Flexible Few-Layered Graphene for the Ultrafast Rechargeable

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Flexible Few-Layered Graphene for the Ultrafast Rechargeable Aluminum-Ion Battery Sung Chul Jung,†,§ Yong-Ju Kang,†,§ Dong-Joo Yoo,‡ Jang Wook Choi,*,‡ and Young-Kyu Han*,† †

Department of Energy and Materials Engineering and Advanced Energy and Electronic Materials Research Center, Dongguk University-Seoul, Seoul 100-715, Republic of Korea ‡ Graduate School of Energy, Environment, Water, and Sustainability (EEWS), and KAIST Institute (KI) NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Fast ion transport is essential for high rate capability in rechargeable battery operation. Recently, an ultrafast rechargeable aluminum-ion battery was experimentally demonstrated through the reversible intercalation/deintercalation of chloroaluminate anions (AlCl4−) in graphitic-foam cathodes. Using first-principles calculations, herein, we report that the unique structural characteristic of graphitic foam, i.e., mechanical flexibility of few-layered graphene nanomaterials, plays a key role for the ultrafast aluminum-ion battery. We found that AlCl4− is stored by forming doubly stacked ionic layers in the interlayer space between graphene sheets, and their diffusivity increases dramatically once graphene film is less than five layers thick; the diffusivity begins to increase when the film thickness reduces below five layers in such a way that the film thickness of four, three, and two graphene layers enables 48, 153, and 225 times enhanced diffusivity than that of the bulk graphite, respectively, and this nanoscale thickness is mainly responsible for the observed ultrafast rate capability of graphitic foam. The faster anion conductivity with the reduced film thickness is attributed to high elasticity of few-layered graphene, providing more space for facile AlCl4− diffusion. This study indicates that even bulky polyanions can be adopted as carrier ions for ultrahigh rate operation if highly elastic few-layered graphene is used as an active material.

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issues, such as poor cycle life,16,18−20 slow ion transport,17,22 cathode material disintegration,20 and low discharge voltage profiles without clear plateaus.16,18,19 These problems are mainly linked to the difficulty of finding suitable cathode materials that allow facile ion transport in a reversible manner.16,23 Remarkably, Lin et al.24 recently reported an ultrafast rechargeable aluminum-ion battery by utilizing graphitic-foam cathode allowing reversible intercalation/deintercalation of chloroaluminate anions (AlCl4−). The reported cell exhibited the unprecedented performance: excellent long-term cycling stability and ultrafast charging/discharging capability at high current densities up to 5000 mA g−1 (75C rate), together with well-defined discharge voltage plateaus near 2.0 V. Such ultrafast performance with long-term cyclability must highly depend on the choice of carbon for the cathode material, as other graphitic materials, such as natural graphite and pyrolytic graphite, showed inferior rate capabilities with lower specific capacities at the given current densities. In a similar context, Cohn et al.25 also reported that graphitic foam plays a crucial role for fast ion transport in Na-ion batteries. It is particularly

INTRODUCTION Rechargeable lithium-ion batteries (LIBs) have played a central role as power sources for portable consumer electronic devices for the last two decades. Nevertheless, the demand for higher energy/power density and longer cycle life has been continuously growing for timely advent of emerging electric vehicles and stationary energy storage systems linked to renewable energy conversion sectors. In developing rechargeable batteries for these large-scale applications, the chronic shortcomings of LIBs represented by the limited reserves and global maldistribution of lithium raw materials can eventually weaken the price competitiveness of LIBs. This concern has promoted research on alternative systems based on sodium (Na), magnesium (Mg), and aluminum (Al) carrier ions to utilize the abundance and low cost of their electrode raw materials.1−12 Among those, Al-ion batteries have lately attracted rising attention owing to the potential advantages, including natural abundance, low cost, trivalence, inflammability, and high volumetric capacity of aluminum metal.13−22 In particular, the theoretical volumetric capacity of aluminum metal is 8.0 Ah cm−3, which is 4 times higher than that of Li metal.16 Despite continued efforts, however, aluminum-ion batteries are yet at the fundamental research stage because of their several chronic © 2016 American Chemical Society

