Dendrite-Free Potassium-Oxygen Battery Based on a Liquid Alloy Anode

Engineering Triple-Phase Interfaces around the Anode toward Practical Alkali Metal-Air Batteries

Alkali metal-air batteries (AMABs) promise ultrahigh gravimetric energy densities, while the inherent poor cycle stability hinders their practical application. To address this challenge, most previous efforts are devoted to advancing the air cathodes with high electrocatalytic activity. Recent studies have underlined the solid-liquid-gas triple-phase interface around the anode can play far more significant roles than previously acknowledged by the scientific community. Besides the bottlenecks of uncontrollable dendrite growth and gas evolution in conventional alkali metal batteries, the corrosive gases, intermediate oxygen species, and redox mediators in AMABs cause more severe anode corrosion and structural collapse, posing greater challenges to the stabilization of the anode triple-phase interface. This work aims to provide a timely perspective on the anode interface engineering for durable AMABs. Taking the Li-air battery as a typical example, this critical review shows the latest developed anode stabilization strategies, including formulating electrolytes to build protective interphases, fabricating advanced anodes to improve their anti-corrosion capability, and designing functional separator to shield the corrosive species. Finally, the remaining scientific and technical issues from the prospects of anode interface engineering are highlighted, particularly materials system engineering, for the practical use of AMABs.

Building Stable Anodes for High-Rate Na-Metal Batteries

Due to low cost and high energy density, sodium metal batteries (SMBs) have attracted growing interest, with great potential to power future electric vehicles (EVs) and mobile electronics, which require rapid charge/discharge capability. However, the development of high-rate SMBs has been impeded by the sluggish Na+ ion kinetics, particularly at the sodium metal anode (SMA). The high-rate operation severely threatens the SMA stability, due to the unstable solid-electrolyte interface (SEI), the Na dendrite growth, and large volume changes during Na plating-stripping cycles, leading to rapid electrochemical performance degradations. This review surveys key challenges faced by high-rate SMAs, and highlights representative stabilization strategies, including the general modification of SMB components (including the host, Na metal surface, electrolyte, separator, and cathode), and emerging solutions with the development of solid-state SMBs and liquid metal anodes; the working principle, performance, and application of these strategies are elaborated, to reduce the Na nucleation energy barriers and promote Na+ ion transfer kinetics for stable high-rate Na metal anodes. This review will inspire further efforts to stabilize SMAs and other metal (e.g., Li, K, Mg, Zn) anodes, promoting high-rate applications of high-energy metal batteries towards a more sustainable society.

Carbon Electrode Materials for Advanced Potassium-Ion Storage

Tremendous progress has been made in the field of electrochemical energy storage devices that rely on potassium-ions as charge carriers due to their abundant resources and excellent ion transport properties. Nevertheless, future practical developments not only count on advanced electrode materials with superior electrochemical performance, but also on competitive costs of electrodes for scalable production. In the past few decades, advanced carbon materials have attracted great interest due to their low cost, high selectivity, and structural suitability and have been widely investigated as functional materials for potassium-ion storage. This article provides an up-to-date overview of this rapidly developing field, focusing on recent advanced and mechanistic understanding of carbon-based electrode materials for potassium-ion batteries. In addition, we also discuss recent achievements of dual-ion batteries and conversion-type K-X (X=O2 , CO2 , S, Se, I2 ) batteries towards potential practical applications as high-voltage and high-power devices, and summarize carbon-based materials as the host for K-metal protection and possible directions for the development of potassium energy-related devices as well. Based on this, we bridge the gaps between various carbon-based functional materials structure and the related potassium-ion storage performance, especially provide guidance on carbon material design principles for next-generation potassium-ion storage devices.

Processable Potassium-Carbon Nanotube Film with a Three-Dimensional Structure for Ultrastable Metallic Potassium Anodes

K metal holds great promise as the ultimate anode candidate for K-ion batteries because of its high theoretical capacity and low operating potential. However, due to its high viscosity and poor mechanical processability, it remains challenging to manufacture potassium anodes with precise parameters by a simple and executable method. In this work, a high-performance potassium-carbon nanotubes (K@CNTs) composite film electrode with a three-dimensional (3D) skeleton and superior processability is prepared by simply incorporating CNTs into molten potassium. The in situ potassiation reaction between CNTs and molten K formed potassium carbide (KC₈) so as to obtain a solid-liquid mixture, which can reduce the surface tension of molten potassium and promote the preparation of the K@CNTs film electrode. The composite electrode can be molded into a variety of shapes and thicknesses in accurate dimensions. The porous, well-conducting CNTs act as a 3D skeleton uniformly distributed in the K metal, providing adequate surface and space to accommodate and attract K metal, thereby inhibiting the growth of the potassium dendrites and the volume expansion upon cycling. As a result, the K@CNTs composite anode exhibits excellent cyclability and rate capability in both symmetric and full cells. The superior processability and excellent electrochemical performance make this composite an ideal anode candidate for commercial applications in potassium metal batteries.

