Gaofeng Shao

Dorian A H Hanaor
email: dorian.hanaor@ceramics.tu-berlin.de

Jun Wang

Delf Kober

Shuang Li

Xifan Wang

Xiaodong Shen

Maged F Bekheet

Aleksander Gurlo

Polymer Derived SiOC Integrated with Graphene Aerogel as Highly Stable Li-ion Battery Anodes

Keywords: polymer derived ceramics, Li-ion Batteries, anodes, silicon oxycarbide, graphene aeroge

Amorphous polymer-derived silicon oxycarbide (SiOC) is an attractive candidate for Li-ion battery anodes, as an alternative to graphite, which is limited to a theoretical capacity of 372 mAh/g. However, SiOC tends to exhibit poor transport properties and cycling performance as the result of sparsely distributed carbon clusters and inefficient active sites. To overcome these limitations, we designed and fabricated a layered graphene/SiOC heterostructure by solvent assisted infiltration of a polymeric precursor into a modified 3D graphene aerogel skeleton. The use of a high-melting-point solvent facilitated the precursor's freeze-drying, which following pyrolysis yielded SiOC as a layer supported on the surface of nitrogen doped reduced graphene oxide aerogels. The fabrication method employed here modifies the composition and microstructure of the SiOC phase. Among the studied materials, highest levels of performance were obtained for a sample of moderate SiOC content, in which the graphene network constituted 19.8wt% of the system. In these materials a stable reversible charge capacity of 751 mAh/g was achieved at low charge rates. At high charge rates of 1480 mA/g the capacity retention was ~95% (352 mAh/g) after 1000 consecutive cycles. At all rates, Colombic efficiencies >99% were maintained following the first cycle. Performance across all indicators was majorly improved in the graphene aerogel/SiOC nanocomposites, compared with unsupported SiOC. Performance was attributed to mechanisms across multiple lengthscales. The presence of oxygen rich SiO4-xCx tetrahedra units and a continuous free-carbon network within the SiOC provide sites for reversible lithiation, while high ionic and electronic transport is provided by the layered graphene/SiOC heterostructure.

