LITHIUM ION BATTERY-SUPERCAPACITOR HYBRID USING BIMATERIAL ELECTRODES

Abstract

The electrochemical energy storage system stores the electrical energy in the form of chemical energy while charging and delivers electrical energy while discharging. This type of device is a green system as it does not generate pollution. One of the distinctive characteristics of the electric power sectors is that the amount of electricity that can be generated is relatively fixed with time, although demand of electricity fluctuates throughout the day. Thus storage of electrical energy is very much required so that it can be available to meet the demand whenever needed. The electrochemical energy storage devices can be very effective for the same. They are also useful for energy storage in large-scale solar or wind-based electricity generations. Electrochemical energy storage systems are also required to enable the widespread use of hybrid electric vehicles (HEV), plug-in hybrids, and all-electric vehicles. These systems had already proven their efficiency in the modern portable electronic devices such as laptops and smart phones. Lithium ion battery is an electrochemical energy storage system, which is characterized by a high specific energy. A lithium-ion battery utilizes lithium insertion materials in both the negative and positive electrodes. Supercapacitor is an electrochemical energy storage device characterized by an inherently high specific power. The electrochemical double layer capacitor typically consists of two activated carbon (AC) based electrodes soaked with an organic electrolyte. The hybridization of supercapacitors with lithium ion batteries, with a view to combining the high specific power of the supercapacitor with the high specific energy of the lithium ion battery has been persuaded in the last decade with different approaches. Readily available batteries and electrochemical capacitors can be externally hard wire connected, in serial or in parallel. By analogy, the same approaches can be proposed at the internal level within one device, leading to the internal serial or the internal parallel hybrids. The internal parallel hybrids provide a specific energy greater than that of the capacitor and a specific power greater than that of the battery. Thus, the internal parallel hybrids improve the capacitor in terms of specific energy and the battery in terms of specific power. This Ph.D. thesis is based on preparation of internal parallel hybrid of lithium ion battery and supercapacitor consisting of bimaterial electrode. Bai et al. has patented different approaches of making bimaterial electrodes. A novel bimaterial electrode configuration is adopted in this ii work based on one of the existing configuration with some modifications which makes the hybrid system a three terminal device in relation to conventional two terminal devices. Li4Ti5O12 (LTO) and AC are chosen as battery material and supercapacitor material respectively. Sol-gel process is adopted for synthesis of the above mentioned materials throughout the work. The properties of LTO are enhanced by making it nanoporous and also by doping with vanadium. The role of calcination atmosphere on the electrochemical performance of vanadium doped LTO is studied as the samples are calcined in air and argon atmospheres separately. A comparative study of LTO, nanoporous LTO and vanadium doped LTO based bimaterial electrodes with AC in the three terminal hybrid system is done. All the hybrid systems are also subjected to pulsating input current to test their performance in power application. The thesis content is presented in seven chapters as given below: Chapter 1 Introduction This chapter presents brief overview on importance of electrochemical energy storage devices, lithium ion batteries and supercapacitors stating their advantages, limitations and the need of hybridization between battery and supercapacitor. This chapter also introduces the objective of the Ph.D. dissertation. Chapter 2 Literature Review This chapter describes properties of lithium ion battery and supercapacitor as energy storage devices. The performance of different types of materials as anode of lithium ion battery is surveyed and presented. LTO is chosen as anode material based on the properties and drawbacks of different anode materials. Efforts made by the researchers to overcome the limitations of LTO to enhance the electrochemical performance as anode of lithium ion battery are also discussed. The need of hybridization due to limitations of lithium ion battery and supercapacitor is discussed and different configurations of hybrid system are described. The problem formulation for thesis and scope of the research work are described. Chapter 3 Experimental Techniques iii The purpose of the present work is to prepare and characterize the hybrid of lithium ion battery and supercapacitor which includes synthesis of LTO, nanoporous LTO and vanadium doped LTO as battery materials. Characterization of synthesized samples using X-ray diffractometer (XRD), Field emission-scanning electron microscope (FE-SEM) along with