Figure 1. Micro- and macroscopic characterization of the chemical make-up of TzG/Si@Cu (TzG/Si = 25/75 wt%) electrodes.a , Synthetic pathway for the triazine-based graphdiyne (TzG) polymer. Spectroscopic characterization showing the correspondingb , 13C cross‐polarization / magic‐angle‐spinning (CP/MAS) solid-state NMR spectrum of the TzG/Si polymer composite, and c , the Raman spectrum with fluorescence background collected with a 532 nm laser. d , Schematic of the TzG/Si@Cu electrode. Scanning electron microscopy (SEM) images showinge , the top-view surface morphology and f , the cross section morphology of TzG/Si@Cu films. g , Low-resolution transmission electron microscopy (TEM) images and energy-filtered mapping images showing the homogenous distribution of carbon and silicon environments, and h , high-resolution TEM images showing Si NPs fully enclosed by TzG polymer.
composition of the film presents a content with up to 43.9 mol% of cumulative Li, F, and P elements (Table 1). 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 a value close to zero (Table 1). The fitting of C 1s, F 1s and Li 1s spectra (Figure S10) indicates 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 2a, iv, Table 1). This finding suggests that SEI formation is completed at an early stage, and that SEI thickness does not increase dramatically throughout cycling.24,25 XPS data from the Si 2p region of TzG/Si@Cu electrodes cycled at C/8 (Figure 2a) shows the emergence of new Si 2p peaks corresponding to LixSi, SiOxFy, and LixSiOy after the first-cycle lithiation.19-22 The presence of SiOxFy presumably results from the non-faradic reaction between the remaining LiPF6 and the SiOx on the surface of the Si NPs, during the opening of the cell for ex-situ measurement.19 The environments of LixSi and LixSiOyoriginate 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 S10b and S10c).20,22 Pristine and delithiated samples of TzG/Si@Cu all show peaks corresponding to Si(0) and SiOx, indicating a fully reversible de-/lithiation process.