REVIEW SUMMARY◥

NANOMATERIALS

Energy storage: The future enabled by nanomaterialsEkaterina Pomerantseva*, Francesco Bonaccorso*, Xinliang Feng*, Yi Cui*, Yury Gogotsi*

BACKGROUND:Nanomaterials offer greatly im-proved ionic transport and electronic conduc-tivity comparedwith conventional battery andsupercapacitor materials. They also enable theoccupation of all intercalation sites availablein the particle volume, leading to high specificcapacities and fast ion diffusion. These fea-tures make nanomaterial-based electrodesable to tolerate high currents, offering a pro-mising solution for high-energy and high-power energy storage. However, there arestill many challenges associated with theiruse in energy storage technology and, withthe exception of multiwall carbon-nanotubeadditives and carbon coatings on silicon par-ticles in lithium-ion battery electrodes, the useof nanomaterials in commercial devices is verylimited. After decades of development, a lib-rary of nanomaterials with versatile chemicalcompositions and shapes exists, ranging from

oxides, chalcogenides, and carbides to carbonand elements forming alloys with lithium. Thislibrary includes various particle morphologies,such as zero-dimensional (0D) nanoparticlesandquantumdots; 1Dnanowires, nanotubes, andnanobelts; 2D nanoflakes and nanosheets;and 3D porous nanonetworks. Combined withlithium and beyond lithium ions, these chem-ically diverse nanoscale building blocks areavailable for creating energy storage solutionssuch as wearable and structural energy stor-age technology, which are not achievable withconventional materials.

ADVANCES: The success of nanomaterials inenergy storage applications has manifold as-pects. Nanostructuring is becoming key in con-trolling the electrochemical performance andexploiting various charge storage mechanisms,such as surface-based ion adsorption, pseudo-

capacitance, and diffusion-limited inter-calation processes. The development of newhigh-performance materials, such as redox-active transition-metal carbides (MXenes) withconductivity exceeding that of carbons andother conventional electrode materials by atleast an order of magnitude, open the doorto the design of current collector–free andhigh-power next-generation energy storagedevices. The combination of nanomaterials inhybrid architectures, such as carbon-siliconand carbon-sulfur, together with the develop-

ment of versatilemethodsof nanostructuring, over-come challenges relatedto large volume changetypical for alloying andconversionmaterials. Theseexamples indicate that

nanostructuredmaterials and nanoarchitecturedelectrodes can provide solutions for designingand realizing high-energy, high-power, andlong-lasting energy storage devices.

OUTLOOK: The limitations of nanomaterials inenergy storage devices are related to their highsurface area—which causes parasitic reactionswith the electrolyte, especially during the firstcycle, known as the first cycle irreversibility—as well as their agglomeration. Therefore, fu-ture strategies aim to develop smart assemblyof nanomaterials into architectures with con-trolled geometry. Moreover, combining nano-materials with complementary functionalities,such as high electronic conductivity of gra-phene or MXenes with high operating voltageand high redox activity of oxides, is necessary.Building sophisticated electrode architecturesrequires innovativemanufacturing approaches,such as printing, knitting, spray deposition,and so on. Already-developed techniques suchas 3D printing, roll-to-roll manufacturing, self-assembly from solutions, atomic layer deposi-tion, and other advanced techniques should beused to manufacture devices from nanomate-rials that cannot be made by conventionalslurry-based methods. Such manufacturingapproaches can also enable long-sought flex-ible, stretchable, wearable, and structuralenergy storage and harvesting solutionsfor Internet of Things and other disruptivetechnologies.▪

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Pomerantseva et al., Science 366, 969 (2019) 22 November 2019 1 of 1

1D materials

2D materials0D materials

Spray coating, ink-jet printing

Applications

Roll-to-roll manufacturingSelf-assembly into complex architectures

3D printing, electrospinning

Nanomaterials for energy storage applications. The high surface-to-volume ratio and short diffusionpathways typical of nanomaterials provide a solution for simultaneously achieving high energy and powerdensity. Furthermore, the compatibility of nanomaterials with advanced manufacturing techniques—suchas printing, spray coating, roll-to-roll assembly, and so on—allows for the design and realization of wearable,flexible, and foldable energy storage devices.

The list of author affiliations is available in the full article online.*Corresponding author. Email: ep423@drexel.edu (E.P.);francesco.bonaccorso@iit.it (F.B.); xinliang.feng@tu-dresden.de (X.F.); yicui@stanford.edu (Y.C.); gogotsi@drexel.edu (Y.G.)Cite this article as E. Pomerantseva et al., Science 366,eaan8285 (2019). DOI: 10.1126/science.aan8285

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REVIEW◥

NANOMATERIALS

Energy storage: The future enabled by nanomaterialsEkaterina Pomerantseva1,2*, Francesco Bonaccorso3,4*, Xinliang Feng5,6*, Yi Cui7*, Yury Gogotsi1,2*

Lithium-ion batteries, which power portable electronics, electric vehicles, and stationary storage, havebeen recognized with the 2019 Nobel Prize in chemistry. The development of nanomaterials and theirrelated processing into electrodes and devices can improve the performance and/or development of theexisting energy storage systems. We provide a perspective on recent progress in the application ofnanomaterials in energy storage devices, such as supercapacitors and batteries. The versatility ofnanomaterials can lead to power sources for portable, flexible, foldable, and distributable electronics;electric transportation; and grid-scale storage, as well as integration in living environments andbiomedical systems. To overcome limitations of nanomaterials related to high reactivity and chemicalinstability caused by their high surface area, nanoparticles with different functionalities should becombined in smart architectures on nano- and microscales. The integration of nanomaterials intofunctional architectures and devices requires the development of advanced manufacturing approaches.We discuss successful strategies and outline a roadmap for the exploitation of nanomaterials forenabling future energy storage applications, such as powering distributed sensor networks and flexibleand wearable electronics.

Energy usage is experiencing a large andfast shift toward electricity as the mainpower source. Reversible storage and re-lease of electricity is an essential technol-ogy, driven by the needs of portable

consumer electronics and medical devices,electric vehicles, and electric grids, as well asthe emerging Internet of Things and wearabletechnologies. These applications and the needto store energy harvested by triboelectric andpiezoelectric generators (e.g., frommusclemove-ments), as well as solar panels, wind powergenerators, heat sources, and moving machin-ery, call for considerable improvement anddiversification of energy storage technology.In this context, materials with nanometer-sized structural features and a large electro-chemically active surface can change theparadigm for energy storage from within theelectrode bulk to surface redox processes thatoccur orders of magnitude faster and allow agreatly improved power and cycle life (1–3).High electronic and ionic conductivities com-bined with intrinsic strength and flexibility oflow-dimensionalmaterials allowultrathin, flex-ible, and structural energy storage solutions.

