One-Pot Synthesized Biomass C-Si Nanocomposites as an Anodic Material for High-Performance Sodium-Ion Battery

Aiming at materializing an excellent anodic source material of the high-performance sodium-ion battery (SIB), we fabricated the biomass carbon-silicon (C-Si) nanocomposites by the one-pot synthesis of facile magnesiothermic reduction using brown rice husk ashes. The C-Si nanocomposites displayed an aggregated morphology, where the spherical Si nanoparticles (9 nm on average) and the C nanoflakes were encapsulated and decorated with each other. When utilizing the nanocomposites as an SIB anode, a high initial discharge capacity (i.e., 378 mAh/g at 100 mA/g) and a high reversible capacity (i.e., 122 mAh/g at 200 mA/g) were achieved owing to their enhanced electronic and ionic conductivities. Moreover, the SIB device exhibited a high cyclic stability in its Coulombic efficiency (i.e., 98% after 100 charge-discharge cycles at 200 mA/g). These outstanding results depict that the one-pot synthesized biomass C-Si nanocomposites are beneficial for future green energy-storage technology.


Introduction
For next-generation green energy-storage device technology, the sodium-ion battery (SIB) has emerged as a favorable alternative to the lithium-ion battery (LIB) because of its huge abundance, low cost, eco-friendliness, non-toxicity, and analogous electrochemical mechanisms [1][2][3]. Despite the similarity in their electrochemical operation mechanisms of both LIBs and SIBs, however, the electrode materials of LIBs are incompatible with those of SIBs because of different ionic radii between the sodium ion (Na + : 0.102 nm) and the lithium ion (Li + : 0.076 nm) [4]. For instance, although crystalline graphite is adequate as a commercial LIB anode [5,6], the weak Na + intercalation into graphite [7,8] makes it inappropriate as an SIB anode [9]. Therefore, developing an excellent anode material is essential for demonstrating high-performance SIBs. Accordingly, several types of SIB anode materials have been proposed in resent studies [10][11][12][13][14]; for instance, tin (Sb) [15], antimony [16], phosphorus (P) [17], porous carbon (C) [18], silicon (Si) [19], titanium dioxide (TiO 2 ) [20], and chalcogenides [21] are typical examples that can move a step closer to the practical application of SIBs. Among them, Si is one of the most distinctive anode materials for rechargeable batteries because of theoretical expectations on its high specific capacity (i.e., 4200 mAh/g for LIBs [5] and 954 mAh/g for SIBs [22,23]). In many experimental attempts [23][24][25], however, the Si anode revealed the poor reversible discharge capacity because of both the formation of metastable NaSi crystallites and the volumetric change of Si matrix during the sodiation-desodiation processes. To avoid this, recently, various Si nanoarchitectures and Si-C nanocomposites (e.g., Si microspheres [19], Si nanocrystals [25], The magnesium (Mg, 99% purity) powder, hydrofluoric acid (HF, 48%), and hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich (St. Louis, MO, USA), and used without further purification. The biomass resource-BRH was collected from Perambalur, Tamil Nadu, India. Figure 1 schematically represents the one-pot synthesis of the C-Si nanocomposites through magnesiothermic reduction using BRH ashes. As an initial task, BRHs were calcinated at 400 • C for 2 h in air to collect the carbonaceous and siliceous resource of the BRH ashes. Then, we mixed the BRH ashes (1.2 g) and the Mg powder (0.3 g) using a mortar and annealed those mixtures at 600 • C for 2 h in Ar atmosphere. During the annealing process, the C-Si, MgO, and Mg 2 Si components are produced via magnesiothermic reduction and the chemical reaction described below:

One-Pot Synthesis of C-Si Nanocomposites
This allows the one-pot synthesis of C-Si nanocomposites from BRH ashes via magnesiothermic reduction. Next, to remove residual impurities and precipitates (e.g., Mg 2 Si, unreacted SiO 2 , MgO residues, etc.), the mixtures were stirred in HCl (1 M) for 360 min, and were subsequently soaked with hydrofluoric acid (5%) for 60 min. Finally, the powder type of the C-Si nanocomposite was obtained

