Nickel-Embedded Carbon Materials Derived from Wheat Flour for Li-Ion Storage

The biomass-based carbons anode materials have drawn significant attention because of admirable electrochemical performance on account of their nontoxicity and abundance resources. Herein, a novel type of nickel-embedded carbon material (nickel@carbon) is prepared by carbonizing the dough which is synthesized by mixing wheat flour and nickel nitrate as anode material in lithium-ion batteries. In the course of the carbonization process, the wheat flour is employed as a carbon precursor, while the nickel nitrate is introduced as both a graphitization catalyst and a pore-forming agent. The in situ formed Ni nanoparticles play a crucial role in catalyzing graphitization and regulating the carbon nanocrystalline structure. Mainly owing to the graphite-like carbon microcrystalline structure and the microporosity structure, the NC-600 sample exhibits a favorable reversible capacity (700.8 mAh g−1 at 0.1 A g−1 after 200 cycles), good rate performance (51.3 mAh g−1 at 20 A g−1), and long-cycling durability (257.25 mAh g−1 at 1 A g−1 after 800 cycles). Hence, this work proposes a promising inexpensive and highly sustainable biomass-based carbon anode material with superior electrochemical properties in LIBs.


Introduction
In recent years, with the pollution of the environment and consumption of energy, renewable energy technologies have attracted worldwide attention. The development of efficient energy storage equipment, such as supercapacitors and rechargeable batteries, has been a focused topic now. Lithium-ion batteries (LIBs) occupy important status in the automotive and portable electronic devices such as wearable microelectronic devices and electric transportation on account of their outstanding properties. In the past years, considerable studies have been proposed around the promising battery anode materials [1][2][3][4][5][6].
Carbon materials as typical anode materials have been investigated proverbially nowadays. Owing to the long durability, excellent specific capacity, and superior rate abilities, the hard carbons have been emerging as promising anode materials. Among these, the biomass-based carbon materials have been extensively studied as anode materials in rechargeable batteries for their nontoxicity and abundance resources in the past decades. The carbon materials derived from various biomass sources [7][8][9], such as rice husk [10], starch [11][12][13], nut shells [14][15][16][17], fruit peels [18,19], flour [20][21][22][23] as electrode materials have exhibited superior electrochemistry properties. For example, the spherical hard carbon derived from potato starch gave an invertible capacity of about 531 mAh g −1 at 0.1C for LIBs [24]. The orange peel-derived hard carbon displayed the capacity of 301 mAh g −1 at 1 A g −1 for LIBs [18]. Sweet potato-derived carbon achieved a capacity of 965 mAh g −1 at 0.1 A g −1 for LIBs [13]. Zhou reported highly graphitic carbon nanosheets derived from the wheat stalk with a high capacity

Results and Discussion
The synthesis process of the nickel@carbon materials is as follows. First, a dough is prepared by repeated malaxation of the mixture of wheat flour, nickel nitrate, and deionized water. After freeze-drying, the dried dough is carbonized under N 2 protection. After the sample was washed by hydrochloric acid and deionized water, the final samples are successfully acquired after drying in the vacuum.
