Study On Structural Control And Electrochemical Properties Of Nickel-cobalt-manganese Ternary Cathode Material

This article aims at improving the structural stability of LiNi_0.6Co_0.2Mn_0.2O₂（NCM622）ternary material,easing the happening of side effects of material surface so as to improve the electrochemical performance of electrode material.Spherical and cocoon NCM622 materials were prepared,and with the reduced graphene oxide（RGO）composited,explored the two kinds of surfactants and RGO effects on the properties of materials.Meanwhile,nano-scale bulk NCM622 materials was prepared by the single factor control variate method.On this basis,the NCM622 material was modified by coating and doping.XRD,XPS,SEM,TEM,HRTEM,Raman,charge-discharge test were used to characterize the crystal structure,element content,morphology and electrochemical performance of the prepared materials.The main studies are as follows:1.Spherical NCM622 materials with a diameter of 11μm were prepared by urea homogeneous precipitation method.After respectively adding polyvinyl pyrrolidone K30（PVP）and hexadecyl trimethylammonium bromide（CTAB）,silkworm cocoon-like materials were generated,of which the material added CTAB was the smallest.In addition,the electrochemical performance of CTAB assisted synthesis material（NCM-CTAB）was the best.The initial discharge specific capacity of NCM-CTAB was 175.9 m Ah·g^-1 at 0.1 C,and the capacity retention rate was 80.9%at 0.2 C after 100 cycles.The material was compounded with graphene oxide（GO）synthesized by improved Hummers method.After hydrothermal reduction,the initial discharge specific capacity of NCM/RGO material was 183.34 m Ah·g^-1 at 0.1 C,and the capacity retention rate was 87.6%at 0.2 C after 100 cycles.So the coating of RGO improved the electrochemical performance of the material.2.NCM622 material was prepared by hydrothermal method with oxalic acid as precipitant.When the hydrothermal time was 16 hours and the hydrothermal solvent was a mixture of water and ethylene glycol,NCM622 particles with size of 300-500 nm were generated,and displaying optimistical morphology structure and electrochemical performance.Co₃O₄was coated on the surface of NCM622 material,and the electrochemical performance of the material was optimized when the coating amount was 3 wt.%.The initial discharge specific capacity of 3 wt.%Co₃O₄ modified material was 188.3 m Ah·g^-1 at 0.1 C,and its capacity retentions after 100 cycles reached 87.9%at 1 C.According to the CV and EIS results,the potential difference of battery made by 3 wt.%Co₃O₄coated material was 0.172 V,the sum of membrane impedance and the charge transfer impedance(R_f+R_ct)was 253.66Ω,which were both significantly lower than bare NCM622 impedance values.3.LiNi_0.6Co_0.2Mn_0.2O₂（x=0,0.01,0.015,0.02）series materials were prepared by doping Mg²⁺of different content into single crystal NCM622.When the doped amount x=0.015,the mixing degree of the material Ni/Li was lower,and its layer structure was stable.What’s more,the electrochemical testing results showed that the charge-discharge specific capacity,rate performance and cycling performance of the materials have been significantly improved.On this basis,LiAlO₂ and LiNi_0.6Co_0.2Mn_0.185Mg_0.015O₂ materials with the same layer structure were compounded to obtain NCM-Mg-LiAlO₂ composite material.The value of(R_f+R_ct)of composite material decreased significantly compared with NCM622 and LiNi_0.6Co_0.2Mn_0.185Mg_0.015O₂.In addition,the Li⁺diffusion coefficient DLi⁺reached 1.93×10^-8cm²/s.At 4.6 V high cutoff voltage,the initial discharge specific capacity of the LiAlO₂modified material reached 203.4 m Ah·g^-1 at 0.2 C,and the capacity retention rate was 91.5% at 1 C after 70 cycles.