Characterization of Material Properties
The crystallographic characteristics of the C-Si nanocomposites were investigated by x-ray diffraction (XRD) using a D2 Phaser system (Bruker, Madison, WI, USA). The vibrational characteristics of the nanocomposites were analyzed by Raman scattering spectroscopy using a LabRAM HR800 system (HORIBA Jobin Yvon Inc., Edison, NJ, USA). The morphological and the compositional properties were examined by field-emission scanning electron microscopy (FE-SEM) and in-situ energy dispersive X-ray (EDX) spectroscopy, respectively, using an Inspect F50 system (FEI Company, Mahwah, NJ, USA). Additionally, the microstructures of the nanocomposites were monitored by transmission electron microscopy (TEM) and in-situ selective area electron diffraction (SAED) using a Titan 80-300 microscope (FEI Company, Hillsboro, OR, USA).

Fabrication of SIB Device Using C-Si Nanocomposites
The 2032 coin-cell type of the SIB device was fabricated to assess the electrochemical performances of the C-Si nanocomposites. For preparing the SIB anode, firstly, the C-Si nanocomposites (80 wt.%) were mixed with carbon black (10 wt.%) and polyvinylidene difluoride (PVDF) (10 wt.%) in n-methyl-2-pyrrolidinone. Then, the prepared slurry was coated onto the Cu foil, and was subsequently dried at 110 °C for 5 h in vacuum. Here, the mass loading of the electrode was 0.7 mg. In addition, we also note that the used electrolyte to involve a mixture of ethylene carbonate and dimethyl carbonate in 1 M NaPF6 solvent at a volume ratio of 1:1. Finally, the coin-cell SIB device was assembled in a glove box under Ar (99.999%) ambience. Here, we also note that sodium foil and Celgard 2400 were used as the counter electrode and as the separator, respectively.

Characterization of Electrochemical Performances
The electrochemical performances of the fabricated SIB device were examined through the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectrometry (EIS) measurements by using a MPG2 potentiostat (BioLogic, Seyssinet-Pariset, France). The CV characteristics were tested at the potential range of 0.01-2.5 V (vs. Na/Na + ) under the scan

Characterization of Material Properties
The crystallographic characteristics of the C-Si nanocomposites were investigated by X-ray diffraction (XRD) using a D2 Phaser system (Bruker, Madison, WI, USA). The vibrational characteristics of the nanocomposites were analyzed by Raman scattering spectroscopy using a LabRAM HR800 system (HORIBA Jobin Yvon Inc., Edison, NJ, USA). The morphological and the compositional properties were examined by field-emission scanning electron microscopy (FE-SEM) and in-situ energy dispersive X-ray (EDX) spectroscopy, respectively, using an Inspect F50 system (FEI Company, Mahwah, NJ, USA). Additionally, the microstructures of the nanocomposites were monitored by transmission electron microscopy (TEM) and in-situ selective area electron diffraction (SAED) using a Titan 80-300 microscope (FEI Company, Hillsboro, OR, USA).

Fabrication of SIB Device Using C-Si Nanocomposites
The 2032 coin-cell type of the SIB device was fabricated to assess the electrochemical performances of the C-Si nanocomposites. For preparing the SIB anode, firstly, the C-Si nanocomposites (80 wt.%) were mixed with carbon black (10 wt.%) and polyvinylidene difluoride (PVDF) (10 wt.%) in n-methyl-2-pyrrolidinone. Then, the prepared slurry was coated onto the Cu foil, and was subsequently dried at 110 • C for 5 h in vacuum. Here, the mass loading of the electrode was 0.7 mg. In addition, we also note that the used electrolyte to involve a mixture of ethylene carbonate and dimethyl carbonate in 1 M NaPF 6 solvent at a volume ratio of 1:1. Finally, the coin-cell SIB device was assembled in a glove box under Ar (99.999%) ambience. Here, we also note that sodium foil and Celgard 2400 were used as the counter electrode and as the separator, respectively.

