Graphene-like Carbon–Nitride Monolayer: A Potential Anode Material

Article Cite This: J. Phys. Chem. C 2018, 122, 2481−2489

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Graphene-like Carbon−Nitride Monolayer: A Potential Anode Material for Na- and K‑Ion Batteries Preeti Bhauriyal,† Arup Mahata,† and Biswarup Pathak*,†,‡ †

Discipline of Chemistry and ‡Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Indore, M. P. 453552, India S Supporting Information *

ABSTRACT: Presently, great attention is being directed toward the development of promising electrode materials for non-lithium rechargeable batteries which have the advantages of low cost, high energy storage density, and high rate capacity for substantial renewable energy applications. In this study, we have predicted that the C3N monolayer is a potential electrode material for Na- and K-ion batteries by firstprinciple calculations. The diffusion barriers are calculated to be as small as 0.03 eV for Na and 0.07 eV for K, which could lead to a very fast diffusion on the C3N monolayer surface, implying high mobility and cycle stability for batteries. The C3N monolayer is predicted to allow a high storage capacity of 1072 mAh/g by the inclusion of multilayer adsorption with an average voltage of 0.13 V for Na2C3N and 0.26 V for K2C3N systems, which is more promising than previously studied anode materials. All of these results ensure that the C3N monolayer could serve as an excellent anode material for Na- and K-ion batteries.

1. INTRODUCTION Development of next-generation batteries has become a main focus in current research due to the increasing demand for renewable and sustainable energy sources.1−4 In this regard, rechargeable Li-ion batteries (LIBs) have emerged as one of the most important energy storage systems, as they have been widely used in portable electronic devices and are expected to power electric vehicles.5−8 However, Li-ion batteries do not seem capable of fulfilling future energy demands due to having high production costs and to there being limited resources necessary to make them.9,10 Thus, the search for new energetic materials to meet the demand for next-generation LIBs has not stopped. Currently, Na- and K-ion batteries are attracting much attention due to the abundance of resources and associated low cost in making them, in addition to having a storage mechanism similar to that of LIBs.11−18 The area of Na-ion batteries has been explored to some extent, while the area of K-ion batteries is still in its initial stages. Potassium is indeed competitive in terms of its use in metal-ion batteries in many aspects. The standard reduction potentials of Na/Na+ and K/K+ are −2.714 and −2.936 V, respectively, with a similar environmental abundance.19 Moreover, among the alkali metal salts, sodium and potassium salts are commonly used as both supporting electrolytes and electroactive species due to their high conductivity, low cost, abundance, and straightforward electrochemical behavior.20,21 Designing a suitable electrode material with good electrochemical performance is necessary for the development of both Na- and K-ion batteries. Compared to the variety of cathode © 2018 American Chemical Society

materials proposed, the search for anode materials is much slower and challenging for both Na- and K-ion batteries due to limited cycle life caused by electrolyte decomposition, low intercalation utility, slow kinetics, and large volume expansion.22−24 Graphite, which is the most commercially used anode material for Li-ion batteries, is not capable of being used as an anode for Na-ion batteries due to the difficulty of Na intercalation into graphite. Similarly, for K-ion batteries the application of graphite is limited by low theoretical capacity and continued fading of capacity with battery cycle.17 In recent years, with the development of material science, 2D materials have gained more attention as promising choices as battery anodes because of their accessible flat surfaces with high surface areas, which lead toward high energy densities and high motilities.25−27 Many 2D materials, such as pristine/defected/ doped graphene,28,29 Mxenes,30−33 transition metal dichalcogenides,34,35 phosphorene,36,37 and electrides,38 have been studied theoretically for Na-ion batteries and for a few K-ion batteries.32,33 Very recently, a 25% N-doped graphene (2D polyaniline) was synthesized with an empirical formula of C3N, which can also be explored as a battery electrode.39,40 Some previous reports revealed the fact that doping with nitrogen is a valuable approach for overcoming anode limitations in metalion batteries. Compared to pristine graphene which does not favor Na adsorption, the introduction of strongly electroReceived: September 23, 2017 Revised: January 18, 2018 Published: January 23, 2018 2481

