Self-Nitrogen-Doped Carbon from Plant Waste as an Oxygen

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Energy, Environmental, and Catalysis Applications

Self-Nitrogen-Doped Carbon from Plant Waste as an Oxygen Electrode Material with Exceptional Capacity and Cycling Stability for Lithium Oxygen Batteries Meiling Wang, Ying Yao, Zhenwu Tang, Tuo Zhao, Feng Wu, Yufei Yang, and Qifei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11282 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces 1

Self-Nitrogen-Doped Carbon from Plant Waste as an Oxygen Electrode Material with Exceptional Capacity and Cycling Stability for Lithium Oxygen Batteries

Meiling Wang1, Ying Yao1*, Zhenwu Tang2, Tuo Zhao1, Feng Wu1, Yufei Yang3, Qifei Huang3*

1. Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China 2. MOE Key Laboratory of Regional Energy and Environmental Systems Optimization, Environmental Research Academy, North China Electric Power University, Beijing 102206, China 3. State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing100012, China

KEYWORDS: Self-nitrogen-doped carbon, plant waste, oxygen electrode, lithium oxygen batteries, catalyst _________________________ Corresponding Authors *Dr. Y. Yao. Email: [email protected]. Tel: 86-10-68918766. *Dr. Q. Huang. Email: [email protected]

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ABSTRACT To promote the development of electric automobile, high energy density and high power batteries are urgently needed. More and more attention has been drawn to look for high-performance cathode catalyst for Li-O2 batteries. However, the sluggish kinetic reaction, the stacking of electrical insulation product of Li2O2 and the undesired parasitic reaction restrict their capacity and present poor cycle performance. Here, we prepared nitrogen self-doped activated carbons (N-PIACs) derived from the plant waste (poplar inflorescence) through activation then slow pyrolysis carbonization method, exhibiting several advantages. The materials presented a three-dimensional interconnecting pores structure and high surface area. Besides, defects and functional groups doped by nitrogen as active sites improved electrochemical catalysis activity. The LiǀǀN-PIACs−O2 battery delivered a high specific capacity of 12060 mAh/g, which was 2.3 times that of the pristine plant waste based Li-O2 battery (N-PICs). In addition, it presented more excellent cycle stability than other common carbon materials. In this study, we developed a functional carbon nanomaterial from cheap natural materials, which might become a highly attractive subject, indicating this strategy could strengthen the property of Li-O2 batteries.


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1. INTRUDUCTION： Chinese white poplar (Populus tomentosa Carr.), a kind of deciduous tree, is widespread throughout the whole country in china with high ecological and economic value.1 However, the pollen and inflorescence produced each spring not only seriously affect the quality of environment and harm the traffic safety, but also carry dust, pathogenic microorganisms and even induce various diseases.2-4 This huge amount of waste is an environmental contaminant; therefore, it is in accord with the globally sustainable development paradigm to develop uses for these waste resources. At present, applications of poplar inflorescence have been limited to those with low added value, such as fuels, stock farming fodder, landfilling or paving materials. To date, there are only few studies focused on poplar catkins (female inflorescence of poplar), which have been used in adsorbent for organic and heavy metal pollution treatment and in energy conversion and storage devices.2,4,5 Discovering and synthesizing high-value materials from poplar inflorescence is much more desirable. Poplar inflorescence is a type of string-like inflorescence with the characterization of a single stout axis on which unisexual sessile or subsessile apetalous flowers are gathered in a spiral arrangement.6 In general, the chemical composition of poplar inflorescence contains carbohydrates (10~20 wt%), proteins (10~20 wt%), lipids (10~16 wt%), lignin (5~15 wt%), fibers (5~13 wt%), ashes (5~9 wt%), and moisture (2~6 wt%) based on dried weight.7,8 On the one hand, high content of carbohydrates basically ensures enough carbon sources for building 3D carbon framework. On the other hand, high protein content guarantees that poplar



