Dendrite-Free Potassium Metal Anodes in a Carbonate Electrolyte

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.

Ultrauniform Plating of Lithium on 10-nm-Scale Ordered Carbon Grids for Long Lifespan Lithium Metal Batteries

Tailorable lithium (Li) nucleation and uniform early-stage plating is essential for long-lifespan Li metal batteries. Among factors influencing the early plating of Li anode, the substrate is critical, but a fine control of the substrate structure on a scale of ≈10 nm has been rarely achieved. Herein, a carbon consisting of ordered grids is prepared, as a model to investigate the effect of substrate structure on the Li nucleation. In contrast to the individual spherical Li nuclei formed on the flat graphene, an ultrauniform and nuclei-free Li plating is obtained on the ordered carbon with a grid size smaller than the thermodynamical critical radius of Li nucleation (≈26 nm). Simultaneously, an inorganic-rich solid-electrolyte-interphase is promoted by the cross-sectional carbon layers of such ordered grids which are exposed to the electrolyte. Consequently, the carbon grids with a grid size of ≈10 nm show a favorable cycling stability for more than 1100 cycles measured at 2 mA cm-2 in a half cell. With LiNi0.8Co0.1Mn0.1O2 as cathode, the assembled full cell with a cathode capacity of 3 mAh cm-2 and a negative/positive ratio of 1.67 demonstrates a stable cycling for over 130 cycles with a capacity retention of 88%.

In situ fluoro-oxygen codoped graphene layer for high-performance lithium metal anode

Because traditional lithium-ion batteries have been unable to meet the energy density requirements of various emerging fields, lithium-metal batteries (LMBs), known for their high energy density, are considered promising next-generation energy storage batteries. However, a series of problems, including low coulombic efficiency and low safety caused by dendrites, limit the application of lithium metal batteries. Herein, fluoro-oxygen codoped graphene (FGO) was used to modify the copper current collector (FGO@Cu). FGO-coated current collector provides more even nucleation sites to reduce the local effective current density. FGO is partly reduced during cycling and helps form stable LiF-rich SEI. Moreover, graphene's oxygen and fluorine functional groups reconstruct the current density distribution, promoting uniform lithium plating. The FGO@Cu current collector demonstrates superior properties than commercial Cu foil. The FGO@Cu delivers a 97% high CE for over 250 cycles at 1 mA cm-2. The FGO@Cu symmetrical battery cycled at 1 mA cm-2 for over 650 h. LiFePO4 fuel cell with a lithium-plated FGO@Cu collector as an anode exhibits superior cycling stability.

Toward maximum energy density enabled by anode-free lithium metal batteries: Recent progress and perspective

Owing to the emergenceof energy storage and electric vehicles, the desire for safe high-energy-density energy storage devices has increased research interest in anode-free lithium metal batteries (AFLMBs). Unlike general lithium metal batteries (LMBs), in which excess Li exists to compensate for the irreversible loss of Li, only the current collector is employed as an anode and paired with a lithiated cathode in the fabrication of AFLMBs. Owing to their unique cell configuration, AFLMBs have attractive characteristics, including the highest energy density, safety, and cost-effectiveness. However, developing AFLMBs with extended cyclability remains an issue for practical applications because the high reactivity of Li with limited inventory causes severely low Coulombic efficiency (CE), poor cyclability, and dendrite growth. To address these issues, tremendous effort has been devoted to stabilizing Li metal anodes for AFLMBs. In this review, the importance and challenges of AFLMBs are highlighted. Then, diverse strategies, such as current collectors modification, advanced electrolytes, cathode engineering, and operation protocols are thoroughly reviewed. Finally, a future perspective on the strategy is provided for insight into the basis of future research. It is hoped that this review provides a comprehensive understanding by reviewing previous research and arousing more interest in this field.

