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

Alumina - Stabilized SEI and CEI in Potassium Metal Batteries

Aluminum oxide (Al2O3) nanopowder is spin-coated onto both sides of commercial polypropene separator to create artificial solid-electrolyte interphase (SEI) and artificial cathode electrolyte interface (CEI) in potassium metal batteries (KMBs). This significantly enhances the stability, including of KMBs with Prussian Blue (PB) cathodes. For example, symmetric cells are stable after 1,000 cycles at 0.5 mA/cm2-0.5 mAh/cm2 and 3.0 mA/cm2-0.5 mAh/cm2. Alumina modified separators promote electrolyte wetting and increase ionic conductivity (0.59 vs. 0.2 mS/cm) and transference number (0.81 vs. 0.23). Cryo-stage focused ion beam (cryo-FIB) analysis of cycled modified anode demonstrates dense and planar electrodeposits, versus unmodified baseline consisting of metal filaments (dendrites) interspersed with pores and SEI. Alumina-modified CEI also suppresses elemental Fe crossover and reduces cathode cracking. Mesoscale modeling of metal - SEI interactions captures crucial role of intrinsic heterogeneities, illustrating how artificial SEI affects reaction current distribution, conductivity and morphological stability.

Recent Progress in Using Covalent Organic Frameworks to Stabilize Metal Anodes for Highly-Efficient Rechargeable Batteries

Alkali metals (e.g. Li, Na, and K) and multivalent metals (e.g. Zn, Mg, Ca, and Al) have become star anodes for developing high-energy-density rechargeable batteries due to their high theoretical capacity and excellent conductivity. However, the inevitable dendrites and unstable interfaces of metal anodes pose challenges to the safety and stability of batteries. To address these issues, covalent organic frameworks (COFs), as emerging materials, have been widely investigated due to their regular porous structure, flexible molecular design, and high specific surface area. In this minireview, we summarize the research progress of COFs in stabilizing metal anodes. First, we present the research origins of metal anodes and delve into their advantages and challenges as anodes based on the physical/chemical properties of alkali and multivalent metals. Then, special attention has been paid to the application of COFs in the host design of metal anodes, artificial solid electrolyte interfaces, electrolyte additives, solid-state electrolytes, and separator modifications. Finally, a new perspective is provided for the research of metal anodes from the molecular design, pore modulation, and synthesis of COFs.

The Enormous Potential of Sodium/Potassium-Ion Batteries as the Mainstream Energy Storage Technology for Large-Scale Commercial Applications

Cost-effectiveness plays a decisive role in sustainable operating of rechargeable batteries. As such, the low cost-consumption of sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) provides a promising direction for "how do SIBs/PIBs replace Li-ion batteries (LIBs) counterparts" based on their resource abundance and advanced electrochemical performance. To rationalize the SIBs/PIBs technologies as alternatives to LIBs from the unit energy cost perspective, this review gives the specific criteria for their energy density at possible electrode-price grades and various battery-longevity levels. The cost ($ kWh-1 cycle-1) advantage of SIBs/PIBs is ascertained by the cheap raw-material compensation for the cycle performance deficiency and the energy density gap with LIBs. Furthermore, the cost comparison between SIBs and PIBs, especially on cost per kWh and per cycle, is also involved. This review explicitly manifests the practicability and cost-effectiveness toward SIBs are superior to PIBs whose commercialization has so far been hindered by low energy density. Even so, the huge potential on sustainability of PIBs, to outperform SIBs, as the mainstream energy storage technology is revealed as long as PIBs achieve long cycle life or enhanced energy density, the related outlook of which is proceeded as the next development directions for commercial applications.

Building Stable Solid-State Potassium Metal Batteries

Solid-state potassium metal batteries (SPMBs) are promising candidates for the next generation of energy storage systems for their low cost, safety, and high energy density. However, full SPMBs are not yet reported due to the K dendrites, interfacial incompatibility, and limited availability of suitable solid-state electrolytes. Here, stable SPMBs using a new iodinated solid polymer electrolyte (ISPE) are presented. The functional ions reconstruct ion transport channels, providing efficient potassium ion transport. ISPE shows a combination of high ionic conductivity, superior interfacial compatibility, and electrochemical stability. In situ alloying and iodinated interlayer increase K metal compatibility for prolonged cycling with low polarization. Moreover, the ISPE enables SPMBs with Prussian blue cathode stable operation at a high voltage of 4.5 V, a superior rate capability, and long-term cycling over 3000 cycles (4.2 V vs K+/K) with an ultra-high coulombic efficiency of 99.94%. More importantly, a classic solid-state potassium metal pouch cell achieves 4.2 V stable cycling over 800 cycles with a high retention of 93.6%, presenting a new development strategy for secure and high-performance rechargeable solid-state potassium metal batteries.

Optimizing cell voltage dependence on size of carbon nanotube-based electrodes in Na-ion and K-ion batteries

Sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) are gaining extensive attention as promising alternatives to lithium-ion batteries owing to their superior energy density and cost-effectiveness. However, the larger ionic radius of Na+ and K+ ions in comparison to Li+ ions poses a challenge in designing anode materials characterized by enduring structures and elevated voltage to facilitate the efficacy of high-performance NIBs and KIBs. Carbon nanomaterials, particularly carbon nanotubes (CNTs), have emerged as a potential candidate in anode materials. Herein, we used density functional theory calculations to study the cell voltage of CNTs in relation to Na-ion and K-ion storage as a function of CNT size. The adsorption energy profiles of both Na+@CNT and K+@CNT systems exhibit a descending trend concomitant with the increase in the CNT diameter, where Na+/K+ ion primarily prefers to adsorb in the interior wall of CNT. Conversely, the cell voltage for the Na and K system gradually increases with the increasing diameter of CNT, which can be attributed to the stronger electrostatic interaction validated by energy decomposition calculation. The voltage of Na-ion adsorbed on the inter wall of (10,10) CNT attains 1.29 V, close to the previously theoretical voltage of Li-ion on the same CNT (1.35 V), while the much lower voltage pertaining to K-ion adsorption on the inter wall of (10,10) CNT just stands at 0.59 V, suggesting the viability of CNT-based electrode for NIBs but not for KIBs. These findings lay a solid foundation for delineating the interrelationship between the voltage properties of CNT as prospective anode material and their structural characteristics, thereby expanding the application of CNT-based optoelectronic devices.

