High-rate Lithium-rich Manganese Li_1.2Ni_0.25Mn_0.55-xCo_xO_{2Research On Preparation And Modification}

With the rapid growth of lithium-ion batteries,the application of lithium-ion batteries is increasingly penetrating into people's daily life.Lithium-rich manganese cathode materials have great application potential of lithium-ion battery field in the future due to the advantages of abundant resources,high specific capacity,low cost and etc.As the application of lithium-ion batteries is unceasingly expanded in electric vehicles,it is urgent to develop cathode materials with high rate charge-discharge performance.In this thesis,the Li_1.2Ni_0.25Mn_0.55O₂ cathode material was used as the basic material to promote the high-rate performance by improving the preparation proCess,adding the element Co and Sr doping,and appending a conductive agent when synthesizing the precursor.The materials werecomprehensively characterizedby modern testing methods such as XRD,SEM,TG-DTA,BET,and electroChemical performance tests.The Li_1.2Ni_0.25Mn_0.55O₂ material obtained by the coprecipitation method exhibited clear layered structure.Under the optimal sintering process of sintering at 950°C for 14 h,the small particle size materials were investigated,demonstrating initial specific capacity of 208.4 mAh/g with specific capacity of 189.3 mAh/g and capacity retention rate of91.0%after 100 cycles at a current density of 1 C;while at a large current density of 3C,the initial specific capacity was 162.9 mAh/g,and the specific capacity was 161.8mAh/g with the retention rate was 99.3%after 200 cycles.The addition of Co element to synthesize Li_1.2Ni_0.25Mn_0.5Co_0.05O₂ material showed the best performance at 910°C sintering temperature for 14 h,which is lower than the optimum sintering temperature of the base material.The Li_1.2Ni_0.25Mn_0.5Co_0.05O₂material exhibited the initial specific capacity of 183mAh/g at a current density of 1 C and maintained 178.3 mAh/g after 100 cycles with a retention rate of 92.5%,while the material adding the element Co delivered a excellent rate preformances with the initial specific capacity of 173.5 mAh/g,and the specific capacity of 151.9mAh/g and the retention rate of 87.5%after 200 cycles at the high current density of 3 C.Compared with the non-added Li_1.2Ni_0.25Mn_0.55O₂,therate performance was improved at the high current density of 3 C.The method of segmentation sintering can reduce the size of primary particles,thereby increasing the specific capacity at high magnification,but the cycle stability is degraded.The Li_1.2(Ni_0.25Mn_0.5Co_0.05)_1-xSr_xO₂?x=0.01,0.02,0.03?materials were synthesized by adding SrCO₃ during the lithium mixing proCess,When the doping amount is 1%and2%,there is no peak in the XRD diffraction peak,indicating that the structure of the material has no change.Due to Sr²⁺ion radius is large,can hinder the migration of transition metal ions and reduce the cation mixing,So slow down the attenuation of the median voltage.But When the doping amount is 3%,Sr and Mn are solid-solved,and a hetero phase appears,indicating excessive doping.In the process of synthesizing(Ni_0.3125Mn_0.6875)?OH?₂ precursor adding 2%SP,due to SP has excellent conductivity,It can reduce impedance and increase specifi c capacity and enhance circulation stability;The initial specific capacity is 210.9mAh/g and the maximum specific capacity is 221mAh/g during the circulation at acurrent d ensity of 1C.It also exhibited excellet cycling stability with the specific capacity of 200.8mAh/g and the retention rate of 90.8%after 100 cycles;At a high current density of 3C,the initial specific capacity was 165.1 mAh/g,and the specific capacitys was 160.5 m Ah/g with the retention rate of 95.5%after the 200 cycle.The addition of different con tent of SP has a great influence on the material,because SP has reducibility,with the increase of addition and the content of Mn⁴⁺converted into Mn³⁺increases,re suiting in crystallinity of the material decreases and loss of specific capacity incr ease.but the impedance was reduced,which is conducive to the stability of the median v oltage and inpedance decreases.