As shown in the XRD patterns (Figure 1a), all the four samples exhibit a broad diffraction peak at~25 • , ascribing to the (002) lattice plane of graphitic carbon. As for the prepared NC-600 and NC-800, three sharp diffraction peaks at 2θ = 44.5, 51.8, and 76.4 • are observed, which could be assigned to the lattice planes of (111), (200), and (220) for nickel metal with (Fm-3m) space group (JCPDS card 65-2865), respectively [36][37][38][39]. The metallic nickel is formed via a reduction reaction of Ni (NO 3 ) 2 by the surrounding carbon atoms under high carbonization temperature [40,41]. It is well-known that the crystalline structure of the carbon anode materials, for example, the (002) lattice plane, significantly influences the intercalation/de-intercalation of Li + . The (002) peaks of the obtained samples are asymmetric, indicating the coexistence of graphitic and amorphous domains [9]. The peak fitting of the (002) lattice plane can be seen in Figure 1b,c and Figure S1, and Table S1. The microspores result from cross-links between smaller domains of graphitic stacks that could increase the average interlayer spacing [8]. According to Table S1, the d-spacing range of the WFC-600, NC-600, WFC-800, and NC-800 above 0.401 nm is ascribed to highly disordered nanocrystallites for Li + insertion freely. The d-spacing of the WFC-600, NC-600, WFC-800, and NC-800 range from 0.379 to 0.398 nm corresponds to "pseudo-graphitic" carbon, and the range from 0.336 to 0.342 nm and is referred to as "graphite-like" carbon [42,43]. Consequently, the Ni 2+ could assist the chemical modification by promoting the formation of the expanded nanographite, which can supply more intercalation active sites for Li + storage. Figure 1d and Figure S2 show the Raman spectra of WFC-600, NC-600, WFC-800, and NC-800. Two main peaks at~1584 cm −1 and~1351 cm −1 correspond to the in-plane symmetric sp 2 C-C bondstretching vibration of graphitic carbon (G band) and the vibration of disordered carbon (D band) at the edges of the graphite sheets, respectively [44]. As is well-known, the intensity ratio of I D /I G demonstrates the degree of disorder and the average size of the sp 2 domains [10,45]. The peak shape of the obtained products is quite similar. The I D /I G values (Table S1) of NC-600 and NC-800 are lower than those of WFC-600 and WFC-800, respectively, indicating the sp 2 content increases and some graphite-like nanocrystalline structure forms in the nickel@carbon materials possibly because of the catalyzation effect of Ni particles [9,46]. temperature reaches 800 °C, the remaining weight of WFC-600 is only 1.19%, suggesting a tiny content of ashes. Comparatively, the remaining weight of NC-600 is 15.9% (mainly NiO) [47,48]. Accordingly, the Ni content of NC-600 could be determined to be 12.5%. N2 sorption analysis was performed to measure the specific surface area and the porosity of the carbon material ( Figure 1f). The large adsorption quantities at the very low relative pressure and the small adsorption quantities at p/p 0 > 0.4 indicate that NC-600 and NC-800 have a dominant microporosity and minor mesoporosity. This is more visible in the pore size distribution plots. The BET surface specific area of NC-600 and NC-800 is 470.5 m 2 g −1 and 321.3 m 2 g −1 (Table 1), respectively. The porous structure is probably formed by the reaction of Ni(NO3)2 and wheat flour in the carbonization process and the removal of Ni metal during the post-treatment of HCl [11]. Comparatively, the specific surface areas of WFC-600 and WFC-800 are very low indicating a nonporous structure.   There are obvious differences between the WFC-600 and NC-600 in thermogravimetric behavior curves above 200 • C. The WFC-600 and NC-600 exhibit weight loss of 13.9% and 6.6% from 270 • C and 400 • C respectively, indicating the latter sample possesses much higher oxidation resistance performance. The possible reason is that the Ni nanoparticles in NC-600 catalyze the graphitization of the carbon materials during the carbonization process, reflected by the results of XRD and Raman spectra. When the temperature reaches 800 • C, the remaining weight of WFC-600 is only 1.19%, suggesting a tiny content of ashes. Comparatively, the remaining weight of NC-600 is 15.9% (mainly NiO) [47,48]. Accordingly, the Ni content of NC-600 could be determined to be 12.5%. N 2 sorption analysis was performed to measure the specific surface area and the porosity of the carbon material ( Figure 1f). The large adsorption quantities at the very low relative pressure and the small adsorption quantities at p/p 0 > 0.4 indicate that NC-600 and NC-800 have a dominant microporosity and minor mesoporosity. This is more visible in the pore size distribution plots. The BET surface specific area of NC-600 and NC-800 is 470.5 m 2 g −1 and 321.3 m 2 g −1 (Table 1), respectively. The porous structure is probably formed by the reaction of Ni(NO 3 ) 2 and wheat flour in the carbonization process and the removal of Ni metal during the post-treatment of HCl [11]. Comparatively, the specific surface areas of WFC-600 and WFC-800 are very low indicating a nonporous structure.
