Letter Cite This: Nano Lett. 2018, 18, 7761−7768
pubs.acs.org/NanoLett
Multifunctional Structural Ultrabattery Composite Chuanzhe Meng,†,∥ Nitin Muralidharan,†,‡,∥ Eti Teblum,§ Kathleen E. Moyer,†,‡ Gilbert D. Nessim,§ and Cary L. Pint*,†,‡ †
Department of Mechanical Engineering and ‡Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, Tennessee 37235, United States § Department of Chemistry, Bar Ilan Institute for Nanotechnology and Advanced Materials, Bar Ilan University, 52900 Ramat Gan, Israel Downloaded via KAROLINSKA INST on January 28, 2019 at 04:04:43 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: Here we demonstrate a composite material exhibiting dual multifunctional properties of a structural material and a redox-active battery. This incorporates three-dimensional aligned carbon nanotube interfaces that weave together a structural frame, redox-active battery materials, and a Kevlarinfiltrated solid electrolyte that facilitates ion transfer. Relative to the total measured composite material mass, we demonstrate energy density up to ∼1.4 Wh/kg, elastic modulus of 7 GPa, and tensile strength exceeding 0.27 GPa. Mechano-electrochemical analysis demonstrates stable battery operation under mechanical loading that validates multifunctional performance. These findings demonstrate how battery materials that are normally packaged under compression can be reorganized as elements in a structurally reinforced composite material. KEYWORDS: Structural battery, multifunctional energy storage, ultrabattery, carbon nanotubes, reinforced composite
T
unpackaged and require functionality under a wide range of mechanical stresses. Numerous reports have emerged in recent years demonstrating materials where supercapacitor devices and less commonly batteries are configured into a solid-state framework that could be amenable for structural energy storage.4−19 Supercapacitors are the most straightforward for this approach, as they rely on energy storage by an electric double layer that forms on the surface of high surface-area materials such as graphene or carbon nanotubes, which consequently also exhibit excellent native mechanical properties. However, the storage of charge only on active surfaces, the requirement of these materials to remain exfoliated in a dense composite matrix, and the open challenges to form both mechanical and charge storage interfaces simultaneously pose practical technology barriers. Currently, the best measured energy density for multifunctional structural composites has been near 10 mWh/kg, over 10 000 times lower than that for Li-ion batteries, and none of these studies validate any energy storage properties under simultaneous mechanical stresses.15,20,21 Ongoing studies focused on fibers and textiles, which are not strictly “structural” materials, have yielded ∼10× improved energy storage performance.19,22 In this regard, batteries enable a route to increase energy density but also bring new challenges, since battery reactions lead to volume
he rechargeable battery ushered forward the technological revolution that “cut the power cord” to enable new mobile device technologies. The next leg of this revolution underway today is the transition of batteries into larger-scale installations including drones, electric vehicles, and the electric power grid. The massive scales of battery fabrication and adoption for this stage will require a synergy between earthabundant raw materials and completely new approaches to store energy in system architectures. One intriguing pathway is to integrate the chemistries enabling energy storage into materials that otherwise look, feel, and function as structural materials.1,2 This can produce materials that are drop-in replacements for existing structures, such as buildings or vehicles, which can either improve local reliability of microgrids or augment the performance of externally situated energy storage. With even a low mass-specific energy density performance in the 1−5 Wh/kg range, such a global integration strategy into less than 0.1% of infrastructure materials would lead to on-demand energy stored at the scale of terawatt-hours. Moreover, this rapidly accessible and ondemand energy could replace fast-ramping peaker plant generators, augment reliability for conventional generators, and provide a landscape for high penetration of renewables without utility-scale energy storage installations or electric grid overhaul.3 Despite this promise, a critical challenge to fabricate such materials is that energy storage devices are packaged and operate under compression, while structural materials are © 2018 American Chemical Society
Received: August 29, 2018 Revised: November 2, 2018 Published: November 13, 2018 7761
DOI: 10.1021/acs.nanolett.8b03510 Nano Lett. 2018, 18, 7761−7768
Letter
Nano Letters
Figure 1. Schematic representation of a multifunctional ultrabattery composite using fiberglass or Kevlar as the separator between the active Faradaic battery materials.
