Self-Supported Carbon Nanofiber Films with High-Level Nitrogen and

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Self-Supported Carbon Nanofiber Films with HighLevel Nitrogen and Phosphorus Co-Doping for Advanced Lithium-Ion and Sodium-Ion Capacitors Cheng Yang, Mengyan Zhang, Nizao Kong, Jinle Lan, Yunhua Yu, and Xiaoping Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00300 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 5, 2019

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ACS Sustainable Chemistry & Engineering

Self-Supported Carbon Nanofiber Films with High-Level Nitrogen and Phosphorus Co-Doping for Advanced LithiumIon and Sodium-Ion Capacitors

Cheng Yang,† Mengyan Zhang,† Nizao Kong,† Jinle Lan,*,† Yunhua Yu,*,† and Xiaoping Yang†

†State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, North Third Ring Road 15, Chaoyang District, Beijing, China

*J.L. e-mall: [email protected] *Y.Y. e-mall: [email protected]

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ABSTRACT: It is a challenge to achieve the high power/energy densities of portable and flexible energy storage devices. Here, an advanced lithium-ion capacitor (LIC) and sodium-ion capacitor (SIC) were constructed using activated carbon cathodes and self-supported N/P dual-doped carbon nanofiber film (NP-CNF) anodes. The NP-CNF was fabricated through electrospinning assisted by preoxidization and carbonization. The dual-N source (melamine polyphosphate and PAN) and P source (melamine polyphosphate) doping enhanced the specific capacity of the NP-CNF by increasing its defects. Additionally, the N/P co-doping increased the interlayer distance of the NP-CNF to improve its rate performance. Moreover, the NP-CNF remained a stable self-supported structure. The N and P contents of the NP-CNF were optimized to be 12.8 at% and 4.1 at%, respectively. The optimal LIC displayed an energy density of 85.0 Wh kg-1, a power density of 20.0 kW kg-1 at 22.2 Wh kg-1, and a capacity retention of 80.7 % after 10000 cycles at 2.0 A g-1. The optimal SIC displayed an energy density of 95.6 Wh kg-1, a power density of 20.0 kW kg-1 at 10.6 Wh kg-1, and a capacity retention of 77.8 % after 10000 cycles at 2.0 A g-1.

KEYWORDS: Carbon nanofiber, Nitrogen, Phosphorus, Lithium-ion capacitor, Sodium-ion capacitor

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 Introduction Lightweight and flexible energy storage devices with high power/energy densities have been developed to address the urgent demand for portable electronics.1,2 Hybrid ion capacitors (HICs) have the potential to integrate the large power density (PD) of capacitors and the large energy density (ED) of batteries.3,4 Thus, intensive researches have been made to design the HICs with high PD and ED.57

According to the equations of PD = ED/t and ED = ∫ V(t)·Idt/m, enhancing the voltage window and

rate performance of the HIC is an effective strategy to achieve the high ED and PD of the HIC. HICs exhibit two energy storage mechanisms: slow deintercalation/intercalation of cations in battery-type anode and fast surface desorption/adsorption of anions in capacitor-type cathode. Therefore, the kinetic discrepancy between the adsorption and insertion electrodes is a serious issue for HICs. Designing high-rate anodes is highly desired to overcome this issue. Carbonaceous anodes of HICs show the overwhelming advantages as follows: (1) carbonaceous anodes with high electrical conductivity exhibit high rate performance, which can reduce the kinetic discrepancy between the adsorption and insertion electrodes to enhance the PD of HICs; and (2) the low Na redox potential (∼0.1 V vs. Na/Na+) and Li redox potential (∼0.01 V vs. Li/Li+) of carbonaceous anodes can enhance the voltage window of HICs to increase the ED.8,9 However, the specific capacity and rate performance of conventional carbonaceous anodes are difficult to satisfy the development of the HICs with high ED and PD. Heteroatom (nitrogen,9-12 phosphorus,13,14 and boron9) doping is a traditional method to improve the Li+ or Na+ storage capacity and rate performance of carbons by fast inducing the pseudocapacitive reactions. Moreover, heteroatom doping of the carbon anode can reduce the kinetic discrepancy between the adsorption and insertion electrodes of HICs to enhance the ED and PD of HICs. For instance, the capacity of carbon nanospheres with high N content for lithium-ion capacitors (LICs) was 638 mAh g-1 at 2.0 A g-1.12 The capacity of N/B dual-doped 3D porous carbon nanofibers for LICs was 3