Strategies toward anode stabilization in nonaqueous alkali metal-oxygen batteries

Alkali metal-O2 batteries exhibit ultra-high theoretical energy density which is even on a par with to fossil energy and expected to become the next generation of energy storage devices. However,...

Prospects of Electrode Materials and Electrolytes for Practical Potassium-Based Batteries

Potassium-ion batteries (PIBs) have attracted tremendous attention because of their high energy density and low-cost. As such, much effort has focused on developing electrode materials and electrolytes for PIBs at the material levels. This review begins with an overview of the high-performance electrode materials and electrolytes, and then evaluates their prospects and challenges for practical PIBs to penetrate the market. The current status of PIBs for safe operation, energy density, power density, cyclability, and sustainability is discussed and future studies for electrode materials, electrolytes, and electrode-electrolyte interfaces are identified. It is anticipated that this review will motivate research and development to fill existing gaps for practical potassium-based full batteries so that they may be commercialized in the near future.

Potassium-Oxygen Batteries: Significance, Challenges, and Prospects

To mitigate a global crisis of Li depletion, potassium-based rechargeable batteries have received significant attention because of their low cost and high specific energy density. In particular, the rechargeable potassium oxygen (K-O₂) battery has been recognized as a promising energy storage technology because of its low overpotential and high round-trip efficiency based on the single-electron redox chemistry of potassium superoxide. Despite these merits, research on the development of K-O₂ batteries is still in its early stages owing to a lack of understanding of the fundamental reaction chemistry and the difficulties encountered in handling, in terms of practical acceptability. Hence, it is necessary to summarize the representative works and provide overall insights on K-O₂ batteries and recommendations for future studies. In this Perspective, we critically review the important scientific aspects of K-O₂ batteries, discuss the current challenges encountered, and provide recommendations from the scientific and practical points of view. We hope that this Perspecitve will be helpful in designing innovative and advanced K-O₂ batteries.

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.

A Graphite Intercalation Composite as the Anode for the Potassium-Ion Oxygen Battery in a Concentrated Ether-Based Electrolyte

Nowadays, alkali metal-oxygen batteries such as Li-, Na-, and K-O₂ batteries have been investigated extensively because of their ultrahigh energy density. However, the oxygen crossover of oxygen batteries and the intrinsic drawbacks of the metal anodes (i.e., large volume changes and dendrite issues) have still been unsolved key problems. Here, we demonstrate a novel design of the K-ion oxygen battery using a graphite intercalation composite as the anode in a highly concentrated ether-based electrolyte. Instead of the metal K anode, the potassium graphite intercalation compound as the anode is depotassiated/potassiated in a binary form below 0.3 V (vs. K⁺/K); correspondingly, the discharged product KO₂ is formed/decomposed at the carbon nanotube cathode, and an all-carbon full cell exhibits impressive cycling stability with a working voltage of 2.0 V. Furthermore, the utilization of graphite intercalation chemistry has been demonstrated to be applicable in Li-O₂ batteries as well. Therefore, this study may provide a new strategy to resolve the key problems of the alkali metal-oxygen 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.

Emerging Potassium Metal Anodes: Perspectives on Control of the Electrochemical Interfaces

