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Unraveling the Positive Roles of Point Defects on Carbon Surfaces in Non-Aqueous Lithium-Oxygen Batteries Haoran Jiang, Peng Tan, Ming Liu, Yikai Zeng, and Tianshou Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04241 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 3, 2016

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The Journal of Physical Chemistry

Unraveling the Positive Roles of Point Defects on Carbon Surfaces in Non-Aqueous Lithium-Oxygen Batteries H.R. Jiang, P. Tan, M. Liu, Y.K. Zeng, T.S. Zhao* Department of Mechanical and Aerospace Engineering The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong, China

Abstract Carbon has been widely used to form cathodes for non-aqueous lithium-oxygen (Li-O2) batteries due to its high specific surface area, high electrical conductivity and cost-effectiveness. The mechanistic understanding of carbon materials, particularly the effect of carbon surface properties on the battery’s performance, however, is limited. In this work, we perform first-principle calculations to study the roles of point defects on carbon surfaces. Five representative defective structures, including SV (single vacancy), DV5-8-5 (two pentagons and one octagon), DV555-777 (three pentagons and three heptagons), DV5555-6-7777 (four pentagons, one hexagon and four heptagons) and SW (Stone-Wales) defects, are considered. Based on the adsorption energies of O2 and Li, the different Li4O4 growing pathways on these structures are identified and free energy diagrams are then obtained. It is found that the presence of DV5555-6-7777 and SW defects is beneficial to non-aqueous Li-O2

*

Corresponding author. Tel.: (852) 2358 8647 E-mail: [email protected] (T.S. Zhao) 1

ACS Paragon Plus Environment


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batteries because i) DV5555-6-7777 and SW defects exhibit zero-band-gap semiconductor behaviors, ensuring an excellent electrical conductivity; ii) DV5555-6-7777 and SW defects lead to a high discharge voltage, but low charge voltage; iii) DV5555-6-7777 and SW defects are stable during battery cycling and do not promote the formation of side product Li2CO3; and iv) by using DMSO as a sample, DV5555-6-7777 and SW defects are not supposed to decompose electrolytes. Hence, carbon materials containing DV555-6-777 and SW defects are desired for non-aqueous Li-O2 batteries.

2


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1. Introduction To keep pace with the rapid development of electronic market, energy storage systems with high energy density, high power density and long cycle life are in urgent demand. Over the past decades, a great success has been achieved in rechargeable lithium-ion batteries (LIBs) 1-7, promoting their widely use in daily life. However, the limited energy density impedes their further applications in future electrical vehicles (EVs). In this regard, non-aqueous Li-O2 batteries 8-12, which were first introduced by Abraham and Jiang in 1996

13

, has attracted much attention recently due to their

comparable energy density (11682 W h kg-1) with gasoline (13000 W h kg-1) 14. This high value is achieved by the facts that the anode uses the lightest metal lithium and cathode reactant O2 is obtained directly from atmosphere without occupying the inside volume. The electrochemical reaction of non-aqueous Li-O2 batteries is described as 2Li + O2 ↔ Li2O2, with solid Li2O2 as the discharge product and accumulating in the cathode. The discharge capacity, therefore, closely related to the specific surface area and porosity of the cathode. In addition, cathode materials should also have a large electrical conductivity to lower the overpotential caused by electronic resistance. Carbon materials, thus, are widely used to form cathode for non-aqueous Li-O2 batteries due to their large specific surface area, high porosity, good electrical conductivity, and low cost 15-19. 3



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However, until now, the mechanistic understanding of carbon materials in non-aqueous Li-O2 batteries is limited and the roles of surface properties in the discharge and charge processes remain controversial. On the one hand, carbon-based cathodes are considered to be not stable during cycling and result in poor rechargeability. For example, McCloskey et al.

20

found a monolayer Li2CO3 formed

at the C-Li2O2 interface by XPS and isotope labeling, resulting in a 10-100 fold decrease in the exchange current density. Thotiyl et al.

