Due to the advantages mentioned above, a large amount of research has been dedicated to this field and multiple reviews on 3D‐printed energy storage devices have been published, with a focus on the development of electrode materials and a very limited discussion on 3D‐printed techniques and electrolyte. 12 , 13 , 17 , 25 Up to now, there lacks a thorough review on summarizing all available 3D‐printed techniques for battery manufacturing. More importantly, much of the latest progress and many breakthroughs for 3D‐printed batteries have not been discussed in previous reviews. Thus, we provide a comprehensive review of the latest advances and progress in the manufacturing and materials development for 3D‐printed batteries ( Figure 1 ). The content of the review is organized as follows: In Section 2 , we discuss the 3D‐printed batteries via different kinds of printing techniques, including lithography‐based 3D printing, template‐assisted electrodeposition (TAE)‐based 3D printing, inkjet printing (IJP), direct ink writing (DIW), fused deposition modeling (FDM), and aerosol jet printing (AJP). The working principles, printing process, advantages, and limitations for each method are then discussed. Afterward, we highlight the printing materials for electrodes and electrolytes of the printed batteries in Sections 3 and 4 , respectively. Finally, conclusions and future opportunities for 3D printing of batteries are discussed in Section 5 .

As an advanced fabrication technique, 3D printing, i.e., additive manufacturing (AM), has been increasingly utilized to fabricate complex 3D objects via digitally controlled deposition of phase change and reactive materials and solvent‐based inks. 15 , 16 It generally begins with the design of a 3D virtual model that is sliced into several 2D horizontal cross sections using special software. By successively printing new 2D layers on top of previous layers, a coherent 3D object can be finally fabricated. 17 , 18 A few major types of 3D printing technologies have been invented and widely utilized: 1) material extrusion, 2) powder bed fusion, 3) vat photopolymerization, 4) material jetting, 5) binder jetting, 6) sheet lamination, and 7) directed energy deposition. 13 3D printing has demonstrated superior advantages in the rapid prototyping of complex structures and devices with high control accuracy. This has significantly simplified the fabrication process and reduced the design period to avoid the troublesome trial‐and‐error process. 15 , 19 , 20 Due to the unique advantages, it has been extensively applied in many fields such as food, 21 medical, 22 electronics, 23 and aerospace. 24 In recent years, 3D printing has also been utilized to manufacture energy devices such as batteries and supercapacitors for specific applications in laboratories. 17 , 19 , 25 Compared with conventional battery fabrication technologies, 3D printing has several significant advantages: 1) enabling the fabrication of desired complex architectures; 2) precise controlling of the shape and thickness of the electrodes; 3) printing solid‐state electrolyte with high structure stability and safer operation; 4) potential for low‐cost, environmental friendliness, and ease of operation; and 5) possibility of eliminating the steps of device assembly and packaging via direct integration of batteries and other electronics. 17 3D printing is able to fabricate novel 3D‐architectured electrodes with larger surface area and higher areal‐loading density, providing shorter diffusion pathways and smaller resistance during the ion‐transport process so as to improve battery energy density and power density. Additionally, 3D printing can dramatically reduce material waste and may enable potential time saving due to the less complex fabrication procedures. Overall, 3D printing opens new avenues for the rapid fabrication of 3D‐structured batteries with complex architectures and high performance.

To generate high capacity, traditional 2D‐structured batteries require a large footprint area, which hampers their integration into portable microelectronics. Another method for enhancing the energy density and areal capacitance is to fabricate thicker electrodes that can significantly raise the active material loading while preserving rapid ion diffusion. 12 With increasing electrode thickness, however, the electron transport distances and overall electrical impedance of the electrode will inevitably increase, resulting in reductions of power density and rate capability. 12 , 13 Compared with conventional 2D planar structures, 3D structures can yield shorter diffusion pathways and lower resistance during the ion‐transport process, as well as providing increased energy density by creating porous structures with larger surface areas that can improve electrode reaction and ion transfer while efficiently using the limited space in a compact battery. 12 , 13 To date, several methods such as templating and chemical activation 14 have been developed to control electrode microstructure and device assembly, but little work has been done to design, optimize, and fabricate microdevices possessing engineered porous architectures. Thus, the manufacturing of devices possessing complex hierarchical nanoarchitectures via a scalable and controllable method remains a significant challenge. 12

Given the rapid development and extensive use of mobile electronics, there is an increasing demand for reliable and cost‐effective energy storage devices to build power‐independent electronics systems. 1 Batteries, as one of the most important and widely used electrical energy devices, have attracted significant attention and have been extensively studied due to their ability to stably store and source electrical energy as well as their availability for a wide range of forms, capacities, and power densities. 2 , 3 For instance, lithium‐ion (Li‐ion) batteries in large form factors exhibit high energy density, low self‐discharging rate, and high current charge/discharge cycling capability. 2 , 4 Over the years, much effort has been put into exploring new electrode materials, electrolytes, cell structures, and novel fabrication approaches so as to improve the electrochemical performance of batteries, to reduce the cost, and to expand their application. Currently, most electrode materials have relatively low electronic conductivity and slow diffusion speeds of lithium ions, resulting in low charge/discharge rate and power density of batteries. In addition, the high cost of Li‐ion batteries is also a concern; for example, Li‐ion batteries based on nickel and cobalt oxides are quite expensive. Due to their advantages of low cost but high energy, lithium–air (Li–air) 5 and lithium–sulfur (Li–S) batteries 6 have been extensively studied to develop new lithium‐based batteries. In addition, new rechargeable battery systems based on abundant resources, such as zinc‐, 7 , 8 calcium‐, 9 aluminum‐, 10 and sodium‐based batteries, 11 are being explored and have captured increasing attention.

