Carbon Nanofibers‐Based Anodes for Potassium‐Ion Battery

Abstract In recent years, with the global warming getting worse and increasing demand for energy, countries around the world are trying to develop new energy storage technologies to solve this problem. Currently, potassium‐ion batteries (PIBs) have attracted tremendous attention from researchers as low‐cost and high‐performance energy storage devices. However, due to the huge ionic radius of K+, PIBs face significant volume expansion during cycling, which can easily lead to the collapse of electrode structures. In addition, the poor diffusion kinetics of K+ seriously affect the electrochemical performance of the battery. Carbon nanofibers (CNFs)‐based materials (including CNFs, metal/CNFs composites, chalcogenide/CNFs composites, and other CNFs‐based materials) are widely used as PIBs electrode anode materials due to their three‐dimensional conductive network, heteroatom doping and excellent mechanical properties. This review discusses in detail the research progress of CNFs‐based materials in PIBs, including material preparation, structural design, and performance optimization. On this basis, this article explores the key issues faced by CNFs‐based materials and future development directions, and proposes improvement suggestions for providing new ideas for the development of CNFs‐based materials.


Introduction 1.Overview on PIBs
Since entering the 21st century, with the rapid development of mobile devices and electric vehicle industries, energy storage devices have entered a period of rapid development. [1,2]Owing to its reusability and high energy density, lithium-ion batteries (LIBs) have been widely used in the current market.However, with the continuous increase in market demand, the raw material prices and production costs of LIBs remain high, seriously affecting their future development. [3]Therefore, there is an urgent need to develop other high-performance and cheap energy storage devices. [4,5]In recent years, potassium-ion batteries (PIBs) have attracted widespread research interest due to their abundant raw materials, low cost, and low redox potential (À 2.93 V vs. standard hydrogen electrode) for K/K + .Although PIBs occupy some unique advantages, the ionic radius of K + (1.38 Å) is larger than that of Li + (0.76 Å), which can cause significant volume expansion and contraction for the cell upon cycling, damaging the internal structure of the electrode material and causing irreversible capacity loss. [6,7]In addition, the poor diffusion kinetics of K + seriously hinder the capacity and rate performances of the battery.Thus, in order to improve the electrochemical performances of PIBs, electrode materials need to be carefully designed and optimized.

Development of PIBs Anode Material
As one of the key components of PIBs, anode electrode materials greatly affect the electrochemical performance and safety performance of the battery.It is worth mentioning that commercial graphite in LIBs generally undergoes three stages [C!KC 24 (stage I)!KC 16 (stage II)!KC 8 (stage III)] during the intercalation of K + , exhibiting high theoretical capacity (279 mAh g À 1 ).However, the large volume expansion (61 %) of graphite during cycling leads to the pulverization of the electrodes and the formation of large quantities of the solid electrolyte interphase (SEI) layer on the electrode surfaces, which eventually cause the continuous decline of the capacity.Therefore, promoting the development of PIBs requires continuous exploration of other high-performance electrode materials.At present, the reported anode electrode materials for PIBs mainly include carbon materials [e.g. grapheme, soft carbon, hard carbon and carbon nanofibers (CNFs)], metals (e. g.Sn and Sb), metal sulfides (e. g.SnS 2 and CoS 2 ), and other materials (e. g.[10] Among them, CNFs have a special research interest on researchers.Table 1 summaries the electrical conductivity, microstructure, structural advantages, and raw material of CNFs.[13][14] However, individual CNFs typically exhibit limited discharge capacity in PIBs.Therefore, CNFs typically appear together with other high-performance materials as anode electrode materials for PIBs.

Advantages and Challenges of CNFs-based Anodes for PIBs
In detail, CNFs-based anodes for PIBs own the following merits: (i) nanostructures can shorten the diffusion distance of K + and increase the contact area with K + ; (ii) three-dimensional (3D) cross-linked heterogeneous conductive network could inhibit the aggregation of nanoparticles, enhance the electronic conductivity, and thus facilitate the electron/ion transport; and (iii) self-supporting flexible electrodes simplify the manufacturing process of batteries, and increase the volume energy density of the battery by reducing the use of metal current collectors, binders and conductive additives. [3,5,15]owever, CNFs-based materials (including CNFs, metal/ CNFs, chalcogenide/CNFs, phosphide/CNFs and other CNFsbased materials) still face many challenges such as low first cycle coulombic efficiency, unclear reaction mechanism, complex synthesis method and high cost, etc.In this review, the research progress on CNFs-based materials were summarized.In addition, this work provides unique insights into the experimental methods, structural design, or performances of almost every typical case, which is believed to stimulate the further development for CNFs-based materials.