21

reported that carbon

electrodes lead to the decomposition of electrolyte and were not stable when charge voltages above 3.5 V, especially for hydrophilic ones. Itkis et al.

22

found carbon

electrodes would be degraded by superoxide and the existence of double bonds activated defects promoted carbonate production. Their results also suggested that it was better to use carbons materials with little defects and little functional groups in non-aqueous Li-O2 batteries. On the other hand, some investigations supported that defective carbon surfaces are effective to enhance the battery performance. For example, Nakanishi et al.

23,24

synthesized cup-stacked carbon nanotubes (CSCNT)

with different surface properties to form cathode in non-aqueous Li-O2 batteries. They found that the cathodes with many reactive edges presented a larger discharge capacity and lower overpotentials. Huang et al.

25

used vertically aligned carbon

nanotube (VACNT) as the cathode with N-Methyl-N-propylpiperidinium bis 4


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(trifluoromethanesulfonyl) imide (PP13TFSI)-based electrolyte, and the results showed that the defective VACNT had a larger discharge capacity, lower overpotentials and better cycling performance than the pristine one. Although understanding the roles of surface properties on carbon is crucial for improving the battery performance and beneficial for the cathode design, the experimental investigation is extremely difficult, due to the fact that the surface properties contain defects, functional groups, and even element-doping, which are hard to be separated. Fortunately, first-principles study offers an effective way to have an in-depth look into surface properties at atomic-level in non-aqueous Li-O2 batteries. For instance, Ren et al.

26

calculated the catalytic activity of X-doped graphene (X =

B, N, Al, Si, and P) and showed P-doped graphene was the best in reducing the charge overpotential. Jing et al.

27

compared the oxygen reduction reaction (ORR)

performance of five N-doped graphene structures and found the in-plane pyridinic N-doped graphene played the most important role in facilitating the nucleation of Li2O2 clusters. Jiang et al.

28

investigated the ORR and oxygen evolution reaction

(OER) performances of B, N-doped graphene and co-doped graphene. They found that the synergetic effect in co-doped graphene was lost in non-aqueous Li-O2 batteries and B-doped graphene was the best candidate among studied samples. In this work, a density functional theory (DFT) based first-principles study is 5



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performed to study the roles of point defects on carbon surface in non-aqueous Li-O2 batteries. Firstly, the structures of pristine graphite (0001) and five defective graphite surfaces are given, and the corresponding electron density contours as well as the density of state (DOS) are calculated. Then, based on the adsorption energies of Li and O2, the different Li2O2 growing paths on these surfaces are studied. Thirdly, the ORR and OER performances in non-aqueous Li-O2 batteries are evaluated by free energy diagrams, and their promotion to the formation of Li2CO3 are revealed by analyzing optimized structures. Finally, the influence of carbon defects on the electrolyte decomposition is further investigated. Our work distinguishes the roles of different point defects on carbon surface in non-aqueous Li-O2 batteries and provides an insight into the future carbon cathode design. 2. Computational methods All DFT calculations were performed using the ABINIT

29

code and the

exchange-correlation functional was dealt with by the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) type

30

. The core electrons

were modeled by the projector augmented wave (PAW) method

31

and the energy

cutoff was set to be 22 Ha. The 4×4×1 supercells were prepared in all cases and the Brillouin zone was sampled on a 4×4×1 k-point Monkhorst-Pack grids. To do a convergence test and confirm the reliability of our model, computations repeated on a 6


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5×5×1 supercell for DV5555-6-7777 defect were also performed. Small differences between the results of 4×4×1 and 5×5×1 supercells further verify the suitability of our model (Table S1 and Figure S1). The DOS was calculated for the optimized structures on a 31×31×1 k-point Monkhorst-Pack grids. A vacuum layer of 30 Bohr along zdirection was added to avoid the interaction between neighboring layers. The convergence criteria of the electron self-consistent loop and structural optimization were 4.0×10-5 Ha Bohr-1 and 4×10-4 Ha Bohr-1, respectively. The carbon structures were all obtained based on a monolayer graphite (0001) surface, the feasibility of which had been proved previously