DOI: 10.1021/acs.jpcc.7b09433 J. Phys. Chem. C 2018, 122, 2481−2489

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Figure 1. (a) Top and side view of the unit cell structure of the C3N monolayer. (b) Calculated phonon dispersion curves of the C3N monolayer. (c) The variation of energy in the AIMD simulations and (d) the corresponding snapshots of the C3N monolayer after the simulations at 600 K.

studied the electronic properties, such as density of states, charge density difference, and Bader charge, along with the required electrochemical properties like average open circuit voltage and storage capacity. Our results show that C3N could be a promising anode candidate that can be used in Na- and Kion batteries to enhance the battery efficiency.

negative nitrogen atoms can enhance the interaction of the carbon matrix with Li, Na, and other metal atoms.41−44 Moreover, the hybridization of nitrogen lone pair electrons with the p electrons in carbon alters the electronic structure and increases the reactivity by producing locally accessible active sites in the graphite lattice.45−47 Notably, recent experimental efforts have also explored 7.78% and 2.2% Ndoped graphene layers for Na-ion and K-ion batteries, which demonstrated the ability to improve the performance of the battery.43,16 These results have provided a direction for further exploration in the field of N-doped graphene for future Na- and K-ion batteries. Moreover, the structure of 2D C3N is quite interesting, as it exhibits a hole-free 2D honeycomb lattice with uniformly distributed nitrogen atoms for multifunctionality. Such properties could make 2D C3N a potential electrode for battery applications. Inspired by these works, we have systematically investigated the potential applicability of a C3N monolayer as an anode material for Na/K-ion batteries using density functional theory (DFT) calculations. Phonon dispersion and ab initio molecular dynamics studies are used to determine the dynamic and thermal stability of the C3N monolayer, respectively. To check the suitability of the C3N monolayer for a Na- or K-ion battery anode, different sites are explored to determine the most stable adsorption sites, and with that, the diffusion characteristic of Na and K ions on the C3N surface is also explored. We have also

2. COMPUTATIONAL DETAILS We used the DFT as implemented in the Vienna Ab initio simulation package (VASP) in all calculations.48−50 The exchange-correlation potential is described by using the generalized gradient approximation of Perdew−Burke−Ernzerhof (GGA-PBE).51 The projector augmented-wave (PAW) method is employed to treat interactions between ion cores and valence electrons.52 The plane-wave cutoff energy was fixed to 470 eV. The total energy was converged to 10−5 eV. All of the optimized structures were obtained by fully relaxing atomic positions until the Hellmann−Feynman forces on all atoms were smaller than 10−6 eV/Å. The underlying structure optimizations were carried out using the van der Waals corrected density functional theory (DFT-D3) proposed by Grimme to overcome the deficiencies of DFT in treating dispersion interactions.53 During the relaxation, the Brillouin zone was represented by a Γ centered k-point grid of 8 × 8 × 1. To avoid the periodic image interaction between the two 2482

DOI: 10.1021/acs.jpcc.7b09433 J. Phys. Chem. C 2018, 122, 2481−2489

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Figure 2. (a) Stable adsorption sites for Na- and K-ion adsorption and (b) the relative adsorption energies for the stable H-CC, H-NC, and T-N sites on the C3N monolayer. The blue ball represents the adsorbed Na or K ion, and RE (in eV) is the relative energy difference.

nearest neighbor unit cells, the vacuum was set to 20 Å in the zdirection. For the calculation of electronic structure, the Brillouin zone was sampled with a k-point grid of 11 × 11 × 1. The Bader charge analysis was performed to measure the charge transfer between Na/K and the C3N monolayer.54−59 The Ab Initio Molecular Dynamic Simulation (AIMD) was performed using the canonical ensemble with a fixed volume, temperature, and particle number. AIMD simulations were performed at 600 K with a time step of 1 fs for 30 ps of time steps. Temperature control was achieved by a Nosé thermostat model.60 A 3 × 3 supercell was used for both density of states (DOS) and AIMD calculations. We calculated the diffusion barriers using the nudged elastic band (CI-NEB) method.61 The minimum energy paths (MEP) were initialized by considering 15 image structures between fully optimized initial and final structural geometries, and the energy convergence criterion of each image was set to 10−3 eV.