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375 mAh g-1 at 5.0 A g-1.9 The capacity of N/P dual-doped porous graphene for lithium-ion batteries (LIBs) was 450 mAh g-1 at 5.0 A g-1.13 The capacity of N/P dual-doped carbon microspheres for sodium-ion batteries (SIBs) was 280 mAh g-1 at 0.1 A g-1.14 It is generally considered that N incorporates into the edge or basal plane of the graphitic framework, but P with larger atomic size only incorporates into the edge plane.15 N/P co-doping can create the active sites and defects of carbons to enhance the capacity, and enhance the interlayer distances of carbons for fast Li+ or Na+ diffusion to enhance the rate performance.10,11 Enhancing the N or P content of carbons can improve the capacity and rate performance of carbons.16-22 Previous studies presented that the N content of N-doped carbons was over 10.0 at% and the P content of P-doped carbons was over 3.0 at%.20-22 However, the N and P contents of N/P dual-doped carbons were hardly over 10.0 at% and 3.0 at%, respectively. Therefore, designing high-level N/P dual-doped carbonaceous anodes for both Li+ and Na+ storage is still a big challenge. In this work, an advanced lithium-ion capacitor and sodium-ion capacitor (SIC) were fabricated with self-supported N and P dual-doped carbon nanofiber film (NP-CNF) anodes and activated carbon cathodes. The NP-CNF was fabricated via an electrospinning method combined by pre-oxidization and carbonization. Melamine polyphosphate was used as a N and P dual-doped precursor due to its high N content (37.5 at%) and P content (13.8 at%). The dual-N sources and P source achieved the high N content (12.8 at%) and P content (4.1 at%) of the NP-CNF. The N and P contents of the NPCNFs were optimized to achieve the high rate capability and specific capacity. The N and P co-doping creates the defects of the NP-CNFs to improve the storage capacity, resulting in the increased energy densities of the LIC and SIC. Furthermore, the N/P co-doping increases the interlayer distances of the NP-CNFs for fast Li+ or Na+ diffusion to improve the rate capability, resulting in the increased power 4


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densities of the LIC and SIC. Additionally, the stable self-supported structure of the NP-CNFs maintains the cycle stabilities of the LIC and SIC.  Experimental Section Materials Fabrication. As shown in Figure 1, first, a solution for electrospinning was fabricated through mixing 1.5 g PAN (Petro China Jilin Petrochemical Co.), different amounts of melamine polyphosphate (C3H9N6O4P, Adamas Reagent Co.), and 15 mL N, N-dimethylformamide (Beijing Chemical Co.) under magnetic stirring at 60 °C for 10 h. Second, electrospinning happened at a flow rate of 1.0 mL h-1 and a voltage of 20 kV. The distance between the fiber collector and the needle was 12 cm. An aluminum foil was used to collect the nanofiber film. Third, the nanofiber film was preoxidized at 250 °C for 1 h with a rate of 2 °C min-1, and then carbonized in N2 at 800 °C for 1 h with a rate of 2 °C min-1 to obtain NP-CNFs. The NP-CNFs with different mass ratios of the melamine polyphosphate and the PAN (1:6, 1:4, and 1:2) were named as NP-CNFs-6, NP-CNFs-4, and NPCNFs-2. For comparison, CNFs were prepared without melamine polyphosphate in the same way. Materials Characterization. The structures of the CNFs and NP-CNFs were performed by Tecnai G2 F30 S-TWIN high-resolution transmission electron microscopy and Supra55 Carl Zeiss field emission scanning electron microscopy. A Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm) was used to perform the phase structures of the CNFs and NP-CNFs. A Raman system (S. A. S. Co.) with a 514.5 nm laser was used to perform the Raman spectra of the CNFs and NP-CNFs. The specific surface areas and pore structures of the CNFs and NP-CNFs were performed by a Micromeritics ASAP 2460 analyzer. The compositions of the CNFs and NP-CNFs were performed using Thermo Fisher Scientific X-ray photoelectron spectroscopy. Electrochemical Measurements. The electrochemical properties of the CNFs and NP-CNFs were 5



