Preparation And Properties Of High-rate Ternary Cathode Materials

The layer structured metal oxide of LiNi_1/3Co_1/3Mn_1/3O₂,LiNi_0.5Co_0.2Mn_0.3O₂ and LiNi0.8Co0.1Mn0.1O₂ as cathode materials show some advantages of high reversible specific capacity,low costs,etc,which have broad commercial application prospect.However,the poor rate capacity and cycle performance of the aforesaid cathode materials seriously limit their application in practical production,which is mainly due to they are semiconductor materials with poor electronic conductivity.In this thesis,the electrochemical performance of the cathode materials have been improved by compositing them with three-dimensional?3D?conductive network composed by graphene and carbon nanotube.The microstructure and electrochemical properties of the composites,and the correlation of the microstructure and electrochemical have been systematically studied.As a result,the mechanisms of the improved electrochemical performance of the composites have also been summarized.The LiNi_1/3Co_1/3Mn_1/3O₂ with 3D conductive network,which was constructed via recombining it with two-dimensional sheetlike GNs and one-dimensional CNTs.In this composite,the GNs could coat on the surface of LiNi_1/3Co_1/3Mn_1/3O₂,and the CNTs could be inserted into the interval of graphene and LiNi_1/3Co_1/3Mn_1/3O₂,which could build a 3D conductive network.The 3D conductive network could effectively improve the rate capacity and cycle stability of the LiNi_1/3Co_1/3Mn_1/3O₂ because of the encouraged synergistic effect of GNs and CNTs.Compared with the single SP,GNs or CNTs forming the composites with LiNi_1/3Co_1/3Mn_1/3O₂,the LiNi_1/3Co_1/3Mn_1/3O₂/CNTs/GNs composite material exhibits obviously more excellent electrochemical perfonnance.At the rate of 1C,the initial discharge specific capacity of LiNi_1/3Co_1/3Mn_1/3O₂/SP,LiNi_1/3Co_1/3Mn_1/3O₂/GNs,LiNi_1/3Co_1/3Mn_1/3O₂/CNTs and LiNi_1/3Co_1/3Mn_1/3O₂/GNs/CNTs are 156 mAh g-1,150 mAh g-1,142 mAh g-1 and 161 mAh g-1,respectively.50 cycles later at the rate of 1C,the discharge capacity are 56 mAh g-1,90 mAh g-1,75 mAh g-1 and 123 mAh g-1,with the capacity retention ratio of 36%,60%,53%and 76%,respectively.In addition,the influence of different types of carbon nanotubes and graphene on the electrochemical properties of the cathode materials have also been studied.The results indicate that the multi-walled carbon nanotubes with smaller diameter has better effect on enhancing the electrochemical performance of cathode material of LiNi_1/3Co_1/3Mn_1/3O₂ than the bigger one.Moreover,the graphene oxide with a large number of functional groups,which has higher reactive activity with the electrolyte that is not beneficial to improve the electrochemical performance of the cathode materials.On the contrary,the reduced oxide graphene shows excellent electronic conductivity and electrochemical inert,which is favourable to improve the electrochemical performance of the cathode material.While the mass ratio of the GNs to CNTs is 2:1,the LiNi_1/3Co_1/3Mn_1/3O₂/CNTs/GNs composite shows the best rate capability and cyclic performance.The cathode material of LiNi_0.5Co_0.2Mn_0.3O₂ and LiNi0.8Co01Mn0.1O₂ have also been prepared by a rheological phase approach,and the electrochemical performance of the above cathode materials combining with the AB,GNs,CNTs and GNs+CNTs also were systematically studied.The results indicate that the LiNi_0.5Co_0.2Mn_0.3O₂/CNTs/GNs and LiNi0.8Co0.1Mn0.1O₂/CNTs/GNs composites also show the best rate capability and cyclic performance.Moreover,the optimum mass ratio of GNs to CNTs is also 2:1 in the composites.These results manifest that the rate capability and cyclic performance of all the LiNi_1/3Co_1/3Mn_1/3O₂,LiNi_0.5Co_0.2Mn_0.3O₂ and LiNi0.8Co0.1Mn0.1O₂ can be improved by forming the composites with GNs and CNTs,which should be mainly ascribed to the synergistic effect of GNs and CNTs to form 3D electronic conductive network in the composite to effectively improve the electronic conductivity of the cathode materials.Furthermore,the optimum mass ratio of the GNs to CNTs is 2:1 for all the composite,this further identifies that the approach adopted in this thesis is valid to improve the electrochemical performances of the cathode materials of LiNi_1/3Co_1/3Mn_1/3O₂,LiNi_0.5Co_0.2Mn_0.3O₂,LiNi0.1Co0.1Mn0.1O₂,which also has potential commercial application value.