RESEARCH ARTICLE◥

ENERGY STORAGE

Three-dimensional holey-graphene/niobia composite architectures forultrahigh-rate energy storageHongtao Sun,1* Lin Mei,1,2* Junfei Liang,3 Zipeng Zhao,3 Chain Lee,1 Huilong Fei,1

Mengning Ding,3,4 Jonathan Lau,3 Mufan Li,1 Chen Wang,3 Xu Xu,1 Guolin Hao,1

Benjamin Papandrea,1 Imran Shakir,5 Bruce Dunn,3,4 Yu Huang,3,4 Xiangfeng Duan1,4†

Nanostructured materials have shown extraordinary promise for electrochemical energystorage but are usually limited to electrodes with rather low mass loading (~1 milligramper square centimeter) because of the increasing ion diffusion limitations in thickerelectrodes. We report the design of a three-dimensional (3D) holey-graphene/niobia (Nb2O5)composite for ultrahigh-rate energy storage at practical levels of mass loading (>10 milligramsper square centimeter). The highly interconnected graphene network in the 3D architectureprovides excellent electron transport properties, and its hierarchical porous structurefacilitates rapid ion transport. By systematically tailoring the porosity in the holey graphenebackbone, charge transport in the composite architecture is optimized to deliver high arealcapacity and high-rate capability at high mass loading, which represents a critical step forwardtoward practical applications.

Batteries and supercapacitors represent twocomplementary electrochemical energy stor-age (EES) technologies (1–4), with thebatteries offering high energy density butlow power density and supercapacitors

providing high power density with low energydensity. Although lithium (Li)–ion batteries cur-rently dominate themarket for powering consumer

electronic devices and are making in-roads intotransportation and grid storage, there is a growingtechnological demand formore rapid energy stor-age (high power) without compromising energydensity. Thus, there is considerable interest increating materials that combine the high energydensity of batterymaterialswith the short chargingtimes and long cycle life of supercapacitors (5–14).

The combination of high energy density andhigh power density requires materials that canstore a large number of charges (such as Li ions)and an electrode architecture that can rapidlydeliver sufficient charges (electrons and ions) ina given charge/discharge duration. This behaviorcan occur in nanostructured materials that in-clude very thin films and materials reduced tonanoscale dimensions—that is, materials withlow mass loading (<1 mg cm−2) (8, 10, 15–17).However, such nanoscale materials cannot bereadily scaled to electrodes that have practicallevels of mass loading (~10 mg cm−2) because ofincreasing ion diffusion limitations in thickerelectrodes. Moreover, the highly promising electro-chemical properties achieved in low-mass loadedelectrodes rapidly diminish in practical deviceswhen other passive components such as currentcollectors and separators (~10 mg cm−2) are in-cluded (18). As a result, the scaled areal capacityor current density rarely exceeds those of today’sLi-ion batteries (~3mAh cm−2, 4mA cm−2) (19, 20).In general, sustaining the same gravimetric

capacity and current density inhighermass-loadedelectrodes (for example, 10 versus 1 mg cm−2)requires proportionally higher ion and electroncurrents across a longer charge transport distance.

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Fig. 1. Illustration ofthe two-step processflow to prepare 3Dhierarchically porouscomposite architecture.The Nb2O5 is uniformlydecorated on the firstportion of GO (~4 wt % ofthe composite) in stepone and then mixed withthe second portion ofGO/HGO (~11 wt % of thecomposite) in solution,followed by a reductionprocess to produce themonolithic free-standingcomposite. HGOs withtailored nanopores wereprepared by etchingin H2O2 for 0, 0.5, 1.0, to2.0 hours and used toprepare Nb2O5/GF, Nb2O5/HGF-0.5, Nb2O5/HGF-1.0,and Nb2O5/HGF-2.0, with various degrees of porosity in the graphene sheets. The samples were annealed at 600°C in argon at the end of each stepin order to produce the orthorhombic Nb2O5 (T-Nb2O5) and further deoxygenate the RGO sheets so as to improve their electron transport properties. Theamount of T-Nb2O5 is controlled to be ~85 wt % in the final composites.

