A Study of the Effects of Cycling Frequency on Lithium-Ion And

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A STUDY OF THE EFFECTS OF CYCLING FREQUENCY ON LITHIUM-ION AND

LITHIUM-POLYMER BATTERIES’ DEGRADATION

______

A Thesis

Presented to

The Faculty of the Graduate School at the

University of Missouri-Columbia

______

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

______

BHAVANA SHARON GANGIREDDY

Dr. Robert O’Connell, Thesis Supervisor

DECEMBER 2020

The undersigned, appointed by the dean of the Graduate School, have examined the thesis entitled

A STUDY OF THE EFFECTS OF CYCLING FREQUENCY ON LITHIUM-ION AND

LITHIUM-POLYMER BATTERIES’ DEGRADATION presented by Bhavana Sharon Gangireddy, a candidate for the degree of master of science, and hereby certify that, in their opinion, it is worthy of acceptance.

______Dr. Robert O’Connell, Ph.D. Committee Chair and Thesis Advisor

______Dr. Naz Islam, Ph.D.

______Dr. Stephen Lombardo, Ph.D.

ACKNOWLEDGEMENTS

First, I would like to wholeheartedly thank and praise God, the almighty for the blessings and opportunity he bestowed on me, so that I have been able to accomplish my thesis.

I would like to sincerely thank my advisor, Dr. Robert O’Connell, for believing in me and for the remarkable support and expert guidance he has provided throughout my time as his student. He was always quick and patient with my numerous questions and doubts, and genuinely cared about my work. Without his persistent help, I would not have accomplished my goal.

I would like to thank Dr. Naz Islam and Dr. Stephen Lombardo for being my committee members and for extending their time and concern.

I would like to thank my parents, Mr. Deva Kumar and Mrs. Jerusha Kantha Kumari for their love and support in all points of my life, especially in my education. I would like to thank my sister, Keerthana Sharon for always being there for me and for her valuable suggestions. I would like to thank my friend, Puneeth Bathula for being there for me through thick and thin. I would also like to thank my relatives and friends who believed in me and supported me at all times.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... ii

LIST OF ILLUSTRATIONS ...... iii

LIST OF TABLES ...... iv

ABSTRACT ...... v

Chapter Page

1. INTRODUCTION

1.1. Background ...... 5

1.2. Energy Storage Systems ...... 8

1.3. Battery Energy Storage Systems ...... 16

2. BATTERIES USED IN BESS

2.1. Working mechanism of a battery ...... 17

2.2. Comparison of different types of batteries ...... 19

2.3. Types of Lithium-ion batteries ...... 21

2.4. Characteristics of Lithium-ion and Lithium Polymer batteries ...... 26

2.5. Factors contributing to the degradation of Lithium-ion and Lithium Polymer

batteries ...... 30

3. EXPERIMENTAL DESIGN AND PROCEDURES

3.1. Equipment used and their specifications...... 33

3.2. Lab design ...... 34

3.3. Operating procedures ...... 35

4. RESULTS AND ANALYSIS

4.1. Experimental data and analysis ...... 37

iii 5. CONCLUSIONS AND FUTURE RESEARCH PROSPECTS

5.1 Conclusions ...... 45

5.2 Future prospects ...... 45

REFERENCES ...... 47

iv LIST OF ILLUSTRATIONS

Figure Page

1.1 Global fossil consumption ...... 6

1.2 Various forms of energy storage ...... 9

2.1 Discharging process of a battery...... 18

2.2 Charging process of a battery...... 19

2.3 U.S. grid scale battery storage capacity by chemistry ...... 29

3.1 Experimental set-up of the equipment ...... 34

4.1 Plot of the charging process of the INR battery ...... 38

4.2 Plot of the discharging process of the INR battery ...... 40

4.3 Behavior of discharge capacity under standard rate of charging/discharging (1.3 A,

0.5 A) ...... 41

4.4 Behavior of discharge capacity under increased rate of charging/discharging (2 A,

2 A) ...... 42

4.5 Comparison of three different batteries with different cycling frequencies under

increased charging/discharging conditions (2 A, 2 A) ...... 43

4.6 Comparison of lithium-ion and lithium-polymer batteries under their standard

conditions ...... 44

LIST OF TABLES

Table Page

2.1 Comparison of different chemistries of batteries ...... 25-26

4.1 Projected cycle life of the batteries ...... 44

ABSTRACT

Conventional energy sources are depleting rapidly and might last for another 50-150 years depending on our current usage. Environmental issues like global warming are also rising quickly. Renewable energy sources can address both these issues and offer various other advantages like stabilizing the load, lower maintenance requirements etc. However, due to their intermittent supply, they are not always reliable. The energy harnessed from renewable sources needs to be stored and readily available for our use. Hence, the power system network is shifting towards energy storage technologies. There are various energy storage technologies available, out of which the battery energy storage systems (BESS) for large-scale energy storage are widely used. Examples of BESS are in the modern electronics & electrical devices like laptops, smartphones, iPad etc. This thesis focuses on various types of batteries used in BESS, their degradation processes and the factors contributing to their degradation. Two types of batteries, lithium-ion and lithium-polymer were tested under different conditions to observe the degradation process and the results obtained were analyzed.

vii Chapter 1

INTRODUCTION

1.1 Background

Global demand for energy has been increasing rapidly due to growing populations and economies. According to the U.S. Energy Information Administration (EIA), the total global energy consumption statistics over the last five decades are found to be 86,005.52

TWh in 1980, 105,204.31 TWh in 1990, 117,927.7 TWh in 2000, 153,531.4 TWh in 2010 and 171,006.38 TWh in 2017 [1]. Such a rise in consumption poses two serious problems to the world — depletion of fossil fuel reserves and rise in global warming.

To begin with, continuous usage of fossil fuels to generate electricity started raising concerns of depleting fossil fuel reserves, creating a need for energy security for future consumers. Out of 171,000 TWh of total global energy consumption in 2017, 132,500 TWh of energy dominantly came from fossil fuel sources of coal, crude oil and natural gas [2] as shown in Figure 1.1. These fossil fuel reserves continue to diminish as demand for them keeps increasing. According to the Statistical Review of World Energy 2019 by BP, one of the world's seven oil and gas supermajors, the coal reserves accounted for only 132 years of current production, oil reserves for only 50 years and gas reserves also for only 50 years

[3]. These figures are only helpful as a static measure, since they vary with time as our levels of consumption rise or fall.

