Amorphous Carbon-Derived Nanosheet-Bricked Porous Graphite as


Jul 19, 2018 - ... CaCl2, and the high crystallinity and thin layer characters facilitate the high capacity and high rate storage of aluminum tetrachl...
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Functional Nanostructured Materials (including low-D carbon)

Amorphous Carbon Derived Nanosheets-bricked Porous Graphite as High Performance Cathode for Aluminum-Ion Batteries Chunyan Zhang, Rui He, Jichen Zhang, Yang Hu, Zhiyong Wang, and Xianbo Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07590 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Amorphous Carbon Derived Nanosheets-bricked Porous Graphite as High Performance Cathode for Aluminum-ion Batteries Chunyan Zhang, Rui He, Jichen Zhang, Yang Hu, Zhiyong Wang, Xianbo Jin*

Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and Molecular Sciences, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan, 430072, P.R. China. *

Email address: [email protected]

Abstract: Graphite is an attractive cathode material for energy storage because it allows reversible intercalation/deintercalation of many compound anions at high potentials. However, because the sizes of the compound anions are greatly larger than the lamellar spacing of graphite, common graphite for cathode uses may suffer from slow kinetics and large volume expansion. Here, it demonstrates that graphite with a high crystallinity and nanosheets-bricked porous structure can be an excellent cathode for the aluminum-ion batteries. This porous graphite is derived from carbon black via a simple electrochemical graphitization in molten CaCl2, and the high crystallinity and thin layer characters facilitate the high-capacity and high-rate storage of aluminum tetrachloride ions. Moreover, the bricked porous structure endows the fabricated cathode with a providential porosity to perfectly match the huge volume expansion of graphite (650% against a charging capacity of 100 mAh g-1), which thus exhibits integrated high gravimetric and volumetric capacities, as well as high structural stability during cycling. Keywords: graphite cathodes; rechargeable aluminum batteries; porous graphite; volumetric capacity; amorphous carbon; graphitization 1 ACS Paragon Plus Environment

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Introduction Graphite, a traditional anode for lithium ion batteries (LIBs), is emerging as a promising cathode for energy storage because it is cheap and allows reversible intercalation/deintercalation of various kinds of compound anions at high potentials.1-10 It can be used to assemble various kinds of batteries against metals or graphite anodes. In particular, Al/graphite batteries, also called as “aluminum-ion batteries” (AIBs), have attracted increasing interest for the safety, high earth abundance and specific capacity (2980 A h kg−1 and 8046 A h L−1) of Al and the ultralong cycling life of the graphite cathode.7, 11-17 It should be pointed out that although Al/graphite AIBs are rechargeable, there are not rocking chair batteries like LIBs. As well-known during discharging, in a LIB Li+ ions leave the anode, transfer through the electrolyte and integrate into the cathode, but the Al/graphite cell discharges of Al2Cl7- and/or AlCl4- ions at both electrodes.7 Recently it was confirmed that V2O5 allows intercalation of direct Al3+ ions,18 thus the Al/V2O5 battery may work similar to traditional LIBs. The intersection of Al/graphite AIBs and present commercial LIBs is the use of graphite electrode, but as cathode for AlCl4- intercalation and anode for Li+ intercalation, respectively. However, conventional graphite that works well in LIBs have proved to suffer from low specific capacity and poor rate performance when use as cathodes. It was reported that the cathode performance could be significantly improved after exfoliating the graphite into few-layer graphene, but it is essential to retain sufficiently large crystallite domains. Common graphene in an amorphous structure is unsuitable for the cathode use unless post recrystallization is carried out at temperatures higher than 2500 oC.19-24 The difficulty of graphite for cathode uses may arise from the fact that the lamellar spacing of graphite (0.335 nm) is too narrow in comparison with the sizes of compound anions.10, 25 AlCl4ion has a size of 0.609 nm, so its intercalation into graphite may encounter a great pressure stress.

