氮磷硫自摻雜竹炭用作鋰離子電池負(fù)極材料

陳前林 吳洪

N， P and S Self-doped Bamboo Charcoal as an

Anode Material for Lithium-ion Batteries

CHEN Qianlin*， WU Hong

（College of Chemistry and Chemical Engineering， Guizhou University， Guiyang 550025， China）

Abstract：

The preparation process of nitrogen， phosphorus and sulfur self-doped bamboo charcoal is simple， safe and environmentally friendly， which has certain guiding significance for other biomass materials to prepare composite materials. In this research， bamboo （rich in N， P and S components） is used as a biomass-derived carbon precursor， and KOH is used as an activator， then porous bamboo charcoal （BDC-800） is prepared by activation and carbonization in a high-temperature nitrogen atmosphere at 800 ℃ while also achieving N， P and S doping. BDC-800 has a high specific surface area of 1 911 m2/g， a large pore volume of 1.21 cm3/g， and a large number of hierarchical porous structures. BDC-800 as an anode material in a lithium-ion battery shows a high storage capacity of 681.4 mAh/g at a rate of 0.50 C （1 C=372 mAh/g）， good cycling stability （390.1 mAh/g at 2.00 C after 700 cycles） and excellent rate performance （754.1， 697.8， 580.2 and 403.2 mAh/g at 0.25， 0.50， 1.00 and 2.00 C， respectively）. The excellent electrochemical performance of BDC-800 is attributed to its high specific surface area and hierarchical porous structure， as well as the capacitive contribution to the total charge caused by the doped N， P and S and the abundance of surface defects.

Key words： lithium-ion battery; anode; bamboo charcoal; self-doped; capacitive contribution

CLC number：TQ152

Document code：A

Energy storage technology occupies an increa-singly important position in our lives and industry. Among this technology， lithium-ion batteries （LIBs） have the advantages of a high working voltage， good cycling performance， high energy density， and environmental friendliness; thus， LIBs have become the most promising portable energy storage tech-nology[1]. LIBs are widely used in computers， watches， electric vehicles and other products， which has helped promote the rapid development of lithium batteries[2]. Graphite is the most successful commercial negative electrode material for LIBs， but it has a low theoretical capacity （372 mAh/g） and a slow charge/discharge rate. Therefore， graphite cannot meet the rapidly deve-loping market demand[3-4]. To meet market demand， researchers have been exploring different carbon materials to replace graphite.

Biomass is a carbon-rich precursor with an abundance of sources， easy access and environmental friendliness. It has high potential for use in the preparation of carbon anode materials[5-6]. The variety of available biomass， the uniqueness of their struc-tures， and their abundance of N， P， S and other components， make biomass an ideal precursor for carbon materials[7]. Ramie[8]， such as loofah[9]， rice husk[10]， banana peel[11] and nut shells[12-13] biomass-derived carbon have shown excellent electrochemical performances as negative electrode materials for LIBs. Bamboo is an ordinary plant， and China has abundant bamboo resources. Notably， bamboo has a short growth cycle， is easy to obtain， and is rich in N， P and S components[14]. However， untreated bamboo charcoal has poor electrochemical performance[15]. S. K. et al. grew SiO2 on commercial bamboo charcoal in situ to synthesize a SiO2/C composite material， which showed high storage capacity; unfortunately， the preparation process was complicated[16]. SnO2 has been grown on the surface of bamboo charcoal fibers， which are then coated with glucose carbon to obtain a BCF/SnO2/C composite material with excellent storage capacity[17]. However， tin-based anode materials demonstrate large volume changes during charge and discharge cycles， leading to the destruction of the electrode material structure and resulting in the electrode material having poor cycling stability. Porous carbon materials can alleviate the crushing effect of tin-based electrode materials， thereby improving cycle stability[18-19]. Nitrogen self-doped bamboo charcoal prepared from bamboo leaves has been used as a negative electrode material for lithium batteries to obtain good electrochemical performance[20]. Sulfur-doped bamboo charcoal has been used as a negative electrode material for potassium-ion batteries， showing good electrochemical performance[15].

When bamboo is activated and pyrolyzed at high temperature， potassium compounds can promote the further decomposition of bamboo charcoal， thereby increasing the specific surface area and porosity[21-22]. An electrode material with a high specific surface area is conducive to the rapid transport of ions and electrons. Additionally， the large pore volume allows the electrolyte to be fully saturated， allowing Li+ to diffuse quickly[23]. Heteroatom （N， P， S， B， etc.） doping can improve the surface chemical properties of carbon materials and increase the number of active sites for the adsorption of Li+ on the surface of carbon materials. Therefore， heteroatom doping can increase the charge/discharge rate and storage capacity of carbon anode materials[24-25]. In addition， biomass-derived carbon electrodes have two storage mechanisms for Li+： diffusion control of Li+ insertion and extraction between carbon material layers and surface induction capacitance provided by the surface defects and pores of the carbon material[26-27].

This paper reports a method for preparing N， P and S self-doped bamboo charcoal. Bamboo （rich in N， P and S components） is used as the precursor of biomass carbon， and KOH is used as the activator. Bamboo is activated and carbonized at different temperatures （600， 700 and 800 ℃） and simul-taneously realizes N， P and S doping （Fig.1）. BDC-800 has a specific surface area of 1 911 m2/g and a pore volume of 1.21 cm3/g， with a large number of micropores and mesopores. BDC-800 as an anode material in a lithium-ion battery shows a high storage capacity of 681.4 mAh/g at a rate of 0.50 C （1 C=372 mAh/g）， good cycling stability （390.1 mAh/g at 200 C after 700 cycles） and excellent rate perfor-mance （754.1， 697.8， 580.2 and 403.2 mAh/g at 025， 0.50， 100 and 200 C， respectively）. In addition， the carbon production yield （14.1% and 26%， respectively） of bamboo at 800 ℃ and 900 ℃ under the catalysis of KOH is explored. The unique microstructure and surface chemistry of BDC-800 is also investigated. The surface-induced capacitance behavior of BDC-800 is caused by surface defects and pores. As the scanning rate increases， the contribu-tion rate of capacitance to the total charge also increases. Among the other contributors， the capa-citance contributes to the total charge when the scan rate is 0.6 mV/s， exhibiting a rate that reaches 52.7%.

