Feasibility of achieving two-electron K-O2 batteries

A deep understanding of the oxygen (O2) reduction and evolution mechanisms is crucial for understanding metal-O2 batteries. It has become evident that the instability of superoxide in the presence of lithium (Li) ions and sodium (Na) ions is the root cause for the poor reversibility and energy efficiency of Li-O2 and Na-O2 batteries. A straightforward yet elegant method is stabilizing superoxide with the larger potassium (K) ions. Superoxide-based K-O2 batteries, invented by our group in 2013, are operated based on one-electron redox of O2/potassium superoxide (KO2) and have high energy efficiencies without any electrocatalysts. Nevertheless, limiting the anionic redox to O2/superoxide affects the capacity output. Therefore, it is attractive to explore the possibility of beyond KO2 in the K-O2 batteries, especially if the use of catalysts can still be avoided. In this research, solid KO2 was used as the condensed O2 source and pre-dissolved in the dimethyl sulfoxide (DMSO)-based electrolyte. It is encouraging to observe two sets of reversible peaks during the three-electrode cyclic voltammetry scan under an argon atmosphere. One pair of peaks is attributed to the KO2/potassium peroxide (K2O2) interconversion. Such redox has superb reversibility and a small overpotential of 239 mV in the absence of explicit electrocatalysts. Notably, it is further revealed that K2O2 reacts with gaseous O2. Therefore, a gas-open system with an O2 supply is unfavorable for realizing the reversible KO2/K2O2 redox, and a closed cell system with a KO2 supply as the starting active material is suggested instead.

Design and Synthesis of Cubic K3-2 x Bax SbSe4 Solid Electrolytes for K-O2 Batteries

Developing K-ion conducting solid-state electrolytes (SSEs) plays a critical role in the safe implementation of potassium batteries. In this work, a chalcogenide-based potassium ion SSE is reported, K3 SbSe4 , which adopts a trigonal structure at room temperature. Single-crystal structural analysis reveals a trigonal-to-cubic phase transition at the low temperature of 50 °C, which is the lowest among similar compounds and thus provides easy access to the cubic phase. The substitution of barium for potassium in K3 SbSe4 leads to the creation of potassium vacancies, expansion of lattice parameters, and a transformation from a trigonal phase to a cubic phase. As a result, the maximum conductivity of K3-2 x Bax SbSe4 reaches around 0.1 mS cm-1 at 40 °C for K2.2 Ba0.4 SbSe4 , which is over two orders of magnitude higher than that of undoped K3 SbSe4 . This novel SSE is successfully employed in a K-O2 battery operating at room temperature where a polymer-laminated K2.2 Ba0.4 SbSe4 pellet serves as a separator between the oxygen cathode and the potassium metal anode. Effective protection of the K metal anode against corrosion caused by O2 is demonstrated.

Hybrid-Solvent Electrolytes for Enhanced Potassium-Oxygen Battery Performance

Rechargeable potassium-oxygen batteries (KOB) are promising next-generation energy storage devices because of the highly reversible O₂/O₂^- redox reactions during battery charge and discharge. However, the complicated cathode reaction processes seriously jeopardize the battery reaction kinetics and discharge capacity. Herein, we propose a hybrid-solvent strategy to effectively tune the K⁺ solvation structure, which demonstrates a critical influence on the charge-transfer kinetics and cathode reaction mechanism. The cosolvation of K⁺ by 1,2-dimethoxyethane (DME) and dimethyl sulfoxide (DMSO) could greatly decrease overpotentials for the cathode processes and increase the cathode discharge capacity. Furthermore, the Coulombic efficiency for the cathode could be significantly improved with the enhanced solution-mediated KO₂ growth and stripping during cycling. This work provides a promising electrolyte design approach to improve the electrochemical performance of the KOB.