2 Major Printing Methods for 3D‐Printed Batteries

As an innovative manufacturing approach, 3D printing technologies are able to facilitate the fabrication of batteries, enable versatile and miniature batteries ranging from microscale to macroscale, and improve the electrochemical performance of batteries.13 Table 1 presents the representative printed batteries by various methods and materials. The geometry (e.g., porosity, dimensioning, and morphology) and the structure of electrodes and electrolytes of printed batteries can be accurately controlled by greatly simplified and low‐cost processes. However, considering the compatibility among the preparation conditions, materials, and processes, not all of the 3D printing technologies and current materials used in conventional batteries are appropriate for manufacturing printed batteries. A qualitative comparison of different 3D printing techniques for printed batteries is shown in Table 2. There are many challenging issues that have to be addressed in the manufacturing of 3D‐printed batteries. In this section, we summarize the major 3D‐printed batteries that have been demonstrated via advanced 3D printing technologies in recent years.

Table 1. Summary and comparison of the 3D‐printed batteries with various kinds of methods and materials Printing method Anode/Cathode Electrolyte Specific capacity Ref. Lithography‐based 3D printing (NiSn/LMO) 0.06 m NiCl 2 , 0.2 m SnCl 2 , 1 m K 4 P 2 O 7 , 0.04 m potassium sodium tartrate, 0.04 m glycine ≈2.9 µAh cm−2 µm−1 at 1 C 27 LiTiO 2 /LFP LAGP LiTFSI‐PYR 14 TFSI 500 mAh cm−2 at 0.1 C 31 LTO/LFP Poly(ethylene glycol) gel polymer 1.4 µAh cm−2 at 2 C 29 Template‐assisted electrodeposition NiSn/LMO 1 m LiClO 4 – 35 (Pt Foil)/NiOOH 1 m LiClO 4 286 mAh g−1 at 385 C 34 Inkjet printing (Li Foil)/LiMn 0.21 Fe 0.79 PO 4 @C 1 m LiPF 6 150.21 mAh g−1 at 10 C 45 (Li Foil)/LFP Composite (C SP , TX100, poly‐acrylic‐co‐maleic acid, carboxymethyl cellulose) 1 m LiPF 6 , 0.5 M LiTFSI 150 mAh g−1 at 1 C 40 (Hybrid MoS 2 –graphene aerogel)/(Na disk) 1 m NaClO 4 429 mAh g−1 at 3.3 C 44 SnO 2 /Li 1 m LiPF 6 812.7 mAh g−1 at 1 C 39 (Li foil)/LFP 1 m LiPF 6 134.7 mAh g−1 at 0.1 C 41 (Li foil)/MWCNT – 1260 mAh g−1 at 0.5 C 76 Ag/Ag 10 m KOH/ZnO – 96 (Li foil)/LiCoO 2 1 m LiPF 6 105 mAh g−1 at 394 µA cm−2 42 (Li foil)/Li–S/S@SWNT 1 m LiCF 3 SO 3 ≈850 mAh g−1 at 0.5 C 77 (Si NPs/PEDOT:PSS)/Li Foil 1 m LiPF 6 2500 mAh g−1 at 0.1 C 43 (Li foil)/Li 1.2+ x Mn 0.54 Ni 0.13 Co 0.13 O 2 TC‐E918 (Tinci) >250 mAh g−1 at 0.1 C 97 Li 4 Ti 5 O 12 /LFP Ion‐gel solid 60 mAh g−1 at 0.1 C 89 Aerosol jet printing (Li foil)/LFP LiPF 6 151 mAh g−1 at 0.067 C 98 Direct ink writing (Na 3 V 2 (PO 4 ) 3 graphene oxide)/(Na 3 V 2 (PO 4 ) 3 graphene oxide) 1 m NaClO 4 , ethylene carbonate (EC)/propylene carbonate (PC) 1.26 mAh cm−2 at 0.2 C 38 (CNF/Li)/(CNF/LFP) 1 m LiTFSI, 1,2‐dimethoxymethane (DME)/1, 3‐dioxolane (DOL) 140 mA h g−1 at 0.2 C 82 LTO/LFP 1 m LiTFSI/PC 133 mAh g−1 at 0.2 mA cm−2 46 LTO/LFP 1 m LiClO 4 , EC/DMC 1.5 mAh cm−2 at 5 C 47 (Li foil)/(S/GO/DIB) 1 m LiTFSI, DOL/DMC 812.8 mAh g−1 70 (Li Foil)/(Ni/rGO) – 1000 mA h g−1 at 100 mA g−1 99 LTO/LFP 1 m LiPF 6 , EC/DEC 10 mAh g−1 at 50 mA g−1 48 Li/Li Li 7 La 3 Zr 2 O 12 – 94 (LTO/CNF/PVDF)/(LFP/CNF/PVDF) Al 2 O 3 , PVDF, NMP/glycerol 154 mAh g−1 at 0.2 C 93 (LTO/CNF/PVDF)/(LFP/LiCoO 2 ) 1 m LiPF 6 , EC/PC 150 mAh g−1 at 0.2 C 85 Zn/MnO 2 PVDF–HFP, 1‐butyl‐3‐methylimidazolium trifluoromethanesulfonat (BMIM+Tf−) 0.98 mAh cm−2 100 (Li foil)/MnO 2 PVDF‐co‐HFP/Pyr 13 TFSI/LiTFSI/TiO 2 127.3 mAh g−1 49 (Li foil)/hGO – 13.3 mAh cm−2 at 0.1 mA cm−2 71 (LTO/GO)/(LTO/GO) 1 m LiPF 6 , EC/PC 185 mAh g−1 at 10 mA g−1 69 (Li foil)/(S/BP2000) 1 m LiTFSI, DOL/DME 1009 mAh g−1 at 5.5 mg cm−2 78 (LFP/AB/CNT)/(LFP/AB/CNT) LiPF 6 , EC/DMC/DEC 150 mAh g−1 at 0.1 C 79 LTO/LFP LiPF 6 , EC/ethylmethyl carbonate (EMC)/DEC/vinylene carbonate (VC) 128 mAh g−1 at 0.2 C 50 (Li foil)/(NC‐Co) 0.5 m LiClO 4 /dimethyl sulfoxide (DMSO) 525 mAh g−1 at 0.8 mA cm−2 51 Zn/(carbon black/MnCo 2 O 4 /rGO/CNT) polyvinyl alcohol/[BMIM]OH 142.8 Ah L−1 at 0.1 mA cm−2 7 Fused deposition modeling (Graphene/PLA)/Li foil 1 m LiPF 6 40 mAh g−1 at 120 C 101 Graphene/Pt 1 m LiCl 248 mAh g−1 at 40 mA g−1 102 (Graphite/PLA)/Li 1 m LiPF 6 215 mAh g−1 at 18.6 mA g−1 58 (Lithium titanate, graphene nanoplatelets)/(lithium manganese oxide/MWNTs) 1 m LiClO 4 7.48 mAh cm−3 at 1 C 59 (LTO/carbon/polyester polylactic acid)/(LFP/graphite/functionalized MWNTs/C 65 /polyester polylactic acid) 1 m LiPF 6 80 mAh g−1 at 2 C 87