Introduction

The development of high-performance anode materials for use in Li-ion batteries has driven significant research into carbon and silicon based nanostructures [START_REF] Yang | Amorphous Tio2 Shells: A Vital Elastic Buffering Layer on Silicon Nanoparticles for High-Performance and Safe Lithium Storage[END_REF][START_REF] Xu | Scalable Synthesis of Spherical Si/C Granules with 3d Conducting Networks as Ultrahigh Loading Anodes in Lithium-Ion Batteries[END_REF][START_REF] Zhu | Engineering the Distribution of Carbon in Silicon Oxide Nanospheres at the Atomic Level for Highly Stable Anodes[END_REF][START_REF] Zhu | Boron Doping-Induced Interconnected Assembly Approach for Mesoporous Silicon Oxycarbide Architecture[END_REF][START_REF] Tu | Hierarchical Nanospheres Constructed by Ultrathin Mos2 Nanosheets Braced on Nitrogen-Doped Carbon Polyhedra for Efficient Lithium and Sodium Storage[END_REF][START_REF] Li | An Integral Interface with Dynamically Stable Evolution on Micron-Sized Siox Particle Anode[END_REF][START_REF] Wu | The Critical Role of Carbon in Marrying Silicon and Graphite Anodes for High-Energy Lithium-Ion Shao[END_REF] . However, the achievement of increasingly higher reversible capacities, improved cycling behaviour and rapid charging remains a challenging objective. The material most commonly used for Li-ion battery anodes is graphite. However, graphite electrodes are limited to a theoretical capacity of 372 mAh/g for fully lithiated graphite (LiC6) [START_REF] Wu | The Critical Role of Carbon in Marrying Silicon and Graphite Anodes for High-Energy Lithium-Ion Shao[END_REF][START_REF] Peled | Improved Graphite Anode for Lithium-Ion Batteries Chemically: Bonded Solid Electrolyte Interface and Nanochannel Formation[END_REF] . In recent years, higher capacities have been achieved in carbon nanotubes and graphene based systems, however purely carbon based anodes are not conducive to large reversible capacities at high current densities [START_REF] Raccichini | Critical Insight into the Relentless Progression toward Graphene and Graphene-Containing Materials for Lithium-Ion Battery Anodes[END_REF] . Silicon has attracted attention owing to its significantly higher theoretical maximum capacity of 3579 or 4200 mAh/g (lithiated to Li15Si4 or Li4.4Si respectively) [START_REF] Wu | The Critical Role of Carbon in Marrying Silicon and Graphite Anodes for High-Energy Lithium-Ion Shao[END_REF][START_REF] Kaspar | Silicon Oxycarbide/Nano-Silicon Composite Anodes for Li-Ion Batteries: Considerable Influence of Nano-Crystalline Vs. Nano-Amorphous Silicon Embedment on the Electrochemical Properties[END_REF][START_REF] Chan | High-Performance Lithium Battery Anodes Using Silicon Nanowires[END_REF][START_REF] Zuo | Silicon Based Lithium-Ion Battery Anodes: A Chronicle Perspective Review[END_REF] . However, performance in silicon materials is often hindered by high volumetric expansion during lithiation as well as poor ionic mobility [START_REF] Zuo | Silicon Based Lithium-Ion Battery Anodes: A Chronicle Perspective Review[END_REF] . The integration of polymer derived ceramics (PDCs) in anode materials for lithium ion batteries is an attractive prospect that has been the focus of numerous studies since early work into lithiation of PDCs, reported in 1994 [START_REF] Wilson | Lithium Insertion in Pyrolyzed Siloxane Polymers[END_REF] . PDCs, which are generally carbide and oxycarbide materials formed by the pyrolysis of silicon based preceramic polymers [START_REF] Colombo | Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics[END_REF][START_REF] Ionescu | Silicon-Containing Polymer-Derived Ceramic Nanocomposites (Pdc-Ncs): Preparative Approaches and Properties[END_REF][START_REF] Stabler | Silicon Oxycarbide Glasses and Glass-Ceramics: "All-Rounder" Materials for Advanced Structural and Functional Applications[END_REF][START_REF] Wen | The Fate and Role of in Situ Formed Carbon in Polymer-Derived Ceramics[END_REF] , are of growing interest towards diverse applications including electrochemical energy storage [START_REF] Kaspar | Stable Sioc/Sn Nanocomposite Anodes for Lithium-Ion Batteries with Outstanding Cycling Stability[END_REF][START_REF] Lee | One-Pot Synthesis of Antimony-Embedded Silicon Oxycarbide Materials for High-Performance Sodium-Ion Batteries[END_REF][START_REF] Vrankovic | Highly Porous Silicon Embedded in a Ceramic Matrix: A Stable High-Capacity Electrode for Li-Ion Batteries[END_REF][START_REF] Krüner | Silicon Oxycarbide Beads from Continuously Produced Polysilsesquioxane as Stable Anode Material for Lithium-Ion Batteries[END_REF][START_REF] Graczyk-Zajac | The Li-Storage Capacity of Sioc Glasses with and without Mixed Silicon Oxycarbide Bonds[END_REF][START_REF] Dubey | Silicon Oxycarbide-Tin Nanocomposite as a High-Power-Density Anode for Li-Ion Batteries[END_REF] , microbial bioelectrochemical systems [START_REF] De Almeida E Silva | Novel Tape-Cast Sioc-Based Porous Ceramic Electrode Materials for Potential Application in Bioelectrochemical Systems[END_REF] , microelectromechanical systems [START_REF] Dalcanale | Influence of Carbon Enrichment on Electrical Conductivity and Processing of Polycarbosilane Derived Ceramic for Mems Applications[END_REF] , electromagnetic shielding [START_REF] Liu | Single-Source-Precursor Derived Rgo/Cnts-Sicn Ceramic Nanocomposite with Ultra-High Electromagnetic Shielding Effectiveness[END_REF] and thermoelectrics [START_REF] Kousaalya | Polymer-Derived Silicon Oxycarbide Ceramics as Promising Next-Generation Sustainable Thermoelectrics[END_REF] .The development of PDCs is motivated by their remarkable chemical stability and the versatile routes with which they can be processed [START_REF] Wang | Additive Manufacturing of Ceramics from Preceramic Polymers: A Versatile Stereolithographic Approach Assisted by Thiol-Ene Click Chemistry[END_REF][START_REF] Wang | Fabrication of Polymer-Derived Ceramics with Hierarchical Porosities by Freeze Casting Assisted by Thiol-Ene Click Chemistry and Hf Etching[END_REF] . Coupled with tuneable properties of electrical conductivity, permittivity and thermal conductivity, PDCs offer utilitarian pathways towards effective energy materials [START_REF] Stabler | Silicon Oxycarbide Glasses and Glass-Ceramics: "All-Rounder" Materials for Advanced Structural and Functional Applications[END_REF][START_REF] Wen | The Fate and Role of in Situ Formed Carbon in Polymer-Derived Ceramics[END_REF][START_REF] Gurlo | The Thermal Conductivity of Polymer-Derived Amorphous Si-O-C Compounds and Nano-Composites[END_REF] .

