Sustainable SiC Composite Anodes, Graphite Accelerated Lithium Storage

Realizing more holistic electri ﬁ cation in society to disengage current dependence on nonrenewable fuels requires balancing between energy storage mechanisms and actual environmental bene ﬁ ts gained from the transition from traditional resources. Given that the majority of greenhouse gas emissions in battery value chains originate from material mining and production, silicon carbide (SiC) derived from the agricultural waste, rice hull ash (RHA), is introduced as an environmentally-benign alternate anode material. SiC with hard carbon (SiC/HC) exhibits capacity increases on long-term cycling, reaching capacities of > 950 mAh g − 1 competitive with elemental Si with complementary porosity. Herein, a relatively low amount ( < 30 wt%) of graphite added to SiC/ HC composites greatly promotes capacity increases while retaining sustainability. Comparison between graphite contents were optimal at ≈ 30 wt% graphite (SiC/HC/30G) boosted performance, doubling capacity increase rates and subsequently saving > 70% time to reach target speci ﬁ c capacities at C/10. At 2C, SiC/HC/30G offers enhanced speci ﬁ c capacities at ≈ 220 mAh g − 1 . The positive effects from the coincidentally formed HC are demonstrated by oxidizing HC to form SiC/O, followed by graphite addition. Experimental post-mortem analyses support that SiC/graphite composites provide a promising solution for implementing agricultural waste-derived material for next-generation lithium storage. © 2023 The Author(s). Published on

From the pre-industrial era to the 2020s, global warming from human-produced greenhouse gas emissions has increased by ≈90%. 1 Transportation activities have surpassed electric power generation and account for the largest share (≈27%) of U.S. greenhouse gas emissions out of all economic end-use sectors since 2017. 2 Greenhouse gas emissions from transportation primarily come from burning fossil fuel for passenger cars, trucks, aircraft, etc Therefore, replacing petroleum-based vehicles with clean, renewable energy-based vehicles (EVs) is a crucial and urgent problem.The recent increase in EVs on the road represents a start.However, more complete penetration of EVs in the market requires reliable batteries with high energy densities, durability, and much diminished carbon footprints. 3EV batteries are widely expected to deliver energy densities of 350 Wh kg -1 and 750 Wh l -1 at cell levels to achieve driving ranges >500 km and cycling performances >1300 cycles. 4,5ioneering work on innovative electrodes targets increasing energy densities (driving range), extending cycle life, 6 while reducing production economics and resource expenditures. 7otential improvements in anodes also offer multiple opportunities.][10] However, its theoretical maximum capacity of 375 mAh g −1 requires one lithium atom incorporated with six carbon atoms with a ΔV ≈ 13%, which hampers progress toward higher performance rechargeable batteries. 11,12n contrast, Si, with a ten times higher theoretical capacity of 3579 mAh g −1 and low working voltage (0.2 V vs Li/Li +, ) is believed to be one of the most promising alloy-type anodes. 13,14owever, direct use of Si has yet to be realized due to low conductivities and the destructively large volume changes (ΔV >300%) during alloying/dealloying with Li. 15,16 Unfavorable consequences include electrode deformation, electrical isolation induced by Si particle pulverization and disconnection from the current collector, 17,18 excessive Li + consumption, and increased internal resistance resulting from accumulated solid-electrolyte interfaceinterphase (SEI) layers on fractured particles.Taken together these continuous processes lead to severe capacity deterioration and even safety concerns due to bulging cell packaging. 19,208][29] Introducing carbonaceous components may buffer electrode disintegration and enhance the conductivities between silicon particles. 5,16However, current silicon/graphite composite anodes typically use low Si contents < 30 wt% to limit total ΔV and retain contacts and conductivities between components, [30][31][32] restraining practical energy density improvements accessible using Si anodes.
