Preparation of renewable lignin-derived nitrogen-doped carbon nanospheres as anodes for lithium-ion batteries

Abstract

For the common concern that general lignin-based colloidal spheres suffer the weakness of spherical structure destruction during the pyrolysis process, novel uniform lignin-derived nitrogen-doped carbon nanospheres were prepared by direct pyrolysis of lignin-based azo polymer (AL-azo-NO₂) colloidal spheres at 750 °C. In addition, a kind of hard carbon with high graphitization degree was also prepared as a control by direct pyrolysis of alkali lignin (AL). The morphology and structure of the as-obtained carbon materials were investigated by field emission scanning electron microscopy, X-ray diffraction patterns, Raman spectra and elemental analysis. They served as anodes in lithium-ion batteries with the aim to investigate the electrochemical performance. These two kinds of lignin-derived carbon materials exhibited different electrochemical performances due to their various structures and functional groups. The nitrogen-doped carbon nanospheres (C_SAN-750) exhibited a high cyclic stable capacity up to 225 mA h g⁻¹ at a current density of 60 mA g⁻¹ after 50 cycles (C_AL-750, 120 mA h g⁻¹) coupled with high first coulombic efficiency up to 66.4% for C_SAN-3000 with a cyclic capacity of 200 mA h g⁻¹, which is more comparable to that of the C_AL-3000 (53.3%, 199 mA h g⁻¹). This research indicates the great promise of lignin applied in producing hard carbon materials and further energy storage systems.

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Introduction

Nowadays, human society is subjected to a severe shortage of resources and various environmental problems. Seeking a green and sustainable development road has increasingly become a problem for enterprise, especially for the energy industry that is highly dependent on non-renewable fossil fuel resources. In this situation, rechargeable lithium-ion batteries have played the most efficient and environmentally friendly role in energy storage devices, which are widely applied in various fields such as electromobiles, portable electric devices and emerging smart grids.^1–4 As a major part of the negative electrode in lithium-ion batteries, carbon materials are dominating and widely investigated due to their quantity, easy processing, excellent reversibility and low discharge plateau.⁵ The already commercial graphite suffers some drawbacks including low capacity, limited stability, react readily with the electrolytic solution. Although several new efforts have been made to develop new carbon materials such as carbon nanotube, graphene, mesoporous carbon and carbon nanofibers, high conductivity and specific surface area endow them excellent electrochemical performance.^6,7 It is hard for these carbon materials to be large applied due to the nonrenewability and high cost of the precursor such coals, petroleum, polyacrylonitrile, as well as the complex production processes.

In addition to the graphite carbon, other carbonaceous materials such as soft carbon and hard carbon also possess high lithium-ion storage capacity. In particular, hard carbon have been attracted wide attention since its lithium-ion storage capacity is higher than graphite.^8–14 To our best knowledge, hard carbon can be obtained by pyrolysis of polymer. But graphitization is so difficult for it due to the three-dimensional cross-linking graphite layer caused by sp³ hybridization at the initial process of carbonization, which prevents the growth of parallel webs. So the d₀₀₂ of hard carbon is large and beneficial for diffusion of Li-ions. Lithium-ion can be inserted into the both side of the graphite layers and nanopores due to the abundant nanopores, causing a higher specific capacity. At the same time, it is more safe and easier to estimate the voltage since there is no obviously charge–discharge voltage platform. Furthermore, hard carbon has better resistance to overcharge performance compared with graphite. Lithium metal is still not precipitate even the amount of lithium-ion insertion is 110%,¹⁵ while lithium metal has been deposited for graphite when the amount of lithium-ion insertion is 105%. Also, hard carbon possesses an excellent compatible performance with electrolyte and good rate performance compared with graphite. Hard carbon is so promising but it is not worthwhile to produce hard carbon by direct pyrolysis of artificial polymers. Seeking cost-effective and renewable carbon precursor resource for fabricating hard carbon materials is promising and encouraged.

