Free-Standing Thin Webs of Activated Carbon Nanofibers by

Dec 21, 2015 - Free-standing activated carbon nanofibers (ACNF) were prepared through electrospinning combining with CO2 activation and then used for ...
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Free Standing Thin Webs of Activated Carbon Nanofibers by Electrospinning for Rechargeable Li-O2 Batteries Hongjiao Nie, Chi Xu, Wei Zhou, Baoshan Wu, Xianfeng Li, Tao Liu, and Huamin Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10088 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 24, 2015

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Free standing thin webs of activated carbon

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nanofibers by electrospinning for rechargeable Li-O2

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batteries

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Hongjiao Nie, a,b Chi Xu, a,b Wei Zhou, a,b Baoshan Wu, a Xianfeng Li, a Tao Liu *a and Huamin

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Zhang *a

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a

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Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian, 116023, China.

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b

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Keywords: free-standing cathodes; electrospinning; Li-O2 battery; physical activation;

Division of energy storage, Dalian National Laboratory for Clean Energy, Dalian Institute of

University of Chinese Academy of Sciences, Beijing, 100039, China.

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hierarchically porous structure

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Abstract: Free-standing activated carbon nanofibers (ACNF) were prepared through

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electrospinning combining with CO2 activation, and then used for non-aqueous Li-O2 battery

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cathodes. As-prepared ACNF based cathode was loosely packed with carbon nanofibers

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complicatedly overlapped. Owing to some micron-sized pores between individual nanofibers,

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relatively high permeability of O2 across the cathode becomes feasible. Meanwhile, the

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mesopores introduced by CO2 activation act as additional nucleation sites for Li2O2 formation,

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leading to an increase in the density of Li2O2 particles along with a size decrease of the

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individual particles, and therefore, flake-like Li2O2 are preferential to be formed. In addition, the

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free-standing structure of ACNF cathode eliminates the side reactions about PVDF. As a result,

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the Li-O2 batteries with ACNF cathodes showed increased discharge capacities, reduced

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overpotentials and longer cycle life in the case of full discharge and charge operation. This

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provides a novel pathway for the design of cathodes for Li-O2 battery.

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1. Introduction

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Electrical energy storage is attracting more and more attention nowadays, considering the

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enormous demand for mobile electronic devices and the mushrooming markets of Electric

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Vehicles (EVs)1. The energy density of conventional power systems such as Li-ion batteries is

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much too low for commercial requirment2-4. And that makes the seeking for an energy storage

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system with higher energy density beyond Li-ion batteries an even more pressing priority. One

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such system is the Li-O2 batteries. In a Li-O2 battery system, the cathode active material O2 is

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not stored in the battery, but can be obtained from the ambient air. Excluding O2 weight, a

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theoretical energy density as high as 11140 Wh kg-1 can be reached3, 5-8, which is nearly 9 times

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higher than that of current Li-ion batteries.

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A typical Li-O2 battery is composed of a Li anode, an electrolyte consisting of dissolved Li

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salt in an organic solvent, and a porous O2-breathing cathode. Among them, cathode has been

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reported to directly determine the battery performance, including the capacity, cycle life and

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power performance9-17. Carbon with high electric conductivity and tunable microstructure are

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presently the most utilized cathode material for Li-O2 battery18, 19. However, there are still many

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limitations to be conquered. During discharge, O2 is reduced to form large toroid-like or thick

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film-like insoluble lithium oxides (mainly Li2O2) on the surface or within the pores of cathode to

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cause serious pore-clogging. This restricts the oxygen diffusion across the cathode structure,

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leading to limited reaction kinetics and premature battery failure10, 12, 20-24

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To solve the above mentioned issues, the carbon cathodes are generally constructed by

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casting slurry of the mixture of carbon and polymer binder (usually PVDF) onto the framework

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of supporting materials, such as Ni foam and so on25, 26. The Li-O2 battery using such an air

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electrode demonstrated remarkably enhanced capacity. However, the extra weight of the

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supporting materials would not be ignored. At the same time, the presence of PVDF in electrodes

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separates many active materials from the electrolyte, which reduces the effective reaction

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interface area of the electrode. Moreover, PVDF has been reported to undergo extensive

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chemical dehydrofluorination with organic and alkaline bases, yielding unsaturated products

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with polyene structures. The strong base abstracts the proton from the PVDF polymer backbone,

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followed by release of F to form a conjugated double bond. In Li-O2 cells, the O2− ion, which is a

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far stronger base than OH-, operates as the hydrogen abstraction agent for PVDF, resulting in the

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decomposition of the PVDF and consequent cathode degradation and cell failure27.

