DOI: 10.1002/cplu.201402044

Hierarchical Graphene-Rich Carbon Materials with TraceNonprecious Metals for High-Performance Li–O2 BatteriesJing Li ,[a, b] Yining Zhang,*[a] Wei Zhou ,[a, b] Hongjiao Nie ,[a, b] Baoshan Wu,[a] andHuamin Zhang*[a]

Introduction

The nonaqueous lithium–oxygen (Li–O2) battery representsa promising energy storage device for electric vehicle applica-tions owing to its high theoretical energy density (as high as11140 Wh kg�1).[1] For Li–O2 batteries, the key component is theoxygen electrode, which often suffers from sluggish kineticcharacteristics and low electrode space utilization for solidLi2O2 product deposition, resulting in poor performance.[2]

Recently, many studies have focused on the development ofhighly efficient catalysts for the oxygen cathode to lower theoverpotential. Cathode catalysts such as carbon-supported Pt,Ir, and Au,[3] heteroatom N-doped carbon and S-dopedcarbon,[4] and carbon-supported nonprecious Co, Mn, Fe, andCu species[5] have been found to exhibit excellent electrocata-lytic performances. In particular, the transition-metal catalystshave received much more attention, and high activities havebeen achieved. In addition, it has been demonstrated that thepresence of the transition metal can also affect the catalyticgraphitization during the carbonization process.[6] A certainnumber of encapsulated metal nanoparticles in the graphiticstructure could remain even after a tedious removal process.As reported previously, even an extremely small amount of

a metal such as iron could exhibit a high activity for theoxygen reduction reaction (ORR).[7]

On the other hand, various novel carbon nanomaterials andnanostructures have been applied as the oxygen electrode forLi–O2 batteries, including carbon nanotubes, carbon fibers, gra-phene, 3D structured carbon materials, and so on.[8] The use ofa hierarchical architecture of cathode has proved an efficientway to improve battery capacity.[9] A high volume of large mes-opores is an essential feature in the cathode to accommodateLi2O2 precipitates and macropores to promote O2 transport.[10]-

Many novel methods have been proposed for pore construc-tion, such as organic gel and template-assisted processes.[11] Al-though metal salts have been added to the carbonization pre-cursor to prepare metal-doped carbon materials with improvedactivities, the role that the transition metal plays in the porestructure has always been ignored.

In previous work, we prepared a micron-sized honeycomb-like carbon material (MHC) with a hierarchical pore structure,using commercial CaCO3 particles as a template and sucrose asthe carbon precursor.[12] The material exhibited excellentoxygen transport ability and improved pore utilization com-pared with commercial carbon. Herein, we report the fabrica-tion of iron-/cobalt-doped micron-sized honeycomb-likecarbon (Fe-MHC, Co-MHC), employing the same methodexcept for the addition of the corresponding metal salts. In ad-dition to the nano-CaCO3 template, the iron or cobalt salts alsoparticipate in the pore construction, contributing to an opti-mized pore structure with higher volume. Compared with theundoped sample, these materials demonstrate higher activitiesand stabilities in ether-based electrolyte, which are attributedto the trace metal as well as the graphene-rich structure. TheFe-MHC cathode exhibits a discharge capacity as high as9260 mAh g�1, which is 1.5 times that of the non-metal-dopedmaterial.

Sluggish kinetic characteristics of the electrode reactions andlow electrode space utilization for solid Li2O2 deposition arelimiting factors for Li–O2 batteries. In this work, iron-/cobalt-doped micron-sized honeycomb-like carbon (Fe-MHC, Co-MHC) with a hierarchical pore structure is prepared by usingnano-CaCO3 as a template. The effect of the transition-metalnanoparticles as a second template on further pore construc-tion and optimization is presented. As graphitization catalysts,

the added transition metals also affect the formation of thegraphene-rich structure in the carbon framework. As a result,the obtained hierarchical carbon materials doped with tracemetals exhibit higher electrochemical catalytic activity and sta-bility than non-doped samples. In particular, an enhanced dis-charge capacity as high as 9260 mAh g�1 is achieved for theFe-MHC cathode, which is attributed to its enlarged porevolume and high utilization of the electrode space.

