Screen-Printed Nickel–Zinc Batteries: A Review of Additive Manufacturing and Evaluation Methods

Abstract

The advent of personalized wearable devices has boosted the demand for portable, compact power sources. Compared with lithographic techniques, printed devices have lower fabrication costs, while still maintaining high throughput and precision. These factors make thick film printing or additive manufacturing ideal for the fabrication of low-cost batteries suitable for personalized devices. This article provides comprehensive guidelines for thick-film battery fabrication and characterization, with the focus on printed nickel–zinc (Ni-Zn) batteries. Ni-Zn batteries are a more environmental-friendly option compared with lithium-ion batteries (LIBs) as they are fully recyclable. In this work, important battery fundamentals have been described, especially terms of electrochemistry, basic design approaches, and the printing technology. Different design approaches, such as lateral, concentric, and stacked, are also discussed. Printed batteries can be configured as series or parallel constructions, depending on the power requirements of the application. The fabrication flow of printed battery electrodes for the laboratory-scale prototyping process starts from chemical preparation, mixing, printing, drying, pressing, stacking to finally sealing and testing. Of particular importance is the process of electrolyte injection and pouch sealing for the printed batteries to reduce leakage. This entire process flow is also compared with industrial fabrication flow for LIBs. Criteria for material and equipment selection are also addressed in this article to ensure appropriate electrode consistency and good performance. Two main testing methods cyclic voltammetry for the electrodes and charge–discharge for the battery are also explained in detail to serve as systematic guide for users to validate the functionality of their electrodes. This review article concludes with commercial applications of printed electrodes in the field of health and personalized wearable devices. This work indicates that printed Ni-Zn and other zinc alkaline batteries have a promising future. The success of these devices also opens up different areas of research, such as ink rheology, composition, and formulation of ink using sustainable sources.

Introduction

The advent of personalized internet of things (IoT) has increased the demand for wearable devices that can capture real-time data from the human body and its surroundings. This growing need, coupled with the recent technological advancements in fabrication, has driven the miniaturization of electronic devices.¹ The emerging field of flexible and stretchable electronics provides new opportunities for novel devices, such as sensors,^2–4 photovoltaics,⁵ paper-thin displays,⁶ in vitro electronics,⁷ e-textiles,⁸ optics,⁹ and soft robotics.¹⁰ Future market trends are expected to focus more on wearable devices for applications in health care, entertainment, and sports.¹¹

This rapid development and extensive use of mobile electronics has created a growing demand for reliable, cost-effective, portable, and independent power sources. The drive for miniature, portable, and personalized electronics will cause conventional bulky batteries to be no longer applicable due to small form factor constraints. These requirements have opened a new research and business opportunity for all the battery manufacturers and researchers around the world to pursue. The market for thin batteries is predicted to grow by up to $471m in 2026.¹

The thin battery, like its bulky counterpart, is an electrochemical system that consists of one or more electrochemical cells connected, whether in series or parallel or both.¹ This technology enables most of the currently available electronic devices to be portable and wirelessly connected to the power sources without the need for a cable to connect to the wall plug. Batteries are typically categorized into two main categories, which are non-rechargeable or rechargeable batteries. Unlike the non-rechargeable battery, a rechargeable battery allows the user to repeatedly charge and discharge the battery without the need to dispose and replace with a new battery. Wearable formats of batteries are an important area of research as it plays a vital role in the field of personalized IoT.¹¹

Lithography and printing methods have traditionally been the primary technologies used to fabricate miniature two-dimensional (2D) batteries. Lithographic methodologies, such as photolithography, electron beam, and ion beam lithography, fabricate reproducible high-performance devices.¹¹ However, these methods are often expensive as they require clean room facilities, costly chemicals, and labor-intensive processes. Printing techniques, however, have lower fabrication costs while still being able to produce devices on a large scale with the desired precision and accuracy.¹¹ Screen printing and flexography are the commonly used methods for printed batteries,^12,13 especially when customized and unique patterns are required. Other methods for battery fabrication include the doctor-blade coating method¹² and slot-die coating,¹³ which are similar to the standard practice for pouch cell battery fabrication. Technological advancements have made it possible to obtain printed batteries that are thinner than a millimeter, lighter than a gram. They can be produced cost-efficiently through a printing process and on a large scale.¹⁴ Companies such as Enfucell, Blue Spark Technologies, Imprint energy, Solicore are among the market players in this field.

A printed battery, also known as a battery cell without casing, is much smaller and thinner compared with coin cells and cylindrical cells.¹ Some printed batteries can also be packaged in a pouch case, especially when it involves aqueous electrolyte. Printed batteries with its thin and small form factors have been considered ideal as portable power sources for emerging applications such as a powered smart card, smart cosmetics, and smart shoes.^11,15

Due to the advantages of using the screen printing for mass production of batteries, this review is written to provide a step-by-step guide for researchers who wish to venture into the field of thick-film battery fabrication and characterization. The Battery Design and Theory section of this article will discuss the design consideration for printed batteries based on particular applications and the design approaches. The Fabrication Methods section discusses the fabrication methods, highlighting important details on battery fabrication such as the overall process flow, the equipment utilized, materials, and issues related to each of the fabrication steps. The Characterization Methods section discusses characterization methods, current and future applications of printed batteries. Finally, the Conclusion section concludes the review and discusses the possibilities of merging both screen-printing and three-dimensional (3D) printing techniques for fabrication of portable batteries to capitalize on the advantages of template-free printing and mass production.

Battery Design and Theory

Currently, rechargeable batteries are the device of choice when considering sustainability and the environmental impact of batteries. The most common rechargeable battery system in the market is the lithium-ion battery (LIB), which has been widely used in smartphones, cameras, and smartwatches; this is followed by nickel–metal hydride (Ni-MH) battery, which is used in some noncompact electronic devices. Alternatively, nickel–zinc (Ni–Zn) battery is also a good option. It has been identified to have excellent high rate discharge capability at a lower cost compared with lithium-ion, and it has a higher nominal voltage compared with Ni-MH. Ni-Zn battery is also reportedly capable in delivering up to 900 cycles at 80% depth of discharge, which can be as competitive as other good rechargeable batteries such as lithium-ion, Ni-MH, and Ni-Cd.

The zinc battery system is also relatively safer, environmentally friendly compared with the LIB.¹⁶ Even though the zinc battery has such advantages over the LIB, there is still a lot more research needed to improve the life cycle for rechargeable zinc batteries. The performance of zinc battery is highly dependent on zinc electrode since most of the issues reported are related to the zinc electrode, such as zinc dissolution, corrosion, shape change, and dendritic growth.¹⁶ This article will generally focus more on a printed rechargeable battery and specifically on Ni-Zn battery.