The short diffusion path can enable the useof nonflammable solid electrolytes, leading tosafer batteries, and large or multivalent ionsfor more affordable grid-scale applications.In addition to active energy-storing nanomate-rials, passive components can benefit from theuse of nanomaterials as well. For example,ultrathin hexagonal boron nitride (h-BN) andmetal oxide separators and graphene or two-dimensional (2D) transition-metal carbide(MXene) current collectors can decreasethe size and weight of the batteries (4, 5).Today, we possess a large library of nano-particles and nanostructured materials with avariety of compositions, electrochemical prop-erties, and morphologies ranging from 0Dnanoparticles and quantum dots (6, 7) to 1Dnanowires, nanotubes, and nanobelts (8, 9), to2D nanoflakes and nanosheets (10–13), and to3D porous networks (14–17) (Fig. 1).However, some of the challenges related to

the reactivity of nanomaterials are due to theirhigh specific surface area (SSA), which leads toparasitic, and often irreversible, reactions andstrong interaction with electrolyte (1, 2); inaddition, cost and manufacturability of nano-materials make the battery community skepti-cal about their promise for practical applications.Although the number of studies of variousphenomena related to the performance ofnanomaterials in energy storage is increasingyear by year, only a few of them—such asgraphene sheets, carbon nanotubes (CNTs),carbon black, and silicon nanoparticles—arecurrently used in commercial devices, primar-ily as additives (18).High-capacity conversion (sulfur and fluo-

rides) and alloying (Si and Sn) materials un-dergo considerable structure changes and

large volume expansion and contraction (19, 20),which can cause mechanical and chemomech-anical instability across the length scales ofindividual nanoparticles, electrodes, and fullelectrochemical cells (21, 22). Coatings withnanoscale thickness obtained via atomic ormolecular layer deposition may be needed tosuppress parasitic interfacial reactions, includ-ing the growth of metal dendrites, and/orform an artificial solid-electrolyte interphase(SEI) layer leading to the improved stability ofelectrochemical cells (23–27). Achieving futureadvancements in this research area will re-quire broadening the compositional chemistryof interfacial layers and developing nano-technology approaches that would allow forpinhole-free coating of 3D architectures withvarying porosity. Advances in the develop-ment of autonomous microsystems and micro-devices call for smaller power sources. As aresult, many of the future energy storage de-vices need to be just several microns or eventens of nanometers thick. Therefore, thin filmelectrode and electrolyte layers need to begrown or printed not only on flat chips butalso on surfaces of various shapes, such aspackaging materials, or created as wearabletattoos, requiring manufacturing processeswhich differ drastically from the current bat-tery manufacturing practice.This review takes a holistic approach to en-

ergy storage, considering battery materialsthat exhibit bulk redox reactions and super-capacitor materials that store charge owingto the surface processes together, becausenanostructuring often leads to erasing boun-daries between these two energy storage solu-tions. We explain how the variety of 0D, 1D,2D, and 3D nanoscale materials availabletoday can be used as building blocks to createfunctional energy-storing architectures andwhat fundamental and engineering problemsneed to be resolved to enable the distributedenergy storage required by the technologies ofthe next decade.

Nanomaterials platform

Figure 1 shows the variety of available 0D to3D nanomaterials (nanoscale building blocks).What emerges is the large family of carbonnanomaterials (Fig. 1, top row). Carbon isinvaluable for energy storage owing to itsproperties, such as low specific weight andhigh abundance, coupled with the high elec-tronic conductivity of graphitic carbons.More-over, because of sp/sp2/sp3 hybridization,multiple carbon structures and morpholo-gies are available. However, nanostructuredcarbons usually provide limited, if any, redoxcapacity and only after functionalization(28, 29). Therefore, they are usually used asa double-layer capacitor material, or as a con-ducting support backbone (28, 29), rather thanas active material for energy storage devices.

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1A.J. Drexel Nanomaterials Institute, Drexel University,Philadelphia, PA 19104, USA. 2Department of MaterialsScience and Engineering, Drexel University, Philadelphia, PA19104, USA. 3Graphene Labs, Istituto Italiano di Tecnologia,16163 Genova, Italy. 4BeDimensional Spa, 16163 Genova,Italy. 5Center for Advancing Electronics Dresden (CFAED),Technische Universität Dresden, 01062 Dresden, Germany.6Faculty of Chemistry and Food Chemistry, TechnischeUniversität Dresden, 01062 Dresden, Germany. 7Departmentof Materials Science and Engineering, Stanford University,Stanford, CA 94305, USA.*Corresponding author. Email: ep423@drexel.edu (E.P.);francesco.bonaccorso@iit.it (F.B.); xinliang.feng@tu-dresden.de(X.F.); yicui@stanford.edu (Y.C.); gogotsi@drexel.edu (Y.G.)

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The exception is graphite, which consists of anordered stack of graphene layers and exhibitsa specific capacity of 372 mA·hour g−1 for lith-ium ion storage in between the layers (30).A more conventional approach to achieving

high specific capacity is to exploit redox re-actions in nanomaterials and thus utilizematerials beyond carbons. High surface areaof transition-metal compounds (oxides, sul-fides, carbides, nitrides, etc.) and pure elementsforming alloys with Li (Si, Ge, Sn, etc.) (31, 32)allow those reactions to occur without solid-

state diffusion limitations. More recently, high-ly crystalline conductive materials—such asmetal organic frameworks (33–35), covalentorganic frameworks (36), MXenes, and theircomposites, which form both 2D and 3Dstructures—have been used as electrodes forenergy storage. They show promise to deliverhigh areal capacity owing to their high porosity,thus allowing the construction of thick elec-trodes (37). Organic nanomaterials, especiallyheteroatom-rich molecules and porous organicmaterials, not only can be directly used as

electrodes for energy storage but can also beused as precursors to develop carbon-richmate-rials for energy storage (38).In addition to chemical diversity, advances

in nanomaterial synthesis enable control ofmorphological dimensionality, ranging from0D to 3Dnanostructures (Fig. 1), each ofwhichhas both advantages and limitations for en-ergy storage applications. To benefit fromtheir useful properties and alleviate short-comings, redox-active 0D nanoparticles canbe decorated on the surface of conductive 1D

Pomerantseva et al., Science 366, eaan8285 (2019) 22 November 2019 2 of 12

Open 2D channels for ion transport; all surface is accessible enabling fast charge storageCompatible with flexible devicesSmall nanoflakes can be used in inks for printing

Small in all dimensionsSurfaces on all sites are accessible to electrolytesNo bulk solid-state diffusionCan be integrated into multiple systemsCan be used in stable inks for printing

0D 1D 2D 3D

Carbon Onion Single Wall Carbon Nanotube Graphene Pillared Graphene

Nanoparticles Multiwall Nanotube Multielement 2D Compounds Metal-Organic Frameworks

Quantum Dots Nanowires Nanoflakes Aerogels

AdvantagesMechanical reliabilityPossibility to integrate with wearable devicesPorous flexible freestanding films

Low packing density; cannot exhibit high volumetric performanceLow yield and high cost ofsynthesisDiffusion pathways can be relatively long

Re-stackingLow out-of-plane electronic and ionic conductivityHigh cost of synthesis

AgglomerationDo not densify and form only low density non-uniform structuresNumerous points of contact lead to high resistancePoor chemical stability

DesignStabilityManufacturing

Can be used to create thick electrodes with large areal and volumetric storage properties

Limitations

Fig. 1. Overview of 0D, 1D, 2D, and 3D nanomaterials. An illustration of the chemical, structural, and morphological diversity of the available nanoscale buildingblocks that can be used to create complex functional architectures for next-generation energy storage devices with improved performance compared with thecurrently available ones. The advantages and challenges related to the application of each class of nanomaterials are summarized in the last two rows. [Metal organicframeworks, pillared graphene, quantum dots, and aerogels images reproduced with permission from (140–143), respectively; nanowires and nanoflakes images byYayuan Liu/Stanford University and Meng-Qiang Zhao/Drexel University, respectively.]

