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.

Potassium Superoxide: A Unique Alternative for Metal-Air Batteries.

Lithium-oxygen (Li-O2) batteries have been envisaged and pursued as the long-term successor to Li-ion batteries, due to the highest theoretical energy density among all known battery chemistries. However, their practical application is hindered by low energy efficiency, sluggish kinetics, and a reliance on catalysts for the oxygen reduction and evolution reactions (ORR/OER). In a superoxide battery, oxygen is also used as the cathodic active medium but is reduced only to superoxide (O2•-), the anion formed by adding an electron to a diatomic oxygen molecule. Therefore, O2/O2•- is a unique single-electron ORR/OER process. Since the introduction of K-O2 batteries by our group in 2013, superoxide batteries based on potassium superoxide (KO2) have attracted increasing interest as promising energy storage devices due to their significantly lower overpotentials and costs. We have selected potassium for building the superoxide battery because it is the lightest alkali metal cation to form the thermodynamically stable superoxide (KO2) product. This allows the battery to operate through the proposed facile one-electron redox process of O2/KO2. This strategy provides an elegant solution to the long-lasting kinetic challenge of ORR/OER in metal-oxygen batteries without using any electrocatalysts. Over the past five years, we have been focused on understanding the electrolyte chemistry, especially at the electrode/electrolyte interphase, and the electrolyte's stability in the presence of potassium metal and superoxide. In this Account, we examine our advances and understanding of the chemistry in superoxide batteries, with an emphasis on our systematic investigation of K-O2 batteries. We first introduce the K metal anode electrochemistry and its corrosion induced by electrolyte decomposition and oxygen crossover. Tuning the electrolyte composition to form a stable solid electrolyte interphase (SEI) is demonstrated to alleviate electrolyte decomposition and O2 cross-talk. We also analyze the nucleation and growth of KO2 in the oxygen electrode, as well its long-term stability. The electrochemical growth of KO2 on the cathode is correlated with the rate performance and capacity. Increasing the surface area and reducing the O2 diffusion pathway are identified as critical strategies to improve the rate performance and capacity. Li-O2 and Na-O2 batteries are further compared with the K-O2 chemistry regarding their pros and cons. Because only KO2 is thermodynamically stable at room temperature, K-O2 batteries offer reversible cathode reactions over the long-term while the counterparts undergo disproportionation. The parasitic reactions due to the reactivity of superoxide are discussed. With the trace side products quantified, the overall superoxide electrochemistry is highly reversible with an extended shelf life. Lastly, potential anode substitutes for K-O2 batteries are reviewed, including the K3Sb alloy and liquid Na-K alloy. We conclude with perspectives on the future development of the K metal anode interface, as well as the electrolyte and cathode materials to enable improved reversibility and maximized power capability. We hope this Account promotes further endeavors into the development of the K-O2 chemistry and related material technologies for superoxide battery research.

Reversible Dendrite-Free Potassium Plating and Stripping Electrochemistry for Potassium Secondary Batteries.

Rechargeable potassium metal batteries have recently emerged as alternative energy storage devices beyond lithium-ion batteries. However, potassium metal anodes suffer from poor reversibility during plating and stripping processes due to their high reactivity and unstable solid electrolyte interphase (SEI). Herein, it is reported for the first time that a potassium bis(fluoroslufonyl)imide (KFSI)-dimethoxyethane (DME) electrolyte forms a uniform SEI on the surface of potassium enabling reversible potassium plating/stripping electrochemistry with high efficiency (∼99%) at ambient temperature. Furthermore, the superconcentrated KFSI-DME electrolyte shows excellent electrochemical stability up to 5 V (vs K/K+) which enables good compatibility with high-voltage cathodes. Full cells with potassium Prussian blue cathodes are demonstrated. Our work contributes toward the understanding of potassium plating/stripping electrochemistry and paves the way for the development of potassium metal battery technologies.

Probing Mechanisms for Inverse Correlation between Rate Performance and Capacity in K-O2 Batteries.

