瀝青衍生軟碳與硬碳纖維復(fù)合實(shí)現(xiàn)高倍率和長(zhǎng)循環(huán)的K+存儲(chǔ)(英文)

摘要：鉀離子電池（KIBs）因其天然豐富的資源、較低的成本以及與鋰離子電池類似的搖椅式運(yùn)行機(jī)制，一直被視為鋰離子電池（LIBs）的替代品.作為KIBs負(fù)極，軟碳與硬碳相比電壓平臺(tái)更低，與石墨相比晶格結(jié)構(gòu)易于調(diào)節(jié).瀝青作為一種簡(jiǎn)單、易于獲得且價(jià)格低廉的軟碳前體備受關(guān)注，但其結(jié)構(gòu)在循環(huán)過(guò)程中很容易受到破壞.文中通過(guò)靜電紡絲技術(shù)在瀝青軟碳表面均勻纏繞網(wǎng)狀碳纖維，制備了柔性薄膜Pitch@CNF，其不僅使瀝青在循環(huán)過(guò)程中能很好地保持結(jié)構(gòu)，緩解K+插層時(shí)的體積膨脹，而且三維網(wǎng)狀結(jié)構(gòu)有利于離子傳輸，碳纖維上豐富的孔隙能提供更多的K+活性位點(diǎn).制備的柔性自支撐薄膜可直接用作電極，無(wú)需添加粘合劑和導(dǎo)電劑.在電流密度為0.1 A·g-1時(shí)，可逆容量為290 mAh·g-1，循環(huán)500次后容量保持率為83%.

關(guān)鍵詞：軟碳；硬碳；鉀離子電容器；碳纖維；瀝青

中圖分類號(hào)：TM 53;TB 34"" "文獻(xiàn)標(biāo)志碼：A"" "文章編號(hào)：1001-988Ⅹ（2024）06-0053-10

Pitch-derived soft carbon composite with hard

carbon fibers for high-rate and long-cycling K+ storage

PENG Jiao，WANG Rui，JIN Jun，HE Bei-bei，GONG Yan-sheng，WANG Huan-wen

（Faculty of Materials Science and Chemistry，China University of Geosciences，Wuhan 430074，Hubei，China）

Abstract：Potassium-ion batteries（KIBs） have been seen as a competitive alternative to lithium-ion batteries（LIBs） due to their natural abundance，low cost and rocking chair-like operating mechanism similar to LIBs.Soft carbon has a lower voltage plateau compared to hard carbon and an easily modulated lattice structure compared to graphite，which provides particular advantages in KIBs anodes.Pitch has attracted much attention as a simple，readily available and inexpensive precursor for soft carbon，but its structure is easily damaged during cycling.Herein，the flexible film Pitch@CNF are prepared by uniformly winding reticulated carbon fibers on the surface of pitch-soft carbon

via electrostatic spinning technique，which not only enables the pitch to maintain its structure well

during cycling and withstand the volume expansion upon K+ insertion，but also is conducive to ionic transport of the three-dimensional reticulated structure.Meanwhile，the abundant pores on the carbon fibers can provide more K+ active sites.The prepared flexible self-supporting films can be used directly as electrodes without the addition of binders and conductive agents.The reversible capacity is 290 mAh·g-1 at a current density of 0.1 A ·g-1，and the capacity retention rate is 83% after 500 cycles.

Key words：soft carbon;hard carbon;potassium ion capacitors;carbon fibers;pitch

Recent years，the demand for lithium-ion batteries（LIBs） in areas such as portable electronics and automobiles has grown exponentially.However，the finite and uneven nature of lithium resources raises concerns about the sustainability of lithium resources and poses a significant challenge to the development of LIBs［1］.Given the abundance of potassium（K with 1.5% in the Earth’s crust and Li with 0.0017%） and the fact that potassium and lithium have similar physicochemical properties，potassium-ion batteries（KIBs） have a very similar basic structure to LIBs（they are based on the insertion/removal of alkaline cations in the electrode materials）［2-3］.More importantly，KIBs have lower redox couples to LIBs（K+/K is 2.93 V， while Li+/Li is 3.04 V，compared to standard hydrogen electrode（SHE）），which ensures a wide voltage window and high energy density for KIBs［5-6］.At the same time，K+-based electrolytes have the highest ionic conductivity among lithium，sodium and potassium，and the lowest desolvation energy due to the lowest specific charge of potassium ions.As a result，KIBs have attracted increasing attention［7-8］.