Application of CNFs in PIBs
In PIBs, due to the large ionic radius of K + , CNFs are difficult to exhibit excellent performance, and careful design is still needed to improve performances.For example, Touja et al. used PAN as raw material to prepare self-supporting CNFs for PIBs anode by electrospinning technology, showing a general discharge capacity (200.0 mAh g À 1 at 0.025 A g À 1 after 50 cycles) (see Figure 1). [16]Adams et al. obtained CNFs by electrospinning and subsequent carbonization. [15]CNFs exhibit ordinary electrochemical performances (170 mAh g À 1 at 0.279 A g À 1 after 1900 cycles; 110 mAh g À 1 at 2.79 A g À 1 ). [15][19]   N-doped hierarchical porous CNFs (HPCNF) using PAN and polystyrene (PS) as raw materials through electrospinning and pyrolysis processes (see Figure 2a). [17]Among them, the decomposition of small molecular organic compounds forms a hierarchical pore structure and in-situ N-doping carbon.The HPCNF composite exhibits excellent rate performance (204.6 mAh g À 1 at 2 A g À 1 ), reversible capacity (282.9 mAh g À 1 at 0.5 A g À 1 ), and long cycle life [196.7 mAh g À 1 at 2 A g À 1 after 2000 cycles (see Figure 2b)], which can be attributed to defects and porous structure, increasing the contact area between the electrode and electrolyte, and improving K-adsorption capacity.However, the HPCNF electrode has significant capacity loss during the initial cycling process, possibly due to the continu-ous formation of SEI layer due to the large number of pore structures.Liu et al. used the metal organic framework as a template to prepare self-supporting porous, and N-doped CNFs (NCNFs) by electrospinning and subsequent carbonization treatment. [18]The NCNFs exhibit good reversible capacity (290 mAh g À 1 at 0.1 A g À 1 ), which can be attributed to the high surface area, tunable nitrogen species and abundant interconnected nanopores.Zhang et al. prepared N-doped mesoporous CNFs (NMCNFs) using acrylic yarn and submicron tin by electrospinning and subsequent carbonization (see Figure 2c). [20]The NMCNFs show a long-term cycling stability [131.2 mAh g À 1 at 0.5 A g À 1 after 21000 cycles (see Figure 2d)].This is mainly because its large interlayer spacing can adapt to . [16]eproduced from Ref. [16] Copyright (2020), with permission from Elsevier.
large volume changes, as well as stable submicro-sized and internal cross-linking porous structure.However, the template method may lead to waste of raw material and increase preparation cost, and further optimization is needed in the future.Chen et al. synthesized a free-standing flexible electrode material (CNT/SNCF) via electrospinning and sulfuration methods, realizing the high discharge capacity (212.5 mAh g À 1 at 1 A g À 1 after 1000 cycles) and cycling stability (100.1 mAh g À 1 at 5 A g À 1 after 5000 cycles) of K + storage. [19]The excellent storage capacity of CNT/SNCF can be attributed to the following: (i) the unique multi-channel one-dimensional structure can promote ion diffusion, and increase the conductivity and active sites of CNT/SNCF; (ii) S and N co-doping improves electrochemical performance; and (iii) the increased interlayer spacing can mitigate volume changes during cycling.Some synthesis methods and electrochemical properties of CNFs for PIBs are listed in Table 2. Obviously, introducing other carbon materials, pore forming agents, or heteroatoms (such as S and N, etc.) can effectively improve the cycling life and rate performance of electrode.These genius strategies can provide reference for the market application of CNFs anode for PIBs in the near future.

