Green and Scalable Fabrication of Sandwich-like NG/SiOx/NG Homogenous Hybrids for Superior Lithium-Ion Batteries

SiOx is considered as a promising anode for next-generation Li-ions batteries (LIBs) due to its high theoretical capacity; however, mechanical damage originated from volumetric variation during cycles, low intrinsic conductivity, and the complicated or toxic fabrication approaches critically hampered its practical application. Herein, a green, inexpensive, and scalable strategy was employed to fabricate NG/SiOx/NG (N-doped reduced graphene oxide) homogenous hybrids via a freeze-drying combined thermal decomposition method. The stable sandwich structure provided open channels for ion diffusion and relieved the mechanical stress originated from volumetric variation. The homogenous hybrids guaranteed the uniform and agglomeration-free distribution of SiOx into conductive substrate, which efficiently improved the electric conductivity of the electrodes, favoring the fast electrochemical kinetics and further relieving the volumetric variation during lithiation/delithiation. N doping modulated the disproportionation reaction of SiOx into Si and created more defects for ion storage, resulting in a high specific capacity. Deservedly, the prepared electrode exhibited a high specific capacity of 545 mAh g−1 at 2 A g−1, a high areal capacity of 2.06 mAh cm−2 after 450 cycles at 1.5 mA cm−2 in half-cell and tolerable lithium storage performance in full-cell. The green, scalable synthesis strategy and prominent electrochemical performance made the NG/SiOx/NG electrode one of the most promising practicable anodes for LIBs.


Introduction
Lithium-ion batteries (LIBs) have attracted extensive attention in the energy storage field, owing to their high energy density, long lifespan, and low self-discharge properties [1][2][3][4][5]. However, present LIB systems based on carbon as anode and lithium metal oxides as cathode with low energy densities cannot satisfy the ever-growing energy demands for electronic devices [6][7][8][9]. Recently, Si and SiO x (0 < x ≤ 2) have caused widespread concern as promising alternative materials to traditional graphite anodes (with a limited specific capacity of 372 mAh g −1 ), due to their high theoretical capacities, relatively low redox potential vs. Li/Li + and natural abundance features [8,[10][11][12][13]. Meanwhile large volumetric variation over repeated delithiation/lithiation processes led to the particle fracture, the formation of unstable SEI (solid electrolyte interphase), loss contact between current collector and electrode materials, which contributed to its fast capacity decay; low ionic and electronic conductivity also restricted the rate capability [3,10,14,15]. For SiO x , the formation of Li 2 O and Li 4 SiO 4 upon initial lithiation/delithiation process (Equations (1) and (2)) could act as the buffer layers to alleviate the volumetric variation Powder X-ray diffraction (PXRD) patterns were captured on a Bruker D8 (Bruker AXS Co., Karlsruhe, Germany) X-ray diffractometer using Cu Kα radiation. Thermogravimetric analysis (TGA) was recorded on a TG/DTA 6300 thermogravimetric analyzer (Perkin-Elmer Inc., Boston, MA, USA). N 2 adsorption/desorption isotherms were conducted by using a Tristar 3000 micrometric apparatus (Micromeritics Corporate, Norcross, GA, USA) at −196 • C. Raman spectra were recorded on an HR-800 µ-Raman system (HORIBA Jobin Yvon, Paris, France) using an Ar laser (514.5 nm). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) were captured on a JEOL JEM-2100F field-emission TEM (JEOL Ltd., Tokyo, Japan). Scanning electron microscopy (SEM) images were obtained on a Zeiss Sigma-500 field-emission SEM (Carl Zeiss AG Oberkochen, Germany). X-ray photoelectron spectroscopy (XPS) spectra were employed to analyze the chemical state of the samples using a PHI-1600 photoelectron spectrometer (ULVAC-PHI Inc., Tokyo, Japan) with Al-Kα X-ray source.