Figure 1a shows the scheme of a Ni-Zn electrochemical battery. When an external load is connected, the battery discharges. Oxidation occurs at the anode, which then creates excess electrons that flow through the external circuit. The electrons collect at the cathode, where it is reduced. Products of the reduction reaction are negative ions, which are present in the electrolyte and then flow to the anode.¹⁷

FIG. 1.

(a) Illustration of a Ni-Zn battery with ion and electron flow during charging (connected to power source) and discharging (connected to load). (b) General fabrication flow of a printed battery. Ni-Zn, nickel–zinc. Color images are available online.

The charging state restores the balance of the system. In this mode, the battery is then connected to a power source wherein the positive terminal is connected to the cathode and the negative terminal to the anode. Electrons are injected into the anode, replacing those that were lost during discharging; reduction then takes place here. Negative ions at the anode terminal now flow to the cathode, restoring its previous state. Oxidation occurs at the anode forming oxidized species or electrons which flow through the external circuit, creating a current flow.¹⁸

The two modes: charging and discharging are specified for a Ni-Zn battery, as shown in Figure 1a. During charging, the anode is the zinc on top of the negative current collector, whereas the cathode is nickel oxyhydroxide (NiOOH) on top of the positive current collector. In the discharge state, the anode is the zinc oxide (ZnO), whereas the cathode is the nickel(II) hydroxide (Ni(OH)₂). Potassium hydroxide of 3 M to 8 M¹⁶ is used as the electrolyte to allow ionic transportation between the electrodes. A separator is placed between both electrodes to prevent the battery from failing due to short circuit.

During discharge, the zinc (Zn) electrode oxidizes and dissolves into the electrolyte as zincate (Zn(OH₄²⁻)) ions. The intermediate reaction concentrates near the surface of the electrode. The mobile zincate ion will then form into zinc oxide precipitate once the zincate ions are supersaturated in the electrolytes. The electron is released through the load via the current collector and reaches the cathode, which is the nickel oxyhydroxide.^17–19 The reversible anode reactions are summarized in Equations (1) and (2):

Anode reaction:

Next, nickel oxyhydroxide is reduced, and nickel(II) hydroxide will then form after it received the electron from anode.^19,20 The reversible cathode reactions have been summarized in Equation (3):

Cathode reaction:

Equation (4) summarized the overall reversible reaction of the Ni-Zn battery system:

Overall reaction:

The measured nominal discharge voltage may be a bit lower, ∼1.65 V,¹⁶ which is within the voltage discharge window limit of between 1 and 1.8 V. This can be attributed to the variations in the formulation, resistance of the current collector, separator, printing quality, and so on, which may increase the internal resistance of the battery. The factors that have been identified to usually contribute to higher internal resistance are high ratio of nonconductive elements (nonmetals, i.e., polymeric binder) over solid content, low conductivity current collector, bad printing quality due to manual printing, usage of thick separators, low electrolyte molarity, and so on. To further explain this phenomenon, we describe the schematic for the battery's internal circuit in Figure 2. When an electrochemical cell converts chemical energy into electrical energy, an electromotive force (ɛ) is generated ∼1.73 V or more [Eq. (4)]. However, the actual terminal or nominal voltage, V_r, is normally lower than ɛ due to the internal resistance, r of the battery within the electrochemical system. The terminal voltage has been determined to be inversely proportional to the internal resistance, as shown in Equation (5). The current flow, i, and the terminal voltage can be measured using a digital multimeter, as described in Figure 2. In contrast, the charging voltage needs to be higher than the terminal voltage; however, due to internal resistance, the current should be increased so that the charging voltage is above the discharge nominal voltage. This theory is based on the famous Ohm's law (V = I × R).

FIG. 2.

Battery internal circuit.

Design considerations

The design of printed batteries is dependent on what are the specifications required, such as the area limitations, power requirements, capacity, cycle life, weight, ambient temperature and condition, durability under mechanical stress, and thickness based on particular applications. The most common design approaches for printed batteries are stacked or sandwiched, as has been reported by Huebner and Krebs.²⁰ The stacked design has electrodes placed on each other with the electrolyte/separator layer in between, as shown in Figure 3a. For the lateral approach, both the electrodes are placed adjacent to each other, without needing a separator.²⁰

FIG. 3.

Design approaches, electrode layout, and parameters for (a) stacked, (b) lateral, and (c) concentric printed batteries. Color images are available online.

Stacking approach, as shown in Figure 3b, has been identified to be a better option for printed battery due to its lower internal resistance, higher interfacial area, capacity over the area, as well as smaller footprints needed.²⁰ However, a separator is needed in between the electrodes to protect them from short circuiting. Despite the presence of the separator, there is still a risk of having short circuits due to potential dendritic growth of the zinc electrode. However, the lateral approach does not need any separator and thus is a safer method due to lesser short circuit, less cell thickness, and also less printing steps required.²⁰ Gel electrolyte is recommended for this configuration.²¹

The concentric approach (Fig. 3c) is also possible for printed batteries, as reported by Meskon et al.²² For this design, the electrodes surround each other, with a common center point.²² The design can be circular, rectangular, or of any shape. This design approach was reportedly better than the lateral approach since it has more interfacial area, better efficiency, lower possibilities of short circuit, as well as no requirements of having a separator. All the design approaches are described in Figure 3. Further improvements have also been made to the stacked design. For example, rather than using an aqueous electrolyte, researchers have developed a printed solid polymer electrolyte, which has advantages in terms of safety, the mechanical durability of the battery pouch and so forth.²³

Battery configuration and electrical specifications

The electrical requirements of the battery, such as the voltage, current, and discharge capacity for a specific application, are often influenced by the active components that are available on-board, such as liquid crystal display (LCD), light-emitting diode (LED), and microcontroller. Most of the small active electronic components can be powered by typical 3.3–5 V direct current (DC) supply, with currents varying from as low as few microamperes to milliamperes. The operating window time will determine how much minimum discharge capacity is needed to enable the device to work according to the requirements.

Technically, a battery's operating voltage varies based on what kind of battery system is used. For example, lithium-ion, Ni-MH, and Ni-Zn batteries operating voltages are 3.6–3.8 V, 1.2 V, and 1.65–1.7 V, respectively. There are two most common techniques used in increasing the voltage and the discharge capacity of the battery, and these are using either the series or the parallel connection of multiple cells. By connecting two or more identical cells in series, the battery's voltage can be increased. For example, two cells of 1.2 V Ni-MH can produce 2.4 V battery with a similar discharge capacity of a cell.

Connecting the cells in parallel meanwhile increases the battery discharge capacity by multiplying the number of cells with the discharge capacity of a cell. Both nominal voltage and discharge capacity can be increased by applying both series–parallel connections. The three different configurations are illustrated in Figure 4. For example, 2.4 V (two cells) Ni-MH battery can be connected parallel to another set of 2.4 V (two cells) Ni-MH battery. These circuit configurations, however, must be performed within the confines of the specified surface area. In another example, a 14 V open-circuit voltage can be generated by connecting ten 1.4 V printed zinc–manganese dioxide (Zn-MnO₂) alkaline cells in series to power a printed complementary five-stage ring oscillator.²⁴

FIG. 4.

