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Metalatedgraphenenanoplateletsandtheirusesasanodematerialsforlithium-ionbatteries

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Metalated graphene nanoplatelets and their uses as anode materials for lithium-ion batteries

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2DMater. 4 (2017) 014002 doi:10.1088/2053-1583/4/1/014002

PAPER

Metalated graphene nanoplatelets and their uses as anodematerials for lithium-ion batteries

Jiantie Xu1,4, In-Yup Jeon2,4, Hyun-JungChoi2, Seok-JinKim2, Sun-Hee Shin2, NoejungPark3,LimingDai1 and Jong-BeomBaek2

1 Department ofMacromolecular Science and Engineering, CaseWestern ReserveUniversity, Cleveland,OH44106,USA2 School of Energy andChemical Engineering/Center forDimension-Controllable Organic Frameworks, UlsanNational Institute of

Science andTechnology (UNIST), UNIST 50,Ulsan 44919, Korea3 School of Natural Science, UlsanNational Institute of Science andTechnology (UNIST), UNIST 50,Ulsan 44919, Korea4 These authors contributed equally.

E-mail: jbbaek@unist.ac.kr, noejung@unist.ac.kr and liming.dai@case.edu

Keywords:metalation, ball-milling, graphene nanoplatelets, anodematerials, lithium-ion batteries

Supplementarymaterial for this article is available online

AbstractA series of post-transitionmetals and semimetals in groups IIIA (Al, Ga, In), IVA (Ge, Sn, Pb) andVA(As, Sb, Bi)were introduced onto graphene nanoplatelets (GnPs) bymechanochemical reaction. Theselectedmetals have a lower electronegativity (χ, 1.61�χM�2.18) but amuch larger covalentatomic radius (dM=120–175 pm) than carbon (χC=2.55, dC=77 pm). The effect of theelectronegativity and atomic radius of themetalatedGnPs (MGnPs,M=Al, Ga, In, Ge, Sn, Pb, As,Sb, or Bi) on the anode performance of lithium-ion batteries was evalusted. Among the series ofpreparedMGnPs, GaGnP (χGa=1.81, dGa=135 pm) in group IIIA, SnGnP (χSn=1.96,dSn=140 pm) in group IVA and SbGnP (χSb=2.05, dSb=141 pm) in groupVA exhibitedsignificantly enhanced performance, including higher capacity, rate capability and initial Coulombicefficiency. Both the experimental results and theoretical calculations indicated that the optimumatomic size (dM∼140 pm)wasmore significant to the anode performance than electronegativity,allowing not only efficient electrolyte penetration but also fast electron and ion transport across thegraphitic layers.

1. Introduction

Today, lithium-ion batteries (LIBs) not only dominatethe market in electronic devices (e.g., MP3, PC,camera), they have become one of the most promisingrechargeable energy sources for electric vehicles (EV)and hybrid EV (HEV) [1]. However, to meet the ever-increasing demand for EVs and HEVs, and providesufficiently high energy and power densities, it iscritically necessary to explore and develop LIB elec-trodematerials [2, 3]. Over the past few decades, manyefforts have been devoted to developing anodemateri-als for LIBs, which can replace commercial graphite,which has low theoretical capacity of 372 mAh g−1 andpoor rate capability. The search for alternatives hasinvolved the study of Li4TiO5O12, Si, Sn, metal oxides,and graphene [4]. Among these alternatives, graphenehas proved to be an ideal new anode material for LIBs,

with much higher capacity and rate capability thangraphite [5]. Graphene’s excellent electrochemicalperformance can be attributed to its particular combi-nation of properties, including electrical conductivity,large surface area, remarkable mechanical flexibilityand thermal stability [6].

Significant efforts have been made to furtherenhance the anode performance of graphene-basedmaterials by controlling their morphology (e.g., dis-ordering their surface morphology, porosity, creatingholes and defects) [5, 7–22] and by introducing het-eroatoms or heteroatom-containing functionalgroups (e.g., B, N, S, P, F, Cl, Br, I, or combinationsthereof) [9, 13, 17, 18, 23–34]. When lithium inter-calates into graphite, lithium ions diffusemainly in thein-plane direction, and then the ions occupy sitesbetween two adjacent graphitic planes. When the gra-phite is in a fully lithiated state, each lithium is

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associated with a hexagonally connected graphitic car-bon (C) to form a Li–C6–Li–C6 sequence along the c-axis [35–37]. Because of this process, the graphiticedges play a key role during the lithiation-delithiation,by providing active sites/portals for lithium ion sto-rage/diffusion into internal graphitic layers [38].