1 Experimental section

1.1 Preparation of the bamboo-derived charcoal （BDC）

First， fresh bamboo was cleaned with ultrapure water （bamboo was taken from Zunyi， China）， then the fresh bamboo was broken down into powder using a breaking machine. The bamboo powder was placed in a blast oven （80 ℃） and dried to a constant weight. The mass ratio of bamboo powder to KOH was 1∶1. An appropriate amount of ultrapure water was added， and then the bamboo mixed with KOH was wet ball milled for 12 h. The bamboo mixed with KOH was placed in a blast oven （80 ℃） and dried to

a constant weight. Then， the bamboo powder added with KOH was placed into a flowing nitrogen tube furnace at a heating rate of 5 ℃/min. Next， the mixture of bamboo powder and KOH was carbonized at 300 ℃ for 2 h to remove the crystal water and volatile substances in the bamboo. After heating to a certain temperature （600， 700 and 800 ℃） the samples were carbonized for 3 h to obtain three bamboo charcoal samples， named BDC-600， BDC-700 and BDC-800， respectively. The bamboo charcoal samples were treated with 2 mol/L HCl solution in a water bath at 60 ℃ for 24 h and then washed with ultrapure water until reaching a neutral pH. Finally， the bamboo carbon samples were placed in a vacuum oven （80 ℃） and dried to a constant weight to obtain self-doped bamboo charcoal anode materials.

1.2 Materials characterization

X-ray diffraction （XRD， Rigaku， Japan） was performed by using a Cu Kα radiation source （λ=1540 6 ，1 =10-10 nm） at 40 kV and 40 mA. Raman spectroscopy （Thermo DXR2xi） was conducted with laser excitation at 532 nm to characterize the structure of the carbon materials. Scanning electron microscopy （ΣIGMA+X-Max20 Zeiss， Germany） and transmission electron microscopy （TEM， JEM 2010 JEOL， Japan） were used to observe the morphology and microstructure of the carbon materials. A NOVA-1000 automatic analyzer （USA） was used to calculate the specific surface area by nitrogen adsorption/desorption isotherms， and the corresponding pore size distribution （PSD） was obtained by density functional theory （DFT） calculations. X-ray photoelectron spectroscopy （XPS， ESCALAB 250XI） was performed using a monochromatic Al Ka X-ray source to measure the surface composition and chemical bonding properties of bamboo charcoal.

1.3 Cell assembly and electrochemical tests

The electrochemical performance of bamboo charcoal doped with N， P and S as anode materials for LIBs was studied. A button battery （CR-2430） was assembled in an argon-filled glove box. The prepared active substance （bamboo charcoal， 80% by mass）， conductive carbon black （acetylene black， 10% by mass） and polyvinylidene fluoride （PVDF， 10% by mass） were mixed; then， N-methyl-2-pyrrolidone （NMP） was added and stirred evenly. After preparing the slurry， it was evenly coated on a copper foil current collector， and the pole pieces were dried in a vacuum oven at 80 ℃ for 12 h. The area of the circular pole piece is 1.54 cm2， and the amount of active substance loaded is 1.16 mg. A Celgard 2 400 polypropylene membrane was used as the separator， lithium metal was used as the counter electrode， and 1 mol/L LiPF6 mixed with an organic solvent （ethylene carbonate （EC）： dimethyl carbonate （DMC）=1∶1 by volume） was used as the electrolyte. The button batteries were tested on a LANHE blue battery test system （CT3001A） for charge/discharge cycle testing over a potential range of 0.01～3.00 V. Additionally， a CHI760E electrochemical workstation was used for cyclic voltammetry （CV）. Electrochemical impedance spectroscopy （EIS） was performed at an amplitude of 5 mV over a frequency range of 0.01～105 Hz.

2 Results and discussion

2.1 Materials characterization

KOH has very high reactivity during biomass pyrolysis， decreasing the yield of solid biomass products as the carbonization temperature increases[28-29]. As shown in Tab.1， the carbon production yield of bamboo carbonized at 800 and 900 ℃ for 3 h in a N2 flow is 24.4% and 23.6%， respectively. The carbon production yields of bamboo after the addition of KOH are 14.1% and 2.6%， respectively. Regarding the production of biomass-derived carbon from bamboo， the decrease in yield is attributed to the further decomposition of solid products， which is promoted by KOH catalysis[30]. Since the carbon production rate during carbonization at 900 ℃ under the catalysis of KOH is low， it is of minor significance to continue exploring the carbon materials produced at 900 ℃; therefore， the products carbonized at 600， 700 and 800 ℃ are explored further.

Fig.2 shows the microstructures of BDC-600， BDC-700 and BDC-800， which were obtained by XRD and Raman spectroscopy. In the XRD patterns of the three samples， 2θ diffraction peaks appear at approximately 23° and 44°， corresponding to the （002） and （100） planes of the graphite carbon material （Fig.2（a））， respectively; this result indicates that the three samples are amorphous carbon[26]. Compared with the BDC-600 and BDC-700 samples， the diffraction peaks of the （002） plane of BDC-800 are shifted to the left， which indicates that the average interlayer spacing of the （002） plane of BDC-800 increases[31]. The increase in the interlayer spacing of the carbon material is conducive to the rapid transport of Li+ on the surface and inside the electrode material[32]. Fig.2（b） shows the Raman diagrams of the three samples. Two peaks appear at approximately 1 340 cm-1 （D band） and 1 590 cm-1 （G band）. The D band represents the absence of an ordered carbon structure defects. The G band represents an ordered carbon structure[33]. The ID/IG ratio is used to measure the degree of disorder of carbon materials， and the ID/IG values of BDC-600， BDC-700 and BDC-800 are 0.96， 0.98 and 1.05， respectively. The results showed that the structural defects of bamboo charcoal increased with the increase of pyrolysis temperature， which was attributed to the enhanced catalytic action of KOH， which promoted the structural defects of bamboo charcoal[34-35]. At the same time， the substitution of some carbon atoms by N， P and S promoted the structural defects of bamboo charcoal， which were the active sites of bamboo charcoal. The increase of active sites for adsorption of Li+ in bamboo charcoal could lead to a larger reversible capacity[36-38].

