Elsevier

Nano Energy

Volume 78, December 2020, 105352
Nano Energy

Mechanistic investigations of N-doped graphene/2H(1T)-MoS2 for Li/K-ions batteries

https://doi.org/10.1016/j.nanoen.2020.105352Get rights and content

Highlights

  • First-principles calculations were employed to investigate Li/K-ions batteries.

  • Graphitic N is favourable to the structural stability of graphene(Gr)/2H(1T)-MoS2.

  • Li+/K+ adsorption and diffusion mechanisms in NGr/2H(1H)-MoS2 were illustrated.

  • NGr/1T-MoS2 is a promising anode material for Li/K-ions batteries.

  • An optimal range of interlayer distance (6.0–6.5 Å) for K+ insertion is recommended.

Abstract

N-doped graphene (NGr) incorporated with 2H-MoS2 and 1T-MoS2 (NGr/2H(1T)-MoS2) composites have been explored as anode materials for Li/K-ions batteries (LIBs/PIBs), however, the electrochemical mechanisms of their performance have not been well probed. In this work, we use first-principles calculations to investigate the atomic mechanisms associated with their high performance and cycling stability. Graphitic N (grN) is found to play a vital role in improving the structural stability of NGr/2H(1T)-MoS2 and the electronic conductivity of NGr/2H-MoS2, while pyridinic N and pyrrolic N are detrimental to the structural integrity of hybrids. Due to small and stable adsorption energies, fast Li+/K+ adsorption can be achieved in grNGr/2H(1T)-MoS2 hybrids at high Li+/K+ contents. Besides, grNGr/2H(1T)-MoS2 composites have low Li+/K+ diffusion energy barriers and large diffusion coefficients. Especially, grNGr/1T-MoS2 displays superior Li+/K+ adsorption and diffusion capabilities as well as high electronic conductivity, making it a promising anode material for LIBs/PIBs. Based on the lattice expansion during K+ insertion, an optimal range of interlayer distance (6.0–6.5 Å) is found. These findings provide an in-depth understanding on the microscale Li+/K+ storage behaviour and are also instructive for optimising NGr/2H-MoS2 composite and designing NGr/1T-MoS2 anode material of LIBs/PIBs.

Introduction

Recently, rapid developments in electric vehicles, consumer electronics and smart electric grids put forward higher requirements for the power and energy density of lithium-ion batteries (LIBs). However, the limited resource and high cost of Li greatly impede the wide application of LIBs. In comparison to Li, K has a large quantity of natural resources (17000 ppm), which is about 850 times more than the former (20 ppm) [1]. Especially, the redox potential of a K/K+ couple is almost the same as Li/Li+ (2.93 V and 3.04 V versus standard hydrogen electrode, respectively), which makes high energy density and voltage plateau reachable in potassium-ion batteries (PIBs) [2]. Therefore, PIBs have been believed as a promising candidate to LIBs for large-scale stationary energy storage. Notwithstanding, a large ionic radius (1.38 Å) of K+ limits its fast intercalation/deintercalation, leading to a poor capacity and rate performance of PIBs [2,3]. Besides, the extensive structural deformation associated with K+ intercalation/deintercalation poses a negative impact on the cyclability of PIBs [4,5].

Because of the interior drawbacks of K+, many anode materials performing well in LIBs are not suitable for PIBs. Recently, two-dimensional transition metal dichalcogenides have been a hotspot in the energy field. Among these potential anode materials for LIBs/PIBs, 2H-MoS2 and 1T-MoS2 (2H(1T)-MoS2) have gained intensive research interests owing to their unique physical and chemical properties. Nevertheless, 2H(1T)-MoS2 suffers from a large volume change during K+ intercalation/deintercalation, which can cause destruction/pulverization of electrode materials during charge and discharge processes. It may lead to a fast capacity decay and poor cycling performance of electrode materials [6,7]. Besides, the electronic conductivity of 2H-MoS2 is poor, which limits its rate performance. In overcoming these problems, hybrid systems comprised of 2H(1T)-MoS2 and various carbon-based materials (carbon cloth [2,8,9], carbon nanotubes [[10], [11], [12]] and graphene (Gr) [13,14]) have been developed to enhance the electronic conductivity and structural integrity of electrode materials, showing remarkably improved capacity and cycling stability.