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inflorescence is a good nitrogen source, and the numbers of active sites can be increased by a certain amount of nitrogen doping. Therefore, poplar inflorescence is a promising biomass precursor to synthesize valuable functionalized carbon materials because it is inexpensive and renewable, available in high quantity, and is an environmentally friendly resource. Lithium-oxygen (Li-O2) batteries have great prospects for development due to its high theoretical energy density, which is more than 10 times than Li-ion batteries. Differing from conventional batteries, it has become one of the most promising power sources for electric vehicles.9 A typical rechargeable non-aqueous Li-O2 battery consists of a Li metal anode (negative electrode), a non-aqueous Li+ conducting electrolyte and a structural cathode (positive electrode).10 In recent years, rapid progress has been made in improving the performance of Li-O2 battery. Yet over-voltage, poor rate capability and poor cyclability are still major hurdles preventing the practical application of such innovative technology.11,12 Theoretically, the combination of lithium ions and reduced oxygen during discharge can form lithium peroxide (Li2O2), it is a prominently decisive factor for a high specific energy of Li-O2 battery, namely oxygen reduction reaction (ORR).13 It suggested that doped-carbon materials can enhance ORR activity in comparison with common carbon materials.19 Li et al. and Zeng et al. have reported nitrogen doped carbon materials could enhance the reaction kinetics of lithium oxygen batteries (i.e., improving energy density, lowering the potential gap and improving the rate performance).14,15 However, an overvoltage is often detected in the opposite charge


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process, or oxygen oxidation reaction (OER), which leads to a poor cycle efficiency. Until now, there is not an effective carbon catalyst for OER, which still has to be identified.10 In recent years, numerous functional carbonaceous materials from biomass have shown significant potential in energy storage fields. For example, ramie fibers and corncobs, rice husks, peanut shell, chicken eggshell, banana peels were used in sodium and lithium ion battery anodes.16-20

There are also some studies for

supercapacitors using seaweed and fungi.21,22 And some researches focus on highly active electrocatalyst for ORR in fuel cells using ginkgo leaves,23 honeysuckles,24 and chitin.25 To date, however, only a few studies have reported the product of a biomass derived carbon cathode catalyst designing for Li-O2 batteries.15,26,27 In this study, we presented an economic and achievable strategy to prepare self-nitrogen doped porous carbon derived from poplar inflorescence with three-dimensional (3D) pores consisting of macro-, meso-, and micro-pores for Li-O2 batteries. The macro-sized voids provided efficient cushion space for O2/Li2O2 conversion and improve O2 diffusion transfer. The micropores provided access for the electrolyte ions to immerse, which was of the essence for high energy storage, while mesopores could facilitate the kinetic process of ion exchange in the electrodes and improve the rate performance at high current densities.28 The porous carbon cathode in this study combined the advantages of high specific surface area, high conductivity, abundant defects and favorable nitrogen species. It exhibited excellent discharge capacity, activity on ORR/OER, superior cycling efficiency and excellent round-trip



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stability. The procedure in this work was facilely conducted using a common and simple process for taking advantage of natural waste resource to get innovative O2 catalytic cathode materials in comparison to the typical metal oxides catalysts or noble metal catalysts for Li-O2 battery applications.

2. EXPERIMENTAL SECTION 2.1 Synthesis of materials Poplar inflorescence was collected in Beijing (Beijing Institute of Technology) in April. After 3 days of natural sunlight drying, bracts on the branches were stored in polyethylene plastic bags. Before use, the inflorescence was soaked with deionized water, dried in a drying oven overnight at 80 °C, and then ground into dregs using pulverizer. The fabrication of self-nitrogen doped poplar inflorescences based activated carbons (labeled as N-PIACs) and self-nitrogen doped poplar inflorescences based nonactivated carbons (labeled as N-PICs) followed the steps outlined below: Firstly, 2 g poplar inflorescence dregs were dispersed in 0.8 g/100 ml KOH solution (the mass ratio of biomass and KOH is 5:2) and stirred for 4 h at 60 °C. Next, the mixture was dried at 80 °C for 24 h, continued by carbonization under nitrogen atmosphere and heating at 700 °C for 2 h with a heating rate of 10 °C min−1 in a tube furnace then kept this temperature 2h to produce desired N-PIACs. N-PICs were obtained by direct carbonization under the same condition with N-PIACs. After the heating stopped and cooled to room temperature, the obtained biochar was removed from the tube furnace, followed by washing several times with DI water until pH≈7.