Application of Metal/CNFs in PIBs
Recently, high-capacity metal anode materials based on alloying reactions have been receiving increasing attention due to low charge/discharge potentials, low cost, and high theoretical capacity.Such materials mainly include metals, such as Sb, Sn and Bi, and their alloys, which have been extensively studied in LIBs and sodium-ion batteries (SIBs). [21]However, metal materials typically face significant volume expansion (> 300 %) during cycling, which may lead to rapid capacity decay. [5]At present, there are two main strategies to solve this problem: the combination of metal materials with carbon materials, such as graphene, CNFs, and porous carbon, etc.; and the design of novel structures, such as reducing size, and introducing porous structures, etc. [5,21] For example, Li et al. used porous SnO 2 and PAN as precursors to prepare porous Sn/N-doped CNFs frameworks (Sn/N-CNFs) by electrospinning and carbonization methods (see Figure 3a). [5]The Sn/N-CNFs exhibits the outstanding electrochemical properties (198.0 mAh g À 1 at 1 A g À 1 after 3000 cycles, the corresponding capacity retention rate is as high as 88.4 %; 316.1 mAh g À 1 at 0.1 A g À 1 after 100 cycles; 168.7 mAh g À 1 at 2 A g À 1 ) as the anode material for PIBs.The excellent storage capacity of Sn/N-CNFs may be due to the following reasons: (i) porous Sn nanospheres provide sufficient space to accommodate volume changes during the alloying process, and improve potassium storage performances; (ii) the amorphous carbon skeleton inhibits the aggregation of metal nanoparticles; and (iii) the N-doped CNFs can ensure the stability of the structure.However, the first coulombic efficiency of Sn/N-CNFs in this work is relatively low, not exceeding 80 %.In order to improve this issue, the authors propose a strategy to optimize the electron/ion transport capacity, which involves increasing the content of Sn and increasing the carbonization temperature to enhance the conductivity of CNFs.Chen et al. used polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer (P123), SbCl 3 and PAN as precursors to prepare Sb@porous N-doped CNFs (Sb@PCNFs) by electrospinning and carbonization methods. [21]Among them, the addition of P123 can be used to manufacture pore structures, which alleviates the volume changes during cycling and improves the transport capacity of electrons/ions.As an anode electrode material for PIBs, Sb@PCNFs hybrid exhibits superior cycling performance (314 mAh g À 1 at 0.5 A g À 1 after 20000 cycles) (see Figure 3b).Considering the high capacities and rate performances of Sb and Bi, respectively.Li et al. used SbCl 3 , Bi(NO 3 ) 3 • 5H 2 O and PAN as raw materials to prepare BiSb nanoparticles/CNFs composites (BiSb@C) as PIBs anode by electrospinning and carbonization methods, showing good cycle stability and capacity (204.8 mAh g À 1 at 0.1 A g À 1 after 500 cycles, the corresponding capacity retention rate is as high as 82.8 %). [22]The excellent electrochemical performances for BiSb@C are due to the carbon coating can mitigate the volume change of BiSb, as well as the CNFs could improve the conductivity and structural stability (see Figure 3c).Ouyang et al. used Bi(NO 3 ) 3 , PAN, and PVP as Table 2. Summary of the preparation methods and potassium storage performances of CNFs.
Self raw materials to prepare Bi nanoparticles/porous CNFs composites (Bi/PCNFs) as PIBs anode by electrospinning and carbonization methods, showing excellent performances (171 mAh g À 1 at 1 A g À 1 after 1000 cycles; 163.3 mAh g À 1 at 5 A g À 1 ). [23]The good cycle life of Bi/PCNFs can be attributed to the following: (i) carbon coating, relieving the mechanical strain (see Fig- the raw materials of CoSb alloy, and PAN as the carbon source. [24]Among them, N and Co can not only optimize the electronic configuration but also alleviate the volume expansion during cycling.As a PIBs anode material, CoSb@C shows excellent performance (413 mAh g À 1 at 1 A g À 1 after 1000 cycles, the corresponding capacity retention rate is as high as 82.8 %). [24]ome synthesis methods and electrochemical properties of metal/CNFs for PIBs are listed in Table 3.In conclusion, the combination of metal materials and CNFs can effectively accommodate the volume change of metal during cycling and improve the long cycle performance of the electrode.However, the above composites usually face the problem of low the first coulombic efficiency and low capacity.Reproduced from Ref. [5] Copyright (2021), with permission from Royal Society of Chemistry.(b) Cycling performance of Sb@PCNFs electrode at 500 mA/g. [19]Reproduced from Ref. [19] Copyright (2021), with permission from American Chemical Society.(c) transmission electron microscopy (TEM) image of BiSb@C composite. [22]Reproduced from Ref. [22] Copyright (2022), with permission from American Chemical Society.(d) TEM image of the Bi/PCNFs. [23]Reproduced from Ref. [23] Copyright (2022), with permission from American Chemical Society.

