Surface-Driven Sodium Ion Energy Storage in Nanocellular CarbonFoamsYuyan Shao,* Jie Xiao, Wei Wang, Mark Engelhard, Xilin Chen, Zimin Nie, Meng Gu, Laxmikant V. Saraf,Gregory Exarhos, Ji-Guang Zhang, and Jun Liu*

Pacific Northwest National Laboratory, Richland, Washington 99352, United States

*S Supporting Information

ABSTRACT: Sodium ion (Na+) batteries have attracted increasedattention for energy storage due to the natural abundance of sodium,but their development is hindered by poor intercalation property ofNa+ in electrodes. This paper reports a detailed study of high capacity,high rate sodium ion energy storage in functionalized high-surface-areananocellular carbon foams (NCCF). The energy storage mechanism issurface-driven reactions between Na+ and oxygen-containing func-tional groups on the surface of NCCF. The surface reaction, ratherthan a Na+ bulk intercalation reaction, leads to high rate performanceand cycling stability due to the enhanced reaction kinetics and theabsence of electrode structure change. The NCCF makes more surfacearea and surface functional groups available for the Na+ reaction. Itdelivers 152 mAh/g capacity at the rate of 0.1 A/g and a capacityretention of 90% for over 1600 cycles.

KEYWORDS: Energy storage, sodium battery, surface driven reaction, oxygen functional group, nanocellular carbon foams

Low-cost, long-lifetime, and highly efficient energy storagesystems are very important for increased use of renewable

energy and electric vehicles.1,2 Lithium ion batteries are alreadywidely accepted in microelectronic and mobile devices and arefinding applications in electrical vehicles and emerging gridenergy storage markets.3 A wide range of suitable electrodematerials have been developed, mostly based on Li+

intercalation chemistry.4−10 Capacitance-like surface reactionmechanisms have also been investigated for high powerapplications by several groups.11−15 Still, there is a great needto significantly reduce the cost and improve the long-termreliability and stability of the energy storage system through thediscovery and optimization of new energy storage mecha-nisms.1,16−20 Sodium batteries have been considered as anattractive alternative to lithium batteries since sodium isabundant. There have been significant efforts on room-temperature sodium ion batteries,21−30 including cathodes,31−36

anodes,37−42 and electrolytes.43−45 Most Na-ion batteryelectrode materials (especially cathodes) are based onintercalation reactions, but development of efficient Na+

intercalation compounds is a challenge because Na+ is muchlarger than Li+. The Na+ insertion/deinsertion in a hostmaterial is much more difficult than that of Li+.37 During Na+

insertion/deinsertion, a larger structure change occurs, leadingto low capacity and poor cycling stability. For cathodematerials, the reversible, stable capacity of bulk Na+

intercalation is usually limited to ∼120 mAh/g,27,30,32,46 farbelow what can be obtained in Li-ion electrode materials. Somehigher capacity Na+ cathode materials have been reported, but

most of these materials have fast capacity fading duringcycling.23

Since Na+ intercalation is difficult in most host materials, it isimportant to consider new storage mechanisms that are notdependent on bulk diffusion and intercalation. In this paper, weexplore a surface-driven Na+ energy storage mechanism basedon the reactions between Na+ and oxygen functional groups ofhigh-surface-area, free-standing, binder-free nanocellular carbonfoam (NCCF) papers. A Na+ energy storage device based onthis new mechanism and the high-surface-area functionalizedNCCF can have significantly enhanced energy storage capacityand rate capability and exceptional cycling stability. (Thesurface storage mechanism indeed looks like a capacitor. Butcompared to regular capacitor materials, as we will present inthe following sections, the specific energy density is muchhigher. So, we used the term “Na+ energy storage device”,instead of “Na+ capacitor” or “Na+ battery”.) In addition, theuse of free-standing binder-free NCCF papers as electrodessimplifies the electrode structure and reduces the parasiticweight from binders and other additives.High-surface-area, free-standing, binder-free NCCF papers

(Marketech International) were first functionalized to formoxygen functional groups on the surface using concentratedH2SO4/HNO3 mixed acid. In brief, an NCCF paper was putinto H2SO4/HNO3 (vol 3:1) at 50 °C under mild mechanical