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Ultimately, the final products were dried in a vacuum oven at 80 °C for 24 h. The obtained N-PIACs and N-PICs were used as active carbon materials on oxygen electrodes. 2.2. Material characterization. SEM images were taken by a JEOL JSM-6400 scanning electron microscope to acquire the surface and structure characteristics of the samples. The surface elemental composition was examined using an energy-dispersive x-ray fluorescence spectroscope (EDS, oxford instruments). XRD was taken on a X-ray diffractometer (Rigaku Ultima IV) with Cu Kɑ radiation (λ =1.5406 Å) to characterize the crystal texture. Raman analysis was implemented using a Raman spectroscopy (Jobin Yvon, HR800) to explore the defects of material. XPS spectra were carried out on a PHI Quantera II system (Ulvac-PHI Inc., Japan) to make sure the elemental composition and valence state of element. BET analysis was measured by a Tristar 3000 surface area and pore size analyzer to ascertain the specific surface area and pore-size distribution. 2.3. O2 cathode preparation and Li−O2 cell assembly. O2 cathode preparation. In the assembling process of O2 electrodes, the mass ratio of active material, polyvinylidene fluoride (PVDF) and acetylene black was 8:1:1 and then well-mixed in N-methyl-2-pyrrolidone (NMP) solvent using mortar. The completely incorporated slurry was coated on a discal carbon paper with 1.1 cm diameter (TGP-H-060 carbon paper, Torray). Later on, the resulting electrode was dried in an oven at 80°C for 12 h to remove the residual NMP solvent.



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Li−O2 cell assembly. The batteries assembling was operated in a glove box (M-Braum, Labstar (1950/780)) filled with pure argon. In the process of assembling Swagelok cell, Separator (GF/D, Whatman) was used to separate oxygen catalytic cathode and Li anode, which dipping with 1 M lithium trifluoromethanesulfonate (LiTFSI) in a tetraethylene glycol dimethyl ether (TEGDME) electrolyte. Afterwards, this LiǀǀseparatorǀǀO2 cathode was sealed with a 0.5 cm2 air tube (corresponding to the effective area of active material) placed on the cathode side to allow the oxygen to flow in and a 0.1 cm2 air hole to maintain gas cycling. Then inflating 12 h using high-purity oxygen (99.99%) after the Li−O2 cell was completely assembled. 2.4 Electrochemical measurements Galvanostatic charge-discharge measurements and controlled capacity at different current densities cycle measurements of these Li-O2 batteries were tested on a battery tester (LAND-CT2001A). METROHM AUT50378 impedance analyzer (PGSTAT204) was used to investigate the electrochemical impedance spectroscopy (EIS). CV measurement was tested on CHI660 electrochemical workstation, using a two-electrode system, the lithium metal sheet was the counter electrode and the reference electrode, and the air electrode was used as the working electrode. All of the results for the current densities and specific capacities were normalized with the loading mass of active materials (the mass was obtained based on the difference between original carbon paper and coated with active material one).

3. RESULTS AND DISCUSSION


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To clarify the morphologies and microstructures of N-PICs and N-PIACs, the samples were imaged by scanning electron microscopy (SEM). Figure 1b and Figure 1c showed a 3D pores structure with size from several hundred nanometers to several micrometers. Meanwhile, it showed nanosheet morphology between adjacent openings. On the contrary, the SEM image (Figure 1a and Figure S1) of the N-PICs (self-nitrogen doped poplar inflorescences based nonactivated carbons) showed a littery blocky morphology of poplar inflorescences without any pores. Thus, KOH activation not only could completely change the structure of the material, but also made the material more uniform.

Figure 1. SEM images of the nonactivated N-PICs (a) and KOH activated N-PIACs (b) and (c) at different magnifications

The N2 adsorption-desorption isotherm and pore size distribution of N-PIACs and N-PICs were determined by BET method and BJH method, as shown in Figure 2. Compared to the N-PICs, N-PIACs had a higher specific surface area and pore volume: 1049 versus 238 m2/g and 0.561 versus 0.142 cm3/g, respectively (Figure 2a). N-PIACs had a typical type I adsorption-desorption isotherm, which was mainly expressed as a microporous structure, indicating of an interconnected pore system