32-36

.To estimate the

stability of point defects, the formation energy was calculated by:

E f = Edefect − m × µC

(1)

where Ef is the formation energy, E defect is the DFT total energy of the point defects; m is the number of carbon atoms; µC is the chemical potential of carbon, which is leveled from pristine graphene. The adsorption energies of Li and O2 on substrates were defined as: Eads = E system − E substrate − E Li /O2

(2)

where Esystem is the DFT energy of adsorption system; E substrate is the DFT energy of the substrate; E Li /O2 is the DFT energy of Li (s) or O2 (g). The standard formation energy of bulk Li2O2 was obtained from: 7



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∆G f = GLixO y ( s ) − xGLi ( s ) − where

y GO ( g ) 2 2

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(3)

GLixOy (s) , GLi ( s ) and GO ( g ) are the free energies of LixOy (s), Li (s) and O2 (g), 2

respectively. The equilibrium potential, therefore, was calculated according to the Nernst equation:

U0 = −

∆G f ne

(4)

where n represents the number of electrons transfer. The optimized lattice constants and structure are presented in Table S2 and Figure S2. The calculated equilibrium potential at 300 K and 1 atm is 2.94 V, which is consistent with the experimental results of 2.96 V 37. Due to the intrinsic complexity of the reactions in non-aqueous Li-O2 batteries, building suitable computational models to describe ORR and OER processes is still challenging. In this work, to evaluate the ORR and OER overpotentials, we used the model built by J. K. Nørskov et al.

38,39

, and the validity was further examined by

other groups in non-aqueous Li-O2 batteries 40-44. Based on this model, many potential catalysts in non-aqueous Li-O2 batteries were predicted by DFT calculations (e.g., PtCo, Pt3Ti, PtCu, PdCu, silicene) and some of them had been confirmed with excellent catalytic effect by experiments. Although the absolute values of overpotentials obtained from selected model are not identical with experimental ones, the relative values are comparable and the calculated tendency is reliable, especially 8


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for the similar samples. The reaction free energies were presented by: step final ∆G = Eslab +∆NO2 ⋅ µO2 +∆NLi ⋅ (µLi − eU) − Eslab

(5)

step final where Eslab is the DFT total energy of the intermediate step, Eslab is the DFT total

energy of the final step, ∆NO2 and ∆N Li are the number of O2 and lithium, and µLi are the chemical potentials of O2 (g) and Li

(s),

µO

2

respectively. The term −eU

is added to present the effect of potential U on electrons. The zero point energy (ZPE) of the intermediate steps is ignored as previous works did

26,45,46

, due to the

negligible difference in comparison with the total energy 27,44. 3. Results and discussion The optimized structures and electron density contours of graphite (0001) surface,

Figure 1. The optimized structures as well as electron density contours of (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The brown ball represents for carbon atom and the unit of electron density is |e| Bohr-3. 9



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SV, DV5-8-5, DV555-777, DV5555-6-7777 and SW defects are shown in Figures 1a-f, respectively. It is found that except the “sinelike” structure of SW defect, all the other samples have a planar surface with all carbon atoms in the same atomic layer. The formation energies of SV, DV5-8-5, DV555-777, DV5555-6-7777 and SW defects in 5×5×1 supercell are 7.83 eV, 8.03 eV, 6.25 eV, 7.73 eV and 5.24 eV, consistently with

previous theoretical investigations 35,47-49. In addition, the electrons

on graphite (0001) surface are uniformly distributed, accumulating between C-C bonds but seldom in the center of hexagons, as presented in Figure 1a. On the contrary, the existence of defects on carbon surfaces promotes the non-uniform distribution of electrons. For example, in the defects with vacancies, low-electron areas are created at where carbon atoms are removed. In addition, the rebuilt bonds show a relatively weaker bonding than that of the original ones, indicating a stronger electrochemical activity. In the SW defect, the sp2 planar structure of graphite (0001) is destroyed and the electrons do not locate at the same layer, suggesting the original inert π electrons is able to be activated. Moreover, the highest electron density values (e.g., 0.328 and 0.365 |e| Bohr-3 for DV5555-6-7777 and SW defects, respectively) in defective surfaces are all larger than that of the pristine one (0.309 |e| Bohr-3). According to previous works, the redistribution of electrons on carbon surface can increase the catalytic effects correspondingly