[a] J. Li , Prof. Y. Zhang, W. Zhou , H. Nie , B. Wu, Prof. H. ZhangDivision of energy storage, Dalian National Laboratory for Clean EnergyDalian Institute of Chemical Physics Chinese Academy of SciencesNo.457 Zhongshan Road, Dalian 116023 (P. R. China)Fax: (+ 86) 411-84665057E-mail : zhangyn@dicp.ac.cn

zhanghm@dicp.ac.cn

[b] J. Li , W. Zhou , H. NieUniversity of Chinese Academy of Sciences,No. 19A Yuquan Road, Beijing 100049 (P. R. China)

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cplu.201402044

. Part of a Special Issue on “Metal–Air and Redox Flow Batteries“. A linkto the table of contents will appear here once the issue is compiled.

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Results and Discussion

Characterization of hierarchical Co-MHC and Fe-MHC

The X-ray diffraction (XRD) patterns of Co-MHC, Fe-MHC, andMHC are shown in Figure 1. Two broad diffraction peaks locat-ed at approximately 238 and 438 can be observed on all the

three samples, corresponding to the (002) and (100) planes ofgraphite. The broad peaks are a typical feature of amorphouscarbon materials. The inductively coupled plasma atomic emis-sion spectroscopy (ICP-AES) analysis indicates that the metalcontents are 0.05 and 0.19 wt % for Co and Fe, respectively,proving the presence of trace metals after the acid wash. Fur-thermore, there are no clear crystalline Co- and Fe-related dif-fraction peaks for either of the samples because of their ratherlow contents in the carbon frameworks.

The physicochemical properties were characterized by N2

sorption measurements. Figure 2 shows that all the samplespresent a meso–macro double-peak pore-size distribution, inparticular, a mesopore-rich structure centered at about 30–50 nm, in accordance with the particle size of the CaCO3 tem-plate. Compared with MHC, Fe-MHC and Co-MHC display in-creased pore distributions throughout the entire pore sizespan, especially for those larger than 10 nm. According to pre-vious results, this demonstrates that these pores are appropri-ate for Li2O2 accommodation, and this change in pore structure

would lead to an improvement in discharge capacity.[13] Over-all, the total pore volume increases from 1.09 to 2.45 and1.87 cm3 g�1 after the addition of Co and Fe salts, respectively(Table 1), and the BET surface areas increase from 785 to 1152and 970 m2 g�1 for Co-MHC and Fe-MHC, respectively.

The morphology of the carbon material and related electro-des were analyzed by scanning electron microscopy (SEM). Asillustrated in Figure 3, both the Co-MHC and Fe-MHC samplesexhibit highly developed interconnected pore networks, whichare composed predominantly of two kinds of pores. Onecomes from the intrinsic honeycomb-like mesopores in theparticles themselves, as characterized by N2 sorption measure-ments. The other is formed by the macroporous large holesbetween the carbon bulks and are of the order of hundreds ofnanometers, in agreement with our previous report.[12] Bothtypes of pores give the electrode a hierarchical morphology.Notably, Fe-MHC appears to exist in the form of more frag-ments of smaller size. After electrode fabrication, the bindermakes the carbon particles aggregate to form a relatively com-pact electrode. Compared with the Co-MHC electrode, the Fe-MHC electrode appears to retain more macropores because ofthe difference in carbon particle size, as seen from Figures 3 cand d.

Figure 1. XRD patterns of the Co-MHC, Fe-MHC, and pristine MHC samples.

Figure 2. Pore-size distributions for pristine MHC, and metal-doped Co-MHCand Fe-MHC samples, as obtained throught the Barrett–Joyner–Halenda(BJH) method.

Table 1. Pore textural properties of the prepared carbon materials.