Lignin, a major component of wood, is the second most abundant biomass in nature after cellulose.^16–20 Normally, lignin mostly appears as a by-product in spent liquor from the pulping and paper making industry. The pulping spent liquor is one of the main environmental hazards, and many methods have been used to treat the spent liquor with the aim to recycle lignin, which has been used as filler, surfactant, and additive in materials applications. Anyway, most of the recycled lignin is burned or treated in other low-value methods, causing environmental secondary pollution and a waste of resource. Thus, methods to convert lignin into high valued product have attracted worldwide attention.^21–33

As the renewable carbon-rich precursor, lignin is a complex network of phenolic monolignol units, resulting in a high carbon yield after pyrolysis treatment.^34–37 The abundant aromatic rings in lignin tends to form graphite-like domains, while linear branches are lost as small molecular containing various hydrocarbons.³⁸ Recently studies demonstrate that lignin-derived carbons are promising materials for lithium-ion and Na-ions batteries due to their unique turbostratic disorder microstructures.^39,40 Especially, conversions of lignin to carbon fibers have been paid much attention and results indicated that lignin-derived carbon fibers are promising materials for lithium-ion batteries.^{37,38,40–42} Unfortunately, lignin carbon fibers have inferior mechanical properties that generally results in low stable rate capacity and bad cycling performance.³⁷ E.g., solvent extracted hardwood has been carbonized at about 2000 °C to prepare carbon fibers and exhibited about only 100 mA h g⁻¹ capacity at the current density of 15 mA g⁻¹ after 40 cycles.³⁸ Recent studies demonstrated that doping of nitrogen heteroatoms can improve the stability and capacitance of the materials.⁴³ Most of the traditional method of doping nitrogen is to use the urea and PAN (polyacrylonitrile) as the nitrogen source, but decomposition of urea can produce NH₃ easily which is harmful to environment and the cost of nitrogen-doped carbon fibers produced from PAN is expensive.^41,42 In addition, the technologies used such as direct carbonization at 2000 °C, melt spinning and electrospun etc. are of high energy consumption. Furthermore, lignin and lignin derivatives were also utilized as additive in electrochemical devices.^44–46 But it is not efficient to use lignin as additive to form composite electrode materials (weight ratio < 50%). Meanwhile, the lignin suffers the limitation of a certain type. In fact, lignin-derived carbon materials have rarely been utilized due to their surface functional groups, inferior mechanical properties and morphologies.⁴⁶ Development of new technologies is promising and necessary to prepare lignin-derived carbon materials for direct application as active materials in lithium-ion batteries. As is known to all, carbon nano/microspheres possess strong advantages for application as anodes for lithium-ion batteries due to its regular morphology, smooth surface and high bulk density. Thus, development of method to prepare carbon spheres has attracted wide attention.¹⁴

Traditionally, carbon spheres could be obtained by direct pyrolysis of their spherical precursor.⁴⁶ Recently, preparation of colloidal spheres based on lignin had been investigated in-depth in our previous work.^17,47,48 But to our best knowledge, most of the prepared colloidal spheres based on lignin or lignin derivative suffer the weakness of spherical structure destruction during the pyrolysis process. Aiming at the problems mentioned above, in this work, uniform lignin-derived nitrogen-doped carbon nanospheres were prepared by directly pyrolysis of lignin based azo polymers (AL-azo-NO₂) colloidal spheres at 750 °C. We prepared lignin-derived nitrogen-doped carbon spheres by fixing the nitrogen source to the skeleton of lignin precursor, and so the content of nitrogen is controllable via diazotization.¹⁶ Preparation of lignin-derived nitrogen-doped carbon nanospheres by direct pyrolysis AL-azo-NO₂ colloidal spheres, and its potential electrochemical performance served as active materials for lithium-ion batteries have not been investigated. In addition, lignin-derived hard carbon prepared by direct pyrolysis of alkali lignin was also investigated as controls. The influence of temperature on the structure, morphology and performance was studied. Due to the introduction of nitrogen, the lignin based colloidal spheres favours thermal stability and can keep spherical structure during the pyrolysis process. Doping of nitrogen not only improve the stability and endow the formation of carbon spheres, but also can further improve the electrochemical performance of the as-obtained materials. Using lignin not only cuts down the cost of preparation of carbon materials, but also reduces the environmental pollution. Results indicated that alkali lignin is a promising green option for preparation of hard carbon with high graphization degree and functional porous nitrogen-doped carbon nanospheres offer potential application for energy storage systems.

