3D printed functional nanomaterials for electrochemical energy storage

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3D printed functional nanomaterials for electrochemical energy storage Cheng Zhu a , Tianyu Liu b , Fang Qian a , Wen Chen a , Swetha Chandrasekaran a , Bin Yao b , Yu Song b , Eric B. Duoss a , Joshua D. Kuntz a , Christopher M. Spadaccini a , Marcus A. Worsley a,∗ , Yat Li b,∗ a b

Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, United States Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064, United States

a r t i c l e

i n f o

Article history: Received 17 May 2017 Received in revised form 16 June 2017 Accepted 21 June 2017 Available online xxx Keywords: 3D printing Energy storage Supercapacitors Lithium-ion batteries

a b s t r a c t Electrochemical energy storage (EES) devices, such as lithium-ion batteries and supercapacitors, are emerging as primary power sources for global efforts to shift energy dependence from limited fossil fuels towards sustainable and renewable resources. These EES devices, while renowned for their high energy or power densities, portability, and long cycle life, are still facing significant performance hindrance due to manufacturing limitations. One major obstacle is the ability to engineer macroscopic components with designed and highly resolved nanostructures with optimal performance, via controllable and scalable manufacturing techniques. 3D printing covers several additive manufacturing methods that enable well-controlled creation of functional nanomaterials with three-dimensional architectures, representing a promising approach for fabrication of next-generation EES devices with high performance. In this review, we summarize recent progress in fabricating 3D functional electrodes utilizing 3D printing-based methodologies for EES devices. Specifically, laser-, lithography-, electrodeposition-, and extrusion-based 3D printing techniques are described and exemplified with examples from the literatures. Current challenges and future opportunities for functional materials fabrication via 3D printing techniques are also discussed. © 2017 Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Laser-based 3D printing for EES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Selective laser melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Direct metal laser sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Lithography-based 3D printing for EES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Projection micro-stereolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Holographic lithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Electrodeposition-based 3D printing for EES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Electrophoretic deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Template-assisted electrodeposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Extrusion-based 3D printing for EES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Inkjet printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Direct ink writing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Other modified printing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

∗ Corresponding authors. E-mail addresses: [email protected] (M.A. Worsley), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.nantod.2017.06.007 1748-0132/© 2017 Elsevier Ltd. All rights reserved.

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Introduction Since the discovery of electricity, many technologies have been sought to effectively store electrical energy as means to bridge both temporal and geographical gaps between energy supply and demand [1,2]. Among them, electrochemical energy storage (EES) devices, with their high efficiency, versatility, and adaptability, have emerged as one of the most promising energy storage systems for utilization of intermittent sustainable energy sources [3,4]. The recent progress in portable and wearable electronics has further pushed development of high performance, rechargeable EES devices that integrate high energy density and power density, fast charging rate, lightweight, and long lifetime, all in a miniaturized package [5,6]. The development trend of these devices includes three aspects: scalable synthesis of nanostructured electroactive materials [7,8], rational design of electrode microstructures [9,10], and high-resolution fabrication of macroarchitected devices [11–13]. Batteries and supercapacitors are two types of EES devices, whose performance is largely determined by their electrode materials [14,15]. To date, a few transition metal oxides [16,17], conjugated polymers [18] and carbon materials [19,20] have been developed as the active energy storage media. Ideally, these electrode materials should be designed to possess porous structures with high surface area, high electrical conductivity, and excellent cycling stability [21]. However, the intrinsically stochastic microporosity and tortuous pore morphology hinder the charge storage performance of most electrodes (especially at ultrafast charging rates), which is due to limited pore accessibility by ions in electrolytes and sluggish ion transport. As electrode pore morphology plays a key role in dictating device performance, – via electrolyte infiltration, electron transport as well as ion diffusion – engineering electrodes with desired pore architectures becomes critical for achieving higher charge storage capacity and charging rate ability [22,23]. Take the micro-sized EES devices as an example. They have customized configurations, as well as excellent mechanical flexibility for easy integration into other micro-electronics [24,25]. To maintain a small areal footprint, these configurations must be realized with ultrahigh resolution to shorten transport distances yet provide sufficient active material to ensure adequate energy output for an extended timeframe. To further improve the areal capacitance and energy density, building thicker electrodes to increase the active material loading without sacrificing fast ion diffusion is key. However, when increasing the thickness of electrodes, the electron transport distances will inevitably increase, leading to reductions of rate capability resulting from the retarded electron transfer and increased overall electrical resistance of the electrode. Although several conventional methods, including templating techniques and chemical activations, have been employed to demonstrate control over the electrodes microstructure and devices assembly [26–28], few attempts have been made to design and build optimized micro-devices with engineered mesoscale and macroscale porous architectures. Therefore, fabrication of electrodes or devices with tailored hierarchical nanoarchitectures involving micro-, meso- and macro-pores via a controllable and scalable method remains a significant challenge [13]. Additive manufacturing (AM) refers to an industrial production technique that builds 3D objects by adding layer-upon-layer of material directly from computer-aid-design (CAD) files. This method, also known as 3D printing is a novel class of freeform fabrication technologies that have large freedom to fast create complex architectures at lower cost than conventional subtractive methods [29]. According to the way the architecture is assembled, 3D printing techniques can be divided into several categories: laser deposition of energy fusible powders, photo-polymerization

