| Due to abundant reserves,easy exploitation and low cost of sodium resources,sodium-ion battery(SIB)has become one scale energy storage technology with great potential.However,the radius of sodium ion is relatively large,which results in the sluggish intercalation/deintercalation kinetics and large volume expansion of the electrodes,finally restricting electrochemical performance of SIBs.Therefore,it is essential for SIB to develop the electrode materials with stable crystal structures and sufficient channels for fast Na-ion diffusion.Among numerous Na-storege materials,vanadium-based phosphates are a class of important electrode materials owing to the variable valence states of vanadium ion and their small volume change during charging and discharging.Nevertheless,these materials always suffer from poor electronic conductivity,serious particle aggregation and unstable microstructure during cycling,which lead to low actual sodium-storage capacity,poor cycling performance and bad rate performance.In this thesis,four different vanadium-based phosphates are taken as the objects.To address the mentioned challenges,the strategies of rational structure design and micromorphology regulation have been adopteded to greatly improve their electrochemical performances.Shown as follows are the main findings:(1)Based on the easy formation of cross-linked structure through hydrogen bonds among chain polymers containing polysaccharide units,agarose is employed as carbon source and morphology-controlled template to prepare Na3V2(PO4)3/C as high-performance cathode for SIBs.The close combination of nanoscale active particles uniformly coated by thin carbon layer.Three-dimensional conductive framework not only improves the conductivity but also enhances combination of active material and cabon matrix,which greatly improves rate performance and long-term cycling stability of Na3V2(PO4)3/C.This electrode displays an initial capacity of 100 m Ah g-1 at 20 C with a retention of 78%after 7000 cycles.Meanwhile,the Na3V2(PO4)3/C||SC full cell is assembled with Na3V2(PO4)3/C as cathode and the carbon material(SC)derived from sulfur atom seletedly doped between(002)planes of carbon matrix as anode,and this full cell shows a reversible capacity of 110 m Ah g-1 and a working voltage of 1.7 V.The prepared Na3V2(PO4)3/C used as cathode material for SIBs has an important application prospect,and this simple preparation method is expected to realize large-scale production.(2)A molecular self-assembly route is proposed to prepare Na VPO4F/C as cathode for SIBs.After constructing a molecular force field,the F-ions in the precursor can be anchored by strong hydrogen bonds,and the formed sol-gel template can reduce the mobility of ions and affect the distribution of fluorine-hydrogen bonds to build a special arrangement of precursor particles.Under optimal sintering condition,the Na VPO4F/C owns an interconnected and porous three-dimensional coral-like structure composed by oriented packing of uniform nanoparticles,which endows Na VPO4F/C with excellent electrochemical performance.This electrode can stably cycle for 2500times at 5 C with an initial capacity of 100 m Ah g-1 and a low decay rate of 0.012%per cycle.Mranwhile,the assembled Na VPO4F/C||CNT@Ti O2full cell with Na VPO4F/C as cathode and tubular-network-structure CNT@Ti O2as anode exhibits an average operating voltage of 2.5 V and a capacity of 90 m Ah g-1.This novel structure design idea for electrode material can be also employed during the synthesis of other type fluorine contained electrode materials.(3)The Na-storage mechanism and the effect of micromorphological regulation on the Na-storage performance of NaV3(PO4)3 as an anode material for SIBs are systematically studied.A type of carbon coated NaV3(PO4)3 endowed with stable porous microstructure is prepared.This electrode can cycle for 4600 times at 750 m A g-1 with reversible capacity gradually increased from 128 m Ah g-1 to 173 m Ah g-1 and the discharge voltage gradually decreased from 1.18 V to 0.85 V.Mechanism studies show that 4e site is gradually activated,more Na ions are involved in reversible de-intercalation and gradual transformation from the crystal structure to an amorphous phase happens to NaV3(PO4)3 during the long-term cycle.In order to further study the influence of micromorphology on the activation process of 4e site,the agarose is used as template to design the structure of NaV3(PO4)3.The results showed that this activation process could be significantly accelerated by the smaller particle size and reasonable porous composite structure.After cycled for 1200 times at 750 m A g-1,its specific capacity increases from 158 m Ah g-1 to 182 m Ah g-1.The assembled Na3V2(PO4)3/C||NaV3(PO4)3/C full battery shows a capacity of 140 m A h g-1 and an average working voltage of 1.8 V,demonstrating the application potential of NaV3(PO4)3 as an anode material for SIBs.(4)In order to further improve the sodium storage capacity of phosphate anode for SIBs,the VPO4/C/CNT endowed with nanoscale particles and stable microstructure is obtained after its microstructure is controlled by cetyltrimethyl ammonium bromide and carbon nanotubes.It is found that a fast increasing process of reversible capacity in its infancy during long cycles which may be caused by the activation of sodium storage sites in the crystal structure.The maximum value can be up to 365 m Ah g-1 which corresponds to about two Na+ions which are involved in the reversible Na-storage process.Referring to the 4e-site activation mechanism inside NaV3(PO4)3 crystal,there is still a lot of room for further improving Na-storage capacity of VPO4.When cycled at 2000 m A g-1,the released capacity is still 231 m Ah g-1.The results demonstrate that the pseudocapacitance effect of this material contribute to a high proportion of its sodium-storage capacity,demonstrating the value of further research and the potential of application. |
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