Silicon Oxycarbide-Graphite Electrodes for High-Power Energy Storage Devices

Herein we present a study on polymer-derived silicon oxycarbide (SiOC)/graphite composites for a potential application as an electrode in high power energy storage devices, such as Lithium-Ion Capacitor (LIC). The composites were processed using high power ultrasound-assisted sol-gel synthesis followed by pyrolysis. The intensive sonication enhances gelation and drying process, improving the homogenous distribution of the graphitic flakes in the preceramic blends. The physicochemical investigation of SiOC/graphite composites using X-ray diffraction, 29Si solid state NMR and Raman spectroscopy indicated no reaction occurring between the components. The electrochemical measurements revealed enhanced capacity (by up to 63%) at high current rates (1.86 A g−1) recorded for SiOC/graphite composite compared to the pure components. Moreover, the addition of graphite to the SiOC matrix decreased the value of delithiation potential, which is a desirable feature for anodes in LIC.


Introduction
Over the past few years, there has been increasing interest in small high-power energy storage devices. For these applications electrochemical capacitors (ECs) are considered to be more suitable than conventional batteries. Unfortunately, ECs possess much lower energy density in comparison to batteries, namely 3-6 Wh kg −1 vs. 150-200 Wh kg −1 , respectively [1,2], which severely limits their broader application. Many attempts have been made to enhance ECs energy density. Realizing an asymmetric system by employing a Faradaic, battery-like anode as a negative electrode is one of the most promising concepts. This idea was first presented by Amatucci in 2001 [3], and it is commonly known as an EC-battery hybrid or lithium-ion capacitor (LIC).
The main advantage of using a battery-like anode instead of the typical EC's electrode is the extension of the overall potential difference. ECs operates from 0 to 2.7 V (in organic electrolytes) in charge/discharge processes, which results in diminishing potential difference between the electrodes during cycling and hence in limited energy density [4]. On the contrary, battery-like anodes hold constant potential during charge/discharge, which allows for keeping a higher potential difference over the entire cycling process. LIC preserves high power of EC thanks to processes separation-lithiation/delithiation occurs in anode and PF − 6 adsorption/desorption on cathode [5][6][7][8][9]. However, due to differences in Figure 1 presents the thermographs of the investigated materials, i.e., SiOC PhTES , SiOC PhTES /C2g, SiOC PhTES /C4g, and SiOC PhTES /C10g. The analysis provides information about mass loss during the pyrolysis process. It shows the influence of graphite addition on a thermal conversion yield of polymer/graphite blends. Pure SiOC PhTES exhibited two significant mass losses: the first one between 50 • C and 350 • C, and the second one in the temperature range of 400-600 • C. The first one is attributed to the release of by-products of the polycondensation reaction and the residual EtOH/H 2 O. The second one is related to the mass loss occurring during the redistribution reaction, i.e., the exchange of Si-O bonds with Si-H and/or Si-C bonds, with the simultaneous release of volatile compounds, mainly H 2 , CH 4 or C 2 H 2 [48,49]. The overall ceramic yield of SiOC PhTES was around 60.9%, which stays in accordance with the literature [47,50]. When graphite was added to the material, we observed a proportional increase of the mass yield during pyrolysis. The mass yield of SiOC PhTES /C2g, SiOC PhTES /C4g and SiOC PhTES /C10g was equal to 77.4%, 79.5% and 83.7%, respectively. This is mostly related to a smaller mass loss in the temperature range of 400-500 • C, which is a consequence of a lower PhTES content in the preceramic polymer/graphite blends. These results indicate that the content of graphite in the preceramic matrix did not change during pyrolysis in an argon atmosphere, and the final mass yield was directly related to the starting material composition. Energy-dispersive X-ray spectroscopy EDS (EDAX, RTEM model SN9577, 134 eV) was used to identify the chemical elements in designated areas. Measurements were made in the TEM mode (bright-field) and the STEM mode (HAADF and EDX detectors). Preparation of the samples was as follows: a few milligrams of the powder were dispersed in ethanol (99.8% anhydrous) with the aid of ultrasounds for 5 s, and a drop of the dispersion (5 mL) was applied on a carbon-coated copper mesh with holes (Lacey type Cu 400 mesh, Plano, TX, USA), and stored in the room temperature until the complete evaporation of solvent. MAS-NMR measurements were performed on the Bruker Avance Ultrashield 500 MHz spectrometer. 29 Si NMR spectra were recorded with the following parameters: single pulse sequence, 29 Si frequency: 139.11 MHz, π/8 pulse length: 2.5 ms, recycle delay: 100 s, 1k scans, external secondary reference: DSS. 3.2 mm zirconia rotors filled with samples were spun at 8 kHz under air flow. Figure 1 presents the thermographs of the investigated materials, i.e., SiOCPhTES, SiOCPhTES/C2g, SiOCPhTES/C4g, and SiOCPhTES/C10g. The analysis provides information about mass loss during the pyrolysis process. It shows the influence of graphite addition on a thermal conversion yield of polymer/graphite blends. Pure SiOCPhTES exhibited two significant mass losses: the first one between 50 °C and 350 °C, and the second one in the temperature range of 400-600 °C. The first one is attributed to the release of by-products of the polycondensation reaction and the residual