Electrochemical Characterization
CR2032 coin-cells were assembled in an Ar-filled glove box (Omni-LAB, Vacuum Atmospheres Company, Lawndale, CA, USA) to survey the electrochemical performance of the prepared composites, metallic Li (Shenzhen Teesky Technology Co., Ltd., Shenzhen, China) or commercial LiFePO 4 (Shenzhen Teesky Technology Co., Ltd., Shenzhen, China) were used as the counter electrode. A slurry consisting of 70 wt.% active composite, 20 wt.% conductive carbon black (Shenzhen Teesky Technology Co., Ltd., Shenzhen, China) and 10 wt.% polyvinylidene fluoride (PVDF, Shenzhen Teesky Technology Co., Ltd., Shenzhen, China) with N-methylpyrrolidone (NMP, Energy Chemical, Shanghai, China) as solvent were coated on Cu/Al (Shenzhen Teesky Technology Co., Ltd., Shenzhen, China) foil and then dried at 120 • C for 24 h. 1 mol L −1 LiPF 6 in a mixture of ethylene carbonate, dimethyl carbonate (1/1 v/v) and 5 wt.% fluoroethylene carbonate (FEC) were used as the electrolyte (Shenzhen Teesky Technology Co., Ltd., Shenzhen, China). The mass loading of the anode composites on Cu foil was~1 mg cm −2 , and the capacity of anode material in half-cells was calculated based on the weight of anode composites. The mass ratio of negative and positive materials in full-cells was~0.12, and the mass capacity of full-cells were calculated based on the weight of LiFePO 4 . The NG/SiO x /NG anode was activated for 10 cycles in half-cells before assembling NG/SiO x /NG//LiFePO 4 full-cell. Galvanostatic charge/discharge (GCD) tests were conducted using a Neware (CT-3008W, Shenzhen Neware Technology Limited, Shenzhen, China) instrument. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) curves were captured on a Parstat 4000+ electrochemical workstation (Princeton Applied Research, Princeton, United States). The voltage windows were set as 3-0.05 V and 2.8-4.2 V for half-and full-cells, respectively.

Results and Discussion
NG/SiO x /NG composite was prepared by a nontoxic, simple, and scalable freezedrying combined thermal decomposition strategy, as illustrated in Figure 1. After mechanical ball-milling, commercial SiO x was crushed and dispersed into ethanol. Subsequently, SiO x suspension was mixed with GO-urea homogeneous solution uniformly. Under ultrasound treatment, SiO x particles were bundled into GO and surrounded by urea. The freeze-drying strategy maintained the sandwich structure during the drying process. During the thermal treatment, GO was reduced to rGO, partial SiO x was decomposed into Si and SiO 2 , urea was decomposed into N-doped carbon and partial N was doped into rGO. No poisonous chemicals and hydrothermal process were used during the fabrication process of sandwich-like NG/SiO x /NG composite, in other words, the fabrication process is green and scalable.
suspension was mixed with GO-urea homogeneous solution uniformly. Under ultrasound treatment, SiOx particles were bundled into GO and surrounded by urea. The freeze-drying strategy maintained the sandwich structure during the drying process. During the thermal treatment, GO was reduced to rGO, partial SiOx was decomposed into Si and SiO2, urea was decomposed into N-doped carbon and partial N was doped into rGO. No poisonous chemicals and hydrothermal process were used during the fabrication process of sandwich-like NG/SiOx/NG composite, in other words, the fabrication process is green and scalable. The crystal structures are determined by PXRD and the results are demonstrated in Figures 2a and S1. For GO, a typical diffraction peak located below 15° could be observed in Figure S1; while after calcination in Ar at 650 °C, the diffraction peak below 15° disappeared and a new diffraction peak located at 26° appeared, which should be attributed to the removal of oxygen containing functional groups and the exfoliation of multiple layers during the thermal reduction of GO [33][34][35]. The variation of PXRD patterns for GO after calcination confirmed the thermal reduction of GO into rGO under Ar. For SiOx, two broad diffraction peaks located around 25° and 45° can be observed (Figure 2a), indicating the amorphous or low crystallinity structure of SiOx in the samples. No diffraction peak corresponding to GO (below 15°) can be detected in Figure 2a, meaning that GO was successfully reduced to rGO in G/SiOx/G and NG/SiOx/NG [19,33]. Therefore, two broad diffraction peaks located around 25° and 45° in G/SiOx/G and NG/SiOx/NG could be assigned to the superimposed diffraction peaks of SiOx and rGO. Generally, SiO would be decomposed into Si and SiO2 under heat treatment [14,36,37]. In this work, no diffraction peaks assigned to Si can be observed, which can be attributed to the small crystalline size of Si. XRD results verify the successful fabrication of SiOx/G composites.
