1. Introduction

Thin-film batteries are nano- to millimeter-sized solid-state batteries comprising the anode, the cathode, the electrolyte and the separator. The anode is the negative electrode that is oxidized after giving up electrons to the external circuit. It is the anode that generates ions that move through the electrolyte. The cathode is the positive electrode that accepts electrons from the external circuit and is reduced in the process. During the charging and discharging process, ions are inserted into and extracted from the cathode. The electrolyte is the medium for charge transfer between the cathode and the anode. Thin-film electrolyte is usually chemically stable, ionically conductive and electrically insulating and is required also to build good contact with the cathode and anode surfaces. The separator prevents physical contact between the anode and the cathode without blocking the transport of ions. Most times in thin-film batteries, the solid electrolyte acts both as an ion transport medium and physically separates the cathode and the anode as shown in Figure 1.

Figure 1.
Thin-film batteries with a solid-state cathode, anode, and electrolyte.

Thin-film batteries are manufactured using physical and chemical deposition techniques [1]. They include magnetron sputtering, pulsed laser deposition, molecular layer deposition [2], atomic layer deposition, vacuum evaporation [3], thermal evaporation, electron beam and sputtering [4]. These techniques follow four pathways called thin-film battery technologies [5].

The mechanism of the thin-film batteries is that ions migrate from the cathode to the anode charging and storing absorbed energy and migrating back to the cathode from the anode during discharge and thereby releasing energy [6].

The recent research in and development of smarter societies have necessitated the integration of smart devices with improved safety, specific energy, power and reduced-size materials [3]. This has given rise to the demand for using thin-film rechargeable batteries for electrical energy storage with good energy and power densities, excellent mechanical strength, good and long cycle life and appreciable temperature tolerance for small portable consumer electronics, especially in cell phones, laptops and notebook computers, smart cards, mobile applications, for electric cars, communication and other electrical equipment [7].

This book chapter reviews the fundamentals of thin-film batteries and the use of these batteries in various applications.

2. Types of thin-film battery technologies

Four major thin-film battery technologies are discussed here. They include (a) printed battery technology, (b) ceramic battery technology, (c) lithium polymer battery technology and (d) nickel-metal hydride (NiMH) button battery technology [5]. The choice of manufacturing of any of the technological paths will depend on the end-use application of the batteries.

2.1 Printed battery technology

Printed batteries can be in the form of rechargeable and non-rechargeable printed batteries. These printed batteries can be classified based on the make of the electrochemical systems as rechargeable and non-rechargeable printed batteries as shown in Figure 2 [8]. The printing paste for batteries is made up of the electroactive material, the solvent, the conductive agent and the binder. The advantages of printed batteries include (1) it is possible to develop more than one layer pattern of design in the battery fabrication [9, 10], (2) Low cost and mechanically flexible batteries [11], (3) Thin and flexible energy storage [12] and (4) compatibility with the printing processes and devices [13]. Printed battery technologies find applications in the Internet of Things (IoT) devices [5], wearable electronic devices, smart cards, remote sensors, medical devices [9], microelectromechanical systems (MEMS) devices, low-power microprocessors, big-data analytics [14], electric vehicles, stationary storage grids [15], renewable energy and smart grid [16], etc.

Figure 2.
Classification of printed battery technology.

2.2 Ceramic battery technology

They are all solid-state batteries made up of ceramic materials as their electrolytes. The demand for high-density batteries has necessitated research into more technologies that will enhance the thermal stability, mechanical strength and wettability of solid-state battery manufacture. Enhancements at the metal electrodes and/or separator boundaries are made possible by the introduction of ceramic batteries to boast some of the desired properties needed for solid-state batteries. Ceramic-based flexible sheet electrolytes have been formed to improve the energy density of solid-state batteries by synthesizing flexible composite Al-doped LLZO sheet electrolyte [17], coating of Al₂O₃, SiO₂ and TiO₂ onto polyethylene membrane seperators [18]. They are used in cogeneration and fuel cell, electric vehicles, mobile devices and stationary storage applications [19].

2.3 Lithium polymer battery technology

This works on the principle of intercalation and de-intercalation of lithium ions from the positive electrode (cathode) to the negative electrode (anode) and vice versa through a solid-state electrolyte that provides the conductive medium. Lithium-polymer batteries are lightweight, long-lasting and powerful solid-state batteries that can guarantee a constant energy supply [20]. They have a high energy density, flat voltage curves, low self-discharge and no memory effect from being discharged and charged again. The technology is used in laptop computers, personal electronics, cellular phones [21], notebooks and digital cameras. The polymer electrolyte may be in form of dry solid polymer electrolyte, polymer in-salt system, single-lithium-ion conducting electrolyte and gel polymer electrolyte [22].

