Sur ﬁ cial nanoporous carbon with high pyridinic/ pyrrolic N-Doping from sp 3 /sp 2 -N-rich azaacene dye for lithium storage †

Rationally designed pyridinic/pyrrolic N-doping of anodic carbonaceous materials is signi ﬁ cant but rare for lithium storage materials. Herein, two highly pyridinic/pyrrolic N-doped carbon materials i.e. the pristine material (NC, N content 10.9 wt%) and template (NPC, N content 12.6 wt%) are predictively achieved by the direct pyrolyzation of the azaacene dye with a high sp 3 /sp 2 -N content (13.6 wt%). Their total amount of pyridinic and pyrrolic content is as high as 7 at% (NC)/6.98 at% (NPC), which is close to the theoretical value (9.1 at%) with less/no graphitic N. Both of them present higher capacities of 1094.79 mA h g (cid:1) 1 and 411.3 mA h g (cid:1) 1 , respectively, by the 3 rd cycle at 100 mA g (cid:1) 1 . Featured by sur ﬁ cial bumps and hollows of NPC particles, the NPC electrode not only exhibits a ﬁ rst reversible speci ﬁ c discharge capacity as high as 1178.9 mA h g (cid:1) 1 at a 100 mA g (cid:1) 1 current density but is also stabilized at 614 mA h g (cid:1) 1 after 200 cycles at a 200 mA g (cid:1) 1 current density. Our results indicate that the sp 3 /sp 2 -N-rich azaacene dye can be a useful highly pyridinic/pyrrolic N-doped carbon source for high performance anodic materials.


Introduction
Carbon-based anode materials have been explored as one of the most key active power components for portable electronic devices due to their high storage capacity, cycling stability, and reversibility in the elds of lithium-ion batteries, [1][2][3] supercapacitors, [4][5][6] oxygen reduction reactions, 7 and H 2 storage. 8urthermore, the intrinsic properties, including low theoretical and actual capacities, and limited application ranges 4,9 of conventional anodic graphite can no longer meet the requirements for high-efficient and convenient power consumption.Moreover, two main strategies i.e. enlargement of the electrolyte-electrode interfacial area and increasing the redox sites with pre-embedded heteroatoms are applied to optimize the electrochemical properties of carbon-based anode materials.
However, there are few reports 17,20 on the rational design of sp 3 /sp 2 -N-rich aromatics as a source of high ratio pyridinic/ pyrrolic N-doped carbon-based anode materials with N-doping ratios over 10 wt%, 3,20 large specic surface areas, and high Li + ion storage capacity. 28,49,50Therefore, it is highly desirable to explore this type of new carbon source due to its advantages of low-cost, simple synthesis, and high output as high-performing carbon-based anodic lithium storage materials.
Herein, we successfully synthesized two novel amorphous carbon materials, i.e.N-doped porous carbon (NPC) and pristine N-doped carbon (NC), by pyrolyzing the low-cost, easilysynthesized N-rich azaacene dye (mixture of trans isomeric Pigment Orange 43 and cis isomeric Pigment Red 194) 51 without/with a micro-SiO 2 template under an inert gas atmosphere.NPC possesses the special characteristic of surcial nanopores, whereas NC has no surcial pores.The novel Ndoped carbons are systematically characterized via elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), Raman spectroscopy, Brunauer-Emmett-Teller surface area measurements (BET), eld emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and electrochemical tests.