eV, which matches with recent experimental and theoretical works.40,62 3.2. Na/K Atom Adsorption. To systematically study the adsorption behavior of Na and K ions on the C3N monolayer, we first examine the potential adsorption sites for Na and K ions on the C3N surface. Six adsorption sites have been considered (Figure S2): hexagonal sites H-CC (center of C6 hexagon) and H-NC (center of C4N2 hexagon), top sites of nitrogen (T-N) and carbon (T-C) atoms, and the bridging BCC (center of C−C bond) and B-NC (center of N−C bond) sites. However, only three adsorption sites, H-CC, H-NC, and T-N, are stable, which are shown in Figure 2 with their relative energy differences. The adsorption energy is defined as the difference between the total energy of the metal-ion adsorbed system and the sum of the total energy of the isolated metal and isolated C3N monolayer.

3. RESULTS AND DISCUSSION 3.1. Structural Properties. The C3N monolayer structure possesses a hexagonal lattice with a flat surface like graphene, where the hexagonal rings are composed of either 4 C and 2 N atoms or only 6 C atoms. The C and N atoms are sp2 hybridized with conjugated π-bonding. Figure 1 shows the unit cell of the C3N monolayer, which is comprised of 6 C and 2 N atoms. The C−N and C−C bond lengths are equivalent, having values of 1.40 Å, which are smaller than the atomic bond lengths of graphene (1.42 Å) and are in good accordance with a previous theoretical report.62 Even though 2D C3N has been successfully synthesized, it is still necessary to investigate the stability of the C3N monolayer as the monolayer is still not experimentally available. First, we investigated the dynamic stability of the C3N monolayer using phonon dispersion calculations.63 Figure 1 shows a very small imaginary frequency of 0.90 cm−1 at the Γ point. Such a small imaginary frequency value ( 0 and Δρ < 0, respectively.

3.3. Diffusion of Na/K Ions on the C3N Monolayer Surface. One of the crucial factors for determining a battery’s performance is evaluating its charge/discharge rate which depends upon the mobility of ions. In particular, a low diffusion barrier and high mobility are much desired in making a promising electrode material. Therefore, by using the NEB method, we have evaluated the Na and K diffusion paths on a 3 × 3 supercell of a C3N monolayer surface and determined the corresponding diffusion barriers. As previously observed, the HCC site is the most stable site, followed by the H-NC and T-N sites. Therefore, we have considered the diffusion of Na between the neighboring H-CC sites. Two diffusion paths have been considered (path-1 and path-2) as shown in Figure 4(a) with the corresponding energy barriers. The first path is the direct pathway marked by the red color, which passes through a H-NC site, and the second path involves an intermediate T-N, marked by the blue color. We observe that path-2 leads to a diffusion barrier that is 0.030 eV lower than that of path-1 (barrier of 0.037 eV). The energy differences between the HCC site and the intermediate sites H-NC (0.037) and T-N (0.028) for path-1 and path-2, respectively, are in accordance with the relative adsorption energies of those sites. Similarly, like Na diffusion, the K ion also has two diffusion paths, as shown in Figure 4(b), between the two neighboring H-CC sites. The corresponding energy profiles determine the lowest diffusion barrier to be 0.07 eV along path-1. The energies of the local minimums for H-NC and T-N along path-1 and path-2 are higher than those of the ground state H-CC site by about 0.049 and 0.039 eV, respectively. The NEB results are in good agreement with our adsorption energy calculations. We observe that the diffusion barrier for K-ion diffusion (0.070 eV) is greater than that of Na-ion diffusion (0.030 eV) on the C3N surface. The trend is also in accordance with the adsorption energy of Na and K ions on the C3N surface, where the adsorption energy of the K ion (−2.230 eV) is larger compared to that of the Na ion (−1.806). The longer adsorption distance of Na (3.28 Å) compared to that of K (2.72 Å) decreases the

