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

Jan 23, 2018 - Development of next-generation batteries has become a main focus in current research due to the increasing demand for renewable and sus...
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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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09433 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Graphene-like Carbon–Nitride Monolayer: A Potential Anode Material for Na and K-ion Batteries Preeti Bhauriyal,† Arup Mahata,† Biswarup Pathak, †,#,* †

Discipline of Chemistry, and # Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Indore, M.P. 453552, India

Email: [email protected]

Abstract Presently, great attention is being directed towards the development of the promising electrode materials for non-lithium rechargeable batteries having 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 Kion batteries by first principles 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 C3N monolayer surface implying high mobility and cycle stability for batteries. The C3N monolayer is predicted to allow high storage capacity of 1072 mAh/g by the inclusion of multilayer adsorption with the average voltage of 0.13 V for Na2C3N and 0.26 V for K2C3N systems, which is more promising than the previously studied anode materials. All these results ensure that C3N monolayer could serve as an excellent anode material for Na and K-ion batteries.

1. Introduction Development of next-generation batteries has become the main focus of the current research due to the increasing demand for renewable and sustainable energy sources.1-4 In this regard,

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rechargeable Li-ion batteries have emerged as one of the most important energy storage systems, and have been widely used in portable electronic devices and are expected to power electric vehicles.5-8 However, the Li batteries do not seem capable of fulfilling future energy demands because of the high production cost and limited resources.9-10 Thus, the search for new energetic materials to meet the demand for next generation LIBs have never stopped. Currently, Na and Kion batteries have been attracting much attention due to the abundant resources and the associated low cost as well as the similar storage mechanism as LIBs.11-18 The area of Na-ion batteries has been explored to some certain extent, while the area of K-ion battery is still at its initial stage. K is indeed competitive in terms of metal-ion battery in many aspects. The standard reduction potential of Na/Na+ and K/K+ are -2.714 V and -2.936 V respectively with the 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 of the 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 materials proposed, the search for the 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 continue 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

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accessibility of flat surfaces with high surface areas, which lead towards 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 electrides38 have been studied theoretically for Na-ion batteries and few for K-ion batteries.32-33 Very recently, a 25% of N-doped graphene (2D polyaniline) is synthesized with an empirical formula of C3N, which could also be explored as battery electrode.39-40 Some previous reports revealed the fact that doping of N is a valuable approach to overcome the anode limitations in metal-ion batteries. Compared to pristine graphene, which does not favour Na adsorption, the introduction of strongly electronegative nitrogen atoms can enhance the interaction of carbon matrix with Li, Na and other metal atoms.41-44 Moreover, the hybridization of nitrogen lone pair electrons with the p electrons in the 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 % of N-doped few graphene layers for sodium ion batteries and K-ion batteries demonstrated the ability to improve the performance of the battery.43,16 These results have given a direction to further explore the field of N-doped graphene for future Na and K-ion batteries. Moreover, the structure of 2D C3N is quite interesting, because it exhibits a holefree 2D honey-comb 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 C3N monolayer as an anode material for Na/K batteries using the DFT calculations. The phonon dispersion and Ab initio molecular dynamics studies are used to determine the dynamic and thermal stability of C3N monolayer, respectively. To check the suitability of C3N monolayer for Na and K-ion battery anode, different sites are explored to determine the most stable adsorption

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sites and with that, the diffusion characteristic of Na and K-ion on C3N-surface is also explored. We have also 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 one of the promising anode candidates that can be used in Na and K-ion batteries to enhance the battery efficiency.

2. Computational Details We have used the density functional theory (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 (GGAPBE).51 The Projector augmented-wave (PAW) method is employed to treat interactions between ion cores and valence electrons.52 The plane wave cut-off energy is fixed to 470 eV. The total energy is converged to 10-5 eV. All the optimized structures are obtained by fully relaxing atomic positions until the Hellmann-Feynman forces on all atoms are smaller than 10-6 eV/Å. The underlying structure optimizations are 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 is represented by Gamma centered k-point grid of 8 × 8 × 1. To avoid the periodic image interaction between the two nearest neighbour unit cells, the vacuum is set to 20 Å in the z-direction. For the calculation of electronic structure, the Brillouin zone is sampled with a k-point grid of 11×11×1. The Bader charge analysis is performed to measure the charge transfer between Na/K and C3N monolayer.54-59 The Ab Initio Molecular Dynamic Simulation (AIMD) is performed using the canonical ensemble with the fixed volume, temperature, and particle number. AIMD simulations are performed at 600 K with the time step of 1 fs for 30 ps time steps. Temperature

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control is achieved by Nosé thermostat model.60 A 3×3 supercell is used for both density of states (DOS) and AIMD calculations. We have calculated the diffusion barriers using the nudged elastic band (CI-NEB) method.61 The minimum energy paths (MEP) are initialized by considering 15 image structures between fully optimized initial and final structural geometries, and the energy convergence criteria of each image is set to 10-3 eV.