Figure 1.1 Global fossil fuel consumption

Another important limit to fossil fuel usage is its environmental impact. Since fossil fuels are carbon-based, their combustion emits carbon dioxide (CO2), which is usually released into the atmosphere. The concentration of carbon dioxide in Earth’s atmosphere is at 414 parts per million (ppm) as of May 2020 which represents an 11 percent increase

[4] since 2000, when it was near 370 ppm . It can be said certainly that the increase in CO2 in the atmosphere is mainly due to burning fossil fuels since CO2 produced from burning fossil fuels has a different isotopic composition from CO2 in the atmosphere, thereby leaving a different imprint on the instruments measured. The CO2 from burning fossil fuels in the atmosphere absorbs infrared radiation and re-radiates it back to the earth, thereby increasing the temperature of the earth, which is known as the greenhouse effect. The

2 emissions from the combustion of fossil fuels are also a major contributor to local air pollution, which has adverse effects on the health of the people.

Fortunately, renewable energy sources provide the necessary solution to these issues as these do not deplete over time and are environmentally friendly.

Renewable energy sources are sources of clean and inexhaustible energy. There are various renewable energy resources like solar, hydropower, wind, tidal, nuclear, biomass and geothermal energy. All these energy sources originate from the sun except for nuclear energy obtained from nuclear fission and geothermal energy, which is the heat generated inside the earth. Even though renewable energy sources have their benefits of inexhaustibility and ability to provide cleaner energy, they are not entirely reliable for electric power generation.

The efficiency of the solar system significantly drops during cloudy and rainy days since the solar panels depend on sunlight to harness the solar energy. Moreover, the solar energy is not available during the night and hence, additional equipment such as batteries are needed to store the energy when it is produced. Wind energy is also not steadily available all the time. It is unpredictable and fluctuating in nature, thus requiring other auxiliary energy storage devices. Electrical power through tidal energy is also limited since the generation can only be done during the high tide period.

Since renewable energy sources are intermittent sources of energy, they cannot provide a continuous and reliable supply of energy to consumers without proper energy storage. Moreover, introduction of these intermittent renewable energy resources makes it difficult to stabilize the power network due to the voltage fluctuations caused by a supply- demand imbalance. There must be a backup power source to avoid frequent blackouts.

3 Even in the case of power generation through conventional energy sources, reliability of supply is needed, since the tripping of even one large generating unit will significantly affect many consumers. One way to solve these issues is by the introduction of energy storage facilities. This facilitates improvements in stability, power quality, reliability of supply, and eliminates the need for new power generation plants by using energy storage units as additional power sources.

1.2 Energy Storage Systems

Since electrical energy cannot be directly stored as electricity, it is converted and stored electromagnetically, electrochemically, thermally, kinetically, or as potential energy depending on the type of the energy storage technology used. The size, cost and capacity of an energy storage system directly depend on the form of the stored energy. These energy storage systems release the electrical energy into the grid during peak load periods or as needed and ensure stable operation of power systems, reliable power supply to consumers, regulation of variable loads on the system, and its frequency control. Various types of energy storage technologies are shown in Figure 1.2.

Figure 1.2 Various forms of energy storage

Different types of energy storage technologies, their merits, and their limitations are briefly discussed below.

Electro-mechanical storage systems:

Electro-mechanical energy storage systems store energy using kinetic or gravitational forces.

Pumped hydro storage is widely used and feasible at a very large scale, operating on the potential energy of water. Water is pumped from a lower reservoir to an upper reservoir, consuming power from the grid during off-peak hours. When energy needs to be injected into the grid, the water from the upper reservoir is released into the lower reservoir through the hydraulic turbines, thereby generating electrical energy. The world’s largest pumped hydro storage plant, the Bath County Pumped Storage Station in Virginia, US has

5 a capacity of 3 GW. Even though pumped hydro storage can be cheaper for very large capacity storage which other storage technologies struggle to match (the installation cost for pumped-storage hydropower varies between $1,700 - $5,100/kW, compared to

$2,500/kW - 3,900/kW for lithium-ion batteries [5]), it offers low energy densities and is not feasible everywhere as special geographic conditions are required.

In compressed air energy storage (CAES) technology, energy is stored as compressed air in an underground storage structure (usually a cave) during off-peak hours.

During the peak load period, the compressed air from the cavern is released back up, heated, and used to drive turbines. The energy of compressed air is converted into kinetic energy, which is then converted into electrical energy by a generator. However, it has a low energy density and can only be used in turbine power plants and it also needs special geographic conditions such as sealed caverns or mines. Only two large-scale, commercial

CAES installations currently exist, one in Huntorf, Germany at 290 MW and the other in

Alabama, US at 110 MW with several more under development.

Flywheels store kinetic energy in a rotating disk, which is coupled to the shaft of an electrical machine, so it acts as motor/generator depending on power demand. During off-peak hours, the machine uses surplus electrical energy from the grid and operates as a motor, storing kinetic energy in the flywheel by accelerating to very high spin rates (up to

50,000 rpm [5]). In peak hours, the flywheel decelerates and the motor runs as a generator, converting kinetic energy of the flywheel to electricity. The acceleration and deceleration of the flywheel are responsible for the storage and release of electrical energy. The world’s largest flywheel energy storage system is in New York, US at 20 MW. Even though they are effective for load-leveling applications, flywheels possess high self-discharge rates,

6 low energy densities, significant amounts of friction and high technical requirements for rotors and bearings. Thorough evaluation of the safety of containment housing is needed to ensure it withstands potential failures (if the rotor breaks apart, the containment must be able to stop free flying projectiles) [6].

In terms of energy storage, these systems have relatively low energy densities, and they are lossy compared to other forms of energy storage. Even though they are efficient to some extent, they are not considered to be the best solution to the issue of grid-level storage.

Electro-magnetic storage systems:

Electromagnetic energy storage systems store the energy in the form of an electric field or as a magnetic field. Technologies include super capacitors and superconducting magnetic energy storage (SMES).

Supercapacitors are high-capacitance double-layer capacitors that can store a thousand times more energy than a typical capacitor. They share the characteristics of both batteries and conventional capacitors [7][8]. In a supercapacitor, both plates are soaked in an electrolytic solution with a thin insulator (like carbon or paper) placed between them. When the plates are charged up, the charge on the electrodes will attract ions with opposite charge in the surrounding electrolyte solution and cause them to attach to the electrode surface.

Then an opposite charge forms on either side of the insulator and an electric double-layer is created. Unlike batteries, there is no chemical reaction in the energy storage process and the process is reversible. However, supercapacitors are very expensive (estimated as 10 times the cost per kWh of flywheels) and they are not well-suited for long-term energy storage due to high self-discharge rates (significantly higher than lithium-ion batteries; they

7 typically lose 10-20 % of their charge per day due to self-discharge) and gradual voltage loss.

