A study on the improved electrochemical properties of Si-based anode and development of FeS2 cathode for the Li-ion battery

Abstract

This thesis studies the new electrode materials that possibly replace the current electrode materials for Li ion batteries. Electrochemical properties of Si based anode materials and FeS2 cathode materials are studied and detailed microstructure observations are performed for the further understanding of the materials. This thesis is comprised of three parts. In the first part, we report the direct observation of microstructural changes of LixSi electrode with lithium insertion. HRTEM experiments confirm that lithiated amorphous silicon forms a shell around a core made up of the unlithiated silicon and that fully lithiated silicon contains a large number of pores of which concentration increases toward the center of the particle. Chemomechanical modeling is employed in order to explain this mechanical degradation resulting from stresses in the LixSi particles with lithium insertion. Because lithiation-induced volume expansion and pulverization are the key mechanical effects that plague the performance and lifetime of high-capacity Si anodes in lithium-ion batteries, our observations and chemomechanical simulation provide important mechanistic insight for the design of advanced battery materials. In the second part, we report a Si-Ti-Ni ternary alloy developed for commercial application as an anode material for lithium ion batteries. Our alloy exhibits a stable capacity above 900 mAh g-1 after 50 cycles and a high coulombic efficiency of up to 99.7% during cycling. To enable a highly reversible nano-Si anode, we employ melt spinning to embed nano-Si particles in a Ti4Ni4Si7 matrix. The Ti4Ni4Si7 matrix fulfills two important purposes. First, it reduces the maximum stress evolved in the nano-Si particles by applying a compressive stress to mechanically confine Si expansion during lithiation. And second, the Ti4Ni4Si7 matrix is a good mixed conductor that isolates nano-Si from the liquid electrolyte, thus preventing parasitic reactions responsible for the formation of a solid electrolyte interphase. Given that a coulombic efficiency above 99.5% is rarely reported for Si based anode materials, our alloys performance suggests a promising new approach to engineering Si anode materials. In the last part, we embed phase pure natural cubic-FeS2 (pyrite) in a stabilized polyacrylonitrile (PAN) matrix. The PAN matrix confines FeS2s electroactive species (Fe0 and Sn2-) for good reversibility and efficiency. Additionally, the stabilized PAN matrix can accommodate FeS2s 160% volume expansion upon full discharge because it is not fully carbonized. At room temperature, our PAN-FeS2 electrode delivers a specific capacity of 470 mAh g-1 on its 50th discharge. Using high resolution transmission electron microscopy (HRTEM) we confirm that FeS2 particles are embedded in the PAN matrix and that FeS2s mobile electroactive species are confined during cycling. We also observe the formation of orthorhombic-FeS2 at full charge, which validates the results of our previous all-solid-state FeS¬2 battery study.