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performed using 2025-type coin cells. The CNFs and NP-CNFs were cut into round slices (12 mm in diameter) and used as self-supported anodes. In a glove box filled with Ar, the half-cells were assembled by using the CNFs and NP-CNFs anodes, polypropylene membranes, Li or Na foils, and a 1 M LiPF6 in dimethyl carbonate/ethylene carbonate (1:1 by volume) or a 1 M NaClO4 in dimethyl carbonate/ethylene carbonate (1:1 by volume). The LIC and SIC were constructed using the AC cathodes and the NP-CNFs-4 anodes. The mass ratios of the NP-CNFs-4 and AC in the LIC and SIC are about 1:4 and 1:3, respectively. A LAND CT2001A system was used to measure the galvanostatic charge-discharge processes. An Autolab PGSTAT 302N system was used to measure the electrochemical impedance spectroscopy and cyclic voltammetry curves.  Results and Discussion Anode Materials. The structure features of the CNFs and NP-CNFs were elucidated using SEM and HR-TEM. Figure 2a-d show that the surfaces of the CNFs and NP-CNFs are smooth, revealing few pores of all the samples. The average diameters of CNFs, NP-CNFs-6, NP-CNFs-4, and NP-CNFs2 are about 180, 170, 160, and 110 nm, respectively. The average diameter of the CNFs and NP-CNFs decreases from about 180 nm to 110 nm with the increase of the content of melamine polyphosphate. The molecular weight of melamine polyphosphate is much smaller than that of PAN. The average molecular weight of the electrospinning solution mixed with melamine polyphosphate and PAN decreases with the increase of the content of melamine polyphosphate, resulting in the decreased average diameter of the NP-CNFs. The EDS elemental mapping of the NP-CNFs-4 (Figure S1) demonstrates the uniform N and P distributions of the NP-CNFs-4. The HR-TEM images of the NPCNFs-4 (Figure 2e and f) present its amorphous structure. The smooth surface and few pores of the NP-CNFs-4 were also observed. The crystalline structures of the CNFs and NP-CNFs were analyzed by XRD (Figure 3a). The (002) 6


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peaks of CNFs, NP-CNFs-6, NP-CNFs-4, and NP-CNFs-2 are located at 24.70°, 24.46°, 24.36°, and 24.30°, respectively. According to the Bragg's formula, the values of d002-spacing were calculated to be 0.360, 0.364, 0.365 and 0.366 nm for CNFs, NP-CNFs-6, NP-CNFs-4, and NP-CNFs-2, respectively. The increased d002-spacing caused by the nitrogen or phosphorus doping is beneficial for fast Li+ or Na+ intercalation/deintercalation.10,11 The weak (002) peaks indicate that the CNFs and NPCNFs are amorphous carbons caused by the nitrogen or phosphorus doping.23-25 As shown in Figure 3b, the G bands (~1580 cm-1) result from the vibration of sp2-C in a 2D hexagonal lattice and the D bands (~1340 cm-1) result from the vibration of sp3-C.26,27 The ID/IG ratio is used to analyze the degree of defects or disorder of the CNFs and NP-CNFs. The ID/IG ratios of CNFs, NP-CNFs-6, NP-CNFs-4, and NP-CNFs-2 are 1.043, 1.063, 1.101, and 1.120, respectively, revealing the numerous defects of the CNFs and NP-CNFs caused by the nitrogen or phosphorus doping. The BET N2 adsorption/desorption isotherms of the CNFs and NP-CNFs (Figure 3c) show II-type isotherms, revealing the low porosities of the CNFs and NP-CNFs. The BET specific surface areas of CNFs, NPCNFs-6, NP-CNFs-4, and NP-CNFs-2 are 6.1, 8.8, 11.8, and 16.4 m2 g-1, respectively. Figure 3d shows the pore size distributions of the CNFs and NP-CNFs, demonstrating almost no micro/mesopores. The pre-oxidation process and the slow heating rates of the pre-oxidation and carbonization processes can reduce the decomposition of the samples, resulting in the low porosities and specific surface areas of the CNFs and NP-CNFs. The N and P of the CNFs and NP-CNFs were further analyzed by XPS. The XPS survey spectra of the NP-CNFs (Figure 4a) show two weak peaks at 133.0 eV and 190.0 eV, a predominant peak at 285.0 eV, a weak peak at 400.0 eV, and a weak peak at 531.0 eV, corresponding to P, C, N, and O, respectively. The P 2p spectra of the NP-CNFs (Figure 4b) display the peaks at 134.3, 133.4, and 132.5 7