Despite the attractive potential of KIBs，the development of KIBs faces a major challenge in finding suitable electrode materials that can stably accommodate K+ over extended cycling.Typically，the successful commercialization of graphite in LIBs has inspired researchers to try it as a KIBs anode，which shows satisfactory performance（263 mAh·g-1） and accommodates K+ through an embedding mechanism similar to that in LIBs［9］.However，the use of graphite as an KIBs anode material for faces major challenge：on the one hand，the radius of K+ is relatively large （0.133 nm for K+ compared to 0.068 nm for Li+），and thus its intercalation/deintercalation in graphite would lead to a huge volume deformation，resulting in graphite structural collapse and battery performance degradation［10-12］.On the other hand，the long-range ordered structure of graphite is not conducive to the rapid insertion/de-insertion of K+，resulting in poor rate performance.To date，researchers have proposed many effective strategies to improve the above problems，such as microcrystalline conditioning，heteroatom doping，morphology tuning and surface interface modification［13-14］.However，due to the high stability of the graphite structure，it is particularly difficult to modify the graphite lattice or dope other atoms，which requires a lot of time and energy.

Meanwhile，disordered carbon has great application prospects due to its high abundance，simple production process and considerable production capacity.Disordered carbon can be divided into hard carbon（non-graphitizable） and soft carbon（graphitizable）［15］.Hard carbon，usually derived from resins or biomass materials such as starch or glucose，has a highly disordered and less crystalline structure，and a certain number of nanopores are present in hard carbon，which largely accommodates volume expansion and improves long-term cycling stability.However，the capacity contribution of hard carbon comes mainly from the high voltage region，which is certainly not conductive to achieving high energy densities.Another important type of disordered carbon，i.e.， soft carbon，has the following advantages that make it more attractive for KIBs［16］.Firstly，it is available in a wide variety of precursors，including pitch，coal and the organic compound perylene tetracarboxylic dianhydride（PTCDA），in which pitch is a CHx-based compound that is readily available as a by-product of the petroleum industry［17-19］.More importantly，due to its highly tunable crystallinity and lattice spacing，the structural flexibility of the soft carbon can be well controlled to meet the high energy density and stability requirements of KIBs.Firstly，the reversible capacity is limited to below 1 V by increasing the crystallinity of the soft carbon so that K+ is mainly intercalated between the lattices and surface adsorption is reduced，which is key to ensuring high energy density.At the same time，it is also possible to prevent the interlayer spacing from being too small，resulting in unfavorable K+ intercalation，by lowering the heating temperature［20-21］.

In this work，a pitch-soft carbon/porous carbon fiber composite electrode（Pitch@CNF） with long cycle life and high mechanical flexibility was presented.The use of electrostatic spinning technology to composite pitch-soft carbon with hard carbon allows the reticular carbon fibers to uniformly encapsulate the pitch，thereby mitigating the structural collapse caused by the intercalation of large-sized K+.The composite electrode remains structurally intact after a long cycle of cyclic testing.The manufacture of such electrodes does not require the addition of binders and conductive agents，which reduces manufacturing costs to some extent.In addition，the composite electrode improves the overall potassium storage performance，with a reversible capacity of 290 mAh·g-1 at a current density of 0.1 A·g-1 and a capacity retention of 83% after 500 cycles.The three-dimensional reticulated electrode structure improves ion transfer efficiency and maintains a capacity of 220 mAh·g-1 even at a current density of 0.6 A·g-1，demonstrating excellent rate performance.The PIHC assembled with AC as the positive electrode and Pitch@CNF as the negative electrode also showed remarkable cycling and rate performance，demonstrating the potential of this material for practical energy storage applications.

