Preparat Ion And Performances Of Transition Metal Phosphides(Sulfides)/carbon As Anode Materials For Lithium Ion Batteries

Due to the high energy density,high voltage,and long cycle life,lithium-ion batteries have been used as a new alternative energy source and are widely used in portable consumer electronics.The anode materials in the market have generally been dominated by graphite carbon.However,due to the defects of its own nature,it has been unable to meet the growing demand for high-efficiency batteries.Now we should look for higher-capacity and better stable anode materials for lithium-ion batteries and enhance the performance of the battery.As a result,the application of lithium-ion batteries could be more extensive.Transition metal phosphides and sulfides have much higher theoretical capacity than that of graphite carbon,an ideal voltage platform as well as environmental friendliness.They are ideal anode materials for lithium ion batteries.However,there are still some problems such as insufficient electron conductivity and volume expansion during charging and discharging process,which makes their actual capacity lower than their theoretical capacity.In view of these defects in transition metal phosphides and sulfides,loose porous carbon skeleton was successfully constructed by a simple synthesis method while CNT grew in situ.The prepared carbon nanotubes enhanced the electronic conductivity of the material and also constrained the structural deformation of the active material by resisting the volumetric stress,thereby improving the electrochemical performance of the material.The main findings are summarized as follows:1.CoP@N-(C/CNTs)were successfully synthesized by autocatalytic method,using metal ions in ZIF-67 as a catalyst under the condition of high temperature and long-time calcination.As a result,the organic ligand was transformed into the carbon nanotubes with the diameter of 3 nm in situ while the carbon framework was constructed under the existence of metal ions as catalysts,forming an interlaced network.Then,through further phosphating,a porous CoP@N-(C/CNTs)formed with carbon nanotubes interwoven together.When the current density was 0.5 A g-1 during the charge-discharge process,the first discharge/charge specific capacity were 1215 mA h g-1/688 mA h g-1,and the electrochemical stability of 600 mA h g-1 could be retained after 200 cycles.The excellent electrochemical performance of CoP@N-(C/CNTs)may be due to the porous structure of the material,which can alleviate the stress deformation during lithium ion insertion/extraction and maintain the stable structure of the product.The in-situ growth of carbon nanotubes can greatly improve the electronic conductivity of materials,provide more channels and paths for the transfer of ions and electrons,and improve battery performance.The method for the in-situ growth of carbon nanotubes is very convenient and makes the fabricated composites show good performance,which proves that this autocatalytic formation of carbon nanotube interlaced network is a very simple and effective structural design strategy.2.Ni-MOF used as precursor,under the low calcination temperature,a simple two-step calcination is used to form a nanostructure in which metal particles are coated by carbon sphere,with carbon nanotubes extended in situ on the surface of the sphere.Further phosphating eventually formed Ni2P@C/CNTs.Ni2P nanoparticles are well encapsulated in loose porous carbon spheres,which can restrict the volume and structural deformation of N2P during the discharge and charging process.The porous structure improves the structural stability;carbon nanotubes which grown on the surface of carbon spheres provide more paths for the movement of Li+/e-,enhance the electronic conductivity of the material,and improves the stability of the electrode.The Ni2P@C/CNTs electrode also has good electrochemical performance.At a current density of 0.2 A g-1,the first discharge/charge specific capacity is 652 mA h g-1/443 mA h g-1,and the reversible capacity remained at 442 mA h g-1 after 300 cycles,showing a very good electrochemical stability.The preparation method can in situ generate carbon nanotubes at a low calcination temperature,which is simple,convenient,economical and environmentally friendly.In addition,it has certain reference significance for the composite of carbon nanotubes and metal compounds.3.In this paper,Ni foam was used as the current collector of the binder-free negative electrode with ZIF-67 loaded on the surface.ZIF-67/Ni foam was used as the precursor to form multilayer nanoparticle array of Co9S8/C/Ni foam by sulfureted and further carbonized.Conclude from the electrochemical performance testing,the multilayer nanoparticle array Co9S8/C/Ni foam exhibited a 1926 mA h g-1 discharge specific capacity and 1371 mA h g-1 charge specific capacity at the first cycle at a current density of 0.2 A g-1.In addition,after 200 cycles,the reversible capacity was still maintained at 1114 mA h g"1,which showed a very good electrochemical performance.The irregular small particle size surface can provide a larger surface area for the transport and storage of Li+ and better buffer space for the volume expansion during charge and discharge process.At the same time,Ni foam as the current collector of the negative electrode can increase the conductivity,promoting the transport and diffusion of electrons.Its loose macroporous skeleton can effectively ensure the full contact of the active material with the electrolyte,which is beneficial to the transportation of ions and electronics between the positive and negative electrodes.