2.4 Nickel-metal hydride (NiMH) button battery technology

This technology is an energy storage system that depends on charge/discharge reactions occurring between the nickel oxide-hydroxide cathode as the active material and the hydrogen-absorbing alloy anode [23]. This battery technology exhibits good tolerance to overcharge/discharge, high power capacity, very safe and compatible [24]. They are mainly used as power sources for hybrid electric vehicles, digital cameras and cell phones [25].

3. Deposition techniques

Thin-film deposition techniques are used to modify the surface properties of solid-state thin-film batteries. The modification affects the battery characteristics such as energy density, conductivity, storage capacity, charging and discharge time, etc.

Thin-film battery deposition techniques are classified into physical and chemical deposition processes as shown in Figure 3 [26]. In the chemical deposition process, a chemical reaction takes place before the product is deposited on the substrate while the physical deposition method involves only physical mixing and deposition of the mixture on the substrate without any chemical reaction [27]. The coating material in a physical deposition is always a solid while the coating materials in chemical deposition are always in gaseous form. Physical deposition techniques include thermal evaporation, sputtering, ion plating and arc vapor deposition while chemical deposition techniques include chemical vapor deposition, plating, sol-gel deposition, chemical bath deposition and spray pyrolysis deposition techniques [28].

3.1 Physical deposition method

This method includes thermal evaporation, molecular beam epitaxy, pulsed laser deposition, ion plating evaporation, cathodic arc deposition and sputtering techniques. In thermal evaporation, molten metals or metal oxides form vapor and are deposited on the substrate as it cools [29, 30]. Molecular beam epitaxy (MBE) is a deposition technique in which a single crystal layer is deposited in a single crystal substrate, using molecular beams in an ultra-high vacuum chamber [31]. It is used in multi-junction solar cell applications [32], Ga-FACE GaN electron devices [33], etc. Pulsed laser deposition (PLD) process is a technique where a high-power pulsed laser beam strikes a target, vaporizes it and deposits it as a thin film on a substrate [34]. It is useful for optical applications, especially for UV laser emission [35] and superconductor devices for electronic and medical applications [36].

3.2 Chemical deposition method

Chemical deposition of thin films involves the generation of the atoms, molecules or ions through a chemical means; transportation of the atoms or ions through a medium and condensation and cooling of the atoms, molecules or ions on the substrate [27]. Chemical methods include chemical vapor deposition (CVD), electrodeposition, electroless deposition, spray pyrolysis, ionic layer deposition, sol-gel technique, chemical bath, spray and spin coating as shown in Figure 2.

Spray pyrolysis is the process of depositing a thin film on a heated surface by spraying a solution on the heated substrate [37]. The solution reacts with the heated substrate to form a thin film on the surface of the substrate. Mostly used for manufacturing semi-conductor alloys and conductive glasses. Electrodeposition is a technique of coating a thin layer of metal on top of a different metal through electrolysis by reducing the cations of the material from the electrolyte and depositing it as a thin film on the substrate [38]. It is a major method for the production of rechargeable lithium-ion batteries [39, 40]. Electroless deposition is a thin-film coating technique without the application of external electric power. It is used for non-conducting substrates and its major application is in the metalizing of printed wiring boards [41]. Atomic layer deposition (ALD) is a chemical deposition technique where chemical precursors introduced on the surface of the substrate react to form ultra-thin film monolayers on the substrate surface. They find application in fuel cells, capacitors, microelectronics and areas where highly uniform ultra-fine mono-layered thin films are required [42]. Sol-gel is a technique where metal alkoxides, used as precursors, are dissolved in a solvent (especially water or alcohol), heated and stirred to form a gel, which is condensed, dried and deposited on the substrate as a thin film [43]. The process involves hydrolysis, polycondensation, gelation, drying and crystallization [44].

3.3 Deposition technique for cathode materials in thin-film batteries

A cathode is a very important component of thin-film batteries. This electrode material facilitates the stability of the electrochemical reactions in the electrode/electrolyte interface. The material composition of the cathode determines the thermal stability and rate capacity of the battery. For a good and efficient battery system, there is always a good ion transport mechanism between the anode and the cathode through the electrolyte and the acquisition and flow of electrons through the external circuit during an electrochemical reaction. Different deposition methods have been used in the design and fabrication of cathode materials for different solid-state battery applications. Production of LiCoO₂ cathode materials using radio frequency (RF) magnetron sputtering deposition technique for micro battery applications was studied by Jullien et al. [45], MoO₃ thin-film cathodes using DC sputter deposition technique for Li-ion rechargeable battery applications [39] and V₂O₅ thin-film cathodes using radio frequency (RF) reactive sputtering technique for high-performance thin-film batteries (TFB) [46].

