Investigation into the Surface Chemistry of Passivated Carbon Fiber/LiMn2O4 Electrodes for Lithium Ion Batteries
Waller, Gordon Henry
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Lithium-ion batteries are one of the most energy dense electrochemical energy storage systems available today and for the foreseeable future will be the dominant secondary battery type for applications needing large energy density and long operational lifetimes. Among the many varieties and applications of lithium-ion batteries, electrode design - and in particular the selection of active materials - is extremely influential in determining overall device specifications. Furthermore, the cathode plays a particularly important role in factors such as cell safety, lifetime, and cost. In applications which require low cost and high safety LiMn2O4 cathodes are an excellent choice; however, the well-known issue of rapid capacity fading has yet to be overcome. In this dissertation composite electrodes are formed by directly coating the LiMn2O4 active material onto carbon fiber current collectors. When tested as positive electrodes for lithium-ion batteries, these electrodes show comparable energy and power density to conventional tape-cast composites, but can be fabricated without the need for organic solvents, binders or metal foil current collectors. To reduce capacity loss from the LiMn¬2O4 active material ultrathin (<1 nm) coatings of aluminum oxide were deposited onto the surface of the LiMn2O4/carbon fiber composites using atomic layer deposition. Aluminum oxide coatings successfully improved capacity retention by over 100% and led to an unexpected increase in rate capability and total lithium diffusivity. To further investigate the mechanisms in which inert oxide coatings prevent capacity loss and influence cycling behavior, thin-film model electrodes were prepared and measurements of surface chemistry, crystal structure and electrochemical impedance were conducted.