We previously reported processing SiC with ≈15 wt% hard carbon (SiC/HC) from rice hull ash (RHA).The accompanying hard carbon can be removed by oxidatively generating (SiC/O) nanocomposites. 33,34The SiC/HC anodes exhibit charge/discharge capacities 3× that of current graphite anodes at >950 mAh g −1 following slow cycling (C/10-C/2), which promotes distinctive capacity increases.The coincidentally formed hard carbon provides positive effects on anode performance.In particular, SiC lithiation/ delithiation occurs without significant ΔV according to XRD and SEM, suggesting that SiC can compete with porous elemental Si.As discussed above, free volumes equivalent to 3× are required to accommodate the accordion-like behavior of Si anodes.
Furthermore, it is worth noting that metallurgical grade Si metal (Si MT ≈ 98% purities) is synthesized via the carbothermal reduction at ≈1900 °C; however, the purities needed for Si anodes are believed to be much higher mandating additional, expensive (CO 2 and equipment-intensive) processing steps.While battery-grade graphite (purities >99.9%) 35 used today is produced in an environmentally harmful and expensive multistep method that involves heating to ≈3000 °C, 7,36 in comparison, nanocomposite silica-depleted RHA (SDRHA) can be heated to 1450 °C/Ar to produce SiC/HC.In this aspect, biowaste-derived SiC/HC offers a more environmentallyfriendly alternate anode.Furthermore, taking into account the gigawatts of electricity generated by RHA combustion and other valueadded materials generated simultaneously from the processes (e.g.precursors for polymer Li + electrolytes, 37 Li + supercapacitors electrodes, 38 etc), this method has considerable potential as a carbon-neutral and even carbon-negative technology.
The current work is inspired by the developed concept of adding graphite to reduce unnecessary electrolyte decomposition, facilitate anode integrity retention, provide pathways for electron transfer, and consequently improve the performance of silicon-based anodes. 30,39,40We find adding graphite to SiC/HC nanocomposites led to a two-fold improvement in rates of capacity increase on cycling, reaching >950 mAh g −1 after 250 cycles.In contrast to the >50 wt% graphite in common Si-based anodes to build sufficient buffer space for silicon and to integrate the multiple species, 30 mixing only 30 wt% graphite with the SiC/HC here results in specific capacities competitive with porous Si.Thus, adding a low amount of graphite to boost SiC/HC performance still retains the superiority of greener production processes.Experimental postmortem studies also investigated the proposed interactions within the SiC/HC + graphite composites.

Experimental
Synthesis of SiC/HC and SiC/O composites with graphite.-Thesynthesis of SiC using RHA as a starting material was reported in detail recently. 33In brief, 30-50 wt% of SiO 2 was extracted from RHA (Wadham Energy Inc.) to recover SDRHA with target C:SiO 2 ratios. 37,38,41The SDRHA was then carbothermally reduced to generate SiC with hard carbon (SiC/HC ≈ 85/15), while the excess hard carbon can be removed by oxidizing at 500 °C/1 h/O 2 (SiC/O).Commercial graphite supplied from Superior Graphite, was used without further purification to mix with the as-prepared SiC/HC or SiC/O in NMP solvent by low-energy ball milling with yttriastabilized zirconia media (∼3 mm) overnight.The composite weight ratios of graphite to SiC/HC or SiC/O as 7:3 are denoted as SiC/HC/ 30G and SiC/O/30G, respectively, while those with weight ratios of 9:1 as SiC/HC/10G and SiC/O/10G.Material characterization.-Basiccharacterization was conducted after removing NMP solvent at 90 °C under vacuum.X-ray diffraction (XRD, Rigaku Miniflex) was measured using Cu-Kα radiation (λ = 0.154 nm) in the 2θ range 10-80°to identify the crystallinity nature and phases.JSM-IT300HR In Touch Scope Scanning electron microscopy (SEM, JEOL USA, Inc.) was used to acquire the microstructure images and EDX maps.Transmission electron microscopy (TEM) images were acquired with JEOL 1400-PLUS at 120 kV field emission. 13C and 29 Si Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was performed with a Bruker Avance 500 MHz spectrometer at a magnetic field of 11.7 T, corresponding to resonance frequencies of 125.7 and 99.4 MHz, respectively.Spinning was performed in 2.5 mm rotors at 30 kHz.X-ray photoelectron spectroscopy (XPS) was done using the Kratos Axis Ultra (Kratos Analytical) at room temperature under 3.1 × 10 −8 Pa using monochromatic Al source (14 kV and 8 mA) to record the core level atoms.The binding energies of all the elements were calibrated relative to C 1 s at 284.8 eV.