Table 2. Comparison of different 3D printing techniques for batteries 3D printing method Available materials Printing resolution Advantage Disadvantage FDM Thermoplastic 50–200 µm User‐friendly, low cost, high speed, large size capabilities and lack of necessity for chemical post‐processing Limited resolution on the z‐axis, weak mechanical properties, high viscosity of the molten materials, and low surface quality DIW Plastic, metal, ceramic, and food 1 µm Affordable cost, easy operation, large material diversity, and no mask requirement Weak mechanical properties, high requirement of ink. TAE Polymers, metals, oxides, and hydroxides 50 nm to10 µm Highly ordered macroporous structure, low cost, high efficiency, simplicity, versatility, and controllability Weak mechanical properties and strong limitation in materials IJP Sol–gel, metal, conductive polymers, and carbon‐based and protein materials 20 µm Low cost, multimaterial printing capability, able to print large areas Low printing speed, not good for high‐volume printing, and print head is less durable AJP Nanoparticles, nanowires, CNTs, 2D materials, dielectric materials 10 µm High‐resolution, high efficiency, compactable with inks in different viscosities. Overspray, printing quality is not stable, high‐cost of equipment SLA Photopolymers 0.25–10 µm High resolution, high surface finish, and high efficiency Strong limitation in multimaterial deposition

2.1 Lithography‐Based Printing Holographic lithography (HL) is based on the multibeam interference phenomenon without using complex photomasks or optical systems.26 It is a simple and low‐cost technique to fabricate 1D, 2D, and 3D periodic geometries via a single laser exposure. Ning and co‐workers used the combination of HL and conventional photolithography to fabricate high‐performance Li‐ion batteries (Figure 2a).27 In their experiments, a SU‐8 3D lattice was fabricated on the surface of indium–tin oxide (ITO)‐coated glass via several interfering laser beams arranged in an umbrella geometry. Then, another photoresist (AZ9260) was infiltrated into the SU‐8 lattice and formed solid straight walls after solidification within the 3D lattice (Figure 2b). Next, Ni was partially electrodeposited onto the porous SU‐8 lattice, followed by the removal of the photoresist template by oxygen reactive ion etching. The final current collectors composed of an interdigitated 3D porous Ni scaffold were obtained (Figure 2c) for further electroplating active materials, MnO 2 (≈100 nm thick) and Ni–Sn (90% tin, ≈70 nm thick) as cathode and anode, respectively (Figure 2d). The resultant cell showed impressive power and energy densities up to 3600 µW cm−2 µm−1 and 6.5 µWh cm−2 µm−1. It was demonstrated that at least 80% of the initial capacity of the cell was retained after 100 cycles at various rates and that the cell could drive a light‐emitting diode (LED) over 200 charge/discharge cycles with only 12% capacity loss. Figure 2 Open in figure viewer PowerPoint 27 29 C‐rates. j) Ragone plot comparing electrochemical performance of three kinds of batteries. Reproduced with permission. 31 Lithography‐based printing of 3D‐printed batteries. a) Schematic illustration of the printing fabrication process for a 3D microbattery. b) SEM image of the cross section of a fabricated AZ9260 structure. c) SEM image of the cross section of a nickel scaffold. Inset: Optical image (right) and microscopic image (left) of the interdigitated nickel current collector. d) SEM image of the 3D microbattery. Inset: Enlarged view of LMO cathode (left) and Ni–Sn anode (right). Reproduced with permission.Copyright 2014, National Academy of Sciences. e) Schematic of the 3D structure for GPE. f) SEM image of PEG membrane with self‐assembled sub‐micrometer scale channels. Reproduced with permission.Copyright 2018, Elsevier. g) Optical