The morphology and microstructure of WFC-600, NC-600, WFC-800, and NC-800 are illustrated in Figure 2 and Figures Figure S3a) exhibits a relatively smooth surface, and no obvious pores are observed on its surface [20]. Comparatively, numerous pores appear on NC-600 (Figure 2c,d and Figure S3b). Similar results are obtained for WFC-800 and NC-800 ( Figure S4).  distributed in the NC-600 carbon (Figure 2g,h). The size of Ni particles ranged from 4.7 to 12 nm and approximately 8.7 nm on average determined by the number frequency histogram (inset in Figure 2g). Figure S5d displays that after carbonization at 600 °C, some graphite-like clusters appear on the edges of the Ni nanoparticles with lower interplanar ordering, and most of them in the material are disorder carbon at 600 °C [9]. Figure 2h shows the HR-TEM image of nickel nanoparticles. The fringes with spacings of 0.206 nm correspond to the (111) face of nickel metal. Compared with NC-600, NC-800 ( Figure S6) shows an obvious graphiticlike structure and more graphitized carbon fragments are distributed around the Ni particles because of the high carbonization temperature. Therefore, the graphitization degree of NC-800 is significantly enhanced. Moreover, the metal lattice could also be found in NC-800 apparently ( Figure S6c,d). The type of elements and their contents of the prepared samples are determined by energy dispersive spectroscopy and are illustrated in Table 1   The X-ray photoelectron spectrum (XPS) for NC-600 (inset in Figure 3a) demonstrates that the NC-600 is composed of C, N, O, and Ni elements. In the case of C-species, the spectrum of NC-600 could be deconvolved into three peaks. The peaks at 284.6, 285.2, and 286.4 eV are assigned to the sp 2 hybridized C-C, C-N, and C-O, respectively. In the case of N- TEM observations demonstrate that the nickel nanoparticles are homogeneously distributed in the NC-600 carbon (Figure 2g,h). The size of Ni particles ranged from 4.7 to 12 nm and approximately 8.7 nm on average determined by the number frequency histogram (inset in Figure 2g). Figure S5d displays that after carbonization at 600 • C, some graphite-like clusters appear on the edges of the Ni nanoparticles with lower interplanar ordering, and most of them in the material are disorder carbon at 600 • C [9]. Figure 2h shows the HR-TEM image of nickel nanoparticles. The fringes with spacings of 0.206 nm correspond to the (111) face of nickel metal. Compared with NC-600, NC-800 ( Figure  S6) shows an obvious graphitic-like structure and more graphitized carbon fragments are distributed around the Ni particles because of the high carbonization temperature. Therefore, the graphitization degree of NC-800 is significantly enhanced. Moreover, the metal lattice could also be found in NC-800 apparently ( Figure S6c,d). The type of elements and their contents of the prepared samples are determined by energy dispersive spectroscopy and are illustrated in Table 1. Moreover, the C, N, and O elements are homogeneously distributed in the WFC-600 ( Figure S7) while the C, N, O, and Ni are uniformly observed in NC-600 (Figure 2i), as confirmed by EDS characterization.