EtOH/H2O. The second one is related to the mass loss occurring during the redistribution reaction, i.e., the exchange of Si-O bonds with Si-H and/or Si-C bonds, with the simultaneous release of volatile compounds, mainly H2, CH4 or C2H2 [48,49]. The overall ceramic yield of SiOCPhTES was around 60.9%, which stays in accordance with the literature [47,50]. When graphite was added to the material, we observed a proportional increase of the mass yield during pyrolysis. The mass yield of SiOCPhTES/C2g, SiOCPhTES/C4g and SiOCPhTES/C10g was equal to 77.4%, 79.5% and 83.7%, respectively. This is mostly related to a smaller mass loss in the temperature range of 400-500 °C, which is a consequence of a lower PhTES content in the preceramic polymer/graphite blends. These results indicate that the content of graphite in the preceramic matrix did not change during pyrolysis in an argon atmosphere, and the final mass yield was directly related to the starting material composition. The TGA results stay in agreement with the results of the elemental analysis shown in Table 1. As expected, the increase in the amount of graphite in the preceramic blends led to a higher amount of carbon in a final ceramic composite. Free carbon content in the pure SiOCPhTES ceramic, calculated using the approach of Soraru et al. [51], was 34.2 wt.%, which represents 91% of the total carbon in the sample. For the SiOC/graphite composites, the free carbon phase constituted from 95% (for SiOCPhTES/C2g) to almost 100% (for SiOCPhTES/C10g) of the total carbon. The TGA results stay in agreement with the results of the elemental analysis shown in Table 1. As expected, the increase in the amount of graphite in the preceramic blends led to a higher amount of carbon in a final ceramic composite. Free carbon content in the pure SiOC PhTES ceramic, calculated using the approach of Soraru et al. [51], was 34.2 wt.%, which represents 91% of the total carbon in the sample. For the SiOC/graphite composites, the free carbon phase constituted from 95% (for SiOC PhTES /C2g) to almost 100% (for SiOC PhTES /C10g) of the total carbon.  Figure 2 shows the corresponding 29 Si MAS-NMR spectra of SiOC PhTES and SiOC PhTES /C2g. The 29 Si MAS-NMR spectrum of the SiOC PhTES /C2g composite exhibits broader peaks at approximately −109, −73 and −48 ppm, and higher noise than the spectrum of the pure SiOC PhTES ceramic sample. However, the share of SiO 4 and mixed bonds SiO 3 C, SiO 2 C 2 tetrahedra is comparable for both samples ( Table 2 and ref. [47]). SiO 4 tetrahedra dominate in both samples (76%-78%), and the mixed bonds constitute from several up to over a dozen percent. This suggests that there was no reaction between the preceramic polymer and graphite at any of the stages of preparation of the composites, namely during hydrolysis and condensation reactions, high-power ultrasound-assisted gelation process, nor pyrolysis at 1000 • C.  29 Silicon Solid-state NMR measurements ( 29 Si MAS-NMR) were conducted in order to determine the change in the redistribution of various SiOxCy tetrahedra units resulting from the addition of graphite. Figure 2 shows the corresponding 29 Si MAS-NMR spectra of SiOCPhTES and SiOCPhTES/C2g. The 29 Si MAS-NMR spectrum of the SiOCPhTES/C2g composite exhibits broader peaks at approximately −109, −73 and −48 ppm, and higher noise than the spectrum of the pure SiOCPhTES ceramic sample. However, the share of SiO4 and mixed bonds SiO3C, SiO2C2 tetrahedra is comparable for both samples ( Table 2 and ref. [47]). SiO4 tetrahedra dominate in both samples (76%-78%), and the mixed bonds constitute from several up to over a dozen percent. This suggests that there was no reaction between the preceramic polymer and graphite at any of the stages of preparation of the composites, namely during hydrolysis and condensation reactions, high-power ultrasound-assisted gelation process, nor pyrolysis at 1000 °C.  XPS results (presented in Supplementary Materials-SM, Figure S1) confirm the collected NMR data. XPS Si2p spectra of pure ceramic and the composite samples look almost the same. A broad peak fitted with the Si2p3/2 and Si2p1/2 doublet at binding energies of 103.2-103.4 and 103.7-103.8 eV, respectively, corresponding to SiO4 tetrahedra [52,53] is observed. On the other hand, the C1s spectra of the pure ceramic sample and SiOC/graphite composite show some differences in the share of particular bonds. Both C1s spectra ( Figure S2) were deconvoluted with four peaks at BE: 284.0-284.4 eV, 285.2-285.4 eV, 286.5-286.7 eV and 288.8-289.2 eV, which may be attributed to C-Si/C=C, C-C/C-H, C-O and C=O bonds, respectively [54][55][56][57]. However, the C1s spectrum of the SiOC/graphite  XPS results (presented in Supplementary Materials-SM, Figure S1) confirm the collected NMR data. XPS Si2p spectra of pure ceramic and the composite samples look almost the same. A broad peak fitted with the Si2p 3/2 and Si2p 1/2 doublet at binding energies of 103.2-103.4 and 103.7-103.8 eV, respectively, corresponding to SiO 4 tetrahedra [52,53] is observed. On the other hand, the C1s spectra of the pure ceramic sample and SiOC/graphite composite show some differences in the share of particular bonds. Both C1s spectra ( Figure S2) were deconvoluted with four peaks at BE: 284.0-284.4 eV, 285.2-285.4 eV, 286.5-286.7 eV and 288.8-289.2 eV, which may be attributed to C-Si/C=C, C-C/C-H, C-O and C=O bonds, respectively [54][55][56][57]. However, the C1s spectrum of the SiOC/graphite composite shows a significantly larger peak corresponding to the C-C bond, and smaller peaks assigned to the C-Si, C-O and C=O bonds, which is related to a high graphite content in the composite.

