Suchergebnisse

Sodium-oxygen batteries (SOBs) have the potential to provide energy densities higher than the state-of-the-art Li-ion batteries. However, controlling the formation of sodium superoxide (NaO2) as the sole discharge product on the cathode side is crucial to achieve durable and efficient SOBs. In this work, the discharge efficiency of two graphene-based cathodes was evaluated and compared with that of a commercial gas diffusion layer. The discharge products formed at the surface of these cathodes in a glyme-based electrolyte were carefully studied using a range of characterization techniques. NaO2 was detected as the main discharge product regardless of the specific cathode material while small amounts of Na2O2⋅2H2O and carbonate-like side-products were detected by X-ray diffraction as well as by Raman and infrared spectroscopies. This work leverages the use of X-ray diffraction to determine the actual yield of NaO2 which is usually overlooked in this type of batteries. Thus, the proper quantification of the superoxide formed on the cathode surface is widely underestimated; even though is crucial for determining the efficiency of the battery while eliminating the parasitic chemistry in SOBs. Here, we develop an ex-situ analysis method to determine the amount of NaO2 generated upon discharge in SOBs by transmission X-ray diffraction and quantitative Rietveld analysis. This work unveils that the yield of NaO2 depends on the depth of discharge where high capacities lead to very low discharge efficiency, regardless of the used cathode. We anticipate that the methodology developed herein will provide a convenient diagnosis tool in future efforts to optimize the performance of the different cell components in SOBs. ; M.E., L.M and N.O.-V. thank the European Union (Graphene Flagship-Core 3, Grant number 881603) for the financial support of this work. J.I.P. acknowledges funding by the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN), Agencia Estatal de Investigación (AEI) and the European Regional Development Fund (ERDF) through project RTI2018-100832-B-I00. R.Y. acknowledges financial support from StandUp for Energy and the Swedish Energy Agency. ; Peer reviewed