Design And Synthesis Of Nanocomposite Electrode Materials And Their Application In Advanced Secondary Batteries

Aiming at the demand for high energy density batteries and inexpensive large scale electrochemical energy storage systems in the future commercial battery market,we hope to develop some new battery systems and electrode materials with much more better performance.After investigating the development history and research status of commercial secondary batteries,we discovered that the development of negative electrode materials with better electrochemical performance is very important in order to build better lithium ion batteries or sodium-ion batteries.For large-scale energy storage devices,aqueous rechargeable zinc ion batteries are one of the most promising energy storage systems.However,the development of cathode materials for aqueous rechargeable zinc ion batteries is currently in its infancy.Due to its high theoretical capacity and various morphologies,transition metal sulfides are considered as a very promising cathode material for lithium ion batteries and sodium ion batteries.However,their low electronic conductivity,large volume expansion and high dissolubility limit their practical applications.To address these issues,nanocrystallization and compositing are two very effective ways to improve their electrochemical performance.Nanocrystallization and compositing can not only increase the charge transfer rate in the material,but also can effectively inhibit the volume expansion and dissolution of the active materials and improve the electrochemical performance of the material.We have found that the hybrid chemical bonds formed in the processes of nanocrystallization and compositing also play a crucial role in improving the electrochemical performance.In order to solve the problem of poor electrochemical activity of zinc ions and lack of available cathode materials for aqueous zinc ion batteries,we also applied the idea of nanocrystallization and compositing to the design and synthesis of aqueous zinc ion batteriy cathode materials.Based on the above,we successfully designed and synthesized three lithium/sodium ion battery anode materials and two aqueous zinc ion battery cathode materials.The obtained main research results were as follows:(1)A graphene@Mo S2@Ti O2 hybrid material was successfully prepared by a multi-step solution chemistry method.Few-layered Mo S2 nanosheets were impregnated into the nanovoids of mesoporous Ti O2 microspheres and the composite was further encapsulated by a graphene layer.When used as a negative electrode material for lithium ion batteries,the nanovoids of Ti O2 reduced aggregation of Mo S2 and suppressed the large volume change of the active material.Moreover,the dissolution and shuttle of polysulfides were effectively suppressed by the hybrid bonding between Mo S2 and Ti O2.The nano sized Mo S2 and Ti O2 particles encapsulated by a high electronic conductive graphene layer improved the charge transfer reaction of the electrode.Due to these merits,the graphene@Mo S2@Ti O2 showed a large discharge capacity of 980 m Ah g-1 at the 0.1 A g-1 current density with capacity retention of 89 % after 200 cycles.Moreover,the material delivered 602 m Ah g-1 at the 2 A g-1 current density,much larger than the 91 m Ah g-1 for the pristine Mo S2.This demonstrated that the hybrid graphene@Mo S2@Ti O2 microspheres have great potential as a high-performance negative electrode material for lithium ion batteries.(2)An integrated WS2@CMK-3 nano composite is prepared using a one-step hydrothermal method and then used as the anode material for lithium ion and sodium ion batteries.Ultrathin WS2 nanosheets are successfully embedded into the mesoporous conductive CMK-3 framework and the thickness is reduced from ~20 nm to ~5 nm.Owing to the few-layered nano structure of WS2,as well as the high electronic conductivity and the volume confinement effect of CMK-3,the material shows larger discharge capacity,better rate capability and improved cycle stability than the pristine WS2.When tested in lithium ion batteries,the material delivers a reversible capacity of 720 m Ah g-1 after 100 cycles at the current density of 100 m A g-1.A large discharge capacity of 307 m Ah g-1 is obtained at the 2 A g-1 current density.When used in sodium ion batteries,the material exhibits a capacity of 333 m Ah g-1 at 100 m A g-1 without capacity fading after 70 cycles.A discharge capacity of 230 m Ah g-1 is obtained at 2 A g-1.We have found that,the electrochemical performance of one electrode material in sodium-ion batteries is much lower than that in lithium-ion batteries,which is largely due to the poor kinetic properties of sodium ions.(3)Size and conductivity of the electrode materials play a significant role in improving the kinetics of sodium ion batteries.Various characterizations demonstrate that size-controllable VS4 nanoparticles are successfully anchored on graphene sheets(GS)surfaces by a simple cationic surfactant-assisted hydrothermal method.When used as an electrode material for sodium ion batteries,these VS4/GS nanocomposites show large specific capacity(349.1 m Ah g-1 after 100 cycles),excellent long-term stability(84 % capacity retention after 1200 cycles),and high rate capability(188.1 m Ah g-1 at 4000 m A g-1).This remarkable electrochemical performance is attributed to synergistic interactions between nanosized VS4 particles and a highly conductive graphene network that provided short diffusion pathways for Na+ ions and large contact areas between the electrolyte and electrode,resulting in much improved electrochemical kinetic properties.(4)Here we developed a composite material comprised of H2V3O8 nanowires(NWs)wrapped by graphene sheets using a simple hydrothermal method and used it as the cathode material for aqueous rechargeable zinc ion batteries.Owing to the synergistic merits of desirable structural features of H2V3O8 NWs and high conductivity of graphene network,the H2V3O8 NW/graphene composite exhibited superior zinc ions storage performances including high capacity of 394 m Ah g-1 at 1/3C(1C=300 m A g-1),high rate capability of 270 m Ah g-1 at 20 C and excellent cycling stability of up to 2000 cycles with a capacity retention of 87 %.The battery offered a high energy density of 168 Wh kg-1 at 1/3C and a high power density of 2215 W kg-1 at 20 C with a energy density of 89 Wh kg-1.Systematic structural and elemental characterization using ex situ XRD,Raman,XPS and STEM confirmed the reversible Zn2+ and water co-intercalation electrochemical reaction mechanism.(5)In order to obtain a promising aqueous zinc ion battery cathode material,a fluorine-doped carbon nanosheet was prepared by a hydrothermal method using fluorinated graphite as a precursor.Various charactorizations showed that most of the fluorine atoms in the fluorinated graphite were removed and the original bulk fluorinated graphite was exfoliated into ultra-thin fluorine-doped carbon nanosheets.The fluorine atoms in the fluorine-doped carbon nanosheets can rapidly and reversibly adsorbed and desorbed Zn2+ ions through C-F-Zn bonds on the carbon nanosheet surface.In thses prepared samples,the CFx-48 h sample after a 48-hour hydrothermal reaction with proper thickness and appropriate amount of fluorine atoms exhibited the best electrochemical performance.The reversible specific capacity can reach ~80 m Ah g-1 at a current density of 0.1 A g-1.At a high current density of 4 A g-1,the specific discharge capacity was 56.7 m Ah g-1.After 14,000 cycles,its capacity retention rate was 82.3 %.This aqueous zinc ion battery with high rate performance and long life-span is a very promising large-scale energy storage device.After completing the above researches,we have designed and developed five new nanocomposite synthesis methods based on the requirements of different battery systems;summarized the influences of particle size,composite methods,chemical bonds and heteroatoms on the physical and chemical properties of the electrode materials;explored the influence of nanocrystallization and compositing on the electrochemical properties of the electrode materials;analyzed and studied the reasons for the improvement of electrochemical performance at the molecular and atomic scales.This doctoral thesis can play a guiding role in the design,synthesis and analysis of electrode materials in various energy storage systems.