Preparation And Modification Of High Nickel Ternary LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Materials

The exhaustion of non-renewable resources and the serious pollution caused by traditional fuel vehicles have led to the further widespread application of electric vehicles.The endurance of electric vehicles has always been a core issue,and the key technology lies in the development of cathode materials.With the merits of high energy density,low cost and environmental friendliness,the ternary anode material with layered structure of LiNi0.8Co0.1Mn0.1O2(referred to as NCM811)has become the anode material developed by major battery factories.However,there are many problems in the application of NCM811 cathode materials,mainly manifested in unstable structure,poor thermal stability,high alkalinity,and harsh storage conditions,which greatly limits further applications.This article is mainly optimized from three aspects of material synthesis conditions,precursor pre-oxidation treatment and anion-cation co-doping.(1)In this paper,the best NCM811 samples were obtained by optimizing the oxygen flow rate during the synthesis process and optimizing the synthesis process.The effects of different oxygen flow rates on the structure,morphology,and electrochemical performance of the samples were explored.It is found that the surface morphology of the sample is not change duing different oxygen flow rates,and the oxygen flow rate of 0.1 L/min has the best performance,the lowest cation disorder,an ordered layered structure,and excellent cycle performance.Especially under high voltage,its cycling performance is also more stable.The sample synthesized under an oxygen flow rate of 0.1 L/min has a discharge capacity of 174 mAh g-1 after 100 times cycle at 1 C,and has the highest capacity retention rate of 98.3%,The capacity retention rate is also 96.8%at 2 C.Even under the rate capability test,its discharge capacity can still be 145.1 mAh g-1 at 5 C.In the voltage range of 3-4.5 V,the capacity retention rate is 85.8%after 100 cycles at 1 C.Therefore,the sample can greatly improve all aspects of material performance by refinement of oxygen flow.(2)In this paper,the precursor was pre-oxidized with K2Cr2O7 as an oxidant.Ni2+ on the surface was immediately oxidized to Ni3+,and obtain precursors with a well-ordered surface structure.then mixed with a lithium source and calcined under oxygen atmosphere.Through XRD and SEM analysis,it was found that there is no new phase formation in the precursor pre-oxidation,the surface morphology of the synthesized material has not changed significantly,and the degree of cation mixing has decreased.After K2Cr2O7 preoxidation of the precursor,the electrochemical performance of the sample has been greatly improved.In particular,The best performance of samples obtained by adding 1%K2Cr2O7 to pre-oxidation After 50 cycles at 1 C and 3-4.3 V,the capacity can still retain 177.7 mAh g-1,and the capacity retention rate is as high as 99.1%.After 50 cycles,the capacity is only 161.6 mAh g-1,and the capacity retention rate is 96.6%.When charged to a high voltage of 4.5 V,the NCM-1 can derive a discharge capacity of 200.9 mAh g-1,which retains 173.2 mAh g-1 after 50 cycles,while the pristine has 165 mAh g-1 after 50 cycles.The improved electrochemical performance is attributed to the formation of a more ordered layered structure on the surface.In addition,the content of Ni2+is reduced,which reduces the degree of cation mixing,increases the kinetics of charge transfer and lithium ion diffusion.(3)In this paper,the structural stability of NCM811 cathode is improved by P-F co-doping.Although the initial capacity is slightly reduced,the cycle performance of NCM811 material is effectively improved.After 100 cycles,a capacity retention rate of 88.8%(159.5 mAh g-1)and 94.4%(162.3 mAh g-1)has been achieved at 0.5 C for the doped NCM811 samples by 0.5%and 1%LiPF6 solution,which is much higher compared with that of the pristine NCM811(77.7%,143.6 mAh g-1).Similar trend also has been obtained at 1 C,in that,the 0.5%sample has a 87.6%of the optimum capacity enhancement.The improved cycling stability is attributed to the enhanced reversibility of hexagonal(H2)-hexagomal(H3)phase transition and reduced electrode polarization,due to the P and F codoping.Overall,the P and F codoped NCM811 material is promising for high energy LIBs application as a cathode material.