1Department of Chemistry and Biochemistry, University ofCalifornia, Los Angeles, CA 90095, USA. 2State KeyLaboratory for Chemo/Biosensing and Chemometrics,College of Chemistry and Chemical Engineering, HunanUniversity, Changsha 410082, China. 3Department ofMaterials Science and Engineering, University of California,Los Angeles, CA 90095, USA. 4California NanosystemsInstitute, University of California, Los Angeles, CA 90095,USA. 5Sustainable Energy Technologies Centre, College ofEngineering, King Saud University, Riyadh 11421, Kingdom ofSaudi Arabia.*These authors contributed equally to this work.†Corresponding author: Email: xduan@chem.ucla.edu on July 15, 2018

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Recent studies focusing on the question of massloading in Li-ion batteries showed that the ioniccurrent was especially critical and defined a pen-etration depth to describe the utilization of theactive electrochemical material for a given elec-trode thickness (19). Therefore, in a thick electrode,the mass transport limit of ions is particularlyimportant because insufficient charge transportcould severely degrade the capacity within a fixedcharge/discharge window because of considera-bly higher overpotential (21). These consider-ations suggest that the mass loading required totranslate the high performance achieved by manynanoscalematerials into practical devices is a fun-damental challenge in electrodedesign rather thana matter of scaling.

We report on the design and mass loadingproperties of an electrode architecture using athree-dimensional holey graphene framework(3D-HGF) as the conductive scaffold for electro-chemical activematerial [for example, orthorhom-bic niobia (Nb2O5)]. The highly interconnectedgraphene network in the 3D-HGF provides ex-cellent electron transport properties, and itshierarchical porous structure, with large-sizedpores (macropores) in the 3D network and tun-ablemicro- tomeso-pores in the graphene sheets,facilitates rapid ion transport and mitigatesdiffusion limitations throughout the entire elec-trode architecture. This morphology produces in-terpenetrating electron transport and ion transportpaths that enable high capacity at high charge/

discharge rates at practical levels of mass load-ing (22, 23).We used Nb2O5 as a model system for evaluat-

ing the effect of mass loading for the 3D-HGFscaffold. Although the Li insertion properties ofNb2O5 have been known for many years, onlyrecently was it observed that the orthorhombicform of Nb2O5 (T-Nb2O5) is able to retain highlevels of charge storage at high rate (for example,110 mAh g−1 at 60C) (24). The charge storage pro-perties of T-Nb2O5 are not controlled by semi-infinite diffusion as usually occurs in batterymaterials. Instead, surface-controlled kineticsoccurring in the bulk of the material lead to itsunusually high-rate capability (24, 25). However,because of the limited electronic conductivity of

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Fig. 2. Material characterization of T-Nb2O5/HGF composites. (A toD) TEM images of graphene sheets with tailored pores obtained by etchingin H2O2 for 0, 0.5, 1.0, and 2.0 hours, respectively. (E) Cross-sectionalscanning electron microscopy image of Nb2O5/HGF composite shows 3Dhierarchically porous structure. (Inset) A free-standing monolithic compositeused to make the electrode. (F) XRD patterns of the as-synthesized Nb2O5/G powders before and after 600°C annealing, and free-standing Nb2O5/HGFcomposite. (G) TEM image of graphene sheets with uniformly decorated

T-Nb2O5 nanoparticles. (H) HR-TEM image of T-Nb2O5 nanoparticles.(I) Raman spectra of Nb2O5/G powder after thermal annealing, and free-standing Nb2O5/GF and Nb2O5/HGF electrodes. The D and G bands arecharacteristic of RGO; The Raman bands at 120, 230, 310, and 690 cm−1

further confirm the orthorhombic phase of T-Nb2O5. (J) Comparison of DFTpore size distributions. The prominent pore size shifts from micropores(~1.5 nm) to mesopores (2.7 nm) for the composite prepared from HGOwith increasing etching time.

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T-Nb2O5, the high rate capability can only be re-alized in thin-film electrodes or at relatively lowmass loading (generally <2.0mg cm−2) despite con-siderable efforts in nanostructure design and theuse of carbon-based hybrid architectures (24, 26–29).In the thicker electrodes, the overall rate capabil-ity is limited by insufficient delivery of ions to theelectrode material surface (29). The susceptibilityof the high rate performance of T-Nb2O5 to massloading makes it an interesting model system forinvestigating the design of electrode architectures.

Synthesis and characterization of thehierarchically porous composites

We prepared the free-standing Nb2O5/HGF com-posites using a two-step process in which grapheneoxide (GO) or holey graphene oxide (HGO) iscombined with Nb2O5 (Fig. 1 and supplemen-tary text) (30). The porous composite electrodesare designed to fill with electrolyte and thusenhance the ion transport kinetics (31, 32). Weprepared the composites from HGO sheets withvarying pore sizes, which were prepared by etchingGO sheets with H2O2 for 0, 0.5, 1.0, and 2.0 hours.