Energy dispersive spectroscopy (EDS), Transmission electron microscope (TEM), X-ray photoelectron spectrometer (XPS), Electrochemical impedance spectroscopy (EIS) analyser and Brunauer–Emmett–Teller (BET) analyser for surface area measurement of porous samples is performed. Electrochemical analysis of the samples using charge-discharge, cyclic voltammetry etc. is done following the steps given below: (i) Preparation of electrodes of lithium ion cell as well as of hybrid cell using AC as supercapacitor material with all above mentioned battery material synthesized. (ii) Formation of cell assembly under the inert gas atmosphere. (iii) Electrochemical tests using Arbin Cycler. Chapter 4 LTO and nanoporous LTO This chapter presents the results and discussion on synthesis and characterization of LTO with pure phase. Occurrence of rutile TiO2 phase in the compound LTO (synthesized with stoichiometric ratio Li: Ti = 4:5) is due to loss of Li during calcination at higher temperature and is independent of synthesis method (Shin, Chung, Ryu, Park, & Yoon, 2012). Pure LTO phase is desired as rutile TiO2 does not contribute to lithium intake process while charging at room temperature due to unique primitive tetragonal packing in rutile TiO2. The cation radius of octahedral sites is 0.40 Å but the Li ion radius is normally 0.68 Å. Christiansen et al. 1988 shows that rutile TiO2 does not have any discharge capacity at room temperature (Christiansen, West, Jacobsen, & Atlung, 1988). Quantitative analysis of the impurity phase (Rutile TiO2 observed by XRD analysis) was done by PDXL software using relative intensity ratio (RIR) method. According to the weight% of both phases obtained, theoretical calculations were performed to estimate the excess Li required to synthesize phase pure LTO free from the TiO2 impurity phase. Li4+xTi5O12 (x = 0.2, 0.4 and 0.6) are synthesized by following sol-gel route (as described in chapter 3). Electrochemical analysis is performed to understand the effect of excess Li on the capacity of LTO. Morphological analysis of LTO is performed using TEM image analysis. This chapter also describes results and discussion on synthesis and characterization of nanoporous LTO using carbon nano beads as pore former. Carbon nano beads were synthesized iv using CVD process and incorporated at precursor level during synthesis of LTO. Since the calcination temperature of LTO is 800°C, carbon nano beads got oxidized (at about 650°C) to become CO2 resulting in porous LTO. The surface area of the particles was measured following BET analysis. Average pore size of the powder was calculated from the SEM micrographs of the powder. Effect of porous morphology on electrochemical performance is also studied. Chapter 5 Vanadium doped LTO LTO with enhanced properties can replace the conventional carbonaceous anode material of lithium ion battery. Vanadium (V) doped LTO (Li4Ti5-xVxO12, x = 0, 0.05, 0.1, 0.15) materials are synthesized by sol-gel process followed by calcination of dried gel in air and argon atmospheres separately to examine the role of calcination atmosphere on the oxidation state of V and Ti. The electronic conductivity and electrochemical performance of the synthesized samples as anode of lithium ion battery are reported in this chapter. It is understood from the literature that both doping and synthesis in reducing atmosphere increase the conductivity of the material and the probable cause of the enhancement is the reduction of Ti4+ to Ti3+ (Yi et al., 2009, 2010a; Yu et al., 2011). However, it is not clear whether the reduction of Ti ion occurs either due to doping or calcination in reducing atmosphere. An attempt to study the oxidation states of V and Ti by XPS is made to understand the fact. EIS is performed to estimate of lithium ion diffusion coefficient. For electronic conductivity measurements, the powder samples are compacted in the form of cylindrical pellets by hydraulic press and the samples are heat treated. Ohmic contact for electrical conductivity measurement were developed by applying silver paste onto flat surfaces of pellets. Chapter 6 Hybrid system having bimaterial electrode of LTO and AC In chapters 4 and 5 LTO, nanoporous LTO and V doped LTO as electrode materials of lithium ion battery are discussed. This chapter describes the electrochemical behavior of AC based supercapacitor. A new internal hybrid system of lithium ion battery and supercapacitor is investigated. This hybrid is a three terminal device. The bimaterial electrodes using LTO, nanoporous LTO or V doped LTO as battery material and AC as supercapacitor material are v explored in terms of its electrochemical properties comparative to both LTO based lithium ion battery and AC based supercapacitor. All the three systems are tested for pulse applications by studying the electrochemical behavior on application of pulsating charge-discharge current. Argon calcined V doped LTO based hybrid system shows superior performance for all the pulsating charge-discharge currents. Chapter 7 presents the major conclusions of the present study and scope of the future work.