Recently, we prepared a series of edge-halogenatedgraphene nanoplatelets (XGnPs, X=F, Cl, Br, I,COOH or H) by ball-milling graphite in the presenceof fluorine (F2), chlorine (Cl2), bromine (Br2), iodine(I2), carbon dioxide (CO2) or hydrogen (H2). TheseXGnPs displayed good electrochemical performancewith cycle stability suitable for use in LIBs [27, 33]. Theenhanced electrochemical performance of XGnPs isattributable to the combination of higher electro-negativity and widened graphitic edges, which cansupport efficient lithiation-delithiation. One of fea-tures of halogens (X=Cl, Br and I) is that they have ahigher electronegativity (χ=2.66–3.16) and largeratomic radius (dX=99–133 pm) than carbon(χC=2.55, dC=77 pm). To date, while intensivestudies have been carried out to verify the positiveeffect of polar heteroatom-doped carbon based mate-rials on anodic performance for LIBs, their effect onthe interlayer distance (d)–(d) of graphitic edges hasstill not been clarified. Given the nature of graphiticanodes, understanding this edge effect is a criticallyimportant step toward designing a new class of anodematerials for high capacity LIBswith stable cycling.

2. Experimental section

2.1.MaterialsMGnPs were prepared by ball-milling graphite (AlfaAesar, Natural, −100 mesh, 99.9995% metals basis,#14735) in the presence of post-transition metal(M=Al, Ga, In, Sn, Pb or Bi) or semimetal (M=Ge,As or Sb) in a planetary ball-mill crusher. In a typicalexperimental set-up, graphite (5.0 g) and M (5.0 g)were placed in a stainless steel container (250 ml).After adding 500 g of stainless steel balls, the containerwas sealed and five cycles of argon charging (70 psi)/discharging (0.05 mmHg) were applied to remove aircompletely. The container was then fixed in a plane-tary ball-mill machine and agitated at 500 rpm for48 h. The resultantMGnPswere treatedwith 1.0 M aq.HCl solution for 48 h to etch off residual free-standingmetallic reactants and impurities and further treatedwith warm concentrated HCl (∼37%), concentratedHNO3 (∼70%), or aqua regia (HNO3+3 HCl) toremove the remaining reactants (post-transitionmetalor semimetal). The solid samples collected by filtrationwere freeze-dried at −120 °C under reduced pressure(0.05 mmHg) for 48 h, to yield dark black MGnPs.The reference materials, hydrogenated GnPs (HGnP)and carboxylated GnP (CGnP), were prepared usingvery similar procedures to those applied for MGnPs,except for the gas charging [39].

2.2. InstrumentationThe field emission scanning electron microscopy (FE-SEM) was performed using an FEI Nanonova 230.Atomic resolution transmission electron microscopy(AR-TEM)was carried out on a TitanG2Cube 60–300microscope. Time-of-flight secondary ion mass spec-trometry (TOF-SIMS) was carried out with a TOF-SIMS V instrument (ION-TOF GmbH, Germany)using a 10 keV Bi+ primary ion-beam source. X-rayphotoelectron spectra (XPS) were recorded on aThermo Fisher K-alpha XPS spectrometer. The sur-face area was measured using nitrogen adsorption/desorption isotherms using the Brunauer–Emmett–Teller (BET)method on aMicromeritics ASAP 2504 NThemogravimetric analysis was conducted on a TAQ200 (TA Instrument) at a heating rate of 10 °Cmin−1

under air.

