1 Introduction

Due to abundant and environmental benignity of sodium source, sodium ion batteries (SIBs) have been recognized as the most potential new battery for the energy storage devices [1, 2]. In recent years, various cathode materials have been reported to show considerable progress for the development of SIBs [3,4,5,6]. However, the development of corresponding negative electrode materials for SIBs has been relatively slow.

As we know, due to their good conductivity, low price and environmental benignity, carbon-based materials have been considered as anode materials for SIBs. An effective approach of accelerating Na+ diffusion is to shorten the diffusion pathways of Na+ in the electrode materials. Especially the porous carbon, can not only facilitate sodium ion transport by reducing the diffusion lengths but also raise the contact areas between the electrode and electrolyte, which are both beneficial for the improvement of the electrochemical properties [7,8,9,10]. Moreover, modification with non-carbon atoms (particularly the N atom), is another effective mean to improve the electrochemical property of carbonaceous materials as the anode for sodium ion batteries [11,12,13,14,15]. Therefore, for the carbonaceous anode materials in SIBs, porous structure and incorporation of nitrogen are both desirable for Na+ storage. Some works about biomass-derived carbons inspired us to develop cheap and high-performance carbon materials for energy-storage applications [16,17,18,19].

In this work, a facile strategy was reported to fabricate porous carbon, which shows high surface area and appropriate N doping. Gelatin with inherent N content, is derived from leathers and animal bones has been employed as a raw material. KOH was selected as a porogen to produce pore structure. After KOH activation, hierarchical porous carbon materials (HPCMs) with suitable nitrogen doping were fabricated. Because of the unique hierarchical porous structure and suitable nitrogen doping, the as-obtained HPCMs delivers high capacity and excellent rate capability, exhibiting a favorable reversible capacity as high as 220 mAh g−1 at 100 mA g−1 after 100 cycles. Compared with the previous works for preparing heteroatoms doped porous carbon, this work has several advantages: (1) gelatin is inherently rich in nitrogen (without any extra N source predecessor), which is an ideal precursor for nitrogen doping carbon materials; (2) compared to the template synthesis methods (such as SiO2 template method, which involves the toxic HF solution), this preparation process is simple and facile, which makes it suitable for mass production; and (3) the as-obtained carbons show superior electrochemical performances for SIBs.

2 Experimental sections

2.1 Preparation of HPCMs

HPCMs were fabricated by chemical activation of gelatin (as illustrated in Fig. 1). Typically, gelatin and KOH was mixed (weight ratio, gelatin: KOH = 1:3), fired to 700 °C with a heat ramping rate of 5 °C min−1 and calcined for 1 h in argon. The obtained composite was then washed with 1 M hydrochloric acid solution and distilled water until pH became neutral. The carbon was finally dried in an oven at 70 °C. The yield of this process was around 30%. For comparison, the direct carbonized gelatin without KOH treatment was denoted as N-HPCMs.

Fig. 1
figure 1

Illustration of preparation of hierarchical porous carbon material

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2.2 Characterization

The transmission electron microscopy (TEM) images were made applying a FEI Tecnai G2 20. XRD patterns of the samples were carried out on a Bruker Powder X-Ray Diffractometer. The Raman spectrum was carried out using a confocal LabRAM HR800 spectrometer, HORIBA Jobin Yvon, France. Nitrogen adsorption/desorption analysis was conducted with an automatic analyzer (Autosorb-IQ, USA). XPS spectra were characterized using a Kratos XSAM 800 spectrometer (Manchester, UK). Electrochemical impedance spectroscopy (EIS) spectra were obtained by the CHI650e between the frequency of 0.01 and 100 KHz.

