Year

2008

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials - Faculty of Engineering

Abstract

Lithium-ion batteries (or rechargeable lithium batteries) are most advanced battery technology for modern portable electronics such as mobile phones, notebook computers and cameracorders. There are also big potentials for lithium-ion batteries to be used for electric vehicles (EVs), hybrid electric vehicles (HEVs) and stationary power storage. In particular, the later will bring a significant contribution to reduce green-house gas emissions and address global warming and climate change. Materials research plays a key role in the development of next generation of advanced lithium-ion batteries with high energy density, high power density, and long cycle life.

This PhD thesis describes my exploration on developing new anode materials and cathode materials for lithium-ion batteries. I firstly investigated silicon based anode materials since silicon has the highest theoretical lithium storage capacity of about 4200 mAh/g when forming Li21Si5 alloys. The reversible lithium storage mechanism is totally different from that of graphite based anode. It relies on a process called alloying and dealloying instead of intercalation and de-intercalation. However, the formation of Li21Si5 alloys can induce more than 400% volume expansion. The repeated expansion and shrinkage of the silicon electrode will cause cracking and eventually failure of the battery system. A general strategy has been employed to solve this problem. Firstly nanosize silicon powders were used to minimize the volume expansion in local domains. Secondly, silicon particles were embedded in carbon matrix to buffer the volume change during the reaction with lithium. Si-mesocarbon microbeads (MCMB) composite anode materials were produced by ball-milling. Si-MCMB composite electrodes demonstrated superior performance (high capacity and satisfactory cyclability), compared to bare MCMB and bare nano-Si electrodes. Silicon-amorphous carbon composite anode materials were also prepared by carbon aerogel method, through which nanosize silicon particles are homogeneous distributed in carbon matrix. A reversible capacity of 1450 mAh/g for Si-C composite anodes was achieved. The good cyclability should be attributed to the usage of nanosize Si powders and their homogeneous distribution in an amorphous carbon matrix.

Carbon nanotubes have many unique and intriguing properties, including as anode materials for lithium-ion batteries. Vertically aligned multiwalled carbon nanotubes (VAMWCNTs) were prepared by chemical vapor deposition method. Nanosize SnO2-MWCNTs composites were also synthesized. The VAMWCNTs have a typical diameter of several tens of nanometers and consist of compartment structures. Cyclic voltammetry measurements show that the carbon nanotubes are electrochemically active to lithium insertion and extraction. A reversible lithium storage capacity of 950 mAh/g has been achieved for CNTs anodes. The solution-based chemical process enables Sn2+ ions to penetrate into the inner cavity of the carbon nanotubes. The SnO2-CNTs composite electrodes exhibited stable cyclability with a lithium storage capacity of 410 mAh/g after fifty cycles.

Transition metal phosphides such as MnP4 and Zn3P2 were discovered to exhibit interesting phenomena with respect to reversible lithium storage. They were conventionally synthesized by solid state sintering at high temperature for long periods. After sintering, the products were purified by acid etching, which is a very tedious process. Crystalline iron phosphide (FeP4) powders were directly prepared by a solvothermal synthesis technique. Cyclic voltammetry measurements demonstrated the reversible reactivity of FeP4 anodes towards lithium insertion and extraction. The FeP4 anode exhibited a stable lithium storage capacity about 700 mAh/g.

Lithium iron phosphate has been emerging as a new cathode material for lithium-ion batteries with low cost. However, lithium iron phosphate has a very lost electronic conductivity, inducing low rate capacity and preventing commercial application. Various cation dopings have been studied with the goal to improve the overall electrochemical performance of lithium iron phosphate. The synthesis, crystal structure refinement, magnetic and electrochemical properties of a series of LiMnxFe1-xPO4 cathode materials was investigated. A number of conductive phosphides and manganese phosphates were found to be formed during the sintering process with the effect of enhancing the electronic conductivity of the materials. The effect of sintering temperature towards the crystal size of LiFePO4 and Li0.95Mg0.05PO4 compounds were systematically investigated. LiFePO4 and Li0.95Mg0.05PO4 samples exhibit a typical antiferromagnetic behaviour. This antiferromagnetism could be induced by long range Fe-O-P-O-Fe triple exchange due to the lack of direct Fe-O-Fe interactions. LiFePO4 and Li0.95Mg0.05PO4 electrodes show specific capacity in the range 150 mAh/g – 160 mAh/g.

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