Unraveling the Nucleation and Growth Mechanism of Potassium Metal on 3D Skeletons for Dendrite-Free Potassium Metal Batteries

Designing three-dimensional (3D) porous carbonaceous skeletons for K metal is one of the most promising strategies to inhibit dendrite growth and enhance the cycle life of potassium metal batteries. However, the nucleation and growth mechanism of K metal on 3D skeletons remains ambiguous, and the rational design of suitable K hosts still presents a significant challenge. In this study, the relationships between the binding energy of skeletons toward K and the nucleation and growth of K are systematically studied. It is found that a high binding energy can effectively decrease the nucleation barrier, reduce nucleation volume, and prevent dendrite growth, which is applied to guide the design of 3D current collectors. Density functional theory calculations show that P-doped carbon (P-carbon) exhibits the highest binding energy toward K compared to other elements (e.g., N, O). As a result, the K@P-PMCFs (P-binding porous multichannel carbon nanofibers) symmetric cell demonstrates an excellent cycle stability of 2100 h with an overpotential of 85 mV in carbonate electrolytes. Similarly, the perylene-3,4,9,10-tetracarboxylic dianhydride || K@P-PMCFs cell achieves ultralong cycle stability (85% capacity retention after 1000 cycles). This work provides a valuable reference for the rational design of 3D current collectors.

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.

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.

Three-Dimensional Octahedral Nanocrystals of Cu2O/CuF2 Grown on Porous Cu Foam Act as a Lithophilic Skeleton for Dendrite-Free Lithium Metal Anode

Metallic-lithium (Li) anodes are highly sought-after for next-generation energy storage systems due to their high theoretical capacity and low electrochemical potential. However, the commercialization of Li anodes faces challenges, including uncontrolled dendrite growth and volume changes during cycling. To address these issues, we developed a novel three-dimensional (3D) copper current collector. Here, we propose a two-step method to fabricate Cu2O/CuF2 octahedral nanocrystals (ONCs) onto 3D Cu current collectors. The resulting Cu foam with distributed ONCs provides active electrochemical sites, promoting uniform Li nucleation and dendrite-free Li deposition. The stable Cu2O/CuF2 ONCs@CF metallic current collector serves as a reliable host for dendrite-free lithium metal anodes. Additionally, the highly porous copper foam with a preconstructed conductive framework of Cu2O/CuF2 ONCs@CF effectively reduces local current density, suppressing volume changes during Li stripping and plating. The symmetric cell using Cu2O/CuF2 ONCs@CF metallic current collector exhibits excellent stability, maintaining over 1600 h at 1 mA cm-2 and a highly stable Coulombic efficiency of 98% over 100 cycles at the same current density, outperforming Li@CuF metallic current collectors. Furthermore, in a full-cell configuration paired with nickel-rich layered oxide cathode materials (Li@Cu2O/CuF2 ONCs@CF//NMC-811), the proposed setup demonstrates exceptional rate performance and an extended cycle life. In conclusion, our work presents a promising strategy to address Li anode challenges and highlights the exceptional performance of the Cu2O/CuF2 ONCs@CF metallic current collector, offering potential for high-capacity and long-lasting lithium-based energy storage systems.

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.

Construction of Synchronous-Deposition K Metal Anodes Via Modulation of Ion/Electron Transport Kinetics for High-Rate and Low-Temperature K Metal Batteries

The construction of conductive scaffolds is demonstrated to be an ideal strategy to alleviate the volume expansion and dendrite growth of K metal anodes. Nevertheless, the heterogeneous top-bottom deposition behavior caused by incompatible electronic/ionic conductivity of three-dimensional (3D) skeleton severely hinders its application. Here, a K2 Se/Cu conducting layer is fabricated on the Cu foam so as to enhance ionic transport and weaken electronic conductivity of the skeleton. Then, an excellent simultaneous deposition behavior of K metal inside the host is obtained for the first time via tuning fast ionic transport and low electronic conductivity. The simultaneous deposition mode can not only utilize the entire 3D structure to accommodate the volume expansion during K deposition but also avoid the formation of K dendrites at high current and ultra-low temperature. Consequently, the symmetric cells present a long cycle lifespan over 1000 h with a low deposition overpotential of 80 mV at 1 mA cm-2 . Furthermore, the full cell matching with the perylene-tetracarboxylic dianhydride (PTCDA) cathode presents an outstanding cycle lifespan over 600 cycles at 5 C at -20°C. The proposed simultaneous deposition strategy provides a new design direction for the construction of dendrite-free K metal anodes.