Silicon oxycarbide, referred to as SiOC or oxycarbide glass, is an amorphous ceramic that is readily formed PDC microstructures that result from most processes lack ion transport pathways and generally yield poor utilisation of the active phase. To address these shortcomings, the composition and structure of applied PDCs can be designed at various scales to provide improved electronic and ionic transport behaviour in electrodes. PDCs have been processed in forms of etched porous particles [START_REF] Dou | Study of the Na Storage Mechanism in Silicon Oxycarbide-Evidence for Reversible Silicon Redox Activity[END_REF] , aerogels [START_REF] Sasikumar | Structural Design of Polymer-Derived Sioc Ceramic Aerogels for High-Rate Li Ion Storage Applications[END_REF] , beads [START_REF] Krüner | Silicon Oxycarbide Beads from Continuously Produced Polysilsesquioxane as Stable Anode Material for Lithium-Ion Batteries[END_REF][START_REF] Weinberger | Submicron-Sized Silicon Oxycarbide Spheres as Anodes for Alkali Ion Batteries[END_REF] , and fibers [START_REF] Tolosa | Continuous Silicon Oxycarbide Fiber Mats with Tin Nanoparticles as a High Capacity Anode for Lithium-Ion Batteries[END_REF][START_REF] Sang | A Graphene-Modified Flexible Sioc Ceramic Cloth for High-Performance Lithium Storage[END_REF] to impart improved structure driven performance. In terms of composition, PDCs with increased free carbon have been designed through control of pyrolysis conditions [START_REF] Kaspar | Lithium Insertion into Carbon-Rich Sioc Ceramics: Influence of Pyrolysis Temperature on Electrochemical Properties[END_REF] and the use of carbonrich precursors or cross-linkers [START_REF] Liu | Electrochemical Performance of Dvb-Modified Sioc and Sicn Polymer-Derived Negative Electrodes for Lithium-Ion Batteries[END_REF][START_REF] Halim | Phenyl-Rich Silicone Oil as a Precursor for Sioc Anode Materials for Long-Cycle and High-Rate Lithium Ion Batteries[END_REF] . Composite materials incorporating carbon nanostructures (CNTs 41 or graphene [START_REF] Sang | A Graphene-Modified Flexible Sioc Ceramic Cloth for High-Performance Lithium Storage[END_REF][START_REF] Feng | Preparation and Improved Electrochemical Performance of Sicn-Graphene Composite Derived from Poly(Silylcarbondiimide) as Li-Ion Battery Anode[END_REF][START_REF] David | Silicon Oxycarbide Glass-Graphene Composite Paper Electrode for Long-Cycle Lithium-Ion Batteries[END_REF][START_REF] Sang | Sioc Nanolayer Wrapped 3d Interconnected Graphene Sponge as a High-Performance Anode for Lithium Ion Batteries[END_REF] ) and /or nanoparticles of Sn [START_REF] Kaspar | Stable Sioc/Sn Nanocomposite Anodes for Lithium-Ion Batteries with Outstanding Cycling Stability[END_REF][START_REF] Dubey | Silicon Oxycarbide-Tin Nanocomposite as a High-Power-Density Anode for Li-Ion Batteries[END_REF] , Sb [START_REF] Lee | One-Pot Synthesis of Antimony-Embedded Silicon Oxycarbide Materials for High-Performance Sodium-Ion Batteries[END_REF][START_REF] Dubey | Silicon Oxycarbide-Antimony Nanocomposites for High-Performance Li-Ion Battery Anodes[END_REF] , or Si [START_REF] Kaspar | Silicon Oxycarbide/Nano-Silicon Composite Anodes for Li-Ion Batteries: Considerable Influence of Nano-Crystalline Vs. Nano-Amorphous Silicon Embedment on the Electrochemical Properties[END_REF][START_REF] Vrankovic | Highly Porous Silicon Embedded in a Ceramic Matrix: A Stable High-Capacity Electrode for Li-Ion Batteries[END_REF]

Experimental

Fabrication of 3D graphene-based aerogels

To produce composite materials, we first synthesised a highly porous skeleton of N-doped reduced graphene oxide aerogel (NRGOA) via a polymerizationreduction process with subsequent freeze drying and thermal reduction. In a typical process, graphene oxide (GO) was prepared from natural graphite by an improved Hummers method [START_REF] Marcano | Improved Synthesis of Graphene Oxide[END_REF] and ultrasonically dispersed (5 mg/ml) in cold deionized water for 2 h.

An aqueous mixture of pyrrole monomer (50 mg) and GO suspension (10 ml) was ultrasonically dispersed for 30 min, and then maintained at 60 ℃ for 24 h in an oven to facilitate the cross-linking of pyrrole with GO, resulting in a black cylindrical reduced graphene oxide hydrogel (RGOH). This RGOH was then rinsed with a Transmission electron microscopy (TEM) was used to confirm the presence of sandwiched multilayer structure of oxycarbide on graphene within the monolithic nanocomposite 3D NGA-SiOC.

Electrical characterization

Electrical resistances of NGA and NGA-SiOC composites were measured by a two-probe method using a digital multimeter (287, Fluke, US) with a sampling frequency of 2 Hz. The electrodes were painted with highly conductive silver paste (~ 0.01 Ω•cm) on both sides, to minimize contact resistance.

The voltage-current (VI) curves were measured by a Probe Station II using Kiethley SCS4200

Semiconductor Parameter Analyzer (SCS4200, Kiethley, US).

Electrochemical measurements

The electrochemical characterization of electrodes based on the NGA-SiOC composites were tested using CR2032 stainless steel coin cells with lithium foils as the counter electrode. The active materials (85 wt%)

were mixed with PVDF (10 wt%) and carbon black (5 wt%) in N-methyl-2-pyrrolidone to form a slurry. The Zahner ZENNIUM Impedance measurement unit under potentiostatic mode with 5mV amplitude between 100 mHz and 1 MHz. First, both cells were activated at 37 mA/g for five cycles. Then they were charged up to 50%SOC and rested there for 2 h before EIS measurement.

