Nitrogen and Phosphorus Dual-Doped Multilayer Graphene as Universal Anode for Full Carbon-Based Lithium and Potassium Ion Capacitors
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Nitrogen and phosphorus dual-doped multilayer graphene (NPG) was prepared by arc discharge process.
NPG exhibits good rate capability and stable cycling performance in both lithium and potassium ion batteries.
Full carbon-based lithium/potassium ion capacitors are assembled and show excellent electrochemical performance.
KeywordsArc discharge Graphene Heteroatom doping Lithium/potassium ion battery Lithium/potassium ion capacitor
Lithium ion batteries (LIBs) are widely used in various types of portable electronic devices and vehicles, due to their high energy density and long cycle life. Nevertheless, the production costs of LIBs show an increasing trend, due to the limited availability of lithium sources . Recently, potassium ion batteries (PIBs) have emerged as promising candidates for next-generation energy storage systems, owing to the larger potassium reserves in the earth’s crust and oceans and the low negative redox potential of the K+/K couple (− 2.931 V) . Unfortunately, both LIBs and PIBs suffer from low power density and unsatisfactory cycling performance. To overcome these drawbacks, metal ion capacitors, a novel type of capacitor-battery hybrid energy storage systems, have been proposed to achieve high power and energy densities. Such devices are assembled by combining a high-energy battery-type anode and a high-power capacitor-type cathode . During the charging process, cations are inserted in the anode by a Faradaic reaction and anions adsorb on the surface of the cathode by a non-Faradaic reaction; cations and anions return to the electrolyte in the discharging process. Considering the slow kinetics of the intercalation/extraction of metal ions, it is extremely important to explore battery-type anodes with high rate capability, to match that of the capacitive cathode.
Graphite is currently commercialized as an anode material for LIBs. However, its low theoretical specific capacity (372 mAh g−1) and poor rate performance cannot meet the increasingly high energy and power density demands of lithium ion capacitors (LICs) . Graphene, a new type of carbon-based materials, displays remarkable rate capability and improved capacity for LIB applications, due to its large surface area and excellent electronic conductivity . Recently, both theoretical and experimental studies showed that doping with heteroatoms is an efficient method to tailor the electronic structure and enhance the lithium ion storage properties [6, 7]. For instance, nitrogen-doped graphene sheets provide a higher reversible discharge capacity as LIB anode materials than pristine graphene . The incorporation of nitrogen in graphene would further enhance the lithium ion storage and diffusion properties during cycling. Phosphorus, while being in the same VA group as nitrogen, has a higher electron-donating ability and a lower electronegativity. Graphene doped with a small concentration of phosphorus exhibits excellent electrochemical conductivity and electrochemical performance . Moreover, Ju et al. reported that dual doping facilitates insertion and extraction of potassium ions . Thus, dual-doped graphene would be a desirable anode material for LICs, as well as a potential anode material for potassium ion capacitors (PICs).
Heteroatom-doped graphene is usually synthesized through post-thermal treatment and direct synthesis. In the case of post-treatment methods, graphene or graphene oxide has to be obtained first, followed by treatment in NH3 or another atmosphere. The rigorous reaction conditions and toxic reactants greatly limit the practical applications of this approach [11, 12]. Due to the strong π–π interactions, irreversible stacking of graphene invariably occurs in the direct thermal annealing process [13, 14]. Therefore, the direct synthesis may represent a better choice. Currently, chemical vapor deposition is the most common direct synthesis method; unfortunately, high costs and low yields limit its application to produce heteroatom-doped graphene as an electrode material for large-scale energy storage systems. The production of doped graphene through a low-cost and high-yield method remains a challenging task.
Herein, a one-step arc discharge method is employed to obtain dual N, P heteroatom co-doped graphene (NPG) under He and H2 atmosphere, with (NH4)3PO4 as the source of nitrogen and phosphorus. The large-scale NPG nanosheets are composed of 2–6 graphene layers, with P and N concentrations of about 1.3 and 3.2 at.%, respectively. Owing to its remarkable structure and uniform heteroatom doping, NPG displays an extraordinary electrochemical performance as an anode material for LIBs. The enhanced lithium storage properties of NPG are attributed to the synergistic effect of the nitrogen and phosphorus dopants. When employed as anode for PIBs, NPG also shows high capacity, good rate capability, and stable cycling performance. Moreover, full carbon-based LICs and PICs are assembled using the as-prepared NPG as anode and active carbon (AC) as cathode. Both LICs and PICs exhibit a high voltage window, high energy density, and remarkable power density, demonstrating their potential applications. This work introduces a facile arc discharge approach to prepare graphene doped with different heteroatoms, with promising potential as an anode material. In addition, the present work may provide a new route to design high-performance electrode materials for not only Li/K ion batteries or capacitors, but also other energy storage systems.
