Syntheses,modifications And Electrochemical Performances Of Several Vanadium-and Nickel-based Layered Cathode Materials

With the development electric vehicles,large-scale energy storage systems and new electrical equipment,the demand for electrochemical energy storage in human society is growing rapidly,and it also puts forward higher requirements for energy storage battery equipment,such as high energy density,rapid charge and discharge,high safety,low cost and so on.The development of new energy storage systems,such as sodium ion battery(SIBs)and aqueous zinc ion battery(ZIBs),is of great significance to meet these needs.The design and research of suitable electrode materials play an important role in the realization of high performance sodium ion batteries and zinc ion batteries.In order to solve the problems faced by the positive electrodes of these secondary batteries,we explore the layered materials by structural modulation,conductive carbon composite and nanoarchitecture.Furthermore,their electrochemical properties and energy storage mechanism are investigated in detail.The research results include the following parts:Firstly,with high capacity and voltage plateau,honeycomb-ordered layered Na₃Ni₂Sb O₆is a promising cathode material for sodium ion batteries but suffers from rapidly decreased capacity because of partially irreversible phase transition.Herein,Rb with a larger ionic radius was used to replace Na in honeycomb layered Na₃Ni₂Sb O₆ to modulate the interlayered structure for the first time.A series of Na₃-_xRb_xNi₂Sb O₆(x=0.00,0.03,0.15,0.30 and 0.35)samples were successfully prepared by high temperature solid phase sintering.The results of structural refinement showed that the interlayer distance and cell volume effectively enlarged with the changes of the crystal structure after the substitution of Rb for Na.The sample with x=0.03 exhibited the best electrochemical performance,with a discharge specific capacity of121.5 m Ah g^-1,improved rate performance and stable cycle performance.In situ XRD revealed a significant improvement in the partially irreversible transformation from the P'3 phase to the O'3 or O1 phase.Moreover,caused by Rb doping,the structural transition behavior of the material was changed.Noticeably,P3–O1 phase transition in Na_2.7Rb_0.3Ni₂Sb O₆ was more in favor of a solid solution process of Na⁺.Due to the differences of the crystal structure and structural transition behavior,the Rb incorporation is beneficial for enhancing the thermodynamic equilibrium voltage and Na⁺intercalation/deintercalation kinetics of the honeycomb layered materials.Secondly,the improved capability of sodium ion intercalation and storage are considered to be available from bilayerd vanadate preintercalated with metal ions and small molecules.We propose a facile and efficient hydrothermal approach to synthesize the composite of Na_xV₂O₅·n H₂O nanowires with ketjen black nanoparticles,and the electrochemical performance of Na_xV₂O₅·n H₂O/KB cathode for SIBs is investigated for the first time.The as-prepared bilayerd Na_xV₂O₅·n H₂O has a large interlayer distance of?10?,high crystallinity and uniform nanowire morphology.Besides,the in-situ composite of ketjen black nanoparticles during the process of material synthesis provides a conductive network and prevents the nanomaterials from serious aggregation in the electrode preparation process.Attributed to these advantages,Na_xV₂O₅·n H₂O/KB nanocomposite delivered a high capacity of 239 m Ah g^-1,an obvious voltage plateau at 3.5 V and an energy density of 597 Wh kg^-1 based on the weight of cathode material.Thirdly,in order to solve the problem of strong electrostatic interaction between Zn ions and host lattices,which makes the migration and storage of zinc ions more difficult,we introduce the preintercalation strategy in the second part to ZIBs.A novel nickel vanadium oxide hydrate(Ni_0.25V₂O₅·0.88H₂O,Ni VO)was designed and prepared.The advantage of bilayered vanadate hydrate is more obvious in aqueous zinc ion batteries due to electrostatic-shielding effect of structural water,diffusion channels provided by enlarged interlayer spacing for Zn ions and pillar effect of inherent nickel ions to stabilize the structure.Ni VO exhibits a reversible capacity of 418 m Ah g^-1 with 155 m Ah g^-1 retained at 20 A g^-1 and a high capacity of 293 m Ah g^-1 in long-term cycling at 10 A g^-1 with 77%retention after 10,000 cycles.In situ/ex situ analytical results uncovered that the sample underwent multi-step phase transition during the charge and discharge process,and the phase transition for the first cycle was irreversible.Furthermore,the M3 structure formed in the second cycle was reversible and improved the electrochemical performance.Combining the analyses of the elementary composition and chemical states,the Zn²⁺insertion/extraction mechanism of the Ni VO cathode for ZIBs can be deduced.Moreover,the morphological evolution and the corresponding formation mechanism during rapid ion insertion and extraction were studied to reveal the self-hierarchical process.Fourthly,based on the studies in the second and third parts,Na_xV₂O₅·n H₂O nanowires was in-situ grew on a carbon cloth collector by hydrothermal method and then carbon nanotubes composite with the nanowires was introduced to prepare CC-NVO/CNT integrated electrode.By unifying the preparation process of active materials and electrodes,the self-aggregation and pulverization of one-dimensional nanostructure in electrodes are effectively avoided.The microstructure characterization showed that NVO nanowires were in-situ grown on the carbon cloth,and nanowires with the length of tens of microns can be interwoven with carbon fibers to form the three-dimensional structural electrode.Besides,the carbon nanotubes distributed evenly between the nanowires constructed a conductive dense-network.This unique structure could provide more active sites,facilitate migration of Zn²⁺and electrons and better sustain the ceaseless strains caused by the structural transition during zinc ion insertion/extraction.Thus,the zinc storage performance of CC-NVO/CNT cathode is superior to that of NVO cathode prepared by traditional process.The electrochemical behavior analysis confirms that CC-NVO/CNT positive electrode has stronger redox activity and lower charge transfer impedance.