Preparation And Electochemical Performance Of S-doped Transtion Metal Phosphide Based Composites

As conversion-type materials,transition metal phosphides are assumed as promising anode on account of the high theoretical capacity and a low working potential.Due to the high theoretical capacity(894 mAh g-1)and thermal stability of Co-P bond,cobalt phosphide(CoP)has become research hotpot.Nevertheless,inherent defects of the low conductivity and the serious volume swelling during cycling process limit the applications of cobalt phosphide.In order to work out the problems above,we have designed two different composites based on cobalt phosphide from the perspectives of heteroatom doping,composite with carbon materials and changing the spatial structure of materials,and their electrochemical performance was measured.(1)We synthesized precursor Co(OH)2/rGO via a simple chemical precipitation method,and sulfur-doped cobalt phosphide was obtained after the low-temperature phosphorization/sulfuration treatment.The CoP particles with a diameter of 400 nm were well-distributed on graphene layer.Comparing the electrochemical performance of different concentrations of Co2+ of composite,we assumed that when tested as anode for sodium-ion battery,the composite with a concentration of Co(NO3)2·6H2O is 1.8125 mg mL-1(S-CoP/rGO-3)exhibits excellent cycling stability and rate performance(a discharge specific capacity of 340 mAh g-1 after 60 cycles at a current density of 500 mA g-1)and when tested as anode for lithium-ion battery,the composite with a concentration of Co(NO3)2·6H2O is 3.625 mg mL-1(S-CoP/rGO-2)displays a long cyclic span(after 1000 cycles reaching a discharge specific capacity of 542 mAh g-1 at 500 mA g-1).The doping of sulfur provides more active sites and the introduction of graphene can effectively disperse CoP nanoparticles,alleviate volume expansion and improve conductivity.Cobalt phosphide particles can also prevent the accumulation of graphene.(2)Precursor Co(CO3)0.5OH·0.11H2O was prepared through a simple hydrothermal process.After dopamine-coating process and the low-temperature phosphorization/sulfuration treatment,the hollow-nanotube like structure of sulfur-doped cobalt phosphide coated by nitrogen-doped porous carbon(S-CoP@NPC)was obtained.CoP nanoparticles were coated uniformly by N-doped carbon coating forming a closed structure.When tested as anode for sodium-ion battery,S-CoP@NPC composite displays a discharge specific capacity of 230 mAh g-1 after 370 cycles at a current density of 200 mA g-1 with a high capacity pretention of 91%.When tested as an anode for lithium-ion battery,S-CoP@NPC composite displays a discharge specific capacity of 696 mAh g-1 after 134 cycles at a current density of 200 mA g-1.The enhanced performance benefit from these reasons:the doping of sulfur provides more active sites;mesoporous structure contributes to insertion/extraction of lithium/sodium ions;N-doped carbon coating limits the agglomeration of CoP nanoparticles,which helps to alleviate the volume expansion and improve the conductivity and ion diffusion coefficient of the material.