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eV, which are attributed to the O=P (P3), O-P (P2), and P-C (P1), respectively.28,29 The N 1s spectra of the CNFs and NP-CNFs (Figure 4c) show the peaks at 401.7 eV, 400.4 eV, 399.6, and 398.4, which are ascribed to oxides of nitrogen (N4), quaternary nitrogen (N3), pyrrolic nitrogen (N2), and pyridinic nitrogen (N1), respectively.28,29 Table 1 shows the chemical compositions of the CNFs and NP-CNFs from XPS data. The high N and P contents of the NP-CNFs can be attributed to the pre-oxidation process and the slow heating rates of the pre-oxidation and carbonization processes, which can reduce the decomposition of the NP-CNFs. The N, P and O contents of the NP-CNFs increase with the increase of the content of melamine polyphosphate. The nitrogen from melamine polyphosphate can incorporate into the edge or basal plane of the graphitic framework, resulting in the enhanced nitrogen contents of the NP-CNFs. The phosphorus with larger atomic size can only incorporate into the edge plane, resulting in the enhanced phosphorus contents of the NP-CNFs. Meanwhile, the phosphorus doping can enhance the oxygen contents of the NP-CNFs due to the increased amount of O-P and O=P bonds. The increased N and P contents of the NP-CNFs can enhance their active sites and defects to improve their capacities and increase their interlayer distances for fast Li+ or Na+ diffusion to enhance their rate performances.10,11 However, the increased oxygen contents of the NP-CNFs decrease their active sites and electrical conductivities, resulting in their decreased specific capacities and rate performances. Therefore, it is necessary to optimize the N, P and O contents of the NP-CNFs. The CNFs and NP-CNFs were used as self-supported electrodes and assembled into half-cells to evaluate the Li+ and Na+ storage performances. Figure 5a shows the CV curves of the NP-CNFs-4 electrode in the LIB at 0.1 mV s-1. During the 1st cycle, a cathodic peak around 0.70 V is ascribed to the formation of SEI films. A cathodic peak around 0.02 V and anodic peak around 0.12 V are ascribed to the intercalation and deintercalation of Li+, respectively. An anodic peak around 1.10 V corresponds 8


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to the Li+ desorption from the defects created by the P doping.30-32 The existence of the anodic peak around 1.10 V enhances the reversible capacity obviously.33 The CV curves of the CNFs electrode in the LIB (Figure S2) without the anodic peak around 1.10 V also indicate the P doping in the NP-CNFs4. The Li+ deintercalation/intercalation processes of the CNFs and NP-CNFs electrodes in LIBs were further investigated by the GCD processes. The GCD curves of the CNFs and NP-CNFs electrodes in LIBs (Figure 5b) show discharge plateau around 0.70 V and charge plateau around 1.10 V. The Coulombic efficiencies of the CNFs and NP-CNFs electrodes in LIBs are nearly 70% during the 1st cycle. The initial irreversible capacities of the CNFs and NP-CNFs electrodes are ascribed to the formation of SEI films, the decomposition of electrolytes, and the irreversible insertion of Li+. The GCD curves of the NP-CNFs-4 electrode in the LIB (Figure S3a) are almost overlapped after 5 cycles, revealing the high cycling reversibility of the NP-CNFs-4 electrode. The increased interlayer distance of the NP-CNFs-4 can improve the ion transfer in the NP-CNFs-4, leading to its enhanced cycle reversibility. Rate performances of the CNFs and NP-CNFs electrodes in LIBs are presented in Figure 5c. The specific capacity of NP-CNFs-4 is the highest. The N/P co-doping can enhance the active sites of the NP-CNFs, which can enhance the absorption of Li+ to enhance the specific capacity. However, the oxygen doping can decrease the active sites, resulting in the decreased specific capacity of the NPCNFs electrode. The oxygen content of the NP-CNFs increases with the increase of the N and P contents (Table 1). The specific capacity of the NP-CNFs-2 is lower than that of the NP-CNFs-4 due to the higher oxygen content of the NP-CNFs-2. Therefore, the NP-CNFs-4 electrode with the optimal contents of N, P and O exhibits the highest specific capacity among all the electrodes. The rate performance of the NP-CNFs-4 is the highest. The N/P co-doping can enhance the interlayer distance 9



