Carbon-based quantum dots/nanodots materials for potassium ion storage

Carbon-based quantum dots/nanodots materials for potassium ion storage

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Zhanheng Yan^*^,^a^,^b, Weiqing Su^c, Weiwei Xu^a^,^b, Qianhui Mao^a, Lisha Xue^a, Huanxin Li^d, Wuhua Liu^e, Xiu Li^*^,^a^,^b, Qiuhui Zhang^*^,^b

Chinese Chemical Letters | 2025, 36(4) : 110217

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Chinese Chemical Letters | 2025, 36(4): 110217

• Review •

Carbon-based quantum dots/nanodots materials for potassium ion storage

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Zhanheng Yan^*^,^a^,^b, Weiqing Su^c, Weiwei Xu^a^,^b, Qianhui Mao^a, Lisha Xue^a, Huanxin Li^d, Wuhua Liu^e, Xiu Li^*^,^a^,^b, Qiuhui Zhang^*^,^b

Affiliations

^aSchool of Science, Henan University of Engineering, Zhengzhou 451191, China

^bInstrumental Analysis Center, Henan University of Engineering, Zhengzhou 451191, China

^cSchool of Mechanical Engineering, Henan University of Engineering, Zhengzhou 451191, China

^dDepartment of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford South Parks Road, Oxford OX1 3QZ, United Kingdom

^eGuizhou Da Long Hui Cheng New Material Co., Ltd., Tongren 554000, China

Published: 2025-04-15 doi: 10.1016/j.cclet.2024.110217

Outline

Abstract

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With the rapid development of electric vehicles, hybrid electric vehicles and smart grids, people's demand for large-scale energy storage devices is increasingly intense. As a new type of secondary battery, potassium ion battery is promising to replace the lithium-ion battery in the field of large-scale energy storage by virtue of its low price and environmental friendliness. At present, the research on the anode materials of potassium ion batteries mainly focuses on carbon materials and the design of various nanostructured metal-based materials. Problems such as poor rate performance and inferior cycle life caused by electrode structure comminution during charge and discharge have not been solved. Quantum dots/nanodots materials are a new type of nanomaterials that can effectively improve the utilization of electrode materials and reduce production costs. In addition, quantum dots/nanodots materials can enhance the electrode reaction kinetics, reduce the stress generated in cycling, and effectively alleviate the agglomeration and crushing of electrode materials. In this review, we will systematically introduce the synthesis methods, K⁺ storage properties and K⁺ storage mechanisms of carbon quantum dots and carbon-based transition metal compound quantum dots composites. This review will have significant references for potassium ion battery researchers.

Key words

Quantum dots / Nanodots / Potassium ion battery / Anode / Composite material

Cite this Article

Zhanheng Yan, Weiqing Su, Weiwei Xu, Qianhui Mao, Lisha Xue, Huanxin Li, Wuhua Liu, Xiu Li, Qiuhui Zhang. Carbon-based quantum dots/nanodots materials for potassium ion storage[J]. Chinese Chemical Letters, 2025 , 36 (4) : 110217 - . DOI: 10.1016/j.cclet.2024.110217

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1 Introduction

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With the development of electric vehicles and portable devices, large-scale, efficient energy storage equipment is needed. Lithium-ion batteries dominate the energy storage market due to their advantages, such as high energy density and long cycle life [1–4]. However, the uneven distribution of lithium resources and high price limit its application in large-scale energy storage. Because of the low cost, environmental benefit, low redox potential (−2.93 V vs. SHE) and rich reserve in the earth's crust (1.5 wt%) of potassium, potassium-ion batteries promise to become next-generation large-scale storage devices to replace lithium-ion batteries [5–8]. Potassium ion battery has a similar concept to the lithium-ion battery. However, when commercial lithium-ion battery anode material graphite is used in potassium ion batteries, the rate performance and cycling stability are not nearly as good as lithium-ion batteries. This phenomenon could be ascribed to K⁺ having a bigger radius, resulting in poor electrode reaction kinetics. To promote the large-scale application of potassium ion batteries, it is urgent to develop new electrode materials for potassium ion batteries [9,10]. The research on potassium ion batteries is at the initial stage. Typical anode materials of potassium ion batteries include carbon materials (graphene, graphite, porous carbon, biomass-derived carbon) [11–15], metal element (Sn, Sb) [16–18] and carbon-based transition metal compound (metal sulfide, metal carbide, metal phosphide) [19–25]. However, due to the large radius of K⁺, accompanied by severe volume expansion and sluggish kinetic behaviors during the charging and discharging process. The K⁺ storage performance of carbon materials modified through activation and element doping has been significantly improved. Still, the capacity is always low (＜300 mAh/g), which is challenging to meet the needs of high-power potassium ion batteries [26,27].

The storage performance of K⁺ in transition metal compounds can be effectively improved by coating carbon materials or combining them with carbon materials. Still, these methods are challenging in fundamentally solving the problems of electrode structure comminution and volume expansion during charge and discharge [28–30]. Quantum dots or nanodots materials are generally spherical-like nanomaterials with a diameter of less than 10 nm [31]. Quantum dots (nanodots) materials have unique advantages compared with nano/micro-materials, such as effectively shortening the transport distance of ions and electrons and promoting the penetration of electrolytes to enhance the reaction kinetics [32–34]. Another apparent benefit is that the utilization of quantum dot materials is always higher than other structures, which could reduce production costs. When used as electrode materials, metal quantum dot-based materials are often combined with carbon materials, which differs from traditional mechanical mixing and carbon coating. Metal quantum dots are frequently embedded with carbon materials. The most significant advantage of this combination is that it can ensure that carbon materials are evenly wrapped to it so that the quantum dots or nanodots structure can reduce the strain change, effectively restrain volume expansion, improve the cyclic stability and reduce the capacity decay, and solve the electrode expansion problem fundamentally [35–38]. In short, compared with traditional nano/micro-materials, quantum dot materials can effectively improve the cycle stability and specific capacity of potassium ion batteries and reduce production costs simultaneously. It is a promising anode material for potassium ion batteries. This review system introduces quantum dots and nanodot materials applied to the anode of potassium-ion batteries, including synthesized methods, electrochemical performance, and storage mechanisms