images of 3D‐printed perforated spherical, cylindrical, and cubic substrates. h) Energy disperse spectroscopy (EDS) image of the cross section of LFP‐LAGP:PEI‐LTO. i) Discharge voltage profiles of 3D‐LFP cell at different‐rates. j) Ragone plot comparing electrochemical performance of three kinds of batteries. Reproduced with permission.Copyright 2017, Electrochemical Society. As a novel 3D fabrication technology, projection micro‐stereolithography (PµSL) has been developed to fabricate high‐resolution 3D polymer structures and devices.28 In PµSL, a digital microdisplay technology is used as a dynamic mask generator to produce a virtual photomask, and a focused ultraviolet (UV) light spot scans over the surface of photocurable resin. Chen et al. utilized the PµSL technology to directly print a 3D microbattery.29 As shown in Figure 2e, a microstructured UV‐curable poly(ethylene glycol) (PEG)‐based gel polymer electrolyte (GPE) was fabricated, which contained two parts: the trench used to contain current collectors and electrode materials, and the center membrane with a zigzag shape acting as the gel electrolyte of the battery. In photopolymerization, there are self‐assembled sub‐micrometer scale channels formed on the surface of the GPE membrane (Figure 2f). Similar to the porous structures in conventional battery separators, the improvement of ion transport brought by the sub‐micrometer scale channels can further reduce interfacial scattering.29 Thanks to the large ionic conductivity of GPE, the microbattery showed a charge and a discharge plateau around 3.4 and 3.5 V (Li/Li+), respectively, which were close to the theoretical plateau values for LiFePO 4 (LFP).29 Despite the many advantages of PµSL, the 3D microbattery manufactured via this method exhibited poor cycle performance and low capacity, which should be improved in the future. Stereolithography (SLA) is another promising 3D printing technique that is suitable for manufacturing porous 3D battery electrodes due to its high spatial resolution.30 In the SLA system, the photosensitized monomer resin or photosensitized monomer solution (mainly acrylic or epoxy based) is selectively converted into a solidified polymer via the application of visible light and UV light. A 3D structure is built layer by layer according to predesigned CAD patterns, and a postprocess treatment such as heat treating or photocuring has to be done for solidifying the remnant monomers within the structure. Recently, Cohen et al. employed SLA to prepare perforated spherical, cylindrical, and cubic polymer substrates with high surface area (Figure 2g).31 By integrating these substrates with a simple and cost‐effective electrophoretic‐deposition (EPD) method, a trilayered structure consisting of LiTiO 2 ‐based anode, polyethylene oxide (PEO)–LiAlO 2 or Li 1.5 Al 0.5 Ge 1.5 P 3 O 12 (LAGP)‐polyethylenimine (PEI) membrane, and LiFePO 4 cathode was developed on the surface of graphene‐filled conducting substrates (Figure 2h). The 3D LFP cell with perforated polymer substrate can undergo cycling with different C‐rates to yield a decent areal discharge capacity of 400–500 µAh cm−2 (Figure 2i), which can be additionally improved by raising the thickness and surface area gain of the cathode and substrate, respectively.31 Additionally, these 3D microbatteries showed great electrochemical performance, comparable with that of microbatteries from perforated silicon chips. Under the same power capability per battery footprint, the areal energy densities of the 3D microbattery on the perforated silicon chip and printed perforated polymer substrate was, respectively, ten and three times that of the commercially available thin‐film battery (Figure 2j), in accordance with the geometrical area gain.31 In conclusion, the aforementioned lithography‐based printing techniques utilize photocurable materials, and 3D structures are built after a polymerization process via the application of visible light and UV light. These technologies have been widely utilized to print complex 3D structures with high resolution and surface finish. However, some of them require the use of masks, which will increase the production cost.

