The Research Of Preparation And Modification Of Li Rich Layered Li_1.2Ni_0.2Mn_0.6O₂ Solid Solution Using For Lithium-ion Battery Cathode

Li-rich layered Li1.2Ni0.2Mn0.6O2 solid solution material which has a high discharge capacity up to 2 times of Li Co O2, and it doesn't contain the expensive cobalt element,thereby dramatically reduce the cost of lithium batteries as a cathode material of lithium-ion batteries. So it has attracted world wide attention and has been studied by various research institutions in the world because of these obvious advantages.In this paper, Li-rich layered Li1.2Ni0.2Mn0.6O2 solid solution materials were prepared by spray-drying method. Firstly Ni-Mn solution spray-drying method, Li-Ni-Mn turbid liquid spray-drying method and Li-Ni-Mn solution spray drying method were explored.We also conducted a series of modification experiments concerning LPAN coating and nickel-manganese oxide coating in terms of its poor stability and low initial Coulombic efficiency. We have achieved some research results as follow:(1) The powder prepared by using Ni-Mn solution spray-drying method contains water, and its water content is 6.75% measured by TG technology. After subtracting the water content and calculate Li which needed accurately, we explored the influence of different Li sources(Li OH, Li NO3, Li2CO3) on the preparation of Li1.2Ni0.2Mn0.6O2 materials. Results showing that samples using Li OH as Li source have a good crystallinity and electrochemical performance. Then we determine to use Li OH as Li source to explored Li quantity added to the material, presintering temperature, final heat treatment temperature and other impact factors, it turns out that sample prepared by 100% Li quantity(stoichiometric ratio) have a high degree of crystallinity which displayed the highest discharge capacity of ~220m Ah/g at a low rate. We conclude that the presintering temperature requires slightly higher than the melting point of lithium hydroxide, lower temperature leads to incomplete reaction, higher temperature causes a stabilized phase product Li2 Mn O3, and brings out a serious Li2 Mn O3 segregation of Li1.2Ni0.2Mn0.6O2 material, thus affect its properties of materials. The higher sintering temperature, the better crystallinity of materials, but when the temperature is above 900 ?, the material maintains a stable electrochemical properties.(2) We found that the materials have the best electrochemical properties prepared by using Li-Ni-Mn-citric acid solution spray-drying method compared to Ni-Mn solution spray-drying method and Li-Ni-Mn turbid liquid spray-drying method. It provides a discharge capacity of 250 m Ah/g at 0.1 C rate and 220 m Ah/g at 0.2 C rate, after 400 charge-discharge cycles with 80% capacity retention at 0.2 C rate. These materials have excellent performance and stability which is a promising high-capacity material of LIBs.(3) Aims at its poor structure stability of Li1.2Ni0.2Mn0.6O2, the Mn element would transform from Mn4+ to Mn3+ during charge and discharge cycles and the Jahn-Teller effect would occurs. What's more, Mn3+ is likely to occur disproportionated reaction to generate Mn2+/ Mn4+ and dissolved in the electrolyte. Study of Li1.2Ni0.2Mn0.6O2 materials coated with LPAN indicates that modification by coated with LPAN can improve the charge-discharge capacity and cycle stability.(4)To overcome the problem of material's low theoretical and practical initial Coulombic efficiency, we explored the nickel-manganese oxide coating modification. Results indicate that initial Coulombic efficiency were improved significantly of those materials after coated with nickel-manganese oxide layer, the highest efficiency is about 82.1% improved from 64.8% of the blank control, and when the coating amount accounted for 30%wt of material, its initial Coulombic efficiency up to 101.3% in a potential range of 2.5-4.7V vs. Li/Li+. The charge capacity of coated materials are slightly decreased with the increase of the coating amount, but the discharge capacity is higher than the blank control, especially when the coating amount accounted for 20%wt of material, its discharge capacity is 22.7% higher than the blank control. I consider that it is due to the efforts of nickel-manganese oxide coating layer, it can accept Li+ and provide many channels to facilitate Li+ transport, thereby enabling outstanding high discharge capacity and superior high-rate capability.