Synthesis of novel high energy density cathode materials for lithium rechargeable batteries

Author

Stephen L. Bewlay, University of Wollongong

Year

2006

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting & Electronic Materials - Faculty of Engineering

Recommended Citation

Bewlay, Stephen L, Synthesis of novel high energy density cathode materials for lithium rechargeable batteries, PhD thesis, Institute for Superconducting & Electronic Materials, University of Wollongong, 2006. http://ro.uow.edu.au/theses/495

Abstract

“Synthesis of Novel High Energy Density Cathode Materials for Lithium Rechargeable Batteries” summarises 4 years of research by Stephen L. Bewlay in ISEM at the University of Wollongong (UoW). A brief historical introduction is given to previous research & understanding of Li battery cathode materials, with references to many published papers and resources.

Lithium Ferrous Phosphate (LFP) materials were prepared as Li battery cathode material and extensively intercompared. Our group was the first in the world to publish on formation of LFP from inexpensive precursors via spray pyrolysis & hydrogelation/aerogellation. Detailed thermodynamic calculations by the author show why spray-pyrolysed LFP can exist in a normal atmosphere without danger of converting Fe2+ (Ferrous) into unwanted Fe3+ (Ferric) ions. Synchrotron XRD measured crystal lattice spacings of sol-gel “pure” (actually slightly carbon-enriched) and Ti-doped LFP, and XANES (X-Ray Absorbtion Near- Edge Scattering) was done on Fe K-band X-radiation to quantify subtle changes in the environment of LFP Fe ions at various stages of charge/discharge. HRTEM was used to show convincingly that carbon residues from sucrose tended to coat the outside of LFP crystallites, enhancing the surface conductivity of LFP powder grains. A calibration chart was onstructed to allow estimation of actual wt.% levels of residual carbon from a simple, single DTA/TGA measurement.\

Dependence of LFP electrical conductivity on temperature was measured for both pure and Mg-doped LFP samples between room temperature and 400 oC.

LFP has been celebrated in scientific journal publications as being more people- and environment-friendly than LiCoO2, but it is proven experimentally by the author that it underachieves its theoretical energy density and instead usually achieves close to the existing LiCoO2 cathode paste capacities. This was despite use of sophisticated techniques such as sucrose-induced carbon enrichment & hydrogel/aerogel methods.

Vanadium Pentoxides (V2O5) from differing synthetic pathways (spray pyrolysis, aerogellation) were verified to be promising cathode pastes, with discharge capacities >200 mAh/g & capacity losses ~0.4%/cycle. Spray-pyrolysed V2O5 was particularly intriguing – having a near-perfect spherical shape and a mass density ~80% of crystalline V2O5.

WS2 nanotubes were synthesized from ammonium tetrathiotungstate by a unique 2- step un-templated furnace technique, with dimensions verified by TEM and HRTEM. WS2 nanotubes electrode pastes yielded a discharge capacity of ~ 200 mAh/g, & capacity loss ~ 0.2%/cycle.

MoS2 nano-material was synthesized and demonstrated by SEM to have unique, porous, parallel nanolayers, a result corroborated by other published research. Attempts to synthesise FeS2 fortuitously resulted in a new synthesis method dubbed “solid state dilution”, yielding nano-magnetite (Fe3O4) powder with 1st discharge capacity ~302 mAh/g when used as anode material.

Current-Voltage curves, Charge/Discharge capacity and cyclability were assessed for many materials, with suggestions for further research on the most promising.