The X-ray photoelectron spectrum (XPS) for NC-600 (inset in Figure 3a) demonstrates that the NC-600 is composed of C, N, O, and Ni elements. In the case of C-species, the spectrum of NC-600 could be deconvolved into three peaks. The peaks at 284.6, 285.2, and 286.4 eV are assigned to the sp 2 hybridized C-C, C-N, and C-O, respectively. In the case of N-species, pyridinic-N, pyrrolic-N, and graphitic-N of NC-600 are found because the peaks are centered at 398.7, 400.4, and 401.4 eV, respectively (Figure 3b) [49]. In the case of O-species, including the C=O quinone group and C-OH phenol group bonds, the peaks are located at 531.3 and 533.7 eV, respectively (Figure 3c) [50]. The peaks in Ni 2p spectrum (Figure 3d) centered at 852.8 and 870.1 eV are referred to as Ni 2p 3/2 and Ni 2p 1/2 signals of metallic nickel Ni 0 . The peaks at 856.1, 873.6, 861.0, and 879.9 eV are associated with the Ni 2+ , which are generated from the surface partial oxidation of nickel [51]. Additionally, the NiO could not be detected by XRD, demonstrating that the major existence form of nickel is metallic Ni 0 [52]. species, pyridinic-N, pyrrolic-N, and graphitic-N of NC-600 are found because the peaks are centered at 398.7, 400.4, and 401.4 eV, respectively (Figure 3b) [49]. In the case of O-species, including the C=O quinone group and C-OH phenol group bonds, the peaks are located at 531.3 and 533.7 eV, respectively (Figure 3c) [50]. The peaks in Ni 2p spectrum (Figure 3d) centered at 852.8 and 870.1 eV are referred to as Ni 2p 3/2 and Ni 2p 1/2 signals of metallic nickel Ni 0 . The peaks at 856.1, 873.6, 861.0, and 879.9 eV are associated with the Ni 2+ , which are generated from the surface partial oxidation of nickel [51]. Additionally, the NiO could not be detected by XRD, demonstrating that the major existence form of nickel is metallic Ni 0 [52]. The initial 3 CV curves (Figures 4a,b and S8a,b) investigate the electrochemical properties of the NC-600 (800) and WFC-600 (800) for LIBs. The CV profiles of the WFC-600 and WFC-800 (Figures 4a and S8a) are typical carbonaceous anode materials [53]. In the initial CV curve of NC-600 (Figure 4b), a wide cathodic current peak present at 0.63 V is unavailable in the 2nd and 3rd scans. The reason could be assigned to the formation of SEI (solid electrolyte interface) film, the Li + insertion into carbon material, and the reduction of NiO to metallic Ni [11,51]. During the subsequent anodic scan, the two weak peaks at 0.24 and 1.25 are ascribed to the Li+ extraction from the graphitic carbon layers, the partial decomposition of the SEI layer, and the oxidation reaction of Ni to NiO [8,13]. Moreover, these subsequent CV curves nearly overlap, suggesting that the electrode supports good stability during the lithiation/delithiation process [54]. In the 1st cycle of NC-800 ( Figure S8b), there are two obvious cathodic peaks at 0.5 and 1.1 V which could be referred to the reduction of NiO with Li + and to the generation of metallic Ni and Li2O, the formation of the SEI layer. Afterward, in the anodic process, two weak anodic peaks occur at 1.6 V and 2.6 V ascribed to the oxidation reaction of Ni to NiO. The two cathodic peaks in the subsequent scan are moved to 0.75 V and 1.5 V because of the modification of the microstructure and the electrolyte decomposition [51].  Figure S8a) are typical carbonaceous anode materials [53]. In the initial CV curve of NC-600 (Figure 4b), a wide cathodic current peak present at 0.63 V is unavailable in the 2nd and 3rd scans. The reason could be assigned to the formation of SEI (solid electrolyte interface) film, the Li + insertion into carbon material, and the reduction of NiO to metallic Ni [11,51]. During the subsequent anodic scan, the two weak peaks at 0.24 and 1.25 are ascribed to the Li+ extraction from the graphitic carbon layers, the partial decomposition of the SEI layer, and the oxidation reaction of Ni to NiO [8,13]. Moreover, these subsequent CV curves nearly overlap, suggesting that the electrode supports good stability during the lithiation/delithiation process [54]. In the 1st cycle of NC-800 ( Figure S8b), there are two obvious cathodic peaks at 0.5 and 1.1 V which could be referred to the reduction of NiO with Li + and to the generation of metallic Ni and Li 2 O, the formation of the SEI layer. Afterward, in the anodic process, two weak anodic peaks occur at 1.6 V and 2.6 V ascribed to the oxidation reaction of Ni to NiO. The two cathodic peaks in the subsequent scan are moved to 0.75 V and 1.5 V because of the modification of the microstructure and the electrolyte decomposition [51]. Moreover, the CV curves of the second and the third cycles are overlapped, suggesting good reversible electrochemical reactions [4,38].     