Important Role of Functional Groups for Sodium Ion Intercalation in

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Chemistry of Materials

Important Role of Functional Groups for Sodium Ion Intercalation in Expanded Graphite Yong-Ju Kang,†,§ Sung Chul Jung,†,§ Jang Wook Choi,*,‡ and Young-Kyu Han*,† †

Department of Energy and Materials Engineering and Advanced Energy and Electronic Materi-

als Research Center, Dongguk University-Seoul, Seoul 100-715, Republic of Korea ‡

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced

Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

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ABSTRACT Expanded graphite oxide (GO) has recently received great attention as a sodium ion battery anode due to its superior characteristics for sodium ion storage. Here, we report that the sodium ion intercalation behavior of expanded GO strongly depends on the amounts and ratios of different functional groups. The epoxide-rich GO shows significantly higher specific capacities than those of the hydroxyl-rich counterpart utilizing strong sodium–epoxide attractions and appropriately enlarged interlayer spacing during sodiation. The epoxide-rich GO also enables fast sodium ion transport on account of the diminishment of interlayer hydrogen bonds that could reduce the free volume. Our calculations suggest that the theoretical capacity of epoxide-only GO with a stoichiometry of Na2.5C6O3 can reach 930 mAh g−1, which is far higher than recent experimental results as well as even those of conventional graphite materials in lithium ion batteries.


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INTRODUCTION Large-scale electrical energy storage systems (ESSs) have garnered remarkable attention in connection to utilizing renewable resources such as wind, tidal, and solar power during the operation of electrical grids. Rechargeable batteries are considered one of the most viable ESS technologies due to their notable advantages including high charge-discharge efficiency, fast storage time, and easy scale control.1–6 Although among the available rechargeable batteries lithium ion batteries (LIBs) are the most well established, there is demand to find alternative systems for ESSs because the raw materials of LIBs are expensive and maldistributed in certain regions worldwide.7,8 Among a number of alternative candidates, sodium ion batteries (SIBs) are considered the most promising due to their redox chemistry based on monovalency, together with the abundance and non-toxicity of sodium resources.9–18 From the viewpoint of electrode materials, between anodes and cathodes, finding active anode materials is a more significant bottleneck in the progress of SIBs, as only a handful of materials have been identified to date.17–20 Graphite, which exhibits highly reversible Li ion storage, does not allow the same storage properties for Na ions due to its insufficient interlayer spacing of 3.4 Å.21,22 Reversible intercalation of Na ions was recently reported23,24 using a solvent cointercalation mechanism. While fundamentally quite interesting, these studies still do not achieve intercalation of bare Na ions. Hence, it is desirable to modify the atomic structure of graphite toward increased interlayer spacing to facilitate Na ion diffusion. Recently, Wang et al.25 reported remarkable experimental results that an expanded graphite prepared by sequential oxidation and partial reduction can effectively accommodate Na ions due to the enlarged interlayer spacing of 4.3 Å, reaching a high 3 ACS Paragon Plus Environment


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reversible capacity of 284 mAh g−1. While the increased channel dimensions play a critical role in the Na ion diffusion, the generated functional groups, mainly epoxide and hydroxyl groups, would affect Na ion diffusion and eventually impact the overall electrochemical performance. Nonetheless, the atomic-level understanding of the Na ion diffusion in the expanded graphite is limited such that the only available information is the oxygen-to-carbon (O/C) atomic ratio, 11.1%, besides the presence of both functional groups.25 In this article, we study the effects of functional groups on the sodiation process of graphite oxide (GO) using first-principles calculations. It was found that the epoxide groups are more critical in enhancing the specific capacity and Na ion transport compared to the hydroxyl groups. A hydroxyl-rich GO structure with an epoxide-to-hydroxyl ratio of 1:2 reproduces the experimentally obtained interlayer spacing and specific capacity well. The calculations suggest that in an extreme case where epoxide groups are solely present, the theoretical capacity can be as high as 930 mAh g−1, a value that has never been approached with graphite-based materials.