Electrochemical measurements.-CR2032-typecoin cells were assembled to evaluate the electrochemical performance of the anodes.The slurries were prepared with 90 wt% SiC/HC/Graphite or SiC/O/Graphite composite powder, 5 wt% C 65 , and 5 wt% poly (vinylidene fluoride) (PVDF) binder.All powders were dried at 80 °C vacuum −1 overnight and dry-mixed by mortar and pestle, which was then mixed with PVDF binder solution (5 wt% solution in NMP) and 1.5 g NMP in a 16 mL vial via low-energy ball milling (3 mm dia.yttria-stabilized zirconia media) overnight.The obtained slurries were then coated onto a copper foil (16 μm thick) using a wire-wound rod coater at a controlled speed of 50 mm s −1 .After drying at 80 °C vacuum −1 /2 h, 18 mm dia.electrodes were punched out.The loading of active materials is ≈0.6 mg cm −2 .
Cells were assembled in an Ar-filled glovebox, with Li metal (16 mm × 600 μm, MTI Corp.) used as the counter electrode.Celgard 2400 (19 mm) was used as a separator, and 1.1 M LiPF 6 in EC:DC:DMC (1:1:1 weight ratio) with 10 wt% FEC additive as the electrolyte.A Bio-Logic SP-300 was used to measure the opencircuit voltage, the AC impedance, cyclic voltammetry (CV), and galvanostatic intermittent titration technique (GITT).The CV tests were conducted in the 0.01-3 V range with a scan rate of 0.5 mV s −1 .The galvanostatic intermittent titration technique (GITT) tests were applied to the half-cells between 0.01 − 3 V at a 0.1C rate for 10 min, followed by resting periods of 20 min.The potential change of the current pulse versus the square root of time (τ 1/2 ) exhibits linear behavior with R 2 of ≈ 0.89.The ion diffusion coefficient (D) was calculated per D where n M is the moles of active material, A e is the contact area between the electrolyte and electrode, E S Δ is the steady-state voltage change, and E Δ τ is the voltage change after eliminating the IR drop during the constant current pulse.The galvanostatic cycling of the half-cells was performed between 0.01 − 2.5 V vs Li/Li + using a multichannel Maccor test system.The applied current densities were calculated from 680 mAh g −1 for SiC and 375 mAh g −1 for graphite.Data shown in the article are the averages of three cells.
The half-cells after completing 250 cycles were decrimped in a dry room.The recovered electrodes were rinsed and soaked in DEC solvent to remove electrolyte salt residues that are not inherent to the SEI before dissolving the PVDF binder in NMP using ultrasonication.The electrodes were then dried overnight under vacuum at room temperature prior to the following characterization.
Scheme 1. Fabrication processes for SiC/HC with graphite composites.
Journal of The Electrochemical Society, 2023 170 070504

Results and Discussion
Since distillative SiO 2 depolymerization methods were developed, our group has been continuously expanding the potential utility derived from silica-depleted RHA (SDRHA), including improving controllability of SiO 2 :C ratios, 41 which enables using RHA as a practical precursor for carbothermal reduction to solargrade silicon (99.999% pure) 42 or silicon non-oxide ceramics without adding external carbon sources; 33 high surface area fumed and precipitated silicas; 43 new precursors for solid Li + electrolytes; 37 high-performance hybrid Li + supercapacitors electrodes; 38 and the aforementioned potential intercalation-type anodes.Scheme 1 illustrates the SiC/HC/graphite composite fabrication process.In brief, SDRHA with appropriate SiO 2 :C ratios was first synthesized by depolymerizing the SiO 2 in RHA for carbothermal reduction producing SiC/HC.The resulting SiC/HC was then blended with commercial spherical graphite by low-energy ball milling to fabricate SiC/HC/graphite composites.
