Two-Dimensional MnO2 as a Better Cathode Material for Lithium Ion

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Two-Dimensional MnO as a Better Cathode Material for Lithium Ion Batteries Shuo Deng, Lu Wang, Tingjun Hou, and Youyong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10354 • Publication Date (Web): 02 Dec 2015 Downloaded from http://pubs.acs.org on December 9, 2015

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The Journal of Physical Chemistry

Two-dimensional MnO2 as A Better Cathode Material for Lithium Ion Batteries Shuo Deng, Lu Wang*, Tingjun Hou and Youyong Li*. a

Functional Nano & Soft Materials Laboratory (FUNSOM) and Collaborative

Innovation Center of Suzhou Nano Science and Technology Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, China.

ABSTRACT Based on the first-principles calculations, we have systematically studied the adsorption and diffusion of Li ions on monolayer MnO2, and compared with other transition metal dichalcogenides (TMDs) and transition metal dioxides (TMOs). Monolayer MnO2 shows a relatively high Li adsorption energy of 4.37 eV and low Li diffusion barrier of 0.148 eV. The electronic analysis indicates the electron transferred from Li to the empty orbital of O atom, and some orbital coupling between the s-orbital of Li atom and the pz-orbital of O atom in MnO2. Due to Li adsorption on both sides of MnO2 layer, the theoretical Li storage capacity reaches as high as 616 mAh/g. Our results demonstrated that, compared to other two-dimensional (2D) nanomaterials, monolayer or few-layer MnO2 exhibits excellent performance on Li storage capacity and diffusion rate, and is believed to be a promising electrode material for high capacity Li ion batteries.

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1. INTRODUCTION

The rapid development of modern society imposes increasing demands for new energy. Lithium ion batteries (LIBs) gradually become a representative of high performance battery with strong electricity storage capacity. Since the cathode material, LiCoO2, has been developed, LIBs was entering the phase of rapid development of commercial applications. As the most important means of storage, LIBs has played an irreplaceable role in computers, cell phones, electric transportation or new energy automobile and aerospace fields. However, at the current stage, only about half of the Li atoms stored in the LiCoO2 crystal lattice could be discharged in the process of recycling because of the limitation of the structure's stabilization.1 To meet the demand of LIBs with better performances, it is urgent to develop advanced electrode materials that can provide satisfactory capacity, cyclic stability, high-rate capability, and safety.2-5

As the rise of graphene, two-dimensional (2D) materials have been drawn great interests and have promising applications in LIBs due to the abundant adsorption sites and short Li diffusion path. As promising anode materials, graphene can accommodate Li atoms on both sides and leads to a higher theoretical capacity.6-7 Similar to graphene, 2D transition metal chalcogenides (TMDs) and transition metal oxides (TMOs) with a typical sandwich structure of three atomic layers have been widely explored in recent years. Some TMDs materials with single layer or few layers have been proposed to be used as electrode materials in LIBs either in experiments or 2 ACS Paragon Plus Environment

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in theories, i.e., MoS2,8-12 VS2,13 WS2,14 TiS215 and SnS2,16 etc. As the thickness of the nanosheets decreases, the electrochemical performance can be enhanced greatly.10 The favorable structure with enlarged interlayer space can accommodate more Li ions insertion than bulk or nanotubes, and further facilitate the fast charge-discharge process. Besides, as cathode, V2O5 exhibits high redox potential of the V5+/V4+ couple versus lithium with 3.5 V,17 and the Li diffusion barrier decreases from 0.51 eV to 0.20 eV from bulk to a single layered structure.18 Recent experiments showed that mesoporous β-MnO2 or nanocomposite exhibits high Li storage capacity of about 320 mAh/g.19 Later, the theoretical study calculated the intercalation voltages of the stable LixMnO2 configurations to be 3.47 V ~ 2.77 V,20 which agrees well with the experiments.21-22