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 or all 6 C atoms. The C and N atoms are sp2 hybridized having 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 1.40 Å values, which is smaller than the atomic bond lengths of graphene (1.42 Å) and is good accordance with the previous theoretical report.62

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Figure 1. (a) Top and side view of the unitcell structure of 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.

Even though the 2D-C3N has been successfully synthesized, it is still necessary to investigate the stability of C3N monolayer as the monolayer is still not experimentally available. Firstly, we have investigated the dynamic stability of C3N monolayer using the phonon dispersion calculations.63 Figure 1 shows that there shows a very small imaginary frequency of 0.90 cm-1 at Γ point. Such a small imaginary frequency value ( 0 and ∆ρ < 0, respectively.

3.3. Diffusion of Na/K ions on C3N monolayer Surface 10 ACS Paragon Plus Environment

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One of the crucial factors for determining the 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 to make 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 C3N-monolayer surface and determined the corresponding diffusion barriers. As previously observed the H-CC site is the most stable site, followed by H-CC and T-N sites. Therefore, we have considered the diffusion of Na between the neighbouring H-CC sites. Two diffusion paths have been considered (path-1, path-2) as shown in Figure 4(a) with the corresponding energy barriers. The first path is the direct pathway marked by red colour, which passes through a H-NC site and the path-2 involves an intermediate T-N, marked by blue colour. We observe that path-2 leads to lower diffusion barrier of 0.030 eV than that in path-1 pathway (barrier of 0.037 eV). The energy difference between H-CC 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 sites. Similarly, like Na diffusion, K ion also has two diffusion paths, as shown in Figure 4(b) between the two neighbouring 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 minimum H-NC and T-N along path-1, and path-2, respectively, are higher than the ground state H-CC site by about 0.049 eV and 0.039 eV. 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 more than that of Na ion diffusion (0.030 eV) on C3N surface. The trend is also in accordance with the adsorption energy of Na and K ion on C3N surface, where the adsorption energy of K ion (-2.230 eV) is more compared to Na ion (-1.806). The higher adsorption distance of Na (3.28 Å) compared K (2.72 Å), decreases energy barrier for Na ion diffusion leading to a

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trouble free diffusion on C3N surface. The C3N monolayer has lower diffusion barrier (0.030 eV) compared to previously studied pristine graphene (0.16 eV), this could be due to higher distance of Na from C3N surface (3.28 Å) than from 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 (∼0.15-0.32eV),28 boron-doped graphene (0.16−0.22 eV),29 and some other monolayers anodes like MoS2 (0.15 eV),35 Mxenes (~0.56 eV),70 the low diffusion barrier value of C3N anode material makes it a valuable choice for Na-ion battery anode which can allow fast charge-discharge reaction rate. In addition, the very few studies which have been carried out for K-ion anode have reported comparatively higher diffusion barrier 0.49 eV (MoN2 monolayer)70 than C3N electrode.

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Figure 4. The Considered diffusion pathways and the corresponding energy barriers for (a) Naion, (b) K-ion.

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 increasing number of Na and K. The 13 ACS Paragon Plus Environment

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charge/discharged reaction of C3N monolayer anode can be written as the common half-cell reaction vs M/M+,   +   +   ↔    We first determine the maximum storage capacity of Na and K on C3N monolayer. The Na/K storage performance of C3N monolayer can be obtained by calculating the average adsorption energy (Eave) of each layer, which is defined as,  = where, 

 

(   −    −  ) 

and    are the total energies of C3N monolayer after adsorption of x Na/K ions

and before adsorption of Na/K. As previously mentioned, the most stable site for Na and K adsorption is H-CC, followed by HNC and T-N sites, which have very little relative energy difference. When the first layer of Na/K adsorbs on C3N-surface, all the possible most stable sites are taken on one side of C3N-surface and the adsorption follows on the other side of 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 and fourth layer of adsorption, respectively and then the fifth metal layer will form by metal ion adsorption. The 2×2 supercell of C3N is taken for the calculations. For the first layer of Na, initially the first layer of Na is adsorbed on the all the available H-CC sites. The adsorption energy is calculated to be -0.24 eV per Na. Next, we have examined the adsorption for the additional layers with the adsorption energies -0.12, -0.13 and -0.14 eV for the second, third, and fourth of Na adsorption, respectively. We observe that the adsorption of fifth layer of Na leads to clustering of Na ions on C3N surface. This kind of clustering of adsorbates is very common and has been reported for the Li adsorption on graphene surface in some previous reports.71-72 Therefore, Na forms two 14 ACS Paragon Plus Environment

