Role and advancement of 3D printing in energy storage devices

Throughout the 1980s and 90s, multiple patent applications were filed for a device that could create three-dimensional objects from metallic, and/or polymeric materials. These patents laid the groundwork for the stereolithography (STL) file format, as well as many different types of 3D printers.¹  In 2009, the first of these patents expired, with fused deposition modeling (FDM) printers becoming available to the consumer market. A diagram of an FDM printer can be seen below in Fig. 1.² 

By 2016, the patents for stereolithography (SL), selective laser sintering, (SLS), and selective laser melting (SLM) also ended, making them widely available as well. The end of these patents led to a massive advancement in additive manufacturing technology. As such, it is highly likely that the 2020s and beyond will be a verifiable golden age of 3D printing. Reflectively, the creation of specialty filaments,³  multi-material printing,⁴  and reactive 4D printing⁵  are all developments that have arisen due to the newfound availability of 3D printers. The most important prospect to arise from the new age of 3D printing is the low cost of introductory model printers, with prices ranging around $200 USD being available for purchase as of 2023. With prices rivaling that of high-grade laser and inkjet printers, many sectors have discovered a newfound ability to take advantage of this technology.

The most basic sector that takes advantage of 3D printing technology is in the hobbyist field. Here 3D printing is treated as a crowd-sourcing activity, with ideas being created and shared between hobbyists. While a good source of fresh print designs, many of the innovations made at this level are often in response to new research. With the hobbyist field remaining a mere curiosity, different fields take up the slack in developing 3D printing into a useful tool. In manufacturing, 3D printing allows for the rapid creation and modification of prototypes. Such prototypes can be tested and modified multiple times, then later cast to create functional molds. 3D printing can also allow for the creation of parts that would normally require hours of machining, saving volumes of time and energy. In the biomedical field, 3D-printed scaffolds and struts are interesting due to the ability to print them with biocompatible materials.⁶  The creation of custom implants and prosthetics are also attractive uses for 3D printing in this field. The transportation industry can benefit from the lightweight objects that result from FDM printing. In fact, there are a variety of vehicles being built and developed that rely on 3D printing for their construction.⁷  Additive manufacturing has even found application in more esoteric fields, such as the printing of food and clothing.^8,9 

One sector that could be massively improved by the addition of 3D printing is the energy sector. The energy sector is indisputably important in modern society for a wide range of reasons. Cities rely on their power grids to supply homes, businesses, and industries with electricity. This electricity spreads to power a variety of devices, from larger HVAC utilities to personal computers and phones. With such a wide range of applications available, additive manufacturing lends use to solving a few of these problems. These issues can be divided between energy generation, energy transportation, and energy storage. Energy generation refers to the point source where electricity is created. This field can pertain to large turbines, power plants, and fuel cells. Energy transportation has to do with the grid-scale powerlines that connect to buildings, and to a lesser extent, the energy stored and moved via physical objects. An example of the generation and transportation of energy can be seen below in Fig. 2.¹⁰  Energy storage is perhaps the most promising avenue for research and development. Energy storage involves both the accumulation of excess energy and the storage of the said energy for later. Common forms of energy storage include batteries, supercapacitors, and fuel cells. While different in mechanism, each of these devices usually has similar parts in their construction. The housings, electrodes, and separators can all benefit from the additive manufacturing process.

Considering the information above, this chapter aims to explore the role of 3D printing in creating, improving, and exploiting energy storage devices. This will begin with an in-depth look into different energy storage technologies. Of particular interest are portable energy storage options, such as batteries and supercapacitors. Fuel cells are also of interest due to their unique method of energy storage and generation. Furthermore, the challenges and limitations of these devices will be explored, alongside the potential solutions additive manufacturing can provide. Afterward, the fundamentals of 3D printing will be elucidated, alongside a more in-depth explanation of its history and application. From here, the improvements to energy storage, costs, time efficiency, and other factors granted by the 3D printing process will be discussed. Finally, advancements in both technologies will be discussed, with the integration of the two, the challenges faced, and future perspectives of the technology all falling under examination.