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Electrospinning is a simple and efficient method of producing 1D carbon structures by

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applying high voltage to a polymer solution or a melt and subsequent thermal treatment28-30.

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Compared with traditional carbon nanofibers, the resultant carbon materials are typically in the

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form of electron-conducting non-woven webs, providing good mechanical stability31. Hence,

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they can be directly used as cathodes for Li-O2 batteries without adding non-active binders. In

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addition, the micron-sized pores between the individual carbon nanofibers would not be blocked

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by Li2O2 formed during the discharge process and thus facilitate uninterrupted access of O2 to

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the cathodes. The elimination of non-active materials (binders and supporting materials) leads to

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high energy and power densities, improved rate performance and longer cycle life of Li-O2

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batteries. However, the discharge capacity is restricted by the relatively low surface area of

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carbon nanofiber (CNF) cathodes by electrospinning. Herein, we introduced a relatively novel

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approach by combining electrospinning with physical activation to fabricate a binder-free carbon

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nanofiber cathode for Li-O2 battery. The obtained cathode materials contain both open O2

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pathways and high surface areas. Surface morphology and pore structure of the electrodes were

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analyzed by means of SEM and BET. Cyclic voltammetry (CV) was employed to evaluate the

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electrochemical activity. Furthermore, the cell performances of the activated carbon nanofiber

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electrodes were studied in comparison with non-activated ones and conventional composite

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electrodes. Finally, the morphology and composition of the discharge products were also

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determined.

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2. Experimental section

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2.1 Preparation of materials

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Preparation of carbon nanofiber (CNF) by electrospinning: Polyacrylonitrile (PAN) and N,

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N-dimethyl formamide (DMF) were used for getting polymer solution (PAN/DMF, 10 wt. %).

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The prepared polymer solution was ejected from syringe tip to carbon felt-covered collector by

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using an electrospinning apparatus with following conditions [syringe rate: 1 mL h-1, collector

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rotation speed: 300 rpm, tip to collector distance: 10 cm, and applied voltage: 15 kV]. Before

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carbonizing the electrospinning fibers, oxidation process is necessary to change the thermoplastic

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character into thermosetting one. Because electrospinning materials cannot keep their fiber form

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at high temperature because they would be soften and melt. Oxidation of the PAN nanofiber was

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carried out by heating to 523 K at a rate of 1 K min-1 and maintained for 1 h under the air

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atmosphere. Carbonization of the stabilized nanofiber was carried out under Ar at 1473 K for 1 h

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with a heating rate of 10 K min-1. Then the carbon nanofiber was obtained and it is noted as

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CNF.

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Preparation of activated carbon nanofiber (ACNF): The as-prepared CNF was activated

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under CO2 for 1 h at 1273 K (heating rate: 10 K min-1). The resulting nanofiber was noted as

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ACNF.

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2.2 Physical Characterization

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Scanning electron microscopy (SEM) micrographs were taken on QUANTA 2000FEG to

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get the surface morphology of electrodes before and after discharge. N2 adsorption isotherms

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were measured at 77.3K using an ASAP2010 system. Surface areas and pore volumes were

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determined using Brunauer-Emmett-Teller (BET) method. The pore size distribution curves were

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calculated from the desorption branches of nitrogen isotherms using the Barrett- Joyner-Halenda

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(BJH) model. X-ray diffraction (XRD) analyses were carried out on a Rigaku Rotalflex (RU-

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200B) diffractometer with a CuKα source (λ=1.54056 Å) and a Ni-filter. The 2θ values of X-ray

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diffractograms varied between 10° and 70° with a scan rate of 6° min-1. XRD analyses of the

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discharged-charged cathodes were performed in an Ar-filled airtight holder to reduce their

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exposure to the ambient atmosphere.

