THESIS SYNTHESIS AND CHARACTERIZATION OF LITHIUM-ION CATHODE MATERIALS IN THE SYSTEM (1-x-y)LiNi1/3Mn1/3Co1/3O2 ∙ xLi2MnO3 ∙ yLiCoO2 Submitted by Rushendra Paravasthu Department of Mechanical Engineering In partial fulfillment of the requirements For the Degree of Master of Science Colorado State University Fort Collins, Colorado Spring 2012 Master’s Committee: Advisor: Susan P. James Amy Prieto Mingzhong Wu ABSTRACT SYNTHESIS AND CHARACTERIZATION OF LITHIUM-ION CAHTODE MATERIALS IN THE SYSTEM (1-x-y)LiNi1/3Mn1/3Co1/3O2∙xLi2MnO3∙yLiCoO2 Considering various technologies for storing energy the usage of lithium (Li) – ion batteries still stands as one of the most promising options, especially for the on-going huge demand for electric and plug-in hybrid vehicles. The main limiting factor in the performance of a Li-ion battery is the cathode material. The current cathode material that is being used in the present market is LiCoO2 cathode which is effective but is expensive and toxic. The objective of this thesis is to find a cathode material which is advantageous and a probable replacement for LiCoO2. Based on the previously studied work on ternary solid solutions and its advantages, this system (1-x-y)LiNi1/3Mn1/3Co1/3O2∙xLi2MnO3∙yLiCoO2 was chosen. This was made using a ternary composition diagram which is a combination of LiNi1/3Mn1/3Co1/3O2, Li2MnO3 and LiCoO2 materials. Points inside the ternary diagram were chosen in an arrangement conducive to mathematical modeling and compositions of the cathodes were processed accordingly. All the 28 samples in the system were synthesized using the sol-gel method, each sample was characterized using X-ray diffraction (XRD) scans and electrochemical testing was performed by using an Arbin BT2000 battery testing system with MITS Pro Arbin software. The XRD results showed that all the samples established a α-NaFeO2 structure and a space group of R3m with varying degrees of purity in the crystal structure. The compounds in this system showed fairly consistent charge/ discharge curves during the electrochemical testing ii with initial discharge capacities varying from 122mAh/g to 240mAh/g and high capacity compositions were found in the region of high Li2MnO3 content. The highest capacity found was Li1.222Mn0.5Ni.056Co0.222O2 composition with a discharge capacity of 242mAh/gin the voltage range 4.6 – 2V at a C/15 rate. A statistical based analysis was carried out using JMP version 7.0.1 with the design of experiments (DOE) procedure for a mixture design to find variations in capacity and cyclability trends over the composition triangle. Inductively couple plasma (ICP) analysis was carried out on 4 samples which were found to be most promising basing on the statistical analysis. ICP results confirmed the formation of optimum compositions for the specified synthesis conditions. All these 4 samples were synthesized and tested again to confirm consistent performance across repeat samples. XPS (X-ray photoelectron spectroscopy) was conducted for these samples to determine the chemical environment of transition metals. The composition with the highest capacity was found to be Li1.222Mn0.5Ni0.056Co0.222O2 electrode. Its high capacity is attributed to +2 +4 +3 +4 the Ni /Ni and Co /Co red-ox couple and the Li-ion extraction form Li2MnO3 at a voltage >4.5V which results in an extended discharge profile. This cathode composition is very promising because it shows high capacity and good cyclability, and would be much cheaper than conventional LiCoO2 cathodes since it has more manganese which is less expensive than cobalt. iii ACKNOWLEDGEMENTS I want to thank my advisor, Dr. Susan P. James for being the one to support my research work in the first place and for giving me invaluable guidance over the last year. I would also like to thank Dr. Amy Prieto and Dr. Mingzhong Wu for agreeing to serve on my committee. Additionally, I would like to thank Dr. Sandeep kohli and Dr. Pat Mccurdy from the department of chemistry for helping me with different characterization techniques. My colleagues Brandon Kelly, Anish Yerramilli, Madhu Channabasappa, Josh Garrett, Bhavin Patel, Mike Yeager and Siddharth Paravasthu all have played an important role in my research work and were always helpful. Finally, I would like to thank my family and friends for always being there for me and for supporting me in whatever I do. iv TABLE OF CONTENTS List of Tables………………………………………………………………………………….….x List of Figures……………………………………………………………………………………xi Chapter 1 Introduction…………………………………………………………….…………1 1.1 Aim and Purpose…………………………………………………………………..………..1 1.2 History of Li-ion batteries…………………………………………………………..………3 1.3 Working Principal……………………………………………………………………..……5 1.4 Cell design and Configuration…………………………………………………………..