When looking at the energy storage technologies currently in use they typically fall into one of two vast categories, grid-scale and portable. Grid-scale energy storage focuses on storing vast amounts of energy through large mechanisms. These mechanisms can include storing energy physically via water pumping or air compression, or chemically through various battery types. When comparing portable scale to grid scale there is a noticeable difference. Portable energy is typically stored for smaller-scale applications like vehicles, electronic devices, and power tools. All these things may share a common energy storage type, but they feature different energy needs. For example, when comparing the batteries used in electronic devices to those used in power tools there is one crucial difference. The difference is that power tools require a high-power output compared to electronic devices which typically require a steady output of power over time. Each of these two examples requires a completely unique style of energy storage, but there is a commonality between these two examples, and that is the concept of power density versus energy density. This concept is illustrated below in Fig. 3.¹¹ 

In reference to the figure above it can clearly be seen that supercapacitors tend to have higher power densities and lower energy densities. Aside from having excellent power density supercapacitors also offer faster charge and longer life cycles when compared to other storage devices such as batteries.^12,13  Supercapacitors can fall into three distinct types depending on their function. The first is the electrical double-layer capacitors (EDLCs). This capacitor type works by electrostatically adsorbing electrolyte ions onto the surface of electrode materials without transferring the charge. The next type of capacitor is the pseudocapacitor. They function by using fast and reversible redox reactions on the surfaces of the electrodes. The final type of supercapacitor works by combining the characteristics of an EDLC with the characteristics of a pseudocapacitor. This combination creates what is known as a hybrid capacitor.¹² 

As an energy storage device, supercapacitors can be fabricated using a variety of materials. One of the most common materials used for supercapacitors is metal oxides, more specifically, transition metal oxides. Transition metal oxides are so often chosen due to their desirable properties which include high specific capacitance and low cost, and sustainable source.¹⁴  Despite having several desirable characteristics transition metal oxides suffer the drawback of low conductivity. Other materials commonly used in the fabrication of supercapacitors are carbon-based materials such as graphene oxide, carbon nanotubes, and activated carbon.¹⁵  Overall, as a material class carbon has a plethora of desirable characteristics including immense specific surface area, well-developed porosity, favorable conductivity, mechanical stability, and sustainability.¹⁶  Because of these characteristics, carbon has established itself as one of the premier materials for energy storage.

As an energy storage device, supercapacitors suffer from one major defect and that is energy density. The main reason for this defect is the electrolyte that most commercially available supercapacitors use. Most commercially available electrolytes only have a working voltage window of ∼2.7 V at room temperature with the window decreasing proportionally with temperature. This means that commercial supercapacitors have a limited maximum thermal operating voltage leading to lower energy densities.¹⁷  One such way researchers are attempting to fix this issue is by using 3D printing to fabricate thick electrodes inspired by wood. These electrodes feature a macroporous structure, which promotes the diffusion of electrolytes into the electrode. This improves the transfer of ions and greatly improves the energy density of the supercapacitor.¹⁸  These electrodes also provide several other advantages outside of improving energy density and these can be seen below in Fig. 4.¹⁸ 

Moving on from supercapacitors, batteries are energy storage devices that form a balance between power and energy density. Batteries function using a simple process that starts at the anode (negative terminal) electrons are produced through an oxidation reaction. These electrons are then accepted by the cathode (positive terminal) and used in a reduction reaction. Between these two reaction sites, an electrolyte helps to facilitate the diffusion of ions.

Several materials can be used for the fabrication of batteries. One commonly selected material is lithium. As a material, lithium offers excellent characteristics such as excellent reversible capacity, high energy density, long service life, and wide operating temperature.¹⁹  However, lithium does suffer from some drawbacks including an extremely limited supply, and detrimental environmental impact.²⁰  Because of these issues, researchers are working to replace lithium with more sustainable materials. Once such material is zinc, in recent years zinc–air batteries has become one of the most promising materials to replace lithium-ion batteries. This is due to high energy density, safe operation, long shelf life (when sealed), and sustainability. However, these batteries suffer from sluggish reaction kinetics that limit their overall efficiency.²¹  One final material used in the fabrication of batteries is potassium. Potassium-ion batteries have several characteristics that set them apart from other materials, and these are high abundance, the possibility to use graphite at the negative electrode, aluminum as the current collector, and high redox potential. However, these batteries do suffer from low specific energy, cycle life which limits their applications.²² 

