Highly Ordered Graphene Solid: An Efficient Platform for Capacitive

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Highly Ordered Graphene Solid: An Efficient Platform for Capacitive Sodium-Ion Storage with Ultrahigh Volumetric Capacity and Superior Rate Capability Hongyun Ma,† Hongya Geng,† Bowen Yao,† Mingmao Wu,† Chun Li,*,† Miao Zhang,† Fengyao Chi,† and Liangti Qu*,†,‡,§ †

MOE Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Key Laboratory for Advanced Materials Processing Technology, Ministry of Education of China, and State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China § School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China S Supporting Information *

ABSTRACT: As an emerging type of electrochemical energy storage devices, sodium-ion capacitors (SICs) are potentially capable of high energy density and high power density, as well as low cost and long lifespan. Unfortunately, the lack of high-performance capacitive cathodes that can fully couple with the well-developed battery-type anodes severely restricts the further development of SICs. Here, we develop a compact yet highly ordered graphene solid (HOGS), which combines the merits of high density and high porosity and, more attractively, possesses a highly ordered lamellar texture with low pore tortuosity. As the capacitive cathode of SICs, HOGS delivers a record-high volumetric capacity (303 F cm−3 or 219 mA h cm−3 at 0.05 A g−1), a superior rate capability (185 F cm−3 or 139 mA h cm−3 even at 10 A g−1), and an outstanding cycling stability (over 80% after 10 000 cycles). The material design and construction strategies reported here can be easily extended to other metal-ion-based energy storage technologies, exhibiting universal potentials in compact electrochemical energy storage systems. KEYWORDS: ordered graphene solid, capacitive cathode, sodium-ion capacitor, ultrahigh volumetric capacity, excellent rate capability

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even completely replace the current lithium-ion based systems since sodium resource is more abundant.11,12 In the past decade, plenty of efforts have been made on sodium-ion batteries (SIBs), including the investigation of new cathode and anode materials and the exploration of new electrolytes.13−16 So far, most electrode materials of SIBs, especially for cathodes, are based on intercalation reactions. Unfortunately, sodium ion has a larger diameter than lithium ion, thus suffering sluggish kinetics during the insertion/desertion process.9,17,18 Worse still, the insertion/desertion of sodium ions results in large structural distortion to host materials, leading to fast capacity fading upon repeated cycling.9,19 As a result, most of intercalation cathode materials for SIBs exhibit a

lectrochemical energy storage (EES) devices, such as high energy density batteries and high power density supercapacitors, play crucial roles in portable electronics, electric vehicles, and large-scale energy storage grids.1,2 As hybrids of batteries and supercapacitors, metal-ion capacitors are emerging as a new type of EES devices that can potentially combine the advantages of the high energy density of batteries and the high power density of supercapacitors.3−5 A successful example is lithium-ion capacitors (LICs), which contain a battery-type anode and a capacitortype cathode and have been demonstrated to be capable of outstanding energy and power performance.6−8 Nevertheless, lithium-ion based energy storage technologies are facing the upcoming cost crisis due to the limited reserve of lithium resource.4,9,10 Therefore, other low-cost, long-lifespan, and high-efficiency EES devices are urgently desired. Sodium-ion based energy storage technologies have been regarded as the most promising alternatives that could partly or © XXXX American Chemical Society

Received: May 6, 2019 Accepted: July 17, 2019 Published: July 17, 2019 A

DOI: 10.1021/acsnano.9b03492 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the preparations of CGS and HOGS. GO sheets in a dilute suspension (3 mg mL−1) can spontaneously produce disordered micro-zone LC phases; upon hydrothermal reduction and capillary densification, porous graphene hydrogel (CGH) and disordered graphene solid (CGS) are obtained (approach 1, green arrows). After the orientation by adding an appropriate amount of KOH (0.12 mol L−1), GO suspension evolves into a long-range ordered lamellar LC phase (GO LCs); upon the same procedure, highly ordered graphene hydrogel (HOGH) and compact yet highly ordered graphene solid (HOGS) can be successfully prepared (approach 2, red arrows).