Sample SBET [m2 g�1][a] Vtotal [cm3 g�1][b] V [cm3 g�1][c]

MHC 785 1.09 0.70Co-MHC 1152 2.45 1.84Fe-MHC 970 1.87 1.32

[a] SBET: Brunauer–Emmett–Teller (BET) surface area. [b] Vtotal : total porevolume obtained from N2 desorption isotherms (pores ranging from 1.7to 300.0 nm). [c] V: volume of pores (>10 nm) from the desorptionbranches on the basis of the Barrett–Joyner–Halenda (BJH) method.

Figure 3. a–d) SEM images of carbon materials and prepared electrodes:a) Co-MHC, b) Fe-MHC, c) Co-MHC electrode, and d) Fe-MHC electrode.

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Transition metal types and roles

To clarify the effects of the metal or metal composite formedduring pyrolysis on the mesoporous structure formation forCo-MHC and Fe-MHC, we examined the crystalline phases ofCaO-Co/C and CaO-Fe/C samples, with reference to CaO andmetal coexisting nanocomposites. The XRD results (Figure 4)

indicate that besides CaO, CaO-Co/C and CaO-Fe/C also con-tain metallic Co and Fe, respectively. The peaks at 44.28 and51.58 can be assigned to the (111) and (200) planes of Co, re-spectively, and that at 43.68 to the (111) plane of Fe, with refer-ence to the diffraction data for Co (JCPDS: 00-015-0806) andFe (JCPDS: 01-089-4186). Thus, thermal treatment at elevatedtemperature (900 8C) may lead to the following process: first,the metal salts form metal oxides, and then, the formedcarbon or uncarbonized precursor leads to the reduction ofthe metal oxides to their metallic state. It is interesting thata solid solution state is also formed in the Fe-doped sample.The XRD analysis for the CaO-Fe/C sample is shown in Fig-ure S2 (Supporting Information). The weak peak at 50.78 forthe CaO-Fe/C sample is in good agreement with the data forthe (200) plane of austenite (Fe,C) (JCPDS: 00-023-0298), whichalso possesses a peak at around 43.48 for the (111) plane. Thismay be because the solubility of carbon in Fe is higher thanthat in Co at high temperatures (20.2 % in Fe and 13.9 % inCo).[14]

The mean particle size of crystallite metal species can be cal-culated by using Scherrer’s equation, as shown in Equation (1),in which l is the wavelength of the X-rays (0.15432 nm), q isthe angle at the position of the peak maximum, and b is thewidth at half-height of the diffraction peak.

L ¼ 0:9l

b cosðqÞ ð1Þ

The average crystallite sizes for the Co and Fe species arecalculated to be 23.8 and 31.7 nm, respectively. Note that thecalculated metal crystallite sizes both lie in the size range ofthose pores with a greatly increased volume after acid treat-ment, as shown in Figure 2. Thus, the transition metals formed

during pyrolysis act as an assisted template, and create moremesopores.

To further explore the effect of the metal, we characterizedthe carbonized samples without acid wash through a TEM test.For comparison, the test was also performed with the samplewithout added metal salts (CaO/C), as shown in Figure 5 a. Thepristine CaCO3 presents nanoparticles of approximately 50 nm.

After pyrolysis at 900 8C, they decompose into CaO and arecovered by carbon, as labeled by the arrows. For the metal-doped samples (Figure 5 c, e), there are no clear morphologydifferences compared with CaO/C. For detection of the metal,CaO-Co/C and CaO-Fe/C were also examined in the dark-fieldmodel, as shown in Figures 5 d and f. Both samples exhibitbright crystalline particles, which could be related to theformed metal (Fe and Co) by carbon reduction at an elevatedtemperature. Conversely, the crystals in CaO-Fe/C are more dis-persed than in CaO-Co/C. This is in good accordance with theresults illustrated by XRD: besides pure Fe, the metal is presentpartly in the form of austenite (Fe,C) solid solution inCaO-Fe/C.

After the acid wash, XRD tests implied the detachment ofthe metal from the carbonized composites. To check if themetal was completely removed from the carbon framework,we investigated the subtle structure and composition of theas-obtained Fe-MHC and Co-MHC samples through high-reso-lution TEM (HRTEM). The results show that graphitic carbon

Figure 4. X-ray diffraction patterns of 1) CaO-Co/C and 2) CaO-Fe/C. Inset isa magnified view of the rectangular region.

