Functionalization: An Effective Approach to Open and Close Channels

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Functionalization: An Effective Approach to Open/ Close Channels for Electron Transfer in Nitrogenated Holey Graphene CN Anodes in Sodium-Ion Batteries 2

Donghai Wu, Baocheng Yang, Eli Ruckenstein, and Houyang Chen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03435 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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The Journal of Physical Chemistry Letters

Functionalization: an Effective Approach to Open/Close Channels for Electron Transfer in Nitrogenated Holey Graphene C2N Anodes in Sodium-ion Batteries

Donghai Wua, Baocheng Yanga, Eli Ruckensteinb, Houyang Chenb*

a

Henan Provincial Key Laboratory of Nanocomposites and Applications, Institute of

Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou 450006, China b

Department of Chemical and Biological Engineering, State University of New York

at Buffalo, Buffalo, New York 14260-4200, USA

_______________ * To whom correspondence should be addressed. Email: [email protected]

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Abstract Electron transfer plays a crucial role in energy storage materials such as metal-ion batteries (MIBs). Numerous approaches are developed to facilitate the electron transfer for MIBs, and most of them are extended the special surface areas, which promote the contact between metal ions and the electrodes. Herein, we report the formation of intramolecular channels for electron transfer to open/close intermolecular channels in sodium-ion batteries (SIBs) through functionalization, modulating the cell performance of two-dimensional (2D) nitrogenated holey graphene C2N anodes. It also activates the inactive metal ions in anodes, reducing their production costs. Upon increasing the concentration of hydrogen atoms, the intramolecular electron transfer increases, lowering the intermolecular electron transfer between C2N and metal ions and thus weakening their interaction and reducing the capacities of anodes. High concentration of hydrogen atoms introduced in C2N would further promote the intramolecular electron transfer and block the channels for intermolecular electron transfer. For functionalized C2N monolayers which can be used as anodes in SIBs, they possess high specific capacities, high conductivities and low open-circuit voltages. This work proposes the fabrication of 2D energy storage materials with tunable macroscale behaviors (e.g. performance) through the relationships between the macroscale and microscale levels and between intramolecular and intermolecular electron transfer.

Keywords: functionalization engineering; hydrogenation; C2N; sodium-ion batteries; first-principles calculations; electron transfer; diffusion.

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Driven by the increasing requirements of advanced sustainable energies, developing energy storage technologies and materials becomes extremely urgent. The rechargeable Li-ion battery (LIB) with outstanding reversible capacity, high energy density, and long cycling life, has been considered as a potential candidate of next-generation batteries. It has been widely adopted in portable electronic devices and is expected to be used in power electric vehicles.1 However, one of its challenges to commercialization is the high production cost, which limits its applications in commercial fields.2-3 In recent years, the sodium-(Na-) ion battery (SIB) has received considerable attention as an attractive alternative to LIBs, due to its low cost in processing, operational safety, abundant reserves, and similar storage mechanism as LIBs.1, 4-10 Reported SIB cathode materials have shown comparable performances to their LIB counterparts.11-13 However, developing an appropriate anode material with high specific capacity and moderate redox potential for SIBs is still one of the major challenges. Simply adopting LIB anode materials to SIBs has not been successful. For instance, graphite, which is a frequently-used commercial LIB anode material,14 does not show any electroactivity for SIBs because of the difficulty for the intercalation of Na atoms.15-16 Other materials, e.g. amorphous carbon, silicon, germanium, and antimony, exhibit low specific capacities and limited cycle life.8, 10-11 The metal alloys possess high capacities, however they have large volume expansion.17-18 Hence, the development of SIBs anode materials with excellent performance is extremely urgent. Since the unique structures of large specific surface areas, which facilitate fast ion loading and vast ion occupation and enhance the intermolecular electron transfer between ions and materials, two-dimensional (2D) nanomaterials are one of the promising anode materials with promoted electrochemical performance,19 e.g., transition metal dichalcogenides (MoS2, VS2),20-21 MXenes,1, 22 SnS,19 phosphorene,11, 4