Battery configurations for multiple cells: (a) series, (b) parallel, and (c) series–parallel.

The device's operating and assembly environment also needs to be taken into account during the design. Critical parameters such as temperature, humidity, pressure, and altitude can affect the battery's safety requirements. For example, a LIB is susceptible to atmospheric pressure, humidity, and temperature. Therefore, researchers can switch to other battery systems that still comply with the requirements. Another alternative is to design the circuit in such a way that the battery can be protected from the risk. For instance, in smart card applications, the circuit needs to go through assembly temperatures of up to 130°C to 150°C and high-pressure lamination processes.¹ In other applications, such as biopatches, wrist bands, and flexible displays, the battery needs to be able to withstand bending, flexion, folding, stretch, or any other mechanical stress.¹

Fabrication Methods

Printing has been known as a method of transferring ink onto the substrate. It can be any substrate, such as metal, plastics, wooden, and paper. A few printing methods have already been well established for the fabrication of devices, such as screen printing, flexography, lithography, gravure, and drop-on-demand (DOD).²⁰ All these printing methods have their unique specifications, such as ink's viscosity and maximum particle size.

Screen printing is a widely used method since it is the easiest and can support ink viscosities within the range of 100,000–1,000,000 cPs.^22,25 It can also produce fine-line printing down to 10 μm for laboratory scale,²⁶ with printing thickness within the range of 10–100 μm, which is deemed suitable for printed battery application and most of the printed electronic applications. Thus, most of the commercial conductive silver inks developed are for screen printing despite relatively slower throughput compared with both flexography and gravure printing.²⁷ However, DOD is also a well-known printing method mainly developed by Epson, Hewlett Packard (HP), and Canon, in the late 1970s. The term inkjet has been widely used in the industry, although it was HPs proprietary term used in describing the heated resistor that generates a bubble of vapor, which in turn creates a pressure spike in ink to eject the ink from an orifice.²⁸ There are two primary types of DOD printers; the first one is called bubble-jet printer, which heats up ink inside the nozzle up to its boiling point to form a bubble that will eventually burst, projecting ink droplets on the paper. The second type of DOD printer uses piezoelectric crystals to build up the pressure and shoot out the ink upon release.²⁹ It was Huebner and Krebs who reported that the DOD printer can be used in printing the current collector; however, it requires nanoparticle-sized active material inks so as not to block the nozzle.²² Special techniques such as the electrohydrodynamic's pulsed cone-jet approach that employ reactive inks can be used to eliminate the necessity of nanoparticle-sized inks.³⁰ Even with these specialized techniques, screen printing is often preferred since it is simple, well established, and more practical, especially when involving multilayer printing either for manufacturing or for laboratory scale.

Overall battery fabrication process flow

The process flow for the printed battery is almost similar to the standard industrial practice for lithium pouch battery fabrication with some adjustments in the process flow and in the equipment selection, which is often aligned to the research scale. Manufacturing Technology and Innovation International has defined the process flow for lithium pouch battery fabrication, as shown in Figure 5a. It starts with the electrode slurry preparation (which consists of material preparation, powder processing, powder grinding mill, and vacuum mixing), followed by coating, roll press, all the remaining assembly process, and battery analyzer for charge–discharge. All these processes are meant for commercial mass production, whereby quality and consistency are deemed strictly controlled.

FIG. 5.

(a) Standard industrial fabrication flow for lithium pouch battery fabrication.⁵² (b) Process flow for printed Ni-Zn electrode fabrication for laboratory-scale prototyping.

FIG. 6.

Screen-printing steps for different battery configurations using printed electrolytes: (a) stacked approach and (b) lateral or concentric approach. Color images are available online.

In contrast, for laboratory-scale prototyping, as shown in Figure 5b, some of the processes such as powder processing and grinding can be skipped as well as processes that involve moisture control such as vacuum drying (for lithium, due to humidity sensitivity of the reactive lithium electrolytes and metals) before pouch forming to simplify the overall process and reduce the setup cost. These processes can be skipped as they have been determined to not affect the functionality of the developed battery. Still, having these processes may further improve the consistency, quality, and performance of the batteries.

The pouch forming process has been defined as a process where vacuum forming is applied to the aluminum pouch case to form a pocket. This process allows thicker batteries, such as the multicell battery system, to fit inside the pouch with its electrolyte. In a single-cell system, this process may not be necessary. The roll press and die-cutting process may not be needed if the electrode is printed on the current collector such as silver ink with plastic film as the substrate. Roll press is used to optimally compress the electrode to improve adhesion of the thick high-energy electrode to the substrate, especially when it is printed on top of the metal foil where surface roughness is lower. If a high-energy density electrode is not needed and adhesion is not an issue, then the roll press can be skipped as well. Similar to die-cutting, if the electrode is already printed with a specified pattern, then regular scissor cutting is sufficient. Ultrasonic welding is also deemed unnecessary for the laboratory-scale prototyping as it is only applicable when the current collector is made from metal foil.

The performance of a printed battery is highly dependent on its electrodes. This is because active materials that adhere on top of the current collector are in charge of carrying the battery discharge capacity. The fabrication process for the two different designs, stacked and lateral, varies slightly in terms of the number of printing steps and usage of the separator, as shown in Figure 5a and b. For the stacked design, a separator must be used to prevent a short circuit between anode and cathode.

For the lateral or concentric approach, separators are optional, depending on the type of electrolyte setup used. Lateral and concentric designs that have the electrode soaked directly in electrolyte within a pouch also do not require any separator. Figure 5a and b shows the fabrication steps of a printed battery with printable electrolytes. The electrolyte is usually printed as a link that bridges both the anode and cathode allowing the ions to migrate between both top and bottom electrodes. Solid-state batteries have been proven to be a safer and more flexible battery system since it is less prone to the risk of failure due to leakages and other mechanical stress compared with the typical aqueous electrolyte-based batteries.

Material preparation

Chemical preparation is the most critical part of the whole battery fabrication process. It determines the individual electrode performance, which encompasses both the material preparation and vacuum mixing process. Material preparation starts with materials selection and formulation of ink composition, followed by composition determination. There are four primary components of an electrode ink,²⁰ and these are: (i) electroactive material (EAM) (more than 50 wt%), (ii) binder, (iii) solvent, and (iv) additives.

