Building K-C Anode with Ultrahigh Self-Diffusion Coefficient for Solid State Potassium Metal Batteries Operating at -20 to 120 °C

Surface Work Function-Induced High-Entropy Solid Electrolyte Interphase Formation for Highly Stable Potassium Metal Anodes

The failure of the solid electrolyte interphase (SEI) layer is a key issue limiting the practical application of potassium metal batteries. Herein, a novel high-entropy SEI layer rich in inorganic components is designed and constructed via in situ electrochemical conversion of the Sn3O4/Sn2S3 interfacial layer on a porous scaffold. Theoretical studies and experimental techniques reveal that the Sn3O4/Sn2S3 heterostructure, with its low work function and weak Sn─O/S bond, significantly enhances reactivity with the electrolyte, thereby facilitating the in situ formation of the high-entropy SEI layer. The in situ generated high-entropy SEI exhibits low surface roughness, low surface potential, fast potassium ion transport characteristics, and excellent mechanical properties (Young's modulus of 20.08 GPa). Leveraging these advantageous properties of the high-entropy SEI, the resulting potassium metal anode achieves an excellent rate performance up to 10 mA cm-2 in symmetric cells and demonstrates outstanding cycling stability for 2500 h at 0.5 mA cm-2. When paired with a perylene-3,4,9,10-tetracarboxylic dianhydride cathode, the potassium metal full battery retains 81.6% of its capacity over 1650 cycles at 10 C. This work underscores a straightforward and effective approach for the establishment of a stable interphase on metallic potassium anodes.

Status and Challenges of Solid-State Electrolytes for Potassium Batteries

Potassium solid-state batteries (K-SSBs), utilizing solid-state K-ion electrolytes (K-SSEs) and potassium metal anodes, are promising candidates for large-scale energy storage due to their low cost, safety, and high energy density. The limited availability of K-SSEs with excellent electrochemical performances and the associated interfacial issues are the primary hurdles in advancing K-SSBs. In this review, the ion conduction mechanism and the key parameters of solid electrolytes are re-examined, and then reviewed typical inorganic and polymer electrolytes for K-ion migration, highlighting the critical role of electrolyte crystal structure, composition, and preparation techniques in achieving optimal ionic conductivity and stability. This study further emphasizes the essential role of strategic interface engineering, including the formation of stable solid-electrolyte interphases at the anode and the construction of efficient K⁺-ion transport channels at the cathode, on mitigating interfacial impedance and enabling stable cycling performance of K-SSBs. Finally, promising research directions are proposed to advance the development of high-performance K-SSEs, anticipating to provide some inspiration and reference for the continued advancement of K-SSBs.

Regulating Interfacial Wettability for Fast Mass Transfer in Rechargeable Metal-Based Batteries

The interfacial wettability between electrodes and electrolytes could ensure sufficient physical contact and fast mass transfer at the gas-solid-liquid, solid-liquid, and solid-solid interfaces, which could improve the reaction kinetics and cycle stability of rechargeable metal-based batteries (RMBs). Herein, interfacial wettability engineering at multiphase interfaces is summarized from the electrolyte and electrode aspects to promote the interface reaction rate and durability of RMBs, which illustrates the revolution that is taking place in this field and thus provides inspiration for future developments in RMBs. Specifically, this review presents the principle of interfacial wettability at macro- and microscale and summarizes emerging applications concerning the interfacial wettability effect on mass transfer in RMBs. Moreover, deep insight into the future development of interfacial wettability is provided in the outlook. Therefore, this review not only provides insights into interfacial wettability engineering but also offers strategic guidance for wettability modification and optimization toward stable electrode-electrolyte interfaces for fast mass transfer in RMBs.

Recent advances in potassium metal batteries: electrodes, interfaces and electrolytes

The exceptional theoretical capacity of potassium metal anodes (687 mA h g-1), along with their low electrochemical potential, makes potassium metal batteries (PMBs) highly attractive for achieving high energy density. This review first provides an overview of potassium metal anodes, including their origin, current development status, and distinctive advantages compared to other metal anodes. Then, it discusses the composition and characteristics of emerging breakthrough PMBs, such as K-S, K-O2, K-CO2 batteries, and anode-free metal batteries. Subsequently, we delve into the pivotal challenges and theoretical research pertaining to PMBs, such as potassium metal nucleation/stripping, dendritic growth in PMBs, and unstable interfaces. Furthermore, we comprehensively examine the latest strategies in electrode design (including alloy, host, and current collector design), interface engineering (such as artificial solid electrolyte interphase layers, barrier layer design, and separator modification), and electrolyte optimization concerning nucleation, cycling stability, coulombic efficiency, and the development of PMBs. Finally, we introduce key characterization techniques, including in situ liquid phase secondary ion mass spectrometry, titration gas chromatography, neutron-based characterization, and computational simulation. This review will propel advancements in electrodes, separators, and electrolytes for innovative PMBs and other similar alkali metal batteries.