XPS spectra were conducted to investigate the chemical state of Si, O, C, and N in the samples and the results are displayed in Figure 2b-f. The very weak signals of Si 2p and Si 2s in Figure 2b can be attributed to the covering of rGO on the surface of SiOx. Three peaks centered at 105.8, 104.8, and 103.9 eV are detected for Si 2p (Figure 2c), indicating the co-existing of Si 4+ , Si 3+ , and Si 2+ in the two samples [19,33,36,38,39]. Different from G/SiOx/G, new binding energies located at 100.6 and 101.6 eV assigned to Si 0 2p3/2 and Si 0 2p1/2 can be observed in NG/SiOx/NG; while the peak corresponding to Si 2+ became lower and Si 4+ became stronger, indicating that partial SiO was decomposed into Si and SiO2 during the thermal treatment [14,19,31,40]. In other words, the gas released during the decomposition of urea or N doping can promote the disproportionation of SiO. According to the valence of silicon, the atomic ratios of O/Si were 1.55 and 1.68 in G/SiOx/G and NG/SiOx/NG, respectively (Table 1). Two Lorentzian peaks of C1s located at 284.7 and 286.4 eV represented the C-C/C=C and C=O bonds, respectively; and the additional peak centered at 285.2 eV in NG/SiOx/NG was assigned to C-N bond ( Figure 2d) [33,39,[41][42][43]. The N 1s spectrum in Figure 2e exhibited three typical peaks at 398.2, 399.2, and 400.8 eV, which could be attributed to pyridinic N, pyrrolic N, and graphitic N, respectively [38,39,[44][45][46]. The XPS results indicated that N doping facilitated the disproportionation of SiO, resulting in the formation of abundant SiO2 shells and high dispersive Si cores in SiOx/NG composite.  Figures 2a and S1. For GO, a typical diffraction peak located below 15 • could be observed in Figure S1; while after calcination in Ar at 650 • C, the diffraction peak below 15 • disappeared and a new diffraction peak located at 26 • appeared, which should be attributed to the removal of oxygen containing functional groups and the exfoliation of multiple layers during the thermal reduction of GO [33][34][35]. The variation of PXRD patterns for GO after calcination confirmed the thermal reduction of GO into rGO under Ar. For SiO x , two broad diffraction peaks located around 25 • and 45 • can be observed (Figure 2a), indicating the amorphous or low crystallinity structure of SiO x in the samples. No diffraction peak corresponding to GO (below 15 • ) can be detected in Figure 2a, meaning that GO was successfully reduced to rGO in G/SiO x /G and NG/SiO x /NG [19,33]. Therefore, two broad diffraction peaks located around 25 • and 45 • in G/SiO x /G and NG/SiO x /NG could be assigned to the superimposed diffraction peaks of SiO x and rGO. Generally, SiO would be decomposed into Si and SiO 2 under heat treatment [14,36,37]. In this work, no diffraction peaks assigned to Si can be observed, which can be attributed to the small crystalline size of Si. XRD results verify the successful fabrication of SiO x /G composites.    Figure 2b can be attributed to the covering of rGO on the surface of SiO x . Three peaks centered at 105.8, 104.8, and 103.9 eV are detected for Si 2p (Figure 2c), indicating the coexisting of Si 4+ , Si 3+ , and Si 2+ in the two samples [19,33,36,38,39]. Different from G/SiO x /G, new binding energies located at 100.6 and 101.6 eV assigned to Si 0 2p 3/2 and Si 0 2p 1/2 can be observed in NG/SiO x /NG; while the peak corresponding to Si 2+ became lower and Si 4+ became stronger, indicating that partial SiO was decomposed into Si and SiO 2 during the thermal treatment [14,19,31,40]. In other words, the gas released during the decomposition of urea or N doping can promote the disproportionation of SiO. According to the valence of silicon, the atomic ratios of O/Si were 1.55 and 1.68 in G/SiO x /G and NG/SiO x /NG, respectively (Table 1). Two Lorentzian peaks of C1s located at 284.7 and 286.4 eV represented the C-C/C=C and C=O bonds, respectively; and the additional peak centered at 285.2 eV in NG/SiO x /NG was assigned to C-N bond ( Figure 2d) [33,39,[41][42][43]. The N 1s spectrum in Figure 2e exhibited three typical peaks at 398.2, 399.2, and 400.8 eV, which could be attributed to pyridinic N, pyrrolic N, and graphitic N, respectively [38,39,[44][45][46]. The XPS results indicated that N doping facilitated the disproportionation of SiO, resulting in the formation of abundant SiO 2 shells and high dispersive Si cores in SiO x /NG composite. Raman spectra of G/SiO x /G and NG/SiO x /NG in Figure 2f displayed two typical broadened bands at 1315.8 and 1587.4 cm −1 , which are assigned to the disordered band (D-band) and graphite band (G-band) of carbonaceous materials, respectively [3,19,44]. The D-band to G-band intensity ratios (I D /I G ) were calculated to be 1.53 and 1.27 for NG/SiO x /NG and G/SiO x /G, indicating that the main component of carbon in both samples were amorphous; the higher I D /I G in NG/SiO x /NG implied that more defects existed in N-doped rGO, which is beneficial to electronic conductivity and the ion storage of NG/SiO x /NG composite [19,44]. TGA results in Figure S2a showed that the carbon content in NG/SiO x /NG and G/SiO x /G were 21.7% and 16.7%, respectively. In addition, the surface area increased from 4 to 34 m 2 g −1 after N doping ( Figure S2b), the higher surface area favored the electrolyte infiltration and the electrochemical kinetics in NG/SiO x /NG [43,47].