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2.3 Electrochemical Evaluation

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Preparation of cathode: The CNF and ACNF cathodes in this study were prepared by

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punched the received CNF and ACNF webs into disks with a diameter of 16 mm and then dried

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at 393 K for 12 h. The BP2000 composite cathodes were prepared by mixing BP2000 and PVDF

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as a binder in ethanol with a weight ratio of 80/20. The mixture was ultrasonic stirred for 30 min

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to get homogeneous slurry. After ethanol evaporation at 353 K, the mixture was compressed and

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punched into disks with a diameter of 16 mm and then dried at 393 K for 12 h.

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Preparation of single cell: The Li-O2 battery was constructed in an Ar-filled glove box

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(H2O<0.1 ppm, O2<0.1 ppm). The cell consists of a 0.45 mm thick Li foil (16 mm in diameter)

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as anode, a carbon electrode as cathode and polypropylene fiber (Novatexx 2471 Freudenberg

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Filtration Technologies KG) soaked with electrolyte as a separator. The electrolyte employed

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here is 1.0 M bis(trifluoromethane) sulfonamide lithium (LiTFSI) in tetraethylene glycol

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dimethyl ether (TEGDME, Aladdin). A stainless steel mesh was used as the current collector. All

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the cell parts were compressed together to ensure good contact and the cell was completely

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sealed except for the O2 entryway.

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Electrochemical characterization: After exposed to flow pure O2 for 5 h, the Li-O2 battery

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was cycled on a LAND 2100 system (Wuhan, China) at room temperature. The discharge

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performance was tested with a lower voltage limit of 2.0 V versus Li+/Li, while the cycle

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experiments were conducted with a capacity limit of 1000 mAh g-1. The cyclic voltammetry (CV)

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tests were conducted in a traditional three-electrode system with carbon cathode as working

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electrode (WE), glass filter protected Li foil as counter electrode (CE), and porous ceramic

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protected Li foil as reference electrode (RE). 1M LITFSI/TEGDME were used as electrolyte.

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The measurements were conducted between 2.0 V to 4.3 V using an AFCBP1 bipotentiostat

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(Pine Research Instrumentation, USA) in an argon-filled glove box (H2O<0.1 ppm, O2<0.1

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ppm) at room temperature.

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3. Results and discussion

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Figure 1 shows SEM images of both pristine and activated carbon nanofiber cathodes and

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conventional BP2000 composite cathodes. The CNF cathode presents a network of randomly

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arranged long, linear nanofibers, with numerous 2-3 µm large pores between the individual

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carbon fibers (Figure 1a). The carbon nanofibers made from pure PAN precursors have relatively

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smooth and regular surface morphology, with an average diameter of 250 nm (Figure 1b). When

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exposed to CO2 at high temperature, surface carbon atoms of CNF are eroded by CO2, as shown

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in Eq. 1.

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Cܱଶ + ‫ → ܥ‬2‫ܱܥ‬

(1)

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As a result, the positions of original carbon atoms are left to form a highly porous structure. It

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can be seen from Figure 1c and d, in which the surface of carbon nanofiber becomes rough and a

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highly porous structure starts to appear. At the same time, the diameters of the ACNF slightly

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decrease, which is due to the burning off of carbon surface (Figure 1d)32. However, it should be

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noted that the micron-sized pores between carbon fibers are well preserved (Figure 1d). When

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used as cathodes in Li-O2 batteries, these micron-sized pores would not be blocked by lithium

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oxides formed during the discharge process and serve as diffusion channels to keep O2 flow

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continuously. In contrast, BP2000 cathodes contain many spherical particles that connect with

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each other to form large aggregations that accumulate closely to make the electrode quite

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compact (Figure 1e and f), just as previously reported.