Application of Chalcogenide/CNFs in PIBs
[27][28] However, chalcogenide compounds are generally facing many challenges including large volume expansion rate and poor conductivity during cycling, which seriously affect the cycle performance, rate performance and discharge capacity of batteries, especially in PIBs.At present, researchers mainly use the following methods to solve this problem: carbon coating, such as CNFs, and graphene, etc; [29] reasonable structural design, such as reducing the size of chalcogenide, introducing porous structure and constructing heterojunction structure, etc; [30][31][32] introducing heteroatoms, such as metal, and O, etc. [26,33,34] Among them, CNFs, as an important substrate material, often have positive effects when combined with chalcogenide compounds.For example, Luo et al. used acetylacetone iron, graphite oxide and PAN as raw materials to prepare FeSe@-Graphene@CNFs (FeSe@G@CNFs) by electrospinning and selenidation. [25]FeSe@G@CNFs hybrid exhibit excellent electrochemical performances (200 mAh g À 1 at 2 A g À 1 after 1700 cycles, the corresponding capacity retention rate is as high as 80.9 %; 409 mAh g À 1 at 0.2 A g À 1 after 400 cycles; 245 mAh g À 1 at 5 A g À 1 ).The good electrochemical performance can be attributed to the fact that with the help of graphene, more FeSe nanoparticles are encapsulated in CNFs, which can alleviate the volume change of FeSe during cycling and shorten the diffusion distance of ions.However, these experimental steps for this work are complicated, and the introduction of graphite oxide also increases the cost of raw materials.Wang et al. synthesized a few layers of Sn-based sulfide-N/P co-doped CNFs hybrid (SnS x -N/P-CNFs) with chlorella as the precursor through electrospinning technology and vulcanization process. [26]The SnS x -N/P-CNFs hybrid shows the outstanding potassium storage properties (468 mAh g À 1 at 0.1 A g À 1 after 100 cycles; 170 mAh g À 1 at 5 A g À 1 after 10000 cycles).It is worth mentioning that the use of biomass algae as a N/P sources can improve the conductivity of the electrode material, increasing the electrochemical performances of the electrode. [26]Lai et al. used ZIF-67 and PAN as raw materials to prepare ultrasmall CoM x (M = S, O, Se and Te)@hierarchical porous CNFs (HCFs) composites by electrospinning, carbonization and subsequent sulfuration/oxidation/selenidation/tellurization methods (see Figure 4a), indicating that this method has good flexibility. [8]mong them, ultrasmall CoS 2 (u-CoS 2 )@HCFs exhibits excellent long cycle performance (268 mAh g À 1 at 0.5 A g À 1 after 1000 cycles).The outstanding potassium storage properties can be ascribed to the synergistic effects of the well-organized ultrasmall CoS 2 nanoparticles with tight coupling to the hierarchically porous CNFs.Inspired by the favorable structure of pea-pod, Xu et al. used SiO 2 as a template to prepare peapod-like porous CNFs through electrospinning, carbonization and activation methods. [35]Subsequently, by a melting liquefaction strategy, Se is infiltrated into the prepared porous CNFs to  obtain self-supporting Se@peapod-like N-doped CNFs (Se@NPCFs) composite materials (see Figure 4b).The Se@NPCFs hybrid has an excellent long cycle life (367 mAh g À 1 at 0.5 A g À 1 after 1670 cycles). [35]The good cycle life of Bi/PCNFs can be attributed to the following: (i) 1D peapod-like structure, facilitating the electrolyte infiltration and K + diffusion; (ii) the abundant micro/mesopores, increasing mass loading of Se, shortening the ion diffusion distance and buffering the volume expansion; and (iii) N-doping carbon, enhancing the chemical affinity between the carbon matrix and discharge products.Besides, this work further analyzed the reaction mechanism of Se during the discharge process through density functional theory (DFT).The theoretical potential corresponding to the K 2 Se and K 2 Se 2 (1.41 and 1.65 V, respectively) formed during the discharge process is shown in Figure 4c, which is higher than the experimental value (1.20 and 1.45, respectively).The possible reason is the overpotential caused by the electrode and electrolyte.Some synthesis techniques and electrochemical characteristics of chalcogenide/CNFs for PIBs are listed in Table 4.The combination of chalcogenide and CNFs is an effective method to improve the long cycle life of chalcogenide.However, the experimental methods involved in the combination of these two are mostly complex and require careful design to reduce the number of steps for future market applications.

