Study On Three-Dimensional Carbon Nanocomposites As Novel Cathode Materials For High-Performance Lithium-Ion Batteries

Layered nickel-rich material Li Ni_0.8Co_0.1Mn_0.1O₂(NCM811)is one of the promising cathode materials for next-generation lithium-ion power batteries with high-energy density and high-power density.However,the poor rate performance and cycling stability of this material severely restrict its practical application.Carbon nanomaterials with high specific surface area,high conductivity and high carrier mobility can be used to modify the electrode materials of lithium-ion batteries to provide them with fast electron and ion transport channels,which will greatly improve the electrochemical performance of the electrode materials.In this thesis,combining single-component or multi-component commercial carbon nanomaterials(two-dimensional graphene(GN),one-dimensional carbon nanotubes(CNTs)),conductive carbon black(Super P),conductive graphite(KS-6))with commercial NCM811,three-dimensional composite electrodes with superior rate capability and cycling stability have been successfully developed.Electrochemical behaviors of the obtained composite electrodes were studied by rate performance test,cycling performance test,cyclic voltammetry test(CV),and electrochemical impedance spectroscopy(EIS).Surface morphology,microstructure,specific surface area and pore size distribution of these composite electrode materials were studied by scanning electron microscope(SEM)and nitrogen adsorption desorption(BET).Furthermore,the effects of the composite composition and the electrode preparation process on the electrochemical performance and physical and chemical properties of the composite electrodes have been systematically investigated:1.By combining a single-component conductive additive,either graphene or CNT,with NCM811 can significantly improve the conductivity of NCM811 electrode so as to effectively improve the rate performance and cycling performance of the resultant composite electrodes.Moreover,when graphene and CNT as a multi-component conductive additive were composited with NCM811,the rate performance and cycling performance of the obtained composite electrodes can be further improved;2.NCM811/GN composite electrodes were prepared by using graphene as a conductive additive and by optimizing the composition of the composite electrode materials.Experimental results showed that a very low content of graphene at 0.05wt.%can improve the internal conductive network structure and reduce the internal impedance of the battery.Upon the addition of graphene,capacity retention of the battery increased from 21.56%to 87.03%at a current density of 1 C and its capacity retention increased from 31.36%to 70.10%after 50 cycles at a current density of 1/3C;3.NCM811/CNT composite electrodes were prepared by using CNT as a conductive additive and by optimizing the composition of the composite electrode materials.Experimental results showed that a very low content of CNT at 0.1 wt.%can improve the internal conductive network structure and reduce the internal impedance of the battery.Upon the addition of CNT,capacity retention of the battery increased from 21.56%to 86.60%at 1 C and its capacity retention increased from 31.36%to68.57%after 50 cycles at 1/3 C;4.NCM811/GN/CNT three-dimensional composite electrodes were prepared by using graphene and CNT as a multi-component conductive additive and by optimizing the composition as well as the preparation process of the composite electrode materials.The conductive network of the three-dimensional composite electrodes constructed by the synergistic effect of multi-dimensional conductive agents enabled three-dimensional channels for rapid transport of electrons and ions,resulting in significantly enhanced rate performance and cycling stability for the electrodes.With the joint introduction of graphene and CNT,capacity retention of the battery reached91.49%at 1 C(while that of the conventional battery without graphene and CNT was21.56%only under the same testing conditions),capacity retention of the battery after50 cycles at 1/3 C was as high as 92.37%(versus that of 31.36%of the conventional battery),clearly demonstrating the excellent rate performance and cycling stability of the NCM811/GN/CNT three-dimensional composite electrodes developed in this thesis.