Pure spherical and elliptical graphite particles with diameters of ≈10 μm were explored as additives, of which the morphology is displayed in Fig. S1.After ball milling, the fibrous SiC whiskers and surrounding agglomerated SiC particles and hard carbon particles retain their morphologies found in as-prepared SDRHA-derived SiC, as seen in Figure 1b.EDX imaging shows no distinct elemental Si areas in the composites, indicating SiC/HC is evenly distributed through the ball milling processes.The SiC structures investigated by TEM (Fig. 1c) show micrometer-sized spherical particles bridged by submicron-sized primary particles and whiskers.
The SiC/HC and SiC/O with/graphite composites were incorporated into half-cells to assess their electrochemical performance in Li storage.Figure 2a compares three CV scans of SiC/HC/30G graphite.In the initial cathodic scan, peaks in the 2-1.5 V range are ascribed to carbonate decomposition.
Peaks at ≈1 V and ≈0.75 V are attributed to SEI formation during the 1st cycle. 19,44As expected, the peak at ≈0.15 V is associated with lithium intercalation into graphite forming LiC x , while the two peaks at ≈0.19 and ≈0.25 V in anodic scans originate from LiC x . 9Meanwhile, SiC shows responses at higher potential at ≈0.9 and 1.2 V in cathodic and anodic scans, respectively, consistent with that reported before. 34igure 2b shows representative charge and discharge profiles for the 1 st and 250th cycle of SiC/HC/30G half-cells between 0.01-2.5 V.In agreement with the CV results, the plateaus that appear between 0.25-0.01V vs Li/Li + are ascribed to intercalation of Li + into graphite. 9In addition, SiC/HC and SiC/O contribute to lithiation between 0.9-0.25 V (Fig. S2), corresponding to increases shown in the curves.Apparently, the typical charge and discharge curves are a superposition of the two components; thus, the presence of SiC improves Li + capacities in the higher potential window for the composite anodes.Meanwhile, >80% capacities are obtained during charge and discharge at 0.01-0.2V, compatible with LiPF 6 dissolved in EC:DMC mixture electrolyte, of which the SEI layer formation takes place mostly within 0.2-1.0V vs Li/Li + . 45This ensures the feasibility of using SiC/HC or SiC/O with graphite composites as anodes and represents an important factor in determining the long-term cycling performance of Li + batteries.
Moreover, capacity increments on extended cycling highlight the performance of SiC as an anode compared to other materials.Here, the SiC/HC/graphite or SiC/O/graphite composites retain their properties.Both lithiation and delithiation curves for their composite electrodes show enhanced capacities at the cut-off voltage at the 250th cycle compared to the 1st cycle (Fig. 2b). Figure 2c shows specific capacities during galvanostatic cycling and provides a better illustration of capacity improvements with time.
SiC/HC/30G half-cells show specific capacities increase from 420 to 480 mAh g −1 after 20 cycles at C/2.At 1C, the specific capacities improve from ≈280 to ≈360 mAh g −1 after 40 cycles.When the C rate is increased to 2C, the specific capacities remain at ≈220 mAh g −1 .The less evident capacity changes at higher C rates are consistent with previous observations with pristine SiC/HC, while the SiC/HC solely delivers 200 mAh g −1 under the same charge/discharge condition at 2C. Cycling at C/10 leads to prominent capacity increases, with specific discharge capacities increasing from ≈750 to ≈950 mAh g −1 over 150 cycles.With pristine SiC/ HC, achieving the same capacities (≈950 mAh g −1 ) from ≈500 mAh g −1 takes 550 cycles at C/10, as seen in Fig. S3.Thus, the SiC/ HC/30G doubles the rate increases, saving >70% time to reach the specific capacities competitive with porous Si-based anodes, [46][47][48] making RHA-derived SiC a more practical alternate anode material.