EAM is the main element of the electrode. It is involved in the redox reaction and carries the discharge capacity. The higher the weight percentage of the EAM, the higher the capacity of an electrode. However, the amount of EAM is also dependent on the other components of the electrode ink. Some researchers prefer to improve the performance of the battery by enhancing the electrolyte with additives so that the EAM wt% of the electrode can be maximized.²⁰

Another vital component of the ink is the binder. The binder physically holds the grains of the active material and additives together after curing or drying by building a kind of matrix that provides mechanical stability to the ink. This helps to improve printability by preventing the solid particles to be embedded in the screen mesh during printing. The amount of binder used has to be optimized as putting a large amount of binder enhances ink adhesion to the substrate but too much of it will increase the internal impedance of the electrode. This can be achieved by via systematic fine-tuning of the ratio of EAM to additives.²⁵

At the same time, the pressure applied by the roll press to the electrode also has to be optimized. The selection of binders plays an essential role in the performance of the electrode. For LIBs, elastomers such as styrene-butadiene rubber²⁶ or any other rubber-based binder are deemed suitable due to its strong adhesion, flexibility, and water solubility. Ni-Zn batteries, however, require chemically resistant binders since they use potassium hydroxide (KOH) as its electrolyte. KOH is a more aggressive electrolyte that causes the binders to degrade faster, thus limiting the battery life cycle. LIBs do not face this problem as it uses lithium salt as its electrolyte.

Taking this into consideration, polytetrafluoroethylene (PTFE) is one of the commonly used binders for Ni-Zn batteries²⁸ mainly because it is both chemical and thermal resistant, allowing it to be stable and not easily dissolved in most of the alkaline-based electrolyte, especially KOH. PTFE, however, has other drawbacks because it is relatively difficult to mix since it has the tendency to coagulate at high temperatures and during long mixing durations, and this disrupts the printability of the ink. This issue can be solved by using the temperature-regulated mixer, using perfluorinated solvent, and applying additives such as dispersants and surfactants to improve mixture quality.

The next important component of ink is the solvent, which is used to dissolve the binder as well as to enable the ink to flow. No solvent works for all types of binders. Compatibility issues can be observed when the binder starts to swell, becomes lumpy, or segregates from the solvent. An ideal binder-liquid phase combination results in a homogeneous paste. Besides that, increasing the amount of solvent can reduce the viscosity of the ink, which, depending on the need, can be used to improve the printability of the electrode ink. Solvents with a higher boiling point and lower vapor pressure can also improve the stability of the ink as it does not dry easily during printing in normal room temperatures. For example, the solvent used for Ni-Zn batteries, which are the perfluorinated solvents, such as perfluorodecalin, perfluoro-methyl decalin, perfluoro-methyl cyclohexane, and perfluoro (1,3-dimethyl cyclohexane), is used to dissolve the uncrosslinked PTFE binder.

The final component, which also plays an important role in obtaining a homogeneous, printable, and high-performance battery, is the additives. Additives are introduced to the system to develop good electrochemical performance and ink mechanical stability, especially for high volume printing. The most commonly used additive is the conductive additive, whereby it helps to enhance the conductivity of the electrode since the active material such as zinc oxide and nickel (II) hydroxide has low electrical conductivity. By reducing the internal resistance, the efficiency of the battery improves, providing less self-discharge and DC voltage drop.²⁸ Typical examples for conductive agents are as follows: carbon black, acetylene black, graphite, or other metal powder additives. All these items may be of different grades and type. As such, it is crucial to test the performance of the materials to ensure its compatibility with the solvent used, since not all of these, especially carbon can be mixed with the same solvent. For zinc-based batteries, it is vital to study the zinc electrode's stability against the alkaline-based electrolyte. The commonly addressed issues are shape change, agglomeration, dendrite formation, and corrosion. Some examples of the additives for the anode are calcium hydroxide, bismuth(III) oxide, and zinc powder.¹⁶ Additives can also be used to improve homogeneity as well as the stability of the ink during printing. Other possible components of the ink are surfactants, dispersants, humectants, and thickeners. Surfactants are used to reduce the surface tension of the solvent so that the active material and other powder additives can be immersed in the mobile phase and not remain at the surface. Meanwhile dispersants are used to avoid the particles from precipitating at the bottom of the container as well to prevent agglomeration during idle states. Finally, to improve printing consistency and quality, humectants are needed to maintain the electrode ink's viscosity as it tends to dry out during printing, which can cause clogging of the mesh and thus affecting the print quality. The viscosity of the ink can be maintained as the humectants control the evaporation of the mobile phase during printing. All these additional components are only added when necessary, not to affect the actual battery's performance. When adding these components into the ink recipe, it is advised to do so systematically as there are many models of each component and their compatibility to the ink may not be the same.

Mixing methods

Mixing is a process where all the physical components of the ink are blended together to produce a homogeneous mixture. A vacuum mixer is typically used as it helps to remove air bubbles from the ink, which improves the ink's dispersion quality. This type of mixing also protects the sample from oxidation by eliminating oxygen from the mixture, thus allowing it to provide a homogenous mixture reasonably quickly. With such efficiency, less amount of heat will be generated, and the sample will be less exposed to the risk of thermal degradation. There are four types of mixers commonly being used, namely the ultrasonic homogenizer, vacuum mixer with shaft and blade, vacuum centrifugal mixer, and high speed shear mixer. An ultrasonic homogenizer preferred for laboratory-scale prototyping of Ni-Zn batteries, especially when it involves the mixing of nanoparticles, carbon nanotubes, graphene, and any other carbon-based particles that are difficult to homogenize. However, for mass production of LIBs, a vacuum mixer with the shaft and blade is used. Another alternative is the vacuum centrifugal mixer, which can mix inks at very high speeds (up to more than 2000 rpm) by rotating the ink container, without direct contact with the ink. The old-fashioned high speed shear mixer can be used for producing a dispersion of particulate solids in a liquid phase, at speeds of up to 20,000 rpm. The examples of mixing equipment are shown in Figure 7a.

FIG. 7.

(a) Examples of mixers: (i) ultrasonic homogenizer,⁵³ (ii) dual-shaft planetary vacuum mixer,⁵⁴ and (iii) vacuum centrifugal mixer.⁵⁵ (b) Mixing process flow.

The mixing flow process is illustrated in Figure 7b. Better electrode performance can be achieved if all the powder is dry mixed first in a separate container and a new container is used in mixing the binder with the solvent separately. Next, both the wet and dry components are mixed to become a complete slurry.²⁹ This method can blend the powder well before mixing, and the uneven distribution of the particles in the ink system can be minimized. Depending on the volume of mixing and the properties of the binder materials used, minor adjustments can be made, such as adding the powder particles and mix portion by portion, to reduce agglomeration, as well as to avoid the problem of having low solvent. Some types of binders may be challenging to be mixed, for example, binders with a high molecular weight such as PTFE and polyvinylidene fluoride, and some may have a jelly-like consistency, which makes it challenging to be handled. Figure 7b shows the proposed flowchart for mixing polyvinyl butyral binder solution by using a centrifugal mixer.

