First principles study on the polyanion electrode materials for lithium rechargeable batteries

Abstract

Electrochemical energy storage devices are attracting tremendous interest due to the recent growing importance of sustainability and environmental concerns. The lithium rechargeable battery is one of the most advanced energy storage systems and serves as a major power source for various small electronic devices. With the growing interest in power sources for large applications such as the HEV (hybrid electric vehicle) or PHEV (plug-in hybrid electric vehicle), lithium rechargeable batteries are finding new opportunities in this emerging area. Intensive research efforts are focused on developing suitable electrode material, the key component of Li rechargeable batteries, for these applications. The electrode material for lithium rechargeable batteries for use in HEVs and PHEVs requires high stability, high power, high energy and low cost. Polyanion based materials such as phosphates, silicates, fluorophosphates/fluorosulfates and borates have been proposed as promising cathodes for Li rechargeable batteries of large scale applications. The variety of combinations between polyanions and transition metals enables intellectual tuning of electrochemical properties of this class of materials. In this thesis, various polyanion electrode materials such as olivine phosphate, lithium metal borates and sodium/lithium vanadium fluorophosphates are investigated with first-principles calculations. The in-depth study of the multi-component effect on the structural and electrochemical properties of olivine cathodes is conducted using state-of-the-art first-principles calculations. The distribution of multiple transition metals in olivine structure alters local crystal structure and electronic structure, affecting its kinetic and thermodynamic properties. We find that local structure change, such as the reduced Jahn-Teller effect of Mn, significantly enhances both Li mobility and electron (polaron) conductivity when the redox Mn element neighbors Fe or Co. The unexpected one-ph...