Recent advances in electrolyte molecular design for alkali metal batteries

In response to societal developments and the growing demand for high-energy-density battery systems, alkali metal batteries (AMBs) have emerged as promising candidates for next-generation energy storage. Despite their high theoretical specific capacity and output voltage, AMBs face critical challenges related to high reactivity with electrolytes and unstable interphases. This review, from the perspective of electrolytes, analyzes AMB failure mechanisms, including interfacial side reactions, active materials loss, and metal dendrite growth. It then reviews recent advances in innovative electrolyte molecular designs, such as ether, ester, sulfone, sulfonamide, phosphate, and salt, aimed at overcoming the above-mentioned challenges. Finally, we propose the current molecular design principles and future promising directions that can help future precise electrolyte molecular design.

Dendrite-Free and High-Rate Potassium Metal Batteries Sustained by an Inorganic-Rich SEI

Potassium metal battery is an appealing candidate for future energy storage. However, its application is plagued by the notorious dendrite proliferation at the anode side, which entails the formation of vulnerable solid electrolyte interphase (SEI) and non-uniform potassium deposition on the current collector. Here, this work reports a dual-modification design of aluminum current collector to render dendrite-free potassium anodes with favorable reversibility. This work achieves to modulate the electronic structure of the designed current collector and accordingly attain an SEI architecture with robust inorganic-rich constituents, which is evidenced by detailed cryo-EM inspection and X-ray depth profiling. The thus-produced SEI manages to expedite ionic conductivity and guide homogeneous potassium deposition. Compared to the potassium metal cells assembled using typical aluminum current collector, cells based on the designed current collector realize improved rate capability (maintaining 400 h under 50 mA cm-2 ) and low-temperature durability (stable operation at -50 °C). Moreover, scalable production of the current collector allows for the sustainable construction of high-safety potassium metal batteries, with the potential for reducing the manufacturing cost.

Insights into Nano- and Micro-Structured Scaffolds for Advanced Electrochemical Energy Storage

Adopting a nano- and micro-structuring approach to fully unleashing the genuine potential of electrode active material benefits in-depth understandings and research progress toward higher energy density electrochemical energy storage devices at all technology readiness levels. Due to various challenging issues, especially limited stability, nano- and micro-structured (NMS) electrodes undergo fast electrochemical performance degradation. The emerging NMS scaffold design is a pivotal aspect of many electrodes as it endows them with both robustness and electrochemical performance enhancement, even though it only occupies complementary and facilitating components for the main mechanism. However, extensive efforts are urgently needed toward optimizing the stereoscopic geometrical design of NMS scaffolds to minimize the volume ratio and maximize their functionality to fulfill the ever-increasing dependency and desire for energy power source supplies. This review will aim at highlighting these NMS scaffold design strategies, summarizing their corresponding strengths and challenges, and thereby outlining the potential solutions to resolve these challenges, design principles, and key perspectives for future research in this field. Therefore, this review will be one of the earliest reviews from this viewpoint.

In Situ Real-Time Dendritic Growth Determination of Electrodeposits on Ultramicroelectrodes

Monitoring the dendritic electrodeposition process is crucial in various fields such as energy storage devices and sensors. A variety of in situ dendritic growth monitoring methods have been developed, especially for battery applications, but they require specialized cells and equipment and are often invasive, making them unsuitable for various electrochemical systems and commercial batteries. To address these challenges, a real-time impedance analysis technique was used to determine dendritic electrodeposition on microelectrodes. The "effective size" of the electrodeposit was extracted from the impedance data, and the dendritic growth was assessed in real-time by comparing "effective size" to a theoretical radius assuming hemispherical growth. The technique was validated using scanning electron microscopy imaging and finite element method simulation. Initially applied to gold electrodeposition, the method was extended to zinc electrodeposition, demonstrating potential utilization for energy storage systems.

Dynamically Evolving Multifunctional Protective Layer for Highly Stable Potassium Metal Anodes

The construction of an artificial protective layer is an effective method to solve the issues, such as uncontrolled dendrite growth and an unstable solid electrolyte interphase, at the K metal anode. This study proposes a new dynamic evolution strategy that integrates the advantages of previous in situ and ex situ fabrication processes. A multifunctional protective layer enriched with K-Ge alloy is prepared on the K metal electrode by simple surface modification and in situ reduction via an electrochemical process. The protective layer has good potassiophilicity, mechanical flexibility, and high ionic conductivity, which can inhibit dendrite growth and reduce side reactions. The protected K electrode with a protective layer exhibits dendrite-free K plating/striping behavior, and the symmetric cell can run stably for over 1000 h at 1 mA cm-2 and 1 mAh cm-2. Notably, full cells based on this electrode also present excellent rate and cycling performance compared to those of the bare K electrode. This peculiar strategy will open a new avenue for metal anode protection and can be extended to other high-energy battery systems.

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.

Superstable potassium metal batteries with a controllable internal electric field

Stable potassium metal batteries (PMBs) are promising candidates for electrical energy storage due to their ability to reversibly store electrical energy at a low cost. However, dendritic growth and large volume changes hinder their practical application. Here, referring to the morphology and structure of a virus, a bionic virus-like-carbon microsphere (BVC) was designed as the anode host for a PMB. A BVC with a three-dimensional structure can not only control the electric field, which can suppress dendrite formation, but can also provide a larger space to accommodate the volume change during the cycle progress. The designed potassium (K) metal anode exhibits excellent cycle life and stability (during 1800 h of repeated plating/stripping of K at a current density of 0.1 mA cm-2, K-BVC can realize a very stable K metal anode with low voltage hysteresis). Stable cyclability and improved rate capability can be realized in a full cell using Prussian blue over 400 cycles. This research provides a new idea for the development of stable K metal anodes and may pave the way for the practical application of next-generation metal batteries.