COMPUTIONAL DETAILS To study the sodiation of GO, we carried out density functional theory (DFT) calculations implemented in the Vienna ab initio simulation package (VASP).26 The Perdew−Burke−Ernzerhof (PBE) exchange and correlation functionals27 and the projector augmented wave (PAW) method28 were employed for the electron–electron and ion–electron interactions, respectively. The electronic wave functions were expanded on a plane wave basis set of 400 eV. In terms of valence electron configurations, we treated 2p63s1 for Na, 2s22p2 for C, 2s22p4 for O, and 1s for H. We


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used Grimme’s DFT-D2 approach29 to accurately describe van der Waals interactions between adjacent layers of graphite. While the PBE functional gives a too large interlayer spacing of 4.5 Å for graphite,30 the PBE functional supplemented with the DFT-D2 method leads to an interlayer spacing of 3.2 Å which is comparable to the experimental value (3.4 Å). GO with differenet oxidation levels was simulated by a tetragoanl supercell which includes 24 C, 2–12 H, and 2–12 O atoms for graphite including epoxide and hydroxyl functional groups. The initial stacking of GO is ABAB but the optimized GO structures lose their original stacking due to the interactions between graphene layers and functional groups, and/or between functional groups. A 6 × 6 × 3 kpoint mesh was used for Brillouin zone integrations. We optimized cell volume and atomic positions until residual forces were less than 0.02 eV Å–1 to get expanded interlayer spacings for different oxidation levels. The nudged elastic band (NEB) method31 was employed to calculate the energy barriers for Na diffusion in fully sodiated GO structures. We considered the vacancy mechanism32 in which a Na atom migrates from the equilibrium position to another unoccupied equilibrium position. The method applied here was successfully used in our previous studies of the lithiation of Si33 and the sodiation of Al2O3.34

RESULTS AND DISCUSSION The structure of GO strongly depends on the synthetic route and the degree of oxidation (i.e., the O/C atomic ratio).35 For a given O/C ratio, we investigated the expanded GO structure by varying the epoxide-to-hydroxyl ratio because detailed structural information, such as the functional group ratio, is not available in the literature.36–38 All the considered structures of pristine and sodiated GO at O/C ratios of 8.3 and 12.5% are presented in Figure S1 and Table S1. Among 5 ACS Paragon Plus Environment


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the numerous possible GO structures, we identified the GO structures that correlate well with the experimental results in terms of the interlayer spacing and sodium storage capacity. We also examined two extreme GO structures with only epoxide or hydroxyl groups to elucidate the role of each functional group. Figure 1 shows the interlayer spacings of the three types of GO structures: the epoxide-only, the hydroxide-only, and the mixture of both. The GO structures that closely reproduce the experimental interlayer spacings exhibit epoxide-to-hydroxyl ratios of 0:1, 1:2, 1:2, 1:3, and 1:5 (black line) with corresponding O/C ratios of 8.3, 12.5, 25.0, 33.3, and 50.0%, respectively. The values are summarized in Table 1. These series of results reveal that hydroxyl groups are more dominant in the experimentally produced expanded GO. As shown in Figure 1, the interlayer spacing of GO increases roughly in a linear relation with the O/C ratio. It is also notable that as the O/C ratio increases, the interlayer spacing increases to different degrees depending on the type of functional groups. Interestingly, the interlayer spacings of epoxide/hydroxyl mixed structures are larger than those of epoxide-only and hydroxylonly structures. For instance, at a high O/C ratio of 50.0%, the interlayer spacings of epoxideonly, hydroxyl-only, and 1:5 epoxide/hydroxyl mixed structures are 4.59, 5.26, and 5.91 Å, respectively. The larger interlayer spacings of the mixed structures are attributed to the sizes of the functional groups and their interlayer interactions. Due to the larger size of the hydroxyl group than the epoxide group, the steric hindrance is more significant in the hydroxyl-only and 1:5 epoxide/hydroxyl mixed structures than in the epoxide-only structure, thereby contributing to the larger interlayer spacings of those two structures. The larger interlayer spacing of the mixed structure relative to that of the hydroxyl-only counterpart can be explained by the weaker interlayer hydrogen bonds of the former. While the hy-


