Study Of High Capacity Cobalt Oxide Anode Materials For Lithium-Ion Batteries

Lithium-ion batteries are considered as the most promising power sources because of their high potential, high energy density, long cycle life, no memory effect and environmental friendliness. The commercialized graphite-based anode materials exhibit excellent charge and discharge cycling performance, but their low specific capacity (300-350 mA h g?1) can't satisfy the demand for the high energy density of batteries. Therefore, it is urgent to develop new anode materials with larger capacity. Owing to their high capacity of 700-1000 mA h g?1, transition metal cobalt oxides (CoO, Co₃O₄) are a new class of promising anode materials for rechargeable Li-ion batteries. As reported, the large irreversible capacity loss (>30%) in the first cycle and relatively fast capacity fading rate during electrochemical cycling of cobalt oxides nanoparticles and nanotubes limit their practical applications. In this thesis, we have successfully synthesized series of lamellar CoO and Co₃O₄ platelets as well as their composites prepared through as-synthesized hexagonalβ-Co(OH)₂ as templates. Moreover, Co₃O₄-carbon nanofiber (Co₃O₄-CNF) composites were also successfully prepared by calcination of Co(OH)2-carbon nanofiber (Co(OH)₂-CNF) precursors using acid-treated carbon nanofiber as templates. As a result, both the first cycle efficency and cycling performances have been greatly improved by using the prepared CoO, Co₃O₄ and their composites. The concrete research contents are summarized as following:1. Preparation and study on electrochemical performance of lamellar CoO materials as anode materials for lithium-ion batteries. Lamellar β-Co(OH)2 platelets with different dimensions were synthesized by a simple hydrothermal route without using surfactants or templates. The influence of reaction conditions, such as: Co(NO3)2 concentrations, composition of solutions, reaction time, reaction temperature, on the morphology and structure of the obtained products was investigated. Lamellar-type CoO platelets were prepared through thermal decomposition of lamellarβ-Co(OH)2 templates. As a typical example, the effect of the morphology and size of lamellar CoO platelets on the capacity and cycle-ability was systematically investigated. Lamellar CoO platelets with average tubular size of 15μm in diameter and 6μm thick showed larger capacities and much better electrochemical performance than monolayer CoO platelets and CoO nanoparticles. Even after 100 cycles, the reversible capacity of lamellar CoO platelets was still kept at 800 mA h g-1.2. Preparation of nanosized Co₃O₄ and lamellar Co₃O₄ and study on their lithium-storage performance. Co₃O₄ nanoparticles were prepared by the calcination ofα-Co(OH)2 precursors formed in isopropyl alcohol - water (1:1, v/v) solution. The electrochemical performance of Co₃O₄ nanoparticles was thoroughly studied. Furthermore, Lamellar Co₃O₄ platelets were prepared through thermal decomposition of lamellarβ-Co(OH)₂ synthesized by simple hydrothermal method. The lamellar Co₃O₄ platelet with average tubular size of 15μm in diameter and 4-10μm thick exhibited an excellent cycling performance, retaining a specific capacity of approximately 600 mA h g-1 after 100 cycles.3. Preparation and study on electrochemical performance of Co₃O₄-CNF composites as anode materials for lithium-ion batteries. Co₃O₄-CNF composites were prepared by the calcination of Co(OH)2-CNF precursors synthesized on acid-treated carbon nanofiber templates. The effects of the calcining temperature on the crystallinity, grain size, specific surface area of Co₃O₄ and phase transformation from Co₃O₄ to CoO were studied in detail. Both the specific surface area and CNF content in CNF-cobalt oxide composites dominated the electrochemical performance of these composites. As anode materials for lithium ion batteries, Co₃O₄-CNF (24.3 wt% CNF) composite showed excellent cyclability and high lithium-storage capacity (881 mAh g-1 after 100 cycles).4. Preparation of lamellar cobalt oxide/carbon nanofiber (CNF) composite and study on their lithium-storage performance. Lamellar CoO/CNF composite or Co₃O₄/CNF composite have been prepared by caicining as-synthesizedβ-Co(OH)₂/CNF precursors under Ar flow or air atmosphere, respectively. The electrochemical lithium-storage performance of these composites was investigated detailedly. As a result, lamellar CoO/CNF composite electrodes showed excellent cyclability and high charge-discharge rate capability. A complete cell was assembled by lamellar CoO/CNF composite anode coupled with LiNi0.5Mn1.5O4 cathode in 1 M LiPF6-EC: DMC (1:1, Vol) electrolyte solution. The voltage profiles of the complete cell showed a long voltage plateau at about 2.8 V during the first discharge step. The first discharge capacity was 450 mA h g-1 based on the weight of the CoO/CNF composite. It could be a promising and high-capacity negative electrode material for advanced lithium-ion batteries.