In Situ Transmission Electron Microscopy Studies Of The Electrochemical Reaction Mechanisms For Sodium-air Batteries

Lithium-ion batteries(LIBs)have the highest energy density among the commercially available chemical energy storage devices.However,due to the use of organic liquid electrolyte,commercial lithium-ion batteries are plagued by its safety risks such as thermal runaway,fire hazard,easy corrosion and poor thermal stability.Rechargeable non-aqueous alkali metal-air batteries are promising candidates for the next-generation energy storage devices due to their significantly higher theoretic energy density than state-of-the-art lithium ion batteries.Among them,sodium oxygen(Na-O₂)batteries have attracted considerable attentions recently due to the abundant sodium resources,the lower potential gap in the round-trip.Unfortunately,the organic liquid electrolytes react with the discharge products of NaO₂ or Na₂O₂,causing decomposition of the electrolytes and formation of sodium carbonates or other parasitic reaction products on the surface of the electrode,which electrically isolate O₂ with the cathode,in turn shutting off the subsequent electrochemical reactions.The advent of all solid state Na-O₂ battery using ceramic solid-state electrolyte can avoid the flammable and fire hazards caused by short circuit,and is expected to replace the traditional liquid electrolyte Na-O₂ battery.However,the electrochemical reaction of all solid state Na-O₂ battery occurs in the non-equilibrium condition during the charging and discharging process,which is very difficult to dectect the actually electrochemical reaction mechanisms in the battery.Nevertheless,fundamental understanding of the reaction mechanisms including oxygen chemistry in the cathode,the morphology,chemical composition and evolution of the discharge products in the all solid state Na-O₂ batteries is critical for the understanding the basic sciences,technical design and practical applications of Na-O₂ batteries.We constructed all solid state Na-O₂ nanobetteries in a Cs-corrected ETEM(FEI,Titan G2,300 k V)using an in situ TEM-STM(Scanning Tunneling Microscopy)holder(Pico Femto FE-F20).The morphology and structure evolution of the carbonless air cathodes(e.g.CuO,Au/?-MnO₂)in Na-O₂ batteries were investigated during the discharge and charge processes.Additionally,Na-O₂/CO₂(O₂ and CO₂ mixture)and Na-O₂ batteries with either carbon nanotubes(CNTs)or Ag nanowires as the air cathode media were studied using the same TEM-STM platform.With regard to the short circuit problem caused by dendrite growth of all solid state sodium battery,we also conducted real-time characterizations of Na dendrite growth with concurrent mechanical property measurements using a home-made environmental transmission electron microscopy-atomic force microscopy(ETEM-AFM)platform.More specific experiments and conclusions are as follows:1.We report real time imaging of the oxygen reduction reactions(ORRs)in all solid state Na-O₂ batteries with CuO nanowires(NWs)as the air cathode in an aberration-corrected ETEM under an oxygen environment.The ORR occurred in a distinct two-step reaction:namely a first conversion reaction followed by a second multiple ORRs.In the former,CuO was first converted to Cu₂O and then to Cu;in the latter,NaO₂ formed first,followed by its disproportionation to Na₂O₂ and O₂.Concurrent with the two distinct electrochemical reactions,the CuO NWs experienced multiple consecutive large volume expansions.It is evident that the freshly formed ultrafine-grained Cu in the conversion reaction catalyzed the latter one-electron-transfer ORR,leading to the formation of NaO₂.Remarkably,no carbonate formation was detected in the oxygen cathode after cycling due to the absence of carbon source in the whole battery setup.2.We report the first in situ imaging of the operation of the electrocatalysis in a Na-O₂battery in an advanced aberration corrected ETEM.In the Au/?-MnO₂ air cathode,the ORR is characterized by the formation of NaO₂ nucleated from the Au catalysts;the NaO₂ quickly disproportionated to Na₂O₂and O₂,causing the formation of nano bubbles and an 18 times volume increase of the?-MnO₂ nanowires.The nanowires shrank after the burst of the nano bubbles.In contrast,no ORR took place in the bare?-MnO₂ nanowire cathode;instead,the?-MnO₂ nanowires only swelled 217%as a result of the Na⁺intercalation.3.We report in situ studies in the Na-O₂/CO₂(O₂ and CO₂ mixture)and Na-O₂ batteries with either carbon nanotubes(CNTs)or Ag nanowires as the air cathode media in an advanced aberration corrected ETEM.In the Na-O₂/CO₂-CNT nanobattery,the discharge reactions occurred in two steps:(1)2Na⁺+2e^-+O₂?Na₂O₂;(2)Na₂O₂+CO₂?Na₂CO₃+O₂;concurrently a parasitic Na plating reaction took place.The charge reaction proceeded via:(3)2Na₂CO₃+C?4Na⁺+3CO₂+4e^-.In the Na-O₂/CO₂-Ag nanobattery,the discharge reactions were essentially the same as the Na-O₂/CO₂-CNT nanobattery,however,the charge reaction in the former was very sluggish,suggesting that direct decomposition of Na₂CO₃ is difficult.In the Na-O₂ battery,the discharge reaction occurred via reaction(1),but the reverse reaction was very difficult,indicating the sluggish decomposition of Na₂O₂.Overall the Na-O₂/CO₂-CNT nanobattery exhibited much better cyclability and performance than the Na-O₂/CO₂-Ag and the Na-O₂-CNT nanobatteries,underscoring the importance of carbon and CO₂ in facilitating the Na-O₂ nanobatteries.4.The parasitic sodium plating reaction in all solid state sodium battery will cause the formation of sodium dendrite,and it grow and flow by creep through large cracks of the solid electrolyte to cause short circuit of the battery.We conducted real-time characterizations of Na dendrite growth with concurrent mechanical property measurements using an ETEM-AFM platform.In situ electrochemical plating produces Na deposits stabilized with a thin Na₂CO₃ surface layer.These Na dendrites have characteristic dimensions of a few hundred nanometers and exhibit different morphologies,including nanorods,polyhedral nanocrystals,and nanospheres.In situ mechanical measurements show that the compressive and tensile strengths of Na dendrites with a Na₂CO₃ surface layer vary from 36 MPa to above 203 MPa,which are much larger than that of bulk Na.These results provide new baseline data on the electrochemical and mechanical behavior of Na dendrites,which have implications for the development of Na metal batteries and Na-Air batteries toward practical energy storage applications.