Studies On Solid State Sodium Ion Electrolytes And Solid State Sodium Batteries

Sodium ion batteries?NIBs?have received researchers'wide concern because sodium element has the advantages of rich resources and low cost.The related enterprises have included NIBs into research and development plan,expecting to realize the industrialization.At present,NIBs use organic liquid electrolyte to support ion transport between electrodes,which has some safety concerns such as easy leakage and inflammability.Solid state batteries?SSBs?with solid state electrolyte?SSE?possess high security,which has become a research hotspot in energy storage area in recent years.Large-scale energy storage system requires energy storage devices to have the characteristics of low cost,high security and long life.Therefore,solid state sodium batteries,having advantage of both low cost and high security,may become an important developing direction in large scale energy storage area.At present,fewer studies on solid state sodium batteries are reported compared with solid state lithium batteries.A series of basic scientific problems and key technologies such as poor contact between solid and solid particle,compatibility between electrodes and electrolyte,mechanism of interfacial ion transport,improvement of the stability,remain to further investigate.Given this situation,the research works carried out in this dissertation were maily focoused on the modification on solid-solid interface,compatibility between polymer solid state electrolyte and electrode,mechanisim on interfacial stability and reaction mechanisim on solid state Na-O₂ batterry.The detailed statement of the above works are described as follows:1.A novel type toothpaste-like electrode was designed to solve the poor contact between solid and solid particles in inorganic ceramic electrolyte based SSBs.This novel type toothpaste-like electrode are composed of active materials,conductive additives and ionic liquid.In which,ionic liquid replaces the electrolyte additives and binder in traditional cathode,acting as both ionic conductor and wetting agent as well as binder.The mixed ionic and electronic conducting network was constructed with the ionic liquid and conductor additives,thus expanding the contact area among the solid particles.In addition,ionic liquid has the property of non-volatilization,Moreover,the new designed toothpaste electrode can keep the interface wet during the working process of the SSB.The new designed solid state sodium battery shows excellent reversibility,rate performance and cycling stability.This design idea have some guiding significance on solving the interfacial problem in solid SSBs.2.PEO based polymer electrolyte was prepared by an evirenmentally friendly method.Firstly,PEO/Na FSI electrolyte system was selected as research object after screening different sodium salts including inorganic salts and organic salts.Na₃V₂?PO4?₃ was adopted as cathode after analyzing different cathode materials.In addition,we modified the aqueous PEO/Na FSI with nano sized Al₂O₃.The capacity retention of the solid state soudium battery NVP|PEO₂₀NaFSI+1 wt.%Al₂O₃|Na was up to 92.8%after 2000 cycles at a current density of 1C with a very stable coulombic efficiency.When used in soft carbon?SC?anode material,the battery SC|PEO₂₀NaFSI+1 wt.%Al₂O₃|Na also showed a good cycling performance.Moreover,we tried to make a full solid state sodium ion battery and proved the feasibility of the bi-polar solid state sodium ion battery.3.The mechanism of solid state sodium battery based on aqueous PEO/NaFSI+Al2O3 electrolyte with excellent cycling stability was analyzed.Different electrolytes were prepared for comparison by controlling single variable.The electrochemical performances of the batteries using different electrolytes vary greatly.The reason can be explained as follows:1)The acid environment on the surface of the PEO₂₀ NaFSI+1 wt.%Al₂O₃ AQ electrolyte can help to neutralize the alkalescence of the cathode's surface.2)The sulfonamides existing on the surface of the electrolyte are beneficial to the formation of the interfacial film which just like SEI formed in the liquid electrolyte.The interfacial film can stabilized the interface of the electrodes and electrolyte.3)The further nanocrystallization of the nano sized Al₂O₃ enlarged the specific surface area,leading to a relative maximum ionic conductivity by adding only 1 wt.%Al₂O₃.4)The in-situ formed AlF₃ is very stable,avoiding side reaction in the interface.Therefore,the interfacial stability was improved.4.In-situ observing the charge and discharge process of Na-O₂ battery in ETEM and ESEM.In ETEM,the beam radiation have a great influence on the process.The Na⁺diffusion coefficient was calculated according to the growing speed of product Na₂O₂ during discharging.The reasons of the formation of Na₂O₂ and irreversible of the battery were also analyzed.In ESEM,solid state Na-O₂ battery can comlete discharge and charge process.Combined with the TEM results,we concluded that we should optimizing electrolyte and catalyst to control the discharge product to be NaO₂rather than Na₂O₂ when studying the Na-O₂ battery,so that to decrease the polarization and improve the battery performance.