Controlling the ink viscosity can also be quite troublesome since the viscosity varies every time even though the same amount of binder solution was added. It is recommended to put the solvent gradually so that the viscosity does not drop rapidly below the required screen-printable viscosity. It is more challenging to increase the ink viscosity, as this needs the solvent to be evaporated out of the ink, which may degrade the quality of the ink. For screen printing, additives that impart thixotropy are often used. These additives have properties that cause the ink viscosity to drop when it is sheared through the screen by the squeegee. Conversely, when the ink is not sheared, the viscosity rises to prevent the ink from flowing around the screen while waiting for the next layer print cycle.

Commercial silver inks have viscosities of 10,000–100,000 cPs at 20 rpm, and this is considered to be suitable viscosities for screen printing. Users, however, also have to take into consideration the percentage of all filler loadings, and different viscosities may also be used.

Electrode printing

Electrode printing is defined as the process where the electrode ink is transferred onto the current collector. The first step for screen printing is flooding or spreading the ink uniformly by using a squeegee on the surface of the screen mesh without any pressure. Next, the squeegee is swept back toward the initial point with applied pressure; this is to uniformly transfer the ink from the mesh to the surface of the substrate. Few major components are required for screen-printing, namely squeegee, screen mesh, emulsion, ink, and substrate. There are a few preparations that need to be done before printing an electrode, which is the selection of squeegee, screen mesh, emulsion, printing equipment, printing parameters, and printing process.

Squeegee selection is crucial for achieving good printing quality, especially for fine-line printing. The squeegee material has to be chemically resistant to both the ink and cleaning solvents used. Commonly used materials for squeegee are polyurethane and silicon. In addition, squeegee hardness has to be selected based on the printing substrate and pattern. The typical squeegee durometer for printed electronics is generally between 60 and 90 shore A hardness. Higher durometer squeegees, of above 70 shore A, are usually used for fine-line printing (line width down to 50–100 μm) or any other precision printing, where a high mesh count screen is involved. Higher durometer squeegees are also recommended for smooth surface substrates and high-speed printing.

Softer squeegees of 60–70 shore A can be used on rough and nonplanar surfaces as well as for low mesh count printing. Soft squeegees also allow thicker layer ink deposition using lower pressure and lower printing speed. These materials are suitable for any base coating or thick coating in the printed battery. There are a few squeegee profiles that can be selected as well, that is, square edge, round edge, and beveled edge. The round edge profile is for very thick, nonprecision printing for textile, whereas beveled edge is suitable for printing rounded surfaces at high definition, is efficient, and ideal for high-speed printing. The standard square edge profile is preferred for the printed battery as it can ensure a balance between printed layer thickness, adhesion, as well as precision.

Besides squegees, the selection of screen mesh is also important to improve the printing quality. There are three types of screen mesh, polyester, nylon, and stainless steel. The advantages of polyester are that it is solvent and water-resistant, can withstand high-temperatures, has low stretchability, and is suitable for high accuracy printing. Its drawback is that it is less resistant to wear compared with nylon. Nylon, however, has high strength, superior wear resistance, and excellent resistance against both water and solvent. The disadvantages of nylon's are that it is easily stretched under high tension, making it unsuitable for precision and fine-line printing.

Stainless steel mesh, considered as the best among the three, has high strength, low stretchability, and is resistant to abrasion. These features make them very durable and ideal for fine-line printing, especially over longer periods of time. The drawbacks of stainless steel mesh are that it is expensive and the fact that it becomes disfigured once overstretched. A more cost-effective solution is polyester, which is less durable than stainless steel, but is suitable if long-term fine-line printing not required.

Selection of mesh count is dependent on both printing accuracy and printing volume. Higher mesh count typically leads to better printing accuracy, making it suitable for fine-line printing, with line widths as small as 50–100 μm. Mesh sizes can be measured in terms of mesh opening (w) and mesh diameter (d). Narrow lines cannot be printed using a large mesh opening. However, higher mesh count normally leads to less printing volume, assuming the emulsion thickness is fixed, because with smaller mesh opening, fewer particles can pass through the mesh for each printing. The minimum mesh opening should be at least three times bigger than the particle size of the ink/paste to be printed. For example, if the biggest particle size in anode ink is 5 μm; the mesh opening should be at least 15 μm to allow successful printing. The calculation for mesh count/size (M) is stated in Equation (6).³¹

A thick battery electrode carries more amount of active material, which theoretically can improve the capacity of the battery. However, the electrode thickness should be optimized to avoid jeopardizing the adhesion strength.

The printing thickness and volume can be controlled by varying the mesh count (M), emulsion thickness (D), mesh opening (w), and diameter (d).³² To simplify the calculation of the printing volume and thickness, the print length (L_printed) should be made at a constant value of 1′′ or 25.4 mm. The thickness of the printing can be measured by calculating the 2D area of the printing cross section (A_printed) using Equation (7). The thickness of the printing, then, can be calculated using Equation (8). Finally, the volume of the overall printing can be calculated using Equation (9), by knowing the overall printing surface area. These equations were derived based on Figure 8.

FIG. 8.

Cross section of the screen mesh during and after printing to illustrate important design variables.

The next step is the roll press, which is done to improve electrode adhesion to the substrate. Roll press is also known as the calendaring process in LIB industries. This process also compresses and compacts more active material in the electrode, thus increasing the capacity per cubic centimeter. In this process, the electrode is pressed by two top and bottom rollers at very high pressure. The gap between both rollers can be controlled to adjust the thickness of the electrode to micrometer precision. Optimization of electrode thickness is important, as reducing the thickness abruptly could result in cracks on the electrode surface, while slower thickness compression lowers electrode adhesion to the substrate. Electrode compression strength and speed also depend on the type of substrate used, whether copper, aluminum, and so on, as this process may damage the substrate if too much pressure is applied. When printing on plastic films, such as polycarbonate (PC) or polyethylene terephthalate substrates, the roll press may not be suitable and can be opted out. An example of the roller press machine is shown in Figure 9a.

FIG. 9.

(a) Example of roller press machine.³³ (b) Aluminum laminated film pouch case for a battery.³⁴

Both cathode and anode electrodes are printed on the current collector, as shown in Figure 9. Once the electrodes are completely dry, the electrode samples can be cut according to the desired shapes. This process can be performed manually using a pair of scissors or a dedicated die cutter. The electrodes are then placed facing each other with the separator layer in between to avoid short circuits. For multicell applications, the cells can be arranged in series by alternately placing the cathode and anode in sequence with a separator layer in between.

Electrolyte injection and battery packaging

The printed battery can be packaged in an aluminum pouch cell. This casing can hold both the electrodes and electrolyte. Aluminum pouch cases dedicated to the battery have five different materials layers, and these are, polyamide, adhesive, aluminum foil, adhesive, and polypropylene, as shown in Figure 9b. These materials help protect the aluminum from being exposed to potassium hydroxide electrolyte. If exposed, the electrolyte can dissolve aluminum over longer periods of time. For a multicell battery, the thickness of the electrodes is higher; thus, the pouch cell case needs to be vacuum formed upfront so that there is a bigger space for the electrodes and electrolyte to fit in. The pouch forming process is not required for really thin batteries.

