BK Precision 8500 is widely used in different tests, including power supply testing, battery testing, and measuring the performance of DC loads. For the battery test mode, the length of time it takes for the battery

59 voltage to drop can be measured. The test ends upon the voltage reduces to the setting minimum voltage value and the integrated current 0f the battery then can be obtained.

Figure 8.2 BK Precision 8500

8.3. Experiment procedure

Three new lithium polymer batteries and three LiFePO4 batteries were prepared. The procedure for the test is as shown below,

Step 1: The six battery samples were fully charged.

Step 2: Pre-test of the batteries.

The battery was connected with the BK Precision 8500. The “I-set” button was

60 pressed to switch the DC load into the constant mode and the current was set to

0.013A for the lithium polymer battery and 0.450A for the LiFePO4 battery. The

“Enter” button was pressed to finish the setting. The “Shift” + “Battery” buttons were then pressed to set the minimum voltage to 2.7 V for the lithium polymer battery and

2.2 V for the LiFePO4 battery. The “Enter” button was pressed again to start the test.

The test ended automatically once the voltage drops below the setting limits.

Step 3: The samples were recharged fully again after finishing the discharge test.

Step 4: Gamma irradiation of the samples.

Both lithium polymer battery and the LiFePO4 battery samples were exposed under gamma irradiation for 18, 70 and 240 h in the Co-60 irradiator at a dose rate of 39.2 krad/hr.

Step 5: The discharge test was repeated with the same operations of the pre-test.

8.4. Results and discussion

The discharge curves (voltage versus time) of the lithium polymer battery and the

LiFePO4 battery are plotted in Figure 8.3. For each exposure period, the non-irradiated and gamma-irradiated samples were compared with each other.

61 For the lithium polymer battery, a noticeable difference in the sharp drop part of the discharge curve between non-irradiated and gamma-irradiated batteries is seen from the figure. The voltage of the gamma-irradiated batteries decreases quicker than the non-irradiated batteries, and hence reaches to the end of the discharge voltage at a shorter time. Since the discharge current is constant through the discharging process and also same for both non-irradiated and gamma-irradiated battery samples, it is easy to find that the specific discharge capacity, calculated by multiplying the discharge current and discharge time, reduces after the gamma irradiation. The time it takes for the lithium polymer batteries to reach the setting end voltage from the initial value is summarized in Table 8.2 and the corresponding capacity loss is calculated.

(a) 18h

(b) 70h

Figure 8.3 Discharge curve of Lithium polymer battery and LiFePO4 battery.

Irradiation time (hour) 18 70 240 Irradiation dose (Mrad) 0.706 2.744 9.408 Discharge time Non-irradiated 4.58 4.87 4.67 (hour) Gamma-irradiated 4.42 4.48 4.10 Discharge current (mA) 13 13 13 Discharge Non-irradiated 59.54 63.31 60.71 capacity Gamma-irradiated 57.46 58.24 53.30 (mAh) Capacity loss 3.5 % 8 % 12.2 % Table 8.2 Capacity loss after irradiation of the lithium polymer battery

As shown in Table 8.2, the capacity loss of the lithium polymer battery exceeds 10% for the maximum irradiation dose (9.408 Mrad), which has been a substantial effect considering that the capacity loss per month due to the self discharge is up to 5% [57].

Furthermore, this result is consistent with the test Ding et al. (2006) carried out, in which approximately 50% capacity fade of a LiCoO2 full cell occurred after irradiated by Co-60 up to 14.4 Mrad dose [40].

Based on the previous discussion about the degradation mechanism of Li-ion battery and XRD results, this capacity loss may be caused by electrode crystal structure change of the Lithium polymer battery after the irradiation, in which deviations of

64 particles from conducting matrix or decreased active material may occur [23-24]. In addition, electrolyte decomposition should also be considered for the capacity fade because the polymer compositions of the lithium polymer battery electrolyte are susceptible to gamma irradiation, and thus decompose.

However, the change of LiFePO4 batteries’ discharge curve prior to and after irradiation is not as evident. Some noise (big fluctuation of the curve) can be seen from Figure 8.3, which may be introduced by measurements. Compared to lithium polymer battery, LiFePO4 battery shows a better stability under gamma irradiation.

One known characteristic of LiFePO4 battery is its excellent thermal and chemical stability over other common Li-ion batteries [58]. From the chemical view of point, the Fe-P-O bond is much stronger than the Co-O. Therefore, crystal structure change of LiFePO4 may not notable as the lithium polymer battery does after irradiation.

Chapter 9: Summary and Outlook

Development in performance of the Li-ion battery, such as high energy density and large capacity, makes it popular in many applications. In addition to working as power source for consumer electronic devices, the Li-ion battery also provides power for electrical vehicle, aircraft, modern rescue, and sampling equipment in nuclear environment. For example, the Li-ion battery is a significant power supply for robots deployed in the aftermath of Fukushima nuclear accident. For aerospace and nuclear environment applications, radiation is an important concern cannot be ignored.