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(Figure 2a). Figure 2b displayed pore size distribution from nitrogen adsorption. It could be obviously observed that macro-pores were ∼50 nm and mescopores were ∼3 nm and ∼10 nm in as-made N-PIACs. The high specific surface area and the large pore volume prepared through our method could speed up electronic and ionic transfer rate and allowed the electrolyte to penetrate sufficiently and rapidly. The crystal structure of the material was characterized by XRD. The broadening peaks in the X-ray diffraction pattern indicated that both N-PIACs and N-PICs were typical disordered glassy carbons with no apparent order (Figure 2c). Both N-PIACs and N-PICs displayed feature at 24° and 43°, which were corresponded to the (002) and (101) lattice planes, respectively. While Comparing the intensity of (002) peaks, it came to a conclusion that the graphitization degrees of N-PIACs were lower than those of N-PICs.29 The defective structure of materials was further characterized using Raman spectroscopy. It could be obviously observed that two remarkable peaks in the Raman spectroscopy of the two samples, the D-band (disordered carbon) lied on 1350 cm−1 corresponding to sp3 hybridized carbon with disordered state while the G-band (graphitic carbon) located at 1591 cm−1 demonstrating the planar vibration of the sp2 carbon atoms. The disorder degree of carbon sample could be quantified by the intensity ratio of D band and G band, ID/IG.30 As we could see, the value of ID/IG (with the peak area of fitting curves) of N-PIACs was higher than that of N-PICs, demonstrating a more defective structure with a lower degree of graphitization. It could be speculated that the activator would corrode the graphitized structure,


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resulting in a more defective texture.

Figure 2. (a) Nitrogen adsorption−desorption isotherms, (b) pore-size distributions, (c) XRD patterns and (d) Raman spectra of N-PIACs and N-PICs. The inset table in (a) is the parameter values obtained from BET measurement.

At the outset, XPS survey spectrum was performed to analyze elemental composition for N-PIACs. Figure 3a showed a major C 1s peak (285 eV), an O 1s peak (533 eV), and an N 1s peak (401 eV). Besides, the EDX mapping elements of carbon, oxygen, and nitrogen of N-PIACs were further used to characterize the element compositions, showing that elements were homogeneously distributed (Figure S2). The content of N element in the sample was calculated to be 19.15 % using atomic percent. In the high-resolution C 1s spectrum of N-PIACs，mainly three



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components were attributed to C=O, C-C, and O-C=O (Figure 3b).31 The high-resolution N 1s spectrum of N-PIACs (Figure 3d) could be fitted into several types of nitrogen-functional groups, pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen, which were typically observed in nitrogen-doped carbons.32 Pyridinic and graphitic nitrogen atoms provided more active sites for oxygen reduction reaction.33 Pyrrolic nitrogen enhanced the adsorption between lithium ions and materials. Hence, the content of pyridinic, pyrrolic, and graphitic nitrogen in the N-PIACs catalyst could improve the electrochemical performance of Li-O2 battery.34

Figure 3. Full-scan XPS (a), and high resolution C 1s, O 1s, N 1s XPS scan (b–d) of N-PIAC. The possible nitrogen positions in the carbon architecture were displayed in


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Figure 4. When nitrogen was doped into the carbon matrix, the nitrogen was electron deficient and acted as electron withdrawing atom since the electronegativity of nitrogen (3.0) was higher than carbon (2.5), which facilitated fast electron transfer.35 Because of its large average adsorption energy, pyrrolic nitrogen could easily adsorb Li ion, thereby accelerating the electrochemical interaction between Li atoms and N-PIACs.36 Besides, ORR occurred at both graphitic nitrogen and pyridine-like N groups.37 This synergistic effect of nitrogen-doping and 3D porous structure provided a super ORR performance with a 2-electron pathway.12

Figure 4. Schematic illustration of the self N-doped functionalized porous carbon.

The initial full discharge/charge profile of Li-O2 battery using N-PICs and N-PIACs as the oxygen electrodes at the same current density of 0.02 mA/cm2 were shown in Figure 5. N-PIACs-based oxygen electrode delivered discharge capacity of 12060 mAh/g with an average voltage of 2.8 V, which was much superior to 5190 mAh/g of N-PICs-based oxygen electrodes with an average voltage of 2.6 V. The initial discharge specific capacity of the N-PIACs oxygen electrode exceeded most of