Results and Discussion

Highly porous NRGOA skeletons were produced via the polymerization-reduction process used here following freeze drying and thermal reduction. As shown in Figure 2e, the formed NRGOA exhibits a 3D structure with interconnected pores of dimensions ranging from tens to hundreds of micrometers, which provides sufficient space for the integration of the preceramic precursor into the NRGOA skeleton (Figure 2b). Monolithic NGA-SiOC composites were then formed through precursor infiltration, freezedrying and high temperature pyrolysis (Figure 1). An The evolution of elemental composition and speciation of functional groups on specimen surfaces were examined using XPS (Figure S9). 4b) [START_REF] Kim | Oxygen-Doped Porous Silicon Carbide Spheres as Electrode Materials for Supercapacitors[END_REF] . It is worth noting that the relative contents of and NGA-SiOC100, respectively, which are several orders of magnitude higher than those of pristine SiOC following pyrolysis at the same temperatures [START_REF] Cordelair | Electrical Conductivity Measurements as a Microprobe for Structure Transitions in Polysiloxane Derived Si-O-C Ceramics[END_REF] and are of a similar magnitude to the conductivity of pure graphene aerogels [START_REF] Worsley | Synthesis of Graphene Aerogel with High Electrical Conductivity[END_REF][START_REF] Zhang | Superelastic, Electrically Conductive, and Flame-Retardant 3d Multi-Nanolayer Graphene/Ceramic Metamaterial[END_REF] . Compared with other reported graphene/PDC composites summarised in Table S1 (SiOC-graphene@1500/1700 50 , SiOC-GO@1700 [START_REF] Yu | Graphene-Reinforced Silicon Oxycarbide Composites Prepared by Phase Transfer[END_REF] , RGO/CNTS/SiCN@1000 [START_REF] Liu | Single-Source-Precursor Derived Rgo/Cnts-Sicn Ceramic Nanocomposite with Ultra-High Electromagnetic Shielding Effectiveness[END_REF] , GO/PSZ@800/1000 51 , SiCN-rGO@1000 [START_REF] Yu | Electrical Conductivity of Silicon Carbonitride-Reduced Graphene Oxide Composites[END_REF] , SiCNO-GO@1000 [START_REF] Yu | Sicno-Go Composites with the Negative Temperature Coefficient of Resistance for High-Temperature Sensor Applications[END_REF] , SiCO/GO@1100 [START_REF] Shen | Bulk Polymer-Derived Ceramic Composites of Graphene Oxide[END_REF] , rGO/Si3N4@1600 [START_REF] Wang | Synthesis of Polymer-Derived Graphene/Silicon Nitride-Based Nanocomposites with Tunable Dielectric Properties[END_REF] , SiCN/rGOA@1000 [START_REF] Shen | Anisotropic Electrical Conductivity in Polymer Derived Ceramics Induced by Graphene Aerogels[END_REF] ), the lightweight, porous reported graphene/PDCs (SiOC-graphene@1500/1700 50 , SiOC-GO@1700 [START_REF] Yu | Graphene-Reinforced Silicon Oxycarbide Composites Prepared by Phase Transfer[END_REF] , RGO/CNTS/SiCN@1000 26 , GO/PSZ@800/1000 51 , SiCN-rGO@1000 46 , SiCNO-GO@1000 48 , SiCO/GO@1100 62 , rGO/Si3N4@1600 63 , SiCN/rGOA@1000 49 ) (e) Schematic of the conductive pathway for electron transport in porous graphene/SiOC monoliths and dense random graphene/SiOC composite. (Note: The unreported density of dense composites is defined as 2g/cm 3 in (d)).

The electrochemical performance of heterolayered graphene/SiOC was evaluated in half cells. Figure 6a shows the initial charge/discharge curves of NGA-SiOC nanocomposites of varied NGA content. Among them, the SiOC-NGA25 (heterolayered structure) delivers a high initial discharge capacity of 1116 mAh/g and a charge capacity of 751 mAh/g, with the highest initial coulombic efficiency of 67.3% (Table 2). Bare SiOC anodes (particles illustrated in Figure S11) show a similar discharge capacity of 1090 mAh/g, however the following charge only delivers 540 mAh/g translating to a coulombic efficiency of 49.5 %. The irreversible first-cycle capacity results from electrochemical formation processes such as solid-electrolyte interphase (SEI) layer formation [START_REF] Ge | Enabling Superior Electrochemical Properties for Highly Efficient Potassium Storage by Impregnating Ultrafine Sb Nanocrystals within Nanochannel-Containing Carbon Nanofibers[END_REF] . The discharge potential is the highest for SiOC- To date, the cycling performance of SiOC based anodes at high-rates has not been significantly S3. It can be seen that the specific capacities attained in NGA-SiOC25 materials at 1480 and 2000mA/g are significantly higher than similar systems studied previously [START_REF] Kaspar | Stable Sioc/Sn Nanocomposite Anodes for Lithium-Ion Batteries with Outstanding Cycling Stability[END_REF][START_REF] Krüner | Silicon Oxycarbide Beads from Continuously Produced Polysilsesquioxane as Stable Anode Material for Lithium-Ion Batteries[END_REF][START_REF] Feng | Preparation and Improved Electrochemical Performance of Sicn-Graphene Composite Derived from Poly(Silylcarbondiimide) as Li-Ion Battery Anode[END_REF][START_REF] David | Silicon Oxycarbide Glass-Graphene Composite Paper Electrode for Long-Cycle Lithium-Ion Batteries[END_REF][START_REF] Sang | Sioc Nanolayer Wrapped 3d Interconnected Graphene Sponge as a High-Performance Anode for Lithium Ion Batteries[END_REF] . The favourable capacities and cycling behaviour, even at high rates, as achieved in the presently reported composite materials can be interpreted across different identifiable lengthscales shown in Figure 9;

