Nitrogen-Doped Carbon for Sodium-Ion Battery Anode by Self-Etching and Graphitization of Bimetallic MOF-Based Composite

•Bimetallic MOF-based nanocomposites enable self-etching and graphitization•A N-doped porous carbon tubule possesses large interlayer spacing up to 0.44 nm•The carbon tubule paper manifests ultralong cycling life for Na-ion batteries Carbon nanofibers (CNFs) and carbon hollow tubules (CHTs) are attractive anode materials for Na-ion batteries. Here, we report a method of self-etching and graphitization of metal-organic-framework-based nanocomposites for synthesizing a series of N-doped porous nanocarbons with a large fraction of graphitic carbon with larger spacing (0.38–0.44 nm). The N-doped porous CHT paper shows an outstanding cycling life over 10,000 cycles with no clear decline in capacity. Such a strategy could provide new avenues for the rational engineering of nanostructured N-doped carbonaceous materials with large graphene interlayer spacing for better Na-ion batteries. The greater availability of sodium (Na) over lithium (Li) motivates development of a Na-ion battery that can compete with a Li-ion battery. In these batteries, both electrodes consist of hosts into which Li+ or Na+ can be inserted reversibly. Graphite has been the anode host for Li-ion batteries, but the Na+ ion is too large to be inserted easily between the flat graphene layers of common graphite. We report the synthesis and electrochemical performance of N-doped carbon nanofibers and tubules with an organic-liquid electrolyte and a large fraction of graphitic carbon and larger spacing (0.38–0.44 nm) between carbon sheets; the carbon hollow tubules yield ultrastable (10,000 cycles), high-rate capabilities of Na+ intercalation and deintercalation with reversible capacities up to 346 mAh g−1. The greater availability of sodium (Na) over lithium (Li) motivates development of a Na-ion battery that can compete with a Li-ion battery. In these batteries, both electrodes consist of hosts into which Li+ or Na+ can be inserted reversibly. Graphite has been the anode host for Li-ion batteries, but the Na+ ion is too large to be inserted easily between the flat graphene layers of common graphite. We report the synthesis and electrochemical performance of N-doped carbon nanofibers and tubules with an organic-liquid electrolyte and a large fraction of graphitic carbon and larger spacing (0.38–0.44 nm) between carbon sheets; the carbon hollow tubules yield ultrastable (10,000 cycles), high-rate capabilities of Na+ intercalation and deintercalation with reversible capacities up to 346 mAh g−1. Lithium-ion batteries (LIBs) open up huge potential markets for rechargeable batteries in powering vehicles and storing electric power for the grid.1Armand M. Tarascon J.M. Building better batteries.Nature. 2008; 451: 652-657Crossref PubMed Scopus (15043) Google Scholar, 2Chen Y.M. Yu X.Y. Li Z. Paik U. Lou X.W. 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First, polyacrylonitrile (PAN) was dissolved in dimethyl formate (DMF; Aldrich) solvent. Then variable ratios of zinc acetate Zn(Ac)2 and cobalt acetate Co(Ac)2 were uniformly mixed with the PAN solution mentioned above to obtain a homogeneous electrospinning solution. The PAN/Zn(Ac)2/Co(Ac)2 composite nanofibers can be easily produced by a simple electrospinning technique, forming a free-standing paper. Next, the composite nanofiber paper was introduced into ethanol solution containing 2-methyl-imidazole; after 12 hr at room temperature, the potent coordination of 2-methylimidazole to both Zn2+ and Co2+ ions32Chen Y.Z. Wang C. Wu Z.Y. Xiong Y. Xu Q. Yu S.H. Jiang H.L. From bimetallic metal-organic framework to porous carbon: high surface area and multicomponent active dopants for excellent electrocatalysis.Adv. Mater. 2015; 27: 5010-5016Crossref PubMed Scopus (1117) Google Scholar, 33Stassen I. Styles M. Grenci G. Gorp H.V. Vanderlinden W. Feyter S.D. Falcaro P. Vos D.D. Vereecken P. 