of states analysis for the systems, where the corresponding metal ions occupy their most stable sites (H-CC). The total and projected density of states are depicted in Figure 3, with and without Na and K metal ions. The C3N monolayer is a semiconductor in nature with a 0.375 eV band gap, which is in accordance with a previous experimental report.40 However, the adsorption of Na and K into C3N introduces sizable density of states at the Fermi level due to significant charge transfer from Na/K to C3N and shifts the Fermi level toward the conduction band, making the Na/K adsorbed C3N monolayer metallic in nature. We observe significant overlap between the s orbital of Na/K and the p orbitals of C and N at the Fermi level, which indicates notable binding between Na/K and the C3N monolayer originating from s-p hybridization interactions. Moreover, the prominent peak of the K s orbital at the Fermi level further proves the strong binding of the K ion compared to the Na ion on the C3N monolayer. Overall, we can say that the Na/K-adsorbed C3N monolayer ensures good electrical conduction, which is crucial for a battery application. Further, to obtain more information about the Na/K-adsorbed C3N system, the charge redistribution is studied using the charge density difference analysis.67−69 Figure 3 shows the charge density difference Δρ plot of the Na/K-adsorbed C3N monolayer system, demonstrating the change in electronic distribution by charge transfer from Na and K to C3N. The significant charge depletion around the Na/K ion and charge accumulation near the C3N surface induce strong Coulombic interactions between Na/K and C3N, which results in a strong binding energy. In addition to this, the Bader charge analysis54−59 is also performed to determine the charge transfer from Na/K to the C3N monolayer (Table S1). These resulting values indicate that the binding energies of Na and K ions on the C3N monolayer are directly related to the charge transfer values. The charge transfer from K to C3N (0.69 |e|) is higher than that of Na to C3N (0.62 |e|), which leads to the stronger binding of K as compared to Na on the C3N monolayer. 2484

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Figure 4. Considered diffusion pathways and the corresponding energy barriers for (a) the Na ion and (b) the K ion.

increasing number of Na and K. The charge/discharge reaction of the C3N monolayer anode can be written as the common half-cell reaction vs M/M+

energy barrier for Na-ion diffusion, leading to trouble-free diffusion on the C3N surface. The C3N monolayer has a lower diffusion barrier (0.030 eV) compared to that of previously studied pristine graphene (0.16 eV); this could be due to the distance of Na from the C3N surface (3.28 Å) being longer than the distance of Na from the graphene surface (2.70 Å).43 Further, in comparison with the diffusion barriers of Na-ion batteries with previously studied graphene-based 2D anode materials, such as bilayer graphene (∼0.15−0.32 eV),28 borondoped graphene (0.16−0.22 eV),29 and some other monolayer anodes like MoS2 (0.15 eV)35 and Mxenes (∼0.56 eV),70 the low diffusion barrier value of C3N anode material makes it a valuable choice for a Na-ion battery anode that can allow for a fast charge−discharge reaction rate. In addition, the very few studies which have been carried out for the K-ion anode have reported a comparatively higher diffusion barrier of 0.49 eV (MoN2 monolayer)70 compared to that of the C3N electrode. 3.4. Storage Capacity and Average Voltage. Other than the electronic characteristic and diffusion behavior, the storage capacity of the electrode material is also important for the practical application of the battery. Therefore, we have calculated the average adsorption energies of Na and K ions layer by layer to determine the storage capacity of Na and K on C3N. We have taken a 2 × 2 supercell of C3N for the calculation by considering both side adsorptions with an

C3N + x M+ + x e− ↔ MxC3N

We first determine the maximum storage capacity of Na and K on the C3N monolayer. The Na/K storage performance of the C3N monolayer can be obtained by calculating the average adsorption energy (Eave) of each layer, which is defined as Eave =

