Study on the microstructure of active materials in lithium ion battery with enhanced life cycle

Abstract

In this thesis studies, we observed the changes of microstructure in various types of Li ion battery anode and cathode materials FIB/SEM/TEM. It was drawn that stable microstructure for enhanced life cycle by correlating electrochemical properties and change of microstructure with cycle progress. This study is comprised of two parts. Part 1, observation of Si anode materials micro structure from various viewpoints. Part 2, analyze of micro structural change of cathode materials according types of electrolyte. Part 1, drawn suitable microstructure for enhanced life properties by analyzing Si based anode materials microstructure from a variety of points. For good life, effective stress control that generated by volume expansion and suppression of SEI layer forming. First point is stress, mechanical fracture occurred on surface and inside of Si particle, new surface became irreversible SEI layer, exposed at fracture area. In order to effectively control the stress from volume expansion, and ultrafine grain size and uniform microstructure are required. Occurred stress decreased with downsizing grain size. Because volume expansion of fine grain size is smaller than large grain, fracture of Si particle occurring is prevented from stress. And uniform microstructure through whole particle is also very important. If microstructure is non uniform even though Si particle has fine grain size, imbalance of stress is occurred. Mechanical alloying (High Energy ball milling) process is suitable for refining grain size and formation uniform microstructure. After MA process, Si and Silicide single grain have of 5 ~ 20nm and mixed very well. During cycle progress, significantly change of the microstructure was not observed. Aggregation is also very important for MA Si alloys. If particles have weak aggregation force, SEI layer should be formed easily by penetration of electrolyte inside of particles, it caused life cycled decreased rapidly. Carbon is added in progress of Mechanical alloying process as bonding phase for improving aggregation force and remaining after 100 cycles. Second point is surface carbon coating, carbon has ionic/electrical conductivity, and easy for applying particle coating process. Carbon coating layer increase conductivity and prevent mechanical fracture of Si particle. After cycle progress, coating layer and the Si nanoparticles display no evidence of cracking nor does the C layer/Si interface show signs of delamination. And, applying the ionic liquid as electrolyte, additional C layer is formed during electrochemical reaction on Si particle surface that improved life cycle by forming more stable SEI layer than general electrolyte. Third is the formation of 3D network structure due to the diffusion of Si in C matrix. Si/AC composite that has Si embedded in C matrix structure is made by mixed with Si nano particle and carbon. The Si nano path connected into the network structure in-situ during the charge and discharge process. Si is diffused in the C matrix to form a Si network for faster Li ion transmission. 3D Si network provides electronic/ionic conductivity and structure stability for the Si/AC composite, Part 2, Micro structural changes of cathode materials using solid and liquid electrolyte are observed during cycle. We can understand difference between solid and liquid electrolyte cells by analyzing both of the two types of the electrolyte. In case of solid electrolyte, since the reaction area is limited, the reaction rate has slowed but microstructures are stable, On the contrary, in case of liquid electrolyte, since the reaction is occurred every area in electrode, the reaction speed is high but micro structure is unstable. First, we report that a solid-state battery with sulfide based electrolyte enables the reversible FeS2 (pyrite) cathode material. We find that nanoparticles of orthorhombic FeS2 are generated upon recharge at 30–60°C which explains a coincident change in rate kinetics. Because orthorhombic structure has bigger unit cell than cubic structure, Li ion pass more easily. In the case of the solid electrolyte, life properties is improved by preventing liberated of Sulfur. Second, we present improvement process of life properties of cathode active materials. In case of NMC, life property is reduced by change of microstructure from single crystal to amorphous according to the progress of the cycle. TEM selected area electron diffraction (SAED) patterns show that NMC Bare electrode undergoes phase transition during 100 cycles from layered structure mother phase (space group: R-3m) to spinel cubic structure (space group: Fd3m). In contrast, the Al2O3-ALD coating layer preserves the original layered structure of NMC/4ALD electrode for the same cycling period