Received: April 11, 2016 Revised: May 23, 2016 Published: June 10, 2016 13384

DOI: 10.1021/acs.jpcc.6b03657 J. Phys. Chem. C 2016, 120, 13384−13389

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The Journal of Physical Chemistry C

layers between the two intercalant layers, as shown in Figure 1. GICs in the examined stages 2, 3, and 4 turned out to be all

remarkable that the ultrafast rate performance in the recent Alion battery was achieved using the bulky AlCl4− ion, which is far bigger than conventional single atom-based cations in conventional battery systems. However, the atomic origin of the observed ultrafast rate performance of the graphitic-foam cathode remains unclarified. In particular, the structure of graphite into which AlCl4− is intercalated involving the anion−π interaction has not yet been elucidated. Here, we study the graphite intercalation compounds (GICs) with AlCl4− intercalated using first-principles calculations. Our calculations show that the GIC at stage 3 that delivers a theoretical capacity of 105 mAh g−1 comparable to the experimental capacity of 90 mAh g−1 exhibits an unexpected doubly stacked intercalant structure, and successfully reproduces the experimental X-ray diffraction (XRD) spectrum24 of the fully charged graphite and the related structural parameters. In light of AlCl4− diffusivity, the AlCl4− diffusivity increases dramatically as film thickness decreases from six to two layers. For example, the film thicknesses of four, three, and two layers lead to 48, 153, and 225 times greater ionic diffusivities than that of the bulk graphite, respectively, and thus play a central role in the ultrafast rate capability of graphitic foam. The increase in AlCl4− ion conductivity can be explained by the low elastic stiffness of few-layered graphene and the resulting increase of free space for AlCl4− diffusion. This study suggests that when few-layered graphene with nanoscale thickness is used as an active material, its structural elasticity can facilitate ultrafast ion transport of even bulky polyanions.

Figure 1. Fully charged GICs with AlCl4− intercalated: (a) stage 2, (b) stage 3, and (c) stage 4 GICs. Red, green, and gray balls represent the Al, Cl, and C atoms, respectively. di and Ic represent the intercalant gallery height and periodic repeat distance, respectively.

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COMPUTATIONAL DETAILS We performed density functional theory (DFT) calculations using 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 used. The electronic wave functions were expanded on a plane-wave basis set of 400 eV. We treated 2s22p2 for C, 3s23p1 for Al, and 3s23p5 for Cl for the valence electron configurations. The graphite was simulated by an orthorhombic supercell with 46, 69, and 92 C atoms for stages 2, 3, and 4, respectively. A graphite model with vacancy defects was employed by considering experimental XRD spectra on the C 1s of fully charged graphitic-foam cathode which exhibits not only sp2 but also sp3 characters.24 The few-layered graphene was simulated by an orthorhombic supercell with 23 × nC atoms for the n-layer graphene film and a vacuum region of 10 Å. We used Grimme’s DFT-D2 approach29 for the description of van der Waals interactions in the GICs. A 3 × 3 × 3 k-point mesh was used for the Brillouin-zone integrations. We optimized the cell volume and atomic positions until the residual forces were within 0.02 eV Å−1. The XRD simulations of the GICs were performed using the DIAMOND package.30 In the simulations, all peaks were rigidly shifted by 0.35° to match the simulated (002) peak of graphite with the experimental peak.24 The firstprinciples calculation methods applied here were successfully employed in our previous studies on sodium ion intercalation into expanded graphite31 and Al2O3 coating layer.32

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thermodynamically stable with negative formation energies (Figure S1). Interestingly, the GICs at all the stages can accommodate similar amounts of AlCl4 anions, implying that the theoretically determined maximum capacities do not strongly depend on the stage of GIC. The most stable GICs at stages 2, 3, and 4 were found to deliver the specific capacities of 116, 105, and 107 mAh g−1, respectively, when calculated based on the mass of graphite, corresponding to one AlCl4− species per 19−21 C atoms (Figure S2). The theoretical capacities of 105−116 mAh g−1 are slightly higher than the experimental value (∼90 mAh g−1)24 for the first charge of graphitic foam at a current density of 1000 mA g−1, which follows a general trend of lithium- and sodium-ion batteries that experimental capacities are lower by 10−30% than their theoretical maximum capacities.33−36 The fully charged GIC structures with AlCl4− intercalated are displayed in Figure 1. The inserted AlCl4− species are arranged to maximize the Alδ+···Clδ− intermolecular electrostatic attraction and minimize Clδ−···Clδ− repulsion. For the single AlCl4− intercalated in graphite, the average atomic charge is calculated to be +2.31, − 0.79, and +0.01 for Al, Cl, and C, respectively, with the resulting overall charge state of AlCl4− being −0.86. Intriguingly, the intercalated AlCl4− forms doubly stacked AlCl4− layers at stages 3 and 4 (see Figure 1). This storage mechanism is useful because the doubly stacked layer intercalant structure in each stage can provide a higher capacity than the single-layer counterpart. This partly explains why, although higher stages usually exhibit lower capacities than lower stages, stages 3 and 4 with the doubly stacked layer structure can offer the comparable specific capacities (105 and 107 mAh g−1) to that (116 mAh g−1) at stage 2 with the singlelayer structure. The similar specific capacities between stages 3

