Electrodes of TzG/Si@Cu are prepared by dissolving the organic monomer 2,4,6-tris(4-ethynylphenyl)-1,3,5-triazine and dispersing Si NPs in pyridine in a 25%:75% weight ratio, respectively. The reaction mixture is then transferred onto a copper foil (Figure ; Supplementary Information Section S1, Scheme S1 and S2). Residual Cu(II) and Cu(I) species on the untreated copper surface initiate the polymerization via a Glasertype oxidative coupling reaction. The polymerization is driven to completion and the pyridine is removed by evaporation. 13 C cross-polarization-magic-angle-spinning (CP-MAS) solid-state NMR (Figure ) shows the characteristic signals of a triazinebased graphdiyne polymer; the triazine carbon at ~170 ppm and the diyne-bridges at 75-85 ppm. An additional signal seen at ~30 ppm is attributed to O2SiMe2 surface groups originating from the preparation of these commercially available Si NPs. This is corroborated by 29 Si single-pulse-magic-angle-spinning (SP-MAS) solid-state NMR (Figure ) and Fourier transform infrared (FT-IR) spectra (Figure ). The Raman spectrum of TzG/Si@Cu (Figure ) shows stretching bands of diyne C≡C at 2209 cm -1 , of triazine C=N at 1411 cm -1 , of phenyl C=C at 1604 cm -1 , and of crystalline Si-Si bonds at 518 cm -1 . X-ray photoelectron spectroscopy (XPS) performed on c-axis oriented layers of TzG/Si@Cu show all expected carbon environments in the C 1s region that are observed for the neat TzG polymer (Figure ). In addition, Si 2p spectra show the presence of surface silicon oxide, SiOx (~34%) and neat silicon (66%) (Figure ), compared to as-received Si NPs that contain 21% of SiOx and 79% of Si(0) environments (Figure ). In summary, spectroscopic analysis confirms the formation of a covalent, conjugated, triazine-based polymer network around chemically unchanged Si NPs. For comparison, we have prepared three different types of electrode systems via the same one-pot method described above but with varying compositions to carefully test the effects of individual components: (i) growing a film of TzG on Cu, we obtain TzG@Cu, (ii) growing TzG in the presence of Si NPs we get TzG/Si@Cu (in ration 25/75 wt%), and (iii) TzG/Si/CB@Cu (in ratio 20/60/20 wt%) is produced by growing TzG around Si NPs and a conventional, conductive additive, carbon black (CB) (Supplementary Information Section S1.5). Scanning electron microscopy (SEM) images reveal the morphology of pristine TzG/Si@Cu electrodes. The material grown on the copper support adopts a porous, sponge-like, and homogeneous morphology as seen top-down (Figure ) and from cross-sections of the electrode film (Figure ). Crosssectional SEM imaging at lower magnifications shows films of TzG/Si with a thickness of ~25 µm that adhere well to the Cu substrate with no apparent gaps (Figure ). More detailed transmission electron microscopy (TEM) energy-filtered mapping on TzG/Si films shows a homogenous distribution of carbon and silicon on the nano-scale (Figure ). On the nanoscale, the electrode film consists of Si NPs homogenously embedded in an organic polymer matrix of TzG. Residual Cu nanoparticles can be seen within the polymer matrix that stem from the TzG polymerization process (Figure ; a comparison of TEM images of TzG@Cu and of pristine Si NPs can be found in Figure ). Overall, individual Si NPs are enclosed by the conjugated, polymer and held cooperatively as a film on the current collector. We have shown previously that neat, unmodified triazine-based graphdiyne polymers are narrow-band gap semiconductors (Eg,elec = 1.84 eV and conductivity of 1.2 µS cm -1 ) with moderate porosities (N2 BET surface area of 124 m 2 g -1 at 77 K). Hence, the composite TzG/Si on Cu foil (TzG/Si@Cu) has a promising combination of chemical, electrical, and structural features for electrochemical storage. In the following (Supplementary Information Section S2.1), we discuss the electric and electrochemical performance of TzGbased electrodes and the effects of the TzG polymer on the formation of the SEI. For the three electrode systems TzG/Si@Cu, TzG/Si/CB@Cu, and TzG@Cu (i) we compared the bulk conductivities of the unlithiated, "as-synthesized" electrodes (Figure ), (ii) we recorded cyclic voltammetry (CV) curves (Figure ), and (iii) we performed ex-situ XPS measurements probing the electrode surfaces to a depth of approx. 10 nm after a number of de-/lithiation cycles (Figure ; Figure , S9, S10 and S13; Table ; details of the XPS spectra fitting method are described in Supplementary Information section S1.7). During the first lithiation of the TzG/Si@Cu electrode, the resulting composition of the film present a content with up to 43.9 mol% of cumulative Li, F. and P elements (Table ). Furthermore, after the first cycle of lithiation and delithiation, we observe that the initial Si 2p signal of the pristine electrode decreases dramatically in intensity from 40.8 mol% to value close to zero (Table ). The fitting of C 1s, F 1s and Li 1s spectra (Figure ) indicate the formation of the SEI during the first lithiation, resulting in the variations of surface elemental concentration mentioned above. Note though that even after the 100th delithiation event, we detect a signal of Si(0) in the Si 2p region (Figure , iv, Table ). This finding suggests that SEI formation is completed at an early stage, and that SEI thickness does not increase dramatically throughout cycling. XPS data from the Si 2p region of TzG/Si@Cu electrodes cycled at C/8 (Figure ) shows the emergence of new Si 2p peaks corresponding to LixSi, SiOxFy, and LixSiOy after the first lithiation event. The presence of SiOxFy results from the non-faradic reaction between the remaining LiPF6 and the SiOx on the surface of the Si NPs, during the opening the cell for exsitu measurement. The environments of LixSi and LixSiOy originate from the electrochemical alloying reaction of silicon and SiOx with the lithium ions in the electrolyte, as further corroborated by emerging signals in the F 1s and Li 1s regions (Figure and). Pristine and delithiated samples of TzG/Si@Cu all show peaks corresponding to Si(0) and SiOx indicating a fully reversible de-/lithiation process. Ex-situ TEM and SEM imaging of TzG/Si@Cu electrodes at different cycling stages (Figure and) reveal that prior to cycling, the TzG polymer acts as a binder that joins Si NPs into a porous TzG/Si composite (i). After the first lithiation event, Figure . Characterization of TzG/Si@Cu electrodes using a, X-ray photoelectron spectroscopy (XPS) data from the Si 2p region, b, transmission electron microscopy (TEM), and c, scanning electron microscopy (SEM). Data is presented for TzG/Si@Cu electrodes in the pristine state (i), after the first lithiation (ii), after the first delithiation (iii) and after the 100 th delithiation (iv). Cycling was performed at a constant current of 0.32 mA cm -2 (i.e. C/8 for the capacity of Si, within 0.01-1.2 V vs. Li/Li + ).