Results and Discussion
The homogenization method applied for blending of the materials requires high power ultrasound, which can have a destructive effect on the graphite structure. To evaluate possible changes in the graphite lattice XRD measurements were performed. Figure 3 [58,59]. In contrast, pure SiOC PhTES diffractograms show only a broad halo typical for amorphous materials [51,60], while for the SiOC/graphite composites the peaks typical for graphite appear. The higher amount of graphite in the composite, the more pronounced reflexes in the diffractograms are detected. The diffractogram of SiOC PhTES /C10g exhibits sharp peaks, which indicate that the skeleton of the graphite structure was preserved despite the usage of high-power ultrasounds during the synthesis.
Materials 2020, 13, x FOR PEER REVIEW 6 of 18 composite shows a significantly larger peak corresponding to the C-C bond, and smaller peaks assigned to the C-Si, C-O and C=O bonds, which is related to a high graphite content in the composite. The homogenization method applied for blending of the materials requires high power ultrasound, which can have a destructive effect on the graphite structure. To evaluate possible changes in the graphite lattice XRD measurements were performed. Figure 3 shows the diffractograms of pure graphite, ceramic, and SiOCPhTES/graphite composites. For the graphite sample sharp peaks at 12°, 19 [58,59]. In contrast, pure SiOCPhTES diffractograms show only a broad halo typical for amorphous materials [51,60], while for the SiOC/graphite composites the peaks typical for graphite appear. The higher amount of graphite in the composite, the more pronounced reflexes in the diffractograms are detected. The diffractogram of SiOCPhTES/C10g exhibits sharp peaks, which indicate that the skeleton of the graphite structure was preserved despite the usage of high-power ultrasounds during the synthesis. More detailed analysis of the carbon microstructure was performed by means of Raman spectroscopy. Figure 4a depicts Raman spectra of the investigated samples recorded in the range of 500-3000 cm −1 after the background subtraction. Two characteristic bands in the first-order spectra, namely the D-band at approximately 1333 cm −1 , and the G-band at 1575 cm −1 , representing disordered and graphitic carbon, respectively, appear for all of the investigated composites. In order to perform a quantitative analysis of the measured spectra, we deconvoluted the first-order Raman signals into five peaks (Figure 4b-e), namely D1, D2, D3, D4 and G according to [61,62]. More detailed analysis of the carbon microstructure was performed by means of Raman spectroscopy. Figure 4a depicts Raman spectra of the investigated samples recorded in the range of 500-3000 cm −1 after the background subtraction. Two characteristic bands in the first-order spectra, namely the D-band at approximately 1333 cm −1 , and the G-band at 1575 cm −1 , representing disordered and graphitic carbon, respectively, appear for all of the investigated composites. In order to perform a quantitative analysis of the measured spectra, we deconvoluted the first-order Raman signals into five peaks (Figure 4b-e), namely D1, D2, D3, D4 and G according to [61,62].