Optimized K+ Deposition Dynamics via Potassiphilic Porous Interconnected Mediators Coordinated by Single-Atom Iron for Dendrite-Free Potassium Metal Batteries

Potassium metal batteries are emerging as a promising high-energy density storage solution, valued for their cost-effectiveness and low electrochemical potential. However, understanding the role of potassiphilic sites in nucleation and growth remains challenging. This study introduces a single-atom iron, coordinated by nitrogen atoms in a 3D hierarchical porous carbon fiber (Fe─N-PCF), which enhances ion and electron transport, improves nucleation and diffusion kinetics, and reduces energy barriers for potassium deposition. Molten potassium infusion experiments confirm the Fe─N-PCF's strong potassiphilic properties, accelerating adsorption kinetics and improving potassium deposition performance. According to the Scharifker-Hills model, traditional carbon fiber substrates without potassiphilic sites cause 3D instantaneous nucleation, leading to dendritic growth. In contrast, the integration of single-atom and hierarchical porosity promotes uniform 3D progressive nucleation, leading to dense metal deposition, as confirmed by dimensionless i2/imax 2 versus t/tmax plots and real-time in situ optical microscopy. Consequently, in situ X-ray diffraction demonstrated stable potassium cycling for over 1900 h, while the Fe─N-PCF@K||PTCDA full cell retained 69.7% of its capacity after 2000 cycles (72 mAh g-1), with a low voltage hysteresis of 0.876 V, confirming its strong potential for high energy density and extended cycle life, paving the way for future advancements in energy storage technology.

Boosting K+-Ionic Conductivity of Layered Oxides via Regulating P2/P3 Heterogeneity and Reciprocity for Room-Temperature Quasi-Solid-State Potassium Metal Batteries

Solid-state potassium metal batteries are promising candidates for grid-scale energy storage due to their low cost, high energy density and inherent safety. However, solid state K-ion conductors struggle with poor ionic conductivity due to the large ionic radius of K+-ions. Herein, we report precise regulation of phase heterogeneity and reciprocity of the P2/P3-symbiosis K0.62Mg0.54Sb0.46O2 solid electrolyte (SE) for boosting a high ionic conductivity of 1.6×10-4 S cm-1 at 25 °C. The bulk ionic conducting mechanism is explored by elucidating the effect of atomic stacking mode within the layered framework on K+-ion migration barriers. For ion diffusion at grain boundaries, the P2/P3 biphasic symbiosis property assists in tunning the SE microstructure, which crystallizes in rod-like particles with lengths of tens of micrometers facilitating long-distance ion transport and significantly decreasing grain boundary resistance. Potassium metal symmetric cells using the modified SE deliver excellent cycling life over 300 h at 0.1 mA cm-2 and a high critical current density of 0.68 mA cm-2. The quasi-solid-state potassium metal batteries (QSSKBs) coupled with two kinds of layered oxide cathodes demonstrate remarkable stability over 300 cycles, outperforming the liquid electrolyte counterparts. The QSSKB system provides a promising strategy for high-efficiency, safe, and durable large-scale energy storage.

Ultrahigh Pyridinic/Pyrrolic N Enabling N/S Co-Doped Holey Graphene with Accelerated Kinetics for Alkali-Ion Batteries