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common oxygen electrodes, such as commercial KB carbon (5180 mAh/g), the CNFs (7200 mAh/g)38, pyrochlore/carbon catalyst (7200 mAh/g),39 graphene nanosheets (GNSs) (8706 mAh/g)40, and N-GNSs (11660 mAh/g) electrodes.41 CV measurement was further tested to present the electrochemical catalytic activity of N-PIACs and N-PICs (Figure S3). The N-PIACs electrode delivered higher peak current, higher oxygen reduction potential and lower oxygen oxidation potential, indicating an excellent catalytic activity. The high discharge capacity and high catalytic activity voltage in this study could be attributed to 3D porous structure of N-PIACs, which could accelerate the electron-transfer process and facilitate Li+ diffusion. The macrospores, surrounded by a large amount of mesoporous, could provide channels for

O2

diffusion,

contribute

to

electrolyte

permeation

and

Li2O2

formation/decomposition.42 Besides, studies indicated that the nitrogen functional groups in the carbon promoted the electron transfer and enhanced the conductivity of active material on oxygen electrode, further improved electrode properties including capacity and discharge stability.41 Besides capacity, rate performance was also presented in Figure 5 at current density of 0.02 to 0.2 mA/cm2. The expansion of voltage during the first discharge is

∼84 mV for N-PIACs-based oxygen electrode. N-PIACs-based oxygen electrode delivered as high as 10363 mAh/g at 0.2 mA/cm2, which was only slightly smaller than the discharge capacity at 0.02 mA/cm2, indicating an outstanding rate capability. The corresponding coulombic efficiency was 99.8 %, indicating that the catalytic stability of N-PIACs preceded that of others.43 In general, high surface area with 3D


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porous structure and highly active reactive sites (e.g., N-doped and defects) could alleviated the polarization voltage of ORR process when current density increased. What called for special attention was that there existed parasitic reactions after the charge capacity over the discharge capacity corresponding to the circle symbol in the Figure 5. Thus, electrolyte reacted with Li2O2 and Li anode or decomposed easily when Li-O2 batteries under overcharge condition with a high current density or a bad architecture of oxygen electrode. As the current density increased, excessive product was formed on the surface of the electrode, which exacerbated the blockage of the electrode channel. Thus the oxygen required for the reaction product could not be effectively diffused to the reaction site inside the battery. While at a small current density, the oxygen supply was sufficient. Taken together, the Li-O2 batteries with N-PIACs at a current density of 0.02 mA/cm2 presented the best performance than any other two.44

Figure 5. Initial full discharge−charge voltage profiles of a LiǀǀN-PICs−O2 battery and a LiǀǀN-PIACs−O2 battery at a current density of 0.02 mA/ cm2 and 0.2 mA/cm2.



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The cycling performance of Li-O2 batteries with N-PIACs was analyzed by galvanostatic charge-discharge test (Figure 6a). The LiǀǀN-PIACs-O2 cell exhibited excellent cycling stability of 220 cycles at 0.02 mA/cm2 with a limited capacity of 1000 mAh/g. During the 220 cycles, the discharge terminal voltage remained above 2.5 V, and the coulombic efficiency kept reaching 99.9 %, indicating an excellent stability during ORR (Figure 6b). Such cycling performance of N-PIACs in O2-electrode was superior to the reported studies, such as Jung et al and Zhao et al.13,45 In addition, the charge plateau voltage remained below 4.0 V during the initial 10 cycles, further suggesting N-PIACs enhanced OER activity. To forward illuminate the cycling stability of N-PIACs as the cathode at a large current density in Li-O2 battery, we applied the same electrochemical testing at high current density (0.2 mA/cm2) with limited capacity of 500 mAh/g and 1000 mAh/g (Figure 6c and 6d, respectively). Obviously, in 410 cycles, the discharge voltage of LiǀǀN-PIACsǀǀO2 battery was still higher than 2.5V, which was much better than the LiǀǀN-PICsǀǀO2 battery (Figure S4) and most of the previous reports.13,16,39,45 As shown in Figure 6d, when the controlled capacity was 1000 mAh/g, the batteries showed 86 cycles at 0.2 mA/cm2. Therefore, the round-trip performance of N-PIACs-based oxygen electrode at a lower cutoff capacity was better than that at a larger capacity under the same current density condition. This was probably due to the fact that the large capacity resulted in large particle size of discharge product of Li2O2, thus electrodes showed sluggish kinetics between the discharge and charge processes, whereas smaller discharge product formed at a relatively small capacity was liable to


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be decomposed and improve the cycling performance.46

Figure 6. (a) Galvanostatic cycling tests and (b) coulombic efficiency and terminal voltage on the number of cycle of LiǀǀN-PIACs-O2 cells at a current density 0.02 mA/cm2 with controlled capacity of 1000 mAh/g. Galvanostatic charge-discharge tests of LiǀǀN-PIACs-O2 cells at 0.2 mA/cm2 with controlled capacity of (c) 500 and (d) 1000 mAh/g, respectively.