2 Carbon quantum dots

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Carbon quantum dots are environmentally and low toxicity, widely applied in sensors, solar cells, organic photovoltaic devices and catalysis [39]. Carbon quantum dots have recently attracted attention in energy storage and conversion. Carbon quantum dots are as wide as the electrode materials of lithium-ion batteries, sodium-ion batteries, supercapacitors and Li-S batteries [40–42]. Carbon quantum dots with high surfaces and plentiful active edge sites could apparently improve the specific capacity of batteries. Apart from this, the ultrasmall sizes of carbon quantum dots could effectively relieve volume expansion during charging and discharging. Hou's group [43] adopted a hard template method to synthesize the honeycomb hard carbon anode material for PIBs. The designed illustration of honeycomb hard carbon is exhibited in Fig. 1a. The researcher used carbon quantum dots as raw material through the calcination and obtained a uniform porous structure with the assistance of the SiO₂ nanosphere. The honeycomb hard carbon possesses plenty of uniform spherical pores on the surface and inner. Thanks to its unique structure, the honeycomb hard carbon exhibits excellent electrochemical performance, the reversible capacity is 195.3 mAh/g at 0.1 A/g and the capacity retention is 67.43% after 150 cycles. Hard carbon has a disordered structure, which could effectively buffer the volume expansion during cycling. Carbon quantum dots could elevate the K⁺ diffusing rate and short ionic diffusion path [44]. Pure carbon quantum dots synthesis and purification were also complex and consuming, which is unsuitable for large-scale applications. Combining hard carbon and carbon quantum dots is a promising strategy to improve the performance of potassium ion batteries. Based on the above strategies, Zhou's group [45] fabricated carbon quantum dots and hard carbon composite material (CQDHC) through a low-temperature pyrolysis process, the designed illustration of CQDHC is exhibited in Fig. 1b. The carbon quantum dots uniform embedding in the hard carbon not only improves K⁺ adsorption performance but also could effectively avoid carbon quantum dots self-aggregation and enhance the stability of CQDHC. Based on the synergy effect of carbon quantum dots and hard carbon, the CQDHC exhibits excellent K⁺ storage performance, the capacity is 237 mAh/g at 100 mA/g and the capacity retention rate is 75.7% after 150 cycles. The performance of CQDHC is better than that of most hard carbon materials and pure carbon quantum dots, suggesting that combining carbon quantum dots and hard carbon is a promising strategy to improve K⁺ storage performance. The hollow structure was beneficial to electrolyte penetration and short ionic transport path and could effectively relieve volume expansion during cycling [46,47]. Ji's group [48] adopted carbon quantum dots and elaborately synthesized hollow nanostructured N-doped carbon (p-HNCs). The hollow N-doped carbon schematic in Fig. 1c is poly pyrrole as the precursor and carbon quantum dots as templates and pore-forming. During the synthesis process carbon quantum dots through hydrophobicity and lipophilicity to construction of hollow structure. Besides, carbon quantum dots decomposition and releasing gases during the calcination process and formation rich micro-tunnels. The structure of hollow and rich tunnels could enhance the reaction kinetics and accommodate the volume expansion during the charging and discharging process, improving the rate performance and cycling stability of potassium ion batteries. Based on the above advantages, when used as the anode of potassium ion batteries, p-HNCs exhibited a specific capacity of 254 mAh/g at 0.1 A/g after 100 cycles, even though at 1.0 A/g the reversible capacity still has 160 mAh/g after 800 cycles. Apart from p-HNCs also show desirable rate performance of 145 mAh/g at 4.0 A/g. This work exhibited a special application of carbon quantum dots in structure construction and applied for potassium ion batteries.

3 Transition metal carbides

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Graphene, carbon nanotubes and other porous carbon are widely used in the anode of potassium ion batteries. In practice, carbon materials always show excellent cycle stability but are inferior in rate performance and capacity [7,49,50]. Transition metal carbide has apparent advantages in energy storage areas, such as high electronic conductivity and chemical and mechanical stability. Unfortunately, the transition metal carbide always suffers from volume expansion and structure collapse during cycling. Based on the above problems, some researchers combined carbon materials and nanostructured transition metal carbide to reduce the size of synthesized carbon-based transition metal carbide quantum dots composite materials [51–54]. Carbon-based transition metal carbide quantum dots composite materials possess inherent advantages, such as higher utilization of quantum dots materials, embedding in carbon matrix could prevent aggregation. The synergistic effect of carbon materials and transition metal carbide quantum dots can significantly improve the performance of potassium ion batteries.

Wang's group adopted an electrospinning technique to synthesize novel flexible and free-standing Mo₂C quantum dots and N-doped carbon nanofibers composites [55]. As shown in Figs. 2a and b, Mo₂C quantum dots with the size of 1–2 nm and embedding in N-doped carbon nanofibers (Mo₂C/NCNFs), the rational structure with some natural advantages, Mo₂C quantum dots embedding carbon fibers can not only improve the conductivity but also alleviated the volume change and pulverization of Mo₂C during the charging-discharging process. Besides, the N-doped carbon fibers can promote K⁺ and electron transport and enhance electrode reaction kinetics. Based on the above advantages, Mo₂C/NCNFs exhibit excellent K⁺ storage performance, and the reversible capacity of the Mo₂C/NCNFs retains 199 mAh/g after 100 cycles at 0.1 A/g, which is higher than NCNFs (Fig. 2c). Besides, the Mo₂C/NCNFs also exhibit excellent rate performance. The reversible capacity could reach 237 mAh/g and 107 mAh/g at the current density of 50 mA/g and 1 A/g, respectively (Fig. 2d). As shown in Fig. 2e, the long cycle performance of Mo₂C/ NCNFs is investigated, with the capacity only a minor decay after 1000 cycles, exhibiting excellent cycling stability. The outstanding electrochemical performance of Mo₂C/ NCNFs is attributed to Mo₂C quantum dots providing most of the capacity and maintaining excellent structural stability during cycling. Besides, the interconnected nanofiber networks promote ion and electron transport.