Conclusion

We have reported here a method for the infiltration-

  an atomic scale SiOC is comprised of short range ordered SiO4-xCx tetrahedra alongside a phase of free carbon. However, two notable obstacles remain towards the application of SiOC and other PDCs in Li-ion battery anodes 31 ; (1) A low intrinsic electronic conductivity mediated by disparate nanosized free carbon clusters impedes Li-ion insertion/deinsertion reactions in PDCs 32 . (2) Dense

  working electrodes were prepared by coating the slurry on the surface of copper foil current collectors and drying at 100°C in a vacuum oven for 10 h to remove N-methyl-2-pyrrolidone. LiPF6 (0.5 M) in ethylene carbonate and dimethyl carbonate (EC/DMC = 1:1) was used as the electrolyte. The cells were assembled in an argon-filled glovebox. The CV and GCD curves were measured using an electrochemical workstation (USA PARSTAT 273A) and a battery test system (LAND CT2001A) with a potential range between 0.005 and 2.5 V. Electrochemical impedance spectroscopy (EIS) measurements were done (as assembled) at 50% state of charging (SOC) with a

  Figure S1 in the supplementary information illustrates the output of the methods used here. Reduced graphene oxide aerogel (RGOA) was synthesized via polymerization-reduction between pyrrole (Py) and graphene oxide (GO). The polypyrrole (PPy) grows preferentially on the surface of GO sheets due to electrostatic interactions between positively charged Py and negatively charged GO surfaces, π-π interactions between Py rings and conjugated segments in GO and hydrogen-bonding interactions 54 .Through these interactions, GO acts as a structuring agent with PPy forming a thin surface layer. The incorporation of PPy also changes the stacking of GO sheets from a 2D closely stacked layer to a 3D porous structure (Figure2a, d), which was attributed to the increased ratio of attraction/repulsion forces that stabilizes the GO hydrogel network[START_REF] Lin | 3d Nitrogen-Doped Graphene Prepared by Pyrolysis of Graphene Oxide with Polypyrrole for Electrocatalysis of Oxygen Reduction Reaction[END_REF] . During further thermal reduction of RGOA, PPy decomposes, yielding ultralight NRGOA with extremely high compressibility, similar to that of RGOA (FigureS2).

  infiltration technique referred to as polymer-solvent solidification-sublimation is used in place of conventional polymer-solvent evaporation in this work. During infiltration, the precursor attaches to NRGOA surfaces due to favourable wetting and adhesion. By using tert-butanol (TBA, melting point: 25ºC) as the solvent instead of other organic solvents (e.g. ethanol or tetrahydrofuran), freeze drying can be conducted following the infiltration process, which overcomes the problem of surface tension at the gasliquid-pore wall. As a result, the 3D graphene skeleton maintains its morphology without shrinkage, collapse or agglomeration during infiltration and drying (FigureS3). After further pyrolysis, the scaffolds present uniform shrinkage and 3D graphene embedded polymer derived ceramic structures are obtained (Figure2c).

Figure 1 .

 1 Figure 1. Schematic illustration of the fabrication of 3D NGA-SiOC porous monoliths.

Figure 2 .

 2 Figure 2. (a-c) Optical and (d-f) SEM images of RGOA, NRGOA and NGA-SiOC25, (g-i) EDS mapping of C, O, Si elements of (f), (j) radiography image of bulk NGA-SiOC25 (k) SEM image of an individual structural unit in the scaffold (l) high resolution cross-sectional TEM image revealing the graphene/SiOC sandwich structure of individual structural units in NGA-SiOC25.

Figure

  Figure 3b presents ATR-FTIR spectra for GO, RGOA, and NRGOA. For the spectrum of GO, the bands at 1732, 1619, 1216, and 1040 cm -1 correspond respectively to vibrations of carbonyl C=O stretching, C=C stretching, C-OH stretching, and epoxy C-O stretching. After polymerization-reduction, bands related to C=O, C-OH and C-O groups disappear almost completely, indicating the effective reduction of GO. For RGOA, the strong band around 1563 cm -1 occurs on account of typical ring vibrations of PPy, and that at 1211 cm -1 is due to the C-N stretching. The band at 1042 cm -1 corresponds to in plane vibrations of C-H bonds, further confirming the existence of PPy in RGOA through interactions hydrogen bonding and π-π stacking between PPy and graphene 56 . ATR-FTIR analysis also confirms transformation of the H62C polymeric precursor, with its -CH3, -CH2=CH2-, Si-C6H5 and Si-H groups, to SiOC ceramic through high temperature pyrolysis (Figure 3c, d)

SiC4,

  SiOC3 and SiO2C2 units decrease after the incorporation of NRGOA, especially with higher NRGOA content, which can be attributed to the fact that Si bonds more readily with O atoms than C atoms at high temperatures and the oxygen content was elevated by the remaining oxygen group from NRGOA (Figure 4c). A pronounced N 1s peak is observed for RGOA and NRGOA apart from the peaks for C and O elements. The high-resolution N 1s spectrum of the RGOA (Figure S10) reveals the presence of pyrrolic N, which further confirms the successful incorporation of PPy into the RGOA. After thermal reduction, N1s spectra in NGA can be deconvoluted into three peaks located at 398.2, 400.9, and 402.8 eV, which are attributed to pyridinic N, pyrrolic N, and graphitic N, respectively, suggesting N atoms are incorporated into the graphene network.