Figure 5. TEM images of a) CaCO3 template, b) CaO/C, c) CaO-Co/C, ande) CaO-Fe/C. Dark-field TEM images of d) CaO-Co/C and f) CaO-Fe/C compo-sites. b) CaO and C; c–f) CaO, C, and metal coexisted in the composite afterpyrolysis at 900 8C.

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structures are formed, revealing that the transition metal spe-cies may catalyze the graphitization process, leading to the for-mation of a graphene-sheet-like nanostructure. The productcatalyzed by Fe exhibits a typical hollow graphitic nanoshellstructure, as shown in Figure 6 b. A graphene-rich structuremay be formed, in which Co or Fe particles are encapsulatedby carbon. The high degree of graphitization means that Fe-MHC and Co-MHC will possess better stability than pristineMHC upon use in electrode reactions.

A trace amount of metal is retained after the acid wash, asdetected by careful searching in HRTEM tests. The measuredlattice spacing (Figure S1 a) is 0.207 nm, which is close to the dspacing of the (111) plane of Fe. Furthermore, the Fe particlesize is around 5 nm, which is much smaller than the mean par-ticle size (31.7 nm) without acid wash calculated from the XRDresults. In addition, trace Co is also detected in the carbonframework by energy-dispersive X-ray (EDX) analysis of the Co-MHC sample indicated in the high-angle annular dark-field(HAADF)-scanning transmission electron microscopy (STEM)images, as shown in Figures S1 b and c.

The Raman spectra of the three samples are compared inFigure 7. All the samples exhibit two clear peaks at approxi-mately 1348 and 1588 cm�1, corresponding to the D and Gbands, respectively. The D band indicates the disorderedgraphite structure, whereas the G band indicates the presenceof crystalline graphitic carbon. The intensity ratio of the D andG bands (ID/IG) is used to evaluate the degree of disorder ofthe materials. The ratios for Co-MHC, Fe-MHC, and MHC sam-ples are 0.87, 0.85, and 0.88, respectively. The lower ID/IG value

for Fe-MHC means a higher degree of graphitization, whichmight be caused by the formation of the Fe-C solid solution.

Electrochemical properties

The ORR catalytic activities for carbon materials were investi-gated through electrochemical cyclic voltammetry (CV) tests.As shown in Figure 8 a, compared with MHC, improvements in

activity are achieved for both Co-MHC and Fe-MHC, as evi-denced by the positive shift in the ORR onset potential. More-over, the ORR peak current density is increased significantly bya factor of approximately 2.4. The ORR activity is mainly deter-mined by the surface chemical composition. In addition toHRTEM tests, X-ray photoelectron spectroscopy (XPS) tests fur-ther confirmed the presence of the metal, and the contents ofCo and Fe at the surface were 0.10 and 0.19 at %, respectively.As reported previously, the retained transition metal after theacid wash will facilitate the ORR process.[15]

The cycling ability was also studied, and the results are com-pared in Figure 8 b. Clear ORR degradation can be observed forall the samples. However, after 50 cycles, the reduction currentdensity for Fe-MHC is slightly higher than that for Co-MHC.The retention ratios of the peak current density after 50 cyclesfor MHC, Co-MHC, and Fe-MHC are 52.3 %, 53.5 %, 62.3 %, re-spectively (obtained from Figure S3). The improved cycling sta-

Figure 6. HRTEM images of a) Co-MHC and b) Fe-MHC.

Figure 7. Raman spectra for Co-MHC, Fe-MHC, and MHC.

Figure 8. a) CV curves of MHC, Co-MHC, and Fe-MHC. b) Cycling stabilitytests after 50 cycles for MHC, Co-MHC, and Fe-MHC. Electrolyte: O2-saturated0.5 m bis(trifluoromethane)sulfonamide lithium (LiTFSI) in tetraethyleneglycol dimethyl ether (TEGDME) solution (t = 25 8C; scan rate = 10 mV s�1).

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bility for the metal-doped samples, especially for Fe-MHC, maybe related to the higher degree of graphitization of the nano-carbon.