RESULTS AND DISCUSSION

In investigating the intercalation of AlCl4 anions into graphite cathode, we first calculated the formation energies of AlCl4−intercalated graphite compounds at different intercalation stages. The stage number n denotes the number of graphene 13385

DOI: 10.1021/acs.jpcc.6b03657 J. Phys. Chem. C 2016, 120, 13384−13389

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The Journal of Physical Chemistry C and 4 are attributed to the densities of AlCl4−; the density of AlCl4− within the intercalant gallery is 3.79 × 10−3, 4.00 × 10−3, and 5.17 × 10−3 Å−3 for stages 2, 3, and 4, respectively, revealing a closer packing of AlCl4− with increasing stage. This doubly stacked intercalant structure is reminiscent of the cointercalation of Na+ and solvent (diethylene glycol dimethyl ether, DEGDME) into graphite, where Na−solvent complexes form a doubly stacking structure.37 The intercalant gallery height (di), the distance between two graphene layers between which the intercalant layer is interposed (see Figure 1), was calculated to be 8.5, 12.1, and 12.2 Å for stages 2, 3, and 4, respectively, reflecting that the di of the doubly stacked GIC is much larger than that of the single-layer GIC. The periodic repeat distance (Ic), the distance between the intercalant layers, was calculated to be 11.8, 18.7, and 22.1 Å for stages 2, 3, and 4, respectively. Lin et al.24 interpreted that the fully charged pyrolytic graphite with AlCl4− is at stage 4 with an intercalant gallery height of di ∼ 5.7 Å by analyzing XRD patterns that exhibited two peaks at 2θ = 23.56° (d ≈ 3.77 Å) and 28.25° (d ≈ 3.15 Å). Figure 2 shows our calculated XRD peaks of the fully charged

is obtained from the following equation37,38 originating from Bragg’s law: 1 l = sin θ 00l + 1 −1 sin θ 00l

The doublet peaks of the GIC are indexed as (00l) and (00l +1). From the above equation, we obtained l = 5.09 (≈ 5) for the experimental peaks (2θ = 23.56° and 28.25°), indicating that the experimental peaks can be assigned as (005) and (006). By applying the same procedure to our calculated XRD peaks, l = 2.95 (≈ 3), 4.94 (≈ 5), and 5.89 (≈ 6) were attained for stages 2, 3, and 4, respectively, revealing that the l value at stage 3 coincides with that (l = 5.09) obtained from the experiment. Separately, from the determined l value of the experimental peaks, the periodic repeat distance turns out to be Ic = 18.9 Å based on the following equation:37,38 Ic = ld00l = (l + 1)d00l + 1

where d00l and d00l+1 are the d-spacing values of the (00l) and (00l+1) planes, respectively. The obtained value of Ic = 18.9 Å is much closer to that (18.7 Å) at stage 3 in our calculation, rather than that (22.1 Å) at stage 4, supporting again that the fully charged state of the pyrolytic graphite observed in the experiment is at stage 3. Also, the intercalant gallery height of di = 5.7 Å reported in ref 24 was rechecked, as this value is much smaller compared with our calculated values of di = 8.5−12.2 Å for all of the stages considered. The size of AlCl4− is estimated to be 6.09 Å from the ionic radii of 0.68 and 1.67 Å of Al3+ and Cl−, respectively.39 When a single AlCl4− species is intercalated into graphite, three Cl ions in AlCl4− are expected to interact with the graphene sheet with a C···Cl distance of 3.03 Å (see Scheme 1). In this Scheme 1. Single AlCl4− Intercalant Species in Graphitea

a

The C···Cl distance is 3.03 Å. The spherical dot surfaces represent the electron clouds of the atoms/ions. The AlCl4− size estimated from the dot surfaces is 6.09 Å.

