| With the rapid development of human society and economy,the energy problem is becoming more and more serious,which is mainly attributed to the excessive exploitation and utilization of fossil fuel by human beings.Therefore,exploring green new energy has become the focus of the world.At present,lithium ion battery is one of the most promising electrochemical energy storage devices in the secondary battery,which has a wide range of applications from portable consume electronics to electric vehicles to large-scale strorage of renewable energy.However,lithium resources are scarce and expensive,so exploring and developing new sodium ion battery electrode materials is a hot topic in the current research field.Compared with lithium ion batteries,sodium ions have a larger radius and a lower potential,so the requirements for electrode materials are higher,especially in terms of structural stability and energy density.Recently,the negative electrode materials studied by researchers mainly include carbon-based materials,alloy-based materials and transition metal oxides.Transition metal oxides have been extensively studied because of their high capacity and safe compared to other types of materials.This research focuses on the controllable construction and morphology of nikel cobaltate nanomaterials and modification via combining with various carbon based materials.Meantime,analysing its modification mechanism.The main research work are listed on below:(1)NiCo2O4 core-shell micro-nanosphere structure(NCO-YS)with unique morphology was successfully prepared by using synthetic carbon spheres as template,which is composed of nanoparticles about 15 nm.There is a cavity in the NiCo2O4 micro-nanosphere which is synthesized by template method.Their outer and core diameters are 450 nm and 200 nm,respectively.The average thickness of the outer layer is about 100 nm.Compare with pure NiCo2O4 nanoparticles(NCO-NP),the initial discharge capacity of NCO-YS anode material is up to 912 mAh g-1 at the current density of 100 mA g-1.Moreover,after 60 cycles,the reversible capacity still has 342 mAh g-1.This is mainly due to the existence of cavaties inside the micro-nanosphere,which can alleviate the volume effect of NiCo2O4 during charge and discharge process.At the same time,NiCo2O4 micro-nanosphere is composed of nanoparticles and the presence of voids between nanoparticles,which could allow the electrolyte to penetrate into the structure and increase the contact area bewteen electrolyte and activity materials.Also,it could shorten the diffusion path of Na+,which is beneficial to improve the charge transfer process of the electrode material.(2)A unique NiCo2O4 micro-nano solid sphere was synthesized via solvothermal method and NiCo2O4 micro-nanospheres were dispersed on the surface of graphene by hydrothermal method to form a composite NCO@GO anode material of NiCo2O4 and graphene.NiCo2O4 micro-nanospheres have a particle size of 400-500 nm and have good crystallinity and chemical stability.Moreover,NiCo2O4 particles are uniformly distributed on the graphene substrate without particle agglomeration.Compared with pure NiCo2O4 micro-nanospheres(NCO),NCO-GO composite materials possess superior electrochemical performance,corresponding to the initial good discharge capcity(1281 mAh g-1)and its CE(70%)at a current density of 100 mA g-1.After 60 cycles,the reversible capacity remains at 485 mAh g-1.However,the NCO anode material has a reversible specific capacity is only about 240 mAh g-1 after cyclic charging and discharging under the same condition.It can be seen that the excellent conductivity of the layered two-dimensional material-graphene as a effective buffering effect on the stress during the de/intercalation of sodium can greatly improve the cycle stability of the electrode material.At the same time,the particles size of NiCo2O4 is relatively small,so the contact area with the electrolyte is large,which could shorten the Na+ transmission path and improve the charge transfer process in the electrode material.(3)A novel two dimensional layered structure-porous g-C3N4(PCN)was prepared via magnesiothermal reaction successfully.The structure of two-dimensional material was controlled by different proportions of magnesium power with g-C3N4,and the effects of samples with different structures on sodium storage performance were analyzed.Compared with the original g-C3N4,the samples after magnesiothermal reaction have superior electrochemical performance,especially cycle performance.Among them,PCN2 sample has the best performance and the initial discharge specific capacity is up to 2095 mAh g-1 and reversibility capacity at a current density of 500 mA g-1 can reach about 300 mAh g-1 after 450 cycles.At the same time,it is found that PCN sample has good stable discharge capacity after rate capcity testment.Subsequently,in-situ preparation of NiCo2O4 nanoparticles and PCN2 composite(NCO-PCN)anode material by hydrothermal method,the average size of NiCo2O4 nanoparticles is around 20 nm and it has good crystallinity and chemical stability.Moreover,NiCo2O4 nanoparticles are distributed on the surface of PCN evenly.The initial discharge capacity is 1611 mAh g-1 at a current density of 500 mA g-1 and the reversible specific capacity is 639 mAh g-1 after 50 cycles.The studies have shown that the NiCo2O4 nanoparticles loading on porous g-C3N4 can greatly improve the electrical conductivity of active material,whike the presence of the carrier can inhibit agglomeration of particles and optimize Na+ transfer diffusion path and kinetic performance. |
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