In the spectra of the pure ceramic and the composites samples, the D band is separated into a main D1 peak and a small shoulder marked as D4. Both peaks, i.e., D1 and D4, result from a graphitic lattice vibration mode with A 1g -symmetry, which is typical for disordered carbons. D1 is assigned to graphene edges [63][64][65] while the D4 is related to Csp 2 -Csp 3 bonds [66] or ionic impurities [67]. The G band was deconvoluted into a G peak, arising from stretching vibrations of the sp 2 -carbon bond in the ideal graphitic lattice (E 2g symmetry), and a D2 peak, related to the defected graphitic lattice (also E 2g symmetry) [61,64,65] and Stone-Wales defects [68], which is also present in the deconvoluted spectrum of the pure graphite sample ( Figure S3a in Supplementary Materials). Moreover, between the D1 and G bands another band, namely a D3 band at~1525 cm −1 , sp 2 amorphous forms of carbon [61,62], is detected. Detailed information on band parameters is given in Table 3 and Table S1 in the Supplementary Materials. More detailed analysis of the carbon microstructure was performed by means of Raman spectroscopy. Figure 4a depicts Raman spectra of the investigated samples recorded in the range of 500-3000 cm −1 after the background subtraction. Two characteristic bands in the first-order spectra, namely the D-band at approximately 1333 cm −1 , and the G-band at 1575 cm −1 , representing disordered and graphitic carbon, respectively, appear for all of the investigated composites. In order to perform a quantitative analysis of the measured spectra, we deconvoluted the first-order Raman signals into five peaks (Figure 4b-e), namely D1, D2, D3, D4 and G according to [61,62]. In the spectra of the pure ceramic and the composites samples, the D band is separated into a main D1 peak and a small shoulder marked as D4. Both peaks, i.e., D1 and D4, result from a graphitic lattice vibration mode with A1g-symmetry, which is typical for disordered carbons. D1 is assigned to graphene edges [63][64][65] while the D4 is related to Csp 2 -Csp 3 bonds [66] or ionic impurities [67]. The G band was deconvoluted into a G peak, arising from stretching vibrations of the sp 2 -carbon bond in the ideal graphitic lattice (E2g symmetry), and a D2 peak, related to the defected graphitic lattice (also E2g symmetry) [61,64,65] and Stone-Wales defects [68], which is also present in the deconvoluted spectrum of the pure graphite sample ( Figure S3a in Supplementary Materials). Moreover, between the D1 and G bands another band, namely a D3 band at ~1525 cm −1 , sp 2 amorphous forms of carbon [61,62], is detected. Detailed information on band parameters is given in Table 3 and Table S1 in the Supplementary Materials. Table 3. Data obtained from the Raman spectra deconvolution of the investigated samples, band positions, band intensities (I) (height of the fitted peak) and intensity ratio (ID1/IG and ID2/IG). With the increasing amount of graphite in the composites one can notice a significant rise in the G band intensity, from 0.36 for the pure ceramic SiOC PhTES to 0.68 for SiOC PhTES /C10g, along with a decrease in the intensity of the D1 and D4 bands, from 0.98 to 0.84 and from 0.084 to 0.075 for these materials, respectively. These differences are even more pronounced when we compare the I D1 /I G and I D2 /I G intensity ratios that decreased with the increasing graphite content in the ceramic matrix. The I D1 /I G ratio dropped from 2.69 for the pure ceramic to 1.24 for the SiOC PhTES /C10g sample, whereas I D2 /I G decreased from 1.61 to 0.76. Moreover, composites with graphite exhibit lower intensity of the D3 band compared to the pure ceramics. These results prove that the addition of graphite to the composites increased the content of the ordered carbon phase in the material, suggesting that the graphitic structure was hardly affected by the ultrasound treatment. Table 3. Data obtained from the Raman spectra deconvolution of the investigated samples, band positions, band intensities (I) (height of the fitted peak) and intensity ratio (I D1 /I G and I D2 /I G ).

D4
D1 Furthermore, we tested the influence of ultrasounds on the size of graphite flakes used for the synthesis of the composites. In that case, we immersed graphitic powder into test tubes with isopropanol and subjected it to ultrasounds for a time of up to 2 h (analogous to the time used for composite blending). After drying, the morphology of the graphite powder was examined by SEM and no changes with respect to the pristine graphitic morphology were identified (see SEM Figure S4a,b, Supplementary Materials). This suggests that the structure of graphite was preserved in the composites.
In the second-order Raman spectra, the bands at 2700 cm −1 (2D) and 2900 cm −1 (D + G) were detected, i.e., the overtones of D-band along with combined D and G bands, respectively. The 2D band correlates with the number of stacked carbon layers that make up graphite clusters. The second-order Raman spectra are presented in more detail in Figure S3b (Supplementary Materials). Graphite exhibits a very pronounced 2D peak at 2702 cm −1 , indicating stacked undamaged graphene layers. In contrast, the SiOC PhTES shows an asymmetric and blurred 2D peak, indicating high disorder in the carbon structure. With increasing graphite content in the composites, the 2D band becomes narrower and more intense, signifying increased content of a more ordered carbon phase. The overall Raman results do not indicate any damages in the graphitic structure provoked by the ultrasound-assisted synthesis method, confirming the results obtained by NMR and XRD analysis.
