Preparation Of Transition Metal Nitride And Research On Their Energy Storage Performances In Secondary Batteries

Due to a series of advantages,including high electronic conductivity,high catalytic activity and high chemical stability,transition metal nitrides received tremendous attention in the research on electrochemical energy storage devices in recent years.In lithium-sulfur batteries,as sulfur hosts at cathode,transition metal nitrides have excellent electronic conductivity which could provide high-speed electron transfer for sulfur.Besides,their strong chemical bonds can effectively trap the dissolved lithium polysulfides,suppressing the shuttle effect,and thus improve the specific capacity and cycling stability of lithium-sulfur batteries.In sodium-ion batteries,as anode materials,transition metal nitrides have low cost that can well match the cost advantage of sodium ion batteries.Meanwhile,the conversion-type reaction mechanisms and the porous nanostructures of them can provide high battery capacity.In this dissertation,we controllably synthesized vanadium nitride?VN?and titanium nitride?Ti N?and optimized the synthesize condition.We took systematical physical/chemical characterizations on these materials,and meanwhile we conducted careful researches on their electrochemical performances in lithium-sulfur batteries and sodium-ion batteries.The details are listed as follows:?1?Conductive hierarchically porous VN was prepared by a hydrothermal-nitridation method,and was used as non-carbon sulfur host to build high tap-density,high sulfur-loading and high energy-density cathode for lithium sulfur batteries.The strong polar bonds of VN can provide strong chemical anchoring effect on lithium polysulfide and thus effectively suppress their excessive dissolution.The excellent electrical conductivity and catalytic activity of VN can promote the conversion between lithium sulfide and soluble lithium polysulfides,and hence greatly improve the electrochemical performances of lithium-sulfur batteries.The hierarchically porous nanostructure of VN can provide more empty space to storage sulfur,enabling both high sulfur loading and high sulfur utilization.As a result,S@VN could retrieve a specific discharge capacity of 790 m Ah g^-1 after 200 cycles at a current density of 1 C and the sulfur loading of 1.0 mg cm^-2.Even at high sulfur loading(12.6 mg cm^-2),the S@VN cathode can still provide a discharge specific capacity of 518.8 m Ah g^-1 after 100cycles at 0.1 C.?2?Porous Ti N was prepared via a simple,efficient and scalable synthesis method,and was used as a sulfur host material to load sulfur for lithium-sulfur batteries.By adjusting and optimizing the nitridizing temperature and particle size,it is concluded that S@Ti N could retrieve the best electrochemical performances with 60 nm particle-size precursors and 1000?nitridizing condition.The continuous high-conductivity framework of porous Ti N can promote the transfer of electrons,while its strong chemical bond can strongly limit the shuttle effect of lithium polysulfide from chemical aspects.Consequently,with a sulfur loading of 0.5 mg cm^-2,the S@Ti N cathode achieved a reversible initial capacity of 1198.4 m Ah g^-1 at a current density of 0.5 C and the capacity can still maintain 784.5 m Ah g^-1 after 80 cycles.Even at a high sulfur loading(3-4 mg cm^-2),the battery still exhibits decent capacity and stable cycle stability,which could deliver a specific capacity of 423.4m Ah g^-1 after 150 cycles at a current density of 0.5 C.?3?Porous VN with different morphologies was synthesized through a simple and scalable hydrothermal synthesis-nitridation strategy,and was then used as the anode material for sodium ion batteries.The continuous nanostructure of VN nanosheets can provide a better path for electron transport,while its porous structure can shorten the diffusion path of sodium ions and provide more active sites for rapid insertion/extraction of sodium ions.Because the multi-layer porous structure has a large internal space,the volume change of VN during charging and discharging can be effectively buffered,and thus the VN nanosheets could deliver much batter sodium storage performances than VN nanowires.The electrochemical tests proved that the multi-layer porous VN as the anode material has a large proportion of capacitive behavior,which well explained the good rate capability of VN.Besides,the multilayer porous VN nanosheet electrode showed good cycle stability.It delivered a discharge capacity of 111.9 m Ah g^-1at 1.0 A g^-1,and it showed a very high capacity retention after2300 cycles and high coulomb efficiency?close to 100%?.