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droxyl-only structure gives rise to only interlayer O⋯H hydrogen bonds between hydroxyl groups on two adjacent graphene layers, the mixed structure has two kinds of hydrogen bonds (see scheme 1): epoxide–hydroxyl O⋯H bonds and hydroxyl–hydroxyl O⋯H bonds. According to our Bader charge analysis of the participating atoms, the epoxide oxygen, hydroxyl oxygen, and hydroxyl hydrogen atoms have atomic charges of –0.8, –1.2, and +0.6 e, respectively (see Table S2). From the electrostatic point of view, the epoxide–hydroxyl O⋯H bonds with an average length of 2.45 Å are weaker than the hydroxyl–hydroxyl O⋯H bonds with an average length of 1.96 Å. This leads to the conclusion that the presence of the epoxide–hydroxyl O⋯H bonds weakens the interlayer interactions as compared to the other case exclusively with the hydroxyl– hydroxyl O⋯H bonds. For reference, the distance of the hydroxyl–hydroxyl O⋯H bond is very similar to that of the O⋯H bond (~2.0 Å) in a water dimer.39 We extended our investigation to the Na ion intercalation into expanded GO structures. We calculated the formation energies of the sodiated GO structures to identify the most energetically favorable structures bearing Na ions (see Figures S2–S7). The specific capacities obtained from the determined optimal compositions are displayed in Figure 2. Remarkably, the calculated GO structures that showed interlayer spacings consistent with the experimental values also exhibited specific capacities in good agreement with the experimental results (Table 1). When calculated based on the mass of graphite, our specific capacities at O/C ratios of 8.3, 12.5, and 50.0% are 93, 372, and 186 mAh g−1, respectively, which are close to the experimental values of 130, 360, and 190 mAh g−1 during the first desodiation (see black circles and green squares in Figure 2). The excellent agreement verifies the validity of our GO structural models.



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Figure 3a shows the fully sodiated structure of the GO with an O/C ratio of 12.5% and an epoxide-to-hydroxyl ratio of 1:2 (NaC6O0.75H0.50). In this structure, more Na ions are found near the epoxide groups than close to the hydroxyl groups, implying preferential Na binding with the epoxide groups. The Na ion bound to the epoxide group increases the charge of the epoxide oxygen atom by 0.3 e, causing one of the two C–O bonds in the epoxide to break. At this fully sodiated state, the interlayer spacing is 4.79 Å, a value 0.55 Å larger than that of the pristine GO. The volume expansion upon full sodiation corresponds to 13%, which is comparable to the range (10−13%) of graphite40,41 upon full lithiation. It is emphasized that the epoxide group in the expanded GO plays a critical role in increasing the Na ion storage capacity. Our two extreme GO structures with only epoxide or hydroxyl groups show strikingly different Na uptake capacities (see Figure 2). As the level of the oxidation increases, the specific capacity of the epoxide-only case increases continuously, eventually reaching a maximum value of 930 mAh g−1 at an O/C ratio of 50.0%, the known maximum oxidation level.42 The corresponding redox reaction can be expressed as 2C6O3 + 5Na+ + 5e– ↔ 2Na2.5C6O3. In contrast, the hydroxyl-only counterpart shows a maximum capacity of only 279 mAh g−1 at an O/C ratio of 12.5% (4C6O0.75H0.75 + 3Na+ + 3e– ↔ 4Na0.75C6O0.75H0.75). Its further oxidation does not lead to a capacity increase, unlike the epoxide-only case. With the increased oxidation level, the experimentally observed capacities are closer to those of the hydroxyl-only case, reconfirming the hydroxyl-rich compositions of the experimental samples upon their progressive oxidation. The current computational results thus suggest that the capacities of the expanded GO can be significantly enhanced by increasing the portion of epoxide groups. Additional evidence of the epoxide-induced capacity enhancement is presented by the epoxide/hydroxyl mixed structures in Table 1, where the specific capacity increases with an increased portion of 8 ACS Paragon Plus Environment

