Study On The Synthesis Of Nickel Phosphide（Ni₂P） Host Material And Its Electrochemical Property In Lithium-sulfur Battery

Sulfur as lithium-sulfur batteries cathode active material shows its mass energy density of 1675 m Ah·g^-1.However,due to the weakness on conductivity and polysulfide shuttle effect,sulfur as cathode in lithium-sulfur batteries facing on its nature disadvantages on reaction depth,reaction kinetics and cycle life,which hinder its application.Polar/non-polar hybrid host materials contribute both high sulfur storage space with high reaction activities and electrostatic adsorption via the combination of transient metal component and carbon materials by formatting porous composite,which dramatically enhance the lifespan for the electrode materials by electrostatic adsorption.The complex multi-phase and multi-step electrochemical reaction for sulfur electrode leads the separation for polar adsorption site and electrochemical reaction active site which is cause by the weak conductive polar material design.The absorbed electrochemical active materials at adsorption site will move to the reaction active site.The additional diffusion step will influence the electrode reaction depth and activity and further hinder the electrode electrochemical properties.As polar compound and good electron conductor,Ni₂P has promising advantages on its application in lithium-sulfur battery as cathode host material.This thesis has focus on the material and structure design by Ni₂P as new advanced host material with high conductivity and catalytic property to boost the reaction capability,limit the polysulfides shuttle and improve the cycle life.For this thesis,a yolk-shell Ni₂P host material（Ni₂P-YS）is first prepared by solvothermal reaction and phosphorization reaction which has internal space for sulfur storage.Benefit by the internal hollow structure,the polysulfides has been trapped in the internal space.Polar Ni₂P as shell and core will contribute electrostatic adsorption which has clearly showed the limitation of polysulfide shuttle effect.By compare with the yolk-shell Ni O material with low conductivity,the Ni₂P host material with high conductivity unify the active adsorption site and electrochemical reaction site by its high conductivity capability.Thus the electrochemical discharge depth and rate performance all shows dramatically improvements.The host material shows initial discharge capacity of 1404.6 m Ah·g^-1 at 0.2C rate.The electrode shows both excellent energy density and cycling performance at 1C,2C and 3C.The initial capacities are1022,853 and 774 m Ah·g^-1 and the batteries cycles for 500 loops.Although vast hollow space shows excellent polysulfides storage capability and hinder the shuttle effects,however,the scarcity on internal electron conductivity frame dramatically limits its electrochemical reaction kinetics especially under high sulfur area loading.The high sulfur area load electrode for lithium sulfur battery requests the organic combination of porous and interconnectivity for host material.This paper has constructed a porous conductive frame structure from the Ni base metal-organic frameworks by following phosphorization reaction,which limit the polysulfide into the nano pore space.At the same time,the internal frame shows advantages on the discharge depth especially under high sulfur area load.The unified distribution of Ni element successfully turns to Ni₂P nanoparticles which offer addition reaction active area and polysulfides adsorption site.Cause by the strengthen internal electron pathway and porous structure,this type of electrode can touch better discharge ability and reaction depth at sulfur areal loading density of 1.3 mg·cm^-2.This electrode material shows initial discharge capacity of 915.7 m Ah·g^-1 at 1C rate.The 0.2C initial discharge is 918 m Ah·g^-1 which based on 4.6 mg·cm^-2 sulfur areal loading density.The electrode specific capacity still remails 684 m Ah·g^-1 after 300 stable cycles.Although the transfer for electron has been dramatically accelerated by high conductive polar material application.However,the interface understanding and connection between the polar and non-polar interface for lithium-sulfur battery property still not clear.The Ni₂P nanoparticles has been anchored on the thermal reduced oxide graphene which originated from the Ni precursor and graphene by one step hydrothermal and following phosphorization reaction.Different thermal treatment construct different interface condition for polar/non-polar composite materials.XPS spectrum confirm that the electrode host material has form a special interface connection between the Ni₂P nanoparticles and the graphene under high temperature treatment in PH₃ atmosphere.Benefit by this type of special connection,the electrocatalyst and electrochemical dynamic for the electrode has been dramatically enhanced.In＿situ EIS spectrum confirms that the charge transfer resistance for the polysulfides at different reaction steps has all been improved during the whole charge-discharge period.The soft-pack lithium-sulfur battery assembled by this electrode shows 24.3 m Ah initial discharge capacity under 0.15C.The soft-pack battery remail71.6%capacity after 150 cycles.The two-plateau capacity contribution rate is 1:2.59（theory value is 1:3）.