TEM imaging was performed in order to investigate more deeply the microstructure of the composites. Figure 5a,b show TEM images of the amorphous organization of SiOC PhTES sample, (typical for pure silicon oxycarbide pyrolyzed at 1000 • C). On the other hand, two separate phases, representing the amorphous SiOC and the more ordered carbon material, are observed for the SiOC PhTES /C10g sample (Figure 5c,d). With the increasing amount of graphite in the composites one can notice a significant rise in the G band intensity, from 0.36 for the pure ceramic SiOCPhTES to 0.68 for SiOCPhTES/C10g, along with a decrease in the intensity of the D1 and D4 bands, from 0.98 to 0.84 and from 0.084 to 0.075 for these materials, respectively. These differences are even more pronounced when we compare the ID1/IG and ID2/IG intensity ratios that decreased with the increasing graphite content in the ceramic matrix. The ID1/IG ratio dropped from 2.69 for the pure ceramic to 1.24 for the SiOCPhTES/C10g sample, whereas ID2/IG decreased from 1.61 to 0.76. Moreover, composites with graphite exhibit lower intensity of the D3 band compared to the pure ceramics. These results prove that the addition of graphite to the composites increased the content of the ordered carbon phase in the material, suggesting that the graphitic structure was hardly affected by the ultrasound treatment.
Furthermore, we tested the influence of ultrasounds on the size of graphite flakes used for the synthesis of the composites. In that case, we immersed graphitic powder into test tubes with isopropanol and subjected it to ultrasounds for a time of up to 2 h (analogous to the time used for composite blending). After drying, the morphology of the graphite powder was examined by SEM and no changes with respect to the pristine graphitic morphology were identified (see SEM Figure  S4a,b, Supplementary Materials). This suggests that the structure of graphite was preserved in the composites.
In the second-order Raman spectra, the bands at 2700 cm −1 (2D) and 2900 cm −1 (D + G) were detected, i.e., the overtones of D-band along with combined D and G bands, respectively. The 2D band correlates with the number of stacked carbon layers that make up graphite clusters. The secondorder Raman spectra are presented in more detail in Figure S3b (Supplementary Materials). Graphite exhibits a very pronounced 2D peak at 2702 cm −1 , indicating stacked undamaged graphene layers. In contrast, the SiOCPhTES shows an asymmetric and blurred 2D peak, indicating high disorder in the carbon structure. With increasing graphite content in the composites, the 2D band becomes narrower and more intense, signifying increased content of a more ordered carbon phase. The overall Raman results do not indicate any damages in the graphitic structure provoked by the ultrasound-assisted synthesis method, confirming the results obtained by NMR and XRD analysis.
TEM imaging was performed in order to investigate more deeply the microstructure of the composites. Figure 5a,b show TEM images of the amorphous organization of SiOCPhTES sample, (typical for pure silicon oxycarbide pyrolyzed at 1000 °C). On the other hand, two separate phases, representing the amorphous SiOC and the more ordered carbon material, are observed for the SiOCPhTES/C10g sample (Figure 5c,d).  Figure 7a,b show CV plots obtained for the SiOCPhTES/graphite composites and pure components for comparison. During the first cycle, all the composites and pristine ceramic materials exhibited a small cathodic peak at 0.7 V (marked as (I) on the plot), which disappear in the following cycles. This corresponds to the formation of the solid-electrolyte interface (SEI) on the boundaries of the ceramic phase [69,70]. Further on the CV curves of the SiOC-based materials, a broad peak between 0 and 0.3 V (II), which corresponds to lithium insertion into the ceramic [29,69,71], is present. The cathodic current in this range decreases with the increasing graphite content in the composites, but there are no pronounced peaks at 0.16 and 0.05 V as registered for pure graphite. The intensity of the peak (II) is dropping in the following cycles (Figure 7b), which corresponds to the stabilization of the lithium insertion [72]. On the anodic site of the CV of the composites, we observe peaks between 0 and 0.25 V (III) corresponding to graphite delithiation [73] and a broad plateau-like peak (IV) characteristic  Figure 7a,b show CV plots obtained for the SiOCPhTES/graphite composites and pure components for comparison. During the first cycle, all the composites and pristine ceramic materials exhibited a small cathodic peak at 0.7 V (marked as (I) on the plot), which disappear in the following cycles. This corresponds to the formation of the solid-electrolyte interface (SEI) on the boundaries of the ceramic phase [69,70]. Further on the CV curves of the SiOC-based materials, a broad peak between 0 and 0.3 V (II), which corresponds to lithium insertion into the ceramic [29,69,71], is present. The cathodic current in this range decreases with the increasing graphite content in the composites, but there are no pronounced peaks at 0.16 and 0.05 V as registered for pure graphite. The intensity of the peak (II) is dropping in the following cycles (Figure 7b), which corresponds to the stabilization of the lithium insertion [72]. On the anodic site of the CV of the composites, we observe peaks between 0 and 0.25 V (III) corresponding to graphite delithiation [73] and a broad plateau-like peak (IV) characteristic  Figure 7a,b show CV plots obtained for the SiOC PhTES /graphite composites and pure components for comparison. During the first cycle, all the composites and pristine ceramic materials exhibited a small cathodic peak at 0.7 V (marked as (I) on the plot), which disappear in the following cycles. This corresponds to the formation of the solid-electrolyte interface (SEI) on the boundaries of the ceramic phase [69,70]. Further on the CV curves of the SiOC-based materials, a broad peak between 0 and 0.3 V (II), which corresponds to lithium insertion into the ceramic [29,69,71], is present. The cathodic current in this range decreases with the increasing graphite content in the composites, but there are no pronounced peaks at 0.16 and 0.05 V as registered for pure graphite. The intensity of the peak (II) is dropping in the following cycles (Figure 7b), which corresponds to the stabilization of the lithium insertion [72]. On the anodic site of the CV of the composites, we observe peaks between 0 and 0.25 V (III) corresponding to graphite delithiation [73] and a broad plateau-like peak (IV) characteristic for a typical lithium extraction from the ceramic [71]. The intensity of the peaks observed on CV curves depends on the composition of the material and follows the trend of increasing graphite content. For graphite-rich composites, one may identify more pronounced peaks (III), while for graphite-poor composites the peak (IV) is more intense.