of ultra-violet (UV) sensitive liquids, electrodeposition of surface charged particle-based suspensions, and extrusion deposition of colloidal gels [29,30]. These technologies enable a growing palette of functional structures that are difficult to fabricate with conventional methods [30]. With highly programmable microstructures over a wide range of length scales, these printed architectures open new avenues for creating a class of materials with outstanding performance [31]. Recently, a couple of these 3D printing methods have been applied for electrochemical or energy-related applications [32–34]. In this work, we will present the latest progress in the fabrication of EES devices, with an exclusive emphasis on the aforementioned 3D printing strategies. Laser-based 3D printing for EES Laser can consistently and precisely deliver high thermal energy to a target using a highly collimated, coherent beam of light. In a laser-based printing process, the irradiation will instantly melt, sinter or chemically convert functional materials into diverse micro-patterns [35–37]. This single-step approach provides high flexibility for arbitrary patterning via non-contact and mask-free fabrication processes, which obviates the need for costly, time-consuming, and labor-intensive lithography and related post-processing operations [38]. Compared to other 3D printing techniques, laser-based 3D printing has two unique features: 1) this method allows a wide selection of materials including polymers, ceramics, and metals; 2) this method doesn’t require complicated material processing to prepare printable inks or postprocessing (e.g., thermal sintering) of printed parts. In short, any thermally fusible powder materials can be directly used for laser printing. These advantages make laser-based printing a preferred technique for rapid prototyping of on-chip EES micro-devices [39–44]. Selective laser melting Selective laser melting (SLM) is a specific 3D printing technique, which utilizes high power-density laser to fully melt and fuse metallic powders to produce near net-shape parts with near full density (up to 99.9% relative density). The printing process is a layer-by-layer scanning and fusion powders based on the pre-designed patterns (Fig. 1a) [45]. Ambrosi et al. printed helicalshaped steel electrodes using SLM (Fig. 1b) [46]. These tailor-made stainless steel electrodes were used as platforms for supercapacitors, catalysts and sensors by means of an effective and controlled deposition of iridium dioxide (IrO2 ) films (Fig. 1c). The 3D printed steel-IrO2 electrodes demonstrated excellent capacitive and catalytic properties in alkaline solutions and Nernstian behavior as potentiometric pH sensors. This work represents a breakthrough in on-site prototyping and fabrication of highly tailored electrochemical devices with complex 3D shapes which facilitate specific functions and properties. Direct metal laser sintering Direct metal laser sintering (DMLS) is another 3D metal printing method, which also uses a precise, high-wattage laser to microweld powdered metals to build objects out of almost any metal alloy [47]. During this process, a laser is slowly and steadily moved across the surface to sinter a very thin layer of spreading metal powders, which means that the particles inside the metal are fused together, even though the metal is not heated enough to allow it to melt completely. In this way, DMLS gradually builds up a 3D object through a series of very thin layers, even porous metal components. This method has also been extended to sinter other ceramic, polymer powders and is renamed as selective laser

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Fig. 1. (a) Schematic illustration of SLM process. (i) High-power laser melts selective areas of the powder bed. (ii) Process is repeats for successive layers. (iii) Loose powder removed and finished part revealed. Printed with permission from AIP Publishing LLC [45]. Schematic of (b) helical-shaped stainless steel electrodes, and optical image of SLM (c) scaffolds for supporting IrO2 film. Printed with permission from Wiley-VCH Verlag GmbH & Co. KGaA [46].

sintering (SLS). The major advantage of DMLS or SLS is that it produces objects free from the residual stresses and internal defects that can plague traditionally manufactured metal/ceramic components. Liu et al. used DMLS to fabricate porous metal scaffolds for subsequent electroactive materials deposition [48]. In this case, the laser power and scan rate were tuned to construct plates and plate-arrays with controlled porosity and surface area through partial sintering of stainless steel powders. 3D hierarchical porous metallic scaffolds are then functionalized via co-electrodeposition of manganese oxides (denoted as MnOx , containing MnO2 and Mn2 O3 ), along with a polystyrene sulfonate (PSS)-doped poly (3,4-ethylenedioxythiophene) (PEDOT: PSS) polymer (Fig. 2). This approach combines 3D printed stainless steel scaffolds with coelectrodeposition of MnOx -PEDOT: PSS to achieve pseudocapacitor with high areal capacitance, low resistance, and long cycling performance. The volume expansion of the electrode during the charge and discharge process can be reduced within the pores of the scaffold and the porous structure can avoid the delamination of active materials and benefit the cycling behavior.

Lithography-based 3D printing for EES Stereolithography is a technique in which a 3D structure is built in a layer-by-layer fashion using photo-polymerization as a means to solidify each layer of light sensitive resins [49]. However, this method is limited by the beam size and scan rate of the incident

UV laser that impact sample resolution (maximum ∼200 ␮m) and fabrication time (as long as a few hours) [50]. Projection micro-stereolithography To overcome traditional stereolithography limitations, mask projection micro-stereolithography (P␮SL) is developed by replacing the scanning UV laser with a UV light source and dynamic photo mask to digitally pattern light and expose an entire layer at once [51]. P␮SL enables the creation of feature size smaller than 10 ␮m, while also reducing building time by an order of magnitude [52,53]. Yang et al. produced a 3D structured dielectric capacitor based on high dielectric polymer/ceramic composites using the P␮SL method [54]. They first prepared a composite slurry by mixing silver decorated lead zirconate titanate (PZT@Ag) particles with Flex resins, which rendered a much higher viscosity and lower curing depth than the original photocurable resin. A thin layer of the composite was cured on the bottom of the substrate through the projected mask images (Fig. 3a). Then, another uniform thin layer was recoated onto previous cured layer for subsequent mask projection stereolithography. The thickness of each layer is controlled by the gap of Z stage movement, which is generally smaller than the cure depth related to the projection time and the photosensitivity of mixture of the particular composite resin. In this work, a single layer thickness of 25 ␮m was achieved by exposing mask images for 10 s. The patterns of the 3D structure were controlled by the light exposure patterns. Varied sizes of hexagonal patterns with a maximum edge width of 1 mm and minimum edge width

