Synthesis And Surface Modification Of Layered Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ Cathode Materials With LiMnPO-_--4 Nanoparticles For Lithium-ion Batteries

Low energy density is the bottleneck problem of t he cathode materials in commercial lithium- ion batteries for their practical applications. Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ with high energy density is considered as a promising cathode material that is selected as research object. However, the practical application of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ still is hampered by its intrinsic shortcomings, such as the low initial charge-discharge efficiency, poor cycling stability and rate-capability. Moreover, the transition metal ions can be dissolved easily in the electrolyte. In order to solve the above problems, in this thesis, co-precipitation method and alcohol-combustion method have been both used to prepare Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ samples. Meanwhile, LiMnPO₄ was introduced to make the surface modification on Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ samples. The main contents are summarized as follows:Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ samples were prepared by co-precipitation method and alcohol-combustion method. The morphology, microstructure and electrochemical performance of the as-prepared two samples were systematically made a comparison. For the co-precipitation route, ammonia solution was chosen as the base solution and complexing agent. And NaOH solution was used as the precipitator. In more detail, NaOH solution and the mixed solution containing Mn（Ac）2·4H2O, Ni（Ac）2 ·4H2O and Co（Ac）2·4H2O were dropwise added into ammonia solution simultaneously to prepare precursor. The pH of mixed solution was kept at 11. The n the precursor mixed with a stoichiometric amount of LiO H·H2O by ball milling. Finally, the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ sample was obtained by calcining the aforementioned mixture. In the procedure of alcohol-combustion method, a stoichiometric of Co（CH3COO）2·4H2O, Mn（CH3COO）2·4H2O, Ni（NO3）2·6H2O and LiNO3（10% excess） were dissolved in the anhydrous ethanol. Then the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ was prepared via calcining the compound. The results demonstrated that the samples prepared by co-precipitation and alcohol-combustion methods both have the same order layered structure, consisting Li2 MnO3 and LiMO2 phases. The sample prepared by alcohol-combustion has the smaller particle size ranging from 50 to 200 nm. However, the particles were seriously aggregated, and the surface area decreased, which will restrict the diffusion of Li- ion. O n the contrary, the sample prepared by co-precipitation has bigger particle size, however, no visibly aggregation can be detected. The electrochemical performance clearly indicated that the initial charge-discharge efficiency of co-precipitated sample is higher than the sample prepared by alcohol-combustion. The first discharge capacity of co-precipitated sample is 287 mAhg-1 at current density of 30 mAg-1, while the capacity of another sample is 267 mAhg-1. But the cyclic performance of two samples is similar. Additionally, the co-precipitated sample exhibited better rate capability that the discharge capacity at 5 C（1500 mAhg-1） remains 90 mAhg-1. According to EIS results, it is easily found that the charge-transfer resistance of the co-precipitated sample is smaller than the sample prepared by alcohol-combustion method. Meanwhile the Li- ion diffusion rate of co-precipitated sample is also larger than the sample prepared by alcohol combustion.In order to further improve the cyclic stability and rate capability of Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂, the olivine LiMnPO₄ nanoparticles were introduced to modify the surface of Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂. The phase, microstructure, morphology and element distribution of the LiMnPO₄@Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ sample were measured by powder X-ray diffraction（XRD）, scanning electron microscopy（SEM） and transmission electron microscopy（TEM） equipped with an energy dispersive spectroscopy（EDS） detector. The morphology of LiMnPO₄@Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ sample was similar with the pure one. And particles size did not change obviously. The electrochemical performance tests demonstrated that that Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ modified by 3 wt% LiMnPO₄ had the best electrochemical properties. Its initial charge-discharge efficiency was improved to 76.1%. Meanwhile its cycling stability also was enhanced. The discharge capacity remained 206 mAhg-1 after 80 cycles at 0.1 C. Even at the current density of 1500 mAg-1, it also delivered a high capacity of 117 mAhg-1. In addition, EIS results also confirmed that the charge transfer resistance and Li- ion migration ability of LiMnPO₄@Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ were better than pure one.