Owing to the formation of potassium superoxide (K+ + O2 + e- = KO2), K-O2 batteries exhibit superior round-trip efficiency and considerable energy density in the absence of any electrocatalysts. For further improving the practical performance of K-O2 batteries, it is important to carry out a systematic study on parameters that control rate performance and capacity to comprehensively understand the limiting factors in superoxide-based metal-oxygen batteries. Herein, we investigate the influence of current density and oxygen diffusion on the nucleation, growth, and distribution of potassium superoxide (KO2) during the discharge process. It is observed that higher current results in smaller average sizes of KO2 crystals but a larger surface coverage on the carbon fiber electrode. As KO2 grows and covers the cathode surface, the discharge will eventually end due to depletion of the oxygen-approachable electrode surface. Additionally, higher current also induces a greater gradient of oxygen concentration in the porous carbon electrode, resulting in less efficient loading of the discharge product. These two factors explain the observed inverse correlation between current and capacity of K-O2 batteries. Lastly, we demonstrate a reduced graphene oxide-based K-O2 battery with a large specific capacity (up to 8400 mAh/gcarbon at a discharge rate of 1000 mA/gcarbon) and a long cycle life (over 200 cycles).

Concentrated Electrolyte for the Sodium-Oxygen Battery: Solvation Structure and Improved Cycle Life.

Alkali metal-oxygen batteries are of great interests for energy storage because of their unparalleled theoretical energy densities. Particularly attractive is the emerging Na-O2 battery because of the formation of superoxide as the discharge product. Dimethyl sulfoxide (DMSO) is a promising solvent for this battery but its instability towards Na makes it impractical in the Na-O2 battery. Herein we report the enhanced stability of Na in DMSO solutions containing concentrated sodium trifluoromethanesulfonimide (NaTFSI) salts (>3 mol kg-1 ). Raman spectra of NaTFSI/DMSO electrolytes and ab initio molecular dynamics simulation reveal the Na+ solvation number in DMSO and the formation of Na(DMSO)3 (TFSI)-like solvation structure. The majority of DMSO molecules solvating Na+ in concentrated solutions reduces the available free DMSO molecules that can react with Na and renders the TFSI anion decomposition, which protects Na from reacting with the electrolyte. Using these concentrated electrolytes, Na-O2 batteries can be cycled forming sodium superoxide (NaO2 ) as the sole discharge product with improved long cycle life, highlighting the beneficial role of concentrated electrolytes for Na-based batteries.

Potassium-Ion Oxygen Battery Based on a High Capacity Antimony Anode.

Recent investigations into the application of potassium in the form of potassium-oxygen, potassium-sulfur, and potassium-ion batteries represent a new approach to moving beyond current lithium-ion technology. Herein, we report on a high capacity anode material for use in potassium-oxygen and potassium-ion batteries. An antimony-based electrode exhibits a reversible storage capacity of 650 mAh/g (98% of theoretical capacity, 660 mAh/g) corresponding to the formation of a cubic K3Sb alloy. The Sb electrode can cycle for over 50 cycles at a capacity of 250 mAh/g, which is one of the highest reported capacities for a potassium-ion anode material. X-ray diffraction and galvanostatic techniques were used to study the alloy structure and cycling performance, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy were used to provide insight into the thermodynamics and kinetics of the K-Sb alloying reaction. Finally, we explore the application of this anode material in the form of a K3Sb-O2 cell which displays relatively high operating voltages, low overpotentials, increased safety, and interfacial stability, effectively demonstrating its applicability to the field of metal oxygen batteries.

Aqueous Lithium-Iodine Solar Flow Battery for the Simultaneous Conversion and Storage of Solar Energy.

Integrating both photoelectric-conversion and energy-storage functions into one device allows for the more efficient solar energy usage. Here we demonstrate the concept of an aqueous lithium-iodine (Li-I) solar flow battery (SFB) by incorporation of a built-in dye-sensitized TiO2 photoelectrode in a Li-I redox flow battery via linkage of an I3(-)/I(-) based catholyte, for the simultaneous conversion and storage of solar energy. During the photoassisted charging process, I(-) ions are photoelectrochemically oxidized to I3(-), harvesting solar energy and storing it as chemical energy. The Li-I SFB can be charged at a voltage of 2.90 V under 1 sun AM 1.5 illumination, which is lower than its discharging voltage of 3.30 V. The charging voltage reduction translates to energy savings of close to 20% compared to conventional Li-I batteries. This concept also serves as a guiding design that can be extended to other metal-redox flow battery systems.