SMES systems store energy in a magnetic field, which is created by the DC current flowing through a superconducting coil maintained at cryogenic temperatures. A power conversion system connects the SMES unit to an ac power system, and it is used to charge/discharge the coil [9]. They can inject/absorb vast amounts of energy in a very short period of time and have high efficiencies over 95%. Since superconductor materials present almost negligible resistance at cryogenic temperatures, very little energy is dissipated by ohmic losses. The biggest drawbacks are high costs and the need for compressors and pumps to maintain the low temperature of the coolant, making the system more complex.

Energy densities obtained are also relatively low. Hence, SMES systems are limited to short-time storage applications, such as improving power quality.

Thermal storage systems:

Thermal energy storage systems absorb, store, and release thermal energy in a controlled manner. The available technologies can be classified into three categories: sensible heat media, latent heat media, and chemical heat media.

Sensible heat storage systems are a widely used form of thermal storage system, in which a storage medium like water, molten salts, sand, rocks etc. [10] is heated or cooled to store the thermal energy. The change in the temperature of the medium can be ‘sensed’ and the medium does not change in phase. The capacity of sensible heat storage depends on the specific heat of the medium, its mass, and the allowed temperature change in operation. It is widely used in concentrated solar power applications where it helps to generate electricity after sunset. The sensible heat storage system is heated when solar energy is

8 available (during daytime), and the heat from the system is used to produce electricity in the absence of the solar energy (during nighttime or in cloudy weather). The energy charge and discharge occur through a storage heat-transfer medium circulating in the piping system. Steam is produced from the heat transfer medium, driving a turbine (thermal to mechanical energy), which in turn is coupled to a generator (mechanical to electrical energy), thereby producing electricity.

Latent heat storage systems store energy by keeping the temperature of the medium constant but changing its phase. Hence, temperature change of the medium cannot be

‘sensed’. They use phase change materials (PCM) as medium. The thermal energy is released or stored by PCM during phase change processes (e.g., from solid to liquid).

Charge and discharge involve phase changes, using the resulting enthalpy. It has roughly three times higher energy density compared to sensible heat storage, but it is more expensive. Since the temperature at which a material changes its phase is constant, different phase change materials are required for different energy storage temperatures.

Thermochemical heat storage systems store energy by using reversible thermochemical reactions. During peak hours, the system discharges energy by an exothermic reaction, thereby separating the substance into respective components and releasing heat energy. During off-peak hours, the above reaction is reversed. The resultant components from the previous chemical reaction are combined again by applying heat, thus performing an endothermic reaction. Heat energy is absorbed in this process and the system is charged.

9 Electro-chemical storage systems:

Electro-chemical storage systems involve hydrogen-based energy storage systems, power-to-gas storage, batteries, and flow batteries.

Surplus electrical energy generated during off-peak hours is used to produce hydrogen through water electrolysis or water reduction with carbon. The most common method is water electrolysis where water molecules are decomposed into hydrogen (H2) and oxygen (O2) by applying electricity. The obtained hydrogen is stored in high pressure cylinders. During off-peak hours, the chemical energy stored in hydrogen is converted into electrical energy using fuel cells. H2 and O2 are combined to produce water (H2O) and electricity [11]. This technology can generate power efficiently with low impact on the environment, and it offers greater cyclability than flow batteries and conventional batteries.

However, the round-trip efficiency of the system is lower than that of other storage technologies. If the energy efficiencies for the electrolyzer and the fuel cell are about 60% and 70%, then the round-trip efficiency falls to 42%. The high flammability of hydrogen gas is also a big safety concern.

The power-to-gas concept refers to converting renewable energy into gaseous energy carriers (hydrogen or methane through water electrolysis) which can be used as fuels to generate power. Water is decomposed into hydrogen and oxygen through electrolysis, and the obtained hydrogen is either used as a fuel to generate power through gas turbines or injected and stored in the existing gas infrastructure. Methane gas can be produced from hydrogen gas by combining it with carbon dioxide in the presence of biocatalysts and it can be used as a direct replacement for natural gas (fossil fuel). The technical potential of power-to-gas technology is very high, and it can integrate large

10 amounts of renewable energy sources into the power system [12]. Hence, it is considered one of the most promising strategies for renewable integration and decarbonization of large interconnected power systems.

Flow batteries are also electrochemical cells, but unlike conventional batteries, the electrolyte is not permanently stored in the cells. Two aqueous electrolytic solutions are contained in separate tanks. Electrical energy is stored or released by means of a reversible electrochemical reaction between these two salt solutions. They possess characteristics between those of a battery and a fuel cell; they can be charged and discharged like a battery and can deliver power when supplied with fuel like a fuel cell, but they use charged electrolytes instead of a fuel. Hence, they can supply electricity when provided with charged electrolytes. Good efficiency is obtained since the electrodes do not undergo any physical and chemical changes during operation (since no active materials). The main disadvantage is the complex system requirements of devices like pumps, sensors, containment vessels and flow-check equipment. Different types of flow batteries are available such as vanadium redox battery (VRB), zinc–bromine battery (ZBB), polysulfide bromide battery (PSB) etc.

Conventional batteries are electrochemical cells grouped together in series or parallel. Each electrochemical cell has two electrodes immersed in an electrolyte. During discharge, ions from the anode (first electrode) are released into the electrolyte and metal oxides are formed on the cathode (second electrode). When the electrical charge through the system is reversed, the chemical reactions are reversed, restoring the battery to its original condition, thereby recharging the battery. Different types of batteries are available based on different chemistries, such as lead-acid, nickel–cadmium (NiCd), sodium– sulfur

11 (NaS), lithium-ion (Li-ion) etc. Batteries have high energy densities and are the best option available for cost-effective energy storage technologies, for storing small to medium quantities of electricity.

1.3 Battery Energy Storage systems (BESS):

Battery energy storage systems use batteries to convert electrical energy into potential chemical energy while charging, and chemical energy to electrical energy while discharging, based on reduction and oxidation reactions. Batteries can discharge energy into the power network whenever there is a sudden voltage/frequency drop, and they can be recharged when the voltage/frequency changes. Large-scale BESS usually use NaS batteries, mid-scale applications primarily use Li-ion batteries, small-scale BESS usually use vanadium redox flow batteries. Large battery energy storage facilities provide significant dynamic operation benefits for electric utilities. They offer various features like good voltage and frequency regulation during load disturbances, renewable source penetration in the power systems, reliability, transmission congestion relief, power quality improvement, and spinning reserve [13]. The cost of implementation of BESS is also relatively low and current advances in battery development offer increased energy densities and improvement of the chemistries used in the batteries.

Batteries used in a BESS, their advantages and limitations, and various battery terminologies are discussed in Chapter 2.