Insight into a Nitrogen-Doping Mechanism in a Hard-Carbon-Microsphere Anode Material for the Long-Term Cycling of Potassium-Ion Batteries

To investigate the alternatives to lithium-ion batteries, potassium-ion batteries have attracted considerable interest due to the cost-efficiency of potassium resources and the relatively lower standard redox potential of K⁺/K. Among various alternative anode materials, hard carbon has the advantages of extensive resources, low cost, and environmental protection. In the present study, we synthesize a nitrogen-doping hard-carbon-microsphere (N-SHC) material as an anode for potassium-ion batteries. N-SHC delivers a high reversible capacity of 248 mAh g⁻¹ and a promoted rate performance (93 mAh g⁻¹ at 2 A g⁻¹). Additionally, the nitrogen-doping N-SHC material also exhibits superior cycling long-term stability, where the N-SHC electrode maintains a high reversible capacity at 200 mAh g⁻¹ with a capacity retention of 81% after 600 cycles. DFT calculations assess the change in K ions’ absorption energy and diffusion barriers at different N-doping effects. Compared with an original hard-carbon material, pyridinic-N and pyrrolic-N defects introduced by N-doping display a positive effect on both K ions’ absorption and diffusion.

Unraveling the Reaction Interfaces and Intermediates of Ru-Catalyzed LiOH Decomposition in DMSO-Based Li-O2 Batteries

Investigation of LiOH decomposition in nonaqueous electrolytes not only expands the fundamental understanding of four-electron oxygen evolution reactions in aprotic media but also is crucial to the development of high-performance lithium-air batteries involving the formation/decomposition of LiOH. In this work, we have shown that the decomposition of LiOH by ruthenium metal catalysts in a wet DMSO electrolyte occurs at the catalyst-electrolyte interface, initiated via a potential-triggered dissolution/reprecipitation process. The in situ UV-vis methodology devised herein provides direct experimental evidence that the hydroxyl radical is a common reaction intermediate formed in several nonaqueous electrolytes; this method is applicable to study other battery systems. Our results highlight that the reactivity of the hydroxyl radical toward nonaqueous electrolyte represents a major factor limiting O₂ evolution during LiOH decomposition. Coupling catalysts restraining hydroxyl reactivity with electrolytes more resistant to hydroxyl radical attack could help improve the reversibility of this reaction.

Versatile Redox-Active Organic Materials for Rechargeable Energy Storage

With the ever-increasing demand on energy storage systems and subsequent mass production, there is an urgent need for the development of batteries with not only improved electrochemical performance but also better sustainability-related features such as environmental friendliness and low production cost. To date, transition metals that are sparse have been centrally employed in energy storage devices ranging from portable lithium ion batteries (e.g., cobalt and nickel) to large-scale redox flow batteries (e.g., vanadium). Toward the sustainable battery chemistry, there are ongoing efforts to replace the transition metal-based electrode materials in these systems to redox-active organic materials (ROMs). Most ROMs are composed of the earth abundant elements (e.g., carbon, nitrogen, oxygen, sulfur), thus are less restrained by the resource, and their production does not require high-energy consuming processes. Furthermore, the structural diversity and chemical tunability of organic compounds make them more attractive for the versatile design of future energy storage systems. Accordingly, the timely development of high-performance ROM-based electrodes would expedite the shift from the current resource-limited battery chemistry to more sustainable energy solutions.In this Account, we provide an overview of the endeavors to employ and develop ROMs as high-performance active materials for various battery systems. Diverse approaches will be introduced starting from the new ROM design mimicking the energy carrying molecules in biological metabolism to the chemical modifications to tailor the properties for specific battery systems. The molecular redesign of ROM, for example, can be carried out by substituting heteroatoms in the redox center, which leads to the enhancement of the redox potential by the inductive effect. Or, tailoring the ROM molecule by removing redox-inactive functionals results in a reduced molecular weight, thereby an increased specific capacity. The intrinsic limitations of ROMs, such as the low electrical conductivity and the dissolving nature, have been under extensive scrutiny; however, they can be partly addressed through efforts including intermolecular fusion and/or nanoscale hybridization with a conducting scaffold. On the other hand, this problematic dissolving nature of ROMs makes them appealing for some new battery configurations such as redox flow batteries that employ the liquid-state active materials. The high solubility and the stability of the ROM were found to be beneficial in attaining the enhanced energy density and the cycle stability of flow batteries, which could be further optimized by the chemical modifications of ROMs. Besides the role of active materials, the redox activity of ROMs has also enabled their use as catalysts to promote the electrode reaction in metal-air batteries. The redox capability of the ROM was often proven to be effective in the solution-based redox mediation that facilitates both the charging and discharging reaction in metal-air batteries. Finally, we conclude this account by proposing the future research directions regarding the fundamental electrochemistry and the further practical development of ROMs for the sustainable rechargeable energy storage.