low reversible capacity (limited to ∼120 mA h g−1), a poor rate capability, and a limited cycling stability (usually ∼200 cycles), much inferior to those can be achieved in lithium-ion ones. Alternatively, considering the low reaction barrier and negligible structural distortion of their capacitive cathodes, sodium-ion capacitors (SICs) are a more fascinating type of sodium-ion based EES devices.20 Moreover, with a similar device structure and electrochemical property to LICs, SICs are potentially capable of high energy and high power densities, as well as low cost and long lifespan.4,9,21−23 Therefore, developing high-performance capacitive cathode materials for SICs is of extreme significance. Carbon materials, especially graphene and carbon nanotubes, are ideal capacitive materials owing to their high specific surface area (SSA), good conductivity, and tunable surface chemistry.24−28 However, compared with inorganic intercalative materials, these nanosized carbon materials often possess low packing densities (usually 0.97) uptake on the isotherms. Pore size distribution (PSD) curves give more details on the pore structures of CGS and HOGS (Figure 3b and Figure S20). The pore diameters are centered around 0.6, 1.5, 2.5, and 3.5 nm, and HOGS has larger amounts of micropores and small mesopores (slightly larger than 2 nm) than CGS. This result is consistent with previous E

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triangular shapes owing to the pseudocapacitive sodium-ion storage mechanism. Even at 10 A g−1, the GCD curve of HOGS still maintains its symmetry and just shows a small IR drop, again reflecting its excellent rate capability. The CV curves at different scan rates and GCD curves at different current densities of CGS are also given in Figure S29. By contrast, HOGS shows a much stronger redox hump on its CV curve (Figure S30a), which is mainly attributed to its higher SSA and larger amounts of electrochemically active sites, namely, O-I type oxygen functional groups. Correspondingly, HOGS shows a longer discharge time on its GCD curve (Figure S30b). The gravimetric capacitances of CGS and HOGS were calculated from their GCD curves and are depicted in Figure 4c. At 0.05 A g−1, HOGS delivers a high gravimetric capacitance of 205 F g−1, which is much higher than that of CGS (154 F g−1). Even at 10 A g−1, HOGS still delivers a gravimetric capacitance of 124 F g−1, being over 2 times higher than that of CGS (60 F g−1). The capacitance retentions of HOGS and CGS were further depicted in Figure 4d. Obviously, HOGS exhibits a better rate capability among all operating current densities, especially at high current densities (2, 5, 8, and 10 A g−1). For example, the capacitance retention of HOGS at 10 A g−1 is calculated to 61%, much higher than that of CGS (39%). Generally, the ordered texture greatly decreases the pore tortuosity of HOGS and thus significantly shortens the ion diffusion pathways from bulk electrolyte to the inside of the electrode material (Figure 4e). As a result, the ion diffusion coefficient of HOGS is much higher than that of CGS (8.5 × 10−11 and 1.3 × 10−11 cm2 s−1, respectively; see more calculation details in Figures S30c, S31, and S32), guaranteeing the fast kinetics of HOGS based electrode. Furthermore, the low-tortuosity feature of HOGS also enables HOGS based cathode to work well upon increasing the areal mass loading from 2 to 10 mg cm−2 (Figure S33). Compared with gravimetric capacitance, volumetric capacitance is more crucial for an electrode material in practical applications.30,33,54 Owing to its high-density feature, HOGS delivers an ultrahigh volumetric capacitance of 303 F cm−3 (Figure 4f). To the best of our knowledge, it is the highest value among those of reported carbon materials for capacitive cathodes of SICs and LICs (Figure 4g).3,4,6,7,9,21,55 When increasing the current density to 10 A g−1, HOGS also delivers a high volumetric capacitance of 185 F cm−3, which is much higher than that of CGS and those of reported carbon materials at the same current density, further highlighting the ordered microstructure of HOGS. Indeed, it is a great challenge to achieve simultaneous high volumetric and rate performances in one electrode material, which needs to carefully balance its packing density and porosity, as well as other parameters including SSA, conductivity, wettability, and surface chemistry. In our case, the highly ordered, compact, yet porous microstructure enables HOGS to maintain excellent volumetric performance at high charge/discharge rates, demonstrating the rationality of the design and construction methodologies for HOGS. Additionally, the HOGS based electrode (containing active material, conductive additive, and binder) possesses a high density of 1.12 g cm−3, demonstrating a high volumetric capacitance of 230 F cm−3 at the level of electrode (Figure S34). Moreover, in order to maintain consistency with those of intercalative battery-type cathodes and/or anodes, the electrochemical performances of CGS and HOGS were also