As for SiC/O/30G half-cells charged/discharged following the same protocols, the specific capacities increase from ≈350 to ≈390 mAh g −1 and ≈270 to 350 mAh g −1 at C/2 and 1C, respectively.The specific capacities of SiC/O/30G at 2C remain at ≈190 mAh g −1 , ≈10 mAh g −1 higher than SiC/O.Cycling at C/10 for 150 cycles results in increases from ≈500 to ≈600 mAh g −1 , corresponding to >50% faster rate increases.Furthermore, graphite halfcells prepared using the same procedures exhibit typical performance, 9 where the specific discharge capacities remained ≈370 mAh g −1 , ≈280 mAh g −1 , and ≈170 mAh g −1 at C/10, C/2 and 1C, respectively.As expected, no capacity increases are observed with pure graphite.At 2C, the capacities decrease rapidly Journal of The Electrochemical Society, 2023 170 070504 to ≈80 mAh g −1 after 20 cycles, demonstrating the consensus that graphite anodes are the limiting factor for fast charging. 49,50he Coulombic efficiencies (CE) of the cells mentioned above are illustrated in Fig. 2d.Yet the initial Coulombic efficiency (ICE) is only ≈60% due to the high surface area SiC/HC nanocomposites and consequent large inrreversible Li + consumption, 34 it was found that the capacity retention of SiC/HC/30G at the 106 th cycle is ≈96% compared to the 2nd cycle (Fig. S4), both at C/10.Benefits from the capacity increases, the retention approaches >120% after another 150 cycles.As for SiC/O/30G, the capacity retention after 150 cycles at C/10 stabilizes at ≈93%.The CEs for SiC/HC/30G and SiC/O/30G are ≈97% at C/2, >99% at 1C, and >99% ∼at 2C.
However, one may question the relatively low CEs at C/10 for SiC/HC/30G (≈75%) and SiC/O/30G (≈80%).As stated above, a half-cell configuration was chosen to offer fundamental electrochemical insights during the early-stage studies, where the different SiC electrodes were paired against Li-metal counter electrodes and tested with excess electrolyte. 51In such half-cells, the CE is not an ideal indicator for the actual cycling performance of those materials in full Li + batteries. 52,53Nonetheless, it is worth noting here that gaps between charge and discharge capacities are also shown in the graphite half-cells under the test conditions used, suggesting a large portion of the discrepancies between charge and discharge may be attributable to parasitic reactions, including electrode material surface properties, induced Li dendrite growth, or the cycling parameters. 54,55eanwhile, it is also observed that the presence of HC generally results in ≈5% lower CE in SiC/O anodes, which is more evident in Fig. 2e when continuously charging/discharging the samples at 1C for 150 cycles.The SiC/HC/30G half-cells show specific discharge capacities increasing from ≈450 mAh g −1 and ≈560 mAh g −1 , with CE maintained at ≈95%, while those for SiC/O/30 G half-cells increase from ≈400 mAh g −1 and ≈490 mAh g −1 with CEs >99%.
It has been demonstrated that low CE remains the primary obstacle to address for practical application of HC anodes.In particular, the high HC specific surface area consumes excessive amounts of electrolyte, and heteroatoms at graphene layer structures both contribute to trapping Li + irreversibly or alkali metal ions in the HC matrix. 56,57Nevertheless, those inherent structural properties also result in smaller ionic transfer distances, enabling fast capacitive ion storage. 58,59This is also reflected in the SiC/HC/30G composites, as discussed above.