Monolayer manganese dioxide (MnO2) was synthesized successfully by Omomo et al. in 2003,23 which is an indirect semiconductor with a band gap of 3.41 eV, and the Mn ions induce intrinsic ferromagnetism with a low Curie temperature.24 Here, we propose monolayer MnO2 to be used as cathode material for LIBs. As an ideal cathode material, it should meet two conditions: 1) the material should react with Li with a high free energy of reaction, leading to a high Li capacity and high-energy storage; 2) the material should react with Li very rapidly both on insertion and removal, leading to high power density. Correspondingly, higher Li adsorption energy and lower diffusion barrier are extremely necessary for cathode material. By first-principles calculations, our results indicate that layered MnO2 shows a higher adsorption energy of 4.37 eV for Li ion and a lower Li diffusion barrier of only 0.148 3 ACS Paragon Plus Environment


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eV, which exhibits the better performance than the other TMD and TMO materials. Due to the adsorption by both sides of Li ions, the theoretical Li storage capacity can reach as high as 616 mAh/g.

2. COMPUTATIONAL METHODS

All the calculations were performed using spin-polarized density functional theory (DFT) and implemented in Vienna Ab initio Simulation Package (VASP) with a plane wave basis set.25 The frozen-core all-electron projector-augmented wave (PAW) method was used for the electron-ion interaction,26 and Perdew-Burke-Ernzerhof (PBE) approximation was used to describe the electron exchange-correlation.27 The cut-off energy for the plane-wave basis expansion is chosen as 450 eV. Conjugated gradient (CG) atomic optimization is performed with a criterion of convergence of 0.02 eV/Å.

To study the properties of adsorption and diffusion of Li ion on MnO2 layer, a 5×5 MnO2 periodic supercell with the lattice of 14.65 Å × 14.65 Å was adopted, where in the direction normal to the MnO2 layer we use a 15-Å-thick vacuum region to separate it from the adjacent images. For the Brillouin zone integration, a 4×4×1 k-point grid was chosen for a 5×5 supercell of MnO2.

To investigate the Li diffusion on the MnO2 layer, the climbing image nudged elastic band (CI-NEB) method was used.28 The CI-NEB method is an efficient method to determine the minimum energy path and saddle points between a given initial and final positions. By inserting a series of intermediate image structures 4 ACS Paragon Plus Environment

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between the optimized initial and final structures, a spring force is introduced to determine the minimum energy path of reaction and locate the transition state structure.

3. RESULTS and DISCUSSION

3.1. Li adsorption on monolayer and bilayer of MnO2.

Initially, we considered the possible adsorption sites of Li atom on monolayer MnO2. MnO2 presents a typical sandwich structure with the Mn layer sandwiched between two O layers. The optimized MnO2 unitcell has the lattice parameters of 2.93Å, and the Mn-O bond length of 1.93Å with an O-Mn-O bond angel of 81.64°, which is shown in Fig. 1(a). The Li adsorption site and adsorption energy on MnO2 are the most important parameters for cathode material in LIBs. The adsorption energy (Ea) for Li ion is defined by the following formula:

Where EMnO2-Li, EMnO2 and ELi are the total energies of Li-adsorbed MnO2, pristine MnO2 and the energy of an isolated Li atom, respectively. According to our definition, a more positive adsorption energy indicates a more favorable exothermic reaction between MnO2 and Li. Similar to Li adsorption on VS2 and MoS2,13,9 there are two adsorption sites for Li atom, as shown in Fig. 1(b). H site is Li locating at the center of a Mn-O hexagonal 5 ACS Paragon Plus Environment


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ring, while T site is Li atom locating at the top site above a Mn atom. Our results show that Li atom prefers to adsorb on the H site with the adsorption energy of 4.37 eV, slightly lower than the adsorption energy of 4.27 eV on the T site. The adsorption distance between Li atom and the three affected O atoms on the H site is 2.02 Å, while that on the T site is 2.07Å. The Li adsorption energy is extremely higher than the reported TMD materials and the bulk MnO2,10,20 and comparable to the Li adsorption energy of 4.09 eV on monolayer MoO3.29

Figure 1. (a) Top view and side view of monolayer MnO2 structures; (b) two sites for Li adsorption on monolayer MnO2 labeled as H site and T site; (c) Octahedral site and (d) tetrahedral site for Li adsorption on bilayer MnO2.