Electrochemical Testing
Materials 2020, 13, x FOR PEER REVIEW 10 of 18 for a typical lithium extraction from the ceramic [71]. The intensity of the peaks observed on CV curves depends on the composition of the material and follows the trend of increasing graphite content. For graphite-rich composites, one may identify more pronounced peaks (III), while for graphite-poor composites the peak (IV) is more intense. The capacity distribution over the potential range of 0-3 V and the Coulombic efficiencies were assessed from charge/discharge profiles. Figure 8a,b show charge/discharge curves of the first and second cycles, respectively, while more detailed information is presented in Table 4. The first cycle lithiation profiles of the SiOCPhTES, SiOCPhTES/C2g and SiOCPhTES/C4g samples are very similar. A different curve shape with a quasi-plateau at around 0.7 V, related to SEI formation, is noticed for the SiOCPhTES/C10g sample. Galvanostatic charge-discharge curves correspond well to the cyclic voltammetry curves. The trends of increasing length of the plateau, corresponding to delithiation of the graphitic phase, and a decrease in the delithiation potential with increasing graphite content in the ceramic matrix can be identified. A low delithiation potential is one of the features expected for LIC anodes. The curves of the SiOCPhTES/C2g and SiOCPhTES/C4g samples exhibit small plateaus in the 0.11-0.15 V voltage range, followed by a more rapid voltage increase, while the plateau recorded for the SiOCPhTES/C10g composite is significantly longer, and the onset of the faster voltage rise is observed at 0.2 V. This results in the higher capacity recovered below 0.5 V for the SiOCPhTES/C10g sample compared to the composites and the pure ceramics.  The capacity distribution over the potential range of 0-3 V and the Coulombic efficiencies were assessed from charge/discharge profiles. Figure 8a,b show charge/discharge curves of the first and second cycles, respectively, while more detailed information is presented in Table 4. The first cycle lithiation profiles of the SiOC PhTES , SiOC PhTES /C2g and SiOC PhTES /C4g samples are very similar. A different curve shape with a quasi-plateau at around 0.7 V, related to SEI formation, is noticed for the SiOC PhTES /C10g sample. Galvanostatic charge-discharge curves correspond well to the cyclic voltammetry curves. The trends of increasing length of the plateau, corresponding to delithiation of the graphitic phase, and a decrease in the delithiation potential with increasing graphite content in the ceramic matrix can be identified. A low delithiation potential is one of the features expected for LIC anodes. The curves of the SiOC PhTES /C2g and SiOC PhTES /C4g samples exhibit small plateaus in the 0.11-0.15 V voltage range, followed by a more rapid voltage increase, while the plateau recorded for the SiOC PhTES /C10g composite is significantly longer, and the onset of the faster voltage rise is observed at 0.2 V. This results in the higher capacity recovered below 0.5 V for the SiOC PhTES /C10g sample compared to the composites and the pure ceramics.
Materials 2020, 13, x FOR PEER REVIEW 10 of 18 for a typical lithium extraction from the ceramic [71]. The intensity of the peaks observed on CV curves depends on the composition of the material and follows the trend of increasing graphite content. For graphite-rich composites, one may identify more pronounced peaks (III), while for graphite-poor composites the peak (IV) is more intense. The capacity distribution over the potential range of 0-3 V and the Coulombic efficiencies were assessed from charge/discharge profiles. Figure 8a,b show charge/discharge curves of the first and second cycles, respectively, while more detailed information is presented in Table 4. The first cycle lithiation profiles of the SiOCPhTES, SiOCPhTES/C2g and SiOCPhTES/C4g samples are very similar. A different curve shape with a quasi-plateau at around 0.7 V, related to SEI formation, is noticed for the SiOCPhTES/C10g sample. Galvanostatic charge-discharge curves correspond well to the cyclic voltammetry curves. The trends of increasing length of the plateau, corresponding to delithiation of the graphitic phase, and a decrease in the delithiation potential with increasing graphite content in the ceramic matrix can be identified. A low delithiation potential is one of the features expected for LIC anodes. The curves of the SiOCPhTES/C2g and SiOCPhTES/C4g samples exhibit small plateaus in the 0.11-0.15 V voltage range, followed by a more rapid voltage increase, while the plateau recorded for the SiOCPhTES/C10g composite is significantly longer, and the onset of the faster voltage rise is observed at 0.2 V. This results in the higher capacity recovered below 0.5 V for the SiOCPhTES/C10g sample compared to the composites and the pure ceramics.   Table 4. Reversible C rev and irreversible C irrev capacity values of the first cycle upon polarization with 0.186 A g −1 , Coulombic efficiency of the first cycle η and the average delithiation capacities CD of graphite, ceramic and composite samples measured at different current rates (average capacity values calculated from data presented in Figure 9).
