Highly Durable and Stable Sodium Superoxide in Concentrated Electrolytes for Sodium–Oxygen Batteries

Abstract

great attention because they exhibit the highest theoretical energy density (≈1100 Wh kg −1 based on NaO 2 as a discharge product) among available sodium rechargeable battery chemistries while also offering the advantages of elemental earth abundance and potential cost efficiency. [1] In particular, their intrinsically high energy efficiency and reversibility make sodium-oxygen batteries strong candidates for next-generation rechargeable batteries. [1,2] Unlike lithium-oxygen batteries, for which a peroxide phase is commonly formed as the main discharge product rather than a superoxide phase, [3] the thermodynamic stability of sodium superoxide and its facile dissociation enable its reversible formation as a main product in sodium-oxygen batteries. [1,2,[4][5][6][7] The promising electrochemical properties of sodium-oxygen batteries have been attributed to the formation of sodium superoxide, which exhibits substantial solubility in electrolytes and thus drives solution-based reactions as a dominant pathway. The finding of a proton as a phase-transfer catalyst [8] and the critical role of electrolyte solvation in the discharge process [9] also strongly support the dominant solution reaction mechanisms in sodium-oxygen batteries, which have been confirmed by experimental evidence of soluble superoxide species [7,10,11] and the low solvation energy of the sodium superoxide estimated in a previous computational study. [12] These intrinsic solution chemistries enable the achievement of high capacity and low polarization for sodium-oxygen batteries without the need for redox mediators or soluble catalysts, which are believed to be necessary for the development of highly reversible lithiumoxygen [13][14][15][16][17][18][19] or lithium-gas (SO 2 , CO 2 …) [20][21][22] batteries.Despite the thermodynamic stability of sodium superoxide in sodium-oxygen batteries, recent studies have reported the chemical instability of sodium superoxide in ether-based electrolytes, in which undesirable self-decomposition occurs. In our previous study, time-resolved characterizations revealed that sodium superoxide transforms into byproducts, mainly sodium peroxide dihydrate (Na 2 O 2 ·2H 2 O), within 12 h of exposure to diethylene glycol dimethyl ether (DEGDME) electrolytes; we proposed the idea that the side reactions were triggered by spontaneous dissolution of sodium superoxide and subsequent Rechargeable sodium-oxygen batteries have attracted considerable interest as promising candidates for next-generation batteries owing to their large energy density, high energy efficiency, and potential cost-effectiveness. The intrinsic stability of the discharge product, sodium superoxide, results in highly reversible solution-based electrochemistry in sodium-oxygen batteries, leading to their higher energy efficiency and reversibility compared with their lithium counterparts. However, recent studies have shown that extended storage of sodium superoxide in ethereal electrolytes induces dissolution of the sodium superoxide and undesirable chemical ...

Strategies Toward Stable Nonaqueous Alkali Metal–O2 Batteries

“…As is well known, the main discharge product NaO 2 in ether‐based electrolyte undergoes disproportionation during storage, resulting in significant cell degradation. It was ascribed to the dissolution and ionization of NaO 2 into the electrolyte as highlighted by Kim et al7c To suppress the dissolution related deterioration, Kang et al applied concentrated NaClO 4 /diethylene glycol dimethyl ether (DEGDME) in Na–O 2 batteries . Benefit from the decreased free solvent molecules, the dissolution of NaO 2 was greatly suppressed, and its lifetime was significantly prolonged to 5 d (<2 d in dilute electrolyte), as shown in Figure d,f.…”

“…f) Cycling performance of the Na–O 2 cells in 3 m concentrated electrolyte with and without aging. Reproduced with permission . Copyright 2018, Wiley.…”

Phosphorous and Nitrogen Dual-Doped Carbon as a Highly Efficient Electrocatalyst for Sodium-Oxygen Batteries

“…36,37,38,39 Therefore, it is important to achieve an in-depth understanding of the in uence of electrocatalysts on the composition of discharge products and to prevent the transformation of NaO 2 , which is a prerequisite for developing e cient catalysts with high-performance and long-life cycling stability. 40,41,42 Owing to the tunability of their structure and surface properties, carbonaceous based materials have been extensively studied as e cient catalysts for various catalytic reactions. 43,44,45,46,47 The catalytic performance of carbon based materials could also be improved through doping with different elements, such as phosphorus, sulfur, or boron.…”

Aprotic Alkali Metal–O2 Batteries: Role of Cathode Surface-Mediated Processes and Heterogeneous Electrocatalysis

“…44,45 This approach has been extended to Na−O 2 batteries by increasing the salt concentration (Na + ion concentration) in the electrolyte to lower the rate of disproportionation of the discharge product. 41,46,47 However, it is also worth noting that the increase in the salt concentration results in higher viscosity, causing lower (i) ionic conductivity and (ii) oxygen solubility in the electrolyte. 48 In the first case, lower ionic conductivity leads to compromised discharge capacities, while the latter results in a surface-mediated discharge product growth mechanism.…”

“…These studies suggest that the solvent in the electrolyte played a critical role in the chemical disproportionation of NaO 2 , consequently resulting in an increase in overpotential losses . Similar conclusions were drawn by other studies. − This highlights the fact that the intrinsic stability of NaO 2 is not the root cause of the observed chemical disproportionation to Na 2 O 2 ·2H 2 O; rather, its surrounding environment plays a more significant role.…”