Next, we examined the adsorption properties of Li atom on bilayer MnO2. Two MnO2 layers have different types of stacking, and AB stacking is energetically favorable, so AB stacking for bilayer MnO2 is considered here. There are two sites for 6 ACS Paragon Plus Environment

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Li atom adsorbed between two MnO2 layers. One is the octahedral site (O site) in which Li atom binds to three O atoms from each layer (Fig. 1c), while the other is the tetrahedral site (T site) forming by three O atoms in the upper layer and one O atom in the lower layer (Fig. 1d). The adsorption energy of Li atom on these two sites is similar with 4.55 eV on O site and 4.50 eV on T site, respectively, which are slightly larger than that of Li atom on monolayer MnO2.

To get a deep understanding on the interaction mechanism between Li atom and MnO2 layer, we calculated the charge density difference and the density of states for Li adsorbed on monolayer MnO2. The calculated total density of states (TDOS) and projected density of states (PDOS) are plotted in Fig. 2(a). There is some coupling between the s-orbital of Li atom and Pz-orbital of O atom in MnO2. The electron of Li atom may transfer to the empty orbital of O atom. The following analysis of Bader charge and charge density difference confirms it.

The definition of charge density difference is defined by:

where ρMnO2-Li, ρMnO2 and ρLi are charge density of the system of Li adsorbed on MnO2, pristine MnO2 monolayer, and the isolated Li atom, respectively. From the analysis of charge density difference, there is a net loss of charge above the Li atom combined with a net charge gain for the nearest O atoms (see Fig. 2b and 2c), indicating a significant charge transfer between Li and O atoms. In order to 7 ACS Paragon Plus Environment


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quantitatively calculate the amount of charge transfer between Li and MnO2, a Bader charge analysis has been performed. After Li adsorbed on MnO2, the charges on Li atom decrease from 0.28 |e| to 0.01 |e|, while the charges on one of the nearest O atoms increase from 7.04 |e| to 7.20 |e|. The charge transfer of about 0.27 |e| suggests a significant electron transfer from Li atom to one nearest O atoms, indicating a dative bond between Li atom and MnO2 surface.

Figure 2. (a) The projected spin polarized density of states of Li atom adsorbed on monolayer MnO2; (b) top view and side view of charge density difference distribution 8 ACS Paragon Plus Environment

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for Li atom adsorbed on monolayer MnO2. Cyan and yellow regions indicate depletion and accumulation of electrons, respectively.

3.2. Li diffusion and intercalation voltage.

The Li ion mobility on the cathode is important for its performance in cyclic LIBs. We studied the Li diffusion on monolayer MnO2 and interlayer MnO2. The H site is the favorable site for Li adsorption on monolayer MnO2 as discussed above, thus we studied the Li diffusion path between two adjacent H sites. As shown in Fig. 3, the diffusion of Li on monolayer MnO2 can occur by migrating from one H site to another, passing through a T site. And, it only needs to overcome a negligible energy barrier of 0.148 eV on monolayer MnO2, which is significantly lower than the diffusion barrier of Li ion in bulk β-MnO2 with 0.26 eV.20 Moreover, we also studied the migration of Li ion between the MnO2 layers. In this case, Li migrated from one octahedral (O) site to another adjacent one by passing through a tetrahedral (T) site. The diffusion barrier between the interlayer MnO2 is 0.114 eV, which is even slightly lower than that on the surface of monolayer MnO2. Compared to the Li diffusing on the monolayer MnO2, the enhanced binding of Li by both two layers lowers the energy of the transition state of Li diffusing between bilayer MnO2, which results in a lower diffusion barrier. Our results indicate that MnO2 layer shows excellent high-rate performance, and is promising to be used as potential cathode materials in LIBs.