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Upon heating the film to 700°C, the BMZIF coating was carbonized; ZIF-8 (zinc coordinated by four imidazolate rings) enables high N doping of a porous carbon structure,34Zhang W. Wu Z.Y. Jiang H.L. Yu S.H. Nanowire-directed templating synthesis of metal–organic framework nanofibers and their derived porous doped carbon nanofibers for enhanced electrocatalysis.J. Am. Chem. Soc. 2014; 136: 14385-14388Crossref PubMed Scopus (511) Google Scholar and ZIF-67 (cobalt coordinated by four imidazolate rings) provides well-graphitized carbon.26Wu R. Wang D.P. Rui X. Liu B. Zhou K. Law A.W. Yan Q. Wei J. Chen Z. In-situ formation of hollow hybrids composed of cobalt sulfides embedded within porous carbon polyhedra/carbon nanotubes for high-performance lithium-ion batteries.Adv. Mater. 2015; 27: 3038-3044Crossref PubMed Scopus (567) Google Scholar, 35Xu J. Wang M. Wickramaratne N.P. Jaroniec M. Dou S. Dai L. High-performance sodium ion batteries based on a 3D anode from nitrogen-doped graphene foams.Adv. Mater. 2015; 27: 2042-2048Crossref PubMed Scopus (744) Google Scholar Inheriting the advantages of carbons from both ZIF-67 and ZIF-8, the BMZIF layer was converted into N-doped porous carbon with high graphitization. The PAN core was turned into carbon that can be etched by ZnO from the decomposition product of Zn(Ac)2 in the composite fiber core,36Jin J.E. Lee J.H. Choi J.H. Jang H.K. Na J. Whang D. Kim D.H. Kim G.T. Catalytic etching of monolayer graphene at low temperature via carbon oxidation.Phys. Chem. Chem. Phys. 2016; 18: 101-109Crossref PubMed Google Scholar, 37Kim C. Ngoc B.T.N. Yang K.S. Kojima M. Kim Y.A. Kim Y.J. Endo M. Yang S.C. Self-sustained thin webs consisting of porous carbon nanofibers for supercapacitors via the electrospinning of polyacrylonitrile solutions containing zinc chloride.Adv. Mater. 2007; 19: 2341-2346Crossref Scopus (388) Google Scholar as shown in Figure 1C, according to the carbothermal reduction ZnO + C → Zn + CO2 or CO. The carbon core can be completely etched out by increasing the content of Zn(Ac)2. The free-standing paper was then treated with acid (HCl) to remove the residual metals within the material to yield a N-doped CHT with spacing of 0.38–0.44 nm between the graphene layers (Figure 1B1–4). The thickness of this free-standing CHT paper is ∼70–80 μm and the nominal density is ∼0.45 g cm−3. The in-plane sheet resistance is ∼3 Ohm/sq from four-probe measurements. The paper also possesses good mechanical integrity on bending, which could be useful for making flexible batteries. The as-synthesized free-standing N-doped porous CHT paper exhibits excellent electrochemical performance with high specific capacity, a remarkable rate capability, and a long cycle life of over 10,000 cycles when evaluated as the anode of a SIB with a liquid electrolyte. Typical field-emission scanning electron microscopy (FESEM) images show that the BMZIFs are grown onto the composite nanofibers (Figures S1A and S1B). A core-shell structure can be clearly identified by transmission electron microscopy (TEM; Figures S1C–S1E). Energy-dispersive X-ray mappings (EDX) show that C, N, Zn, and Co are uniformly distributed throughout the prepared materials (Figures S1F–S1J), whereas O from the metal acetate is located only in the core of the composite (Figure S1K), consistent with a PAN/Zn(Ac)2/Co(Ac)2 core with a shell of BMZIF. The X-ray diffraction (XRD) pattern of the synthesized hybrids indicates typical diffraction peaks in the ZIF phase (Figure S2A).33Stassen I. Styles M. Grenci G. Gorp H.V. Vanderlinden W. Feyter S.D. Falcaro P. Vos D.D. Vereecken P. Ameloot R. Chemical vapour deposition of zeolitic imidazolate framework thin films.Nat. Mater. 2016; 15: 304-310Crossref PubMed Scopus (402) Google Scholar, 34Zhang W. Wu Z.Y. Jiang H.L. Yu S.H. 