Characterization of Electrochemical Performances
The electrochemical performances of the fabricated SIB device were examined through the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectrometry (EIS) measurements by using a MPG2 potentiostat (BioLogic, Seyssinet-Pariset, France). The CV characteristics were tested at the potential range of 0.01-2.5 V (vs. Na/Na + ) under the scan rate (r s ) of 0.2 mV/s, and the GCD characteristics were examined at 0.01-2.5 V (vs. Na/Na + ) by applying the constant current of 100-2000 mA/g. The charge-transfer impedance characteristics were measured by EIS at the frequency range from 0.01 Hz to 100 kHz.  [36,50,51], respectively; and the broad peak at 22 • and the small peak at 42.4 • arise from the (002) and (100) phases of activated carbon [52][53][54], respectively. Using the Scherer formula [55][56][57], the average crystal size of the C-Si nanocomposites was determined to be approximately 14 nm. From the Raman scattering spectroscopy measurement, the sample also revealed its intrinsic vibration properties from only Si and C. As displayed in Figure 2b, the sample showed the four predominant Raman bands at 514, 959, 1341, and 1590 cm −1 from Si and C. The former two bands at 514 and 959 cm −1 originate from the first and the second order transversal optical (TO) mode of crystalline Si [58,59], respectively; and the latter two vibration modes at 1341 and 1590 cm −1 come from D (sp 3 type) and G (sp 2 type) bands of graphitized C, respectively [60,61]. Here, it should be noticeable that the C nanoflakes exhibited the high intensity area ratio of I D /I G (i.e., Asp 3 /Asp 2 0.99), indicative of the high graphitization (i.e., ultrathin C layers) and large amount of sp 2 carbon exists in the composite system [39,53,60,[62][63][64]. The absence of extra lattice phases and their vibrations depict that the high purity C-Si nanocomposites were well-crystallized with their intrinsic carbonaceous and siliceous resources via the one-pot synthesis of magnesiothermic reduction using biomass BRH ashes.   [36,50,51], respectively; and the broad peak at 22° and the small peak at 42.4° arise from the (002) and (100) phases of activated carbon [52][53][54], respectively. Using the Scherer formula [55][56][57], the average crystal size of the C-Si nanocomposites was determined to be approximately 14 nm. From the Raman scattering spectroscopy measurement, the sample also revealed its intrinsic vibration properties from only Si and C. As displayed in Figure  2b, the sample showed the four predominant Raman bands at 514, 959, 1341, and 1590 cm −1 from Si and C. The former two bands at 514 and 959 cm −1 originate from the first and the second order transversal optical (TO) mode of crystalline Si [58,59], respectively; and the latter two vibration modes at 1341 and 1590 cm −1 come from D (sp 3 type) and G (sp 2 type) bands of graphitized C, respectively [60,61]. Here, it should be noticeable that the C nanoflakes exhibited the high intensity area ratio of ID/IG (i.e., Asp 3 /Asp 2 ≅ 0.99), indicative of the high graphitization (i.e., ultrathin C layers) and large amount of sp 2 carbon exists in the composite system [39,53,60,[62][63][64]. The absence of extra lattice phases and their vibrations depict that the high purity C-Si nanocomposites were well-crystallized with their intrinsic carbonaceous and siliceous resources via the one-pot synthesis of magnesiothermic reduction using biomass BRH ashes. Next, the morphological properties of the C-Si nanocomposites were monitored by FE-SEM measurements. Figure 3a and b display the low-and high-magnification FE-SEM images of the C-Si nanocomposites, respectively. The sample exhibited an aggregated nanocomposite morphology, where the spherical Si nanoparticles were interconnected with the C nanoflakes. From the EDX spectrum (Figure 3c), one can also confirm that the biomass BRH-derived C-Si nanocomposites involved their intrinsic species of Si (83.12 wt.%) and C (15.25 wt.%). This corroborates that the C-Si nanocomposites were effectively synthesized from the biomass carbonaceous and siliceous resource of BRH ashes. The small portion of Pt (1.63 wt.%) is thought as sprouting from conductive-coating for the FE-SEM measurement. Next, the morphological properties of the C-Si nanocomposites were monitored by FE-SEM measurements. Figure 3a and b display the low-and high-magnification FE-SEM images of the C-Si nanocomposites, respectively. The sample exhibited an aggregated nanocomposite morphology, where the spherical Si nanoparticles were interconnected with the C nanoflakes. From the EDX spectrum (Figure 3c), one can also confirm that the biomass BRH-derived C-Si nanocomposites involved their intrinsic species of Si (83.12 wt.%) and C (15.25 wt.%). This corroborates that the C-Si nanocomposites were effectively synthesized from the biomass carbonaceous and siliceous resource of BRH ashes. The small portion of Pt (1.63 wt.%) is thought as sprouting from conductive-coating for the FE-SEM measurement.  The TEM images display the further insight into the microstructures of the C-Si nanocomposites ( Figure 4). As represented in high-and low-magnification TEM images, the nanocomposites were constructed in the form of the aggregated structure, consisting of the Si nanoparticles and the C nanoflakes (Figure 4a,b). Additionally, from the high-resolution TEM image, one can also observe that the spherical Si nanoparticles were entrenched with the ultrathin C nanoflakes (Figure 4c). The average size of the Si nanoparticles was determined to be 9 nm, and the interlayer spacing of the Si lattice fringes was confirmed to be 0.31 nm, corresponding to the lattice constant of crystalline (111) Si. The SAED patterns of the C-Si nanocomposites further elucidate the effective aggregation of wellcrystallized Si nanoparticles with well-graphitized C nanoflakes.  The TEM images display the further insight into the microstructures of the C-Si nanocomposites ( Figure 4). As represented in high-and low-magnification TEM images, the nanocomposites were constructed in the form of the aggregated structure, consisting of the Si nanoparticles and the C nanoflakes (Figure 4a,b). Additionally, from the high-resolution TEM image, one can also observe that the spherical Si nanoparticles were entrenched with the ultrathin C nanoflakes (Figure 4c). The average size of the Si nanoparticles was determined to be 9 nm, and the interlayer spacing of the Si lattice fringes was confirmed to be 0.31 nm, corresponding to the lattice constant of crystalline (111) Si. The SAED patterns of the C-Si nanocomposites further elucidate the effective aggregation of well-crystallized Si nanoparticles with well-graphitized C nanoflakes.  The TEM images display the further insight into the microstructures of the C-Si nanocomposites ( Figure 4). As represented in high-and low-magnification TEM images, the nanocomposites were constructed in the form of the aggregated structure, consisting of the Si nanoparticles and the C nanoflakes (Figure 4a,b). Additionally, from the high-resolution TEM image, one can also observe that the spherical Si nanoparticles were entrenched with the ultrathin C nanoflakes (Figure 4c). The average size of the Si nanoparticles was determined to be 9 nm, and the interlayer spacing of the Si lattice fringes was confirmed to be 0.31 nm, corresponding to the lattice constant of crystalline (111) Si. The SAED patterns of the C-Si nanocomposites further elucidate the effective aggregation of wellcrystallized Si nanoparticles with well-graphitized C nanoflakes.