The oxidative-etching process initiates from thechemically more active oxygenic defect sites andpropagates in the basal plane of GO to form in-creasingly larger pores with increasing etchingtime (23, 33, 34), as demonstrated through high-resolution transmission electron microscopy(HR-TEM) studies (Fig. 2, A to D). The two-step syn-thesis produce free-standing 3D porous compositeswith graphene framework (GF) or holey grapheneframework (HGF) as the conductive scaffoldsfor Nb2O5 nanoparticles (Fig. 2E and fig. S1), whichare identified as Nb2O5/GF for the material witha nonholey GO as the graphene source, and Nb2O5/HGF-0.5, Nb2O5/HGF-1.0, and Nb2O5/HGF-2.0 forthose materials using HGO as the graphene sources(obtained by etching in H2O2 for 0.5, 1.0, and2.0 hours), respectively.The as-synthesized Nb2O5 on GO in the first

step is amorphous and can be effectively convertedinto the orthorhombic phase (JCPDS 30-873)upon annealing at 600°C in argon, as indicatedby the x-ray diffraction (XRD) studies (Fig. 2F andfig. S2). The crystallite size of T-Nb2O5 is ~15 nm asdetermined by the Scherrer equation based on

XRD peak width (35). The TEM image (Fig. 2G)further confirms that T-Nb2O5 nanoparticlesare homogeneously decorated on the graphenesheets, with a size of around 10 to 15 nm,which isconsistent with that determined from the XRDstudies. The HR-TEM image shows a latticespacing of 0.39 nm (Fig. 2H), corresponding tothe (001) plane of the orthorhombic phase.Raman spectroscopy studies show the expectedD and G bands of reduced GO (RGO) in thecomposites, and the characteristic Raman bandsat 120, 230, 310, and 690 cm−1 for the ortho-rhombic phase T-Nb2O5 (Fig. 2I). In addition, N2

adsorption/desorption isotherms (fig. S3) showvarious cumulative volumes of pores, and thedensity functional theory (DFT) analysis indi-cates that the prominent pore sizes in the 3DNb2O5/HGF composites nearly double from ~1.5to 2.7 nm with increasing etching time (Fig. 2J),which is consistent with HR-TEM studies (Fig. 2,A to D). The specific surface area of the com-posites also increases from 63 to ~83 m2 g−1 withthe increasing etching time from 0 to 2.0 hours(table S1).The two-step synthesis approach effectively

produces mechanically strong 3D porous com-posites with high mass loading of Nb2O5 nano-particles and sufficient electrical conductivityfor high power performance. In contrast to othersynthesis methods (figs. S4 and S5), the two-stepapproach decouples the active material loadingstep from the formation of the 3D architecture,thus offering a general strategy to incorporate awide range of active materials into the 3D ar-chitecture without affecting the overall structure.

Tuning electrochemical propertiesby porosity

In order to ensure a proper comparison for thedifferent Nb2O5/HGF composite electrodes, theamount of T-Nb2O5 is controlled to be around 85weight % (wt %) for all composites [fig. S6,thermogravimetric analysis (TGA)]. The porosityis controlled at 0.60 ± 0.03, with a tap density of1.54 ± 0.09 g cm−3 for all the composite electro-des after compression for coin cell assembly(supplementary text). To probe the effect of struc-tural features on the charge transport kinetics, wefirst conducted electrochemical impedance spec-troscopy (EIS) measurements on composites byusing a symmetric cell with two identical electro-des, which can provide more accurate impedancespectra of the electrode-electrolyte interface thanthat of typical EIS measurement of asymmetriccell with a counter electrode (such as a lithiummetal electrode) (36, 37). Before lithiation, theNyquist plots (Fig. 3A and fig. S7) of a symmetriccell at a practical mass loading of 11 mg cm−2

describe a nonfaradaic process with a state ofcharge (SOC) at 0%. These plots exhibit a 45°slope in the frequency region between ~5 and100 Hz and quasi-vertical lines at lower frequency(<1 Hz). These features indicate a nonfaradaicprocess that can be further validated analyticallyby an equivalent circuit by using a transmissionline model (TLM) for porous electrodes (fig. S8A)(36, 37). The projection of the 45° slope to the real