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15, 23,

borophene,24 and graphene and its derivatives or analogues.5, 7, 25-35 Recently, the nitrogenated holey graphene, i.e. C2N, was successfully synthesized

using a bottom-up wet-chemical reaction.36 The single layer37 and layered25 C2N are promising anode materials for use in LIBs and/or SIBs. Prior study37 identified that intermolecular electron transfer between C2N and metal ions promotes the capacity of metal ions in C2N. On the other hand, functionalization is widely used for property modification in materials.38-42 In this work, we report the formation of intramolecular channels for electron transfer to open/close intermolecular channels in SIBs through hydrogenation (functionalization), modulating the cell performance of 2D nitrogenated holey graphene C2N anodes. The density functional theory (DFT) calculations are carried out by using VASP (Section S1).43 The DFT results show that, the increase of the concentration of hydrogen atoms promotes the intramolecular channels for electron transfer and decreases the intermolecular electron transfer, reducing the capacities of C2N anodes. The capacities of functionalized anodes are high and their average opencircuit voltages (OCVs) are low. Additionally, functionalization actives the inactive metal ions in C2N anodes, decreasing their production costs. The C2N with high concentration of hydrogen atoms are not available for use in metal-ion battery anodes. This work provides a crucial insight to design energy storage materials with tunable macroscale behaviors (e.g. performance) from microscale levels (e.g. the electronic level). Functionalization: an effective approach for formation intramolecular channels for electron transfer to open/close the intermolecular channels between C2N and Na ions. In our previous work,37 we found that intermolecular channels between metal ions and C2N are formed for electron transfer when the metal ions are adsorbed on the C2N monolayer. In this paper, we find that functionalization engineering is an effective 5



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approach for formation of intramolecular channels for electron transfer to open/close intermolecular channels between C2N and metal ions. To investigate this issue, we take hydrogenation as an example of functionalization, and introduce 1, 2, 3, or 6 hydrogen atom(s) in each nitrogenated hole of C2N (Section S2 of the supporting information). The hydrogen atom(s) distribute uniformly. For simplification, the hydrogenated C2N is denoted by Hn–C2N, and n is the number of hydrogen atoms in each hole of C2N, n = 1, 2, 3, and 6. Results of their adsorption energies show that the Na atoms can be adsorbed on Hn–C2N with n = 1 and 2, whereas they cannot be adsorbed on the Hn–C2N with n = 3 and 6 (Section S2 and Table S1 of the supporting information). We calculated the charge density differences of the most stable adsorption structures with one and two Na atoms as well as the maximum capacity of Na atoms (Figure 1). The electrons mainly accumulate around the H2–C2N substrates, and the Na and H atoms almost lose their electrons, which demonstrates that the charges are transferred from Na and H atoms to the N atoms of the H2–C2N monolayer and indicates the new formation of the intramolecular channels between the functional group of hydrogen and the N atoms of H2–C2N. In addition, the Bader charge analysis manifests that the charge transfer from Na atoms to C2N are approximately 0.840 and 0.739 |e| per Na atom for the most stable adsorption structures with one (Figure S3a) and two Na adatoms (Figure S4a), respectively, which are much less than that (0.932 and 0.899 |e| for the structures with one and two Na atoms, respectively) between Na atoms and the pristine C2N monolayer.37 This is because the hydrogen atoms also donate electrons to nitrogen atoms (intramolecular electron transfer), which partially block the intermolecular channels for electron transfer between C2N and Na atoms. Based on the atomic properties, this occurs because the lone electrons of nitrogen atoms become weakened by the functionalization of the hydrogen atoms, lowering the adsorption 6