2.1 Preparation of Nitrogen- and Phosphorus-Doped Graphene Sheets
The NPG material was prepared by a one-step arc discharge method. In a typical procedure, graphite powders were first compressed into a rod, and an opening was drilled in the center of the rod. (NH4)3PO4 powders were then loaded into the cavity of the graphite rod as the raw materials, and the resulting rod was used as the cathode. At the same time, a pure graphite rod served as the anode. The cathode and anode were placed in a vacuum chamber pumped to below 1 Pa. The arc discharge process was carried out in a water-cooled stainless-steel chamber under a steady current of 120 A and a voltage of 15 V. Hydrogen and helium were introduced into the pre-vacuumed chamber as working atmospheres at a volume ratio of 1:3; both gases had 99.99% purity. NPG was obtained after cooling the cavity to room temperature. For comparison, N-doped graphene (NG) was obtained by introducing NH3 into the reactant gas in the absence of (NH4)3PO4, while P-doped graphene (PG) was obtained by introducing the same number of moles of phosphate instead of (NH4)3PO4. Pure graphene was obtained under the same conditions, without (NH4)3PO4. Graphene sheets were exfoliated by arc discharge in a buffer gas mixture of H2 (10 kPa) and He (90 kPa).
2.2 Structure Characterization
X-ray diffraction (XRD) patterns were obtained with a Siemens D500 diffractometer, to investigate the crystal structure of the samples. The surface morphologies and detailed microstructures were analyzed by scanning electron microscopy (SEM, Nova NanoSEM 230) and transmission electron microscopy (TEM, FEI Tecnai G2 F20). The Raman spectra were obtained using a LabRAM HR800 confocal microscopic spectrometer with a laser excitation wavelength of 532 nm. The surface elemental composition and bonding configuration were determined by X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Scientific™). Thermogravimetric analysis (TGA) was carried out with a Netzsch STA 449C thermal analyzer.
2.3 Electrochemical Measurements
The electrochemical performances were tested using CR2032 coin-type cells. The electrode materials were prepared by mixing the active material, Super P conductive additive, and the polyvinylidene fluoride binder at 8:1:1 weight ratio in the N-methylpyrrolidinone solution. The mixtures were mixed evenly, coated onto copper foil substrates and dried at 110 °C for 24 h in a vacuum oven; the cells were then assembled in an argon-filled glove box. The mass loading of the active material was about 1 mg cm−2. The electrolytes used for LIBs and PIBs were 1 M LiPF6 and 1 M KPF6, respectively, in ethylene carbonate/diethyl carbonate (1:1, v/v). Polypropylene film and lithium foil were used as the separator and counter electrode, respectively, for LIBs. A glass microfiber filter film and potassium foil were used as the separator and counter electrode, respectively, for PIBs. Charge/discharge tests were performed in the 0.01–3.0 V voltage window at various current densities, using a Neware battery tester. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements of the electrodes were carried out on a Bio-Liogic VMP3 workstation. The CV measurements were conducted with a sweep rate of 0.1 mV s−1 in the range of 0.01–3.0 V. The frequency range of the EIS tests was set to 0.01–100 kHz, with a 5 mV amplitude.
Before assembling the LICs and PICs, the NPG half-cells were tested by lithiation at 0.02 V followed by delithiation at 3.0 V and 100 mA g−1 over five cycles, with a final lithiation at 1.0 V. LICs and PICs were then assembled with AC as the cathode and activated NPG as the anode in cathode/anode mass ratios of 1:1, 2:1, 4:1, 6:1, and 8:1.
3 Results and Discussion
To investigate the electrochemical performance, CV tests were carried out at a scan rate of 0.1 mV s−1, as shown in Fig. 4a. In the first cycle, the CV curve exhibits a large cathodic peak at about 0.55 V, which disappears during the following cycles. The irreversible peak can be attributed to the formation of a solid electrolyte interphase (SEI) layer and to the irreversible lithium ion storage on defect/edge sites of NPG during the initial cycle. The exact match between the areas of the second to the fourth cycle indicates a highly reversible lithium ion insertion/deintercalation process and a good capacity retention of NPG during the following cycles [26, 27]. Figure 4b shows the galvanostatic discharge–charge profiles of NPG at the current density of 100 mA g−1 in the voltage range from 0.01 to 3 V, measured in the 20th, 50th, 100th, 200th, and 500th cycle. In the first cycle, the NPG electrode delivers a lithiation capacity of 1383 mAh g−1 and a charge capacity 859 mAh g−1 at a current density of 100 mA g−1. The irreversible capacity is due to the SEI formation and to the irreversible lithium storage, in agreement with the CV results. For comparison, NG shows initial discharge and charge capacities of 982 and 636 mAh g−1, respectively, while the values obtained for PG are 913 and 576 mAh g−1, respectively (Fig. S6), which are much lower than those of NPG. This suggests that co-doping with heteroatoms could provide a higher number of lithium ion storage sites and an improved capacity. In addition, the profiles largely maintain their shape with increasing number of discharge–charge cycles, demonstrating a good structural stability. Figure 4c displays the lithiation/delithiation curves of NPG under different current densities, ranging from 100 to 2000 mA g−1. A reversible discharge capacity of 758 mAh g−1 is obtained