50,51

. Therefore, it is reasonable to speculate that the 10


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defective graphite surfaces may also lead to a higher discharge voltage and a lower charge voltage in non-aqueous Li-O2 batteries, but further detailed investigations are needed in latter parts. To investigate the electronic properties of defective surfaces, the DOS of selected samples is shown in Figure 2, and the graphite (0001) surface is also calculated for comparison. It is seen that the Dirac point locates at the Fermi energy for monolayer graphite (0001), suggesting its zero-band-gap semiconductor behavior, as presented in Figure 2a. For defective carbon surfaces, DV5555-6-7777 and SW defects exhibit a zero-band-gap semiconductor behavior, and DV5-8-5 defect has a small band gap. In addition, SV and DV555-777 defects display metallic behaviors. Therefore, the existence of carbon defects would not deteriorate the electrical conductivity of

Figure 2. The DOS of (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The Fermi levels have been shifted to zero. 11



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cathode. The initial growing process of Li2O2 on graphite (0001) and defective surfaces is simulated by considering two possible pathways, which is dependent on the adsorption energies of O2 and Li

27

. One pathway is O2 → LiO2 → Li2O2 with O2

adsorption is preferred first, and the other is Li → LiO2 → Li2O2 with Li adsorption is preferred first. The adsorption energies are summarized in Table 1. The calculated O2 and Li adsorption energies on graphite (0001) surface are -0.09 and +0.36 eV, respectively, consistently with previous works

52,53

. From the calculated results, it is

found that the first growing step of Li2O2 on graphite (0001) surface, SV and SW defects is the adsorption of O2 due to its larger adsorption energy than that of Li. On the contrary, the first growing step on DV5-8-5, DV555-777 and DV5555-6-7777 defects is the adsorption of Li. Moreover, for the further growing process from Li2O2

Table 1. The adsorption energies of O2 and Li on graphite (0001) surface, SV defect, DV5-8-5 defect, DV555-777 defect, DV5555-6-7777 defect and SW defect.

Graphite (0001)

SV defect

surface

DV5-8-5

DV555-777

DV5555-6-7777

defect

defect

defect

SW defect

Eads(O2) (eV)

-0.09

-4.46

-0.03

-0.12

-0.04

-0.10

Eads(Li) (eV)

+0.36

-1.12

-0.71

-0.86

-0.63

-0.07

12


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Figure 3. The schematics of Li4O4 growing pathways on (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The brown, red and green balls represent for carbon, oxygen and lithium atoms, respectively. 13



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to Li4O4, two pathways of Li2O4 → Li3O4 → Li4O4 and Li3O2 → Li3O4 → Li4O4 are also considered, and the corresponding adsorption energies between substrates-Li2O2 nanostructures and O2/Li are summarized in Table 2. It is shown that the first growing step from Li2O2 to Li4O4 on graphite (0001) surface, SV, DV5-8-5 and DV555-777 defects is the adsorption of O2, while that on DV5555-6-7777 and SW defects is the adsorption of Li. Therefore, for the entire growing process of Li4O4 on substrates, graphite (0001) surface and SV defect follow the pathway of O2 → LiO2 → Li2O2 → Li2O4 → Li3O4 → Li4O4; DV5-8-5 and DV555-777 defects follow the pathway of Li → LiO2 → Li2O2 → Li2O4 → Li3O4 → Li4O4; DV5555-6-7777 defect follows the pathway of Li → LiO2 → Li2O2 → Li3O2 → Li3O4 → Li4O4; and SW defect follows the pathway of O2 → LiO2 → Li2O2 → Li3O2 → Li3O4 → Li4O4. The schematics of Li4O4 growing pathways on graphite (0001) and defective surfaces are shown in Figure3.

