Article pubs.acs.org/accounts
Electrochemical Thin Layers in Nanostructures for Energy Storage Published as part of the Accounts of Chemical Research special issue “Nanoelectrochemistry”. Malachi Noked,†,‡,§ Chanyuan Liu,†,‡ Junkai Hu,§ Keith Gregorczyk,†,‡ Gary W Rubloff,†,‡ and Sang Bok Lee*,†,§ †
Department of Materials Science & Engineering, University of Maryland, College Park, Maryland 20742, United States Institute for Systems Research, University of Maryland, College Park, Maryland 20742, United States § Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States ‡
CONSPECTUS: Conventional electrical energy storage (EES) electrodes, such as rechargeable batteries, are mostly based on composites of monolithic micrometer sized particles bound together with polymeric and conductive carbon additives and binders. The kinetic limitations of these monolithic chunks of material are inherently linked to their electrical properties, the kinetics of ion insertion through their interface and ion migration in and through the composite phase. Redox chemistry of nanostructured materials in EES systems offer vast gains in power and energy. Furthermore, due to their thin nature, ion and electron transport is dramatically increased, especially when thin heterogeneous conducting layers are employed synergistically. However, since the stability of the electrode material is dictated by the nature of the electrochemical reaction and the accompanying volumetric and interfacial changes from the perspective of overall system lifetime, research with nanostructured materials has shown often indefinite conclusions: in some cases, an increase in unwanted side-reactions due to the high surface area (bad). In other cases, results have shown significantly better handling of mechanical stress that results from lithiation/delithiation (good). Despite these mixed results, scientifically informed design of thin electrode materials, with carefully chosen architectures, is considered a promising route to address many limitations witnessed in EES systems by reducing and protecting electrodes from parasitic reactions, accommodating mechanical stress due to volumetric changes from electrochemical reactions, and optimizing charge carrier mobilities from both the “ionic” and “electronic” points of view. Furthermore, precise nanoscale control over the electrode structure can enable accurate measurement through advanced spectroscopy and microscopy techniques. This Account summarizes recent findings related to thin electrode materials synthesized by atomic layer deposition (ALD) and electrochemical deposition (ECD), including nanowires, nanotubes, and thin films. Throughout the Account, we will show how these techniques enabled us to synthesize electrodes of interest with precise control over the structure and composition of the material. We will illustrate and discuss how the electrochemical response of thin electrodes made by these techniques can facilitate new mechanisms for ion storage, mediate the interfacial electrochemical response of the electrode, and address issues related to electrode degradation over time. The effects of nanosizing materials and their electrochemical response will be mechanistically reviewed through two categories of ion storage: (1) pseudocapacitance and (2) ion insertion. Additionally, we will show how electrochemical processes that are more complicated because of accompanying volumetric changes and electrode degradation pathways can be mediated and controlled by application of thin functional materials on the electrochemically active interface; examples include conversion electrodes, reactive lithium metal anodes, and complex reactions in a Li/O2 cathode system. The goal of this Account is to illustrate how careful design of thin materials either as active electrodes or as mediating layers can facilitate desirable interfacial electrochemical activity and resolve or shed light on mechanistic limitations of electrochemical processes related to micrometer size particles currently used in energy storage electrodes.
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INTRODUCTION Over the past few decades, the science of nanostructures has been a central focus across many disciplines. New capabilities to characterize and precisely control the architecture and composition of a material on the atomic/molecular scale have quickly led to demonstrations of new anomalous behavior and increased performance metrics related to nanomaterials, nanostructures, and their applications (quantum dots, surface plasmons, nanocarbon allotropes, etc.). Constructing materials © 2016 American Chemical Society
from their most elementary unit, in a bottom up approach, is inspiring for chemists and materials scientists, hence even after decades, there is no sign of any reduction in the extent of research and level of interest in nanomaterials. One of the fields in which nanomaterials have attracted particular attention is energy storage and conversion by Received: June 24, 2016 Published: September 16, 2016 2336
DOI: 10.1021/acs.accounts.6b00315 Acc. Chem. Res. 2016, 49, 2336−2346
Article
Accounts of Chemical Research
Figure 1. Thin electrode material in nanostructure vs monolithic thick particle.
electrochemical means.1,2 Synthesis of nanostructured materials is considered an effective strategy to overcome kinetic bottlenecks in electrochemical processes found in the electrodes for various energy storage devices (i.e., batteries, pseudocapacitors, fuel cells, etc.), and energy harvesting systems (i.e., solar cells, water splitting, etc.). Conformal application of electrochemically active thin materials in nanostructures renders them readily accessible to ions (through high aspect ratio electrode/ electrolyte interfaces), which enables faster electrochemical response of the active material compared to currently used electrode architectures (Figure 1).1−3 Harnessing the ability to tune composition and design flexibility enabled by new synthesis methods allows optimization of the material’s physicochemical properties on the atomic scale, thus enabling applications of thin materials on various high aspect ratio substrates with the desired electrical properties and morphologies. In this Account, we show how synthesized functional nanostructured materials with well-controlled morphologies, composition, and structure are extremely useful model systems for mechanistically elucidating the physicochemical response of materials in the context of energy storage. We utilize atomic layer deposition (ALD) and templated electrodeposition for the growth of nanowires, nanotubes, thin films, and arrays of nanostructures. The various active materials are fabricated in or onto diverse templates to build the desired heterogeneous morphology for electrochemical studies. Previously, the utilization of these complementary techniques enabled fabrication of operational arrays of metal−insulator− metal (MIM) electrostatic nanocapacitors by ALD inside porous anodic aluminum oxide (AAO) films (Figure 2A), with equivalent planar capacitances up to 100 mF cm−2.3 Similarly, electrochemical deposition (ECD) of active materials into AAO enabled synthesis of coaxial nanowires of MnO2/PEDOT using a single step coelectrodeposition4 (Figure 2B), with precise control over the structures of the coaxial nanowires by applied potential. Here we show how these synthesis techniques are used to carefully design thin electrochemically active materials and how these materials are used to explore the link between dimensionality
and mechanism of the electrochemical reaction on or through their interface. We illustrate how a wise choice in the structure and physicochemical properties of the interface is a key strategy to achieve control over the electrochemical response. Such control enables facilitation of desirable kinetics (high power), improved utilization of the active material (high energy), mediation of desirable electrochemical processes (catalysis), and suppression of parasitic interfacial reactions (protection). In general, electrode materials can be categorized based on the electrochemical process that takes place through or on their interface. In non-Faradaic processes (capacitive), the response will be dominated by the electrical double layer (EDL), whereas in Faradaic (reduction/oxidation) processes, there is a real charge transfer through the electrode/electrolyte interface. This discussion will focus on findings related to thin electrodes that go through or mediate a Faradaic reaction. In the first section, we categorize scientific findings by the electrochemistry related to ion storage under two categories of electrochemical reaction, pseudocapacitance and insertion. In the second section, we review recent findings related to thin electrochemically functional layers (e.g., protection and electrocatalytic thin films) that facilitate desirable interfacial electrochemical response on/through interfaces with higher reactivity and complicated degradation pathways as conversion electrodes, lithium metal, and Li/O2 cathode.