2.3. Electrochemical analysisThe electrochemical characterization of all the sampleswas carried out using coin cells. The two-electrodeelectrochemical cells were fabricated by blending theactive materials with acetylene black carbon andPVDF, at a weight ratio of 8:1:1, respectively. N-methyl-2-pyrrolidone (NMP)was used as the blendingsolvent for the mixture. The obtained slurry wascoated on Cu foil, dried at 90 °C for 12 h, and pressedat 2 MPa.N-methyl-2-pyrrolidone (NMP)was used asthe blending solvent for the mixture. The electrodewas punched from as-prepared electrode paper in around disk and the mass loading of an electrode wascontrolled to ∼3 mg cm−2. CR 2032 coin-type cellswere assembled in an Ar-filled glove box using the as-prepared electrode as the working electrode, lithiumfoil as the counter electrode and reference electrode,porous polypropylene film as separator, and 1MLiPF6(Sigma-Aldrich) in a 1:1:1 (v/v/v)mixture of dimethylcarbonate, ethylene carbonate, and diethyl carbonateas the electrolyte. The cells were galvanostaticallycharged and discharged using an automatic batterytester system (Land®, China) at various currentdensities in the range 0.02–3 V (assumption:1 C=500 mA g−1). It should be noted that, given thevarious theoretical capacities of MGnPs, for simpledescription the normalized current density (C) in 1 hfor allMGnPswas assumed to be 500 mA g−1. Electro-chemical impedance spectroscopy (EIS) measure-ments were performed using a computer-controlledpotentiostat (CHI 760 C, CH Instruments, USA).Impedance plots were collected for the cells from105 Hz to 10 mHz. The cyclic voltammograms (CV)and EIS were measured using a Biologic VPM3electrochemical workstation.

2.4.Ab initio calculationmethodFor computations, we used the Vienna Ab initioSimulation Package to calculate the ground state of amany-electrons system in the frame work of density

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2DMater. 4 (2017) 014002 J Xu et al

functional theory (DFT) [40–43]. The plane-wavebasis set, with an energy cut-off of 400 eV, and thegradient-corrected exchange-correlation potentialsuggested by Perdew, Burke, and Ernzerhof (PBE-type), were employed [44].

3. Results and discussion

To clarify how the effect of graphitic edges canminimize the contribution of electronegativity, a seriesof metalated GnPs (MGnPs, M=Al, Ga, In, Ge, Sn,Pb, As, Sb or Bi) were, for the first time, systematicallyintroduced by a mechanochemical reaction driven bythe ball-milling technique. As schematically repre-sented in figure 1, post-transitionmetal (Al, Ga, In, Sn,Pb or Bi) or semimetal (Ge, As or Sb)-functionalizedgraphene nanoplatelets (MGnPs, M=Al, Ga, In, Ge,Sn, Pb, As, Sb or Bi) were prepared simply by dry ball-milling graphite in the presence of the correspondingmetal (details are described in the Experimentalsection). The metal (M) atoms in the MGnPs have alarger atomic size (covalent radius,dM=120–175 pm) (figure S1), but a much lowerelectronegativity (χM=1.67–2.05) than carbonatoms (dC=77 pm, χC=2.55). For further compar-ison, (HGnP, dH=37 pm, χH=2.20) and CGnPwere also prepared by ball-milling graphite in thepresence of hydrogen (H2) and dry ice (solid CO2),respectively.

SEM images of graphite before and after ball-mil-ling in the presence of the corresponding metals illus-trate the dramatic changes in grain sizes produced bythe milling (figure S2). While the pristine graphite hasa flake-type morphology with large grain size

(<150 μm, figure S2(a)), the dimensions of all the pre-pared MGnPs were dramatically reduced (<1 μm,figures S2(b)–S2(l)), which confirms that mechan-ochemical reactions occurred. After a complete work-up to remove unreacted metals and other impurities(as described in Experimental section), the presence ofthe essential elements was clearly observed in theMGnPs by SEM energy dispersive x-ray spectroscopy(EDS) (figure S3). The corresponding elemental con-tents were in the range of approximately 0.44–19.27at% (table S1). Furthermore, high resolution trans-mission microscopy (HR-TEM) and atomic-resolu-tion TEM (AR-TEM) with scanning TEM (STEM)images of the prepared AlGnP showed that the Al ele-ment in the AlGnP had atomic level distributionrather than forming a bulk solid state aggregated clus-ter (figure S4). In accordance with our previousreports [45, 46], these results provide evidence thatcarbon (C) and metal (M) bonds were formed as aresult of mechanochemical reactions during the ball-milling.