One way 3D printing has helped to improve batteries is by re-designing the current collectors used in batteries. These printed current collectors are found to help increase the rate performance and cycle life of lithium metal anodes helping to increase the viability of lithium batteries in long-term applications.²³  3D printing can also be used to print unique battery housings. Since 3D printing allows the creation of highly customized parts and the printing of unique housing batteries. This would allow batteries to be used in a wider variety of unique applications.²⁴ 

Trending up towards higher energy densities, fuel cells are an extremely unique energy storage device. Fuel cells produce and store energy. Since fuel cells can be used to produce energy, they have the potential to be used in a wide range of applications. Because of the broad range of applications, there are several fuels used in these cells. The most common of these fuels is hydrogen, however, methanol, methane, and carbon monoxide are also used.²⁵  Even though these cells all depend on a different fuel source they all function in a relatively similar manner. In the case of a hydrogen cell, hydrogen is then pumped into the cell near the anode. The hydrogen atoms then undergo an oxidation reaction to form electrons and hydrogen cations. These electrons travel to the cathode to complete the electron flow. Meanwhile, at the cathode oxygen is pumped and undergoes a reduction reaction to produce oxygen anions. These anions then bond with the hydrogen cations to form water as a by-product.²⁵ 

Fuel cells use a variety of different materials outside of those used as fuel sources. One such material used is platinum. The main reason for this is the abundant active site allowing it to act as a catalyst helping to induce the oxygen reduction reaction (ORR). However, platinum is an extremely rare metal making it enormously costly to use.^26,27  Another material used in fuel cell applications is nickel. It plays a crucial role in the anode as it is an extremely effective catalyst in the hydrogen oxidation reaction (HOR) in hydrogen fuel cells. This is due to its ability to absorb the energies of HOR intermediates. However, when used in other fuel cell types it is relatively ineffective, but when alloyed with other metals such as iron, copper, and cobalt it experiences increased efficacy.²⁸ 

Currently, 3D printing for fuel cell applications is a relatively new idea and has hardly been explored. However, it has the potential to completely revolutionize energy storage. One method that is under investigation, is the integration of 3D printing into fuel cells by the additive manufacturing of ceramic electrolytes. These electrolytes have improved mechanical properties while keeping the desired electrochemical properties intact. This expands the horizons of fuel cells as it allows them to be used in more rugged applications.²⁹ 

3D printing, also known as additive manufacturing, is a manufacturing technique that fabricates an object layer-by-layer.³⁰  3D printing use in manufacturing has only come about in the last few years as the technology was recently devised about 50 years ago. The first 3D printing technology was discovered by Charles Hull from his work in fabricating plastic devices from photo polymers. While working on these devices, Hull noticed finished devices had high rates of design imperfections accompanied by a lengthy fabrication process. Wanting to improve on the imperfections, Hull developed the technology for stereolithography in 1886.¹  In that same year, Hull also developed the .STL file to help bridge the gap between computer-aided design (CAD) software and printers.¹  Following the discovery of these technologies in 1988 a patent for SLS printing technology was filed by Carl Deckard. This printing technology was unlike anything else seen at the time as instead of resin SLS printing used powders fused by a laser rather than resins.³¹  After the establishment of the SLS technology the technology for FDM printing was finally patented. This technology was patented by Scott Crump in 1990 and helped to make it known that 3D printing was poised to take manufacturing and research by storm.

All types of 3D printing typically follow the same basic steps. The first of which is the creation of what is to be printed. This typically takes place in CAD software which allows the user to create a complex 3D model of what they wish to print. The next step in the process is the slicing of the model. This is where the model is converted into a file the printer can read. This file is typically a .STL file however, there are other file types that can be used such as .OBJ and .PLY. Once these files are uploaded to the printer the process can begin. How the actually accomplished printing varies from printer to printer. With an FDM printer, the process involves feeding a spool of filament into an extrusion nozzle. This nozzle then heats the material and extrudes it layer by layer onto the build plate. Now this can look different depending on what printer is used. Some may have the nozzle move along the XYZ axis to print, others may have the plate and head move along a specific axis. When looking at the materials used in this type of printing, polylactic acid (PLA) filament is the most common. However other materials used include polycaprolactone (PCL), polypropylene (PP), polyethylene (PE), polybutylene terephthalate (PBT), and acrylonitrile butadiene styrene (ABS). Furthermore, more complex printer materials such as wood, carbon fiber and metals can be used in printing.²  When choosing an SLA printer, the print process involves loading the photopolymer resin into a basin. The printer will then lower the build plate into the resin. Once the plate is in the resin, a laser is then directed onto the surface of the resin to build the object onto the plate.³²  A diagram depicting an SLS printer can be seen below in Fig. 5.¹ 

