Improving The Low-temperature Performance Of Layered Lithium-rich Cathode Materials For Lithiumion Batteries

Layered cathode materials are the hotspot of research and application in the field of cathode materials for Li-ion batteries.The further applications of materials are facing many difficulties, along with the progress of science and technology, the improvement of the low temperature performance of the material application should be paid more attention. In this thesis, Li1.2Mn0.6Ni0.2O2 was chosen as the research object, and the electrochemical properties of the materials were improved by doping and surface coating. Experiments are as follows.We prepared materials Li1.2Ni0.2-xCo2xMn0.6-xO2(x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) by a sol-gel method. X-ray diffraction studies show that the main structure of modified materials has hardly changed, preserving the layered structure with well crystallinity. With the increase of Co doping amount, the lattice parameters a and c decrease, which results from the fact that the radius of the cobalt ion is similar to that of the manganese ion, while smaller than that of the lithium and nickel. It was proved that the Co was successfully doped into the material and occupied the position of the Ni or Mn.The EDS test shows that the proportion of the experimental materials and the theory of the composite material is the same. The SEM images further exhibited that material morphology did not change before and after Co doping. Electrochemical test results show that the electrochemical properties of the Co doped materials have been improved, with the decrease of temperature, the effect is more obvious. At-20°C, the initial discharge capacity of sample with x = 0 could retain only22.1%(57.3/259.2 mAh·g-1) of that at 30°C, while sample with x = 0.05 could maintain39.4%(111.3/282.2 mAh·g-1). Discharge differential capacity(dQ/dV) plots are employed to further investigate the influence from Co doping. The results show that the reduction peaks at about 3.85 V, which is generally considered as the reduction of Ni4+or Co4+ions in the rhombohedral phase, is hardly affected by the testing temperature in the samples with high Co contents. To further interpret this finding, impedance spectra of all the samples were measured at different temperatures. The results show that the charge transfer impedance of sample with x = 0.05 is significantly less than that of sample with x = 0. The activation energy(Ea) was also calculated through Arrhenius equation. And the results reveal that the interface reaction of Ni4+or Co4+reduction at 3.85 V becomes easier after Co doping. In the future research, we can also try to improve the low temperature performance andelectrochemical performance of the material by reducing their activation energy at low temperature through a simple ion-doping method.Li1.2Ni0.2Mn0.6O2 was prepared by a sol-gel method, which was further coated by different amounts of lithium boron oxide. Coated quantity was 0.1 wt %, 0.3 wt % and 0.5wt %. X-ray diffraction studies show that the main structure of modified materials has hardly changed, preserving the layered structure with well crystallinity. No impure peaks were found in XRD patterns, indicating that the coating layer is amorphous structure. The SEM results further exhibited that material morphology did not change much before and after coating, but a little agglomeration appeared when coating amount increased. TEM diagram shows that the bare material has a clear boundary and the LBO-coated is surfaced by colorless transparent thin layer. Electrochemical tests showed that the sample with 0.3wt%exhibited the best electrochemical performance and of its initial discharge capacity was287.8 mAh·g-1, which is higher than that of the bare material at the same temperature. In conclusion, the coating layer can alleviate the contact between cathode material and electrolyte and prevent the side reaction so as to enhance the stability of the electrode. The coating layer brings good compatibility with electrolyte and improves lithium ion conduction between electrode and electrolyte, thus enhancing electrochemical activity and structure stability of cathode material; Consequently, dissolution of manganese ions was effectively inhibited and cycle and rate performanceswere also greatly improved.