Figure 3 .

 3 Figure 3. (a) XRD patterns of GO, RGOA, NRGOA, SiOC and NGA-SiOC nanocomposites. ATR-FTIR spectra of(b) GO, RGOA and NRGOA, (c) preceramic polymer and (d) SiOC and NGA-SiOC25

Figure 4 .

 4 Figure 4. High resolution XPS spectra for (a) C1s of GO, RGOA, NRGOA, NGA-SiOC25 and SiOC, (b) Si2p of SiOC and NGA-SiOC nanocomposites, (c) Transformation of nanodomain structure and silicon coordination in SiOC after the introduction of RGOA.

  graphene/SiOC composites here present outstanding electrical properties at equivalent levels. The electronic conduction in NGA-SiOC is schematically illustrated in Figure5e. The sandwiched multi-layer graphene sheets of the NGA scaffold provide well-interconnected pathways for electron transport at a large mobility. In contrast, graphene/PDC composites in which graphene is dispersed in the form of isolated particles in the ceramic phase lack such interconnectivity, and thus rely on free carbon in the ceramic matrix to facilitate electron percolation. In the aerogel infiltration approach applied here the NGA framework is retained intact, being free from damage due to its high mechanical robustness during the infiltration process, which effectively reduces possible deterioration (e.g., bubbles, weak interfaces, and cracks) in the microstructure. Each graphene sheet in the NGA provides a pathway for electron migration and contributes simultaneously to mechanical reinforcement. In this respect such conductive NGA-SiOC nanocomposites possess promising functionalities for electromagnetic shielding, as well as for application as thermoelectric / conductive ceramic composites.

Figure 5 .

 5 Figure 5. (a-c) SEM images of NGA-SiOC15, NGA-SiOC50 and NGA-SiOC100, respectively. (d) The electrical properties of the 3D porous NGA-SiOC monoliths (NGA-SiOC15, NGA-SiOC50 and NGA-SiOC100) and the

  NGA25 and decreases with decreasing NGA content with the lowest value for SiOC, indicating that the overpotential is lowered due to the NGA matrix. EIS measurements of NGA-SiOC25 and SiOC electrodes (Figure S12) give further insight into the single polarization processes. The ohmic part (summarized resistance of current collectors, electrolyte and electrodes, SEI) is similar for both samples showing values of 1.5 and 2.5 Ohm, mainly due to the electrolyte resistance. The IR-drop for a current at 37 mA/g corresponds to 0.6mV, which is negligible with respect to the reaction and diffusion polarization. The reaction polarization (charge transfer at the electrode/electrolyte interface), described by a semicircle in the Nyquist-plot, is around 30 Ohm for NGA-SiOC25 and ca. 150 Ohm for SiOC. According to the Butler-Volmer-equation the charge transfer reaction is related to the active surface area and exchange current density. Hence, the lower measured transfer resistance for NGA-SiOC25 indicates that the embedded multilayer graphene network imparts a higher exchange current density and improved electron transport. This directly influences the cell potential. This is emphasized by the reduced relaxation time constant of =75 s for SiOC25 compared to =734 s for SiOC. This relaxation time constant (=RC) describes the time it takes a process to return to equilibrium in the absence of the electric field. Smaller  values correspond to faster processes. The diffusion polarization is related to the sloping line in the low frequency range of the Nyquist-plot and describes the diffusion of Li + ions in electrode's vicinity. A rough estimation of the Warburg diffusion W gives 50 Ohms -0.5 and 150 Ohms -0.5 for NGA-SiOC25 and SiOC, respectively. Lower W corresponds to a lower diffusion polarization, due to a higher diffusion coefficient, surface area and/or ion concentration. The lower reaction and diffusion polarizations explain the higher discharge potential and lower charge potential for NGA-SiOC25 compared to SiOC. The higher dis-/charging overpotentials of the pristine SiOC anodes lead to a reduced capacity, with the designated cut-off potentials of 5mV and 2.5 V (for discharging and charging, respectively) being reached earlier.

Figure 6 .

 6 Figure 6. Galvanostatic charge-discharge profiles in the 0.005-2.5 V window (versus Li/Li + ) for (a) 1st cycle of NGA-SiOC nanocomposites and SiOC at 37mA/g (b) 2nd to 6th cycles of NGA-SiOC25 and SiOC at 37mA/g. (c) Rate performances of NGA-SiOC nanocomposites at various current density, (d) Cycling performances of NGA-SiOC25 at current density of 1480 and 2000mA/g, and SiOC at current density of 1480 mA/g. (37 mA/g for the first 3 cycles)

Figure 6c presents the

  Figure 6c presents the galvanostatic cycling performance of the NGA-SiOC nanocomposites and SiOC at different charge/discharge current densities, illustrating the current rate capabilities and cycling performances for materials of different NGA contents. NGA-SiOC25 shows a superior rate capability among the investigated NGA-SiOC anode materials. It exhibits discharge capacities of 628, 565, 462 and 351mAh/g in the 10th-cycle at 74, 148, 370 and 740 mA/g, which are much higher than the SiOC anode (164, 102, 49 and 24 mAh/g, respectively). The discharge capacity of NGA-SiOC25 is significantly enhanced compared to that of SiOC and other NGA-SiOC nanocomposites at the same current density.More importantly, for NGA-SiOC25, when the current density is reset to 74 mA/g after cycling at different current densities, the discharge capacity is restored to

Figure 7 .Figure 8 .