12 Chapter 2

BATTERIES USED IN BESS

2.1 Working mechanism of a battery

A battery is a device consisting of one or more electrochemical cells whose chemical reactions create a flow of electrons in a circuit. These electrochemical cells store chemical energy and transform it into electrical energy as needed. To be precise, a battery is a multi-cell array, but many single cells are called batteries in common usage. A car battery is an example of multi-cell battery, with most of the cars having six cells [14]. This practice of using the term battery to include both single and multi-cell devices is followed throughout the thesis.

An electrochemical cell consists of a positive electrode, a negative electrode, an electrolyte (insulator) which conducts ions between the electrodes, a separator and a container. Each electrode is made up of an active material which undergoes chemical reaction. The electrical energy is converted into potential chemical energy while charging the battery, and chemical energy is converted into electrical energy while discharging the battery, based on reduction and oxidation reactions (redox reactions). To enable the flow of ions and electrons between the areas in which these redox reactions occur, the battery has two circuits. One is the internal circuit (the battery itself) which provides a path for the ions to flow, and the other is the external electrical circuit which provides the path for the electrons to flow. The external path is provided by either a load or an energy source to which battery is connected. The flow of ions is enabled by an electrolyte in the battery which may be a solid or a liquid insulating material. A separator (thin insulating material) prevents the two electrodes from contact with each other, since there is an electric potential

13 between them. This prevents the occurrence of a short-circuit. The container provides a safe and controlled environment for the cells in the battery.

There are two types of batteries- primary and secondary batteries. Primary batteries are the batteries which are not rechargeable, whereas the secondary batteries are rechargeable.

During the discharging process, where the battery is connected to a load, a chemical reaction called the oxidation reaction takes place between the anode (negative electrode) and the electrolyte, where the negative ions of the electrolyte move to the negative electrode and react with its active material, resulting in the release of electrons. The electrons then pass through the external load, thereby doing useful work, and flow to the positive electrode, creating a DC current. The sign convention is contradictory to the standard idea that the anode is the terminal into which current flows. A reduction reaction occurs at the positive electrode due to the gain of electrons. This process continues until the battery is discharged and there is no more chemical energy left. The active substances at both the electrodes are diminished and so is the electric potential between them. The discharge process of a battery is shown in Figure 2.1.

Figure 2.1 Discharging process of a battery

14 During the charging process, in which the battery is connected to an external energy source, the two electrodes are restored by reversing the flow of current. The anode becomes the cathode and vice-versa. Electrons move towards the cathode through the external circuit. Therefore, a potential difference is created between the anode and cathode and the battery is charged. The flow exists as long as there is an energy difference between the active substances involved in the reactions. The charging process of a battery is shown in

Figure 2.2.

Figure 2.2 Charging process of a battery

2.2 Characteristics of batteries

Batteries are selected based on the required characteristics/specifications, which are detailed below [15].

Open-circuit voltage: The electric potential difference between the positive and negative electrodes of the cell as a result of the chemical reactions occurring in the cell. It is measured in full charge state of the cell when disconnected from any circuit (no net current flow). It is not the voltage immediately obtained after the charging process is completed,

15 but rather after the battery is allowed to relax from the charging process. Different active substances provide different open-circuit voltages.

Cut-off voltage: Defines the usable voltage range of the cell. The voltage continues decaying from its maximum value when fully charged until it reaches a voltage called the cut-off voltage. From the cut-off voltage, the voltage starts decaying drastically, limiting the use of the device. Hence, a battery should not be discharged below the cut-off voltage.

Nominal voltage: The default resting voltage of the battery (under equilibrium conditions).

It is the average of the maximum voltage when fully charged and the cut-off voltage. For instance, lithium polymer (LiPo) batteries are fully charged at 4.2V/cell, with their cut-off voltage at 3.0V/cell, and their nominal voltage is 3.7V/cell.

Battery capacity: The value of the charge stored by the battery (Ampere-hours) and depends on the amount of active material present in the battery, discharging current, cut- off voltage and temperature. The capacity determines how long a battery can run before recharging is needed. The higher the capacity, the greater the run time of the battery.

Specific energy: The energy per unit mass of the battery (Watt-hour/kg).

Energy density: The energy per unit volume of the battery (Watt-hour/cubic meter).

C-rate: A value numerically equal to the Ampere-hour rating of the cell. Charge and discharge currents are typically expressed in fractions or multiples of the C-rate. C-rate can be defined as a measure of the rate at which a battery is charged or discharged with respect to its capacity. It can be formulated as follows.

C-rate (expressed in h ) = [(charge or discharge current) ÷ (battery capacity)] [16][17]

For example, if a battery is rated at 10Ah at a 2C-rate, then the battery can discharge safely at a discharge current of 10×2= 20A. This current can discharge two such batteries in one

16 hour (2C-rate means 2 batteries/hour with each battery taking 30 mins). If the same battery is rated at 10Ah at a C/5 rate, then the battery can discharge safely at a discharge current of 2A. This current can discharge 1 battery in 5 hours. All batteries have a ‘C’ charge rate in addition to the ‘C’ discharge rate, where the charge rate is usually smaller than the discharge rate.

State of Charge (SoC): The ratio of energy capacity remaining in the battery to the rated capacity at a defined discharge rate.

End of Discharge voltage (EODV): The measured cell voltage at the end of its operating life.

Self-discharge: Determines the shelf life of a battery and highly depends on temperature.

It increases as the battery temperature increases.

Slow charging: A charging current that can be safely applied to a battery without any kind of monitoring or charge termination method (also called trickle charging).

Fast charging: Usually defined as a 1-hour recharge, it requires more complex charging circuitry but gives the customer faster charging time.

Battery cycle life: Number of cycles a battery can be run for, before its capacity falls below

80% of its initial capacity (in other words, after 20% loss of the initial capacity)

2.3 Comparison of different types of batteries

Different types of battery chemistries are available today, each with its own advantages and limitations. The main battery chemistries are discussed below.