Received: June 1, 2013Revised: July 18, 2013Published: July 23, 2013

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stirring for 1 h; the functionalized NCCF was washed withdeionized water and dried in vacuum (80 °C, 72 h) before use(denoted as “NCCF-Acid”). For comparison, the KOHactivation was carried out under N2 at 700 °C for 2 h (denotedas “NCCF-KOH”). As expected, after functionalization, themeasured electric conductivity drops from 105 S/cm (NCCF)to 82 S/cm (NCCF-KOH) and 58 S/cm (NCCF-Acid), butboth are still very conductive.Figure 1 presents scanning electron microscope (SEM) and

high-resolution transmission electron microscopy (HR-TEM)images of NCCF and NCCF-Acid. SEM images show highlyporous morphology and HR-TEM images show disorderedstructure of both NCCF and NCCF-Acid. There is no obviouschange in the morphology and structure of NCCF after acidfunctionalization (also see Supporting Information Figure S1).The Brunauer−Emmett−Teller (BET) test results show similarpore size (∼18 nm) and pore size distribution among NCCF,NCCF-KOH, and NCCF-Acid (Supporting Information FigureS2). The BET surface area is almost the same for NCCF and

NCCF-Acid with an enhanced specific surface area for NCCF-KOH (Table 1). X-ray photoelectron spectroscopy (XPS)

surface analysis (Supporting Information Figure S3) showsenhanced signal in the binding energy range of 275.5−290 eV(C1s) for NCCF-Acid, which is attributed to carbon bondswith oxygen (O−CO/CO),47−50 while NCCF andNCCF-KOH show no obvious peaks in that range. The threeelectrode materials, NCCF, NCCF-Acid, and NCCF-KOHmake a good model system to study the factors (surface

Figure 1. SEM (a,c) and high-resolution TEM images (b,d) of nanocellular carbon foam (NCCF) papers before and after acid functionalization.(a,b) NCCF; (c,d) NCCF-Acid.

Table 1. BET Test Results for NCCF-Acid, NCCF-KOH,and NCCF

NCCF-Acid NCCF-KOH NCCF

surface area (m2/g) 513.4 1082 537.4pore size (nm) 17.9 17.7 17.7pore volume (cc/g) 0.75 1.11 0.79

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chemistry, morphology, surface area, etc.) that influencesurface-driven Na+ energy storage behaviors.We first investigate surface redox reactions on functionalized

NCCF electrodes in Na+ cells using cyclic voltammetry (CV).NCCF papers and sodium foil were used as cathode and anodein coin cells, respectively, with a Celgard K1640 separator. Theelectrolyte was 1.0 M NaPF6 in EC/DMC (3:7). Figure 2a

shows the CV on an NCCF-Acid electrode. Broad redox peaksoccur in the middle of the CV (1.5−4.2 V); however, no redoxpeaks can be observed on NCCF-KOH and NCCF electrodes,and their current responses are also much lower than that of theNCCF-Acid electrode (Supporting Information Figure S4).Beyond that potential range, both the oxidation current (>4.2V) and the reduction current (<1.5 V) start to increase; theseincreases are attributed to the oxidation and reduction ofelectrolyte, respectively. Therefore, the potential range of 1.5−4.2 V (shadowed region in Figure 2a) was chosen for furtherelectrochemical investigations. Figure 2b shows CVs at variousscanning rates and the linear relationship between peakcurrents and scanning rates, which indicates the redox reactionis confined to the surface and not diffusion limited.11 Wetentatively attribute the surface redox reaction to the onebetween carbon−oxygen functional groups and Na+, that is,−CO + Na+ + e ↔ −C−O−Na, similar to that for Li+.11,13

These preliminary CV results indicate that using oxygenfunctional groups for Na+ surface energy storage showspromise.Figure 3 shows the performance of a Na+ cell using an

NCCF-Acid cathode in comparison with others. The specificcapacity is 152 mAh/g (0.1 A/g) based on the total weight ofthe whole electrode, while NCCF and NCCF-KOH havespecific capacities of only 46 and 70 mAh/g, respectively(Figure 3a). This is consistent with CV results, which show thehighest current response for an NCCF-Acid electrode, whileNCCF-KOH and NCCF show rectangular CVs with lowercurrent response that are characteristic of electrochemicaldouble-layer capacitors (Supporting Information Figure S4).Figure 3b shows the discharge/charge profiles of an NCCF-Acid/Na cell at various discharge/charge rates. The rateperformance is excellent; the specific capacity is ∼100 mAh/gat the discharge rate of 1.0 A/g and ∼50 mAh/g at 5.0A/g.The NCCF-Acid electrode in fact shows superior power/

energy capability. The Ragone plot of an NCCF-Acid/Nacathode is presented in Figure 3c together with two otherrepresentative Na-ion battery cathodes (Na4Mn9O18,

