Preparation And Electrochemical Properties Of Transition Metal Compounds

The rapid development of the electric device industry has prompted researchers to develop a new generation of high-energy density electrochemical energy storage devices to meet the needs of fast charge and slow discharge,and long cycle life.Lithium-sulfur batteries?LSBs?have attracted extensive attention from researchers due to its theoretical energy density(2600 Wh kg^-1)and specific capacity(1675 mAh g^-1),which are much higher than that of commercial lithium-ion batteries?LIBs?.However,the practical application of LSBs still faces great challenges:?1?the insulation of sulfur and Li₂S(5*10^-30 S cm^-1);?2?the severe volume expansion of sulfur?about 80%?during charging and discharging;?3?the shuttle effect of dissolved polysulfide,which lead to continuous loss of active materials.To solve the above problems,different sulfur carriers were prepared by spray drying and hydrothermal reaction,and their electrochemical properties were also studied.The main research contents are as follows:?1?The N-doped mesoporous TiO₂/C?MTC?composite microspheres were produced by spray-drying method.The N-doped amorphous C around TiO₂ could provide a conductive matrix,and can buffer the volume change during the charge and discharge progress.When evaluated as electrodes for LIBs and LSBs,the MTC microsphere exhibits excellent capacity retention and rate capability.As anode for LIBs,after 200 cycles,the reversible capacity can achieved 230 mAh g^-1 at current density of 1 C(100 mA g^-1).As a sulfur carrier material,MTC can promote the electrostatic adsorption of S-Ti-O and inhibit the shuttle effect of polysulfide,thus improving the utilization rate of active materials.A specific capacity can retain about 1317.7 mAh g^-1 at 1 C and the capacity retention is 73.8%after 500 cycles.?2?The precursor microspheres were prepared by the spray drying method with PS as template.When the volume ratio of PS and TiO₂ was 3:1,the honeycomb-like TiO₂microspheres were obtained.And the TiN hosts were obtained by further nitriding.Finally,the electrochemical properties of TiN/S as cathode in LSBs were tested.The honeycomb-like architecture can shorten the diffusion path of lithium ions,and buffer the volume change of sulfur,which can maintain the stability of material structure.In addition,TiN can not only improve the conductivity of electrode,but also act as adsorbent to inhibit the shuttle of polysulfide.As a result,the TiN/S electrode with honeycomb-like structure exhibit an excellent cycling performance with a stable capacity of 650.4 mAh g^-1 after 500 cycles at 100 mA g^-1.?3?A freestanding tungsten nitride nanorod/carbon cloth?WN/CC?interlayer was prepared by hydrothermal growth to function as both current collector and physicochemical barrier to soluble lithium polysulfides?Li₂S_x?.Thanks to the combination of physical barrier?carbon cloth?and chemical barrier?WN?,the initial discharge capacity of the cells with WN/CC interlayer were significantly improved to1337 mAh g^-1,and the reversible discharge capacity remained above 814.2 mAh g^-1 after500 cycles at 100 mA g^-1.The enhanced performance is attributed to effective recycling of active materials by the WN nanorods entrapping polysulfides along with electrically conducting CC.The density functional theory?DFT?calculations prove important roles of the WN?200?surfaces in entrapping polysulfides through their adsorption energies?3.21-4.67 eV?with Li₂S_x and S₈.