Preparation And Interface Optimization Of Na₃Zr₂Si₂PO₁₂ Solid State Electrolyte

The abundant sodium element promotes the development of sodium-ion batteries and sodium-metal batteries.In order to meet the requirement of high safety,it is necessary to develop suitable solid-state electrolyte.NASICON-type(Na₃Zr₂Si₂PO₁₂)solid electrolyte with wide electrochemical stability window,good thermal stability,excellent chemical and electrochemical stability,high mechanical strength and high room temperature ionic conductivity is considered to be one of the important candidates for sodium-ion solid-state electrolytes.However,Na₃Zr₂Si₂PO₁₂electrolyte has a series of issues,such as the lower ionic conductivity than traditional liquid electrolyte,the poor contact of electrolyte/sodium anode interface,and the growth of sodium dendrite.In this paper,focusing on the composition regulation of the Na₃Zr₂Si₂PO₁₂ electrolyte and the interfacial compatibility between Na₃Zr₂Si₂PO₁₂electrolyte and sodium metal anode,and taking the preparation science of point and surface defect engineering regulation of Na₃Zr₂Si₂PO₁₂ crystal as the clue,a series of studies were carried out from the aspects of new preparation method,aliovalent doping,the construction of interfacial atomic alloy layer and the bonding of interfacial molecular layer.The following innovative research results have been achieved:(1)Considering the disadvantage of the formation of impurity phases caused by component volatilization in high temperature solid-state reaction,a new method for preparing Na₃Zr₂Si₂PO₁₂ electrolyte by microwave calcining-microwave sintering was proposed.The effects of sintering temperature and sintering duration on the phase composition,micromorphology and ionic conductivity of the electrolyte were studied respectively.The components in ceramic embryo could directly absorb microwave energy by material's dielectric loss,so as to realize the uniform heating of the electrolyte as a whole.The phase-pure Na₃Zr₂Si₂PO₁₂ electrolyte with a relative density of 96%and an ionic conductivity of 2.51×10^-4S cm^-1 at room temperature could be prepared by microwave sintering at 850? for only 30 minutes.(2)In view of the low ionic conductivity of electrolyte,a doping scheme of using lanthanide rare earth ions to replace Zr⁴⁺in Na₃Zr₂Si₂PO₁₂ electrolyte was proposed.The effects of the doping amounts on the phase composition,microstructure and electrical conductivity of the electrolyte were studied.Low valence ions with large radius could broaden the migration channel of Na⁺and increase the gap Na⁺concentration.At the same time,the number and size of pores in the electrolyte were reduced.The dense microstructure was conducive to the movement of Na⁺at the grain boundary.The grain boundary phases of Na₃Eu(PO₄)₂ and Na₃Pr(PO₄)₂ in the electrolyte could not only improve the occupancy of Na⁺by optimizing the Si(14)P ratio,but also hinder the transmission of electrons at the grain boundary,so as to reduce the electronic conductivity of the electrolyte.After optimization,the ionic conductivity and the critical current density of Pr³⁺and Eu³⁺doped Na₃Zr₂Si₂PO₁₂ electrolyte were developed significantly.(3)An ultrathin Sn S₂ coating was constructed on the surface of Na₃Zr₂Si₂PO₁₂electrolyte by thermal decomposition method to solve the interface issue between electrolyte and sodium.Sn S₂ filled the grain boundaries and pores on the surface of the electrolyte pellet,effectively preventing the growth of sodium dendrites at the interface defects and reducing the interface impedance.During the cycle,the in-situ Na-Sn alloy with electronic compound characteristics and Na₂S ionic conductor could improve the wettability of metal sodium on the electrolyte and reduce the overpotential of Na deposition/stripping.The critical current density of assemble sodium symmetrical cell was increased to 0.9 m A cm^-2 at room temperature,and could cycle stably for a long time at the current density of 0.1 m A cm^-2 for 800 hours with a low potential less than 25 m V.(4)A room temperature ultrasonic welding strategy was proposed to approach the issue of poor interface contact between sodium metal and Na₃Zr₂Si₂PO₁₂electrolyte.The high-frequency ultrasonic vibration energy converted into slip deformation energy of metallic sodium atoms,friction work and limited temperature rise,which promoted the rapid diffusion of sodium atoms at the ceramic interface and the formation of tight bonding interface.Friction could destroy the oxide film on the metal surface and the gas film on the ceramic surface quickly,promoting the wettability between metal and ceramics.Under the synergistic effect of the interfacial physical close contact and the Na₂Si O₃ ion conductor formed in-situ at the interface,the interfacial impedance was reduced.The assembled sodium symmetrical cell could cycle stably for 400 hours at a current density of 0.2 m A cm^-2,and the critical current density was increased to 0.6 m A cm^-2 at room temperature.In addition,the interface between lithium/sodium metal and various oxide electrolytes was constructed by ultrasound solid welding,proving the ultrasonic solid welding had universality in improving the interface contact performance.(5)An Au-Na alloy interface layer was formed between Na₃Zr₂Si₂PO₁₂electrolyte and sodium anode at room temperature by ion sputtering and ultrasonic welding to solve the interface issue of poor contact.This interface construction process eliminated the heating and melting steps in conventional operation.The Au-Na alloy with the ordered structure of electronic compounds could provide abundant natriophilic sites for efficient Na plating and promote fast and uniform charge transfer.The stable layer constructed by Sn S₂ ensured smooth and uniform Na plating/stripping during cycling,thereby avoiding the form of sodium dendrites.The impedance of modified interface was reduced,and the sodium symmetrical cell could stably cycle for 900 hours at a current density of 0.3 m A cm^-2.The critical current density was increased significantly.The assembled full cell showed favorable cycle stability and rate performance.(6)In view of the poor interfacial contact of Na₃Zr₂Si₂PO₁₂/electrodes,PVDF-HFP/PMMA/TPU@Na₃Zr₂Si₂PO₁₂ composite gel electrolyte was prepared by solution casting method to develop ionic conductivity as well as improve interface compatibility with anode and cathode.The addition of oxide particles improved the thermal stability of polymer membrane.The composite gel electrolyte was easier to absorb and store liquid electrolyte because of the decrease of crystallinity and the increase of porosity.The ionic conductivity of modified gel electrolyte could reach2.83×10^-3 S cm^-1,and the electrochemical stability window was widened.The interface stability between gel electrolyte and metal sodium was improved.