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the epoxide group. The GO samples with epoxide-to-hydroxyl ratios of 0:1, 1:2, 1:3, and 1:5 exhibit specific capacities of 93, 372, 279, and 186 mAh g−1, respectively (see Figure 2 and Table 1). These findings naturally give rise to the question why the epoxide-only structures can accommodate larger amounts of Na ions despite their smaller interlayer spacings compared to the hydroxyl-only and epoxide/hydroxyl mixed structures. First, the Na ions show a higher binding affinity towards the epoxide group than the hydroxyl group. The binding of a single Na ion to an isolated epoxide group is energetically preferred to an isolated hydroxyl group by 7.61 kcal mol−1. The stronger Na–epoxide interaction brings an additional advantage of shielding the Na ion from Coulomb repulsion with other Na ions during the (de)intercalation, as shown in Figure 3b. Note that, when the highest capacities are reached, the epoxide-only structure (Na2.5C6O3) with the formation energy of –23.76 kcal mol–1 is much more stable than the hydroxyl-only structure (Na0.75C6O0.75H0.75) with the formation energy of –3.69 kcal mol–1. Second, the interlayer spacings of the epoxide-only structures are easily enlarged during sodiation due to the absence of interlayer O⋯H hydrogen bonds. Thus, more epoxide groups can participate in bonding with Na ions by taking advantage of this structural benefit. As a result, the epoxide-only structures undergo maximum volume expansion of 30% at an O/C ratio of 50.0%. This volume expansion is within the safe range before undesirable exfoliation43 while enabling facile Na ion (de)intercalation. For the epoxide-only structure with an O/C ratio of 50.0%, our calculations show that the exfoliation energy per carbon required to separate the GO layers increases from 0.83 kcal mol−1 in the pristine GO to 5.81 kcal mol−1 in the fully sodiated GO (see Table S3). This indicates that the sodiation significantly reduces the viability of exfoliation by reinforcing the interlayer coupling strength. 9 ACS Paragon Plus Environment


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To elucidate the Na ion diffusion kinetics in the expanded GO structures, we examined the Na ion vacancy diffusion in fully sodiated states with O/C ratios of 8.3, 12.5, and 33.3% (see Table S4 and Figure S8). The calculated Na ion diffusivity decreases as the O/C ratio increases (Figure 4), implying that large amounts of functional groups in the interlayer space could impede Na ion transport to some degree. In particular, the hydroxyl groups showed this effect significantly. At an O/C ratio of 33.3%, the sodium diffusion in the epoxide-only structure is 3 × 102 times faster than that in the epoxide/hydroxyl mixed structure. By contrast, the diffusion in the hydroxyl-only structure is 7 × 107 times slower than that in the epoxide/hydroxyl mixed structure. One of the main reasons for the slow diffusion in the hydroxyl-only structure is its complex networks of interlayer O⋯H hydrogen bonds, which provide less free volume for Na ion diffusion as compared with the epoxide-only counterpart, which is devoid of interlayer bonding interactions. We anticipate that the epoxide-functionalized GO can similarly boost the capacity for Li intercalation. However, to the best of our knowledge, there has been no study dealing explicitly with the effect of epoxide and hydroxyl groups on Li intercalation into GO. Instead, the capacity increase has been reported for the lithiation of other types of carbon structures such as graphene based on preferable Li−O interactions.44−47 For example, graphene oxide with epoxide and hydroxyl groups was found to theoretically store up to 2.25 Li ions per C6.47

CONCLUSIONS In conclusion, our first-principles study demonstrates that, when functionalized with epoxide groups rather than the commonly observed hydroxyl groups, expanded GO can improve the spe-


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cific capacity and sodium transport significantly. This prediction is well explained by a combined outcome of the enhanced binding affinity of Na ions to the epoxide group, the consequent charge shielding with other Na ions, and the mitigation of interlayer hydrogen bonding interactions that could reduce the free volume. The superior electrochemical performance, reaching a high capacity of 930 mAh g−1, envisaged from the present investigation would stimulate researchers to develop various procedures that can increase epoxide groups during the modification of pristine graphite.

ASSOCIATED CONTENT Supporting Information Atomic structures, Bader populations, formation energies, exfoliation energies, and Na diffusivities. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: (J.W.C) [email protected] 11 ACS Paragon Plus Environment


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*E-mail: (Y.K.H) [email protected] Author Contributions §

Y.J.K. and S.C.J. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning of Korea (2010-C1AAA001-0029018) and the Energy Efficiency & Resources Core Technology Program of the KETEP granted financial resource from the Ministry of Trade, Industry & Energy (No. 20132020000260). J.W.C. acknowledges the support by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2012-R1A2A1A01011970).