A host potassiophilicity strategy for unprecedentedly stable and safe K metal batteries

Creating high-performance host materials for potassium (K) metal anodes remains a significant challenge due to the complex preparation process and poor K reversibility. In our work, we developed a potassiophilicity strategy using an oxygen-modified carbon cloth (O-CC) network as a host for K metal anodes. The O-CC network exhibited superior potassiophilic ability, and this improvement was also observed in other carbon hosts using the same process. The oxygen-induced epoxy group in the carbon network regulates interface electrons and enables strong binding of K adatoms through orbital hybridization, resulting in fewer side reactions with the electrolyte and promoting K-ion desolvation and uniform deposition. These factors result in unprecedented stability of the carbon network host, with a long lifespan of over 5500 hours at 0.5 mA cm-2/0.5 mA h cm-2 and 3500 h at 1 mA cm-2/0.5 mA h cm-2 in symmetric cells for K metal anodes, surpassing the cycle life of all previously reported K metal anodes. Furthermore, a high average coulombic efficiency of over 99.3% is demonstrated in O-CC//K cells during 210 cycles. The O-CC also exhibited a stable electrochemical performance, with a capacity retention of 73.3% in full cells coupled with a perylene-3,4,9,10-tetracarboxylic dianhydride cathode. We believe that this new strategy holds great promise for metal anodes in battery applications.

Polyolefin-Based Separator with Interfacial Chemistry Regulation for Robust Potassium Metal Batteries

Potassium metal batteries (KMBs) are ideal choices for high energy density storage system owing to the low electrochemical potential and low cost of K. However, the practical KMB applications suffer from intrinsically active K anode, which would bring serious safety concerns due to easier generation of dendrites. Herein, to explore a facile approach to tackle this issue, we propose to regulate K plating/stripping via interfacial chemistry engineering of commercial polyolefin-based separator using multiple functional units integrated in tailored metal organic framework. As a case study, the functional units of MIL-101(Cr) offer high elastic modulus, facilitate the dissociation of potassium salt, improve the K+ transfer number and homogenize the K+ flux at the electrode/electrolyte interface. Benefiting from these favorable features, uniform and stable K plating/stripping is realized with the regulated separator. Full battery assembled with the regulated separator showed ∼19.9 % higher discharge capacity than that with glass fiber separator at 20 mA g-1 and much better cycling stability at high rates. The generality of our approach is validated with KMBs using different cathodes and electrolytes. We envision that the strategy to suppress dendrite formation by commercial separator surface engineering using tailor-designed functional units can be extended to other metal/metal ion batteries.

Nanoelectrochemistry in electrochemical phase transition reactions

Electrochemical phase transition is important in a range of processes, including gas generation in fuel cells and electrolyzers, as well as in electrodeposition in battery and metal production. Nucleation is the first step in these phase transition reactions. A deep understanding of the kinetics, and mechanism of the nucleation and the structure of the nuclei and nucleation sites is fundamentally important. In this perspective, theories and methods for studying electrochemical nucleation are briefly reviewed, with an emphasis on nanoelectrochemistry and single-entity electrochemistry approaches. Perspectives on open questions and potential future approaches are also discussed.

Building electrode skins for ultra-stable potassium metal batteries

In nature, the human body is a perfect self-organizing and self-repairing system, with the skin protecting the internal organs and tissues from external damages. In this work, inspired by the human skin, we design a metal electrode skin (MES) to protect the metal interface. MES can increase the flatness of electrode and uniform the electric field distribution, inhibiting the growth of dendrites. In detail, an artificial film made of fluorinated graphene oxide serves as the first protection layer. At molecular level, fluorine is released and in-situ formed a robust SEI as the second protection "skin" for metal anode. As a result, Cu@MES | | K asymmetric cell is able to achieve an unprecedented cycle life (over 1600 cycles). More impressively, the full cell of K@MES | | Prussian blue exhibits a long cycle lifespan over 5000 cycles. This work illustrates a mechanism for metal electrode protection and provides a strategy for the applying bionics in batteries.

Optimized Solid Electrolyte Interphase and Solvation Structure of Potassium Ions in Carbonate Electrolytes for High-Performance Potassium Metal Batteries

The introduction of electrolyte additives is one of the most potential strategies to improve the performance of potassium metal batteries (PMBs). However, designing an additive that can alter the K⁺ solvation shell and essentially inhibit K dendrite remains a challenge. Herein, the amyl-triphenyl-phosphonium bromide was introduced as an additive to build a stable solid electrolyte interphase layer. The amyl-TPP cations can form a cation shielding layer on the metal surface during the nucleation stage, preventing K⁺ from gathering at the tip to form K dendrites. Besides, the cations can be preferentially reduced to form K_x P_y with fast K⁺ transport kinetics. The Br^- anions, as Lewis bases with strong electronegativity, can not only coordinate the Lewis acid pentafluoride to inhibit the formation of HF, but also change the K⁺ solvation structure to reduce solvent molecules in the first solvation structure. Therefore, the symmetrical battery exhibits a low deposition overpotential of 123 mV at 0.1 mA cm^-2 over 4200 h cycle life. The full battery, paried with a perylene-tetracarboxylic dianhydride (PTCDA) cathode, possesses a cycle life of 250 cycles at 2 C and 81.9% capacity retention. This work offers a reasonable electrolyte design to obtain PMBs with long-term stablity and safety.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

Porous Metal Current Collectors for Alkali Metal Batteries

Alkali metals (i.e., Li, Na, and K) are promising anode materials for next-generation high-energy-density batteries due to their superior theoretical specific capacities and low electrochemical potentials. However, the uneven current and ion distribution on the anode surface probably induces undesirable dendrite growth, which leads to significant safety hazards and severely hinders the commercialization of alkali metal anodes. A smart and versatile strategy that can accommodate alkali metals into porous metal current collectors (PMCCs) has been well established to resolve the issues as well as to promote the practical applications of alkali metal anodes. Moreover, the proposal of PMCCs can meet the requirement of the dendrite-free battery fabrication industry, while the electrode material loading exactly needs the metal current collector component as well. Here, a systematic survey on advanced PMCCs for Li, Na, and K alkali metal anodes is presented, including their development timeline, categories, fabrication methods, and working mechanism. On this basis, some significant methodology advances to control pore structure, surface area, surface wettability, and mechanical properties are systematically summarized. Further, the existing issues and the development prospects of PMCCs to improve anode performance in alkali metal batteries are discussed.

