Triazine-Based Graphitic Carbon Nitride Thin Film as a Homogeneous Interphase for Lithium Storage

Triazine-based graphitic carbon nitride is a semiconductor material constituted of cross-linked triazine units, which differs from widely reported heptazine-based carbon nitrides. Its triazine-based structure gives rise to significantly different physical chemical properties from the latter. However, it is still a great challenge to experimentally synthesize this material. Here, we propose a synthesis strategy via vapor-metal interfacial condensation on a planar copper substrate to realize homogeneous growth of triazine-based graphitic carbon nitride films over large surfaces. The triazine-based motifs are clearly shown in transmission electron microscopy with high in-plane crystallinity. An AB-stacking arrangement of the layers is orientationlly parallel to the substrate surface. Eventually, the as-prepared films show dense electrochemical lithium deposition attributed to homogeneous charge transport within this thin film interphase, making it a promising solution for energy storage.


Table 1 | EELS analysis of the spectrum in Figure S9b
Table 2 | Atomic ratio on surface (by XPS) for t-CN@Cu sample Reference Figure S1 | UV-vis-NIR absorbance spectrum of t-CN@Cu.

Note:
The multiple oscillations in the wavelength range of 400-1200 nm are interference fringes due to the constructive and destructive interferences of various internally reflected wavelengths of light inside t-CN layer.This phenomenon confirms the parallel orientation of t-CN layer stacking and further represents the homogeneity of the t-CN phase throughout the thickness.S1.
Particularly, d is the thickness of t-CN layers, which is approximated to the diffusion distance across a 2D film.

Note:
The differences between the CN@Cu samples prepared at different temperatures (450 ℃, 500 ℃ and 550 ℃) were investigated.As shown in the SEM images of the CN@Cu samples prepared at different temperatures (Figure S18), the CN layers at different temperatures are all evenly covered on the substrates, while the functional groups and degree of polymerization are varied.According to the FT-IR spectra in the supporting information (Figure S15), the CN prepared at 450 ℃ is lowpolymerized as the weak vibrations at the range of 1100~1800 cm -2 , which are ascribed to the C−N of condensed triazine-based carbon nitride phase.As shown in the Figure S19, more contribution from C≡N terminal groups is observed in the XPS spectra of C1s and N1s of the sample prepared at 450 ℃, indicating the existence of ring-open structures and the low polymerization of CN.At a higher deposition temperature of 550 ℃, the change in the vibrations as shown in the FT-IR spectra reveals the depolymerization of t-CN phase (Figure S15).The XPS results show differences in the peak contributions between the sample prepared at 500 ℃ and 550 ℃, confirming the chemical environment changed at the high temperature.In addition, the thicknesses of these three samples are also different, which is confirmed by their varied capacities of initial lithiation.As shown in the Figure S20a, these three samples show different plateau potentials and specific areal capacities.Less condensation can be considered as less oxidization, which is translated into a low redox potential of the sample prepared at 450 ℃.The depolymerization of the sample prepared at 550 ℃ creates complicate chemical structures at more oxidized state, exhibiting a different electrochemical behavior from t-CN prepared at 500 ℃.As shown in the Figure S20b, the t-CN sample performs the best cycling stability among all these samples.Therefore, 500 ℃ was selected as the optimal condition for the synthesis of t-CN material in this study.

Note:
The current density is 0.1 mA cm -2 .The t-CN@Cu electrode exhibits a specific capacity of 0.23 mAh cm -2 with a plateau at around 0.8 V (vs Li + /Li), which is absent in the case of the bare Cu electrode.The formation of Li−N=C or Li−C=N species were observed in the activated electrode by FT-IR spectroscopy as a set of strong vibration bands centred near 2000 cm -1 , which are similar to the spectral features in this region of lithium nitrile. 2 We cannot exclude that the active N sites react with Li + and electrons, contributing partially to the irreversible capacity in the plateau region, which occurs in the first charging process.

Figure S1 |
Figure S1 | UV-vis-NIR absorbance spectrum of t-CN@Cu Figure S2 | AFM images and roughness of bare Cu substrates and t-CN@Cu samples Figure S3 | AFM analysis of t-CN film Figure S4 | SEM images of bare Cu substrate and t-CN@Cu sample Figure S5 | HR-TEM and SAED images of different t-CN sheets Figure S6 | HR-TEM image of t-CN sheet with corresponding FFT image of the selected region Figure S7 | Pole figure plots of (002) diffraction of t-CN@Cu sample Figure S8 | SEM image and EDX mapping of t-CN@Cu sample Figure S9 | The STEM image and corresponding spectra of selected regions Figure S10 | ToF-SIMS spectrum of t-CN@Cu sample Figure S11 | Three-dimensional overlay figure of CuN x C y fragments Figure S12 | ADF-STEM image and corresponding EELS spectrum of the t-CN layer Figure S13 | The XPS spectrum of t-CN@Cu Figure S14 | TGA-MS results of melamine Figure S15 | The comparison of FT-IR spectra of precursor and samples prepared at different condensation temperatures Figure S16 | The electrochemical activation of the t-CN@Cu electrode Figure S17 | GITT measurements of t-CN@Cu electrode Figure S18 | SEM images of CN@Cu samples prepared at different temperatures Figure S19 | XPS spectra of C 1s and N 1s of CN@Cu samples prepared at different temperatures Figure S20 | The comparison of electrochemical Li storage behavior of CN@Cu electrodes prepared at different temperatures Figure S21 | FIB-SEM images of the cross-section view of activated t-CN@Cu Figure S22 | EDX mapping of the cross-section view of t-CN@Cu electrode after 0.5 mAh cm -2 Li deposition Figure S23 | FIB-SEM images of the cross-section view of t-CN@Cu electrodes after Li stripping Figure S24 | The activation of t-CN@Cu electrode Figure S25 | Galvanostatic charge/discharge curves of bare Cu and t-CN@Cu electrodes at various discharging current densities Figure S26 | Galvanostatic charge/discharge curves of bare Cu and t-CN@Cu electrodes with various specific areal capacities Figure S27 | The cycling performance of bare Cu||Li and t-CN@Cu||Li cells after the multicapacity cycling tests.Figure S28 | Full cell performance of LFP||Cu and LFP||t-CN@Cu batteries.