Carbonaceous materials hold great promise for K-ion batteries due to their low cost, adjustable interlayer spacing, and high electronic conductivity. Nevertheless, the narrow interlayer spacing significantly restricts their potassium storage ability. Herein, hierarchical N, S co-doped exfoliated holey graphene (NSEHG) with ultrahigh pyridinic/pyrrolic N (90.6 at.%) and large interlayer spacing (0.423 nm) is prepared through micro-explosion assisted thermal exfoliation of graphene oxide (GO). The underlying mechanism of the micro-explosive exfoliation of GO is revealed. The NSEHG electrode delivers a remarkable reversible capacity (621 mAh g-1 at 0.05 A g-1), outstanding rate capability (155 mAh g-1 at 10 A g-1), and robust cyclic stability (0.005% decay per cycle after 4400 cycles at 5 A g-1), exceeding most of the previously reported graphene anodes in K-ion batteries. In addition, the NSEHG electrode exhibits encouraging performances as anodes for Li-/Na-ion batteries. Furthermore, the assembled activated carbon||NSEHG potassium-ion hybrid capacitor can deliver an impressive energy density of 141 Wh kg-1 and stable cycling performance with 96.1% capacitance retention after 4000 cycles at 1 A g-1. This work can offer helpful fundamental insights into design and scalable fabrication of high-performance graphene anodes for alkali metal ion batteries.

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.

In Situ Ions Induced Formation of KxF-Rich SEI Layers toward Ultrastable Life of Potassium-Ion Batteries

Engineering F-rich solid electrolyte interphase (SEI) layers is regarded as an effective strategy to enable the long-term cycling stability of potassium-ion batteries (KIBs). However, in the conventional KPF6 carbonate electrolytes, it is challenging to form F-containing SEI layers due to the inability of KPF6 to decompose into KxF. Herein, AlCl3 is employed as a novel additive to change the chemical environment of the KPF6 carbonate electrolyte. First, due to the large charge-to-radius ratio of Al3+, the Al-containing groups in the electrolyte can easily capture F from PF6 - and accelerate the formation of KxF in SEI layer. In addition, AlCl3 also reacts with trace H2O or solvents in the electrolytes to form Al2O3, which can further act as a HF scavenger. Upon incorporating AlCl3 into conventional KPF6 carbonate electrolyte, the hard carbon (HC) anode exhibits an ultra-long lifespan of 10000 cycles with a high coulombic efficiency of ≈100%. When coupled with perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), the full cell exhibits a high capacity retention of 81% after 360 cycles-significantly outperforming cells using conventional electrolytes. This research paves new avenues for advancing electrolyte engineering towards developing durable batteries tailored for large-scale energy storage applications.

Heteroatoms-Doped Mesoporous Carbon Nanosheets with Dual Diffusion Pathways for Highly Efficient Potassium Ion Storage

The high potassization/depotassization energy barriers and lack of efficient ion diffusion pathways are two serious obstacles for carbon-based materials to achieve satisfactory potassium ion storage performance. Herein, a facile and controllable one-step exfoliation-doping-etching strategy is proposed to construct heteroatoms (N, O, and S)-doped mesoporous few-layer carbon nanosheets (NOS-C). The mixed molten salts of KCl/K2SO4 are innovatively used as the exfoliators, dopants, and etching agents, which enable NOS-C with expanded interlayer spacing and uniformly distributed mesopores with the adjusted electronic structure of surrounding carbon atoms, contributing efficient dual (vertical and horizontal) K-ion diffusion pathways, low potassization/depotassization energy barriers and abundant active sites. Thus, the NOS anodes achieve a high reversible capacity of 516.8 mAh g-1 at 0.05 A g-1, superior rate capability of 202.8 mAh g-1 at 5 A g-1 and excellent long-term cyclic stability, and their practical application potential is demonstrated by the assembled potassium-ion full batteries. Moreover, a surface-interlayer synergetic K+ storage mechanism is revealed by a combined theoretical and experimental approach including in situ EIS, in situ Raman, ex situ XPS, and SEM analysis. The proposed K+ storage mechanism and unique structural engineering provide a new pathway for potassium-ion storage devices and even beyond.

Nitrogen-Doped Carbon Nanothin Film as a Buffer Layer between Anodic Graphite and Solid Electrolyte Interphase for Lithium-Ion Batteries

Lithium-ion batteries are essential batteries for electric vehicle drive systems. Such batteries must provide stable performance over a long period of time. Therefore, the degradation or aging of the battery capacity must be improved. In the case of the current graphite anodes, graphite coated with an amorphous layer is used. It is known that the amorphous layer can reduce the irreversible capacity loss caused by the solid electrolyte interphase (SEI) layer. The amorphous carbon layers reduce the initial capacity due to higher electrical resistance. In this study, we aim to develop a buffer layer using nitrogen-containing graphene that would prevent the increase in electrical resistance while maintaining the amorphous structure. Coatings with different film thicknesses were prepared by using the solution plasma method. The thinnest sample was oven sintered to optimize the structure, especially the surface and interface of the layer. The battery capacity from charge-discharge experiments and the resistance change of each part from electrochemical impedance measurements were evaluated. The results showed that the coating layer increased the electrical resistance of the graphite anode. On the other hand, the resistance of the SEI layer was reduced by the coating layer. It can be predicted that the addition of the coating layer will increase the total charge transfer resistance (R ct) of the cell but will also improve the period average capacity in the long run. To be used as a practical material, the film thickness would need to be further reduced, and the balance between the loss of charge transfer resistance and the gain of SEI layer resistance would need to be further optimized.

