| Nowdays, layered Li-Ni-Co-Mn-O materials have been the focus of cathode materials due to the high capacity,excellent cycle performance,safty and low toxicity,etc. With the requirement of high energy density,improving the working voltage of the cathode material has become an important method.However,the electrochemical performance of the Li-Ni-Co-Mn-O materials will decay under high working voltage.So it will be significantly important to improve the electrochemical properties of Li-Ni-Co-Mn-O cathode material under high working voltage.In this work, Li[Ni1/3Co1/3Mn1/3]O2 and Li[Ni0.5Co0.2Mn0.3]O2 cathode materials were studied and their electrochemical performance under high voltage(4.7V) were improved with modifying the electrolyte and electrode materials. Scanning Electron Microscopy(SEM), Energy Dispersive Spectrometer(EDS), Transmission Electron Microscope(TEM) and X-ray diffraction(XRD) were used to characterize the crystal structures and particle morphologies of the materials. The electrochemical performance of the cathode materials were examined by cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS) and galvanostatic charge/discharge performance.Li2CO3 and Li2 Si O3 were used as additives in the electrolyte to improve the electrochemical performance under high voltage of Li[Ni1/3Co1/3Mn1/3]O2 cathode. The optimum content of Li2CO3 to enhance the electrochemical performance of the cathode materials was demonstrated to be about 2wt.%. The specific capacity and the capacity retention of the cathode materials are 171.2 mAh/g and 81.21% respectively, after charged/discharged 50 times,between 2.8V and 4.7V, at 0.2C. While the materials in the electrolyte without Li2CO3, exhibited only 156.2 mAh/g and 73.75% respectively.With 0.3 wt.% Li2SiO3 in the electrolyte,the discharge capacity of the cathode materials is 172.7 mAh/g,and the capacity retention is 79.73% after charged/discharged 70 times.AlF3 and MgF2 were introduced for the surface modification of the Li[Ni1/3Co1/3Mn1/3]O2 materials via liquid-phase reaction. 1wt.%AlF3 +1wt.%MgF2-coated Li[Ni1/3Co1/3Mn1/3]O2 delivers 177.6 mAh/g(79.21% of its intial discharge capacity), after 95 cycles between 2.8V and 4.7V, at 0.2C.While the bare Li[Ni1/3Co1/3Mn1/3]O2 delivers only 130.3 mAh/g whih 59.51% capacity retention left. Meanwhile, the cells were charged/discharged between 2.8V and 4.7V at 0.2C, 0.5C, 1C, 2C, 5C, respectively, then cycled at 0.2C again, the rate capability of the 1wt.%AlF3 +1wt.%MgF2-coated Li[Ni1/3Co1/3Mn1/3]O2 is enhanced, especially at 5C(discharge capacity is 114.5 mAh/g).The Li[Ni0.5Co0.2Mn0.3]O2 was surface modified by AlF3 and MgF2, then charged/discharged between 2.8V and 4.7V, at 0.5C. The 0.5wt.% AlF3-coated Li[Ni0.5Co0.2Mn0.3]O2 materials exhibit high discharge capacities of 155.2 mAh/g after 100 cycles, and the capacity retention rate is 73.45%. 0.5 wt.% MgF2-coated discharged 151.3 mAh/g after 100 cycles, maintained 70.53% of the initial discharge capacitythe. The 0.5 wt.% AlF3+ 0.5 wt.% MgF2-coated Li[Ni0.5Co0.2Mn0.3]O2 show the best performance than the previous two cases.The materials’ specific capacity remains 164.4 mAh/g,and the capacity retention is 75.56%,after 100 cycles, however, bare samples discharge only 116.9 mAh/g with 55.99% capacity retention. The rate capability are also enhanced comparing to the pristine Li[Ni0.5Co0.2Mn0.3]O2.In summary,with the modification of the electrolyte and electrode material, the electrochemical performances of Li[Ni1/3Co1/3Mn1/3]O2 and Li[Ni0.5Co0.2Mn0.3]O2 materials are improved under high voltage. The oxidative decomposition of the electrolyte are suppressed.The surface coating can protect the electrode materials from attacks by HF in the electrolyte. And the reduction reaction between electrolyte and active materials, stripping of transition metal ion can be prevented.Therefore the materials’ s structurures and electrochemical performance can be stabled. |
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