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strength between C2N and Na atoms. Besides, the contribution of charge transfer from Na and H atoms to other Hn–C2N (n= 1, 3, and 6) monolayers with one Na atom adsorption are also provided. We found that, as the concentration of hydrogen atoms increases, the electrons (per atom) donated from a Na/H atom to the N atoms (0.845/0.452, 0.840/0.409, 0.827/0.407, 0.461/0.310 |e| for H1–C2N, H2–C2N, H3–C2N and H6–C2N, respectively) decrease, implying that the Na–N and H–N interactions are weakened gradually. The charge density differences of the most stable configurations with one Na atom adsorption on the Hn–C2N monolayers (Figure S5) also show that the charge lost from Na atom is reduced with increasing hydrogen atoms. It should be mentioned that, for Hn–C2N with n = 3 and 6, one can find that electron transfer occurs if a Na atom is adsorbed on the sheet. However, the results of adsorption energies indicate that the complex structures of Hn–C2N (n = 3 and 6) and Na atoms are unstable. Thus, the values of the electron transfer between Hn–C2N (n = 3 and 6) and Na atoms are meaningless. In a word, functionalization is an effective approach for the formation of intramolecular channels of electron transfer to open/close intermolecular channels between C2N and Na atoms. The electron localization functions (ELFs) are also adopted to analysis the interaction between Na atomic layers and the H2–C2N monolayer (Figures 2 and S6). In the H2–C2N sheet, the electron clouds are mainly accumulated between C, N and H atoms, thus the C–C, C–N, and N–H bonds show the covalent bonds feature. The directional localized electron clouds between the first/second Na atomic layers and the H2–C2N substrate suggest an ionic bond character of Na–N/C. Obviously, there is a little electron cloud enveloping the Na atoms, indicating that they are mostly ionized forming Na+ cations. The valence electrons of Na atoms almost spread out in the entire layers of Na atoms, which could neutralize the repulsion between Na+ cations and 7



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stabilize the adsorption layers.1 Such ionic bonds could make a contribution to the adsorption of Na layers. Assuming that four layers (the 0th, 1st, 2nd and 3rd layers) of Na atoms can be adsorbed on an H2–C2N sheet. The electron clouds located in the 0th and 1st layers remain nearly constants, whereas the electron clouds located in the 2nd and 3rd layers sharply drop (Figures 2c and S6c). Apparently, little electron cloud spreads over the second and third layers of Na atoms, which is disadvantageous to adsorb the third layer of Na atoms on H2–C2N. To further explore the charge distribution of Na atoms in the third layer, we analysis the Bader charge and find that charge transfer occurs among Na atoms in the third layer. Thus, the H2–C2N monolayer could hold at most three layers (the 0th – 2nd layers) of Na atoms, and no more layer can be adsorbed. 5 Performance of functionalized C2N monolayer anodes. Now, we investigate the battery performance of the functionalized C2N anodes. As one of the crucial performances of electrode materials, the storage capacity of a H2–C2N monolayer with Na ions is calculated. The maximum storage capacity is thirteen Na atoms (Figure S4c, including the 0th, 1st and 2nd layers) adsorption on the H2–C2N monolayer, corresponding to the stoichiometric ratio of Na13(C2N)6H2. The calculated maximum specific capacity of H2–C2N monolayer as an anode for use in a SIB is 1515 mA h g-1, which is approximately four times larger than that in the bilayer graphene (382 mA h g-1),34 and is much larger than numerous 2D electrode materials, such as Ti3C2 (352 mA h g-1),22 phosphorene (433 mA h g-1),6 VS2 (466 mA h g-1),44 BxCyNz(810 mA h g-1),45 maximum defective graphene (1450 mA h g-1),7 and penta-graphene (1489 mA h g-1).5 Such a high specific capacity is attributed to the intermolecular channel formation for the electron transfer between Na atoms and the Hn–C2N monolayer. Our ab initio molecular dynamics (AIMD) simulation results (Section S9 in the supporting information) indicate that the structure of the H2–C2N monolayer with the maximum 8