Table 2. The continuous adsorption energies of O2 and Li on graphite (0001) surface-Li2O2, SV defect-Li2O2, DV5-8-5 defect-Li2O2, DV555-777 defect-Li2O2, DV5555-6-7777 defect-Li2O2 and SW defect-Li2O2. Graphite (0001)

SV

DV5-8-5

DV555-777

DV5555-6-7777

SW

surface-Li2O2

defect-Li2O2

defect-Li2O2

defect-Li2O2

defect-Li2O2

defect-Li2O2

Eads(O2) (eV)

-1.51

-1.27

-1.35

-0.99

-1.31

-1.37

Eads(Li) (eV)

-1.18

+1.07

-0.80

+0.02

-1.88

-1.41

14


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To investigate the ORR and OER performances of defective surfaces in non-aqueous Li-O2 batteries, free energy diagrams are given in Figure 4 and that of graphite (0001) surface is also calculated for comparison. The free energies of intermediates adsorbed on defective carbon surfaces are summarized in Table S3-8. Only the intermediates formed electrochemically are included due to the thermal

Figure 4. The free energy diagrams of (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect.

15



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barriers are independent of potential 39, and * indicates the adsorbed states. Based on the growing process, the elementary steps considered for graphite (0001) surface, SV, DV5-8-5 and DV555-777 defects are (a) 4 ( Li+ + e- ) + 2O2 → LiO2* + 3 ( Li+ + e- ) + O2, (b) LiO2* + 3 ( Li+ + e- ) + O2 → Li2O4* + 2 ( Li+ + e- ), (c) Li2O4* + 2 ( Li+ + e- ) → Li3O4* + ( Li+ + e- ), (d) Li3O4* + ( Li+ + e- ) → Li4O4*; while the elementary steps considered for DV5555-6-7777 and SW defects are (a) 4 ( Li+ + e- ) + 2O2 → LiO2* + 3 ( Li+ + e- ) + O2, (b) LiO2* + 3 ( Li+ + e- ) + O2 → Li2O2* + 2 ( Li+ + e- ) + O2, (c) Li2O2* + 2 ( Li+ + e- ) + O2 → Li3O4* + ( Li+ + e- ), (d) Li3O4* + ( Li+ + e- ) → Li4O4*. Here, the UDC/UC represents for the highest/lowest voltage that makes the free energy of each ORR/OER step still goes downhill. For the ORR process, the discharge voltage decreases in the sequence of DV555-777 defect (2.08 V) > SV defect (1.89 V) > DV5-8-5 defect (1.60 V) > DV5555-6-7777 defect (1.53 V) > SW defect (1.30 V) > graphite (0001) surface (1.21 V). These findings suggests that all the defective surfaces here can benefit the ORR process in non-aqueous Li-O2 batteries, which is well consistent with previous experimental results that defective carbon cathode can lead to high discharge capacity and high discharge voltage 23,24. For the OER process, the charge voltage increases in the sequence of DV555-777 defect (2.60 V) < DV5555-6-7777 defect (2.79 V) = SW defect (2.79 V) < graphite (0001) surface (2.97 eV) < DV5-8-5 defect (5.52 V) < SV defect (6.61 V), showing that the DV555-777, 16


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DV5555-6-7777 and SW defects benefit the OER process, while DV5-8-5 and SV defects deteriorate the OER process. In addition, it is also found that some calculated charge voltages (e.g., 2.79 V for DV5555-6-7777 and SV defects) are smaller than the calculated equilibrium potential (2.94 V). This phenomenon is attributed to the facts: i) The equilibrium potential is calculated by the bulk phase of Li2O2, but the discharge voltages are calculated by surface-molecule phase, showing surface-effect and size-effect; ii) The properties of some defective carbon surfaces would benefit the desorption of Li2O2 and show catalytic effects, which lowers the charge voltage; iii) Some factors may influence the ORR and OER processes are not considered in the present model, such as electrolyte, lithium salt and geometrical structures. Therefore, the calculated charge voltage is a predictor to find which voltage is lower instead of the real charge voltage in experiments. Thus, although SV and DV5-8-5 defects have excellent ORR performance, they greatly limit the OER performance for carbon cathodes. Importantly, DV555-777, DV5555-6-7777 and SW defects can benefit both ORR and OER processes simultaneously, leading to the high discharge voltage and low charge voltage in non-aqueous Li-O2 batteries. However, previous works indicated that defective carbon surfaces lead to the formation of side products (especially, Li2CO3) in non-aqueous Li-O2 batteries and limit the battery performances 22. From the composition perspective, the total breakup 17