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Figure 3. Li diffusion paths on the monolayer MnO2 (upper) and between interlayer MnO2 (lower).

To further study the electrochemical properties of MnO2 layer as cathode materials, the theoretical intercalation voltage for LixMnO2 has been calculated, which could well imply the performances of cathode in LIBs. As previously reported, the theoretical intercalation voltage for Li can be computed as the difference of Gibbs free energy for the intercalation reaction divided by the number of transferred Li ions, and the Gibbs free energy can be approximately calculated by the total energy with volume and entropy both neglected.29,30


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The electrochemical reaction is as following:

where x refers to the number of Li+ ions transferred, and the average intercalation voltage is calculated by

where ELix1MnO2, ELix2MnO2, and ELi are the total energies of the system of MnO2 with x1 and x2 Li adsorbed, and the isolated Li atom, respectively. We have examined the intercalation voltage for different Li intercalation stage with x=0.67, 1.33 and 2. Our results have shown that the discharge voltage for Li intercalation dropped from 3.54 V to 2.93 V, comparable to some reported cathode materials.8,20,31-34 Thus, the appropriate intercalation voltage provides feasibility for MnO2 monolayer to be used as a promising cathode material in LIBs.

3.3. Comparison with other TMD and TMO materials



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Table 1. Summary of the Li adsorption energies, diffusion barriers, favorable adsorption sites, adsorption distance (dLi-O/S) and Li storage capacity on different TMD and TMO materials.

Materials

Ea (eV)

Barrier(eV)

MnO2 MoO2 WO2 MoS2 WS2 VS2

4.37 3.05 2.34 1.96 1.56 2.13

0.148 0.095 0.122 0.216 0.220 0.208

Favorable adsorption site H

T

dLi-O/S (Å) 2.02 2.00 1.99 2.37 2.36 2.44

Storage Capacity (mA h/g) 616 419 248 335 216 466

In order to emphasis the superiority of MnO2 monolayer in LIBs, a series of TMD (MoS2, WS2 and VS2) and TMO (MoO2 and WO2) monolayer materials have been compared. The adsorption energies and diffusion barriers of Li atom on these materials are calculated and summarized in Table 1 and Fig. 4(a). The adsorption energies of Li atom on the surface of TMO materials are systematically higher than that on the TMD materials, implying a stronger interaction between Li and O atoms. The shorter distance between Li atoms and the O atoms in TMO (~2.0 Å) comparing with that between Li atom and the S atoms in TMD (~2.4 Å) also demonstrated it. Among them, MnO2 possesses the largest Li adsorption energy. It is worthy to note that, TMO and TMD materials prefer different Li adsorption sites. H site is the favorable site for all the TMO materials, while T site is the favorable site for all the TMD materials. The reason is that Li shows favorable interactions with oxygen atoms. On the contrary, the Li diffusion barriers on monolayer TMO materials are 12 ACS Paragon Plus Environment

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lower than that on TMD materials. Among them, MoO2 possesses the lowest Li diffusion energy of only 0.1 eV. Our results indicate that the TMO is a promising material for electrodes of LIBs, which is better than the TMD materials.

Figure 4. (a) Adsorption energies and diffusion barriers of Li atom on monolayer MnO2 compared to the other TMD and TMO materials; (b) the variations of adsorption energies with increasing Li content in different TMD and TMO materials. 13 ACS Paragon Plus Environment