The next important procedure is injecting the electrolyte, which is shown in Figure 10. Once the electrodes and separator are stacked together, a hot melt adhesive tape needs to be applied on the base or neck of the terminal tab. This tape acts as a sealant for the pouch cell case and the terminal tab since the hot sealer cannot seal if silver ink is in between the lips of the pouch cell case. This process is illustrated in Figure 10(i) and (ii). Next, the electrodes are placed on top of an unsealed pouch cell case, which will be folded into two and sealed at the top and on one side of the pouch [Fig. 10(iii), (iv)]. This step is carried out to allow some space to inject the electrolyte into the pouch [Fig. 10(v), (vi)]. After electrolyte injection, the top side of the pouch is sealed at the edge, giving 2′′ clearance for second sealing. The battery is then charged and discharged for a few cycles to release the gas. The pouch is pressed firmly to ensure that all the gas exits the pouch, whereas the second sealing process is performed just outside the electrolyte/electrode area [Fig. 10(vii), (viii)]. Finally, excess regions are trimmed, and the battery is ready.

FIG. 10.

Electrolyte injection and sealing procedures for printed batteries (i)–(ix). Color images are available online.

Screen printing and additive manufacturing

The screen printing process is widely used in a variety of industries such as textiles, electronics manufacturing, and labeling. The desired pattern can be printed by modifying the design of the emulsion layer on the screen. The emulsion creates an impermeable layer that blocks ink from going through the mesh and vice versa, blank areas that are not coated with emulsion allow ink to pass through the mesh on to the substrate. To prepare the emulsion layer, photosensitive emulsion liquid is coated on to the screen. Next, the desired pattern is printed on a transparent film using a laser printer and placed on top of the screen, as shown in Figure 11. The pattern is then exposed to ultraviolet light and is transferred on to the screen. The emulsion layer hardens during ultraviolet (UV) exposure and unexposed emulsion regions are peeled off during washing, leaving the desired pattern.

FIG. 11.

Screen mesh emulsion preparation (left) and screen-printing process flow (right).

To print, the ink is first “flooded” or covered on the entire screen mesh area using a squeegee. The second sweep of the squeegee applies more pressure to transfer the ink to the substrate. The printing gap or gap between the mesh and the substrate has to be adjusted based on the viscosity and the tackiness of the ink. Inks that are more viscous require larger printing gaps to ensure good printing quality. Once printing is complete, the substrate is dried in the oven for a specific period of time to cure the ink. Figure 11 explains the screen mesh preparation and screen printing process.³⁵

Another widely popular printing technique is 3D printing, in particular, extrusion printing. This technique is commonly used to create 3D objects using thermoplastic materials by heating the plastic into a semi-molten state before extrusion through the nozzle dispenser. The deposited plastics are then solidified to create a 2D layer; then, the 3D structure can be produced by stacking more layers on top of the previous 2D layer. The extrusion printing is commonly used for printing thermoplastics, such as polylactic acid, acrylonitrile butadiene styrene, and PC. Recent research has demonstrated the use of this technique for battery fabrication.³⁶ For example. Sun et al. developed LIBs by using extrusion techniques. Lithium titanium oxide and lithium iron phosphate colloidal inks (with solid loadings of 57% and 60%, respectively) were developed with the rheological parameters tuned for 3D filamentary printing.³⁶

This article will be mostly focused on screen-printing techniques for printed Ni-Zn batteries rather than the conventional 3D printing methods for 3D printing, such as photopolymerization, extrusion, powder-based, and lamination. Screen-printing techniques can also be considered as a type of additive manufacturing, whereby the combination of 2D layers of printing can produce a 3D structure. However, unlike 3D printing, the screen-printing can be more practically applicable for a simple 3D structure where the 2D layers are similar across the height of the 3D structure, such as cuboid and cylinder. Besides that, although screen-printing can produce a 2D layer with thickness up to 100 μm, the thickness for multiple stacking of 2D screen-printing is limited since the adhesion strength and flexibility of the screen-printed ink will reduce with increasing thickness.

Thus, screen printing is highly recommended for thin form factor devices such as powered smart cards, and wearable devices, which do not require such thickness to power up the devices. For high volume manufacturing, screen printing and extrusion printing can be equally quick, depending on how many layers of screen-printing involve and how fast the ink can be dried, since the 3D extrusion printing may take more to print bigger surface area pattern.

Characterization Methods

Many factors, such as the process, materials, and equipment, can affect the performance of the battery. There are three important tests to evaluate the cell performance, namely cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge–discharge analysis. The sequence of tests for a printed rechargeable battery is summarized as a flowchart in Figure 12a. It is recommended to start with the three-electrode CV test for each individual electrode (cathode and anode) to find their redox peaks. This test ensures that each electrode is functioning well before it is assembled together as a battery.

FIG. 12.

(a) Measurement flow for printed rechargeable battery. (b) Cyclic voltammetry setup: (i) three-electrode setup, (ii) two-electrode setup.³⁶

Next, the two-electrode CV test, which connects both the printed anode and cathode together is used to understand the cell behavior within a specific voltage window. This approach is very crucial in determining which voltage range can be used to operate the cell safely. After that, EIS can be used to evaluate the electrode further. Finally, the data gathered from the two electrode CV can be applied to program the operating voltage window for the charge–discharge test. The charge–discharge test captures the actual battery performance. Typically, before using a rechargeable battery, relevant information such as capacity (mAh), nominal voltage (operating voltage under load), maximum charge voltage, end voltage (for discharging purposes), maximum and standard charging–discharging current, C-rate, operating temperature, and its cycle life are specified by the manufacturer. Ideally, a high-performance rechargeable battery should have a long life cycle, high energy density, high power, and energy output, high coulombic, voltaic, and energy efficiency, high capacity retention, minimum self-discharge, and high charge–discharge rate.^16,37 In practical applications, the performance of the battery is defined by its ability to meet the requirements of the application taking into consideration other factors such as cost, ambient condition, temperature, space allocation, and durability under mechanical stress.

Cyclic voltammetry

CV is one of the widely used techniques to study the electrochemical performance of the electrode and battery.³⁸ The CV test allows important data such as peak potentials, peak separation, peak current, and peak reversibility to be gathered. These information are beneficial to predetermine the electrochemical behavior of the electrode or the cell before the actual charge–discharge test. There are two different setups for CV testing, as shown in Figure 12b. A three-electrode CV consists of three different electrodes, which are the working electrode (WE), the counter electrode (CE), and the reference electrode (RE). The three-electrode setup can be used to examine the electrochemical performance of each individual electrode before being assembled as a complete cell. In this setup, the WE is connected to the sample, whereas the CE is connected to an inert electrode, preferably platinum or any other alternative such as graphite [Fig. 12b(i)]. To run the three-electrode setup for anode CV, the RE is Hg/HgO in NaOH solution, the CE is a platinum electrode, and the WE is the printed ZnO. For cathode CV, the WE is connected to the printed Ni(OH)₂, whereas the other two electrodes are unchanged.^39,40

The electrochemical behavior of the full cell at a specific potential window can be observed using the two electrode setup. In this configuration, WE is connected to the sample under test, whereas the CE and RE are connected together as a single node [Fig. 12b(ii)]. In this case, the ZnO anode is connected to WE, and CE/RE is connected to Ni(OH)₂ cathode.