….7 1.5 Battery Specifications……………………………………………………………………..12 1.5.1 Capacity and Rate capability…………………………………………………………13 1.5.2 Cyclability…………………………………………………………………………....14 1.5.3 Safety…………………………………………………………………………………15 1.5.4 Cost…………………………………………………………………………..……….15 1.6 Recent Developments……………………………………………………………...………16 Chapter 2 Cathode Materials…………………………………………………………..……18 2.1 Layered Metal Oxides…………………………………………………………………...…18 2.1.1 LiCoO2…………………………………………………………………………………19 2.1.2 LiMnyNi1-yO2………………………………………………………………………..…21 2.1.3 LiNi1/3Mn1/3Co1/3O2……………………………………………………………………22 2.1.4 LiMnO2 (layered)/LiMn2O4 (Spinel)……………………………………………..……23 v 2.1.5 Li2MnO3……………………………………………………………………..…………25 2.2 Binary and Ternary Solid solutions containing Li2MnO3…………………………….……26 2.2.1 Li[Lix/3Co1-xMn2x/3]O2…………………………………………………………….……26 2.2.2 Li2MnO3-LiNi1/3Co1/3Mn1/3O2- LiNiO2………………………………………….…….27 2.2.3 Li[Li(1-x)/3Mn(2-x)/3Nix/3Cox/3]O2………………………………………………………...28 2.2.4 Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2………………………………………………………..….28 2.2.5 (1-x-y)LiNi1/2Mn1/2O2∙xLi[Li1/3Mn2/3]O2∙yLiCoO2………………………………........29 Chapter 3 System Analysis……………………………………………………………………30 3.1 Research Statement…………………………………………………………………...……30 3.2 Objective………………………………………………………………………………...…30 3.3 Approach………………………………………………………………………………...…30 Chapter 4 Experimental Methods……..……………………………………………….……34 4.1 Synthesis………………………………………………………………………………...…34 4.2 Material Characterization………………………………………………………………..…35 4.2.1 X-ray Diffraction (XRD)………………………………………………………………36 4.2.3 Inductively Coupled Plasma (ICP)………………………………………………….…37 4.2.4 X-ray Photoelectron Spectroscopy (XPS)…………………………………………..…37 vi 4.2.5 Statistical Analysis…………………………………………………………………..…38 4.3 Cathode Preparation…………………………………….....................................................39 4.4 T-cell Assembly………………………………………………………………………...…40 4.5 Electrochemical Testing…………………………………………………………………...41 Chapter 5 Results and Discussion……………………………………………………………42 5.1 (1-x-y)LiNi1/3Mn1/3Co1/3O2∙xLi2MnO3∙yLiCoO2………………..………………………….42 5.1.1 System Overview…………………………………………………..……………………42 5.1.2 XRD………………………………………………………………………..……………45 5.1.3 Battery Testing……………...………………………………………………………...…49 5.2 Statistical Analysis……………………………..………………………..……….…………57 5.2.1 Optimization………………………………………………………………….…………57 5.2.2 Repetition………………………………………………………………………..………61 5.2.3 ICP………………………………………………………………………………………63 5.2.4 XRD…………………………………………………………………………..…………64 5.2.5 XPS…………………………………………………………………...…………………66 5.2.6 Battery Testing………………………………….……………………………………….69 5.3 Results and Discussion……………………………………………………………………71 vii Chapter 6 Conclusions……………………………………..…………………..………………………...…76 References………………………………..………………………………………….…..………78 viii LIST OF TABLES 1. Material compositions of all the 28 samples in the system…………………………...…44 2. Results for the 28 samples in the system after testing…………………………………..56 3. Input data for regression analysis……………………………………………………..…58 4. Material compositions of selected samples for repetition……………………….……….61 5. ICP data analysis………………………………………………………………...……….63 6. Results from ICP calculations…………………………………………………..………..64 7. Results analysis for 4 repeated samples……………………………………………….…72 8. Comparison between capacities of original and repeated samples…………………..…..73 ix LIST OF FIGURES 1. Comparison of various battery technologies in terms of energy density……………….....2 2. Dendrite formation on lithium anode………………………………………………….…..3 3. Structure of graphite intercalated with lithium……………………………………...…….4 4. Diagram of Li-ion cell during charging and discharging……………………………….....6 5. Electrode and cell reactions in a lithium ion battery……………………………………....6 6. Battery configurations cylindrical, coin, prismatic and thin/flat polymer………………...7 7. Coated foil layout for Sony 18650 LiCoO2 battery……………………………………….8 8. Typical small-scaled coating machine…………………………………………………….9 9. Calendaring machine…………………………………………………………………….10 10. Winding machine………………………………………………………………………...10 11. Battery manufacturing process flow chart…………………………………………….…11 12. Capacity plot ( Voltage v. Capacity)……………………………………………………..13 13. Comparison of the rate capability at constant current for 18650 batteries………………14 14. Voltage v. capacity for positive and negative electrodes for next generation Li-cells….17 15. Layered structured transition metal oxides……………………………………………....19 16. Layered structure of LiCoO2……………………………………………………………..20 17. Unit cell of LiMnyNi1-yO2 showing li-filled and Ni, Mn-filled layers…………………...21 18. Comparison between layered LiMnO2 and spinel LiMn2O4 structure………………...…24 19. The rock salt structure of Li2MnO3……………………………………………………....25 20. Ternary composition triangle proposed by Zhang et al………………………………….31 21. Ternary composition triangle proposed by