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Using few-layer graphene may help to alleviate this stress, and a relative high gravimetric capacity can be achieved.19-24 On the other hand, the intercalation of large compound anions into graphite may lead to a huge volume expansion,7 which has been recognized as a main issue to address for the practice uses. It should be pointed out that there is still no experimental data of volume expansion determined for the graphite cathode. It is important for the graphite material to have a proper nanoporous structure to alleviate the intercalation stress and maintain a moderate porosity in the cathode. Ideally, the porosity should be high enough to buffer the volume change, so to avoid the cell failure as a result of volume expansion; but too high a porosity will lead to not only a low volumetric capacity, but also a low gravimetric capacity because the electrode would take in large amounts of superfluous electrolyte.26,27 Here we report that a nanosheets-bricked porous graphite (NSPG) derived from carbon black through a simple electrochemical graphitization can be a high performance cathode material for AIBs in an [Emim]Cl/AlCl3 (1-ethyl-3-methylimidazolium chloride/aluminum chloride) electrolyte. The NSPG has many advantageous features for the intercalation of large anions. The high crystallinity and thin layer nanosheet characters endow the NSPG with a large gravimetric capacity and high rate performance. The nanosheets-bricked porous structure helps to maintain the cathode an appropriate porosity (~87%), which guarantees a high volumetric capacity while well matches the about 650% volume expansion of graphite upon intercalation about 100 mAh g-1 of AlCl4- ions as we demonstrated. Experimental Section Preparation of nanosheets-bricked porous graphite (NSPG)

Typically, 0.5 g Carbon black (Vulcan XC-72, Cabot, USA ) was die-pressed into a cylindrical pellet and then packed with porous nickel foam to form the cathode, which was polarized in molten CaCl2 (1093 K) against a graphite anode by applying a cell voltage of 2.6 V for 30 min, 3 ACS Paragon Plus Environment

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60 min or 120 min. Then, the cathode was taken out, cooled to room temperature, washed and dried for use.28 Preparation of electrolyte for AIBs

The ionic liquid electrolyte is prepared by mixing [EMIm]Cl (97%, Acros Chemicals) and anhydrous aluminum chloride (AlCl3) (99.999%, Sigma Aldrich) (molar ratio of 1.3:1) in an argon-atmosphere glove box ([O2] < 0.1 ppm, [H2O] < 0.1 ppm). Before the mixing, the as received [EMIm]Cl was baked at 100°C under vacuum for 36 h to remove residual water. The resulting light yellow and transparent electrolyte was allowed to stand for at least 12 h before used. Electrochemical tests of AIBs

Swagelok-type cells were assembled and tested in the glove box, and a comparison between NSPG and commercial nanosheet graphite (CNG, graphite nanosheet, D50<400nm, 99.95%, Aladdin) was also carried out. The cathode membranes (ca. 2 mg cm-2) comprising 80 wt% active material, 10 wt% acetylene black, and 10 wt% polytetra-fluoroethylene (PTFE) were prepared by the rolling-press method. Before use, the membranes were dried in a vacuum oven over night at 110 °C. The anode is an Al foil (34mg, 16um, purity 99.5%), which was separated from the cathode by one layer of glass fiber paper (whatman, GF/D). Tantalum foil (99.95%) is used as current collector. Galvanostatic charge–discharge of the cell was performed at room temperature engaging the BTS 7.6.x Neware Battery Testing System (Shenzhen, China). Cyclic voltammograms (CVs) and electrochemical impedance spectra (EISs) were recorded using an Iviumstat workstation (Ivium Technologies BV Company). Material Characterizations

X-ray diffraction (XRD) analysis were performed on a Bruker D8-advanced instrument. The structure and morphology of the samples were characterized by Scanning Electron Microscopy 4 ACS Paragon Plus Environment

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(SEM, Zeiss Merlin Compact, 6kV) and Transmission Electron Microscopy (TEM, JEM-2100, HR). Raman spectra were recorded at an incident laser wavelength of 514.5 nm by Confocal Raman Microspectroscopy (CRM, Renishaw, RM-1000). The sample pore structure was analysed by using a Micromeritics ASAP 2020 Analyzer (Norcross, GA) with nitrogen adsorption. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method.