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adsorption layers on C3N monolayer surface on each side. Next, for the first layer of K adsorption, the adsorption energy is -0.34 eV, and it changes to -0.20, -0.17, and -0.16 eV for the second, third and fourth layer, respectively. In a similar way like Na, the adsorption of K to form the fifth layer results into metal clustering of K atoms, which means further adsorption, is not favourable. The layer by layer adsorption patterns are shown in Figure S3, and S4, (Supporting Information) for Na and K ions, respectively. Therefore, the maximum number of 16 Na/K can adsorb on 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 systems) and the unstable five-layer clustered Na/K ion adsorbed C3N system. We observe that for the Na adsorption, the net charge transfer value decreases as more layers of Na ions are formulated on C3N surface with the values 0.24 |e|, 0.23 |e|, 0.11 |e|, 0.08 |e| for one, two, three, and four layer Na/K ion adsorption and the charge transfer is 0.02 |e| for Na/K clustered system. Whereas, for K ion adsorption, the values are 0.27 |e|, 0.25 |e|, 0.13 |e|, 0.10 |e|, and 0.03 |e|. Therefore, the threshold values of charge transfer for Na and K ion adsorption are 0.08 |e| and 0.10 |e|, respectively, and beyond these values adsorption would not be favourable. Even though the 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 useful for guiding and interpreting experimental studies. Thus 2 × 2 C3N monolayer can accommodate upto 16 of Na and K ions, which corresponds to the stoichiometry of Na2C3N and K2C3N. The average theoretical capacity is estimated using the given equation,73

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 =

!" #

Where, x and n are the electronic charge and number of Na/K adsorbed in the formula unit, respectively. F is Faraday constant, and Mf is the molecule mass of the formula unit. For the C3N monolayer system, both Na and K lead to same formula unit which is Na2C3N and K2C3N respectively. Therefore, the corresponding storage capacity is estimated to be 1072 mAh/g for Na/K adsorbed C3N system. The calculated value is much higher than some experimentally reported N-doped anode materials45,47 and theoretically predicted 2D electrodes such bilayer graphene (382 mAh/g),28 B-doped graphene (762 mAh/g),29 Mo2C (146 mAh/g),74 and MoN2.70 In case of K-ion batteries, very few works are reported with lower storage capacity like experimentally graphite (278 mAh/g),15 N-doping of few-layered graphene (350 mAh/g)16 and Sn-C composite (150 mAh/g)75 are studied with capacity, and theoretically MoN2 monolayer (432 mAh/g).70 Next, to evaluate the voltage profile of C3N monolayer, we have calculated the formation energies of MxC3N systems, where x varies from 0 to 2. Convex hull is drawn by the tie lines connecting all the lowest energy structures. Four MxC3N systems appear in the tie lines of the convex hull, x = 0, 0.25, 0.50, 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 equation,76,77 )*) (' ) − )*) (% ) − ∆.  $(% , ' ) = − ( ∆.  where, Etot(x2) and Etot(x1) are the total energies of the MxC3N compound at two adjacent lowenergy concentrations x2 and x1 along the computed convex hull shown in Figure 5 (a), and EM is the energy per atom of Na/K metal atom in their bulk structure. Figure 5 (b) shows the

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concentration-dependent profile of the voltage for both Na and K ions. The voltage for Na and K varies in a range of 0.11-0.15 V and 0.11-0.35 V, respectively with a decreasing trend with increasing capacity. The average voltage is calculated to be 0.13 V and 0.26 V for Na and K ion respectively. The average voltage for Na and K ranges between the desired potential ranges (0.10 V-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.

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

4. Conclusions

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In this work, the geometric, electronic and electrochemical properties of 2D C3N monolayer have been systematically studied based on the first-principles calculations. It is established that the C3N monolayer is one of the promising anode material for Na and K-ion batteries. Through the calculated phonon spectra and AIMD simulation, we first confirmed the thermal and dynamical stabilities of the C3N monolayer. It is observed that the S1 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 C3N monolayer can possess high charge/discharge rate with the diffusion of 0.03 eV 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 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 some previous reports, the obtained results clearly lead towards the possibility of the C3N monolayer as Na and K-ion battery anodes.

5. Supporting Information The supporting information file contents are Total and projected density of states of C3N monolayer, considered adsorption sites for Na and K-ion adsorption on C3N monolayer surface, adsorption energies and charge transfer values for Na and K adsorption on C3N surface, adsorption structures of Na adsorption layer on C3N surface, and adsorption structures of K adsorption layer on C3N surface.

6. Acknowledgments

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We thank IIT Indore for the lab and computing facilities. This work is supported by DST-SERB, (Project Number: EMR/2015/002057) New Delhi. P.B. and A.M. thank MHRD for research fellowships.

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