Electrochemical Performances of C-Si Nanocomposites as an SIB Anode
The conformation of the C-Si nanocomposite system would be beneficial for improving the SIB anode performances because of the synergetic effects from the Si nanoparticles (i.e., high specific capacity and low discharge potential) and the C nanoflakes (i.e., high electrical conductivity and large surface area) [36,38,65]. We, therefore, assessed the electrochemical performance of the C-Si nanocomposites as aa SIB anode. Figure 5a shows the CV curves of the fabricated SIB device that comprised the C-Si nanocomposite anode. Here, the CV characteristics were measured at the potential range of 0-2.5 V (vs. Na/Na + ) under r s = 0.2 mV/s. The device clearly exhibited the charge-discharge characteristics at the potential range below 1 V. For the first CV sweep, the sample revealed a wide reduction in the sodiation regime because of the formation of the solid electrolyte interphase (SEI) layer. After the formation of the SEI layer, however, the CV curve became stable with maintaining a distinctive area. These indicate that the sodiation-desodiation effectively took place in the C-Si nanocomposite SIB anode [28,66].
The anodic performances of the C-Si nanocomposites were further examined by GCD measurements. As shown in Figure 5b, the SIB device clearly revealed the charge-discharge characteristics in its GCD curves. For the first cycle, the specific charge capacity and the discharge capacity were obtained to be 334 and 378 mAh/g, respectively, under the injection current density of 100 mA/g. This represents the C-Si nanocomposites to hold a high initial Coulombic efficiency up to 88%, attributing to the encapsulation of Si nanoparticles with C nanoflakes. Namely, since the ultrathin C nanoflakes covered the surface of the Si nanoparticles, the large number of Na + ions could be intercalated and de-intercalated with the C-Si composites. Because of the capacity loss due to the inevitable formation of NaSi [14,19,30], the discharge capacity was decreased to 161 mAh/g at the 2nd cycle under the current injection of 200 mA/g. From the 5th to 100th cycles, however, the discharge capacity became stable at 122 mAh/g. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 12