32 P2-Na2/3[Fe1/2Mn1/2]O2

23) and lithium batteries. To our best

knowledge, P2-Na2/3[Fe1/2Mn1/2]O223 presents the highest

energy storage capacity while Na4Mn9O18 shows the bestcycling stability32 in the literature. Our NCCF-Acid electrodeshows energy s torage per formance super ior toNa2/3[Fe1/2Mn1/2]O2 and Na4Mn9O18, especially at highpower. Since LiFePO4 is widely investigated as a Li-ion batterycathode material for stationary energy storage,1 the Ragoneplots of the LiFePO4/Li cell and LiFePO4/TiO2 cell arepresented for comparison.51 The properties of NCCF-Acid/Naare impressive even compared with the Li-ion chemistry.The cycling stability of NCCF-Acid is presented together

with NCCF-KOH and NCCF in Figure 3d. The capacity of theNCCF-Acid electrode drops slightly during the early cycles andthen becomes flat with 90% capacity retention after 1600 cycles.In comparison, the capacity of Na2/3[Fe1/2Mn1/2]O2 drops byover 20% within 30 cycles23 and that of Na4Mn9O18 drops byover 20% within 500 cycles.32

It is important to understand the surface reactionmechanisms and their relation to the high Na+ charge storagecapacity. The XPS surface chemistry analysis provides directevidence of the reaction between Na+ and oxygen functionalgroups on the NCCF-Acid electrode. After acid functionaliza-tion, C1s XPS (Supporting Information Figure S3) shows apeak on the NCCF-Acid electrode in the binding energy (BE)range of 287−290 eV which is attributed to carbon−oxygendouble bond groups (O−CO/CO).14,47−50 Oxygencontent also increases significantly from 2.8% for NCCF and4.6% for NCCF-KOH to 22.6% for NCCF-Acid (Table 2).Figure 4a shows the C1s XPS of an NCCF-Acid electrodebefore and after discharge/charge. After discharge, the carbon−oxygen double bond peak (O−CO/CO) decreases and anew bump (peak) appears in the BE range of 285.5−287.5 eV,which is attributed to a carbon−oxygen single bond (C−

Figure 2. Cyclic voltammograms for functionalized carbon paper(NCCF-Acid) cathode in an NCCF-Acid/Na cell: (a) 1.0 mV/s, (b)0.2−5 mV/s. (inset) Linear relationship between redox peak currentand scanning rates.

Figure 3. (a) Comparison of discharge−charge curves of NCCF-Acid/Na, NCCF-KOH/Na, and NCCF/Na cells at the rate of 0.1 A/g; (b)discharge−charge curves of NCCF-Acid/Na cells at rates from 0.1A/gto 5A/g ; (c) Ragone plot of various Na cathodes (data for cathodematerials outside this work are drawn from the literature:Na4Mn9O18,

32 P2-Na2/3[Fe1/2Mn1/2]O223) and LiFePO4, LiFePO4−

TiO2;51 (d) cycling stability of NCCF-Acid, NCCF-KOH, and NCCF

electrodes (0.1 A/g).

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O).11,52−54 This correlates perfectly with the discharge reaction−CO + Na+ + e → −C−O−Na, which involves thetransformation from carbon−oxygen double bond to carbon−oxygen single bond. After charge, the C1s XPS is restored to acurve almost the same as that for the original NCCF-Acid. Thisagain correlates very well with the charge reaction −C−O−Na→ −CO + Na+ + e. This also indicates that thetransformation between carbon−oxygen double bond andcarbon−oxygen single bond is in fact very reversible. Otherfunctional groups such as C−OH or C−O−C might alsoparticipate in the reaction and contribute to the capacity butprobably very little.14

The surface analysis result for Na content on NCCF-Acid isalso consistent with carbon−oxygen bond change duringdischarge/charge. In the wide-scan XPS (Figure 4b), the Nasignal increases significantly on the NCCF-Acid electrode afterdischarge and then disappears after charge. The results fromhigh-resolution XPS, which are summarized in Table 2, providequantitative information: after discharge, Na content increasesfrom 0 for original NCCF-Acid to 6.5%; after charge, Nacontent decreases back to ∼0 (0.2%). This is again consistentwith the redox reaction mechanism between oxygen functionalgroups and Na+.The above XPS analysis helps to elucidate the surface redox

reaction mechanism behind the outstanding charge storagebehavior of NCCF-Acid. However, we could not exclude thecontributions of other mechanisms such as a double-layercapacitor mechanism, because the NCCF electrode, which doesnot exhibit a surface redox reaction, also has a capacity of 30%of that of the NCCF-Acid electrode even though they bothhave the same surface area and similar morphology. Here, weconsider three potential reaction mechanisms that mightcontribute to the capacity of NCCF-Acid: (1) surface reactionbetween Na+ and oxygen functional groups (−-CO + Na+ +e ↔ −C−O−Na); (2) the adsorption/desorption of anegatively charged PF6

