Why the Shift from Lithium-ion?

Traditionally, lithium-ion batteries have been the workhorse of energy storage, meeting society´s promising energy demands. Unfortunately, they cannot achieve the required transformative scale of improvement required because of the: (i) limited geographical supply of Lithium (thus increase in cost), (ii) scarcity of cobalt and (iii) limited energy storage capacity.

Their widespread adoption faces significant hurdles, dampening their potential for large-scale implementation. Herein enters the attraction of Sodium-air/O₂ batteries.

Unveiling the Potential of Na-air/O₂ batteries

Na-air/O₂ batteries offer a paradigm shift in energy storage dynamics. Na-air/O₂ batteries directly address the issues pointed above due to: (i) the large abundance of sodium (Na) and its low cost (~30 times cheaper than their Lithium counterparts), (ii) the lack of requirement for cobalt, and (iii) their high theoretical energy density as a result of using a lightweight air cathode.

Na-air/O₂ batteries present ca. 5–10 times higher theoretical capacities than that of Li-ion and Na-ion batteries (1,605/1,108 vs 100–265 and ~150 Wh kg^-1, respectively) since the active material is taken from the surrounding environment (O₂).

Operating Principles and mechanisms of Na-air/O₂ batteries

A Na-air/O₂ battery comprises three key elements: a sodium metal anode, a porous air cathode, and an electrolyte acting as a separator between them. The functionality of superoxide-based Na-O₂ batteries primarily follows equations (I) and (II):

Na⁺ + O₂ + e⁻ ↔ NaO₂ (Eº = 2.27 V) (I)

2Na⁺ + O₂ + 2e⁻ ↔ Na₂O₂ (Eº = 2.33 V) (II)

The initiation of the oxygen reduction reaction (ORR) typically involves a one-electron reduction process, yielding superoxide anions (O₂^-) that react with sodium metal cations (Na⁺) from the anode oxidation reaction to form the discharge product (NaO₂). During discharge, the sodium metal anode oxidizes, releasing Na⁺ ions, while the cathode facilitates the ORR using available oxygen at the triple phase boundaries (air electrode/O₂/electrolyte).

Two main mechanisms, the solution-mediated and surface-mediated mechanism, are proposed for NaO₂ discharge product formation. Additionally, the selection of electrolyte is widely recognized as critical, as the shape, size, and formation mechanism of NaO₂ discharge product strongly rely on the physicochemical properties of the electrolyte.

Addressing Challenges Head-On

The challenges facing Na-air/O₂ batteries are remarkable but not impossible. Issues such as liquid electrolyte instability, O₂/O₂^- crossover, sodium anode passivation, and dendritic growth have been identified and are actively being 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 being pursued as potential solutions.

The Role of OXBLOLYTE

Herein lies the significance of the OXBLOLYTE project, spearheaded by CIC energiGUNE. This European Postdoctoral Fellowship endeavors to revolutionize NaBs through the development of solid-state electrolytes. Dr. Mohamed Yahia, an expert in polymer membranes, and Dr. Nagore Ortiz-Vitoriano, a specialist and a research line manager in metal-air batteries, will lead the OXBLOLYTE project aiming to mitigate oxygen crossover and enhance battery stability.

The primary objective of OXBLOLYTE is to identify and demonstrate a solid membrane electrolyte, a pivotal step towards stable and high-performing solid-state Na-air/O₂ batteries. By unraveling the mechanistic understanding of solid-state Na-air/O₂ battery systems, the project ventures into uncharted territories, pushing the boundaries of energy storage research.

Navigating Future Challenges

While progress is being made, future challenges in advancing Na-air/O₂ battery technology loom large. Fundamental mechanisms such as discharge product chemistry, dendritic growth, O₂ crossover, electrolyte properties, and battery chemistry stabilization must be thoroughly understood and addressed. 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.

As we stand at the limit of a renewable energy revolution, the significance of energy storage cannot be overstated. Na-air/O₂ batteries, with their potential to transcend the limitations of traditional lithium-ion batteries, offer a glimpse into a future powered by sustainable energy solutions. Through pioneering endeavors like OXBLOLYTE, we inch closer towards realizing this vision, lighting the path forward towards a greener, more sustainable tomorrow.