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of O-O bonds is the basic requirement for the formation of CO3 groups. Therefore, we phenomenally investigate the promotion of defective carbon surfaces to Li2CO3 by analyzing the optimized structures in the growing process, as shown in Figure 3. It is found that graphite (0001) surface is relatively stable during the whole growing process and does not break the O-O bonds (Figure 3a), which is consistent with previous experimental findings

22

. On the contrary, SV defect is extremely reactive

under O2 atmosphere and O2 molecules decompose on it with no energy barrier (Figure 3b), indicating CO3 groups are easily formed when SV defect is exposed to O2. Although DV5-8-5 and DV555-777 defects are stable under O2 atmosphere (Figure S3), they promote the breakup of O-O bonds when Li atoms involved. The corresponding intermediates are more like OLiO rather than LiO2, as shown in Figures 3c and 3d, which further forms Li2CO3 during battery cycling. The formation of Li2CO3-like species was considered to result in a high charge voltage by previous work 54, and the similar phenomenon is also found in the free energy diagrams of SV and DV5-8-5 defects in Figures 4b and 4c. Interestingly, even though the O-O bonds are broken in DV555-777 defect, the charge voltage in this case is still as low as 2.60 V, implying DV555-777 defect can effectively catalyze the OER process from O2- to O2. Therefore, it is speculated that DV555-777 defect may be a candidate for traditional ORR and OER process with H atoms involved. But in non-aqueous Li-O2 18


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batteries, the existence of DV555-777 defect would better be avoided due to its promotion to the formation of side product Li2CO3. More importantly, DV5555-6-7777 defect and SW defect are stable during the whole discharge and charge processes, and can effectively increase the discharge voltage as well as decrease the charge voltage, indicating their positive roles in non-aqueous Li-O2 batteries. Besides promoting the formation of side products, defective carbon surfaces are also considered to be the origin of the decomposition of electrolytes

21

. Here, we

examine the thermal stability of electrolyte molecule on defective carbon surfaces to investigate the decomposition of electrolytes. Dimethyl sulfoxide (DMSO) is chosen as the sample solvent due to its widely usage in non-aqueous Li-O2 batteries 55-57. The optimized structures of DMSO molecule on graphite (0001) surface, SV, DV5-8-5,

Figure 5. The optimized structures of DMSO molecule on (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The brown, yellow, red and white balls represent for carbon, sulfur, oxygen and hydrogen atoms, respectively. 19



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DV555-777, DV5555-6-7777 and SW defects are presented in Figures 5a-f, respectively. The calculated adsorption energies of DMSO molecule on graphite (0001) surface, SV defect, DV5-8-5 defect, DV555-777 defect, DV5555-6-7777 defect and SW defect are -0.04, -2.00, -0.05, -0.03, -0.03 and -0.04 eV, respectively. It is found that DMSO molecule is decomposed on SV defect, but it is relatively stable on other five samples, which is similar like the case that O2 molecules adsorbed on defective surfaces. The decomposition of DMSO molecule mainly happens on the S-O bond, and the O atom would bond together with carbon atoms at SV defect while -S-(CH3)2 group is pushed away from the surfaces. From the structures of defective carbon surfaces in Figure 1, it is seen that SV defect has dangling carbon atoms but other do not, which may be the origin of its large decomposition ability to DMSO and O2. Therefore, the decomposition of electrolyte on carbon surfaces in non-aqueous Li-O2 batteries is mainly caused by those with dangling carbon atoms. And it is expected that except SV defect, other defective surfaces (e.g., DV5555-6-7777 defect, SW defect) are relatively stable towards DMSO. 4. Conclusion In summary, a DFT based first-principles study is performed to investigate the positive roles of point defects on carbon surfaces in non-aqueous Li-O2 batteries. Five representative point defects, including SV, DV5-8-5, DV555-777, DV5555-6-7777 20