2.3 Electrochemical test

The electrochemical performance of the fabricated HPCMs was tested in 2032-coin cells. Na metal was applied as anode, 1 M NaClO4 in a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1, v/v) as electrolyte, and Celgard 2400 as separator. The HPCMs active material was mixed with acetylene black (AB) and polyvinylidene fluoride (PVDF) binder with a weight ratio of 8:1:1 in the N-methyl pyrrolidinone (NMP) solvent. The electrode was made by coasting the slurry onto a Cu foil and then drying at 120 °C for 5 h. The galvanostatic charge/discharge tests were conducted between 0.01 and 2.8 V versus Na+/Na on a Newware CT-3008W battery cycler (Guangdong, China). Cyclic voltammetry (CV) was tested by a CHI650e electrochemical workstation in the range of 0.01–2.8 V versus Na+/Na at a scan rate of 0.1 mV s−1.

3 Results and discussion

Figure 2a displays the XRD pattern of the obtained carbon sample after KOH treatment at 700 °C and being washed with HCl solution. Two peaks located at 23.6° and 43.6° can be attributed to the (002) and (100) reflections of graphitic carbon, respectively, suggesting the amorphous feature of the as-obtained carbon. Furthermore, a high intensity at (002) peak indicated that the as-obtained carbon showed a high crystallinity. According to the (002) peak, the value of d(002) spacing was calculated to be 0.376 by Bragg’s formula. The large d(002) spacing may be favorable to the insertion and extraction of sodium ion during the charge/discharge process [20, 21]. The Raman spectrum of HPCMs (Fig. 2b) exhibits a G-band at 1592 cm−1 (related to graphitic carbon) and a D-band at 1359 cm−1 (associated with defects). Generally, the value of ID/IG is commonly applied to determine the graphitization of carbon materials. In this case, The ID/IG value of HPCMs is estimated to be 0.92, demonstrating a highly amorphous structure of the HPCMs, which can be agreed with the XRD result. The disordered structure may be more beneficial for reversible storage of the Na-ion [12, 22, 23].

Fig. 2
figure 2

a XRD pattern and b Raman spectra of the as-synthesized HPCMs

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The morphology of the gelatin-derived porous carbons is displayed in Fig. 3. As depicted in Fig. 3a (red arrow represents some mesopores), b, the TEM and HRTEM images clearly show that the as-obtained carbon delivers a hierarchical pore structure [micro- and mesopores (marked by the red arrow)]. This unique pore structure may originate from the KOH activation during the high temperature and is different from the single pore structure, which may derive from the template of SiO2, MgO and CaCO3 [24,25,26]. To further determine the porosity of the HPCMs, the nitrogen adsorption/desorption isotherm was carried out and shown in Fig. 3c. The N2 adsorption/desorption isotherms show a much higher N2 sorption capacity at a low relative pressure (P/P0, range from 0 to 0.1), revealing a microporous property. With the increase of the relative pressure, the sorption capacity tends to be smooth, which indicate that the as-obtained HPCMs possess a hierarchical porous structure (micro- and mesopores). Furthermore, a large specific surface area of 1006 m2 g−1 can be obtained, which is much higher than that of N-HPCMs (32 m2 g−1). According to the TEM and N2 adsorption/desorption isotherm, the carbon with micro-, mesopore and high specific surface area can provide a large electrode/electrolyte interface for ion or charge accumulation and facilitate the electrolyte penetration for sodium ion storage [27, 28]. The pore size distribution of the HPCMs is calculated applying the conventional density functional theory (DFT) method on the N2 adsorption isotherm part as presented in Fig. 3d. The open system integrated a well-developed micro- and mesopore size distribution in the HPCMs. The total pore volume of HPCMs is found to be 0.54 cm3 g−1 (micropore volume was 0.39 cm3 g−1 and the mesopore volume was 0.16 cm3 g−1) and the average pore size diameter 4.3 nm. Various pore structures may play different roles in the transportation of the ions. The mesopores can act as reservoirs for storing electrolyte, resulting in reduced ion diffusion resistance and rapid ion transfer pathways. Large amount of micropores were favorable for the diffusion of sodium-ion and electrolyte in the carbon materials. This hierarchical pore structure (consisting of micropores and mesopores) could offer an effective path for penetration and transportation of electrolyte ions [29,30,31]. Such carbon with a unique hierarchical pore structure and a high surface area is expected to be a superior anode material for SIBs.