CV can be performed to examine electrode stability, efficiency, and impedance to improve the cell's performance. There are also variations in the numbers of the theoretical specific energy of Ni-Zn, which is 334 Wh/kg compared with an actual specific energy of the Ni-Zn battery, which is in between 70 and 110 Wh/kg, or around 21% and 33% of the theoretical specific energy.²⁸ Another important feature for rechargeable batteries is the reversibility of the electrode. This feature can also be studied using CV. Ideally, the output of cyclic voltammograms should be in the form of a “duck shape,” as shown in Figure 13(i), whereby the anodic (i_pa) and cathodic (i_pc) peak currents are equal. Besides that, the peak-to-peak separation potential for both anodic (E_pa) and cathodic (E_pc) peak must be within 57 mV⁴¹ to signal that the electrode has a reversible redox reaction. The location of the anodic and cathodic peaks can sometimes be interchanged depending on the accepted convention (United States or International Union of Pure and Applied Chemistry) for anodic and cathodic reactions.⁴¹

FIG. 13.

(i) Duck shape cyclic voltammogram. (ii) Example of cyclic voltammogram for zinc oxide converted to IUPAC convention.³⁹ (iii) Example of cyclic voltammogram for nickel hydroxide (IUPAC convention).⁴⁰ IUPAC, International Union of Pure and Applied Chemistry. Color images are available online.

In reality, an ideal cyclic voltammogram is very difficult to achieve since it requires an entirely reversible electrochemical system that consists of electrodes, electrolytes, and other items such as separator and additives. Other factors such as cost-effectiveness, practicality, safety, environment friendliness, and availability of resources need to be considered before the optimization of the electrodes to fit the ideal CV curve can be done. The printed zinc oxide anode's cyclic voltammogram should be close to the curve of the conventional zinc oxide CV curve, as shown in Figure 13(ii)³⁹ for both zinc and zinc oxide electrodes with a few additives added to the KOH such as bismuth(III) oxide (Bi₂O₃), lithium hydroxide (LiOH), and sodium carbonate (Na₂CO₃).

The potential sweep was from −1.6 to 0 V with a sweep rate of 100 mV/s. The peak potential for the anodic reaction is located somewhere around −1 V, whereas the cathodic potential peak is located around 1.5 V. Validation of the printed zinc oxide anode can be done by comparing the measurement results to this curve.

Similar methods can be applied to the anode,⁴⁰ where the CV analysis can be compared with measurements conducted for Ni(OH)₂, as shown in Figure 13(iii). The sweep potential was set to be between 0 and 0.6 V with a scan or sweep rate of 10 mV/s. Different scan rates can affect the peak current output for the CV curve without horizontally shifting the peak potential.

Thus, it is a good practice to just stick with a single scan rate that is found to be the most convenient. CV curves at lower scan rates are time-consuming and can be challenging to be analyzed, whereas higher scan rates are deemed faster but may lose some essential details of the curve, such as the noise and minor fluctuations. The anodic peak potential of Ni(OH)₂ is located somewhere around 0.5 V and cathodic peak potential around 0.4 V. Thus, it is expected that the printed nickel hydroxide cathode should exhibit similar characteristics.

The maximum battery charging and nominal voltages can be calculated using the oxidation–reduction peak potentials after both half-cell CV results for anode and cathode have been obtained,. The oxidation peak potential of cathode, E_opc, and reduction peak potential of the anode, E_rpa, can be used in Equation (10) to calculate the estimated charging voltage. The estimated nominal voltage can be obtained by subtracting the oxidation peak potential of the anode, E_opa, from the reduction peak potential of the cathode, E_rpc, as shown in Equation (11).

Besides the use of CV, there is another test method that can be used to characterize the batteries before subjecting them to charge–discharge testing, known as EIS. This is a more sophisticated method of testing an electrochemical cell and is used to gain more data regarding the cell, such as electrode resistance, electrolyte resistance, charge-transfer resistance, polarization resistance, diffusion, and double layer capacitance. These information are determined from the measured Nyquist plot and by fitting the data to an equivalent circuit. The data obtained using EIS can be used to complement the CV data.⁴²

Charge–discharge cycles

The last and most frequently used test for both laboratory-scale prototyping and manufacturing is the charge–discharge test. This test is conducted using a battery tester to determine the life cycle of the cell, its capacity, capacity retention, ohmic loss, self-discharge rate, and coulombic efficiency. This test simulates the actual cell performance under specific charge–discharge cycle conditions specified in the battery tester. Useful information from CV can be applied in the charge–discharge test, such as the safe operating voltage window for the cell without deteriorating its physical properties or packaging. Cell deterioration can be in the form of electrode decomposition and pressure build-up in the cell due to gas release. The minimum information needed to conduct the charge–discharge test are as follows: the upper and lower voltage limits, the C-rate, amount of current in the cell, rest period duration, and number of charge–discharge cycles.

The charge–discharge setup is shown in Figure 14a. Using this setup, eight batteries can be tested where each printed battery is connected to the battery tester via its cathode and anode terminals. This setup can also be used to de-gas the pouch before sealing the cell. There are several essential parameters for the charge–discharge test of a battery system. For the standard Ni-Zn battery, the expected nominal voltage is around 1.65 V.¹⁶ The upper voltage charging limit can be set to be a bit higher (1.8–2 V), while taking into consideration that extending the upper voltage limit may cause degradation of the battery performance over the time due to gas evolution.²⁸ The lower voltage limit is between 0.8 and 1.1 V.

FIG. 14.

(a) Charge–discharge setup for eight printed batteries when connected to a battery tester. (b) Charge–discharge curve of aqueous flexible Ni-Zn battery versus flexible quasi-solid-state Ni-Zn battery.⁴⁶ Color images are available online.

Similarly, extending the discharge limit may cause degradation of the battery life cycle, capacity retention, and performance.^28,43,44 An appropriate charge and discharge current also has to be selected. Higher charge current is preferred, and the current value may be increased to push the battery to charge up to its upper voltage limit. Meanwhile, the discharge current can be determined based on its charge capacity. For example, if the battery is charged at a constant charging current of 1 mA for 1 h, the discharging current can be 0.5 mA for 2 h to be within the upper and lower voltage limits. The estimated battery capacity can be calculated based on the theoretical capacity of the battery using Equation (12)¹⁶:

where Q_p (mAh) is the practical capacity of the battery, m_e is the mass of the electrode (g), A is the percentage of the active material (%), and Q_t is the theoretical capacity (mAh/g). Theoretical capacity values for ZnO is 820 mAh/g ⁴⁵ and Ni(OH)₂ is 292 mAh/g.¹⁶ The measured battery capacity is dependent on lowest capacity between the two electrodes.