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of the NP-CNFs, which can provide numerous big channels to improve their rate performance. According to the XRD and XPS results, the interlayer distance and the oxygen content of the NP-CNFs increase with the increase of the N and P contents. The interlayer distance of the NP-CNFs-4 (0.365 nm) is similar to that of the NP-CNFs-2 (0.366 nm), but the higher oxygen content of the NP-CNFs-2 decreases the electrical conductivity, leading to the decreased rate capability. Therefore, the NP-CNFs4 electrode with the optimal contents of N, P and O displays the highest rate performance among all the electrodes. It exhibits specific capacities of 507.3, 451.9, 365.3, 312.0, 218.1, 177.4, and 124.7 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 A g-1 in the LIB, respectively. The reason for the high rate performance of the NP-CNFs-4 electrode in the LIB was further verified by the EIS measurements. As shown in Figure 5d, the small diameter of the semicircle of the NP-CNFs-4 electrode reveals the low charge-transfer resistance, demonstrating the fast charge transfer at the electrolyte/electrode interface because of the N/P co-doping. The large slope of the inclined line of the NP-CNFs-4 electrode presents its low Li+ diffusion resistance, resulting from the increased interlayer distance of the NPCNFs-4. The low Li+ diffusion resistance and charge-transfer resistance can enhance the rate performance of the NP-CNFs-4. The CV curves of the NP-CNFs-4 electrode in the SIB are shown in Figure 6a. A cathodic peak around 0.70 V during the 1st cycle is ascribed to the formation of SEI films. A cathodic peak around 0.10 V and anodic peak around 0.20 V are ascribed to the intercalation and deintercalation of Na+, respectively. The GCD curves of the CNFs and NP-CNFs electrodes in SIBs (Figure 6b) show that their Coulombic efficiencies are nearly 50% during the 1st cycle. The initial irreversible capacities of the CNFs and NP-CNFs electrodes result from the formation of SEI films, the decomposition of electrolytes, and the irreversible insertion of Na+. As shown in Figure S3b, the nearly overlapped GCD 10


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curves of the NP-CNFs-4 electrode in the SIB during the following cycles demonstrate the high cycling reversibility of the NP-CNFs-4 electrode. This can be attributed to the increased interlayer distance of the NP-CNFs-4 caused by the N/P co-doping. The rate performances of the CNFs and NP-CNFs electrodes in SIBs are shown in Figure 6c. The Li+ storage capacities of the CNFs and NP-CNFs electrodes are higher than the Na+ storage capacities because the ionic radius of Li+ (0.76 Å) is smaller than that of Na+ (1.02 Å). The large ionic radius of Na+ limits the intercalation of Na+ into the carbon interlayer. After cycling, the specific capacities of the NP-CNFs at 0.1 A g-1 increase because a number of active sites created by the N/P co-doping were activated upon charge and discharge. This phenomenon is observed difficultly in the Li-ion storage system because Li+ with smaller ionic radius compared to Na+ can fast activate the active sites created by the N/P co-doping. The NP-CNFs-4 electrode displays the highest rate performance among all the electrodes due to its optimal N, P and O contents. The specific capacities of the NP-CNFs-4 electrode in the SIB are 260.3, 210.9, 195.5, 159.2, 133.0, and 81.4 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g-1, respectively. The N/P co-doping of the NP-CNFs-4 not only increases its specific capacity by creating its defects, but also enhances its rate performance by increasing its interlayer distance.10,11 The oxygen contents of the NP-CNFs increase with the increase of the N and P contents. The rate performance of the NP-CNFs-2 is lower than that of the NP-CNFs-4 because the high oxygen content of the NP-CNFs-2 decreases its electrical conductivity. The Nyquist plot of the NP-CNFs-4 electrode (Figure 6d) displays a small semicircle, revealing the low charge-transfer resistance because the N/P co-doping is beneficial for the fast charge transfer at the electrolyte/electrode interface. The Nyquist plot of the NP-CNFs-4 electrode exhibits a large slope of the inclined line, demonstrating the fast Na+ diffusion because of the increased interlayer distance of the NP-CNFs-4. The fast Na+ diffusion and 11