Insights into Cationic Transference Number Values and Solid Electrolyte Interphase Growth in Liquid/Solid Electrolytes for Potassium Metal Batteries

Liquid/solid battery electrolytes make separators dispensable and enable a high cationic transference number with liquid-like room temperature ionic conductivity. This work gives insights into electrochemical behavior (galvanostatic polarization and time-dependent impedance spectroscopy) of liquid/solid electrolytes containing potassium salts in battery cells enclosing potassium metal anodes. Very high potassium transference numbers (t _K = 0.88) are observed in carbonate-based electrolytes, linked with long-term mechanical instability of the solid electrolyte interphase on the potassium anode. In the case of glyme-based electrolytes, electrochemical behavior indicates the existence of the highly porous solid electrolyte interphase and additional surface porosity of the potassium electrode.

Codoped porous carbon nanofibres as a potassium metal host for nonaqueous K-ion batteries

Potassium metal is an appealing alternative to lithium as an alkali metal anode for future electrochemical energy storage systems. However, the use of potassium metal is hindered by the growth of unfavourable deposition (e.g., dendrites) and volume changes upon cycling. To circumvent these issues, we propose the synthesis and application of nitrogen and zinc codoped porous carbon nanofibres that act as potassium metal hosts. This carbonaceous porous material enables rapid potassium infusion (e.g., < 1 s cm−2) with a high potassium content (e.g., 97 wt. %) and low potassium nucleation overpotential (e.g., 15 mV at 0.5 mA cm−2). Experimental and theoretical measurements and analyses demonstrate that the carbon nanofibres induce uniform potassium deposition within its porous network and facilitate a dendrite-free morphology during asymmetric and symmetric cell cycling. Interestingly, when the potassium-infused carbon material is tested as an active negative electrode material in combination with a sulfur-based positive electrode and a nonaqueous electrolyte solution in the coin cell configuration, an average discharge voltage of approximately 1.6 V and a discharge capacity of approximately 470 mA h g−1 after 600 cycles at 500 mA g−1 and 25 °C are achieved.

Reduced Graphene Oxide-Coated Separator to Activate Dead Potassium for Efficient Potassium Batteries

Potassium (K) metal batteries (KMBs) have the advantages of relatively low electric potential (-2.93 V), high specific capacity (687 mAh g^-1), and low cost, which are highly appealing to manufacturers of portable electric products and vehicles. However, the large amounts of "dead K" caused by K dendrite growth and volumetric expansion can cause severe K metal anode deactivation. Here, a thin layer of conductive reduced graphene oxide (rGO) was coated on a GF separator (rGO@GF) to activate the generated dead K. Compared with the batteries adopting an original separator, those adopting a modified separator have significantly improved specific capacity and cycling stability. The life of full-cell of KMBs combining an rGO@GF separator with synthesized K_0.51V₂O₅ is expected to exceed 400 cycles, with an initial capacity of 92 mAh g^-1 at 0.5 A g^-1 and an attenuation rate per cycle as low as 0.03%. Our work demonstrates that a composite separator of high conductivity is beneficial for high performance KMBs.

Cyclic-anion salt for high-voltage stable potassium metal batteries

Electrolyte anions are critical for achieving high-voltage stable potassium-metal batteries (PMBs). However, the common anions cannot simultaneously prevent the formation of ‘dead K’ and the corrosion of Al current collector, resulting in poor cycling stability. Here, we demonstrate cyclic anion of hexafluoropropane-1,3-disulfonimide-based electrolytes that can mitigate the ‘dead K’ and remarkably enhance the high-voltage stability of PMBs. Particularly, even using low salt concentration (0.8 M) and additive-free carbonate-based electrolytes, the PMBs with a high-voltage polyanion cathode (4.4 V) also exhibit excellent cycling stability of 200 cycles with a good capacity retention of 83%. This noticeable electrochemical performance is due to the highly efficient passivation ability of the cyclic anions on both anode and cathode surfaces. This cyclic-anion-based electrolyte design strategy is also suitable for lithium and sodium-metal battery technologies.

An Anode-Free Potassium-Metal Battery Enabled by a Directly Grown Graphene-Modulated Aluminum Current Collector

Potassium (K)-metal batteries have emerged as a promising energy-storage device owing to abundant K resources. An anode-free architecture that bypasses the need for anode host materials can deliver an elevated energy density. However, the poor efficiency of K plating/stripping on potassiophobic anode current collectors results in rapid K inventory loss and a short cycle life. Herein, commercial Al foils are decorated with an ultrathin graphene-modified layer (Al@G) through roll-to-roll plasma-enhanced chemical vapor deposition. By harnessing strong adhesion (10.52 N m^-1 ) and a high surface energy (66.6 mJ m^-2 ), the designed Al@G structure ensures a highly smooth and ordered K plating/stripping process. Consequently, during K-metal plating/stripping, Al@G can operate at a current density of up to 4.0 mA cm^-2 and cyclic capacity of up to 4.0 mAh cm^-2 , with an ultralong lifespan of up to 1000 h at 0.5 mA cm^-2 and stable cycling of up to 750 h under periodic current fluctuations of 0.1-2.0 mA cm^-2 . In addition, a novel anode-free K-metal full-cell prototype enabled by Al@G anode current collectors is constructed, demonstrating ameliorative cyclic stability.

Highly Potassiophilic Graphdiyne Skeletons Decorated with Cu Quantum Dots Enable Dendrite-Free Potassium-Metal Anodes

Employing an Al foil current collector at the potassium anode side is an ideal choice to entail low-cost and high-energy potassium-metal batteries (PMBs). Nevertheless, the poor affinity between the potassium and the planar Al can cause uneven K plating/stripping and, hence, an undermined anode performance, which remains a significant challenge to be addressed. Herein, a nitrogen-doped carbon@graphdiyne (NC@GDY)-modified Al current collector affording potassiophilic properties is proposed, which simultaneously suppresses the dendrite growth and prolongs the lifespan of K anodes. The thin and light modification layer (7 µm thick, with a mass loading of 500 µg cm^-2 ) is fabricated by directly growing GDY nanosheets interspersed with Cu quantum dots on NC polyhedron templates. As a result, symmetric cell tests reveal that the K@NC@GDY-Al electrode exhibits an unprecedented cycle life of over 2400 h at a 40% depth of discharge. Even at an 80% depth of discharge, the cell can still sustain for 850 h. When paired with a potassium Prussian blue cathode, the thus-assembled full cell demonstrates comparable capacity and rate performance with state-of-the-art PMBs.

