The Synthesis Of Bimetallic Compounds/Graphene Materials For Lithium Storage Performance

Lithium-ion batteries are widely used in various electronic devices due to their high capacity,long service life and safety without pollution.Anode materials are an important part of lithium-ion batteries.At present,commercial anode materials can not meet people's demand for future energy.Therefore,it is imperative to develop high-capacity anode materials.Bimetallic compounds generally have better electrical conductivity and certain complementary effects,and tend to exhibit better electrochemical performance than single metal compounds when used as negative electrode materials.In this paper,cobalt ferrite and tin disulfide/zinc sulfide are used as the main energy storage materials,and two different bimetallic based graphene composites are obtained by combining two-dimensional graphene which with excellent electrochemical properties.The composites were tested and characterized as anode materials for a series of material structures and electrochemical properties.In the preparation of two bimetallic-based graphene composites,the introduction of the graphene can not only solve the problem of poor conductivity,low cycle stability and large volume expansion of cobalt ferrite and tin disulfide/zinc sulfide as electrode materials,but also can improve the electrochemical performance of the whole composites.Both bimetallic-based graphene composites exhibit better cycle stability and higher rate performance compared to a single electrode material.The main contents of this paper are as follows:?1?Coordination polymers derived three-dimensional hierarchical CoFe₂O₄hollow spheres as high-performance lithium ion storage.In this experiment,the 1,1'-ferrocenedicarboxylic acid ligand contains iron ions,and the ligand still contains iron metal after being calcined,so the morphology of the precursor can still be maintained.First,the hierarchical CoFe₂O₄ hollow spheres were obtained by solvothermal method and calcination treatment using the cobalt based ferrocenyl coordination polymers as a template.Then,in order to alleviate the problem of large volume change of CoFe₂O₄during charge/discharge processes,reduced graphene oxide was introduced during the synthesis of CoFe2O4@rGO composite.The introduction of reduced graphene oxide in this experiment could alleviate the volume change of CoFe2O4@rGO during the lithiation/delithiation processes,which is beneficial to the structural integrity and stability.In addition,the reduced graphene oxide sheets provide continuous conductive paths between CoFe2O4 hollow spheres and reduce the resistance of the particle-particle interface,thereby improving the conductivity of the electrode and the electrochemical performance of the CoFe₂O₄@rGO composite.The obtained CoFe₂O₄@rGO composite has a specific discharge capacity of 933.1 mA h g^-1 after 100cycles at a current density of 100 mA g^-1.And it still has a discharge specific capacity of 498.9 mA h g^-1 after 1200 cycles at a current density of 1000 mA g^-1,which exhibited excellent cycle performance.?2?A facile approach for construction of uniform SnS₂/ZnS heterostructure nanosheets embedded in graphene for high performance lithium batteries.Most of the bimetallic sulfides are not prepared from the same material,so the two sulfides can't compound well.In this experiment,a bimetallic hydroxide was used as a precursor,so that a bimetallic sulfide having a uniform heterojunction structure can be prepared after one-step vulcanization.The ultrathin SnS₂/ZnS heterostructure nanosheets with graphene network?SnS₂/ZnS-rGO?was synthesized by a simple hydrothermal method using thioacetamide?TAA?and ZnSn?OH?₆ cube in a graphene oxide solution.SnS₂/ZnS-rGO composite have a fast electron/ion transfer rate due to the combination of two-dimensional ultra-thin nanosheets and heterostructures.In this work,the SnS₂/ZnS-rGO heterostructure nanosheets graphene composite was prepared by introducing reduced graphene oxide,which not only ensure the structural integrity of the SnS₂/ZnS material,but also improve the conductivity of the composite,so that improve electrochemical performance.The obtained three-dimensional SnS₂/ZnS-rGO composite also exhibited excellent cycle performance,rate performance,and long cycle stability.The SnS₂/ZnS-rGO composite has a specific discharge capacity of 1458.3 mA h g^-1 after 200 cycles at a current density of 0.2 A g^-1,and still has a specific capacity of 432.4 mA h g^-1 after 4000 cycles at a current density of 10 A g^-1.?3?Pseudocapacitive capacitance and reaction mechanism of composite materials.We can linearly fit the cyclic voltampere curve at different voltages to obtain the capacitive response current,so that the relationship between the capacitive response current and voltage at different scanning rates can be obtained.The contribution of the capacitance-control can be calculated by integrating the relationship between the capacitance response current and voltage fitted at a certain scanning rate and the cyclic voltammogram at the same scanning rate.At the same time,the lithium storage mechanism was further studied by in-situ X-ray diffraction?XRD?and ex-situ X-ray diffraction?XRD?techniques during charge/discharge of the electrode materials.In the in-situ XRD test of CoFe₂O₄@rGO composite,the diffraction peak of the standard metal Co at 47.2^o appears and disappears periodically,which corresponds to the conversion storage mechanism of CoFe₂O₄.In the ex-situ XRD test of SnS₂/ZnS-rGO composites,the peak of the?111?crystal plane of ZnS located near 28.6°appears and disappeares with the periodicity of charge and discharge,demonstrating the conversion storage mechanism of ZnS.At the same time,the in-situ XRD test further demonstrates the conversion storage mechanism of ZnS and the conversion mechanism and alloying storage reaction mechanism of SnS₂.