Owing to high electrical conductivities, outstanding chemical stability, and cheap precursors, vanadium carbide is widely applied in lithium-ion batteries [56–58]. Qu's group adopted a solution combustion and subsequent carbothermal reduction method synthesis non-stoichiometric VCx quantum dots embedding 3D foam-like macro porous carbon micro sheets (CVCx-QDs/nFCM) as an anode of PIBs [59]. The fabrication process of CVCx-QDs/nFCM is exhibit in Fig. 3a. Fig. 3b shows the quantum dots with a lattice distance of 0.206 nm, corresponding to the (200) plane of cubic vanadium carbide. VCx quantum dots coated by carbon are stable and can efficiently avoid collapse during cycling. The composite materials contain high vacancy concentrations, which can increase ion conduction and provide abundant active sites for energy storage. Besides, the foam-like carbon micro sheets can increase electrode and electrolyte connect areas and K⁺ mobility. Based on the above advantages, CVCx-QDs/nFCM possess excellent rate performance. As shown in Fig. 3c, the reversible capacity of CVCx-QDs/2FCM is 301, 256, 184, and 161 mAh/g at 0.1, 0.2, 1, and 2 A/g, respectively. Apart from, CVCx-QDs/2FCM also exhibit outstanding cycle stability; the capacity retention reaches up to 89.9% after 3000 cycles at 0.4 A/g (Fig. 3d). The electrochemical performance of CVCx-QDs/2FCM is better than that of reported anode materials of potassium ion batteries. These results indicate that transition metal carbide quantum dots have broad application prospects in potassium storage.

4 Transition metal sulfides/selenides

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Owing to high theoretical capacity, excellent chemical stability and appropriate interlayer spacing, transition metal sulfides/selenides are widely used in secondary batteries [60,61]. At present, transition metal sulfides such as MoS₂, FeS₂, SnS, ZnS and VS₂ and transition metal selenides such as CoSe₂/FeSe₂, NbSe₂, and NiCo₂Se₄ have been applied in PIBs [62–69]. Guo's group report a two-step hydrothermal method synthesized the CoS quantum dot and graphene composite(CoS@G) [70]. Fig. 4a displays an SEM image of CoS@G-25 (containing 25% graphene). CoS quantum dot nanoclusters are uniformly anchored on graphene nanosheets in these composites. Through the high-resolution TEM images of CoS@G-25 (Figs. 4b and c), we see that the CoS nanocluster is composed of interconnected quantum dots with a diameter of 10–20 nm. With the help of interactions between metal ions of the quantum dot materials and graphene, the CoS quantum dot could be uniformly anchored on the surface of graphene nanosheets without aggregation. Quantum dot materials have a large specific surface, high utilization and short ion transfer distance; graphene possesses a large surface area and high conductivity: The CoS quantum dot and graphene synergy effect endow CoS@G-25 with excellent electrochemical performance. Fig. 4d shows the cycling performance of composites with different quantum dot content at 0.5 C. CoS@G-25 electrode exhibits excellent cycling performance, the capacity is 438 mAh/g after 100 cycles and the capacity retention reaches up to 80%. However, the capacity of pure CoS electrodes causes severe fading, which is only 126.3 mAh/g after 60 cycles. CoS@G-25 also exhibits outstanding rate performance for PIBs (Fig. 4e).

As shown in Fig. 5a, Kuang's group adopted a simple self-template method to synthesize FeS nanodots embedded in S-doped porous carbon (FeS@SPC) [71]. The TEM images of FeS@SPC show that the sample consists of cross-linked ultra-thin porous carbon (Fig. 5b). The monodispersed FeS nanodots with an average diameter of 5 nm were uniformly embedded into the S-doped porous carbon without apparent aggregation (Fig. 5c). FeS@SPC's unique cross-linked porous structure can enhance the electrode's stability, relieving the volume expansion of FeS during cycling. S-doped can expand the lattice distance of carbon, which is beneficial to K⁺ transport and enhances the electrode reaction kinetics. Besides, nanodot structure materials possess high utilization; FeS@SPC exhibited excellent performance in the lower content of FeS nanodot when used as the anode of PIBs. Based on the synergistic of cross-linked S-doped porous carbon and FeS nanodot, the FeS@SPC exhibits an excellent rate performance and cycling stability. As shown in Fig. 5d, the capacity of FeS@SPC is 388 ± 127.5, 200 ± 2.7 and 140 ± 3.2 mAh/g at the current densities of 0.1, 1, and 5 A/g. After 100 cycles, the reversible capacity of FeS@SPC is 309 mAh/g at 0.1 A/g, even though in high current density (1 A/g) (Fig. 5e), the FeS@SPC also keeps a reversible capacity of 232 mAh/g after 3000 cycles (Fig. 5f). Due to the absence of the FeS nanodots, the capacity of SPC is lower than FeS@SPC. To search for the reasons for the stability of FeS@SPC, the researcher investigated the structure of FeS@SPC after 3000 cycles at 1 A/g. As shown in Fig. 6a, the initial petal structure of FeS@SPC is broken and partially occurred aggregation, but the FeS nanodots remain intact without accumulation. The result manifests that the nanodot structure has excellent stability. Besides, K⁺ adsorption behavior on the FeS@SPC was investigated by DFT (Fig. 6b). The research results indicated that the adsorption energy of K on FeS@SPC is higher, suggesting embedded FeS nanodots make FeS@SPC more beneficial to K storage. Xie's group also adopted the solvothermal method to synthesize morphology-controllable ZnS quantum dots loading on graphene (ZnS QDs-rGO) (Fig. 6c) [72]. TEM analysis suggests that ZnS quantum dots with a diameter of 2.8 nm have no apparent aggregation (Fig. 6d). Due to the large surface of graphene and excellent stability of ZnS quantum dots, ZnS QDs-rGO has excellent electrochemical performance as the anode of PIBs. The capacity of ZnS QDs-rGO is 350.4 mAh/g after 200 cycles at 0.1 A/g, and capacity retention reaches 99.8% (Fig. 6e). Besides, the rate performance of ZnS QDs-rGO is also outstanding. As shown in Fig. 6f, the capacity of ZnS QDs-rGO is 340.9 and 132.2 mAh/g at 0.05 and 1 A/g, respectively. Yu's group embedding Sn₂S₃ quantum dots with the diameter of 1.6 nm in an N, S co-doped carbon fiber network (Sn₂S₃—CFN). As an anode of potassium ion battery, Sn₂S₃—CFN exhibits outstanding rate performance (165.3 mAh/g at 5 A/g) and cycling stability (166.3 mAh/g after 5000 cycles at 2 A/g), which could attributed to the synergistic effect of the Sn₂S₃ quantum dots and the carbon network [73].

