SYNTHESIS AND ELECTROCHEMICAL STUDIES OF SPINEL TYPE ANODE MATERIALS FOR LITHIUM-ION BATTERIES

Abstract

With the advancement in technology, the emission of greenhouse gases as well as the depletion of natural resources is escalating and imparting worse effects on the environment. Therefore, today the major concern is how to preserve natural resources. One of the ways is to shift towards renewable, clean, and non-polluted resources using energy storage and conversion technologies such as solar cells, rechargeable batteries, super-capacitors, and many more. Rechargeable batteries are one of the advancing and rapidly growing technology. Moreover, the development of electronic devices such as laptops, iPads, and mobile phones and transportation industries like electric vehicles (EVs), and hybrid electric vehicles (HEVs), are innovating. All these sectors need good storage and conversion devices with high energy density, high power density, durability, and environmental friendliness. Among the energy storage devices, Lithium ion batteries (LIBs) are fulfilling the demand for renewable and non-renewable energy resources. The development of LIBs has been started in 1990-1991 with graphite as an anode and oxide-based (LiCoO2) as cathode materials. Till now, in commercialized batteries, graphite has been used as an anode electrode, but still, there is a need to improve the limiting properties of graphite and develop alternative materials. Since the graphitic anode has several disadvantages such as volume expansion (~10%), unstable at high temperatures, and formation of dendrites at higher current rates which restricts its high power application and escalates the safety issues of LIBs. Recently, Ti-based spinel materials such as Li2ZnTi3O8 and other spinel-type transition metal oxides such as NiMn2O4 have been analyzed as alternative anodes for LIBs. Hence, the research work

vii reported in this thesis mainly focuses on the synthesis, physicochemical characterizations, and electrochemical analysis of Ti-based and transitional metal oxide-type anode materials. The research conducted within the thesis framework demonstrates that spinel-type anode material from the Ti-based family, Lithium Zinc Titanate (Li2ZnTi3O8), and AB2O4 type, Nickel Manganese Oxides (NiMn2O4) are promising alternate as anode materials in LIBs. The low-cost and facile synthesis routes are used to optimize the structural and morphological properties which leads to stable and advanced anode materials with superior electrochemical performance. These proposed anode materials have the capability to replace the commercialized materials in LIBs. The results of the current research work have been divided into seven chapters with the following brief details: Chapter 1 includes the introduction of rechargeable batteries and an overview of Lithium-ion batteries (LIBs) with an explanation of different types of anode materials explored for LIBs. In this chapter, a literature review of spinel-type materials specifically Li2ZnTi3O8 and NiMn2O4 has been described as alternate anode materials for this research work. Chapter 2 contains the experimental details such as synthesis routes and characterization techniques used in this research work. Synthesis of pristine Li2ZnTi3O8, pristine NiMn2O4, Cr-doped Li2ZnTi3O8, MoS2, and NiMn2O4- NiMnO3@MoS2 samples using solid-state, sol-gel and hydrothermal methods have been attempted in this study. This chapter includes a detailed analysis and experimental conditions of the characterization techniques used such as XRD, SEM, TEM, EDX,