XPS analysis has been widely utilized to identifythe chemical bonding and/or heteroatom-dopinglevel of carbon materials [27–30]. Pristine graphiteexhibits a major C 1 s peak from the sp2 C–C bond,along with a minor O 1 s peak, due to physical absorp-tion of oxygen and moisture (figure 2) [47]. TheMGnPs display characteristic peaks for each element(Al 2p, Ga 3d, In 3d, Ge 3d, Sn 3d, Pb 4f, As 3d, Sb 4dor Bi 4f) [45, 47–57], in addition to themain C 1 s peakalong with relatively stronger O 1 s peaks. The pre-sence of the O 1 s peak(s) in each sample is due to ter-mination reactions of the remnant active carbonspecies upon contact with air moisture (i.e., O2, CO2

and H2O) upon exposing the sample after processing.

Figure 1.A schematic representation of themechanochemically driven physical breaking of graphitic C–Cbonds andmetallicM–Mbonds, and the formationC–Mbonds, by ball-milling graphite in the presence of post-transitionmetal (Al, Ga, In, Sn, Pb or Bi) orsemimetal (Ge, As or Sb) in solid state.

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2DMater. 4 (2017) 014002 J Xu et al

As determined by XPS, the content of ‘M’ elements intheMGnPs were in the range 0.15–3.74 at% (table S2).The large discrepancy between the SEM EDS and XPSresults is due to the difference between the two meth-ods. It is generally appreciated that XPS significantlyunderestimatesmetallic elements [58, 59]. In addition,the formation of C–M bonds could be confirmed byexamplary HR XPS spectra of MGnPs (figure S5).Deconvoluted XPS spectra of Bi 4f (figure S5(a)), In3(d) (figure S5(b)) and Pb 4f (figure S5(c)) suggest theformation of C–Mbonds.

To further verify the formation of C–M bonds intheMGnPs, pristine graphite and each type of MGnPswere compared one-by-one using TOF-SIMS. Thepositive ion spectra of the MGnPs show the presenceof ions as the isotopes of each element (M=Al, Ga,In, Ge, Sn, Pb, As, Sb or Bi) (figure 3), while pristinegraphite shows only the hydrocarbon peak, withoutany peaks related to post-transition metals orsemimetals.

The Raman spectra for all of the MGnPs showed aD band at around 1350 cm−1 and a G band at around1560 cm−1 (figure S6). The ratios of the D- to G-bandintensities (ID/IG) of the MGnPs are in the range of0.83–1.04, due to significant edge contributions caused

by the reduction in grain size (<1 μm, seefigure S2) andmetalation. The specific surface areas of the MGnPswere determined by BET plots of an N2 adsorption iso-therm (table S3). All of the MGnPs showed muchhigher specific surface areas (117–463m2 g−1) thanpristine graphite (2.8m2 g−1) [47]. This indicates thatthe MGnPs had been significantly exfoliated intoa few graphitic layers (average number of layers=2630m2 g−1, divided by the specific surface area of theMGnPs). In addition, theMGnPs exhibited a high porevolume (0.18–0.46ml g−1) and a mesoporous nature(pore size<5 nm), which can contribute to electro-chemical performance (vide infra).

Given this structural information, the MGnPswere then evaluated as anode materials for LIBs. Thedischarge/charge profiles of the MGnPs (M=H, C,Al, Ga, In, Ge, Sn, Pb, As, Sb or Bi) were measured atvarious C-rates from 0.1 to 10 C in the voltage range of0.02–3 V, and the results are shown in figure 4 and S7,respectively. Together with the discharge/charge pro-files of CGnP and HGnP (figures 4(a)–(c)), all of theother MGnPs show typical electrochemical behaviorsof carbonaceous materials, which is in agreement withprevious results [15, 22, 29, 31].

Figure 2.XPS survey spectra: (a) pristine graphite, CGnP andHGnP; (b) group IIIA (B-familyMGnP,M=Al, Ga or In), (c) groupIVA (C-familyMGnP,M=Ge, Sn or Pb); (d) groupVA (N-familyMGnP,M=As, Sb or Bi).