Transition-metal selenides with the merits of high theoretical capacity, narrow bandgap, and intrinsic safety, are promising alkali-ion batteries anode materials. At the same time, their intrinsic poor conductivity and serious volume change during the K⁺ intercalation inhibit extensive application. Reducing the size of transition-metal selenides to quantum dot scale is an effective method to address volume expansion. Apart from, encapsulation by carbon materials is another effective method to improve the stability and conductivity [20,74,75]. Based on the above synthetic strategy, Wang's group adopted the solvothermal approach of synthetic Co₃Se₄ quantum dots (QDs) encapsulated by N-doted carbon as the anode of PIBs [76]. The schematical of the Co₃Se₄ QDs synthesis process is exhibited in Fig. 7a. TEM was used to research the microstructure of the composite. The result of TEM images (Figs. 7b and c) indicated that Co₃Se₄ QDs were homogenously encapsulated by a carbon frame with an average diameter of 5 nm. This unique core-shell structure could effectively decrease the aggregation and exfoliation of QDs during the cycling, remarkably enhancing the stability of electrode materials. Due to the synergistic effect, the reversible capacity of Co₃Se₄ QDs encapsulated by N-doped carbon (CSC) could maintain 407 mAh/g after 500 cycles at 0.1 A/g (Fig. 7d). The rate performance of CS (Co₃Se₄ nanosheets) and CSC (Co₃Se₄ QDs encapsulated by N-doped carbon) was expressed in Fig. 7e. The capacity of CSC was 370, 348, 241 and 202 mAh/g at 0.1, 0.2, 1.0 and 2.0 A/g, respectively, which is better than CS. The excellent rate performance of CSC is attributed to the unique core-shell nanostructure. Besides, density functional theory calculation results show that the K atom intercalation reaction is more favorable and Co₃Se₄ QD possesses a larger reaction energy and lower K atom migration than bulk Co₃Se₄ (Figs. 7f and g). Li's group also adopted a hydrothermal method to synthesize the composite of metallic cobalt selenide quantum dots (Co_0.85Se-QDs/C) [77]. The synthesized path of Co_0.85Se-QDs/C is in Fig. 8a. Fig. 8b shows Co_0.85Se-QDs exhibit a hollow polyhedral framework consisting of carbon petals flake.

This unique tertiary structure, including quantum dots, petals flakes and hollow micro polyhedron framework, can take advantage of the synergistic effect of microscale features and nanoscale, which is beneficial to potassium ion and electron transport and could effectively restrain the volume expansion and structural collapse. Owing to the unique structure, Co_0.85Se-QDs/C exhibits excellent potassium ion storage performance. As shown in Fig. 8c, Co_0.85Se-QDs/C-20 (the mount of hydrazine hydrate is 20 mL) indicates reversible capacities of 369, 296, and 253 mAh/g at 100, 500, and 1000 mA/g, respectively. The result of the cycle stability test in Fig. 8d shows that the Co_0.85Se-QDs/C-20 delivers a reversible capacity of 228 mAh/g at a current density of 1 A/g after 500 cycles. The excellent cycle stability is attributed to the stable tertiary structure. More importantly, the synthesized method can promote the synthesis of other metal selenide quantum dot composites.

Copper antimony sulfide (Cu₁₂Sb₄S₁₃) with the advantages of unique cell arrangement and multivalent states of copper ions, has been widely applied in energy fields [78,79]. When used as the anode of potassium ion battery, Cu₁₂Sb₄S₁₃ always confronts serious volume expansion and insufficient electrode dynamic. To solve the above problems, Yang's group synthesized the composite of Cu₁₂Sb₄S₁₃ quantum dots and few-layered Ti₃C₂ nanosheets (CAS-Ti₃C₂) [80]. The scheme for the fabrication of CAS-Ti₃C₂ is shown in Fig. 9a. Firstly, through a hot injection method, synthesized Cu₁₂Sb₄S₁₃ quantum dots (CAS). Subsequently, the formation of the Ti-S bond makes Cu₁₂Sb₄S₁₃ quantum dots tightly anchored on the surface of Ti₃C₂ nanosheets. The high-resolution TEM analysis results indicate that CAS with an average diameter of 5.7 nm (Fig. 9b) is distributed on the surface of Ti₃C₂ nanosheets in a monodisperse state (Fig. 9c). Cu₁₂Sb₄S₁₃ quantum dots can shorten ion and electron transport paths and remission volume expansion during charging and discharging. The high specific surface of Ti₃C₂ nanosheets provides abundant active sites for potassium ion storage and, more importantly, effectively avoids the accumulation of Cu₁₂Sb₄S₁₃ quantum dots. Thanks to the synergistic effect of Cu₁₂Sb₄S₁₃ quantum dots and Ti₃C₂ nanosheets, CAS-Ti₃C₂ exhibits reversible capacity of 378.7, 241.8, 163.3 mAh/g at the current density of 0.5, 1.0 and 5.0 A/g, respectively, superior to bare Cu₁₂Sb₄S₁₃ quantum dots or Ti₃C₂ nanosheets (Fig. 9d). Besides, CAS-Ti₃C₂ also possesses excellent cyclic stability. The reversible capacity maintains 175.6 mAh/g after 1800 cycles at 1.0 A/g, much better than most transition metal disulfide-based anode materials (Fig. 9e).