Fig.3（a） shows the microscopic morphology of bamboo， and Fig.3（b） shows the microscopic mor-phology of bamboo charcoal obtained by the pyrolysis of bamboo.

As shown in Fig.4， the morphology and micro-structure of the BDC-800 sample， as well as the distribution of O， N， P and S in the carbon material， were further studied. There was no obvious change in the morphology of bamboo before and after pyrolysis. Fig.3（c）， Fig.3（f） and Fig.4（a） show the morphologies of bamboo after activation and pyrolysis by the addition of KOH; notably， BDC-600， BDC-700 and BDC-800 exhibit interconnected pore struc-tures. As the pyrolysis temperature increases， the pore size expands. Furthermore， the increase in temperature enhances the catalytic effect of KOH， thus enhancing the pore formation process[29]. The interconnected porous structure shortens the Li+ and electrolyte migration path， reduces the diffusion resistance， and promotes the rapid transport of Li+ and electrons inside and on the surface of the electrode material[25]. Fig.4（b） shows the TEM images of the edge of the BDC-800 carbon material. The carbon material has disorderly stacked graphite-like band fringes， and its pitch is 0.413 nm （Fig.4（c））. Fig.4（e）—（i） shows the elemental mapping （EDS） of BDC-800， revealing that N， P， S and O are uniformly distributed on the carbon material.

As shown in Fig.5， the specific surface area and pore structure of BDC-600， BDC-700， and BDC-800 were studied by nitrogen adsorption/desorption isotherms. Fig.5（a） shows that the nitrogen adsor-ption/desorption isotherms of BDC-600 and BDC-700 have type I characteristics， indicating that these samples have a characteristic microporous structure; the microporous structure plays a role in storing charges[34]. The nitrogen adsorption/desorption isotherm of BDC-800 has the comprehensive characteristics of a type I/IV material， which means that BDC-800 has both microporous and mesoporous composite structures. The mesoporous structure in BDC-800 allows the electrolyte to fully infiltrate the material and promotes the rapid diffusion of Li+ [23]. Fig.5（b） shows that the pore sizes of BDC-600 and BDC-700 are less than 2 nm， and the pore size of BDC-800 is in the range of 5.5 nm. The specific surface areas of BDC-600， BDC-700 and BDC-800， which were calculated according to Brunauer-Emmett-Teller （BET） theory， are shown in Tab.2. The bamboo charcoal specific surface area and porosity increase as the temperature increases during the activation and pyrolysis of bamboo; this result is due to the enhanced pore formation under the catalysis of KOH[29].

The reaction process of KOH etched bamboo charcoal at different temperatures. When the pyrolysis temperature reached 400 ℃， the bamboo charcoal and KOH started the activation reaction （1）. The decomposition of KOH was completed at about 600 ℃， and the formed K was inserted into the carbon layer. When the pyrolysis temperature reached 700 ℃， K2CO3 began to decompose to produce K2O and CO2 （2）. At the same time， the generated CO2 reacted with bamboo charcoal （3）， resulting in escaped CO to increase the specific surface area porosity of bamboo charcoal[39]. When the pyrolysis temperature reached 800 ℃， K2CO3 and K2O were thermally reduced with bamboo charcoal to metal K （4） and （5）. At the same time， the bamboo charcoal was further etched to increase the specific surface area and pore volume of bamboo charcoal[29， 40].

6KOH+2C→2K+3H2+2K2CO3（1）

K2CO3→K2O+CO2（2）

CO2 +C→CO（3）

K2CO3+2C→2K+3CO（4）

C+K2O→2K+CO（5）

With the increase of pyrolysis temperature， the specific surface area and porosity of bamboo charcoal increased， which was completely consistent with the results of nitrogen adsorption/desorption isotherm. The electrode material with high specific surface area is conducive to the rapid transmission of Li+ and electrons[23]. The larger pore volume allows the electrolyte to fully permeate the electrode material， thus reducing the diffusion resistance of Li+ in the electrode material and improving the storage capa-city[41-42]. Compared with BDC-600 and BDC-700 electrode materials， BDC-800 electrode material has the highest specific surface area and pore volume， so BDC-800 electrode material shows better electroche-mical performance.

As shown in Fig.6， XPS was used to analyze the surface composition and chemical bond properties of BDC-600， BDC-700 and BDC-800 （Tab.3）， N， P and S were all doped in the three kinds of bamboo charcoal. The amount of doped N and S increases as the pyrolysis temperature increases; in contrast， the amount of doped P decreases as the pyrolysis temperature increases. Due to the high thermal stability of graphite， nitrogen and nitrogen oxides， as the pyrolysis temperature increases， the amount of doped N increases[43-44]. The phosphorus-doped structural formulas （C-P， P-O， C-P-O） have poor thermal stability， and the relative content of P decreases with an increasing temperature[45]. The bond energies of S-O （521.7 kJ/mol） and S=O （523 kJ/mol） are higher than that of C-C （347 kJ/mol）.

