Development of KPF6/KFSA Binary-Salt Solutions for Long-Life and High-Voltage K-Ion Batteries

Long-life potassium metal batteries enabled by anion-derived solid electrolyte interphase using concentrated ionic liquid electrolytes

Potassium metal batteries (PMBs) show great potential as next-generation energy storage systems yet face challenges such as the dendritic growth of the potassium anode, leading to issues with cycle life and safety. This study reports a potassium salt-concentrated ionic liquid electrolyte (PCIL) consisting of potassium bis(fluorosulfonyl)imide (KFSI) and 1-methyl-1-propyl pyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI) to achieve long-life and, safe PMBs. PCIL presents several advantages including outstanding oxidation stability (≈5.2 V), decent ionic conductivity (4.0 mS cm-1 at 25 °C), and negligible flammability. Moreover, PCIL promotes the development of anion-derived solid-electrolyte interphase (SEI) with high inorganic content. This not only hinders the growth of potassium dendrites but also facilitates facile interfacial charge transfer kinetics. Benefiting from these advantages, PMBs (K||KVPO4F) employing PCIL exhibit remarkable cycle performances at both ambient and elevated temperatures (capacity retention after 300 cycles: 74.8% at 25 °C and 82.9% at 45 °C), surpassing the performance of conventional carbonate (1 M KPF6 EC/PC) and dilute potassium ionic liquid electrolyte (PIL). This work demonstrates the tangible capability of PCIL in realizing practical PMBs.

Strategies for developing layered oxide cathodes, carbon-based anodes, and electrolytes for potassium ion batteries

Lithium-ion batteries (LIBs) have become the most popular portable secondary energy storage facilities. However, the limited lithium resource results in possible unsustainable development. Potassium-ion batteries (PIBs) are considered promising alternatives to LIBs because of their high resource availability, low cost, and environmentally friendly features. In this field, high energy density layered cathodes and carbon-based anodes are also the main research objectives. However, compared to the most appealing alternative sodium-ion batteries (SIBs), despite having various theoretical advantages, PIBs exhibit poorer electrochemical performance in practice. Their poor capacity retention and narrow working voltage range seriously limit their applications. The performance of the electrodes is usually considered an important factor for battery performance, life, and safety. To solve these problems, many significant research studies have been carried out in the last decade, achieving numerous breakthroughs. Nevertheless, there are still many drawbacks and unclear mechanisms. In this comprehensive review, we examine the current state of high-performance layered oxide cathodes, electrolytes, and carbon-based anodes, to identify potential candidates for PIBs. Our focus lies on their structural characteristics, interface properties, underlying mechanisms, and modification techniques. The viewpoints of these advanced strategies are integrated, and concise development suggestions and strategies are subsequently proposed.

Impact of electrolyte decomposition products on the electrochemical performance of 4 V class K-ion batteries

In the pursuit of long-life K-ion batteries (KIBs), half-cell measurements using highly reactive K metal counter electrodes are a standard practice. However, there is increasing evidence of electrolyte decomposition by K metal impacting electrode performance. Herein, we systematically explored the K metal-treated electrolytes KPF6, KN(SO2F)2 (KFSA), and their combination in ethylene carbonate/diethyl carbonate (EC/DEC), referred to as K-KPF6, K-KFSA, and K-KPF6:KFSA, respectively, after storage in contact with K metal. Through mass spectrometry analysis, we identified significant formation of carbonate ester-derived decomposition products such as oligocarbonates for K-KPF6, while K-KFSA predominantly generates anions combining FSA- with the solvent structures. Using three-electrode cells, we delineated the positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes. The interactions between the decomposition products and the electrodes were further evaluated using density functional theory calculations. Full cell measurements using K-KPF6:KFSA showed an improved energy density and capacity retention of 78% after 500 cycles compared with an untreated electrolyte (72%). Hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which was not observed in K metal-free cells. Overall, this work indicates further complexities to consider in KIB measurements and suggests the potential application of decomposition products as electrolyte additives.

Electrolyte Design Strategies for Non-Aqueous High-Voltage Potassium-Based Batteries

High-voltage potassium-based batteries are promising alternatives for lithium-ion batteries as next-generation energy storage devices. The stability and reversibility of such systems depend largely on the properties of the corresponding electrolytes. This review first presents major challenges for high-voltage electrolytes, such as electrolyte decomposition, parasitic side reactions, and current collector corrosion. Then, the state-of-the-art modification strategies for traditional ester and ether-based organic electrolytes are scrutinized and discussed, including high concentration, localized high concentration/weakly solvating strategy, multi-ion strategy, and addition of high-voltage additives. Besides, research advances of other promising electrolyte systems, such as potassium-based ionic liquids and solid-state-electrolytes are also summarized. Finally, prospective future research directions are proposed to further enhance the oxidative stability and non-corrosiveness of electrolytes for high-voltage potassium batteries.