Results and Discussion Preparation and characterization of NSPG samples

As schematically shown in Figure S1, NSPG samples were prepared from carbon black XC-72 via a new electrochemical graphitization process in molten CaCl2. Specifically, about 0.5 g of XC-72 was die-pressed into a cylindrical pellet, which was packed with a porous nickel foam to form the working electrode. A graphite rod served as the counter electrode, which has been proved to be inert during the graphitization process. The cathodic polarization of carbon black was carried out under argon protect by applying a cell voltage of 2.6 V at 820 oC. It has been confirmed that the electrochemical graphitization involves mainly two steps. One is the electrochemical deoxygenation of the amorphous carbon; the other is the long-distance rearrangement of carbon atoms.28,29 Good graphitization can be achieved in 120 minutes with a yield of about 80%, and the resultant is denoted as NSPG-120. To study the influence of graphitization degree on the cathode performance of NSPG in the AIB electrolyte, the 30-minute and 60-minute polarization products were also collected, and denoted as NSPG-30 and NSPG-60, respectively. According to scanning electron microscopy (SEM) images, XC-72 consists of nanospheres about 20~100 nm in diameter (Figure 1a). After the cathodic polarization for 30 minutes, the resulting 5 ACS Paragon Plus Environment

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NSPG-30 appears to be a mixture of original XC-72 nanospheres and newly generated two dimensional nanoflakes (Figure 1b). Compared to original XC-72 nanospheres, the nanoflakes are thinner but much larger in plane dimension, indicating occurrence of long distance rearrangement of carbon atoms during the graphitization. In NSPG-60, the original nanospheres disappear, and the nanoflakes become larger, with lateral sizes of 200-500 nm (Figure 1c). Under transmission electron microscope (TEM), these nanoflakes resemble the image of graphene, but crystalline graphite structure can be clearly recognized (Figure S2).

Figure 1. Morphology and structure and spectroscopy characterization of NSPG samples. SEM images of a) XC-72, b) NSPG-30, c) NSPG-60 and d) NSPG-120. e) TEM and HRTEM images of XC-72. f) TEM and HRTEM images of NSPG-120. g), h) and i) are XRD patterns, Raman spectra and pore size distribution curves of indicated samples.

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The nanoflakes grow further with the polarization time, mainly in the plane orientation. NSPG-120 comprises nanosheets with a lateral size of about 1 µm and thickness of about 10 to 20 nm. These nanosheets are cambered and mutually supported, constructing a porous framework as shown in Figure 1d. Figure 1e and 1f compare TEM images of XC-72 and NSPG-120. It is clear that XC-72 spheres comprise randomly aligned graphene debris, but NSPG-120 consists of two-dimensional graphite sheets in high crystallinity. A typical high resolution TEM image of NSPG-120 sheets was inset in Figure 1f, which has a thickness of about 12.5 nm or 37~38 layers of graphitic lattice planes, suggesting a lamellar spacing of about 0.34 nm, matching well with the graphite structure. The degree of graphitization was investigated by X-ray diffraction (XRD) analysis. As shown in Figure 1g, NSPG-30 shows a small but typical 002 peak (2θ=26.2°) of graphite. This peak becomes sharper and stronger with the increase of graphitization time, indicating an increasing graphitization degree from NSPG-30 to NSPG-120. Using the Scherrer equation, the crystallite domain dimensions of NSPG-30, NSPG-60 and NSPG-120 along the c-axis are calculated to be 9, 10 and 12 nm respectively, in line with the above SEM and TEM observations. Raman spectra were also recorded to analyze the graphitization degree. In a carbon, the bond stretching of sp2 atom pairs give rise to the G peak at about 1600 cm−1; defects and disorders cause a D peak at about 1350 cm−1, and the intensity ratio of the D to G peak (ID/IG) can be an indicator of relative graphitization degree. In addition, the 2D peak at about 2710 cm−1 is another typical feature of graphitic carbon.30 As displayed in Figure 1h, XC-72 shows a ID/IG of 0.87 and no 2D peak. After cathodic polarization in molten CaCl2, the 2D peak appears in all the NSPG products, evidencing that graphitization of XC-72 occurred. With the graphitization time increases, the D peak

