Structural Design And Electrochemical Performance Of Transition Metal Phosphorus (sulfide) Compounds

Under the background of the rapid consumption of fossil fuels and increasing environmental pollution,the development of renewable,clean and efficient energy storage technologies has become a worldwide problem.Lithium-ion batteries have been widely used in portable electronic devices and electric vehicles as important power sources due to their high capacity,long life,and environmental friendliness.The rapid development of electric vehicles has also raised increasingly needs on the overall performances of lithium-ion batteries.However,the current commercial lithium-ion batteries are more and more difficult to meet these requirements,and there is an urgent need to develop electrode materials with better performance.On the other hand,the reduction of lithium resource reserves has become a bottleneck for the further development of lithium-ion batteries.Sodium-ion batteries have similar working principles to lithium-ion batteries,and have great advantages in cost.Although the capacity and cycle life are relatively low,they have received more and more attention in recent years and are promising to replace lithium-ion batteries.As a commercial anode material for lithium-ion batteries,graphite has excellent cycle stability,but its relatively low capacity cannot meet current needs.In addition,graphite cannot be used as anode material for sodium-ion batteries,because the volume of sodium ion is much larger than that of lithium ions.Therefore,it is important to develop new anode materials with low cost and high capacity.Among many anode electrode materials,transition metal phosphide and transition metal sulfide have attracted much attention due to their advantages such as high specific capacity and low price.However,the low conductivity of metal phosphides and metal sulfides results in poor rate performance.At the same time,the inevitable volume change during the charge and discharge process causes the destruction and pulverization of the electrode materials,speeding up the attenuation of its capacity and cycling life.In this article,we will improve the lithium/sodium storage performance of the material from aspects of microstructure design.（1）In a typical synthesis,a flower-like Fe precursor was prepared by a solvothermal method,and then coated with dopamine hydrochloride to form a nitrogen-doped carbon layer.Finally,the self-assembled flower-like FeP hybrid coated with nitrogen-doped carbon was obtained（Fe P@N,C）by phosphating.In this hybrid,flower-like structure can allow the rapid transfer of lithium ions,which would increase the reaction rate.Nitrogen-doped carbon can provide more active sites and promote the decomposition of Li₃P.Moreover,it wolud effectively improve the electrical conductivity of the material and further maintain the stability and activity of electrode materials.In addition,the flower-like FeP@N,C hybrid material has a larger surface area,increases the contact area between the electrode and the electrolyte,and provides a larger buffer space,so that its structure is well maintained during the cycle.Therefore,the flower-like Fe P@N,C hybrid shows good electrochemical performance.At a current density of 0.5 A g^-1,after 300cycles,its capacity can be maintained at 564.1 mA h g^-1,and the flower-like structure is maintained well.（2）Using ZIF-8 as a precursor,ZnS nanoparticles composited with nitrogen-doped carbon were successfully prepared after coating dopamine hydrochloride,high temperature calcination and hydrothermal vulcanization strategies.In this way,ZnS can be encapsulated in nitrogen-doped carbon derived from ZIF-8,limiting further growth and agglomeration of ZnS,and obtaining uniformly dispersed ZnS ultra-small nanoparticles with a particle size of about 10 nm.The ultra-small nanoparticles can shorten the diffusion distance of lithium ions and increase the specific surface area.In addition,after calcination,the dopamine-coated ZIF-8 precursor will produce a high content of nitrogen-doped carbon.The nitrogen-doped carbon will provide a channel for the transfer of lithium ions and buffer the volume expansion during the cycle.As a result,the ZnS@N,C composite shows excellent performance.At a current density of 200 mA g^-1,after 250 cycles,its capacity can be maintained 859.3 mA h g^-1.Even at a large current density of 500 m A h g^-1,it can maintain a high capacity of 627.9 mA h g^-1 after400 cycles of stable circulation.（3）A flower-like SnS₂@CoS₂–rGO composite was synthesized through a simple hydrothermal method and used as an anode material for sodium-ion batteries.The composite is assembled by SnS₂ nanosheets,CoS₂ nanoparticles and rGO.SnS₂ nanosheets have a large layer spacing on the horizontal axis,which provides an effective diffusion path for the sodium ions,and Co generated by CoS₂ during the reaction can improve the reversibility of the SnS₂.At the same time,rGO can buffer the mechanical stress generated during the reaction and keep the structure intact.Therefore,the synergistic effect of SnS₂,CoS₂ and rGO makes SnS₂@CoS₂–rGO nanoflowers presentexcellent sodium storage performance.At a current density of 200 mA g^-1,after100 cycles,it can maintain a high capacity of 514.0 mA h g^-1 and good rate performance.