The NG/SiO x /NG and G/SiO x /G composites both exhibited 2D nanosheet flake morphologies, which is the typical morphology of rGO (Figures 3a,b and S3a,b). The enlarged pictures in Figure 3a,b showed that internal particles popped up many protrusions on the surface of rGO, indicating that SiO x was stacked and encapsulated into rGO nanosheets. More importantly, no agglomeration and unfavorable exposure of SiO x or rGO could be seen in Figures 3a,b and S3a,b, indicating the formation of sandwich-like SiO x /G homogeneous hybrids. TEM results in Figure S3c,d further proved the formation of sandwich-like structure of the prepared composites. In Figure 3d, lattice fringes with a d-spacing of 0.31 nm corresponding to Si (111) can be observed, indicating the partial decomposition of SiO into Si in NG/SiO x /NG, which is consistent with the XPS results. In accordance with the SEM and TEM results, the elements distributions in Figure 3e,f also indicated that the SiO x particles were trapped and encompassed by the conductive carbon tightly in G/SiO x /G and NG/SiO x /NG. The uniform distribution of N and C in Figure 3f further confirmed the successful doping of N into carbon in NG/SiO x /NG, agreeing with the XPS results. Morphological characterizations verified the formation of sandwich-like structure of SiO x /G and SiO x /NG homogeneous hybrids; in other words, NG/SiO x /NG and G/SiO x /G homogeneous hybrids were successfully fabricated by the green, simple, and scalable method.   (1) and (2)) [36,44,48,49]. And the distinct cathodic peak below 0.25 V can be assigned to the lithiation of Si into LixSi alloys (Equation (3)) [3,21,36,50]. The charging branch showed broad peaks at 0.3-0.6 V, which can be attributed to the multi-step de-alloying processes of LixSi alloys [10,17,21,36,51]. The CV areas increased in the following cycles, which was associated with the activation of the electrode material and the improved contact between SiOx particles and coated carbonaceous [17,21,36]. The GCD curves of SiOx, G/SiOx/G, and NG/SiOx/NG at 0.1 A g −1 are demonstrated in Figure S4. Bulk SiOx delivered a high initial discharge capacity of 2718.3 mAh g −1 with a low coulombic efficiency of 49.6%. The G/SiOx/G electrode exhibited initial discharge and charge capacities of 1678.5 mAh g −1 and 1070.8 mAh g −1 with a coulombic efficiency of 63.8%. N doping would create more defects and more active sites for ion adsorption, deservedly, the NG/SiOx/NG electrode delivered a higher initial charge capacity of 1182.3 mAh g −1 with a higher coulombic efficiency of 66.6%. The overlapped GCD curves in the following cycles indicated the high  (1) and (2)) [36,44,48,49]. And the distinct cathodic peak below 0.25 V can be assigned to the lithiation of Si into Li x Si alloys (Equation (3)) [3,21,36,50]. The charging branch showed broad peaks at 0.3-0.6 V, which can be attributed to the multi-step dealloying processes of Li x Si alloys [10,17,21,36,51]. The CV areas increased in the following cycles, which was associated with the activation of the electrode material and the improved contact between SiO x particles and coated carbonaceous [17,21,36]. The GCD curves of SiO x , G/SiO x /G, and NG/SiO x /NG at 0.1 A g −1 are demonstrated in Figure S4. Bulk SiO x delivered a high initial discharge capacity of 2718.3 mAh g −1 with a low coulombic efficiency of 49.6%. The G/SiO x /G electrode exhibited initial discharge and charge capacities of 1678.5 mAh g −1 and 1070.8 mAh g −1 with a coulombic efficiency of 63.8%. N doping would create more defects and more active sites for ion adsorption, deservedly, the NG/SiO x /NG electrode delivered a higher initial charge capacity of 1182.3 mAh g −1 with a higher coulombic efficiency of 66.6%. The overlapped GCD curves in the following cycles indicated the high electrochemical reversibility of NG/SiO x /NG [1,23,43]. The improved coulombic efficiency from 49.6% for SiO x to 66.6% for NG/SiO x /NG and the high reversibility verified the NG/SiO x /NG electrode to be a promising anode for LIBs (Table S1). omaterials 2021, 11, 2366 7 of electrochemical reversibility of NG/SiOx/NG [1,23,43]. The improved coulombic efficien from 49.6% for SiOx to 66.6% for NG/SiOx/NG and the high reversibility verified t NG/SiOx/NG electrode to be a promising anode for LIBs (Table S1). The rate performance and cyclic performance in Figure 4c-f further verified the s perior electrochemical performance of NG/SiOx/NG. The capacities of the G/SiOx/G el trode decreased quickly with increasing densities (Figure 4c,d) and, typically, the avera capacities were 928.4, 619.6, 479.2, 463.8, 362.1, 276.5 mAh g −1 at 0.1, 0.2, 0.3, 0.5, 1, and A g −1 , respectively; when the current density recovered to 0.2 A g −1 from 2 A g −1 , the pacity was 561.1 mAh g −1 . After 100 cycles at 0.5 A g −1 , the specific capacity of G/SiOx decreased dramatically, and only a stable capacity of 212.5 mAh g −1 was retained after 4 cycles (Figure 4f). In contrast, the NG/SiOx/NG demonstrated higher capacities of 1197 1002.3, 978.9, 803.3, 643.1, and 545.8 mAh g −1 at above current densities; when the curre density recovered to 0.2 A g −1 from 2 A g −1 , a high capacity of 1072.6 mAh g −1 was obtaine indicating the high reversibility of the NG/SiOx/NG electrode (Figure 4c,e). Moreover, ter 450 cycles at 0.5 A g −1 , the NG/SiOx/NG electrode maintained a high reversible capac of 798.9 mAh g −1 with a loading amount of 1 mg cm −2 ( Figure 4f) and a comparable rever ble capacity of 696.9 mAh g −1 with a higher loading amount of 3 mg cm −2 ( Figure S5). T comparison of the electrochemical performance between the state-of-the-art SiOx el trodes and the NG/SiOx/NG electrode also verified the superior lithium storage propert of NG/SiOx/NG (Table S1).