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and 2D materials with a double function—that is, preventing their aggregation andrestacking—while at the same time contrib-uting to charge storage. Examples of suchstructures are demonstrated by graphene-oxide (6, 39, 40) or MXene-oxide hybrids withenhanced energy storage capabilities enabledby the redox activity of oxide nanoparticlesand thehigh electronic conductivity of graphene(41) or MXene (42). When produced as nano-scale particles, even typical battery materialscan show pseudocapacitive behavior (pseudo-linear charge-discharge), as was demonstratedfor 6-nm nanoparticles of LiCoO2 (43) and willbe discussed in detail later. The self-assemblyof 0D nanoparticles into denser systems ispossible if the particles are monodisperse, asshown by the assembly of oxide nanoparticleson dichalcogenide sheets into ordered struc-tures (44). Another limitation of 0D nanopar-ticle electrodes is a low electrical conductivity,owing to the contact resistance created byeach contact point. An electrode containingmultiple nanoparticle contact points can beconsidered as many resistors in series. Ifnanoparticles are conductive, as in the caseof carbon onions, this problem is mitigated,but for most less conductive materials, thisrepresents a severe limitation. Even in thecase of conducting particles, Schottky junc-tions can form (45). In addition, nanopar-ticles of certain material classes, such asdichalcogenides, are not very stable (46) andrequire protective treatment during both syn-thesis and handling.Many semiconductor andmetal nanoparticles form a passivating coat-ing layer on the surface (47), creating a barrierfor the charge transport, which either requiresa further annealing step or encapsulation.A key feature of 1D structures relies on the

possibility of forming highly conducting con-tinuous networks, by assembling conducting1D nanomaterials (e.g., CNTs andmetallic nano-wires) over a large area and thickness of theelectrode, leading to fast electron transport (20).CNTs are already widely used as conductiveadditives in battery electrodes (20). Addition-ally, many of the 1D nanostructures, especial-ly CNTs, provide high mechanical strength(e.g., Young’s modulus of 0.6 TPa) (48), en-suring mechanical integrity of the electrodes(20). Unlike the 0D particles, 1D nanomate-rials do not require a binder to produce flex-ible structures, a feature that is particularlyattractive for wearable applications. More-over, although 1D nanomaterials form bundles(49), they still produce porous electrodes en-abling electrolyte penetration (20, 35). Porousfree-standing membranes were formed usingAl2O3 nanowires through direct transforma-tion of bulk aluminum alloy into alkoxidenanowires, followed by heating in air (8).These membranes were used to produce cera-mic separators for high-rate batteries, which

are less prone to catching fire (8). The emptyspace between 1D structures allows the designof zero-expansion electrodes, leading to in-creased lifetime and decreased mechanical,thermal, and resistive losses. This property isparticularly important for alloying anodes,such as Si, Ge, or Sn, in accommodating vol-ume changes associated with interaction withup to 4.4 Li+ ions per atom (3). The downside of1D nanostructures is their low packing density,which makes the design and realization of1D nanomaterial–based electrodes with highvolumetric performance challenging (3). Astrategy to overcome this limitation could becombining 1D nanostructures with 0D nano-particles, thereby filling gaps between thenanotubes or nanowires and increasing thevolumetric performance. The large-scale pro-duction of 1D nanostructures is another chal-lenge. 1D nanomaterials such as single-wallCNTs or silicon nanowires are difficult to pro-duce in large quantities at low cost. Anotherlimitation of 1D nanomaterials is their intrin-sic structure-morphology relationship, whichaffects the electrochemical performances. Forexample, for tunnel manganese oxide nano-wires (9, 50), thediffusionpathways are orientedalong the length, not across the 1D nanowire,and can be relatively long, up to 100 mm.For 2D nanomaterials, their primary advan-

tage is related to their intrinsic structure, thatis, the entire 2D surface can be accessible toelectrolyte ions, enabling fast charge storage(11, 42). Similar to 1D nanostructures, 2Dsheets can form flexible electrodes withoutbinders (51) or other additives, but they havethe added value of achieving a much higherpacking density and volumetric performancecompared with the 1D counterparts. Although2D sheets can be grown on some metal sub-strates, and progress is being made towardlarge-area single crystals (52), the large-scalebottom-up production of 2D materials is tooexpensive for the majority of energy storageapplications, with the exception of small on-chip devices. By contrast, the direct liquid-phase exfoliation (LPE) of bulk layered crystals(53–55), wet chemical synthesis, and selectiveetching and dealloying (transformative syn-thesis) are much more relevant strategies forthe scalable production and processing of 2Dnanomaterials (56). The LPE process enablesthe formulation of inks of 2D materials in dif-ferent solvents (10) for printing devices (10, 11).2D structures can provide high electronic con-ductivity in-plane, but there might be low elec-tronic conductivity and slow transport of ionsin the out-of-plane direction (57, 58). Thus, the2D morphology hinders the achievement ofhigh (electro)chemical performance of thickplanar electrodeswith 2D sheets aligned alongthe surface of the current collector (37). Inthis respect, defect (pinhole) generation in2D sheets would help, if it did not sacrifice

the intrinsic electronic and electrochemicalproperties. Moreover, restacking of 2D struc-tures limits electrolyte penetration and ionictransport (57). Smart architectures should havesufficient porosity and enlarged interlayer dis-tance, as well as vertical alignment of 2D sheetsenabling fast electrolyte penetration, while stillenabling dense packing of nanoflakes. Hybrid-ization of 2D sheets with 0D and 1D nano-materials can either minimize or completelyeliminate the restacking issue. Preventing re-stacking is also important to boost the per-formance of catalytically active 2D materialsin order to make their surface accessible toreagents and increase the number of reactionsites. Layered materials and pillared clays of-fer opportunities for a tailored nanodesign,including tunable interlayer spacing andmod-ification of the interlayer chemistry often ac-companied by improved stability, leading totheir diverse applications as multifunctionalcatalysts (59). Efficient strategies demonstra-ted in the catalysis area can also be exploredfor energy storage application and vice versa.Many 3D nanomaterials, such as carbon