When compared to the previous printing methods SLS printing has a noticeable difference. Instead of using filament or resin SLS printers use powders to print objects. This is accomplished by spreading a powder onto the basin of a printer. The printer will then heat the powder close to the specific powder’s melting point. Once the powder is heated the printer will then direct a laser onto the powder to melt it into the shape of the object to be printed,³³  as seen below in Fig. 6.³⁴ 

With the flexibility inherent in a 3D printer’s varying methods and materials, it comes as little surprise that they have established an important position in various industries. One industry that has taken full advantage of 3D printing is the medical industry. Since the 3D printing processes are all highly customizable, 3D printing enjoys a variety of applications inside the medical industry. These applications include the fabrication of drug-delivery devices and bio-printing for synthetic skin, heart valves, and orthopedic implants.^35–37  Another industry that 3D printing has established a role in, is the energy sector. 3D printing in the energy sector is most often used in the fabrication of electrodes. This is because 3D printing allows greater control over characteristics such as structure, rigidness, porosity, and size, as well as using specific printing techniques for the use of various precursors.³⁸  Aside from electrodes, 3D printing also can be used in the fabrication of electrolytes. With the push for flexible electronics gaining increasing support, the 3D printing of hydrogels for application as an electrolyte has found increased attention This is due to the extreme flexibility and high porosity that these hydrogels possess.^39,40 

As 3D printing technology continues to advance, 3D printers have become a focal point in the fabrication of devices for electronic applications. Examples of these devices include but are not limited to electrodes for various electrochemical devices, electrolytes for these devices, and even the housing for batteries.⁴¹  These devices can be fabricated using more basic forms of printing such as FDM, SLA, and SLS. However, there are more distinct forms of printing that are emerging. These methods include direct ink writing (DIW), inkjet printing, and selective laser melting (SLM).⁴²  3D printing has presented researchers with the ability to produce devices with unique architectural advantages. With 3D printers allowing for a great deal of creativity, energy storage devices can be fabricated into almost any shape possible. This can help to tremendously improve the capabilities of wearable and flexible electronics, as geometry will not be a limiting factor in their fabrication.⁴³  Another architectural advantage provided by 3D printers is the ability to produce electrodes with high surface area and porosity. These features are desired because they promote the infiltration of the electrolyte into the active material, which in turn increases the capacitance of the capacitor.⁴⁴  This is illustrated in a study conducted by Tagliaferri et al. where micro-supercapacitors with high capacitance were printed using DIW printing in combination with graphene ink.⁴⁵  To accomplish this, the ink used was first synthesized by dissolving sodium carboxymethyl cellulose in DI water. Graphene platelets were then subsequently added to the solution. This solution was then homogenized using a planetary mixer, after which 1-octanol was added stepwise into the solution. This ink was then loaded into 3 mL polypropylene syringes with blunt nozzles and mounted onto the three-axis stage of the 3D printer, then connected to displacement-controlled plungers. The electrodes were then printed onto graphite foil.⁴⁵  The resulting electrodes were then air-dried overnight and treated in a tube furnace. The filament used to produce the electrodes was then analyzed to determine the porosity. The filament was scanned using X-ray nano-computed tomography. The resulting data was then used to calculate porosity, wherein the electrodes produced featured a high porosity with an average pore size of ∼1.4 µm. Furthermore, micro-supercapacitors were fabricated using the printed electrodes alongside a LiOH gel electrolyte. Testing of these devices demonstrated that they possessed a capacitance of about 1.57 F cm². This is a great improvement compared to other, non 3D printed electrodes which demonstrate lower capacitance values.⁴⁶ 

Another area where 3D printing has shown electrochemical properties is in the electrodes of batteries. In a study published by Wang et al., Lithium-ion batteries with high areal energy and power density were fabricated using 3D-printed electrodes.⁴⁷  This was accomplished by first mixing LFP nanoparticles, acetylene black (AB), and multiwall carbon nanotubes. This mixture was subsequently ground together. Following this, PVDF-HFP powder was dissolved in 1200 mg 1-methyl-2-pyrrolidinone. This was then added to the powder as a binder and mixed to obtain an LFP paste for printing. The paste was then loaded into an FDM printer, and electrodes varying from two to eight layers were printed. An illustration of this process can be seen below in Fig. 7.⁴⁷ 