EC3NMx − EC3N − xEM x

where, EMxC3N and EC3N are the total energies of the C3N monolayer after adsorption of x number of Na/K ions and before adsorption of Na/K, respectively. As previously mentioned, the most stable site for Na and K adsorption is the H-CC site, followed by the H-NC and T-N sites, which have a very little relative energy difference. When the first layer of Na/K adsorbs on the C3N surface, all of the possible, most stable sites are taken on one side of the C3N surface, and the adsorption follows on the other side of the C3N surface to form the second layer. After the complete occupation of stable sites of the C3N monolayer, the adsorption will occur on the top of the next stable site to form the third 2485

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Figure 5. (a) Calculated formation energies at different Na/K concentrations including the convex hull, and (b) voltage calculated using the concentration of the tie lines on the convex hull of NaxC3N and KxC3N systems.

systems) and the unstable five-layer clustered Na/K-ionadsorbed C3N system. We observe that for the Na adsorption the net charge transfer value decreases as additional layers of Na ions are added to the C3N surface with values of 0.24, 0.23, 0.11, and 0.08 |e| for one-, two-, three-, and four-layer Na/K-ion adsorption, respectively, and the charge transfer is 0.02 |e| for the Na/K-clustered system. Whereas for K-ion adsorption, the values are 0.27, 0.25, 0.13, 0.10, and 0.03 |e| for each respective layer. Therefore, the threshold values of charge transfer for Naand K-ion adsorption are 0.08 |e| and 0.10 |e|, respectively, and beyond these values adsorption would not be favorable. Even though adsorption interactions in experiments can be affected by many factors, such as the quality and morphology of electrodes and the type of electrolyte and its concentration, the theoretical DFT result is still widely accepted as being useful for guiding and interpreting experimental studies. Thus, the 2 × 2 C3N monolayer can accommodate up to 16 Na or K ions, which corresponds to the stoichiometry of Na2C3N and K2C3N. The average theoretical capacity is estimated using the given equation73

and fourth layers of adsorption, and then the fifth metal layer will form by metal-ion adsorption. A 2 × 2 supercell of C3N is used for the calculations. Initially the first layer of Na is adsorbed on all of the available H-CC sites. The adsorption energy is calculated to be −0.24 eV per Na. Next, we have examined the adsorption of the additional layers with adsorption energies of −0.12, −0.13, and −0.14 eV for the second, third, and fourth layer of Na adsorption, respectively. We observe that the adsorption of the fifth layer of Na leads to clustering of Na ions on the C3N surface. This kind of clustering of adsorbates is very common and has been reported for the Li adsorption on a graphene surface in some previous reports.71,72 Therefore, Na forms two adsorption layers on the C3N monolayer surface on each side. Next, for the first layer of K adsorption, the adsorption energy is −0.34 eV and changes to −0.20, −0.17, and −0.16 eV for the second, third, and fourth layer, respectively. Similar to the Na adsorption, the adsorption of K to form the fifth layer results in metal clustering of K atoms, which means further adsorption is not favorable. The layer by layer adsorption patterns are shown in Figures S3 and S4 for Na and K ions, respectively. Therefore, a maximum number of 16 Na/K ions can adsorb on the C3N monolayer surface. To determine the threshold value of charge transfer for both Na- and K-adsorbed systems, we have calculated the net charge transfer from Na/K ions in the stable Na/K-adsorbed C3N systems (having one layer, two layers, three layers, and four layers of Na/K-ion-adsorbed C3N

C=

xnF Mf

where x and n are the electronic charge and number of Na/K adsorbed in the formula unit, respectively. F is the Faraday constant, and Mf is the molecule mass of the formula unit. For the C3N monolayer system, both Na and K lead to the same 2486

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the possibility of using the C3N monolayer for Na- and K-ion battery anodes.