−

Figure 2. Simulated XRD peaks of fully charged GICs with AlCl4 intercalated: (a) stage 2, (b) stage 3, and (c) stage 4 GICs. All the stages show doublet peaks of (00l) and (00l+1) where l is 3, 5, and 6 for stages 2, 3, and 4, respectively. The dashed lines represent the positions of the experimental XRD peaks for fully charged pyrolytic graphite (2θ = 23.56° and 28.25°).24 The black peak at 2θ = 26.55° represents the (002) peak for graphite.

interaction, the electron clouds of AlCl4− and graphite do not overlap, implying that noncovalent interaction accounts for the anion−π interaction40,41 between AlCl4− and graphite. We examined the adsorption of a single AlCl4− species on graphene and found that the equilibrium C···Cl distance is 3.18 Å (see Figure 3); once the C···Cl distance is below 3 Å, the energy substantially increases due to the exchange repulsive interaction.42 From the AlCl4− size and C···Cl distance, it is likely that di should be at least as large as 8 Å. It is noted that Rüdorff et al.43 reported di values of about 9.5 Å for AlCl3 intercalation into graphite, which is within the range identified for AlCl4− intercalation into graphite in the current study (di = 8.5−12.2 Å). Ion conductivity is an important factor to determine the rate capability of electrodes in rechargeable batteries. Based on the

GIC structures with AlCl4− intercalated. All the stages exhibited the two dominant peaks, which is reflective of the staging phenomenon. Unexpectedly, however, our simulation indicates that the experimental peaks at 2θ = 23.56° and 28.25° are rather closer to the peaks at stage 3 (2θ = 23.51° and 28.36°) than to those at stage 4 (2θ = 23.71° and 27.81°) proposed in the original experimental report. In addition, stage 3 can reproduce the structural parameters (e.g., l and Ic) derived from the experimental XRD peaks more accurately, where l is the index of (00l) plane along the stacking orientation. The index l 13386

DOI: 10.1021/acs.jpcc.6b03657 J. Phys. Chem. C 2016, 120, 13384−13389

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Figure 4. AlCl4− diffusivities D in graphite and six-, five-, four-, three-, and two-layer graphene films at T = 300 K, where the film thicknesses are 2.56, 2.24, 1.97, 1.64, and 1.33 nm, respectively.

Figure 3. Binding energies of a single AlCl4− on graphene as a function of the C···Cl distance, defined as Eb = Etot(AlCl4/graphene) − Etot(AlCl4) − Etot(graphene), where Etot(AlCl4/graphene) is the total energy of the AlCl4 adsorbed graphene, Etot(AlCl4) is the total energy of an AlCl4 molecule, and Etot(graphene) is the total energy of the graphene.

10−8 cm2 s−1) is 10.2 times faster than that in graphite (2.2 × 10−9 cm2 s−1), indicating that AlCl4− diffusivity in graphitic foam starts to increase when the number of graphene layers crosses five. The thickness of five-layer graphene film including intercalated AlCl4− ions is 2.2 nm. Following the trend, the diffusivities in the four- and three-layer graphene films (1.1 × 10−7 and 3.4 × 10−7 cm2 s−1) are 48 and 153 times faster than that in graphite, respectively. The diffusivity of 5.0 × 10−7 cm2 s−1 in the thinnest two-layer graphene film is 225 times higher than that in graphite, manifesting the extreme case with regard to ionic conductivity. Hence, we interpret that the experimentally observed ultrafast rate capability of graphitic foam is mainly a combined result of AlCl4− (de)intercalation (from) into 2−4 layered graphene films with 1.3−2.0 nm thicknesses. It is notable that the calculated AlCl4− diffusivities are comparable to those of the cases where large carrier ion complexes intercalate into graphitic foam; a diffusivity of 2 × 10−7 cm2 s−1 was recently reported for the cointercalation of Na+ and solvent (diglyme) into few-layer graphene foam using the galvanostatic intermittent titration technique.25 This diffusivity fits in our calculated range from 1.1 × 10−7 cm2 s−1 in the four-layer graphene film to 3.4 × 10−7 cm2 s−1 in the three-layer graphene film. The increase in AlCl4− ion conductivity with decreasing number of graphene layer can be explained by the mechanical properties of few-layer graphene films, namely elastic stiffness coefficient, as this property largely affects ionic transport. Figure 5 shows the calculated elastic stiffness coefficients along the c-

observed rapid charge−discharge operation at a current density up to 5000 mA g−1 (75C rate), which is about 75 times higher than that of the pyrolytic-graphite cathode, along with an impressive cycling stability over 7500 cycles (∼100% capacity retention),24 the graphitic-foam cathode in the aluminum-ion battery is expected to allow considerably fast ion conductivity. The graphitic foam has an internal structure44,45 consisting mainly of few-layered graphene (usually less than ten layers).25,44−46 To elucidate the origin of the superior rate capability of graphitic foam, we correlate the finite number of layers in the graphene films with AlCl4− ion conductivity. To this end, we investigated the diffusion barriers of AlCl4− in bulk graphite and multilayer graphene films using the nudged elastic band method.47 We considered the diffusion of AlCl4− species in the doubly stacked intercalant structure of the fully charged stage 3 GIC (Figure S3). AlCl4− diffusivity was attained using the following equation: D = a 2ve−E b / kBT