ConspectusPotassium metal serves as the anode in emerging potassium metal batteries (KMBs). It also serves as the counter electrode for potassium ion battery (KIB) half-cells, with its reliable performance being critical for assessing the working electrode material. This first-of-its-kind critical Account focuses on the dual challenge of controlling the potassium metal-substrate and the potassium metal-electrolyte interface so as to prevent dendrites. The discussion begins with a comparison of the physical and chemical properties of K metal anodes versus the much oft studied Li and Na metal anodes. Based on established descriptions for root causes of dendrites, the problem should be less severe for K than for Li or Na, while in fact the opposite is observed. The key reason that the K metal surface rapidly becomes dendritic in common electrolytes is its unstable solid electrolyte interphase (SEI). An unstable SEI layer is defined as being non-self-passivating. No SEI is perfectly stable during cycling, and all SEI structures are heterogeneous both vertically and horizontally relative to the electrolyte interface. The difference between a "stable" and an "unstable" SEI may be viewed as the relative degree to which during cycling it thickens and becomes further heterogeneous. The unstable SEI on K metal leads to a number of interrelated problems, such as low cycling Coulombic efficiency (CE), a severe impedance rise, large overpotentials, and possibly electrical shorting, all of which have been reported to occur as early as in the first 10 plating/stripping cycles. Many of the traditional "interface fixes" employed for Li and Na metal anodes, such as various artificial SEIs, surface membranes, barrier layers, secondary separators, etc., have not been attempted or optimized for the case of K. This is an important area for further exploration, with an understanding that success may come harder with K than with Li due to K-based SEI reactivity with both ether and ester solvents.The second critical problem with K metal anodes is that they do not thermally or electrochemically wet a standard (untreated) Cu foil current collector. Published experimental and modeling research directly highlights the weak bonding between the K atoms and a Cu surface. Existing surface treatment approaches that achieve improved K wetting are discussed, along with the general design rules for future studies. Also discussed are geometry-based methods to tune nucleation as well dual approaches where nucleation and SEI structure are tuned through complementary schemes to achieve extended half-cell and full battery stability. We hypothesize that K metal never achieves a planar wetting morphology even at cycle one, making the dendrites "baked-in". We propose that classical thin film growth models, Frank van der Merwe (F-M), Volmer-Weber (V-W), and Stranski-Krastanov (S-K), can be employed to describe early stage plating behavior. It is demonstrated that islandlike V-W growth is the applicable description for the natural plating behavior of K on pristine Cu. Moving forward, there are three inter-related thrusts to be pursued: First, K salt-based electrolyte formulations have to mature and become further tailored to handle the increased reactivity of a metal rather than an ion anode. Second, the K-based SEI structure needs to be further understood and ultimately tuned to be less reactive. Third, the energetics of the K metal-current collector interface must be controlled to promote planar wetting/dewetting throughout cycling.

Room-Temperature Fabrication of a Liquid NaK Alloy-Based Membrane Electrode for Sodium-Ion Batteries

The room-temperature liquid anode is a feasible method for building dendrite-free alkali-metal-based batteries. The Na-K phase diagram shows a eutectic point as low as 260.53 K with a long liquid range below 298 K with the molar fraction of potassium ranging from 30.48 to 84.99%. However, the NaK alloy exhibits a very high surface tension preventing it from wetting the current collector surface. Herein, a novel homogeneous dual solid-liquid composite in which the liquid alloy is fixed by the solid Na₁₅Sn₄ phase and perfectly stuffed into the grid of the mesh has been designed and fabricated. Based on the liquid range of the NaK alloy, the Na-K-Sn mixture possesses a theoretical specific capacity of 768 mAh g^-1. The symmetric cells of the Na-K-Sn@mesh electrodes cycled at 2.0 mA cm^-2 with 1.0 mAh cm^-2 showed little fluctuations with the stable overpotential of ∼200 mV for 550 h, and the full cell coupled with Na₃V₂(PO₄)₃ showed an initial discharge capacity of 103 mAh g^-1 at 2 C with a retention of 90% after 800 cycles. When the high-loading Na₃V₂(PO₄)₃ electrode is applied in the full cell, a stable cycling life is still maintained with a good capacity retention of 86% over 190 cycles (2.7 mAh cm^-2) and 91% over 60 cycles (5.2 mAh cm^-2).

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.

Novel Polyimide Separator Prepared with Two Porogens for Safe Lithium-Ion Batteries

A porous polyimide (PI) membrane is successfully prepared via nonsolvent-induced phase separation with two porogens: dibutyl phthalate and glycerin. The as-prepared uniform porous PI membrane shows excellent separator properties for lithium-ion batteries (LIBs). Compared with the commercial polyethylene (PE) separator, the PI separator exhibits significant thermal stability, better ionic conductivity, and wettability both in carbonate and ether electrolytes for LIBs. The battery coin-cells assembled with the PI separator is more robust and still works even after heating at 140 °C for 1 h, while the cells with the commercial PE separator could not charge any more due to the shrinkage of the PE under the same condition.

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.

Approaching high-performance potassium-ion batteries via advanced design strategies and engineering

Potassium-ion batteries (PIBs) have attracted tremendous attention due to their low cost, fast ionic conductivity in electrolyte, and high operating voltage. Research on PIBs is still in its infancy, however, and achieving a general understanding of the drawbacks of each component and proposing research strategies for overcoming these problems are crucial for the exploration of suitable electrode materials/electrolytes and the establishment of electrode/cell assembly technologies for further development of PIBs. In this review, we summarize our current understanding in this field, classify and highlight the design strategies for addressing the key issues in the research on PIBs, and propose possible pathways for the future development of PIBs toward practical applications. The strategies and perspectives summarized in this review aim to provide practical guidance for an increasing number of researchers to explore next-generation and high-performance PIBs, and the methodology may also be applicable to developing other energy storage systems.