Experimental

Materials

Alkali lignin (AL), separated from pine pulping black liquor, was purchased from Xiangjiang Paper (Hunan, China). The performed lignin used in this research was purified by acidification, filtration and washing for at least three times with the aim to remove inorganic components. p-Nitroaniline was analytic grade purchased from Aladdin Chemistry Co. Ltd. Deionized water (resistivity of ≥18 MΩ cm⁻¹) was obtained from a Millipore water purification system. High-performance liquid chromatography grade tetrahydrofuran (THF) was purchased from Kernel Chemical Reagent Co., Ltd. (Tianjin, China). Other regents are of analytic, and solvents were used as received without further purification.

Preparation of lignin-based azo polymers colloidal spheres

The lignin-based azo polymers (AL-azo-NO₂) were synthesized by diazotization reaction according to a similar procedure that has been described in our previous work.¹⁶ The lignin-based azo polymers colloidal spheres were obtained as follows: A 5.0 g L⁻¹ sample solution was obtained by dissolving AL-azo-NO₂ in THF overnight. The colloidal spheres were obtained by adding 2 L deionized water via a peristaltic pump at a speed of 20.0 rpm. Then THF and excess water were removed by rotary evaporation, and products were dried by freeze-drying. Similarly, the AL was used to prepare AL colloidal spheres as control at the same condition.

Preparation of lignin-derived nitrogen-doped carbon materials

The lignin-derived nitrogen-doped carbon nanospheres were obtained by direct pyrolysis of the as-obtained AL-azo-NO₂ colloidal spheres. The carbonization program was set as follows: the raw colloidal spheres were transferred into a temperature-controlled tube furnace and heated to 750 °C with a heating rate of 5 °C min⁻¹ under an argon flow, and then maintained at the desired temperature for 2 h. After the tube furnace was cooled naturally to ambient temperature under an argon flow. Simultaneously, the AL colloidal spheres mentioned above were pyrolyzed at the same condition to prepare the control sample. Thus, the samples were obtained and labeled as C_SAN-750 for AL-azo-NO₂ colloidal spheres and C_SAL-750 for AL colloidal spheres, respectively. Furthermore, the AL-azo-NO₂ colloidal spheres were also carbonized at 3000 °C in an ultra-high temperature graphite furnace under an argon flow and labeled as C_SAN-3000. AL powders without micellization were considered as controls at the same condition, and the as-obtained samples were labeled as C_AL-750 and C_AL-3000, respectively. The black powders were ultimately obtained and served as anode materials in lithium-ion batteries without further treatment.

Characterization and measurement

SEM imaging. The morphology of the lignin-derived carbon materials was performed by field emission scanning electron microscope (FE-SEM, Merlin, Carl Zeiss AG, German).

X-ray powder diffraction. Wide-angle X-ray powder diffraction (XRD) patterns for samples were measured on a X-ray model diffractometer (D8 ADVANCE, Bruker, German) with Ni-filtered Cu Kα (V = 40 kV, I = 40 mA) radiation at a scanning rate of 10° min⁻¹ in a reflection mode over a 2θ range from 10° to 90°.

Raman spectra. Raman spectra were recorded on a spectrograph micro-Raman system (LabRAM Aramis, HORIBA Jobin Yvon, France).

Elemental analysis. Elemental analysis was performed on an Elementar analyzer (vario EL cube, Germany).

BET surface area measurements. The specific surface area and pore structure were performed by using the Brunauer–Emmett–Teller (BET) method with a Tristar II 3020 automated surface area and pore size analyzer (Micromeritics Corp., USA). Prior to analysis, samples were outgassed at 200 °C for 12 h under a vacuum in the degas port of the analyzer.

Electrical conductivity measurement. The sheet electrical conductivity of a pressed wafer with a diameter of 12 mm for the carbon materials were measured by a four-point probe method on the probe station at room temperature.

