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Printed Batteries: An Overview
Juliana Oliveira1, Carlos Miguel Costa1,2 and Senentxu Lanceros-Méndez1,3
1 Center of Physics, University of Minho, Gualtar campus, Braga, Portugal
2 Center of Chemistry, University of Minho, Gualtar campus, Braga, Portugal
3 BCMaterials, Basque Center for Materials, Applications and Nanostructures, Spain
1.1 Introduction
Increasing technological development leads to the question of how to efficiently store energy for devices in the fields of mobile applications and transport that need power supply [1, 2]. Energy storage is thus not only essential but also one of the main challenges that it is necessary to solve in this century [2, 3].
Further, energy storage systems are also increasingly needed, among others, to suitably manage the energy generated by environmentally friendly energy sources, such as photovoltaic, wind and geothermal [4, 5].
Batteries are the most-used energy storage systems for powering portable electronic devices due to the larger amounts of energy stored in comparison to related systems [2, 6]. Among them, the most widely used battery type is lithium-ion batteries, with a market share of 75% [7].
Anode, cathode and separator/electrolyte are the basic components of a battery, the cathode (positive electrode) being responsible for the cell capacity and cycle life. The anode (negative electrode) should show a low potential in order to provide a high cell voltage with the cathode [8-10].
The separator/electrolyte is placed between the electrodes as a medium for the transfer of lithium ions and also to control the number of lithium ions and their mobility [11].
Advances in the area of batteries in relation to printed technologies is expected to have a large impact in the growing area of small portable and wearable electronic devices for applications such as smart cards, RFID tags, remote sensors and medical devices, among others. This in fact originated in the development and proliferation of smart and functional materials and microelectromechanical systems (MEMS) needing on-board power supply to provide capacities of 5 to 10 mAh.cm-2 with overall dimension of <?10?mm3 [12-14].
The technological advances of the past years and the need for low-cost and simple processing leads to the potential replacement, in some areas, of conventional processing technologies by printed technologies, as evidenced in applications such as sensors, light-emitting devices, transistors (TFT), photodiodes, flat panel display solar cells and batteries, among others. Printed technology characteristics such as low cost, large area, high volume, light weight, and the processing of multilayered functional structures on rugged and flexible substrates, pave the way for new production paradigms for specific application areas [15-17].
In fact, it is expected that the global market for printed electronics will reach $45 billion in 2017 and is estimated to exceed $300 billion over the next 20 years [15, 16, 18].
This fact is also evidenced by the many articles published in scientific journal about inks and printed electronics, as shown in Figure 1.1.
Figure 1.1 Research articles published related to inks and printed electronics. Search performed in Scopus database with the keywords "inks" and "printed electronics" on 19 June 2017.
Printed materials for electronics can be applied on different substrates such as paper, plastics and textiles, giving origin to the term "flexible electronics". Typically, the most frequently used printing techniques for printed electronics are ink-jet and screen-printing [19], but related cost-efficient and high-throughput production techniques such as solution-processing techniques including spin, spray, dip, blade and slot-die have been used, as well as gravure, flexographic and offset printing technologies [20, 21].
The different printing techniques require the use of specific inks with accurate control of viscosity and surface tension, among other things [22, 23]. Further, for specific printing techniques, the ink properties should be adjusted taking into account the specific pattern to be printed [24].
Printed electronics requires the use of different types of inks such as dielectric, semi-conductive or conductive, which are used to print the different active layers of the devices. Further, inks with piezoelectric [25], piezoresistive [26], and photosensitive [27] properties, among others, have been developed for the fabrication of sensor devices. Typically, inks can be defined as colloidal solutions as the result of a dispersion of organic and/or inorganic particles with specific size into a polymer solution [28]. Moreover, these inks must be cheap, reliable, safe to human health, and processable at temperatures below 50?°C. Further, the inks should preferentially show mechanical robustness, flexibility and recyclability [29].
Independent of the printing process, the ink should be distributed on the substrate with a specific pattern in a reproducible way, which strongly depends on its rheological properties [30].
The rheological properties (flow behavior, flow time and tack) of the ink can be evaluated by using the rotational viscosimeter to measure the viscosity as a function of shear rate, as the material is subjected to multiple shear rates during material processing.
In particular, it is important to prevent the agglomeration or sedimentation of the particles through attractive/repulsive forces, which depends on processing shear rate, as this will strongly affect the final properties of the printed layer [31].
At low shear rate, the viscosity of the inks is higher due to the attraction between particles, which induces their flocculation and immobility. At higher shear rates, the viscosity of the inks decreases through the low flocculation and higher mobility of solvent entrapped between particles [32, 33]. However, the viscosity of printing inks is not only a function of the shear stress but also of time, which plays an important role in the flow process of the ink for each printed element [30].
Further, the physical and chemical stability of the inks is affected by the different fabrication steps (stirring, dispersion, etc.), in which the energy input and mixing time influence both particle stability and degree of dispersion [34].
The combination of printing and battery technologies gives rise to printed batteries; for this at least one of the components should be processed and deposited through printing techniques in order to keep that designation [12, 35].
Figure 1.2 shows the origin of the denomination and the main applications of printed batteries.
Figure 1.2 An overview of printed batteries and main applications.
Further, flexible/stretchable batteries [36, 37] and solid-state microbatteries [38] can be included within the printed battery area when one or more components are produced by printing technologies. In addition, there are usually non-printed components such as the current collector, which also serves as support for the printed structure.
Inks for printed batteries are typically composed of a polymer binder, a solvent and suitable fillers, depending on the layer type: electrodes and separator/electrolyte [35]. Suitable fillers are in the form of micro/nanoparticles, nanoplates, nanowires, carbonaceous matter or ionic liquid, among others [29]. The proper transfer of the ink from the printing plate to the substrate is the main function of a printing process [30].
In the field of printed batteries, ink rheology is one of the key issues, due to the high active material loading that may be necessary for proper battery performance. This ink rheology depends mainly on particle size, solid loading concentration and solvent type [39, 40], with adequate ink showing moderate viscosity and weak sedimentation behavior resulting in an homogeneous particle system within a polymer network [31].
The main printed battery component is the electrode (anode and cathode) [22], and different inks have been reported in the literature based on different active materials such as lithium cobalt oxide (LiCoO2) [41] and lithium iron phosphate (LiFePO4) [40] for the cathode, and graphite [42], mesocarbon microbeads (MCMBs) [43] and tin oxide (SnO2) for the anode [44]. The active material content of the electrode affects its thickness, which in turn influences battery capacity: increasing electrode thickness leads to mass transport limitations of lithium ions in the electrolyte phase leading to a reduction in the capacity of the cell [45, 46]. Also the porosity of the electrodes has a strong impact on battery performance as it influences the effective electronic and ionic conductivity values [47].
On the other hand, the separator/electrolyte has not been printed very often due to the necessary low ionic conductivity, which leads to the use of composite gel electrolytes to achieve ionic conductivity values closer to those of conventional electrolytes [35]. The separator/electrolyte component of printed batteries is mainly based on composite gel electrolytes where the separator layer is soaked in an organic liquid electrolyte (salt dissolved into an organic solvent or ionic liquid to produce an ion-conducting solution in an inert porous polymeric membrane) in which it is important to control the swelling process [35, 48,...