1 Materials and methods

1.1 Materials synthesis

First，0.3 g of asphalt was dissolved in 10 mL of N，N-dimethylformamide（DMF），and after complete dissolution，1 g of PAN was added to the above solution，then dispersed by ultrasonic dispersion for 30 min，and then placed on the magnetic stirring table for 12 h.The dark brown homogeneous suspension was finally obtained by homogeneous dispersion.The solution was then transferred to a 10 mL syringe，ensuring that the humidity in the spinning machine was lt;25%，the distance between the needle and the receiver was 18 cm，and the operating DC voltage was set at 20 kV to advance the solution at a rate of 1 mL·h-1.The resulting films were dried in an oven at 80 ℃ for 12 h，followed by annealing in a muffle furnace at an elevated temperature rate of 0.5 ℃·min-1 to 180 ℃ for 2 h for curing.Carbonization was performed by annealing the black flexible film（denoted as Pitch@CNF） at 1100 ℃ for 2 h under Ar atmosphere.Carbon fiber films without added pitch （referred to as CNF） were prepared using the same procedure.The pitch was annealed at 180 ℃ for 2 h in air and then carbonized at 1100 ℃ for 2 h to obtain pitch-soft carbon（denoted as PSC）.

1.2 Materials Characterization

X-ray diffraction（XRD，Philips PC-APD，Cu-Kα radiation） was used to identify the crystalline characteristics of the precursors and hard carbon samples.The structure of the carbon material was characterized by Raman spectroscopy（Renishaw RM-1000， 532 nm excitation） in the range 200-3500 cm-1.X-ray photoelectron spectroscopy（XPS，Kratos AXIS Ultra） was used to characterize the surface chemistry and chemical composition of the materials.The microstructural information of the obtained samples was observed by scanning electron microscope（SEM， model JSM-7600F） and transmission electron microscope（TEM， model JEM-2100）.The Brunauer-Emmett-Teller（BET） surface areas of the samples were determined using N2（77 K） on a Micromeritics ASAP 2460 analyzer.

1.3 Electrochemical tests

PSC，super P and polyvinylidene fluoride（PVDF） binder were mixed in a mass ratio of 8∶1∶1 in N-methylpyrrolidone（NMP） and the paste was coated on copper foil to prepare anode and was dried in a vacuum oven at 80 ℃ for 12 h and then cut into small discs of 12 mm diameter to be used as electrode pieces.The average active substance loading of the negative electrode was about 1.2 mg·cm-2.Pitch@CNF and CNF can be used directly as anode electrode.Electrolyte used in this work is using 0.8 mol·L-1 KPF6/EC/DEC.

Galvanostatic charge-discharge and galvanostatic intermittent titration technique（GITT） tests were performed using a LAND battery test system over a voltage range of 0.01～3.0 V（vs K+/K）.For the GITT tests，cells were discharged at 25 mA·g-1 for 20 min，followed by open-circuit relaxation for 2 h，and this process was continued until the potential

was below 0.01 V.Cyclic voltammogram （CV） profiles were scaled in a potential range of 0.01～2.5 V（vs.K+/K）on a CHI750E electrochemical workstation.Electrochemical impedance spectroscopy（EIS） tests were performed on a Reference 3000 Gamry with a recording range of 0.1 Hz～100 kHz and an amplitude of 0.5 mV.All tests were conducted at room temperature.

2 Results and discussion

2.1 Structural analysis

The experimental steps for the preparation of Pitch@CNF are shown in Fig.1（a）.Firstly，the pitch and PAN were uniformly mixed in DMF，and the film was made by using the electrostatic spinning equipment，and then it was pre-oxidized in the air and carbonized at high temperature in Ar to obtain the flexible thin film（Pitch@CNF）.It can be observed through the SEM that the carbon fibers are randomly oriented and densely arranged and no obvious fracture，the pitch is uniformly wrapped by the carbon fibers，and the pitch is changed from granular to molten state after high-temperature carbonization，and is more uniformly dispersed in the carbon fiber network（inset in Fig.1（a））.The resulting Pitch@CNF films show excellent mechanical flexibility after several folding，twisting，and curling without fracture，crushing，and recovery（Fig.1（b）～（e））.In contrast，the carbon powder obtained by direct pre-oxidation of pitch at 180 ℃ and carbonization at 1100 ℃ is mainly present as large micron-sized particles with rough surfaces and irregular shapes（Fig.2（a））.