EIS and XRD tests were carried out on N-PIACs-based oxygen cathodes. As shown in Figure 7a, the value of resistance increased significantly after discharge, and decreased almost to the initial value after recharge. This phenomenon was probably the formation of discharge products (Li2O2) at the oxygen electrode side, which has poor electronic conductivity. XRD patterns in Figure 7b clearly indicated the dominate discharge product on the N-PIACs cathode was Li2O2 with the characteristic



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peaks at 32.9°, 35°, 40.6°, and 58.7°, which accorded with the lattice planes of Li2O2. After the battery recharged to 4.5 V these characteristic peaks fully disappeared, suggesting the good reversibility of the N-PIACs cathode.

Figure 7. (a) Electrochemical impedance spectra (EIS) and (b) XRD patterns of the N-PIACs electrode at 10 minutes’ standing before discharge, after discharge to 2.0 V and recharge to 4.5 V at a current density of 0.2 mA/cm2.

The morphologies of the discharge products on N-PIACs electrodes were shown in Figure 8. Spherical-like discharge products were 50-500 nm thick, which were confirmed to be Li2O2.41,47 The conclusion was based primarily on Density Functional Theory (DFT) calculations, which has indicated that discharge products most likely to nucleate and grow around the defective sites with functional groups on N-PIACs.48 Raman spectrum (Figure 8c) of the fully discharged cathodes presented a protruding peak at 789 cm−1 indicating the formation of Li2O2, which was in accord with XRD results in Figure 7b. Besides, there was a small and broad shoulder (∼1150 cm−1 to 1190 cm−1). Yang et al. had indicated that it was the fluctuation of the superoxide-like species, which was part of the surface structure of Li2O2 particles.49 Inevitably, a


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small Li2CO3 peak appeared at 1089 cm-1, probably due to reaction with CO2 during Raman analysis test at air condition rather than electrolyte decomposition. The morphologies of recharge cathodes for N-PIACs electrodes were shown in Figure S5. When the charge voltage was higher than 4.5 V, there were almost none of the discharge products on the oxygen electrode, which suggested that the discharge product on the oxygen electrode could be sufficiently decomposed and further demonstrated that this Li-O2 battery has good cycling performance.

Figure 8. (a) and (b) SEM images (Signal A = InLens) of the N-PIACs as the oxygen electrode discharged until the discharge voltage below 2.0 V, (c) Raman spectrum of the discharged N-PIACs-based cathodes.

4 CONCLUSIONS We prepared a self-N-doped carbon cathode with poplar inflorescence biomass as precursor by KOH activation method (N-PIACs) in lithium oxygen batteries. This N-PIACs by our method presented a 3D interconnecting pores structure, high surface area (1049 m2/g), and excellent ORR/OER catalytic activity. The Li-O2 batteries using N-PIACs as oxygen electrodes exhibited high power and energy capacity, long circular life, and excellent rate performance. This work was characterized by its



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convenient and low cost; it could provide a facile strategy for preparing high-performance Li-O2 batteries. The eco-friendly fabrication process made our N-PIACs-based cathode promising for new development ideas of green secondary batteries.

SUPPORTING INFORMATION SEM images for N-PICs; Low-magnification SEM image of N-PIACs and SEM-EDS mapping of C, O and N; Continuous current cycling tests of LiǀǀN-PICsǀǀO2 batteries; SEM images of the oxygen electrode with N-PIACs when it first recharge.

ACKNOWLEDGEMENTS This research was supported by the National Key Program for Basic Research of China through Grant 2015CB251100 and the Graduate Technological Innovation Project of Beijing Institute of Technology through Grant 2018CX10024. The use of Swagelok cell was supported by Cunzhong Zhang.

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Graphical Abstract


Self-Nitrogen-Doped Carbon from Plant Waste as an Oxygen

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