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In the CV of theMGnPs (M=C,H,Ga, Sn or Sb),the first cathodic peaks appear at around 0.9 V in thefirst cycle and then almost disappear in the followingcycles, as measured during the initial three scansbetween 0.02 and 3 V at a scan rate of 0.2 mV s−1

(figure S8). This indicates the formation of a solid–electrolyte interphase (SEI) layer on the sample sur-faces in the first cycle. After the formation of the SEIfilm, the starting cathodic peak at around 0.5 V is rela-ted to the insertion of Li+ into the graphitic layers,which is a key indicator of lithium storage in graphene.

During anodic scans, the MGnPs (M=Ga, Sn, orSb), and particularly, the SbGnP, clearly showed par-tially reversible redox peaks at around 0.5, 1, and2.5 V, which are most likely due to the interactionbetween lithium ions and the defects/oxygen contain-ing functional groups at the terminal ends of graphiticlayers. This indicates that electroactivity was higher forthe MGnPs (M=Ga, Sn or Sb) than for HGnP andCGnP. Moreover, the high reversibility and stablestructure of the MGnPs (e.g., M=Sb and Ga) wasalso further confirmed by ex situ XRD. The mech-anism of lithium ions stored inMGnPs was studied byXRD. As shown in figure S9, the ex situ XRD patternsof MGnPs (e.g., M=Sb or Ga) at different dischargevoltage of 2.7 V (open circuit voltage), 0.8 V (SEI for-mation), 0.02 V (full discharge) indicate various

degrees of lithium storage in the graphitic layers ([002]‘peak left’ at∼25°) during the discharge process. Inthe charge process, the [002] peak has a slight ‘rightshift’ for MGnPs from 0.02 to 3.0 V (full charge)with-out any distinct impurity peaks, indicating the highreversibility and stable structure ofMGnPs.

In order to investigate their rate and cycling perfor-mance, the MGnP cells were discharged and chargedfor 60 cycles between 0.02 and 3 V at various currentrates (figure 5(a) and S10). In the periodic table, the Melements are divided into three different groups (GroupIIIA: B-family, Group IVA: C-family, and Group V:N-family). As shown in figure S10, in the B-family,GaGnP exhibited a higher initial capacity(1689mAh g−1) than AlGnP (882.15 mAh g−1) andInGnP (1159.44 mAh g−1) at 0.1 C. The reference sam-ples of CGnP had a higher initial discharge capacity(1828.3 mAh g−1) thanHGnP (1720.08 mAh g−1). Thisis because oxygenated groups (e.g., −COOH and−OH) were richly present in the CGnP. The higheroxygen content (table S2, XPS) for CGnP, which resul-ted from ball-milling graphite in the presence of dry ice(the solid form of CO2), can favor higher lithiumadsorption during the initial discharge process. Com-pared to HGnP and CGnP, all B-family MGnPs(M=Al, Ga or In) showed a lower initial dischargecapacity. The decreased discharge capacity of these

Figure 3.Comparison of time-of-flight secondary ionmass spectra (TOF-SIMS) of pristine graphite andMGnPs: (a)AlGnP; (b)GaGnP; (c) InGnP; (d)GeGnP; (e) SnGnP; (f)PbGnP; (g)AsGnP; (h) SbGnP; (i)BiGnP.

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2DMater. 4 (2017) 014002 J Xu et al

MGnPs may be attributed to stable M3+ (e.g., Al3+),which cannot be further oxidized but reduces the activesites available for lithium ions storage. Similar caseshave also been observed in intensive works on inactivealuminum doped cathode materials, in which alumi-num only stabilizes its own structure [60]. Despite thereduced capacity of some MGnPs (M=Al, Ga, or In),their Coulombic efficiencies (figure 5(b), 63.2% forInGnP and 69.5% for GaGnP) were much higher thanthe reference samples CGnP (54.9%) and HGnP(50.1%). In addition, the rate capabilities of AlGnP andGaGnP (figure 5(c) and S11a) were significantlyenhanced. This is thought to be due to improved

charge-transfer kinetics associated with the greater dM–dM entry of lithium ions during diffusion, which occursduring the lithium insertion/extraction processes. Itshould be noted that the lowest Coulombic efficiency(28.5%) for InGnP (with the largest dIn–dIn,figure S1) ismost likely related to the formation of extensiveagglomerations (figure S2(f)), low In content (table S2),and large charge-transfer resistance. Even so, InGnPshowed a higher initial capacity thanAlGnP.