Hydroxyl-Decorated Carbon Cloth with High Potassium Affinity Enables Stable Potassium Metal Anodes

Highly anticipated potassium metal batteries possess abundant potassium reserves and high theoretical capacity but currently suffer from poor cycling stability as a result of dendritic growth and volume expansion. Here, carbon cloths modified with different functional groups treated with ethylene glycol, ethanolamine, and ethylenediamine are designed as 3D hosts, exhibiting different wettability to molten potassium. Among them, the hydroxyl-decorated carbon cloth with a high affinity for potassium can achieve molten potassium perfusion (K@EG-CC) within 3 s. By efficiently inducing the uniform deposition of metal potassium, buffing its volume expansion, and lowering local current density, the developed K@EG-CC anode alleviates the dendrite growth issue. The K@EG-CC||K@EG-CC symmetric battery can be cycled stably for 2100 h and has only a small voltage hysteresis of ≈93 mV at 0.5 mA cm-2 . Moreover, the high-voltage plateau, high energy density, and long cycle life of K metal full batteries can be realized with a low-cost KFeSO4 F@carbon nanotube cathode. This study provides a simple strategy to promote the commercial applications of potassium metal batteries.

Cobalt/Nitrogen Co-Doped Carbon Materials Enhance the Reaction Rate of Sodium-Potassium Alloy Electrodes

Sodium-potassium (NaK) alloy electrodes are ideal for next-generation dendrite-free alkali metal electrodes due to their dendrite-free nature. However, issues such as slow diffusion kinetics due to the large K+ radius and the loss of active potassium during the reaction severely limit its application. Here a novel cobalt/nitrogen-doped carbon material is designed and it is applied to the construction of a NaK alloy electrode. The experimental and theoretical results indicate that the confining effect of the nitrogen-doped graphitic carbon layer can protect the cobalt nanoparticles from corrosion leaching, while the presence of Co─Nx bonds and cobalt nanoparticles provides more active sites for the reaction, realizing the synergistic effect of adsorption-catalytic modulation, lowering the K+ diffusion energy barrier and promoting charge transfer and ion diffusion. The application of this electrode to a symmetrical battery can achieve more than 1800 stable cycles under a current density of 0.4 mA cm-2 and a charge/discharge specific capacity of 122.64 mAh g-1 under a current of 0.5C in a full battery. This finding provides a new idea to realize a fast, stable, and efficient application of NaK alloy electrodes.

Li, Na, K, Mg, Zn, Al, and Ca Anode Interface Chemistries Developed by Solid-State Electrolytes

Solid-state batteries (SSBs) have received significant attention due to their high energy density, reversible cycle life, and safe operations relative to commercial Li-ion batteries using flammable liquid electrolytes. This review presents the fundamentals, structures, thermodynamics, chemistries, and electrochemical kinetics of desirable solid electrolyte interphase (SEI) required to meet the practical requirements of reversible anodes. Theoretical and experimental insights for metal nucleation, deposition, and stripping for the reversible cycling of metal anodes are provided. Ion transport mechanisms and state-of-the-art solid-state electrolytes (SEs) are discussed for realizing high-performance cells. The interface challenges and strategies are also concerned with the integration of SEs, anodes, and cathodes for large-scale SSBs in terms of physical/chemical contacts, space-charge layer, interdiffusion, lattice-mismatch, dendritic growth, chemical reactivity of SEI, current collectors, and thermal instability. The recent innovations for anode interface chemistries developed by SEs are highlighted with monovalent (lithium (Li+ ), sodium (Na+ ), potassium (K+ )) and multivalent (magnesium (Mg2+ ), zinc (Zn2+ ), aluminum (Al3+ ), calcium (Ca2+ )) cation carriers (i.e., lithium-metal, lithium-sulfur, sodium-metal, potassium-ion, magnesium-ion, zinc-metal, aluminum-ion, and calcium-ion batteries) compared to those of liquid counterparts.