The Preparation And Properties Of High Capacity Lithium-Rich Ternary Materials

This paper aims at the research and development of new high-capacity lithium-rich cathode materials,focusing on the synthesis and modification of lithium-rich material xLi2MnO3.(1-x)LiNi0.6Co0.2Mn0.2O2.The relationship between the structure and properties of the prepared material were characterized by using x-ray diffraction,scanning electron microscopy,charge and discharge tests,and AC impedance etc.,and the following results were obtained:The studies on the preparation of xLi2MnO3·(1-x)LiNi0.6Co0.2Mn0.2O2 show that it is easier to prepare cathode materials with good layered structure and excellent electrical properties by using the hydroxide precursor.The pH and ammonia concentration affect the atomic ratio of nickel,cobalt and manganese as well as the grain and particle morphology of the prepared precursor.The calcination temperature and time have a significant effect on the grain development,layered structure integrity and electrical properties of xLi2MnO3·(1-x)LiNi0.6Co0.2Mn0.2O2 materials.The optimized preparation conditions is using NaOH solution as precipitant,pH value of 11.5 and ammonia concentration of 0.5 mol/L for preparation of hydroxide precursor,and calcining at 850? for 12 h for preparation of xLi2MnO3·(1-x)LiNi0.6Co0.2Mn0.2O2 after mixing lithium.Li1.167Mn0.433Ni0.3Co0.1O2 prepared under the optimized conditions has the established atomic ratio of nickel,cobalt and manganese,and excellent layered structure and electrochemical performance.Its initial discharge capacity is 251.5 mAh/g in the first cycle,and the efficiency is 77.5%.Its capacity maintains at 180 mAh/g,and the capacity retention rate is over 89%after charge-discharged at 0.5 C for 50 cycles.The investigation on the effect of the composition of xLi2MnO3·(1-x)LiNi0.6Co0.2Mn0.2O2(x=0.1-0.5)on its structure and electrochemical performance shows that the materials synthesized by the hydroxide coprecipitation method are irregular secondary particles formed by the sintering of small grain agglomerates,and has pure phase hexagonal a-NaFeO2 layered structure.With the increase of x value,the crystals of the prepared material are well-developed,the mixing of cations is reduced,the layered structure tends to be perfect,and the structural stability under high-voltage deep charge increases,the charge transfer impedance decreases significantly,and the discharge capacity,rate performance,cycle stability are gradually improved.Li1.167Ni0.433Mn0.3Co0.1O2(x=0.5)exhibits smaller electrode polarization,faster charge transfer process and better electrochemical properties.The studies on the modification of Li1.167Ni0.433Mn0.3Co0.1O2 material show that the trace K doping has no obvious effect on the morphology of as-prepared material,but the mixing degree of cations increases and the 2D layered structure deteriorates,which leads to an increase in the interfacial impedance and a significant decrease in the discharge capacity.The morphology of Li1.167Ni0.433Mn0.3Co0.1O2 material is also not significantly affected by AlPO4 coating,but there was trace amount of Al3+ doping and Li3PO4 forming on the surface,which did not destroy the integrity of the 2D layered structure.The appropriate amount of AIPO4 coating can slow down the electrolyte decomposition and HF erosion,reduce the interface impedance,improve the discharge capacity,cyclablity and rate performance.The modified sample with 4%AIPO4 coating shows the lowest impedance and the best electrochemical performance.Its 0.1 C discharge capacity is 261.1 mAh/g,and the capacity at 2 C and 5 C reaches 137.5 mAh/g and 52.3 mAh/g,respectively.The capacity retention rate reaches 96.9%after charge-discharged at 0.5 C for 50 cycles.In a word,under the optimal conditions,the prepared 4%AIPO4 coated Li1.167Ni0.433Mn0.3Co0.1O2 material exhibits better electrochemical performance,which is considered to be a promising new type of lithium-rich cathode material.