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Ideal Three-Dimensional Electrode Structures for Electrochemical Energy Storage Sakineh Chabi, Chuang Peng,* Di Hu, and Yanqiu Zhu*
1. Introduction Driven by the fast development of portable electronics, standalone renewables, the smart grid, and the transport sector, there is an ever increasing demand for electrochemical energy storage (EES) technology, including batteries and supercapacitors. Their performances are evaluated in terms of three parameters: power capability, cycle life, and energy density. The electrode kinetics and mass transport (EKMT) are the S. Chabi, Dr. C. Peng,[+] College of Engineering Mathematics and Physical Sciences University of Exeter Cornwall Campus, Penryn, Cornwall, TR10 9EZ, UK E-mail: [email protected] Dr. D. Hu Department of Chemical and Environmental Engineering Faculty of Engineering University of Nottingham University Park, Nottingham, NG7 2RD, UK Prof. Y. Zhu College of Engineering Mathematics and Physical Sciences University of Exeter The Queen’s Drive, Exeter, Devon, EX4 4QJ, UK E-mail: [email protected] [+]Present address: Department of Chemical and Environmental Engineering, Faculty of Science and Engineering, University of Nottingham Ningbo Campus, Ningbo, 315100 China
DOI: 10.1002/adma.201305095
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key to the performance of an EES technology because an improved EKMT can: i) directly enhance the power capability, ii) improve the system’s reversibility and thus the cycle life, and, iii) improve the material utilisation and hence the energy density. Reducing the electron transport length and ion diffusion distance during the electrode process is an effective strategy to improve the EKMT. More specifically, the reduction of electron transport length can be achieved by using thin layers of redoxactive material. Fine particle materials are also beneficial because of the high surface area and reduced electron transport lengths within the particle. To reduce the ion diffusion distance, thin films of redoxactive material and porous electrodes are highly favored. Most of the existing synthesis routes yield discrete powdery materials[1–3] and binders are often required to prepare the electrodes. The commonly used binders, such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), are insulators and thus adversely affect the electronic conductivity of the electrode.[4,5] Recent research suggests that some specially engineered 3D structures are self-supported, and therefore offer additional advantages over powdery materials.[5–7] The common features of ideal 3D structured electrodes are discussed in the following text. We also review the synthesis routes and energy storage performance of these 3D structures.
Three-dimensional electrodes offer great advantages, such as enhanced ion and electron transport, increased material loading per unit substrate area, and improved mechanical stability upon repeated charge–discharge. The origin of these advantages is discussed and the criteria for ideal 3D electrode structure are outlined. One of the common features of ideal 3D electrodes is the use of a 3D carbon- or metal-based porous framework as the structural backbone and current collector. The synthesis methods of these 3D frameworks and their composites with redox-active materials are summarized, including transition metal oxides and conducting polymers. The structural characteristics and electrochemical performances are also reviewed. Synthesis of composite 3D electrodes is divided into two types — templateassisted and template-free methods — depending on whether a pre-made template is required. The advantages and drawbacks of both strategies are discussed.
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2. Ion Diffusion and Electron Transport in 3D Batteries and 3D Electrodes Energy-storage electrodes conventionally have a 2D structure with a layer of redox-active material attached to a planar current collector. This structure suffers from a trade-off between the energy and power densities on a given apparent area of the current collector (i.e., the footprint area).[8] This is because the kinetics of thicker films of redox-active material becomes poor, leading to a low power capability. The adoption of a 3D structure offers a good strategy to achieve both high energy and power densities on a given footprint area or space. Existing 3D structures for electrochemical energy storage include both 3D batteries and 3D electrodes, each addressing different issues and challenges. As illustrated in Figure 1, a 3D battery employs an interdigitated positive and negative electrode configuration, while a 3D electrode normally consists of hybrid materials with
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a whole must be monolithic or self-supported so that no organic binder is needed for electrode fabrication. The 3D electrode structure in Figure 1b is for purposes of illustration. Apart from the vertically aligned core–shell pillars, various other structures can be considered ideal as long as they possess the above-mentioned structural features. It is worth noting that some specially engineered 3D electrodes have an interpenetrating hierarchical structure. The narrow pore size Figure 1. Schematic illustration of a typical 3D battery (a) and an ideal 3D electrode (b). The distribution together with the hierarchical black and red double-headed arrows in (b) represent the solid-state ion diffusion and electron structure offers the benefits of both uniform transport lengths, respectively. ion diffusion rate and short diffusion distance. Considering the size of the solvated ions, the ideal pore size is between 2 and 5 nm diameter for a core–shell structure. The shell is a thin layer of redox-active the mesopores.[10] In addition, the presence of micropores can material and the core material is a good electron conductor. As improve the electrode volumetric capacity.[11] Recent endeavors a result, the solid-state ion diffusion lengths of both structures have achieved vast success in the synthesis of hierarchical are reduced. However, the electron transport in the two strucstructures with a combination of meso- and micropores.[12–19] tures is quite different. In the 3D battery, the electron transThe synthesis and performance of various 3D or architecport lengths vary enormously for different redox sites, simply tured electrodes have recently been reviewed.