As the pyrolysis temperature increases from 600 to 800 ℃， the relative content of S increases[43]. Porous carbon with residual oxygen can improve the wettability of the carbon electrode and increase its storage capacity， but it will reduce the rate performance to a certain extent[46-47]. With the increase of N， P and S contents， the structural defects of porous bamboo charcoal increased， which was completely consistent with Raman’s results. The structural defects and pores of porous bamboo charcoal lead to the capacitive control behavior and the capacitive diffusion control can contribute to the reversible capacity[19， 26-27]. The increase of active sites for adsorption of Li+ in bamboo charcoal increases the reversible capacity of the bamboo charcoal electrode material.

Fig.7（a） shows that there are three peaks in the high-resolution C 1s spectrum of BDC-800： C=C/C-S （284.6 eV）[48] ， C-C （284.8 eV）[49]， and C-O/C-N/C-P （285.5 eV）[26，36]. Fig.7（b） shows that the high-resolution N 1s spectrum of BDC-800 has four peaks： pyridinic N （398.6 eV）， pyrrolic N （400.1 eV）， graphitic N （401.7 eV） and oxidized N （403.3 eV）[35-38]. Hydrocarbons have a negative impact on the electrochemistry of carbon materials. Fortunately， N doping can inhibit the formation of hydrocarbons[50]. The electronegativity of N（3.04） is higher than that of C（2.55）. Furthermore， N doping is beneficial to the adsorption of Li+， which increases the conductivity and storage capacity of the carbon material[20]. Fig.7（c） shows that there are three peaks in the high-resolution P 2p spectrum of BDC-800. These peaks are at 130.2， 134.2 and 133.1 eV and correspond to C-P， P-O and C-P-O[51-53]， respectively. P acts as a bridge between C and O to inhibit unstable quinone and promotes the formation of carboxyl groups to maintain the electrochemical stability of the carbon surface[45]. The P group has excellent cation exchange properties， which enhances the ability of carbon materials to adsorb Li+， thus increasing their conductivity and storage capacity[54]. Fig.7（d） shows that the high-resolution S 2p spectrum of BDC-800 has two peaks： 164.1 eV （S 2p3/2） and 165.0 eV （S 2p1/2） correspond to thiophene sulfur （C-S-C） and 168.6 eV and 169.8 eV correspond to sulfoxide （C-SOx-C， x=2～4）[35，49，55]. The adsorption energy of S-doped sites is lower than that of oxygen sites; thus， S-doped sites are more conducive to Li+ adsorption than O sites[15]. The self-doping of N， P and S improves the chemical properties of the bamboo charcoal surface and improves the conductivity and storage capacity of the bamboo charcoal[56-57].

2.2 Electrochemical characterization and battery performance

Fig.8（a） shows that in the potential range of 001～300 V， the battery is charged/discharged at a rate of 0.20 C for the first 3 cycles to activate the electrode material and then the battery is charged/discharged at a rate of 0.50 C until the end of the 150th cycle. The BDC-600， BDC-700 and BDC-800 samples retain storage capacities of 301.4， 368.6 and 681.4 mAh/g， respectively. The storage capacity of bamboo charcoal increases with an increasing pyrolysis temperature， which is attributed to the increase in the specific surface area and porosity of the carbon material. The self-doping of N， P and S improves the chemical properties of the bamboo charcoal surface， and the number of active sites to store Li+ on the bamboo charcoal increases[24-25]; in addition， the surface defects and pores of the carbon material contribute to its capacitive behavior[26]. Compared with previously reported biomass carbon materials， BDC-800 has excellent electrochemical performance （Tab.4）. The Fig.8（b） shows the first charge/discharge curves of BDC-600， BDC-700， and BDC-800. The platform observed in the charge and discharge curve is consistent with the result of the CV curve.

The first discharge specific capacity of BDC-600 electrode material is 433.4 mAh/g， the charge specific capacity is 107.0 mAh/g， and the first coulomb ef-ficiency is 24.69%.The first discharge specific capa-city of BDC-700 electrode material is 956.1 mAh/g， the charging specific capacity is 375.2 mAh/g， and the first coulomb efficiency is 39.24%.The first discharge specific capacity of the BDC-800 electrode material is 2 406.0 mAh/g， the charge specific capacity is 1 029.7 mAh/g， and the first Coulomb efficiency is 42.8%. During the first discharge process， the larger specific surface product leads to the formation of a larger SEI layer， which consumes more lithium ions and increases the discharge specific capacity. At the same time， due to the increase of active sites on the N， P and S doped surface， the specific discharge capacity of the porous structure electrode material is more favorable to increase[19]. The pore volumes of BDC-600， BDC-700 and BDC-800 are 0.19， 0.39 and 1.21 cm3/g， respectively （This is shown in Tab.2）. The larger pore volumes can store more lithium ions， thus improving the storage capacity of electrode materials. The experimental results verified that the synergistic effect of higher specific surface area and pore volume， as well as higher content of N， P and S doped porous bamboo charcoal improved the first Cou-lomb efficiency of porous bamboo charcoal.

Fig.8（c） shows the rate performance of the BDC-600， BDC-700 and BDC-800 electrode materials. When BDC-800 is charged/discharged at rates of 025， 0.50， 100 and 200 C， the specific discharge capacities of the electrode materials are 754.1， 697.8， 580.2 and 403.2 mAh/g （Fig.8（d））， respectively， indicating that BDC-800 has good rate performance. When the charge/discharge rate is restored to 0.25 C， the specific discharge capacities of the BDC-600， BDC-700 and BDC-800 electrode materials are 3508， 411.3 and 790.9 mAh/g， respectively. Compared with BDC-600 and BDC-700， the BDC-800 sample has a higher specific capacity. Notably， there is no constant voltage plateau. This curve is caused by the capacitive control behavior driven by the surface of the carbon material， indicating that there is capacitive control behavior during the charge/discharge cycles[65].