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Fig. 3. Evolution of kinetic properties and electrochemical characteristics with porosity.(A) Comparison of Nyquist plots obtained from potentiostatic EIS of a symmetric cell using two identicalelectrodes (11 mg cm−2) at a SOC 0%. The projection of the 45° slope in the high-frequency region isused to determine the ionic resistance for the electrolyte-filled porous architectures in a nonfaradaicprocess (fig. S4, A to D). The open symbols and solid lines represent the experimental and simulationresults, respectively. (B) The ionic resistance (Rion) as a function of the porosity of the electrodematerials. With increasing porosity, the ionic resistance is reduced substantially. (C) Galvanostaticcharge-discharge curves of electrodes with tunable nanoporosity at a rate of 10C in the voltage window1.1 to 3.0 V (versus Li/Li+). (D) Comparison of specific capacities (normalized by the total mass of theelectrodes) at various rates (1 to 100C) for composite electrodes with tunable nanoporosity.

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axis reflects the ionic resistance for the electrolyte-filled pores inside the 3D electrode structures (Fig.3A and fig. S7), which is a key parameter for char-acterizing the rate capability of a porous HGFelectrode during the charge/discharge process(37). This projection is defined as Rion/3, asderived from a TLM for cylindrical pores (thecomplete derivation is available in the supple-mentary text) (36, 37). The determination ofRion/3 for each composite electrode in Fig. 3Ais shown in fig. S7, A to D. In our simulations,we followed the approach of Ogihara et al., usingthe finite length Warburg element open circuitterminus (Wo) for the TLM at 0% SOC (36). Weadded another circuit element (fig. S7E) to ac-count for an additional resistance at high fre-quency because the EIS showed a depressedsemicircle in the frequency region >100 Hz. Thiscontribution (Rhigh) is believed to arise from aninterfacial charge transfer resistance of thegraphene component in the composites (38, 39).Another parameter derived from the TLM is theelectrolyte resistance, Rsol. The simulations (fig.S7) enable us to obtain values for Rion, Rhigh, andRsol (table S2) for the various electrodes shown inFig. 3A.The gradual changes in projection length values

for the different electrodes show a decrease inionic resistance (Rion) from 27.1 to 8.8 ohm cm2,with increasing pore size in the HGF scaffold (Fig.3B and table S2). Additionally, the variation in theimaginary part of the capacitance with frequencyindicates that the optimized porous architecture(Nb2O5/HGF-2.0) exhibits a much shorter timeconstant (fig. S9) (40). These studies demonstratethat the ion transport kinetics can be improved bytailoring the pore size in the holey graphenesheets that form the 3D graphene scaffold. Here,the in-plane pores in the holey graphene sheetfunction as ion transport shortcuts in the hier-archical porous structure to facilitate rapid iontransport throughout the entire 3D electrodeand greatly improve ion access to the surface ofthe T-Nb2O5. As expected, neither Rhigh nor Rsol

shows significant variation with electrode poros-ity (table S2).We have carried out a series of galvanostatic

studies to examine the effect of C-rate andcomposite architecture at a mass loading of6 mg cm−2. The results indicate that the Nb2O5/HGF-2.0 electrode consistently exhibits highercapacity for a given C-rate (Fig. 3, C and D, andfigs. S10 and S11). The continuous decrease involtage with increasing capacity is observed in allelectrodes, suggesting a pseudocapacitive behav-ior. Another apparent characteristic of Nb2O5/HGF electrodes is that there is a relatively highcut-off voltage of 1.1 V, which is consistent withprevious studies (41).The 3D porous composites with tailored nano-

pores exhibit enhanced specific capacity com-pared with the macroporous electrode withouttailored nanopores (Nb2O5/GF), and this differencewidens with increasing C-rate (Fig. 3D). Forexample, at 100C, the Nb2O5/HGF-2.0 electrodedelivers a specific capacity of 75 mAh g−1, which ismore than two times that of the Nb2O5/GF elec-

trode (35 mAh g−1). Sweep voltammetry exper-iments are consistent with these results becausesurface-controlled kinetics dominates these elec-trodes (figs. S12 and S13 and supplementarytext). Hence, the tailored porosity is essential forsupporting the high rate capability of T-Nb2O5.Last, because the Nb2O5/HGF electrodes containno additional conductive additive or binder,their gravimetric capacities are considerably high-er than other Nb2O5 electrodes when normal-ized by the total mass of the electrode material(table S3); Moreover, the loading of the Nb2O5/HGF

electrodes is 6 and 11 mg cm−2, whereas thatof the other electrodes is in the range of 1 to2 mg cm−2.