Robust 3D Copper Foam Functionalized with Gold Nanoparticle as the anode for High Performance Potassium Metal battery

As a possible anode for the next generation of electrochemical energy storage battery, potassium metal has attracted a great deal of attentions in recent years. However, potassium metal batteries suffer from poor stability and low coulombic efficiency due to unfavorable dendrite growth and severe volume expansion of potassium metal anodes during charge and discharge processes. In this paper, a strategy is developed to inhibit dendrite growth, reduce volume expansion and improve the stability of the potassium metal anode by using three-dimensional (3D) copper foam chemically loaded with a thin layer of uniform gold particles (Au/Cu) as the anode substrate. As the host of potassium deposition for forming K/Au/Cu foam anode, such a 3D copper foam with large specific surface area can reduce the potassium deposition current density. The gold particles on the Cu foam surface with a good affinity to potassium can reduce the potassium deposition overpotential, provide deposition sites and realize uniform and dense deposition. As a result, a stable cycle of more than 500 cycles is achieved by using such a K/Au/Cu foam anode. A full cell with K/Au/Cu foam anode and Prussian blue cathode delivers much better coulombic efficiency, cycle stability, and low-temperature performance when compared to that without gold deposition anode (K/Cu). Both the experimental characterization for material structure/morphology/composition and the theoretical DFT calculations are carried out for fundamental understanding of the performance enhancement mechanism. The main advantages of this 3D K/Au/Cu foam are its potassiophilicity and less volume expansion during charge/discharge processes, which can reduce or eliminate the potassium dendrite growth through forming favorable and stable solid electrolyte interface.

Electrolyte formulation strategies for potassium-based batteries

Potassium (K)-based batteries are viewed as the most promising alternatives to lithium-based batteries, owing to their abundant potassium resource, lower redox potentials (-2.97 V vs. SHE), and low cost. Recently, significant achievements on electrode materials have boosted the development of potassium-based batteries. However, the poor interfacial compatibility between electrode and electrolyte hinders their practical. Hence, rational design of electrolyte/electrode interface by electrolytes is the key to develop K-based batteries. In this review, the principles for formulating organic electrolytes are comprehensively summarized. Then, recent progress of various liquid organic and solid-state K⁺ electrolytes for potassium-ion batteries and beyond are discussed. Finally, we offer the current challenges that need to be addressed for advanced K-based batteries.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Optimizing the Interlayer Spacing of Heteroatom-Doped Carbon Nanofibers toward Ultrahigh Potassium-Storage Performances

Precise control over the interlayer spacing for K⁺ intercalation is an effective approach to boost the potassium-storage performances in carbonaceous materials. Herein, we first found that the optimal interlayer spacing for K⁺ intercalation is around 0.38 nm for N, O codoped carbon nanofibers (NOCNs), displaying a reversible capacity of 627 mAh g^-1 at 0.1 A g^-1 after 200 cycles, excellent rate capability (123 mAh g^-1 at 20 A g^-1), and ultrastable cycling stability (262 mAh g^-1 at 5 A g^-1 after 10 000 cycles). Such good potassium-storage performances have never been reported in carbonaceous materials. The theoretical calculations and electrochemical studies reveal that the optimal interlayer spacing and N, O heteroatom-induced active sites work together to provide an intercalation-adsorption mechanism for storing K⁺ in carbonaceous materials. This work facilitates the understanding of the role of the critical interlayer spacing for K⁺ intercalation in carbonaceous materials.

Multifunctional Separator Allows Stable Cycling of Potassium Metal Anodes and of Potassium Metal Batteries

This is the first report of a multifunctional separator for potassium-metal batteries (KMBs). Double-coated tape-cast microscale AlF₃ on polypropylene (AlF₃ @PP) yields state-of-the-art electrochemical performance: symmetric cells are stable after 1000 cycles (2000 h) at 0.5 mA cm^-2 and 0.5 mAh cm^-2 , with 0.042 V overpotential. Stability is maintained at 5.0 mA cm^-2 for 600 cycles (240 h), with 0.138 V overpotential. Postcycled plated surface is dendrite-free, while stripped surface contains smooth solid electrolyte interphase (SEI). Conventional PP cells fail rapidly, with dendrites at plating, and "dead metal" and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF₃ @PP display enhanced capacity retention (91% at 100 cycles vs 58%). AlF₃ partially reacts with K to form an artificial SEI containing KF, AlF₃ , and Al₂ O₃ phases. The AlF₃ @PP promotes complete electrolyte wetting and enhances uptake, improves ion conductivity, and increases ion transference number. The higher of K⁺ transference number is ascribed to the strong interaction between AlF₃ and FSI^- anions, as revealed through ¹⁹ F NMR. The enhancement in wetting and performance is general, being demonstrated with ester- and ether-based solvents, with K-, Na-, or Li- salts, and with different commercial separators. In full batteries, AlF₃ prevents Fe crossover and cycling-induced cathode pulverization.

Moderate Specific Surface Areas Help Three-Dimensional Frameworks Achieve Dendrite-Free Potassium-Metal Anodes

The inevitable problem of dendrites growth has hampered the further development of K metal anodes. Constructing a three-dimensional anode framework and potassiophilic nanocoating is an effective way to enlarge the specific surface area, reduce the local current density, and inhibit the formation of K dendrites. However, the effects of the electrochemically active surface area (ECSA) of the framework on deposition behavior have not been clarified. Hence, SnS₂ nanosheets with different sizes are loaded on the surface of carbon paper (SnS₂@CP) to improve the potassiophilicity and realize dendrite-free K-metal anodes. Experiments reveal that the size of SnS₂ nanosheets would determine the ECSA of the framework, while the ECSA reveals the relative sizes of specific surface areas of frameworks. Excessive or limited specific surface areas will cause morphological collapse or weak potassiophilicity during potassiation, respectively, thus leading to high nucleation overpotential. The moderate specific surface area and abundant and stable potassiophilic sites prompt the SnS₂@CP framework to achieve uniform electrodeposition of K. A low nucleation overpotential of 11.2 mV and a cycle life of more than 800 h are exhibited at a current density of 0.25 mA cm^-2, indicating the directional strategy for stable and safe K metal anodes.