1.065 and 1.007, respectively, implying more structural defects, oxygen functional groups in this case, on the graphene sheets of HOGS. This result further confirms the etching effect of KOH on graphene sheets during the hydrothermal treatment, which not only enlarges the SSA of HOGS but also introduces plenty of oxygen functional groups on its graphene sheets. X-ray photoelectron spectroscopy (XPS) studies provide more details about the chemical structures of CGS and HOGS. Compared with CGS, HOGS exhibits a stronger O 1s peak on its XPS survey spectrum (Figure S21), and the oxygen atomic content of HOGS was measured to be 18.8%, much higher than that of CGS (15.0%) and those of other rGO-based carbon materials. Furthermore, the O 1s core level spectra of CGS and HOGS were carefully studied to detect more details of their oxygen functional groups. As shown in Figure 3d, each spectrum can be divided into two types of oxygen functional groups: CO quinone type groups (O-I, 531.6 ± 0.2 eV) and C−OH phenol and/or C−O−C ether groups (O-II, 533.2 ± 0.2 eV).52,53 Between them, O-I type oxygen functional groups are reported to be the active sites that can electrochemically react with sodium ions and thus contribute pseudocapacitance.9,21 After normalizing the spectra with respect to the intensity of O-II, the intensity of O-I of HOGS was distinctly stronger than that of CGS. Thus, the extra oxygen functional groups of HOGS can be assigned to O-I. Threrefore, from the point of chemical structure, HOGS is more capable of storing sodium ions owing to its larger amounts of active sites. As described above, HOGS possesses an ordered microstructure as well as abundant oxygen functional groups on its graphene sheets, both of which make HOGS a versatile cathode material for SICs. The electrochemical performance was tested in a half-cell configuration by employing HOGS (or CGS) as the cathode, sodium foil as the anode, and 1 mol L−1 NaClO4 in EC/DMC (1:1) as the electrolyte. Cyclic voltammetry (CV) was first used to investigate the sodiumion storage behaviors of both HOGS and CGS. The potential window was set to be 1.5−4.0 V, within which both electrolyte decomposition and cation intercalation can be avoided.9 As shown in Figure 4a, the CV curves of HOGS exhibit a quasirectangular shape with a couple of broad redox humps, indicating a combined behavior of electrical double-layer capacitor (EDLC) and Faraday pseudocapacitor. The EDLcapacitance is attributed to the adsorption/desorption of Na+ and/or ClO4−, while the pseudocapacitance originates from the reversible redox reaction between oxygen functional groups and Na+. Given the sharp capacitance loss and slight SSA increase after annealing HOGS in argon atmosphere (Figures S22 and S23), we can conclude that the main charge storage mechanism for HOGS is Faraday pseudocapacitive reaction.21 Specifically, the mechanism can be expressed as >CO + Na+ + e− ↔ >C−O−Na, as revealed by the ex situ XPS and SEM measurements (Figures S24−S26). Additionally, the average redox potential of this Faraday pseudocapacitive reaction was detected to be 2.9 V vs. Na/Na+ (Figure S27). Upon increasing scan rate from 1 to 10 mV s−1, the CV curves of HOGS show negligible shape distortion and the peak current has a good linear relationship with scan rate (Figure S28). This result reflects the excellent rate capability of HOGS, which mainly results from the well-constructed unimpeded ion diffusion channels inside HOGS as well as the high reversibility of the pseudocapacitive reaction. Figure 4b shows the galvanostatic charge/discharge (GCD) curves of HOGS; they are symmetrical and slightly distorted from ideal F

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Figure 5. Electrochemical performances of CGS and HOGS detected from the viewpoint of batteries in half-cell configurations. (a) Cycling performance of CGS and HOGS at the current density of 0.1 A g−1. (b) Rate performance of CGS and HOGS at various current densities (A g−1). (c) Long-term cycling performance of HOGS at a current density of 2 A g−1. (d) Comparison of cycling performance of HOGS with those of capacitive cathode materials of SICs and LICs, as well as those of intercalative cathode materials of SIBs (Table S2). (e) Comparation of energy/power densities of HOGS-based SIC with those of state-of-the-art SICs, LICs, and SIB (Table S2).

capability of HOGS. Notably, the extraordinary electrochemical performances achieved by HOGS, including simultaneous high volumetric/gravimetric capacities and excellent rate capability, are superior to most state-of-the-art intercalative battery-type cathodes for SIBs, illuminating the promising potential of HOGS based capacitive cathode for practical applications. The long-term cycling stability of HOGS was also investigated at a high current density of 2 A g−1. As shown in Figure 5c, even after 10 000 cycles, HOGS still delivers a high specific capacity of 121 mA h cm−3 (or 82 mA h g−1), with a capacity retention of 80.6% and a small average fading rate of 0.0019% per cycle. The slow capacity fading is mainly caused by the gradual degradation of the surface oxygen functional groups as revealed by ex situ XPS measurement (Figure S36). Additionally, the Coulombic efficiency increased rapidly during the first few cycles and stayed at almost 100% afterward. The outstanding cycling stability of HOGS was further highlighted by comparing it with those of capacitive cathodes of SICs and LICs, as well as intercalative battery-type cathodes of SIBs (Figure 5d and Table S2). Usually, the cycling tests for the cathode materials of SICs and/or SIBs are