2.2 Template‐Assisted Electrodeposition Highly ordered macroporous materials have swiftly become competitive candidates for electrode fabrication owing to their large specific surface areas. The TAE technique exists as the most typical approach in the synthesis of macroporous materials with tunable pore sizes and structures.32 It is low cost, versatile, and convenient to control the shape and size of the structure by just changing the electrodeposition parameters and choosing templates with different characteristics.33 The Braun group first fabricated high‐performance batteries using a TAE technique.34 They designed a self‐assembled 3D bicontinuous bulk cathode, which consisted of conductive pathways for ion and electron transport by sandwiching an electrolytically active material (Figure 3a). Compared with conventional electrodes, the designed bicontinuous electrodes showed superior advantages in four aspects: i) fast ion transport enabled by an interconnected electrolyte‐filled pore network in these electrodes, ii) minimal effect of sluggish solid‐state ion transport due to a short ion diffusion distance, iii) high electrode surface area, and iv) excellent electron conductivity in the electrode assembly (Figure 3b).34 In the fabrication process, self‐assembled opal templates were prepared by deposition of polystyrene (PS) spheres (Figure 3c) and a thin layer of Ni was then electrodeposited through the void space of the opal template. The electrolytically active materials were finally deposited onto the highly porous nickel scaffold (Figure 3d). The obtained battery possessed ultrahigh charge/discharge rate capability with minimal capacity loss, such that the rates for nickel‐metal hydride chemistries and Li ion could reach up to 1000 and 400 C, respectively, and it could be charged to 75% after 60 s or 90% after 120 s (Figure 3e). Figure 3 Open in figure viewer PowerPoint 2 /nickel composite cathode. e) The charging/discharge curves of a prototype lithium‐ion battery. Reproduced with permission. 34 2 cathode (right). h) A top‐view SEM image of the interdigitated electrodes. Reproduced with permission. 35 Template‐assisted electrodeposition of printed batteries. a) Schematic of a battery structure with a bicontinuous cathode. b) Schematic illustration of four primary resistances in a battery electrode. c) Schematic illustration of the fabrication process for bicontinuous electrode. d) SEM and schematic images (inset) of the lithiated MnO/nickel composite cathode. e) The charging/discharge curves of a prototype lithium‐ion battery. Reproduced with permission.Copyright 2011, Springer Nature Group. f) Schematic of a microbattery design. g) SEM image of the interdigitated electrodes. Inset: NiSn anode (left) and LiMnOcathode (right). h) A top‐view SEM image of the interdigitated electrodes. Reproduced with permission.Copyright 2013, Springer Nature Group. Following this pioneering work, the same group further reported a high‐power Li‐ion microbattery made of interdigitated 3D bicontinuous nanoporous electrodes (Figure 3f).35 In this microbattery, a thin coating of lithiated manganese oxide (LMO) or nickel–tin was applied upon interdigitated highly porous metallic scaffolds to form the electrode (Figure 3g,h), which can decrease electron transport lengths and electrical resistance while sustaining a high volume of active material. Thanks to the 3D bicontinuous nanoporous electrodes, the microbattery exhibited a maximum energy density of 15 µWh cm−2 µm−1 and a maximum power density of 7.4 mW cm−2 µm−1. This work illustrated the potential of integrating materials and microengineered architecture to improve battery performance for broad applications in sensor networks and medical implants.35 In conclusion, TAE is one of the few 3D printing techniques that can fabricate nanostructured electrodes. It has been proven to be an attractive method to fabricate microbatteries with supercapacitor‐like charge/discharge rate while retaining comparable battery‐like storage capacity. However, large‐scale production is yet to be verified and the mechanical properties of the printed battery electrodes are poor due to their high porosity.