Tribo-electrochemistry induced artificial solid electrolyte interface by self-catalysis

Potassium (K) metal is a promising alkali metal anode for its high abundance. However, dendrite on K anode is a serious problem which is even worse than Li. Artificial SEI (ASEI) is one of effective routes for suppressing dendrite. However, there are still some issues of the ASEI made by the traditional methods, e.g. weak adhesion, insufficient/uneven reaction, which deeply affects the ionic diffusion kinetics and the effect of inhibiting dendrites. Herein, through a unique self-catalysis tribo-electrochemistry reaction, a continuous and compact protective layer is successfully constructed on K metal anode in seconds. Such a continuous and compact protective layer can not only improve the K⁺ diffusion kinetics, but also strongly suppress K dendrite formation by its hard mechanical properties derived from rigid carbon system, as well as the improved K⁺ conductivity and lowered electronic conductivity from the amorphous KF. As a result, the potassium symmetric cells exhibit stable cycles last more than 1000 h, which is almost 500 times that of pristine K.

Design Principles of Sodium/Potassium Protection Layer for High-Power High-Energy Sodium/Potassium-Metal Batteries in Carbonate Electrolytes: a Case Study of Na2 Te/K2 Te

The sodium (potassium)-metal anodes combine low-cost, high theoretical capacity, and high energy density, demonstrating promising application in sodium (potassium)-metal batteries. However, the dendrites' growth on the surface of Na (K) has impeded their practical application. Herein, density functional theory (DFT) results predict Na₂ Te/K₂ Te is beneficial for Na⁺ /K⁺ transport and can effectively suppress the formation of the dendrites because of low Na⁺ /K⁺ migration energy barrier and ultrahigh Na⁺ /K⁺ diffusion coefficient of 3.7 × 10^-10 cm² s^-1 /1.6 × 10^-10 cm² s^-1 (300 K), respectively. Then a Na₂ Te protection layer is prepared by directly painting the nanosized Te powder onto the sodium-metal surface. The Na@Na₂ Te anode can last for 700 h in low-cost carbonate electrolytes (1 mA cm^-2 , 1 mAh cm^-2 ), and the corresponding Na₃ V₂ (PO₄ )₃ //Na@Na₂ Te full cell exhibits high energy density of 223 Wh kg^-1 at an unprecedented power density of 29687 W kg^-1 as well as an ultrahigh capacity retention of 93% after 3000 cycles at 20 C. Besides, the K@K₂ Te-based potassium-metal full battery also demonstrates high power density of 20 577 W kg^-1 with energy density of 154 Wh kg^-1 . This work opens up a new and promising avenue to stabilize sodium (potassium)-metal anodes with simple and low-cost interfacial layers.

Towards Dendrite-Free Potassium-Metal Batteries: Rational Design of a Multifunctional 3D Polyvinyl Alcohol-Borax Layer

K metal is the optimal anode for K-ion batteries because of its high capacity and low operating potential, but it suffers from fast capacity fading and safety issues due to an unstable solid electrolyte interphase (SEI) and continuous K-dendrite growth. Herein, to obtain promising potassium-metal batteries, a 3D polyvinyl-alcohol (PVA)-borax layer is designed, which enables a dendrite-free K-plating/stripping process. The protective layer possesses good wettability, high K-ion diffusivity, and good structural stability, which enables a "uniform and underneath plating" behavior, therefore exhibiting a stable electrochemical performance. As a result, Cu current collector with PVA-borax (PVA-borax@Cu) exhibits a stable cycling lifetime for 700 h at 0.5 mA cm^-2 and 500 h at 1 mA cm^-2 at 10 % depth of discharge (DOD) without dendrite formation. Even at a high utilization of 25 % DOD and 50 % DOD, the PVA-borax@Cu shows a stable cycle for 180 h and 100 h, respectively.

Mechanistic Insights into the Intercalation and Interfacial Chemistry of Mesocarbon Microbeads Anode for Potassium Ion Batteries

Mesocarbon microbeads (MCMB) are highly desirable as anode materials for rechargeable potassium ion batteries (PIBs) due to their commercially availability, high stability and low-cost. However, their charge storage and interfacial mechanisms are still unclear. In this work, the intercalation mechanisms and the solid-electrolyte-interphase (SEI) formation of the MCMB in four different electrolytes is comprehensively studied. The MCMB anodes exhibit superior rate and cycle performances via a naked K-ions sequentially staging intercalation mechanism, realizing the complete transformation from graphite to KC₈ . Whereas a solvated K-ions co-intercalation mechanism of the MCMB occurs in ether-based electrolytes, which might induce graphite exfoliation and result in unsatisfied specific capacity and capacity decay. Nevertheless, this co-intercalation behavior could be effectively suppressed by a highly concentrated electrolytes. Interfacial analyses unveil the distinct SEI components, which vary with the electrolyte chemistries. These SEI components also varies from surface to bulk and especially attention should be paid to the accurate control of the concentration of the fluoroethylene carbonate additives. This work provides a panoramic understanding of the intercalation and interfacial mechanisms on the MCMB anodes for PIBs.

High-Performance Cathode Materials for Potassium-Ion Batteries: Structural Design and Electrochemical Properties

Due to the obvious advantage in potassium reserves, potassium-ion batteries (PIBs) are now receiving increasing research attention as an alternative energy storage system for lithium-ion batteries (LIBs). Unfortunately, the large size of K⁺ makes it a challenging task to identify suitable electrode materials, particularly cathode ones that determine the energy density of PIBs, capable of tolerating the serious structural deformation during the continuous intercalation/deintercalation of K⁺ . It is therefore of paramount importance that proper design principles of cathode materials be followed to ensure stable electrochemical performance if a practical application of PIBs is expected. Herein, the current knowledge on the structural engineering of cathode materials acquired during the battle against its performance degradation is summarized. The K⁺ storage behavior of different types of cathodes is discussed in detail and the structure-performance relationship of materials sensitive to their different lattice frameworks is highlighted. The key issues facing the future development of different categories of cathode materials are also highlighted and perspectives for potential approaches and strategies to promote the further development of PIBs are provided.