Sample 1st Cycle C rev /mAh g −1 1st Cycle C irrev /mAh g −1 η Average CD  Table 4. Reversible Crev and irreversible Cirrev capacity values of the first cycle upon polarization with 0.186 A g −1 , Coulombic efficiency of the first cycle η and the average delithiation capacities CD of graphite, ceramic and composite samples measured at different current rates (average capacity values calculated from data presented in Figure 9).  The electrochemical contribution of both components, namely SiOC and graphite, is unambiguously exposed on charge-discharge curves of the investigated composites, and the contribution of each part is proportional to its content. For graphite-poor samples, a small plateau originating from graphite, and a long ascending curve typical for ceramics can be noticed, while in the case of graphite-rich sample a much larger plateau is observed.

Sample
The charge/discharge profile allows one to evaluate the Coulombic efficiency of the material. One could expect that the first cycle efficiency (FCE) should increase with the addition of graphite to the composites. However, the graphite flakes used for composite preparation exhibit relatively low Coulombic efficiency of the first cycle (lower than reported by electrode suppliers [74,75] or in literature [76,77]). This may be caused by lower crystallinity and higher surface area of the flakes than of graphite used in commercial batteries. Addition of small quantities of graphite seems to slightly increase the FCE (59.6% and 57.8% for the SiOCPhTES/C2g and SiOCPhTES/C4g, respectively, compared to 54.7% for the pure ceramic). However, the sample with the highest graphite content, i.e., the SiOCPhTES/C10g, showed the FCE of only 53.9%. This may be caused by higher activity at 0.7 V observed on the CV and GCD curves during the first cycle. In the following cycles, all studied materials show efficiency of over 99%. Extended cycling of the electrodes at the C/2 and 5C current rates (C = 372 mA g −1 ) is presented in Figure 9. All the SiOC-based materials show better electrochemical performance than graphite. The difference is even more pronounced at high current rates. Note that for the C/2 rate the highest capacity is recorded for the composite with the lowest graphite content (423 mAh g −1 for SiOCPhTES/C2g), while for the 5C rate, the graphite-rich composite has the highest capacity (293 mAh g −1 for SiOCPhTES/C10g compared to 218 mAh g −1 for the pure ceramic SiOCPhTES and only 108 mAh g −1 for the graphite electrode). High capacity is another feature of anodes that are suitable for high power energy devices. Better electrochemical performance of the SiOCPhTES/C10g sample compared to the SiOCPhTES one may follow from the carbon content in these materials. According to elemental analysis (Table 1), the amount of carbon increases from 37.4 wt.% for pure ceramic to 68.7 wt.% for the composites with the highest graphite content. A higher amount The electrochemical contribution of both components, namely SiOC and graphite, is unambiguously exposed on charge-discharge curves of the investigated composites, and the contribution of each part is proportional to its content. For graphite-poor samples, a small plateau originating from graphite, and a long ascending curve typical for ceramics can be noticed, while in the case of graphite-rich sample a much larger plateau is observed.
The charge/discharge profile allows one to evaluate the Coulombic efficiency of the material. One could expect that the first cycle efficiency (FCE) should increase with the addition of graphite to the composites. However, the graphite flakes used for composite preparation exhibit relatively low Coulombic efficiency of the first cycle (lower than reported by electrode suppliers [74,75] or in literature [76,77]). This may be caused by lower crystallinity and higher surface area of the flakes than of graphite used in commercial batteries. Addition of small quantities of graphite seems to slightly increase the FCE (59.6% and 57.8% for the SiOC PhTES /C2g and SiOC PhTES /C4g, respectively, compared to 54.7% for the pure ceramic). However, the sample with the highest graphite content, i.e., the SiOC PhTES /C10g, showed the FCE of only 53.9%. This may be caused by higher activity at 0.7 V observed on the CV and GCD curves during the first cycle. In the following cycles, all studied materials show efficiency of over 99%. Extended cycling of the electrodes at the C/2 and 5C current rates (C = 372 mA g −1 ) is presented in Figure 9. All the SiOC-based materials show better electrochemical performance than graphite. The difference is even more pronounced at high current rates. Note that for the C/2 rate the highest capacity is recorded for the composite with the lowest graphite content (423 mAh g −1 for SiOC PhTES /C2g), while for the 5C rate, the graphite-rich composite has the highest capacity (293 mAh g −1 for SiOC PhTES /C10g compared to 218 mAh g −1 for the pure ceramic SiOC PhTES and only 108 mAh g −1 for the graphite electrode). High capacity is another feature of anodes that are suitable for high power energy devices. Better electrochemical performance of the SiOC PhTES /C10g sample compared to the SiOC PhTES one may follow from the carbon content in these materials. According to elemental analysis (Table 1), the amount of carbon increases from 37.4 wt.% for pure ceramic to 68.7 wt.% for the composites with the highest graphite content. A higher amount of carbon in the ceramic matrix leads to a higher electronic conductivity and a larger number of diffusion paths for lithium ions, crucial upon polarization with high currents. Slightly lower capacity showed by the SiOC PhTES /C10g composites compared to the pure ceramic at the C/2 rate is probably due to a lower share of the active sites for lithium storage present in the ceramics [78]. Low capacity of pure graphite at a high current rate is often explained by material exfoliation caused by electrolyte penetration along with lithium ions [79].