Synthesis
The p-conjugated azaacene dye carbon precursor was synthesized via a facile cyclocondensation route in high yield. 52As shown in Scheme 1, NC was prepared by the direct carbonization of the azaacene dye at high temperatures for 2 h under an Ar atmosphere.Considering that a low temperature will result in incomplete carbonization, whereas a high temperature will lead to a low heteroatom doping content, 17d we have nally chosen 700 C as the ideal temperature.As the temperature increased, the dye powder melted at 497-504 C (ref.52e) and decomposed accompanied by swelling.The high melting point led to a high carbon yield.Subsequently, the melted azaacene dye combusted and carbonized into intact, bulky, crisp, and black shiny 3D-like self-supported carbon due to the self-sealed pressure from the released waste gas.For NPC, the p-conjugated azaacene dye was granulated with micro-SiO 2 for 15 min and then carbonized under the former conditions.Then, the organic powder melted on the surface of the neighbouring micro-SiO 2 particles and then decomposed with waste gas released through the capillary space between micro-SiO 2 particles.This led to the formation of an NPC-SiO 2 mixture as a dark black powder, which is different from the shiny black 3D-like appearance (Scheme S1 †) of NC.Then, the as-prepared NPC-SiO 2 mixture was immersed in excess aqueous NH 4 HF 2 and sodium hydroxide in sequence to get rid of SiO 2 .Finally, NPC was obtained as a black powder with a large surface area.

Morphology
The shiny black 3D-like appearance of the self-supported carbon NC (Fig. S1a †) matched well with the smooth surface morphology shown in the FE-SEM images of the ground NC particles.The size of these particles ranged from several to tens of micrometers without well-dened structures, as shown in the magnied images (Fig. 1a inset).Moreover, NPC is presented as unshaped particles (Fig. 1b).From another magnied image (Fig. 1b inset), it was found that the surface of the NPC particles had large quantities of bumps and hollows in the range of tens of nanometers acting as effective Li + intercalation sites at the liquid-solid interface.The XRD patterns of both NC and NPC are inserted as white curves in Fig. 1a and b  graphitization degree for both NC and NPC.According to the Bragg's Law 2d sin q ¼ nl (l ¼ 1.5405 Å), the values of the weak d 002 and d 100 bands are 0.34-0.32nm (26-28 ) and 0.22-0.20 nm (41-46 ), respectively, indicating the low crystallinity 53,54 of these two new carbon materials.
TEM and HR-TEM analyses were further conducted to investigate the microstructures of the as-prepared carbon materials.In Fig. 1c and d, NPC displays a similar disordering of its inner crystalline lattice as NC; this indicates that their structural defects are favourable for Li + ion diffusion and effective contact between active materials and electrolyte. 55The HR-TEM images, as shown in Fig. 1e and f, also reveal that both NC and NPC are amorphous carbon materials without a long range order and straight crystalline lattices.However, a few large and distinguishable short lattice fragments were observed, which were marked by white arrows and bars.The measured width of the lattice fragments of NC was as large as 0.38/ 0.41 nm, and that for NPC was as large as 0.55/0.64nm, which was much rougher than NC.It was proposed that the surface pores originated from the interfacial etching reactions among the surcial corrosive matters and groups (-OH, -COOH, etc) on the template.Moreover, for NPC, the micro-SiO 2 template interdicted macroscale 3D-like cross-linkage and thermal agglomeration of the melting and combusting azaacene dye during the pyrolysis process.
Therefore, N 2 adsorption-desorption isotherms were obtained, as shown in Fig. 2. From the analysis of the BET data shown in Fig. 2a, the specic surface area of NPC was found to be 456.8m 2 g À1 , which was almost 264 times that of NC (1.73 m 2 g À1 ).Moreover, NPC possesses both micropores (peaked at 0.415 nm) and mesopores (peaked at 5.10 nm) (Fig. 2b).The pores of NPC supply a number of commodious reservoirs for potential Li + ion transport and accumulation under electrochemical test conditions.

Raman spectroscopy
The Raman spectra in Fig. 3a exhibit two distinct peaks attributed to the D band (ca.1353/1367 cm À1 ) and G band (ca.1600/ 1571 cm À1 ) for NC and NPC, respectively.The intensive G bands indicate that the degree of graphitization decreases from NC to NPC aer the addition of the micro-SiO 2 template.Compared with that of NC, the intensity ratio of the D-band versus G-band (I D /I G ) in NPC was calculated to be 1.02, which was larger than 0.86 calculated for NC and suggested much more structural defects and disorder [56][57][58] for NPC.This was further conrmed by its widely distributed surcial pores (Fig. 1b inset).