To understand the dynamic electrochemical properties of the NG/SiOx/NG electrod the electrochemical kinetics and charge storage mechanism were investigated and demo strated in Figures 5 and 6. Semicircles in the high-frequency region corresponding to t SEI resistance (Rf) and charge transfer resistance (Rct) could be observed in the Nyqu diagrams of the G/SiOx/G and NG/SiOx/NG electrodes after 450 cycles ( Figure 5 [3,52,53]. In the corresponding fresh cells, no semicircles assigned to Rf were detected sin the absence of SEI (Figure 5a) [52]. Smaller Rf and Rct values (Table S2)   The rate performance and cyclic performance in Figure 4c-f further verified the superior electrochemical performance of NG/SiO x /NG. The capacities of the G/SiO x /G electrode decreased quickly with increasing densities (Figure 4c,d) and, typically, the average capacities were 928.4, 619.6, 479.2, 463.8, 362.1, 276.5 mAh g −1 at 0.1, 0.2, 0.3, 0.5, 1, and 2 A g −1 , respectively; when the current density recovered to 0.2 A g −1 from 2 A g −1 , the capacity was 561.1 mAh g −1 . After 100 cycles at 0.5 A g −1 , the specific capacity of G/SiO x /G decreased dramatically, and only a stable capacity of 212.5 mAh g −1 was retained after 450 cycles (Figure 4f). In contrast, the NG/SiO x /NG demonstrated higher capacities of 1197.6, 1002.3, 978.9, 803.3, 643.1, and 545.8 mAh g −1 at above current densities; when the current density recovered to 0.2 A g −1 from 2 A g −1 , a high capacity of 1072.6 mAh g −1 was obtained, indicating the high reversibility of the NG/SiO x /NG electrode (Figure 4c,e). Moreover, after 450 cycles at 0.5 A g −1 , the NG/SiO x /NG electrode maintained a high reversible capacity of 798.9 mAh g −1 with a loading amount of 1 mg cm −2 ( Figure 4f) and a comparable reversible capacity of 696.9 mAh g −1 with a higher loading amount of 3 mg cm −2 ( Figure S5). The comparison of the electrochemical performance between the state-of-the-art SiO x electrodes and the NG/SiO x /NG electrode also verified the superior lithium storage properties of NG/SiO x /NG (Table S1).
