Potassium-ion hybrid capacitors (PIHCs) shrewdly combine a battery-type anode and a capacitor-type cathode, exhibiting an energy density close to that of potassium ion batteries and a comparable power density of supercapacitors. However, the rosy scenario is compromised by the sluggish kinetics in the PIHCs device. Herein, the kinetics enhanced nitrogen-doped hierarchical porous hollow carbon spheres (NHCS) are synthesized and successfully applied to PIHCs. As for the K half-cell, NHCS anchored with sodium alginate delivers excellent electrochemical performance. Further evaluation shows that the binder can significantly affect the potassium storage performance of NHCS by adjusting the coatability and ionic conductivity of the NHCS anode. Moreover, kinetic analysis and density functional theory calculations reveal the origin of the superior electrochemical properties of NHCS. As expected, an advanced PIHC device is assembled with a NHCS anode and an activated NHCS cathode, demonstrating an exceptionally high energy/power density (114.2 Wh kg −1 and 8203 W kg −1 ), along with a long-life capability. The successful construction of high-performance PIHCs in this work opens a new avenue for the development and application of PIHCs in the future.

kind of secondary batteries will find promising applications in large-scale energy storage. [3] Owing to the low-cost [4] and low standard potential [5] of potassium (K), KIBs could in principle offer cost-effectiveness and higher energy density, which would lead to a substantial advance in energy storage technology. The fast K + ion diffusion property provides the possibility of saving the charging time of KIBs as the next generation energy storage and transport devices. [6] Then compared with Na-ion battery (NIB), a huge advantage lies in the application of graphite anode for KIB, [7] which is not feasible for Na + insertion into the graphite. [8] And also, the weaker Lewis acidity of K + ions ensures smaller solvated ions, leading to the higher mobility and larger transference number in electrolytes and lower desolvation energy, which offers faster K + ion diffusion kinetics with regard to that of Li + or Na + ions. [4,9] Based on these advantages, many investigations on the available anode materials for KIBs, such as carbonaceous materials, [8a,10] oxides, [11] sulfides, [12] organic materials, [13] alloy, [5] prussian blue analogues, [14] etc., have been reported continuously in the past few decades, [2a,15] however, fewer studies are devoted to the development of cathode. Present cathodes could be summarized as the layered transition metal oxides, [16] polyanions and pyrophosphates, [17] prussian blue analogs, [18] nanostructured iron composite, [19] organic materials, [20] and other potential materials. Up to now, most of the available studies are working on screening for new electrode materials with enhanced electrochemical performance, while the underlying K storage and transport mechanism in the electrode materials is lack of exploration and comprehension. [2a,21] Therefore, studying the electrochemical behavior of potassium ions is very important for building a better KIBs system.In specific, alkali transition metal oxides (AMO 2 : A = Li, Na, K; M = Ni, Co, Mn, Fe, etc.) with layered structure have been widely studied as promising cathodes for rechargeable batteries, [22] because of the topotactic de/intercalation feature. However, the accommodation of the bulky K + ion is less favorable than that of Li + and Na + ion in such compact structures, [2a] which will plausibly hinder the K + ion diffusion kinetics. [23] Previous reports [16e,24] have shown the sluggish diffusion performance of K + ions in KIBs with alkali transition Novel and low-cost rechargeable batteries are of considerable interest for application in large-scale energy storage systems. In this context, K-Birnessite is synthesized using a facile solid-state reaction as a promising cathode for potassium-ion batteries. During synthesis, an ion exchange protocol is applied to increase K content in the K-Birnessite electrode, which results in a reversible capacity as high as 125 mAh g −1 at 0.2 C. Upon K + exchange the reversible phase transitions are verified by in situ X-ray diffraction (XRD) characterization. The underlying mechani...

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Potassium-ion hybrid capacitors (PIHCs) have attracted tremendous attention because their energy density is comparable to that of lithium-ion batteries, whose power density and cyclability are similar to those of supercapacitors. Herein, a pomegranate-like graphene-confined cucurbit[6]uril-derived nitrogen-doped carbon (CBC@G) with ultra-high nitrogen-doping level (15.5 at%) and unique supermesopore-macropores interconnected graphene network is synthesized. The carbonization mechanism of cucurbit[6]uril is verified by an in situ TG-IR technology. In a K half-cell configuration, CBC@G anode demonstrates a superior reversible capacity (349.1 mA h g −1 at 0.1 C) as well as outstanding rate capability and cyclability. Moreover, systematic in situ/ex situ characterizations, and theory calculations are carried out to reveal the origin of the superior electrochemical performances of CBC@G. Consequently, PIHCs constructed with CBC@G anode and KOH-activated cucurbit[6]uril-derived nitrogen-doped carbon cathode demonstrate ultra-high energy/power density (172 Wh kg −1 /22 kW kg −1) and extraordinary cyclability (81.5% capacity retention for 5000 cycles at 5 A g −1). This work opens up a new application field for cucurbit[6]uril and provides an alternative avenue for the exploitation of high-performance PIHCs.

It is a highly expected avenue to construct dual-carbon sodium-ion hybrid capacitors (SIHCs) using hierarchical porous carbon with interconnected pores, high accessible surface area, and disordered carbon frameworks for ameliorating the sluggish kinetics of SIHCs. In this work, a novel dual-carbon SIHCs system with homologous enhanced kinetics hierarchical porous hollow carbon spheres (HPCS) and hierarchical porous hollow carbon bowls (HPCB) as the anode and cathode is constructed for the first time. In a Na half-cell configuration, the HPCS anode synthesized through a facile one-pot in-situ template route demonstrates a superior reversible capacity as well as outstanding rate capability and cycleability, and the HPCB cathode fabricated by chemical activation of HPCS exhibits excellent capacitive behaviors. Thanks to superior properties and structures of the anode and cathode, the constructed novel dual-carbon SIHCs present an exceptionally high energy/power density (128.5 Wh kg–1 and 11.9 kW kg–1), along with a long cycling lifespan with retained morphology. This study on the kinetics of enhanced dual-carbon SIHCs opens a new avenue for optimizing the microstructure of hierarchical porous carbon and constructing new type of high-performance SIHCs systems.

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The sulfur doping strategy has been attracting extensive interest in potassium-ion battery carbon anodes for the dual potential of improving the capacity and kinetics of carbon anodes. Understanding the doping and potassium storage mechanism of sulfur is crucial to guide the structural design and optimization of high-performance sulfur-doped carbon anodes. Herein, presenting a laboratory-synthesized sulfur-doped hard carbon (SHC) with a sulfur content of 6.4 at. % as an example, we clarify the sulfur doping mechanism and reveal the role of sulfur in potassium storage. The high sulfur content of SHC stems from the selective substitution of sulfur for carbon and the residual trace of sulfur molecular fragments after sulfurization. As a result, thanks to the multifaceted roles of doped sulfur in potassium storage, about twice as much capacity, rate capability, and cycling stability is achieved for SHC against S-free hard carbon at the same test conditions. Furthermore, potassium-ion hybrid capacitors assembled based on an SHC anode demonstrate high energy/power density (139 Wh kg–1/7.3 kW kg–1), along with an extraordinary cycling stability.