Electrochemical measurement

The electrochemical performance tests of the as-obtained carbon materials were performed in CR2032-type coin cells. The work electrodes were prepared by mixing the active materials (80 wt%), conductive carbon black (10 wt%) and polyvinylidene fluoride (10 wt%) uniformly in n-methyl-pyrrolidinone (NMP). Then the electrodes paste were coated on copper foil current collector and dried in a vacuum oven at 120 °C for 12 h. Lithium foil was served as the counter electrode, and Celgard 2325 was used as the separator. The electrolyte was a mixture containing 1 M LiPF6 in a blended ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1 : 1 : 1, v/v/v). All the cells used were assembled in a glove box filled with high purity argon and evaluated at room temperature. Cyclic voltammetry (CV) measurement was tested on Shanghai Chenhua CHI650D electrochemical workstation (Shanghai, China). The charge and discharge performances of the cells were performed on the LAND Battery Test System (Wuhan, China). The test voltage ranges from 0.001 V to 2.0 V (versus Li/Li⁺).

Results and discussion

Graphitization of alkali lignin

In order to explore the electrochemical performance of the lignin-derived carbon materials served as anodes for lithium-ion batteries, conversion of the stabilized AL into carbon materials was performed by direct pyrolysis of AL under an argon flow at 750 °C and 3000 °C, resulting a 68% and 75% reduction in mass along with a volumetric expansion, respectively. The SEM images of C_AL-750 and C_AL-3000 were shown in Fig. 1. It was obviously found that both of C_AL-750 and C_AL-3000 caked (10–20 μm) in the process of pyrolysis. Obvious edges were observed between the particles which were distinctive feature of the pyrolytic hard carbon. Moreover, a portion of pores and holes on the surface was reasonable due to the gas escaping, such a structure is a “double-edged sword” in terms of this kind of carbon materials. On the one hand, it is beneficial to increase the storage capacity of lithium ions. On the other hand, it will lead to “dead lithium” generation, causing a high irreversible capacity and reduction of the first coulombic efficiency. In particular, there were a small amount of one-dimensional carbon nanofibers in C_AL-750, probably demonstrated a growth tendency of AL during the low temperature pyrolysis process. While C_AL-3000 exhibited a more obvious faultage in partial enlarged detail (Fig. 1(d)), and the size of the particles was more uniform due to high degree of crystallinity.

Fig. 1 SEM images at different magnification of C_AL-750 (a, b) and C_AL-3000 (c, d).

To determine the structure of C_AL-750 and C_AL-3000, XRD measurements were conducted, as is shown in Fig. 2(a). The main weak peak located at about 2θ = 24.0° of C_AL-750 can be attributed to the (002) diffraction of hexagonal graphite. The broadening peaks is indicative of the long-range disordered structure which can also be reflected by indistinguishability of the (100) and (101) centered at 43.8°.⁴⁹ Compared with C_AL-750, a strong sharp diffraction peak at 2θ = 26.4° was observed, demonstrating the good crystallinity of C_AL-3000. Obviously, the (002) plane of C_AL-3000 had moved towards a bigger Bragg angle. The distance between the (002) planes, d₀₀₂ was calculated by using Bragg equation (λ = 2d₀₀₂ sin θ). The d₀₀₂ of the C_AL-750 and C_AL-3000 was 0.371 and 0.337 nm, respectively, indicating a high graphization degree of the as-obtained hard carbon. The smaller d₀₀₂ will provide shorter and more continuous pathways for electron transportation and thus reduce the electrochemical resistance of the electrode. The electrical conductivity of C_AL-750 and C_AL-3000 was 0.014 and 5.644 S cm⁻¹, respectively.

Fig. 2 XRD pattern (a) and Raman spectrum (b) of the as-obtained carbon materials of AL under different pyrolysis temperature.

Raman spectra were also performed to identify the bonding and structure of the as-obtained C_AL-750 and C_AL-3000, as is shown in Fig. 2(b). The two peaks centered at about 1328 cm⁻¹ and 1580 cm⁻¹ are ascribed to the D peak for disordered carbon and G peak for graphite carbon, respectively. The special peak centered at 2661 cm⁻¹ is ascribed to the 2D peak for double phonon resonance Raman peak, and the peak intensity reflects the degree of crystallinity and accumulation of graphite.⁵⁰ The peaks for C_AL-3000 are more sharp and distinguishable in comparison to C_AL-750. Typically, the intensity ratio of I_D/I_G is a useful parameter to evaluate the degree of graphitization of carbon materials. The calculated value of I_D/I_G is 1.14 for C_AL-750 and 0.14 for C_AL-3000, indicating the amorphous structure of C_AL-750 and graphite structure with a high graphization degree of C_AL-3000. Both XRD and Raman spectra results demonstrated that the degree of graphization of lignin-derived materials increased with the increase of pyrolysis temperature.