Covalent Organic Frameworks and Their Derivatives for Better Metal Anodes in Rechargeable Batteries

Metal anodes based on a plating/stripping electrochemistry such as metallic Li, Na, K, Zn, Ca, Mg, Fe, and Al are recognized as promising anode materials for constructing next-generation high-energy-density rechargeable metal batteries owing to their low electrochemical potential, high theoretical specific capacity, superior electronic conductivity, etc. However, inherent issues such as high chemical reactivity, severe growth of dendrites, huge volume changes, and unstable interface largely impede their practical application. Covalent organic frameworks (COFs) and their derivatives as emerging multifunctional materials have already well addressed the inherent issues of metal anodes in the past several years due to their abundant metallophilic functional groups, special inner channels, and controllable structures. COFs and their derivatives can solve the issues of metal anodes by interfacial modification, homogenizing ion flux, acting as nucleation seeds, reducing the corrosion of metal anodes, and so on. Nevertheless, related reviews are still absent. Here we present a detailed review of multifunctional COFs and their derivatives in metal anodes for rechargeable metal batteries. Meanwhile, some outlooks and opinions are put forward. We believe the review can catch the eyes of relevant researchers and supply some inspiration for future research.

Crowning Metal Ions by Supramolecularization as a General Remedy toward a Dendrite-Free Alkali-Metal Battery

Alkali metals have low potentials and high capacities, making them ideal anodes for next-generation batteries, but they suffer major problems, including dendrite growth and low Coulombic efficiency (CE). Achieving uniform metal deposition and having a reliable solid electrolyte interphase (SEI) are the basic requirements for overcoming these problems. Here, a general remedy is reported for various alkali-metal anodes by the supramolecularization of alkali-metal cations with crown ethers that follows a size-matching rule. The positively charged supramolecular complex provides electrostatic shielding layers to regulate metal deposition and suppress dendrite formation. More promisingly, it reforms electric double layers and drives the production of organic-dominated SEIs with improved flexibility that can accommodate large volume changes. The high flexibility of SEIs during metal deposition and dissolution reduces the amount of dead metal and improves CE and cycling stability. Specifically, a 200% excess Li-based full cell has a capacity retention of ≈100% after 100 cycles. This crown-like supramolecularization strategy is a new chemistry that may be used for the production of dendrite-free metal-anode-based batteries not limited to the cases with alkali metal. It is also expected as a practical technology to improve the uniformity of coatings produced in the electrodeposition industry.

Directionally Tailoring Macroporous Honeycomb-Like Structured Carbon Nanofibers toward High-Capacitive Potassium Storage

Constructing high-capacitive potassium storage materials can avoid the sluggish and unstable bulk diffusion process via a surface-induced process, which is conducive to swift and frequent potassium storage. Herein, we demonstrated the use of macroporous honeycomb-like carbon nanofibers (MHCNFs) as an excellent anode material for high-capacitive potassium storage. The as-made MHCNFs feature abundant well-controlled macropores, an amorphous structure, and a large specific surface area of around 595.9 m² g^-1. These structural characteristics could significantly reduce the transferring distance of electrons/ions, offer abundant active sites, enable high-capacitive contribution, and thus substantially improve the kinetics and structural stability of MHCNFs. Experimental investigation demonstrated that MHCNFs enable ultrahigh potassium storage ability (329.1 mAh g^-1 at 100 mA g^-1) and competitive rate capability (168.5 mAh g^-1 at 5000 mA g^-1). More impressively, even when cycled at 1000 mA g^-1, the robust structure of MHCNFs can still enable the electrodes a capacity of 252.6 mAh g^-1 over repeating 2500 cycles. This work offers a promising strategy that macropore engineering coupled with amorphous structure can make effectively elevated K⁺ diffusion kinetic performance and promoted K⁺ adsorption/intercalation storage possible.

Strongly Coupled 2D Transition Metal Chalcogenide-MXene-Carbonaceous Nanoribbon Heterostructures with Ultrafast Ion Transport for Boosting Sodium/Potassium Ions Storage

HIGHLIGHTS

Unique "Janus" interfacial assemble strategy of 2D MXene nanosheets was proposed firstly. Ternary heterostructure consisting of high capacity transitional metal chalcogenide, high conductive 2D MXene and N rich fungal carbonaceous matrix was achieved for larger radius Na/K ions storages. The highly accessible surfaces and interfaces of the strongly coupled 2D based ternary heterostructures provide superb surficial pseudocapacitive storages for both Na and K ions with low energy barriers was verified. Combining with the advantages of two-dimensional (2D) nanomaterials, MXenes have shown great potential in next generation rechargeable batteries. Similar with other 2D materials, MXenes generally suffer severe self-agglomeration, low capacity, and unsatisfied durability, particularly for larger sodium/potassium ions, compromising their practical values. In this work, a novel ternary heterostructure self-assembled from transition metal selenides (MSe, M = Cu, Ni, and Co), MXene nanosheets and N-rich carbonaceous nanoribbons (CNRibs) with ultrafast ion transport properties is designed for sluggish sodium-ion (SIB) and potassium-ion (PIB) batteries. Benefiting from the diverse chemical characteristics, the positively charged MSe anchored onto the electronegative hydroxy (-OH) functionalized MXene surfaces through electrostatic adsorption, while the fungal-derived CNRibs bonded with the other side of MXene through amino bridging and hydrogen bonds. This unique MXene-based heterostructure prevents the restacking of 2D materials, increases the intrinsic conductivity, and most importantly, provides ultrafast interfacial ion transport pathways and extra surficial and interfacial storage sites, and thus, boosts the high-rate storage performances in SIB and PIB applications. Both the quantitatively kinetic analysis and the density functional theory (DFT) calculations revealed that the interfacial ion transport is several orders higher than that of the pristine MXenes, which delivered much enhanced Na+ (536.3 mAh g-1@ 0.1 A g-1) and K+ (305.6 mAh g-1@ 1.0 A g-1 ) storage capabilities and excellent long-term cycling stability. Therefore, this work provides new insights into 2D materials engineering and low-cost, but kinetically sluggish post-Li batteries.