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Figure 2a presents the XRD patterns of BP 2000, CNF and ACNF cathodes respectively. In

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comparison with CNF cathodes, the disordered degrees of ACNF cathodes are increased. It is

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further confirmed by the Raman spectra in Figure 2b. The relative intensity of signals at 1346

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cm-1 and 1588 cm-1, often referred to be as D band and G band, illustrates the disordered degree

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of samples. The calculated ID/IG of ACNF is much higher than that of CNF, demonstrating

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more disorder structure of ACNF. It is because that high temperature activation can efficiently

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produce a large number of defective bits in ACNF cathode and increase the disorder degree.

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Figure 1. SEM images of CNF cathodes (a and b), ACNF cathodes (c and d) and BP2000

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composite cathodes (e and f).

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Nitrogen adsorption-desorption measurements were performed at 77 K to determine the

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surface area and pore volume of the three samples, and the results were shown in Figure 3. The

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adsorption/desorption isotherms in Figure 3a show that both ACNF and BP2000 possess a

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typical type-IV isotherm with hysteresis loop following the IUPAC classication, indicating that

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they all have highly mesoporous structure. In comparison, the pore structure of CNF is less

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developed.

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Figure 2. a. XRD patterns of CNF, ACNF and BP 2000 cathodes before discharge, b. Raman

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spectra of ACNF and CNF cathodes.

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The detailed pore parameters are summarized in Table 1. It is seen that both the specific

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surface area and pore volume of non-activated CNF are small. It is expectable because carbon

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nanofibers prepared through electrospinning exhibit solid interior and smooth surface, and hence

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the total surface area is determined by the fiber diameter33. After activation, ACNF exhibits a

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specific surface area of 709 m2 g-1 and a pore volume of 0.93 cm3 g-1, which is higher than that of

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CNF, illustrating that the activation process develops abundant pores in ACNF. This is in

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accordance with the SEM images in Figure 1c and d. As for BP 2000, both the specific surface

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area and pore volume are the highest, which are 2.1 and 2.5 times that of ACNF respectively.

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Figure 3. Pore structure of three different samples, a. N2 adsorption/desorption isotherms, b.

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PSD curves.

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The pore size distribution (PSD) curves are derived using the BJH method from the

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desorption branch of the isotherm and shown in Figure 3b. In spite of their vastly different

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surface area and pore volume, the majority of pores in both ACNF and BP 2000 are in the

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similar range of mesopores. This is preferred for lithium oxides deposition. Because as previous

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studies reported, pores in this range are inclines to achieve a high utilization34, 35.

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Table 1. Porosity parameters of CNF, ACNF and BP 2000. Surface area/ m2 g-1

Pore volume/ cm3 g-1

CNF

20.2

0.07

ACNF

709

0.93

BP 2000

1483

2.37

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The galvanostatic experiments were conducted to evaluate the discharge behavior of CNF,

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ACNF and BP 2000 cathodes in Li-O2 batteries at a current density of 200 mA g-1. All of the

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results for the specific capacities and current densities are calculated with the total mass of the

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cathodes. As shown in Figure 4a, CNF cathode delivers a discharge capacity of 4166 mAh g-1.

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Subsequent activation by CO2 makes the discharge capacity further increased. ACNF cathode

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activated at 1273 K shows the highest discharge capacity of 6099 mAh g-1, which is higher than

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that of BP2000 composite cathode (3538 mAh g-1) under similar experiment conditions. In the

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meanwhile, the discharge voltages of Li-O2 cells with ACNF cathodes are also significantly

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improved. In detail, the discharge voltages of Li-O2 cells with ACNF cathode are higher than

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that with CNF and BP 2000 cathodes by 140 mV and 240 mV respectively. The electrochemical

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processes of ORR are also investigated using cyclic voltammetry (CV). Figure 4b presents the

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first cycle CV response at a constant scan rate of 10 mV s-1. It reveals that the ACNF cathode

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exhibits a higher current density compared with the CNF and BP 2000 composite cathode.

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Figure 4. Electrochemical performance of CNF, ACNF and BP 2000 cathodes, (a) the first cycle

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discharge curves at 200 mA g-1, (b) the first cycle CV curves for the reduction of O2-saturated

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1.0 M LiTFSI/TEGDME at sweep rate 10 mV s-1.