[6–8,20–23] This because the distances between the current collector and indimini-review focuses only on the structures that satisfy all the vidual redox site are different. As a result, electrons at the tip criteria of ideal 3D electrode. The following are intentionally of the pillar have to be transported far before reaching the curexcluded in this paper: interdigitated batteries, 3D electrodes rent collector. In addition, 3D interdigitated battery electrodes with non-uniform redox material distribution, architectures suffer from a non-uniform current distribution.[6,8] Due to both without a true 3D current collector, and electrodes fabricated the non-uniform current density and finite electronic conducwith powdery redox material and organic binder. tivity of the electrodes, all 3D interdigitated batteries are highly susceptible to stress,[8] which has an adverse effect on their cycle life. In general, the main advantage of a 3D battery is the reduced internal impedance and the improved material loading 3. Ideal 3D Electrode Structures on a given footprint area,[6,8,9] while improvements in rate per3D structures have been extensively reported as next-generation formance are modest, because their structures mainly address electrodes for electrochemical energy storage. However, not all ion- but not electron-transport issues. of them satisfy the criteria of an ideal 3D structure. In this secIn comparison, a 3D electrode is characterised with a thin tion, we review various recently reported ideal electrode struccoating of redox-active material on a 3D current electrode to tures, their preparation method, and their performance. ensure a short electron transport length for all the redox-active material. In Figure 1b, red and black double-headed arrows are used, respectively, to depict the electron transport and solid3.1. Monolithic and Self-Supported Architectural Carbon state ion diffusion lengths of different redox sites. An ideal 3D electrode requires the redox-active material to be a thin coating Unlike conventional powdered carbon materials for EES, on a 3D current collector. Due to the presence of this 3D curcarbon monoliths are porous, self-supported, and do not rent collector, the electrons of any redox site only need to reach require any binder or conductive agent (e.g., carbon black) the conducting core. In addition, the nano/microscale current during electrode preparation.[24] Elimination of binders could collector is beneficial for uniform current distribution and significantly reduce the polarization by lowering the electrical hence reduces mechanical stresses during repeated charge/ and mass-transport resistance. Furthermore, handling carbon discharge processes.[9] Another important feature of an ideal monolith is much easier than handling conventional porous 3D structure is that the electrode as a whole is continuous and carbons, such as activated carbon, carbon nanotubes (CNTs), porous so no added binder is needed. Thus, the space within and ordered mesoporous carbon.[24,25] Different methods have the electrode can accommodate large volume changes during been employed to prepare monolithic carbon, including seleccharge/discharge. Therefore, an ideal 3D electrode has not only tive etching of carbide, nanocasting with silica template, and a high energy capacity per unit footprint area, but also much mold conforming.[26–29] As shown in Figure 2a, monolithic improved power performance as well as cycle life. carbon is porous, continuous, and self-supported. These feaAs discussed above, an ideal 3D electrode has to satisfy tures ensure a maximum utilization of the carbon surface area the following criteria: i) it must have a porous interconnected for EES applications. Electrochemical measurements have indiframework with high electronic conductivity to serve as a 3D cated that the carbon monoliths are high in both volumetric current collector; ii) the redox-active material must form a thin and mass specific capacitances, reported as 180 F cm−3 [27] and coating on the 3D current collector, which ensures an improved 334 F g−1 [29] in organic and aqueous electrolytes, respectively. electrode kinetics and mass transport; and, iii) the material as
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Furthermore, they preserve the desirable properties of their building blocks. A typical example is graphene foam[31] prepared by the CVD method with a Ni-foam template. The as-prepared graphene foam is highly flexible and mechanically stable.[31] Scanning electron microscopy (SEM; see Figure 2b) suggests the GN foam is free of defects and intersheet junctions and does not suffer from stacking.[31] This porous interconnected 3D structure endows the foam with excellent conductivity and ultrahigh surface area of about 850 m2 g−1.[31] Similar methods have been used to prepare self-supported graphene–CNT foam architectures.[32,33] The architectures have revealed mass specific capacitance as high as 387 F g−1[32] and a capacitance retention of 99.34% after 85 000 charge–discharge cycles.[33] Using graphene or CNT as the building block, similar interconnected architectural carbon electrodes with good electrochemical performance have also been prepared by freeze-drying[13] and hydrothermal[18] methods. 3.2. Composite Electrodes Prepared by Template-Assisted Methods Monolithic and self-supported architectural carbons store electrical charge by the doublelayer process. Their unique structures provide excellent power and cycle performance. In order to improve the energy density, it is Figure 2. 3D current collectors and composite electrode prepared by template methods. a) essential to incorporate redox-active mate[ 28 ] SEM image of carbon monolith prepared with a silica replica Reproduced with permission. Copyright 2007, Wiley. b) SEM image of a graphene foam. Reproduced with permission.[31] rial in the electrode. Composites of the two Copyright 2011, Nature Publishing Group. SEM image of nickel inverse opal (c) and NiOOH possess the structural merits of the intercoating on nickel inverse opal (d). e) Discharge curves of NiOOH/nickel cathode at various connected carbon framework, while a thin C-rates. Reproduced with permission.[34] Copyright 2011, Nature Publishing Group. f) SEM coating of redox-active material enhances the image of LiFePO4–graphene foam. The inset shows the LiFePO4 nanoparticle coating on the energy capacity. Compared to metal-foam graphene foam. Reproduced with permission.