2.3 Inkjet Printing Inkjet printing is a representative droplet‐based deposition technique that can directly deposit materials through nozzles onto plastic, paper, or other substrates to create complex patterns with high resolution and tunable thickness in accordance with the number of droplets discharged.36 It has been explored to print sol–gel, metal, conductive polymers, carbon‐based, and protein materials.37 The ink for IJP usually has specific requirements in surface tension, density, and dynamic viscosity. In the past decade, IJP has also been adopted to fabricate electrochemical storage devices.38-42 For example, Lawes et al. fabricated thin‐film silicon anodes for Li‐ion batteries via a desktop inkjet printer.43 The ink was uniformly mixed by conducting agent (carbon black), electrochemically active material (silicon nanoparticles, SiNPs), and polymer binder in water through sonication (Figure 4a). Their results showed that poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was the most effective binder for battery electrodes, possessing large initial and reversible capacity due to its conductive nature. The inkjet‐printed anodes demonstrated excellent electrochemical performance with a capacity of >1700 mAh g−1 for 100 cycles. Furthermore, when a limited depth of discharge measurement of 1000 mAh g−1 was carried out, the battery exhibited a very stable cycling performance of over 1000 successful cycles. This work shows that inkjet printing can fabricate thin‐film SiNP electrodes of high capacity, and can be extended to print other electrode materials.43 Figure 4 Open in figure viewer PowerPoint 43 2 –rGO printed on Ni foam. Reproduced with permission. 44 45 Inkjet printing of microbatteries. a) Fabrication process for SiNP anodes on a copper foil. Reproduced with permission.Copyright 2017, Elsevier. b) Schematic of the 3D “drop‐on‐demand” ink jet printer. c) Schematic illustration of ice template formation. d) SEM image of MoS–rGO printed on Ni foam. Reproduced with permission.Copyright 2019, Elsevier. e) Schematic illustration of a battery with 3D‐printed electrodes. f) Cycling performance of 3D‐printed and traditional electrodes at 10 and 20 C rates for 1000 cycles. Reproduced with permission.Copyright 2016, Wiley‐VCH. Significant progress has also been made to improve the specific surface area of printed electrodes. For instance, the Li group developed a 3D freeze‐printing method to fabricate porous sodium‐ion battery anodes.44 Graphene oxide (GO) and ammonium thiomolybdate (ATM) were dispersed in deionized (DI) water and served as the precursor ink, followed by printing onto a porous Ni foam as the current collector at −30 °C (Figure 4b). The ATM–GO droplets were frozen into ice crystals to produce a continuous matrix (Figure 4c). The MoS 2 – reduced graphene oxide (rGO) hybrid aerogel was finally prepared by thermally annealing the precursor aerogel at 600 °C for 2 h. The scanning electron microscopy (SEM) image of the printed hybrid MoS 2 –rGO aerogel revealed that many micrometer‐sized pores formed in wrinkled rGO flakes (Figure 4d), which were notably different from those obtained via traditional 3D‐printed/freeze‐dried materials. Experimental testing showed that the electrical energy storage capability of the sodium‐ion battery with hybrid MoS 2 –rGO aerogel anodes was reasonable and a specific capacity of 800 mAh g−1 could be achieved in the first cycle at 100 mA g−1 current density. However, the specific capacity value dramatically reduced to 429 mAh g−1 in the first ten cycles due to irreversible conversion of MoS 2 into Mo and reversible two Na+ insertion. When the current density was enhanced in following cycles, the conversion reaction became noticeably lower and the stability also significantly increased.44 Recently, Hu et al. studied 3D inkjet printing of Li‐ion batteries based on LiMn 0.21 Fe 0.79 PO 4 @C (LMFP) nanocrystal cathodes.45 In their printing, a slurry made of carbon black, LMFP, and poly(vinylidene fluoride) (PVDF) dissolved in N‐methylpyrolidone (NMP) was prepared and extruded by a micronozzle onto an aluminum foil, with well‐controlled width and height of the printed LMFP lines (Figure 4e). The Li‐ion battery was then assembled together with lithium foil, LMFP cathode, and electrolyte. Compared with the batteries with electrodes coated the same materials, the printed battery demonstrated excellent electrochemical performance. For example, at rates of 1–100 C, the gravimetric discharge capacity of a 3D‐printed electrode varied from 161.36 to 108.45 mAh g−1, while that of a traditionally coated electrode changed from 141 to 70 mAh g−1, respectively, demonstrating an improved discharge rate via 3D printing of the electrode. In addition, after 1000 cycles with current rates of 10 and 20 C, the specific capacities of the 3D‐printed electrode were 150.21 and 140.67 mAh g−1, respectively. In contrast, under the same conditions, the specific capacities of the coated electrode only reached 103.38 and 90.64 mAh g−1, respectively (Figure 4f). This research clearly demonstrates the advantages of the 3D‐printed battery over the traditionally battery with coated electrodes and provides helpful guidance for the future design of Li‐ion batteries. In summary, IJP is a relatively new technology for producing low‐cost printed batteries and has the potential for broader applications at an industrial scale. Furthermore, IJP has good multimaterial capability, high material utilization, and exceptional resolution, which are a great merit in printing various designed patterns, beneficial to improving the performance of batteries. The major limitations of IJP are its relatively low printing speed and high requirements for the formulation of inks. Moreover, the printing head is less durable and prone to clogging and damage.