Figure S8c,d delivers the galvanostatic charge/discharge profiles for WFC-600, NC-600, WFC-800, and NC-800 samples at 1st, 2nd, and 3rd cycles at 0.1 A g −1 . The first coulombic efficiencies of the WFC-600 and NC-600 are 56.4% and 57.6%, respectively, while those of the WFC-800 and NC-800 are only approximately 37.5% and 50%, respectively. The reason probably is the electrolyte decomposition at the electrode/electrolyte interface [55]. Figure 4e depicts the cyclic performance of the prepared samples at 0.1 A g −1 . The capacity of the NC-600 is 700.8 mAh g −1 at the current density of 0.1 A g −1 after 200 cycles, better than that of WFC-600 (342.9 mAh g −1 ). A similar result is obtained for NC-800 and WFC-800, with the discharge capacities of 464.8 and 297.4 mAh g −1 at 0.1 A g −1 , respectively ( Figure S9). These facts demonstrate the significant positive effect of the Ni embedding on the Li + storage performance. First, the Ni catalyzes the formation of the graphite-like carbon microcrystalline structure, thus providing more stable intercalation sites for Li + [42]. Second, the activation reaction between Ni(NO 3 ) 2 and carbon and the acid-etching post-treatment introduce abundant micropores into the Ni@carbon, leading to additional defect sites for Li + storage and better ionic diffusivity [8,9,56,57]. Additionally, the charge transfer may be improved because of the existence of Ni nanoparticles proved by EIS spectra next. Figure 4f exhibits the rate property of the WFC-600 and NC-600 at 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 A g −1 for LIBs. The NC-600 can deliver reversible capacities of 609.6, 547.95, 476.4, 369.9, 264.1, 188.6, 92.6, and 51.3 mAh g −1 for LIBs, which are higher than those of WFC-600 obtained at the same current densities. The specific capacity of NC-600 remains 51.3 mAh g −1 at 20 A g −1 , and then the capacity quickly recovered when the current density reduced to 0.1 A g −1 , demonstrating good charge-discharge transfer kinetics within the NC-600 architecture. The specific charge capacities of NC-800 ( Figure S9) are lower than those of NC-600 because of the higher content of graphitic fragments for NC-800 limits the insertion of Li + . WFC-600 and NC-600 exhibit good long cycle performance at 1A g −1 for LIB. After 800 cycles, the capacity of WFC-600 and NC-600 is 162.8 and 257.25 mAh g −1 , respectively (Figure 4g). Previously reported biomass-derived carbons and metal@carbon materials are sweet potato-derived carbon (320 mAh g −1 at 0.1 A g −1 after 200 cycles), NiO/C hollow microspheres (628 mAh g −1 at 0.1 A g −1 after 100 cycles), C/Ni700 (205 mAh g −1 at 1C after 50 cycles), NiO-graphene (646 mAh g −1 at 0.1 A g −1 after 35 cycles) and Ni-graphene (675 mAh g −1 at 0.1 A g −1 after 35 cycles), etc., (Table 2). Comparatively, NC-600 exhibits the advantages in rate capacity (51.3 mAh g −1 at 20 A g −1 ) and cycling stability (700.8 mAh g −1 at 0.1 A g −1 after 200 cycles; 257.2 mAh g −1 at 1 A g −1 after 800 cycles). The unique structure of the NC-600 with nanocrystalline structure and the introduced defect sites lead to good Li + storage and better ionic diffusivity. Therefore, NC-600 is a material with high performance and good application prospects.
The electronic transport properties of the materials are explored by the electrochemical impedance spectra (Figure 4h and Figure S9c). Typically, the straight line is ascribed to the ion diffusion process (R w ), the following semicircle is associated with the charge transfer resistance (R ct ), and the high-frequency region reflects the electrode resistance (R e ). The lower charge transfer resistance of NC-600 (NC-800) than WFC-600 (WFC-800) is because the microstructure of the former electrode materials with more defects can provide additional sites, and the anchored nickel nanoparticles can initiate fast electron transfer and better ionic diffusivity during the cycling process [7,8,23,58].