[37] current collectors, porous carbons not only have higher specific surface area (m2 g−1), but Low-dimensional carbons, including CNTs and graphene also offer additional advantages, such as low density and a wide (GN), are at the forefront of EES research due to their unique electrochemical potential window. A common method to prestructures and high surface areas. The theoretical maximum pare this type of composite is electrochemical deposition, which surface areas are 1315 and 2600 m2 g−1 for CNT and GN, takes place at the interface between the 3D current collector and respectively. Assuming a specific capacitance of 60 mF cm−2 the electrolyte. This heterogeneous reaction is highly beneficial for the graphite basal plane,[30] the theoretical maximum mass for preparing thin and uniform coatings of redox-active matespecific capacitances would be 789 and 1560 F g−1 for CNT and rial.[34,35] In a typical example, porous 3D nickel was prepared GN, respectively. However, pure single-walled CNTs and singlewith an opal template. The nickel was further used as a 3D curlayer graphene are very difficult to obtain. Moreover, in pracrent collector for the electrodeposition of NiOOH and MnO2. tical applications, their surface areas are largely reduced due to As shown in Figure 2c and d, the redox-active NiOOH formed agglomeration and the use of polymeric binders. As a result, a thin coating on the porous and bicontinuous nickel current the double-layer capacitances of both CNTs and graphene are collector. The composite as a whole is self-supported, uniform, far below the theoretical maximum values. and porous, showing the characteristics of ideal 3D electrode. Recent studies have shown that self-supported carbon Electrochemical tests unveil high capacity retention at ultrafast architectures can be synthesised with low-dimensional nanocharge–discharge rates or C-rates up to 1000C (Figure 2e) and carbon as the building block. These sponge-like structures 400C, respectively, for NiOOH and lithiated MnO2 on porous are continuous and porous, resembling monolithic carbon. nickel.[34] These ultrafast C-rates suggest very high power and
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3.3. Composites Prepared by Template-Free Methods The template-assisted approach yields well-defined 3D structures controlled by the template and synthesis method. However, the synthesis processes are normally tedious and expensive, and hence pose difficulties for commercial-scale production. Some recent studies have made success in temple-free synthesis of ideal 3D electrodes.
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Conducting polymers such as PANI,[44,45] polypyrrole (PPY)[44,46] and poly(3,4-ethylenedioxythiophene) (PEDOT)[44,47] are a group of commonly used redox-active materials in the template-free synthesis of 3D electrodes. Their corresponding monomers are dissolved in a solution that also contains dispersed anionic CNTs or graphene. By passing a DC current, the monomers are oxidized and deposit at the anode as a polymer while the anionic CNT or graphene move towards the anode under the electric field.[44,45] This one-step co-deposition method combines the electropolymerisation of the redox-active material with the electrophoresis of the low-dimensional carbon building-blocks. After the co-deposition, a conducting scaffold is constructed by the CNTs or graphene and the redox-active material forms a thin coating on the conducting scaffold. Another method is in situ chemical polymerization by adding an oxidizing agent to the aforementioned mixture solution.[46] Taking PPY–CNT composite as an example, the SEM images of in situ polymerized (Figure 3a) and electro-co-deposited (Figure 3c) PPY–CNT both exhibit a porous interconnected network structure composed of the randomly arranged nanotubes. The TEM image (Figure 3b) reveals a thin coating of PPY on individual nanotubes. Compared to the template-assisted synthesis, template-free methods normally yield a less-ordered architecture. However these structures satisfy the criteria of ideal 3D electrode. They also exhibit good power performance in cyclic voltammetry (CV) tests at high current densities. As shown in Figure 3d, an electro-deposited PPY–CNT composite electrode presented rectangular CV curves at scan rates as high as 500 and 1000 mV s−1. The capacitance values were calculated as 72.0 and 69.0 mF cm−2, respectively. Therefore, the electrode retained 95.8% of its capacity as the scan rate increased from 500 to 1000 mV s−1. Figure 3e shows a thick PAN–CNT monolith on a graphite disc, electrodeposited with a high deposition charge. Clearly this electrode has a very high area density (g cm−2) and capacity compared to a conventional 2D electrode, which is normally less than 100 μm in thickness. The cyclic voltammogram of a PAN–CNT symmetrical supercapacitor with the same deposition charge is shown in Figure 3f; it displays good capacitive behavior with capacitance values calculated as 253.42 and 241.25 mF, respectively, at moderate scan rates of 5 and 10 mV s−1. Apart from conducting polymers, attempts have also been made to synthesize 3D electrodes with various transition-metal oxides or sulfides using template-free methods. For instance, a NiO–graphene hierarchical structure prepared by hydrothermal synthesis displayed discharge capacities of 1098 and 615 mA h g−1 at current densities of 100 mA g−1 and 4 A g−1, respectively.[48] In another study of hydrothermal synthesis, a freeze-drying process post-treatment was used to preserve the porous 3D architechture of graphene network with a coating of MoS2 (Figure 3g) or FeOx.[49] These centimeter-scale monoliths exhibited high capacities of up to 1200 mA h g−1 at discharge rate of 0.5C. Moreover, the capacity loss was only negligible after 3000 cycles for MoS2– graphene and 1500 cycles for FeOx–graphene.[49] The hydrothermal and freeze-drying combined method has also been used to produce 3D NiOH–graphene[50] and SnO–graphene[51] porous interconnected frameworks (Figure 3h and i) with high capacity and cycling stability. The 3D electrode prepared by the templatefree methods generally has a less-ordered structure (Figure 3g–i) compared to those prepared by the template methods. However,