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Fig. 2. DMLS of the plate-array scaffold with MnOx -PEDOT: PSS. X-ray tomography reconstruction of (a) fresh full body scan and (b) an orthogonal slice with color coding indicating the electroactive material and steel phases (purple: active material; gray: steel). (c) High resolution X-ray computed tomography (XCT) reconstruction of stainless steel and MnOx-PEDOT: PSS with the inset showing the stainless-steel phase. (d) Orthogonal slice of the high resolution XCT reconstruction showing the electrode morphology, cracks, and current collector bonding. Printed with permission from Wiley-VCH Verlag GmbH & Co. KGaA [48].

of 200 ␮m were fabricated (Fig. 3b). The dielectric permittivity of Flex/PZT@Ag composite reached as high as 120 at 100 Hz with 18 vol% filler (Fig. 3c), which is about 30 times higher than that of pure

Flex. Furthermore, the dielectric loss is as low as 0.028 at 100 Hz. The calculated specific capacitance of the 3D printed capacitor is about 63 F g−1 at a current density of 0.5 A g−1 . Cyclic voltammetry

Fig. 3. P␮SL of (a) high dielectric material patterns including (b) 3D hexagonal bulges patterns (1 and 3), and hexagonal hole patterns (2 and 4) of Flex/PZT@Ag (18 vol%) composites in relation to a quarter dollar for comparison. (c) SEM images of the fracture surface of 3D printed hexagonal bulges patterns Flex/PZT@Ag capacitor. Printed with permission from Elsevier [54].

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(CV) curves indicated that the 3D printed capacitors possess low resistance and good capacitive properties. This work not only provides a tool to fabricate capacitors with complex shapes but also lays the groundwork for creating highly efficient polymer-based composites via 3D printing. Holographic lithography Holographic lithography (HL) is developed based on optical interference [55,56] of multiple laser beams with a single light exposure, which enables patterning regular arrays of fine features, without the need of complex optical systems [57,58]. Ning et al. combined HL with conventional photolithography to fabricate 3D mesostructured Li-ion micro-batteries based on lithium manganese dioxide (LMO, cathode material) and nickel-tin (NiSn, anode material) (Fig. 4a) [59]. First, a thick 3D holographically defined negative photo-resistor lattice was created on a piece of indium tin oxide (ITO) coated glass. Then, a positive photoresist was infiltrated and the electrode distribution was photolithographically defined with this positive photoresist (Fig. 4b). Ni was then partially electrodeposited through a porous SU8 lattice. After removal of the photoresist, highly porous Ni scaffolds were patterned for subsequent Ni-Sn and MnO2 electroplating (Fig. 4c). Finally, the active materials, namely, Ni-Sn (90% Sn, ∼70 nm thick) and MnO2 (∼100 nm thick) were sequentially electroplated onto the Ni scaffold as anode and cathode, respectively, followed by a selective lithiation process to produce the LMO (Fig. 4d). The resultant devices exhibit impressive performance. For example, a light-emitting diode (LED) could be powered with a 500-␮A peak current generated from a 10-␮m-thick micro-battery with an area of 4 mm2 . The fabricated battery was able to perform for 200 cycles with only 12% capacity fade.

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polymer separators, improves performance because of the ease with which ions can access the active material. Their volumetric power density is comparable to electrolytic capacitors, but the achieved capacitance is four orders of magnitude higher, and the volumetric energy density is of an order of magnitude higher. Additionally, they are capable of discharging with rates that are three orders of magnitude higher than conventional supercapacitors. Liu et al. also used this EPD method to fabricate graphene quantum dots (GQDs)-based micro-supercapacitors [70]. Both a symmetric supercapacitor (with the GQDs as both the cathode and anode) and an asymmetric supercapacitor (with GQDs as the anode and MnO2 as the cathode) were made. Their work demonstrated that symmetric micro-supercapacitors have superior rate capability, excellent power response with very short relaxation time constant, and excellent cycling stability. While the asymmetric supercapacitor exhibits two times higher specific capacitance and energy density than the symmetric micro-supercapacitor. Du et al. fabricated a multi-wall carbon nanotube (MWNT) based supercapacitor electrode by EPD [71]. These supercapacitors exhibit small equivalent series resistance yet high specific power density. They also show superior frequency response, with a knee frequency more than 70 times higher than the highest reported knee frequency for supercapacitors. Niu et al. fabricated flexible, compact, and ultrathin, solid-state micro-supercapacitors using graphene [72]. They combined photolithography with EPD to build micro-patterned reduced graphene oxide (RGO) interdigitated electrodes coated by gel electrolytes. These lateral ultrathin RGO micro-patterned electrodes shortened the diffusion path length in the normal and parallel directions of the RGO electrodes, leading to effective utilization of the electrochemical surface area of RGO layers. Compared to conventional capacitors, these microsupercapacitors demonstrate a combination of enhanced specific capacitance, columbic efficiency, and knee frequency.

