Green Manufacture Of Li[Ni_0.6Co_0.2Mn_0.2]O₂ Cathode Material For Li-ion Batteries And Its Energy Storage And Failure Mechanism

The layered LiNi_1-x-yCo_xMn_yO₂(NCM)cathode materials have attracted much attention from researchers of new energy materials due to the synergistic effect of three atoms.Among them,LiNi_0.6Co_0.2Mn_0.2O₂(NCM622)cathode material still has high capacity retention rate and high thermal stability with high discharge specific capacity.This study systematically explored the green manufacturing process of NCM622 cathode material for high specific energy lithium-ion battery,optimized the processing parameters,explored the coating modification research and explored the electrochemical performance of the material in extreme environments.Finally,in lithium intercalation,the relationship among the mass transfer process,temperature rise effect and stress of cathode materials are investigated by simulation calculation.The Relevant research and main innovative conclusions are as follows:(1)Thermodynamic and dynamics models of hydroxide coprecipitation were constructed to reveal the coprecipitation mechanism of complexing agent and transition metal ions in alkaline environment.Thermodynamic calculation and analysis of coprecipitation reaction of various green complexing agents were carried out by the software of Visual MINTEQ 3.1.The calculation show that lactate has the best coprecipitation effect at p H=11.15,and is the best complexing agent for coprecipitation reaction of NCM precursor materials.(2)Ni_0.6Co_0.2Mn_0.2(OH)₂precursor was prepared by coprecipitation with sodium lactate as complexing agent.The effects of coprecipitation process parameters on the structure of precursor and the electrochemical properties of cathode materials were systematically studied.The mechanism of the effect of coprecipitation p H and temperature on the properties of NCM622 cathode materials was revealed.It was found that the optimum co-precipitation p H value and temperature of Ni_0.6Co_0.2Mn_0.2(OH)₂ precursor were 11.0 and 60?respectively.Because of its special phase structure of?+?-(Ni_0.6Mn_0.2Co_0.2)(OH)_2,the cathode material with p H=11.0 and 60?has the best electrochemical performance,and its lithium ion diffusion coefficient is much higher than that of single-phase structure(?or?-(Ni_0.6Mn_0.2Co_0.2)(OH)₂).The influence mechanism of sintering process on NCM622 cathode material was revealed through systematic study of sintering process.SEM,XRD and other characterization methods show that when the sintering temperature is too low or the time is too short and the growth of particles is insufficient at one time,if the sintering temperature is too high or the time is too long,the grain boundary will melt and the grains will adhere.It is pointed out that the optimum sintering temperature is 800?and the sintering time is 15 hours.(3)Ti₃C₂(OH)₂was successfully coated on NCM622 cathode material by wet chemical method.The coating mechanism of Ti₃C₂(OH)₂ was discussed.It was found that Ti₃C₂(OH)₂ coating had no effect on the structure of cathode material,but the rate capability and cycle stability of the modified material were improved significantly.The study showed that 3.0wt.%Ti₃C₂(OH)₂ coated NCM622could change the interface structure of electrolyte and cathode material and reduce charge-discharge.And the Ti₃C₂(OH)₂coating can enhance the diffusion ability of lithium ions by reducing the charge transfer impedance R_ct.When the amount of coating materials is increased to 5.0wt.%,the thick Ti₃C₂(OH)₂coating reduces the contact between electrolyte and cathode material and the discharge capacity of NCM622.(4)The effect of tested temperature on the charge and discharge failure mechanism of NCM622cathode material was systematically investigated.It is found that the influence mechanism of different test temperatures on the electrochemical properties of NCM622 cathode materials is different.The failure of cathode materials at high temperature is mainly caused by the decomposition of electrolyte and the side reaction between the decomposition products and cathode materials.The electrochemical performance degradation of cathode materials at low temperature is mainly caused by the sharp increase of charge transfer impedance R_ct.It is found that the variation of R_ct^-1 and temperature is in accordance with Arrhenius formula.(5)The lithium ion transfer reaction during lithium ion intercalation into NCM622 cathode material is simulated by finite element method.The dynamic relationship between temperature rise and stress distribution with time is analyzed.It is found that the lithium ion intercalation rate tends to dynamic equilibrium after the lithium ion intercalation rate changes from fast to slow.The variation of stress state with time in cathode material is consistent with the temperature-time curve.The coupling relationship between chemical reaction,temperature and stress(strain)in cathode materials was revealed.