The lattice fringes of the samples were then observed by TEM.As shown in Fig.2（b）～（d），the lattice fringes of PSC are regular and arranged with almost fixed layer spacing，similar to that of graphite;the lattice fringes of CNF are similar to that of hard carbon，showing a typical turbo structure with disorder.In Pitch@CNF，there are not only graphite-like stripes，representing soft carbon，but also disordered structures，representing CNF.The proportion of ordered and disordered structures is similar and evenly distributed.

XRD data were collected to investigate the crystal structure of the three materials.As shown in Fig.2（e），all three samples show two peaks at 2θ=23° and 43°，corresponding to the（002） and（100） crystal planes of amorphous carbon.The （002） peak of pitch is sharp，whereas the（002） peaks of the CNF and Pitch@CNF samples are broader，suggesting that

Pitch@CNF is less graphitized.It is also noteworthy that the diffraction angle of the （002） crystal plane of CNF is smaller than that of Pitch@CNF.According to

the Bragg formula：2dsin θ=nλ，the smaller the diffraction angle，the larger the layer spacing.This suggests a successful combination of soft and hard carbon in Pitch@CNF，resulting in a spacing

that is larger than that of PSC but smaller than that of CNF.

Meanwhile，the Raman spectra（Fig.2（f）） both showed the characteristic peak（D peak） located at 1350 cm-1 representing disordered structure or crystal defects，and the telescopic vibration peak（G peak） located at 1580 cm-1 representing ordered graphitic carbon，and the sharp G peak suggests that PSC has a higher degree of graphitization，and the 2D peak at 2700 cm-1 with a larger half-peak width is found，suggesting that the soft carbon of pitch firing is mainly composed of multilayered graphite stacks.Compared to PSC，the G-band of CNF has a much lower intensity，similar to typical hard carbon，indicating the presence of a large amount of disordered structure.Pitch@CNF has less sharp D and G peaks due to the combination of pitch-soft carbon and CNF，and the 2D peak is barely observed.The surface composition of Pitch@CNF was then investigated by XPS（Fig.2（g）～（h））.The peaks of C 1s near the binding energies of 284.7，290.1 and 285.4 eV belong to C-C，sp2 C and sp3 C，respectively，indicating the presence of graphitized carbon in Pitch@CNF.The results of XRD，Raman and XPS are in good agreement with those of SEM and TEM，confirming the introduction of pitch-soft carbon into the 3D network formed by PAN using electrostatic spinning.

The specific surface area and pore size distribution of Pitch@CNF were then analyzed and compared with that of PSC to investigate the effect of carbon nanofiber incorporation on the pore structure of the material.Fig.2（i） shows the N2 isothermal adsorption and desorption curves and the pore size distribution of the two samples.Pitch@CNF has a higher specific surface area of 21.49 m2·g-1，which is much larger than that of PSC，and the pore sizes are mainly distributed below 5 nm，which is probably due to the fact that many small gas molecules are generated in the PAN nanoparticles in the process of calcination and decomposition to produce more mesopores in the CNF.This abundant mesopore structure can provide a large number of active centers and act as a buffer reservoir for K+，thus greatly enhancing the kinetic effect of ion diffusion and potentially improving rate performance.