Electrodeposition-based 3D printing for EES Template-assisted electrodeposition Electrodeposition-based methods use an applied electric field to create conformal coatings of a desired material from a target colloidal suspension or chemical solution [60,61]. This method allows exquisite control over layer thickness and compositional gradients of materials normal to the substrate surface [62,63]. It has been employed to conformally coat complex 3D electrodes [64,65] with state-of-the-art morphologies for batteries, supercapacitors, and fuel cells [66,67]. This method has the largest flexibility in choosing candidate materials. For example, many candidate materials can be procured as powders which can be easily processed into stable suspensions. The geometry of the resultant thin films is only limited by the shape of the deposition electrodes. Electrophoretic deposition Electrophoretic deposition (EPD) is a technique that deposits layers of colloidal particles onto a substrate. The driving force for EPD is an electric field that carries charged particles to the electrode possessing the opposite charge (e.g. positively charged particles are driven to the negatively charged electrode) [61,68]. Pech et al. fabricated ultrahigh-power micrometer-sized supercapacitors based on onion-like carbon (OLC) particles using this EPD method [69]. OLC particles were deposited from colloidal suspensions onto an interdigitated gold (Au) current collector patterned on silicon wafers (Fig. 5a). An adherent layer of OLC was obtained on the Au current collectors, with a well-defined pattern free of short circuits between the electrodes (Fig. 5b). The micro-supercapacitors consisted of a several-micron-thick layer of nanostructured OLC with individual diameters of 6–7 nm (Fig. 5c). Integration of these nanoparticles in a micro-device with a high surface-to-volume ratio, without the use of organic binders and

To tune the morphology and structure of deposited films, electrodeposition is frequently combined with pre-engineered templates to obtain electrodes with more complex microstructures. For example, Zhang et al. combined electrodeposition with a traditional templating method to fabricate 3D ultrafast graphene and silicon (Si) battery electrodes. The Si electrode consists of Si sandwiched between bi-continuous, highly conductive ion and electron transport pathways (Fig. 6a) [73,74]. This electrode design provides (i) an interconnected electrolyte-filled pore network that enables rapid ion transport, (ii) a short solid-phase ion diffusion length minimizing the negative impact of sluggish solid-state ion transport, (iii) a large electrode surface area, and (iv) high electron conductivity that facilitates electron transport in the electrode assembly (Fig. 6b). The synthesis process is outlined in Fig. 6c. Ultrahigh charge and discharge rates with minimal capacity loss were obtained from the printed device. Rates of up to 400C and 1000C for lithium-ion and nickel-metal hydride chemistry, respectively, were achieved (1C rate refers to a current density that can fully charge or discharge a battery or in one hour), enabling fabrication of a lithium-ion battery that can be 90% charged in two minutes. Along these lines, Pikul et al. used a similar method to assemble these electrodes into interdigitated 3D lithium ion batteries [75]. Self-assembled opal templates of polystyrene spheres served as the lithium-ion cathode template (Fig. 7a). After electrodepositing Ni into the interstitial space of the template, followed by template removal, a nickel inverse opal was obtained. Finally, the as-prepared nickel template was conformally coated with either a thin layer of Ni-Sn (anode) or lithiated manganese oxide particles (cathode) as shown in Fig. 7b. The thickness of the active materials was varied between 17 nm and 90 nm. The nickel scaf-

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Fig. 4. HL combined with conventional photolithography fabrication of (a) 3D micro-batteries with SEM cross-section of (b) the photo-patterned second photoresist AZ9260 embedded inside the 3D holographic lattice. (c) A single digit of the interdigitated Ni scaffold. (Insets) Top-down optical micrograph (right) and an enlarged view (left) of the interdigitated Ni current collector and (d) the interdigitated electrodes that alternate between LMO cathode (left inset) and Ni-Sn anode (right inset). Printed with permission from National Academy of Sciences [59].

fold had well-defined pores with diameters of 330 nm and 500 nm (Fig. 7c). The resultant interdigitated electrodes have a width of 30 ␮m and a spacing of 10 ␮m, and the full battery cell has a volume of about 0.03 mm3 (Fig. 7d). The architecture based on interdigi-

tated porous electrodes allows control over the disparate length scales necessary for high power. The design also overcomes the challenge of fabricating full cells on a single substrate by allowing independent electrodeposition of the active materials onto their

Fig. 5. EPD of onion like carbon onto gold current collector: (a) schematic illustration of the micro-device. (b) Optical image of the interdigital fingers with 100 ␮m spacing. (c) SEM image of the cross-section of the OLC electrode. Printed with permission from Nature Publishing Group [69].

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Fig. 6. (a) Schematic of the printed battery with a cathode equipped with a bi-continuous pathway for ions and electrons. (b) Illustration of the four primary resistances in a battery electrode: (1) ion transport in the electrolyte, (2) ion transport in the electrode, (3) electrochemical reactions in the electrode, and (4) electron conduction in the electrode and current collector. (c) Bicontinuous electrode fabrication process: the electrolytically active phase is yellow and the porous metal current collector is green. The electrolyte fills the remaining pores. Printed with permission from Nature Publishing Group [73].

