Over the last decade researchers worldwide have focused their efforts on developing “beyond lithium” battery technologies to augment, or in certain situations replace, Li-ion batteries. Energy demand is increasing and new solutions are necessary to allow the use of renewable energies, thus enabling a clean transition towards decarbonisation. In this scenario, metal-oxygen/air (M-O₂/air) batteries have the capability to play an important role in the development of both stationary and mobile applications, due to their high theoretical energy density compared to current systems.

The inherent advantage of metal-air batteries is that the active material in the cathode is absorbed from the surrounding environment (O₂), giving metal-air batteries some of the highest theoretical energy densities of all battery systems.

Among the different chemistries, the Na-air/O₂ battery (NAB) is a promising candidate with an energy density of up to 6 times higher than Lithium-ion batteries (LIBs) (~1600 Wh kg^-1 vs 250 Wh kg^-1). In addition, sodium is the 6^th most abundant element in the earth´s crust, which is a significant economic and resource advantage, especially when combined with the option of employing an aluminium current collector, which lowers overall production costs and battery weight.

Peled et al. reported the first NAB in 2011 and since then the research efforts have focused on understanding the mechanisms behind this chemistry which rely on the oxygen reduction reaction (ORR) during discharge and oxygen evolution reaction (OER) during charge, apart from investigating novel electrodes and electrolytes.

Unlike Li-ion batteries which rely on intercalation chemistry (a cation is intercalated in the electrodes), a NAB store its energy by the so-called conversion reaction. This means that a new solid is formed, i.e., an oxide. Typically, the configuration of the battery consists of a sodium-containing anode, a sodium-conducting organic electrolyte, and an air cathode (see Figure). The reaction mechanism involves oxygen gas dissolving into the electrolyte and forming O₂^2-, which then combines with Na⁺ to form sodium superoxide (NaO₂) (Na⁺ + O₂ + 1e- ↔ NaO₂). Other discharge products, such as sodium peroxide (Na₂O₂) and peroxide hydrate (Na₂O₂·H₂O), have also been described.

So far, two mechanisms have been reported for the formation of these oxides. The surface-mediated mechanism where ORR takes places at the electrolyte/cathode interface, thus leading to the direct formation of discharge products on the air cathode. This process often induces the formation of a film on the air cathode which passivates the surface causing limited battery performance.

By contrast, in the solution-mediated mechanism, the formation of the oxides occurs in the electrolyte where superoxide is dissolved (from reduction of oxygen at the cathode). First small nuclei form in the electrolyte and as they grow, they end up precipitating at the electrolyte/cathode interface forming the discharge products. The electrolyte plays therefore an important role in these deposition mechanisms.

The main challenges regarding the electrolyte selection are:

• Understanding the solvation, coordination and interactions between the solvent and Na⁺: by modifying this parameters discharge product chemistry can be tuned; for example, the formation of Na₂O₂ over NaO₂ would lead to much higher energy density.
• Tuning mass transport properties: conductivity and viscosity are very important; in fact, the latter is key for the O₂
• Enabling a stable solid electrolyte interphase (SEI) using electrolyte additives so Na metal can be used as anode (achieving higher energy density).
• Minimizing the parasitic chemistry which occurs because of the reactivity of the oxygen radicals with the electrolyte. The use of hybrid electrolytes composed by a glyme and ionic liquid has proven to be a great strategy.

Currently, the vast majority of the studies have focused on liquid electrolytes which suffer from the above-mentioned limitations, in addition to their flammability and volatility which need to be considered for practical implementations.

The use of ionic liquids overcomes some of these challenges but due to their higher viscosity, a co-solvent is needed. The near future research, therefore, should focus on developing environmentally friendly co-solvents that can overcome the limitations of the ionic liquids.

Another venue of research is the replacement of liquid electrolytes by solid-based, both polymeric and ceramic. Few works have been reported in literature but we can acknowledge the use of ionogels as a promising strategy. Gel polymer electrolytes offer a wide variety of alternatives that can boost the performance of NABs and enable the transition towards more environmentally friendly solutions than currently used solvents.

At CIC energiGUNE, within metal-air research line, we first investigated currently used electrolytes in order to understand the critical parameters of NAB electrolytes, in collaboration with Oak Ridge National Laboratory. Afterward, we have developed novel hybrid electrolyte formulations based on ionic liquids with outstanding properties, in collaboration with Deakin University. Computational efforts have been key for the general understanding of the properties (solvation, coordination, etc.) in which we have worked in collaboration with the Modelling and Computational Simulation group at CIC energiGUNE. Now, the challenge we are facing is to combine the excellent properties of these hybrid electrolytes into a gel matrix to develop solid state NABs.

NAB are still in their early stages of development. However, great progress has already been made in the development of electrodes. It is now where we need to take a step forward and investigate solid electrolytes that allow to overcome the present challenges.