The CV profiles of NC-600 at various scan rates from 0.2-10 mV s −1 are shown to investigate the contributions of different mechanisms for LIBs (Figure 5a). The Equations (1) and (2) can express the charge storage mechanism between the response currents (i) and the scan rates (v) [44].
Here, b is an indicator of capacitive process free of diffusion limitations (when b = 1), diffusion-controlled process (when b = 0.5), or mixed mechanism (0.5 < b < 1). The b value could be obtained from the slope of the log (i) versus log (v) plots. For carbon anode materials, the diffusion-controlled reaction suggests the intercalation reactions while the capacitive process free of diffusion limitations indicates the electrochemical double layer (EDL) and pseudocapacitive storage processes [65][66][67]. The b values for NC-600 is 0.74 (Figure 5b), suggesting a surface capacitive process domination, a combination of diffusion capacitance and surface capacitance lithium storage mechanism. The capacitive contribution ratios gradually grow with the increase of scan rates, reaching up to 46.1% for NC-600 (Figure 5c) at the scan rate of 10 mV s −1 . The b values of NC-600 indicate the good distribution of the main intercalation mechanism for Li + storage which also identifies the formation of the graphite-like carbon structure. Materials 2020, 13, x FOR PEER REVIEW 10 of 14 Figure 5. Cyclic voltammograms of NC-600 at various scan rates (a); the relationship between peak currents and scan rates (b); survey of the capacitive and diffusion-controlled contribution ratio at different scan rates; Normalized contribution ratio of diffusion and capacitive capacities at different scan rates of NC-600 (c).

Conclusions
In summary, we produced a novel hard carbon embedded with nickel nanoparticles (nickel@carbon) by carbonizing the dough which is prepared by mixing wheat flour and the nickel nitrate. The introduced Ni is proved to play a vital role in the catalyzed graphitization and regulated the carbon nanocrystalline structure, and made a positive effect on the Li + storage performance. Because of the unique structure, the NC-600 sample shows the favorable electrochemical performance, with a reversible capacity of up to 700.8 mAh g −1 at 0.1 A g −1 after 200 cycles, longterm cyclic performance of up to 257.25 mAh g −1 after 800 cycles, and good rate capability with 51.3 mAh g −1 at 20 A g −1 . The unique inexpensive NC-600 and their environment-friendly synthesis approach can be applied to develop advanced transition metal/oxides-embed hard carbon materials for future electrochemical energy storage and conversion applications.

Conclusions
In summary, we produced a novel hard carbon embedded with nickel nanoparticles (nickel@carbon) by carbonizing the dough which is prepared by mixing wheat flour and the nickel nitrate. The introduced Ni is proved to play a vital role in the catalyzed graphitization and regulated the carbon nanocrystalline structure, and made a positive effect on the Li + storage performance. Because of the unique structure, the NC-600 sample shows the favorable electrochemical performance, with a reversible capacity of up to 700.8 mAh g −1 at 0.1 A g −1 after 200 cycles, long-term cyclic performance of up to 257.25 mAh g −1 after 800 cycles, and good rate capability with 51.3 mAh g −1 at 20 A g −1 . The unique inexpensive NC-600 and their environment-friendly synthesis approach can be applied to develop advanced transition metal/oxides-embed hard carbon materials for future electrochemical energy storage and conversion applications.
Author Contributions: Conceptualization, W.D. and S.Z.; methodology, software, investigation, data curation, writing-original draft preparation, W.D.; validation, formal analysis, visualization, S.W., Y.L. and W.D.; resources, writing-review and editing, supervision, project administration, funding acquisition, S.Z. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding:
We are grateful for the financial support by the National Natural Science Foundation of China (NSFC21978159, 51907110) and the Opening Fund of State Key Laboratory of Heavy Oil Processing (SKLOP202002004).

Conflicts of Interest:
The authors declare no conflict of interest.