X-ray photoelectron spectroscopy (XPS)
XPS was used to analyse the elemental distribution and the related bonding species in NC and NPC.As shown in Fig. 3b, both NC and NPC mainly contain C, N, and O dopants with three characteristic peaks at $285 eV, $400 eV, and $532 AE 1 eV, corresponding to C 1s, N 1s, and O 1s.For NC, the total content of C, N, and O elements was 85.07 at%, 7.48 at%, and 7.45 at%, respectively.For NPC, the content of C, N, and O elements was 85.6 at%, 7 at%, and 5.57 at%, respectively.Moreover, a certain amount of residual uorine (1.64 at%) and Si (0.17 at%) in NPC was detected, which most likely originated from the F À of NH 4 HF 2 and SiO 2 particles, respectively, buried and trapped in the amorphous carbon during the de-SiO 2 process.
In Fig. 3c, the C 1s spectra for NC and NPC can be deconvoluted into several individual peaks.The tted peaks at 284.6 eV, 286.1 eV, and 288.5 eV are related to the bonding congurations of C]C, C]N/C-O, and C]O, respectively. 59It should be noted that the ratios of sp 2 -Cs in NC and NPC occupied a large percentage (61.58at% and 58.99 at%, respectively).The other types of C elements occupied 23.49 at% for NC and 26.61 at% for NPC.In Fig. 3c, the three deconvoluted peaks for N element in NC at 398.3 eV, 400.8 eV, and 402.8 eV belong to pyrrolic-N, pyridinic-N, and graphitic N/oxidized N species, occupying 3.67 at%, 3.31 at%, and 0.5 at%, respectively. 29,60,61owever, in Fig. 3d, only pyrrolic-N and pyridinic-N peaks in NPC were detected, which were 3.41 at% and 3.59 at%, and they originated from the pre-embedded sp 2 -N and sp 3 -N, respectively.As shown in Fig. S3b, † the spectra of O element in NC were also deconvoluted into two peaks at 531.5 eV and 532.8 eV, occupying 5.11 at% and 2.34 at% from O]C and O-C, respectively.For NPC, the corresponding two tted peaks at 531.5 eV and 532.8 eV of O element occupied 3.55 at% and 2.02 at%, respectively.The FE-SEM mapping images exhibit the homogenous distribution of C, N, and O for both the NC (Fig. S2d-S2f †) and NPC (Fig. S2h-S2j †) particle samples.Elemental analysis disclosed that the N content weight ratios in both NC and NPC were as high as 10.9 wt% and 12.6 wt%, respectively, which were higher than those in other N-contained carbonaceous materials for LIBs. 19,62,63he investigation on NC and NPC suggested that employment of effective templates and heteroatom pre-embedded precursors is an efficient strategy to produce highly N-doped porous carbon materials, 33,64,65 which are expected to display effective Li + ion insertion/de-insertion behaviour according to the proposed lithiation mechanism (Scheme 1). 17