As such, the ratio of SiC/HC to graphite within the composites must be optimized to balance between reversible capacities, capacity rate increases, and rate capabilities.SiC/HC/30G and SiC/O/30G were selected as they showed more promising electrochemical performance while keeping the graphite amount in accord with the sustainable perspectives of agricultural waste-derived materials.However, investigating the performance of composites with various ratios provides insights into understanding the interaction between components.Figure 3a illustrates the galvanostatic cycling performance of SiC/HC + 10 wt% graphite (SiC/HC/10G) and SiC/O + 10 wt% graphite (SiC/O/10G).Specific capacities for SiC/HC/10G are ≈400, ≈390, and ≈215 mAh g −1 at C/2, 1C, and 2C, respectively, whereas for SiC/O/10G they are ≈380, ≈310, and ≈170 mAh g −1 at C/2, 1C, and 2C, respectively.
The differential capacity (dQ/dV) profiles from the 2nd cycle of the half-cells in Figs.3b and 3c provide more in-depth analyses of the reaction mechanisms of the composite electrodes with different ratios.The dQ/dV profiles were normalized by graphite mass such that graphite Li + intercalation and deintercalation peaks at 0.2, 0.1, and 0.07 V vs Li/Li + , and at 0.23, 0.1, and 0.15 V vs Li/Li + , respectively, were used as indicators of the composite interaction effects.In SiC/HC/10G and SiC/O/10G half-cells, the featured peak locations shift to higher voltage during charge and lower voltage during discharge.In addition, these peaks generally reduce in magnitude and become broader in composites compared to pristine graphite, suggesting the SiC/HC or SiC/O with 10 wt% graphite hindered graphite from complete lithiation/delithiation, presumably due to significant resistive and diffusion polarization effects at graphite in the composite electrodes. 60,61In contrast, the SiC/HC/ 30G and SiC/O/30G show less evident peak changes and more intense responses than graphite, suggesting less localized polarization and that SiC components probably facilitate graphite lithiation/ deliathiation processes.At present, these effects remain to be identified.
The Li + diffusion coefficients calculated from the galvanostatic intermittent titration technique (GITT) for SiC/HC/30G as a function of the state of charge (SOC) are illustrated in Fig. 3d.It is seen that Li + diffusion coefficients are 10 −16 -10 −14 cm 2 s −1 , while those for SiC/HC half-cells are 10 −18 -10 −16 cm 2 s −1 . 34The trends between charge and discharge are nearly identical.During discharge, the initial diffusion coefficients are ≈10 −12 cm 2 s −1 , increasing to ≈10 −9 cm 2 s −1 at ≈10% SOC.Then, the diffusion coefficients gradually decreased to ≈10 −13 cm 2 s −1 at SOC ≈15%, followed by increasing to ≈10 −11 cm 2 s −1 at SOC ≈20%.The fluctuations of Li + diffusion coefficients originate from phase transformations that occur in SiC and/or graphite.It was reported that significant increases in spacing (0.1 Å) provide sufficient space for Li + diffusion, subsequently leading to increased diffusion coefficients. 62,63Thereafter, the Li + diffusion coefficients decrease to ≈10 −15 cm 2 s −1 on further charging/discharging, attributed to the increased Li + concentrations in the anode and possible formation of intermediate phases that hinder Li + transport. 