Studies On Interface Modification And Electrochemical Performance Of Na₃PS₄ Solid Electrolyte

Sodium elements are widely distributed and abundant,which can effectively reduce the cost of batteries.Solid-state sodium battery can further improve the safety and energy density,which could apply in large-scale energy storage and makes it one of the promising next-generation batteries.Nevertheless,there are complex issues of interfacial engineering worth studying:chemical stability,electrochemical stability,mechanical stress,etc.The bad interface compatibility will increase the interfacial impedance and degrade the electrochemical performance.Among solid electrolytes,sulfide electrolytes stand out because of some advantages,such as high room-temperature ionic conductivity,low Young’s modulus,and low grain-boundary resistance and etc.Therefore,the thesis adopts Na₃PS₄electrolyte and focuses on interfacial modification.The thesis includes the following two parts:1)The decomposition behavior of solid electrolyte at high voltage and its influence on electrochemical performance are studied by adopting the oxide cathode Na_0.78Ni_0.2Fe_0.38Mn_0.42O₂（NNFM）and Na₃PS₄electrolyte.Through analyzing the electrochemical performance and the charge/discharge curve,it was speculated that the electrolyte decomposes during the first charge,which affects the performance of the battery.X-ray photoelectron spectroscopy（XPS）characterization confirms the hypothesis.In addition,electrochemical impedance spectrum（EIS）was adopted to analyze the influence of the ratio（solid electrolytes:cathode）on ion/electron transport properties in the composite electrodes,which shows that the design of a positive electrode is vital for the electrochemical performance.This work explores the importance of the electrochemical stability of sulfide electrolyte and the ion/electron transport properties of composite electrode on solid-state battery.2)By material and interfacial modification based on Na₃PS₄ solid electrolytes（SEs）,room-temperature solid-state sodium-sulfur（Na-S）battery is achieved.First,ionic liquid N-butyl-N-methylpyrrolidinium bis（fluorosulfonyl）imide(Pyr₁₄FSI)is employed to modify anode/electrolyte interface.A stable in-situ solid electrolyte interphase（SEI）layer is formed between the interface of Na Sn and Na₃PS₄,proved by XPS measurements.An overpotential of 0.55 V after 900 h of a symmetrical battery indicates the enhanced interfacial stability.Furthermore,Se-doped S@p PAN is used to boost the ionic and electronic conductivity of the sulfur cathode.As a result,the Na-S battery using Se_0.05S_0.95@p PAN cathode and the interfacial modification delivers improved cycle performance,which displays an initial capacity of 708.5 m Ah g^-1 and stable cycling of 50 cycles at a current density of 0.3 A g^-1.This work demonstrates that interface engineering and electrode design play an important role on electrochemical performance.