3.4 Deposition technique for anode materials in thin-film batteries

A good anode material should be a good conductor and reducing agent and possess a high electrical energy density. They are made from carbon (graphite), lithium metals, lithium alloys, titanium and its alloys, silicon and other metals. Several methods have been developed on good anode materials for solid-state thin-film batteries for micro-mechanical system applications. Reto Pfenninger et al. [47] studied the electrodeposition of Li-garments on solid state anode. The group worked on metal oxide anodes of Li₄Ti₅O₁₂ with good ionic conductivity and less Li-dendrite formation for microbattery applications [47]. Pulsed electrodeposition of Sn-Cu anodes has been studied and developed for enhanced cycle performance in micro battery applications [48, 49]. Spray deposition of silicon anode materials with high energy density for solid-state battery application was developed by Shin Kimura et al. [50]. Many researchers have fabricated different anode materials and their reports have shown that anodes fabricated with good properties have shown to exhibit better battery performances. Li₄Ti₅O₁₂ anode thin film was deposited on a magnesium oxide (MgO) substrate using the pulse laser deposition (PLD) technique [47], pulsed laser deposition method was used to fabricate a silicon thin-film anode with iron sulphide for solid-state battery applications [51]. DC magnetron sputtering technique was reported by L. Baggetto et al. for Cu₂Sb thin-film deposition for sodium ion (Na-ion) batteries because of excellent storage capacity, high-rate capacity, good reaction potential and decent cycling capacity retention [52]. Silicon/carbon (SiC) thin-film anode was fabricated using a radio frequency (RF) magnetic sputtering technique with improved cycling performance and high current density for thin-film battery applications.

3.5 Deposition technique for electrolyte materials in thin-film batteries

A solid-state electrolyte is a solid ionic and electron-insulating material that promotes the movement of ions from the cathode to the anode during charging and the moving of ions from the anode to the cathode during discharging conditions. There are different types of electrolytes (glass, crystalline and ceramic) depending on the primary material makeup [53]. Several works have been done on solid-state electrolytes and more researchers keep working to get an electrolyte that will improve the thin-film battery characteristics such as reduced package size, increased safety and enhanced power and energy density [54]. Solid polymer electrolytes made of polyvinylpyrrolidone (PVP) with dopant p-amino benzoic acid (PABA) and poly p-amino benzoic acid (PPA) were developed by Sangeetha et al. for improved dielectric property and ionic conductivity applications [55]. Solid-state electrolytes have been fabricated using pulsed laser deposition (PLD) technique [56, 57], sol-gel techniques [58], atomic layer deposition [59, 60], chemical solution deposition (CSD) [61], etc. for microbattery and other solid-state battery applications.

3.6 Deposition technique for current collector materials

Current collectors are made of copper and/or aluminum foils that collect electrons from the electrochemical reaction of the electrodes and external circuit and at the same time support the material layers of the cathode and the anode. For a material to function efficiently as a current collector, it must be cheap and available, very light material, a good electrical conductor and chemically stable and resistant to corrosion [62]. The importance of current collectors in the solid-state battery (SSB) industry has necessitated research focus on the materials for its fabrication and applications. One-step galvanostatic electrodeposition of lithium in 3D porous copper-based nanoflake structures (3D Cu-NF@Cu foam) as anode current collector for high energy density lithium metal batteries was reported by Yuanyuan Xia et al. [63], electrode polymer-carbon composite current collector foil for bipolar lithium-ion battery applications was reported by Fritsch et al. [64]. Also, Zhen Hou et al. designed a current collector using a nanostructured silver lipophilic layer on a copper foil through an electroless plating electrodeposition process for a stable lithium metal anode [65], fabrication of nickel-phosphorous-modified copper current collector using facile electroless plating electrodeposition method aimed at superior coulombic efficiency for microbattery applications [66], etc.

4. Mechanism

Solid-state thin-film batteries have solid components for the electrodes (cathode and anode) and the electrolyte. They are made by stacking a thin-film electrolyte on the cathode and anode in a vacuum state as shown in Figure 1. The principal operation of thin-film batteries works in the same way rechargeable batteries work. The lithium ions migrate from the cathode to the anode and generate electrical energy when the battery is charged and stored in a current collector. During discharge, the lithium ions move from the anode back to the cathode, the electrons move from the cathode to the anode and a current between the cathode and the anode is created as a result of the potential difference between the electrodes [6].