64o gain a better understanding of the composite anodes' working and aging mechanisms, post-mortem analyses were conducted after the above discussed 250-cycle galvanostatic cycling tests.The cycled SiC/HC/30G and SiC/O/30G half-cells were de-crimped by the methods described in the experimental section.Figure 4a illustrates the XRD of SiC/HC/30G electrode after 250 cycles.Compared with the pristine composites shown in Fig. 1a, peaks indexed to graphite and 3C-SiC are still observed.Additional weak peaks at ≈38°, 41°, 64°2θ are attributed to (1 0 3), (1 0 4), (1 0 9) planes of 6H-SiC.Doublet peaks at ≈48°and 49°2θ can be assigned to lithiated graphite (LiC x ), indicating residual lithium insertion into graphite layers. 8 C and 29 Si MAS NMR spectroscopy was used to further probe structural changes in the composites with graphite.The 13 C spectra of pristine and cycled SiC/HC/30G in Fig. 4b are dominated by broad C=C units (sp 2 carbon) peaks.Spectral deconvolution indicates at least two resolved signals at 140 and 220 ppm, corresponding to graphite and HC in the composites, respectively. 65,66In the cycled composites (recovered after discharging to 2.5 V), both peaks show slight broadening and shifts to lower values of 110 and 190 ppm, respectively.The downfield shifts indicate incomplete Li deintercalation with some Li + residue in the composites, consistent with the XRD results and the observed CE discussed above.Meanwhile, peaks for SiC, usually centered at −15 ∼ −23 ppm are absent in the 13 C spectra.Vyalikh et al. 67 subjected possible unobservable 13 C spins in MAS NMR to the reduced resolution in low sample volume and demagnetizing fields within graphitic regions. 68n contrast to the single −16.5 ppm 29 Si NMR peak for cubic SiC observed in pristine SiC/HC, a second broad peak centered around −34 ppm is present in the SiC/HC/30G composites (Fig. 4c).The latter peak hints at mixed C/O environments around Si.This unexpected phenomenon can be understood first from the experimental material preparation method, by which the SiC components may be partially oxidized during ball-milling with graphite in the solvent.It has been reported that SiC oxidizes at comparatively lower temperatures with smaller particle sizes, with nanosized material showing an oxidation activation energy of ≈80 kJ mol −1 . 67On the other hand, introducing conductive graphite also induces demagnetizing local fields that generally result in a mismatch and broadening or even quenching of the resonance signals. 69,70Nonetheless, Li insertion minimizes the demagnetizing effects and enhances field homogeneity, resulting in improved NMR spectral resolution. 68Therefore, in the cycled 29 Si spectra, the narrower peak shifts to lower ppm and deconvoluted to reveal a peak at −8.3 ppm for irreversible Li x SiC and a peak at −21 ppm for hexagonal SiC, agreeing well with the pristine SiC/HC systems. 71igure 5 SEM images reveal the morphologies of SiC/HC/30G and SiC/O/30G before and after cycling.As expected, the electrode surfaces are covered with SEI layers after cycling.Nonetheless, SiC whiskers are still visible, indicating that the SEI is not excessively thick even with the high SiC/HC surface areas (>200 m 2 g −1 ) and SiC/O (>40 m 2 g −1 ) components.
The synergistic effects of mixed components lead to electrodes consisting of multiple interfaces, improving stress relaxation and mechanical stability. 72This also demonstrates that the SEI layers on the composite electrodes are robust and stable, suppressing deleterious Li + loss and rise in the charge transfer resistance and interface impedances, 73,74 which could be anticipated to correlate with the long-term cycling performance of the composite electrodes.Moreover, the cycled electrodes retain integrity with no cracking observed from the top view or volume changes from the crosssection view, ensuring the fundamental precondition for the cyclabilities of the composite electrodes.