To understand the dynamic electrochemical properties of the NG/SiO x /NG electrode, the electrochemical kinetics and charge storage mechanism were investigated and demonstrated in Figures 5 and 6. Semicircles in the high-frequency region corresponding to the SEI resistance (R f ) and charge transfer resistance (R ct ) could be observed in the Nyquist diagrams of the G/SiO x /G and NG/SiO x /NG electrodes after 450 cycles (Figure 5b) [3,52,53]. In the corresponding fresh cells, no semicircles assigned to R f were detected since the absence of SEI (Figure 5a) [52]. Smaller R f and R ct values (Table S2) of the Li//NG/SiO x /NG cell than those of the Li//G/SiO x /G cell indicated the enhanced conductivity of the NG/SiO x /NG composite by N doping, which resulted in its fast electrode kinetics [3,10,17,52]. Slope lines in the low-frequency region are ascribed to Warburg impedance (Z w ), which is associated with Li + diffusion in bulk electrode materials [18,47,54]. And the lithium ion diffusion coefficient (D Li+ ) could be calculated with Equation (4) through linear fitting between Z' and ω −1/2 (ω = 2πf) in low frequencies of Nyquist diagrams [1,3]. In Equation (4), R, T, A, n, F, and C are the gas constant, absolute temperature, surface area of the electrode, number of transferred electrons per molecule in the material, Faraday's constant and concentration of Li + , respectively; while σ represented the Warburg coefficients and could be determined by the slopes of Z − ω −1/2 plots (Figure 5c,d) [3,29,52]. According to Equation (4), for a given electrode, the σ −2 value is proportional to D Li+ while all the other parameters are constants [1,18,55]. The much smaller σ −2 values (Table S2) of the NG/SiO x /NG electrode indicated the faster diffusion of the Li + in NG/SiO x /NG electrode [53,54,56]. The lower electrode impedance and faster diffusion of Li + contributed to the enhanced electrochemical capacity and rate capability of the NG/SiO x /NG electrode.  (Figure 5c,d) [3,29,52]. According to Equation (4), for a given electrode, the σ −2 value is proportional to DLi+ while all the other parameters are constants [1,18,55]. The much smaller σ −2 values (Table S2) of the NG/SiOx/NG electrode indicated the faster diffusion of the Li + in NG/SiOx/NG electrode [53,54,56]. The lower electrode impedance and faster diffusion of Li + contributed to the enhanced electrochemical capacity and rate capability of the NG/SiOx/NG electrode. To have thorough insights into the underlying mechanism of the superior electrochemical performance of the NG/SiOx/NG electrode, CV curves at scan rates ranging from 0.1 to 2 mV s −1 were measured to estimate the charge storage process. The anodic and cathodic peaks exhibited similar shapes with smaller potential shifts with increasing scan rates (Figure 6a,b) in the NG/SiOx/NG electrode, indicating its faster reaction kinetic and lower polarization during the lithiation/delithiation process [1,57]. To date, two charge storage mechanisms of the diffusion-controlled intercalation/alloying/conversion process and the capacitance-controlled pseudocapacitive process are involved in various electrode materials for LIBs [23,43,58]. The power-law relationship between the measured current (i) and sweep rates (v) is generally expressed as Equation (5), where a and b are adjustable parameters [1,43,57,59]. The b values could be determined by slopes of log (v) − log (i) plots, and two typical b values of 1 and 0.5 indicated that the electrode was dominated by capacitive behaviors and diffusive behaviors, respectively [45,54,57]. The calculated b values at low potentials are 0.72 and 0.55 in the NG/SiOx/NG and G/SiOx/G electrodes ( Figure  6c), indicating the hybrid contributions of diffusion-controlled behavior and capacitancecontrolled behavior to the total capacity; and the larger b value for the NG/SiOx/NG electrode suggested that the charge storage was mainly dominated by surface capacitancecontrolled behavior, which would result in a faster electrode kinetic [23,29,52].
To have thorough insights into the underlying mechanism of the superior electrochemical performance of the NG/SiO x /NG electrode, CV curves at scan rates ranging from 0.1 to 2 mV s −1 were measured to estimate the charge storage process. The anodic and cathodic peaks exhibited similar shapes with smaller potential shifts with increasing scan rates (Figure 6a,b) in the NG/SiO x /NG electrode, indicating its faster reaction kinetic and lower polarization during the lithiation/delithiation process [1,57]. To date, two charge storage mechanisms of the diffusion-controlled intercalation/alloying/conversion process and the capacitance-controlled pseudocapacitive process are involved in various electrode materials for LIBs [23,43,58]. The power-law relationship between the measured current (i) and sweep rates (v) is generally expressed as Equation (5), where a and b are adjustable parameters [1,43,57,59]. The b values could be determined by slopes of log (v) − log (i) plots, and two typical b values of 1 and 0.5 indicated that the electrode was dominated by capacitive behaviors and diffusive behaviors, respectively [45,54,57]. The calculated b values at low potentials are 0.72 and 0.55 in the NG/SiO x /NG and G/SiO x /G electrodes (Figure 6c), indicating the hybrid contributions of diffusion-controlled behavior and capacitance-controlled behavior to the total capacity; and the larger b value for the NG/SiO x /NG electrode sug-gested that the charge storage was mainly dominated by surface capacitance-controlled behavior, which would result in a faster electrode kinetic [23,29,52].
