Study On Sodium/Lithium Storage Properties Of Vanadium/Molybdenum Based Carbon Nanocomposites

The development of high efficiency,low cost and high safety sodium ion batteries(SIBs)and lithium ion batteries(LIBs)is an efficient way to solve energy storage problem.The electrode materials is the key to improve electrochemical performance of ion batteries.Vanadium-based and molybdenum-based anode materials have been extensively studied in SIBs/LIBs due to their high theoretical capacities,low cost and environmental friendly properties.However,vanadium-based and molybdenum-based materials suffer from low capacity of bulk materials,slow ion diffusion rate,and large volume expansion during the cycling process.In this paper,four strategies of constructing nanostructures,combining with carbon materials,heterogeneous interface construction and heteroatoms doping of the composite carbon materials are adopted to solve the above problems.The high-performance SIBs/LIBs anode material based on vanadium/molybdenum materials are developed,with in-depth studies on the reasons for its performance improvement and energy storage mechanism.The main finding are as follows:Firstly,we report a facile approach to prepare the anodes with VC_0.75 nanodots wrapped by N and P co-doping carbon nanosheets(VC_0.75@NPC)for SIBs and LIBs,via a simple wet chemical process followed by a high-temperature calcination.The as-obtained VC_0.75@NPC exhibits excellent sodium storage performances with high reversible capacity(250 m Ah g^-1/0.1 A g^-1)and excellent rate capability(88 m Ah g^-1/5.0 A g^-1).In addition,when used as anode for LIBs,the VC_0.75@NPC also delivers high lithium storage capacity(532.2 m Ah g^-1 after 200 cycles at 0.1 A g^-1)and high rate capability(91%capacity retention at 1.0 A g^-1 after 500 cycles).The excellent electrochemical properties of the composite can be attributed the hierarchically structure in VC_0.75@NPC electrode,since the VC_0.75 nanodots are homogenously grown on the loosely stacked N,P-doped carbon nanosheet scaffolds,which possess numerous voids to relieve volume variation of VC_0.75 and generate3D conductive network;the average diameter of?2-10 nm for the as-synthesized VC_0.75 nanoparticles could greatly shorten the Li and Na ions diffusion distance,thus facilitates electrochemical kinetic performances.MoC nanoparticles anchored to N-doped carbon Nanosheets(denoted as MoC@NC)are established to shed light into the contributions of different N species,namely:pyridinic N,pyrrolic N,graphitic N,and Mo-N.At the same time,X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),electron paramagnetic resonance(EPR),and the electrochemical analysis of MoC@NC electrodes are systematically applied to elucidate the relationship between various N dopants and electrochemical behaviors.The increases in N-doping level will not only lead to the better conductivity of the electrodes,but also improve the cycling stability by pinning NPs on carbon NSs via Mo–N bonds.Moreover,both pyridinic N and pyrrolic N can create additional defects and active sites in carbon NSs,which are of benefit for ion storage.Accordingly,the performance of MoC@NC-1.0 is the best,delivering specific capacities of 390 and 252 m Ah g^-1for SIBs,as well as 1062 and768 m Ah g^-1 for LIBs at 0.1 and 1.0 A g^-1,respectively.The composites of MoO₂@C,MoS₂@C and MoO₂/MoS₂@C are designed and synthesized.Electrochemical performance show the MoO₂/MoS₂@C composite exhibites excellent sodium/lithium storage properties including good rate capability and outstanding long-term cycling performance.When used as anode for SIBs and LIBs,the MoO₂/MoS₂@C delivers 482 m Ah g^-1 and 727 m Ah g^-1 reversible specific capacities at 0.1 A g^-1,and the capacity retention rates are 90%and 89%after 200cycles,respectively.At 1.0 A g^-1,the sodium storage capacity can be retained at 324m Ah g^-1 after 1000 cycles.The excellent electrochemical performance of the MoO₂/MoS₂@C composite can be ascribed to the three-dimensional structure formed by foamy carbon,which not only benefits the infiltration of electrolyte and shortens the Li and Na ions diffusion distance,but also buffers the volume expansion of the material and generates 3D conductive network.The constructed MoO₂/MoS₂ heterostructure can generate an internal electric field conducive to charge transfer at the interface,thus improving the electrochemical kinetic performances.In summary,we not only explore three vanadium-and molybdenum-based electrode materials as SIBs and LIBs negative electrode materials,but also study the roles of carbon composite,nano structure,heterostructure and heteratomic doping on the electrochemical properties,which provides the new strategies and experimental basis for the further design and construction of novel transition metal-based carbon nanocomposites for SIBs,LIBs and other areas.