Figure 2. Deposition of active materials and characterization. (a) Schematic representation of the electrodeposition process for depositing NiOx and FeOx on the surface of CNTs grown on stainless steel (SEM micrographs). (b, c) SEM micrographs of the deposited NiOx and FeOx on CNTs. (d−f) SEM-EDS elemental maps showing the presence of nickel-rich species on CNTs. (g−i) SEM-EDS elemental maps showing the presence of iron-rich species on CNTs. (j) Raman spectra of the deposited material on CNTs.
expansion in active materials and hence mechanical stresses across working battery interfaces.23−26 Currently, there remain no routes described in the literature where an unpackaged material can operate in air under mechanical stresses and exhibit reversible performance as a battery, even though efforts have evaluated packaged batteries or solid-state polymer-based electrode architectures in this context.27−29 Importantly, one of the key challenges in modern liquid-electrolyte lithium battery
manufacturing is the delamination at the electrode-current collector interface,30−32 which sets a heavy precedent for an unpackaged structural battery composite. Although many faradaic battery chemistries have been reported in literature,33−37 research into the century-old Ni− Fe battery system with energy densities of ∼30−50 Wh/kg and power densities of ∼3−50 W/kg continues today owing to its durability and robust energy storage capabilities even under 7762
DOI: 10.1021/acs.nanolett.8b03510 Nano Lett. 2018, 18, 7761−7768
Letter
Nano Letters
Figure 3. Electrochemical performance of the individual electrodes. (a) Cyclic voltammograms at various scan rates. (b) Capacity and capacitance variation with scan rates. (inset) Schematic representation of the ultrafast electron conduction pathways as a result of the CNT-SS framework. (c) Galvanostatic discharge curves for the deposited nickel hydroxide on CNT-SS electrode. (d) Cyclic voltammograms at various scan rates. (e) Capacity and capacitance variation with scan rates. (inset) Schematic representation of the ultrafast electron conduction pathways as a result of the CNT-SS framework. (f) Galvanostatic discharge curves for the iron oxide on CNT-SS electrode.
harsh conditions.38,39 Recently, Wang et al.40 employed inorganic/carbon hybrid pseudocapacitive electrode materials to increase the power density greater than 15 W/kg1000 times higher than conventional Ni−Fe cells. This is the basis of an ultrabattery: a device building on conventional redox chemistry associated with batteries but utilizing nanoengineering approaches to achieve high power cycling capability. Whereas reports on Ni−Fe ultrabattery chemistries focus on electrode fabrication approaches, binder free approaches, and electrolyte optimization approaches,41−44 the integration of these robust battery systems into structural composites remains unexplored so far. In this Letter we demonstrate the design, fabrication, and multifunctional testing of a structural ultrabattery composite. This design builds upon the use of carbon nanotube (CNT) arrays that are connected both into a structural frame (stainless steel (SS)) and a solid-state ion conducting matrix and which are coated with nickel- or iron-based active materials. This ensures an interconnected system with integrated active material and mechanical interfaces in the design. We then
use a wet lay-up process combining these electrodes with fiberglass or Kevlar separators and perform simultaneous in situ mechano-electrochemical tests that demonstrate stable electrochemical energy storage performance until mechanical failure. This work stands out as the first practical design combining a reinforcement strategy across heterogeneous material interfaces integrated with battery materials, with the highest reported energy density of ∼1.4 Wh/kg relative to the total mass of all materials making up the multifunctional composite. The schematic representation of the ultrabattery composite is shown in Figure 1a. This ultrabattery composite reflects a battery chemistry capable of fast charging and discharging that is not prone to the low-energy density limitation of doublelayer capacitive energy storage devices. In these devices, CNTs are grown onto SS meshes using chemical vapor deposition (CVD),45,46 and oxides/hydroxides of iron and nickel are deposited onto the CNT surfaces to produce the anode and cathode, respectively. The composite is assembled using wet lay-up process using potassium hydroxide−poly(vinyl alcohol) 7763
DOI: 10.1021/acs.nanolett.8b03510 Nano Lett. 2018, 18, 7761−7768
Letter
Nano Letters
Figure 4. Electrochemical performance of the ultrabattery composite. (a) Cyclic voltammograms at various scan rates. (inset) Schematic representation of the iron-based anode and nickel-based cathode for the ultrabattery composite. (b) Capacity and capacitance variation with scan rates. (c) Galvanostatic discharge curves. (d) Capacity and capacitance variation with different applied current densities. (e) Energy and power density of the Ni−Fe ultrabattery composite compared to state-of the art structural energy storage composites in literature. (f) Energy and power density projections for the Ni−Fe ultrabattery composite (based on active mass and highest reported values) compared to state-of-the-art structural energy storage composites in literature.