Electrochemical performance
Fig. 4a and S4b † present the rst three voltage curves of the NC and NPC electrodes measured through cyclic voltammogram (CV) under ambient conditions between 0.005 and 3.0 V at a scan rate of 0.5 mV s À1 .The rst discharge semi-cycle did not overlap the subsequent cycles.The strong but broad peaks at 0.4-1 V were mainly contributed by the newly generated solid electrolyte interphase (SEI) lm, [66][67][68][69][70][71] especially for the enhanced peak at 0.4 V for NPC with an improved surface area.
In Fig. 4b, the NC and NPC electrodes present rate performance curves at different current densities ranging from 100, 200, 400, 800, and 1600 to 3200 mA g À1 .At a current density of 100 mA g À1 , initial irreversible discharge capacity of the NPC electrode reached as high as 2283 mA h g À1 , which was thrice the corresponding value of 767 mA h g À1 for the NC electrode.It should be noted that these values are almost double of their initial reversible charge capacities of 1107 mA h g À1 and 448 mA h g À1 .The irreversibility is most likely due to the SEI effect and unidirectional Li + ion trapping or consumable reaction.However, in the rst reversible cycle, the specic discharge capacity of the NPC electrode reached as high as 1178.9mA h g À1 and stabilized at 1014 mA h g À1 by the end of the 11 th cycle, which were both nearly 2.7 times the corresponding value of 432 mA h g À1 , and stabilized at 387 mA h g À1 for the NC electrode.0][71][72][73] Even at the strongest current density of 3.2 A g À1 , the NC and NPCbased electrodes still delivered 155 and 207 mA h g À1 at the 57 th cycle.Signicantly, when the current density returned to 100 mA g À1 in the 70 th cycle, the discharge capacity was still over 976 mA h g À1 and 389 mA h g À1 for NPC and NC, respectively, which recovered to the initial value although slightly lower than that in the rst cycle at a current density of 100 mA g À1 .Fig. 4c shows the cyclability and coulombic efficiency (CE) at a current density of 200 mA g À1 for the NC and NPC electrodes.The initial irreversible discharge capacity of the NPC electrode reached as high as 1648 mA h g À1 .The NPC electrode exhibited good and stable cyclability with a specic capacity ranging from 700 to 614 mA h g À1 up to the 200 th cycle.In contrast, the NC electrode presented a much lower specic capacity ranging from 325 to 364 mA h g À1 up to the 200 th cycle.However, both their coulombic efficiencies from the second cycle up to the 200 th cycle were almost over 93%.In this situation, the corresponding galvanostatic charge/discharge proles (Fig. 4d) of the better performing NPC electrode were obtained at six different current densities ranging from 100, 200, 400, 800, 1600, to 3200 mA g À1 at the 10 th , 20 th , 30 th , 40 th , 50 th , and 60 th cycles, respectively, which indicated that the NPC electrode exhibited matching electrochemical properties, almost coinciding with the charge/discharge voltage-capacity curves.To further understand the charge-transfer ability and illustrate the advantages of the NPC electrode over the NC electrode, EIS tests on NPC and NC electrodes were conducted under the same conditions.The charge transfer resistances are depicted in Fig. S5.† It can be seen that both electrodes present highfrequency semicircles, representing the charge-transfer process.The EIS plots in the low-frequency region suggest that the NPC electrode possesses a lower Li + ion diffusion resistance than the NC electrode.From the CV measurement and calculation, the charge-transfer resistance of the NPC electrode was calculated to be about 42.5 U, smaller than that of NC (55.7 U), indicating its better conductivity.
Compared to the NC electrode, the improved high capacity and rate capability of the NPC electrode could be explained by its special porous structure and high-level N-doping.As abovementioned, the carbonaceous NPC electrode plays a key role as conductive channels 1,9b,18 for electron transport, whereas its large number of pores possibly act as Li + ion storage cisterns. 71urthermore, the enlarged electrode/electrolyte interface of the NPC electrode promotes the rapid absorption and release of Li + ions with a fast charge-transfer process.

Conclusions
In summary, two novel pyridinic/pyrrolic N-doped carbon materials have been rationally designed and successfully obtained via the pristine and template pyrolyzation of an sp 2 /sp 3 -N-rich azaacene dye with a complete p-conjugated framework and high melting point.The anodic NPC electrode exhibited high Li + ions storage performance due to its high pyridinic/ pyrrolic N-content and electrolyte and liquid interface.Its specic capacity was as high as 614 mA h g À1 aer 200 cycles even at 200 mA g À1 .Our results indicate that the pyrolyzation of azaacene dye rich in sp 3 /sp 2 -N is an effective strategy to achieve more highly monospecies or concomitant pyridinic/pyrrolic Ndoped carbon materials.

Synthesis of NC and NPC
Azaacene was synthesized, ltered, washed, and dried as a red powder in 64% yield by reuxing DMF in o-benzene-diamine and naphthalene-1,4,5,8-tetracarboxylic acid dianhydride.The mixture of azaacene dye (1.5 g) with micro-SiO 2 (3 g) was mixed and ground for 10 min.Azaacene (1 g) and the ground mixture were put into corresponding Al 2 O 3 crucibles, in an OTF-1200X tube furnace with SiO 2 glass and protected under an Ar atmosphere and heated to 700 C at a heating rate of 3 C min À1 and kept for 0.5 h under a nitrogen atmosphere.The activated mixture was then washed with the NH 4 HF 2 solution (1 mol L À1 ), NaOH solution (1 mol L À1 ), and deionized water in sequence until the ltrate became neutral.The sample was nally dried overnight at 100 C in an oven.The carbonization yields of NC and NPC were 65% and 40%, respectively.

