Preparation And Electrochemical Performance Of Fe-doped Lithium Phosphorus Oxynitride Solid Electrolyte Films

Solid-state thin-film batteries have the unique advantages of high safety performance,high energy density,ultra-long cycle performance and flexible bending.They are widely used in microelectronic systems,sensors and wearable devices,etc.,and are a new generation of battery systems with high-density and long-term cycle.As the most widely used solid electrolyte films,lithium phosphorus oxynitride(LiPON)is favored by researchers because of its wide electrochemical window,high thermal stability and good electrode compatibility.However,the low ionic conductivity(only 2?10^-6 S/cm at room temperature)of LiPON solid electrolyte severely restricts the reversible capacity and cycle stability of solid-state thin film batteries.In order to improve the ionic conductivity of LiPON solid electrolyte,the researchers optimized the process parameters to increase the N content or achieved heterogeneous element doping by regulating the target composition.Because the improvement of N content is limited by the limitation of plasma technology,and the doping elements have a certain electrostatic confinement effect on Li⁺,the ionic conductivity of LiPON has not been greatly improved.Therefore,it is a great challenge to develop the LiPON solid electrolyte with high ionic conductivity and low interface resistance.In this paper,the high efficiency ion transfer of LiPON thin films is realized by Fe doping(Fe-LiPON)modification by RF magnetron sputtering.The Fe doping concentration in LiPON films is controlled by optimizing process parameters and changing the target composition.Electrochemical performance tests show that the Li⁺conductivity of Fe-LiPON at room temperature is as high as 1.08×10^-5 S/cm,the Li⁺migration number is 0.974,and the Li⁺activation energy is only 0.42 e V,showing excellent Li⁺transport kinetics.The promising ion conductivity of Fe-LiPON films comes from three aspects:First,the polyvalence properties of Fe determine that it can form various types of Fe-N bridge bonds,connect more adjacent PO₃N^4-tetrahedrons to produce Li⁺transport channels,and enhance the network forming body effect.Second,the high electronegativity of Fe causes the redistribution of the electron density field in the Fe-LiPON local structure,forming a high electron density center around Fe-N bonds,while PO₃N^4-tetrahedrons form a low electron density center,stabilizing PO₃N^4-tetrahedrons structure and reducing the electrostatic force on Li⁺.Thirdly,the larger bond length of Fe-N bond reduces the distribution density of nearby atoms and lowers the Li⁺migration energy barrier,which is conducive to the Li⁺transport dynamics.In addition,the work function of Li Co O₂/Fe-LiPON interface is tested and analyzed.The results show that Fe doping increases the work function of LiPON films to 3.90 e V and reduces the interfacial equilibrium potential difference to 0.78 V,which weakens the built-in electric field in the space charge layer accompanied by the lower carrier concentration and interfacial resistance.As a result,the dynamic diffusion of Li⁺in the depletion layer is promoted.