Effects of mass loading onelectrode performance

Three different levels of mass loading were inves-tigated, corresponding to a typical loading amountfor research studies (1 mg cm−2), one that isrepresentative of practical levels of loading (11 mgcm−2) and an intermediate level of loading (6 mgcm−2). The studies also considered different C-rates

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Fig. 4. Effects of mass loading on electrochemical characteristics. (A and B) Galvanostaticcharge-discharge curves for (A) the Nb2O5/G control electrode and (B) the Nb2O5/HGF-2.0 electrode at arate of 10C for the mass loadings of 1, 6, and 11 mg cm−2. (C) Comparison of the rate performancebetween 1C and 100C for Nb2O5/HGF-2.0 (open) and Nb2O5/G (solid) electrodes under different massloadings (1, 6, and 11 mg cm−2). (D and E) Retention of specific capacity at (D) 10C and (E) 50C as a functionof mass loading for three different electrodes. The properties of Nb2O5/HGF-2.0 and Nb2O5/GFelectrodes were normalized by the total mass of the electrode materials (free of conductive additivesand binders); the control Nb2O5/G electrodes were normalized by the total mass of the electrode materials(including binders and conductive additives).

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and different electrode architectures. For thelatter, the electrochemical behavior of one ofthe optimized 3D porous HGF composites withtailored nanopores (Nb2O5/HGF-2.0) was com-pared with two other composites: 3D porousNb2O5/GF composite with no in-plane nanoporesand Nb2O5/G composite with a randomly stackedgraphene network (the different synthesis pro-cesses are described in the supplementary mat-erials, materials and methods, and the structuralcomparison is provided in fig. S14).For the Nb2O5/G control electrodes, it is ap-

parent that the mass loading can dramaticallyalter the galvanostatic charge-discharge char-acteristics (Fig. 4A). The voltage-capacity curvesexhibit an increasingly steeper slope and largervoltage drop with increasing mass loading. Thisresponse results from an increasingly larger in-ternal resistance that in turn leads to higher over-potentials and lower capacities (21). For example,at 10C the capacity of the Nb2O5/G control elec-trode decreases from 85 mAh g−1 at mass loadingof 1 mg cm−2 to 25 mAh g−1 at 11 mg cm−2 (Fig. 4,A and D). Such rapidly degrading performancewith increasing mass loading highlights the chal-

lenges in delivering sufficient ion current den-sities to retain the samegravimetric energy storageperformance in thicker electrodes.The charge-discharge curves of the optimized

Nb2O5/HGF-2.0 electrode show a relatively smallvoltage drop and capacity loss, with increasingmass loading (Fig. 4B). This indicates that the 3Dhierarchically porous HGF architecture leads toa much lower internal resistance. As a result, theNb2O5/HGF-2.0 electrode shows much less ca-pacity degradation induced by mass loadingat various C-rates (Fig. 4C), and a capacity of139 mAh g−1 is maintained with 11 mg cm−2 load-ing at a high rate of 10C. This value is only 7% lessthan the 1 mg cm−2 mass loading for the sameelectrode architecture (Fig. 4D). Even at the highrate of 50C, the Nb2O5/HGF-2.0 electrode with amass loading of 11 mg cm−2 retains a capacity of~74 mAh g−1, a decrease of only 28% from thatof the 1 mg cm−2 electrode (Fig. 4E). In contrast,increased mass loading for Nb2O5/G control elec-trodes (Fig. 4C) leads to a significant decreasein energy storage properties with essentially di-minished capacity at high C-rate (only 7 mAh g−1

at 50C). The Nb2O5/HGF-2.0 electrode also ex-

hibits stable cycling performance (fig. S15). After10,000 cycles at 10C, the capacity is ~125 mAh g−1

(90% retention), and Coulombic efficiency isabove 99.9%, demonstrating the robust porousarchitecture of the 3D composites.