Application of Phosphide/CNFs in PIBs
Phosphides attract the interest of researchers due to its high theoretical capacity (for example, the theoretical capacity of boron phosphide is 570 mAh g À 1 ) and low voltage platform (~0.4 V).However, the large volume expansion, poor conductivity (for example, ~10 À 14 S cm À 1 for red phosphorus) and sluggish kinetics during charging/discharging prevent its practical application. [36,37]As a stable carbon substrate, CNFs have been used to improve the electrochemical performance of phosphide.For example, Sun et al. used MIL-88A (Fe) and PAN as raw materials to obtain self-supporting Fe 2 P nanoparticle doped CNFs (Fe 2 P-CNFs) composites through electrospinning, phosphorylation and carbonization (see Figure 5a). [38]The Fe 2 P-CNFs hybrid has good reversible capacity (379.2 mAh g À 1 ), rate performance (211.8 mAh g À 1 at 2 A g À 1 ), and long cycle life Table 4. Summary of the preparation methods and potassium storage performances of chalcogenide/CNFs.
To investigate the reaction mechanism of MoP@NPCNFs, Raman spectra was applied to follow structural evolution after different cycling (see Figure 5b).From Figure 5b, it can be observed that the characteristic D (~1350 cm À 1 ) and G (~1585 cm À 1 ) bands of the pristine MoP@NPCNFs electrode.The intensity ratio of I D /I G decreased after the 1 st discharge, indicating more defects and disorders in the carbon structure, which may be caused by K + insertion.After fully charged, the value of I D /I G is restored again.The same situation is also observed for the 3 rd discharge and charge, indicating the stability of the electrode structure.Note that the first coulombic efficiency of MoP@NPCNFs is inefficient (not more than 40 %), which needs to be further improved.
Zhang et al. prepared Sn 4 P 3 @CNFs as PIBs anode using Sn, P and PAN as raw materials by ball milling and electrospinning. [39]n this work, the authors found that potassium bis(fluorosulfonyl)imide can effectively inhibit the growth of potassium dendrites, reduce the impact of polarization, ensure the stable SEI layer, and allow for long-term stable operation of the electrode.Thus, the composite material exhibits good cyclic stability (160.7 mAh g À 1 at 0.5 A g À 1 after 1000 cycles).In addition, the authors also found that the potassiation process could be described by the following equations based on synchrotron X-ray diffraction (XRD) results (see Figure 5c): Some preparation methods and electrochemical performances of phosphide/CNFs for PIBs are listed in Table 5.The use of CNFs to encapsulate phosphides seems to effectively improve the conductivity of electrodes and alleviate the volume expansion of nanoparticles.

Application of Other Materials/CNFs in PIBs
In addition to the materials mentioned above, there are other materials combined with CNFs, such as quasi-metals (Te, etc.), and carbides (Mo 2 C, etc.).[42] However, the alkali metal-Te cells are affected by the unfavorable shuttling effect of the polytetrafluoroethylene intermediate, leading to the loss of active materials and severely weakening the electrochemical stability of the two electrodes.In addition, Te material undergoes huge volume expansion (398 % for K 2 Te) during charging and discharging processes, resulting in rapid capacity decay. [40,42]In order to solve the above problems, an effective approach is to confine Te into porous carbon.For example, Yu et al. used zeolite imidazolate backbone material (ZIF-8) and PAN as raw materials, and prepared Te@hierarchical porous CNFs (Te@HPCNFs) composite material as PIBs anode electrode by electrospinning method and melt-diffusion method (see Figure 6a), showing excellent electrochemical properties (231.7 mAh g À 1 at 7C after 4500 cycles; 207.1 mAh g À 1 at 14C). [42]The unprecedented cycle life of Te@HPCNFs can be attributed to the space confinement of HPCNFs, which accommodates the volume expansion of amorphous Te during cycling.Besides, Te@HPCNFs hybrid also has a superior capacity in K-ion full cell (170.3mAh g À 1 at 0.1 A g À 1 after 100 cycles).In this work, the authors comprehensively examined the electrochemical performances of Te@HPCNFs, which help readers understand the performances of Te-based materials in PIBs.
Other uncommon electrode materials have also been reported to be combined with CNFs for PIBs, such as Mo 2 C. For example, Min et   MoP@NPCNFs Electrospinning, carbonization and phosphating 320/0.1 220/2/100 [36]   Sn 4 P 3 @CNFs Ball milling and electrospinning 160.7/0.5/1000[39]  spinning and carbonization methods (see Figure 6b). [43]The electrode material exhibits general properties (88 mAh g À 1 at 1 A g À 1 after 1000 cycles; 211 mAh g À 1 at 0.1 A g À 1 ).In this work, the average diameter of Mo 2 C quantum dots is 1-2 nm (see Figure 6c and 6d), which can shorten the electron/ion migration distance and improve charge transfer kinetics.However, the loading amount of Mo 2 C in this material is found to be 17 wt %, resulting in lower levels of performance.
In conclusion, when CNFs are compounded with quasimetals (Te, etc.), and carbides (Mo 2 C, etc.), etc, these hybrids can be guaranteed to have good cycle stability.However, the loading capacity of these materials in CNFs is not high, which affects the volume energy density of the entire battery.Therefore, the next focus can be on increasing the proportion of active substances in hybrid.In addition, how to reduce the specific surface area of the electrode material and improve the first coulomb efficiency of the electrode should also focus.