5 Transition metal nitrides

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Due to their high safety, high theoretical capacity and low cost, vanadium-based materials are attracting attention in energy storage and conversion [81–83]. Due to their high capacity and environmental benefits, vanadium-based nitrides have been applied in potassium-ion batteries. However, serious volume expansion and dissolution of vanadium-based nitrides during the charging and discharging hinder their large-scale application. To solve the above abstracts, Qian's group adopted electrospinning and subsequently ammonia reduction method fabricated vanadium nitride quantum dots and carbon fiber composite (VN/CNF) [84]. The synthesis roadmap of VN/CNF is shown in Fig. 10a. The TEM analysis indicates that the VN quantum dots with an average diameter of 5 nm are uniformly embedded into the carbon fibers (Fig. 10b). Carbon nanofiber structures possess many advantages, including excellent stability and electrode conductivity. Cross-linked carbon fibers can provide high-speed ion and electron transport channels, improving electrode reaction dynamics. Furthermore, carbon fibers can effectively restrain VN aggregation and structure collapse, improving the cycling stability of electrode. Based on unique structure and composition, VN/CNF exhibits excellent electrochemical performance in potassium ion batteries. As shown in Fig. 10c, VN/CNF delivers a reversible capacity of 270, 225, 205 and 187 mAh/g at 100, 500, 1000 and 2000 mA/g, respectively. The method of synthesis VN/CNF is simple and suitable for mass production. Therefore, we have enough reasons believe that VN/CNF has bright prospect in potassium ion batteries. Besides, Qu's group through solution combustion and ammonia reduction method successfully embedding VN quantum dots with the average diameter of 5 nm encapsulated in N-doped mesoporous micro sheets (VN-QDs/CM) [85]. The synthesis route of VN-QDs/CM is shown in Fig. 10d, all process takes only 2 min, making it ideal for mass production. The ultra large N-doped carbon sheets not only improve the transport of potassium ion but also restrain the quantum dots volume expansion and break away from the electrode. Based on the structure advantages VN-QDs/CM deliver a reversible capacity of 261, 215, 187 and 152 mAh/g at 0.1, 0.5, 1 and 2 A/g, respectively (Fig. 10e). Subsequently, researchers investigated the structure change of VN-QDs/CM-600 through TEM. As shown in Fig. 10f, after 100 cycles the morphologies of VN quantum dots almost no change. The result indicated that quantum dots have excellent cycle stability. Through the above two papers, we found that the quantum dot structure can well solve the problem of short cycle life of transition metal nitrides, and this discovery will promote the large-scale application of transition metal nitrides in potassium ion batteries.

6 Transition metal phosphides

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Owing to high theoretical capacity and environmentally friendly, transition metal phosphides have been considered promising anode materials for potassium ion batteries. However, in the practical application transition, metal phosphides always cause irreversible structure collapse, resulting in their inferior rate and cycle performance. Besides, the synthesis method of transition metal phosphides is complicated and always accompanied by toxic byproducts [86–88]. Deng's group adopted a facial and green strategy and synthesized a novel “bubble-in-bowl” (BIB) structured transition metal phosphide nanodot and N, P co-doped carbon (NPC) composites [89]. In the synthesized process, bio-organisms (oryzae spores) were adopted as bio-fuels and phosphorus sources. The benefit to the multiple functional groups from oryzae spores could efficiently avoid transition metal ion aggregation. Through a facile strategy, the researcher succeeded embedded CoP nanodots in N, P co-doped carbon (CoP@NPC). The schematic illustrations of “bubble-in-bowl” (BIB) structured CoP@NPC are exhibited in Fig. 11a. As shown in Fig. 11b, the structure of the CoP@NPC BIB composite is the hollow center and possesses a wrinkled surface. The TEM images of CoP@NPC (Figs. 11c and d) showed that CoP nanodots with an average diameter of 2 nm were evenly depressed on the surface and inside of the particle shell. This unique structure could effectively remission volume expansion during charging and discharging, improving durability. This design is very clever. Currently, the design of composites of metal quantum dots and carbon is mainly focused on the morphology and the size of the metal. Still, there is a lack of design and research on the fine structure of the material surface. Biomimetic design has many advantages. Materials prepared by mimicking cell models and skin can effectively reduce the occurrence of side reactions and produce stable SEI films during charging and discharging. Biological models such as neural networks, cells, synapses and other organisms provide new ideas for preparing quantum dot materials [90,91]. CoP@NPC exhibits excellent potassium ion storage prosperity based on the bubble-in bowl structure. To examine CoP@NPC's practical ability, potassium ion hybrid capacitors (PIHCs) were fabricated by bio-mass derived carbon cathode and CoP@NPC anode (Fig. 11e). This kind of PIHCs can supply power to a small fan in a curved state, showing a broad application prospect (Fig. 11f). In addition, its charge-discharge curve shows that the energy storage mechanism is a mixed mechanism of faraday reaction and non-faraday reaction (Fig. 11g). The capacitor possesses superior low-temperature resistance. As shown in Fig. 11, Fig. 11, the capacity remains at 82% when the temperature is low at −15 ℃. The traditional synthesis transition metal phosphorus method always needs extra phosphorus sources. Kuang's group adopted a self-templated method through reduced phosphate synthesized mono-dispersed MoP nanodots and porous carbon composited (MoP@PC) [92]. The synthetic routes of MoP@PC are exhibited in Fig. 12a; mixing ammonium phosphomolybdate trihydrate, sodium citrate and annealing treatment, successfully fabricated MoP nanodots evenly dispersed on porous carbon. As shown in Fig. 12b, the morphology of MoP@PC is cross-linked nanosheets. High-resolution transmission microscope results show MoP nanodots with an average diameter of 4 nm homogeneous distribution in porous caron (Fig. 12c).

Benefiting from the unique structure, MoP@PC exhibited superior electrochemical properties for potassium ion batteries. The rate performance of MoP@PC is shown in Fig. 12d. MoP@PC exhibited reversible 460, 250, and 208 mA/g capacity at the current density of 0.1, 1 and 5 A/g, respectively. The rate performance of MoP@PC is superior to most materials at present. It is worth noting that MoP nanodots exhibit excellent structure stability after long cycles at high current density and still maintain complete structure without any aggregation and collapse (Fig. 12e). To promote potassium ion batteries practically, Kuang's group select green and cheap raw materials and synthesize transition metal phosphide nanodot composite materials. The schematic diagram of the Ni₂P nanodot composite is shown in Fig. 12f [93]. In this process, gelatin and phytic acid are carbon, nitrogen and phosphorus sources, respectively. The several groups in the gelatin provide rich anchor points of metal ions, which could effectively avoid the aggregation of metal ions in the solution [94,95]. After calcined and HCl etching, Ni₂P nanodots and N, P co-doped porous carbon (Ni₂P@NPC) composited was obtained. The TEM image of Ni₂P@NPC was shown in Fig. 12g, monodisperse Ni₂P nanodots with an average diameter of 4 nm uniformly embedded in carbon material. The Ni₂P@NPC exhibits excellent reversible capacity of 335, 203 and 158 mAh/g at 0.1, 1 and 2 A/g, respectively (Fig. 12h). Ni₂P@NPC also exhibits excellent cycle stability; the capacity remains 212 mAh/g after 5000 cycles at the current density of 1 A/g (Fig. 12i). The results of the above manifest nanodot structure have excellent structural stability. Cheap raw materials, simple synthesis routes and excellent electrochemical give Ni₂P@NPC broad prospects.