Electrochemical Performances of C-Si Nanocomposites as an SIB Anode
The conformation of the C-Si nanocomposite system would be beneficial for improving the SIB anode performances because of the synergetic effects from the Si nanoparticles (i.e., high specific capacity and low discharge potential) and the C nanoflakes (i.e., high electrical conductivity and large surface area) [36,38,65]. We, therefore, assessed the electrochemical performance of the C-Si nanocomposites as aa SIB anode. Figure 5a shows the CV curves of the fabricated SIB device that comprised the C-Si nanocomposite anode. Here, the CV characteristics were measured at the potential range of 0-2.5 V (vs. Na/Na + ) under rs = 0.2 mV/s. The device clearly exhibited the chargedischarge characteristics at the potential range below 1 V. For the first CV sweep, the sample revealed a wide reduction in the sodiation regime because of the formation of the solid electrolyte interphase (SEI) layer. After the formation of the SEI layer, however, the CV curve became stable with maintaining a distinctive area. These indicate that the sodiation-desodiation effectively took place in the C-Si nanocomposite SIB anode [28,66]. The anodic performances of the C-Si nanocomposites were further examined by GCD measurements. As shown in Figure 5b, the SIB device clearly revealed the charge-discharge characteristics in its GCD curves. For the first cycle, the specific charge capacity and the discharge capacity were obtained to be 334 and 378 mAh/g, respectively, under the injection current density of 100 mA/g. This represents the C-Si nanocomposites to hold a high initial Coulombic efficiency up to Next, we tested the rate performance and the cyclic stability of the SIB device. As represented in Figure 5c, the device showed the reversible discharge capacity of 265, 122, 101, 88, and 59 mAh/g when the injection current density was 100, 200, 500, 1000, and 2000 mA/g, respectively. Here, one can observe that the discharge capacity was effectively recovered to the specific value at the certain current injection conditions. Namely, the magnitude of the specific discharge capacity is nearly identical at both the beginning stage and the final stage under the current injection of 200 mA/g. Such a superb reversibility is also associated to the cyclic stability. As aforementioned, due to the phase transition period (i.e., SEI formation) [19], the device lost 13% of the Coulombic efficiency during the initial three cycles (Figure 5d). After the initialization, however, the SIB device maintained its high Coulombic efficiency up to 98% even after 100 cycles under the current injection of 200 mA/g. Such an excellent cyclic stability is comparable to literature values, and is even greater than other Si-based SIB anodes that were fabricated with commercial resources [19,[25][26][27][28][29][30][31] (See Table 1). Finally, we discuss the charge-transfer characteristics of the C-Si nanocomposites as a SIB anode. As shown in the Nyquist plots (Figure 6a), the sample displayed both semicircles and long tails at low and high frequency regions, respectively. According to the equivalent circuit model [67], the former ones are attributed to the charge transfer resistance (R ct ) [28], and the latter ones are associated with the Warburg impedance (W 0 ), corresponding to Na + intercalation into the electrode material [8]. Through adopting this model to our SIB device structure, we established the equivalent circuit model (Figure 6b) and fitted the experimental data to the circuit simulator to estimate the key device parameters. We, here, note that two additional resistance components of R s and R SEI in the equivalent circuit are the electrolyte solution resistance and the SEI resistance, respectively. CPE1 and CPE2 means the constant phase elements of the SEI layer and the electrochemical double layer in the composite, respectively [67]. Using the established equivalent circuit model, R SEI and R ct were determined to be 134 and 206 Ω, respectively, at the first cycle (i.e., before cyclic stability test). However, after the cyclic stability test (i.e., 100 charge-discharge cycles at 200 mA/g), R SEI and R ct were decreased to 79 and 174 Ω, respectively, because of the SEI formation. These low R SEI and R ct values infer that the C-Si nanocomposites possess high ionic and electronic conductivities, which can enhance the sodium ion intercalation into the SIB anode material via the electrode/electrolyte interface. Consequently, the excellent SIB anode performances could be thought as resulting from the formation of the C-Si nanocomposite solid-state system. In other words, the encapsulation of Si nanoparticles (i.e., high charge capacity resource) with C nanoflakes (i.e., high conductivity) could help enhance the intercalation/de-intercalation of Na + ion with the anodic material of the C-Si nanocomposites. Nanomaterials 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials formation of the C-Si nanocomposite solid-state system. In other words, the encapsulation of Si nanoparticles (i.e., high charge capacity resource) with C nanoflakes (i.e., high conductivity) could help enhance the intercalation/de-intercalation of Na + ion with the anodic material of the C-Si nanocomposites.