(graphene) aerogels (14) and metal organicframeworks (MOFs), are a result of the assem-bly process of nanoparticles. Few 3D nanoma-terials have intrinsic nanoscale features (e.g.,thin walls of porous templated or carbide-derived carbons); most of them are built bycombining 0D, 1D, and 2D nanomaterials ofthe same kind (carbons). To produce thickelectrodes (e.g., 100 to 200 mm) with large arealand volumetric storage properties, it is neces-sary to develop 3D architectures optimizedfor both ion and electron transport. Thesearchitectures would minimize the amount ofpassive materials in cells, such as current col-lectors and separators that occupy additionalvolume and add dead weight. Examples of 3Delectrodes with porous architectures that en-able advances in energy storage have alreadybeen reported in literature (60–62). Buildingon these approaches, as well as developingnew ones, is important for moving closer tonanomaterials-enabled energy storage.Despite exciting diversity, none of the avail-

able nanomaterials are perfect, and none ofthem can solve all the problems of the currentenergy storage technologies. Carbon materialsoffer high electrical conductivity and chemicalstability but a limited charge-storage capabil-ity. Transition-metal oxides and redox-activeorganic materials can often offer much largercharge storage, but most of them have lowelectrical conductivity. The latter issue can beovercome by combining the aforementionedactive materials in a physical mixture or ahybrid structure with carbon or other conduct-ing materials (20). 2D transition-metal car-bides, nitrides, and carbonitrides, classifiedas MXenes, possess high electronic and highionic conductivities (42). However, for this

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class ofmaterials, cost, stability, and an under-standing of the electrochemical mechanismsare still open issues. This field is rapidly grow-ing, with more than 30 highly conductivematerials available, of which Ti3C2 has alreadydemonstrated exceptional values of capaci-tance in aqueous electrolytes and excellentperformance at rates up to 1000 V s−1 (57). Over-all, the availability of nanoscale building blocksis continuously increasing. This provides a port-folio of materials with properties not achieva-ble with the conventional materials used forbatteries and supercapacitors.

Fundamental processes governingenergy storage

The fundamental processes that control electro-chemical performance can be somewhat differ-ent from bulk battery materials. For example,the transport of ions in nanoscale systemstypically occurs in confinement between orat the surfaces of electrode materials (Fig. 2A),as in supercapacitors with porous carbon elec-trodes (63), instead of the transport in bulkelectrolyte and solid electrodes in conven-tional batteries. Examples of extreme confine-ment could be narrow 2D slit pores between2D sheets or narrow 1D channels in the struc-ture of tunnel oxides or nanotubes. Althoughan understanding of such transport propertiesis largely missing thus far, there are data thatshow anomalous fast transport of water inhydrophobic channels of CNTs (64) or fasttransport of protons by the Grotthuss mech-

anism between MXene nanosheets (65). Thepresence of confinedwater or electrolytewithinnanochannels is known to facilitate the trans-port of ions (66), including large or multiva-lent ions, such as Mg2+ and Al3+ (67). Water orelectrolyte confinement also allows the trans-port of complex ions—for example, AlCl4

− ororganic ions—which is challenging and/orcannot be achieved in conventional batteryelectrodes (68), at least not for the intercala-tion mechanism. In the case of nanomaterials,we can explore fast storage mechanisms, suchas intercalation pseudocapacitance, which is aprocess determined as non–diffusion-limitedinsertion of ions into the active electrodemate-rial (69). Confined structural water can be bene-ficial to improving charge-transfer resistance,especially in the case of aqueous energy stor-age systems (70, 71). Moreover, it was shownthat there was no detrimental effect on theperformance of nonaqueous Li-ion cells overseveral hundreds of cycles when a small amountof water was added to the electrolyte (72).However, a considerable amount of water pres-ent in the electrode structure can be harmfulto the device operation by causing parasiticreactions, which lead to irreversible chemicaltransformations of the device components (73).As of now, there is no clear understanding ofhow the kind of confinement or the amountof water in thematerial structure, the strengthof bonds, crystallographic positions, and otherparameters affect the electrochemical storageproperties. A more in-depth study of these as-

pects is important to gain knowledge of therole of confined water in charge storage prop-erties of nanomaterials.It is also necessary to study the transport of

electrons, because for some low-dimensionalmaterials—such as CNTs, graphene, or Nb2C—quantum capacitance (74) can become a lim-iting factor. The quantum capacitance is linkedwith the variation of the density of states of thematerials (i.e., the effect of band filling andband emptying), which modifies the capaci-tance, imitating a second capacitor in series(74). More severely, many nanomaterials, suchas oxides, are poor electronic conductors, rais-ingmultiple questions. How canwewire them?How can we inject electrons? Can electronstunnel through thin mono-, double-, or triple-layers of ions of electrolyte between the sheets?What are the rate-limiting factors?Hybridization of dissimilar nanomaterials—

that is, the combination of oxide nanoparticlesand carbons—maximizes heterointerfaces atwhich new phenomena can occur, as schemat-ically illustrated in Fig. 2B. As a result of dif-ferent work functions of carbon (e.g., graphene,CNTs, or carbon onions) and oxide nanopar-ticles (e.g., MnO2 or Nb2O5), electrons may beinjected from the carbon into the oxide, in-creasing the electrical conductivity of thelatter. Consequently, formation of holes ingraphene leads to an increase in the numberof charge carriers and its quantum capaci-tance. Faster ionic transport and different ion-insertion potentials have been reported forhybrid 2D materials (75), such as grapheneand MoS2 (76) and graphene and Ti3C2 com-pared with the individual components (77).Therefore, the synergistic effects between dif-ferent nanomaterials can be achieved by com-bining materials with different properties.One challenge is the avoidance of unwanted

chemical reactions associated with the highSSA of nanomaterials, which reaches to morethan 2000 m2 g−1 for porous carbon andgraphene and is between 100 and 1000 m2 g−1

for most other nanomaterials, such as CNTs,dichalcogenides, or MXenes (42). High SSAfavors chemical interactions and the forma-tion of SEI-like structures, often resulting ina very large first-cycle irreversible capacityor parasitic reactions during cycling, limitingthe device’s lifetime (Fig. 2C). Therefore, reac-tions between nanoparticles and electrolyteduring cycling need to be studied in depth tofully understand and control them. An effec-tive strategy to solve this problem is to as-semble small nanostructures into secondaryparticles—that is, aggregates or clusters—oflarger dimensions. In these structures, theelectrolyte only contacts the outer surface ofthe secondary particles, thus reducing the ef-fective contact surface area without losingthe advantages of nanoscale dimensions ofelectrode material. This has been seen in

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Transport of Ions Transport of Electrons

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Apex

Fig. 2. Fundamental properties governing the performance of nanostructured materials for energystorage application. (A) Transport of ions. (B) Transport of electrons. (C) SEI formation and parasitic reactionsbetween electrode and electrolyte. Blurry areas represent reaction products, such as SEI. (D) Connectivity andtransport in 3D space.

