Synthesis And Lithium Storage Performance Of Ni,Co-Based Bimetal Nitrides

The demand of high-effieciency energy storage devices in small electronic devices,electric cars,hybrid electric cars and large-scale smart grids has steadily increased in recent years and lithium-ion batteries(LIBs)are currently the most commercially used device to storage energy in various occasions.As an important part of LIBs,anode material plays a significant role in determining the properties of LIBs.Hence,it is vital to probe suitable anode materials for the development of high-performance LIBs.In various type of anode materials,transition metal nitrides(TMNs)have been considered as promising anode materials for LIBs due to their much higher theoretical specific capacity and electronic conductivity,relatively lower conversion reaction potential.However,there still exist some shortcomings of these TMNs anode materials:On one hand,most TMNs will experience much large volume variation during the electrochemical reaction process,resulting in shortened cycle life,which is bad for its practical application.On the other hand,the electrochemical reaction mechanism of these TMNs during the cycling process,especially the bimetal nitride materials,is still unclear at present,which is not conducive to the further modification of these materials.In view of these problems,this thesis tries to improve the performance of bimetal nitride materials through the optimization of the material synthesis and preparation processes,modification of its morphology and structure,and then several bimetal nitride anode materials with favorable electrochemical properties were obtained.Moreover,the electrochemical reaction mechanism of these bimetal nitride materials was comprehensively studied by a series of optics and electrochemistry characterization methods.This thesis is specified in the following aspects:(1)Thin carbon layer coated NiCo2N film with three-dimensional porous structure was deposited on nickel-foam substrate through electrostatic spray deposition(ESD)technique followed by a post-annealing process under ammonia atmosphere.The threedimensional porous structure is of benefit to the diffusion of Li+ions in the material,accelerating the electron transfer,as well as increasing the number of active sites for lithiation/delithiation reaction.Moreover,the coated carbon layer can confine the overlarge volume expansion of the active material in the time of the repeated cycle process.Based on these qualifies,the NiCo2N film electrode shows excellent electrochemical performance with high-capacity value of 1574.6 mAh g-1 on 400th cycle at 1 A g-1,and the specific capacity can maintain a high value of 955.4 mAh g-1 at high current density of 10 A g-1.In addition,the reaction mechanism of the NiCo2N material was deduced to be conversion-type through the ex-situ XRD and TEM characterization methodes.(2)NiCo2N powder material with hollow nanosphere shell structure was synthesized via a simple solvothermal route and the nanosphere was constructed with interconnected nanosheets.When it was assembled into LIBs for testing,the crosslinked nanosheets can provide large number of active sites for electrochemical reactions,endows a large contact area between the active materials and electrolytes.In addition,the hollow structure can provide a smoother path for the diffusion of Li+ions and electrons inside the electrode material,effectively alleviate the volume expansion of materials during the cycling process.Thanks to these features,the NiCo2N electrode shows excellent electrochemical performance with high specific capacity of 1244.5 mAh g-1 at 1 A g-1 after 400 cycles.Similar to the film material,the NiCo2N powder material also has the conversion-type reaction mechanism.(3)Amorphous layer wrapped Ni0.2Mo0.8N/Ni3N heterostructure powder material with tremella-like morphology was fabricated through one-pot hydrothermal process.The research results shown that the Ni3N particles evenly distributed in the materials and worked as pillars to support the Ni0.2Mo0.8N active material.It could also enhance the electronic conductivity of heterostructure material.The wrapped amorphous layer was demonstrated to be Ni0.2Mo0.8N phase with massive defects and vacancies,which can provide more Li+ion storage sites and speed up the transfer of Li+ions and electrons.It can also effectively alleviate the volumic change and structural stress generated in the cycles.Profiting from these advantages,the Ni0.2Mo0.8N/Ni3N heterostructure anode displays an excellent cyclic performance and rate capability(695.5 mAh g-1 at 2 A g-1 after 600 cycles,595.3 mAh g-1 at 5 A g-1).Different to general nitride material,the Ni0.2Mo0.8N/Ni3N heterostructure exhibits a unique intercalation-type reaction mechanism from various in-/ex-situ characterizations.(4)Mo2N/CoN heterostructure powder material constructed with massive hollow nanotubes was prepared by one-pot hydrothermal method.It is found that the synthesized material has hybrid mechanism with conversion-and intercalation-type reactions when used as anode material for LIBs.It has high specific capacity similar to the conversion-type material,and high stability similar to the intercalation-type anode.In addition,the special hollow nanotubes structure of the active material greatly promotes the ion transport during the battery operation,and is also more conducive to the release of the strain,which caused by the material volume change,further stabilizing the material structure.Under the dual influence of intrinsic structural characteristics and unique morphology design,the material posess excellent performance:Prominent cycle stability with high remained reversible capacity of 1091.8 mAh g-1 at 2 A g-1 after 600 cycles,Meanwhile,outstanding rate property also appears with a high-capacity value of 597.5 mAh g-1 at 10 A g-1.