7 Metal element

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Owing to higher energy density, Bi, Sb, and Sn metal possess broad prospects in potassium ion storage [96–99]. Bi with high theoretical capacity, large lattice spacing, environment friendly and nontoxicity, is an ideal anode material for potassium ion batteries. Unfortunately, Bi alloy/dealloying reaction with K always accompanies severe volume expansion and structural collapse. Bi as an anode of potassium ion batteries exhibits poor rate performance, inferior cycling life and slow reaction kinetics [97,100,101]. To solve the above problems, Tai's group by reducing sodium bismuthate/graphene composites, obtained hierarchical bismuth nanodots/graphene composites(BiND/G) [102]. The TEM image of BiND (Fig. 13a) displays sheet structures. The high-resolution TEM result (Fig. 13b) shows that isolated ultrafine Bi nanodots with a diameter of 3 nm are uniformly confined in graphene sheet structures. These unique structures can relieve volume expansion and enhance reaction kinetics during the alloying and dealloying. The hierarchical graphene structures in the composites could enhance Bi nanodots conductivity. Based on the synergistic effect of Bi nanodots and graphene, the BiND/G composites exhibit excellent electrochemical properity in potassium ion batteries. As shown in Fig. 13c, the reversible capacity of BiND/G is 320, 230, 225, and 215 mAh/g at 0.1, 1, 2, and 5 A/g, respectively. When the current density increased to 10 A/g, still maintain 200 mAh/g reversible capacity, exhibits outstanding rate performance. Besides, the capacity retention rate of BiND/G is 99% and 88% after 500 cycles at 5 A/g and 10 A/g (Fig. 13d), respectively, which is superior to most anode materials in PIBs at present. To resolve Bi pulverization during cycling, Tong's group combined Bi with metastable alloys synthesized metastable Bi:M (M = Co, Fe) alloys [103]. Metastable alloys could stop the re-alloying reaction upon charging process, effectively remission structure pulverization of electrode material. As shown in Fig. 13e, researchers through a simple solvothermal and subsequently annealing their metal-organic frameworks (MOF) precursors method synthesized metastable Bi:Co and Bi:Fe alloys nanodots and carbon composites (Bi:Co and Bi:Fe@C). The TEM pictures of Bi:Co@C manifest the Bi:Co nanodots are uniformly distributed in the carbon matrix and the carbon is amorphous nature (Fig. 13f). In the composite material, the rules of Co are important including as conductive binders, prevent Bi aggregation, accelerate the reaction kinetics during cycling. Based on above advantages, Bi:Co@C exhibit broad prospect in potassium ion battery. As shown in Fig. 13g, the reversible capacity of Bi_0.85Co_0.15@C is 364, 258 and 178 mAh/g at 0.2, 5 and 20 A/g, respectively. When the current density returned 0.2 A/g, the capacity no obvious decrease. To further research application potential of Bi_0.85Co_0.15@C, as shown in Fig. 13h, the full potassium ion batteries compose of PTCDA-0C (3,4,9,10-perylene-tetracarboxylic acid-dianhydride and hydrazine) cathode, Bi_0.85Co_0.15@C anode and 1 mol/L KPF₆/DME electrolyte. The assembled potassium ion full battery exhibits outstanding electrochemical performance, the capacity retention up to 86% after 100 cycles at 0.5 A/g (Fig. 13i). The experiment results manifest Bi:Co@C nanodots composite is promising for potassium ion battery and the method of proposed metastable alloys could provide reference for synthesis new type anode materials for potassium ion battery.

Sb is a promising anode material for potassium ion batteries because of its high theoretical capacity (660 mAh/g). However, as a typical alloying type anode material, Sb metal always causes volume expansion and structural collapse during charging and discharging, which resulting poor rate performance and cycling stability [104–106]. To solve the above troubles, Wang's group designed a hierarchical antimony single atoms (SAs), quantum dots (QDs) and Ti₃C₂T_x MXene-based aerogel composite (Sb SQ@MA) [107]. The synthesis routes of Sb SQ@MA are exhibited in Fig. 14a. The functional groups on the surface of Ti₃C₂T_x nanosheets provide an amount sits to absorb Sb³⁺ and anchor the Sb atoms. The citric acid can control Sb growth, ensuring the synthesis of Sb single atoms and quantum dots. Through facile hydrothermal and subsequent calcination steps, they successfully synthesized Sb SQ@MA. The high-resolution TEM image of Sb SQ@MA (Fig. 14b) exhibits the Sb quantum dots isolate embedding in the Ti₃C₂T_x nanosheets with a diameter of about 5 nm. The high-angle annular dark-field scanning TEM (HAADF-STEM) image in Fig. 14c proves the existence of Sb single atoms. The electrochemical performance of Sb SQ@MA is exhibits in Figs. 14d and e. The reversible capacity of Sb SQ@MA is 447, 364, 299 and 246 mAh/g at 0.2, 0.8, 1.6 and 3.2 A/g, respectively. The capacity does not obviously decay when the current density reverses to 0.1 A/g (Fig. 14d). Apart from, the Sb SQ@MA also presents excellent cycling stability; the capacity retention reached 94% after 1000 cycles at 1 A/g (Fig. 14e). To survey the K⁺ storage mechanism of Sb SQ@MA, the researcher adopted the operando XRD technique, and the results are exhibited in Fig. 14f. The Sb phase became wake and disappeared during the first discharge process, which proves Sb transformed to K_xSb. During the charging process, the peaks of K_xSb gradually disappeared, and no related peaks belonged to hexagonal Sb, indicating that the Sb was transformed into an amorphous phase. The second cycle peaks are similar to the first cycle, suggesting that the Sb SQ@MA anode possesses excellent reversibility. Electrospinning is another common method for synthesizing nanomaterials. Tang's group adopted the electrospinning technique in situ embedding Sb nanodots in N-doped nanowires (Sb@NCNWs) [108]. The synthetic routes of Sb@NCNWs are exhibited in Fig. 14g. The SEM and TEM results indicate that Sb@NCNWs possess a cross-linked three-dimensional structure (Fig. 14, Fig. 14), the Sb nanodots with a diameter of 5 nm uniformly embedding in N-doped carbon nanowires with a diameter of 200–300 nm. The nanowire structure could effectively resume volume expansion during cycling and improve stability. The conductivity of Sb nanodots was enhanced through a coated carbon layer. Electro spun fibers can provide transport channels for K⁺ and significantly enhance the conductivity of electrode materials [109]. Based on the above advantages, Sb@NCNWs have outstanding K⁺ storage performance. In addition, an integrated flexible electrode can be prepared by electrospinning technology to improve the electrode's mechanical properties, thermal stability and wettability [110]. To verify practicality, the potassium dual-ion batteries (PDIBs) were composed with Sb@NCNWs anode and expanded graphite cathode (Sb@NCNWs//EG). The working mechanism of PDIBs is shown in Fig. 14j. Fig. 14k exhibits the rate performance of the PDIBs; the reversible capacity of the PDIBs is 218, 152 and 94 mAh/g at the current density of 0.2, 0.6 and 1.0 A/g, respectively. The outstanding performance of PDIBs can be ascribed to Sb unique nanodot structure and cross-linked carbon matrix. This work could promote the application of PDIBs.