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pomegranate-like Si nanoparticle–assemblyanodematerials (78) (Fig. 3A) and concentration-gradient LiNiMnCoO2 cathode materials (79).Finally, assembly into interconnected 3D

electrode architectures with different lengthscales (from nanometers to microscale) canprovide electronic and ionic conductivity inthree dimensions. This requires pore engineer-ing (80) and controllable assembly of differentclasses of nanomaterials (Fig. 2D). A goal isto generate a self-assembled 3D bicontinuousnanoarchitecture consisting of electrochemi-cally active material with rapid ion and elec-tron transport pathways (81). For building truly3D architectures that integrate anode, cathode,and electrolyte together, which shortens theionic diffusion length compared with conven-tional electrodes, the interdigitated types ofelectrode configurations have been proposed(82), although their experimental realizationat a large scale is still challenging. A true 3Darchitecture was demonstrated using layer-by-layer self-assembly of interdigitated thin filmson the surface of an open-cell aerogel sub-strate and other examples (83–85). An alter-

native strategy relies on the infiltration of a 3Dscaffold that has amacroporous structure—forexample, aerogel, graphene, or CNTs—withnanoparticles of active material. However,this approach has a number of limitations,such as limited volumetric performance. Fun-damentally, it is necessary to learn how tobuild 3D architectures by self-assembly duringelectrode manufacturing. Relatively little hasbeen done in this direction, but there are al-ready encouraging demonstrations of trulyintegrated 3D electrodes (86) (Fig. 3B). Electro-chemical or vapor-phase deposition of a sepa-rator and a counter electrode on a meso- ormacroporous scaffold is promising as well.

Advances and phenomena enabled bynanomaterials in energy storage

Nanostructuring often enables the use of con-ventional materials that cannot be used in themicrocrystalline state as either cathodes oranodes. Classical examples are alloying anodes—such as silicon, germanium, or tin—that ex-perience large structure and volume changesduring cycling (31, 32). Bulk silicon,which has a

theoretical capacity of up to 3579 mA·hour g−1,considering Li15Si4 formation, cannot work asstand-alone anode in a Li-ion battery. The lifecycle of silicon-based anodes is limited by thepulverization of the active material, which isdetermined by the volume swelling of siliconupon lithiation (up to 400 volume %) andsubsequent shrinkage upon delithiation (87).However, reducing the particle size below~150 nm (3) limits the electrode cracking uponthe insertion of Li+ ions, which mitigates theanodemechanical failure (Fig. 4A). There havebeen designs proposed to overcome the issuesof large volume expansion and mechanicalfailure, including the use of nanowires (1, 8),nanotubes (88), graphene flakes (19), hollowspheres, and core-shell and yolk-shell struc-tures (89). To build a stable SEI for nano-materialswith large volume change, the conceptof nanoscale double-walled hollowed struc-tures was demonstrated. In this structure, theouter wall confines the expansion of the innerwall toward the hollow space inside andtherefore generates a static outer surface forstable SEI formation (31).

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Fig. 3. Strategies developed to overcome performance limitations of nanomaterials in energy storage applications. (A) Nanoscale coatings on the surfaceof conversion and alloying electrode materials need to avoid mechanical instability caused by large-volume change and loss of the surface area as a result ofagglomeration (78). D, diameter of pomegranate microparticle; t, thickness of the conducting framework; 2a, void dimension; d, diameter of the active materialprimary particles. (B) Nanostructured 3D electrode architecture can be realized through a scalable block-copolymer self-assembly process (86). [Images adapted withpermission from (78, 86)]

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A large family of conversionmaterials—suchas oxides, sulfides, and fluorides—offer poten-tial for storing a large amount of charge, butthey have poor cyclability coupled with phasetransformation and large volume change (90).Benefits of nanostructures have been fully de-monstrated on these materials as well (20).For example, through a conversion reaction,lithiation and sodiation of FeF2 electrodesgenerate a composite material consisting ofultrafine (1 to 4 nm) Fe nanoparticles, whichare further fused into a continuous conduc-tive network, and a fluoride phase. Metal nano-particle networks in the resulting structuresenable fast electron transport during furthercycling (91, 92). A similar effect is observedduring lithiation of a Ag2VO2PO4 cathode (93).High rate capability demonstrated by thismaterial is attributed to the 15,000-fold in-crease in electronic conductivity associated within situ formation of Ag nanoparticles duringelectrochemical reduction of Ag2VO2PO4 (93).The sulfur cathode is another important ex-ample (94). Sulfur has a high theoretical ca-pacity of 1673 mA·hour g−1 and offers a hightheoretical specific energy of 2500W·hour kg−1

for Li-S batteries. However, sulfur cathodeshave three critical problems: (i) dissolution oflithium or sodium polysulfides into the or-ganic electrolyte and their shuttling across theseparator, (ii) lack of the electronic and ionicconductivity of sulfur and lithium sulfide re-sulting in poor utilization of active material,and (iii) large (80%) volume expansion of sul-fur upon lithiation. The nanomaterials ap-proach represents the most powerful solutionto the aforementioned problems (89, 95). Thinlayers of 2D materials, such as MXene (42), orelectrospun carbon nanofibers (96) at theseparator on the cathode side can serve asbarriers for polysulfide transfer across the

separator. Encapsulation of sulfur in mesopo-rous carbon or MXene (95), S-TiO2 yolk-shellstructures (89), hollow sulfur spheres, and am-phiphilic binding of sulfur and lithium sulfidespecies by oxide and nitrides offer solutions.In the case of metal-S batteries, nanomaterialswith strong affinity to Li-polysulfides, such ascarbides and nitrides of transitionmetals withhigh metallic conductivity (97), are preferredfor building a scaffold for sulfur. The samematerials with nanofiber or nanosheet mor-phology can be used for coating separators toprevent polysulfide shuttle. Another type ofnanomaterial in the form of 0D or 2D particlesor porous scaffolds can be used to prevent Lidendrite growth on the anode side (98, 99).Such battery architecture highlights the im-portance of the use of nanomaterials in allthe battery components but also requiresa holistic approach toward selecting nano-materials that can perform different functionswithin an energy storage device. This area ofresearch is currently under active investigation,and specific material solutions are yet to befound for each individual energy storage sys-tem. Nevertheless, MXenes and graphene havealready shown promise in all the three keyaforementioned battery components.The continued pursuit of high–energy den-

sity battery chemistries, such as Li-S, recent-ly revived considerable interest in Li metalanodes. Li metal has the theoretical specificcapacity of 3860mA·hour g−1 and the lowestpotential as an anode, which maximizes thespecific energy. However, Li metal has a longlist of problems, including extremely highchemical reactivity and large volume changesduring Li metal plating and stripping, whichcreate phenomenological problems of Limetaldendrites and mossy Li formation, instabilityof SEI, low coulombic efficiency, battery short-

ing, and fire hazard (100). The interfacial sta-bility was recently improved with additives orcoatings of nanodiamonds, h-BN, and othernanomaterials (4, 101). The issue of large vol-ume change can be addressed by designinghost materials to house Li metal plating andstriping, including hollow carbon nanospheres,graphene oxide, MXene, and polymer nano-fiber scaffolds (102).Many conventional cathode materials, such

as LiFePO4 or LiCoO2, when downsized to thenanometer scale, can provide faster energystorage compared with the bulk counterparts(43). However, the energy storage mechanismchanges, with the surface redox reaction be-coming a dominant process. Large surface areacreates a variety of sites for redox reactions, eli-minatesdiffusion, andveryoften leads tochangesin the electrochemical behavior, as has beenshown, for example, for LiCoO2 (43) (Fig. 4B). Adecrease in the particle size leads to capacitor-likebehavior, almost linear (supercapacitor-like)galvanostatic charge-discharge curves, and adecrease in capacity (43). Changes in electro-chemical behavior induced by nanostructuring,similar to those observed for LiCoO2, are likelyto be exhibited by other intercalation cathodes,including high-capacity materials from thelithium nickel cobalt aluminum oxide (com-monly LiNi0.8Co0.15Al0.05O2 or NCA) and lith-ium nickel cobalt manganese oxide (oftenLiNi0.6Co0.2Mn0.2O2 or LiNi0.8Co0.1Mn0.1O2 orNCM) families. This behavior is not neces-sarily exhibited by all nanomaterials, but it isnecessary to consider that capacity can be bothincreased or decreased, with the shrinking ofcrystal or particle size, depending on the typeof material and charge-storage mechanism.Examples are the increase in capacity in thecase of silicon (3) and the decrease in capacityin the case of LiCoO2 (43) (Fig. 4B).

