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Cite this: J. Mater. Chem. A, 2019, 7, 4134
A novel porous C4N4 monolayer as a potential anchoring material for lithium–sulfur battery design† Tongtong Li,a Cheng He
*a and Wenxue Zhang*b
Lithium–sulfur (Li–S) batteries have attracted considerable attention due to high theoretical specific energy and environmental friendliness. However, the shuttle of lithium polysulfides (LiPSs) has become a major obstacle for developing Li–S batteries. In this work, we explored a new C4N4 monolayer, which could be easily prepared from its bulk form using a similar mechanical exfoliation approach to that of graphene due to its smaller exfoliation energy. The C4N4 monolayer could suppress the shuttling of LiPSs and produce significant improvement in the cycling performance of Li–S batteries. Moreover, adsorption calculations revealed that polysulfide captured by C4N4 was chemisorbed with a suitable range of adsorption energies from 1.931 to 3.119 eV. Furthermore, excellent surface diffusions of LiPSs on C4N4 resulted in a fast charge/discharge rate. Moreover, to improve the electrical conductivity of C4N4,
Received 14th November 2018 Accepted 24th January 2019
graphyne was selected to construct a C4N4/graphyne heterostructure. Based on these remarkable results, we conclude that C4N4 is a highly promising anchoring material for Li–S batteries. We hope that
DOI: 10.1039/c8ta10933h
our studies will inspire more experimental and theoretical studies on exploring the potential of other 2D nanostructures as lithium–sulfur battery hosts.
rsc.li/materials-a
1. Introduction Lithium–sulfur (Li–S) batteries are promising and attractive candidates for next-generation portable or stationary power supplies due to their high theoretical energy density of up to 2600 W h kg1, which is 3–5 times higher than that of traditional lithium batteries.1–3 Unlike the “rocking chair” mechanism, the charging and discharging process of a Li–S battery is a multi-step redox process, in which sulfur and Li bind to each other. The whole redox process is S8 + 16Li+ + 16e 5 8Li2S; this unique electrode mechanism and reaction can lead to high theoretical specic capacity. Moreover, sulfur as an active material has the advantages of low cost, natural abundance, and environmental friendliness. Therefore, a Li–S battery can become one of the best candidates for energy storage devices. However, although Li–S batteries have been extensively studied during the past three decades,4–7 their practical application is still hindered by the rapid capacity decay and serious selfdischarge caused by dissolving and shuttling of soluble lithium polysuldes (LiPSs) in electrolytes. Generally, when
a
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China. E-mail:
[email protected]
b
School of Materials Science and Engineering, Chang'an University, Xi'an 710064, China. E-mail:
[email protected] † Electronic supplementary 10.1039/c8ta10933h
information
4134 | J. Mater. Chem. A, 2019, 7, 4134–4144
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interactions between LiPSs and commonly used electrolyte solvents are stronger than those between LiPSs and electrode materials, LiPSs dissolve in electrolyte solvents, which causes loss of active materials. The shuttle effect is unavoidable. These dissolved LiPSs shuttle back and forth between the cathode and anode, leading to capacity fading and low coulombic efficiency.8 Importantly, dissolution results in reactions of LiPSs with the anode, forming an insulating layer and causing signicant polarization.9,10 To avoid the shuttling effect, an effective way is to trap LiPSs by adopting suitable anchoring materials. Due to the high surface-volume ratio and unique electronic properties, twodimensional (2D) materials have shown unique advantages as anchoring materials in Li–S batteries.11 Moreover, because extensive research has conrmed that carbon-based materials can improve the specic capacity and cycling performance of Li–S batteries,12–14 carbon materials have been widely used in Li– S batteries as conductive additives and anode protectors. Therefore, some researchers have started to focus on graphene, a typical representative of 2D carbon materials. However, the adsorption energies of S8 clusters and LiPSs on graphene are less than that of S8 clusters and LiPSs on electrolyte molecules.15 Thus, pure graphene is not an ideal anchoring material for Li–S batteries. Fortunately, it was recently found that N-doped carbon can anchor LiPSs,16,17 thus suppressing their shuttling and improving the cycling performance of Li–S batteries. Zhang et al.16,18 reported that N-doped graphene can bind LiPSs even more strongly with larger adsorption energies than pristine