The quantitative contribution of surface capacitance-controlled behavior and diffusioncontrolled behavior could be calculated by separating the total current response at a fixed potential according to Equation (6), where i (V), k 1 v and k 2 v 1/2 represented the total current, capacitance contributed current and diffusion contributed current, respectively [29,45,52]. According to Equation (6), k 1 values at different voltages could be determined by the slopes of i (V)/v 1/2 − v 1/2 plots, and then k 1 v − V plots at different scan rates could be obtained. The pseudocapacitive contribution ratios at different scan rates could be quantified by the area ratios of k 1 v − V plots (which were filled with a blue color in Figure 6d,e) and CV curves. The calculated capacity contribution from the diffusion-controlled behavior and surface-capacitive behavior (blue area) at 1 mV s −1 are demonstrated in Figure 6d,e. The surface-capacitive behavior contributed 60.7% and 70.6% to the total storage capacity at 1 mV s −1 for the G/SiO x /G and NG/SiO x /NG electrodes, respectively; and the capacitive contribution ratios to total stored charge increased with increasing scan rates, implying that capacitive charge-storage mechanism mainly dominated the total charge storage; especially for the NG/SiO x /NG electrode, which possessed higher capacitive contribution ratios at all scan rates [1,45,57]. The above electrochemical charge storage mechanism analysis further revealed the fast electrochemical kinetics and explained the enhanced rate capability of the NG/SiO x /NG electrode exhibited in Figure 4. The probable reasons might be that N-doped rGO with high conductivity provided an affluent high-speed path to lowering the electrode impedance for ion diffusion. The quantitative contribution of surface capacitance-controlled behavior and diffusion-controlled behavior could be calculated by separating the total current response at a fixed potential according to Equation (6), where i (V), k1v and k2v 1/2 represented the total current, capacitance contributed current and diffusion contributed current, respectively [29,45,52]. According to Equation (6), k1 values at different voltages could be determined by the slopes of i (V)/v 1/2 − v 1/2 plots, and then k1v − V plots at different scan rates could be obtained. The pseudocapacitive contribution ratios at different scan rates could be quantified by the area ratios of k1v − V plots (which were filled with a blue color in Figure 6d,e) and CV curves. The calculated capacity contribution from the diffusion-controlled behavior and surface-capacitive behavior (blue area) at 1 mV s −1 are demonstrated in Figure 6d,e. The surface-capacitive behavior contributed 60.7% and 70.6% to the total storage capacity at 1 mV s −1 for the G/SiOx/G and NG/SiOx/NG electrodes, respectively; and the capacitive contribution ratios to total stored charge increased with increasing scan rates, implying that capacitive charge-storage mechanism mainly dominated the total charge storage; especially for the NG/SiOx/NG electrode, which possessed higher capacitive contribution ratios at all scan rates [1,45,57]. The above electrochemical charge storage mechanism analysis further revealed the fast electrochemical kinetics and explained the enhanced rate capability of the NG/SiOx/NG electrode exhibited in Figure 4. The probable reasons might be that N-doped rGO with high conductivity provided an affluent high-speed path to lowering the electrode impedance for ion diffusion. The morphological stability of the G/SiOx/G and NG/SiOx/NG electrodes after 450 cycles were carried out to deeply understand the enhanced electrochemical performance by N doping into G/SiOx/G. The thickness of the cycled NG/SiOx/NG only increased from 10.7 μm to 11.6 μm with a 17.7% volume expansion; while the G/SiOx/G electrode displayed a larger swelling rate of 146.5% and a peel-off phenomenon of the electrode materials from Cu foil, resulting in loss of the electrical contact and the fast capacity fading during cycling ( Figure S6a-d) [10,20]. Furthermore, after 450 cycles, the sandwich struc- The morphological stability of the G/SiO x /G and NG/SiO x /NG electrodes after 450 cycles were carried out to deeply understand the enhanced electrochemical performance by N doping into G/SiO x /G. The thickness of the cycled NG/SiO x /NG only increased from 10.7 µm to 11.6 µm with a 17.7% volume expansion; while the G/SiO x /G electrode displayed a larger swelling rate of 146.5% and a peel-off phenomenon of the electrode materials from Cu foil, resulting in loss of the electrical contact and the fast capacity fading during cycling ( Figure S6a-d) [10,20]. Furthermore, after 450 cycles, the sandwich structure of NG/SiO x /NG maintained, while that of G/SiO x /G obviously collapsed, the stable sandwich structure relieved the mechanical stress caused by volumetric variation during the lithiation/delithiation process and contributed partially to the stable electrochemical performance of the NG/SiO x /NG nanocomposites ( Figure S6e-h) [10,20,47].
