Research On Doping Modification And Stability Of NASICON Structure Solid Electrolyte

With the large scale application of traditional lithium ion batteries in various fields,the shortage of raw materials leads to the increase of costs.Moreover,the lithium–ion batteries have potential safety hazards such as ignition and explosion.Compared with traditional lithium ion batteries,the sodium secondary rechargeable batteries with sodium based solid–state electrolyte have the advantages of the high energy density,high safety,and low cost,have great potential application in the field of energy.However,low ionic conductivity and low stability of sodium based solid–state electrolyte has limited the application of sodium secondary rechargeable batteries.NASICON,composed of Na₃Zr₂Si₂PO₁₂,is one of the most promising sodium based solid–state electrolytes with high ionic conductivity and high stability.However,NASICON solid-state electrolyte is usually prepared by high temperature solid phase method,the precipitation of ZrO₂ phase produced with the volatilization of Na and P at high temperature,reducing ionic conductivity.And the stability of NASICON solid–state electrolyte applied in secondary sodium batteries has been questioned.In this thesis,the NASICON solid–state electrolyte was prepared by optimizing calcination parameters.The best calcination condition?1225?,15 h?was determined with the cold isostatic pressing–high temperature solid phase method.The high purity NASICON solid–state electrolyte was prepared,and the ionic conductivity was 2.64×10^–3 S/cm at room temperature.Ulteriorly,the rare earth elements of La and Y were doped in the structure of NASICON solid–state electrolyte.It was found that NASICON solid–state electrolyte doped by rare earth elements can increase the density of NASICON,increase carrier concentration,improve the sodium ion channel,thereby increasing the ionic conductivity of these materials.The ionic conductivity of NASICON solid–state electrolyte after doping with La and Y were increased to 6.71×10^–3 S/cm and 4.35×10^–3 S/cm respectively.The stability of NASICON solid–state electrolyte was studied in alkaline solution?pH=14,11,9,7?.The corrosion mechanism of NASICON solid–state electrolyte in the alkaline solution is that the H ₃O⁺occupied the transport space of Na⁺,and blocked the sodium ion transport channels,resulting the increased grain impedance,the grain boundary and new grain boundary impedance increased with deformation and distortion of grain.With the increase of immersed time,the lower of pH value,the more serious corrosion.The ionic conductivity of NASICON solid–state electrolyte was reduced to 1.73×10^-3 S/cm after 50 days of immersion in solution with pH value of 7.The reserch about NASICON solid–state electrolyte with high ionic conductivity provides the basis for its wide application in the field of energy at room temperature.The corrosion mechanism of NASICON solid–state electrolyte in alkaline solution is studied,which provides the theoretical basis for the study of methods to improve the stability of inorganic solid–state electrolyte.