respective metallic scaffolds. Moreover, it improves power density and enables microelectronics integration of lithium-ion batteries. The high-power density is achieved by simultaneously reducing ion diffusion length and electrical resistance of the entire micro-battery system. Extrusion-based 3D printing for EES Extrusion-based 3D printing employs a three-axis motion stage to draw patterns by robotically squeezing “ink” through a micronozzle [76]. This technique [77] can be divided into droplet-based approaches (e.g., ink-jet printing [78] and hot-melt printing [79]) and filamentary-based approaches (e.g., robocasting [80,81] and fused deposition [82,83]), based on the rheology properties of the ink materials [84,85]. Inkjet printing Inkjet printing (IJP) is a representative extrusion-based deposition technique, which ejects ink droplets through nozzles to fabricate thin films or patterns with uniform thickness. By controlling the amount of droplets discharged, the film thickness can be tuned [86]. The typical ink materials for IJP should consist of a solute (or solutes) dissolved or dispersed in a solvent [87] with dynamic viscosity of less than 20 ␮Pa s [78], surface tension value below 80 ␮N m−1 [88]. Ho et al. first demonstrated 3D zinc-silver oxide (Zn-AgO) micro-batteries fabricated using a custom-built super inkjet printer (SIJP) [89]. As shown in Fig. 8a, the SIJP uses electrohydrodynamic actuation to print features with dimensions less than ∼1 ␮m. The drop size generated by this SIJP is three orders of magnitude smaller than conventional inkjet printers. Due to the small size of the resulting droplets, the ink dries during its flight from the print head to the substrate. Thus, successive drops can be

printed accurately on a fixed location to create high-aspect-ratio features without ink spreading [90]. Li et al. developed another efficient and mature inkjet printing technology for mass production of high-quality graphene patterns with high resolutions [91]. To prepare high-concentration graphene dispersions, graphene was first produced from exfoliating graphite flakes in dimethylformamide (DMF). A small amount of polymer (ethyl cellulose) was added to prevent the graphene flakes from agglomeration. Afterwards, the toxic solvent DMF with low viscosity was exchanged by environmentally friendly terpinol with high-viscosity through distillation. The combination of solvent exchange and polymer stabilization resulted in stable graphene inks with compatible fluidic characteristics for efficient and reliable inkjet printing. A mixture of high- and low-contact-angle solvents together with polymers in the inks suppressed the coffeering effect (particles accentuated at the edges causing non-uniform ring-like patterns) through unpinning of the contact lines during solvent evaporation. After printing, the polymers used for capping graphene flakes were removed through a simple annealing process (baking on a hot plate at 300–400 ◦ C in air for about 1 h). Fig. 8b–d shows the printed droplets, lines, and films, respectively. All patterns are of great uniformity and free errant drops, confirming the excellent printing reliability and efficiency. Furthermore, after drying the solvent, the patterns still retained their uniformity, as shown by the optical micrographs (Fig. 8e and h) and the atomic force microscopy (AFM) images (Fig. 8f and i). The cross-sectional profile analyses of the AFM images (Fig. 8g and j) quantitatively confirm the pattern uniformity. The patterns are globally uniform in any direction. The local roughness on the patterns is likely caused by some protruding graphene flakes as shown by the scanning electron microscopy (SEM) images below. The graphene was printed onto the silver fingers to assemble electrodes in an interdigitated structure. Several passes of printing and

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Fig. 7. Template assisted electrodeposition of interdigitated 3D lithium ion batteries. (a) Schematic of the fabrication process. (b) Micro-battery design. (c) SEM cross-section of the interdigitated electrodes spanning two periods. The interdigitated electrodes alternate between anode and cathode. The insets show the magnified electrodes with the nickel scaffold coated with Ni-Sn on the left and LMO on the right. Scale bar represent 50 ␮m. Scale bars in the inset are 1 ␮m. (d) A top-down SEM image of the interdigitated electrodes with the anode electrodes connecting at the top, cathode electrodes connecting at the bottom and the anode and cathode electrodes overlapping in the middle. Scale bar is 500 ␮m. Printed with permission from Nature Publishing Group [75].

annealing allow the integration of micro-supercapacitors into flexible electronic systems.

Direct ink writing Direct ink writing (DIW) is another type of extrusion-based printing technique that is typically carried out at room temperature. The ink materials are extruded from a nozzle head to form a continuous, self-standing filament without spreading, collapsing, or sagging. One main advantage of DIW is its diversity of printable materials. Polymers, waxes, hydrogels, ceramics and even metals are all printable candidate materials [77]. However, these materials must be processed into gel-based viscoelastic inks possessing shear-thinning behavior (viscosity decreases under shear strain) to be printable via DIW. This rheology property is mandatory to facilitate ink extrusion under pressure and to allow a rapid pseudoplastic-to-dilatant recovery for shape retention after deposition [84,85]. Sun et al. fabricated the first interdigitated lithium-ion microbatteries via DIW [92]. In this work, highly concentrated cathode, lithium iron phosphate (LFP), and anode, lithium titanate (LTO), inks were prepared by dispersing the respective nanoparticles into an aqueous solvent system containing glycerol, ethylene glycol, and cellulose-based viscosifiers (hydroxypropyl cellulose and hydroxyethyl cellulose). The anode and cathode ink materials were directly printed onto a gold current collector layer-by-layer to stack

a high-aspect-ratio wall structure (Fig. 9a and b). 1 M lithium perchlorate (LiClO4 ) in 1:1 ratio of ethylene carbonate to dimethyl carbonate by volume was used as the electrolyte. Then, the 3D interdigitated electrodes were sealed with a small case (Fig. 9c and 9d). The apparent viscosity of both inks as a function of shear rate exhibits an obvious shear-thinning behavior (Fig. 9e). The storage modulus of both inks is a function of shear stress because of a high yield stress (Fig. 9f). The optimized rheological properties of these inks were achieved by controlling solid loading fraction, solvent content, and the concentrations of cellulose-based viscosifiers. The resultant ink enabled reliable filament formation and provided robust structural stability during filament stacking and solidification. Following this work, Fu et al. modified the ink recipe by introducing graphene oxide (GO) into both LFP and LTO electrodes. After printing, a gel-based solid electrolyte was used to complete the interdigitated lithium-ion batteries [93]. In this work, GO sheets and electrode active materials were dispersed in water to make aqueous based inks. The GO composition in the ink can be reduced to RGO to produce 3D-printed electrodes with high electrical conductivity, which significantly improved battery’s electrochemical performance. In addition, GO sheets tended to align along the extrusion direction by the extrusion shear stress. This configuration provided a continuous electron transfer pathway. The porous GO structure also opens channels for electrolyte infiltration. Nevertheless, clogging due to large agglomerates in the ink is a major