The common problem for charge–discharge testing of rechargeable batteries is self-discharging. For excellent performance, the discharging curve should have a plateau that can prolong the capacity of the battery. Self-discharging can be attributed to high internal impedance, binder instability, poor adhesion, depleted electrolyte, short circuit due to dendritic growth, and so on. Figure 14b shows the charge–discharge curve for two different types of Ni-Zn batteries. The aqueous, flexible Ni-Zn battery shows a better discharge curve plateau, whereby the discharge curve plateau is longer and less steep compared with the flexible quasi-solid-state Ni-Zn battery.⁴⁶ Thus, to get an excellent printed Ni-Zn battery, a few cycles of the charge–discharge test should be performed before the actual high cycle charge–discharge.

For troubleshooting, it is simpler to test the battery as half-cell charge–discharge to determine which electrode is not performing well. For example, the printed Zn electrode can be discharged with a commercial air electrode as the cathode, and the printed NiOOH electrode can be discharged with Zn metal as the anode. The half-cell electrode needs to be charged upfront and then discharged. Charging the printed electrode is performed by using the conventional CE setup. For example, printed ZnO needs to be paired with the commercial Ni(OH)₂ as the CE for charging. The parameters of the few cycles of the successful trial runs can be copied and applied for the actual charge–discharge cycle run of up to more than ten, hundreds, or beyond to evaluate the battery cycle life, capacity retention, coulombic efficiency, and so on.

Printed battery applications

The extensive use of mobile electronics has given rise to a wide variety of applications for printed batteries. A popular application is as a smart card. The powered smart card is an active smart card that can power up a LCD, has an interactive button for navigation, a fingerprint sensor for security, has improved radio frequency identification (RFID) range, and can also generate One Time Pin number for transactions.¹ A commercial example of the smart card is a product developed by Mereal Biometrics, the MeReal Biometrics card, which is a multiapplication smart card. Unlike the conventional smart card, the patented product features near-field communication, RFID, acoustics, EMV (Europay, MasterCard, and Visa), LED lights, and a fingerprint sensor, which is powered by an embedded printed battery. These features provide ease of use and added security for the users. The printed battery was designed to be able to power up all the inputs and outputs and can last for at least 24 h without charging.⁴⁷ The battery is compact and thin and can withstand high temperatures of up to 140°C–190°C,⁴⁸ taking into consideration that the smart card undergoes a lamination process.

Smart skin patch is another emerging application for printed batteries. These wearable devices have a variety of functions, from stretchable skin sensors to detect harmful UV rays, smart insulin patches, which can both detect blood sugar elevations and deliver insulin, and also sweat analyzers, which can noninvasively detect cystic fibrosis, nutritional deficiencies, and ion imbalances from a person's sweat.⁴⁹ More complex, wearable sensors are also in the pipeline, especially for applications in the health care sector.

Recently, researchers have developed a Smart Integrated Miniaturised Sensor System, which consists of printed cholesterol sensors, circuitry, a battery, and an electrochromic display. The printed battery was designed with 4.5 V nominal voltage by connecting three 1.5 V printed cells in series with 1 mA current flow for up to 600 s to power up all the components on-board.⁵⁰ In another research, printed batteries have also been used in a Bluetooth-enabled high-accuracy skin temperature measurement patch developed by Texas Instruments.⁵¹ The product consists of Bluetooth low energy (BLE) and a skin temperature sensor. The product was designed to read skin temperature every 1 second, with average current consumption roughly 230 μA. The product contains a printed battery that supplies 35 mAh allowing the patch to be on for 5 days and has a shelf life of up to 3 years. The patch operates with a 3 V flexible battery, which is sufficient to power up both the skin temperature sensor and the BLE microcontroller unit.⁵¹ It can be said that due to the increasing popularity of wearable devices, there will be a growing market demand for printed batteries.

Conclusions

Technological improvements in manufacturing have accelerated the growth of flexible and stretchable electronics, creating the next generation of mobile sensors, energy harvesters, photovoltaics, paper-like displays, e-textiles, optics, and soft robotics. All these components require thin, compact, and portable energy sources, making printed batteries the ideal candidate. Printed battery technology is expected to be in high demand primarily to cater to future wearable devices, which integrates IoT with miniature sensors and devices that are placed on the human body.

This article highlights the commonly used screen-printing technique as a manufacturing process ideal for fabricating wearable devices of the future. Critical parameters and techniques needed to print electrodes, especially for Ni-Zn batteries and other alkaline batteries have been explained in detail. Ni-Zn batteries are a viable and more sustainable alternative to LIBs as Ni-Zn batteries do not contain harmful substances such as mercury, lead, or cadmium or metal hydrides, which are arduous to recycle. Both nickel and zinc can be fully recycled. Each fabrication step, including mixing, printing, and packaging, was explained in detail.

Next, the experimental steps and characterization methods for electrochemical cells were also elaborated so that researchers can be provided with a step-by-step method to validate their battery electrodes for functionality. CV graphs were also explained, including methods to calculate the maximum charging and nominal voltage. Complete battery charge–discharge testing methods were also illustrated. This article provides a comprehensive guide for new researchers in the field of screen-printed battery fabrication and intends to stimulate more research and promote screen-printing techniques for Ni-Zn batteries.

It is predicted that in the future, advanced battery fabrication techniques will integrate the nontemplate advantages of 3D printing with the mass production capabilities of screen printing. Additive manufacturing and 3D printing are advantageous due to its ability to perform digitally controlled deposition of different materials to form complex 3D objects. Novel 3D electrodes that provide larger surface area, higher areal-loading density, shorter diffusion pathways, and lower resistances during the ion-transport process will improve the energy and power densities of the battery. Screen printing techniques, however, offer low cost, high-throughput production of devices, and have wide database of inks that are biocompatible.

To merge these two manufacturing techniques, a new class of 3D printable inks that are not only biocompatible but are also flexible enough to conform to the human body is needed. These novel high-performance materials will be subjected to the normal conditions exposed to the human body, and as such, it should be able to perform reliably under a range of stresses, mechanical deformations, and environmental conditions. An added feature of this material would be to be biodegradable, and this would be extremely useful for implantable devices. It is evident that the current screen printing techniques provide essential building blocks for the additive manufacturing techniques of the future. Novel ink formulations for 3D printing and development in the field of screen printing will pave the way for low-cost, mass produced, 3D printing devices.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This project is co-funded by Jabil Circuit Sdn Bhd and International Islamic University Malaysia under grant P-RIGS18-038-0038.