viii TGA, XPS, and A.C./D.C. conductivities. The preparation of electrodes and coin-cell fabrication followed by detailed parameters used for electrochemical characterizations such as Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetry (CV), Galvanostatic Charge-Discharge (GCD), and rate performance testing are also reported. Chapter 3 includes the observed physicochemical and electrochemical results of both pristine Li2ZnTi3O8 (LZTO), and NiMn2O4 (NMO). Both of these spinel-type materials have been prepared using a simple and facile solid-state reaction route followed by calcination at high temperatures in the air atmosphere. XRD results confirm the proper phase of LZTO and NMO is formed with cubic spinel structures showing P4332 and Fd3m space group respectively. Morphological studies are carried out by scanning electron microscope and energy dispersive X-rays, revealing the irregular shape particles and uniform distribution of elements respectively. Conductivity measurements indicate that the LZTO, samples of NMO calcined at 700 ºC and 800 ºC (NMO_700, and NMO_800) attain the conductivity of the order of 10-5 , 10-7 , and 10-6 S/mm respectively. Electrochemical characterizations such as CV, EIS, and GCD depict the potential of both materials as alternative anodes. Chapter 4 describes the effect on the electrical and cycling performance of NiMn2O4 (NMO) hexagonal-shaped nanoparticles synthesized using different synthesis routes such as solid-state and sol-gel with varied calcination temperatures. XRD results confirm the highly crystalline cubic spinel structure with zero impurities for all samples, except NMOS_700, which indicates the presence of a slight NiMnO3 phase. Morphological results by SEM and TEM micrographs confirm the formation of hexagonal shape particles of size less than <0.5μm. Electrical measurements depict the

ix strong dependence of conductivities (σAC and σDC) on grain size, grain boundary, and operating temperature. All the samples exhibit conductivities between 10-7 - 10-3 S/mm with the varied calcination temperature. Electrochemical performances are studied using EIS, CV, and GCD profiles. Sample NMOB_700 and NMOB_800 exhibit the initial discharging capacity of 1104 mAh g-1 and 1188 mAh g-1 at 100 mA g-1 current density. All the samples exhibit above 98% columbic efficiency after two initial cycles and show the reversible nature of NiMn2O4 and excellent cyclability. The electrochemical results confirm that preparation methods and calcination temperature greatly impact the grain properties of materials. Chapter 5 deals with the effect of Cr-doping on the Li2ZnTi3O8 (LZTO) anode to enhance the electrochemical properties of LIBs. Cr-doped and undoped lithium zinc titanate, Li2ZnCrxTi3-xO8 (x =0, 0.1, 0.3, 0.5) has been prepared using a solid-state ball milling route followed by calcination in the air atmosphere. The cubic spinel structure with the P4332 space group is confirmed through X-ray Diffraction (XRD) for all compositions indicating doping of Cr3+ does not cause any changes within the lattice. SEM and TEM revealed the formation of polyhedron-shaped spherical facets in the sub-micron to the nanometer range. The particle size reduces towards the nanometer range as the amount of Cr doping increases and it also decreases the agglomeration of particles. Sample Li2ZnCr0.3Ti2.7O8 doped with Cr content 0.3 displays higher discharge capacity, better cyclability, and lower polarization among all other Cr-doped samples. Li2ZnCr0.3Ti2.7O8 delivered excellent rate performances attaining capacities of 251.73±4, 184.34±5, 157.42±5, and 119.03±4 mAh g-1 at 0.1C, 0.5C, 1C, and 2C, respectively which is ~10-18% higher than the pristine LZTO sample.Chapter 6 deals with the synthesis of MoS2, and NiMn2O4-NiMnO3@MoS2 composites using the hydrothermal method and wet chemical mixing. Physicochemical properties such as phase, morphology, and distribution of elements are confirmed using XRD, SEM, and EDX respectively. Electrochemical properties are analyzed using CV, EIS, and GCD analysis. Firstly, MoS2 is prepared using the hydrothermal method, and its electrochemical properties are studied in brief. In the second section, a composite of NiMn2O4 is prepared with MoS2. In the MoS2 type of composite, the layered structure of MoS2 provides intercalation of lithium ions without any major volume expansion of NiMn2O4-NiMnO3 material prepared at 80 ºC using simple and facile wet chemical mixing. Furthermore, NMO nanoparticles occupy the spaces between the MoS2 nanosheets making both faces accessible to electrolyte penetration. The resulting composite material displayed a stable cyclic voltammogram profile and discharge capacity of 361.54 mAh g-1 even after 200 cycles at the current density of 500 mA g-1 . Chapter 7 includes the conclusion and summary of the work done in this research work. The results of the optimized samples have been mentioned in this section. This section also includes the outline of the future scope of the present investigation.