even at a high current density of 2000 mA g−1, denoting a high rate performance. A potential plateau is observed at under 0.5 V, which can enhance the working voltage of the full cell. Figure 4d displays the rate performances of G, NG, PG, and NPG at various current densities. NPG shows reversible capacities of 889 and 758 mAh g−1 at current densities of 100 and 2000 mA g−1, respectively, which is higher than those of undoped G, NG, and PG, respectively. Owing to the similar morphology and structure of NPG, G, NG, and PG, the enhancement of the reversible electrochemical performance observed for NPG should be attributed to the N and P doping. Furthermore, all samples exhibit a small capacity loss at each current density (100, 200, 300, 500, 1000, and 2000 mA g−1) and a nearly 100% recovery of discharge and charge capacities when the rate is restored to 100 mA g−1, demonstrating a remarkable rate capability. Moreover, the stable capacity of NPG obtained at each current rate is also higher than that of the G, NG, and PG samples. The enhanced capacity of NPG is ascribed to the synergistic effect of phosphorus and nitrogen co-doping in graphene. The capacity enhancement is closely related to the doping content of nitrogen or phosphorus. As shown in Fig. S7, the NPG sample displays not only an enhanced capacity upon doping with nitrogen and phosphorus, but also a further increase in capacity due to the synergistic effect of N and P co-doping. Figure 4e shows the cycling performance and corresponding coulombic efficiency of NPG at a current density of 1000 mA g−1. A reversible capacity of 798 mAh g−1 is achieved in the initial cycles. After 1000 cycles, NPG retains a capacity of 787 mAh g−1, corresponding to a retention of 98.6% and a fading of 0.0014% per cycle. Moreover, NPG displays a stable coulombic efficiency of ~ 100% during the whole cycling process, with the exception of the initial cycles. For comparison, Fig. S6 shows that the discharge capacities of G, PG, and NG are about 280, 570, and 627 mAh g−1, respectively, at the same current density, confirming that heteroatom doping can improve the capacity of graphene. The AC impedance of NPG before and after cycling is shown in Fig. 4f. After cycling, the charge-transfer resistance of NPG decreases from 140.7 to 58.3 Ω (Table S2), due to the full penetration of the electrolyte and the activation of active materials, which promote fast lithium ion diffusion and efficient charge transfer at the electrolyte/electrode interface. Moreover, the comparison of the electrochemical performance of various N-doped, P-doped, and N, P co-doped carbon anodes in Table S3 highlights the competitive electrochemical performance of the as-prepared NPG.
The high reversible capacity, rate performance, and cycling ability of NPG as the anode material for both LIBs and PIBs can be attributed to the intrinsic properties of graphene and the synergistic effect of N and P doping. First, the arc discharge method could produce high-quality and few-layer graphene nanosheets. The small thickness could provide shorter diffusion paths for lithium/potassium ions, and the large surface area enhances the contact between active materials and electrolyte. Second, doping with nitrogen atoms introduces active sites in graphene, providing additional storage sites for lithium/potassium ions [30, 31]. At the same time, nitrogen doping would enhance the electronic conductivity, leading to a high rate capability. Third, doping phosphorus atoms with large atomic radius can suppress the agglomeration of graphene [16, 32, 33]. The presence of phosphorus leads to the adsorption of lithium/potassium ions . Therefore, NPG displays outstanding electrochemical performance.
An anode material based on nitrogen and phosphorus dual-doped graphene has been successfully designed and synthesized via a one-step arc discharge method. NPG can be employed as a universal anode material for lithium ion and potassium ion batteries. As anode for LIBs, NPG displays a high specific capacity of 889 mAh g−1 and outstanding cyclability over 1000 cycles at the current density of 1000 mA g−1. Moreover, NPG also shows remarkable electrochemical performance as the anode material for PIBs. A capacity of 194 mAh g−1 is obtained at the current density of 1000 mA g−1. Even at a high current density of 500 mA g−1, NPG exhibits a high reversible capacity along with a stable cycling performance after 500 cycles. In addition, full carbon-based NPG‖LiPF6‖AC LICs and NPG‖KPF6‖AC PICs show capacities of 98 and 56 mAh g−1 at 1 A g−1, respectively. Maximum energy densities of 195 and 104.4 Wh kg−1 and power densities of 14,983.7 and 14,976 W kg−1 can be achieved for the LICs and PICs, respectively, demonstrating the potential applications of NPG anodes. Thus, the arc discharge method is an effective way to prepare graphene doped with various heteroatoms, which can be used as promising electrode materials for high-performance energy storage systems. Moreover, multiple doping represents a smart option to improve the electrochemical performance of electrode materials, including not only graphene, but also other carbon-based materials.
Mrs. Luan thanks Mr. Hao Wu for the graphic design. This work was supported by National Natural Science Foundation of China (Nos. 51672056 and 51702063), Natural Science Foundation of Heilongjiang (LC2018004), China Postdoctoral Science Foundation (2018M630340), and the Fundamental Research Funds for the Central University (HEUCFD201732).
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