ANALYTICAL MODELING AND EXPERIMENTAL CHARACTERIZATION OF CATHODE MATERIAL FOR LITHIUM-ION BATTERIES

Abstract

Lithium-ion battery technology is proving to be one such technology that can meet the requirements of high energy density, good power density, non-toxicity, good cyclability, environmental friendly, and cost-effectiveness. It is being extensively used to power the range of products or systems including portable electronic products, large-scale power storage in smart grids, computing and communications devices, military and automotive as well as Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs), etc. The cathode material present in lithium-ion batteries, together with the anode, separator, and electrolyte, plays a crucial role in the energy and power density factor. The fabrication of a lithium-ion cell with high energy content, low price, lightweight, long life, and environmental compatibility strongly depends on the crystallographic structure of the cathode materials. Cathode materials currently used or processed can be classified into three different types depending on the dimensions in which lithium ions can move such as layered type (2-D), Spinel type (3D), and Olivine type (1-D). This research work is focused on the spinel and olivine-type structure of cathode material. Spinel-type structure cathodes allow the lithium intercalation pathways in all three dimensions. Spinel compounds are generalized as AB2O4 (A = Li, B = Mn, Ni). LiMn2O4 is the most studied material in this category. It is a low-cost material and has better cycling stability at 4V redox potential, but delivers lower discharge capacity compared to the layered cathode material. Similarly, Olivine type structure cathodes lie in the category of “polyanion compounds” which contain (XO4) n- (X = P, Si, etc.) tetrahedral anion structural units with a strong covalent X-O bonding network. This structure proves a very promising material in terms of structure stability, a flat voltage plateau, and high theoretical capacity. LiFePO4 is one such cathode material that falls in olivine-type materials. However, its sluggish lithium-ion diffusion kinetics and poor conductivity limit its commercial use. In spite of the rigorous research work on spinel-LiMn2O4 and olivine-LiFePO4 cathode material, a comprehensive study of the change in electrical and electrochemical studies still lacks which prompted to carry out the present research work on spinel and olivine-based cathodes. In order to enhance the characteristics of cathode material, multiple solid solutions of the AB2O4- ABO2 type have gained impressive attention from the research community. Therefore, to provide optimum solution, a series of novel spinel-olivine dual composites (LiFePO4- LiMn2O4) have been included in the current research work. 16 This thesis presents a brief introduction to battery technology, focusing on different types of cathode materials for rechargeable lithium-ion batteries. Various types of existing cathode materials have been extensively studied. Among these cathode materials, spinel and olivine type cathode material and their composites have been identified as the potential cathode material for the present study. Hence, literature review on different cathode materials, synthesis, and characterization techniques used for LiFePO4, LiMn2O4, and their composites has been presented. In addition, the modeling and simulation approach was also demonstrated to analyze the thermal, electrical, electrochemical, and mechanical properties of different cathode materials. This modeling approach provides a benchmark for new researchers to validate and investigate the electrical, electrochemical, thermal and controlling the internal impedance parameters of the battery. For the synthesis of the pure phase cathode materials, sol-gel and planetary ball milling methods were used. This work provides details on experimental conditions and parameters used for various physicochemical characterization techniques such as thermogravimetric analysis (TGA), X-ray diffraction (XRD), SEM/TEM, FTIR, I-V, and activation energy measurements. The instrument and conditions used in the electrochemical analysis, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge tests through the coin cell assembly are also given. Furthermore, the modeling and simulation of cathode material with spinel and olivine structure was performed and the various internal parameters such as electrolyte concentration, battery potential, thermal runaway, internal impedance control parameters, and many more were analyzed. In addition, a new computational model for dual composites was simulated to study the properties of the cathode material at high temperatures. In addition, after evaluating the computational results, the experimental characterization was performed to develop a spinel and olivine structure cathode material. The synthesized composition for spinel-type cathode material, LiMn2O4 found that XRD results show that the synthesized sample was crystalline in nature and also reveals the formation of the spinel phase and cubic structure of the LMO material. The SEM characterization reveals the spherical morphology with the average grain size of 8-10 µm grain size of the LMO material. FTIR study helps to determine the vibrational modes across the metal-oxygen bonds of the LMO material and the electrical characterization reveals better electronic conductivity as the resistance was low which is evaluated with the help of activation energy. Similarly, the olivine type structure, 17 LiFePO4 composition was characterized and evaluated with all the physio-chemical and electrochemical performances. Therefore, after evaluating both the materials, a novel olivine spinel dual composite is synthesized. The olivine part is represented by LiFePO4 (LFP) while the spinel part includes the LiMn2O4 (LMO). Different weight ratios of LFP and LMO were mixed by means of ball-milling named 1LFP/1LMO, 3LFP/1LMO, and 1LFP/3LMO, which refers to different mass ratios 1:1, 3:1, and 1:3, respectively, with olivine and spinel structure to form the intercalation cathode. It has been found that 1LFP/3LMO shows good cycling stability and also contributes to the enhancement of ionic conductivity which can be attributed to all the electrochemical and electrical characterization. 1LFP/3LMO also shows a good discharge capacity curve as compared to other solid solutions at a 1C rate. After evaluating the simulation and experimental results, the hardware prototype was designed to evaluate the electrical characteristics of the lithium-ion coin cell. This coin cell was fabricated in a research laboratory and used to develop a battery bank. However, this coin cell was used to develop a battery bank for a hardware prototype. This hardware was interfaced with software through the NI-6001 DAQ card and the results were optimized with the help of data acquisition software. The main focus of this experiment is to optimize the electrical characteristics such as battery voltage and current during discharge conditions and also perform battery external short circuit tests for boosting battery characterization. The capacitor bank has been connected in parallel fashion with the battery bank to reduce the fluctuation as well as for enhancement in electrical characteristics. After performing all the simulation and experimental research work, the results helps to conclude that the dual composite cathode material of 1LFP/3LMO is having superior thermal, electrical and conductive properties as compare to other cathode material. This dual composite cathode material also proofs good electrical properties as a battery bank on hardware system which clearly demonstrates that the dual composite 1LFP/3LMO cathode material is used for high power practical application. Also, in the future, all the simulation, experimental, and hardware set-up results will be tested for prototype and large-scale battery devices.