Highly Stable Potassium Metal Anodes with Controllable Thickness and Area Capacity

K metal battery is a kind of high-energy-density storage device with economic advantages. However, due to the dendrite growth and difficult processing characteristics, it is difficult to prepare stable K metal anode with thin thickness and fixed area capacity, which severely limits its development. In this work, a multi-functional 3D skeleton (rGCA) is synthesized by simple vacuum filtration and thermal reduction, and K metal anodes with controllable thickness and area capacity (K content) can be fabricated by changing the raw material mass and graphene layer spacing of rGCA. Moreover, the graphene sheet layer of rGCA can relax stress and relieve volume expansion; carbon nanotubes can serve as the fast transport channel of electrons, reducing internal impedance and local current density; Ag nanoparticles can induce the uniform nucleation and deposition of K+ . The K metal composite anodes (rGCA-K) based on the conductive skeleton can effectively suppress dendrites and exhibit excellent electrochemical performance in symmetric and full cells. The controllable fabrication process of stable K metal anode is expected to help K metal batteries move toward the stage of commercial production.

Regulating the Solvation Structure of Electrolyte via Dual-Salt Combination for Stable Potassium Metal Batteries

Batteries using potassium metal (K-metal) anode are considered a new type of low-cost and high-energy storage device. However, the thermodynamic instability of the K-metal anode in organic electrolyte solutions causes uncontrolled dendritic growth and parasitic reactions, leading to rapid capacity loss and low Coulombic efficiency of K-metal batteries. Herein, an advanced electrolyte comprising 1 M potassium bis(fluorosulfonyl)imide (KFSI) + 0.05 M potassium hexafluorophosphate (KPF₆ ) dissolved in dimethoxyethane (DME) is introduced as a simple and effective strategy of regulated solvation chemistry, showing an enhanced interfacial stability of the K-metal anode. Incorporating 0.05 M KPF₆ into the 1 M KFSI in DME electrolyte solution decreases the number of solvent molecules surrounding the K ion and simultaneously leads to facile K⁺ de-solvation. During the electrodeposition process, these unique features can lower the exchange current density between the electrolyte and K-metal anode, thereby improving the uniformity of K electrodeposition, as well as potentially suppressing dendritic growth. Even under a high current density of 4 mA cm^-2 , the K-metal anode in 0.05 M KPF₆ -containing electrolyte ensures high areal capacity and an unprecedented lifespan with stable Coulombic efficiency in both symmetrical half-cells and full-cells employing a sulfurized polyacrylonitrile cathode.

3-D nitrogen-doped carbon cage encapsulated ultrasmall MoC nanoparticles for promoting simultaneous ZnIn2S4 photocatalytic hydrogen generation and organic wastewater degradation

Simultaneous redox reactions on photocatalysts make it possible to use wastewater for hydrogen production. The controlled synthesis of ultrasmall metal carbides effectively enhances the photocatalytic efficiency under this system. Here, we report a new type of cocatalyst in which a three-dimensional (3-D) nitrogen-doped carbon cage (NGC) of metal-organic framework derivatives encapsulates ultrasmall MoC nanoparticles (MoC@NGC), promoting simultaneous degradation of organic pollutants and hydrogen production by ZnIn₂S₄ (ZIS). Characterization analyses showed that MoC accelerated the separation of the photogenerated carrier and effectively reduced the overpotential of hydrogen evolution, while NGC promoted the good dispersion of MoC particles and provided sufficient sites. The MoC@NGC/ZIS composite exhibited a high hydrogen (H₂) evolution rate of 1012 µmol g^-1h^-1, which exceed that of ZIS loaded with platinum. In the coupled system, where the electron donor was replaced with rhodamine B (RhB), the mechanism analysis showed that RhB and the as-generated intermediates consumed holes and facilitated hydrogen evolution. In addition, we designed a combined photocatalytic anoxic and oxic sequence process to achieve the recovery of hydrogen energy during the treatment of dye wastewater. This study provides a new way for cooperation between energy development and environmental protection.

Stable Anode-Free All-Solid-State Lithium Battery through Tuned Metal Wetting on the Copper Current Collector