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The above results show that the binder-free ACNF cathode exhibits superior

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electrochemical performance. Generally speaking, carbon electrodes with large surface areas and

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pore volumes are expected to obtain better discharge performance, because they could serve

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more ORR active sites and accommodate more discharge products5, 11. Based on the porosity

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analysis in Table 1, the total surface area and pore volume of ACNF are lower than 50 % that of

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BP 2000. Surprisingly, Li-O2 cell with the ACNF cathode delivers a discharge capacity nearly

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1.7 times that with the BP 2000 cathode. It can be ascribed to two main reasons: the first reason

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is the hierarchical structure of the self-supported network of ACNF cathode consisting of inter-

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fiber micron-sized pores and mesopores on the individual fibers. The former act as O2 diffusion

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pathways, through which O2 can penetrate to react with Li+ ions. After discharged to 2 V, the

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carbon nanofibers are uniformly covered with lithium oxides formed during the discharge

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process (Figure 5c and d). The micron-sized pores present between individual carbon fibers are

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not blocked by lithium oxides, which guarantee uninterrupted O2 flow. As a result, the inefficient

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surface utilization related to O2 hungry will be restrained. In comparison, the surface of BP 2000

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composite cathode is covered by a film-like discharge product, with almost all the pores being

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filled up (Figure 5e and f). The resultant discharge product film would block subsequent O2

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access to the cathode and lead to undesirable battery failure. The uniform and complete coverage

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of cathode surface with lithium oxides without disturbing O2 diffusion is necessary to attain

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maximum discharge capacity. On the other hand, the mesopores introduced by CO2 activation

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act as additional nucleation sites for lithium oxides crystallization during discharge process,

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leading to an increase in the density of lithium oxides particles, which results in increased

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discharge capacity36. The other origin of excellent electrochemical behavior of Li-O2 cells with

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ACNF cathodes is the absence of PVDF in free-standing ACNF structure. This allows the active

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materials to be thoroughly exposed to the electrolyte, which increases the effective reaction

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interface area of the electrode. As a result, the discharge performance will be enhanced.

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It should also be noted that there is a clear difference in the morphology of discharge

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products of the ACNF cathodes (Figure 5c) compared to that of the CNF cathodes (Figure 5a).

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After full discharge, small flake-like discharge products are closely arranged along carbon

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nanofibers in ACNF cathodes. However, in the case of CNF cathodes, these products were

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agglomerated together, resulting in bulky discharge products along the carbon nanofibers. It

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seems that the mesopores on the surface of carbon fibers confine the growth of lithium oxides in

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ACNF cathodes, resulting in size decrease of the individual lithium oxides particles. It has been

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reported that the adsorption energies of Li2O2 on the defect model (such as mesopores in ACNF)

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were much lower than that of Li2O2 on the basal model, which implies that the adsorbed Li2O2 is

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difficult to migrate on a defect surface37. For ACNF cathode, the carbon nanofiber surface is

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composed of many mesopores, Li2O2 will be toughly adsorbed and then will homogeneously

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cover the surface of carbon nanofiber, resulting in the layer-like morphology. While in the case

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of CNF cathode, carbon nanofiber surface is relatively smooth. So Li2O2 will be softly adsorbed

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and will easily migrate on the surface, reforming the aggregates of products.

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Figure 5. SEM images of CNF cathodes (a and b), ACNF cathodes (c and d) and BP 2000

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composite cathodes (e and f) after discharge.

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Figure 6 shows the XRD patterns of discharged ACNF, CNF and BP 2000 cathodes. It can

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be seen that all the three discharged cathodes show the characteristic diffraction lines of Li2O2.

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The peaks at 32.8, 34.9, 40.6, 48.8, and 58.6°represent (100), (101), (102), (004) and (110)

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reflections respectively. These results confirm that Li2O2 are the dominant products in all the

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three cathodes, despite the different morphology.

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Figure 6. XRD patterns of the discharged CNF, ACNF and BP 2000 cathodes.