The resulting electrodes were then tested in 2032 coin cells with Li foil as a counter and reference electrode and 1 M LiPF₆ dissolved in a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate as an electrolyte. These batteries were then tested using a LAND test system in the voltage range of 2.5–4.2 V to evaluate the galvanostatic charge–discharge performance. After testing, the eight-layer electrode was determined to have superior electrochemical properties as it produced the highest areal capacity, power density, and energy density which had values of ∼7.5 mA h cm⁻², ∼2.99 mW cm⁻², and ∼69.41 J cm⁻² respectively. These values are comparable to electrodes fabricated through other methods showing how these printed electrodes have great potential for future applications.⁴⁷ 

Aside from improving the electrochemical properties of energy storage devices, 3D printing is helping to reduce the overhead cost of component fabrication for energy storage devices. One of the first ways 3D printing helps the cost in the manufacture of energy storage devices is low machine and material costs. In recent years the cost of 3D printers has decreased tremendously since they first came on the market. Subsequently, as the cost of printers decreases, the cost of materials decreases as well. This in turn opens the possibility of 3D printing to more companies. Along with cost efficiency, 3D printing also helps with time efficiency. Since 3D follows a relatively simple process, printers can print extraordinarily complex and unique parts in a short amount of time. 3D printers also allow for the simple creation of prototypes extremely fast, at a low cost, which can save manufacturers vast amounts of money and time.

As 3D printing technology advances, researchers have devised methods for the possible use of advanced materials in the fabrication of energy storage devices. Because of the limitations of the current 3D printers, the fabrication of liquid-metal embedded elastomer-based electric devices exclusively relies on the use of stencil lithography and screen printing.⁴⁸  However, recently researchers have broken the barrier and produced liquid-metal embedded elastomers that can be used in a 3D printer. In a study published by Won et al., a process for producing 3D printable liquid-metal embedded elastomers with high electrical conductivity was devised.⁴⁹  This was accomplished by first mixing a eutectic gallium indium alloy with polydimethylsiloxane in an overhead mixer to produce a liquid-metal embedded elastomer (LMEE). This LMEE ink had a volume of 70% liquid metal as this was determined to have the most uniform dispersion of the metal throughout the ink. This ink was then loaded into a DIW printer to print a usable electrode. After the print, the electrodes then underwent an additional activation process to improve the electrode’s conductivity. This was done by placing a PVA-based hydrogel over the electrode. The gel was then compressed onto the electrode which applied internal stress breaking the droplets of liquid-metal inside the elastomer in turn activating the conductivity within the LMEE. A more detailed description of this activation process can be found below in Fig. 8.⁴⁹  These activated electrodes were shown to have an electrical conductivity as high as 6 × 10⁴ S cm⁻¹. This high conductivity shows how with some refinements these 3D printed electrodes could have a future place in energy storage devices.

In addition to the materials, advances in 3D printing have revealed new architectures that can be used to improve the electrochemical properties of energy storage devices. In a study by Muñoz-Perales et al., electrodes with a lung-inspired flow field design were successfully able to improve the electrochemical properties of the electrode.⁵⁰  These electrodes were printed using a clear acrylate-based UV-curing resin through the SLA printing process. After printing and curing the electrodes were then coated with a conductive silver paint. Following this, the painted electrodes were sputtered with a platinum coating.⁵⁰ 

These electrodes then underwent extensive electrochemical analysis. First, the mass transfer resistance of the traditional electrode was tested and found to be 0.5 Ω cm² (at ve ≈ 0.5 cm s⁻¹) to 0.25 Ω cm² (at ve ≈ 5 cm s⁻¹). Both the 2L and 3L electrodes were found to have mass transfer resistance less than the traditional electrode. To further analyze the electrodes the current density of each was analyzed. At all electrolyte velocity conditions (0.5, 1.5, 3.5, and 5 cm s⁻¹). At the highest velocity, the 2L electrode had a current density of 350 mA cm⁻² while the traditional electrode had a current density of 200 mA cm⁻². These values were all found at 50 mV of iR_Ω-corrected potential. As for the 3L electrode at this highest velocity, it produced a current density of about 250 mA cm⁻². Even though at the highest velocity it outperformed the traditional electrode at lower velocities it performed the worst. The reason for this is most likely at lower velocities the flow rate is not high enough to completely fill all the channels of that electrode. Overall, though in all the testing the 2L electrode performed this best. This is most likely due to the 2L electrode having a larger electrode reaction volume and the lowest mass transfer resistance.