where a is the hopping distance, υ is the vibrational prefactor (1012 Hz),48,49 Eb is the activation energy barrier for AlCl4− diffusion obtained from the calculations of the nudged elastic band method, kB is the Boltzmann constant, and T is the temperature. The AlCl4− diffusivities in graphite and six-, five-, four-, three-, and two-layer graphene films at T = 300 K were obtained from the calculated activation energy barriers (Table S1). Table 1 summarizes the absolute and relative diffusivities with respect to graphite. Notably, AlCl4− diffusivity increases markedly as the number of graphene layers decreases (see Figure 4). The diffusivity in the five-layer graphene film (2.3 × Table 1. AlCl4− Diffusivities D in Graphite and Few-Layer Graphene Films at T = 300 Ka system graphite few-layer graphene film

a

N

D (cm2 s−1)

6 5 4 3 2

2.2 3.3 2.3 1.1 3.4 5.0

× × × × × ×

10−9 10−9 10−8 10−7 10−7 10−7

D/Dgraphite 1.0 1.5 10.2 47.8 153.2 225.2

Figure 5. Elastic stiffness coefficients along the c-axis (C33) of graphite and six-, five-, four-, three-, and two-layer graphene films.

N represents the number of layers in graphene film. 13387

DOI: 10.1021/acs.jpcc.6b03657 J. Phys. Chem. C 2016, 120, 13384−13389

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The Journal of Physical Chemistry C axis (C33) of graphite and few-layer graphene films. The C33 value was derived from the second derivative of the potential energy surface on the c axis according to

Author Contributions §

S.C.J. and Y.J.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

2

C33 =

co ∂E A ∂c 2

where c0 and A are the equilibrium interlayer distance and cross-sectional area of graphite, respectively (Figure S4).50 The C33 elastic constant in the five-layer graphene film (21.1 GPa) is smaller by 23% than that of graphite (27.5 GPa), and this value goes down further to 12.1 GPa, which is only 44% of that of graphite, once the number of layer decreases to two. This series of results suggest that the structural flexibility of graphitic foam begins to increase when the number of graphene layers reduces to five. This elasticity trend is in good correlation with the diffusivity behavior displayed in Figure 4. For example, the thickness change from six layers to five layers that causes the largest change in the C33 value (21% reduction) leads to the largest diffusivity increase (6.9 times) (Table 1). Figure 5 demonstrates a great enhancement of the structural flexibility of graphitic foam with decreasing the layer thickness. As a result of the enhanced structural flexibility, the intercalant gallery height di increases from 12.4 Å in graphite to 13.3 Å in the two-layer graphene film (Table S1), providing more free volume for AlCl4− transport and thus lowering the activation energy barrier. Thus, the low elastic constants of few-layered graphene sheets serve as the origin of the ultrafast rate capability of graphitic foam and successfully explain the recent experimental observation.

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CONCLUSIONS In conclusion, we have investigated the energetics, structure, ion conductivity, and mechanical property of AlCl4−-intercalated graphitic materials using first-principles calculation. By comparison with the experimental XRD spectrum, we reveal that the fully charged graphitic cathode in the aluminum-ion battery is at stage 3 with a doubly stacked intercalant structure. The AlCl4− diffusivity in few-layer graphene films increases significantly as the number of graphene layer decreases below five. In particular, the low elastic stiffness of few-layered graphene that allows to provide more free volume for AlCl4− diffusion turned out to be the origin of the ultrafast rate performance of graphitic foam. Our findings suggest that even bulky polyanions can be carrier ions for batteries with ultrafast rate performance if nanothickness few-layered graphene with enhanced elasticity is used as active materials, opening up a new research direction: developing various battery systems based on polyatomic carrier ions. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03657. Additional results related to formation energies, AlCl4− diffusivities, and elastic stiffness coefficients (PDF)

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ACKNOWLEDGMENTS

This work was partly supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning of Korea (2010-C1AAA0010029018 and 2016R1A2B4013374) 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). This work was also supported by the Dongguk University Research Fund of 2013.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Phone: +82 42 350 1719. *E-mail: [email protected]; Phone: +82 2 2260 4975. 13388

DOI: 10.1021/acs.jpcc.6b03657 J. Phys. Chem. C 2016, 120, 13384−13389

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DOI: 10.1021/acs.jpcc.6b03657 J. Phys. Chem. C 2016, 120, 13384−13389