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Another advantage of Li-O2 cells with ACNF cathodes is the much lower over potentials

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during oxygen evolution reaction (OER) processes. The cells were charged at 200 mA g-1 with a

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limited capacity of 1000 mAh g-1. It can be seen from Figure 7a that the voltage obtained at the

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charge terminal of the ACNF cathode in the Li-O2 battery is only 4.3 V, which is 200/300 mV

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lower than that of CNF/BP 2000 cathode respectively. The relatively lower charge overpotential

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of ACNF cathode in this experiment may be related to its 3-D interconnected microstructure and

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the unique flake-like structure of Li2O2. Unlike conventional composite cathodes, the 3-D

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interconnected microstructures of ACNF cathodes allow uniform O2 and electrolyte distribution

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around the discharge products and promote the decomposition of the products during charge. In

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addition, as above mentioned, the micron-sized O2 diffusion channels between nanofibers are not

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blocked by Li2O2 after full discharge. In the following charge process, the open channels help

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easy O2 release during Li2O2 decomposition, which results in improvement of the overall charge

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behavior of the Li-O2 battery. Moreover, in contrast to the Li2O2 film on discharged CNF and BP

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2000 cathodes (Figure 5a and e), the formation of loosely packed flake-shaped Li2O2 particles on

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the surface of individual carbon fibers in ACNF cathodes (Figure 5c) provide sufficient Li2O2-

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electrolyte interfaces during the charge process38,39, and consequently enhance the charge

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performance of the Li-O2 battery.

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We further investigated the cycling stability of the Li-O2 batteries with BP 2000, CNF and

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ACNF cathodes. The current density is 200 mA g-1 and the capacity was restricted to 1000 mAh

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g-1. Figure 7d shows that the cut-off voltage of Li-O2 batteries with ACNF cathode remains

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higher than 2.0 V until 51 cycles. In contrast, the cut-off voltages of the CNF and BP 2000

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cathodes degrade to lower than 2.0 V after only 35 and 19 cycles, respectively (Figure 7b and c).

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That is, the cycle performance of Li-O2 battery with ACNF cathode has been considerably

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improved.

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Previous studies report that for carbon cathodes without effective catalysis, the complete

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oxidation of lithium oxides usually happens only above 4.5 V40, 41. On such a high voltage,

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electrolyte and carbon cathodes are unstable. They would suffer from severe decomposition42,43.

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Although cannot be totally avoided, the relatively lower charge voltage in Li-O2 battery with

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ACNF cathode can effectively restrain the decomposition reaction of carbon materials and

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electrolyte, leading to improved cycling stability of the corresponding Li-O2 batteries. In addition,

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the detrimental side reactions involving PVDF binder are eliminated in ACNF cathode due to the

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self-supporting structure, which may be another contributor to better cycle performance.

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Figure 7. (a) First charge-discharge curves of Li-O2 cells at a current density of 200 mA g-1 and

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a capacity limit of 1000 mAh g-1, cycle performance of Li-O2 cells with (b) BP 2000 cathodes, (c)

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CNF cathodes and (d) ACNF cathodes.

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4. Conclusions

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The free-standing activated carbon nanofiber (ACNF) material can be readily prepared

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through CO2 activation assisted electrospinning. A hierarchically porous structure with both

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micron-sized pores and mesopores was obtained for ACNF. The as-prepared ACNF can be

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directly used as cathodes material in Li-O2 battery, eliminating the detrimental effect of PVDF.

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The micron-sized pores between carbon nanofibers can effectively improve the permeability of

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O2 across the cathode, and thus enhance the utilization rate of cathode. At the same time, the

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mesopores introduced by CO2 activation increase the number of nucleation sites for the growth

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of Li2O2 particles and reduce the size of the formed particles. It is found that flake-like Li2O2 has

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been formed. The electrochemical measurements demonstrate that Li-O2 batteries with ACNF

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cathodes show higher discharge capacity, reduced over potentials and improved cycling stability

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compared to non-activated CNF cathodes and conventional BP 2000 composite cathodes.

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Corresponding Author

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Email:[email protected]; [email protected]

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval

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to the final version of the manuscript. ‡These authors contributed equally.

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Acknowledgements

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The authors acknowledge financial support from National Natural Science Foundation of China

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(No. 21506210).

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References

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