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Fig. 8. (a) Schematic illustration of a drop-on-demand IJP system. Printed with permission from Royal Society of Chemistry [90]. (b-d) Optical images of as-printed patterns on glass slides: (b) droplet matrix, (c) lines, and (d) a film corner; (e, f) The dried patterns of (b) and (c), respectively; (f) An AFM image of a dried droplet in (e); (i) An AFM image of a dried line segment in (g); (g, j) Cross-sectional profiles along three directions in (f) and (i), respectively; All unmarked scale bars are 100 ␮m. Printed with permission from Wiley-VCH Verlag GmbH & Co. KGaA [91].

limitation of this work. The GO-based inks only flow through large nozzles and hence limit the resolution of these 3D-printed electrodes, and thus, pose a challenge on the electrochemical performance enhancement of these batteries. The high-aspect ratio architecture can make optimal use of the vertical dimension by increasing the active materials without increasing the footprint area. However, most of the current thick 3D porous EES devices are exhibit low volumetric capacitance, power and energy density due to non-optimized 3D configura-

tions. Recently, Zhu et al. developed a novel aqueous-based GO ink for the first time and demonstrated printing of graphene aerogel micro-lattices using the developed ink (Fig. 10) [94]. They further assembled these graphene-based composite aerogels into a sandwiched supercapacitor [95,96]. Through rational design of electrode architecture, the 3D-printed graphene structure could achieve high mass loading and tailored structure for high energy density. The inks were prepared by adding graphene nanoplatelets (GNPs) and silica fillers into aqueous GO. GNPs can increase

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Fig. 9. DIW of 3D interdigitated micro-battery architectures (3D-IMA) by printing. (a-b) LTO and LFP inks through small nozzles with (c) SEM images of printed and annealed 16-layer interdigitated LTO-LFP electrode architectures. (d) Optical image of the whole battery packaging. (e) Apparent ink viscosity as a function of shear rate and (f) storage modulus as a function of shear stress for both LTO and LFP inks. Printed with permission from Wiley-VCH Verlag GmbH & Co. KGaA [92].

the electrical conductivity of the aerogel while maintaining its high surface area, which are essential for capacitive performance. Hydrophilic silica fillers serve as a viscosifier to tailor the rheology of the ink to render the shear-thinning behavior [84,85]. The 3D graphene was extruded into an organic bath (isooctane) that was immiscible with the aqueous inks to prevent ink drying during the printing process. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight combined with high electrical conductivity and excellent electrochemical properties. The typical morphology of the 3D-GCA is shown in Fig. 10b. In particular, the 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A g−1 , Fig. 10c) and power densities (>4 kW kg−1 ) that rival those of reported devices made with electrodes 10–100 times thinner. The excellent electrochemical behavior and rate capability are attributed to the 3D-printed macro-architecture that enables fast ion diffusion through the thick electrode, gel electrolyte infiltration, and low resistance, due to introduction of the GNPs with varying concentrations. This work provides a benchmark example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrated energy storage devices that can be optimized for a broad range of applications. Other modified printing techniques It is quite challenging to formulate printable inks with desired rheology suitable for DIW process. To simplify the ink preparation, a few modified printing techniques have been recently developed for 3D printing of other carbon-based materials [97–100] into designed structures for electrochemical applications. For example, Kim et al. printed RGO nanowires by local growth of GO at the meniscus formed at a micropipette tip followed by reduction of GO through thermal or chemical treatment (Fig. 11a) [101]. These 3D-printed RGO nanowires, with diverse and complex forms, demonstrated

their ability to print in any direction at the selected sites. They also used this meniscus-guided 3D printing technique to fabricate highly conductive multi-walled nanotube microarchitectures by rapid solidification of a fluid ink meniscus during pulling a micronozzle (Fig. 11b) [102]. Zhang et al. printed a graphene aerogel with overhang structures by combining drop-on-demand 3D printing and freeze casting [103]: the water-based GO ink was directly ejected onto a cold sink (–25 ◦ C) and instantly frozen. The extruded filaments were frozen layer by layer, assembled into the designed 3D structures (Fig. 11c). Outlook In this article, we have reviewed the recent progress of 3Dprinting technologies for EES device fabrication. The key features of these methods are summarized in Table 1. Each type of the discussed 3D printing method has its own strengths and limitations. Laser-based 3D printing utilizes a highly-focused laser beam with ultrahigh energy to induce fully melting and fusion of a printed material, and produce pre-designed 3D architectures by moving the laser point along a programmed path. Materials qualified for this type of 3D printing are restricted to powders with relatively high boiling points (preventing vaporization upon illuminated by laser) and the ability to rapidly solidify when cooled to room temperature. Lithography-based 3D printing constructs structures by pre-patterned and layer-by-layer curing of photo-polymer resins with the aid of UV radiation. It has the largest flexibility in designing and producing complex microarchitectures, but applicable materials are primarily limited to photo-curable resins with low viscosity. Electrodeposition-based 3D printing relies on an electrophoresis mechanism to deposit materials onto pre-made 3D electrodes. However, this method has a limited control over the size and the shape of the printed objects since the process is limited to the geometry of the electrodes. Extrusion-based 3D printing is straightforward for building up 3D designs via layer-by-layer stacking of

