Designing High-Donicity Anions for Rechargeable Potassium Superoxide/Peroxide Batteries.

A battery cathode based on the superoxide/peroxide redox not only inherits the advantage of oxygen (O2 ) batteries in high capacities and low costs but also overcomes the disadvantages in O2 storage, electrolyte evaporation, and anode deactivation due to O2 crossover. Herein, we report an enhanced potassium superoxide (KO2 )/peroxide (K2 O2 ) conversion by adopting a high-donicity anion additive in the ether-based electrolyte. Such an anion was synthesized via a "Solvent-in-Anion" strategy and validated to enhance the electron donicity of the electrolyte. The use of high-donicity anion could lead to enhanced KO2 utilization (≈90.2 %) by retarding electrode passivation and allow the full charging back of K2 O2 through the solution-mediated pathway without electrocatalysts. No apparent cell degradation is observed during the first 120 cycles by controlling the reversible depth-of-discharge capacity at 292 mAh g-1 KO 2 ${{_{{\rm KO}{_{2}}}}}$ within an O2 -free region. The K-KO2 cell delivers a high energy efficiency (>84.4 %) and a lifespan of over 1440 hours.

Unusual Melting Trend in an Alkali Asymmetric Sulfonamide Salt Series: Single-Crystal Analysis and Modeling.

Developing low-melting alkali salts is of interest for both battery electrolytes and inorganic ionic liquids. In this study, we report a series of asymmetric alkali-metal sulfonamide salts based upon the (3-methoxypropyl)((trifluoromethyl)sulfonyl)amide (MPSA) anion. This family of salts features an unusual melting point trend, where the melting point of the salts decreases as the cation increases in size from Li to K but then the melting point increases as the cation further increases in size from K to Cs. Analyses of single crystals reveal that the unusual higher melting points of RbMPSA and CsMPSA in comparison to KMPSA can be attributed to the greater cation-cation distances as well as the increased rigidity of anion-cation coordination due to an increase in cyclic structures in comparison to KMPSA. Exceptionally, KMPSA features a very low melting point of only 50.79 ± 0.31 °C. This low melting point can be attributed to a relatively high degree of disorder, an unusual uncoordinated ether moiety, and a very short K-K distance of only 3.4348(7) Å among other factors, which is supported by the low cohesive energy and small elastic moduli among the rest according to density functional theory (DFT) calculations. The low melting point of KMPSA makes it interesting for low-temperature ionic liquids.

Superoxide-Based K-O2 Batteries: Highly Reversible Oxygen Redox Solves Challenges in Air Electrodes.

In the past 20 years, research in metal-O2 batteries has been one of the most exciting interdisciplinary fields of electrochemistry, energy storage, materials chemistry, and surface science. The mechanisms of oxygen reduction and evolution play a key role in understanding and controlling these batteries. With intensive efforts from many prominent research groups, it becomes clear that the instability of superoxide in the presence of Li ions (Li+) and Na ions (Na+) is the fundamental root cause for the poor stability, reversibility, and energy efficiency in aprotic Li-O2 and Na-O2 batteries. Stabilizing superoxide with large K ions (K+) provides a simple but elegant solution. Superoxide-based K-O2 batteries, invented in 2013, adopt the one-electron redox process of O2/potassium superoxide (KO2). Despite being the youngest metal-O2 technology, K-O2 is the most promising rechargeable metal-air battery with the combined advantages of low costs, high energy efficiencies, abundant elements, and good energy densities. However, the development of the K-O2 battery has been overshadowed by Li-O2 and Na-O2 batteries because one might think K-O2 is just an analogous extension. Moreover, due to the lower specific energy and the high reactivity of K metal, K-O2 is often underestimated and deemed unsuitable for practical applications. The objective of this Perspective is to highlight the unique advantages of K-O2 chemistry and to clarify the misconceptions prompted by the name "superoxide" and the judgment bias based on the claimed theoretical specific energies. We will also discuss the current challenges and our perspectives on how to overcome them.

Alkynyl-Based Covalent Organic Frameworks as High-Performance Anode Materials for Potassium-Ion Batteries.

The development of high-performance organic electrodes for potassium-ion batteries (KIBs) is attracting interest due to their sustainability and low costs. However, the electrolyte systems and moieties that generally proved to be successful in high-performance Li-ion batteries have found relatively little success in KIBs. Herein, two alkynyl-based covalent organic frameworks (COFs) containing 1,3,5-tris(arylethynyl)benzene (TAEB) and dehydrobenzoannulene (DBA) units are utilized as bulk anode materials for KIBs in a localized high-concentration electrolyte. TAEB-COF provides a high capacity value of 254.0 mAh g-1 at ∼100% efficiency after 300 cycles, and DBA-COF 3 provides a capacity of 76.3 mAh g-1 with 98.7% efficiency after 300 cycles. DFT calculations suggest that the alkynyl units of TAEB-COF facilitate the binding of K-ions through both enthalpic and geometric driving forces, leading to high reversible capacities.

Anchoring an Artificial Protective Layer To Stabilize Potassium Metal Anode in Rechargeable K-O2 Batteries.

Rechargeable potassium batteries, including the potassium-oxygen (K-O2) battery, are deemed as promising low-cost energy storage solutions. Nevertheless, the chemical stability of the K metal anode remains problematic and hinders their development. In the K-O2 battery, the electrolyte and dissolved oxygen tend to be reduced on the K metal anode, which consumes the active material continuously. Herein, an artificial protective layer is engineered on the K metal anode via a one-step method to mitigate side reactions induced by the solvent and reactive oxygen species. The chemical reaction between K and SbF3 leads to an inorganic composite layer that consists of KF, Sb, and KSb xF y on the surface. This in situ synthesized layer effectively prevents K anode corrosion while maintaining good K+ ionic conductivity in K-O2 batteries. Protection from O2 and moisture also ensures battery safety. Improved anode life span and cycling performance (>30 days) are further demonstrated. This work introduces a novel strategy to stabilize the K anode for rechargeable potassium metal batteries.