The charge transfer kinetics was further confirmedby EIS. Two of the MGnPs (M=Al or Ga) showed alower charge resistance (diameter of semicircle of EISspectra) [27] than CGnP, HGnP and InGnP (figure

Figure 4. (a) Initial discharge/charge profiles ofMGnPs (M=C,H,Ga, Sn, or Sb) at 0.1 C.Discharge/charge profiles: (b)CGnP; (c)HGnP; (d)GaGnP; (e) SnGnP; (f) SbGnP at various C-rates in the voltage range 0.02–3 V.

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2DMater. 4 (2017) 014002 J Xu et al

S12(a)), indicating fast ion transport for AlGnP andGaGnP.

Compared to the transition metal M3+ in theB-family which had no higher oxidation states (Al, Gaand In); the M elements in the C-family (Ge, Sn andPb) and the N-family (As, Sb and Bi) showed variousoxidation states for many M or M-containing com-pounds [53]. This indicates potentially strong interac-tions between the C–OM or C–M groups and lithiumions. As shown in figure 5(a) and S10(b), the C-familyMGnPs (M=Ge, Sn or Pb) exhibited the initial capa-cities of 1865.3, 1767.3, and 1727.45 mAh g−1 at 0.1 C,with a Coulombic efficiency of 45.4, 67.7, and 53.2%,respectively. The N-familyMGnPs (M=As, Sb or Bi)(figure 5(a) and S10(c)) exhibited initial capacities of

1905.7, 1671.8, and 1832.4 mAh g−1, respectively, at0.1 C; with a Coulombic efficiency of 56.1, 67.2 and60.3%, respectively.

As can be seen, the initial capacities of the MGnPs(M=Ge or Sn) and MGnPs (M=As or Bi) werehigher than those references CGnP (1828.3 mAh g−1)and HGnP (1720.1 mAh g−1). In addition, the higherinitial Coulombic efficiencies of theMGnPs (M=Sn,Sb or Bi) as compared with the references (CGnP:54.9%,HGnP: 50.1%) indicate the high reversibility ofthe lithium ion storage in the first cycle (figure 5(b)).The high initial CE is most likely related to stronginteractions between lithium ions and C-OM or C–Mat the termini, leading to the formation of a thin,strong SEI film and the fast, reversible diffusion of

Figure 5. (a)Rate capabilities ofMGnPs (M=C,H,Ga, Sn or Sb) at from0.1 to 10 C; (b) initial Coulombic efficiency ofMGnPs(M=C,H, Al, Ga, In, Ge, Sn, Pb, As, Sb or Bi ); (c) discharge capacity retention versus C-rates ofMGnPs (M=C,H,Ga, Sn or Sb);(d) long cycling performance ofMGnPs (M=C,H,Ga, Sn or Sb) at 0.5 C in the voltage range 0.02–3 V.

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2DMater. 4 (2017) 014002 J Xu et al

lithium ions (vide infra). In a comparison of all thesamples, SbGnP showed the best rate capability(figure 5(c), S11 and table S4). Consistent with its elec-trochemical performance, the lower charge-transferresistance of SbGnP (among the MGnPs) was alsoconfirmed by EISmeasurements (figure S12).

To further compare their relatively long-termcycling performance, the MGnPs were measured at0.5 C in the voltage range of 0.02–3 V for 500 cycles.The MGnPs exhibited the different cycling stability(figure 5(d) and S13) with different discharge capa-cities in the initial cycles and the 500th cycles (figureS14 and table 1). Generally, it was found that the bestrelatively long-term cycling stability occurred inAlGnP (AlGnP>GaGnP>InGnP), SnGnP(SnGnP>PbGnP>GeGnP) and SbGnP(SbGnP>BiGnP>AsGnP) in the B-, C- andN-families, respectively.