Exploration of bifunctional Vanadium-based Metal-Organic framework with double active centers for Potassium-ion batteries

Exploration of suitable electrode hosts with large open channels that can reversibly accommodate K⁺ with large radius have been extensively investigated. Nevertheless, the reported inorganic counterparts were inevitably restricted by the difficulty of large K⁺ diffusion capability in crystal structure and the huge volume change. Herein, we report a bifunctional vanadium-based metal-organic framework (MIL-47) with double active centers and larger lamellar spacing that could serve as both cathode and anode material, respectively by controlling the redox potential range in potassium-ion batteries. The results suggest that the stable K-storage mechanism is the reversible rearrangement of the conjugated carboxyl groups of organic terephthalic acid into enolate and the reversible redox activity of V ions, with the specific capacity of 272 mAh g^-1 (0.01-1.5 V) and 50 mAh g^-1 (1.5-3.8 V) at the current density of 10 mA g^-1 for MIL-47 anode and MIL-47 cathode, respectively. The unsaturated functional group of MIL-47 and the intermediate bridged V atom not only provide multi-dimensional channels for electron and ion transport but also stabilize its crystal structure. Additionally, a symmetric full-cell in potassium-ion batteries based on MIL-47 was constructed successfully by avoiding the utilization of K metal with safety concerns. Our results provide new insight into structure design for next-generation large-scale energy storage applications.

Active material and interphase structures governing performance in sodium and potassium ion batteries

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

Superconcentrated NaFSA-KFSA Aqueous Electrolytes for 2 V-Class Dual-Ion Batteries

Superconcentrated aqueous electrolytes containing NaN(SO2F)2 and KN(SO2F)2 (for which sodium and potassium bis(fluorosulfonyl)amides (FSA), respectively, are abbreviated) have been developed for 2 V-class aqueous batteries. Based on the eutectic composition of the NaFSA-KFSA (56:44 mol/mol) binary system, the superconcentrated solutions of 35 mol kg-1 Na0.55K0.45FSA/H2O and 33 mol kg-1 Na0.45K0.55FSA/H2O are found to form at 25 °C. As both electrolytes demonstrate a wider potential window of ∼3.5 V compared to that of either saturated 20 mol kg-1 NaFSA or 31 mol kg-1 KFSA solution, we applied the 33 mol kg-1 Na0.45K0.55FSA/H2O to two different battery configurations, carbon-coated Na2Ti2(PO4)3∥K2Mn[Fe(CN)6] and carbon-coated Na3V2(PO4)3∥K2Mn[Fe(CN)6]. The former cell shows highly reversible charge/discharge curves with a mean discharge voltage of 1.4 V. Although the latter cell exhibits capacity degradation, it demonstrates 2 V-class operations. Analysis data of the two cells confirmed that Na+ ions were mainly inserted into the negative electrodes passivated by a Na-rich solid electrolyte interphase, and both Na+ and K+ ions were inserted into the positive electrode. Based upon the observation, we propose new sodium-/potassium-ion batteries using the superconcentrated NaFSA-KFSA aqueous electrolytes.

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.

Strategies for Harnessing High Rate and Cycle Performance from Graphite Electrodes in Potassium-Ion Batteries

Potassium-ion batteries (PIBs) have been lauded as the next-generation energy storage systems on account of their high voltage capabilities and low costs and the high abundance of potassium resources. However, the practical utility of PIBs has been heavily encumbered by severe K metal dendrite formation, safety issues, and insufficient electrochemical performance during operations─indeed critical issues that underpin the need for functional electrolytes with high thermal stability, robust solid-electrolyte interphase (SEI)-forming capabilities, and high electrochemical performance. In a bid to establish a knowledge framework for harnessing high rate capabilities and long cycle life from graphite negative electrodes, this study presents the physical properties and electrochemical behavior of a high K⁺ concentration inorganic ionic liquid (IL) electrolyte, K[FSA]-Cs[FSA] (FSA^- = bis(fluorosulfonyl)amide) (54:46 in mol), at an intermediate temperature of 70 °C. This IL electrolyte demonstrates an ionic conductivity of 2.54 mS cm^-1 and a wide electrochemical window of 5.82 V. Charge-discharge tests performed on a graphite negative electrode manifest a high discharge capacity of 278 mAh g^-1 (0.5 C) at 70 °C, a high rate capability (106 mAh g^-1 at 100 C), and a long cyclability (98.7% after 450 cycles). Stable interfacial properties observed by electrochemical impedance spectroscopy during cycling are attributed to the formation of sulfide-rich all-inorganic SEI, which was examined through X-ray photoelectron spectroscopy. The performance of the IL is collated with that of an N-methyl-N-propylpyrrolidinium-based organic IL to provide insight into the synergism between the highly concentrated K⁺ electrolyte at intermediate temperatures and the all-inorganic SEI during electrochemical operations of the graphite negative electrode.

