Surface Modification And Electrochemical Properties Of Lithium-and Manganese-rich Layered Metal Oxides Cathode Materials And LiNi_0.8Co_0.15Al_0.05O₂

With the rapid development of new energy automobile industry,it is particularly important to develop cathode materials with more potential than traditional commercial LiCoO₂.Li-and Mn-rich layered oxides and layered Ni-rich cathode materials NCA(LiNi_0.8Co_0.15Al_0.05O₂)have attracted much attention due to their high specific capacity,good thermal stability,low toxicity and low cost.However,for the first time,the problems of low Coulomb efficiency,poor cycle stability,fast voltage attenuation and poor rate performance still need to be solved.In this paper,the surface modification of Li-and Mn-rich layered oxides was carried out by Co₃O₄ and SnO₂,and the surface modification of NCA by Li₃PO₄.The morphological structure and electrochemical properties of the two materials were studied.The main research contents are as follows:?1?Co₃O₄ successful modification on the surface,XRD and SEM analysis showed that nano-sized Co₃O₄ particles were uniformly distributed on the surface of LMCNO without changing the original structure and morphology of the matrix material.In comparison with pristine LMCNO,all the Co₃O₄-modified LMCNO composites show higher capacity,better cycling stability and rate capability.Among them,1.5 wt.%Co₃O₄-modified LMCNO shows high initial capacity of 253.3 mAh g^-1,good capacity retention of 73.3%after 80 cycles at 0.2C and much improved rate capability especially at high rates.The enhanced electrochemical properties can be attributed to protecting LMCNO bulk materials from the electrolyte attack by Co₃O₄ surface layer.Electrochemical impedance spectroscopy reveals that the Co₃O₄-modified LMCNO sample shows much smaller charge transfer resistance than pristine sample.?2?SnO₂ modified layers with different contents have been prepared on the surface of Li-and Mn-rich layered oxides cathode materials using SnCl₄ as raw material.Co₃O₄nanoparticles distribute uniformly on the surface of LMCNO,the structure and morphology of matrix material remain unchanged.0.5wt.%SnO₂-LMCNO can deliver the capacity of 207.7 mAh g^-1 after 80 cycles at 0.2C(60 mA g^-1),and the capacity retention rate is 81.8%,compared to 63.3%in pristine LMCNO.Results above indicate remarkable improved cyclic stability.When the current density increased to 2C,1 wt.%SnO₂-LMCNO exhibits the discharge capacity of 96.2 mAh g^-1 after 400 cycles,while pristine LMCNO can only deliver 57.7 mAh g^-1,corresponding to better rate capability of SnO₂ modified LMCNO.?3?Li₃PO₄ modified layers have been prepared on the surface of NCA using LiOH and NH₄H₂PO₄ as raw materials.Investigation results show that Li₃PO₄ modified layers distributed on the surface of NCA can efficiently protect matrix material from being corroded by HF arising from electrolyte,thus improve the cycling properties of NCA.1wt.%Li₃PO₄-NCA can deliver the capacity of 151.7 mAh g^-1 after 100 cycles at 0.1C(18 mA g^-1),and the capacity retention rate is 79.6%,higher than pristine NCA.After300 cycles at high current density of 2C,the capacity retention rate is 66.0%,which is attributed to the high ionic conductivity and stable structure of Li₃PO₄ modifier.