Study On Preparation And Surface Coating Of Li-rich Layered Cathode Material And Its Electrochemical Performance

With the need of social sustainable development and the increasing attention to environmental pollution,people are forced to seek for efficient,renewable and pollution-free green energy.Lithium-ion batteries are widely used in portable electronic equipment,electric vehicles and hybrid vehicles as green energy storage device,which can effectively reduce the emission of carbon dioxide and other gases.Lithium rich layered cathode material in the chemical formula of xLi2MnO3·(1-x)LiMO2(M=Ni,Co and Mn)has been considered to be an ideal cathode material for electric vehicle power lithium ion batteries for its advantages of high specific capacity,low price and friendly environment,which has attracted wide attention.However,lithium-rich layered cathode materials have problems such as low initial coulombic efficiency,poor rate performance,and significant capacity decay during cycling,which severely limits their commercial applications.In this thesis,lithium-rich layered cathode materials were synthesized by a simple oxalate method,and their surface modification was carried out.The phase composition,microstructure and electrochemical properties of the samples were characterized by X-ray diffraction(XRD),specific surface area measurement,field emission scanning electron microscope(FESEM),transmission electron microscopy(TEM),inductively coupled plasma torch emission spectrometer(ICP),energy-dispersive X-ray spectroscopy(EDX),cyclic voltammetry(CV),galvanostatic charge-discharge(GCD),and electrochemical impedance spectroscopy(EIS).The main research contents and results were summarized as follows:(1)Li1.2Mn0.54Ni0.13Co0.13O2(denoted as LMNCO)with different morphology was prepared by solid-phase oxalate method and liquid-phase(including aqueous medium and ethanol medium)oxalate method,and the relations between its morphology,structure and electrochemical performance were investigated.The results of structural analysis showed that all three samples have a good layered structure with accurate chemical compositions.The samples synthesized by the solid-phase oxalate method are well-dispersed ultrafine particles with a primary particle size of 150-200 nm.And the sample synthesized by oxalate method in ethanol medium is composed of micro-rods structure with a length of 1-2 μm and a diameter of 100-300 nm,and the primary particle size is 100-200 nm.The sample prepared by oxalate method in aqueous medium presents porous hierarchical micro-cuboids with a size of 2-5 μm,and the primary particle size is 50-200 nm.Among them,the sample synthesized by oxalate method in aqueous medium has the highest degree of order of cation distribution,the smallest specific surface area(1.148 m2/g),and the best electrochemical performance.Its first discharge capacity was determined to be 261.9 mAh/g,and its 1st-cycle coulombic efficiency to be 74.6%at a current rate of 0.1 C(1 C=200 mA/g).It delivers a discharge capacity of 108.9 mAh/g when increasing discharge rate to 5 C.After cycled for 100 cycles at 1 C,it still relains a discharge capacity of 159.2 mAh/g,and its capacity retention is as high as 80.9%.(2)Based on the above results,Li1.2Mn0.54Ni0.13Co0.13O2 cathode material was prepared by oxalate synthesis in aqueous medium,and the influence of precursor synthesis conditions on its microstructure and electrochemical performance was studied.The results showed that the samples prepared at 50 ℃,70 ℃ and 90 ℃(referred to as LMNCO-50,LMNCO-70,and LMNCO-90,respectively)had similar morphologies,exhibiting micron-sized porous rectangular aggregates.And the primary particle size is about 200 nm.Besides,it also exhibited accurate stoichiometry,well-developed layered structure and high chemical homogeneity.However,detailed structural analysis showed that the rectangular aggregates in the LMNCO-70 samples were more compact,the pore size was smaller and narrower distributions,and the order degree of cationic distribution in layered structure was higher.Electrochemical analysis showed that the electrochemical properties of LMNCO-70 samples were significantly better than LMNCO-50 and LMNCO-90 samples.The initial discharge capacity of the LMNCO-70 sample at a current density of 0.1 C was determined to be 261.9 mAh/g,and the first Coulomb efficiency was 74.6%.It delivers a discharge capacity of 108.9 mAh/g when increasing discharge rate to 5 C.After cycled for 100 cycles at 1 C,it still remains a discharge capacity of 159.2 mAh/g,and its capacity retention is as high as 80.9%.(3)Based on the above research results,the spinel-structured LiNi0.5Mn1.5O4 was coated on the surface of Li1.2Mn0.54Ni0.13Co0.13O2 by the sol-gel method,and the effects of LiNi0.5Mn1.5O4 coating amount(4 wt%,8 wt%and 12 wt%)on the morphology,structure and electrochemical performance of Li12Mn0.54Ni0.13Co0.13O2 were further analyzed.The results showed that LiNi0.5Mn1.5O4 was successfully coated on the surface of Li1.2Mn0.54Ni0.13Co0.13O2,and the primary particle size of the lithium-rich cathode material Li1.2Mn0.54Ni0.13Co0.13O2 has not changed significantly.With the increase of coating amount,the rectangular aggregates of the sample were all more compact and the porosity was reduced.The results of structural analysis showed that the layered structure of the coated samples became more intact and the cationic mixing degree decreased.Electrochemical tests showed that the electrochemical properties of Li1.2Mn0.54Ni0.13Co0.13O2 samples were improved most obviously when the coating amount was 8 wt%.It shows a 1st-cycled coulombic efficiency of 82.1%.Its discharge capacity is 282.7 mAh/g at 0.1 C,and decreases to 140.0 mAh/g when increasing current rate to 5 C.After cycled for 100 cycles at 1 C,it retains a discharge capacity of 169.6 mAh/g with a capacity retention of 84.1%.