spectroscopy (EDS) elemental mapping (Figure 2d−i) indicates the presence of nickel and iron species on the CNT surfaces. This is further corroborated using Raman spectroscopic analysis (Figure 2j), which shows peak signatures of α-nickel hydroxide and iron oxide.49−51 Overall, this demonstrates an architecture that is ideal for a structural battery where the CNTs reinforce the composite and structurally interconnect the electrolyte matrix, the active material, and the structural frame. Electrochemical characterization of the redox-active materials in KOH aqueous electrolytes (1 M) was performed to assess the redox properties of the individual electrodes. Cyclic voltammograms at scan rates from 5 to 100 mV/s for the nickel hydroxide−CNT/SS electrode (Figure 3a) reveals oxidation and reduction reactions corresponding to the reversible conversion of Ni(OH)2 to NiOOH. The anodic and cathodic peaks are centered around ∼0.3 V versus standard calomel electrode (SCE). At scan rates of 5 mV/s the electrode delivers a high capacity of 86 mAh/g (capacitance of
(PVA-KOH) gel, which forms a solid ion conducting matrix to function as the electrolyte (see Figure S1a,b).41,47,48 Electrical separation between the electrodes is ensured by intermediate layers of either fiberglass or Kevlar (aramid fiber). Images of the ultrabattery composites employing both fiberglass and Kevlar separators used in this study are shown in Figure 1a. Figure 2 demonstrates characterization of the CNTs grown on the stainless steel and the coating of the CNTs with redoxactive battery materials. Figure 2a shows scanning electron microscopy (SEM) micrographs demonstrating the morphology of the vertically aligned CNTs grown using CVD (also see Figure S2). Raman spectra shown in Figure S3 reveal both sp3 and sp2 hybridized carbons characterized by the D and G peaks, respectively, indicative of both multiwalled carbon nanotubes and the presence of active sites for electrochemical deposition. Electrochemical deposition of nickel and iron onto CNTs was performed using nitrate precursor solutions (see methods) to obtain NiOx- and FeOx-coated CNTs as observed in the SEM micrographs (Figure 2b,c). Energy-dispersive X-ray 7764
DOI: 10.1021/acs.nanolett.8b03510 Nano Lett. 2018, 18, 7761−7768
Letter
Nano Letters
Figure 5. In situ mechano-electrochemical performance of the ultrabattery composite. (a) Images of the ultrabattery composites based on fiberglass and Kevlar clamped in tensile mode in an Instron mechanical tester. (b) In situ mechano-electrochemical measurements of the fiberglass composite during simultaneous charge−discharge measurements along with dynamic tensile tests. (c) Galvanostatic charge/discharge profiles from a region showing stable cycling performance of the fiberglass composite. (d) Galvanostatic charge/discharge profiles from the region showing the failure of the fiberglass-based ultrabattery composite. (e) In situ mechano-electrochemical measurements of the kevlar composite during simultaneous charge−discharge measurements along with dynamic tensile tests. (f) Galvanostatic charge/discharge profiles from a region showing stable cycling performance of the Kevlar composite. (g) Galvanostatic charge/discharge profiles from the region showing the failure of the Kevlar-based ultrabattery composite.
