Synthesis And Modification Of Ternary Cathode Material LiNi_0.8Co_0.1Mn_0.1O₂

Layered lithium transition metal oxide LiNi_0.8Co_0.1Mn_0.1O₂(NCM811),which has been increasingly studied due to its ultrahigh capacity,low price,and environmental friendliness,is considered to be one of the most promising cathode materials for the next generation.However,some inherent problems of NCM811,such as cation mixing,transition metal dissolution,surface residual lithium side reaction,irreversible phase change,structural damage and so on,are the bottleneck of its further development.Aiming at the main problems existing in NCM811,this paper explored the optimal sintering conditions,optimized its crystal structure by doping coating co-modification and cation and ions co-doping to improve its electrochemical performance under high cut-off voltage of 4.5 V.The specific research contents of this paper are as follows:(1)By comparing and analyzing the phase structure and electrochemical properties of NCM811 synthesized by different calcination temperatures,calcination times and lithium content,the optimal sintering conditions were determined.The results show that the cathode material prepared under the conditions of 750? calcination temperature,15h calcination time and 1:1.05 lithium content has the best layered structure,low Li⁺/Ni²⁺mixing degree,the discharge specific capacity of 169.6 mAh g^-1 at 0.5 C in the first cycle,and the capacity retention rate of 79.24%after 200 cycles.(2)The dual modification of MnO₂ coating and Mn⁴⁺doping was achieved by the synthesis of NCM811 from KMnO₄ pretreatment Ni_0.8Co_0.1Mn_0.1(OH)₂precursor in air.The capacity retention rate of the 1.0 wt.%KMnO₄ pre-oxidized sample in the 3.0-4.5V potential range of 0.2 C for 100 cycles is 79.15%,which is much higher than that prepared in the air and prepared in oxygen without any treatment.The capacity retention of 82.27%is achieved at 1 C.Cyclic voltammetry and electrochemical impedance tests show that the pretreatment significantly enhances the diffusion capacity of Li⁺and inhibits irreversible phase changes during the cycle.The MnO₂ coating inhibits the corrosion of the active substance by the electrolyte,and the Mn⁴⁺doping stabilizes the crystal structure,so that NCM811 still has excellent electrochemical performance under the high cut-off voltage of 4.5 V.(3)By introducing Na⁺and Br^-to achieve co-doping of anion and cation,the crystal structure of NCM811 is optimized,and the electrochemical performance under the high cut-off voltage of 4.5 V is improved.Na⁺doped at Liposition acts as a pillar ion to expand the spacing of lithium layers,and Br^-doped at O position forms stronger covalent bonds with transition metal elements.Co-doping inhibits Li⁺/Ni²⁺mixing degree,reduces residual lithium on the surface and stabilizes the crystal structure.The0.02 mol Na⁺and Br^-co-doping have the best cycling and scaling properties.In the voltage range of 3.0-4.5 V,the capacity retention rates after 100 cycles at 0.2 C and 1C rates reached 87.68%and 94.41%,respectively,which were far better than 69.02%and 73.95%of the original NCM811.Co-doping increases the diffusion coefficient and decreases the charge transfer resistance,and the volumetric voltage curve shows that co-doping inhibs the irreversible phase transition.