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Fig. 4. Effect of nanostructuring on the electrochemical performance of energy storage materials in Li-ion batteries. (A) Schematic showing the effect ofparticle size on the lithiation of silicon anode material (144). (B) Effect of grain size on the galvanostatic discharge curve of LiCoO2 cathode material (43).[Images adapted with permission from (43, 144)]

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Solid-state batteries, exhibiting substantiallyimproved safety compared with the traditionalones, are considered to be one of the mostpromising storage technologies. In this con-text, there are a few considerations that makenanomaterials important for advancing thistechnology. First, in case of the solid-state bat-teries with planar geometry, nanostructuringpromises to control 2D interfaces betweenbattery components by means of the incorpo-ration of specifically designed interface layerswith nanoscale thickness and the ability tosuppress parasitic reactions between electrodeand electrolyte or metal dendrite growth(23–27). Additionally, nanomaterials can beused to create specific battery components. Forexample, nanoflakes of conductive 2D mate-rials, such as graphene or MXene, can be as-sembled to form an exceptionally thin currentcollector layer (103). An exciting opportunityexists in the area of creating a highly conduc-tive, uniform, and pinhole-free solid-state elec-trolyte layer with nanoscale thickness, whichcan potentially be achieved, for example, byusing atomic layer deposition (104). Alterna-tively, Li-conducting nanofibers or nanowirescould be embedded into the Li-conductingpolymermatrix to produce a hybrid reinforcedelectrolyte layer with both high ionic conduc-tivity and improvedmechanical properties com-pared with the pristine one (105). Second, incase of the solid-state batteries with 3D archi-tectures, the aforementioned nanomaterials-enabled advances are also important. However,in this case, nanomaterials can be used to con-struct 3D electrodes. Indeed, 3D elements inelectrodes, such as pillars or cavities, often havemicron-sized geometry to ensure mechanicalstability. However, they can be hierarchicallybuilt out of nanoscale particles of variousgeometries to form pores for electrolyte pene-tration and to shorten diffusion distances toachieve fast transport of ions and electrons(106). Of course, building such architectureswould require advanced manufacturing ap-proaches, which we discuss below.In addition to higher cost, compared with

conventional materials, the remaining chal-lenges related to the use of nanomaterials inenergy storage devices include dealing withthe sloping charge-discharge and the toxicity.For the former, the supercapacitor communityknows how to handle this issue (107). Concern-ing the toxicity, it is important to ensure thatnanomaterials have no or low toxicity and willnot be harmful either during manufacturingor in the environment, once disposed. Nano-materials, such as carbons, silicon, MXenes, orTiO2, are nontoxic. Moreover, nanomaterialscan be degraded in the environment fasterthanmacroscopicmaterials with the same com-position. For example, Ti3C2 degradation pro-duces TiO2 and CO2, both nontoxic products,and the 2D morphology of MXene will lead to

fast biodegradationwhen exposed towater andair. However, it is important to study thetoxicity and environmental fate of new nano-materials to ensure that they can safely beintroduced into large-volume manufacturing.Minimization of the side reactions caused by alarge surface area of many nanomaterials isanother major challenge. Surface chemistrypassivation, electrode materials design thatminimizes exposed SSA (e.g., yolk-shell par-ticles), preconditioning of electrodes, and useof electrolytes that produce stable SEI can beused separately or together to mitigate thenegative effects of side reactions.

Nanomaterials with fast ion andelectron transport

Low-dimensional materials can combine highelectronic and ionic conductivities by usinga mechanism that is usually referred to aspseudocapacitive or surface redox energy stor-age (69). It was realized more than 20 yearsago by Conway et al. that ruthenium oxide(RuO2), having a capacitor-like behavior, hasredox energy storage (faradaic process) be-hind its large capacitance values (108). Butthe cost of the rare element ruthenium coupledwith the fact that this material can only ope-rate in very thin layers has limited its prac-tical use (109). Additionally, the amount ofstructural water in ruthenium oxide is a keyfactor for its electrochemical performance(Fig. 5A), and its control during device ope-ration represents a major challenge (110).Transition-metal atoms on the surface ofMXenes can participate in redox reactionswith fully electrochemically reversible redoxwave in cyclic voltammetry curves overlaidon the large rectangular area correspondingto the double-layer capacitive charge storagemechanism (57) (Fig. 5B). The example ofMXenes (42, 57) has shown that both double-layer and redox capacitance can be used atvery high current rates, with just ~20% elec-trochemical performance loss when goingfrom 10 to 100,000 mV s−1 cycling (Fig. 5B).This rate would be impossible for conven-tional redox electrodes, which have lowconductivity and a diffusion-limited charge-storage mechanism. MXenes have shown acharging time in the 1- to 10-ms range (57).At the same time, the capacitance of MXenes(up to 500 F g−1 and 1500 F cm−3 in thinfilms in acidic electrolyte, Fig. 5B) exceedsthe capacitance of double-layer capacitormaterials, such as carbons, which have 100to 200 F g−1 or F cm−3, while volumetricallyrivaling that of ruthenium oxide thin films(1500 F cm−3) (110). Thick electrodes can alsowork well, if restacking of the 2D sheets isprevented. In fact, vertical alignment of 2Dsheets, achieved by exploiting their liquidcrystalline behavior or through templating,would allow the development of MXene elec-

trodes with tens of milligrams per square cen-timeter. In this context, thickness-independent(up to 200 mm) capacitance of vertically alignedMXene flakes has been demonstrated (37).In many cases, however, it is necessary to

combine different materials to achieve fasttransport of both electrons and ions. A goodexample is the design and realization of hybridstructures, which have been reported for num-erous oxides (Nb2O5, TiO2, MoO3, etc.) on avariety of carbon supports, such as nanotubes,graphene-based materials, and porous carbons(13). The carbon affects the electronic proper-ties of both materials because it not only actsas a channel for electrons but also forms aheterojunction between the oxide and car-bon surface. As a result, a higher capacity(~1000 mA·hour g−1) has been achieved fora graphene–iron oxide electrode comparedwith both only oxide (~600 mA·hour g−1),which cannot operate at high rates, and onlycarbon material (~400 mA·hour g−1) (111)(Fig. 5C). When combined, these materialscan operate at current densities exceedingseveral amperes per gram. Moreover, with thecorrect design of the electrode architecture, avery high rate performance can also be achieved,as demonstrated for Nb2O5 supported on gra-phene or MXene (112). Building such compo-site architectures can also allow the use ofconversion electrodematerials, such as FeF3,CuCl2, or S undergoing phase transforma-tions (21).Another way to enable fast transport of