Therefore, understanding the radiation effects on the Li-ion battery is necessary for its further developments.

Radiation effects on the Li-ion battery include many aspects, including the inner compositions of the battery and electrical properties. Cathode, important component of the battery, which is made up by lithium metal oxide and determine the active lithium ions number (affect the electrical properties directly), should be paid more

66 attention for studying the radiation effects. Many advanced characterization techniques, such as AFM, XRD, and TEM provide access to the cathode study.

In this study, AFM height images of the Li-ion battery cathode showed the surface morphology characteristics of the cathode before and after irradiation treatment. For both neutrons and gamma rays irradiation, there was an increase in the cathode particle size, and this change became noticeable with the bigger irradiation dose. Also, fast neutrons showed a bigger influence on the particle size than the fast plus thermal neutrons with same neutrons fluence. Roughness data collected during AFM scanning process further proved this particle coarsening. Based on studying on crystal structure characteristics, crystal boundary migration after irradiation was thought to be responsible for this change. Internal stress produced after the collisions between the neutrons and cathode atoms and driven the crystal boundary to migrate and release deposited energy. Crystal structure disordering due to gamma ray also caused the internal stress and lead to the coarsening finally. XRD characterizations of the cathode demonstrated the assumption about the particle size change mechanism.

Based on XRD patterns, peaks change, such as less peak split and deceased peak

67 intensity ration (I003/I104), showed that crystal structure disordering occurs after irradiation. Since crystal structure change of the Li-ion battery cathode may cause direct influences on electrical properties, such as capacity and power, electrical test was carried. From the discharge curve, substantial capacity loss was found after gamma irradiation, proving the radiation effects on the cathode crystal structure.

In the future work, some other characterization techniques should be utilized to study the Li-ion battery cathode to obtain more parameters and information. Also, considering the complex compositions of the Li-ion battery, other parts should also be considered under radiation. By carrying radiation on specific part, then installing them back and measuring the electrical properties of the whole battery, more targeted and convincing results can be obtained for the radiation effects on the Li-ion battery.

Appendix A: PDF Card 70-2685

PDF#70-2685: QM=Calculated; d=Calculated; I=Calculated

Lithium Cobalt Oxide Li Co O2

Radiation=CuKa1 Lambda=1.54060 Filter= Calibration= d-Cutoff=17.7 I/Ic(RIR)=4.43 Ref= Calculated from ICSD using POWD-12++

Rhombohedral - (Unknown), R-3m (166) Z=3 mp= Cell=2.816x14.054 Pearson=hR4 (?) Density(c)=5.051 Density(m)=4.71A Mwt=97.87 Vol=96.5 F(19)=999.9(.0000,19) Ref= Akimoto, J., Gotoh, Y., Oosawa, Y. J. Solid State Chem., 141 298 (1998)

FIZ=051182: REF Journal of Solid State Chemistry. CLAS -3m (Hermann-Mauguin) - D3d (Schoenflies). PRS hR12. ANX ABX2. WYCK c b a. REM TEM 27 C. Synthesis and structure refinement of Li Co O2 single crystals See ICSD 51381.

Strong Line: 4.68/X 2.00/5 2.40/3 1.41/1 1.43/1 2.30/1 1.55/1 1.84/1 1.35/1 2.34/1

19 Lines, Wavelength to Compute Theta = 1.54056A(Cu), 1%-Type = Peak Height

# d(A) I(f) h k I 2-Theta Theta

1 4.6847 100.0 0 0 3 18.928 9.464

2 2.4028 28.2 1 0 1 37.395 18.698

3 2.3423 3.7 0 0 6 38.398 19.199

4 2.3039 9.0 0 1 2 39.064 19.532

5 2.0034 50.5 1 0 4 45.223 22.612

6 1.8420 7.5 0 1 5 49.438 24.719

7 1.5616 1.0 0 0 9 59.112 29.556

8 1.5500 8.4 1 0 7 59.597 29.798

9 1.4254 10.3 0 1 8 65.420 32.710

10 1.4080 10.4 1 1 0 66.333 33.166

11 1.3484 6.7 1 1 3 69.675 34.837

12 1.2177 1.2 1 0 10 78.482 39.241

13 1.2148 2.1 0 2 1 78.704 39.352

14 1.2068 1.8 1 1 6 79.331 39.665

15 1.2014 0.9 2 0 2 79.755 39.877

16 1.1712 1.1 0 0 12 82.250 41.125

17 1.1520 3.7 0 2 4 83.929 41.946

18 1.1317 2.1 0 1 11 85.784 42.892

19 1.1186 0.8 2 0 5 87.037 43.518

Reference

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