2.4 Direct Ink Writing Direct ink writing has been the most widely used 3D printing method to fabricate batteries due to its advantages of affordable prices, easy operation, material diversity, and maskless process.46-52 This method is based on the extrusion of ink materials at room temperature, and its resolution is determined by the nozzle diameter.53 The printing process begins with preparation of the gel‐based viscoelastic inks with the shear‐thinning property. Then, the designed pattern is produced by extruding a continuous filament of the ink material on the platform. By moving down the stage or up the nozzle, the next layer is then printed on the former layer. This process is repeated in accordance with the model design until the required pattern is completed. In the recent work by the Lewis group, printed Li‐ion batteries with thick electrodes were fabricated using the DIW technique.46 All components in the battery, including the packaging, anode, separator, and cathode, were printed by the DIW method (Figure 5a). The four printing inks were tailored to obtain the desired properties of shear‐thinning and viscoelastic response required for DIW (Figure 5b). To assess the effect of electrode thickness, they studied the areal energy density and areal power density of the battery with different electrode thicknesses ranging from 50 µm to 1 mm, exhibiting that full LFP/Li 4 Ti 5 O 12 (LTO) cells with ultrathick electrodes had higher areal energy density and could retain the areal power density of thin electrodes (Figure 5c). Furthermore, the areal capacity of the fully 3D‐printed battery could reach up to 4.45 mAh cm−2 at a current density of 0.14 mA cm−2. The ability to develop high‐performance Li‐ion batteries in customized forms should have tremendous potential to directly integrate batteries with other 3D‐printed devices. Figure 5 Open in figure viewer PowerPoint 46 2 QDs/GO ink. e) SEM image of the 3DP‐SnO 2 QDs/G porous structure. f) Cycling stability of 3DP‐SnO 2 QDs/G, SnO 2 QDs/G, and SnO 2 QDs' architectures. Reproduced with permission. 52 2 battery. Reproduced with permission. 51 Direct ink writing of microbatteries. a) Optical (left) and schematic (right) images for printing the four kinds of functional inks. b) Apparent viscosity behavior of these four functional inks. c) Areal energy density as a function of areal power density for the batteries with different electrode thicknesses. Reproduced with permission.Copyright 2018, Wiley‐VCH. d) Optical image of the different patterns printed with the SnOQDs/GO ink. e) SEM image of the 3DP‐SnOQDs/G porous structure. f) Cycling stability of 3DP‐SnOQDs/G, SnOQDs/G, and SnOQDs' architectures. Reproduced with permission.Copyright 2018, Royal Society of Chemistry. g) Schematic illustration of a novel designed cathode. h) Specific power as a function of specific energy of conventional supercapacitors, Li‐ion, Li–S batteries, and the fabricated 3D‐printed Li–Obattery. Reproduced with permission.Copyright 2018, Wiley‐VCH. Most recently, Zhang et al. employed DIW to print 3D electrodes with SnO 2 quantum dot (QD) inks.52 A sol–gel approach was used to prepare a mass of monodispersed and ultrafine SnO 2 QDs of 2–4 nm size, which were then mixed with graphene oxide and water to yield ink for DIW. Various complex architectures such as zigzag lines, spiral rectangles, periodic microlattices and mosquito coils were printed without clogging issues (Figure 5d), confirming the capability of the ink to print 3D features. The filaments possessed substantial micropores formed in the freeze‐drying process (Figure 5e), and the 3D‐printed SnO 2 QDs/graphene electrodes exhibited an ultrahigh charge capacity of 991.6 mAh g−1 at 50 mA g−1 in the first cycle and the reversible capacity extended up to 1004.9 mAh g−1 after 50 cycles (Figure 5f), indicating an excellent cycling stability and superior performance over pure SnO 2 QDs. Intensifying attention has also been directed to Li–O 2 batteries in recent years due to their remarkable theoretical specific energy (3505 Wh kg−1).54 However, the practical specific energy density and electrode capacity of Li–O 2 batteries are not meeting the application requirements. To address this issue, Lyu et al. employed DIW to fabricate a novel metal–organic framework (MOF)‐derived hierarchically porous framework for Li–O 2 batteries.51 The printed hierarchically porous network was composed of the micrometer‐scale pores generated among Co‐MOF‐derived carbon flakes and the meso‐ and micropores within the flakes.51 The novel structure allowed the deposition of Li 2 O 2 particles inside the framework and promoted the decomposition of these insulating Li 2 O 2 particles to improve the electrochemical performance of a Li–O 2 battery (Figure 5g). At a current density of 0.05 mA cm−2, the discharge capacity of the 3D‐printed cathode reached up to 1124 mAh g−1. From the Ragone plot for the cells with the 3D printing Co nanoparticles assembled in nitrogen‐doped mesoporous carbon flake (3DP‐NC‐Co) cathode, the specific energy of the Li–O 2 battery was as high as 798 Wh kg−1, far beyond that of the well‐established Li–S batteries (<500 Wh kg−1) and Li‐ion batteries (<300 Wh kg−1) (Figure 5h), albeit at a lower maximum in specific power.51 Li et al. proposed a novel multiscale process to fabricate high‐performance Li‐ion batteries via DIW combined with an applied electric field treatment.55 They studied the physical properties and battery performance of the paste with different solid loadings (SLs). The as‐printed electrode was further cured by applying an electric field to regulate its internal structure for achieving the so‐called “chain effect,” leading to the improvement of electrode surface (200%) and battery performance (7%). Finally, a macro–microcontrolled 3D structure was fabricated and demonstrated superior properties and advantages than those structures with randomly distributed materials. In summary, DIW is a commonly used technique in the fabrication of 3D‐printed batteries. By optimizing printing parameters, its resolution can reach down to 1 µm.56 However, DIW has high requirements for the gel‐based viscoelastic inks, requiring sufficiently high yield stress and storage modulus. Moreover, the weak mechanical strength between the layers is a burning problem needing to be solved. Thus, greater efforts need to be taken to improve the application of the DIW technique in battery manufacturing.