8 Summary and outlook

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As a new type of secondary battery, potassium ion batteries are expected to replace lithium-ion batteries in large-scale energy storage, such as electric vehicles and smart grids, so potassium ion batteries are a hot spot in the field of energy storage research today. Electrode materials play a crucial role in the performance of potassium ion batteries, and it is very significant to develop suitable electrode materials for the large-scale application of potassium ion batteries. As a unique nanomaterial, quantum dots/nanodot materials have been widely studied for potassium-ion batteries. This paper summarizes the synthesis, electrochemical performance testing and storage mechanism of quantum dots/nanodots in potassium-ion batteries, which will positively promote the research of potassium-ion batteries. Although quantum dots/nanodots materials have many advantages in potassium ion storage, there are still some challenges: (1) The synthetic routes of quantum dot material are relatively complex, and it is not easy to achieve large-scale production. At present, quantum dot-based materials have the problem of single structure and irregular shape. If quantum dot-based materials can be designed into bionic structures, such as synapses or neural networks, according to bionic principles, potassium storage performance will be further improved. (2) It is difficult to accurately control the size of quantum dots/nanodots. (3) The potassium-ion storage mechanism of quantum dots/nanodots materials is still unclear, and advanced characterization techniques need to be developed to research the principle of potassium-ion storage. Through the research literature and the author's research experience, quantum dots/nanodot materials will have several development directions in the potassium ion storage field. (1) The synthesis process of quantum dots/nanodot materials tends to be simplified and convenient for large-scale production and the structure is diversified, more benefit to the transport of electrons, and has better mechanical properties. (2) The raw materials for synthesizing quantum dots/nanodots are green and pollution-free. No harmful substances are produced during the synthesis process. (3) Currently, the storage mechanism of K⁺ in quantum dot materials/nanodots is relatively vague, so it is of great significance to reveal the storage mechanism of K⁺ in quantum dot materials for large-scale applications. (4) Advanced characterization and analysis techniques (in situ Raman, in situ TEM and DFT) will be used to investigate the interfacial reactions, structure and energy changes of quantum dots/nanodot materials during charging and discharging.

Declaration of competing interest

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I would like to stress that: (ⅰ) the manuscript has not been published, or is not under consideration for publication elsewhere including the internet, (ⅱ) there are no intellectual property and/or conflict of interest issues, and (ⅲ) all co-authors have read and approved the manuscript.

CRediT authorship contribution statement

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Zhanheng Yan: Writing – review & editing. Weiqing Su: Visualization. Weiwei Xu: Data curation. Qianhui Mao: Data curation. Lisha Xue: Data curation. Huanxin Li: Supervision. Wuhua Liu: Supervision. Xiu Li: Funding acquisition. Qiuhui Zhang: Funding acquisition.

Acknowledgments

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This work received financial support from the Doctoral Foundation of Henan University of Engineering (No. D2022025); National Natural Science Foundation of China (No. U2004162); National Natural Science Foundation of China (No. 52302138); Key Project for Science and Technology Development of Henan Province (No. 232102320221).

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Year 2025 volume 36 Issue 4

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doi: 10.1016/j.cclet.2024.110217

Receive Date：2024-05-13
Online Date：2025-11-19
Published：2025-04-15

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Received：2024-05-13
Revised：2024-06-13
Accepted：2024-07-04

Affiliations

^aSchool of Science, Henan University of Engineering, Zhengzhou 451191, China

^bInstrumental Analysis Center, Henan University of Engineering, Zhengzhou 451191, China

^cSchool of Mechanical Engineering, Henan University of Engineering, Zhengzhou 451191, China

^dDepartment of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford South Parks Road, Oxford OX1 3QZ, United Kingdom

^eGuizhou Da Long Hui Cheng New Material Co., Ltd., Tongren 554000, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Fig. 1 (a) The synthesized illustration of hard carbon from carbon quantum dots. Reproduced with permission [43]. Copyright 2019, Elsevier. (b) The designed illustration of CQDHC. Reproduced with permission [45]. Copyright 2022, Elsevier. (c) Schematic of the synthesis of N-doped carbon from carbon quantum dots. Reproduced with permission [48]. Copyright 2019, Elsevier.

Fig. 2 (a) SEM image and (b) TEM image of Mo₂C/NCNFs. (c) Cycle performance (0.1 A/g) and (d) rate performance of Mo₂C/NCNFs. (e) Cycle performance of Mo₂C/NCNFs at 1 A/g. Reproduced with permission [55]. Copyright 2021, Elsevier.