Potassium Fluoride and Carbonate Lead to Cell Failure in Potassium-Ion Batteries

While Li-ion is the prevailing commercial battery chemistry, the development of batteries that use earth-abundant alkali metals (e.g., Na and K) alleviates reliance on Li with potentially cheaper technologies. Electrolyte engineering has been a major thrust of Li-ion battery (LIB) research, and it is unclear if the same electrolyte design principles apply to K-ion batteries (KIBs). Fluoroethylene carbonate (FEC) is a well-known additive used in Li-ion electrolytes because the products of its sacrificial decomposition aid in forming a stable solid electrolyte interphase (SEI) on the anode surface. Here, we show that FEC addition to KIBs containing hard carbon anodes results in a dramatic decrease in capacity and cell failure in only two cycles, whereas capacity retention remains high (> 90% over 100 cycles at C/10 for both KPF₆ and KFSI) for electrolytes that do not contain FEC. Using a combination of ¹⁹F solid-state nuclear magnetic resonance (SSNMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS), we show that FEC decomposes during galvanostatic cycling to form insoluble KF and K₂CO₃ on the anode surface, which correlates with increased interfacial resistance in the cell. Our results strongly suggest that KIB performance is sensitive to the accumulation of an inorganic SEI, likely due to poor K transport in these compounds. This mechanism of FEC decomposition was confirmed in two separate electrolyte formulations using KPF₆ or KFSI. Interestingly, the salt anions do not decompose themselves, unlike their Li analogues. Insight from these results indicates that electrolyte decomposition pathways and favorable SEI components are significantly different in KIBs and LIBs, suggesting that entirely new approaches to KIB electrolyte engineering are needed.

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.

Multiscale Investigation of the Diffusion Mechanism within the Solid-Electrolyte Interface Layer: Coupling Quantum Mechanics, Molecular Dynamics, and Macroscale Mathematical Modeling

The solid-electrolyte interface (SEI) layer has a critical role in Li-ion batteries' (LIBs) life span. The SEI layer, even in modern commercial LIBs, is responsible for more than 50% of capacity loss. Due to the inherent complexity in studying the SEI layer, many aspects of its performance and characteristics, including diffusion mechanisms in this layer, are unknown. As a result, most mathematical models use a constant value of the diffusion coefficient, instead of a variable formulation, to predict LIBs' properties and performance such as capacity fading and the SEI growth rate. In this work, by employing a multiscale investigation using a combination of quantum mechanics, molecular dynamics, and macroscale mathematical modeling, some equations are presented to evaluate the energy barrier against diffusion and the diffusion coefficient in different crystal structures in the inner section of the SEI layer. The equations are evaluated as a function of temperature and concentration and can be used to study the diffusion mechanism in the SEI layer. They can also be integrated with other mathematical models of LIBs to increase the accuracy of the latter.

Effect of Particle Size and Anion Vacancy on Electrochemical Potassium Ion Insertion into Potassium Manganese Hexacyanoferrates

Potassium manganese hexacyanoferrate (KMnHCF) can be used as a positive electrode for potassium-ion batteries because of its high energy density. The effect of particle size and [Fe(CN)₆ ]^n- vacancies on the electrochemical potassium insertion of KMnHCFs was examined through experimental data and theoretical calculations. When nearly stoichiometric KMnHCF was synthesized and tested, smaller particle sizes were found to be important for achieving superior electrochemical performance in terms of capacity and rate capability. However, even in the case of larger particles, introducing a suitable number of anion vacancies enabled KMnHCF to exhibit comparable electrode performance. Electrochemical tests and density functional theory calculations indicated that anion vacancies contribute to the enhancement of K⁺ ion diffusion, which realizes good electrochemical performance. Structural design, including crystal vacancies and particle size, is the key to their high performance as a positive electrode.

The Features and Progress of Electrolyte for Potassium Ion Batteries

Nowadays, Li-ion batteries have achieved great success and are widely used in various fields. However, the scarcity and uneven distribution of lithium resources together with the increasing cost may hamper the sustainable development of Li-ion batteries in the future. Hence, many researchers have turned to potassium ion batteries due to their abundant raw materials, low price, and high energy density. Although great progress has been made in recent years, there are still existing many challenges, especially the severe side reaction between electrolyte and K metal, which leads to an unstable solid-liquid interface and low coulombic efficiency. Hence, an excellent electrolyte may be the key to development of K-ion batteries in the future. Unfortunately, no systematic research has been conducted to study the electrolyte and its role on the performance yet. In order to compensate for this limitation, in this paper, the status and progress of electrolytes for K-ion batteries are reviewed, the issues and challenges existing in the development of electrolyte are clarified, and the future development is prospected.