5. Fundamentals of thin-film batteries

Electrodes (both positive and negative) and electrolytes make up the bulk of a current battery’s physical components. From an electrical engineering perspective, their properties can be deduced from the first principles. A chemical reaction is what gives a battery its ability to provide electrical energy (the battery converts chemical energy to electrical energy) [67]. Electrodes are required to conduct electrons for electrical energy to be realized in the external circuit. On the other hand, the electrolyte need not. Because if it did, the battery’s internals would be accessible to the electrons, rather than just the external circuit, leading to self-discharge and, ultimately, no usable energy at the battery terminals [68]. There is no such thing as an “exceptional” all-solid-state thin-film battery (ASSTFB) [69]. A thin film version of the same three components is used in an ASSTFB, as shown in Figure 1.

According to A.G. Buyers [70], the potential energy measured between electrodes for any electrochemical cell, as depicted in Figure 3, is the difference in the standard free energy as defined in Eq. (1).

∆G=−nFEE1

where ∆G = change in free energy, n = number of electrochemical equivalents transferred in the cell reaction, E = measured voltage (EMF) and F = Faraday constant.

For a material to conduct electricity, the charge must be transferred from one location to another. This charge transfer can take the form of electrons, holes (the lack of an electron), ions or ion vacancies [71]. Interstitial hopping mechanisms, such as the Frenkel defect reaction [67], allow metal ions such as Li + to migrate as defined in Eq. (2).

Mmx+Vix→Mi•+Vm′E2

Metal ions like Li continue to occupy the same site (denoted by Mmx), while vacated sites (denoted by Vix), newly occupied sites (denoted by Mi•) and vacated sites (denoted by Vm′) are created. Neutral, positive and negative charges are represented by the superscripts x, • and ′. It follows that the electrolytes need to have enough charge carriers for proper ion movement, including that of lithium ions. As a result, according to the definition in (3), ionic conductivity, σ, is proportional to the density of the charge carrier, c.

σ=qxμxcE3

where q = charge of the ions and μ = mobility of ions according to the Nernst-Einstein equation, which is defined in Eq. (4).

μ=DxqkTE4

From (4) and according to constable [71], it is determined that diffusivities, D, fluctuate with temperature as a thermally activated Boltzmann process defined in Eq. (5)

D=Doe−Ed/kTE5

where k, T and Ed represent the Boltzmann constant, absolute temperature, and diffusivity activation energy, respectively. The interstices should have enough space for the Li + ions to hop around. Metal ion conductivity, in this case, Li+, σ, can be calculated using Fick’s second law and the Nernst-Einstein equation.

σT=σoe−Ed/kTE6

σo is the pre-exponential constant.

Charge transfer occurs at the interface between the electrodes (cathode and anode) and the electrolyte simultaneously with ion migration through the electrolyte. Therefore, the electrode needs to be highly conductive not only in terms of ions but also electrons. Unfortunately, the electronic conductivity of bare electrode materials is quite low. To provide reasonably good electronic conductivity, some conductive materials such as carbon and carbon derivatives should be incorporated with the electrode materials, and also electrode particles sometimes are coated with carbon layers.

6. Advantages over other battery types

Thin-film lithium-ion batteries offer improved performance due to their higher average output voltage, lighter weights, higher energy density, long cycling life (1200 cycles without degradation) and ability to operate in a wider temperature range (between −20 and 60°C) when compared with the standard lithium-ion batteries [72, 73].

Lithium-ion transfer cells stand out as the most promising systems for meeting the need for high specific energy and high power at a low manufacturing cost [74].

Each electrode in a thin-film lithium-ion battery can accept lithium ions in either direction, creating a Li-ion transfer cell. The components of a battery, including the anode, solid electrolyte, cathode and current leads, must be fabricated into multi-layered thin films using the appropriate technologies to build a thin-film battery [75, 76].

The electrolyte in a thin-film-based system is often a solid electrolyte that can take on the form of a battery. This differs from traditional lithium-ion batteries, which typically use a liquid electrolyte [77]. If the liquid electrolyte is not suitable for use with the separator, its use can be complicated. As a general rule, liquid electrolytes necessitate a larger battery, which is not ideal when trying to get a high energy density in the final product.