electrons and ions is through the creation of2D heterostructures (12), which allow thecombination of highly conducting and high–energy density 2D materials. Because at leastone material in the hybrid structure shouldhave good electronic conductivity, graphenehas been the primary material of choice. Thisapproach is rather universal, with a very largenumber of metallically conductive and redox-active materials available (113). It has been im-plemented in several different systems forapplications ranging from pseudocapacitorsto Li-ion and Li-S batteries (113). As of now,the governing assembly principles of integrat-ing dissimilar nanomaterials into desired arc-hitectures are poorly understood. Moreover, itis not yet known how the transport of electronsand ions occurs between dissimilar 2D sheetsand through the separating electrolyte orconfined fluid. What is the optimal spacingbetween the sheets? Is the physical contactbetween the particles an essential requirementfor electron transport or can the hopping ortunneling serve as the dominant transportmechanism? Machine learning should allowfor optimization of those systems and forunderstanding the guiding principles for theselection of the optimum combinations of 2Dmaterials to achieve the best electrochemicalperformance (114).

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Manufacturing of the nanomaterial-enabledenergy storage devicesFor large-scale applications, we need to buildbatteries and supercapacitors in a convent-ional format, but exploiting nanomaterialswill allow faster operation, higher power, andlonger lifetime compared with the currenttechnology. For example, replacing graphitewith nanostructured silicon (115) can lead toa substantial increase in the energy densityover conventional batteries. One of the keyadvantages of nanoscale materials is that theycan be used to manufacture electrodes of anysize, shape, or form factor. For example, we areused to seeing conventional batteries as sepa-rate units. But there is no reason why theycannot be combined with structural elementsand distributed, when electrodes can conformto any shape and be made strong, robust, andflexible. As a result, instead of occupying spaceunder the car body or in a trunk, they can be-come part of the automobile structure per se,for example, forming the car body or filling theempty space in doors and under the hood. All

these properties can be achieved by using 1Dand 2D materials with high mechanical prop-erties and electronic conductivity. Beyond con-ventional energy storage devices for portableelectronics and vehicles, there is increasingdemand for flexible energy storage devicesneeded to power flexible electronics, includingbendable, compressible, foldable, and stretch-able devices. Wearable electronics (116) willrequire the incorporation of energy storagedevices. This means that we need energy stor-age fibers, fabrics, and textiles and the abilityto incorporate energy-storing materials intoclothes. This involves the manufacture of non-toxic, strong, stretchable, and even washableconductive fibers, capable of both ionic andelectronic transport. For the implantable de-vices, instead of traditionally used coin cells orpatches, stretchable and biodegradable or bio-compatible batteries could be incorporated in-side thehumanbodyor battery-powereddevices.The design and realization of these devices

require the incorporation of nanomaterials intofunctional architectures. Several strategies have

been established by (i) using flexible substrates(117) and separators (118); (ii) designing newdevice patterns (119) and configurations—forexample, fiber-like and spring-like devices (120);(iii) compositing electrode materials into flex-iblematrices (32, 121), and so on. Flexible energystorage devices, including Li-ion battery (122),Na-ion battery (7), and Zn-air battery (123);flexible supercapacitors, including all-solid-statedevices (124); and in-plane and fiber-likemicro-supercapacitors (125) have been reported.How-ever, the packaged microdevice performance isusually inferior in terms of total volumetric orgravimetric energy density comparedwith con-ventional batteries of supercapacitors. Nano-materials will use different manufacturingmethods (Fig. 6). Spray coating and other high-throughput manufacturing techniques (e.g.,doctor blade, dip coating, electrodeposition,layer-by-layer deposition, vacuum filtration, andink-jet printing) (10) may make these devicessufficiently inexpensive for commodity appli-cations. For example, electrochemically exfo-liated graphene has been used to formulate

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Fig. 5. Achieving fast transport of both electrons and ions in nanomaterials. (A) Schematic illustration of RuO2·nH2O demonstrating the ability to controlelectronic conductivity and proton transport by tuning the material hydration and cyclic voltammetry (CV) data obtained at scan rates from 2 to 500 mV s−1 (145).dQ dE−1, capacitance; RHE, reversible hydrogen electrode. (B) Schematic illustration of a typical M3C2 MXene structure and cyclic voltammetry data collectedat scan rates from 10 to 100,000 mV s−1 for a 90-nm-thick Ti3C2Tx film. Ti3C2Tx possesses excellent electronic conductivity owing to an electronically conductivetransition metal carbide layer. Intercalated water molecules enable accessibility of protons to the redox-active TiO2-like surface (57). (C) Schematic illustrationof a composite material synthesized via decoration of graphene sheets with Fe3O4 nanoparticles and rate performance of the obtained material at current densitiesranging from 35 to 1750 mA g−1 as compared with that of commercial Fe3O4 particles and nanoparticles of Fe3O4 synthesized using a similar approach butwithout the addition of graphene sheets (146). GNS, graphene nanosheets. (D) Schematic illustration of a 2D heterostructure consisting of the alternate MoS2 andnitrogen-doped graphene (NDG) layers and its rate performance in a Li-S battery at current densities ranging from 0.1 to 5 A g−1 (147). [Images adapted withpermission from (57, 145–147)]

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inks and manufacture in-plane micro-super-capacitors on flexible substrates and wafersby spray coating (126). Ink-jet printing hasshown promise in the fabrication of flexiblethin-film energy devices with large area andreadily controllable thickness (127). Centrifu-gal casting can produce large-area sheets of 2Dmaterials much faster than vacuum-assistedfiltration. Roll-to-roll manufacturing can trans-form the assembly of battery-powered devicesinto a process similar to printing a newspaper.It is important to mention that conductingcurrent collectors and insulating separators(in the case of sandwich-device architecture)need to be printed by the samemethod. Also,interdigitated and other non-sandwich energystorage device architectures become possiblewith printing technologies and polymer gelelectrolytes, which both conduct ions andseparate the electrodes.The quality of printed films depends on the

stability of active nanomaterial-based inks (10).In this context, functionalized nanoparticlesare typically used for ink formulation becauseof their good dispersibility in solvents (bothaqueous and organic). To increase the arealenergy density of devices and their capacitanceor capacity, it is necessary to build 3D deviceswith increased thickness and hierarchy of theelectrodes. Therefore, it is necessary to develop3D printing techniques and adjust them tospecific conditions; for example, 0D particlesand 2D sheets are easier to print comparedwith CNTs, but the latter can be more readilyincorporated into fibers, providing mechanicalstrength and electrical conductivity requiredin this application. A 3D printed, interdigi-tated Li-ion microbattery was demonstratedusing Li4Ti5O12 (LTO) and LiFePO4 (LFP) as