Apart from establishing heterostructure, element doping (e.g., N, Co and Mn) is also applied to optimise the charge transfer and structural stability of electrode materials [2,[15], [16], [17], [18]]. N-doped graphene/2H-MoS2 (NGr/2H-MoS2) composites display superior electrochemical performance as anode materials for LIBs/PIBs, much better than Gr/2H-MoS2 [8,10,14,16,19,20]. Many experimental studies have been reported on NGr/2H-MoS2 via different synthesis methods with various structures. For example, Tan et al. [14] prepared an integrated towel-like NGr/2H-MoS2 structure using a hydrothermal method. The heterostructure presents a high electronic conductivity and an enhanced structural stability and contributes to the excellent rate performance and cycling stability. Zhang et al. [17] obtained a strongly coupled NGr/2H-MoS2 composite by a scalable dopamine-assisted hydrothermal technique to show a Li storage capacity of 1102 mAh g‒1 at 0.1 A g‒1. Similarly, Wang et al. [10] developed bamboo-like hollow tubes with NGr/MoS2 interface as an anode material for PIBs, which exhibits a high capacity of 330 mAh g‒1 at 0.05 A g‒1 after 50 cycles and good capacity retention at a large current density (151 mAh g‒1 at 0.5 A g‒1 after 1000 cycles). These works unveil that the unique structure of NGr/2H-MoS2 can effectively alleviate the mechanical stress induced by Li+/K+ intercalation/deintercalation during long-term cycling, and thus improve the structural stability of electrode materials. The heterogeneous interface can also largely enhance the charge transfer rate and inhibit the continuing growth of the solid electrolyte interphase in particles, leading to reduction in the irreversible capacity loss.

Although many theoretical studies have been made on Li+ storage mechanisms in Gr/2H(1T)-MoS2 hybrids [[21], [22], [23]], 2H(1T)-MoS2 monolayer [24,25] and bulk 2H(1T)-MoS2 [26,27], the atomic-scale storage mechanism of Li+/K+ in NGr/2H-MoS2 has not been well understood. In contrast to single Li+ [23] and multiple Li+ storage [21] at Gr/2H-MoS2 interface, multiple Li+ adsorption on MoS2 and Gr surfaces of the hybrid has not been explored [23]. Especially, relevant researches on the electrode materials for PIBs are still in an infant stage. To the best of our knowledge, there has been no report on the K+ intercalation behaviours in Gr/2H-MoS2 [1], let alone NGr/2H-MoS2. Therefore, it is necessary to have a comprehensive study on the Li+/K+ storage mechanism in NGr/2H-MoS2, which is essential to the development of hybrids in LIBs/PIBs.

In this paper, by using first-principles calculations, we systematically investigate the lithiation/potassiation mechanism in NGr/2H-MoS2 at an atomic scale. In addition to NGr/2H-MoS2, Li+/K+ storage in NGr/1T-MoS2 composite is also studied by considering the much better electrochemical performance of Gr/1T-MoS2 than Gr/2H-MoS2 [28,29]. It is expected to reveal whether NGr/1T-MoS2 could be utilized as a suitable anode material for LIBs/PIBs. Thus, we first explore the structural, mechanical, and electronic properties of NGr/2H(1T)-MoS2. The effects of different N dopants (graphitic N (grN), pyridinic N (pyN), and pyrrolic N (prN)) on the structural stability and electronic conductivity of the hybrids are studied. It is found that, in contrast to pyN and prN, grN is quite helpful to enhance the structural integrity of Gr/2H(1T)-MoS2 and electronic conductivity of Gr/2H-MoS2. Then, we focus on the simulations of Li+/K+ adsorption and kinetic diffusion in grN-doped Gr/2H(1T)-MoS2 (grNGr/2H(1T)-MoS2) heterostructures. Finally, the influence of interlayer spacing on vertical lattice expansion upon potassiation is discussed for the optimal design of electrode materials for PIBs.

All simulations have been performed using the Vienna Ab initio Simulation Package (VASP) [30,31]. The projected augmented wave method and the generalized gradient approximation were used to deal with the ion-electron interaction and the electron exchange-correction energy, respectively [32]. In calculations, the kinetic cut-off energy was set to be 400 eV. The optimised lattice constants of graphene and MoS2 monolayers are 2.468 and 3.184 Å, respectively, which are consistent with the corresponding experimental and theoretical values (2.46 and 3.13 Å, respectively) [33,34].