2.2 Electrochemical performance

To further investigate whether the hard carbon nanofiber coating of pitch-soft carbon could effectively improve the electrochemical performance，half-cells were assembled using PSC，CNF and Pitch@CNF as the working electrodes and potassium metal as the counter electrode，respectively，and the electrochemical performances of the batteries were investigated under the constant current charge-discharge test.As shown in Fig.3（a）～（c），the GCD curves of the half-cells assembled by the three electrodes are shown at a current density of 0.1 A·g-1.In the PSC electrode，due to the formation of intercalation compounds from the K+ intercalation to the graphite-like layer in the low voltage interval，a clear voltage plateau appears in the curves and shows a discharge capacity of about 300 mAh·g-1（discharge process in the second cycle），not only the ICE is just 66.4%，but also the Coulombic efficiency in the third turn is lower than 85%，suggests that the intercalation process has irreversibly depleted the proportion of K+.The CNF electrode exhibits capacitive behavior as it relies mainly on the adsorption/desorption of K+ by the mesopores to store K+，and therefore the corresponding GCD curve exhibits a large number of slope regions，which is favorable for fast charging and discharging，and has a capacity of 240 mAh·g-1，and inevitably its large specific surface area gives it a low ICE.It is worth noting that in the Pitch@CNF electrode，due to both the porous structure of CNF and the graphite-like layer of PSC，a small portion of K+ can be adsorbed in the pores of CNF and another portion of K+ is intercalated between the graphite-like layers，and thus the GCD curve shows a small number of sloped regions with a very distinct plateau region，resulting a capacity of about 300 mAh·g-1.Although the introduction of this porous structure inevitably reduces the ICE（only 59.75%），the Coulombic efficiency is close to 100% from the second cycle onwards.

We then performed a long-cycle test on the battery at a current density of 0.1 A·g-1 to compare the cycling stability of the three electrodes（Fig.3（d））.It can be seen that the capacity retention rate of PSC is only 23% after 200 cycles，while that of Pitch@CNF is 86%，and the capacity retention rate can be as high as 83% even after 500 cycles.The results demonstrate that the Pitch@CNF material can effectively mitigate the structural damage of pitch during cycling and has a longer cycling life compared to PSC.

To further evaluate the potassium storage performance of the Pitch@CNF electrode，we performed charge/discharge tests at different current densities.As shown in Fig.3（e），Pitch@CNF exhibits excellent rate performance，providing reversible capacities of 292，282，268，237， 234， and 220 mAh·g-1 at current densities of 0.1，0.2，0.3，0.4，0.5，and 0.6 A·g-1，respectively，which are significantly better than those of PSC.The capacity of the PSC electrode has decayed to below 200 mAh·g-1 at a current density of 0.2 A·g-1，and as the current density is further increased，the capacity shows a cliff-like decline，with a capacity of only 106 mAh·g-1 at 0.6 A·g-1.This can be attributed to the rapid insertion/degradation of K+ at high current densities，which severely damages the graphite-like structure，leading to a significant capacity drop.The coating of hard carbon nanofibers can not only mitigate the volume expansion and structural collapse caused by K+ intercalation in pitch，but also improve the electrochemical performance of PSC through the three-dimensional network structure，which is conducive to ionic transport，and the rich pore structure of carbon nanofibers can provide more active sites for K+ storage.

To elucidate the K+ storage mechanism，the CV curves of Pitch@CNF at different scanning rates were further plotted（Fig.3（f）） and the b values were calculated according to the function of peak current（i） and scanning rate（v）：i=avb，to determine the electrochemical behavior of K+ during the cycling process.We know that if the b value is close to 0.5，the process is dominated by diffusion;and if the b value is close to 1，the process is dominated by capacitive behavior.Illustration in Fig.3（f） shows the b values of 0.35 and 1.06 calculated by fitting the O1 and O2 peaks selected from the CV curves of Pitch@CNF.The results show that in the low-potential plateau region，K+ is mainly intercalated and enters the graphite interlayers to form KC8，whereas in the high-potential slope region，K+ mainly exhibits capacitive behaviour and adsorbs/desorbs at pores or defects on the surface of Pitch@CNF.This combination of“intercalation-adsorption”mechanism is very favourable for high rate and fast charge/discharge performance.

Immediately following this，we delved into the mechanism behind the excellent rate performance of Pitch@CNF and further investigated its kinetic properties using the Gitt test，which was performed in the first discharge/charge cycle at 50 mA·g-1 with a pulse duration of 20 min at 2 h intervals.As shown in Fig.3（g），the voltage versus time curve obtained can be substituted into Fick’s second law to calculate the K+ diffusion coefficient.Plotting log D versus voltage （inset in Fig.3（g）），it can be seen that the diffusion coefficient of Pitch@CNF is larger than that of CNF during both discharging and charging，indicating that K+ has a faster diffusion rate in Pitch@CNF，which is beneficial to its rate performance.EIS tests were then performed and Nyquist plots of the two electrode materials were plotted （Fig.3（h））.It can be seen that the resistance of Pitch@CNF before and after cycling is smaller than that of PSC，indicating that K+ has a faster transfer rate in Pitch@CNF.