Preparation of lignin-derived nitrogen-doped carbon nanospheres

The AL and AL-azo-NO₂ colloidal spheres were successfully prepared via self-assembly when water was added dropwise into the as-obtained solution. As is shown in Fig. 3, the surface morphology of these two kinds spheres were different, the AL-azo-NO₂ colloidal spheres were particularly more smooth than AL colloidal spheres, which is mostly attributed to the introduction of hydrophobic rigid azo benzene groups. After the diazotization treatment, the concentration of the rigid segment around hydrophilic segments increased significantly. Large amount of rigid azo benzene segments tended to arrange in parallel with each other, which provided the driving force for the formation of uniform and tight colloidal spheres.¹⁶ As is shown in Fig. 3, the average diameters of the AL and AL-azo-NO₂ uniform colloidal spheres were estimated to be about 300 and 200 nm, respectively.

Fig. 3 SEM images of colloidal spheres prepared by AL (a) and AL-azo-NO₂ (b) in THF/H₂O, illustrations are the higher magnification enlarged images.

The colloidal spheres were pyrolyzed at 750 °C and 3000 °C in an argon flow with the aim to prepare carbon nanospheres, resulting a 69% for C_SAL-750, 52% for C_SAN-750 and 65% for C_SAN-3000 reduction in mass, respectively. Fig. 4 displays the SEM images of the as-obtained products. As is shown in Fig. 4(a), many inhomogeneous carbon fragments (1–2 μm) and one-dimensional fibers (100 nm) of C_SAL-750 were observed, demonstrating the destruction and a growth tendency to fibers of AL colloidal spheres in the pyrolysis process. Compared with C_SAL-750, it can be found that the whole materials were composed of large amounts of uniform carbon spheres of C_SAN-750 in Fig. 4(b) after pyrolysis due to the introduction of nitrogen heteroatom. From a higher magnification enlarged SEM images of Fig. 4(c), it can be estimated that the average diameter of the as-obtained carbon spheres was 580 nm. In general, the presence of a mixed oxygen/nitrogen ligand favours thermal stability and can keep the intact structure.^51,52 Moreover, there were obvious small holes on the surface of C_SAN-750 due to the gas escaping, simultaneously causing a volumetric expansion. When the AL-azo-NO₂ colloidal spheres were pyrolyzed at 3000 °C, bowl-shaped (600 nm), lamellar wafer (1–2 μm) and other shape fragment were observed in Fig. 4(d). With the increase of pyrolysis temperature, the nitrogen heteroatom was no longer stable and lost. Finally, destruction of spherical structure happened.

Fig. 4 SEM images of the as obtained C_SAL-750 (a), C_SAN-750 (b, c) and C_SAN-3000 (d), illustration is the higher magnification enlarged SEM image of C_SAN-750.

Wide-angle X-ray diffractograms of C_SAN-750 and C_SAN-3000 were investigated in Fig. 5(a), there were two broad diffraction peaks at around 24.5° and 43.5° due to the long-range disordered structure of C_SAN-750, corresponding to (002) and (100) planes of micrographites. Compared with C_SAN-750, a sharp diffraction peak at 2θ = 26.5° signified that the good crystallinity of C_SAN-3000. The d₀₀₂ of the C_SAN-750 and C_SAN-3000 was 0.363 and 0.336 nm, respectively, indicating the hard carbon structure of the as-obtained nitrogen-doped carbon materials.

Fig. 5 XRD pattern (a) and Raman spectrum (b) of the as-obtained lignin-derived nitrogen-doped carbon materials under different pyrolysis temperature.