Lead acid batteries:

A lead-acid battery is the oldest type of rechargeable battery, which uses lead dioxide as cathode and porous lead as anode with sulphuric acid as the electrolyte. The

17 electric potential between the two electrodes of the cell as a result of internal chemical reactions is around 2.04 Volts. These are low-cost, reliable batteries used in heavy duty applications. However, these batteries suffer from the problem of sulphation, which occurs when they do not undergo periodic full charge processing. During prolonged charge deprivation, lead sulfate from the chemical reactions in the cell converts to a stable crystalline form and deposits on the negative plates. These large crystals of lead sulphate cannot be reversed to lead and lead dioxide in the electrodes. This reduces the battery’s active material, thereby reducing its performance and battery life. Hence, these batteries must be periodically charged for 14–16 hours to attain full saturation. Corrosion also occurs, which is often seen at the terminals of the battery as white powder, as a result of oxidation between two different metals connecting the poles. This can eventually lead to an open electrical connection. The corrosion process is a characteristic of a lead-acid battery; repeated, controlled charging can reduce the corrosion to a low level [18]. Their dependence on hazardous element (lead) poses a problem during their disposal. These batteries have the poorest cycle life with just 200-1800 cycles, depending on the depth of discharge (DoD) [19] and the operating temperature, when compared to other available batteries. Despite these disadvantages, lead-acid batteries are widely used for stationary applications such as lights, alarms, and uninterrupted power supply (UPS) for computers, due to their low cost and low self-discharge rates. They are also used in grid energy storage.

Nickel-cadmium batteries:

A nickel–cadmium battery (NiCd battery or NiCad battery) is a type of rechargeable battery that uses nickel oxyhydroxide as cathode and cadmium as anode with potassium hydroxide as the electrolyte. The electric potential between the two electrodes

18 of the cell as a result of internal chemical reactions is around 1.2 Volts, lower than that of lead-acid batteries. These batteries can maintain the cell voltage and hold charge when not in use. These offer good life cycle (>3500 cycles and even 50,000 cycles at 10% DoD) at low temperatures with a fair capacity and deliver their full rated capacity at high discharge rates. The cycle life and efficiency depend on the type of construction used (pocket type or sinter type). However, they suffer from the problem of memory effect; when a partially charged battery is recharged before being fully discharged, a sudden voltage drop is seen.

The lower voltage is not sufficient for proper management of power conversion, thereby lowering the future capacity of the battery. Moreover, their cost is approximately 10 times higher than that of lead-acid batteries [20]. Nickel and cadmium are also hazardous.

Sodium sulfur:

A sodium sulfur battery (NaS) is a high-temperature battery. The cell construction is different from the previously mentioned construction shown in Figures 2.1 and 2.2. The electrodes are in the liquid state (liquid sodium as negative electrode and liquid sulfur as positive electrode) while the electrolyte is a solid (ceramic beta-alumina). For the battery to operate, the electrodes need to be melted to their liquid state; thus, the battery operates at around 350°C. The electric potential between the two electrodes of the cell as a result of internal chemical reactions is around 2.075 Volts at 350°C. These batteries have high cycle life and high specific energy. However, the power output is very small at room temperature

(limited power to energy ratio) and the operating temperature must be kept at around 350

ºC through insulation or heating through the cell’s own power. It uses around 10% of its own capacity per day to maintain its temperature when not in use, which lowers the energy

19 density. Also, cracking of the ceramic electrolyte tube may occur, and corrosion may also take place due to the sulfur.

Lithium based batteries (lithium ion and lithium polymer):

Lithium is highly suited to be used as the negative electrode in a battery, since it has a high electrochemical reduction potential of 3.045 V which gives a sure 3 V battery when combined with a positive electrode and is the lightest metal available, giving rise to a cell of high specific energy. Since lithium is highly reactive with water, it cannot be used with an aqueous electrolyte; thus, non-aqueous electrolytes are used in lithium batteries, such as solutions of lithium salts in polar organic/inorganic liquids, fused lithium salts, ionically conducting polymers and ionically conducting ceramics. Based on these alternatives, many chemistries of lithium batteries are possible. Polymers with conductivity similar to that of lithium ions have also been under development.

Rechargeable lithium batteries include lithium-ion (Li-ion) batteries and lithium polymer (Li-po) batteries.

In Li-ion batteries, an intercalated lithium compound is used as the positive electrode and graphite is used as the negative electrode with a liquid chemical electrolyte.

These batteries have a high energy density, no memory effect (except lithium iron phosphate cells) and low self-discharge. However, they can pose a safety hazard since they contain flammable electrolytes that can lead to explosions and fires if damaged or incorrectly charged. Also, the liquid electrolyte can become unstable at extreme temperatures and cause thermal runaway.

In Li-po batteries, polymer electrolyte is used instead of a liquid electrolyte. High conductivity semisolid (gel) polymers are used. Li-po battery technology is newer than Li-

20 ion technology and the polymer batteries have higher specific energy than other lithium battery types. They are more flexible and safer because of a lower chance of leaking electrolytes, which can result in thermal runaway. However, they have a higher manufacturing cost and store less energy than the same sized Li-ion battery.

Comparison of the above-mentioned battery chemistries is shown in Table 2.1 below [19][20].

Battery type Energy density Energy Cycle life Self-discharge

(Wh/kg) efficiency rate (%)

(%)

Lead acid [19][20] 30 85-90 A) valve 2-5

regulated: 200-

300 at 80% DoD

B) flooded type:

1000-2000 at

70% DoD

Nickel cadmium 50 60-83 based on 3000 5-20

[20] the type of

construction

Sodium sulfur 100 89-92 2500 No

[19][20]

21 Lithium ion [20] 80-150 90 and higher 3000 at 80% ~1

DoD

Lithium polymer 100-150 90 and higher 500- 600 ~5

Table 2.1 Comparison of different battery chemistries

2.4 Types of Lithium-ion batteries

Different chemistries of lithium-ion batteries, based on the materials of electrodes and electrolytes, are discussed below.

Lithium cobalt oxide (LCO):

Lithium cobalt oxide (LiCoO2) batteries use cobalt oxide as cathode and graphite as anode. They have stable capacities with high energy densities, thus making them a popular choice for cellphones, laptops, and digital cameras. However, these batteries have a relatively short life cycle and low thermal stability. Their internal resistance increases with cycling and aging and after 2-3 years of use, they become of no use due to a large voltage drop caused by high internal resistance. Thus, they offer only a relatively low discharge current with their safety circuit limited to a charge and discharge rate of about

1C. This means that a 2500 mAh 18650 cell can only be charged and discharged with a maximum current of 2.5 A. Forcing a fast charge or applying a load higher than 2.5 A could cause overheating and undue stress.

Lithium Manganese oxide (LMO):

Lithium manganese oxide batteries use a spinel structure with lithium manganese oxide (LiMn2O4) as the cathode material. Another version of lithium manganese oxide is also available, which uses the compound Li2MnO3 as the cathode. The spinel arrangement improves ion flow in the battery, thereby lowering the internal resistance of the battery.

22 Due to this, its current handling capability increases (enables fast charging/discharging) and hence, an 18650-LMO battery can be discharged at high currents of 20–30 A while not getting unreasonably hot. These are high power, low-capacity batteries, often used for medical devices, electric vehicles, and power tools. High thermal stability and enhanced safety are also obtained, but their cycle life is relatively limited.

