Unveiling the Role of Tetrabutylammonium and Cesium Bulky Cations in Enhancing Na-O-2 Battery Performance

The use of two types of bulky cations, tetrabutylammonium (TBA(+)) and Cs+, as electrolyte additives in Na-O-2 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-O-2 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.