The electrochemical performance of N-， P-， and S-doped porous bamboo charcoal was studied at a scan rate of 0.1 mV/s， showing the potential change of Li+/Li insertion and extraction in the potential range of 0.01～300 V （Fig.9（a））. The cyclic voltammetry curves of the BDC-800 lithium battery almost overlap， indicating that BDC-800 has good cyclic reversibility. It delivers a typical CV behavior of carbon materials in lithium batteries[66].

Fig.9（b） shows the EIS results of the BDC-600， BDC-700 and BDC-800 samples. The Nyquist plots are composed of a semicircle and diagonal lines， and the equivalent circuit model from these results is illustrated. In the equivalent circuit model， Rs is the resistance of the electrolyte; Rct is the charge transfer resistance， which is represented by the semicircle; Zw represents the Warburg impedance， and CPE represents the capacitance of the double-layer capacitor and passivation film. The charge transfer resistances of the BDC-600， BDC-700 and BDC-800 samples are 76， 64 and 46 Ω， respectively， which indicates that the charge transfer resistance decreases as the pyrolysis temperature increases. This is because the pore structure of the carbon material increases， the pore size becomes larger， and the interlayer spacing of the carbon material increases as the pyrolysis temperature increases. These changes shorten the migration path of Li+ and the electrolyte[34]， as well as promoting the doping of N， P and S. Impurities improve the electrochemical properties of a carbon material surface and increase the conductivity of the carbon material[56-57].

battery applications in industry and life. Fig.10 shows that within the potential range of 0.01～3.00 V， BDC-800 has 700 charge/discharge cycles at a high rate of 2.00 C. The first 3 charge/discharge cycles are at 020 C to activate the electrode material and then the charge/discharge cycling is conducted at a high rate of 2.00 C until the end of the 700th cycle; notably， BDC-800 still retains a storage capacity of 390.1 mAh/g. Generally， when an electrode material is charged/discharged at a high rate， the electrode structure is destroyed， resulting in a rapid decrease in the storage capacity of the electrode[67].

The structure change of BDC-800 electrode material during charge/discharge cycle is given. As can be seen from Fig.11， the shape of BDC-800 after 150 charge/discharge cycles still maintains the shape before the charge/discharge cycles. The results show that the storage capacity of the BDC-800 electrode material hardly attenuates after being charged/discharged at a high rate， indicating that BDC-800 has good cycling stability. Due to the expanded interlayer spacing， high specific surface area and hierarchical porous structure of BDC-800， the rapid transport of Li+ is promoted on the surface and inside this carbon material[34]. The BDC-800 electrode material has high potential in high-rate charge/discharge applications.

Biomass-derived carbon anode materials have two Li+ storage mechanisms： diffusion behavior and capacitive behavior[68]. The contribution of capacitive behavior to the storage capacity during charge/discharge cycles is analyzed. As shown in Fig.12（a）， within the potential range of 0.01～3.00 V， the BDC-800 lithium battery undergoes an increasing scan rate from 0.2 to 1.0 mV/s. Under these conditions， the CV curve of BDC-800 has a similar expansion， and the redox peak current increases as the scan rate increases， indicating that BDC-800 can quickly respond to CV scans. According to the pseudo-capacitance theory calculation proposed by P. Simon et al.[69]， Fig.12（b） shows the current peak value of the redox reaction during the charge/discharge process （Fig.12（a））， and the slope of the scan rate curve （b value） is 0.84 and 0.80 （when 0.50

Fig.12（c） shows the BDC-800 sample at a scan rate of 0.6 mV/s. The capacitance area presents a contour similar to the CV curve， indicating that the demonstrated capacitance is pseudo-capacitance. As shown in Fig.12（d）， when the scan rate is 0.2， 0.4， 0.6， 0.8 and 1.0 mV/s， the contribution of the capacitance behavior to the total charge is 42.5%， 46.8%， 52.7%， 56.9%， and 62.3%， respectively. As the rate increases， the contribution rate of the capacitance behavior to the total charge increases， which is one of the reasons for the excellent electrochemical performance of BDC-800.

3 Conclusions

In summary， N-， P-， and S-doped bamboo charcoal with a porous structure （BDC-800） was successfully prepared by a simple method. KOH was used as an activator. As the temperature was increased during the activation and pyrolysis of bamboo， the specific surface area and porosity of the bamboo charcoal increased; moreover， the bamboo charcoal had a large number of hierarchical porous structures. The specific surface area of BDC-800 was 1 911 m2/g， and the pore volume was 1.21 cm3/g. At the same time， N， P and S doping improved the chemical properties of the carbon material surface and increased the number of active sites for storing Li+. BDC-800 as an anode material in a lithium-ion battery showed a high storage capacity of 681.4 mAh/g at a rate of 0.50 C （1 C=372 mAh/g）， good cycling stability （390.1 mAh/g at 2.00 C after 700 cycles） and excellent rate performance （754.1， 697.8， 580.2 and 403.2 mAh/g at 0.25， 0.50， 1.00 and 2.00 C， respectively）. In addition， the oxidation-reduction reaction process of the BDC-800 electrode material was controlled by two behaviors： diffusion and capacitance behaviors. The capacitance behavior was caused by the surface defects and pores， and as the scanning rate increased， the contribution of the capacitance behavior to the total charge increased. BDC-800 had excellent electrochemical performance when used as a negative electrode material for LIBs. This performance was attributed to its high specific surface area and rich hierarchical porous structure， as well as the capacitive behavior caused by the doped N， P and S and the abundance of surface defects. The BDC-800 electrode material has the potential to become a next-generation lithium-ion battery anode material.

References：

[1]TARASCON J M， ARMAND M.Issues and challenges facing rechargeable lithium batteries[J]. Nature， 2001， 414： 359-367.

[2] GEIM A K， NOVOSELOV K S. The rise of graphene[J]. Nature Materials， 2007， 6： 183-191.

