Preparation And Sodium Storage Performance Of Metal Sulfide(Phosphide)/Carbon Nanotubes Composites Anode

Sodium-ion batteries（SIBs）are promising energy storage devices for large scale energy storage systems due to the low cost and rich abundance of sodium sources.However,the practical application of SIBs is limited by the lack of suitable electrode materials,especially the anode materials with high performance.Various anode materials,including carbon-based materials,metal oxides and metal sulfides/phosphides have been widely investigated for application in SIBs.Among these materials,metal sulfides/phosphides have attracted wide attention due to their large theoretical capacity and weak M-S and M-P bonds,favoring multi-electron conversion reactions.Especially,cobalt sulfides（Co S,Co S₂ and Co₉S₈）and cobalt phosphides（Co P,Co₂P,Co P₃）materials are considered as promising electrode candidates for SIBs.However,cobalt sulfides/phosphides usually suffer from poor cycling performance due to huge volume change during the sodiation/desodiation processes and poor rate performance because of sluggish electrode kinetics.Many efforts have been made to solve these problems,such as nano-size tailoring and carbon matrixes modification of cobalt sulfides/phosphides.Unfortunately,the capacity and cycling stability of these cobalt sulfides/phosphides is still not satisfied.How to design and develop cobalt sulfides/phosphides anodes with high capacity and cycling stability is still a big challenge.In this paper,we fabricated high performance Co₉S₈/N-doped carbon nanotubes（NCNTs）and Co P/NCNTs anodes for SIBs with high capacity and cycling stability by sulfiding and phosphiding Co-metal organic frameworks（ZIF-67）-derived Co₃O₄/NCNTs.In general,the reaserches in this paper will provide new strategy for the design and exploration of high performance SIBs electrodes.The major work is summarized as the followings:1. We fabricated the Co₉S₈/NCNTs composite material with Co₉S₈ nanoparticles homogenously embedded in hollow and multi-dimensional three-dimensional porous NCNTs by sulfiding Co₃O₄/NCNTs derived from Co-metal organic frameworks grown on copper nanowires tempalte via hydrothermal method using thiourea as sulfur source.The morphology and structure of Co₉S₈/NCNTs composite material are regulated by changing the annealing temperature in the hydrothermal reaction.The as-fabricated Co₉S₈/NCNTs composite material with annealing temperature of 600℃presented good sodium storage properties performance as anode materials for SIBs.At a current density of 0.1 A·g^-1,Co₉S₈/NCNTs composite anode can exhibit a capacity of470.6 m Ah·g^-1,and remain a capacity of 325.5 m Ah·g^-1 after 100 cycles,with a capacitance retention of 69.2%.Even at a high current density of 20 A·g^-1,the discharge capacity still reaches152.9 m Ah·g^-1,and exhibits a capacitance value of 108.9 m Ah·g^-1 after 10000 cycles.In the Co₉S₈/NCNTs composite anode,Co₉S₈ nanoparticles are uniformly embedded on the high nitrogen doped NCNTs with one-dimensional nanotubular and multi-stage pore structure,which is advantageous for improving the diffusion and permeation between the Co₉S₈ active material and the electrolyte,shorting the sodium ion transmission path and providing an effective channel for the diffusion,penetration and charge transfer of sodium ions.The high-conductive NCNTs with hollow one-dimensional structure provides a good charge transfer path for the sodium storage process of Co₉S₈ nanoparticles,and can inhibit the agglomeration and volume expansion of Co₉S₈nanoparticles during the sodiumation/desodiumization process.2. Co₃O₄/NCNTs derived from ZIF-67 grown on copper nanowires is prepared as precursor and then phosphated by sodium hypophosphite at 300℃under argon atmosphere to obtain Co P/NCNTs composite material.The Co P/NCNTs composite material is used as anode for SIBs and exhibits high cycle performance.Co P/NCNTs composite anode can deliver capacitance of 398.8 m Ah·g^-1 at a current density of 0.1 A·g^-1 after 100 cycles.At a high current density of 20 A·g^-1,the capacity of Co P/NCNTs composite anode can remain 211.4 m Ah·g^-1 after 7000 cycles.The one-dimensional hollow structure of the highly conductive NCNTs matrix can provide a highly conductive path for the electrochemical sodium ion insertion/extraction process of Co P nano particles,shorten the sodium ion transport channels and suppress the agglomeration and volume expansion and of Co P nanoparticles during the sodium storage.Therefore,Co P/NCNTs anode exhibits excellent electrochemical sodium storage performance.