Highly Potassiophilic Carbon Nanofiber Paper Derived from Bacterial Cellulose Enables Ultra-Stable Dendrite-Free Potassium Metal Anodes

Potassium-metal batteries are attractive candidates for low-cost and large-scale energy storage systems due to the abundance of potassium. However, K metal dendrite growth as well as volume expansion of K metal anodes on cycling have significantly hindered its practical applications. Although enhanced performance has been reported using carbon hosts with complicated structure engineering, they are not suitable for mass production. Herein, a highly potassiophilic carbon nanofiber paper with abundant oxygen-containing functional groups on the surface and a 3D interconnected network architecture is fabricated through a facile, scalable, and environmental-friendly biosynthesis method. As a host for K metal anode, uniform K nucleation and stable plating/stripping performance are demonstrated, with a stable cycling of 1400 h and a low overpotential of 45 mV, which are much better than all carbon hosts without complicated structure engineering. Moreover, full cells pairing the carbon nanofiber paper/K composite anodes with K₄Fe(CN)₆ cathodes exhibit excellent cycle stability and rate capability. The results provide a promising way for realizing dendrite-free K metal anodes and high-performance potassium-ion batteries.

Microbe-Assisted Assembly of Ti3C2Tx MXene on Fungi-Derived Nanoribbon Heterostructures for Ultrastable Sodium and Potassium Ion Storage

As a typical family of two-dimensional (2D) materials, MXenes present physiochemical properties and potential for use in energy storage applications. However, MXenes suffer some of the inherent disadvantages of 2D materials, such as severe restacking during processing and service and low capacity of energy storage. Herein, a MXene@N-doped carbonaceous nanofiber structure is designed as the anode for high-performance sodium- and potassium-ion batteries through an in situ bioadsorption strategy; that is, Ti₃C₂T_x nanosheets are assembled onto Aspergillus niger biofungal nanoribbons and converted into a 2D/1D heterostructure. This microorganism-derived 2D MXene-1D N-doped carbonaceous nanofiber structure with fully opened pores and transport channels delivers high reversible capacity and long-term stability to store both Na⁺ (349.2 mAh g^-1 at 0.1A g^-1 for 1000 cycles) and K⁺ (201.5 mAh g^-1 at 1.0 A g^-1 for 1000 cycles). Ion-diffusion kinetics analysis and density functional theory calculations reveal that this porous hybrid structure promotes the conduction and transport of Na and K ions and fully utilizes the inherent advantages of the 2D material. Therefore, this work expands the potential of MXene materials and provides a good strategy to address the challenges of 2D energy storage materials.

A highly concentrated electrolyte for high-efficiency potassium metal batteries

We report a highly concentrated electrolyte consisting of 4 M potassium bis(fluorosulfonyl)imide (KFSI) in diethylene glycol dimethyl ether (DEGDME). This new electrolyte enables stable cycling of K metal anodes with a high CE (over 98% over 400 cycles), and excellent capacity retention (99.7% after 500 cycles) of K||potassium Prussian blue (KPB) batteries.

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.

Sulfur-Rich Graphene Nanoboxes with Ultra-High Potassiation Capacity at Fast Charge: Storage Mechanisms and Device Performance

It is a major challenge to achieve fast charging and high reversible capacity in potassium ion storing carbons. Here, we synthesized sulfur-rich graphene nanoboxes (SGNs) by one-step chemical vapor deposition to deliver exceptional rate and cyclability performance as potassium ion battery and potassium ion capacitor (PIC) anodes. The SGN electrode exhibits a record reversible capacity of 516 mAh g^-1 at 0.05 A g^-1, record fast charge capacity of 223 mA h g^-1 at 1 A g^-1, and exceptional stability with 89% capacity retention after 1000 cycles. Additionally, the SGN-based PIC displays highly favorable Ragone chart characteristics: 112 Wh kg^-1at 505 W kg^-1 and 28 Wh kg^-1 at 14618 W kg^-1 with 92% capacity retention after 6000 cycles. X-ray photoelectron spectroscopy analysis illustrates a charge storage sequence based primarily on reversible ion binding at the structural-chemical defects in the carbon and the reversible formation of K-S-C and K₂S compounds. Transmission electron microscopy analysis demonstrates reversible dilation of graphene due to ion intercalation, which is a secondary source of capacity at low voltage. This intercalation mechanism is shown to be stable even at cycle 1000. Galvanostatic intermittent titration technique analysis yields diffusion coefficients from 10^-10 to 10^-12 cm² s^-1, an order of magnitude higher than S-free carbons. The direct electroanalytic/analytic comparison indicates that chemically bound sulfur increases the number of reversible ion bonding sites, promotes reaction-controlled over diffusion-controlled kinetics, and stabilizes the solid electrolyte interphase. It is also demonstrated that the initial Coulombic efficiency can be significantly improved by switching from a standard carbonate-based electrolyte to an ether-based one.

Effect of the electrolyte on K-metal batteries

The comparison of different electrolytes showed that both salt concentration and anion are key parameters for controlling the performance of K-metal batteries. Among the different tested electrolytes, 5 M KTFSI in DME exhibits the best stability at high potential and good performance in K|Prussian blue cells.

Stable Potassium Metal Anodes with an All-Aluminum Current Collector through Improved Electrolyte Wetting

This is the first report of successful potassium metal battery anode cycling with an aluminum-based rather than copper-based current collector. Dendrite-free plating/stripping is achieved through improved electrolyte wetting, employing an aluminum-powder-coated aluminum foil "Al@Al," without any modification of the support surface chemistry or electrolyte additives. The reservoir-free Al@Al half-cell is stable at 1000 cycles (1950 h) at 0.5 mA cm^-2 , with 98.9% cycling Coulombic efficiency and 0.085 V overpotential. The pre-potassiated cell is stable through a wide current range, including 130 cycles (2600 min) at 3.0 mA cm^-2 , with 0.178 V overpotential. Al@Al is fully wetted by a 4 m potassium bis(fluorosulfonyl)imide-dimethoxyethane electrolyte (θ_CA  = 0°), producing a uniform solid electrolyte interphase (SEI) during the initial galvanostatic formation cycles. On planar aluminum foil with a nearly identical surface oxide, the electrolyte wets poorly (θ_CA  = 52°). This correlates with coarse irregular SEI clumps at formation, 3D potassium islands with further SEI coarsening during plating/stripping, possibly dead potassium metal on stripped surfaces, and rapid failure. The electrochemical stability of Al@Al versus planar Al is not related to differences in potassiophilicity (nearly identical) as obtained from thermal wetting experiments. Planar Cu foils are also poorly electrolyte-wetted and become dendritic. The key fundamental takeaway is that the incomplete electrolyte wetting of collectors results in early onset of SEI instability and dendrites.