 78 Figure 7. The cycling performace (capacity retention) of the NGA-SiOC25 anode from this work, compared with performance of similar materials reported in earlier studies. (Full data in supplementary material)

( 1 )

 1 Molecular scale: the elemental composition of SiO4-xCx tetrahedral units is tailored by oxygen groups of reduced graphene oxide from carbon-rich to oxygen-rich SiO4-xCx. As carbon rich SiOC3 units react irreversibly with lithium to form SiC4 units when lithium is inserted, the increasing oxygen content manifests in higher reversible capacities as shown here. (2) Nanoscale: the presence of increased levels of free carbon domains within the PDC provide higher levels of Li-ion intercalation sites and raise the intrinsic conductivity of the SiOC component in the composite. (3) Microscale: the graphene network in which the SiOC is embedded facilitates high electrical conductivity and ion mobility, shortens diffusion distances and enables the full utilisation of the SiOC material.

Figure 9 .

 9 Figure 9. Schematic illustration of structure / performance relationships across multiple length-scales.

  based fabrication of monolithic composites consisting of a polymer derived SiOC ceramic on surfaces of a 3D porous graphene structure. The highly porous and mechanically stable N-doped reduced graphene oxide aerogel, which was formed by polymerizationreduction and freeze drying, provides sufficient space for the integration of a preceramic precursor into its 3D skeleton. Facilitated using a relatively high-melting-point solvent, 3D porous NGA-SiOC monoliths were then fabricated via a polymer-solvent solidification-sublimation infiltration followed by high temperature pyrolysis. In 3D NGA-SiOC materials, thin SiOC layers are homogeneously coupled with multilayer graphene sheets, forming heterolayered graphene/SiOC structures. NGA-SiOC nanocomposites fabricated here demonstrate low density and high electrical conductivity, which is Shao, Gaofeng, et al. "Polymer-Derived SiOC Integrated with a Graphene Aerogel As a Highly Stable Li-Ion Battery Anode." ACS Applied Materials & Interfaces 12.41 (2020): 46045-46056. attributed to the NGA skeleton as well as the presence of increased levels of free carbon within the oxycarbide phase. It is postulated that the excellent cycling performance and high charging rates found here are also facilitated by these structures, which provide rapid and extensive reversible lithiation sites. The encouraging results found in this work motivates further investigations into graphene network based PDC composites and their optimisation with a view towards Li-ion battery materials.

2.2 Fabrication of 3D porous graphene/ silicon oxycarbide monoliths

	Shao, Gaofeng, et al. "Polymer-Derived SiOC Integrated with a Graphene Aerogel As a Highly Stable Li-Ion
	Battery Anode." ACS Applied Materials & Interfaces 12.41 (2020): 46045-46056.
	varied PDC loadings. Towards this end, the polymeric	microfocus X-ray source with a flat panel detector
	precursor was combined with tert-Butanol (TBA,	(Hamamatsu, Japan) with an area of ˜ 120 x 120 mm²
	(CH3)3OH, Merck, Germany) at concentrations of 15,	and a pixel size of 50 μm. Microstructural analysis was
	25, 50, 100, and 300 mg/ml. Then NRGOA, which	conducted by Scanning Electron Microscopy (SEM)
	exhibits good wettability, was immersed into this	using a Leo Gemini 1530 (Zeiss, Germany).
	solution under vacuum for 2 h to facilitate precursor
	penetration into the aerogel and eliminate bubbles.
	Subsequently, the wetted samples were taken out of
	the solution and freeze-dried for 12 h to remove the
	TBA. The resulting polymeric precursor loaded
	NRGOA (NRGOA-P) maintained their structure
	without visible changes as shown in Figure. 1, which
	was attributed to the use of high melting-point-solvent
	assisted infiltration and freeze drying instead of a more
	conventional solvent followed by vacuum drying.
	Subsequently, NRGOA-P cylinders were cross-linked
	at 250 ℃ for 2 h and pyrolyzed at 1000 ℃ for 3 h in a
	tube furnace under flowing argon at a heating rate of
	100℃/h to obtain the final 3D N-doped graphene
	aerogel supported SiOC (NGA-SiOC) materials.	The reduced graphene oxide aerogel (RGOA) was
	Samples are denoted here as NGA-SiOCX, where	subsequently obtained by freeze-drying of the RGOH
	X=15, 25, 50, 100, 300, corresponding to the values of	for 24 h.
	precursor concentration. For comparison, unsupported
	SiOC particles were prepared by the pyrolysis of
	preceramic polymer H62C at the same heat-treatment	RGOA with attached nitrogen bearing pyrrole groups
	conditions.				was further thermally reduced at 900℃ for 1 h in a
	2.3 Characterization			tube furnace under flowing argon at a heating rate of
	X-ray diffraction (XRD) patterns were acquired using	180℃/h to combust the organic content and form N-
	a Bruker D8 diffractometer (Bruker, Billerica, MA,	doped reduced graphene oxide aerogel (NRGOA). The
	USA) with Co Kα radiation (λ=0.1789 nm) over a 2θ	nitrogen doping here is conducted in order to reduce
	range from 10° to 90°. Fourier transform infrared	the systems oxygen content and further enhance
	spectroscopy (FTIR) in Attenuated Total Reflection	electronic conductivity in the materials 53 . A solution
	(ATR) mode was carried out in an Equinox 55 (Bruker,	of commercial preceramic polymer precursor,
	Germany) in the range of 500 -4000 cm-1 to	methylphenylvinylhydrogen polysiloxane (SILRES
	characterize	bonding.	X-ray	photoelectron	H62C, Wacker Chemie, Burghausen, Germany) was
	spectroscopy (K-Alpha TM, Thermo Scientific) was	infiltrated into as-prepared NRGOA frameworks
	used to examine surface elemental composition. X-ray	under low vacuum (200 mbar) to fabricate 3D
	imaging (radiography) was performed using a	graphene/polymer-derived ceramic architectures with