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rapid recharge. NiOOH and lithiated MnO2 are widely used cathodes in commercial nickel–metal hydride and lithium-ion batteries. Therefore, the superior rate capabilities must have originated from their unique architecture, i.e. the thin coating on the nickel scaffold can greatly reduce the electron transport length and ion diffusion distance. In another example, polyaniline (PANI) electrodeposited on porous monolithic carbon exhibited a notably high capacitance of 2200 F g−1.[35] This record high specific capacitance of PANI gave rise to a high energy density of 300 W h kg−1.[35] This value is comparable to the best commercial lithium-ion batteries, though PANI is normally considered as a high-power but low-energy supercapacitor electrode. The exceptionally high capacitance and energy density are because the thin coating of PANI on monolithic carbon has improved the electrode kinetics and material utilization. The above two examples have demonstrated that both power and energy density can be notably improved in ideal 3D structured electrodes. Conventional battery electrodes exhibit the power capabilities of a supercapacitor[34] while conventional supercapacitor electrodes have energy density as high as batteries.[35] Bridging the gaps between batteries and supercapacitors represents a new focus in EES research and the construction of ideal 3D electrodes is a very effective strategy to this end. Other methods to synthesize thin coatings of redox-active material on a porous template include magnetron sputtering,[36] solution casting,[37–39] solution casting followed by thermal decomposition,[40] chemical vapor deposition,[41] physical vapor deposition by evaporation,[42] hydrothermal methods,[43] etc. In template-assisted synthesis, various redox-active materials have been used, such as NiOOH,[34] MnO2,[34] conducting polymers,[35] LiFePO4,[36–38] Li4Ti5O12,[37,39] SnO2,[40] silicon,[41] germanium,[42] and Co3O4.[43] It is worth noting that, apart from conducting polymer, all the other redox-active materials are very poor electron conductors. Conventionally, a considerable amount of conductive fillers and binders have to be used during electrode preparation and the electrode materials have to be sufficiently thin to achieve good electrode kinetics. Thus, a 2D electrode has inherent disadvantages of limited energy and power density on a given footprint area. By combining a thin coating of redox-active material with self-supported 3D current collectors, not only are the conductive fillers and organic binders eliminated, but also the energy and power performance are significantly improved. The 3D electrodes prepared by the template-assisted methods normally have an ordered architecture due to the presence of the hard template. As shown in Figure 2d and f, the ordered structure of the nickel template and graphene foam is well preserved in the electrode. However, the template-assisted methods are tedious, energy consuming, and difficult to scale up.
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Figure 3. Ideal 3D electrodes prepared by template-free methods. SEM (a) and TEM (b) images of PPY-CNT prepared by in situ polymerisation. Reproduced with permission.[46] Copyright 2007, Elsevier. c) SEM image of PPY–CNT prepared by electro-co-deposition. Reproduced with permission.[44] Copyright 2007, Elsevier. d) CV of electro-co-deposited PPY–CNT electrodes at potential scan rates of 500 and 1000 mV s−1. e) Digital photograph of an electro-co-deposited PPY–CNT on graphite disc. Deposition charge: 10.6 C cm−2. Reproduced with permission.[44] Copyright 2007, Elsevier. f) CV of a symmetrical supercapacitor at voltage scan rates of 5 and 10 mV s−1. g) SEM image of 3D monolithic MoS–graphene. Inset: photograph of a centimeter-scale monolith. Reproduced with permission.[49] Copyright 2013, Wiley. h) SEM image and photograph (inset) of 3D monolithic Ni(OH)2– graphene. Reproduced with permission.[50] Copyright 2013, Springer Verlag. i) SEM image and photograph (inset) of 3D monolithic SnO2–graphene. Reproduced with permission.[51] Copyright 2013, Wiley.
both the ion diffusion and electron transport lengths are minimized and these binder-free electrodes satisfy the criteria of ideal 3D electrode for electrochemical energy storage. All of the above temple-free methods use a starting solution or dispersion containing redox-active materials and discreet CNTs or graphene as the building block. These solution-based processes do not require a pre-made template and are thus less tedious and more suitable for scale-up and commercial production.
4. Conclusions and Outlook In electrochemical energy storage, an ideal 3D electrode should possess a structure that minimizes the solid state electron transport
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length and ion diffusion distance. Typical structural features include: i) a porous interconnected framework as the backbone with high electronic conductivity, ii) a thin coating of redox-active material on the conducting backbone, and, iii) a monolithic or selfsupported electrode structure, so that no organic binder is needed. The most commonly used conducting backbones are porous monolithic carbons or carbon architectures made of CNTs or graphene. In composites of a carbon backbone with a thin coating of redoxactive material, the number of redox sites is maximized while the ion and electron transport is optimized. Both template-assisted and template-free methods have shown success in making ideal 3D electrodes. The template-assisted approach involves coating redox-active material on a pre-made 3D framework or backbone.
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Acknowledgements This work received financial support from the new academic startup fund and a studentship from the University of Exeter. Received: October 12, 2013 Revised: November 3, 2013 Published online: December 16, 2013
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This method usually yields ideally ordered 3D structures with high power capabilities and cycle stability. The main drawback of this approach is the tedious synthesis procedures and high production cost. In contrast, the template-free approach normally produces less-ordered electrode structures. However, the solutionbased methods offer advantages of simple synthesis procedures, lower production costs, and easy scale-up. The ideal 3D electrode structures have achieved tremendous success in numerous laboratory-based studies in pursuit of an energy storage system with high energy and power density, as well as long cycle life. Various reports indicate the energy and power performance of a few ideal 3D electrodes could bridge the gaps between supercapacitors and batteries. It is foreseeable that ideal 3D electrode structures will play a vital role in the next generation of high-performance electrochemical energy storage technology.