2.5 Fused Deposition Modeling Fused deposition modeling has been one of the most widely used 3D printing technologies to manufacture complex objects with almost no material waste. During the printing process, the thin filaments of thermoplastic materials are first heated at the nozzle head with resistive heaters to reach their semiliquid state, after which they are extruded and solidified immediately onto the build platform to get a cross section according to the sliced model design. The thermoplastic materials used by FDM include polylactic acid (PLA), polycarbonate (PC), PS, acrylonitrile butadiene styrene (ABS), polyphenylsulfone (PPSF), and polyamide (PA). The main advantages of FDM are its user friendliness, affordable prices, high speed, large size capabilities, and avoidance of chemical postprocessing. Wei et al. used this method to print graphene composite structures for the first time.57 In their work, graphite was first oxidized and exfoliated into GO sheets, then dissolved in NMP with ABS polymer and mixed using a homogenizer. This formulation was then followed by the chemical reduction of GO to form G–ABS dispersion.57 To enable compatibility with FDM printing heads, the graphene–polymer composites were generally extruded into filaments. As demonstrated in Figure 6a, the graphene–polymer composite filaments were first melted in the FDM 3D printer nozzle and then extruded onto a heated platform (80 °C) layer by layer to fabricate conducting 3D objects.57 Owing to the high accuracy and controllability, the filaments can be printed with an FDM printer to manufacture various conductive 3D models (Figure 6b), which have potential applications in printed batteries. Figure 6 Open in figure viewer PowerPoint 57 58 59 Fused deposition modeling (FDM) manufacturing of Li‐ion batteries. a) Schematic illustration for the printing process of FDM. Inset: The graphene‐based filament used in FDM. b) 3D objects printed via FDM. Reproduced with permission.Copyright 2015, Springer Nature Group. c) Optical image and d) SEM image of a homogeneous filament with 40% PEGDME500. e) Optical image of a high‐resolution complex “3DBenchy” boat. Reproduced with permission.Copyright 2018, American Chemical Society. f) Decomposed assembly of a 3D‐printed coin cell. g) 3D‐printed glasses with an electronic darkening LCD lens and 3D‐printed batteries integrated into the side temples. h) 3D‐printed bangle battery lighting up an LED. Reproduced with permission.Copyright 2018, American Chemical Society. Maurel et al. reported a 3D‐printable graphite/PLA filament, which can be used to print negative electrodes for Li‐ion batteries through an FDM 3D printer.58 In this work, a homogeneous negative electrode disk was printed at a thickness of ≈250 µm (Figure 6c). They found that 40% plasticizer (PEGDME500) mixed into the PLA matrix can effectively improve the ductility and reduce the stiffness of the filament. The graphite grains were homogeneously dispersed within the PLA polymer matrix (Figure 6d).58 With a high graphite and carbon black loading, an unprecedented reversible capacity of 200 mAh g−1 after six cycles at a current density of 18.6 mA g−1 (C/20) was obtained from a printed negative electrode disk.58 The printability of the filament was verified by printing a high‐resolution complex 3DBenchy boat (Figure 6e). The integration of different 3D‐printed electronics with various shapes and volumes together with conventional batteries is a challenging task and has limited the development of new power‐independent electronic systems. The direct 3D printing approach for batteries is able to facilitate the creation of such kinds of systems. For example, Reyes et al. developed a fully printed and customized Li‐ion battery with an FDM 3D printer to accommodate a given product design.59 By screening nine combinations of carbonate solvents and three electrolytes for commercial lithium‐ion batteries, they successfully improved the ionic conductivity of PLA to 0.085 mS cm−1, about four times higher than the previously reported values.60 Remarkably, they also fabricated a 3D‐printed full cell including anodes, cathodes, separators, current collectors, and cases (Figure 6f). Furthermore, they integrated the full printed batteries with a pair of 3D‐printed liquid crystal display (LCD) sunglasses (Figure 6g) and an LED bangle (Figure 6h) to demonstrate the capability of 3D printing to produce customizable wearable electronics. At present, FDM has become the most common 3D printing technique and has been widely used in many fields. However, there are still many problems for its application in the fabrication of batteries. Different from other techniques, the electrochemical active materials must be mixed with thermoplastics and extruded to be a filament for subsequent printing, which severely increases the overall procedural complexity and manufacturing time. Moreover, the thermoplastic filaments need to be heated to their glass transit state with high temperature. In addition, the printing resolution of FDM is usually low, especially along the Z‐axis, remaining typically in the range of 50–200 µm,13 which will affect the printing accuracy and controllability.