Study On Gas Production And Electrochemical Performance Of Lithium-rich Manganese-based Battery

As the proportion of electric vehicles in the market increases year by year,the development of high-energy-density power batteries is urgent.Lithium-rich manganese-based materials are expected to become one of the next generation cathode materials due to its high energy density,excellent thermal stability,and low costs.However,after the first charging voltage of Li-rich batteries exceeds 4.5 V,the electrochemical activation reaction of the Li2MnO3 phase in the material will cause the lattice O which in the crystal to participate in the charge compensation,resulting in the gas generation inside the battery.The generation of gas in the battery will cause the electrochemical performance to decline and the appearance of swelling and deformation,which will seriously affect the use of the battery.This paper mainly adopts methods such as change the formation process and cation doping to study its influence on the gas production and electrochemical performance of the Li-rich system batteries.The main research contents and results are as follows:First of all,the gas production in each voltage section of the lithium-rich manganese-based system battery was measured during the formation process,and we found that the 4.7 V constant voltage charging section was the main stage where the gas production increased sharply.Therefore,we designed the pulsed formation process,the battery can be charged and discharged stepwise and repeatedly in the low potential area.The results show that compared with the traditional formation process,the gas production of the battery during the formation period is reduced by about 37%after the pulse formation process is adopted,and there is no obvious difference in the composition and proportion of the gas.This is mainly because the pulse formation process reduces the battery in the high-voltage area.The charging time is reduced,thereby reducing the occurrence of gas generating reactions.In addition,the pulse formation process can also effectively save the formation time,shortening the time from 102.6 h to 81.5 h.At the same time,the electrochemical stability of the battery during the long cycle is improved.After 500 cycles,the battery capacity retention rate of the pulsed battery is 14.1%higher than that of the ordinary battery,and the median voltage is increased by 0.048 V.Analysis of the formed battery with X-ray photoelectron spectroscopy(XPS),scanning electron microscope(SEM)and electrochemical impedance spectroscopy(EIS)shows that the pulsed formation process can form a dense and uniform layer on the surface of the positive and negative active materials.The membrane structure is more conducive to the deintercalation of Li+during the charging and discharging process,and avoids the side reaction between the active material and the electrolyte,relieves the stress of the cell structure during the formation process,and stabilizes the electrode structure.Secondly,the cation doped Li-rich cathode material was selected to study the effect of the doping modification on the battery performance.The raw material characterization results show that the doped material has a good layered structure,the average thickness of the lamella is reduced,and the particle size of the material is reduced,and the specific surface area is increased.In addition,the crystallinity of the crystal is improved,and the Li+/Ni2+mixing degree is reduced.The pulse formation process was used to form the lithium-rich manganese-based battery before and after the modification.The total gas production of the battery before and after the doping was 23.06 ml and 31.76 ml,respectively.The more gas produced by the modified material is due to the cation doping.The reduction of the primary particle thickness of the material helps the Li2MnO3 component to be more fully activated during the formation.Subsequently,the electrochemical performance of the battery was tested under different cycling conditions.The results showed that the capacity retention rate of the undoped material after 500 weeks cycle at the voltage of 2.0-4.6 V,2.0-4.55 V,and 2.0-4.5 V at 25℃ are respectively 36.17%,42.27%,47.22%,the median voltage are 2.48 V,2.50 V,2.55 V.And the capacity retention rate of the doped materials are 71.78%,79.25%,83.59%,and the median voltage are 2.84 V,2.96 V,3.13 V.It can be seen that the doping modification can greatly improve the cycling electrochemical performance of the batteries,and it also shows that the electrochemical performance of the battery decreases with the increase of the cycle cut-off voltage.In addition,the growth of the DCR,the Rs,and the Rct of the doped material during the cycle are effectively inhibited.The analysis of the cathode after cycling shows that cation doping can stabilize the layered structure of the material,slow down the tendency of(003)characteristic peak to shift to a low angle,and at the same time inhibit particle breakage during cycling,and stabilize transition metal elements,thereby improving the cycle electrochemical performance of the batteries.