Raman spectra were also performed to evaluate the carbonization process. As is shown in Fig. 5(b), the two peaks centered at about 1333 cm⁻¹ and 1585 cm⁻¹ are ascribed to the D peak for disordered carbon and G peak for graphite carbon, respectively. As is mentioned above, the calculated value of I_D/I_G of C_SAN-3000 is lower than C_SAN-750, because more defect structure formed with the loss of nitrogen heteroatom. And such kind of Raman spectra peak of C_SAN-3000 corresponds to the turbostratic stacked graphite carbon.⁵³ Compared to the peak positions of the materials before N-doping, blue shift was observed with both the D-band (from 1328 cm⁻¹ to 1333 cm⁻¹) and G-band (from 1580 cm⁻¹ to 1585 cm⁻¹) of the carbon materials after N-doping. The upshift of both bands could be attributed to doping with nitrogen, which introduces more topological defects and structural disorder to the carbon materials during the carbonization process, and causes a change in vibration energy levels. Similar changes were also observed in nitrogen-doped graphene and carbon nanotubes.^54,55 Doping with heteroatoms (such as N, B) has proven to be an enabling strategy to improve electrochemical reactivity and electrical conductivity of carbonaceous materials.^56,57 The electrical conductivity of C_SAN-750 and C_SAN-3000 was 0.008 and 28.95 S cm⁻¹, respectively. It is reasonable for C_SAN-750 due to the porous structure and big d₀₀₂. Non-interconnected structure provides longer and more discontinuous pathways for electron transportation, and thus increases the electrochemical resistance of the electrode.

The elemental distributions of the as-obtained products were listed in Table 1. Notably, the content of nitrogen of AL-azo-NO₂ increased significantly compared with AL due to the introduction of –N=N– and –NO₂. After pyrolysis treatment, the content of carbon element of C_AL-750 and C_AL-3000 was of 99.06% and 99.85%, while the content of carbon of C_SAN-750 and C_SAN-3000 was of 82.15% and 87.23%, respectively. This indicates that the AL-azo-NO₂ colloidal spheres favours thermal stability, which matches well with the SEM results (Fig. 4).

Table 1 Elemental analyses of AL, AL-azo-NO₂ and the as-obtained carbon materials

Samples	Element mass (%)
	Carbon	Nitrogen	Oxygen	Hydrogen
AL	56.12	0	38.46	5.42
AL-azo-NO2	59.23	5.73	30.14	4.90
CAL-750	99.06	0	0	0.94
CAL-3000	99.85	0	0	0.15
CSAN-750	82.15	3.85	12.36	1.64
CSAN-3000	87.22	0.80	6.68	5.30

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In addition, the content of nitrogen element of C_SAN-750 and C_SAN-3000 was 3.85% and 0.80%, respectively, which was lower than the nitrogen content of original AL-azo-NO₂ (5.73%). The reduction of nitrogen was due to the gas escaping of nitrogen-containing small molecule. It could be speculated that the C_SAN-750 with higher content of nitrogen might possess the better performance as the anodes for lithium-ion batteries.

The specific surface area and pore structure of the carbon materials were investigated by Bruauer–Emmett–Teller (BET) equation, and the related textural properties were summarized in Table 2. The surface areas of C_AL-750 and C_AL-3000 were 17.08 and 38.50 m² g⁻¹, respectively. Notably, as is shown in Fig. 1, the C_AL-3000 with small size and rough surface had big surface areas. Surprising, it was discovered that the surface areas of C_SAN-750 increased significantly up to 419.24 m² g⁻¹ due to the spherical and porous structure, which was caused by the loss of small molecules during the pyrolysis process. But the value rapidly decreased to 4.48 m² g⁻¹ for C_SAN-3000 due to the spherical structure destruction. In addition, C_SAN-750 possessed the most total pore volume, and almost 90% of pore belongs to micropores, indicating an excellent energy storage performance.

