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Sodium–air batteries (SABs) are receiving considerable attention for the development of next generation battery alternatives due to their high theoretical energy density (up to 1105 W h kg−1). However, most of the studies on this technology are still based on organic solvents; in particular, diglyme, which is highly flammable and toxic for the unborn child. To overcome these safety issues, this research investigates the first use of a branched ether solvent 1,2,3-trimethoxypropane (TMP) as an alternative electrolyte to diglyme for SABs. Through this work, the reactivity of the central tertiary carbon in TMP towards bare sodium metal was identified, while the addition of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyr][TFSI]) as a co-solvent proved to be an effective strategy to limit the reactivity. Moreover, a Na-β-alumina disk was employed for anode protection, to separate the TMP-based electrolyte from the sodium metal. The new cell design resulted in improved cell performance: discharge capacities of up to 1.92 and 2.31 mA h cm−2 were achieved for the 16.6 mol% NaTFSI in TMP and 16.6 mol% NaTFSI in TMP/[C4mpyr][TFSI] compositions, respectively. By means of SEM, Raman and 23Na NMR techniques, NaO2 cubes were identified to be the major discharge product for both electrolyte compositions. Moreover, the hybrid electrolyte was shown to hinder the formation of side-products during discharge – the ratio of NaO2 to side-products in the hybrid electrolyte was 2.4 compared with 0.8 for the TMP-based electrolyte – and a different charge mechanism for the dissolution of NaO2 cubes for each electrolyte was observed. The findings of this work demonstrate the high potential of TMP as a base solvent for SABs, and the importance of careful electrolyte composition design in order to step towards greener and less toxic batteries. [ABSTRACT FROM AUTHOR]

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]

The use of two types of bulky cations, tetrabutylammonium (TBA+) and Cs+, as electrolyte additives in Na‐O2 batteries is investigated. These cations facilitate the stabilization of sodium superoxide in the electrolyte, promoting the solution‐mediated pathway. Both the addition of TBA+ and Cs+ favor the growth of larger NaO2 cubes than in the case of the electrolyte containing only sodium salt, particularly in the case of Cs+. In terms of full discharge capacity, both additives lead to an increase in the discharge capacity, which is greater in Cs+ (50% enhancement). TBA+ also provides an improved stabilization of superoxide; nevertheless, the interaction is not as strong as in the case of Cs+ due to the steric hindrance set by the alkyl groups. The presence of these additives not only affects the NaO2 formation mechanism, but also influences the nature of the solid electrolyte interphase. The presence of Cs+ generates a more stable solid‐electrolyte interphase, which increases the cycle life of Na‐O2 batteries. Overall, new insights are provided to control the growth of the discharge products, modify the oxygen reduction reaction mechanism, and protect the Na metal anode surface by adding bulky monovalent cations in the electrolyte formulation. [ABSTRACT FROM AUTHOR]

This perspective points out the potential of solid‐state Na‐air/O2 batteries for powering next‐generation storage devices, highlighting their high energy density, efficiency, and cost‐effectiveness. The challenges faced by Na‐air/O2 batteries, including liquid electrolyte instability, O2/O2− crossover, Na anode passivation, and dendritic growth are addressed. Strategies such as exploring solid‐state electrolytes (SSE), mitigating O2/O2− crossover, protecting the Na anode, and enhancing the solid electrolyte interphase (SEI) are discussed as potential solutions. Future challenges in advancing the technology are reviewed, emphasizing the need for addressing and understanding the fundamental mechanisms of (quasi)solid‐state Na‐air/O2 batteries (i.e., discharge product chemistry, dendritic growth, O2 crossover, the electrolyte properties, the stabilization of battery chemistry and Na+ transport). A roadmap outlining future directions is presented to overcome these challenges, with the goal of fully harnessing the potential of Na‐air/O2 batteries as a viable option for renewable energy and industrial‐scale commercialization. [ABSTRACT FROM AUTHOR]

The morphology and size of the discharge products in Na−O2 batteries are largely dominated by the interactions of the electrolyte with superoxide and sodium ions; therefore, an in‐depth understanding is essential for achieving high performance batteries. Herein, we report a framework designed to understand solvation and coordination in the recently discovered hybrid electrolytes, based on glyme and a pyrrolidinium ionic liquid. FTIR and NMR spectroscopic techniques, coupled with molecular dynamics simulations, have been used to characterize these systems. We demonstrate that the presence of ionic liquid in the hybrid electrolyte affects the superoxide coordination environment by weakening the glyme‐Na+ interactions, and generating solvent separated ion pairs. All these factors lead to different deposition mechanisms, which will determine the battery performance. The Na+ solvation shell compositions, anion conformers and relative free glyme content are also evaluated. The combinatorial approach used in this study‐experimental and computational – can be applied for further design of these hybrid electrolytes and other metal – O2 chemistries and electrochemical systems. [ABSTRACT FROM AUTHOR]