investigated from the viewpoint of batteries in half cells. As shown in Figure 5a and Figure S30d, HOGS delivers a high specific capacity of 209 mA h cm−3 (or 141 mA h g−1) at 0.1 A g−1 and remains at 178 mA h cm−3 (or 120 mA h g−1) after 500 cycles, much higher than that of CGS (135 mA h cm−3 or 87 mA h g−1) cycled with the same procedure. Correspondingly, the Coulombic efficiency increased rapidly from the first cycle (82%) and stayed at almost 100% afterward (Figure S35). The rate performance of HOGS was tested at the current densities ranging from 0.05 to 10 A g−1, as shown in Figure 5b. A higher specific capacity can be achieved for HOGS (219 mA h cm−3 or 148 mA h g−1) by decreasing the operating current density to 0.05 A g−1, at which the pseudocapacitive reaction between sodium ions and oxygen functional groups can be carried out more completely. Even at 10 A g−1, HOGS still maintains a high volumetric capacity of 139 mA h cm−3, as well as a high gravimetric capacity of 94 mA h g−1, much higher than those of CGS (79 mA h cm−3 and 51 mA h g−1, respectively). In addition, when the current density was switched back to 0.1 A g−1, the reversible capacity of HOGS was recovered to 195 mA h cm−3 (or 132 mA h g−1), 95% of the initial value, further demonstrating the excellent rate G

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sliced by using the technique of ultrathin sections. The specimens had a typical thickness of ∼70 nm and were directly transferred onto copper grids for TEM and STEM observations. Nitrogen adsorption was carried out on an Autosorb-iQ gas absorption analyzer (Quantachrome Instruments, USA). The SSAs were calculated via the Brunauer−Emmett−Teller (BET) method. The PSDs were calculated by using density functional theory (DFT). Conductivities were measured on a KDY-1 four-point probe instrument (Kunde Science and Technology Co. Ltd., China). UV−visible spectra were collected on a Lambda 35 spectrophotometer (PerkinElmer, USA). Zeta potentials were measured on a 32BIT zeta potential analyzer (Brookhaven Instruments Corporation, USA). Rheological tests were conducted on an AR-G2 rheometer (TA Instruments, USA). Raman spectra were performed by using a LabRAM HR Evolution Raman microscope with a 532 nm laser (Horiba Jobin Yvon, France). XRD patterns were carried out on a D8 Advanced X-ray diffractometer with Cu Ka radiation (Bruker, Germany). XPS spectra were collected by using an ESCALAB 250XI photoelectron spectrometer (ThermoFisher Scientific, USA). The packing densities were determined via Archimedes principle on a DH-300 electronic densimeter (Dongguan Hongtuo Instruments Co., Ltd., China). Electrochemical Measurements. Typically, the electrode was prepared by mixing the fine powder of HOGS (or CGS) with carbon black (Super P) and poly(vinylidene fluoride) (PVDF) in Nmethylpyrrolidone (NMP) with a weight ratio of 8:1:1 to form a slurry. Then the slurry was coated onto carbon-coated Al foil and vacuum-dried at 60 °C for 12 h; the mass loading of the active material is about 2 mg cm−2. Notably, though HOGS was ground into fine powders during the electrode fabrication process, its highly ordered microstructure was well preserved (Figure S37), which guarantees the excellent rate capability of HOGS based electrode. Half-cell configurations were assembled with CR2032-type coin cells by using HOGS (or CGS) as the cathode, sodium foil as the anode, glass fiber as the separator, and 1 mol L−1 NaClO4 in EC/DMC (volume ratio, 1:1) as the electrolyte. CV, GCD, and EIS tests were carried out on a CHI 660E electrochemical workstation (CH Instruments Inc. Shanghai, USA). EIS analyses were performed at the amplitude of 5 mV with the frequency ranging from 105 to 10−2 Hz. The galvanostatic cycling performances were tested on LANDCT2001A battery testers (LAND, Wuhan, China). All the electrochemical performance reported here are based on active materials (i.e., HOGS or CGS), unless otherwise stated (see more calculation details in the Supporting Information).