394 F/g) based on active material mass. The electrode exhibits high rate capability maintaining ∼90% of the capacity observed at 5 mV/s at high scan rates of 100 mV/s as shown in Figure 3. We attribute this high capacity maintenance at high scan rates to the design where the CNTs are directly coupled to the conductive stainless steel meshes (Figure 3b), providing efficient electron conduction pathways for the active materials
Galvanostatic tests (Figure 3c) indicate capacity over 50 mAh/g that is maintained even at high current densities of 20 mA/cm2 (active area) indicating the good energy storage capability of the nickel hydroxide electrode. Similar tests were performed on the iron oxide CNT-SS electrode in aqueous 1 M KOH electrolyte. The cyclic voltammograms (Figure 3d) from 5 to 100 mV/s show anodic and cathodic peaks corresponding to reversible redox reaction of FeOOH ↔ 7765
DOI: 10.1021/acs.nanolett.8b03510 Nano Lett. 2018, 18, 7761−7768
Letter
Nano Letters Fe(OH)2 centered around ∼0.9 V versus SCE. Here, the iron redox electrode shows high capacity maintenance with over 130 mAh/g at a scan rate of 100 mV/s (Figure 3e), where the CNTs play the same role as in the Ni electrodes. Galvanostatic tests (Figure 3f) indicate capacity maintenance of over 130 mAh/g at a high current density of 20 mA/cm2. These half-cell tests emphasize the individual redox properties of these electrodes relevant to the structural composite materials, while cycling studies and full cell evaluation of the liquid (nonstructural) battery configuration is included in the Supporting Information (Figures S4 and S5, respectively). To fabricate structural ultrabatteries, the NiOx and FeOx CNT/SS meshes were assembled in a sandwich configuration with fiberglass or Kevlar separator using PVA-KOH (1 M) gel electrolyte in a wet lay-up process (see Supporting Information). The total mass loading of the active materials was 16 mg/cm2. Higher mass loadings of 24 mg/cm2 were studied and found to have poorer energy density performance attributed to a thicker coating of active materials that cannot be electrically addressed by the CNT reinforcing medium as produced in our experiments (Figure S6). Since our design reflects a coordination between the reinforcing layer (CNTs) and active material thickness, we expect thicker CNT layers combined with thicker active material depositions to accommodate a higher accessible mass loading. The methodology of wet lay-up composite processing for our structural batteries correlates to the widespread use of this technique in conventional fiber-based composite manufacturing, while integration of structural fibers such as fiberglass and aramid fibers (Kevlar) into the composite battery matrix capitalizes on materials used in traditional structural composites. Electrochemical performance of the ultrabattery composite using a fiberglass separator layer was determined using cyclic voltammetry and galvanostatic charge−discharge tests. Voltammograms at various scan rates (5 to 100 mV/s) shown in Figure 4a indicate the redox reaction corresponding to Ni2+→3+ and Fe3+→2+ and the reverse reactions in each forward and reverse sweep. The fabricated full cell composite exhibits a good capacity of ∼17 mAh/g at a scan rate of 5 mV/s as well as maintaining good rate capability when increasing the scan rates from 5 to 100 mV/s (Figure 4b). The slight capacity loss at high rates in the structural battery versus the half-cell tests are attributed to series electrolyte resistance through the fiberglass/Kevlar material that is not present in three-electrode measurements in half cell configurations. Nevertheless, the composite shows good galvanostatic charge/discharge characteristics (Figure 4c) with discharge plateaus indicative of faradaic charge storage unlike linear discharge behavior of double-layer systems. The composite exhibits a high capacity of ∼24 mAh/g at a current density of 0.5 A/g with good capacity maintenance at high rates (Figure 4d). Cycling performance of the fiberglass-based composite and the electrochemical performance of the ultrabattery composite with the Kevlar separator is provided in the Supporting Information (Figures S7 and S8, respectively). In terms of energy and power performance, our ultrabattery composite delivers a peak energy of 1.4 Wh/kg at power densities of 29 W/kg and 290 mWh/kg at 170 W/kg, respectively, based on total mass of all the components in the composite (Figure 4e). When comparing this performance to other structural energy storage devices and approaches in literature, the ultrabattery evidently overcomes the energy density limitations of electrostatic double-layer capacitor
