The Preparation And Electrochemical Performance Study Of Transition Metal Oxide Heterostructure Anode Materials

Lithium ion batteries（LIBs）with the high energy and power densities are considered as the most important for energy storage and conversion technologies.However,the low theoretical capacity of the current commercial graphite anode still hinders the further large-scale application of next-generation LIBs.It is imperative to development of new anode materials with reduce costs and extend the cycle life yet retaining high capacities.Recently,transition metal disulfides（TMOs）have been considered as promising candidate for LIBs.However,owing to the thermodynamic restrictions and kinetic limitations,these electrode materials often suffers from serious volume changes during cycling,leads to fast capacity fading and low initial coulombic efficiency.These drawbacks greatly plague its electrochemical performance.Based on the above considerations,designing a stable electrode structure with high conductivity and buffer volume expansion during cycling is an effective way to boost the energy storage and conversion properties.In this paper,transition metal oxides heterostructure are designed by solvothermal method,electrospinning technology and special calcination process.The main research results are as follows:（1）We utilize a simple and bender-free method to realized 2D nanosheets of ZnO-Co₃O₄ heterostructure attached to the surface of highly conductive 3D carbon cloth（CC）for Li/Na ion cells and flexible cells.The FE-SEM image shows that the morphology of ZnO-Co₃O₄@CC composite is regular,and the two-dimensional nanosheets are evenly supported on the fibers,showing s abundant space among adjacent nanosheets and abundant pores.Typical TEM images suggest that the nanosheets are composed of crystalline particles and consisted of interconnected nanoparticles with diameters of 10 nm.XPS results indicate that the formation of heterostructures in the composite causes the oxide electron cloud density to recombine,resulting in energy level shifts.When employed as lithium ion batteries anode to provide an excellent reversible capacity of 1785 mAh g^-1 at a current density of 200 mA g^-1.Even at a high current density of 2000 mA g^-1,the ZnO-Co₃O₄@CC composite showed a reversible capacity of 491 mAh g^-1 after 400 cycles.For sodium-ion batteries,the composite has a reversible specific capacity of 684 mAh g^-1at a current density of 200 mA g^-1 and a capacity of 265 mAh g^-1 at a current density of 1000 mA g^-1 after 500 cycles.Additionally,flexible half-cells were also fabrication to lightting the LED lamp and exhibit high flexibility and excellent electrochemical performance.The AC impedance proves that the formation of this heterogeneous interface could greatly facilitate charge transfer,thereby promoting surface reaction kinetics.Besides,Ex-situ XRD and FESEM were used to demonstrate phase transformation during cycling and changes in electrode structure after cycling.The outstanding performance is attributed to the in-situ growth of ZnO-Co₃O₄heterostructures on carbon cloth,which provided a large contact area,ultimately facilitating rapid electron transport and preventing the aggregation of large surface area nanosheets.（2）A simple and efficient method was developed to synthesized CuO_x-Co₃O₄heterostructure attached on three-dimensional porous nitrogen-doped carbon nanofibers（PNCNF）.FE-SEM demonstrates that CuO_x-Co₃O₄ nanowires are uniformly coated on the surface of porous carbon nanofibers.This 1D nanowire feature not only enhances electron transfer along its 1D geometry,but also provides a large number of active sites for the reaction.TEM image shows that the CuO_x-Co₃O₄nanowires are composed of crystalline particles are uniformly modified on 3D nitrogen-doped porous carbon fibers.XPS analysis were performed to understand the electronic interaction between CuO_x and Co₃O₄,and the energy band offset of each component of the composite material is due to the heterogeneous interface.This unique hetero-junction would greatly favour for obtaining much more electroactive sites for efficient electrochemical energy storage and promoting charge transport,thus improving the surface reaction kinetics as well as enhancing the electrochemical performance.Ex-situ XPS was applied to explore the phase transformation in electrochemical processes.At the same time,the mechanism of synergy is explained by summarizing the phase transformation process.EIS and CV with different sweep speeds were used to study the surface reaction kinetics and determine the pseudocapacitive contribution of the lithium storage process.Ex-situ SEM was used to study structural transformation after cycling.Remarkably,the combined effect of the 1D heterostructure and the 3D substrate structure features provides excellent electrochemical performance.For binder free and self-supporting LIBs anode,this smart electrodes offered an excellent reversible capacity rate,about 1122 mAh g^-1 at a current density of 200 mA g^-1.Even at a charge-discharge rate of 2 A g^-1,the CuO_x-Co₃O₄@PNCNF composite revealed a reversible capacity of 668 mAh g^-1 after1000 cycles.The excellent performance is attributed to the rational design of the electrode structure and the fascinating synergy of the CuO_x-Co₃O₄ heterostructure.（3）The MoS₂ nanoflowers were encapsulated into containing amorphous SnO₂carbon fibers.It is clearly see that the binder free electrode was composed of highly uniform nanofibers with a diameter of 500 nm.TEM and HRTEM were further performed to examine the inner structure of MoS₂-SnO₂@NCNF nanofibers.In addition,the MoS₂ nanoflowers were highly dispersed within carbon nanofibers.Besides,the interstitial spaces between the nanoflowers are retained,and the insertion of the carbon layer during the calcination process increases the interlayer spacing.This structural feature is beneficial for alleviating the volume expansion and pulverization of MoS₂ during cycle.The interaction of electrons in the material is analyzed by XPS.Compared with the single component,the characteristic peak of Mo in the composite is shifted to the low binding energy direction after the introduction of SnO₂.This may be due to the increased density of electron cloud around MoS₂.Correspondingly,the characteristic peak of Sn in the composite shifts toward the high binding energy could be attributed to the electron cloud density drop around SnO₂.When used as a LIBs anode,such a clever structure possesses superior performance:exhibited discharge capacity is 983 mAh g^-1 at current density of 200 mA g^-1following 100 cycles and revealed a reversible capacity of 710 mAh g^-1 at 2 A g^-1 after800 cycles.To further explain the reaction kinetics of the electrode,CV curves at different scan rates of 0.2,0.4,0.6 and 0.8 mV s^-11 were recorded.As shown in Fig.4c,the CV curves at different scan rates hold similar shapes reflects a sign of pseudocapacitive behavior.With increasing the scan rates,the capacitive component increased from 20.5%to 78.3%.The AC impedance demonstrates that the MoS₂-SnO₂@NCNF composite has faster charge transfer kinetics.The offer excellent performance is attributed to the rational design of electrode structure:the high electrical conductivity of carbon fiber and the carbon in favor of alleviating volume expansion of the two types of materials,such structural features greatly enhance the surface reaction kinetics and promote charge transport.In addition,the assembly and application of flexible full cells demonstrates the potential of MoS₂-SnO₂@NCNF composites in flexible devices.