A stable anode-free all-solid-state battery (AF-ASSB) with sulfide-based solid-electrolyte (SE) (argyrodite Li₆ PS₅ Cl) is achieved by tuning wetting of lithium metal on "empty" copper current-collector. Lithiophilic 1 µm Li₂ Te is synthesized by exposing the collector to tellurium vapor, followed by in situ Li activation during the first charge. The Li₂ Te significantly reduces the electrodeposition/electrodissolution overpotentials and improves Coulombic efficiency (CE). During continuous electrodeposition experiments using half-cells (1 mA cm^-2 ), the accumulated thickness of electrodeposited Li on Li₂ Te-Cu is more than 70 µm, which is the thickness of the Li foil counter-electrode. Full AF-ASSB with NMC811 cathode delivers an initial CE of 83% at 0.2C, with a cycling CE above 99%. Cryogenic focused ion beam (Cryo-FIB) sectioning demonstrates uniform electrodeposited metal microstructure, with no signs of voids or dendrites at the collector-SE interface. Electrodissolution is uniform and complete, with Li₂ Te remaining structurally stable and adherent. By contrast, an unmodified Cu current-collector promotes inhomogeneous Li electrodeposition/electrodissolution, electrochemically inactive "dead metal," dendrites that extend into SE, and thick non-uniform solid electrolyte interphase (SEI) interspersed with pores. Density functional theory (DFT) and mesoscale calculations provide complementary insight regarding nucleation-growth behavior. Unlike conventional liquid-electrolyte metal batteries, the role of current collector/support lithiophilicity has not been explored for emerging AF-ASSBs.

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.

Catalytic Effects of Electrodes and Electrolytes in Metal-Sulfur Batteries: Progress and Prospective

Metal-sulfur (M-S) batteries are promising energy-storage devices due to their advantages such as large energy density and the low cost of the raw materials. However, M-S batteries suffer from many drawbacks. Endowing the electrodes and electrolytes with the proper catalytic activity is crucial to improve the electrochemical properties of M-S batteries. With regard to the S cathodes, advanced electrode materials with enhanced electrocatalytic effects can capture polysulfides and accelerate electrochemical conversion and, as for the metal anodes, the proper electrode materials can provide active sites for metal deposition to reduce the deposition potential barrier and control the electroplating or stripping process. Moreover, an advanced electrolyte with desirable design can catalyze electrochemical reactions on the cathode and anode in high-performance M-S batteries. In this review, recent progress pertaining to the design of advanced electrode materials and electrolytes with the proper catalytic effects is summarized. The current progress of S cathodes and metal anodes in different types of M-S batteries are discussed and future development directions are described. The objective is to provide a comprehensive review on the current state-of-the-art S cathodes and metal anodes in M-S batteries and research guidance for future development of this important class of batteries.

In Situ Constructing Ultrathin, Robust-Flexible Polymeric Electrolytes with Rapid Interfacial Ion Transport in Lithium Metal Batteries

Safety of lithium metal batteries (LMBs) has been improved by using the solid-state polymer electrolytes, but the performance of LMBs is still troubled by the poor interface of solid electrolytes/electrodes, leading to insufficient interfacial Li⁺ transport. Here, a novel ultrathin, robust-flexible polymeric electrolyte is achieved by in situ polymerization of 1,3-dioxolane in soft nanofibrous skeleton at room temperature without any extra initiator or plasticizer, leading to the electrolyte with rapid interfacial ion transport. This facilitated Li⁺ transportation is demonstrated by molecular dynamics simulation. Consequently, the as-prepared electrolyte exhibits excellent cycling performance. The results indicate that the electrolyte works well in the LiFePO₄ //Li cell at elevated temperature up to 90 °C, and further matches with the high-voltage LiNi_0.8 Mn_0.1 Co_0.1 O₂ cathode. This study provides an effective approach to constructing a practical polymeric electrolyte for fabrication of safe, high performance LMBs.

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.

CoZn Nanoparticles@Hollow Carbon Tubes Enabled High-Performance Potassium Metal Batteries

Potassium-metal batteries (PMBs) are attractive candidates for low-cost and large-scale energy storage systems due to the abundance of potassium. However, its application is hampered by large volume change and serious dendrite growth. Herein, a CoZn semicoherent structure nanoparticle-embedded nitrogen-doped hollow carbon tube (CoZn@HCT) electrode is prepared via coaxial electrospinning. Due to the high potassiophilic CoZn semicoherent structure nanoparticles and large potassium metal storage space, the free-standing CoZn@HCT host for K metal exhibits uniform K nucleation and stable plating/stripping (stable cycling 1000 h at 1 mA cm^-2 with 1 mA h cm^-2). Furthermore, enhanced electrochemical performance with good cycling stability and rate capability is achieved in (CoZn@HCT@K||PTCDA) full batteries. Our results highlight a promising strategy for dendrite-free K metal anodes and high-performance PMBs.

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.

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.