Li-O2 battery performance

The Co-MHC and Fe-MHC electrodes delivered initial dischargecapacities of 7440 and 9260 mAh g�1, respectively, as shown inFigure 9. In particular, the Fe-MHC cathode shows a capacity

about 1.5 times that of MHC (5860 mAh g�1). From our previ-ous study, the capacity of the best commercial carbon material,KB (KetjenBlack 600), is just 3214 mAh g�1 under the same testconditions.[12] Although the total pore volume of Co-MHC islarger, Fe-MHC delivers higher electrode space utilizationowing to its abundant macropores that can facilitate oxygentransport, as indicated by the SEM images of the electrodes.Moreover, the Co-MHC and Fe-MHC cathodes exhibit averagedischarge plateaus approximately 60 mV higher than that ofMHC (2.77 V vs. 2.71 V). The decreased discharge overpotentialis in line with the improved ORR activity obtained in CV tests.

Conclusion

In summary, hierarchical meso–macro porous carbon materialswith a graphene-rich structure and trace incorporated metal(Fe, Co) components were developed successfully. The additionof metal salts improved the activity and stability of the ob-tained carbon. Moreover, Fe-MHC as a cathode material exhib-ited a greatly improved discharge capacity for Li–O2 batteries.The outstanding performance of the metal-doped carbon is at-tributed to: 1) the metal nanoparticles formed during carboni-zation acting as a second template for pore construction, lead-ing to a higher pore volume and discharge capacity; and2) the encapsulated metal catalyzing the surrounding carboninto a graphene layer, and simultaneously, the trace amount ofmetal acting as a catalyst for electrode reactions. This novelstrategy also provides a promising candidate giving an excel-

lent electrochemical performance in fuel cells, lithium–sulfurbatteries, supercapacitors, and so on.

Experimental Section

Synthesis of hierarchical Fe-MHC and Co-MHC

Commercial nano-CaCO3 powder (ca. 30–50 nm, Henan Keli NewMaterial Co. Ltd) was used as a hard template to synthesize thesample according to procedures reported in our previous study.[12]

Briefly, Fe(NO3)3·9 H2O (0.404 g, 1 mmol) or Co(NO3)2·6 H2O (0.291 g,1 mmol), and sucrose (2 g) were dissolved in deionized water(15 mL), in which CaCO3 nanoparticles (4 g) were dispersed at 60 8Cunder vigorous stirring until a slurry mixture was obtained. Themixture was vacuum-dried at 60 8C for 24 h. The final solid was car-bonized at 900 8C in Ar for 5 h. The sample after pyrolysis was la-beled CaO-Co/C or CaO-Fe/C. Then, the above material waswashed with 2 m HCl to remove CaO and any metallic or metaloxide crystallites that might have formed during thermolysis. Theproduct thus obtained after acid treatment was labeled Co-MHC orFe-MHC. CaO/C and pure MHC were used as the blank samples forcomparison, respectively.

Cathode preparation

The cathode was prepared through a dry rolling process describedin our previous work. The obtained Co-MHC or Fe-MHC and PTFE(Teflon, solid content = 61.5 %, DuPont) were mixed in the weightratio of 4:1 and rolled into a thin film. After drying at 80 8C undervacuum for over 8 h, the film was punched into disks of diameter15 mm as cathodes, and the carbon loadings were approximately(6.3�0.4) mg.

Characterization

Nitrogen sorption measurements were performed on a Micromerit-ics ASAP 2010 at �196 8C. The specific surface areas were calculat-ed by using the Brunauer–Emmett–Teller (BET) method. The poresize distributions (PSDs) were found from the desorption branchesbased on the Barrett–Joyner–Halenda (BJH) method.

X-ray diffraction (XRD) patterns were collected on a PANalyticalX’Pert PRO powder X-ray diffractometer using a CuKa radiationsource (l= 0.15432 nm) operating at 40 kV and 40 mA. The contin-uous mode was used for data collection at a scanning speed of108min�1.