Implications of high mass loadingIncreasing the areal capacity can reduce therelative overhead from the current collectors andseparators and is thus critical for achieving highercell level energy and power density and loweringcost (19). For this reason, the areal capacity pro-vides an important measure for assessing theperformance of an EES system (18, 19, 21).The relation between areal capacity (mAh cm−2)

and areal mass loading per (mg cm−2) gives us thespecific capacity (mAh g−1). As indicated in Figs. 3and 4, the specific capacity depends on C-rate, themass loading, and the electrode architecture; thus,we can determine their influences on areal ca-pacity. The areal capacity versus mass loadingfor the optimized Nb2O5/HGF-2.0 compositeelectrode and various control electrodes at acharge-discharge rate of 10C is plotted in Fig.5A. The Nb2O5/HGF-2.0 electrode has a much

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Fig. 5. Performance metrics for electrodes with high mass loading.(A) Dependence of areal capacity on mass loading at 10C for Nb2O5/HGF-2.0, Nb2O5/GF, and Nb2O5/G electrodes. (B) The correlation of areal capacitywith mass loading (1 to 22 mg cm−2) at various C-rates for the Nb2O5/HGF-2.0electrode. (C) The effect of current density and mass loading on the arealcapacity of the Nb2O5/HGF-2.0 electrode. (D) The comparison of areal

performance metrics of Nb2O5/HGF-2.0 electrode with variouscommercial and research anodes, including graphite anodes, high-capacitySi anodes, and high-rate Nb2O5 anodes. (E) Translation of specificcapacities at various current densities when the mass of current collectors(~10 mg cm−2) is included for the Nb2O5/HGF-2.0 electrode with massloadings of 1 and 11 mg cm−2.

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higher slope at the 10C as compared with otherarchitectures. The linear response for the Nb2O5/HGF-2.0 composite electrode suggests that nolimitation has been reached in mass loadingat the rate of 10C (which corresponds to a cur-rent density of 22 mA cm−2), whereas the othercontrol electrodes exhibit apparent plateausor even a decrease in capacity, with increasingmass loading due to the worsening charge tran-sport characteristics in thicker electrodes.A second relationship evident from Figs. 3 and

4 is the dependence of specific capacity on C-ratefor the Nb2O5/HGF-2.0 electrode. In Fig. 5B, theareal capacity increases linearly, with massloading at C-rates of 5 and 10C up until 11 mgcm−2. At higher mass loading levels, deviationsfrom linearity occur as the areal capacity beginsto reach a plateau at the 5C rate and experiencesa slight decrease in areal capacity at 10C. Inthe linear region, there is a relatively small dif-ference in slope, which is expected because thespecific capacities vary only by <10% for thedifferent levels of mass loading. The plateau-likebehavior at 5C has been interpreted as reachinga limiting condition. Specifically, Gallagher et al.proposed that the plateau was an indication thatthe penetration depth for the ionic current wasbeing reached (19). The transition between thelinear response and the plateau suggests that thehigher loading becomes progressively less bene-ficial. The decrease in areal capacity (for exam-ple, 10C at loadings above 11 mg cm−2) wasinterpreted as being the result of concentrationgradients being generated. It would seem thatunder the conditions involved here—thick elec-trodes and high current density—concentrationeffects are likely to develop.The data in Fig. 5B are rearranged in Fig. 5C to

show the effect of both loading and current den-sity on the areal capacity of the Nb2O5/HGF-2.0electrode. The highest loading (22mg cm−2 here)leads to an areal capacity reaching 3.9mAh cm−2.The curve shape for each mass loading level issigmoidal: The high areal capacities will level offto a maximum value at sufficiently low currentdensity as all the electrode material becomesaccessible to electrolyte penetration (19), andall the samples with different mass loadingsshow a fairly steep decrease in areal capacitywhen the current density is increased beyond20 mA cm−2. This response suggests that chargetransport within the electrolyte is becoming thelimiting factor at such high current density (19).That is, ion transport in the electrolyte-filledporous electrode is responsible for the fall-offin areal capacity, indicating that the capacity ofthe active electrode is not being fully used.We have also compared the areal capacity

versus current density with state-of-the-art com-mercial graphite anodes (19, 20, 42) and repre-sentative research anodes (such as Nb2O5 and Si)(Fig. 5D) (15–17, 24, 26, 27, 43–45). In general, incontrast to the rapid decay in areal capacity withincreasing current density observed in typical

commercial or research devices, our compositeelectrode exhibits a much more gradual change.This is particularly evident in the linear plot ofthe areal capacity versus current density (fig.S16). At >5 mA cm−2, the optimized Nb2O5/HGF-2.0 electrode delivers amuch higher arealcapacity at a given current density. Moreover,the composite electrodes continue to provideenergy storage at high current density beyond20mA cm−2, at which no other battery materialsystem seems to perform.The advantage of having electrodes with high