Polyacrylonitrile-Reinforced Composite Gel Polymer Electrolytes for Stable Potassium Metal Anodes

The potassium-ion battery (PIB) is an emerging energy storage technology due to its potential low cost and reasonably high energy density. However, PIB suffers from severe potassium (K) dendrites growth and even short circuiting caused by the high reactivity of K metal. To address these challenges, this work develops a robust composite gel polymer electrolyte (CGPE), named poly(vinylidene fluoride-hexafluoropropylene) potassium bis(fluorosulfonyl)imide polyacrylonitrile (PVDF-HFP-KFSI@PAN). It is found that the introduction of PAN nanofibers not only improves the mechanical properties but also widens the electrochemical stability window of the CGPE. As a result, the CGPE is much more effective in regulating K stripping/plating and enabling stable K metal anode. K metal symmetrical cells with CGPE exhibit a lifetime of over 1200 h at the current density of 0.5 mA cm^-2 , in contrast to 22 h for PVDF-HFP-KFSI and 185 h for a conventional glass fiber separator. The main reasons for the excellent performance of K metal cells with CGPE are attributed to the suppressed K dendrite growth, good structural integrity electrochemical stability of the CGPE, and stable KF-rich solid electrolyte interphase layer at the electrode-CGPE interface. It is expected that this facile strategy to stabilize K metal will pave the way for safer and more durable K metal batteries.

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.

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.

Response and Implication of NASICON Solid-State Electrolytes to Local Electrical Stimulation: From Surface Engineering to Interfacial Manipulation

The surface feature of solid electrolytes fundamentally governs their own physical properties and significantly affects the interaction with the electrode materials. The evaluation of interfacial contact between the electrolyte and the metallic anode is largely relied on the macroscopic contact angle measurement, which is influenced by the intrinsic wettability and the microstructure of the electrolyte. In this work, the surface chemistry of the solid electrolyte is first regulated via facile thermal treatments. Then, scanning probe microscopy (SPM)-based techniques are comprehensively adopted to study the interaction between the electrolyte and metallic anode at the nanoscale. By manipulating the overpotential applied on the SPM tip, the mobile sodium ions at the subsurface of the solid electrolyte can be extracted toward the surface, and the eventual topography of the products is deliberately correlated with the sodium wettability. In this context, the impact of surface treatment on the sodium wettability of the surface layer is systematically evaluated based on the topographic evolution at the nanoscale. Furthermore, the local electrochemical reaction dynamics is revealed by correlating the surface ionic activity and current-voltage (I-V) curves. This work presents a new methodology to effectively evaluate the sodium wettability of the solid electrolyte, and these findings can provide meaningful implications to the surface engineering of ceramic electrolytes for high-performance solid-state 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.

Red phosphorus embedded in TiO2/C nanofibers to enhance the potassium-ion storage performance

TiO₂-red phosphorus/C nanofibers (TiO₂-RP/CN) have been synthesized via electrospinning and then annealed with red phosphorus sublimation. Benefiting from the high electronic/ionic conductivity and robust stability of the unique structure, the TiO₂-RP/CN show high reversible capacities, as well as an outstanding cycling ability. In K half cells, the capacity decay of the TiO₂-RP/CN electrode mainly occurs in the first few cycles, and at 0.05 A g^-1 it delivers a high specific capacity of 257.8 mA h g^-1 after 500 cycles. K full cells were fabricated; these are well-matched with PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) and also exhibited a good electrochemical performance (62 mA h g^-1 after 100 cycles). Therefore, the TiO₂-RP/CN are potential anode materials for use in K-ion batteries.

Optimal utilization of fluoroethylene carbonate in potassium ion batteries

This work provides a novel strategy of optimal utilization of fluoroethylene carbonate to generate a uniform and compact solid electrolyte interface film, enhancing the cycle life of potassium ion batteries. With K foil being treated with fluoroethylene carbonate prior to use, enhanced cycling performance up to 1200 hours was achieved. Combining in situ electrochemical impedance spectroscopy with the distribution of relaxation time analysis and XPS analysis, the solubility of KF in the electrolyte is proposed as a crucial factor to determine the quality of a solid electrolyte interface. Our work contributes to understanding the role and manipulating the usage of the fluoroethylene carbonate additive in potassium ion batteries.

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.

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.