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(40) Koyama, Y.; Chin , T. E.; Rhyner, U.; Holman, R. K.; Hall, S. R.; Chiang, Y.-M. Harnessing the Actuation Potential of Solid-State Intercalation Compounds. Adv. Funct. Mater. 2006, 16, 492–498. (41) Lee, J. H.; Lee, H. M.; Ahn, S. Battery Dimensional Changes Occurring during Charge/Discharge Cycles—Thin Rectangular Lithium Ion and Polymer Cells. J. Power Sources 2003, 119, 833–837. (42) Johns, J. E.; Hersam, M. C. Atomic Covalent Functionalization of Graphene. Acc. Chem. Res. 2013, 46, 77–86. (43) Kim, Y.; Kim, Y.; Choi, A.; Woo, S.; Mok, D.; Choi, N.-S.; Jung, Y. S.; Ryu, J. H.; Oh, S. M.; Lee, K. T. Tin Phosphide as a Promising Anode Material for Na-ion Batteries. Adv. Mater. 2014, 26, 4139–4144. (44) Bhardwaj, T.; Antic, A.; Pavan, B.; Barone, V.; Fahlman, B. D. Enhanced Electrochemical Lithium Storage by Graphene Nanoribbons. J. Am. Chem. Soc. 2010, 132, 12556– 12558. (45) Uthaisar, C.; Barone, V.; Fahlman, B. D. On the Nature of Thermally Reduced Graphene Oxide and Its Electrochemical Li Intake Capacity. Carbon 2013, 61, 558–567. (46) Kuo, S.-L.; Liu, W.-R.; Kuo, C.-P.; Wu, N.-L.; Wu, H.-C. Lithium Storage in Reduced Graphene Oxides. J. Power Sources 2013, 244, 552–556. (47) Stournara, M. E.; Shenoy, V. B. Enhanced Li Capacity at High Lithiation Potentials in Graphene Oxide. J. Power Sources 2011, 196, 5697–5703. 18 ACS Paragon Plus Environment

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Table 1. Interlayer spacings and epoxide-to-hydroxyl ratios of the GO structures a this work

a

x (%)

d (Å)

0

3.20

8.3

3.82

12.5

experiment O:OH

x (%)

d (Å)

3.7

3.4

0:1

8.7

3.7

4.24

1:2

11.1

4.3

25.0

4.70

1:2

33.3

5.07

1:3

50.0

5.91

1:5

51.1

6.1

x, d, and O:OH represent the O/C ratio, interlayer spacing, and epoxide-to-hydroxyl ratio, re-

spectively. The experimental data are taken from ref. 25.



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Scheme 1. Three types of GO structures with epoxide (O) and hydroxyl (OH) functional groups: (a) O only, (b) OH only, and (c) O/OH mixed. Red and white balls represent the O and H atoms, respectively. Dashed lines represent the O⋯H hydrogen bonds.


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Figure 1. Interlayer spacings of different GO structures with epoxide (O) and/or hydroxyl (OH) functional groups. The experimental values are taken from ref. 25.



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Figure 2. Sodium capacities of different GO structures with epoxide (O) and/or hydroxyl (OH) functional groups. The capacity was calculated based on the mass of graphite. The experimental values are taken from ref. 25.


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Figure 3. Fully sodiated stages of GO structures: (a) the 1:2 epoxide/hydroxyl mixed structure with an O/C ratio of 12.5% and (b) the epoxide-only structure with an O/C ratio of 50.0%, corresponding to specific capacities of 372 and 930 mAh g−1, respectively. Yellow, red, white, and grey balls represent the Na, O, H, and C atoms, respectively. The interlayer spacings are 4.79 and 6.02 Å in (a) and (b), respectively.



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Figure 4. Sodium diffusivities D in fully sodiated GO structures at T = 300 K. The diffusion barriers are calculated based on the sodium ion vacancy diffusion mechanism.


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TOC GRAPHICS

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Important Role of Functional Groups for Sodium Ion Intercalation in

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