## 2 Materials and Methods  
### Synthesis of SiOC/C/Sn Composites  
Starting materials: Polymethylhydrosiloxane (PMHS, MW \(\approx\) 1900), divinylbenzene (DVB, technical grade, 80%), graphite (flakes, 20 \(\upmu\)m), tin octaate (Sn(Oct)\({}_{2}\), 92.5-100.0%, Sigma-Aldrich), [PERSON]'s catalyst (2% of Pt element in xylene), and acetone (\(\geq\)99.5%) were purchased from Sigma Aldrich, Buchs, Switzerland. All materials were used as received. Synthesis of the active materials: The composites were synthesised using the ultrasound-enhanced hydrosilylation method. First, 5 uL of [PERSON]'s catalyst was added to 2 g of DVB. Then, after 2 min of intense stirring, 2 g of PMHS, 2.5 g of acetone, and different amounts of graphite and tin octaate were added to the mixture. After a couple of minutes of stirring, the suspension was treated with an ultrasonic probe sonicator for 3-5 min to obtain a solid gel. The gels of various compositions were dried for two days at 80 \({}^{\circ}\)C to complete crosslinking. Finally, the green bodies were pyrolyzed using the following programme: heating to 250 \({}^{\circ}\)C with a rate of 100 \({}^{\circ}\)C h\({}^{-1}\), dwelling at 250 \({}^{\circ}\)C for 2 h, heating to 1000 \({}^{\circ}\)C with a rate of 150 \({}^{\circ}\)C h\({}^{-1}\), dwelling at 1000 \({}^{\circ}\)C for 1 h, cooling to room temperature with a rate of 60 \({}^{\circ}\)C h\({}^{-1}\). The compositions of the preceramic blends are presented in Table 1. The matrix was composed either of the pure ceramic (SiOC) or the ceramic mixed with graphite (SiOC:C\({}_{x}\), where the \"x\" value corresponds to the mass fraction of graphite replacing the preceramic polymer). The tin precursor content is denoted as Sn-y%, where \"y\" corresponds to the weight percentage of the tin precursor with regard to the mass of the whole preceramic polymeric blend. Characterization techniques: Raman analysis was conducted using a micro-Raman spectrometer (InVia, Renishaw, Wotton-under-Edge, UK) equipped with an Ar ion laser (514 nm) within 100-3200 cm\({}^{-1}\). The analysis of obtained spectra was performed using OriginPro2016 software after background subtraction with custom settings. The modes were fitted according to ([PERSON], 2016), using Lorentzian fitting for D4, D1, G, and D2 peaks, and Gaussian fitting for the D3 peak. XRD reflexes were collected using the EDAX, RTEM model SN9577. The morphology of obtained materials was characterised using a scanning electron microscope (Tescan VEGA, Tescan, Brno, Czech Republic) and a transmission electron microscope (FEI, G2 F20X-Twin 200 kV, FEG), while the surface elemental distribution was assessed using a Phenom XL Scanning Electron Microscope (ThermoFisher Scientific, Warsaw, Poland) equipped with an energy dispersive X-ray (EDX) microanalyser. Brunauer-Emmett-Teller (BET) surface area analysis was performed on a BET analyser (SA 3100, Beckman Coulter, Brea, California, USA). Magic angle spinning nuclear magnetic resonance (MAS NMR) measurements for silicon \({}^{29}\)Si NMR were conducted on the Bruker Avance Ultrashield 500 MHz spectrometer (Billerica, MA, U.S) and the Avance Ultrashield 500 MHz spectrometer (Waltham, MA, USA). The parameters were as follows: single pulse sequence, \({}^{29}\)Si frequency: 139.11 MHz, \(\pi\)/8 pulse length: 2.5 ms, recycle delay: 100 s, 1 k scans, external secondary reference: DSS. Under air flow, 3.2 mm zirconia rotors filled with samples were \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Composite**} & \multirow{2}{*}{**Tin Octoate**} & \multicolumn{2}{c}{**Matrix**} \\ \cline{3-4}  & & **Graphite** & **PMHS/DVB; 1:1 w/w Ratio** \\ \hline SiOC & - & - & 5 g \\ SiOC/Sn-40\% & 3.33 g & - & 5 g \\ SiOC:C\({}_{0.2}\)/Sn-40\% & 3.33 g & 1 g & 4 g \\ SiOC:C\({}_{0.2}\)/Sn-60\% & 7.5 g & 1 g & 4 g \\ SiOC:C\({}_{0.1}\)/Sn-60\% & 7.5 g & 0.5 g & 4.5 g \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Compositions of the preceramic blends. spun at 8 kHz.