In combination with the results of the rate cap-ability measurements for all MGnPs (figure 5(a) andtable S4), the MGnPs (M=Ga, Sn and Sb) deliveredthe best electrochemical performance in each familycompared to the references HGnP and CGnP. Coin-cidently, the electronegativity (χ) and atomic size (d)of the M’s were approximately 2.0 and 140 pm, whichcould also be associated with the high surface area, theoptimum dM–dM, the moderate oxygenated groups,the low SEI resistance and the low charge transferresistance.

Furthermore, SbGnP showed a higher reversiblecapacity over 500 cycles, than all the other MGnPs(figure 5(d)). To further investigate the kinetics ofMGnPs, we also measured EIS for MGnPs (e.g.,M=Ga, Sn, Sb or C) at 0.5 C at different cycles. Ascan be seen, the Rct’s of MGnPs decrease from the 0th

cycle to the 25th cycle (Figure S15). The decrease in Rct

ismainly attributed to the electrode–electrolyte activa-tion, leading to the enhanced ionic conductivity andefficient utilization of active sites of electrode materi-als. Then, along with Rs, Rct almost remain constant at100th and 200th cycles, indicating the formation ofstrong SEI film and stable diffusion of lithium ions inthe deep cycles. The observed kinetics of MGnPs asanode materials for LIBs is in agreement with thechange trend of cycling performance. In spite of thedifferent capacities and capacity retention, the averageCoulombic efficiency of all the MGnPs approached99% at 0.5 C (figure S16), indicating that the MGnPsmaintain their structural integrity during the lithiumion insertion/extraction process.

Previously, we also reported excellent electro-chemical results for fluorinated GnP (FGnP) as an LIBanode material, which was prepared by the samesynthesis protocol as the present study [33]. Com-pared to the FGnP, whose F has amuch higher electro-negativity (χF=3.98) and smaller atomic size(dF=72 pm) than carbon (χC=2.55, dC=77 pm),SbGnP (χSb=2.05, dSb=141 pm) showed higherinitial Coulombic efficiency and reversible capacity

than FGnP. This was further indication of the strongereffect of the dM–dM than χM on the anode perfor-mance for LIBs.

To understand the effect GnPs metalation on Listorage, we performed first-principle DFT calculationsfor the properties of Li binding to the edges ofMGnPs.In a previous study, the binding energies of a Li atomto a single pure graphene surface and to graphite inter-stitial sites were determined to be about 1.2 eV and2.0 eV, respectively [61], indicating that the insertion/extraction of lithium ions in graphene was relativelyeasier. Based on the same computation parameters, itwas found that the Li adsorbers interactedwith a singlepure graphene at the edges with stronger bindingenergy than at the surface (figure S17).

In our samples, hydrogen passivated grapheneedges were selectively decorated by metal atoms withor without oxygenated species, such as C–OH and –

COOH functional groups. As expected, themain focuswas on the Li binding energetics of the metal-deco-rated termini, in comparison with the pure graphenesurface. Moreover, the functionalized edges of theMGnPs were aggregated into a glassy form, and somepart of them interfaced with the electrolyte. To simu-late whether the metal-decorated edge can accom-modate a higher concentration of Li atoms than thegraphite interstitials within the desired window, there-fore, we also calculated the binding energetics byincreasing the number of Li adsorbates.

For all of the model geometries with oxygen spe-cies, Li atoms developed strong Li–O bonds duringgeometric optimization by DFT total-energy mini-mization. This indicates that the Li coming to the oxy-gen sites is vulnerable to the formation of segregatedlithium-oxide clusters. However, when the edges werefree of oxygen atoms, each metal-decorated site couldaccommodate 3–4 Li atoms in the desired energy win-dow. Figure 6 presents the Li binding energetics of theAs and Sb decorated sites of armchair-type grapheneedges (figure 6(a)) and DFT-optimized Sb-decoratededges and its Li-adsorption configurations(figures 6(b)–(d)). The metal-decorated grapheneedges without oxygen atoms can lead to a higher Liconcentration than for bulk graphite at a bindingenergy lower than 2.0 eV (figure 6(a)). This is almostconsistent with the results of the higher capacity ofMGnPs than references CGnP and HGnP, as well ashigh initial CE forMGnPswith less oxygenated groupsat the termini. It should be noted that the slope of thebinding energy, as shown in figure 6(a), is slightlymilder but similar to that for bulk graphite [61]. Inaddition, we also performed similar investigation ofthe Bi-decorated armchair edge, but found that bis-muth is easily detached from the graphene edges whenthree or more Li atoms concentrate at the site. It isthought that the strength of a C–Bi bond is weakerthan one between C–As or C–Sb; thus, Li could sub-stitute for Bi, thereby effectively segregating Bi fromthe active graphene edge. A few other metals with

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2DMater. 4 (2017) 014002 J Xu et al

Table 1. Initial capacity and capacity retention ofMGnPs after 500 cycles.