Figure S2 |
Figure S2 | AFM images and roughness of bare Cu substrates and t-CN@Cu samples.The AFM image and 3D version of (a) bare Cu substrate and (b) t-CN@Cu sample; (c) The roughness statistics of six samples of bare Cu and t-CN@Cu each (box: 25~75%).

Figure S3 |
Figure S3 | AFM analysis of t-CN film.(a) The AFM image of a piece of t-CN sheet and (b) the height plot of selected cutting line.

Figure S4 |
Figure S4 | SEM images of (a) bare Cu substrate and (b) t-CN@Cu sample.

Figure S5 |
Figure S5 | HR-TEM images of different t-CN sheets with corresponding SAED and FFT images of the selected region.

Figure
Figure S6 | (a) HR-TEM image of t-CN sheet with corresponding (b) FFT image of the selected region.

Figure S8 |
Figure S8| SEM image and EDX mapping of t-CN@Cu sample.The C/N atomic ratio is 0.76 close to that of ideal triazine-based graphitic carbon nitride (C 3 N 4 ).

Figure S9 |
Figure S9 | The STEM image and corresponding spectra of selected regions.(a) the t-CN sheet of the collected EDX mappings in Figure 2a; (b) Line profile and (c) EDX spectra of the selected regions.The sample was loaded on a gold microgrid.

Figure
Figure S11 | ToF-SIMS spectrum of t-CN@Cu sample.The main signals are assigned to triazine-based units and C x N y Cu z fragments, but the typical fragments of heptazine-based units are not pronounced.This result indicates the triazine-based structure and uniform dispersion of Cu atoms. 1

Figure S14 |
Figure S14 | TGA-MS results of melamine.The first mass loss region before 300 o C is attributed to the sublimation of melamine; the second mass loss between 420 o C and 500 o C is for the condensation of melamine.The ionic currents of NH 2 + (m/z=16, light blue), NH 3 + (m/z=17, dark blue), NO + (m/z=30, light green) and N 2 O + (m/z=44, dark green) corresponding to the release of NH 3 and the reactions between NH 3 and Cu oxides on the surface of bare Cu.

Figure S15 |
Figure S15 | The comparison of FT-IR spectra of precursor and samples prepared at different condensation temperatures.

Figure S16 |
Figure S16| The electrochemical activation of the t-CN@Cu electrode.(a) The initial lithiation of bare Cu and t-CN@Cu electrodes to 0 V (vs Li + /Li) at a constant current density of 0.1 mA cm -2 ; (b) EIS response of bare Cu and t-CN@Cu electrodes after activation cycles.

Figure S17 |
Figure S17| GITT measurements of t-CN@Cu electrode.(a) GITT curves of t-CN@Cu electrode upon the initial lithitation cycle as a function of time in the potential range of 0.0-2.0V (vs Li + /Li); (b) A selected single titration of GITT measurement at around 0.55-0.60V (vs Li + /Li) upon lithiation process; (c) The chemical diffusion coefficients (D Li + , cm 2 s -1 ) versus potential (V, vs Li + /Li) of t-CN@Cu electrode in the initial lithitation cycle.

Figure S18 |
Figure S18 | SEM images of CN@Cu samples prepared at different temperatures.The surface and cross-section view (inset) of the samples prepared at (a) 450 o C and (b) 550 o C.

Figure S19 |
Figure S19 | XPS spectra of CN@Cu samples prepared at different temperatures.The XPS spectra of (a) C 1s and (b) N 1s.(top: 450 o C; middle: 500 o C; bottom: 550 o C)

Figure S21 |
Figure S21| The FIB-SEM images of the cross-section view of activated t-CN@Cu.The thickness of the t-CN interphase increases from 0.70 μm to 1.20 μm after activation.The corresponding elemental mapping images show that the electrolyte-derived SEI (containing O) mainly distributes on the interface between t-CN and electrolyte.This result indicates that the desolvation of Li + occurs out of the t-CN interphase and the continuous decomposition of electrolyte can be suppressed upon cycling.

Figure S24 |
Figure S24 | The activation of t-CN@Cu electrode.(a) Galvanostatic charge/discharge curves of initial activation process; (b) FT-IR spectra of pristine and lithiated t-CN@Cu electrode.

Figure S25 |
Figure S25 | Galvanostatic charge/discharge curves of (a) bare Cu and (b) t-CN@Cu electrodes at various discharge current densities.

Figure S26 |
Figure S26 | Galvanostatic charge/discharge curves of (a) bare Cu and (b) t-CN@Cu electrodes with various specific areal capacities at the current density of 0.5 mA cm -2 .

Figure S27 |
Figure S27 | The cycling performance of bare Cu||Li and t-CN@Cu||Li cells after the multicapacity cycling tests.Inset: potential vs time plot of corresponding charge/discharge cycling.

Table S2
| Atomic ratio on surface (by XPS) for t-CN@Cu.