In-situ Tarnsmision Electron Microscopy Study Of Na-O₂ Batteries

The development of low-cost,safe and reliable secondary batteries is currently a hot research direction,and it is also one of the important stragies to solve energy problems and achieve efficient energy utilization.Sodium-air battery has the advantages of low cost,good safety performance and strong stability,which has attracted widespread attention from researchers.However,the presence of moisture and CO₂in the positive electrode of sodium-air battery will cause a series of side reactions,leading to the formation of insulating materials（For example,alkali metal hydroxide and alkali metal carbonate）,which deteriorates the electrochemical performance of the battery,and even causes the battery to fail.The development of sodium pure oxygen batteries（Na-O₂）has become an inevitable trend.The current research is still limited to the traditional macro batteries,anddirect observation of the electrochemical reaction mechanisms of Na-O₂batteries has not been possible.In this thesis,an in-situ TEM-STM platform（Pico-Femto FE-F20）has been used to construct an all-solid-state Na-O₂battery in a spherical aberration-corrected environmental transmission electron microscope（FEI,Titan G2,300 k V）.The changes in the morphology and structure of the battery during discharging and charging using carbon-free air cathode catalysts（Au/CuO,C/CuO and Au/CNT）were studied.The specific research is as follows:In-situ observations on the charging and discharging reactions in the all-solid Na-O₂battery with Au/CuO and C/CuO nanowires as the cathode catalyst were carried out.It is observed that the volume expansion ration of Au/CuO and C/CuO nanowires is 153.7%;and 79.2%,respectively,and both discharge rezctions experienced only one time volume expansion;In contrast,the volume expansion of pure CuO nanowires has occurred more than three times discharge,and the volume expansion ratioees of the first three discharge reactions were 70%,340%and 580%respectively.In comparison,the volume expansion ratios of Au/CuO and C/CuO nanowires during discharge were significantly lower than that of pure CuO nanowires.The charge and discharge reaction in the all-solid Na-O₂battery with Au/CNT nanowires as the cathode catalyst were observed in-situ,and the reaction mechanism was studied.The characteristics of the reaction of CNT nanowires coated with Au nanoparticles（Au/CNT）as the cathode catalyst of Na-O₂battery are:Na O₂was stable during the discharge process,which was completely decomposed during charge,and the battery xhibited good cyclability.During the discharge and charging process,it was observed that the carbon nanowires were corroded and consumed;The possible reasons for the carbon corrosion are:（1）Na O₂was produced during discharge,and the O₂^-in Na O₂corroded the carbon nanowires;（2）Na₂O₂was decomposed during the charging process,and the decomposition of Na₂O₂needs to consume C.The reaction mechanism is:2Na₂O₂+C→4Na⁺+CO₂+O₂+8e^-.The pure CNT nanowire reaced only at the front end of the nanowire during the discharge,and the pure CNT nanowire still maintained its integrity after discharge and charging.Compared with pure CNT nanowires,the cycle performance of Au/CNT nanowires has been significantly improved.