A high-capacity cathode for rechargeable K-metal battery based on reversible superoxide-peroxide conversion

As a promising low-cost energy storage device, the development of a rechargeable potassium-ion battery (KIB) is severely hindered by the limited capacity of cathode candidates. Regarded as an attractive capacity-boosting strategy, triggering the O-related anionic redox activity has not been achieved within a sealed KIB system. Herein, in contrast to the typical gaseous open K-O₂ battery (O₂/KO₂ redox), we originally realize the reversible superoxide/peroxide (KO₂/K₂O₂) interconversion on a KO₂-based cathode. Controlled within a sealed cell environment, the irreversible O₂ evolution and electrolyte decomposition (induced by superoxide anion (O₂⁻) formation) are effectively restrained. Rationally controlling the reversible depth-of-charge at 300 mAh/g (based on the mass of KO₂), no obvious cell degradation can be observed during 900 cycles. Moreover, benefitting from electrolyte modification, the KO₂-based cathode is coupled with a limited amount of K-metal anode (merely 2.5 times excess), harvesting a K-metal full-cell with high energy efficiency (∼90%) and long-term cycling stability (over 300 cycles).

Recent Progress on Two-Dimensional Carbon Materials for Emerging Post-Lithium (Na+, K+, Zn2+) Hybrid Supercapacitors

The hybrid ion capacitor (HIC) is a hybrid electrochemical energy storage device that combines the intercalation mechanism of a lithium-ion battery anode with the double-layer mechanism of the cathode. Thus, an HIC combines the high energy density of batteries and the high power density of supercapacitors, thus bridging the gap between batteries and supercapacitors. Two-dimensional (2D) carbon materials (graphite, graphene, carbon nanosheets) are promising candidates for hybrid capacitors owing to their unique physical and chemical properties, including their enormous specific surface areas, abundance of active sites (surface and functional groups), and large interlayer spacing. So far, there has been no review focusing on the 2D carbon-based materials for the emerging post-lithium hybrid capacitors. This concept review considers the role of 2D carbon in hybrid capacitors and the recent progress in the application of 2D carbon materials for post-Li (Na+, K+, Zn2+) hybrid capacitors. Moreover, their challenges and trends in their future development are discussed.

Electrolytes and Interphases in Potassium Ion Batteries

Potassium ion batteries (PIBs) are recognized as one promising candidate for future energy storage devices due to their merits of cost-effectiveness, high-voltage, and high-power operation. Many efforts have been devoted to the development of electrode materials and the progress has been well summarized in recent review papers. However, in addition to electrode materials, electrolytes also play a key role in determining the cell performance. Here, the research progress of electrolytes in PIBs is summarized, including organic liquid electrolytes, ionic liquid electrolytes, solid-state electrolytes and aqueous electrolytes, and the engineering of the electrode/electrolyte interfaces is also thoroughly discussed. This Progress Report provides a comprehensive guidance on the design of electrolyte systems for development of high performance PIBs.