completed as early as 1000 cycles, rarely being performed beyond 5000 cycles. By contrast, HOGS keeps working well throughout 10 000 charging/discharging cycles and, more importantly, exhibits a high capacity retention over 80%. The energy/power densities of HOGS-based SIC were also calculated by assuming that the anode material coupled with HOGS possesses an identical specific capacity and an identical packing density with HOGS, as well as the same operating voltage with sodium foil (see more details in the Supporting Information). As seen the Ragone plot depicted in Figure 5e, a superb volumetric energy density of 416 W h L−1 was achieved at a power density of 148 W L−1; even at an ultrahigh power density of 36200 W L−1, a high energy density of 253 W h L−1 was retained. These values are fully competitive with those of state-of-the-art supercapacitors,31,45,53,56 SICs,9,12 LICs,6−8 SIBs,14,15,17 and even LIBs57,58 (Figure 5e and Table S2). This result again highlights the attractiveness of HOGS based capacitive cathode for practical applications.

CONCLUSIONS Highly ordered, compact, and oxygen-rich graphene solid, HOGS, was successfully prepared by using long-range ordered lamellar GO LCs as the precursor, followed by hydrothermal reduction and capillary densification. The material preparation strategy reported here can well balance the packing density and porosity of the obtained graphene solid and, more importantly, optimize its pore structure and surface chemistry. As a result, when employed as the capacitive cathode of SICs, HOGS delivers an ultrahigh volumetric and/or gravimetric capacity, an excellent rate capability, and an outstanding cycling stability. These superior electrochemical performances make HOGS based SICs fully competitive with other types of state-of-the-art EES devices, such as LICs, SIBs, organic electrolyte based supercapacitors, ionic liquid electrolyte based supercapacitors, and even LIBs, demonstrating the rationality of our material design and construction methodologies for HOGS. EXPERIMENTAL SECTION Preparation of HOGS. GO was synthesized from natural flake graphite (325 mesh) via the modified Hummers method (see more details in the Supporting Information). HOGH was prepared through an improved hydrothermal reaction by employing highly oriented GO LCs as the precursor. Typically, a well-dispersed GO suspension (30 mL, 3 mg mL−1) containing appropriate amounts of KOH (0.12 mol L−1) was sealed in a 50 mL Teflon-lined autoclave and treated at 180 °C for 6 h to yield HOGH. The obtained HOGH was rinsed with a HCl aqueous solution (1 mol L−1) repeatedly to remove residual KOH, followed by dialyzing to remove HCl. Then the capillary densification was carried out and HOGH was dried in a vacuum oven at 55 °C for 24 h to obtain HOGS. For comparison, CGS was prepared through the same procedure by using a pure GO suspension (30 mL, 3 mg mL−1) as the precursor. Additionally, HOGS800 was prepared by annealing HOGS at 800 °C for 2 h in an argon atmosphere. Characterizations. POM images were taken on a Nikon LVUEPI system (Nikon, Japan). AFM images were taken on a SPM 9600 atomic force microscope (Shimadzu, Japan). SEM images were taken on a FEI Sirion-200 field-emission scanning electron microscope (FEI, USA). HRSEM images were taken on a FEI Nova NanoSEM450 field-emission scanning electron microscope (FEI, USA). TEM and STEM images were taken on a FEI TECNAI TF20 transmission electron microscope (FEI, USA). Specifically, to observe the ordered texture of HOGS, a combined method of embedding and slicing was employed. HOGS was first embedded in epoxy resin with desired section being exposed. Then the embedded HOGS was

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b03492. Supplementary experimental section, including preparation of graphene oxide, capillary densification of HOGH, and electrochemical calculation; additional AFM, POM, SEM, TEM, and STEM images; additional BET, XRD, XPS, UV−vis, zeta potential, and apparent viscosity data; additional electrochemical data, including CV curves, GCD curves, Nyquist plots, cycle stability, and Coulombic efficiency (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongyun Ma: 0000-0003-2412-0536 Chun Li: 0000-0002-3132-3756 Liangti Qu: 0000-0002-0161-3816 H

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H.M. and L.Q. conceived the idea. L.Q. supervised the entire project. H.M. carried out the majority of experimental measurements. H.G., B.Y., M.W., M.Z., and F.C. helped to characterize the materials and fabricate the devices. H.M., C.L., and L.Q. analyzed the data and discussed the results. H.M. and L.Q. wrote the paper, and C.L. gave advice and reviewed the manuscript. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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