Surface elemental analyses of the SiC/HC/30G and SiC/O/30G electrodes before and after cycling are shown in Fig. 6.Within XPS analyses, at depths of ≈10 nm, F, O, C, and Si elements were found in all electrode samples.Figure 6a presents the deconvoluted C 1s spectra of the SiC/HC/30G and SiC/O/30G electrodes before and after cycling.The pristine electrodes show C-Si bonds at ≈283 eV.The prominent peak at 284 eV is ascribed to sp 2 C-C, and that at 285 eV to sp 3 C-C overlap with C-O from carbon components (HC, graphite, C 65 ).The peak at ≈289 eV is assigned to F-C-H and F-C-F species from the PVDF binder. 75After cycling, peaks for lithiated graphite species Li x C 6 and/or lithium carbide (Li 2 C 2 ) are detected at 282-283 eV, 76,77 consistent with the XRD results.The C-C peaks are broader due to carbonate decomposition.Specifically, both EC and DMC decomposition release C-H species at ≈284.6 eV, while Li 2 CO 3 and metastable alkyl lithium carbonates ROCO 2 Li show signals at ≈289.5 and ≈291 eV, respectively. 78he Si 2p spectra in Fig. 6b are similar to those of SiC/HC and SiC/O without graphite. 71The intense peak at ≈103 eV is from the oxidized SiC surface, forming C-Si-O/Si-C-O and O-Si-O. 79After cycling, the Si-C peaks at ≈101 eV shift slightly to lower binding energies due to Li incorporation.As previously reported, 71 conversion and alloying are not prominent lithiation mechanisms for SiC anodes; thus, no elemental Si (≈99.5 eV) or Li x Si (≈95-97 eV) components are observed in the spectra.
As expected, the F1s XPS spectrum shown in Fig. 6c indicates the main PVDF component in pristine electrodes.On deconvolution, the F-C-F peak appears at ≈685.5 eV, along with a peak at ≈684.5 eV associated with H-C-F. 75Note that the convoluted peaks for the pristine SiC/O/30G electrodes appear at higher binding energy, probably induced by surface interfacial interactions between PVDF H-C-F dipoles and oxygen on the SiC/O surface that shows higher electronegativity.The peak at ≈684.5 eV broadens and is more intense after cycling due to decomposition products from LiPF 6, e.g.LiF and Li x PF y O z . 80The Li 1s spectra (Fig. 6d) present intense peaks for formation of irreversible Li x Si y C and LiF species in the SEI.Distinctive peaks corresponding to the Li x C components from lithiated graphite are in agreement with the C 1s spectra, XRD, and 13 C MAS NMR results discussed above.

Conclusions
SDRHA-sourced SiC offers potential as an alternate anode that delivers competitive specific capacities and eliminates the structural  13 C and (c). 29Si MAS NMR spectra of pristine SiC/HC/30G powder and cycled (250 cycles) electrode.
Journal of The Electrochemical Society, 2023 170 070504 change obstacles found for silicon metal anodes.SiC/graphite composites were designed and fabricated to promote performance.Adding small amounts of graphite to SiC/HC composites still results in extraordinary sustainability compared to the environmentally harmful graphite production but boosts the performance by doubling rates of capacity increase.The optimized graphite content was found to be 30 wt% to address graphite diffusion polarization effects and balance various electrochemical performance indices for the composite electrodes.
The specific discharge capacities of SiC/HC/30G reach ≈950 mAh g −1 over 150 cycles at C/10, which saves >70% time to reach the specific capacities found for pristine SiC/HC and enables more practical composites as alternatives to Si anodes.Detailed analyses of SiC/O/30G composites demonstrate that the coincidentally formed HC also facilitates anode performance in the composites with graphite.The high surface areas and inherent structural properties of HC result in relatively lower CE shown in the halfcell configuration; the shorter ionic transfer distance also promotes the fast capacitive Li + storage performance.
Moreover, post-mortem analyses provide a glance at interactions within the SiC/HC/graphite composites.XRD reveals partial cubic to hexagonal SiC phase changes, while lithiated graphite was observed to form from XRD and XPS measurements. 13C MAS NMR suggests demagnetizing local field effects from the conductive graphite, while the enhanced resolution after cycling also proves residual Li in the graphite layers.Consistent with the previously observed cubic to hexagonal SiC phase transition on charging/ discharging, the same phenomenon was also demonstrated by XRD and 29 Si MAS NMR, explaining the capacity increments.The SEIs observed by SEM are rather thin after long-term cycling, even with the high surface areas of the SiC/HC and SiC/O components, indicating that the synergistic effect of multiple component interfaces improves stress relaxation and mechanical stability.The robustness of the composite electrodes allows maintenance of structural integrity during cycling and shows minimal volume expansion, as expected.