Research On The Sodium-based Solid-state Electrolytes And Solid-state Batteries

The rapid development of consumer electronics,new energy vehicles,and large-scale energy storage fields urgently require secondary batteries with higher safety and higher energy density.However,the energy density of currently widely used lithium-ion batteries is gradually increasing.As a result,the problem of shortage of lithium resources has slowly emerged.Moreover,forcibly increasing the energy density of lithium-ion batteries brings serious safety hazards and poor electrochemical stability,and the abundance of lithium also limits its coverage in all areas.In this context,sodium-ion batteries have good application prospects in large-scale energy storage and other fields due to their potential cost advantages and have been extensively researched and developed.In addition,solid-state batteries,as the next generation of battery systems with both high safety and high energy density,have also received widespread attention.However,the development of sodium-ion batteries and solid-state batteries will inevitably require high-voltage and high-capacity cathode materials.Under high-voltage conditions,the electrolyte interface and the electrode are often unstable and prone to severe decomposition reactions.In addition,the development of solid-state batteries is inseparable from solid electrolytes with excellent comprehensive performance.However,dendrite problems,interfacial contact problems,and electrochemical stability problems are often not solved in a single solid electrolyte.Based on the above background analysis,this thesis has carried out a systematic study on the three aspects of sodium-based high-voltage,high-capacity cathode,sodium-based solid electrolyte,and the interface between solid electrolyte and electrode.1.To reduce the preparation energy consumption,save costs,and realize large-scale preparation of Na₃(VOPO₄)₂F(NVOPF),this paper has developed a new low-temperature liquid phase method.By optimizing the concentration of the raw materials in the liquid phase synthesis process,the p H of the solution,and the reaction time,the low-temperature controllable large-scale synthesis of NVOPF is realized.The prepared NVOPF is assembled into Ah-grade batteries with hard carbon as the anode.The assembled 26650 cylindrical cell rated capacity is 1.78 Ah,the initial discharge capacity at 5C rate is 0.99 Ah,and the capacity retention rate reaches 95.15%after 2000 cycles.The above preliminary experimental results show that the new method for large-scale preparation of NVOPF at low temperature and liquid phase developed by us has strong practicability and an excellent industrial application prospect.2.Based on the NVOPF prepared above,we studied its electrochemical performance in PEO-based polymer solid electrolytes.We first conducted a constant voltage charging test on the prepared PEO-based polymer solid-state electrolyte,which can be dynamically stabilized to 4.2 V(vs.Na⁺/Na).The charging curve of the NVOPF-based high-voltage solid-state half-cell also verifies this result.However,the capacity retention rate after 100 cycles is only 53.53%.To further improve the cycle stability of the solid-state battery,we coated a layer of(NH₄)₃Al F₆ in situ on the surface of the NVOPF,whose capacity retention rate increased significantly to 83.35%after 100cycles.The above experimental results show that the solid electrolyte can fully work outside its electrochemical stability window,which depends on the nature of the interface layer between the solid electrolyte and the electrode.3.Based on the NVOPF prepared above,we studied the influence of different heat treatment atmospheres and temperatures on the structure of NVOPF and its electrochemical performance in PEO-based solid-state and organic liquid batteries.The results show that the heat treatment temperature and atmosphere significantly impact the structure of NVOPF and its electrochemical performance in solid-state and organic liquid batteries.Therefore,under the appropriate combination of temperature and atmosphere,the heat-treated NVOPF can perform better electrochemical performance than the original NVOPF.4.To optimize the PEO-based polymer solid electrolyte itself,we verified the feasibility of Na PF₆ as a sodium salt.We assembled a solid-state battery based on the prepared PEO₁₆-Na PF₆,and the capacity retention rate of the Na₃V₂(PO₄)₃|PEO₁₆-Na PF₆|Na solid-state battery was 85.8%under 2C magnification for 200 cycles at 80?,indicating the feasibility of Na PF₆ used as a sodium salt in PEO-based polymer solid electrolyte.5.To broaden the operating temperature range of the PEO-based polymer solid electrolyte,making it work at high temperatures and room temperature,22 wt.%TEGDME ether plasticizer was introduced into the PEO₂₀-Na FSI-1 wt.%Al₂O₃polymer solid electrolyte.Based on the plasticized PEO-based solid-state polymer electrolyte,the assembled solid-state sodium batteries exhibited excellent cycling stability at room temperature and 80?,indicating that TEGDME has dramatically broadened the operating temperature range of the PEO-based polymer solid electrolyte.6.To optimize the NASICON solid-state electrolyte,we doped NASICON with Mg element,and the research results show that the Mg element mainly exists in Na Mg PO4 and is distributed in the NASICON grain boundary.To realize the practical application of NASICON solid-state electrolyte,we prepared the Na_3.3Zr_1.85Mg_0.15Si₂PO₁₂ powder with the highest ionic conductivity among the Mg-doped NASICON solid-state electrolyte.We coated it on the surface of the PE separator,which significantly improved the wettability of the liquid electrolyte.Based on the NASICON-coated PE separator,Ah-level 26650 cylindrical cells are assembled,whose capacity retention rate is 88%at a rate of 1C for 2000 cycles.7.The reason why the NASICON-type oxide ceramic solid electrolyte is difficult to practically apply,in addition to its brittleness and interface contact problems,another critical problem is the short circuit risk caused by the penetration of sodium dendrites.We observe the formation and growth of sodium dendrites in NASICON ceramic wafer by the ex-situ SEM and in-situ optical microscopy analysis.To suppress sodium dendrites,we prepared a layer of fluorinated amorphous carbon material in situ on the surface of the NASICON ceramic wafer.After the interface optimization,NASICON's ability to inhibit sodium dendrites is significantly improved.