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An alternative anodeto metallic sodium based on the high capacity of Sn4P3 alloy is investigated for the first time in Na–O2 batteries. This alloy is synthesised by an easy and scalable method based on ball milling which is further optimized in order to obtain the most suitable morphology while avoiding agglomeration of the material. First, the stability of Sn4P3/C composite within the battery environment is analysed and compared to that of metallic Na. Second, the electrochemical study reveals the formation of sodium superoxide on both the metallic anode and Sn4P3 alloy for the first time, which evidences the existence of O2/O2− crossover processes. Complementary impedance studies, at various voltages and as a function of cycle number, demonstrate a higher stability of the solid electrolyte interphase (SEI) throughout cycling in addition to presenting lower reactivity and greater stability than the metallic Na system. Regarding the cycling performance, although it is lower than that of metallic Na, it surpasses that exhibited by other alternative anodes under similar conditions. This work therefore demonstrates the viability of a new family of anodes, paving the way for the development of more stable and safer Na–O2 batteries. Moreover, the importance of oxygen crossover is highlighted which is a common problem regardless of the anode used. [ABSTRACT FROM AUTHOR]

Carbon coated tin phosphide is synthesized by an easily scalable ball milling method. The electrochemical performance of Sn4P3/C electrodes is explored using different combinations of carbonate and ether-based solvents. Our experiments reveal that a remarkable improvement in the specific charge of Sn4P3 anodes can be obtained by using a combination of NaPF6 salt and diethylene glycol dimethyl ether electrolyte. A reversible specific charge of 900 mA h g−1 and initial coulombic efficiency of around 90% have been achieved. We carefully examine the origin of the improved capacity and cycle life when using glyme-based electrolytes and compare with carbonate electrolytes. Moreover, the reaction mechanism of the Sn based anode is further discussed. [ABSTRACT FROM AUTHOR]

Due to the increase in global energy demand and the unsustainability of burning fossil fuels, the search for optimized energy systems has become a priority in recent years. In 1990, lithium batteries revolutionized the energy market; however, the unceasing increase in demand and the limited lithium supplies induce the need for alternative energy storage systems. Li–O2 batteries are promising to make a difference in applications such as those related to electromobility. In the last few years the interest in metal–air batteries has increased exponentially; however, several challenges must be overcome to improve their efficiency. For this system to be viable, it is necessary to solve key problems such as the high polarization of the charge reaction. To overcome this limitation, facilitate the oxidation of the Li2O2 discharge product, and increase the efficiency of these batteries, the use of redox mediators has been widely investigated. In the present work, the mediators reported in the Li–O2 literature are classified into four groups according to their charge state in both their reduced and oxidized forms and their performance is discussed. Finally, a summary of the redox mediators used in analogous Na–O2 batteries is presented. This work provides a clear view about the use of redox mediators as a powerful strategy for the commercialization of these metal–O2 batteries, discussing the advantages and disadvantages of this type of homogenous catalyst, and suggesting future directions. [ABSTRACT FROM AUTHOR]

Fundamental understanding of the physical phenomena and electrochemical reactions occurring in metal-air batteries is critical for developing rational approaches towards high-performing Na-O2 battery cathodes. In this context, air cathode porosity plays a key role in battery performance, influencing oxygen supply and hence oxygen reduction and evolution reaction kinetics (ORR/OER). Graphene-based aerogels offer great versatility as air-cathodes due to their low density, high electronic conductivity and adjustable porosity. Reduced graphene aerogels with different porosities are examined where high meso-macroporosity and a narrow macropore size arrangement exhibit the best electrode performance among all studied materials (6.61 mA h cm-2). This is ascribed to the particular macroporous 3D structure of graphene-based electrodes, which favours the diffusion of oxygen to the defect sites in graphene sheets. An outstanding cycle life is achieved by using the pore-tuned cathode, leading to 39 cycles (486 h) at 0.5 mA h cm-2 with very low overpotential (250 mV) and efficiency over 95%. The cyclability is further increased to 745 h (128 cycles) by decreasing the capacity cut-off. This study shows that tuning of material porosity opens a new avenue of research for achieving Na-O2 batteries with high performance by maximizing the effective area of the electrodes for the ORR/OER. [ABSTRACT FROM AUTHOR]