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charge transfer of the NP-CNFs-4 electrode are favorable for improving its rate performance. The optical photographs of the NP-CNFs-4 electrode in the LIB and NP-CNFs-4 electrode in the SIB after cycling (Figure S4a,f) reveal the stable self-supported structure of the NP-CNFs-4 electrode. The structure stabilities of the CNFs and NP-CNFs electrodes in LIBs and SIBs after cycling were further investigated by SEM. As shown in Figure S4, all the electrodes show the integrate network structures constructed from the carbon nanofibers, indicating the good structure stabilities of the CNFs and NP-CNFs electrodes. The pre-oxidation process can reduce the decomposition, specific surface areas and porosities of the CNFs and NP-CNFs, resulting in the stable self-supported structure of the CNFs and NP-CNFs. Li-Ion and Na-Ion Capacitors. Activated carbon was used as a capacitive-type cathode material for LICs and SICs because of its large specific surface area and high electrical conductivity. The AC cathodes were assembled into half-cells to evaluate the Li+ and Na+ storage performances. The GCD curves and rate performances of the AC cathodes in the LIB and SIB (Figure S5a,b and Figure S6a,b) show the high capacitances and rate performances of the AC cathodes in the LIB (123.5 F g-1 at 0.1 A g-1 and 20.0 F g-1 at 10.0 A g-1) and SIB (113.4 F g-1 at 0.05 A g-1 and 35.1 F g-1 at 10.0 A g-1). The LIC and SIC were constructed using the AC cathodes and the NP-CNFs-4 anodes. The CV curves of the AC/NP-CNFs-4 LIC (Figure 7a) show a little distortion from triangular shape due to the deintercalation/intercalation of Li+ in battery-type anode and the surface desorption/adsorption of PF6in capacitor-type cathode.8 Meanwhile, the dual energy storage mechanisms of the AC/NP-CNFs-4 LIC result in the non-linear slopes of the GCD curves (Figure 7b).8 The PD (W kg-1) and ED (Wh kg1)

of the LIC and SIC were calculated according to the equations as follows: PD = 3600ED/t

(1) 12


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ED = ∫ V(t)·Idt/3.6m

(2)