As 3D printing continues to rise in the energy sector it is becoming increasingly popular to combine printing with other manufacturing techniques. A technique that is often used is the 3D printing of a precursor to an electrode. A great example of this is the fabrication of 3D-printed hydrogel that is then supercritically dried to form an aerogel.⁵¹  This was done in a study by Chandrasekaran et al. where graphene oxide–MoS₂ hydrogels were fabricated using the previously mentioned process. These electrodes featured an extremely high surface area and high HER activity when compared to a non-3D printed graphene aerogel.⁵²  Another manufacturing technique that is often combined with 3D printing is the postproduction coating of fabricated electrodes. This coating can be put on in several ways, but the two most common ways are paint coating or electrodeposition.^53,54  In a study by Hüner et al., a commercially available conductive graphene-based PLA filament was used to 3D print electrodes. These electrodes were then coated with nickel and cobalt via electrodeposition. These electrodes were then tested and found to have much improved electrochemical properties compared to the 3D printed electrodes using unmodified filament.⁵⁵ 

Since 3D printing can continually replicate electrodes with extreme accuracy and precision multiple 3D printers could be used to mass produce one type of complex electrode.⁵⁶  This would go perfectly with horizontal scaling as 3D printers could be used to fabricate complex electrodes while other manufacturing methods could be used to fabricate the other components of a device. This would save manufacturers time as by employing multiple methods of manufacturing, all the components of one device can be fabricated at the same time. Manufacturers could also use vertical scaling for 3D printers. An example of this would be starting with a FDM printer for simple manufacturing, but then as the manufacturing gradually becomes more and more complex you phase out the FDM printer and replace it with a more advanced printer such as an SLS machine. Despite the ability of 3D printers to be scaled vertically and horizontally they still are unable to compete with machines that can manufacture high volume parts at one time.

There are several challenges associated with 3D printing, with a primary factor being the limited number of conductive materials that can be used. This fact is extremely prevalent when using FDM and SLA printers. This is due to the fact that they typically print using thermoplastic materials that are non-conductive and require postproduction processing to make them viable.^53,57  In the case of SLS printers, the issue of conductivity is not relevant as they can be metallic materials. However, the cost of the machine and materials is extremely high. Another challenge that is facing 3D printing is its effectiveness on a mass production scale. In comparison to modes of fabrication, 3D printing has a significantly lower up-front cost. Meaning that up front in mass scale production 3D printers may cost more, but in the long run yield a much lower return on investment. This is because a printer can only make one part at a time. Whereas an injection molder can continually produce several parts at a time. Because of this fact 3D printers will experience better results in smaller scale applications rather than mass scale.

Despite these challenges, there is one area of 3D printing that has an extremely bright future, and that is the realm of DIW printing. This is due to factors including a simple printing process and high fabrication efficiency.⁵⁸  Another reason is the wide range of materials that can be used in the printing process. This allows DIW printers to be exploited in a broad range of electrochemical applications. An image showing some of these materials and applications can be seen below in Fig. 9.⁵⁸ 

Given that the energy needs of the world are vitally important to everyday life, the need for new technology to help improve current technologies is more vital than ever. Because of this 3D printing technology has arisen as a possible solution. When using 3D printing, there is a wide variety of printing processes, designs, and materials that can be used to fabricate components, feeding into the idea that 3D printing allows for the extreme customization of components that perform better than components that were not 3D printed. Additionally, as the technology for printing continues to advance, new component architectures are more easily printed fixing the problems that certain energy storage devices suffer from. Along with architecture, more advanced printing technology has enabled the use of complex materials. These complex materials help to push the capabilities of current energy storage devices to the limit. Even though printing technology is advanced enough to experiment with these complex architectures and materials they are not widespread enough to be used commercially. This is why the future of 3D printing of energy is extremely focused on the capabilities of DIW printing as it has the brightest future in the application of energy storage.