Synthesis Process And Surface Modification Of Nickl Rich Ternary Cathode Materials LiNi_0.6Co_0.2Mn_0.2O₂

Nowadays,due to the increasing demand for the lithium-ion batteries with high energy density,NCM-based electrode materials,especially nickel-rich NCM materials,have attracted great attention as alternative cathode materials due to their excellent electrochemical performance such as high specific capacity,good cycling stability,and good safety.In this thesis,a series of Ni-rich electrode materials with the component of LiNi_0.6Co_0.2Mn_0.2O₂?NCM622?with good electrochemical performance were synthesized through coprecipitation.The influence of the main parameters in synthesis process,such as excess of lithium element,sintering temperature,and surface modifications with freeze-dried reduced graphene oxide?FD-rGO?or polypyrrole?PPy?,on the electrochemical performance of the obtained LiNi_0.6Co_0.2Mn_0.2O₂ cathode material.And the phase composition,microstructure,morphology,and electrochemical lithium-storage performance were characterized by XRD,SEM,cyclic voltammograms?CV?and charge-discharge techniques.The main obtained results are as follows:?1?When the excess of lithium is ca.5 wt.%,the obtained NCM622 electrode material shows the best electrochemical lithium-storage performance.With a voltage range of 2.75-4.6 V and the current density of 1 C,the obtained electrode material shows a discharge capacity of 154.2 m Ah g^-1 for the first cycle and 140.1 mAh g^-1 for the 50th cycle,indicating a capacity retention of 90.9%.?2?Based on the results of the majorization of the sintering temperatures,the NCM622 electrode material obtained at 470?×5 h+800?×5 h shows the best elelctrochemical performance.With a voltage range of 2.75-4.6 V at 1C,the as-obtained NCM622 cathode material delivers a discharge capacity of 183.1 mAh g^-1for the first cycle and 153.4 m Ah g^-11 for the 50 cycle,indicating a capacity retention of 83.8%.Attributing to the good layer microstructure and low cation mixed arranging,the the obtained cathode material also shows good rate capability,and could deliver high capacity over 104.7 m Ah g^-1 for 50 cycles even at 10 C.?3?Surface modification with FD-rGO could enhance the lithium storage capacity for NCM622 cathode material.When coated by 2 wt.%FD-rGO,the modified NCM622 delivers a discharge capacity of 209.5 m Ah g^-1 for the first cycle and 169.0 m Ah g^-11 after the 50 cycles in a voltage range of 2.75-4.6 V at 1C,which are higher than that of the samples without FD-rGO.It is due to the surface modification with FD-rGO could reduce the direct contaction between electrode material and electrolyte which caused side reactions.?4?Surface modification with PPy could enhance the cycling stability for NCM622 cathode material at a certain degree.When coated by 1 wt.%PPy,the modified NCM622 delivers a discharge capacity of 205.8 m Ah g^-1 for the first cycle and 141.4 m Ah g^-11 after the 50 cycles in a voltage range of 2.75-4.6 V at 1C,attributing to the enhance of electroconductivity by the surface modification with PPy.