Study On Charge/Discharge Process And Mechanisms Of Positive Electrode Materials For Aqueous Rechargeable Lithium Batteries

Energy is the material basis for the progress of human civilization and an indispensable basic condition for the development of modern society. As the important material foundation of human survival and development, coal, oil, natural gas and other fossil fuels have supported the progress of human civilization and economic and social development from the 19th century to the 20th century which last nearly 200 years. However, since fossil energy is non-renewable and enormously consumpted by human beings, it is becoming gradually depleted. On the other hand, the use of fossil fuel has been producing a lot of greenhouse gases and other polluting fumes, which will threaten global ecology. Thus, the development of cleaner, renewable energy is the future direction of development. Wind and solar energy are not only the representatives of renewable energy sources, but also the focus of various coutries at present. However, the output of these energy is strongly dependent on natural conditions, and in some cases it does not match with the energy requirements, therefore, a load shifting energy storage system is needed, which can store electricity during the low valley of electricity using period and transfer electricity during the high valley of electricity using period.Due to its excellent properties such as high safety, non-toxicity, long cycling time, high power, and low cost, aqueous rechargeable lithium battery becomes one of the most suitable energy storage and conversion devices of short-range electric cars and solar energy, and wind power plants. Current researches are widely focused on positive electrode materials in aqueous rechargeable lithium batteries, but it is far from enough on the research of electrochemical behavior in aqueous rechargeable lithum batteries. Therefore, electrochemical quartz crystal microbalance was used to explore electrochemical behivors of several positive electrode materials for aqueous rechargeable lithium batteries.In addition, some lithium iron phosphate/carbon nanotube composites with several advantages were synthesized for aqueous rechargeable lithium batteries. A series of characterization and electrochemical properties tests were carried out in aqueous electrolyte. The main contents of this thesis include:Firstly, by using electrochemical quartz crystal microbalance, studies on electrochemical behaviors of several positive electrode materials for aqueous rechargeable lithium batteries were carried out. Sauerbery formula were used to estiminate the intercalation and de-intercalation of Li+ions in the positive electrode materials such as LiCoO2, LiMn2O4 and LiFePO4 during the charging and discharging process in lithium salt aqueous solution. The three positive electrodes exhibit almost similar behaviors as in the organic electrolyte.Next, composites of lithium iron phosphate with carbon nanotube were prepared by a template method, and their electrochemical properties were also studied. SEM micrographs shows a three-dimensional ordered macroporous structure, and TEM micrographs clearly show the existence of the carbon coating layer and the doped carbon nanotubes. Their discharge capacity can reach 114,110,105,99 and 94 mAh g"1 at the charging/discharging rate of 500,1000,2000,5000 and 10000 mA g-1 respectively, which shows a good rate performance. The main reasons are as follows: (1) carbon nanotubes and the carbon coating layer significantly improve the intrinct electron conductivity; (2) three-dimensional porous structure improves the wettablity of electrolyte; (3) nanoparticles which constitute macroporous walls shorten the transmission path of Li+ions, accelerate the charge transfer reaction, and speed up the intercalation and de-intercalation of Li+ions even in the high current density. Because of these improvements, the composites exhibit excellent rate performance. The subsequent fast charging-slow discharging tests which discharge at a fixed current density of 3 C, charge at different current rates of 120 C,300 C,600 C furtherly show this composite material has an ultra-fast charging performance at the second-level.