Sodium Storage Performance Of Phosphate Framework Materials For Na-Ion Batteries

Sodium ion batteries (SIBs) have attracted increasing research attention for energy storage systems due to its wide availability of sodium resources and low cost of sodium. However, because of larger ionic radius of sodium and sluggishness of electrochemical kinetics, it becomes a huge challenge to develop suitable electrodes for high-performance SIBs. Though various materials have been investigated for electrode material of SIBs, they are still hard to meet the requirements for large scale electrical energy storage systems. It becomes the key point to develop promising electrode materials to construct high-performance SIBs. Due to the very stable frameworks, and potential high cycling stability and safety, the phosphates became potential materials for high-performance sodium ion batteries. This thesis was aimed at exploring high performance electrodes based on phosphates fiameworks with appropriate microstructure and high conducting network and investigating their sodium storage mechanism. The main results and conclusions are as follows:(1) Fe-based phosphates:Olivine NaFePO4 was successfully prepared by a facile aqueous electrochemical displacement method from LiFePO4 precursor, and the NaFePO4 electrode exhibits excellent electrochemical performance (high discharge capacity of 111 mAh g-1, excellent cycling stability with 90% capacity retention over 340 cycles at 0.1 C, high rate capacity of 46 mAh g-1 at 2 C). Through XRD, CV and TEM measurements, the Na2/3FePO4 intermediate is detected during the Na+ intercalation process, and the electrochemical mechanism is also investigated. As the reversible capacity of NaFePO4 is a little low and huge valumetric mismatch is emerged during charge/discharge process, we synthesized mesoporous amorphous FePO4 nanospheres successfully through a simple chemically induced precipitation method. The amorphous nature of FePO4 can avoid the lattice stress and provide continuous Na ion diffusion pathways and more insertion sites, so as to greatly improve the reversible capacity and structural stability. The mesoporous amorphous FePO4/C electrode exhibits a high initial discharge capacity of 151 mAh g-1 and very stable cyclability (94% capacity retention ratio over 160 cycles). This structural design also provides a novel idea for tailoring the structure of materials to obtain high capacity and good stability.(2) Na3V2(PO4)3 electrodes:Hierarchical graphene decorated Na3V2(PO4)3 micro sphere is synthesized through a facial spray-drying method. Structural and morphological characterizations reveal that Na3V2(PO4)3 particles are homogeneously encapsulated in a 3D conductive graphene network. The optimal architecture ensures the effective diffusion of electrons and sodium ions, thus to much benefitting electrochemical utilization and rate capability of the Na3V2(PO4)3 electrode (high discharge capacity of 115 mAh g-1, excellent cycling stability with 81% capacity retention over 3000 cycles at 5 C, high rate capacity of 44 mAh g-1 at 50 C). To improve the rate capability of the Na3V2(PO4)3 electrode, Na3V2(PO4)3 is synthesized by a simple pre-reduction and subsequent calcination process, and then introduced into a CVD furnace to establish in situ a hierarchically carbon coated Na3V2(PO4)3, in which graphene-like coating carbon layers are interconnected via graphitic carbon nanofibers. The as prepared Na3V2(PO4)3/C electrode exhibits a high reversible capacity (115 mAh g-1), ultra-long cycling stability (54% of capacity retention over 20000 cycles at 30 C) and superior high-rate capability (38 mAh g"1 at 500 C).(3) NaTi2(PO4)3:the spray-drying method for constructing hierarchical graphene decorated NASICON-type material is also applied to synthesize graphene decorated NASICON-type NaTi2(PO4)3 anode. The NaTi2(PO4)3/graphene material demonstrates a high initial coulombic efficiency (98%), high initial charge capacity (130 mAh g-1 at 0.1 C rate), superior high rate capability (38 mAh g-1 at 200 C) and excellent cycling stability (77% capacity retention over 1000 cycles at 20 C). In addition, a NaTi2(PO4)3//Na3V2(PO4)3 full cell also shows high rate performance (88 mAh g-1 at 50 C) and long-term cycling life (80% capacity retention ration over 1000 cycles at 10 C). The all NASICON-type full sodium ion battery with superior electrochemical performance can give a reference for practical sodium ion batteries.(4) NaVOPO4:all the above phosphates have 3D tunnel structures, in which Na ions are zigzaged to transport through the lattice frameworks, leading to a slow kinetics. We report a new layered NaVOPO4 and its electrochemical behavior as a cathode host for SIBs the first time, due to the high sodium ion diffusion rate in 2D nanosheets and inductive-effect of strong P-O covalent bonds, the NaVOP04 electrode can deliver a high voltage of-3.5 V (vs. Na/Na+), high discharge capacity of 144 mAh g-1 at 0.05 C, long term cycle life with 67% capacity retention ratio over 1000 cycles at 0.5 C as well as strong rate capability of 58 mAh g-1 at 0.5 C. We believe that the novel structure obtained in this thesis may offer a new avenue to develop advanced cathode materials for sodium ion batteries.