mass loading becomes more apparent when themass of the inactive components (such as currentcollectors ~10 mg cm−2) are taken into account(18, 21). As demonstrated in Fig. 5E, an electrodewith amass loading of 11 mg cm−2 will experience~50% decrease in both the specific capacity andcurrent density if the mass of the current col-lector (~10 mg cm−2) is considered. By com-parison, the corresponding performance for anelectrode with a mass loading of 1 mg cm−2 willbe reduced >90%. Thus, even with a high per-formance material, when the mass loading islow, the performance of the active materialhas much less influence on the final devicebecause the mass of the passive componentsdominates the total electrode mass. This anal-ysis only considers the additional weight fromthe current collectors. The inclusion of sep-arators and packaging (table S4) could furtherdiminish the performance of the device and fur-ther establish the importance of achieving highmass loading for practical devices.Together,ourstudiesdemonstrate thattailored

porosity in the 3D conductive scaffold is essentialfor achieving optimized charge transport andhigh-rate energy storage at practical levels ofmass loading. We showed that the Nb2O5/HGFcomposite electrode with optimized porosity isable to support a significant increase in massloading with little decrease in electrochemicalperformance. At rates as high as 10C, there islittle difference in specific capacity for massloadings ranging from1 to 11mg cm−2, the latterbeing relevant to practical device applications.The achievement of high area capacity with high-ratecapabilityat largemass loadingrepresentsanessential step toward practical electrochemicalenergy storage devices.

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ACKNOWLEDGMENTS

H.S., H.F., and I.S. extend sincere appreciation to the Deanshipof Scientific Research at King Saud University for its funding ofthis research through grant PEJP-17-01 (material preparationand electrochemical studies). X.D. and M.L. thank the U.S.Department of Energy (DOE) Office of Basic Energy Sciences,Division of Materials Science and Engineering, award DE-SC0008055 (structural characterizations). Y.H. M.D. and Z.Z.appreciate the support from the National Science Foundationthrough award DMR-1437263 (ion transport studies). B.D. andJ.L. greatly appreciate the support by the Office of NavalResearch (N00014-16-1-2164) (impedance analysis). L.M., J.L.,X.X., and G.H. thank the Chinese Scholar Council scholarship forthe financial support (materials preparation of electrochemicalstudies). L.M. also thanks a postdoctoral fellowship from HunanUniversity. Holy-graphene/niobia composite are available fromthe University of California, Los Angeles (UCLA) under amaterials transfer agreement with the University. A provisionalpatent application has been filed by UCLA (UCLA Case 2017-216) that covers the subject described here.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/356/6338/599/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S16Tables S1 to S4References (46–48)

13 December 2016; accepted 29 March 201710.1126/science.aam5852

Sun et al., Science 356, 599–604 (2017) 12 May 2017 6 of 6

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storageThree-dimensional holey-graphene/niobia composite architectures for ultrahigh-rate energy

Wang, Xu Xu, Guolin Hao, Benjamin Papandrea, Imran Shakir, Bruce Dunn, Yu Huang and Xiangfeng DuanHongtao Sun, Lin Mei, Junfei Liang, Zipeng Zhao, Chain Lee, Huilong Fei, Mengning Ding, Jonathan Lau, Mufan Li, Chen

DOI: 10.1126/science.aam5852 (6338), 599-604.356Science 

, this issue p. 599Sciencediffusion limitations.transport, and the hierarchical porous structure in the graphene sheets facilitated rapid ion transport and mitigated higher charge transport in the composite architecture. The interconnected graphene network provided excellent electronmass loading and improved power capability were reached by tailoring the porosity in the holey graphene backbone with

. A high5O2The three-dimensional, hierarchically porous holey graphene acted as a conductive scaffold to support Nb/holey graphene framework composite with tailored porosity.5O2 developed a Nbet al.usually opposing objectives. Sun

Improving the density of stored charge and increasing the speed at which it can move through a material areAs with donuts, the holes matter

ARTICLE TOOLS http://science.sciencemag.org/content/356/6338/599

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2017/05/10/356.6338.599.DC1

CONTENTRELATED http://science.sciencemag.org/content/sci/356/6338/582.full

REFERENCES

http://science.sciencemag.org/content/356/6338/599#BIBLThis article cites 47 articles, 9 of which you can access for free

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