Study On The Key Materials And Interfacial Film Optimization For High Voltage Lithium Ion Batteries

The electric vehicles will become the main development direction of the future automobile industry because of the oil resources drying up and environment pollution from vehicle emissions.In order to better meet users' needs,solve the problem of mileage anxiety,we develop higher energy density battery to prolong the vehicles mileage.There are two ways to improve the energy density of battery,one is improved capacity of the active material,the other one is broadened voltage windows of the battery.So,the development of high voltage lithium ion battery becomes inevitable.This paper based on the development of high voltage lithium ion battery as the goal.We studied the key materials and process parameter optimization.Firstly,we developed the lithium-rich layered oxide materials which decide the capacity of the battery and the resistance oxidation decomposition electrolyte which decide the voltage windows of the battery.Secondly,for the high voltage lithium ion battery having more excellent performance,we optimized the formation process parameter which is suitable for high voltage battery.At positive,we controlled growth the surface of membrane at high potential by parameter optimization.At negative,we studied the effects of constant voltage formation at low potential to form good SEI film.Moisture is inevitable in processing and manufacturing,we studied the effect of moisture content on the electrochemical performance of battery.Finally,the lithium-rich layered oxide materials,the resistance oxidation decomposition electrolyte and the optimization of formation process were organically combined.We developed the high voltage lithium ion battery,and???died the reason of capacity fading,further optimized the electrochemistry tem of battery.The lithium-rich layered oxide material has a high capacity.But this material the rapid degradation of performance due to the valence of Mn element will pen as similar as phase change of the spinel structure in charge and discharge.his paper,the Cu²⁺ was multi-doped in lithium-rich layered oxide material and laced the Mn⁴⁺,so as to improve the structure of the stability and electrical ductivity,further improve the electrochemical properties of materials.The Li2MnO3·0.5Li?Ni0.5Mn0.5-xCux?O2 was synthesized by coprecipitation and ball mill.The XRD,SEM,electronic conductivity and electrochemical charge-discharge performance was used.It shows that:after the Cu²⁺ multi-doping,the morphology of material particles are changed from granular to rod-shaped,the layer spacing increases,the capacity decreases.When the doping ratio of Cu²⁺increased,the material electronic conductivity and the capacity increases,the cycle performance is better.The multi-doped could obviously increase the cell size and the layer spacing of lithium-rich layered oxide material,as a result,the diffusing speed of Li+ in material and the structure of the stability increase,and then improve the cycle performance.In order to broaden the battery voltage windows,we studied the resistance oxidation decomposition electrolyte which based on fluorinated solvent named D2.The different solvent composition and lithium salt concentration in organic electrolyte were studied by testing the conductivity,vapor pressure,thermal stability,electrochemical windows,and the stability on positive and negative.The result shows that:as the lithium salt concentration increasing,the conductivity first increases than decreases,the vapor pressure and thermal stability decreased.When the ratio of fluorinated solvent D2 in electrolyte increased,the conductivity decreases gradually,the vapor pressure increases.The fluorinated solvent D2 significantly promotes the resistance oxidation of the electrolyte decomposition characteristics,and participates in reduction reaction with carbonate solvent on the anode.For the high voltage lithium ion battery has more excellent performance,we studied the formation process parameter in order to optimize the surface film on positive and negative in the first charge and discharge.For the anode,we studies constant voltage formation at low potential process,it found that the formation protocol of 0.05 C to 3.3-V constant voltage 60 min is the best.On this formation process,the reaction speed between electrolyte and negative radually slows down,result in the SEI film being carefully repaired and the passivating layer becoming thin and denser.For the cathode,the additive VC in electrolyte was oxidized at 4.63V in first charge of Li-rich/graphite battery,the product of oxidized decomposition deposits on the surface of cathode and formed the SEI film.To control the content of VC in electrolyte and the time of constant potential at 4.63V,the oxidized decomposition reaction of VC is slowly and carefully,the passivating layer becomes thin and denser.It found that the formation protocol of constant voltage 40 min at 4.63V and with 0.8%wt.VC in electrolyte is best parameter.We studied the mechanism of the effect of moisture in lithium battery.The moisture content influences the oxidized decomposition of electrolyte on the cathode.The higher moisture content leads a lower oxidized decomposition potential.The product of oxidized decomposition deposits on the surface of positive,the positive surface film resistance increases.The material structure is stable with change in moisture content.Moisture joined in the formation reaction and changed the components of the solid electrolyte interphase film in negative.The high moisture content resulted in a thick,porous,non-dense solid electrolyte interphase film.Cycle testing shows that the moisture content in the negative electrode influences capacity fading more significantly than in the positive electrode.The reason for capacity fading was studied by three-electrode technology.The increase in negative impedance is the main reason for an increase in battery impedance.The battery impedance increased rapidly when the moisture content in the negative electrode was high.This resulted in accelerated capacity fade.At the end,we designed the high voltage battery with the lithium-rich layered oxide materials,the resistance oxidation decomposition electrolyte and the optimization of formation process.Then we studied the reason of capacity fading,and further optimized the electrochemistry system of high voltage battery.