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decreases and the G peak increases. The ID/IG values are 0.45, 0.3 and 0.11 for NSPG-30, NSPG-60 and NSPG-120 respectively, revealing a gradual increase of the graphitization degree. In particular, the ID/IG of NSPG-120 is almost the same as that of commercial graphite powder, suggesting the high crystallinity of the NSPG-120. Brunauer–Emmett–Teller (BET) analysis indicates that the specific surface area decreases from 218 m2 g-1 of XC-72 to 87 m2 g-1 of NSPG-120 (Figure S3), in agreement with the morphology evolution from nanoparticles to micron level nanosheets. However, the pore volume remains almost unchanged (0.38 cm3 g-1 of XC-72 vs. 0.37 cm3 g-1 of NSPG-120, Figure 1i), indicating that the nanosheets-bricked structure can effectively construct large pores in the resultant graphite. Influence of graphitization degree on the cathode performance of NSPG

The cathode performance of different NSPG samples were evaluated using Swagelok-type cells, where an Al foil served as the counter electrode and tantalum foil served as current collector. Figure 2a shows the cyclic voltammograms (CVs) of the tantalum foil, XC-72 and NSPG samples at a scan rate of 10 mV s-1 in a potential range of 0.1–2.55 V vs. Al. It can be seen that the background current is very small, indicating that the tantalum foil is basically inert. The XC-72 shows mainly a capacitor behavior with a low capacitance. All NSPG samples show an apparent oxidation (charging) peak at about 2.25 V with a shoulder at 2.0 V. The backward scan branches are relatively complicated, exhibiting three reduction peaks at about 1.9, 1.7 and 1.5 V, and two shoulders at about 2.2 and 1.2 V. These results suggest that the charging/discharging of NSPG may involve 2-5 stages of graphite intercalation compounds.31 The CVs also indicate that the higher is the graphitization degree of NSPG, the larger is the charging/discharging capacity. This is confirmed by galvanostatic charging/discharging tests as shown in Figure 2b. The XC-72 shows no charging/discharging platform, featuring a capacitive 8 ACS Paragon Plus Environment

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behavior. In contrast, all NSPG samples have apparent voltage platforms in both charging and discharging branches, and the higher the graphitization degree, the longer the voltage platforms, indicating that the graphite structure is essential for the occurrence of anion intercalation.

Figure 2. Electrochemical characterization of different samples as indicated in the [Emim]Cl/AlCl3 electrolyte. a) CVs in a potential window of 0.1-2.55 V vs. Al3+/Al. b) Charging and discharging profiles at a current density of 2 A g-1 in a potential window of 0.1-2.46 V. c) Rate performance while charging at 2 A g-1 and discharging at various current densities from 0.15 to 2 A g-1. d) Cycling performance at a current density of 10 A g-1 in a potential range of 0.1-2.5 V.

The NSPG-120 can be rapidly charged, for example at 2 A g-1, to a capacity of 103 mAh g-1, which can then be fully released at different discharging rates (0.15-2 A g-1, Figure 2c). Even at much higher charging/discharging rates, the NSPG-120 can still deliver a high reversible capacity and exhibit excellent cycling stability. For example, it delivers a capacity of 104 mAh g-1 after 3000 charging/discharging cycles at a current density of 10 A g-1 (Figure 2d), which is

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much higher than those of XC-72 (11 mAh g-1), NSPG-30 (58 mAh g-1) and NSPG-60 (83 mAh g-1). These results suggest that NSPG-120 is a promising cathode material for AIBs. Compare NSPG-120 with commercial nanosheet graphite