[3] WU H B， CHEN J S， HNG H H， et al. Nanostructured metal oxide-based materials as advanced anodes for lithium-ion batteries[J]. Nanoscale， 2012， 4（8）： 2526-2542.

[4] BUQA H， GORS D， HOLAZPFEL M， et al. High rate capability of graphite negative electrodes for lithium-ion batteries[J]. Journal of The Electrochemical Society， 2005， 152（2）： A474-A481.

[5] SONG X Y， MA X L， LI Y， et al. Tea waste derived microporous active carbon with enhanced double-layer supercapacitor behaviors[J]. Applied Surface Science， 2019， 487： 189-197.

[6] ZHU X M， JIANG X Y， LIU X L， et al. A green route to synthesize low-cost and high-performance hard carbon as promising sodium-ion battery anodes from sorghum stalk waste[J]. Green Energy & Environment， 2017， 2（3）： 310-315.

[7] JIANG L L， SHENG L Z， FAN Z J. Biomass-derived carbon materials with structural diversities and their applications in energy storage[J]. Science China Materials， 2017， 61（2）： 133-158.

[8] JIANG Q， ZHANG Z H， YIN S Y， et al. Biomass carbon micro/nano-structures derived from ramie fibers and corncobs as anode materials for lithium-ion and sodium-ion batteries[J]. Applied Surface Science， 2016， 379： 73-82.

[9] WU Z R， WANG L P， HUANG J ， et al. Loofah-derived carbon as an anode material for potassium ion and lithium ion batteries[J]. Electrochimica Acta， 2019， 306： 446-453.

[10]CHAIKAWANG C， HONGTHONG R， KAEWMALA S， et al. Surface modification of rice husk ash as anodes for lithiumion batteries[J]. Materials Today： Proceedings， 2018， 5（6）： 13989-13994.

[11]LI F Q， QIN F R， ZHANG K K， et al. Hierarchically porous carbon derived from banana peel for lithium sulfur battery with high areal and gravimetric sulfur loading[J]. Journal of Power Sources， 2017， 362： 160-167.

[12]DEMIR E， AYDIN M， ARIE A A， et al. Apricot shell derived hard carbons and their tin oxide composites as anode materials for sodium-ion batteries[J]. Journal of Alloys and Compounds， 2019， 788： 1093-1102.

[13]LIU J， LIU B， WANG C W， et al. Walnut shell-derived activated carbon： synthesis and its application in the sulfur cathode for lithium-sulfur batteries[J]. Journal of Alloys and Compounds， 2017， 718： 373-378.

[14]LIU Z G， LI Z L， LI D H， et al. ent-Abietane-type diterpenoids from the roots of Euphorbia ebracteolata with their inhibitory activities on LPS-induced NO production in RAW 264.7 macrophages[J]. Bioorganic & medicinal chemistry letters， 2016， 26： 1-5.

[15]TIAN S， GUAN D C， LU J， et al. Synthesis of the electrochemically stable sulfur-doped bamboo charcoal as the anode material of potassium-ion batteries[J]. Journal of Power Sources， 2020， 448： 227572.1-227572.11.

[16]KUANG S J， XU D H， CHEN W Y， et al. In situ construction of bamboo charcoal derived SiOx embedded in hierarchical porous carbon framework as stable anode material for superior lithium storage[J]. Applied Surface Science， 2020， 521： 146497.1-146497.11.

[17]HAN Q G， YI Z， WANG F X， et al. Preparation of bamboo carbon fiber and sandwich-like bamboo carbon fiber@SnO2@carbon composites and their potential application in structural lithium-ion battery anodes[J]. Journal of Alloys and Compounds， 2017，709：227-233.

[18]CHENG Y， WANG Z M， CHANG L M， et al. Sulfur-mediated interface engineering enables fast SnS nanosheet anodes for advanced lithium/sodium-ion batteries[J]. ACS Applied Materials & Interfaces， 2020，12： 25786-25797.

[19]CHENG Y， WANG S H， ZHOU L， et al. SnO2 quantum dots： rational design to achieve highly reversible conversion reaction and stable capacities for lithium and sodium storage[J]. Small， 2020， 16（26）： 2000681.1-2000681.12.

[20]YAN Z H， YANG Q W， WANG Q H， et al. Nitrogen doped porous carbon as excellent dual anodes for Li- and Na-ion batteries[J]. Chinese Chemical Letters， 2020， 31（2）： 583-588.

[21]DING Y M， HUANG B Q， WU C B， et al. Kinetic model and parameters study of lignocellulosic biomass oxidative pyrolysis[J]. Energy， 2019，181： 11-17.

[22]YUAN R，YU S L， SHEN Y F. Pyrolysis and combustion kinetics of lignocellulosic biomass pellets with calcium-rich wastes from agro-forestry residues[J].Waste Management， 2019， 87（11）： 86-96.

[23]XU G Y， DING B， SHEN L F， et al. Sulfur embedded in metal organic framework-derived hierarchically porous carbon nanoplates for high performance lithium-sulfur battery[J]. Journal of Materials Chemistry A， 2013， 1： 4490-4496.

[24]WANG S B， XIAO C L， XING Y L， et al. Carbon nanofibers/nanosheets hybrid derived from cornstalks as a sustainable anode for Li-ion batteries[J]. Journal of Materials Chemistry A， 2015， 3（13）： 6742-6746.

[25]WANG D W， LI F， LIU M， et al. 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage[J]. Ange-wandte Chemie， 2008， 47： 373-376.

[26]LI R Z， HUANG J F， LI J Y， et al. Nitrogen-doped porous hard carbons derived from shaddock peel for high-capacity lithium-ion battery anodes[J]. Journal of Electroanalytical Chemistry， 2020， 862： 114044.1-114044.8.

[27]SUN L， MA T T， ZHANG J， et al. Double-shelled hollow carbon spheres confining tin as high-performance electrodes for lithium ion batteries[J]. Electrochimica Acta， 2019， 321： 134672.1-134672.8.

