Free-Standing Thin Webs of Activated Carbon Nanofibers by

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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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ACS Applied Materials & Interfaces

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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

ACS Paragon Plus Environment

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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)

4

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

5

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

9

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

10

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

11

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

12

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

13

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

15

interconnected microstructures of ACNF cathodes allow uniform O2 and electrolyte distribution

16

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

2

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

4

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

5

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

6

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

7

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

8

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

10

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

11

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

12

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

13

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

14

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

15

improved.

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

17

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

18

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

19

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

20

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

21

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

22

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

23

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

3

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

4

CNF cathodes and (d) ACNF cathodes.

5

4. Conclusions

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

2

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

3

Corresponding Author

4

Email：[email protected]; [email protected]

5

Author Contributions

6

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

7

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

8

Acknowledgements

9

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

10

(No. 21506210).

11

References

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Table of Contents

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Free-Standing Thin Webs of Activated Carbon Nanofibers by

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