Structural Regulation of Nickel Sulfide/heteroatom Doped Carbon for Electrochemical Sodium Storage

Author:Li Jia Bao

Supervisor:pan li kun


Degree Year:2019





Due to their high energy density and long life span,lithium-ion batteries(LIBs)play an important role in efficient energy storage and conversion.However,the limited resource and high cost restrict their further development.Considering the natural abundance,low cost and wide distribution in the earth crust of sodium resource as well as the similar physical and chemical properties between lithium and sodium,sodium-ion batteries(SIBs)are currently considered as the most promising alternative to LIBs and have attracted extensive attention.At present,exploring novel and high-performance electrode materials for SIBs is highly desirable since the performance of SIBs is basically determined by the electrode materials employed.Currently,interest in nickel sulfides for efficient energy storage and conversion has grown due to their rich types,natural abundance and high theoretical specific capacities.Unfortunately,nickel sulfides based electrodes often suffer from deteriorated cycling performance,resulting from their intrinsically poor electrical and ionic conductivity,aggregation of active materials during preparation and large volume change upon conversion reactions.In order to solve these issues mentioned above,Ni3S2 with stable crystal structure and NiS2 with high theoretical capacity were selected as research targets to optimize their electrochemical performances.Improved sodium storage performances can be received through reducing the particle size and combining with heteroatom doped carbon,where shortened Na+diffusion length and increased electrical conductivity can be obtained.Besides,the relationship between micro-structure and electrochemical performance was discussed,and the sodium storage mechanisms of the obtained materials were also analysed with the help of various characterizations,electrochemical tests and ex-situ technologies.The major contents of this thesis are summarized as following:1.Ni3S2 based electrodes often suffer from degraded cycling performance and low specific capacity when employed as anode materials for SIBs,which mainly results from their poor ionic and electrical conductivity.In this context,Ni3S2 particles decorated on nitrogen doped reduced graphene oxide(Ni3S2/NrGO)composites were synthesized through a two-step method,including freeze-drying and in-situ sulfidation.The characterization results show that the in-situ generated Ni3S2 particles with size of500-600 nm were dispersed uniformly on the NrGO.The introduction of NrGO can enhance the electrical conductivity of the Ni3S2/NrGO hybrid,and simultaneously mitigate the structure change of Ni3S2 upon cycling.Besides,the combination of Ni3S2and NrGO can alleviate the aggregation of both Ni3S2 and NrGO.After optimizing the content of NrGO in the resultant samples,the Ni3S2/NrGO displays improved sodium storage performance with a large reversible capacity(290.6 mAh g-1 after 100 cycles at a current density of 0.1 A g-1)and a high initial Coulombic efficiency(76.83%).However,the rate performance and cycling of Ni3S2/NrGO at the higher current density(0.5 A g-1)are not satisfied,and further structural regulation is necessary.2.Although the electrochemical performance of Ni3S2 has been improved after introduction of NrGO,the sodium storage performances are still not satisfied,especially the rate performance and cycling performance at higher current densities.Reducing the particle size of Ni3S2,fabricating heteroatom doped carbon wrapped Ni3S2 and three-dimensional conductive networks should be effective approaches to further improve its electrochemical performance.In this context,Ni3S2 particles wrapped by nitrogen doped carbon(Ni3S2@NC)were prepared though a freeze-drying and in-situ sulfidation method,using NaCl and citric acid as template and carbon source,respectively.The results demonstrate that appropriate addition of citric acid is critical for the dispersion of Ni3S2 particles and formation of three-dimensional conductive networks.Additionally,the Ni3S2 particles with size ranging from 100 to 300 nm were wrapped by NC tightly in the Ni3S2@NC hybrid.Improved sodium storage performance of Ni3S2@NC hybrid is obtained after optimizing the weight ratio of Ni3S2and NC,and a high reversible capacity of 402.8 mAh g-1 can be obtained at 0.1 A g-1after 100 cycles with a high initial Coulombic efficiency of 75.33%.Further increasing the current density to 0.5 A g-1,a reversible capacity of 245.4 mAh g-1 can still be achieved after 500 cycles.3.In order to simplify the preparation of Ni3S2/heteroatom doped carbon and further improve its sodium storage performance,a novel metal-chelate was synthesized by using nickel acetate tetrahydrate and dithiooxamide as starting materials in this section.After a simple pyrolysis process,nitrogen and sulfur codoped carbon confined Ni3S2(Ni3S2@NSC)were obtained,and nanoscaled Ni3S2 particles can be observed from the TEM images of Ni3S2@NSC.Besides,the NSC can not only enhance the electrical conductivity of the hybrid but also maintain its structure integrity.Moreover,the high sodium storage activity of NSC helps to increase the specific capacity of Ni3S2@NSC hybrid.Through optimizing the ratio of Ni2+and dithiooxamide in the precursor,the resultant Ni3S2@NSC exhibits excellent sodium storage performance with high reversible capacities of 458.1 mAh g-1 at 0.1 A g-1 after 100 cycles and 392.6mAh g-1 at 0.5 A g-1 after 300 cycles,respectively.Moreover,the charge diffusion kinetics and sodium storage mechanism of Ni3S2@NSC were also discussed via pseudocapacitive analysis and ex-situ XRD measurements,respectively.4.Through optimizing the preparation approach and micro-structure as well as hybridizing with heteroatom doped carbonaceous agent,stable and highly-efficient sodium storage performances of Ni3S2 based electrodes have been obtained.In this section,we developed a metal-organic framework(MOF)strategy to synthesize pomegranate-like clusters with small NiS2 nanoparticles embedded in nitrogen doped porous graphitic carbon networks(NiS2@NC)via successive carbonization and sulfidation.Benefitting from the unique structure features inherited from the MOF precursors and the in-situ fabrication approach,the NiS2 nanoparticles tightly embedded in the N-doped carbon networks can avoid the direct contact between active material and electrolyte,which reduces the irreversible reactions and improves the sodium storage activity.The electrochemical tests demonstrate that a high reversible capacity of 505.7 mAh g-11 can be received at 0.1 A g-1 after 100 cycles with a high initial Coulombic efficiency of 82.5%.Furthermore,a reversible capacity of 285.3 mAh g-1can be maintained at 0.5 A g-1 after 500 cycles.