the anode and cathode materials, respectively(128). This approach can produce distributedenergy storage devices integrated with otherelectronic components.The use of structural or printable energy stor-

age (Fig. 6) adds additional requirements to theenvironmental and temperature stability of allcomponents of the device. If a battery is loca-ted close to the hot part of an engine or in-corporated into a part of the car body that willbe subjected to sunshine over a prolongedperiod of time, the use of thermally stableelectrolytes (e.g., ionic liquids) and other com-ponents (e.g., ceramic separators, such as cera-mic nanofibers, boron nitride, or clay) may berequired. In printable devices for Internet ofThings and future miniaturized electronics,the use of nanomaterials should be considerednot only as active components but also asbinders, current collectors, sealants, and en-capsulating elements. For example, the lattercan be constructed using micrometer-thickpolymer films with insulating 2D nanopar-ticles, decreasingwater and oxygen permeability.In-plane micro-supercapacitors made of

carbide-derived carbons and 2D materials—including graphene,MXene,metal oxides, andconductive MOFs—are among the most pop-ular flexible and integrated energy storagedevices. Carbide-derived carbon films thatpossess a high SSA and narrow pore size canalso be fabricated on a silicon wafer withoutcracking by two key steps: sputtering of metalcarbides and chlorine-gas etching (129). 2Dheterostructures can provide improved electron-and ion-transport pathways (12). However,most 2D materials–based in-plane micro-supercapacitors are fabricated by photo-lithography. 2D metal–organic coordination

framework graphene- and MXene-based in-plane micro-supercapacitors with ac line-filtering performance were fabricated by insitu layer-by-layer growth of active materialon prefabricated current collectors (34). Suchan approach, coupled with the exploitation of2D materials, offers a pathway for the scalablefabrication of in-plane micro-supercapacitors.Smart energy storage devices, which can de-

liver extra functions under external stimulibeyond energy storage, enable a wide rangeof applications. In particular, electrochromic(130), photoresponsive (131), self-healing (132),thermally responsive supercapacitors and bat-teries have been demonstrated. However, thefade of the performance under stimuli stillhinders their practical applications. Anotherpathway to achieve stimuli is manipulatingelectrolyte—for example, by using thermallyresponsive polymer gels—to control the iontransport between the electrodes, which caneventually cause the on and off switchingof the device. Although several prototypeshave already been demonstrated, consid-erable challenges—for example, balance ofhigh performance and extra functions andthe integration of such smart devices intofully functioning systems—still need to beresolved (133).Parasitic reactions between electrode nano-

materials and electrolytes (3) can cause thedecompositionof electrolyte andmetal consump-tion for metal-ion batteries and consequentlyresult in poor energy storage performance,including low Coulombic efficiency, cycle life,and energy density, compromised safety, andso on. Many efforts—such as the developmentof coated electrodematerials (134), electrolytesand additives (135), membranes (136), and

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Fig. 6. Nanomaterials enable the production of next-generation energy storage systems by different manufacturing methods. [Supercapacitor array imageby Husam N. Alshareef/King Abdullah University of Science and Technology (KAUST); figure wearing smart textiles image by Kristi Jost/Drexel University]

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metal-electrolyte interfaces (137)—have beenmade to suppress parasitic reactions. For ex-ample, fluoroethylene carbonate additive hasbeen used to improve the cycle life of Li-ionbatteries with Si nanoparticles (60 nm) bysuppression of parasitic reactions, avoiding theformation of metastable c-Li15Si4 phase (138).The use of nanomaterials and newmaterials, ingeneral, may require the development of newelectrolytes compatible with those materials,especially in confinement (139).

Conclusions

Despite certain skepticism within the batterycommunity related to the use of nanomaterialsin commercial devices, several examples inwhich nanostructuring led to breakthroughsin performance, such as in the case of silicon-carbon composite anodes, indicate that nano-structured materials can provide solutions tocreate high-energy, high-power, and long-lastingenergy storage devices. Research carried outover the past decade has shown that a device’slifetime increases as a result of nanostructur-ing. Indeed, overcoming the strain induced bycyclic expansion and contraction of macroscaleparticles can compensate for lifetime limita-tions resulting from electrolyte breakdown ona large surface of nanomaterials. Design of en-capsulated nanoparticle assemblies can fur-therminimize the contact areawith electrolyte,decreasing the irreversible processes of thefirst charge-discharge cycle. Large-scale imple-mentation of Si nanoparticles in Li-ion batteryanodes by Sila Nanotechnologies and othercompanies is a convincing demonstration ofthe scalability of nanomaterials for large-volume battery production. The use of hund-reds of tons of multiwall CNTs as conductingand reinforcing additives in battery electrodesis an excellent example of nanoscale additiveuse. There are other nanomaterials—such assingle-wall CNTs, graphene, and so on—usedin small-volume or small-size batteries andsupercapacitors. Decreased prices and increasedconfidence in safety (health, environmental,and operational) will open doors for a widerimplementation of nanomaterials in energystorage technology. To reach full potential,nanomaterials need to be combined in so-phisticated architectures that enable multiplefunctionalities related to the transport of elec-trons and ions as well as interactions betweenvarious device components or suppression ofsuch interactions. However, enabling complexarchitectures requires the use of advanced pro-cessing and manufacturing techniques compa-tible with nanomaterials, such as 3D printing,knitting, spray and/or spin coating, roll-to-rollassembly, and others.

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ACKNOWLEDGMENTS

Funding: E.P. acknowledges support from National ScienceFoundation award no. DMR-1752623. F.B. acknowledgesfunding from the European Union’s Horizon 2020 research andinnovation program under grant agreement no. 785219–GrapheneCore2 and thanks A. E. Del Rio Castillo and H. Sun foruseful discussions. X.F. thanks EU Graphene Flagship, ERCT2DCP, Coordination Networks: Building Blocks for FunctionalSystems (SPP 1928, COORNET), and the Center of AdvancingElectronics Dresden (cfaed). Y.G.’s research on energystorage was supported through the Fluid Interface Reactions,Structures, and Transport (FIRST) Center, an Energy FrontierResearch Center funded by the U.S. Department of Energy,Office of Science, and Office of Basic Energy Sciences.Competing interests: None declared.

10.1126/science.aan8285

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Energy storage: The future enabled by nanomaterialsEkaterina Pomerantseva, Francesco Bonaccorso, Xinliang Feng, Yi Cui and Yury Gogotsi

DOI: 10.1126/science.aan8285 (6468), eaan8285.366Science 

, this issue p. eaan8285Scienceas that found in the battery community, to the use of these materials.materials and briefly explore potential manufacturing processes. The authors also consider some of the skepticism, such

review fundamental processes of charge storage that apply specifically to nanostructuredet al.context, Pomerantseva enhanced energy storage, although there are also challenges relating to, for example, stability and manufacturing. In thismaterials continue to grow. Materials that have at least one dimension on the nanometer scale offer opportunities for

From mobile devices to the power grid, the needs for high-energy density or high-power density energy storageThinking small to store more

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