where I (A) is the discharge current, V(t) (V) is the discharge voltage excluding the IR drop, t (s) is the discharge time, and m (g) is the total mass of the NP-CNFs-4 and AC. Figure 7c shows that the ED and PD of the AC/NP-CNFs-4 LIC are as high as 85.0 Wh kg-1 and 20.0 kW kg-1 (achieved at 22.2 Wh kg-1), respectively. The AC/NP-CNFs-4 LIC still displays high ED and PD compared with other previous LICs including BNC/BNC LIC (82.2 Wh kg-1 at 17.8 kW kg-1),9 ANCS/ANCS LIC (91.2 Wh kg-1 at 17.8 kW kg-1),12 AC/NixFeyOz@rGO LIC (27.0 Wh kg-1 at 2.5 kW kg-1),34 AC/HTPC LIC (31.3 Wh kg-1 at 0.2 kW kg-1),35 AC/Nb2O5-Graphene LIC (15.0 Wh kg-1 at 18.0 kW kg-1),36 C/SnO2-C LIC (45.0 Wh kg-1 at 3.0 kW kg-1),37 AC/TiO2-rGO LIC (8.9 Wh kg-1 at 8.0 kW kg-1),38 Graphene/LTOGraphene LIC (32.0 Wh kg-1 at 3.0 kW kg-1),39 AC/V2O5-CNT LIC (6.9 Wh kg-1 at 6.3 kW kg-1),40 and AC/MnO-PC LIC (35.0 Wh kg-1 at 2.6 kW kg-1).41 After 10000 cycles at 2.0 A g-1, the AC/NPCNFs-4 LIC show a reasonable retention of 80.7 % (Figure 7d), demonstrating the superior cycle stability of the AC/NP-CNFs-4 LIC. As shown in Figure 7e, a single coin-type AC/NP-CNFs-4 LIC can lighten three blue light-emitting diodes (LEDs, 3.0-3.2 V), suggesting its high voltage output. The non-rectangular CV curves of the AC/NP-CNFs-4 SIC (Figure 8a) result from the surface desorption/adsorption of ClO4- in capacitor-type cathode and the deintercalation/intercalation of Na+ in battery-type anode.8 The GCDs curves of the AC/NP-CNFs-4 SIC (Figure 8b) display non-linear slopes caused by the dual energy storage mechanisms.8 The ED of 95.6 Wh kg-1 and PD of 20.0 kW kg-1 (achieved at 10.6 Wh kg-1) are achieved in the AC/NP-CNFs-4 SIC (Figure 8c). In addition, the AC/NP-CNFs-4 SIC exhibits comparable ED and PD when not only compared with other previous SICs such as AC/V2O5-CNT SIC (7.5 Wh kg-1 at 5.0 kW kg-1),42 AC/TiO2 mesocages@rGO SIC (25.8 Wh kg-1 at 1.4 kW kg-1),43 AC/NiCo2O4 SIC (13.8 Wh kg-1 at 0.3 kW kg-1),44 AC/Nb2O5@C-rGO SIC 13



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(6.0 Wh kg-1 at 20.0 kW kg-1),45 rGO film/Na2Ti3O7 SIC (21.7 Wh kg-1 at 3.0 kW kg-1),46 but also compared with other previous LICs such as AC/NixFeyOz@rGO LIC (27.0 Wh kg-1 at 2.5 kW kg-1),34 AC/TiO2-rGO LIC (8.9 Wh kg-1 at 8.0 kW kg-1),38 AC/Nb2O5-Graphene LIC (15.0 Wh kg-1 at 18.0 kW kg-1),36 AC/V2O5-CNT LIC (6.9 Wh kg-1 at 6.3 kW kg-1),40 and MWCNT/TiO2-B nanotubes LIC (19.3 Wh kg-1 at 2.5 kW kg-1).47 After 10000 cycles at 2.0 A g-1, the AC/NP-CNFs-4 SIC exhibits exceptional cycle stability with a retention of 77.8 % (Figure 8d). As shown in Figure 8e, a “C❤Y” pattern formed by 20 yellow LEDs (1.8-2.0 V) can be lightened by two coin-type AC/NP-CNFs-4 SICs, demonstrating the great feasibility for practical application.  Conclusions In summary, self-supported N and P dual-doped carbon nanofiber film (NP-CNF) was successfully fabricated using melamine polyphosphate and PAN. A high-performance LIC and SIC were constructed using the activated carbon cathodes and the NP-CNFs-4 anodes. The optimal N and P contents of the NP-CNFs are 12.8 at% and 4.1 at%, respectively. The maximum ED of 85.0 Wh kg-1 and 95.6 Wh kg-1 can be achieved in the LIC and SIC, respectively. The ED of 22.2 Wh kg-1 and 10.6 Wh kg-1 can be maintained at a high PD of 20.0 kW kg-1 in the LIC and SIC, respectively. Moreover, the capacitance retentions of the LIC and SIC sill remain 80.7 % and 77.8 % after 10000 cycles at 2.0 A g-1, respectively. The high ED, high PD and excellent cycle stability of the LIC and SIC can be ascribed to the reasons as follows: (1) the defects created by the N/P co-doping enhance the storage capacity of the NP-CNFs-4, which improves the energy densities of the LIC and SIC; (2) the N/P codoping increases the interlayer distance of the NP-CNFs-4, which enhances the power densities of the LIC and SIC; and (3) the stable self-supported structure of the NP-CNFs-4 retains the cycle stabilities of the LIC and SIC. This work offers a promising self-supported anode for advanced LICs and SICs. 14


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


ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM and elemental mapping images of NP-CNFs-4, and GCD curves and rate performances of the activated carbon cathodes in the LIB and SIB (PDF) 

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This study was supported by the National Natural Science Foundation of China (51072013, 51402010, 51272021, and 51772016).