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storage capacity is thermally stable, which may suggest the good recycle stability of a H2–C2N monolayer anode for SIBs.46 It should be mentioned that the specific capacity of a H2–C2N monolayer is lower than that of the pristine C2N. 37 This is because that the hydrogenation, which generates the intramolecular electron transfer, weakens the intermolecular electron transfer between Na atoms and Hn–C2N monolayers, thus decreasing their interaction and reducing the capacity. The pristine C2N holds 5 layers (the 0th – 4th layers) of Na atoms, whereas H2–C2N only adsorbs three (the 0th – 2nd layers) layers. Further, when n equals to 3 or 6, no Na atom can be adsorbed on Hn–C2N, indicating that Hn–C2N (n = 3 and 6) are not available for use in SIB anodes. In a word, intermolecular electron transfer between C2N and Na atoms, which can be modulated by the intramolecular electron transfer, tunes the cell performance of functionalized C2N anodes. To examine the decrease of the capacity of Na atoms in C2N when functionalization is introduced, the layered Bader charge is adopted. For Na13(C2N)6H2, the calculated layered Bader charge (per Na atom) of the 0th, 1st and 2nd layers of Na atoms adsorption on the H2–C2N monolayer are 0.752, 0.324 and 0.275 |e|, respectively. The values of Bader charge for the first and second layers are not equal. This happens because a small stratification occurs after the first layer of Na atoms adsorption on the H2–C2N monolayer, resulting in a small decrease of the charge transfer. Assuming that four layers of Na atoms are adsorbed (see Na17(C2N)6H2 in Figure S7d), the layered Bader charge for the 0th, 1st, 2nd and 3rd layers are 0.749, 0.263, 0.163, and 0.108 |e|, respectively, which are much smaller than those of Na13(C2N)6H2, especially for the second layer of Na atoms (almost halved). Thus, the interaction between Na layers and the H2–C2N substrate becomes weakened correspondingly. In Na17(C2N)6H2, the layered Bader charge of the outer layer (the 3rd layer) is much lower than those of 9



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internal layers (the 0th, 1st, and 2nd layers), implying that the charge transfer is mainly contributed by the internal Na layers and the external layer makes a little contributions. Therefore, the interactions between the H2–C2N substrate and internal layers are much stronger than that between the H2–C2N substrate and the external layer. Besides, Figure S7d shows that the four Na atoms located in the 3rd layer slightly divided into two layers, meaning that the configuration is unstable. Furthermore, the charge transfer of hydrogen atoms was also investigated. The charge transfer from the two hydrogen atoms to the C2N for Na17(C2N)6H2 is 0.573 |e|, which is much larger than that for Na13(C2N)6H2 with 0.181 |e|, indirectly signifying that the interaction between Na in Na17(C2N)6H2 and H2–C2N is weaker than that between Na in Na13(C2N)6H2 and H2– C2N. The above discussion again confirms that the functionalization weakens the interaction between Na atoms and C2N, reducing the capacity of metal ions in C2N. The open circuit voltages (OCVs, see the calculation in Section S3 of the supporting information) monotonously decrease as the specific capacity increases. The trend is the same as the case of adsorption energies. The average OCV for a H2–C2N anode is 0.332 V, which belongs to the ideal potential ranges of 0.10–1.00 V for an anode material.4 The average OCV is lower than that of the pristine C2N.37 The superior specific capacities occur with low OCVs, suggesting that the H2–C2N monolayer is a desirable anode material for SIBs. Our DOS results (Figure 3) showed that, as the concentration of Na atoms increases, the sub-gaps around the Fermi level shrink gradually until seven Na atoms are adsorbed when they vanish. Correspondingly, the electronic conductivity of the H2– C2N monolayer is enhanced with the increase of the concentration of Na ions. Further analyzing the projected density of states (PDOS), one can see the distinct overlap between the Na–s orbital and the p orbitals of C and N at the Fermi level. Such 10