A Non-aqueous H3 PO4 Electrolyte Enables Stable Cycling of Proton Electrodes

A non-aqueous proton electrolyte is devised by dissolving H₃ PO₄ into acetonitrile. The electrolyte exhibits unique vibrational signatures from stimulated Raman spectroscopy. Such an electrolyte exhibits unique characteristics compared to aqueous acidic electrolytes: 1) higher (de)protonation potential for a lower desolvation energy of protons, 2) better cycling stability by dissolution suppression, and 3) higher Coulombic efficiency owing to the lack of oxygen evolution reaction. Two non-aqueous proton full cells exhibit better cycling stability, higher Coulombic efficiency, and less self-discharge compared to the aqueous counterpart.

Electrochemical Insights, Developing Strategies, and Perspectives toward Advanced Potassium-Sulfur Batteries

The boosting demand for high-capacity energy storage systems requires innovative battery technologies with low-cost and sustainability. The advancement of potassium-sulfur (K-S) batteries have been triggered recently due to abundant resource and cost effectiveness. However, the functional performance of K-S batteries is fundamentally restricted by the vague understanding of K-S electrochemistry and the imperfect cell components or architectures, facing the issues of low cathode conductivity, intermediate shuttle loss, poor anode stability, electrode volume fluctuation, etc. Inspired by considerable research efforts on rechargeable metal-sulfur batteries, the holistic K-S system can be stabilized and promoted through various strategies on rational physical regulation and chemical engineering. In this review, first an attempt is made to address the electrochemical kinetic concept of K-S system on the basis of the emerging studies. Then, the classification of performance-improving strategies is thoroughly discussed in terms of specific battery component and prospective outlooks in materials optimization, structure innovations, as well as relevant electrochemistry are provided. Finally, the critical perspectives and challenges are discussed to demonstrate the forward-looking developmental directions of K-S batteries. This review not only endeavors to provide a deep understanding of the electrochemistry mechanism and rational designs for high-energy K-S batteries, but also encourages more efforts in their large-scale practical realization.

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.

Carbon Dots@rGO Paper as Freestanding and Flexible Potassium-Ion Batteries Anode

Carbonaceous materials, especially with graphite-layers structure, as anode for potassium-ion batteries (PIBs), are the footstone for industrialization of PIBs. However, carbonaceous materials with graphite-layers structure usually suffer from poor cycle life and inferior stability, not to mention freestanding and flexible PIBs. Here, a freestanding and flexible 3D hybrid architecture by introducing carbon dots on the reduced graphene oxide surface (CDs@rGO) is synthesized as high performance PIBs anode. The CDs@rGO paper has efficient electron and ion transfer channels due to its unique structure, thus enhancing reaction kinetics. In addition, the CDs provide abundant defects and oxygen-containing functional groups, which can improve the electrochemical performance. This freestanding and flexible anode exhibits the high capacity of 310 mAh g-1 at 100 mA g-1, ultra-long cycle life (840 cycles with a capacity of 244 mAh g-1 at 200 mA g-1), and excellent rate performance (undergo six consecutive currents changing from 100 to 500 mA g-1, high capacity 185 mAh g-1 at 500 mA g-1), outperforming many existing carbonaceous PIB anodes. The results may provide a starting point for high-performance freestanding and flexible PIBs and promote the rapid development of next-generation flexible 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.

Review of Emerging Potassium-Sulfur Batteries

This is the first review on potassium-sulfur (K-S) batteries (KSBs), which are emerging metal battery (MB) systems. Since KSBs are quite new, there are fundamental questions regarding the electrochemistry of S-based cathode and of K metal anode, as well as the holistic aspects of full-cell performance. The manuscript begins with a critical discussion regarding the potassium-sulfur electrochemistry and on how it differs from the much better-known lithium-sulfur. Cathodes are discussed next, focusing on the role of sulfur structure, carbon host chemistry and porosity, and electrolytes in establishing the reversible potassium sulfide K₂ S_n phase sequence, the parasitic polysulfide shuttle, pulverization-driven capacity fade, etc. Following is a discussion of solid-state electrolytes (SSEs), including of hybrid solid-liquid systems that show much promise. Potassium metal anodes are then critically reviewed, emphasizing electrolyte reactions to form stable versus unstable solid electrolyte interphase (SEI), covering the current understanding of potassium dendrites, and highlighting the deep-eutectic K-Na alloying approaches for room temperature liquid anodes. The manuscript concludes with K-S batteries, focusing on cell architectures and providing quantitative performance comparisons as master plots. Unanswered scientific/technological questions are identified, emerging research opportunities are discussed, and potential experimental and simulation-based studies that can unravel these unknowns are proposed.