In order to further investigate whether carbon nanofiber wrapping could effectively address the problem of pitch fragmentation，we analyzed the morphology of the pole pieces after cycling.First，the electrode sheets were removed after 20 cycles in an Ar-filled glove box and the excess potassium salts on the surface were removed with EC.The surface morphology of the electrodes after cycling was then observed using SEM（Fig.3（i））.It can be clearly seen that the pitch structure is decomposed and the surface is cracked due to the repeated intercalation of K+，whereas the pitch in Pitch@CNF is still encapsulated by carbon nanofibers with a well-maintained structure and smooth surface，which is conducive to the formation of a homogeneous SEI.

2.3 Pitch@CNF//AC full cell

Given the excellent performance of the Pitch@CNF electrode in half-cells，the Pitch@CNF//AC full cell was assembled for performance testing using AC as the cathode material to verify its prospects for practical application，and the charge storage mechanism is shown in Fig.4（a）.During charging，K+ is inserted between the graphite layers under the action of the electric field to form an intercalation compound，while PF- are adsorbed on the surface of the AC，whereas during discharging，both K+ and PF- are released from the electrode material to re-enter the electrolyte.By combining the operating voltages of the two electrodes，the voltage window of the full cell was set between 1 and 4 V，facilitating the provision of higher energy densities.To compensate for the loss of K+ due to partial irreversible chemistry，both electrodes were pre-potassified prior to assembly of the full cell.

The rate performance test was performed in the current density range of 0.1～25 A·g-1（Fig.4（b）），and the capacity is 32.2 mAh·g-1 at an initial current density of 0.1 mA·g-1，while when the current density is increased to 10 A·g-1， the capacity is still 11.9 mAh·g-1.With the increase in current density throughout the process，the capacity maintains a steady decrease without a precipitous drop and when the final current density returns to 0.1 A·g-1，there is still a capacity of 33.8 mAh·g-1，demonstrating good reversibility.In the constant current charge/discharge test（Fig.4（c）），a reversible capacity of 43.2 mAh·g-1 is demonstrated at a current density of 0.1 A·g-1，and after 400 cycles a reversible capacity of 27.9 mAh·g-1 is still available，which is a good stability.

The energy and power densities of the Pitch@CNF//AC PIHC device are obtained based on the total mass of the anode and the cathode and the results are shown in the Ragone plot from Fig.4（d）.Remarkably，a maximum energy density of 87.33 Wh·kg-1 could be available at a power density of 80 W·kg-1.With increasing power density to 10000 W·kg-1，the PIHC can still achieve a high energy density of 25 Wh·kg-1，demonstrating its powerful potential practical application in energy storage equipment.

3 Conclusions

In this article，we have used a simple electrostatic spinning technique to embed bitumen in carbon nanofibers to produce pitch-soft carbon-carbon nanofiber composite electrodes（Pitch@CNF） with excellent mechanical flexibility.Pitch@CNF can mitigate the structural deformation and fragmentation of bitumen particles due to repeated intercalation/ deintercalation of K+ during cycling，and thus has ultra-long cycle life and excellent rate performance.The assembled Pitch@CNF//AC has been cycled 400 times and retained over 65% of its capacity.It provides a reliable strategy to improve the compatibility of soft carbon anodes with KIBs.

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（責(zé)任編輯 陸泉芳）

收稿日期：2024-10-20

基金項(xiàng)目：國(guó)家自然科學(xué)基金資助項(xiàng)目（22279122）；深圳市自然科學(xué)基金資助項(xiàng)目（JCYJ20220530162402005）

作者簡(jiǎn)介：彭姣（1999—），女，湖北襄陽(yáng)人，碩士研究生.主要研究方向?yàn)橛蔡钾?fù)極在鈉離子電池中的應(yīng)用.

E-mail：1202210282@cug.edu.cn

通信聯(lián)系人，男，教授，博士，博士研究生導(dǎo)師.主要研究方向?yàn)樾履茉措姵夭牧?

E-mail：wanghw@cug.edu.cn