Fig. 3
figure 3

TEM (a), HRTEM (b), nitrogen adsorption/desorption isotherm (c), pore size distribution (d) and enlarged micropore size distribution (e) of the obtained HPCMs

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In order to evaluate the chemical states of the elements in the as-obtained carbons, XPS measurements are performed and displayed in Fig. 4. As presented in Fig. 4a, for the survey spectrum, the as-prepared HPCMs delivers three peaks located at 284.8, 400.0 and 533.6 eV, which related to C 1s, N 1s and O 1s, respectively. The calculated N content was about 2.85%. The high-resolution C 1s spectra (Fig. 4b) can be fitted by three peaks, located at 284.8 eV due to the graphitic (sp2 hybridized) carbon, at 286.1 eV corresponds to the carbons on the C–N and at 288.8 eV attributes to carbon on the C=O bonds. The N 1s spectrum (Fig. 4c) can be deconvoluted into two peaks, located at 398.3 and 400.7, respectively, which are attributed to two nitrogen components, namely, pyrrolic (N-5) and pyridinic (N-6) (as displayed in Fig. 4d), respectively. Furthermore, the oxygen, which mainly originated from the thermally stable functional groups of the carbon, was calculated to be about 9.35%, resulting in the enhancement of capacity by surface redox reactions (–C=O + Na+ + e → –C–O–Na) [32, 33]. According to the literatures [34, 35], N atoms at different positions in the carbon play different roles and affect the electrochemical performance of the as-fabricated carbon in different ways. The pyrrolic and pyridinic nitrogen atoms seated at the verge of the graphene layer, are beneficial for adsorbing exotic atoms and improving the sodium ions storage [36, 37]. Meanwhile, the N doping can increase the interlayer distance and the electrochemical activity of porous carbon, which offer more active sites for sodium ion storage [38]. Therefore, nitrogen doping in the carbon would be expected to offer more active sites for Na-ion storage, not just contributing to increase the electric conductivity of the material.

Fig. 4
figure 4

a XPS total spectrum, b C 1s, c N 1s and d schematic structure of the binding conditions of nitrogen of HPCMs

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The electrochemical performance of the as-achieved carbons was carried out by cyclic voltammetric (CV) method at a scan rate of 0.5 mV s−1 between 0.01 and 2.8 V (vs. Na/Na+). Figure 5a displays the CV profiles of the carbon electrode the first several cycles. In the first reduction process, the peak located at about 0.7 V is ascribed to the electrolyte decomposition and generation of a solid electrolyte interphase (SEI) film, which disappeared in the following cycles, suggesting a good reversibility of the carbons [39]. After the first scan, the second and following CV profiles almost overlap, suggesting good cycle property of HPCMs during Na+ insertion and extraction. Figure 5b displays the charge/discharge profiles of the as-obtained HPCMs within 0.01–2.8 V, at the current density of 100 mA g−1. The first charge/discharge cycle indicates the initial discharge and charge capacities to be 829 and 261 mAh g−1, respectively, delivering an initial coulombic efficiency of 31.4%. The large irreversible capacity loss may result from the irreversible reaction between Na-ion and surface functional groups as well as with the electrolyte generating a SEI film [40, 41]. After the first cycle, the as-obtained carbons show exhibit good reversibility (236 mAh g−1 after 50 cycles). Once a stable SEI film is formed during the first cycle, the coulombic efficiency quickly reaches up to ca. 100% in the following cycles (Fig. 5c). As seen in Fig. 5c, the carbon delivers a favorable capacity of 220 mAh g−1 at 100 mA g−1 even after 100 cycles. Furthermore, as shown in Fig. 5d, the carbon shows superior rate capability. In detail, the carbon can deliver reversible capacities of 240, 199, 179, 156, 127 and 104 mAh g−1 at the current densities of 100, 200, 500, 1000, 2000, and 5000 mA g−1, respectively. When it reduced back to 100 mA g−1, the reversible capacity can recover to 226 mAh g−1. Meanwhile, the SEM image and electrochemical performance of the direct carbonized gelatin can be seen in the Fig. 6. As shown in Fig. 6a, the N-HPCMs show a smooth surface, indicating a nonporous characteristic (which is in conformity with the result of the Fig. 3c). Figure 6b displays the electrochemical performance of the N-HPCMs at the current density of 0.1 A g−1 between 0.01 and 2.8 V. Specifically, the carbon shows capacities of 161, 150, 142 and 121 mAh g−1 at 1st, 2nd, 10th and 30th cycle, respectively. The electrochemical performance of the N-HPCMs was inferior to that of HPCMs, mainly derived from the large specific surface area of carbon material. This superior performance may be derived from the hierarchical porous structure and suitable N doping effects. First, the porous structure with a high surface area can facilitate the penetration of the electrolyte into the carbon electrode and ensure a reduced sodium ion diffusion pathway for rapid charge transport. Second, the suitable nitrogen doping can provide extra sites for Na-ion storage, thus further enhancing its rate-performance.