Table 2 Characteristic of Pore structures of the obtained lignin-derived carbon materials

Samples	SBET^a (m^2 g^−1)	Vt^b (cm^3 g^−1)	Vmicro^c (cm^3 g^−1)	Vmeso^d (cm^3 g^−1)	Da^e (nm)
a Specific surface area determined according to the BET equation.b Total pore volume.c Volume of micropores.d Volume of mesopores.e Adsorption average pore diameter (4 V A^−1 by BET).
CAL-750	17.08	0.0680	0.0005	0.0675	15.97
CAL-3000	38.50	0.0646	0.0050	0.0596	6.71
CSAN-750	419.24	0.2178	0.1938	0.0240	2.08
CSAN-3000	4.48	0.0200	0.0042	0.0158	17.87

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Electrochemical performances

Li-storage performance of the as-obtained carbon materials probed by cyclic voltammogram, galvanostatic measurement and rate charge–discharge at different current densities are displayed in Fig. 6 and 7. The CV profiles of C_AL-750 and C_AL-3000 in the initial three cycles are shown in Fig. 5(a). The obvious reduction peak centered at 0.7 V and 0.05 V in the first cycle of C_AL-750 can be assigned to the formation of SEI (solid electrolyte interphase) and the insertion of Li-ions into the interlayers of the carbon, respectively. There is no obvious irreversible reduction peak for C_AL-3000 at 0.7 V due to the full pyrolysis of functional groups at 3000 °C. Moreover, the oxidation peak of C_AL-3000 at 0.25 V is more sharp and obvious, demonstrating the rapid Li-ions extraction performance of C_AL-3000. The CV curves of C_AL-3000 are more stable with overlaps from the second cycle, demonstrating a more excellent stability performance.

Fig. 6 Electrochemical performance of the as obtained C_AL-750 and C_AL-3000 for lithium-ions batteries (the capacities were calculated based on C_AL mass). (a) Cyclic voltammogram profiles at a scan rate of 0.1 mV s⁻¹. (b) First cycle of discharge/charge voltage profiles. (c) Cycling performance and coulombic efficiency at a current density of 60 mA g⁻¹. (d) The rate performance under different current densities between 0.001 and 2.0 V.

Fig. 7 Electrochemical performance of the as obtained C_SAN-750 and C_SAN-3000 for lithium batteries (the capacities were calculated based on C_AL mass). (a) Cyclic voltammogram profiles at a scan rate of 0.1 mV s⁻¹. (b) First cycle of discharge/charge voltage profiles. (c) Cycling performance and coulombic efficiency at a current density of 60 mA g⁻¹. (d) The rate performance under different current densities between 0.001 and 2.0 V.

As is shown in Fig. 6(b), the first charge–discharge cycle curves of C_AL-750 and C_AL-3000 exhibited a discharge capacity of 440 and 366 mA h g⁻¹, respectively, and a charge capacity of 171.2 and 195.4 mA h g⁻¹, respectively. A larger irreversibility occurred in the electrode of C_AL-750, which matches well with the shape of the CV curves. The discharge capacity and coulombic efficiency of C_AL-750 and C_AL-3000 at a current density of 60 mA g⁻¹ in 50 cycles were displayed in Fig. 6(c). The first coulombic efficiency of these two kinds of samples was of 38.9% and 53.3%, respectively. However, the coulombic efficiency went up to 90.8% and 92.8% from the beginning of the second cycle, respectively. As the cycles continue, the coulombic efficiency almost reached 100%. After 50 cycles, the discharge capacity of C_AL-750 and C_AL-3000 were 141 and 194 mA h g⁻¹, respectively. The C_AL-3000 exhibits a better cycling performance than C_AL-750 at low current density.

Fig. 6(d) shows the capacity of the C_AL-750 and C_AL-3000 under different current density from 37.2 mA g⁻¹ to 744 mA h g⁻¹. Notably, the C_AL-3000 exhibited much higher and stable capacity at each stage. Even at a very high current density of 744 mA h g⁻¹, a satisfying capacity value of ∼200 mA h g⁻¹ for C_AL-3000 is still achieved. When the current density recovered to 37.2 mA g⁻¹ after cycling at different rates, the reversible capacity can be almost recovered to 199 mA g⁻¹ without notable decrease. Whereas the charge capacity of C_AL-750 had an obvious decrease at each stage, the discharge capacity decreased to 120 mA h g⁻¹ and cannot be recovered when the current density was turned back to 37.2 mA g⁻¹.