(EDLC)-based devices without sacrificing power density demonstrating 10× better energy density than other state-ofthe-art systems in literature.7,12,15,17,18,20,22,52 The performance relative to active battery material mass (Figure 4f) demonstrates peak energy density of 23 Wh/kg and peak power density of 2.8 kW/kg (see Supporting Information Figures S9−S11 for active mass and total mass energy densities of fiberglass and Kevlar-based Ni−Fe ultrabattery composite, respectively). As numerous reports commonly refer to “projected performance” for structural energy storage architectures, the projected energy density based on active mass for our system is ∼120 Wh/kg40 versus CNT fiber-based composites, which are projected to ∼11 Wh/kg.18 This emphasizes the value of Faradaic structural energy storage composites and indicates room for improvements to our ultrabattery composite, even though the appropriate developmental benchmark for structural energy storage materials should be the total energy measured relative to the total composite mass. Moreover, our design approach is versatile enough to allow other high rate redox chemistries such as Ni− Zn, Cu−Fe, and Cu−Zn to be incorporated. Further improvements to energy densities can be achieved by incorporating alkali metal ion battery chemistries such as Liion systems in our structural load-bearing configuration. Finally, as the basic function of a structural energy storage material requires the simultaneous capability to maintain energy storage and mechanical properties simultaneously, we evaluated this using in situ mechano-electrochemical tests. Importantly, measuring the mechanical properties independently from the electrochemical response does not evaluate multifunctional performance. This point is underlined by noting battery degradation due to delamination at the active material-current collector interface is an ongoing challenge in battery manufacturing, even when devices are packaged under compressive stress. Therefore, care must be taken to properly evaluate energy storage properties under mechanical load to support the structural energy storage performance. To perform the in situ mechano-electrochemical tests, the ultrabattery composite using either fiberglass or Kevlar separator layers was subjected to galvanostatic charge/discharge cycles clamped in an Instron (tensile mode) mechanical tester (Figure 5a). In this case, the force is axially applied on the entire composite cross-sectional area within the gauge length of these dog-bone samples, allowing mechanistic insight into the correlation between the stress applied and the resulting effect on the electrochemical response. This test-to-failure methodology is necessary to evaluate the role of the structural interfaces on the dual load-bearing and energy storing properties of the material, and insight gained from this technique can be extrapolated to more specialized static and dynamic mechanical tests, such as bend, torsion, impact, and so forth. The composite was subjected to tensile stresses at the rate of 1 mm/min (Figure 5b) after several stable charge/discharge cycles (Figure 5c). The different stages observed in the stress−strain curve during testing is provided in the Supporting Information (Figure S12). Resilience and toughness are also useful performance metrics, especially for structural applications pertaining to automobiles. The resilience and toughness values obtained from the stress−strain measurements of the ultrabattery composite that correspond to the ability of these composites absorb energy in the elastic and plastic regimes, respectively, is also provided in the Supporting Information (Figure S13). Importantly, in all tests, total device failure occurs at the onset 7766
DOI: 10.1021/acs.nanolett.8b03510 Nano Lett. 2018, 18, 7761−7768
Letter
Nano Letters