Lithium Iron Phosphate (LFP):

Lithium iron phosphate (LiFePO4) batteries use iron phosphate as cathode and graphite as anode. These batteries are more tolerant at full-charge conditions and are less prone to thermal stress than other Li-ion batteries under prolonged high voltages. They have low resistance and high discharge rate capability [21], which increase their safety and thermal abilities, and make them ideal for electric vehicle applications. Their benefits are high current rating, long cycle life, good thermal stability, enhanced safety and thermal tolerance. However, they possess low-voltage capacities and offer less energy than other types of Li-ion batteries. They also have a higher self-discharge rate than other Li-ion batteries. Even though this can be reduced by using other techniques, the cost incurred increases. Also, they must be free from moisture, and cold temperatures reduce their performance.

Lithium Nickel Manganese Cobalt oxide (NMC):

Lithium nickel manganese cobalt oxide (LiNiMnCoO2) batteries use a combination of nickel, manganese, and cobalt for the cathode (usually 60% nickel, 20% manganese, and

20% cobalt), and graphite for the anode. These batteries also can have either high-specific energy or high-specific power, but not both. For example, NMC designed for optimal specific energy in an 18650 cell at 2,800 mAh can deliver 4 A to 5 A, whereas NMC for

23 optimal specific power in the same cell has only 2,000 mAh capacity but delivers a continuous discharge current of 20 A. Nickel provides the cell with high-specific energy

(but reduced stability) and manganese provides low internal resistance (but reduced specific energy). However, the combination makes them suitable for electric powertrains and cordless power tools.

Lithium Nickel Cobalt Aluminum oxide (NCA):

Lithium nickel cobalt aluminum oxide batteries (LiNiCoAlO2) are similar to NMC batteries and provide high energy and power densities, along with good cycle life. Due to higher energy densities, they are suitable for applications in electric powertrains and grid storage. Also, the addition of aluminum gives better stability to the battery compared to the

NMC However, these batteries are expensive, and their safety is not reliable.

Lithium Titanate (LTO):

Lithium titanate (Li2TiO3) batteries replace the graphite in the anode with lithium- titanate and use LMO or NMC as cathode. Replacing graphite with LTO provides it with a larger surface area compared to carbon, allowing the electrons to enter and exit the anode very quickly, which makes them one of the fastest load-charging batteries available

(delivers a high discharge current of 10C). The cycle count is said to be higher than that of a non-LTO Li-ion battery. They are safer in terms of thermal tolerances; thus, they are widely used in electric vehicle and military applications. However, they have lower inherent voltage and lower specific-energy ratings (only 65 Wh/kg) than conventional lithium technologies, and they are expensive.

When selecting a battery, terms like IMR, INR, ICR etc. are used in the market instead of LMO, NMC, LCO etc. creating a lot of confusion. IMR refers to Li-ion

24 Manganese Rechargeable, INR refers to Li-ion Nickel Rechargeable and ICR refers to Li- ion Cobalt Rechargeable. IMR refers to any Li-ion chemistry containing manganese (could be LMO or NMC), INR refers to any Li-ion chemistry containing nickel (could be NMC or NCA) and ICR refers to any Li-ion battery containing cobalt (could be LCO, NMC or

NCA).

BESS widely use Li-ion batteries rather than other available options because technological advances and improved manufacturing capacity have resulted in a sharp decline of over 70% in the price of Li-ion chemistries from 2010-2017 [22], as shown in

Figure 2.3.

Figure 2.3 US grid scale battery storage capacity by chemistry; Data source: U.S. Energy Information Administration, Form EIA-860.

25 2.5 Factors contributing to the degradation of Lithium-ion and Lithium Polymer Batteries

Batteries suffer from significant problems of degradation (reduced cycle life) and ageing (reduced calendar life). To obtain an efficient performance from a battery, it is required to have knowledge of various factors which affect its performance adversely and contribute to its degradation. It is also necessary to have a good understanding of its ageing mechanisms. Rate of degradation depends on various external stress factors like overcharging, rates of charging/discharging (cycling), depth of discharge (DOD) of battery cycles, extreme temperatures, and also on the battery’s life cycle [23]. Ageing or self- discharge [39] occurs due to various internal processes and their interactions which are difficult to understand individually [24]. These factors are described briefly below.

Cycling: Cycling (charging and discharging) eventually reduces the active material of the battery (permanent change), thereby resulting in increased internal resistance and permanent capacity loss [33]. The magnitude of this loss is dependent on the number of cycles, the depth of discharge (DoD) that the battery is subjected to during these cycles, and the rates of charging and discharging (C-rates). If the battery is discharged at high C- rates (high discharge currents), the cycle life decreases and hence, the best operation can be achieved with low load currents (around 0.2C-0.5C rate). The Li-po cycle life is shorter, and the batteries store less energy than the same sized Li-ion.

Self-discharge: This is a phenomenon which reduces the stored charge of the battery even if it is not used, that is, during its storage, thereby leaving it with less than a full charge when it is initially used. This affects the battery’s calendar life, and it decreases the energy efficiency of the electricity storage process if the battery must be maintained in its fully charged state for long time periods. The rate at which self-discharge occurs differs for

26 different batteries, since it depends on the battery’s chemistry, its construction, and most importantly on the temperature at which it is stored. Most Li-ion batteries self-discharge about 5% in the first 24 hours after manufacture and then lose 1–2% per month (protection circuitry adds another 3% loss per month). Li-po batteries self-discharge about 5% per month [25] [26].

Solid Electrolyte Interphase (SEI) film: This is a major cause of degradation of the Li- ion and Li-po batteries. This process occurs mainly during the first cycle of the battery but also occurs to a lesser extent during subsequent cycles. When a new battery starts to operate, a certain number of active lithium ions are consumed irreversibly at the electrode/electrolyte interface when the electrode is in the charged state. Resultant products from the chemical reaction cover the electrode’s surface in protective layers. Since SEI layers are only permeable for lithium ions but impermeable for other electrolyte components and electrons, they mitigate the process of further reduction of electrolyte compounds and the corrosion of the charged electrode [24]. Eventually, the SEI penetrates the pores of the electrode and the pores of the separator, which decreases the accessible active surface area of the electrode and increases electrode impedance. The rate at which this ﬁlm is formed decreases when a stable ﬁlm has been formed. However, this process leads to gradual decrease in the amount of active material of the battery, thereby leading to capacity fade.