Li-storage performance of the as-obtained nitrogen-doped carbon spheres was investigated, as shown in Fig. 7. The CV profiles of C_SAN-750 and C_SAN-3000 in the initial three cycles are shown in Fig. 7(a). The CV curves for Li-ions storages are different from the profiles of C_AL-750 and C_AL-3000. The CV of C_SAN-750 is more sluggish than that of C_AL-750 due to the abundant nanoporous structure and edge carbon atoms, which caused a typical voltage hysteresis. Notably, for C_SAN-3000, there is a small reduction peak at 0.1 V compared to C_AL-3000. It may correspond to the reduction of oxygen-containing functional groups, which is consistent with the result of elemental analysis. The cathodic peak at 0.25 V is much sharp and distinguishable than that of C_AL-3000, indicating a more rapid lithium-ions extraction performance of C_SAN-3000.

As is shown in Fig. 7(b), the first discharge/charge cycle curves of C_SAN-750 and C_SAN-3000 exhibited a discharge capacity of 728.3 and 301.3 mA h g⁻¹, respectively. It was much larger for C_SAN-750 in compassion to C_AL-750 due to the abundant nanoporous structure and structural defects caused by nitrogen heteroatom, which provide abundant active site for insertion of Li-ions. The charge capacity of C_SAN-750 was 302.5 mA h g⁻¹ with a first coulombic efficiency of 41.5%. As to C_SAN-3000, because of the destruction of spherical structure and the loss of nitrogen, the charge capacity of C_SAN-3000 is of 200 mA h g⁻¹ with an excellent first coulombic efficiency up to 66.4%. The discharge capacity and coulombic efficiency at a current density of 60 mA g⁻¹ in 50 cycles were displayed in Fig. 7(c). Notably, both of the first coulombic efficiency of the two samples was higher than C_AL-750 and C_AL-3000, respectively. The coulombic efficiency went up to 88.5% and 93.5% from the beginning of the second cycle, respectively. As the cycles continue, the coulombic efficiency almost reached 100%. After 50 cycles, the discharge capacity of C_SAN-750 and C_SAN-3000 were 225 and 201 mA h g⁻¹, respectively, which is reasonable for C_SAN-3000 due to the loss of nitrogen and destruction of the regular spherical structure, causing a big decrease of nanoporous structure. Both of these two samples exhibited a better cycling performance than C_AL-750 and C_AL-3000 at low current density.

The capacity of the C_SAN-750 and C_SAN-3000 under different current density from 37.2 mA g⁻¹ to 744 mA h g⁻¹ were also investigated, as is shown in Fig. 7(d). The C_SAN-3000 exhibited much stable capacity due to a higher graphitization degree. A satisfying capacity value of ∼190 mA h g⁻¹ and ∼200 mA h g⁻¹ can be achieved for C_SAN-750 and C_SAN-3000, respectively. Even the charge capacity has an obvious decrease at each stage for C_SAN-750, the discharge capacity can be recovered when the current density was turned back to 37.2 mA g⁻¹. In summary, both C_SAN-750 and C_SAN-3000 showed more excellent cycling and rates performance in comparison to C_AL-750 and C_AL-3000 without nitrogen-doped modification.

Conclusions

In summary, lignin has been utilized as a low-cost and sustainable carbon precursor for fabricating carbon materials. Uniform lignin-derived nitrogen-doped carbon nanospheres were successfully prepared, and direct pyrolysis of alkali lignin powder was also performed. Results demonstrated that nitrogen-doping not only generates the formation of uniform nitrogen-doped carbon nanospheres with high specific surface area, but also can further improve their specific capacity as well as the first coulombic efficiency when served as anode for lithium-ion batteries. The nitrogen-doping and porous carbon nanospheres structure endow them with excellent potential application in energy storage. Direct pyrolysis of alkali lignin at different temperature also exhibited potential application for preparation of hard carbon materials with high graphitization degree. Results indicated the technologies used in this research not only is of great significance for value-added utilization of biomass lignin, but also provide a green sustainable option for converting inexpensive industrial lignin into high-value carbon materials.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (21374032), Guangdong Province Science and Technology Research Project of China (2014B050505006) and Guangdong Province Natural Science Foundation of China (2016A030311031).

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