The Research Based On NASICON Anode Materials In Aqueous Rechargeable Batteries

Sodium superionic conductor?NASICON?material has its many advantages like open and stable 3D framework structure,facile sodium ion conductivity.So these materials are favored by researchers in the application of solid electrolytes and the selection of sodium ion electrode materials.However,this type of material has low electronic conductivity,which makes its electrochemical performance poor.The coating methods to increase their conductivity are generally used.NaTi₂?PO₄?₃,a promising anode electrode materials candidate for aqueous rechargeable sodium ion battery,belongs to the NASICON structure.Based on NASICON anode NaTi2?PO4?3 in aqueous rechargeable batteries,it is the possibility of its practical application to find a cathode material in aqueous rechargeable batteries.Due to the low cost?the stabe structure and abundance of raw materials,sodium manganate has become the most attractive cathode material in aqueous rechargeable batteries.This paper focuses on the positive electrode material Na_0.7MnO_2.05and the negative electrode material NaTi₂?PO₄?₃ and the modified material NaTi₂?PO₄?₃@C/Ag.The characteristics of the electrode materials are explored through a series of material characterization,electrochemical performance tests and a series of kinetic calculations.The specific research contents are as follows:Firstly,NaTi₂?PO₄?₃ was synthesized via a facile sol-gel method.Then,polyethylene glycol and silver nitrate used as carbon source,silver source were toally mixed with NaTi2?PO4?3,and finally heat treated to synthesize NaTi₂?PO₄?₃@C/Ag composite.X-ray diffraction?XRD?and scanning electron microscopy?SEM?were performed on NaTi₂?PO₄?₃ and NaTi₂?PO₄?₃@C/Ag,respectively.It was proved that these materials prepared by this method had a spherical shape and the particle sizes were nanometer.Thermogravimetric analysis?TGA?,transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?were used to further characterize the composite.It was concluded that the surface of NaTi₂?PO₄?₃ had a relatively uniform coating layer.Conducting a series of electrochemical tests on NaTi₂?PO₄?₃ and NaTi₂?PO₄?₃@C/Ag,the NaTi₂?PO₄?₃@C/Ag electrode material demonstrated higher rate performance and better cycle stability.It could deliver an initial discharge specific capacity of 126.5 mA h g^-1 at a current rate of2 C with the capacity retention of 94.5%after 100 cycles.When the long cycle test was carried out at a current density of 5C,the composite electrode material had an initial discharge specific capacity of 97.5 mAh g^-1 and a coulombic efficiency of 85%.After 400 cycles,the electrode still delivered a discharge specific capacity of 81.2 mAh g^-1.Secondly,micron-sized Na_0.7MnO_2.05 materials were synthesized by a sol-gel method using sodium nitrate,manganese nitrate hexahydrate as the raw material and citric acid as the chelating agent.The morphology of the materials were found to be flat cakes through XRD,SEM,TEM and other characterization measurements.According to the XPS and EDS tests,the content of Mn²⁺and Mn³⁺in Na_0.7MnO_2.05 was analyzed,respectively.At a current density of 50 mA·g^-1,the electrode material delivered an initial charge specific capacity of 42.6 mAh g^-1.Due to the occurrence of battery activation,the electrode material reached a maximum charge specific capacity of 52 mAh g^-1 after 100 cycles and the capacity retention rate was 90.1%after 600 cycles.Similarly,when the current density was increased to 500 mA·g^-1,the electrode material reached a maximum reversible specific capacity of 36.8 mAh g^-1 after 200 cycles and the capacity retention rate was still as high as 90.5%after 1200 cycles.Finally,multi-sweep cyclic voltammetry and electrochemical impedance spectroscopy were usually taken to obtain the diffusion coefficient of Na⁺in the electrode material.For the electrode materials tested by cyclic voltammetry,during the anodization/cathation process,D Na+in the NaTi₂?PO₄?₃@C/Ag electrode material were 3.33×10^-9cm²/s and 2.86×10^-9 cm²/s,respectively;During the cathodic/anodic reaction,the D_Na+of the Na_0.7MnO_2.05 electrode material were 3.57×10^-10 cm² s^-1 and 6.35×10^-10 cm² s^-1,respectively.However,the diffusion coefficient of Na⁺measured by EIS was smaller.During one discharge,the value of D _Na+in the electrode material NaTi₂?PO₄?₃@C/Ag varied from 2.43×10^-11 cm²/s to 3.11×10^-10 cm²/s.When the voltage was around-0.8V,the value was the highest.That was,the diffusion of Na⁺was the fastest in the electrode material.The measurement of Na_0.7MnO_2.05.05 under the initial OCV showed that the value of D _Na+was 3.80×10^-12 cm² s^-1.Compared with the diffusion coefficient of Na⁺in the organic electrolyte,these two materials could be used as high-performance materials in aqueous rechargeable sodium ion batteries.