Methods
The morphology of the samples was observed via FE-SEM (Hitachi S-4800, Tokyo, Japan).TEM observation was carried out using a JEOL 2100F microscope operating at an accelerating voltage of 100 kV.HR-TEM observation was carried out using an FEI TECAI G2F20 microscope operating at an accelerating voltage of 200 kV.X-ray diffraction (XRD) patterns were obtained using a Bruker D8 X-ray diffractometer with Cu Ka radiation (l ¼ 1.5405 Å).Raman spectroscopy was carried out suing a WITec alpha300M+ micro-Raman confocal microscope.CHN elemental analysis was performed using a Thermo Scien-tic FLASH 2000 elemental analyzer.XPS measurements were performed using a Thermo Scientic ESCALAB 250XI system with a monochromatic Al Ka X-ray source.Nitrogen adsorption and desorption isotherms were determined by nitrogen physisorption at 77 K using a V-Sorb 2800P BET surface area and pore volume analyser.

Electrochemical tests
The electrochemical properties of NC and NPC were measured with 2032 coin cells using pure lithium foil as the counter electrode.The working electrodes were constructed by mixing the as-prepared azaacene dye samples of 80 wt% NC or NPC, 10 wt% polyvinylidene uoride (PVDF), 10 wt% acetylene black, and N-methyl-2-pyrrolidone as the dispersing solvent to obtain slurries, which were coated homogeneously onto the copper foil and dried at 100 C overnight in a vacuum oven.Circular electrodes (12 mm in diameter) were prepared using a punching machine and weighed with an electronic balance within 0.01 mg.The electrolyte was a 1.0 mol L À1 LiPF 6 in ethylene carbonate and dimethyl carbonate solution (1 : 1 (w/w)).Celgard 2300 was applied as the separator.Coin cells with both electrodes were assembled in an inert Ar gas-lled glove box.Both the moisture and oxygen contents were kept below 0.1 ppm.The galvanostatic charge/discharge parameters of the cells were measured using a multichannel NEWARE batterytesting system with the voltage ranging from 0.005 to 3.0 V vs. Li + /Li at different rates.The calculated capacity values were determined based on the weight of the loaded carbon materials on the collectors.Galvanostatic cycling was performed using a NEWARE multichannel battery testing system.Alternative current (AC) impedance measurements were conducted using a CHI 760D electrochemical workstation (CH Instruments, Inc.) at room temperature.EIS was performed with an AC voltage of 5 mV amplitude in the frequency range from 100 kHz to 0.1 Hz.

Fig. 1
Fig. 1 FE-SEM images of NC (a) and NPC (b), TEM (c, d) and HR-TEM (e, f) images of NC (c, e) and NPC (d, f).Inset images: magnified surficial morphologies of NC and NPC.Inset white curves: XRD patterns of NC and NPC.White arrows and bars: distances of the crystal lattices.

Fig. 3
Fig. 3 Raman spectra (a) of NC and NPC synthesized from p-conjugated azaacene dye at 700 C. C 1s (c) and N 1s (d) XPS spectra (b) with the deconvoluted curves of NC and NPC.

Fig. 4
Fig. 4 (a) Electrochemical performance of the NPC electrode: the first, second, and third CV profiles at a scan rate of 0.5 mV s À1 over the potential window of 0.005-3 V (vs.Li/Li + ).(b) Rate performance at different current densities from 100 to 3200 mA g À1 .(c) Cycling performance and CE at 200 mA g À1 d À1 ) Galvanostatic chargedischarge curves of NPC at different current densities ranging from 100 to 3200 mA g À1 in the 10 th -60 th cycles.