Three dimensional Ti3C2 MXene nanoribbon frameworks with uniform potassiophilic sites for the dendrite-free potassium metal anodes

Potassium (K) metal batteries hold great promise as an advanced electrochemical energy storage system because of their high theoretical capacity and cost efficiency. However, the practical application of K metal anodes has been limited by their poor cycling life caused by dendrite growth and large volume changes during the plating/stripping process. Herein, three-dimensional (3D) alkalized Ti₃C₂ (a-Ti₃C₂) MXene nanoribbon frameworks were demonstrated as advanced scaffolds for dendrite-free K metal anodes. Benefiting from the 3D interconnected porous structure for sufficient K accommodation, improved surface area for low local current density, preintercalated K in expanded interlayer spacing, and abundant functional groups as potassiophilic nuleation sites for uniform K plating/stripping, the as-formed a-Ti₃C₂ frameworks successfully suppressed the K dendrites and volume changes at both high capacity and current density. As a result, the a-Ti₃C₂ based electrodes exhibited an ultrahigh coulombic efficiency of 99.4% at a current density of 3 mA cm^-2 with long lifespan up to 300 cycles, and excellent stability for 700 h even at an ultrahigh plating capacity of 10 mA h cm^-2. When matched with K₂Ti₄O₉ cathodes, the resulting a-Ti₃C₂-K//K₂Ti₄O₉ full batteries offered a greatly enhanced rate capacity of 82.9 mA h g^-1 at 500 mA g^-1 and an excellent cycling stability with high capacity retention (77.7% after 600 cycles) at 200 mA g^-1, demonstrative of the great potential of a-Ti₃C₂ for advanced K-metal batteries.

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.

Stable, yet "naked", azo radical anion ArNNAr- and dianion ArNNAr2- (Ar = 4-CN-2,6-iPr2-C6H2) with selective CO2 activation

Azo radical anion 1˙- and dianion 12- have been isolated by one- and two-electron reduction of the azo compound 1 (ArNNAr, Ar = 4-CN-2,6-iPr2-C6H2) with alkali metals, respectively. The reduced species have been characterized by single-crystal X-ray analysis, EPR, UV and FT-IR spectroscopy, as well as SQUID measurements. The filling of one and two electrons in the π* orbital of the N-N double bond of 1 leads to a half-double N-N bond in 1˙- and a single N-N bond in 12-. The uncoordinated nature of these reduced species enables them to activate CO2. The exposure of 1˙- solution to CO2 led to the formation of oxalate anion C2O42-, while that of 12- solution to CO2 afforded the hydrazine dicarboxylate dianion [1-2CO2]2-, which reversibly dissociated back to 1 and CO2 upon oxidation.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO₂ emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O₂ cells but include Na-O₂, K-O₂, and Mg-O₂ cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O₂ reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O₂ cells.

Current Challenges and Routes Forward for Nonaqueous Lithium-Air Batteries

Nonaqueous lithium-air batteries have garnered considerable research interest over the past decade due to their extremely high theoretical energy densities and potentially low cost. Significant advances have been achieved both in the mechanistic understanding of the cell reactions and in the development of effective strategies to help realize a practical energy storage device. By drawing attention to reports published mainly within the past 8 years, this review provides an updated mechanistic picture of the lithium peroxide based cell reactions and highlights key remaining challenges, including those due to the parasitic processes occurring at the reaction product-electrolyte, product-cathode, electrolyte-cathode, and electrolyte-anode interfaces. We introduce the fundamental principles and critically evaluate the effectiveness of the different strategies that have been proposed to mitigate the various issues of this chemistry, which include the use of solid catalysts, redox mediators, solvating additives for oxygen reaction intermediates, gas separation membranes, etc. Recently established cell chemistries based on the superoxide, hydroxide, and oxide phases are also summarized and discussed.

In situ imaging of electrocatalysis in a K-O2 battery with a hollandite α-MnO2 nanowire air cathode

Using α-manganese dioxide (α-MnO2) nanowires as the air electrode, a K-O2 nanobattery is assembled in an aberration corrected environmental transmission electron microscope. It is found that the α-MnO2 nanowires are reduced into Mn3O4 and MnO during discharge; meanwhile, KO2 is formed on the surface of the α-MnO2 nanowires.