Polymer electrolytes, which are commonly used in thin-film flexible Li-ion batteries, can serve multiple functions, including those of electrolyte, separator and binder. Since the problem of electrolyte leakage is thus avoided, flexible systems can be built [78].

Finally, unlike traditional liquid lithium-ion batteries, solid systems can be packed together densely to maximize energy density. Thin-film batteries production have the advantage of high energy densities [79].

7. Application

More and more uses, with varying demands for energy storage, are finding it difficult to stockpile electricity. Printed batteries are only ever mentioned in the literature when the energy demand of a thin-film battery application is less than 1 Ah [5]. Since the battery thickness needed to power a gadget may be drastically reduced with thin-film lithium-ion batteries, slimmer portable electronics can be created with them. These power sources could be used in a wide variety of implantable medical devices, including; Implanted Cardioverter Defibrillators (ICD), Cochlear Implants, Implanted Drug Pumps, Implanted Pacemakers etc.

Thin-film batteries are an efficient means of storing the intermittently produced electricity from solar and other renewable energy sources. It is possible to design these batteries with a negligible self-discharge rate, allowing them to be stored for extended periods without suffering a serious loss of energy capacity [80]. When completely charged, these batteries may also provide more consistent energy for a smart electrical grid [81].

Similar in size to a conventional credit card, “smart cards” actually contain a microchip that can be read, written to, authorized or used to process information. These cards are manufactured in fairly severe environments, with temperatures reaching as high as 150°C. At this severe temperature, other batteries could suffer damage because their internal parts could degas or simply malfunction. Thin- film lithium-ion batteries, however, may operate in temperatures ranging from −40 to 150°C [82]. In addition, the durability of thin film lithium-ion batteries may be advantageous in other applications that involve temperatures that the human body cannot withstand [81].

Radiofrequency identification (RFID) tags are employed in logistics and stock management and are frequently included in discussions of the Internet of Things (IoT) [83, 84]. RFID tags have several applications, some of which include authentication, identification and security. Some RFID tags have built-in sensing technologies that can pick up data about their immediate surroundings. With a higher output battery, the RFID tag may be read from greater distances [85]. As these tags become more sophisticated, the battery demands will have to keep up, and thin-film batteries can be incorporated into these tags due to the battery’s versatility in terms of size and shape, as well as the ability to power the tag’s functions. The disposable applications of RFID technology may be made possible by the low production costs of thin-film batteries [81].

As these tags become more sophisticated, the battery demands will have to keep up, and thin-film batteries can be incorporated into these tags due to the battery’s versatility in terms of size and shape, as well as its ability to power the tag’s functions.

Batteries for implantable medical devices must be reliable and able to give power for an extended period. These batteries must have a low self-discharge rate while not in use and a high-power rate when it is being used, particularly for usage in an implantable defibrillator [86]. Batteries for implantable medical devices should have a high capacity for charge and discharge cycles so that the devices can go longer between servicing and replacement [87]. The power needs of implantable medical devices are ideally met by thin-film batteries. Since the electrolyte in thin-film batteries is solid rather than liquid, they may be shaped in a wide variety of configurations without the risk of leakage, and it has been found that certain types of thin-film batteries can withstand charging and discharging for up to 50,000 times.

The global applications of thin-film batteries cannot be fully enumerated here. They also find applications in smart cards, touch screens, wireless sensors, laptops, astronomical mirrors, photovoltaic energy generation and storage, RFID tags, implantable devices and many others.

8. Conclusion

In this chapter, the overview of types of thin-film batteries, deposition techniques, mechanisms, fundamentals, advantages and applications of thin-film batteries were given. We summarized the types of thin-film manufacturing technologies, and the choice of any technological pathway will depend on the end-use applications of the thin-film batteries. Physical and chemical methods are used to prepare thin-film batteries. These techniques are used to modify the surface properties of the batteries. In chemical vapor deposition, films are deposited from various chemical reactions to generate the vapor that is deposited on the substrate, while the physical deposition technique involves physical mixing and deposition of the mixture on the substrate without any chemical reaction. The need to address the current energy crisis using the most fascinating next-generation energy storage systems and vast applications of thin-film batteries has driven research focus on thin-film batteries. Thin-film batteries have a wide area of applications covering the Internet of Things (IoT), implantable medical devices, integrated circuit cards, smart watches, radio-frequency identifier (RFID) tags, remote sensors, smart building control, astronomical mirrors and other wireless devices.

Acknowledgments

This work was not supported by any fund and was self-funded by the authors.

Conflict of interest

The authors declare no conflict of interest.

Arshad Ali