NGr/2H(1T)-MoS2 and Gr/2H(1T)-MoS2 heterostructures were established by placing a (4 × 4) NGr or Gr unit cell on a (3 × 3) 2H(1T)-MoS2 monolayer with lattice mismatches of ~ 3%. For K+ intercalations at the interlayer of 2H(1T)-MoS2, the effect of NGr on K+ intercalation can be neglected due to a larger distance between the outer NGr and inner MoS2. Thus, the (3 × 3) 2H(1T)-MoS2/(3 × 3) 2H(1T)-MoS2 supercells were used with the experimental interlayer spacing of 1 nm [3,28]. A 20 Å vacuum layer was applied in the z direction to remove the interaction between two neighbouring images. Since the density functional theory (DFT) can only locate a local energy minimum, and thus during structural relaxation, the (N)Gr monolayer was first moved along z, y and x directions relative to the bottom 2H(1T)-MoS2 layer. The initial structure of each step was based on the most stable structure from the previous step. This provides an optimal interlayer spacing between (N)Gr and 2H(1T)-MoS2 (see Fig. S1) and the initial interfaces with the global energy minimum. Then, the initial structures were further optimised by using the conjugate gradient method until residual forces were less than 0.03 eV/Å. A 3 × 3 × 1 k-point mesh was chosen for interface calculations and simulations of electronic properties were undertaken by using a 5 × 5 × 1 k-grid. The van der Waals interaction was corrected by the DFT-D3 method of Grimme [35]. The visualisation of crystal structures was conducted via the VESTA program [36].

Because neutral K atom can easily lose its outer valence electron and change into K+ when adsorbed on an electrode material, K atom is generally used to represent K+ + e in the K+ adsorption reaction (K+ + e + MoS2 → KMoS2). The same method is used to calculate Li+ adsorption. Here, the adsorption energy (Eads) and formation energy (ΔHf) of Li+/K+ adsorption are calculated by Eqs. (1), (2)), respectively.Eads=(Ehybrid+Li/KEhybridnEisolated Li/K)/nΔHf=(Ehybrid+Li/KEhybridnEbulk Li/K)/nwhere, Ehybrid and Ehybrid+Li/K are the total energies of a pristine and a Li/K-adsorbed hybrid, respectively, Eisolated Li/K and Ebulk Li/K are the energies of an isolated Li/K atom and a Li/K atom in the bulk state, respectively, and n is the number of Li/K atoms adsorbed in a system. Here, a negative Eads implies that the adsorption reaction can take place. The negative ΔHf refers to a thermodynamically stable structure.

The interface strength is measured by the work of separation (Wsep) as belowWsep=(E(N)Gr+EMoS2Ehybrid)/S,where E(N)Gr and EMoS2 are the total energies of isolated (N)Gr and MoS2 layers in an interface model, respectively, and S is the total interface area. A greater positive Wsep suggests stronger interfacial cohesion while a negative value refers that a heterointerface cannot form [37].

Under small strains (ϵx and ϵy), the in-plane stiffness (C2D) along x and y directions (Cx and Cy) is defined by the following equation [38].C2D=[2Eδ2]/S,where E is the total energy of strained hybrid and δ is the applied uniaxial strain.

The climbing image nudged elastic band (CINEB) method was employed to simulate Li/K diffusion paths and energy barriers in hybrids [39]. Based on the transition-state theory [40,41], the reaction rate k of Li/K hopping can be described byk(T)=v0exp(EakBT),where kB and Ea are the Boltzmann constant and activation energy of diffusion, respectively, v0 is the attempt frequency (1013 Hz), and T is temperature (K) [42]. Since the 0-K migration barrier is used to describe the kinetic diffusivity in the CINEB method, the change of entropy can be neglected and the free energy is approximated by the 0-K activation energy (Ea) [43,44]. Then, according to the Li/K hopping distance (l) and k(T), the diffusion coefficient (D) can be obtained by [44]D=l2k(T).

Section snippets

Structural, mechanical and electronic properties of NGr/2H(1T)-MoS2

To reveal the influence of grN-doping, we investigated the structures of (grN)Gr/2H-MoS2 and (grN)Gr/1T-MoS2 hybrids. As seen in Fig. 1a and c, the interlayer distances (d) of pristine Gr/2H-MoS2 and Gr/1T-MoS2 are 4.96 Å. After grN-doping, the d values rise to 5.00 and 4.97 Å, respectively, which are close to the expanded d in experiments [2,3,10,28]. The enlarged d is favourable to Li+/K+ intercalation in the interface region [2,3]. From the relaxed structures of (grN)Gr/1T-MoS2 (Fig. 1c and

Conclusions

In this paper, we have systematically explored the excellent electrochemical performance of NGr/2H(1T)-MoS2 anode materials for LIBs/PIBs in terms of molecular structure. The first-principles calculations show that grN can largely strengthen the structural stability, with interface strengths of Gr/2H-MoS2 and Gr/1T-MoS2 increased by about 7% and 18%, respectively. In contrast, pyN and prN may weaken the interface strength and electronic conductivity of Gr/1T-MoS2. Besides, grNGr/2H(1T)-MoS2

CRediT authorship contribution statement

Panpan Zhang: Conceptualization, Writing - original draft. Yangyang Yang: Data curation. Xiaoguang Duan: Visualization, Investigation. Shu Zhao: Software, Data curation. Chunsheng Lu: Writing - review & editing. Yonglong Shen: Validation. Guosheng Shao: Validation. Shaobin Wang: Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The simulations were performed on resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia.