Tuning anion solvation energetics enhances potassium-oxygen battery performance

The oxygen reduction reaction (ORR) is a critical reaction in secondary batteries based on alkali metal chemistries. The nonaqueous electrolyte mediates ion and oxygen transport and determines the heterogeneous charge transfer rates by controlling the nature and degree of solvation. This study shows that the solvent reorganization energy (λ) correlates well with the oxygen diffusion coefficient [Formula: see text] and with the ORR rate constant [Formula: see text] in nonaqueous Li-, Na-, and K-O₂ cells, thereby elucidating the impact of variations in the solvation shell on the ORR. Increasing cation size (from Li⁺ to K⁺) doubled [Formula: see text], indicating an increased sensitivity of k to the choice of anion, while variations in [Formula: see text]were minimal over this cation size range. At the level of a symmetric K-O₂ cell, both the formation of solvent-separated ion pairs [K⁺-(DMSO)_n-ClO₄ ^- + (DMSO)_m-ClO₄ ^-] and the anions being unsolvated (in case of PF₆ ^-) lowered ORR activation barriers with a 200-mV lower overpotential for the PF₆ ^- and ClO₄ ^- electrolytes compared with OTf^- and TFSI^- electrolytes with partial anion solvation [predominantly K⁺-(DMSO)_n-OTf^-]. Balancing transport and kinetic requirements, KPF₆ in DMSO is identified as a promising electrolyte for K-O₂ batteries.

Dye-sensitized photocathodes for oxygen reduction: efficient H2O2 production and aprotic redox reactions

Dye-sensitized photoelectrochemical cells (DSPECs) can be used to store solar energy in the form of chemical bonds. Hydrogen peroxide (H2O2) is a versatile energy carrier and can be produced by reduction of O2 on a dye-sensitized photocathode, in which the design of dye molecules is crucial for the conversion efficiency and electrode stability. Herein, using a hydrophobic donor-double-acceptor dye (denoted as BH4) sensitized NiO photocathode, hydrogen peroxide (H2O2) can be produced efficiently by reducing O2 with current density up to 600 μA cm-2 under 1 sun conditions (Xe lamp as sunlight simulator, λ > 400 nm). The DSPECs maintain currents greater than 200 μA cm-2 at low overpotential (0.42 V vs. RHE) for 18 h with no decrease in the rate of H2O2 production in aqueous electrolyte. Moreover, the BH4 sensitized NiO photocathode was for the first time applied in an aprotic electrolyte for oxygen reduction. In the absence of a proton source, the one-electron reduction of O2 generates stable, nucleophilic superoxide radicals that can then be synthetically utilized in the attack of an available electrophile, such as benzoyl chloride. The corresponding photocurrent generated by this photoelectrosynthesis is up to 1.8 mA cm-2. Transient absorption spectroscopy also proves that there is an effective electron transfer from reduced BH4 to O2 with a rate constant of 1.8 × 106 s-1. This work exhibits superior photocurrent in both aqueous and non-aqueous systems and reveals the oxygen/superoxide redox mediator mechanism in the aprotic chemical synthesis.

DABCOnium: An Efficient and High-Voltage Stable Singlet Oxygen Quencher for Metal-O2 Cells

Singlet oxygen (1 O2 ) causes a major fraction of the parasitic chemistry during the cycling of non-aqueous alkali metal-O2 batteries and also contributes to interfacial reactivity of transition-metal oxide intercalation compounds. We introduce DABCOnium, the mono alkylated form of 1,4-diazabicyclo[2.2.2]octane (DABCO), as an efficient 1 O2 quencher with an unusually high oxidative stability of ca. 4.2 V vs. Li/Li+ . Previous quenchers are strongly Lewis basic amines with too low oxidative stability. DABCOnium is an ionic liquid, non-volatile, highly soluble in the electrolyte, stable against superoxide and peroxide, and compatible with lithium metal. The electrochemical stability covers the required range for metal-O2 batteries and greatly reduces 1 O2 related parasitic chemistry as demonstrated for the Li-O2 cell.

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.