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Ideal Three-Dimensional Electrode Structures for Electrochemical Energy Storage Sakineh Chabi, Chuang Peng,* Di Hu, and Yanqiu Zhu*
1. Introduction Driven by the fast development of portable electronics, standalone renewables, the smart grid, and the transport sector, there is an ever increasing demand for electrochemical energy storage (EES) technology, including batteries and supercapacitors. Their performances are evaluated in terms of three parameters: power capability, cycle life, and energy density. The electrode kinetics and mass transport (EKMT) are the S. Chabi, Dr. C. Peng,[+] College of Engineering Mathematics and Physical Sciences University of Exeter Cornwall Campus, Penryn, Cornwall, TR10 9EZ, UK E-mail: [email protected] Dr. D. Hu Department of Chemical and Environmental Engineering Faculty of Engineering University of Nottingham University Park, Nottingham, NG7 2RD, UK Prof. Y. Zhu College of Engineering Mathematics and Physical Sciences University of Exeter The Queen’s Drive, Exeter, Devon, EX4 4QJ, UK E-mail: [email protected] [+]Present address: Department of Chemical and Environmental Engineering, Faculty of Science and Engineering, University of Nottingham Ningbo Campus, Ningbo, 315100 China
DOI: 10.1002/adma.201305095
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key to the performance of an EES technology because an improved EKMT can: i) directly enhance the power capability, ii) improve the system’s reversibility and thus the cycle life, and, iii) improve the material utilisation and hence the energy density. Reducing the electron transport length and ion diffusion distance during the electrode process is an effective strategy to improve the EKMT. More specifically, the reduction of electron transport length can be achieved by using thin layers of redoxactive material. Fine particle materials are also beneficial because of the high surface area and reduced electron transport lengths within the particle. To reduce the ion diffusion distance, thin films of redoxactive material and porous electrodes are highly favored. Most of the existing synthesis routes yield discrete powdery materials[1–3] and binders are often required to prepare the electrodes. The commonly used binders, such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), are insulators and thus adversely affect the electronic conductivity of the electrode.[4,5] Recent research suggests that some specially engineered 3D structures are self-supported, and therefore offer additional advantages over powdery materials.[5–7] The common features of ideal 3D structured electrodes are discussed in the following text. We also review the synthesis routes and energy storage performance of these 3D structures.
Three-dimensional electrodes offer great advantages, such as enhanced ion and electron transport, increased material loading per unit substrate area, and improved mechanical stability upon repeated charge–discharge. The origin of these advantages is discussed and the criteria for ideal 3D electrode structure are outlined. One of the common features of ideal 3D electrodes is the use of a 3D carbon- or metal-based porous framework as the structural backbone and current collector. The synthesis methods of these 3D frameworks and their composites with redox-active materials are summarized, including transition metal oxides and conducting polymers. The structural characteristics and electrochemical performances are also reviewed. Synthesis of composite 3D electrodes is divided into two types — templateassisted and template-free methods — depending on whether a pre-made template is required. The advantages and drawbacks of both strategies are discussed.
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2. Ion Diffusion and Electron Transport in 3D Batteries and 3D Electrodes Energy-storage electrodes conventionally have a 2D structure with a layer of redox-active material attached to a planar current collector. This structure suffers from a trade-off between the energy and power densities on a given apparent area of the current collector (i.e., the footprint area).[8] This is because the kinetics of thicker films of redox-active material becomes poor, leading to a low power capability. The adoption of a 3D structure offers a good strategy to achieve both high energy and power densities on a given footprint area or space. Existing 3D structures for electrochemical energy storage include both 3D batteries and 3D electrodes, each addressing different issues and challenges. As illustrated in Figure 1, a 3D battery employs an interdigitated positive and negative electrode configuration, while a 3D electrode normally consists of hybrid materials with
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a whole must be monolithic or self-supported so that no organic binder is needed for electrode fabrication. The 3D electrode structure in Figure 1b is for purposes of illustration. Apart from the vertically aligned core–shell pillars, various other structures can be considered ideal as long as they possess the above-mentioned structural features. It is worth noting that some specially engineered 3D electrodes have an interpenetrating hierarchical structure. The narrow pore size Figure 1. Schematic illustration of a typical 3D battery (a) and an ideal 3D electrode (b). The distribution together with the hierarchical black and red double-headed arrows in (b) represent the solid-state ion diffusion and electron structure offers the benefits of both uniform transport lengths, respectively. ion diffusion rate and short diffusion distance. Considering the size of the solvated ions, the ideal pore size is between 2 and 5 nm diameter for a core–shell structure. The shell is a thin layer of redox-active the mesopores.[10] In addition, the presence of micropores can material and the core material is a good electron conductor. As improve the electrode volumetric capacity.[11] Recent endeavors a result, the solid-state ion diffusion lengths of both structures have achieved vast success in the synthesis of hierarchical are reduced. However, the electron transport in the two strucstructures with a combination of meso- and micropores.[12–19] tures is quite different. In the 3D battery, the electron transThe synthesis and performance of various 3D or architecport lengths vary enormously for different redox sites, simply tured electrodes have recently been reviewed.