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Fig. 10. (a) Schematic showing the DIW processing of graphene aerogel based 3D supercapacitors. Fumed silica powder, GNPs, and resorchinol-formaldehyde (RF) solution were added into the as-prepared aqueous GO suspension. After mixing, a homogeneous GO ink with designed rheological properties was received. The GO ink was extruded through a micronozzle in an isooctane bath to prevent structural shrinkage during printing. The printed lattice was gelled at 85 ◦ C overnight and then dried using supercritical carbon dioxide. Afterward, the structure was heated to 1050 ◦ C in nitrogen atmosphere for 3 h. Finally, the silica fillers were etched away using diluted hydrofluoric acid aqueous solution (5 wt%). Scale bar is 10 mm. (b) SEM images of the 3D-GCA (scale bar: 1 ␮m). The inset shows the cubic pattern of the printed architecture (scale bar: 250 ␮m). (c) Rate capability of the 3D-GCA electrodes fabricated by direct ink writing. Printed with permission from American Chemical Society [94].

Fig. 11. (a) Schematic of GO nanowire fabrication by pulling a micropipette filled with an aqueous GO suspension and stretching the meniscus during water evaporation. Printed with permission from Wiley-VCH Verlag GmbH & Co. KGaA [101]. (b) Schematic of 3D-printing process based on meniscus-guided printing of CNT ink with water evaporating from the ink meniscus during the nozzle pulling. Printed with permission from American Chemical Society [102]. (c) Freeze-drying 3D printing of graphene aerogel in a cold sink. Printed with permission from Wiley-VCH Verlag GmbH & Co. KGaA [103].

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C. Zhu et al. / Nano Today xxx (2017) xxx–xxx Filament-based squeezing metal, ceramic, polymer gels 1 ␮m

Droplet-based squeezing metal, ceramic, and polymer suspension Ink viscosity and surface tension

Inkjet printing (IJP) Direct ink writing (DIW)

20 ␮m

Particles conformal coating Colloidal metals, ceramics, polymer particles Electrophoresis Electrophoretic deposition (EPD) Electrodeposition

5 ␮m

Photo-polymerization UV-curable polymers Digital dynamic masks Optical interference of laser beams Projection micro-stereolithography (P␮SL) Holographic lithography (HL) Lithography

10 ␮m 0.25 ␮m

Particles fusion Polymer, metal, ceramic powders 250 ␮m 200 ␮m High power laser deposition

the extruded ink filaments according to programmed patterns. The prerequisite for this technology is obtaining inks that possess a unique shear-thinning rheological behavior that makes them printable. 3D-printed architectures with well-defined morphologies and diverse features will only continue to emerge and establish a significant and pervasive impact on energy storage. 3D printing offers tremendous flexibility which is simply not possible with conventional fabrication techniques. Nonetheless, numerous challenges must be addressed before 3D printing can be ubiquitously employed for EES device fabrication. First, the versatility and customizability of many 3D printing methods is limited. Most developed 3D printing techniques are capable of making only a single component of EES devices (e.g., electrodes, electrolytes, and packaging). One-step printing of the whole devices, which is essential for fully integrated manufacturing, is still challenging for 3D printing communities. Perhaps collaborative utilization of different 3D printing techniques is a key to address this obstacle by combining the strengths of each technique. In addition, fabricating advanced structures, such as core-shell-shell structures that can be engineered directly into electrode-electrolyte-electrode device configurations, is a promising route towards full integration. Second, most 3D printing enables controllable patterning of highly ordered large pore (micron scale), but very few techniques [104] can manipulate the distribution of pores in the sub-micron regime (e.g., mesopores and micropores), which also significantly influence device performance. Hierarchical pore systems involving orderly distributed macropores, mesopores, and micropores are particularly critical for ultrafast energy storage applications, as they render large electrolyte accessible surface area as well as low tortuosity for ion diffusion. Pushing the 3D printing resolution to higher levels will address the aforementioned limitation. However, it is a nontrivial process since a number of technical issues, including the redesign of the printer set-up, will manifest and require joint efforts with academia and industry to resolve them. Finally, the availability of printable materials, especially electrochemically active materials for EES, is still limited. Polymer resins that are electrically insulating and metals that are chemically unstable require post-treatments or modifications before serving as electrode materials for EES. Such post-treatments are usually time consuming and may not be compatible with all components in the EES device. Nevertheless, it is encouraging to see some electrochemically active materials, such as graphene aerogels, have already been successfully fabricated as printable inks. Additionally, a diverse array of printable materials will open new opportunities in materials design, as well as, broader applications for 3D printing technologies (e.g., catalysts, electronics and bio-applications). We envision that, with the continuous development of 3D printing techniques aimed towards high-speed, low-cost, versatile production capabilities, and ultrafine resolution, 3D printing will eventually become an essential part of future manufacturing touching both fundamental research and industry. Acknowledgements This work was done under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344, through LDRD award 16-ERD-051. Tianyu Liu acknowledges the financial support from the Chancellor’s Dissertation-year Fellowship granted by University of California, Santa Cruz (LLNL-JRNL-732865). References

Extrusion

Laser

Selective laser sintering (SLM) Direct metal laser sintering (DMLS)

Material(s) Resolution Prerequisite(s) 3D printing techniques Categories of 3D printing

Table 1 Key features of various 3D printing techniques for fabrication of EES devices.