To demonstrate the potential application of NG/SiO x /NG, full-cells using NG/SiO x /NG as anode and commercial LiFePO 4 as cathode were assembled. The LiFePO 4 cathode delivered high discharge capacities of 147.6 mAh g −1 , 150 mAh g −1 , and 140 mAh g −1 at the 1st, 20th, and 50th cycles at 0.5 C (1 C = 170 mA g −1 ), respectively (Figure 7a). More importantly, the NG/SiO x /NG//LiFePO 4 full-cell outputted a stable voltage platform at 3.3 V, and the red-light-emitting diode bulb could be easily powered by the prepared NG/SiO x /NG//LiFePO 4 full-cell (Figure 7b). The results strongly indicated that the NG/SiO x /NG electrode was a potential practical anode for LIBs. To demonstrate the potential application of NG/SiOx/NG, full-cells using NG/SiOx/NG as anode and commercial LiFePO4 as cathode were assembled. The LiFePO4 cathode delivered high discharge capacities of 147.6 mAh g −1 , 150 mAh g −1 , and 140 mAh g −1 at the 1st, 20th, and 50th cycles at 0.5 C (1 C = 170 mA g −1 ), respectively (Figure 7a). More importantly, the NG/SiOx/NG//LiFePO4 full-cell outputted a stable voltage platform at 3.3 V, and the red-light-emitting diode bulb could be easily powered by the prepared NG/SiOx/NG//LiFePO4 full-cell (Figure 7b). The results strongly indicated that the NG/SiOx/NG electrode was a potential practical anode for LIBs.

Conclusions
Sandwich-like NG/SiOx/NG was successfully fabricated through a freeze-drying combined thermal decomposition strategy. When used as the anodes for LIBs, a high specific capacity of 545 mAh g −1 at 2 A g −1 , stable capacity of 799 mAh g −1 after 450 cycles at 0.5 A g −1 and high areal capacity of 2.06 mAh cm −2 after 450 cycles at 1.5 mA cm −2 were obtained in half-cells; NG/SiOx/NG//LiFePO4 full-cells exhibited a high output voltage of 3.3 V and a tolerable specific capacity. The superior electrochemical performance can be attributed to the following features of the NG/SiOx/NG electrode: (i) the uniform and agglomeration-free package of SiOx into rGO efficiently improved the electric conductivity of the electrode, lowering the electrode impedance and favoring the fast electrochemical kinetics; (ii) the stable sandwich structure relieved the mechanical stress caused by volumetric variation during the lithiation/delithiation process, resulting in the prolonged cyclic life; (iii) nano Si produced from the disproportionation reaction of SiOx and defects originated from N doping created more active sites for ion storage, resulting in the high specific capacity; (iv) N doping induced a strong capacitive contribution to ion storage, improving the electrochemical kinetics. The green, scalable fabrication strategy and superior electrochemical performance indicated that the NG/SiOx/NG electrode should be a promising practicable electrode for LIBs. In addition, the simple and universal strategy can also be expanded to the fabrication of other electrodes (Si, Sn and Ge etc.) with high densities.
Supplementary Materials: The following are available online at www.mdpi.com/article/10.3390/nano11092366/s1, Figure S1: PXRD patterns of GO obtained from thermal drying of GO dispersion and rGO from calcination of GO in Ar at 650 o C. Figure S2: TGA curves and N2 adsorption-desorption isotherms of G/SiOx/G and NG/SiOx/NG. Figure S3: SEM and TEM pictures of G/SiOx/G and NG/SiOx/NG. Figure S4: GCD curves of SiOx, G/SiOx/G and NG/SiOx/NG at 0.1 A g −1 . Figure S5: Cyclic performance of NG/SiOx/NG electrode with loading amount of 1 mg cm −2 , 3 mg

Conclusions
Sandwich-like NG/SiO x /NG was successfully fabricated through a freeze-drying combined thermal decomposition strategy. When used as the anodes for LIBs, a high specific capacity of 545 mAh g −1 at 2 A g −1 , stable capacity of 799 mAh g −1 after 450 cycles at 0.5 A g −1 and high areal capacity of 2.06 mAh cm −2 after 450 cycles at 1.5 mA cm −2 were obtained in half-cells; NG/SiO x /NG//LiFePO 4 full-cells exhibited a high output voltage of 3.3 V and a tolerable specific capacity. The superior electrochemical performance can be attributed to the following features of the NG/SiO x /NG electrode: (i) the uniform and agglomeration-free package of SiO x into rGO efficiently improved the electric conductivity of the electrode, lowering the electrode impedance and favoring the fast electrochemical kinetics; (ii) the stable sandwich structure relieved the mechanical stress caused by volumetric variation during the lithiation/delithiation process, resulting in the prolonged cyclic life; (iii) nano Si produced from the disproportionation reaction of SiO x and defects originated from N doping created more active sites for ion storage, resulting in the high specific capacity; (iv) N doping induced a strong capacitive contribution to ion storage, improving the electrochemical kinetics. The green, scalable fabrication strategy and superior electrochemical performance indicated that the NG/SiO x /NG electrode should be a promising practicable electrode for LIBs. In addition, the simple and universal strategy can also be expanded to the fabrication of other electrodes (Si, Sn and Ge etc.) with high densities.