As the NSPG-120 shows the best cathode performance, it was further compared to commercial nanosheet graphite (CNG) which has stronger XRD peaks (Figure S4) thus should be of higher degree of crystallinity. The CV of CNG is displayed in Figure 2a, showing an anodic branch similar to that of NSPG-120 but a lower current, indicating that the crystallinity degree is merely one of those factors that may significantly influence the cathode performance of graphite. When compare the cathodic (negative scan) branches, not only the current of CNG is lower, but there is a lag discharge at potentials ranging from 1.1 to 0.3 V in the case of CNG, indicating that it is difficult for the deintercalation of AlCl4- ions from the CNG electrode. The discharge capacities of NSPG-120 and CNG at different rates (1 - 100 A g-1) are compared in Figure 3a, with the voltage profiles shown in Figure 3b. For high rate charging, Ohmic drop compensation was made according to the cell resistance determined by AC impedance analysis (Figure S5). It can be seen that at 1 A g-1, the discharge capacities of NSPG-120 and CNG are comparable, but the CNG undergoes a rapid capacity decrease when the charging/discharging rate increases (from 98 mAh g-1 at 1 A g-1 to 59 mAh g-1 at 10 A g-1). However, the NSPG-120 shows an amazing high rate performance, and it delivers a capacity of 104, 103 and 90 mAh g-1 at 1, 10 and 100 A g-1 respectively. The performance of NSPG-120 is comparable to the best result reported previously (Table S1).7, 11-24 To understand the performance difference between the NSPG-120 and CNG electrodes, electrochemical impedance spectra (EISs) were recorded after cycling the electrodes to reach a stable state. EISs of uncharged electrodes (Figure 3c) were recorded at 1.85 V, i.e. the onset potential of AlCl4- intercalation (Figure 2a); EISs of charged electrodes (Figure 3d) were 10 ACS Paragon Plus Environment

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recorded at 2.35 V. All the Nyquist plots feature a combination of a semicircle in the high-frequency region and a sloping line in the low-frequency region. Usually, the diameter of the semicircles represents the charge transfer resistance (Rct), and the smaller is the diameter, the higher is the charge transfer speed; the sloping line contains the information of Warburg impedance, and the larger is the slope, the quicker is the diffusion process. It can be seen that under same conditions, the NSPG-120 electrode exhibits both higher charge transfer speed and faster AlCl4- diffusion than the CNG electrode, in line with the observed superior performance of NSPG-120 over CNG.

Figure 3. Comparative studies of NSPG and CNG as cathodes for AIBs. a) Rate performance at various charging/discharging current densities from 1 to 100 A g-1. b) Charging and discharging profiles at 5, 10, 20, 50 and 100 A g-1. c) and d) Nyquist plots of the cells at a bias voltage of 1.85 V (c) and 2.35V (d) after charging/discharging for 500 cycles at 10 A g-1 (frequency range: 0.1-100 kHz); e) Schematic illustration of the change of electrode porosity after intercalation of AlCl4- into NSPG and CNG. Insets in (c) and (d) are local zooms of the corresponding Nyquist plots.

EISs of each electrode before and after charging are also compared. It can be seen that for both NSPG-120 and CNG, after charging the slope of the Warburg line in the low frequency region 11 ACS Paragon Plus Environment