Temperature: Li-ion and Li-po batteries suffer from stress when exposed to heat. A temperature above 30°C (86°F) is considered elevated temperature and for most Li-ion batteries, a voltage above 4.10 V/cell is considered as high voltage. Exposing the battery to high temperature can be more stressful than cycling.

27 Many experiments have examined the cycling performance and aging of commercial Li-ion and Li-po cells [27][28][29][30]. However, each of these studies typically focuses on a single chemistry under a limited subset of conditions in order to understand the influence of a particular variable, such as C-rate [31], mechanical stress [32] or temperature [33][37]. Hence, in this project, several experiments were designed to understand the performance and degradation of the batteries under the influence of one variable in each test, which are described in Chapter 3.

28 Chapter 3

EXPERIMENTAL DESIGN AND PROCEDURES

3.1 Equipment used and their specifications

Experiments were designed to evaluate the performance of batteries of several different chemistries. Li-ion (INR/NMC) and Li-po chemistries were considered for comparison. The batteries selected were a Li-ion INR LG HE2 18650 battery and a 505068

Li-po battery. The experimental set-up involved an iCharger 1010B+ to charge/discharge the batteries, an input power supply for the iCharger, a mini-USB data line, the batteries, a battery holder, and ‘logview’ software to display, plot and analyze the charge and discharge data.

The iCharger 1010B+ is a synchronous balance charger/discharger which supports both Li-ion and Li-po batteries. It has various charging/discharging settings such as fast charging, normal charging, balance charging, cycling, storage etc., thereby making it possible to experiment with different settings of various variables and to evaluate their performance by observing the data collected.

The Li-ion LG HE2 18650 battery is a cylindrical battery with its first four numeric digits indicating its dimensions of 18 mm width and 65 mm length, and the last digit indicating its cylindrical shape. LG refers to the LG Electronics company which manufactured the battery and HE2 refers to the model number. It is an INR/NMC battery.

The Li-po 505068 battery is a rectangular flat-topped battery with dimensions of 5 mm thickness, 50 mm width and 68 mm height.

Logview software was used to display and plot the charge and discharge data of each battery.

29 3.2 Lab design

An input supply of 5 V was provided to the iCharger. The positive pole of the main charging port of the iCharger is connected to the positive of the battery and the negative pole of the port is connected to the negative pole of the battery. The iCharger offers protection for reverse polarity. It is then connected to a laptop with logview software installed via a USB cable. Pairing of the laptop and the iCharger is done in the logview window. The iCharger offers different program modes for different batteries, and the desired battery was selected from the options offered (lithium battery for both Li-ion and

Li-po in this context). Charging mode, discharging mode or cycling mode were selected based on the experiment being conducted. The experimental set up is shown in Figure 3.1.

Figure 3.1 Experimental set-up of the equipment

30 3.3 Operating procedures

Test 1: The Li-ion INR LG HE2 battery was first tested under its standard charging and discharging conditions to observe how well it performs within the standard limits of the C- rates and rest time between cycles. The battery is rated at 2500 mAh with a standard charging current of 1.25 A (0.5C-rate) and discharging current of 0.5 A (0.2C-rate) with 2 minutes between charging and discharging processes, and 20 minutes between the cycles.

Since the iCharger does not allow setting the current values to the second decimal place, a charging current of 1.3 A is taken instead of 1.25 A as the standard charging current. After the iCharger is set with the value of charging current, the charging process is started. The charging process involves two stages - constant current charging followed by constant voltage charging [36]. The battery is charged by constant current charging until the battery gets to 4.2 V, followed by constant voltage charging until the current decreases to 0.05 A, which is clearly shown in the results section in Figure 4.1. The procedure was performed at ambient room temperature so that no extreme temperatures were involved. The iCharger is provided with an in-built fan which is switched on automatically if the temperature increases. Then the battery was allowed to rest for 2 minutes before the discharging process began. The discharge current is set on the iCharger, along with the cut-off voltage, below which the battery is not supposed to be discharged. The battery is then discharged through the constant current method at 0.5 A until the voltage reaches its cut-off voltage of 2.5 V, clearly shown in Figure 4.2. The recorded data appears in the logview window in both graphic and tabular forms. One charging process and one discharging process constitute one cycle of the battery. The battery was tested for 100 consecutive cycles with 20 minutes rest after each cycle.

31 Test 2: A second Li-ion INR battery was then tested under increased charging and discharging rates of 2 A each, with the same rest time of 2 minutes between the charging and discharging processes, and 20 minutes between the cycles, to observe the effect of higher charging/discharging rates on the battery performance.

Test 3: Later, three Li-ion INR LG HE2 batteries were tested, each with different rest times between the cycles to observe their performances. These are charged and discharged at the same higher current of 2 A (as in test 2) with different rest times of 2 minutes, 20 minutes and 40 minutes. These batteries were cycled for several cycles, and their data was extrapolated and graphed using Excel.

Test 4: A Li-po 505068 battery was later tested in the same way as in test 1 under its standard charging and discharging conditions to evaluate its performance [40][41]. The battery is rated at 2000 mAh and tested under its standard charging and discharging currents of 1.5 A with the same rest time of 2 minutes between the charging and discharging processes, and with 20 minutes between the cycles. Standard condition performances of the two batteries were compared by comparing the results of test 1 and test 4.

32 Chapter 4

RESULTS AND ANALYSIS

4.1 Experimental data and analysis

Test 1: A plot of the charging process of a Li-ion INR LG HE2 18650 battery under its standard charging conditions is shown in Figure 4.1. The blue, red, pink, green and purple plots in the figure denote, respectively, the capacity, voltage, power, current and internal temperature characteristics of the battery throughout the charging process. The two stages of the charging process can be clearly seen in the figure. The constant current charging lasted for around 1 hour 32 minutes into the charging process, with a constant current plot until then, and voltage plot increasing gradually to 4.2 V. After the voltage reached 4.2 V, the charging mode switched to constant voltage charging. The charging process was continued with a constant voltage of 4.2 V for the rest of the process with gradually decreasing current. The current characteristics (green plot) for the constant voltage charging mode are not visible in the figure, since the power characteristics (pink plot) follow the current characteristics, and hence, the power plot was plotted over the current plot. The internal temperature of the battery increased around 2.6C during the charging process. The current, voltage and temperature plots obtained during the charging process are similar with the results obtained from other experiments in this field [38]. The blue plot represents the capacity change in the battery (slope of the capacity plot is still positive even in the discharging process since it denotes the change in the capacity, not specifically an increase or a decrease), implying the change to be rising in the case of charging. The entire charging process was completed in 2 hours 7 minutes.