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THIN ELECTRODE MATERIALS FOR ION STORAGE
Pseudocapacitance
Pseudocapacitor (PC) materials store charge through interfacial Faradaic electrochemical reactions; the kinetics of this reaction are fast enough to facilitate high power capacitor like behavior but also energetic enough to enable high specific energy densities compared to electrical double layer capacitors. Compared to batteries, PCs have been reported to show significant cycling stability with high power performances and nearly 100% Coulombic efficiency due to interface only or near interface changes through the Faradaic process. In most PC electrodes, material utilization is higher when fabricated as a thin film on a high aspect ratio structure facilitating 2337
DOI: 10.1021/acs.accounts.6b00315 Acc. Chem. Res. 2016, 49, 2336−2346
Article
Accounts of Chemical Research
Figure 2. Nanostructures enabled by (a) ALD and (b) ECD. (a) MIM nanocapacitor fabricated by ALD. Adapted with permission from ref 3. Copyright 2009 Nature Publishing Group. (b) MnO2/PEDOT coaxial nanowire supercapacitor by ECD. Adapted with permission from ref 4. Copyright 2008 American Chemical Society.
absence of those sites (as in monoclinic crystallinity) will decrease its PC performances. Furthermore, by changing the thickness of the V2O5 ALD and testing the electrochemical response, a characteristic length scale for pseudocapacitance response of orthorhombic V2O5 was found to be on the order of 7 nm. This work establishes two design rules for building an ideal PC architecture: (1) a high surface area conductor with pores/voids just large enough to enable conformal coating of a high aspect ratio scaffold (similar to Figure 1b,c) and (2) the coating thickness of the PC material should be carefully selected by considering the thickness under which the material actually behaves as a PC rather than diffusion limited insertion material. Design rules are one significant aspect of nanostructured PC. Another is related to anomalous solvent dependent behaviors of nanostructured PC or mixed electrochemical responses that hybridize insertion and PC reactions as demonstrated with MnO2 nanowires. The authors note that the work on various MnO2 polymorphs in the context of energy storage is incredibly broad,11 and it is significantly beyond the scope of this Account; here we focus on our findings with amorphous nanostructures of MnO2 (as depicted in Figure 1b) as a model system. The Lee group specializes in synthesizing amorphous MnO2 nanostructures by electrodeposition into AAO,4,12 (Figure 5a). The electrode material can then be studied as an array of 1D nanowires (Figure 5b), further treated (through various techniques) to form a desired feature of interest in the surface of an electrode (Figure 5c) or synthesis of special interconnected 3D architecture by electrochemical manipulation of the AAO template (Figure 5d). We used these arrays as a model system to investigate the electrochemical response of heterogeneous nanostructured arrays when issues of electron and ion transports and tortuosity converge.
the highest possible surface area and rendering more material availability for the interfacial reaction. It was recently demonstrated that a PC response can occur in the bulk of an electrode when lithium ions are inserted into a host material with enhanced ionic conductivity or with thin electrode materials. In such cases the PC is referred to as an insertion PC.5,6 Major synthesis efforts for the ALD process for V2O5 and postsynthesis thermal treatments enabled Rubloff’s group to carefully control the level of material crystallinity and thickness and to explore the effect of material’s composition and dimensions on its electrochemical response.7,8 As demonstrated in Figure 3, conformal thin films of amorphous/crystalline V2O5 ALD were grown on various substrates to form heterogeneous morphologies with core−shell structures (Figure 1c) and integrated current collectors as MWCNT9 (a), nanocrystalline ITO (b), Ni coated TMV (c), and AAO (d). Rauda et al.8 showed that when applying an extremely thin layer of active material, the mechanism for ion storage and the performance of the material is dictated by its crystallinity. For example, when 2 nm low ordered V2O5 was grown on mesoporous ITO scaffolds (Figure 3b), it doubled the capacity compared to the same film thickness of V2O5 with a monoclinic crystal structure. However, when films with the same thickness were grown by an ozone based ALD process to form orthorhombic crystallinity, the capacity was even higher compared to the less ordered film (Figure 4). Interestingly, when the electrochemical response of the various thin films of V2O5 was carefully analyzed, it was confirmed that the mechanism for Li ion insertion in the high capacity thin films (