pathways to study and evaluate performance in these systems. However, at the core of such a discussion rests a multitude of new opportunities to study phenomena at the intersection between interface mechanics and redox chemistry and hence forge a foundation for future advances and applications in multifunctional structural or wearable energy storage devices. In summary, our work demonstrates a technique where three-dimensional aligned carbon nanotube interfaces can be the threading component between a solid-state electrolyte, a conductive steel structural frame, and redox-active battery material. This gives rise to a structural battery architecture with energy density measured ∼1.4 Wh/kg relative to the total composite material mass and with elastic modulus exceeding 7 GPa. Whereas these metrics stand out well-ahead of existing reports on structural energy storage approaches, mechanoelectrochemical testing demonstrates robust Faradaic energy storage function under tensile loading, until mechanical failure is achieved. Moving forward, approaches to enhance reinforcement across multifunctional interfaces, minimize packaging without compromising mechanical and chemical stability, and develop electrolytes with improved mechanical properties are all needed. However, as the demands for energy technology steepen with a greater global technology footprint and the world population becomes more expecting of energy resources accessible remotely and on-demand, transforming structural materials into stable, multifunctional, and energy-dense batteries presents a strategy to address this. Our findings broadly emphasize a meaningful energy storing material for this purpose based on an Fe−Ni ultrabattery chemistry.
of mechanical failure (Figure 5b−f). A critical advantage of the in situ mechano-electrochemical tensile testing methodology, where the composites are subjected to complete fracture, lies in elucidating and highlighting the failure mechanisms of electrochemical processes that occur before the total mechanical failure of the multifunctional device. Further analysis of the in situ mechano-electrochemical test correlating the fluctuations in capacity and voltage drop (direct current internal resistance) in the various regimes of the stress strain curve for the Kevlar Ni−Fe ultrabattery composite is provided in the Supporting Information (Figure S14). Additional in situ mechano-electrochemical tests showing stable capacity retention properties during continuous loading/unloading cycles as well as during twisting the composite are provided in the Supporting Information (Figures S15 and S16). These results support the general concepts learned from our primary in situ mechanical loading tests. The best mechanical performance is measured from the Kevlar-based composites demonstrating ultimate tensile strength of ∼0.28 GPa with an elastic modulus of 7.3 GPa. Whereas the mechanical properties with the fiberglass composites are poorer, the fiberglass separator minimizes cell resistance to facilitate the best energy storage performance. Nonetheless, all of these tests demonstrate the basic function of carbon nanotubes as a mediator to interconnect a solid-state electrolyte matrix, the stainless steel current collector, and the metal oxide-based redox-active energy storage materials with mostly invariant energy storage behavior observed until composite failure. In turn, this presents a dual purpose design strategy to achieve interface reinforcement and accommodate active material host architecture. This marriage between the design rules of batteries and structural composites is necessary to overcome interface challenges that arise when stacking current collectors, electrode materials, and electrolyte materials together into a structural materialwhere interface integrity under stress defines the dual functionality of this system for simultaneous load bearing and energy storing modes. Building from this design scheme, we demonstrate this approach could be straightforwardly integrated into other applications outside of purely structural (infrastructure) materials. One specific area is in the field of wearable energy storage materials. In this application, one could design a yarn or fiber architecture that incorporates the reinforcement strategy described by our work to produce woven fabrics with robust built-in energy storage function. Much of the progress in the area of wearable battery fabrics and materials has been limited by a combination of performance-related challenges such as the sensitivity of solid electrolyte media to air and the poor adhesion of interfaces when all components are microsized to fibers. Our approach may be able to overcome such limitations through combination of structural design and the use of the Fe−Ni battery chemistry. However, unlike load-bearing structural materials, which often have large mass and volume footprint in infrastructure systems, an article of clothing incorporating battery textiles may only have mass of a few hundred grams, which translates to a small quantity (