[28]BISWAS B， PANDEY N， BISHI Y， et al. Pyrolysis of agricultural biomass residues： Comparative study of corn cob， wheat straw， rice straw and rice husk[J]. Bioresource Technology， 2017， 237： 57-63.

[29]SHEN Y F， ZHANG N Y， ZHANG S. Catalytic pyrolysis of biomass with potassium compounds for co-production of high-quality biofuels and porous carbons[J]. Energy， 2020， 190： 116431.1-116431.12.

[30]HU S， JIANG L， WANG Y， et al. Effects of inherent alkali and alkaline earth metallic species on biomass pyrolysis at different temperatures[J]. Bioresource Technology， 2015， 192： 23-30.

[31]LIU J L， ZHANG Y Q， ZHANG L， et al. Graphitic carbon nitride （g-C3N4）-derived N-rich graphene with tuneable interlayer distance as a high-rate anode for sodium-ion batteries[J]. Advanced Materials， 2019， 31： 1901261.1-1901261.10.

[32]PAN D Y， WANG S， ZHAO B， et al. Li storage properties of disordered graphene nanosheets[J]. Chemistry of Materials， 2009， 21（14）： 3136-3142.

[33]ELIZABETH I， SINGH B P， TRIKHA S， et al. Bio-derived hierarchically macro-meso-micro porous carbon anode for lithium/sodium ion batteries[J]. Journal of Power Sources， 2016， 329： 412-421.

[34]ZHANG D D， ZHAO J H， FENG C， et al. Scalable synthesis of hierarchical macropore-rich activated carbon microspheres assembled by carbon nanoparticles for high rate performance supercapacitors[J]. Journal of Power Sources， 2017， 342： 363-370.

[35]OU J K， YANG L， ZHANG Z， et al. Honeysuckle-derived hierarchical porous nitrogen， sulfur， dual-doped carbon for ultra-high rate lithium ion battery anodes[J]. Journal of Power Sources， 2016， 333： 193-202.

[36]SARAVANAN K R， KALAISELVI N. Nitrogen containing bio-carbon as a potential anode for lithium batteries[J]. Carbon， 2015， 81： 43-53.

[37]LI X N， ZHU X B， ZHU Y C， et al. Porous nitrogen-doped carbon vegetable-sponges with enhanced lithium storage performance[J]. Carbon， 2014， 69： 515-524.

[38]MA C C， SHAO X H， CAO D P. Nitrogen-doped graphene nanosheets as anode materials for lithium ion batteries： a first-principles study[J]. Journal of Materials Chemistry， 2012， 22（18）： 8911-8915.

[39]GU X X， WANG Y Z， LAI C， et al. Microporous bamboo biochar for lithium-sulfur batteries[J]. Nano Research， 2014， 8： 129-139.

[40]BALAHMAR N， JUNMIALY A S， MOKAYA R. Biomass to porous carbon in one step： directly activated biomass for high performance CO2 storage[J]. Journal of Materials Chemistry A， 2017， 5： 12330-12339.

[41]ZHENG F C， LIU D， XIA G L， et al. Biomass waste inspired nitrogen-doped porous carbon materials as high-performance anode for lithium-ion batteries[J]. Journal of Alloys and Compounds， 2017， 693： 1197-1204.

[42]ZHENG F C， YANG Y， CHEN Q W. High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework[J]. Nature Communications， 2014， 5： 5261.1-5261.10.

[43]MIAO L， DUAN H， LIU M X. Poly（ionic liquid）-derived， N， S-codoped ultramicroporous carbon nanoparticles for supercapacitors[J]. Chemical Engineering Journal， 2017， 317： 651-659.

[44]WANG A， SUN K， LI J H， et al. Nitrogen and oxygen dual-doped activated carbon as electrode material for high performance supercapacitors prepared by direct carbonization of amaranthus[J]. Materials Chemistry and Physics， 2019， 231： 311-321.

[45]BI Z H， HUO L， KONG Q Q， et al. Structural evolution of phosphorus species on graphene with a stabilized electrochemical interface[J]. ACS Applied Materials & Interfaces， 2019， 11： 11421-11430.

[46]LIU H Y， SONG H H， CHEN X H， et al. Effects of nitrogen- and oxygen-containing functional groups of activated carbon nanotubes on the electrochemical performance in supercapacitors[J]. Journal of Power Sources， 2015， 285： 303-309.

[47]ZHANG D D， WANG J L， HE C， et al. Rational surface tailoring oxygen functional groups on carbon spheres for capacitive mechanistic study[J]. ACS Applied Materials & Interfaces， 2019， 11： 13214-13224.

[48]LIU J L， ZHU Y R， CHEN X H， et al. Nitrogen， sulfur and phosphorus tri-doped holey graphene oxide as a novel electrode material for application in supercapacitor[J]. Journal of Alloys and Compounds， 2020， 815： 152328.1-152328.11.

[49]XIAO-QING H Q， LI G， LI M J， et al. Biomass-derived nitrogen-doped hierarchical porous carbon as efficient sulfur host for lithium-sulfur batteries[J]. Journal of Energy Chemistry， 2020， 44： 61-67.

[50]PHUM T V， KIM J G， JUNG J Y， et al. High areal capacitance of N-doped graphene synthesized by arc discharge[J]. Advanced Functional Materials， 2019， 29： 1905511.1-1905511.9

[51]ROSAS J M， BEDIA J， RODRIGUEZ-MIRASOL J， et al. HEMP-derived activated carbon fibers by chemical activation with phosphoric acid[J]. Fuel， 2009， 88： 19-26.

[52]PUZIY A M， PODDUBNAYA O I， SOCHA R P， et al. XPS and NMR studies of phosphoric acid activated carbons[J]. Carbon， 2008， 46： 2113-2123.