[6–8,20–23] This because the distances between the current collector and indimini-review focuses only on the structures that satisfy all the vidual redox site are different. As a result, electrons at the tip criteria of ideal 3D electrode. The following are intentionally of the pillar have to be transported far before reaching the curexcluded in this paper: interdigitated batteries, 3D electrodes rent collector. In addition, 3D interdigitated battery electrodes with non-uniform redox material distribution, architectures suffer from a non-uniform current distribution.[6,8] Due to both without a true 3D current collector, and electrodes fabricated the non-uniform current density and finite electronic conducwith powdery redox material and organic binder. tivity of the electrodes, all 3D interdigitated batteries are highly susceptible to stress,[8] which has an adverse effect on their cycle life. In general, the main advantage of a 3D battery is the reduced internal impedance and the improved material loading 3. Ideal 3D Electrode Structures on a given footprint area,[6,8,9] while improvements in rate per3D structures have been extensively reported as next-generation formance are modest, because their structures mainly address electrodes for electrochemical energy storage. However, not all ion- but not electron-transport issues. of them satisfy the criteria of an ideal 3D structure. In this secIn comparison, a 3D electrode is characterised with a thin tion, we review various recently reported ideal electrode struccoating of redox-active material on a 3D current electrode to tures, their preparation method, and their performance. ensure a short electron transport length for all the redox-active material. In Figure 1b, red and black double-headed arrows are used, respectively, to depict the electron transport and solid3.1. Monolithic and Self-Supported Architectural Carbon state ion diffusion lengths of different redox sites. An ideal 3D electrode requires the redox-active material to be a thin coating Unlike conventional powdered carbon materials for EES, on a 3D current collector. Due to the presence of this 3D curcarbon monoliths are porous, self-supported, and do not rent collector, the electrons of any redox site only need to reach require any binder or conductive agent (e.g., carbon black) the conducting core. In addition, the nano/microscale current during electrode preparation.[24] Elimination of binders could collector is beneficial for uniform current distribution and significantly reduce the polarization by lowering the electrical hence reduces mechanical stresses during repeated charge/ and mass-transport resistance. Furthermore, handling carbon discharge processes.[9] Another important feature of an ideal monolith is much easier than handling conventional porous 3D structure is that the electrode as a whole is continuous and carbons, such as activated carbon, carbon nanotubes (CNTs), porous so no added binder is needed. Thus, the space within and ordered mesoporous carbon.[24,25] Different methods have the electrode can accommodate large volume changes during been employed to prepare monolithic carbon, including seleccharge/discharge. Therefore, an ideal 3D electrode has not only tive etching of carbide, nanocasting with silica template, and a high energy capacity per unit footprint area, but also much mold conforming.[26–29] As shown in Figure 2a, monolithic improved power performance as well as cycle life. carbon is porous, continuous, and self-supported. These feaAs discussed above, an ideal 3D electrode has to satisfy tures ensure a maximum utilization of the carbon surface area the following criteria: i) it must have a porous interconnected for EES applications. Electrochemical measurements have indiframework with high electronic conductivity to serve as a 3D cated that the carbon monoliths are high in both volumetric current collector; ii) the redox-active material must form a thin and mass specific capacitances, reported as 180 F cm−3 [27] and coating on the 3D current collector, which ensures an improved 334 F g−1 [29] in organic and aqueous electrolytes, respectively. electrode kinetics and mass transport; and, iii) the material as
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Furthermore, they preserve the desirable properties of their building blocks. A typical example is graphene foam[31] prepared by the CVD method with a Ni-foam template. The as-prepared graphene foam is highly flexible and mechanically stable.[31] Scanning electron microscopy (SEM; see Figure 2b) suggests the GN foam is free of defects and intersheet junctions and does not suffer from stacking.[31] This porous interconnected 3D structure endows the foam with excellent conductivity and ultrahigh surface area of about 850 m2 g−1.[31] Similar methods have been used to prepare self-supported graphene–CNT foam architectures.[32,33] The architectures have revealed mass specific capacitance as high as 387 F g−1[32] and a capacitance retention of 99.34% after 85 000 charge–discharge cycles.[33] Using graphene or CNT as the building block, similar interconnected architectural carbon electrodes with good electrochemical performance have also been prepared by freeze-drying[13] and hydrothermal[18] methods. 3.2. Composite Electrodes Prepared by Template-Assisted Methods Monolithic and self-supported architectural carbons store electrical charge by the doublelayer process. Their unique structures provide excellent power and cycle performance. In order to improve the energy density, it is Figure 2. 3D current collectors and composite electrode prepared by template methods. a) essential to incorporate redox-active mate[ 28 ] SEM image of carbon monolith prepared with a silica replica Reproduced with permission. Copyright 2007, Wiley. b) SEM image of a graphene foam. Reproduced with permission.[31] rial in the electrode. Composites of the two Copyright 2011, Nature Publishing Group. SEM image of nickel inverse opal (c) and NiOOH possess the structural merits of the intercoating on nickel inverse opal (d). e) Discharge curves of NiOOH/nickel cathode at various connected carbon framework, while a thin C-rates. Reproduced with permission.[34] Copyright 2011, Nature Publishing Group. f) SEM coating of redox-active material enhances the image of LiFePO4–graphene foam. The inset shows the LiFePO4 nanoparticle coating on the energy capacity. Compared to metal-foam graphene foam. Reproduced with permission.