| Traditional layered cathode materials with high capacity,good electrochemical properties,and mature production processes have been widely used in a variety of lithium-ion battery,this includes portable devices,energy storage and electric vehicles.The performance of cathode materials determines the upper limit of lithium-ion battery performance,however,NMC(LiNixMnyCozO2,x+y+z=1,x≥0.8)has been the research focus in the field of lithium-ion battery.However,NMC materials also face several severe challenges:the scarcity of Co resources,environmental pollution and high cost.Therefore,it is necessary to study a material that can replace Co.In order to solve the above problems,the feasibility of using NMA(LiNixMnyAlzO2,x+y+z=1)as a Co-free cathode material was investigated,the main research work of this paper includes:1、NMA and NMC materials were synthesized by coprecipitation method and molten salt method respectively,and the synthesis process was optimized,the variation law of purity and size of morphology in the process of calcination was mastered.2、The secondary heat treatment of NMC and NMA synthesized by molten salt method was carried out,and the in-situ XRD monitoring was carried out.The regulation of the secondary heat treatment on the mixed arrangement mechanism of Li/Ni was revealed.The optimal temperature treatment parameters were obtained,the treatment of residual salt and other impurities on the surface of the material improved the electrochemical properties of the material.The results show that 500-600℃ is the optimum temperature range for the secondary heat treatment,at which the mixed arrangement of Li/Ni is the lowest,and the specific capacity is obviously increased compared with the untreated material.3、The effect of Al and cation vacancy on the reversibility of Co-free laminates was investigated by in situ X-ray diffraction(XRD).The results show that Al can effectively improve the structure stability and cation disorder of layered materials,and decrease the lattice strain between H2 and H3 phases,but has a negative effect on the structure reversibility.The combination of Al and cation vacancy can not only reduce the lattice strain of H2/H3,but also improve the re versibility of the structure.4、The diffusivity of lithium ion in Al-free,Al-containing and Al-associated cation vacancy materials was compared.The migration barrier of Li ions in materials with or without cation vacancies is calculated by first-principles,the effect of metal cations on the reversibility of interlayer Li ion migration was investigated by combining diffusion coefficient with transport barrier.The results show that under different SOC,the DLi of the materials with cation vacancy is always obviously higher than that of other positive materials without cation vacancy.The calculated results are in agreement with the experimental ones.The cation vacancy reduces the transport energy barrier of lithium ion by an order of magnitude.The experimental results show that in the design and preparation of Co-free layered cathode materials,the synergistic effect of Al and cation vacancies can improve the structure irreversibility,phase transition and Li/Ni mixed arrangement of Co-free layered cathode materials.In this paper,the NMA-δ material with the substitution of Al for Co and the introduction of cation vacancy shows high capacity(0.5 C up to 180 mAh·g-1)and good retention capacity(81%capacity retention after 100 weeks),furthermore,the Coulomb efficiency of NMA is always at a high level(up to 99%),which proves that NMA materials are likely to replace NMC materials in the future market. |
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