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At 300°C, the sample was found to be almost identical to the original (Figures 2B and 2C). Some black nanoparticles were observed in the composite nanofibers after heating at 500°C (Figures 2D and 2G). High-resolution TEM (HRTEM) analysis of these particles (inset of Figure 2G) demonstrated that they have an interplanar distance of 0.26 nm, corresponding well to that of the ZnO (002) plane,41Wang G. Li Z. Li M. Chen C. Lv S. Liao J. Aqueous phase synthesis and enhanced field emission properties of ZnO-sulfide heterojunction nanowires.Sci. Rep. 2016; 6: 29470Crossref PubMed Scopus (15) Google Scholar indicating that the Zn(Ac)2 was converted into ZnO nanoparticles during the heating process. These ZnO nanoparticles are only localized in the fiber core, because of the initial distribution of O. TEM and EDX mapping further prove that ZnO/carbon composite nanofibers can be achieved by calcination of PAN/Zn(Ac)2/Co(Ac)2 composite nanofibers in inert gas at 500°C (Figure S3). All the diffraction peaks in the XRD pattern can be well indexed to hexagonal ZnO (JCPDS card no. 36-1451). No peak for the Co-based materials can be observed because of its low content in the composite. Interestingly, some pores can be clearly identified in the CNFs at 700°C (Figures 2E and 2H), formed by the reaction ZnO + C → Zn + CO2↑ or CO↑. Some of the Zn materials formed are evaporated and some can react with Co metal from the decomposition of Co(Ac)2 to form zinc-cobalt metals such as Zn13Co and Zn3Co.37Kim C. Ngoc B.T.N. Yang K.S. Kojima M. Kim Y.A. Kim Y.J. Endo M. Yang S.C. Self-sustained thin webs consisting of porous carbon nanofibers for supercapacitors via the electrospinning of polyacrylonitrile solutions containing zinc chloride.Adv. Mater. 2007; 19: 2341-2346Crossref Scopus (388) Google Scholar Zn3Co is not stable at high temperature and will decompose into Co and Zn13Co. An interplanar distance of 0.244 nm corresponding to the CoZn13 (130) plane can be seen (Figure 2I). Figure S4 shows that porous metal/carbon composite nanofibers can be obtained by heating composite nanofibers in inert gas at 700°C. No black particles can be observed inside the carbon particles after carbonization of the BMZIFs. Co and Zn elements can still be detected everywhere (Figure S5). Electron-diffraction and HRTEM analysis were also carried out to study the formation process. Figures 2J–2M show the evolution of selected-area electron-diffraction (SAED) patterns and HRTEM images with increasing temperature. From 25°C to 300°C, no diffraction ring was observed, indicating that the precursors are amorphous, consistent with the HRTEM result (Figures 2J and 2K). When the temperature was increased to 500°C, blurred diffraction rings appeared and a 2- or 3-layer-stacked graphene layer (generally curved) was observed on the HRTEM images (Figure 2L). The SAED pattern in the inset of Figure 2L can be well indexed to carbon, indicating the initiation of graphitization at this stage.22Chen Y. Lu Z. Zhou L. Mai Y.W. Huang H. Triple-coaxial electrospun amorphous carbon nanotubes with hollow graphitic carbon nanospheres for high-performance Li ion batteries.Energy Environ. Sci. 2012; 5: 7898-7902Crossref Scopus (176) Google Scholar, 42Chen Y. Lu Z. Zhou L. Mai Y.W. Huang H. In situ formation of hollow graphitic carbon nanospheres in electrospun amorphous carbon nanofibers for high-performance Li-based batteries.Nanoscale. 2012; 4: 6800-6805Crossref PubMed Scopus (82) Google Scholar The diffraction rings became brighter and sharper with further increase in temperature, implying enhanced graphitization as further confirmed by HRTEM (Figure 2M). As a result, porous CNFs with high graphitization can be produced. The decomposition of Zn(Ac)2 in the precursor solution to form ZnO plays an important role in the formation of a porous structure, because etching by ZnO leaves porosities. To verify the morphology evolution of the N-doped porous CHTs, electrospinning solutions with different amounts of zinc acetate were prepared (with the content of the other ingredients fixed) and investigated by TEM (Figure 3). As shown in Figure 3A, the resulting material reveals a fibrous structure, containing a few small pores with low content of Zn(Ac)2. An