[37] current collectors, porous carbons not only have higher specific surface area (m2 g−1), but Low-dimensional carbons, including CNTs and graphene also offer additional advantages, such as low density and a wide (GN), are at the forefront of EES research due to their unique electrochemical potential window. A common method to prestructures and high surface areas. The theoretical maximum pare this type of composite is electrochemical deposition, which surface areas are 1315 and 2600 m2 g−1 for CNT and GN, takes place at the interface between the 3D current collector and respectively. Assuming a specific capacitance of 60 mF cm−2 the electrolyte. This heterogeneous reaction is highly beneficial for the graphite basal plane,[30] the theoretical maximum mass for preparing thin and uniform coatings of redox-active matespecific capacitances would be 789 and 1560 F g−1 for CNT and rial.[34,35] In a typical example, porous 3D nickel was prepared GN, respectively. However, pure single-walled CNTs and singlewith an opal template. The nickel was further used as a 3D curlayer graphene are very difficult to obtain. Moreover, in pracrent collector for the electrodeposition of NiOOH and MnO2. tical applications, their surface areas are largely reduced due to As shown in Figure 2c and d, the redox-active NiOOH formed agglomeration and the use of polymeric binders. As a result, a thin coating on the porous and bicontinuous nickel current the double-layer capacitances of both CNTs and graphene are collector. The composite as a whole is self-supported, uniform, far below the theoretical maximum values. and porous, showing the characteristics of ideal 3D electrode. Recent studies have shown that self-supported carbon Electrochemical tests unveil high capacity retention at ultrafast architectures can be synthesised with low-dimensional nanocharge–discharge rates or C-rates up to 1000C (Figure 2e) and carbon as the building block. These sponge-like structures 400C, respectively, for NiOOH and lithiated MnO2 on porous are continuous and porous, resembling monolithic carbon. nickel.[34] These ultrafast C-rates suggest very high power and
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3.3. Composites Prepared by Template-Free Methods The template-assisted approach yields well-defined 3D structures controlled by the template and synthesis method. However, the synthesis processes are normally tedious and expensive, and hence pose difficulties for commercial-scale production. Some recent studies have made success in temple-free synthesis of ideal 3D electrodes.
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Conducting polymers such as PANI,[44,45] polypyrrole (PPY)[44,46] and poly(3,4-ethylenedioxythiophene) (PEDOT)[44,47] are a group of commonly used redox-active materials in the template-free synthesis of 3D electrodes. Their corresponding monomers are dissolved in a solution that also contains dispersed anionic CNTs or graphene. By passing a DC current, the monomers are oxidized and deposit at the anode as a polymer while the anionic CNT or graphene move towards the anode under the electric field.[44,45] This one-step co-deposition method combines the electropolymerisation of the redox-active material with the electrophoresis of the low-dimensional carbon building-blocks. After the co-deposition, a conducting scaffold is constructed by the CNTs or graphene and the redox-active material forms a thin coating on the conducting scaffold. Another method is in situ chemical polymerization by adding an oxidizing agent to the aforementioned mixture solution.[46] Taking PPY–CNT composite as an example, the SEM images of in situ polymerized (Figure 3a) and electro-co-deposited (Figure 3c) PPY–CNT both exhibit a porous interconnected network structure composed of the randomly arranged nanotubes. The TEM image (Figure 3b) reveals a thin coating of PPY on individual nanotubes. Compared to the template-assisted synthesis, template-free methods normally yield a less-ordered architecture. However these structures satisfy the criteria of ideal 3D electrode. They also exhibit good power performance in cyclic voltammetry (CV) tests at high current densities. As shown in Figure 3d, an electro-deposited PPY–CNT composite electrode presented rectangular CV curves at scan rates as high as 500 and 1000 mV s−1. The capacitance values were calculated as 72.0 and 69.0 mF cm−2, respectively. Therefore, the electrode retained 95.8% of its capacity as the scan rate increased from 500 to 1000 mV s−1. Figure 3e shows a thick PAN–CNT monolith on a graphite disc, electrodeposited with a high deposition charge. Clearly this electrode has a very high area density (g cm−2) and capacity compared to a conventional 2D electrode, which is normally less than 100 μm in thickness. The cyclic voltammogram of a PAN–CNT symmetrical supercapacitor with the same deposition charge is shown in Figure 3f; it displays good capacitive behavior with capacitance values calculated as 253.42 and 241.25 mF, respectively, at moderate scan rates of 5 and 10 mV s−1. Apart from conducting polymers, attempts have also been made to synthesize 3D electrodes with various transition-metal oxides or sulfides using template-free methods. For instance, a NiO–graphene hierarchical structure prepared by hydrothermal synthesis displayed discharge capacities of 1098 and 615 mA h g−1 at current densities of 100 mA g−1 and 4 A g−1, respectively.[48] In another study of hydrothermal synthesis, a freeze-drying process post-treatment was used to preserve the porous 3D architechture of graphene network with a coating of MoS2 (Figure 3g) or FeOx.[49] These centimeter-scale monoliths exhibited high capacities of up to 1200 mA h g−1 at discharge rate of 0.5C. Moreover, the capacity loss was only negligible after 3000 cycles for MoS2– graphene and 1500 cycles for FeOx–graphene.[49] The hydrothermal and freeze-drying combined method has also been used to produce 3D NiOH–graphene[50] and SnO–graphene[51] porous interconnected frameworks (Figure 3h and i) with high capacity and cycling stability. The 3D electrode prepared by the templatefree methods generally has a less-ordered structure (Figure 3g–i) compared to those prepared by the template methods. However,