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significant s–p hybridization indicates notable interactions between metal atoms and the H2–C2N monolayer. The charge/discharge rate is another one of the crucial indexes for the rechargeable batteries, which is related to the ion mobility. The paths for single atom diffusion and for atom pair diffusion37 are prepared (Figure 4). The single atom diffusion model showed that the calculated diffusion barriers of paths I, II and III (Figure 4a) for a Na ion are 1.196, 1.263 and 1.117 eV, respectively. They are significantly reduced after hydrogenation, in compared with that on the pristine C2N of 3.627 eV.37 The atom pair diffusion approach indicated that the calculated diffusion barriers of Na ions for paths i and ii (Figure 4b) are 0.147 and 0.307 eV, respectively, which are much lower than that of a single Na ion diffusion on the H2–C2N monolayer. It should be mentioned that four initial paths (paths i, ii, iii, and iv) are prepared. However, paths iii and iv do not work and they changed to path i or ii after the CI-NEB processes. From both results of the single atom diffusion and atom pair diffusion approaches, the comparatively low diffusion barriers of Na ions on a H2–C2N monolayer would bring high mobility of metal ions for battery applications. In conclusion, by adopting first-principles calculations, we have systematically studied the effect of hydrogenation (functionalization) on the modulation of electron transfer between C2N and metal ions and on the performance of functionalized C2N anodes in SIBs. Our results showed that functionalization is an effective strategy for the formation of intramolecular channels of electron transfer to open/close the intermolecular channels for electron transfer, tuning the performance of functionalized C2N anodes. With the increase of the concentration of hydrogen atoms, the intramolecular channels increase, the intermolecular electron transfer between C2N and metal ions becomes weak and the capacity becomes small. Further, the results indicated 11



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that H3–C2N and H6–C2N cannot be used as anodes in SIBs. For functionalized C2N monolayers which can be used as anodes in SIBs (H1–C2N and H2–C2N), they possess high specific capacities, high conductivities and low OCVs. Taking H2–C2N as an example, its capacity is 1515 mA h g-1, which is approximately 4 times larger than the bilayer graphene, and its OCV is 0.332 V. The hydrogenated C2N has a little volume expansion and excellent cycling stability during the intercalation of Na atoms. More importantly, the hydrogenation on C2N actives the inactive metal ions in the pristine C2N anodes, which would decrease their product costs. The hydrogenation engineering also reduces the diffusion barriers of Na ions on C2N monolayers because (1) the dangling bonds of N atoms could be partially saturated by the hydrogen atoms, decreasing the adsorption strength of Na atoms in holes, and (2) it increases the steric hindrance of holes in C2N, extruding the Na atoms out of the sheets. This work provides a significant insight for modulating the macroscale behaviors (e.g. battery performance) of energy storage materials from microscale levels (e.g. the electronic level), and proposes a strategy to the innovation of 2D materials with tunable performance through the relationships between the macroscale behaviors and microscale views and between intramolecular and intermolecular electron transfer.

Conflicts of interest There are no conflicts to declare. Acknowledgments This work was supported by the University at Buffalo and by the National Natural Science Foundation of China (NSFC) (Grant No. 21206049) and by the Innovation Scientists and Technicians Team Construction Projects of Henan Province 12


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(CXTD2017002).

Supporting Information Available Computational methods; structures, volume expansion and thermal stability of hydrogenated C2N monolayers; definition of adsorption energy; adsorption energies of a single Na atom adsorption on each site of Hn–C2N monolayers; the atom pair adsorption model; average adsorption energy; calculations of storage capacity and open circuit voltage; structures and electronic properties of Na adsorption on a H2-C2N; diffusion behavior of Na on a H2-C2N; the atom pair diffusion method; total density of states of Hn–C2N; charge density difference for the most stable adsorption of a single Na atom on Hn–C2N monolayers; 3D ELFs of a H2–C2N monolayer with multiple layers of Na atoms; snapshots of an AIMD simulation of thirteen Na atoms adsorption on a H2–C2N monolayer; and the definition of layers of metal ions in an asymmetric substrate.

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Figure 1. Top (upper) and side (below) views of charge density difference for the most stable adsorption of one (a), two (b) and thirteen (c) Na atoms on the H2–C2N monolayers. Cyan and yellow regions represent the electron loses and gains, respectively.

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Figure 2. The 2D ELFs projected onto the (110) section of a H2–C2N monolayer with (a) two (0th and 1st layers), (b) three (0th – 2nd layers), and (c) four (0th – 3rd layers) layers of Na adatoms.

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Figure 3. Density of states for a H2–C2N monolayer (a) and the H2–C2N monolayers with one (b), seven (c), and thirteen (d) Na adatoms. The Fermi level is set as 0 eV and labeled with red vertical dash lines.

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Figure 4. Diffusion paths and corresponding energy barriers of one (a) and two (b) Na atoms on the H2–C2N monolayers.

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