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graphene. Besides experiments, rst-principle calculations also play an important role for exploring effective anchoring materials. Li et al.19 calculated the anchoring effect of various Ndoped graphene samples and showed that only clustered pyridinic N-dopants can attract lithium polysuldes with larger adsorption energies. Zhang et al. and Sun et al.20,21 reported that C3N, C2N and C3N4 could trap LiPSs via stronger interfacial interaction and alleviate the interactions between LiPSs and solvents as well as the consequent dissolution. However, 2D carbonized nitrogen have different stoichiometric ratios and specic congurations. It is difficult to validate experimentally what kinds of stoichiometric ratios and conguration in carbonized nitrogen works could be an effective sulfur immobilizer for Li–S batteries. Therefore, it is practical to theoretically investigate the interactions between carbonized nitrogen with different stoichiometric ratios and soluble LiPSs to optimize suitable anchoring materials. Herein, we chose a carbonized nitrogen monolayer as a representative to explore the effect of nitrogen content on carbon-based anchoring materials on the performance of Li–S batteries. Our extensive structural search was performed for seven stoichiometric (CmN8m, m ¼ 1–8) carbonized nitrogen monolayers through a rst-principles swarm structural search.22,23 Meanwhile, the dynamic and thermodynamic stabilities of C4N4 monolayer were identied. To explore its potential as an anchoring material, a van der Waals (vdW) correction was incorporated into DFT calculations to calculate the adsorption and diffusion of S8 clusters and various LiPSs (Li2S, Li2S2, Li2S4, Li2S6, and Li2S8) on C4N4 monolayer and C4N4/graphyne heterostructure.24,25 The adsorption energies toward S8 clusters and LiPSs on the C4N4 monolayer and C4N4/ graphyne heterostructure were larger than those for electrolyte solvents. Besides, the diffusion barriers of S8 clusters and LiPSs on the C4N4 monolayer and C4N4/graphyne surfaces were very small, which was benecial for the charge and discharge process of lithium batteries. These characteristics clearly indicated that the C4N4 monolayer is an excellent anchoring material for Li–S batteries. Our work opens up an avenue to explore a more effective carbonized nitrogen material for Li–S batteries, preventing undesirable LiPS shuttling.
2.
Theoretical approach
We performed a structural search by employing a particle swarm optimization (PSO) algorithm as implemented in the crystal structure analysis by particle swarm optimization (CALYPSO) code,26 which could efficiently nd the ground or metastable structures by just depending on the given chemical compositions. Its validity was conrmed by the application of a diverse variety of bulk27 and two-dimensional materials.28–30 A ˚ was used to avoid interaction between vacuum distance of 25 A adjacent layers. The structural relaxations were performed using the density functional theory method31 within the generalized gradient approximation of Perdew–Burke–Ernzerhof (GGA-PBE)32 as implemented in the Vienna ab initio simulation package (VASP)33 code. Moreover, the electronic property calculations were performed using the CASTEP code.34,35 A
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projector-augmented wave (PAW)36 technique was implemented as part of VASP to solve the Kohn–Sham equations.37 Space charge distribution was performed using the CASTEP code, which was used for ultraso pseudopotentials33 to describe ionic cores. The cut-off energy of 450 eV was employed. The atomic positions were completely relaxed until the maximum ˚ 1. Also, van der forces on each atom were less than 105 eV A Waals interaction was considered using the semi-empirical DFT-D2 approach.38 For the diffusion process, the minimum energy path (MEP) proles were studied using the climbing image nudged elastic band (CI-NEB) method39,40 as implemented in transition state tools for VASP (VASP-VTST). The energy difference between the saddle point and the initial state was dened as the diffusion barrier.
3.
Results and discussion
3.1. Structures and stabilities of C4N4 monolayer, S8 cluster and LiPSs Aer an extensive structural search for eight stoichiometric (CmN8m, m ¼ 1–8) carbonized nitrogen monolayers through the CALYPSO code, to determine the relative stability of our predicted C4N4, we completely relaxed the other CmN8m (m ¼ 1–8) monolayers and calculated the relative formation energies, as shown in Fig. S1 (in the ESI†). Based on these results, the C4N4 monolayer with C1 symmetry exhibited the lowest energy, indicating a global minimum for the two dimensional space. From Fig. 1(a), we observe that the C4N4 monolayer has a puckered structure with two atomic layers and the layer ˚ The unit cell was rectangular and the thickness is 0.411 A. ˚ and 6.103 A, ˚ respeclattice constants of a and b were 3.587 A tively. Moreover, the C4N4 monolayer was mainly composed of C3N2 ve-membered rings. Each row of ve-membered rings was connected by two nitrogen atoms. The bond lengths of CA– ˚ and 1.373 A, ˚ respectively. Different CB and CB–CC were 1.395 A bond lengths in the ve-membered rings were due to the existence of wrinkles. These distances were slightly shorter than the ˚ of graphene.41 In addition, the bond C–C bond length (1.420 A) ˚ and 1.350 A, ˚ lengths of NA–NB and NC–ND were 1.403 A respectively. The phonon vibration frequency was calculated to further conrm the thermodynamic stability, as shown in Fig. 1(b). The blue lines represent acoustic branches; it can be seen that all branches of the phonon spectrum are positive and no imaginary phonon mode exists, suggesting the dynamic stability of the C4N4 monolayer. Then, the calculated electron localization function (ELF) was used to analyze the bonding character.42 ELF is a position-dependent function ranging from 0 to 1. Generally, a large ELF value (>0.5) corresponds to a covalent bond or core electrons, whereas the ionic bond is represented by a smaller ELF value (