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Figure 1. The schematic diagram of the process to obtain the NP-CNFs.

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Figure 2. SEM images of (a) CNFs, (b) NP-CNFs-6, (c) NP-CNFs-4, and (d) NP-CNFs-2. (e) and (f) HR-TEM images of NP-CNFs-4.

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Figure 3. (a) XRD patterns, (b) Raman spectra, (c) nitrogen adsorption-desorption isotherms, and (b) pore size distributions of the CNFs and NP-CNFs.

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Figure 4. (a) XPS survey spectra and high-resolution XPS spectra of (b) P 2p and (c) N 1s of the CNFs and NP-CNFs.

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1 2 3 4 Table 1. Chemical compositions of the CNFs and NP-CNFs from XPS data and specific discharge 5 6 capacities of the CNFs and NP-CNFs electrodes at 0.5 A g-1 between 0.005 and 3.0 V. 7 8 9 N/ P/ O/ N1/% N2/% N3/% N4/% P1/% P2/% P3/% CLi/ CNa/ 10 Sample 11 at% at% at% mAh g-1 mAh g-1 12 13 CNFs 11.6 0 4.2 33.6 21.0 21.9 23.5 — — — 271.0 91.0 14 12.3 1.5 7.2 40.0 22.8 27.5 9.7 29.9 34.2 35.9 302.7 147.6 15 NP-CNFs-6 16 NP-CNFs-4 12.8 4.1 9.2 36.5 27.9 22.8 12.8 34.3 35.3 30.4 365.3 195.5 17 18 NP-CNFs-2 13.7 4.4 11.0 35.4 29.1 22.1 13.4 33.1 37.4 29.5 277.7 109.0 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 27



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Figure 5. (a) CV curves of the NP-CNFs-4 electrode in the LIB at 0.1 mV s-1, (b) GCD curves of the CNFs and NP-CNFs electrodes in LIBs during the 1st cycle at 0.1 A g-1, (c) discharge capacity versus cycle number plots of the CNFs and NP-CNFs electrodes in LIBs, and (d) Nyquist plots of the CNFs and NP-CNFs electrodes in LIBs after 100 cycles at 1.0 A g-1 in the frequency range 100 kHz to 0.1 Hz. All measurements were conducted in the voltage range of 0.005-3.0 V vs. Li+/Li.

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Figure 6. (a) CV curves of the NP-CNFs-4 electrode in the SIB at 0.1 mV s-1, (b) GCD curves of the CNFs and NP-CNFs electrodes in SIBs during the 1st cycle at 0.1 A g-1, (c) discharge capacity versus cycle number plots of the CNFs and NP-CNFs electrodes in SIBs, and (d) Nyquist plots of the CNFs and NP-CNFs electrodes in SIBs after 100 cycles at 1.0 A g-1 in the frequency range 100 kHz to 0.1 Hz. All measurements were conducted in the voltage range of 0.005-3.0 V vs. Na+/Na.

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Figure 7. (a) CV curves, (b) GCD curves, (c) Ragone plots versus other reported LICs, (d) cycling performance of AC/NP-CNFs-4 LIC at 2.0 A g-1, and (e) three blue LEDs lightened by a single cointype AC/NP-CNFs-4 LIC. All measurements were conducted in the voltage range of 0-4.0 V vs. Li+/Li.

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Figure 8. (a) CV curves, (b) GCD curves, (c) Ragone plots versus other reported SICs and LICs, (d) cycling performance of AC/NP-CNFs-4 SIC at 2.0 A g-1, and (e) “C❤Y” pattern formed by 20 yellow LEDs and lightened by two coin-type AC/NP-CNFs-4 SICs. All measurements were conducted in the voltage range of 0-4.0 V vs. Na+/Na.

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Table of Contents

An advanced lithium-ion capacitor and sodium-ion capacitor were constructed using activated carbon cathodes and self-supported N/P dual-doped carbon nanofiber anodes.

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Self-Supported Carbon Nanofiber Films with High-Level Nitrogen and

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