Preparation And Performance Study Of Nickel-rich Ternary Cathode Materials For Lithium-ion Batteries

LiNi_xCo_yMn_1-x-yO₂?0<x,y<1,NCM?materials have been widely used in lithium-ion power batteries due to their high specific capacity,long cycle life and low cost.As the requirements of energy density increase,LiNi_0.6Co_0.2Mn_0.2O₂?NCM622?,LiNi_0.8Co_0.1Mn_0.1O₂?NCM811?,and other materials with higher nickel content will be favored by major battery manufacturers.However,the nickel-rich cathode materials have drawbacks,such as complex preparation process,insufficient structural and air stability,to limit their application to a large extent.Therefore,in this paper,we investigated the nickel-rich cathode materials from two aspects of preparation process and air instability,in order to provide a reference for the production and application.The precursor of NCM811 was synthesized by the coprecipitation method,the four factors of p H,stirring speed,reaction temperature and ammonia salt ratio were explored.In the calcination stage,the calcination temperature,calcination time and ratio of lithium source and precursor were explored;the preliminary synthesis parameters were obtained.On the basis of this,the process parameters were further optimized.The particle size of the material was moderate?the median size is about 10?m?,the sphericity and uniformity were improved,and the electrochemical performance was satisfactory.The initial 1 C(180 m A g^-1)discharge specific capacity was 180.2 m Ah g^-1 in the range of 2.8-4.3 V at room temperature.The capacity retention rate is 90.2%after 100 cycles,and the discharge specific capacity at 10 C is 128 m Ah g^-1.In order to improve material production efficiency.The precursor of NCM811 was prepared by a combination of rapid coprecipitation and spray drying.After further calcination with lithium source,the high sphericality secondary microsphere NCM811material was obtained.The material prepared by this route has excellent high voltage and high temperature cycle stability.The initial discharge capacity at 1 C is 178.7 m Ah g^-1 at room temperature with the range of 2.8-4.5 V,and the capacity retentions are 93.3%and86.0%after 100 cycles and 200 cycles,respectively.In compare,the materials by rapid coprecipitation methode are 45.0%and 36.6%,respectively.At 60°C with the range of2.8-4.3 V,the initial specific capacity at 1 C(180 m A g^-1)is 193.5 m Ah g^-1,and the capacity retention rate is as high as 93.4%after 100 cycles,whereas the comparative material is just 52.7%.The nickel-rich cathode material is sensitive to humid air and shows poor air stability.When the material is stored in the air for a period of time,lithium-containing impurities are formed on the surface of the particles,and a phase transition occurs on the near surface region,resulting in deterioration or even complete failure of the material properties.After storing the commercial LiNi_0.70Co_0.15Mn_0.15O₂?NCM701515?material under 60?and80%relative humidity?RH?environment for 30 days,a Li₂CO₃ impurity layer about 90nm was formed on the particle surface,accompanying by the formation of a NiO inert phase.The stored material is simply calcined in an oxygen atmosphere?800°C for 3 h?,the material properties can be completely recovered.During the calcination process,Li₂CO₃ impurities are decomposed and Li⁺is re-inserted back into the crystal lattice.At the same time,NiO cubic phase is oxidized and converted into the original hexagonal layered structure.