Preparation,Modification And Electrochemical Properties Of The Ni-Rich And Low-Co Cathode Materials

In order to alleviate environmental problems such as global warming,electric vehicles(EVs)instead of traditional vehicles can effectively reduce greenhouse gas emissions such as CO2.However,one of the biggest challenges for the commercialization of electric vehicles is to increase the drive mileage,so it is imperative to increase the energy density of power batteries.For ternary NCM series cathode materials(LiNixCoyMnl-x-yO2),the energy densities of NCM111,NCM622 and NCM811 can reach 600,690 and 760 Wh/kg,respectively,and none of these materials can reach the threshold for large-scale commercialization of electric vehicles(300-mile drive mileage,indicating an energy density of at least 800 Wh/kg).If the Ni content of LiNixCoyMn1-x-yO2 is further increased to above 90%,the energy density is anticipated to meet the goal of 800 Wh/kg.Moreover,in order to reduce costs and alleviate pollution,lowering the Co content to less than 3%is also an industrial demand.In short,the study of the Ni-rich and low-Co layered oxide cathodes(Ni>0.9,Co≤0.03)is of great practical significance.However,such extremely Ni-rich cathodes are accompanied with some severe challenges.For example,structural instability originating from phase transitions,chemical degradation arising from the parasitic surface reactions,chemical instability against ambient air,increase of electrode polarization during prolonged cycling,and so on.In order to solve the above problems,synthesis optimization,element doping,surface coating and the use of electrolyte additives are effective methods to improve electrochemical performance.In this paper,sintering optimization and Al-doping are combined to improve the electrochemical performance of Ni-rich and low-Co cathodes,and further explore the limiting effect of Al-doping on polarization increase upon cycling.In ternary NCM cathode materials,the synthesis conditions will change with the variation of Ni/Co/Mn ratio.However,in the existing literature,there are relatively few studies on the synthesis process optimization of extremely Ni-rich cathodes(Ni>0.9).In this paper,self-synthesized Ni0.94Co0.03Mn0.03(OH)2 precursor and LiOH·H2O are used as raw materials,and the performance of Ni-rich and low-Co cathode LiNi0.94Co0.03Mn0.03O2(NCM94)is optimized by adjusting the sintering protocol.Different sintering temperatures,sintering methods and sintering times are set to study the effects of various sintering conditions on the morphology,crystal structure and electrochemical properties of NCM94 cathode.The optimized sintering conditions are as follows:the Ni0.94Co0.03Mn0.03(OH)2 precursor and LiOH·H2O are mixed by grinding,pre-sintered at 500℃ for 10 h,and then sintered at 680℃ for 30 h to obtain the final NCM94 cathode.However,sintering process optimization has limited improvement on cycle stability of NCM94.Therefore,the Al-doped LiNi0.92Co0.03Mn0.03Al0.02O2(NCMA92)cathode is prepared to further improve the electrochemical performance of Ni-rich and low-Co cathodes.It is worth noting that NCM94 shows obvious polarization increase during cycling,while NCMA92 displays restrained polarization increase.In order to understand the mechanism of electrode polarization increase of NCM94 and explore the function of Al-doping,electrochemistry test,In-situ XRD,GITT and EIS are combined to provide new insights for understanding the polarization increase process.During cycling,the decrease of electrochemical reaction kinetics is the main reason for electrode polarization increase of NCM94.As for NCMA92,the stabilizing effect of Al-doping on the structure inhibits the reduction of the electrochemical reaction kinetics,thus effectively restraining the increase of electrode polarization during cycling.In detail,Al-doping reduces the accumulation of internal stress and enhances the structural stability at the end of the charging,thereby reducing the generation of microcracks and the subsequent erosion of the cathodes by electrolyte.The improved structural stability is favorable to maintain connectivity between primary particles and reducing the expansion of rock salt phase,which suppresses the reduction of Li-ions transport kinetics and the increase of the charge transfer impedance during cycling.As a result,NCMA92 cathodes display lower electrode polarization increase during cycling.