5:1 water/ethanol solution (by volume) for 24 h to remove residual pyrrole and polypyrrole oligomer.

Table 1 .

 1 Shao, Gaofeng, et al. "Polymer-Derived SiOC Integrated with a Graphene Aerogel As a Highly Stable Li-Ion Battery Anode." ACS Applied Materials & Interfaces 12.41 (2020): 46045-46056. Atomic concentration of each constituent from the survey scans of GO, RGOA, NRGOA, NGA-SiOC

	nanocomposites and SiOC
	Sample	Si (atom %) O(atom %) C (atom %) N (atom %)
	GO	-	34.39	65.61	-
	RGOA	-	14.33	77.94	7.73
	NRGOA	-	4.03	92.31	3.66
	NGA-SiOC15	11.03	24.85	61.69	2.44
	NGA-SiOC25	11.14	22.64	64.03	2.19
	NGA-SiOC50	12.83	27.27	58.31	1.59
	NGA-SiOC300 15.08	32.35	51.48	1.09
	SiOC	18.6	34.57	46.83	-
	Figures 5a-d show the porous microstructures and
	electrical conductivity of monolithic NGA-SiOC
	materials, demonstrating conductivities as high as 2.7,
	8.4, and 85.5 S/m for NGA-SiOC15, NGA-SiOC50

Table 2 .

 2 Overview of the NGA content, first cycle discharging and charging capacity and initial coulombic efficiency for bare SiOC and NGA-SiOC composites electrodes

	Sample	NGA content	1st Cdis (mAh/g)	1st Cch (mAh/g)	Initial coulombic efficiency (%)
		(Wt.%)
	NGA-SiOC15	29.9	796	479	60.2
	NGA-SiOC25	19.8	1116	751	67.3
	NGA-SiOC50	11.8	807	537	66.5
	NGA-SiOC300	2.5	878	539	61.3
	SiOC	0	1090	540	49.5

  Shao, Gaofeng, et al. "Polymer-Derived SiOC Integrated with a Graphene Aerogel As a Highly Stable Li-Ion Battery Anode." ACS Applied Materials & Interfaces 12.41 (2020): 46045-46056.

	explored. In this work, the cycling performance of	and modified SiOC anodes reported in previous
	bulk SiOC and the NGA-SiOC25 nanocomposites was	studies, as shown in Figure 7 and Table S2. As shown
	further examined with 1000 galvanostatic	in Figure 8, there are no obvious cracks among layered
	charge/discharge cycles at high current densities of	graphene-SiOC structures and no segregation between
	1480 and 2000 mA/g, as shown in Figure 6d and S13.	graphene and SiOC after 1000 cycles at 1480mA/g,
	The composite possesses a reversible capacity of 352	indicating a good cycling lifetime. Even at a current
	mAh/g with a coulombic efficiency of 99.97% after	density of 2000 mA/g, a reversible capacity of 196
	1000 consecutive cycles. The capacity retention was	mAh/g is attained in the nanocomposite with a
	~95% of the second cycle capacity, which is	coulombic efficiency of 99.96% after 1000
	significantly improved in comparison to that of pure	consecutive cycles.

Acknowledgements

This work was supported by China Scholarship Council (CSC, 201608320159, 201604910900), Deutsche Forschungsgemeinschaft (GU 992/17-1), The Startup Foundation for Introducing Talent of NUIST. We also thank Dr. Paul H. Kamm for conducting the X-ray imaging measurement, Dr. Franziska Schmidt for conducting the SEM measurement, and Dr. Sören Selve for conducting the TEM measurement, respectively.

Supporting information:

Supplementary graphics providing further information relating to the macrostructure, microstructure, composition and performance of materials fabricated in the present work Supplementary tables providing a comprehensive comparison of the conductivity and cycling performance of anode materials studied here with related studies reported in the literature.