To examine the phenomenon of high capacity at high currents for graphite rich composite, we took a closer look at the charge/discharge profiles collected upon polarization at a 5C rate ( Figure 10). GCD curves presented in Figure 10 were recorded after 20 cycles at a C/2 rate in order to show a stable response of the electrodes with Coulombic efficiency of over 99%. The shape of the composite curves at 5C exhibits the same tendency as the curves recorded at a C/2 rate (more extended plateau and a lower delithiation voltage for graphite-rich samples), except for the capacity value, which is the highest for the SiOC PhTES /C10g electrode. These features make the material promising for the application in LIC. What is also essential for the potential application in LIC, is that this composite delivers the highest capacity below 0.5 V.
Materials 2020, 13, x FOR PEER REVIEW 12 of 18 of carbon in the ceramic matrix leads to a higher electronic conductivity and a larger number of diffusion paths for lithium ions, crucial upon polarization with high currents. Slightly lower capacity showed by the SiOCPhTES/C10g composites compared to the pure ceramic at the C/2 rate is probably due to a lower share of the active sites for lithium storage present in the ceramics [78]. Low capacity of pure graphite at a high current rate is often explained by material exfoliation caused by electrolyte penetration along with lithium ions [79].
To examine the phenomenon of high capacity at high currents for graphite rich composite, we took a closer look at the charge/discharge profiles collected upon polarization at a 5C rate ( Figure 10). GCD curves presented in Figure 10 were recorded after 20 cycles at a C/2 rate in order to show a stable response of the electrodes with Coulombic efficiency of over 99%. The shape of the composite curves at 5C exhibits the same tendency as the curves recorded at a C/2 rate (more extended plateau and a lower delithiation voltage for graphite-rich samples), except for the capacity value, which is the highest for the SiOCPhTES/C10g electrode. These features make the material promising for the application in LIC. What is also essential for the potential application in LIC, is that this composite delivers the highest capacity below 0.5 V. In Table 5, the capacity values of the most popular potential anodes for LIC reported in the literature are presented. The focus was on the capacity measured at a 2 A g −1 current rate typical for testing of LIC [4], and voltage range below 0.5 V. To the best of our knowledge, the SiOCPhTES/C10g composite exhibits the highest capacity upon these conditions among all popular LIC anode materials. Our composite exhibits twice as high capacity as the competitive materials. Considering the potential range (0-3 V), our composite also shows a satisfactory capacity of almost 300 mAh g −1 .  In Table 5, the capacity values of the most popular potential anodes for LIC reported in the literature are presented. The focus was on the capacity measured at a 2 A g −1 current rate typical for testing of LIC [4], and voltage range below 0.5 V. To the best of our knowledge, the SiOC PhTES /C10g composite exhibits the highest capacity upon these conditions among all popular LIC anode materials. Our composite exhibits twice as high capacity as the competitive materials. Considering the potential range (0-3 V), our composite also shows a satisfactory capacity of almost 300 mAh g −1 .  Our work AC a -Activated carbon; b measurements started above 0.5 V, so we assume negligible capacity below 0.5 V. "~" sign appears for the values assessed from the graph presented in the cited paper.

Conclusions
In this work, we evaluated various graphite-ceramic based composites with different graphite content for potential application as anodes in LIC. The materials were prepared by a novel method utilizing high power ultrasounds. Sonication facilitates the gelation process and uniform distribution of graphite flakes within the preceramic polymer. Silicon oxycarbide is an electrochemically active component contributing to the capacity of composites and plays the additional role of a matrix for graphitic flakes, causing the stabilization of the electrochemical response at high current rates. Moreover, the addition of graphite to SiOC shifts lithiation and delithiation processes towards lower potentials in comparison to the pure SiOC. The best performing material seems to be the SiOC PhTES /C10g one, i.e., the composite with the highest investigated graphite content. This material is characterized by a high capacity of 294 mAh g −1 at a 5C current rate, among which over 160 mAh g −1 is recovered below 0.5 V vs. Li/Li + . This makes the SiOC PhTES /C10g material a potential candidate for anodes used in high power energy storage devices, e.g., lithium-ion capacitors.

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