A high-rate and long-life organic-oxygen battery

Alkali metal-oxygen batteries promise high gravimetric energy densities but suffer from low rate capability, poor cycle life and safety hazards associated with metal anodes. Here we describe a safe, high-rate and long-life oxygen battery that exploits a potassium biphenyl complex anode and a dimethylsulfoxide-mediated potassium superoxide cathode. The proposed potassium biphenyl complex-oxygen battery exhibits an unprecedented cycle life (3,000 cycles) with a superior average coulombic efficiency of more than 99.84% at a high current density of 4.0 mA cm^-2. We further reduce the redox potential of biphenyl by adding the electron-donating methyl group to the benzene ring, which successfully achieved a redox potential of 0.14 V versus K/K⁺. This demonstrates the direction and opportunities to further improve the cell voltage and energy density of the alkali-metal organic-oxygen batteries.

Use of Polarization Curves and Impedance Analyses to Optimize the "Triple-Phase Boundary" in K-O2 Batteries

K-O₂ superoxide batteries have shown great potential for energy-storage applications due to the unique single-electron redox processes in the oxygen or gas-diffusion electrode. Optimization of the 'triple-phase boundary', the region of the cathode where the O₂, electrolyte, and electrode surface are in immediate contact, is crucial for maximizing their power performance, but one that has not been explored. Herein, we demonstrate an efficient method for maximizing the power capabilities of the K-O₂ battery system by optimizing the interface using polarization and impedance analyses. At the one extreme, an electrolyte volume-deficient state decreases access to the electrochemically active surface area resulting in a limitation of the maximum power output of the K-O₂ battery, whereas an excess electrolyte volume state increases the diffusion path to the active surface area for the dissolved O₂ inducing mass-transfer limitations sooner, which results in a decrease in the current and power output. Finally, we show that the optimal electrolyte volume closely matches the void volume of the internal cell materials (separators, cathode) resulting in a maximization of the electrochemically accessible surface area while minimizing the O₂ diffusion path.

A Hybrid Na//K+-Containing Electrolyte//O2 Battery with High Rechargeability and Cycle Stability

Na-O₂ and K-O₂ batteries have attracted extensive attention in recent years. However, the parasitic reactions involving the discharge product of NaO₂ or K anode with electrolytes and the severe Na or K dendrites plague their rechargeability and cycle stability. Herein, we report a hybrid Na//K⁺-containing electrolyte//O₂ battery consisting of a Na anode, 1.0 M of potassium triflate in diglyme, and a porous carbon cathode. Upon discharging, KO₂ is preferentially produced via oxygen reduction in the cathode with Na⁺ stripped from the Na anode, and reversely, the KO₂ is electrochemically decomposed with Na⁺ plated back onto the anode. The new reaction pathway can circumvent the parasitic reactions involving instable NaO₂ and active K anode, and alternatively, the good stability and conductivity of KO₂ and stable Na stripping/plating in the presence of K⁺ enable the hybrid battery to exhibit an average discharge/charge voltage gap of 0.15 V, high Coulombic efficiency of >96%, and superior cycling stability of 120 cycles. This will pave a new pathway to promote metal-air batteries.

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.

Superoxide Stabilization and a Universal KO2 Growth Mechanism in Potassium-Oxygen Batteries

Rechargeable potassium-oxygen (K-O₂ ) batteries promise to provide higher round-trip efficiency and cycle life than other alkali-oxygen batteries with satisfactory gravimetric energy density (935 Wh kg^-1 ). Exploiting a strong electron-donating solvent, for example, dimethyl sulfoxide (DMSO) strongly stabilizes the discharge product (KO₂ ), resulting in significant improvement in electrode kinetics and chemical/electrochemical reversibility. The first DMSO-based K-O₂ battery demonstrates a much higher energy efficiency and stability than the glyme-based electrolyte. A universal KO₂ growth model is developed and it is demonstrated that the ideal solvent for K-O₂ batteries should strongly stabilize superoxide (strong donor ability) to obtain high electrode kinetics and reversibility while providing fast oxygen diffusion to achieve high discharge capacity. This work elucidates key electrolyte properties that control the efficiency and reversibility of K-O₂ batteries.