SEM experiments were performed with a FEI QUANTA 200F elec-tron microscope operating at 20 kV. TEM was conducted ona Tecnai G2 Spirit electronic microscope with an accelerating volt-age of 120 kV, and HRTEM was performed with a Tecnai G2 F30 S-Twin transmission electron microscope operating at 300 kV. Thesamples for TEM and HRTEM observations were dispersed ultrason-ically in ethanol and dropped onto a holey carbon film ona copper grid. Energy-dispersive X-ray spectroscopy (EDX) analysiswas also performed.

ICP-AES analysis of the dissolved sample was performed to deter-mine the bulk Co and Fe contents. Raman scattering spectra wererecorded with a Renishaw inVia Raman Microscope (514 nm laser).

The surface chemical compositions of the samples were deter-mined by X-ray photoelectron spectroscopy (XPS) on a VG ESCA-LAB250 spectrometer (Thermo Electron, U.K.) using AlKa radiation

Figure 9. Li–O2 single-cell discharge voltage curves with Fe-MHC, Co-MHC,and MHC as cathodes, respectively. Cathode: 15 mm in diameter. Carbonloading: (6.3�0.4) mg. Electrolyte: 1.0 m LiTFSI in TEGDME. Separator: poly-propylene fiber separator (Novatexx 2471, Freudenberg Filtration Technolo-gies KG, 22 mm in diameter). Anode: Li foil (16 mm in diameter, 0.45 mmthick). Operating conditions: PO2 = 1.2 atm; I = 30 mA g�1

carbon ; t = 25 8C.

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under operation at 300 W. Spectra were calibrated against the C 1s(284.6 eV) peak.

Electrochemical measurements

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 1 m) in tetra-ethylene glycol dimethyl ether (TEGDME) was used as the electro-lyte. A polypropylene fiber separator (Novatexx 2471, FreudenbergFiltration Technologies KG) (22 mm in diameter) was used as theseparator. All Li–O2 single cells were discharged galvanostatically(ArbinBT-2000) at a rate of 30 mA g�1 carbon with a cutoff voltageof 2.0 V in an O2 atmosphere (1.2 atm) at 25 8C. The specific capaci-ty of the cell was normalized to the mass of carbon materials inthe cathode.

Electrochemical measurements were conducted with an AFCBP1bipotentiostat (Pine Research Instrumentation, USA) at room tem-perature. A three-electrode electrochemical system was employed,incorporating a Li foil as the counter electrode, a Li foil immersedinto electrolyte and connected to the main solution by a glass fritas the reference electrode, and a glassy carbon electrode (GCE,area: 0.1256 cm2) as the working electrode. The CV tests were per-formed in O2-saturated 0.5 m LiTFSI in TEGDME electrolyte. The po-tential range was from 4.3 to 2.0 V, and the scan rate was10 mV s�1. The electrochemical stability tests were performed onthe basis of continuous potential cycling for 50 cycles at 10 mV s�1

in O2-saturated solution at room temperature. The thin-film layeron the working electrode was prepared as follows: Fe-MHC (1 mg)or Co-MHC (1 mg), 2-propanol (0.5 mL), and lithiated Nafion (15 mL,Du Pont Corp., 5 wt %) were mixed and blended ultrasonically for20 min to obtain a homogeneous ink, and then, 5 mL of the inkwas coated on a clean GCE surface with an Fe-MHC or Co-MHCloading of 77 mg cm�2.

Keywords: carbon · catalysis · hierarchical structures · lithium–oxygen batteries · nanoparticles

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Received: March 2, 2014Revised: May 26, 2014Published online on && &&, 0000

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J. Li , Y. Zhang,* W. Zhou , H. Nie , B. Wu,H. Zhang*

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Hierarchical Graphene-Rich CarbonMaterials with Trace NonpreciousMetals for High-Performance Li–O2

Batteries

Improving cathodes : Added transition-metal components, as assisting tem-plates and graphitization catalysts, havea great effect on the pore optimizationand graphene-rich structure formationin carbon frameworks (see figure). Theobtained hierarchical carbon materialsdoped with trace metals exhibit en-hanced discharge capacities, electroca-talytic activities, and stabilities for use inLi–O2 batteries.

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