− ion; and (3) the bulk insertion/deinsertion of PF6

−. In fact, in the case of LiPF6, the

intercalation of PF6− in bulk carbon materials usually occurs

in the potential range of 4.5−5.4 V (Li/Li+)55 or even higher.56

We expect similar behavior for NaPF6 in this work; thereforethe working potential of the NCCF-Acid electrode (1.5−4.2 Vvs Na/Na+, that is, 1.8−4.5 V vs Li/Li+) is not within the rangefor the insertion/deinsertion of PF6

−. We did not see obviousPF6

− insertion in the NCCF-Acid electrode from X-raydiffraction analysis either because the diffraction peak is thesame before and after discharge/charge (Supporting Informa-tion Figure S5), which means the d-value between graphenelayers does not change or changes very little. From XPSelement analysis, we can conclude that it is not due to theadsorption of PF6

− either, because the ratios of P/F are 1/52and 1/29 for the discharged and charged electrode respectively(Table 2), which differ significantly from the stoichiometry of1/6 for PF6

−. The P/F surface chemistries of discharge/chargedelectrodes are quite different from PF6

− (SupportingInformation Figure S6). Therefore, the charge storagemechanism of the NCCF-Acid electrode is mainly the surfaceredox reaction between carbon−oxygen functional groups (C/O double bonds) and Na+. The fact that NCCF-KOH, evenwith much higher surface area but much less oxygen groups, hasmuch lower capacity than NCCF-Acid also tells the critical roleof functional groups. The highly porous structure and highsurface area of NCCF makes more surface and surfacefunctional groups available to react with and store Na+. Wehave also studied other high surface area functionalized carbonmaterials, such as graphene and mesoporous carbon. They alsoshow similar behaviors.In summary, we have reported a new surface-driven Na+

energy storage method. The functionalized NCCF-paper-basedcathode presents high specific capacity and cycling stability.The superior performance comes from the new surface-reactiondriven charge storage mechanism on the high-surface-areaNCCF. The surface redox reaction kinetics on the highlyconductive carbon substrate is much faster than the Na+ bulkintercalation reaction because there is no bulk diffusion of Na+

like that in the intercalation compounds; and there is noelectrode structure change during the surface reaction. Theselead to high rate performance and cycling stability. Furtherimprovement could be done by tuning functional groups,morphology, and structure of carbon materials.14,16,57 However,this new cathode does not contain sodium (it is in the chargedstate). An anode in the charged state is needed for practicalapplication of this cathode. Sodium metal may be a candidate,but there is probably a safety concern. Recent progresses inlithium metal protection might provide guidance for sodiummetal anodes.58,59 New anode materials with novel electrodedesign are also under development and will be reported later.As is known, nanomaterials are often penalized by lowvolumetric energy density.5 Efforts are also needed to increasethe volumetric energy density in future research.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional information about experimental details and addi-tional figures. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E- mail: (J.L.) jun.liu@pnnl.gov; (Y.S.) yuyan.shao@pnnl.gov.

Table 2. Atomic Percentages of Na, P, F, C, and O onCarbon Paper Electrodes (Calculated from the High-Resolution XPS)

% Na P F C O

NCCF 0 0 0 97.2 2.8NCCF-KOH 0 0 0 91.1 4.6NCCF-acid (Original) 0 0 0 77.4 22.6NCCF-acid (after discharge) 6.5 0.3 15.7 51.3 23.2NCCF-acid (after charge) 0.2 0.5 14.5 56.6 26.5

Figure 4. XPS of NCCF-Acid electrodes before and after discharge/charge in NCCF-Acid/Na cells. (a) C1s, (b) wide-scan XPS thatshows the presence of Na after discharge and its disappearance aftercharge.

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NotesThe authors declare no competing financial interests.

■ ACKNOWLEDGMENTS

This work was primarily supported by the U.S. Department ofEnergy, Office of Basic Energy Sciences, Division of MaterialsSciences and Engineering. We are grateful for the financialsupport from the Office of Electricity Delivery and EnergyReliability of the U.S. Department of Energy (DOE) fordeveloping the Na-ion storage battery technology. The XPS,TEM, and SEM work was performed using EMSL, a nationalscientific user facility sponsored by the Department of Energy’sOffice of Biological and Environmental Research and located atPacific Northwest National Laboratory (PNNL). PNNL is amultiprogram national laboratory operated for DOE byBattelle.

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Nano Letters Letter

dx.doi.org/10.1021/nl401995a | Nano Lett. 2013, 13, 3909−39143914