Figure 4.1 Plot of the charging process of a Li-ion INR battery

A plot of the discharging process of a Li-ion INR LG HE2 18650 battery under its standard discharging conditions is shown in Figure 4.2. The battery was discharged at a constant current of 0.5 A (green plot), until its voltage reached its cut-off voltage of 2.5 V (red plot).

The rise in the capacity plot (even though it is actually decreasing) denotes the change in capacity, implying that the battery’s capacity changed around 2300 mAh, or in other words, decreased around 2300 mAh by the end of the discharge process. The discharging process took around 4 hours 35 minutes.

The plots in Figures 4.1 and 4.2 constitute one cycle of the battery, which took around 6 hours 40 minutes to be completed. After testing for 100 cycles under standard charging/discharging conditions, the battery behavior of the change in discharge capacity

(from the blue lines in the Figures 4.1 and 4.2) as a function of cycle number [35] is as shown in Figure 4.3.

The projected number of useful cycles (cycle life) was calculated using the discharge capacity of the battery. The slope of the discharge capacity plot (Figure 4.3) was calculated for the last 25 cycles run on the battery and the cycle number at which the battery reaches 80% of its initial capacity was estimated. Since 80% of initial capacity is 1911 mAh, this battery is estimated to have a cycle life of around 1493 cycles.

Figure 4.2 Plot of the discharging process of a Li-ion INR battery

2500 2450

) 2400 2350 2300 2250 2200 2150

Discharge Discharge capacity (mAh 2100 2050

2000

1 5 9

21 33 13 17 25 29 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97 Number of cycles

Figure 4.3 Behavior of discharge capacity under standard rate of charging/discharging (1.3 A, 0.5 A)

Test 2: A Li-ion INR LG HE2 18650 battery was tested for 100 cycles under increased charging and discharging conditions of 2 A charging and 2 A discharging. The behavior of the change in discharge capacity as a function of cycle number is plotted in Figure 4.4 along with the standard condition results from Figure 4.3 for comparison. It can be seen in

Figure 4.4 that the two graphs begin to separate after approximately 80 cycles, with the increased charge/discharge data degrading faster.

2500 2450 2400 2350 2300 2250 2200 2150 2100

Discharge Discharge capacity (mAh) 2050

2000

1 5 9

13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97 Number of cycles

Standard conditions with 20 minutes rest time Increased conditions with 20 minutes rest time

Figure 4.4 Behavior of discharge capacity under increased rate of charging/discharging (2 A, 2 A)

The projected cycle life was calculated using the discharge capacity of the battery.

The slope of the discharge capacity plot was calculated for cycles 70-80, run on the battery and the cycle number at which the battery reaches 80% of its initial capacity is estimated. Since 80% of initial capacity is 1902 mAh, this battery is estimated to have a cycle life of around 1188 cycles, implying that the higher charge/discharge rates reduce the cycle life of the battery.

Test 3: Three new Li-ion INR LG HE2 18650 batteries were tested with different rest times between cycles, to observe the cycle frequency dependence of discharge capacity. The batteries had 2 A/ 2 A charge/discharge rates, rest times and cycle numbers of 2 minutes,

150 cycles; 20 minutes, 100 cycles; and 40 minutes, 75 cycles, respectively. The results are shown in the Figure 4.5.

2500

2400

2300

2200

2100

Dishcrage Dishcrage capacity (mAh) 2000

1 7

91 97 13 19 25 31 37 43 49 55 61 67 73 79 85

103 109 115 121 127 133 139 145 Number of cycles

Increased conditions with 2 minutes rest time Increased conditions with 20 minutes rest time Increased conditions with 40 minutes rest time

Figure 4.5 Comparison of three different batteries with different cycling frequencies under increased charging/discharging conditions (2 A, 2 A).

The projected cycle life was calculated for each battery using the same method used in Figures 4.3 and 4.4. The battery with 2 minutes rest time is estimated to have a cycle life of around 1061 cycles, the battery with 20 minutes rest time is estimated to last for around

1188 cycles, and the battery with 40 minutes rest time for around 1959 cycles. These results suggest that the batteries will degrade faster if they are not given sufficient time between cycles to fully recover.

Test 4: The behavior of the discharge capacity of a Li-po 505068 battery under its standard charging/discharging conditions over 100 cycles is shown in Figure 4.6 along with the standard condition data for the Li-ion battery from Figure 4.3 for comparison. Using the same method as above, the battery is estimated to have a cycle life of about 610 cycles.

The cycle life projections from the four tests are shown in Table 4.1 for convenience.

2600 2400 2200 2000 1800 1600 1400

Discharge capacity(mAh) 1200

1000

1 5 9

13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97 Number of cycles

Standard conditions of Li-ion Standard conditions of Li-po

Figure 4.6 Comparison of lithium-ion and lithium-polymer batteries under their standard Conditions

Battery type/testing conditions Number of cycles

Li-ion under standard conditions (1.3 A, 0.5 A) with 20 minutes rest time 1493

Li-ion under increased conditions (2 A, 2 A) with 2 minutes rest time 1061

Li-ion under increased conditions (2 A, 2 A) with 20 minutes rest time 1188

Li-ion under increased conditions (2 A, 2 A) with 40 minutes rest time 1959

Li-po under standard conditions (1.5 A, 1.5 A) with 20 minutes rest time 610

Table 4.1 Projected cycle life of the batteries

Chapter 5

CONCLUSIONS AND FUTURE RESEARCH

5.1 Conclusions

1) Standard charging/discharging processes, which involve relatively lower currents to charge and discharge batteries, preserve them to an extent, when compared with the higher rates of current used to charge and discharge the battery. Capacity fade is observed with higher C-rates.

2) The tested INR battery had a longer cycle life than the Li-po battery, suggesting an advantage of Li-ion over Li-po.

3) Li-ion battery capacity fade increases with cycling frequency. As the rest time between the cycles of a battery increases, it has more time to relax from the thermal and mechanical stresses from its internal electro-chemical reactions; and hence, the capacity fade observed is less and its cycle life is improved.

4) The ratings of the lithium-ion INR battery and the lithium polymer battery are not the same. At the time of testing the batteries, a Li-po battery rated at 2500mAh was not immediately available, so the next available option of 2000mAh was chosen. Even though comparison between them is possible by just observing the slopes of the plots, comparison of the batteries might be more useful if both batteries were rated at the same capacity.

5.2 Future Research Prospects

1) Various batteries of other chemistries are available besides the INR and Li-Po batteries, which could be tested and compared with the above obtained results.

2) The fast-charging option of the iCharger was not explored in this work. It could be done to observe the behavior of the battery without compromising the battery’s safety since the iCharger is equipped with protection circuitry.

3) Testing of Li-ion and Li-po batteries with identical ratings could be done.

4) Different temperatures can be considered.

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