[53]PUZIY A M， PODDUBNAYA O I， MARTINEZ-ALONSO A， et al. Surface chemistry of phosphorus-containing carbons of lignocellulosic origin[J]. Carbon， 2005， 43： 2857-2868.

[54]HAN L， CUI X Y， LIU Y Y， et al. Nitrogen and phosphorus modification to enhance the catalytic activity of biomass-derived carbon toward the oxygen reduction reaction[J]. Sustainable Energy & Fuels， 2020， 4 ： 2707-2717.

[55]ZHAO X C， ZHANG Q， CHEN C M， et al. Aromatic sulfide， sulfoxide， and sulfone mediated mesoporous carbon monolith for use in supercapacitor[J]. Nano Energy， 2012， 1： 624-630.

[56]LIU H， JIA M Q， YUE S F， et al. Creative utilization of natural nanocomposites： nitrogen-rich mesoporous carbon for a high-performance sodium ion battery[J]. Journal of Materials Chemistry A， 2017， 5： 9572-9579.

[57]LI Z， XU Z W， TAN X H， et al. Mesoporous nitrogen-rich carbons derived from protein for ultra-high capacity battery anodes and supercapacitors[J]. Energy & Environmental Science， 2013， 6： 871-878.

[58]ZHANG D W， WANG G， XU L， et al. Defect-rich N-doped porous carbon derived from soybean for high rate lithium-ion batteries[J]. Applied Surface Science， 2018， 451： 298-305.

[59]YAN P， YE H B， HAN Y， et al. Dual-templating approaches to soybeans milk-derived hierarchically porous heteroatom-doped carbon materials for lithium-ion batteries[J]. ChemistryOpen， 2020， 9： 582-587.

[60]MURALI G， HARISH S， PONNUSMY S， et al. Hierarchically porous structured carbon derived from peanut shell as an enhanced high rate anode for lithium ion batteries[J]. Applied Surface Science， 2019， 492： 464-472.

[61]WAN H R， HU X F. Nitrogen doped biomass-derived porous carbon as anode materials of lithium ion batteries[J]. Solid State Ionics， 2019， 341： 115030.1-115030.8.

[62]CHEN T J， ZHANG J， WANG Z Q， et al. Oxygen-enriched gasification of lignocellulosic biomass： syngas analysis， physicochemical characteristics of the carbon-rich material and its utilization as an anode in lithium ion battery[J]. Energy， 2020， 212： 118771.1-118771.9.

[63]YOKOKURA T J， RODRIGUEZ J R， POL V G. Waste biomass-derived carbon anode for enhanced lithium storage[J]. ACS Omega， 2020， 5： 19715-19720.

[64]YU H Y， LIANG H J， GU Z Y， et al. Waste-to-wealth： low-cost hard carbon anode derived from unburned charcoal with high capacity and long cycle life for sodium-ion/lithium-ion batteries[J]. Electrochimica Acta， 2020， 361： 137041.1-137041.8.

[65]SONG M Y， KIM N R， CHO S Y， et al. Asymmetric energy storage devices based on surface-driven sodium-ion storage[J].ACS Sustainable Chemistry & Engineering， 2016， 5： 616-624.

[66]KUMAGAI S， ABE Y， SAITO T， et al. Lithium-ion capacitor using rice husk-derived cathode and anode active materials adapted to uncontrolled full-pre-lithiation[J]. Journal of Power Sources， 2019， 437： 226-924.

[67]XIA H， LU L， MENG Y S， et al. Phase transitions and high-voltage electrochemical behavior of LiCoO2 thin films grown by pulsed laser deposition[J]. Journal of the Electrochemical Society， 2007， 154（4）： A337-A342.

[68]ZOU G Q， WANG C， HOU H S， et al. Controllable interlayer spacing of sulfur-doped graphitic carbon nanosheets for fast sodium-ion batteries[J]. Small， 2017， 13： 1700762.1-700762.10.

[69]AUGUSTYN V， COME J， LOWE M A， et al. High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance[J]. Nature Materials， 2013， 12： 518-522.

[70]ASIF M， RASHAD M， SHAH J H， et al. Surface modification of tin oxide through reduced graphene oxide as a highly efficient cathode material for magnesium-ion batteries[J]. Journal of Colloid and Interface Science， 2020， 561： 818-828.

（責(zé)任編輯：周曉南）

摘 要：氮磷硫自摻雜竹炭的制備工藝簡(jiǎn)單、安全、綠色環(huán)保，這對(duì)于其他生物質(zhì)材料制備復(fù)合材料具有一定的指導(dǎo)意義。以竹子（富含N、P、S成分）為碳源，KOH為活化劑，在氮?dú)鈿夥障?00 ℃高溫活化和熱解制備成多孔竹炭（BDC-800），同時(shí)實(shí)現(xiàn)了N、P、S摻雜; BDC-800表現(xiàn)出1 911 m2/g的表面積和1.21 cm3/g的孔體積，且具有大量的分級(jí)多孔結(jié)構(gòu)。BDC-800作為鋰離子電池負(fù)極材料，在0.50 C速率下充電/放電可以提供681.4 mAh/g高儲(chǔ)存容量;即使在2 C高速率下充電/放電循環(huán)700次，仍然保留390.1 mAh/g儲(chǔ)存容量，具有良好的循環(huán)穩(wěn)定性;在不同充電/放電速率下（在0.25， 0.50， 1.00和2.00 C對(duì)應(yīng)的放電比容量分別 為754.1， 697.8， 580.2 and 403.2 mAh/g），表現(xiàn)出優(yōu)異的倍率性能。BDC-800出色的電化學(xué)性能歸因于高的表面積和分級(jí)多孔結(jié)構(gòu)，以及N、P、S摻雜和眾多表面缺陷引起的電容行為貢獻(xiàn)。

關(guān)鍵詞：鋰離子電池;負(fù)極材料;竹炭;自摻雜;電容貢獻(xiàn)