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decreases significantly, indicating that ion diffusion in the charged electrode are more difficult.32 In particular, the charged CNG electrode shows a 45o line, resembling the Warburg response of a plate electrode.33 On the other hand, the Rct of the charged CNG electrode is significantly larger than that of the uncharged (45 vs. 45 Ω). But the NSPG-120 electrode before and after charging exhibits almost the same Rct (about 5 Ω), indicating that the big Rct increase of the CNG electrode could not be a result of the change in chemical state of graphite. Therefore, it is speculated that the increase of Rct and the decrease of diffusion speed, are caused by the change of the electrode structure upon intercalation of anions. It is believed that the unique nanosheets-bricked porous structure of NSPG can help to maintain pores during the electrode fabrication. But the CNG has not such a nanosheets-bricked construction to prevent the sheets from closely stacking, so its nanosheets aggregate seriously (Figure S7). As a result, the density of NSPG electrodes is about 0.3 g cm-3, much lower than that of the CNG electrode (0.7 g cm-3). The ion diffusion in the NSPG-120 electrode is fast because of its high porosity so plenty of pores to serve as electrolyte passageways. The small Rct of NSPG-120 can be explained by its large electrochemical interface (Figure S3). Charging the electrodes will decrease their porosity significantly, as schematically shown in Figure 3e, because of the volume expansion of the active material upon intercalation of the big AlCl4- ions. Therefore, the ion diffusion in the charged electrodes becomes more difficult. However, as the charged CNG electrode resembles a plate electrode in diffusion behavior, it is speculated that the volume expansion would have eliminated most of the electrolyte passageways in the electrode (Figure 4d), so the electrode/electrolyte interface decreases significantly, leading to a remarkable increase of Rct. By contrast, the NSPG electrode may have a sufficiently large porosity, and the charging may only narrow those constructed big pores but not reduce the electrochemical

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interface, so the Rct remains almost unchanged. The assumptions described in Figure 3e have been evidenced by the SEM observation of the electrodes before and after charging (Figure S8). The volume expansion effect

The volume expansion of graphite upon intercalation of AlCl4- ions was further investigated using dense pellet electrodes. These pellets were cut from a graphite plate followed by polishing. Typically, they have an area of about 2 mm2 and a thickness of about 0.15 mm (Figure 4a). The dimensions of the charged pellet were measured initially in the glove box using a rule and then under SEM observation (Figure 4b and S9-11). Figure 4c plots the volume change of the graphite pellet against the charging capacity, showing a proximately linear relationship. The volume expansion ratio was then calculated to be about 6.5% per intercalation of 1 mAh g-1 AlCl4- ions, or 650% for an intercalation capacity of 100 mAh g-1. It should be pointed out that the volume expansion of graphite as an anode for the intercalation of Li+ ions is insignificant (~ 20%),34 which is understandable because the size of Li+ ion (0.152 nm) is much smaller than the lamellar spacing of graphite. However, the size of AlCl4- ion (0.609 nm) is significantly larger, so great volume expansion of the graphite as cathode is expected. Some computational studies predicted that the expansion of graphite upon intercalation of AlCl4- ions is not remarkable (from 40% ~ 80%),35-37 but the prediction is far below experimental observation.7 The reason could be that the structure of the intercalation product assumed for the calculation is not the exact one in practice. To fully buffer the 650% volume change, the graphite cathode should have a porosity of about 87%. Coincidently, the NSPG electrode has such a high porosity according to the electrode density (0.3 g cm-3); the CNG electrode has a porosity of 69%, which is high but not enough. To check it, the volume expansion of the NSPG and CNG membrane cathodes (about 1 cm× 1 cm in geometric area) was investigated. Before charging, SEM images of the electrode were recorded (Figure 4d and 4e). It is interesting that after preparation of the electrode membrane through the 13 ACS Paragon Plus Environment

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rolling-press process, the NSPG still retains its original nanosheets-bricked porous structure (Figure 1d, Figure 4d), which may provide not only sufficient electrolyte channels, but also enough space to buffer the volume expansion of graphite nanosheets. By contrast, the CNG in the membrane appears a stacking morphology, and the electrode is denser (Figure 4e).