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rapid recharge. NiOOH and lithiated MnO2 are widely used cathodes in commercial nickel–metal hydride and lithium-ion batteries. Therefore, the superior rate capabilities must have originated from their unique architecture, i.e. the thin coating on the nickel scaffold can greatly reduce the electron transport length and ion diffusion distance. In another example, polyaniline (PANI) electrodeposited on porous monolithic carbon exhibited a notably high capacitance of 2200 F g−1.[35] This record high specific capacitance of PANI gave rise to a high energy density of 300 W h kg−1.[35] This value is comparable to the best commercial lithium-ion batteries, though PANI is normally considered as a high-power but low-energy supercapacitor electrode. The exceptionally high capacitance and energy density are because the thin coating of PANI on monolithic carbon has improved the electrode kinetics and material utilization. The above two examples have demonstrated that both power and energy density can be notably improved in ideal 3D structured electrodes. Conventional battery electrodes exhibit the power capabilities of a supercapacitor[34] while conventional supercapacitor electrodes have energy density as high as batteries.[35] Bridging the gaps between batteries and supercapacitors represents a new focus in EES research and the construction of ideal 3D electrodes is a very effective strategy to this end. Other methods to synthesize thin coatings of redox-active material on a porous template include magnetron sputtering,[36] solution casting,[37–39] solution casting followed by thermal decomposition,[40] chemical vapor deposition,[41] physical vapor deposition by evaporation,[42] hydrothermal methods,[43] etc. In template-assisted synthesis, various redox-active materials have been used, such as NiOOH,[34] MnO2,[34] conducting polymers,[35] LiFePO4,[36–38] Li4Ti5O12,[37,39] SnO2,[40] silicon,[41] germanium,[42] and Co3O4.[43] It is worth noting that, apart from conducting polymer, all the other redox-active materials are very poor electron conductors. Conventionally, a considerable amount of conductive fillers and binders have to be used during electrode preparation and the electrode materials have to be sufficiently thin to achieve good electrode kinetics. Thus, a 2D electrode has inherent disadvantages of limited energy and power density on a given footprint area. By combining a thin coating of redox-active material with self-supported 3D current collectors, not only are the conductive fillers and organic binders eliminated, but also the energy and power performance are significantly improved. The 3D electrodes prepared by the template-assisted methods normally have an ordered architecture due to the presence of the hard template. As shown in Figure 2d and f, the ordered structure of the nickel template and graphene foam is well preserved in the electrode. However, the template-assisted methods are tedious, energy consuming, and difficult to scale up.
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Figure 3. Ideal 3D electrodes prepared by template-free methods. SEM (a) and TEM (b) images of PPY-CNT prepared by in situ polymerisation. Reproduced with permission.[46] Copyright 2007, Elsevier. c) SEM image of PPY–CNT prepared by electro-co-deposition. Reproduced with permission.[44] Copyright 2007, Elsevier. d) CV of electro-co-deposited PPY–CNT electrodes at potential scan rates of 500 and 1000 mV s−1. e) Digital photograph of an electro-co-deposited PPY–CNT on graphite disc. Deposition charge: 10.6 C cm−2. Reproduced with permission.[44] Copyright 2007, Elsevier. f) CV of a symmetrical supercapacitor at voltage scan rates of 5 and 10 mV s−1. g) SEM image of 3D monolithic MoS–graphene. Inset: photograph of a centimeter-scale monolith. Reproduced with permission.[49] Copyright 2013, Wiley. h) SEM image and photograph (inset) of 3D monolithic Ni(OH)2– graphene. Reproduced with permission.[50] Copyright 2013, Springer Verlag. i) SEM image and photograph (inset) of 3D monolithic SnO2–graphene. Reproduced with permission.[51] Copyright 2013, Wiley.
both the ion diffusion and electron transport lengths are minimized and these binder-free electrodes satisfy the criteria of ideal 3D electrode for electrochemical energy storage. All of the above temple-free methods use a starting solution or dispersion containing redox-active materials and discreet CNTs or graphene as the building block. These solution-based processes do not require a pre-made template and are thus less tedious and more suitable for scale-up and commercial production.
4. Conclusions and Outlook In electrochemical energy storage, an ideal 3D electrode should possess a structure that minimizes the solid state electron transport
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length and ion diffusion distance. Typical structural features include: i) a porous interconnected framework as the backbone with high electronic conductivity, ii) a thin coating of redox-active material on the conducting backbone, and, iii) a monolithic or selfsupported electrode structure, so that no organic binder is needed. The most commonly used conducting backbones are porous monolithic carbons or carbon architectures made of CNTs or graphene. In composites of a carbon backbone with a thin coating of redoxactive material, the number of redox sites is maximized while the ion and electron transport is optimized. Both template-assisted and template-free methods have shown success in making ideal 3D electrodes. The template-assisted approach involves coating redox-active material on a pre-made 3D framework or backbone.
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Acknowledgements This work received financial support from the new academic startup fund and a studentship from the University of Exeter. Received: October 12, 2013 Revised: November 3, 2013 Published online: December 16, 2013
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RESEARCH NEWS
This method usually yields ideally ordered 3D structures with high power capabilities and cycle stability. The main drawback of this approach is the tedious synthesis procedures and high production cost. In contrast, the template-free approach normally produces less-ordered electrode structures. However, the solutionbased methods offer advantages of simple synthesis procedures, lower production costs, and easy scale-up. The ideal 3D electrode structures have achieved tremendous success in numerous laboratory-based studies in pursuit of an energy storage system with high energy and power density, as well as long cycle life. Various reports indicate the energy and power performance of a few ideal 3D electrodes could bridge the gaps between supercapacitors and batteries. It is foreseeable that ideal 3D electrode structures will play a vital role in the next generation of high-performance electrochemical energy storage technology.
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