Preparation And Performance Of NASICON-type NaTi₂?PO₄?₃ And Na₃V₂?PO₄?₃ As Electrode Materials For Sodium-ion Batteries

Sodium ion batteries(SIBs)have attracted increasing attention of researchers due to its advantages of abundant resources,low price,wide distribution and green environmental protection.However,sodium ion has a larger radius and molar mass than lithium ion and the performances of SIBs are still not comparable to those of LIBs.In order to promote the application of SIBs in real life,it is necessary to develop a kind of suitable electrode materials for rapid Na+ insertion/extraction.In many electrode materials,polyanionic compounds with sodium superionic-conductors(NASICON)structure have great potential in the application of SIBs.However,this kind of materials have poor electronic conductivity,which seriously affects the electrochemical properties under high current density.In this paper,taking two NASICON-type materials of NaTi2(PO4)3(NTP)and Na3V2(PO4)3(NVP)as the objects,we expect to improve their sodium storage properties by microstructure engineering,combination with highly conductive carbon and fabrication of self-supporting composite.The main results are as follows:(1)A free-standing 3D nanocomposite constructed by reduced graphene oxide(rGO),thermally-treated protein(TP)and mesoporous NaTi2(PO4)3 nanocrystals(denoted as MNTP-TP@rGO)is reported.The fabrication includes an electrostatic self-assembly,freeze-drying,mechanical pressing and thermal treatment.In the MNTP-TP@rGO nanocomposite,3D interconnected carbon network of rGO and TP acts as both a support for the anchored well-distributed MNTP nanocrystals and a current collector.When the free-standing MNTP-TP@rGO nanocomposite is used directly as anode in coin-type half-cell,it delivers a high-rate capacity(52.8 mAhg"1 at 50C)and robust cycling stability with the capacity retention of 80%after 1000 cycles at 5C.Furthermore,a full Na-ion battery is constructed using Na3V2(PO4)3/C(NVP/C)as a cathode and free-standing MNTP-TP@rGO as an anode and it exhibits a high specific capacity(58 mAhg-1 at 1C)and outstanding cycling stability(98%capacity retention over 100 cycles at 1C).Our results suggest great potential of the MNTP-TP@rGO nanocomposite in high-performance SIBs.(2)A self-supporting composite composed of CNTs modified Na3V2(PO4)3 and CNTs was successfully prepared by electrostatic self-assembly,vacuum filtration and thermal treatment.In this composite,3D interconnected carbon network of CNTs greatly improves the conductivity of the self-supporting composite.When it is used directly as cathode in coin-type half-cell,the self-supporting composite delivers a discharge capacity of 110 mAhg-1 at 1C,and reversible capacity of 79 mAhg-1 at high rate of 50C.As anode in coin-type half-cell,the self-supporting composite has a discharge specific capacity of 76 mAhg-1at 1C,and robust cycling stability with the capacity retention of 93%after 100 cycles.In addition,a symmetric full cell is fabricated using the self-supporting composite as both cathode and anode.The full cell delivers good electrochemical performance with a discharge capacity of 55 mAhg-1 at 0.5C and 76%of capacity retention over 100 cycles at 0.9C.