Fig. 3 (a) The diagram of synthesis CVCx-QDs/nFCM. (b) HRTEM image of the CVCx-QDs/nFCM. (c) Rate performance and (d) cycle performance (400 mA/g) of CVCx-QDs/nFCM. Reproduced with permission [59]. Copyright 2021, Elsevier.

Fig. 4 (a) SEM image of CoS@G-25. (b) TEM image and (c) dark field image of CoS@G-25. (d) Cycle performance and (e) rate performance of CoS, CoS@G-25, CoS@G-10, and CoS@G-15. Reproduced with permission [70]. Copyright 2017, Wiley.

Fig. 5 (a) The synthesized routs of FeS@SPC. (b) TEM image and (c) high-resolution TEM image of FeS@SPC. (d) Rate performance of FeS@SPC. Cycling performance of FeS@SPC at (e) 100 mA/g and (f) 1 A/g. Reproduced with permission [71]. Copyright 2021, Elsevier.

Fig. 6 (a) TEM image of FeS@SPC after 3000 cycles at 1 A/g. (b) The models of K⁺ adsorption behavior on FeS@SPC. Reproduced with permission [71]. Copyright 2021, Elsevier. (c) Synthesis diagram of ZnS QDs-rGO. (d) TEM image of ZnS QDs-rGO. (e) Cycle performance at 0.1 A/g and (f) rate performance of ZnS QDs-rGO. Reproduced with permission [72]. Copyright 2021, Elsevier.

Fig. 7 (a) Schematic illustration of synthesis Co₃Se₄ quantum dots encapsulated by N-doped carbon (CSC). (b) TEM and (c) HRTEM image of CSC. (d) Cycling performance at 0.1 A/g and (e) rate performance of CSC. (f) K⁺-intercalation reaction energy for Co₃Se₄ QDs. (g) K⁺ migration energy barrier and reaction energy for Co₃Se₄ QDs. Reproduced with permission [76]. Copyright 2021, Wiley.

Fig. 8 (a) The synthesis process of Co_0.85Se-QDs/C composite. (b) TEM image of Co_0.85Se-QDs/C-20. (c) Rate performance of Co_0.85Se-QDs/C composite. (d) Cycling stability of Co_0.85Se-QDs/C-20 at 1 A/g. Reproduced with permission [77]. Copyright 2019, The authors.

Fig. 9 (a) Schematic illustration for synthesis process of Cu₁₂Sb₄S₁₃ quantum dots (CAS) and Ti₃C₂ nanosheets composite (CAS-Ti₃C₂). TEM image of (b) Cu₁₂Sb₄S₁₃ quantum dots and (c) CAS-Ti₃C₂ composite. (d) Rate performance and (e) cycling stability of CAS-Ti₃C₂ composite, Cu₁₂Sb₄S₁₃ quantum dots and Ti₃C₂ nanosheets. Reproduced with permission [80]. Copyright 2021, Wiley.

Fig. 10 (a) Synthesis routes of the VN/CNF. (b) TEM image of VN/CNF composite. (c) Rate performance of VN/CNF. Reproduced with permission [84]. Copyright 2020, Royal Society of Chemistry. (d) Schematic illustration of the synthesis of the VN-QDs/CM composites. (e) Rate performance of VN-QDS/CM-600. (f) High-resolution TEM image of VN-QDS/CM-600 after 100 cycles. Reproduced with permission [85]. Copyright 2019, Elsevier.

Fig. 11 (a) Schematic illustration of the synthesis of the “bubble-in-bowl” (BIB) structured CoP@NPC (CoP@NPC BIB). (b) SEM image and TEM images (c, d) of CoP@NPC BIB. (e) Schematic illustration of the potassium ion hybrid capacitors (PIHCs). (f) Photos of the PIHCs powering an electronic fan. (g) Charging/discharging curves of the PIHCs. (h) Scene diagram of PIHCs working in low-temperature environment. (i) Rate performance of PIHCs at −15 ℃. Reproduced with permission [89]. Copyright 2021, Royal Society of Chemistry.

Fig. 12 (a) Schematic illustration of the synthesis of the MoP@PC. (b) SEM image and (c) high-resolution image of MoP@PC. (d) Rate performance of MoP@PC at various current densities from 0.1 A/g to 5 A/g. (e) TEM image of MoP@PC after 1000 cycles at 5 A/g. Reproduced with permission [92]. Copyright 2021, Elsevier. (f) The synthesis routs of Ni₂P@NPC. (g) High-resolution TEM image of Ni₂P@NPC. Rate performance (h) and long cycling stability (i) at 1 A/g of Ni₂P@NPC. Reproduced with permission [93]. Copyright 2021, Elsevier.

Fig. 13 (a) TEM and (b) high-resolution TEM of BiND. (c) Rate performance of BiND/G at various current densities from 0.1 A/g to 10 A/g. (d) Comparison of capacity retention rate between BiND/G and other anode electrode materials for potassium ion battery. Reproduced with permission [102]. Copyright 2019, Wiley. (e) The synthesis routes of Bi:Co and Bi:Fe alloys nanodots and carbon composites. (f) HRTEM image of Bi_0.85Co_0.15@C composites. (g) Rate performance of Bi:Co@C. (h) Schematic diagram of the PTCDA-0C//Bi_0.85Co_0.15@C potassium ion full battery. (i) Cycling stability of the full potassium ion batteries. Reproduced with permission [103]. Copyright 2022, Springer.

Fig. 14 (a) Schematic illustration of the Sb SQ@MA composite. (b) HRTEM image and (c) high-angle annular dark-field (HAADF) image of Sb SQ@MA composite. (d) Rate performance and (e) cycling stability at 1 A/g of Sb SQ@MA electrode. (f) In situ XRD patterns of Sb SQ@MA electrode. Reproduced with permission [107]. Copyright 2022, American Chemical Society. (g) Schematic illustration of the synthesis routes for Sb@NCNWs. (h) SEM image and (i) HRTEM image of Sb@NCNWs. (j) The working mechanism of potassium dual-ion batteries (PDIBs) composed of Sb@NCNWs anode and EG cathode. (k) Rate performance of the PDIBs at various current densities. Reproduced with permission [108]. Copyright 2022, Elsevier.

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