Conclusions
Currently, LIBs have been widely used, and SIBs batteries are also in the preparation stage for marketization.It is believed that PIBs will also be applied in the near future.In this paper, the research progress and key problems of CNFs-based materials (including CNFs, metal/CNFs composites, chalcogenide/CNFs composites, phosphide/CNFs composites and other CNFs-based materials) in PIBs are mainly discussed on the basis of predecessors.Among them, although CNFs-based materials have received widespread attention from researchers due to their excellent structural stability, conductivity and long cycle life.However, CNFs-based materials still face many challenges such as low capacity, low first cycle coulombic efficiency, unclear reaction mechanism, complex synthesis method and high cost.Future research may be conducted as follows: (i) It is evident that introducing heteroatoms, pore structures, and other components, etc, can significantly improve the electrochemical performance of CNFs.However, those unique structures can only be prepared in the laboratory.How to simplify production processes and reduce production costs to meet the needs of industrial production is the focus of the next step of research.In summer, with the continuous iteration and upgrading of technology and the increasing market popularity, it is believed that CNFs-based materials can play a huge role in PIBs.
For example, Sun et al. successfully constructed in-situ Chao Li received his Ph.D. from Jilin University in 2021.He is currently working as a postdoctoral fellow at Research Institute of Sinopec Maoming Company.His research interests focus on energy conversion and energy storage materials.Wen-jun Jiang obtained a Master degree from Hunan University of Science and Technology in 2008.He is currently working as a senior engineer at Research Institute of Sinopec Maoming Company.His research interests focus on development and utilization of organic fine chemicals.Zhen-yu Liu earned his Ph.D. from Beijing University of Chemical Technology in 2013.He is currently working as a senior engineer at Research Institute of Sinopec Maoming Company.His research interests focus on organic fine chemicals, olefin polymerization catalysts, and new product development.
ure 3d); (ii) the unique vessel like pores, enhancing the contact area between the electrode and electrolytes; and (iii) the optimal ratio between the Bi and the carbon, improving capacity and cycle life.Han et al. prepared N-doped CoSb@carbon nanofiber (CoSb@C) composite materials by electrospinning and carbonization with Co(CH 3 COO) 2 • 4H 2 O and SbCl 3 as
(ii) Advanced characterization techniques (such as in-situ XRD, TEM, Raman, etc.) and theoretical methods (such as, DFT calculation, etc.) are needed to deeply analyze the structural characteristics of CNFs-based materials, guide their composition and structural design, and analyze the structural changes of electrode materials during cycling.(iii) There is relatively little research on the coordination of CNFs-based materials with other battery components (such as positive electrodes, separators, electrolytes, etc.).More research should focus on these directions to improve the overall performances of battery.

Table 1 .
Comparison of the electrical conductivity, microstructure, structural advantages, and raw material of CNF synthesis methods.

Table 3 .
Summary of the preparation methods and potassium storage performances of metal/CNFs.

Table 5 .
Summary of the preparation methods and potassium storage performances of phosphide/CNFs.