Fast Redox Kinetics in Bi‐Heteroatom Doped 3D Porous Carbon Nanosheets for High‐Performance Hybrid Potassium‐Ion Battery Capacitors

Abstract

instance, lithium-ion hybrid capacitors (LIHCs) consisting of an activated carbon (AC) cathode and a prelithiated graphite anode have been successfully commercialized and widely applied in small portable electronics. [2a] Nevertheless, the limited reserves and high cost of Li resources likely hinder them from applying in large-scale energy storage devices. As a new class of hybrid capacitors, potassium-ion hybrid capacitors (PIHCs) show great potential as alternative to LIHCs due to the profusion and low cost of potassium resources. [3] In addition, the redox potential of the K + /K (−2.936 V vs standard hydrogen electrode) is quite close to that of Li + /Li (−3.040 V), indicating PIHCs can afford a considerably high working voltage and energy density. [4] Unfortunately, the relatively large radius of K + (1.38 Å) tends to behave sluggish redox reaction kinetics for most of potassium-ion batteries (PIBs) anodes.As compared to LIHCs, the research regarding PIHCs is still in its infant stage and thus calls for extensive exploration on newly favorable electrode materials, in the hope to enhance the kinetics of K + insertion/extraction so that it can match with the fast kinetics of capacitor-style cathodes. To date, various electrode materials, such as carbonaceous materials, [5] metal alloys, [6] oxides, [7] sulfides, [8] and MXene, [9] have been extensively studied as battery-type anodes. Among them, carbon-based material, thanks to the merits of abundant resources, low cost, and high conductivity, has been regarded as one of the most promising electrode materials for the practical application of PIHCs. [10] However, the implementation of carbon-based electrode material for high-performance PIBs requires high conductivity and a rather large interlayer spacing for facilitating insertion and extraction of bulky K + and improving the redox reaction Kinetics. Therefore, it still remains a grand challenge to address the major issues existed in carbon-based anodes, including limited reversible capacities, unsatisfactory cycling stability, and poor rate performance, which result from the large volume change (61% caused by KC 8 vs 10% by LiC 6 ). [5] Heteroatom-doped carbonbased materials have exhibited the enhanced electrochemical properties for potassium-ion storage due to the increased electric conductivity and more active sites for potassium-ion storage by generating extrinsic defects. [11] In comparison with nitrogen, sulfur has a relatively large size but small electronegativity, Potassium-ion hybrid capacitors (PIHCs) hold the advantages of high-energy density of batteries and high-power output of supercapacitors and thus present great promise for the next generation of electrochemical energy storage devices. One of the most crucial tasks for developing a highperformance PIHCs is to explore a favorable anode material with capability to balance the kinetics mismatch between battery-type anodes and capacitortype cathode. Herein, a reliable route for fabricating sulfur and nitrogen codoped 3D porous carbon nanosheets...

Rational Construction of Nitrogen‐Doped Hierarchical Dual‐Carbon for Advanced Potassium‐Ion Hybrid Capacitors

“…This excellent performance is comparable and even better than the rGO||AC and NMCP||AC PIHCs (Figure S24, Supporting Information) and some representative hybrid capacitors or other batteries reported in the literature (Figure 5e). [ 3–8,44–48 ] This NMCP@rGO||AC full cell can light up a “star‐like” bulb, verifying its promising practical potential, and it can maintain ≈84.7% capacity retention compared with its initial value after 12 000 cycles at 2.0 A g −1 , as well as a nearly perfect CE (Figure 5f). These results reflect a promising future storage device with high energy/power density and long cycling life.…”

“…[ 37 ] Compared with other carbon materials with a high SAA, the high initial CE of 50.8% in NMCP@rGO is ascribed to its unique “dual‐carbon” structure. [ 3,6,8 ] Subsequently, the CE of the NMCP@rGO electrode rapidly rises over 90% after three cycles and approaches ≈100% after 20 cycles, owing to the gradual stabilization of the formed SEI. The proper SAA, as well as the unique hierarchical structure, reduces the side effects and ensures intimate contact between the porous‐based electrode material and the K + ‐electrolyte that facilitates excellent performance.…”

“…From the CV curves, the AC shows a typical double‐layer capacitance behavior. [ 3,5 ] The mass of the AC electrode should be controlled to approximately four times the mass of the NMCP@rGO material, according to the reversible specific capacity balance theory (NMCP@rGO: 236.3 mAh g −1 , in Figure S21, Supporting Information; AC: 58.7 mAh g −1 , in Figure S20f, Supporting Information). These values stimulate the maximum power density and energy density as well as ultralong‐cycling performance of the overall PIHCs.…”

“…[ 1,2 ] Potassium‐ion hybrid capacitors (PIHCs), which integrate the merits of high‐power density as well as ultralong life‐span of supercapacitors and high energy density of potassium ion batteries (PIBs), have attracted much attention and shown great potential. [ 3,4 ] Concerning device structure, PIHCs shrewdly combine a capacitor‐type cathode and battery‐type anode, which means that the properties of the cathode and anode determine the practical application of the PIHCs. [ 5,6 ] Commercial activated carbon (AC) has been selected as the capacitor‐type cathode and has shown favorable performance.…”

“…[ 5,6 ] Further optimization has been accomplished using heteroatom‐doped porous carbon nanosheets, as an enhanced pseudocapacitance effect can be induced in a two‐dimensional (2D) sheet structure. [ 3,6 ] Although significant progress has been made in capacity and kinetics, work focused on cycling stability has been scarce because it relies on the design of a sophisticated multi‐dimensional structure.…”