New Insights into the Electrochemistry Superiority of Liquid Na-K Alloy in Metal Batteries

The significant issues with alkali metal batteries arise from their poor electrochemical properties and safety problems, limiting their applications. Herein, TiO₂ nanoparticles embedded into N-doped porous carbon truncated ocatahedra (TiO₂ ⊂NPCTO) are engineered as a cathode material with different metal anodes, including solid Na or K and liquid Na-K alloy. Electrochemical performance and kinetics are systematically analyzed, with the aim to determine detailed electrochemistry. By using a galvanostatic intermittent titration technique, TiO₂ ⊂NPCTO/NaK shows faster diffusion of metal ions in insertion and extraction processes than that of Na-ions and K-ions in solid Na and K. The lower reaction resistance of liquid Na-K alloy electrode is also examined. The higher b-value of TiO₂ ⊂NPCTO/NaK confirms that the reaction kinetics are promoted by the surface-induced capacitive behavior, favorable for high rate performance. This superiority highly pertains to the distinct liquid-liquid junction between the electrolyte and electrode, and the prohibition of metal dendrite growth, substantiated by symmetric cell testing, which provides a robust and homogeneous interface more stable than the traditional solid-liquid one. Hence, the liquid Na-K alloy-based battery exhibits to better cyclablity with higher capacity, rate capability, and initial coulombic efficiency than solid Na and K 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.

K–O2 electrochemistry: achieving highly reversible peroxide formation

Differential electrochemical mass spectrometry and classical electrochemical methods reveal that electrochemically produced K₂O₂ can be reversibly reoxidized to O₂.

Exploring Self-Healing Liquid Na-K Alloy for Dendrite-Free Electrochemical Energy Storage

The development of high-performance dendrite-free liquid-metal anodes at room temperature is of great importance for the advancement of alkali metal batteries. Herein an intriguing self-healing liquid dendrite-free Na-K alloy, fabricated by a facile room-temperature alloying process, aiming for application in potassium-ion batteries is reported. Through extensive investigation, its self-healing characteristics are rooted upon a thin solid K₂ O layer (KOL) coated on the liquid Na-K alloy. The KOL not only acts as a protective layer to prevent the Na-K alloy from making contact with the electrolyte, but also greatly improves the wetting capability and adhesion between the liquid alloy and the carbon matrix (e.g., carbon fiber cloth (CFC)) to form a stable interface. Consequently, the as-prepared CFC/KOL@Na-K alloy anode exhibits prominent electrochemical performance with smaller hysteresis (less than 0.3 V beyond 140 cycles at 0.4 mA cm^-2 ), better capacity retention, and higher Coulombic efficiency than the CFC/bare Na-K alloy counterpart. When coupled with a potassium Prussian blue (PPB) cathode, the full cell manifests higher capability retention and improved cycling stability. This research deepens the understanding of self-healing Na-K alloys and opens a new way to achieve high-performance dendrite-free alkali metal anodes for application in rechargeable 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.

Controllable Electrochemical Fabrication of KO2-Decorated Binder-Free Cathodes for Rechargeable Lithium-Oxygen Batteries

Understanding the electrochemical property of superoxides in alkali metal oxygen batteries is critical for the design of a stable oxygen battery with high capacity and long cycle performance. In this work, a KO₂-decorated binder-free cathode is fabricated by a simple and efficient electrochemical strategy. KO₂ nanoparticles are uniformly coated on the carbon nanotube film (CNT-f) through a controllable discharge process in the K-O₂ battery, and the KO₂-decorated CNT-f is innovatively introduced into the Li-O₂ battery as the O₂ diffusion electrode. The Li-O₂ battery based on the KO₂-decorated CNT-f cathode can deliver enhanced discharge capacity, reduced charge overpotential, and more stable cycle performance compared with the battery in the absence of KO₂. In situ formed KO₂ particles on the surface of CNT-f cathode assist to form Li₂O₂ nanosheets in the Li-O₂ battery, which contributes to the improvement of discharge capacity and cycle life. Interestingly, the analysis of KO₂-decorated CNT-f cathodes, after discharge and cycle tests, reveals that the electrochemically synthesized KO₂ seems not a conventional electrocatalyst but a partially dissolvable and decomposable promoter in Li-O₂ batteries.

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.

Room-temperature liquid metal-based anodes for high-energy potassium-based electrochemical devices

A dendrite-free CM@NaK electrode was fabricated via room-temperature adsorption of liquid Na–K onto a super-aligned CNT membrane driven by capillary force.