Dr. Panpan Zhang received her PhD in the School of Civil and Mechanical Engineering from Curtin University (Australia) in 2019. Currently, she is a visiting scholar in the School of Chemical Engineering and Advanced Materials at The University of Adelaide (Australia). Her research interest is the application of computational material science in exploring the structure-properties relationship of functional materials for energy storage and conversion.

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    Dr. Panpan Zhang received her PhD in the School of Civil and Mechanical Engineering from Curtin University (Australia) in 2019. Currently, she is a visiting scholar in the School of Chemical Engineering and Advanced Materials at The University of Adelaide (Australia). Her research interest is the application of computational material science in exploring the structure-properties relationship of functional materials for energy storage and conversion.

    Mr Yangyang Yang is currently a PhD candidate in the School of Chemical Engineering and Advanced Materials at The University of Adelaide (Australia). He focuses on the design and fabrication of nanomaterials and micro/nanomotors applied in the environmental remediation.

    Dr. Xiaoguang Duan received his PhD in Chemical Engineering from Curtin University (Australia) in 2016. He currently works as a University Research Fellow in the School of Chemical Engineering and Advanced Materials, The University of Adelaide (Australia). His research focuses on the design and development of functional nanomaterials for environmental remediation and facilitating the mechanistic innovations of metal-free carbocatalysis and advanced oxidation processes. He was awarded the ACS Catalysis Early Career Award and Journal of Materials Chemistry A Emerging Investigators.

    Miss Shu Zhao received her master degree in the School of Material Science and Engineering at Xiangtan University (China) in 2019. She is currently a PhD candidate in Westlake University (China). Her research interest is the exploration of promising cathode materials for metal-ion batteries by using theoretical simulations based on density functional theory.

    Dr. Chunsheng Lu received his PhD from the Institute of Mechanics, Chinese Academy of Sciences, in 1993, and subsequently worked at Tohoku University in Japan, Victoria University of Wellington in New Zealand, University of Leoben in Austria, and The University of Sydney in Australia as a postdoc or research fellow. He is now an Associate Professor in the School of Civil and Mechanical Engineering at Curtin University in Australia. His research activities mainly focus on multi-scale modelling and size effects of advanced materials and structures.

    Dr. Yonglong Shen received his B.S. degree and M. S. degree in Materials Science from Zhengzhou University in 2006 and 2009, respectively. He came to the UK in 2010 and received a PhD degree in Materials Science from the University of Bolton (with Prof. Guosheng Shao) in 2015. He then spent two years as a postdoctoral fellow in Prof. Guosheng Shao's lab at Zhengzhou University and has been a lecture since 2018. His research interests include semiconductors, thin films, electrical and optical properties of metal oxides, and advanced characterization techniques (TEM and EELS).

    Prof. Guosheng Shao obtained his PhD in Materials Science at the University of Surrey in 1995 and thereupon worked as a research fellow and senior research fellow, until transferring to Brunel University as Reader in Materials in 2005. He joined the University of Bolton as a Professor of Materials Modelling & Simulation in 2007. He has been the Director of CDLCEM at Zhengzhou University, the Founding Director of the Zhengzhou Materials Genome Institute (ZMGI, 2016), and Visiting Professor to the University of Surrey, UK (2018-). His academic interest is “designer” materials, application devices and advanced thin film coating technologies.

    Prof. Shaobin Wang obtained his Ph.D. in Chemical Engineering from the University of Queensland, Australia. He is now a Professor at the School of Chemical Engineering and Advanced Materials, The University of Adelaide, Australia. His research interests focus on nanomaterial synthesis and application for adsorption and catalysis, fuel and energy conversion and environmental remediation. He has published more than 400 refereed journal papers with citation over 39,000 and H-index of 111. He is a Global Highly Cited Researcher in Chemical/Environmental Engineering for 2016, 2017, 2018, and 2019.

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