Figure 4. Volume expansion of different graphite cathodes upon intercalation of AlCl4- ions. a) and b), Top and sectional SEM images of a dense pellet electrode before (a) and after (b) charging for a capacity of 80 mAh g-1. c) Volume change of the dense pellet electrode against the charging capacity. d) and e) Sectional SEM images of NSPG-120 (d) and CNG electrodes (e) before and after charging for a capacity of 50 mAh g-1. All the electrode were charged at a rate of 2 A g-1

During charging, it was found that the change in geometric area of both NSPG-120 and CNG membranes is insignificant (Figure S12). The CNG membrane expands by about 40% in the thickness direction after the first charging of a capacity of 50 mAh g-1 (Figure 4e), and the 14 ACS Paragon Plus Environment

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expansion rate increases to 103% for a charging capacity of 84 mAh g-1 (Figure S13). It should be pointed out that the observed volume expansion ratio of the CNG electrode is in well agreement with that of the dense graphite pellet: according to Figure 4c, the volume expansion ratio of the 69%-porosity CNG electrode will be 32% and 100% for a charging capacity of 50 and 84 mAh g-1 respectively. By contrast, the NSPG-120 membrane does not expand even against a charging capacity of 102 mAh g-1 (Figure 4d and S14). Therefore, the NSPG-120 can deliver a volume capacity of about 31 mAh cm-3 without volume expansion to the electrode. This value should be significantly higher than those recently reported with ultralight (0.001-0.004 g cm-3) graphene materials.19, 23 On the other hand, although the CNG at low rates can deliver a higher volumetric capacity of 67 mAh cm-3, but this value is calculated based on the original electrode volume. However, the huge volume expansion may limit the application of CNG in practice. Finally, although a higher porosity can prevent the electrode from expansion, too high a porosity is not recommended, because not only the volumetric capacity will decrease, but also the actual gravimetric capacity by full electrode weight (graphite + electrolyte residing in electrode pores) will be significantly lowered. For example, when the electrode porosity increases from 87% to 95%, the volumetric capacity will decrease by 62%. At the same time, the actual gravimetric capacity will decrease by more than 59%, because the mass ratio of graphite to electrolyte within the electrode decrease from 0.25 to 0.088 (a density of 1.32 g cm-3 for the [Emim]Cl/AlCl3 electrolyte is used for the calculation38). This again indicates the structural advantage of NSPG-120, which can perfectly match the volume change, endowing the cathode with integrated high gravimetric and volumetric capacities (Figure S15). Moreover, two Al/NSPG-120 single cells (3 mg NSPG per cell) in series have been tested as the power source of a violet light

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emitting diode (LED) (ignition voltage, 3.2-3.4V) as shown in Figure S16. The cells were fully charged in about 3 min, and lighted up the LED for about 50 min, indicating good application potential of the NSPG-120. However, to optimize the graphitization process, further investigations, for example, understanding the influence of the graphitization temperature and time on the performance of NSPG, may be needed. Conclusion In conclusion, we have demonstrated that a nanosheets-bricked porous graphite (NSPG), derived from carbon black through a simple, one-step electrochemical graphitization in molten CaCl2, could be an excellent cathode material for AIBs. The high crystallinity and thin layer characters endow the NSPG with a high specific capacity and high speed of anion intercalation, and it can deliver a capacity of 104 mAh g-1 at a charging/discharging rate of 10 A g-1 and 90 mAh g-1 at 100 A g-1, and stably cycle over 3000 cycles. Moreover, the volume expansion of graphite upon charging has been experimentally determined to be about 6.5% per intercalation of 1 mAh g-1 AlCl4- ions, or 650% for a charging capacity of 100 mAh g-1. It is interesting that the nanosheets-bricked porous structure of NSPG can maintain an appropriate porosity in the cathode to well match this volume change, which means that the cathode has no redundant pores and does not expands, thus has integrated high gravimetric and volumetric capacities, as well as high structural stability during cycling. Our findings may prove useful for the development of graphite cathodes for high performance AIBs and other energy devices.

Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx/xxxxx.

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Acknowledgements We appreciate the funding support from NSFC (21673164, 21173161), and the Large scale Instrument and Equipment Sharing Foundation of Wuhan University. References 1.

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