Controlling the Three‐Phase Boundary in Na–Oxygen Batteries: The Synergy of Carbon Nanofibers and Ionic Liquid

Abstract

A series of electrospun binder‐free carbon nanofiber (CNF) mats have been studied as air cathodes for Na–oxygen batteries using a pyrrolidinium‐based electrolyte and compared with the commercial air cathode Toray 090. A tenfold increase in the discharge capacity is attained when using CNFs in comparison with Toray 090, affording a discharge capacity of 1.53 mAh cm−2 at a high discharge rate of 0.63 mA cm−2. The good specific discharge and charge capacities of these CNFs are determined by the void space and the highly accessible surface of the carbon fiber. Furthermore, a threefold increase has been attained in terms of specific capacity by controlling the flooding of the air cathode and hence the location of the three‐phase boundary within the CNF mat. The enhancement in performance has been correlated to the morphology, composition, distribution, and location of the discharge products. Sodium superoxide and peroxide were identified as the discharge products and, more importantly, the common side reaction discharge products, which are known to be detrimental to battery performance (including sodium fluoride, sodium hydroxide, and formate), were not observed, exemplifying the stability of the pyrrolidinium‐based electrolyte and these binder‐free CNF air cathodes.

Highly Homogeneous Sodium Superoxide Growth in Na–O2 Batteries Enabled by a Hybrid Electrolyte

“…This specific ionic liquid was selected because of previous research on both stable sodium plating and stripping in Na−O 2 batteries. 9,11,12 This improvement was related to the formation of a three-dimensional morphology of the discharge product on the air cathode, as opposed to a film-like deposit, governed by a weak interaction of the electrolyte with the discharge species. The correlation between discharge product morphology and battery performance has also been reported in glymebased electrolytes.…”

Unveiling the Role of Tetrabutylammonium and Cesium Bulky Cations in Enhancing Na‐O2 Battery Performance

“…Higher current densities lead to more filmlike growth dominated by the surface-mediated mechanism, while low current densities lead to a solution-mediated type mechanism. [8] In addition, materials parameters such as the surface properties of the oxygen electrode, [9][10][11] the nature of solvent, [18,[12][13][14] and salt [15,16] which formed the electrolyte, the salt concentration in the electrolyte [17] or the use of certain additives [18,19] can also modify the dominant mechanism of the ORR. The presence of traces of water in the electrolyte can also alter the mechanism of formation and growth of the discharge products.…”

Functional Binders Based on Polymeric Ionic Liquids for Sodium Oxygen Batteries Using Ionic Liquid Electrolytes

“…The discharge products included NaO 2 and irreversible products formed during the ORR which resulted in low Coulombic efficiency (CE), determined by the ratio charge capacity/discharge capacity. The CE 35,36 value only reflects the measured charge and discharge magnitudes within the voltage limits; the true efficiency of the O 2 /O 2…”

“…The discharge products included NaO 2 and irreversible products formed during the ORR which resulted in low Coulombic efficiency (CE), determined by the ratio charge capacity/discharge capacity. The CE , value only reflects the measured charge and discharge magnitudes within the voltage limits; the true efficiency of the O 2 /O 2 •– reaction is hidden by irreversible processes including electrolyte and electrode degradation as well as the inherent problems of deposit growth/disconnection and electrode passivation. These processes all contributed to poor extended cycling performance of these cells.…”