Printing style

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ARTICLE IN PRESS C. Zhu et al. / Nano Today xxx (2017) xxx–xxx

Cheng Zhu is currently an engineer at Lawrence Livermore National Laboratory (LLNL). He received his Ph.D. in Chemical Engineering from Oklahoma State University (2010). He worked as a postdoctoral research fellow at LLNL from 2011 to 2014. His research focuses on the 3D printing techniques development, additive manufacturing of structural and functional materials, colloidal processing of particulate materials and rheology of complex fluids.

Yu Song received his B.S. in chemistry from Jilin University, China and received his Ph.D. in physical chemistry from Northeastern University in China. During 2015–2017, he joined Prof. Yat Li’s laboratory as a visiting student at University of California, Santa Cruz, United States. His research interest focuses on graphene-, metal oxide- and conducting polymer-based energy storage and conversion devices.

Tianyu Liu received his B.S. in chemistry from the University of Science and Technology, Beijing, China in 2012 and joined the Prof. Yat Li’s lab at University of California, Santa Cruz, United States thereafter. He is now a Ph.D. in Chemistry. His research focuses on development of functional materials for sustainable energy harvesting, conversion and storage with an emphasis on supercapacitors.

Eric B. Duoss is currently a Member of the Technical Staff at Lawrence Livermore National Laboratory. He received his B.S. in mathematics and chemistry from St. Norbert College (2003). He completed his Ph.D. in materials science and engineering from UIUC with Prof. Jennifer A. Lewis. His research focuses on the areas of advanced materials and manufacturing combined with micro-architected design.

Fang Qian is currently a staff scientist at Lawrence Livermore National Laboratory. She received her B.S. in Chemistry from University of Science and Technology of China (2002). She completed her Ph.D. in physical chemistry from Harvard University with Prof. Charles M. Lieber (2008). Her research focuses on the area of advanced materials, biosensing, and lab-on-a-chip device design and fabrication.

Joshua D. Kuntz is currently the Deputy Group Leader, Functional Material Synthesis Integration Group (2012present), previously a member of the Advanced Materials Synthesis Group. He received his B.S. (1997) and Ph.D. (2005) in materials science and engineering from UC Davis. He joined LLNL in 2005 as a postdoctoral researcher. His research focuses on materials processing, especially ceramics. He also works in the development of additive manufacturing (LDRD, WCI, & external sponsors) transparent ceramics (DHS & WCI) and energetic materials (DTRA & JMP).

Wen Chen is currently working as a Postdoctoral Research Staff Member in Center for Engineered Materials and Manufacturing at Lawrence Livermore National Laboratory. He completed his Ph.D. degree in Mechanical Engineering and Materials Science with Prof. Jan Schroers at Yale University. His research interests lie in mechanical behavior of materials, materials processing and additive manufacturing, architectured materias, and metallic glasses. He is the recipient of many academic awards including Yale Pierre W. Hoge Fellowship Award, Acta Student Award, and Chinese Government Award for Outstanding Oversea Graduate Students.

Christopher M. Spadaccini is currently the Director, Center for Engineered Materials, Manufacturing and Optimization at LLNL. He received his B.S. (1997), M.S. (1999) and Ph.D. (2004) from MIT. He has been a member of the technical staff in the Materials Engineering Division for the past ten years. He is currently the Principal Investigator for several advanced materials and additive manufacturing projects. He is also the founder and director of a new additive manufacturing, process development, and architected materials center. The work in these laboratories focuses on developing next-generation additive processes that are capable of micro- and nano-scale features and have the ability to create components with mixtures of materials ranging from polymers to metals and ceramics. Development of these processes also involves the synthesis and materials science of feedstocks such as photopolymers and nanoparticles.

Swetha Chandrasekaran is currently a Postdoctoral Research Staff Member in Materials Science Division at Lawrence Livermore National Laboratory. She received her Ph.D. from Institut für Kunststoffe und Verbundwerkstoffe − Technische Universität Hamburg-Harburg, Germany in 2014. Her research focus on the synthesis and characterization of carbon aerogels, and preparation and testing of carbon nanoparticle based polymer composites.

Bin Yao received his B.S. and M.S. in material science from Wuhan University of Technology, China. During 2012–2015, he was a visiting student in Wuhan National Laboratory for Optoelectronics and Huazhong University of Science and Technology. He is currently a graduate student in chemistry at University of California, Santa Cruz under the supervision of Prof. Yat Li. His research interest includes flexible energy storage and conversion devices based on metal oxides and nitrides.

Marcus A. Worsley received his BS in chemical engineering at Michigan State University (2001). He completed his M.S. (2003) and Ph.D. (2006) in chemical engineering at Stanford University. He was a Postdoctoral Fellow at Lawrence Livermore National Laboratory (2006–2008) before his current position as a Staff Scientist in the Advanced Materials Synthesis group. Currently, his research focuses on nanostructured and porous materials (e.g., aerogels and functional nanocomposites) for a wide range of applications, such as energy storage, sensing, and catalysis. His research includes both the development of materials with novel properties and the development of feedstock materials for various additive manufacturing (a.k.a. 3D printing) techniques. Yat Li received his B.S. and Ph.D. in chemistry from the University of Hong Kong. He was a postdoctoral research fellow at Harvard University from 2003 to 2007 under the supervision of Prof. Charles M. Lieber. He joined the University of California, Santa Cruz in 2007 and is now an associate professor of chemistry. His research focuses on the design and synthesis of semiconductor nanostructures, investigation of their fundamental properties, and exploration of their potential for energy conversion and storage.

Please cite this article in press as: C. Zhu, et al., 3D printed functional nanomaterials for electrochemical energy storage, Nano Today (2017), http://dx.doi.org/10.1016/j.nantod.2017.06.007