formula unit, which is Na2C3N and K2C3N, respectively. Therefore, the corresponding storage capacity is estimated to be 1072 mAh/g for the Na/K-adsorbed C3N system. The calculated value is much higher than the values for some of the experimentally reported N-doped anode materials45,47 and theoretically predicted 2D electrodes, such as bilayer graphene (382 mAh/g),28 B-doped graphene (762 mAh/g),29 Mo2C (146 mAh/g),74 and MoN2.70 In the case of K-ion batteries, very few works have reported lower experimental storage capacities like those of graphite (278 mAh/g),15 N-doping of few-layered graphene (350 mAh/g),16 Sn−C composite (150 mAh/g),75 and theoretical MoN2 monolayer (432 mAh/g).70 Next, to evaluate the voltage profile of the C3N monolayer, we have calculated the formation energies of MxC3N systems, where x varies from 0 to 2. A convex hull is drawn by the tie lines connecting all of the lowest energy structures. Four MxC3N systems appear in the tie lines of the convex hull, where x = 0, 0.25, 0.50, and 2 for both Na and K ions, and any other concentrations lie higher in energy. From the slopes of the convex hull a voltage profile is obtained between adjacent Na/K concentrations along the convex hull. The voltage is calculated using the equation76,77

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09433. Total and projected density of states of a C3N monolayer and considered adsorption sites for Na- and K-ion adsorption, adsorption energies and charge transfer values for Na and K adsorption, and adsorption structures of Na and K adsorption layers, all on the C3N monolayer surface (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Biswarup Pathak: 0000-0002-9972-9947 Notes

The authors declare no competing financial interest.

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⎛ E (x ) − Etot(x1) − ΔxEM ⎞ V (x1 , x 2) = −⎜ tot 2 ⎟ ⎠ ⎝ Δxe

ACKNOWLEDGMENTS We thank IIT Indore for use of its lab and computing facilities. This work was supported by DST-SERB (EMR/2015/002057) and CSIR (01(2886)/17/EMR-II), New Delhi. P.B. and A.M. thank MHRD for research fellowships.

where Etot(x2) and Etot(x1) are the total energies of the MxC3N compound at two adjacent low-energy concentrations x2 and x1 along the computed convex hull shown in Figure 5(a), e is the electronic charge, and EM is the energy per atom of Na/K metal atom in their bulk structure. Figure 5(b) shows the concentration-dependent profile of the voltage for both Na and K ions. The voltage for Na and K varies in the range of 0.11−0.15 and 0.11−0.35 V, respectively, with a decreasing trend in voltage with increasing capacity. The average voltage is calculated to be 0.13 and 0.26 V for Na and K ions, respectively. The average voltage for Na and K is within the desired potential ranges (0.10−1.00 V) for an anode material. Therefore, we can say that the C3N monolayer can be utilized as a promising anode material for Na- and K-ion batteries.

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REFERENCES

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4. CONCLUSIONS In this work, the geometric, electronic, and electrochemical properties of a 2D C3N monolayer have been systematically studied based on first-principle calculations. It is established that the C3N monolayer is one of the promising anode materials for Na- and K-ion batteries. Through calculated phonon spectra and AIMD simulations, we first confirmed the thermal and dynamical stabilities of the C3N monolayer. It is observed that the H-CC site is the most stable site for Na- and K-ion adsorption on the C3N monolayer. The density of states calculations show that the C3N monolayer becomes metallic on Na/K-ion adsorption, which is crucial for a battery application. Moreover, our diffusion calculations conclude that the C3N monolayer can possess a high charge/discharge rate with a diffusion of 0.03 and 0.07 eV for Na- and K-ion batteries, respectively. In addition, we have obtained a considerably high storage capacity of 1076 mAh/g for Na and K adsorption. The average open circuit voltage for the C3N monolayer is 0.13 V for Na and 0.26 V for K, which will give a high full-cell voltage when combined with a suitable cathode material. In conclusion, we can say that, on comparing every required step of our data with previous reports, the obtained results clearly lead toward 2487

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.7b09433 J. Phys. Chem. C 2018, 122, 2481−2489

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