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and SW defects, are considered, while the graphite (0001) surface is also included for comparison. From the structures of graphite (0001) and defective surfaces as well as electron density contours, we found that defective surfaces lead to the non-uniform distribution of electrons. Further DOS displays defective surfaces have zero-band-gap semiconductor or metallic characters, ensuring an excellent electrical conductivity. The growing pathways for Li4O4 are then clarified based on the adsorption energies of O2 and Li. There are two pathways for the formation of Li2O2, one is O2 → LiO2 → Li2O2, the other is Li → LiO2 → Li2O2. Similarly, two continuously pathways are also considered from Li2O2 to Li4O4 of Li2O4 → Li3O4 → Li4O4 and Li3O2 → Li3O4 → Li4O4. Thirdly, free energy diagrams are given to evaluate the ORR and OER performances of defective surfaces. It is found that all the defective surfaces lead to a high discharge voltage compared with graphite (0001) surface. It is also found that DV555-777, DV5555-6-7777 and SW defects contribute to a low charge voltage, while SV and DV5-8-5 defects lead to an ultra-high ones. Although DV555-777 defect presents a satisfactory ORR and OER performances, it can promote the formation of side product Li2CO3. Finally, DMSO molecule is used as a sample to investigate the decomposition of electrolyte on defective surfaces. Except SV defect, all the other samples do not decompose DMSO molecule. Thus, it is proposed that the dangling atoms in carbon surfaces are the origin of the decomposition of electrolytes 21



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in non-aqueous Li-O2 batteries. Based on our theoretical results, the existence of DV5555-6-7777 and SW defects is beneficial for non-aqueous Li-O2 batteries, which increase discharge voltage, decrease charge voltage, does not promote the formation of side product Li2CO3, and does not promote the decomposition of electrolytes. Therefore, it is strongly recommended that the carbon cathode containing DV5555-6-7777 and SW defects should be used in non-aqueous Li-O2 batteries. Our work unravels the positive roles of point defects, promotes the in-depth understanding on carbon electrode, and provides valuable suggestions for the future cathode design in non-aqueous Li-O2 batteries. Supporting Information The adsorption energies and free energy diagrams of DV5555-6-7777 defect in 4 × 4 × 1 supercell and 5 × 5 × 1 supercell, the optimized structure and lattice constants of Li2O2, the free energies of Li-O intermediates adsorbed on different substrates, the optimized structures of Li and O2 adsorbed on different substrates. This material is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgements The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 16213414). 22


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Table of contents (TOC)

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Figure 1. The optimized structures as well as electron density contours of (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The brown ball represents for carbon atom and the unit of electron density is |e| Bohr-3. 175x96mm (300 x 300 DPI)



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Figure 2. The DOS of (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The Fermi levels have been shifted to zero. 175x89mm (300 x 300 DPI)


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Figure 3. The schematics of Li4O4 growing pathways on (a) graphite (0001) surface, (b) SV defect, (c) DV58-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The brown, red and green balls represent for carbon, oxygen and lithium atoms, respectively. 140x230mm (300 x 300 DPI)



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Figure 4. The free energy diagrams of (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. 177x183mm (300 x 300 DPI)


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Figure 5. The optimized structures of DMSO molecule on (a) graphite (0001) surface, (b) SV defect, (c) DV5-8-5 defect, (d) DV555-777 defect, (e) DV5555-6-7777 defect and (f) SW defect. The brown, yellow, red and white balls represent for carbon, sulfur, oxygen and hydrogen atoms, respectively. 175x65mm (300 x 300 DPI)



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Table of contents 85x33mm (300 x 300 DPI)


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