increasingly hollow structure can be identified in Figure 3B as the amount of zinc acetate increases. The morphology of the inner hollow tube can be seen more clearly with an increasing amount of Zn(Ac)2 in the precursor solution (Figures 3C and 3D). More importantly, when a significant amount of carbon was etched out, CHTs with few carbon nanosheets were formed. The corresponding final materials in Figures 3E and 3F exhibit a bamboo-like structure that is internally wired by segments of carbon layers with a thickness of ∼3.5 nm. Thus, the porosity of CHTs can be tuned by controlling the content of Zn(Ac)2 (Figure S6). In addition, the heating temperature also plays an important role in the structure of the final samples. A low calcination temperature is only able to produce porous carbon nanofibers (Figure S7) but not tubules. Figure 4A shows that the N-doped porous carbon paper has high mechanical flexibility, which could be useful for making flexible batteries. The morphology of the N-doped porous CHTs was characterized by FESEM and HRTEM (Figures 4B–4D). The N-doped porous CHTs show 1D tubular morphology with a length up to several micrometers and a diameter of ∼170 ± 20 nm (Figure 4B). The hollow tubular structure and rough surface can be observed in Figures 4C and 4D, facilitating the effective transport of ions.43Wu H. Chan G. Choi J.W. 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Single crystals of single-walled carbon nanotubes formed by self-assembly.Science. 2001; 292: 1136-1139Crossref PubMed Scopus (182) Google Scholar Raman-spectrum analyses show that the ratio (R ≡ IG/ID) of the prepared samples is much higher than that of their counterpart from the carbonization of pure ZIF-8, but is lower than that of graphite, confirming the much enhanced graphitization of the resulting carbonaceous materials after adding Co(Ac)2 in the precursors, but some defects still remain in the prepared samples (Figure S8).22Chen Y. Lu Z. Zhou L. Mai Y.W. Huang H. Triple-coaxial electrospun amorphous carbon nanotubes with hollow graphitic carbon nanospheres for high-performance Li ion batteries.Energy Environ. Sci. 2012; 5: 7898-7902Crossref Scopus (176) Google Scholar, 24Chen Y. Li X. Zhou X. Yao H. Huang H. Mai Y.W. Zhou L. Hollow-tunneled graphitic carbon nanofibers through Ni-diffusion-induced graphitization as high-performance anode materials.Energy Environ. Sci. 2014; 7: 2689-2696Crossref Google Scholar We also conducted a nitrogen adsorption/desorption isotherm experiment for the N-doped porous CHTs. The specific Brunauer-Emmett-Teller surface area of the N-doped porous CHTs is 438 m2 g−1, much larger than that of the CNFs (35 m2 g−1; Figure S9). The strong N2 adsorption below the relative pressure of 0.1 implies the existence of micropores. The following increase of the sorption isotherms ranging from 0.1 to 1 is characteristic of mesopore (0.1–0.6) and macropore (0.6–1) filling in the CHTs (Figures 4H).47Zhang L.L. Zhao X. Stoller M.D. Zhu Y. Ji H. Murali S. Wu Y. Perales S. Clevenger B. Ruoff R.S. Highly conductive and porous activated reduced graphene oxide films for high-power supercapacitors.Nano Lett. 2012; 12: 1806-1812Crossref PubMed Scopus (808) Google Scholar Pore-size distribution in the inset of Figure 4H shows N-doped porous CHTs with micropores peaking at ∼0.6 and 1.5 nm, and mesopores peaking at 3.5 nm. These results are in agreement with the TEM and HRTEM observations of a multi-level or hierarchical porous structure.48Zhang F. Yao Y. Wan J. Henderson D. Zhang X. Hu L. High temperature carbonized grass as a high performance sodium ion battery anode.ACS Appl. Mater. Interfaces. 2017; 9: 391-397Crossref PubMed Scopus (101) Google Scholar To examine the electrochemical performance of the N-doped CHTs, we set up coin cells with Na metal as counter electrode. Figure 5A shows the representative galvanostatic charge-discharge profiles of the as-synthesized porous CHTs in the voltage range of 0–3 V versus Na/Na+ at the rate of 0.12 A g−1. The first discharge and