Controlling Synthesis And Content Optimization Of Lithium-rich Layered Oxides As Lithium-ion Battery Cathodes

Nowdays, lithium ion battery (LIB) have been widely used in portable electronic devices such as mobile phone and computer. However, they are difficult to meet the demands of high power and energy densities for electric and hybrid electric vehicles. It is well known that, the electrochemical performance and cost of LIB are determined by the cathode materials. Commercial cathode materials such as LiCoO2, LiMn2O4and LiFePO4only have a specific discharge capacity within100-160mAh g-1, which limits the further development of LIB. The exploration and development of cathode materials with higher specific discharge capacity are of great importance. In recent years, the lithium-rich layered oxides in the chemical formula of xLi2MnO3·(1-x)LiMO2(M=Ni, Co, Mn, Fe, Cr, Ni1/2Mn1/2, Ni1/3Co1/3Mn1/3...) have attracted specialists more and more attentions and have been recognized as one of the most promising next generation cathodes due to their high specific discharge capacities of230-300mAh g-1. However, their high irreversible capacity loss in the initial cycle, low rate capability and decrease of discharge midpoint voltage upon cycling still exist. In order to solve or alleviate these shortages, preparation of well-constructed and nanosized materials, optimization of the chemical compositions of lithium rich layered oxides, doping with other ions and surface modification with functional compounds have been successfully used. Herein, a promising lithium-rich layered oxides Li1.2Ni0.13Co0.13Mn0.54O2(0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2) has been selected as research target, and a serie of controlling synthesis and content optimization studies have been carried out, shown as below:(1) Lithium-rich layered oxides Li1.2Ni0.13Co0.13Mn0.54O2have been successfully prepared by a template directed route using β-MnO2nanorods as sacrificed templates. The influence of calcination temperature on the structures and electrochemical properties of as-prepared materials are systematically studied. The elevated calcination temperature helps to improve the layered structures and particle sizes. At the low temperatures (i.e.,750and800℃), rod-like Li1.2Ni0.13Co0.13Mn0.54O2can be obtained, and which will be transformed into polyhedral Li1.2Ni0.13Co0.13Mn0.54O2nanoparticles at a higher temperature (i.e.,850or900℃). The electrochemical performances of as-prepared Li1.2Ni0.13Co0.13Mn0.54O2are determined by their different particle sizes and layered structures. At the optimal calcination temperature of850℃, the resulting Li1.2Ni0.13Co0.13Mn0.54O2shows the highest discharge capacity of239.2mAh g-1at20mA g-1within2.0-4.7V, and a stable discharge capacity of92.8mAh g-1at1000mA g-1. The good electrochemical performances of850℃sample should be attributed to the better layered structure and/or the more appropriate particle size comparing with those of other samples.(2) Porous and solid Li1.2Ni0.13Co0.13Mn0.54O2spheres have been successfully prepared by using templates of spherical Ni0.13Co0.13Mn0.54(CO3)0.8and MnO2. Porous Li1.2Ni0.13Co0.13Mn0.54O2spheres obtained from carbonate precursor are composed of well-defined primary nanoparticles with the size of100-300nm, and the porous feature can be visually determined. While the solid Li1.2Ni0.13Co0.13Mn0.54O2spheres are made of tightly clustered nanoparticles. The former porous structures come from the generation of CO2, and the latter results from the excessive ions insertion during calcination. Correspondingly, the porous spheres have higher BET surface area than solid spheres. As lithium ion battery cathodes, the porous spheres exhibit a higher initial discharge capacity of255.7mAh g-1at0.1C between2.0and4.8V. After50cycles, a discharge capacity of177.7mAh g-1could be retained at0.5C. Even at a high charge-discharge rate of5C (1000mA g-1), a specific value of121.4mAh g-1can be reached. By comparison, the solid spheres only deliver initial discharge capacity of159.9mAh g-1at0.1C.(3) A combination of the primary polymerization of acrylic acid (AA) monomers, the subsequent pyrolysis of PAA polymer gels at450℃and the final high-temperature crystallization (850℃) is used to prepare Li1.2Ni0.13Co0.13Mn0.54O2nanoparicles. The as-prepared Li1.2Ni0.13Co0.13Mn0.54O2nanoparicles have an average particle size of173.2±66.2nm and a good layered structure. HRTEM and SAED results show that, these nanoparticles possess a single-crystalline nature and a superlattice ordering. As LIB cathodes, these single-crystalline nanoparticles can deliver a high initial discharge capacity of290.7mAh g-1and the first cycle coulombic efficiency of78.3%at20mA g-1between2.0and4.8V, remain a reversible value of182.7mAh g-1at100mA g-1over60charge-discharge cycles and reach a capacity of122.5mAh g-1at a high current rate of1000mAg-1.(4) A series of cathode materials0.5Li2MnO3·0.5LiNi1/3+xCo1/3-2xMn1/3+xxO2(-1/12≤x≤1/12) are prepared by a PAA polymerization-pyrolysis-assisted crystallization route. All powders possess a two-dimensional sheet-like superstructure composed of crystalline nanoparticles, and the added Co content helps to increase the particle size. As LIB cathodes, the samples with an increasing Co content can slightly increase the diacharge capacity, but are unfavorable to the initial coulombic efficiency and cycling stability. The optimal Co contents correspond to the x value of1/12and0, which is closed to the Co content of most used and commercial lithium-rich layered oxides.(5) The influences of Mn-Ni contents on the structures and electrochemical performances of Li1.2Ni0.13+xCo0.13Mn0.54-xO2(-0.06≤x≤0.06) are clearly studied. A citric acid-assisted sol-gel route has been successfully used for the nanofabrication of these materials. The elevated Mn contents can increase the content of Li2MnO3component and particle sizes, and the elevated Ni contents lead to more Li/Ni cation mixing. As LIB cathodes, the x=0sample shows the highest discharge capacity and coulombic efficiency, the elevated Mn content helps to improve the cycling stability, and the elevated Ni content are favorable to the rate capability, electrochemical conductivity and discharge voltage. The optimal x value should be between-0.03and0.03. Furthermore, the upper cutoff voltage (4.5-4.8V) studies are also carried out for Li1.2Ni0.13Co0.13Mn0.54O2nanoparticles. Based on the total considerations of discharge capacity, discharge energy, cycling stability and rate capability, the optimal upper cutoff coltage should be4.6-4.7V.