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In order to determine the thermodynamic stability of MnO2 at a high Li concentration, we calculated the Li adsorption energies on monolayer MnO2 as the increasing of Li concentration, and compared to other TMO and TMD materials. A series of configurations (LixMO2 or LixMS2, x=0.125, 0.222, 0.5, 0.667, 1 and 2) with Li adsorption on both sides of monolayer surface in the 4×4, 3×3, 2×2, √3×√3, 1×2, and 1×1 supercell were constructed, respectively. On the surface of these configurations, Li atoms distributed uniformly on the favorable H sites for both sides of MnO2 layer. The same supercells were calculated for other TMD and TMO materials and all the results were summarized in Fig. 4(b). The Li adsorption energy decreases gradually with the increasing of x since the larger Li concentration introduces more repulsive interaction. Encouragingly, compared to others, when MnO2 monolayer reaches the highest Li concentration corresponding to the case of x=2 in LixMnO2, the Li adsorption energy is still as high as 2.89 eV. The results suggest that the LixMnO2 structure has high stability even with high Li concentration.

As x=2 in LixMnO2 represents the highest storage capacity, we can deduce the theoretical capacity of 616 mAh/g for MnO2 monolayer, which is almost double higher than the capacity of nanocomposite and mesoporous MnO2 (320 mAh/g).19 Except for monolayer MoO2 and VS2 with a Li storage capacity of ~400 mAh/g, WO2 and WS2 exhibit a relatively low capacity of ~200 mAh/g. Compared with other TMD and TMO materials considered here, MnO2 shows an excellent performance on storage capacity and diffusion rate of Li ions. Our results suggest that monolayer MnO2 can be a promising cathode material for applications in LIBs. 14 ACS Paragon Plus Environment

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3.4. Conclusions

In summary, by employed the first-principles calculations we systematically studied the adsorption and diffusion of Li atom on monolayer and interlayer MnO2. Our results show that, different from Li adsorption on TMD surface with T site, Li atom prefers to adsorb on the H site with a adsorption energy of 4.37 eV, and even a larger adsorption energy of 4.55 eV on O site between the interlayer of MnO2. Electronic analysis has been performed to clarify the interaction between Li and MnO2. A dative bond has been formed between the Li atom and the O atoms, which is deduced from the significant electron transfer from Li atom to the nearest O atoms and the orbital coupling between the s-orbital of Li atom and Pz-orbital of O atom. As an ideal cathode material for LIBs, the high-rate performance is also important, so we have calculated the Li diffusion on the surface of monolayer MnO2 and between the interlayer of MnO2. Our results demonstrated that it only needs to overcome a negligible energy barrier of ~0.15 eV, which is significantly lower than the diffusion barrier of Li ion in bulk MnO2. In order to emphasis the advantages of MnO2 as cathode materials, we considered some other TMD and TMO materials for comparison. Among them, MnO2 possesses the highest adsorption energy and lowest diffusion barrier of Li ion. Moreover, when MnO2 monolayer reaches the highest Li concentration, the Li adsorption energy can be still as high as 2.89 eV with a theoretical capacity of 616 mAh/g. Our results strongly suggest monolayer or few-layer of MnO2 to be promising cathode materials for LIBs with excellent electrochemical performance in the near further. 15 ACS Paragon Plus Environment


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]. ACKNOWLEDGEMENT

The work is supported by the National Basic Research Program of China (973 Program, Grant No. 2012CB932400), the National Natural Science Foundation of China (Grant No. 21403146, 91233115, 21273158 and 91227201), Natural Science Foundation of Jiangsu Province (Grant No. BK20140314), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is also a project supported by the Fund for Innovative Research Teams of Jiangsu Higher Education Institutions, Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Collaborative Innovation Center of Suzhou Nano Science and Technology. REFERENCES (1) Bai, Y.; Shi, H.; Wang, Z.; Chen, L. Performance improvement of LiCoO2 by molten salt surface modification. J. Power Sources 2007, 167, 504-509. (2) Hu, M.; Pang, X. L.; Zhou, Z. Recent progress in high-voltage lithium ion batteries. J. Power Sources 2013, 237, 229-242 (3) Tarascon, J. M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359-367. 16 ACS Paragon Plus Environment

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Two-Dimensional MnO2 as a Better Cathode Material for Lithium Ion

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