MGnPs

M=Post-transitionmetal or semimetalM=C M=H

B-family C-family N-family

Al Ga In Ge Sn Pb As Sb Bi C H

1st cycle (mAhg−1) 377 820.9 532.4 1088.5 1234.7 1046.7 1335.3 1212.3 955.8 1260.8 1064.1

500th cycle (mAhg−1) 213.8 183.7 82.8 218 413.9 260 173.8 521.5 402.3 296.5 249.8

Retention 56.7% 22.4% 15.5% 20% 33.5% 24.8% 13% 43% 42.1% 23.5% 23.5%

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(2017)014002JX

uetal

similar configurations are summarized in figure S18.Hence, it could be noted that the optimum atomic size(dM) of MGnPs is closely related to electrochemicalperformance of LIBs.

4. Conclusion

We prepared a series of metalated GnPs (MGnPs,M=Al, Ga, In, Ge, Sn, Pb, As, Sb or Bi). The metals(M’s, post-transition metals and semimetals) havelower electronegativity (χ, 1.61�χM�2.18) thancarbon (χC=2.55, dC=77 pm) but much largeratomic size (d, 120�dM�175 pm). The exper-imental results and quantum mechanics calculationsclearly revealed that the optimum atomic size (dM)wasmore important than electronegativity (χM). Forexample, GaGnP (χGa=1.81, dGa=135 pm),SnGnP (χSn=1.96, dSn=140 pm), and SbGnP(χSb=2.05, dSb=141 pm) exhibited better anodeperformance in LIBs, due to higher capacity, ratecapability, and initial Coulombic efficiency than all theother MGnPs (M=Al, In, Ge, Pb, As, or Bi) as well asthe references HGnP (χH=2.20, dH=37 pm) andCGnP (χC=2.55, dC=77 pm). The enhanced

electrochemical performance is mainly attributed tothe optimum dM∼140 pm, which not only enablesefficient electrolyte penetration but also facilitates fastelectron and ion transport across the graphitic layers.Moreover, the stronger interaction between lithiumions and MGnPs via Li-M-C, than via Li-MO-C,accommodates greater lithium ion storage anddecreases the irreversible reaction between lithiumions and OM (or oxygenated groups) at the activetermini, leading to high capacity and high initialCoulombic efficiency, respectively. In addition, giventhat the use of heteroatom (especially nitrogen)-dopedcarbon materials in various energy storage fields is ofgreat interest, the strategy for enhancing anodeperformance by employing MGnPs for LIBs using thepositive effect of dM at their termini, could be widelyextended to other energy-related fields.

Acknowledgments

The authors are grateful for financial support from theCreativeResearch Initiative (CRI, 2014R1A3A2069102),BK21 Plus (10Z20130011057) and Science ResearchCenter (SRC, 2016R1A5A1009405) programs through

Figure 6. (a)DFT-calculated binding energetics for Li atoms onto armchair edges ofMGnPs (M=As or Sb); (b)model geometry forarmchair edges of SbGnP.DFToptimized geometries; (c) two Li atoms on SbGnP; (d) three Li atoms on SbGnP. Small gray andwhiteballs represent carbon and hydrogen atoms, respectively. Larger purple and blue balls indicate Li and Sb atoms, respectively.

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2DMater. 4 (2017) 014002 J Xu et al

the National Research Foundation (NRF) of Korea, aswell as from the AFOSR (FA9550-12-1-0037, FA-9550-12-1-0069, FA 9550-10-1-0546), NSF (NSF-CMMI-1000768, NSF-DMR-1106160). Jiantie Xu and In-YupJeon contributed equally.

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