Fig. 5
figure 5

a CV curve, b galvanostatic discharge/charge profile, c cycling performance and d rate performance of as-obtained HPCMs

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Fig. 6
figure 6

SEM image (a) and charge/discharge profiles (b) (between 0.1 A g–1 in 0.01–2.8 V) of the N-HPCMs

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In order to investigate the kinetics of sodium ions in electrode, EIS analysis was conducted and displayed in Fig. 7. EIS spectra were obtained during the various charge/discharge cycles (the current density was 0.1 A g−1). The low frequency region showed a straight line, suggesting a Warburg element (Zw) [42]. Higher frequencies delivered two features: Rs, indicative of resistance of electrode materials and contact resistance between the electrode and electrolyte; Rct, referring to the charge transfer resistance [43]. After fitted by the equivalent circuit (as shown in the inset of Fig. 7), the Rs and Rct of the carbon electrode in the before test, 10th and 30th were 202 and 1285 Ω, 67 and 496 Ω and 46 and 243 Ω, respectively. This result indicating the improved charge transfer with the increase of the cycling, this may be due to the activation of the electrode during the repeated sodium-ion insertion and extraction processes.

Fig. 7
figure 7

EIS spectra of the HPCMs electrode in the before test, 10th and 30th (the current density was 0.1 A g−1)

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A comparison of the rate performance of HPCMs with several carbons reported in literature is also shown in Table 1. It can be clearly seen that the as-prepared HPCMs shows a much higher reversible capacity and better rate capability at high current densities. The improved electrochemical performances of the gelatin-based carbons can be attributed to the unique hierarchical pore structure and appropriate N doping effects. More specifically, the material with large surface area delivers a reduced Na+ diffusion pathway and a high electrode/electrolyte contact area, which can be beneficial to facilitate sodium ion diffusion and ultimately improve rate performance. Meanwhile, the unique porous structure can provide enough space to accommodate volume changes that result in superior cycling performance. Furthermore, suitable nitrogen doping can produce a large amount of defects in the carbon and form an amorphous structure, which further improves sodium absorption performances.

Table 1 Electrochemical property comparison of HPCMs versus anode carbons of SIBs reported in literature
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4 Conclusions

In summary, HPCMs were successfully fabricated from gelatin via a facile approach of KOH activation. Thanks to the unique hierarchical porous structure and appropriate N doping, the HPCMs as anode for sodium ion batteries (SIBs) deliver large reversible capacity (261 mAh g−1 at 100 mA g−1), superior cycling durability (220 mAh g−1 at 100 mA g−1 after 100 cycles), and good rate performance (104 mAh g−1 at 5 A g−1). Considering their excellent electrochemical performances and the facile preparation approach, HPCMs should be regarded as one of the most potential candidates as anode material for SIBs.