| With the progress of the world's economy,searching and exploiting renewable energy and new ways of energy conversion and storage becomes the key to the global sustainable development,and it has been listed among strategic policies in many countries to secure energy.Transportation,however,makes up a larger and larger part of energy consumption in the world,so efficient and new energy vehicles are becoming the significant approach to improve diversified development of automobile energy structure,to mitigate the contradiction between supply and demand of fossil fuels,to ameliorate energy consumption proportion as well as living condition of human.The first commercial lithium-ion battery was invented by Japanese company SONY and greated attention have been received as soon as it came out.Due to non-environmental pollution,large specific capacity,high operating voltage and long cycle life,lithium-ion battery have been widely used in 3C market.With fast growth of electric vechicle,lithium-ion battery have become the kernel component of new energy automobiles.Energy density that is an vital index orients directly the endurance mileage of EVs,among different aspects of battery performance,which influences greatly the market recognition of the EVs.Thus,research on cells of high energy density bears on the prospect of the entire new energy vehicles industry.The purpose of this essay is to develop 230Wh·kg-1 lithium battery which can meet performance of EVs.The main research contents and conclusions are shown as below:(1)Designed DOE(Design of Experiment)scheme with different cathodes,anodes and electrode coating weight.Set the formula ratio as factor(input)while battery performance as response(output),and explored relationship between materials usage,coating weight and battery performance,thus got the optimal formula.Batteries tested with optimal formula indicated that:energy density got 236Wh·kg-1;no explode and no fire after nail penetration,over charge,over discharge and short circuit test;DCR(Direct Current Resistance)and power density at 50%SOC(state of charge)are 2.2m? and 1732W·kg-1 respectively;constant current charge capacity ratio at 1C,2C,3C and 4C are 91.4%?85.3%?81.4%?74.4%respectively;discharge capacity ratio at 2C?3C?4C?5C are 99.0%?98.9%?98.7%?99.1%respectively;capacity retention for 1C,25? after 2000 cycles is 92.1%while for 1C,45? after 1000 cycles is 92.4%.Every tested items meet design target.(2)There are three types of visible surface stains on the lithium-ion cell graphite anode.These are dark spots,bubble spots,and lithium plating.Dark spots are formed by a different mechanism from the other two types of stains,while bubble-spots and lithium-plating can be well demonstrated.This essay aimed at exploring mechanism of dark spot forming and solving the problem.X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy,nuclear magnetic resonance,and Raman spectroscopy used to analyze dark spots and normal anode regions.These analyses indicated that dark spots are mainly caused by imperfect SEI(solid electrolyte interface)film formation.In the dark spot,the content of Li,O and F in the SEI film are elevated,chemical composition is abnormal,and graphite is floating from the electrode unlike in normal areas of the anode.All these problems may interfere with the stability of electrode,as a result,cell capacity will reduce and safety will get worse.As temperature in formation of SEI,yet,is associated with the shape of black spots afterwards,higher precharge temperature can solve the problem radically.(3)A study was performed to determine the cause and mechanism of abnormal direct current resistance(DCR)during high-temperature storage of a commercialized lithium-ion battery designed for an electrical vehicle.Physical,chemical and electrochemical analysis were used to analyze batteries with abnormal and normal DCRs.The results of these analyses show that:a high temperature and SOC shift the anode energy level,inducing violent side reactions between H2O and the electrolyte.Lithium salt(LiPF6)spontaneously decomposes into LiF and PF5,which reacts with H2O to form POF3 and HF.LiF and POF3 have poor electronic and Li+conductivity and deposit on the surfaces of the graphite,thus hindering the lithiation/delithiation reaction and increasing the charge transfer impedance,which affects the electrochemical performance and produces an abnormal increase in the DCR.Therefore,the humidity during battery production must be strictly controlled.Diagram[65]Form[30]Reference[180]... |
PDF Full Text Request