Summary and Conclusions
The biomass C-Si nanocomposites were synthesized by the one-pot synthesis method through the facile magnesiothermic reduction process using the BRH ashes. The nanocomposites showed an interconnected and aggregated morphology, where the spherical Si nanoparticles were densely packed and encapsulated with the C nanoflakes. For the electrochemical SIB anode characteristics, the nanocomposites exhibited the high initial discharge capacity of 378 mAh/g (at 100 mA/g) as well as the prominent reversible capacity of 122 mAh/g (at 200 mA/g). Additionally, the excellent cyclic stability (i.e., 98% capacity retention after 100 cycles at 200 mA/g) was observed from the nanocomposites. These excellent anodic performances could be attributed to the encapsulation of the highly intercalative Si nanoparticles with the highly-conductive ultrathin C nanoflakes. The results suggest that the biomass BRH-derived C-Si nanocomposites hold great potential as a highperformance SIB anode material.

Summary and Conclusions
The biomass C-Si nanocomposites were synthesized by the one-pot synthesis method through the facile magnesiothermic reduction process using the BRH ashes. The nanocomposites showed an interconnected and aggregated morphology, where the spherical Si nanoparticles were densely packed and encapsulated with the C nanoflakes. For the electrochemical SIB anode characteristics, the nanocomposites exhibited the high initial discharge capacity of 378 mAh/g (at 100 mA/g) as well as the prominent reversible capacity of 122 mAh/g (at 200 mA/g). Additionally, the excellent cyclic stability (i.e., 98% capacity retention after 100 cycles at 200 mA/g) was observed from the nanocomposites. These excellent anodic performances could be attributed to the encapsulation of the highly intercalative Si nanoparticles with the highly-conductive ultrathin C nanoflakes. The results suggest that the biomass BRH-derived C-Si nanocomposites hold great potential as a high-performance SIB anode material.