Development of air-stable polyethylene oxide based polymer electrolytes for printable batteries

Since numerous objects will be wirelessly connected to each other through the Internet of Things (IoT), new emerging miniaturized and customized electronics such as wearable applications and bio-implantable devices have attracted great interest. To meet the growing demand for unconnected portable electronic devices, rechargeable Li-ion batteries (LIBs), one of the most promising power sources, are expected to be miniaturized while maintaining high energy density and long cycle life, and offering scalable, lightweight, and unlimited form factors. Furthermore, the high design flexibility of LIBs and their ability to be directly integrated into arbitrarily shaped target applications will be advantageous. To address these demands, various printable technologies such as screen printing, spray coating, inkjet printing, transfer printing, 3D printing, and 4D printing have been well developed. These techniques play a critical role in the production of flexible and compact battery systems by enabling the production of electrode and electrolyte components in the desired geometries, thin film form, and multi-layered structures. Considering the flexibility and high area loading of printable micro-scale energy storage devices, screen printing can flexibly print on various substrates. Compared to spray coating and 3D printing, it can improve high area loading with a low-cost repeatable printing process. Additionally, with the discovery of a wide range of printable binders and solvents, screen printing can achieve increased precision compared to inkjet printing and transfer printing. These features make screen printing a promising technology for the scalable production of microelectronics. Furthermore, eco-sustainable screen printing technology offers a compatible approach with the "green ink" concept, formulated using environmentally friendly solvents and binders that are easily biodegradable in nature. The term "green ink" refers to ink systems composed of non-toxic, biodegradable, or environmentally harmless ingredients. Therefore, water-based screen printing ink is more suitable for the large-scale production of wearable microelectronics. In order for printing techniques to become applicable in battery technologies, not only the electrodes but also all other components such as the electrolyte, current collector, and separator must be designed to be compatible with this method for production. Each component must be chemically, rheologically, and morphologically compatible with the printing process, which is critical for overall system performance. Therefore, for this technology to be successful, all components of the battery must be considered and developed. Among these, the most important are the electrolytes. The electrolyte is one of the fundamental components of lithium-ion batteries and is responsible for ionic transport. The specific capacity, cycle performance, operating temperature range, and safety issues of batteries are all significantly influenced by the selected electrolyte. Armand, in his first publication in 1970, proposed the use of polymer electrolytes to enhance the energy density and efficiency of LIBs. Polymer electrolytes possess distinctive qualities such as dimensional flexibility, transparency, lightweight, ease of production, flexibility, and the ability to establish efficient contact between the electrode and electrolyte. Polymer-based electrolytes (PEs) are relatively free from safety issues such as explosions and fires compared to liquid electrolytes. Most importantly, they possess good processability and flexibility that can be easily applied to the screen printing process. The selection of an appropriate polymer, inorganic salt, and ionic liquid is crucial for preparing good gel polymer electrolytes in printable systems. To date, poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA), and copolymer poly(vinylidene fluoride hexafluoropropylene) (PVDF-HFP) have been complexed with various inorganic salts and doped with various ionic liquids to form gel polymer electrolytes, and their detailed properties have been investigated. Poly(ethylene) oxide (PEO)-based polymer electrolytes have been extensively studied because PEO has a high dielectric constant (ε: 10⁻¹⁵) and can easily form coordination complexes with lithium salts. Additionally, the ability of ether oxygen to coordinate with lithium is highly effective for achieving fast Li-ion movement. PEO has low lattice energy, a low glass transition temperature, high solubility for alkali metal salts, and ease of forming flexible films. However, PEO's semi-crystalline structure leads to low ionic conductivity in PEO-based electrolytes at room temperature. Additionally, due to the lower mobility of Li+ cations compared to anions, PEO-based electrolytes typically exhibit a low Li-ion transfer number (TLi+), which leads to a strong electric field and irregular lithium electrodeposition according to the charge concentration model, resulting in irregular lithium dendrite growth and even battery short-circuiting. To address these issues with PEO-based electrolytes, various attempts have been made, including the use of cross-linking agents and functional fillers. It is well known that glims (of various lengths) form complexes with metal ions through their multiple ether-like oxygen atoms. Tetraglim (TEGDME) exhibits excellent properties in terms of compatibility with lithium metal electrodes and suppression of dangerous dendrite growth. TEGDME contains methylene groups that can undergo hydrogen abstraction and subsequent radical reactions to form oligomers or bind to adjacent PEO chains. The ionic conductivity in a PE is always dependent on the polymer and salt concentrations calculated using stoichiometric calculations. The properties that salts must possess include: low lattice energy and high ionic conductivity, high mobility and a wide voltage stability window, lower cation radius and larger anion radius, high thermal and chemical stability, high lithium transfer number, and inertness toward cell components. LiTFSI has been selected as a doping salt due to its volumetric structure, which enhances electrochemical stability. It dissolves well in low dielectric solvents, is non-corrosive to electrodes, and exhibits excellent thermal stability with a decomposition temperature of 360 °C. The TFSI anion also possesses a significant amount of delocalized electrons. The performance of PEO-based gel polymer electrolyte systems is not only related to the structural properties of the polymer, but also directly influenced by factors such as the volatility, polarity, dielectric constant, and interaction with the electrode interface of the solvent systems used. Therefore, in this thesis studies, the effects of 1,3 dioxolane (DIOX) and acetonitrile (ACN) additives on ionic conductivity, electrochemical stability, and film-forming properties were evaluated in the solvent system, with a particular focus on comparative analysis. Additionally, succinonitrile (SN) was investigated in detail as an additional additive component. DIOX stands out as an effective solvent that supports stable film formation, particularly in casting processes carried out in open atmospheres, thanks to its low vapor pressure and high solubility compatibility with PEO. Its moderate dielectric constant provides a balanced environment in the solvent–polymer–salt triple interaction. DIOX's controlled evaporation rate facilitates even ink distribution during screen printing, reduces screen clogging caused by sudden drying, and supports the formation of homogeneous layers. In contrast, ACN has the ability to increase salt solubility due to its low viscosity and high polarity. However, its low boiling point (~82 °C) and high vaporization rate negatively affect film continuity when used as a solvent in open atmosphere conditions and disrupt the stability of the ink system during printing. Additionally, ACN's strong interactions with electrode surfaces can limit electrochemical stability during prolonged cycles and may lead to decomposition under high voltage conditions. For these reasons, ACN has been found to be disadvantageous for integration with screen printing processes. Succinonitrile (SN) was evaluated in this thesis not as a solvent but as an additive to support the ionic environment and enhance the transport of Li⁺ ions. Thanks to its high dielectric constant (ε > 50), it positively contributes to the overall ionic conductivity by supporting the ionic dissociation of lithium salts such as LiTFSI. Its low viscosity may enhance segmental chain mobility, allowing ions to move more freely within the polymer matrix. However, the fact that SN has a solid crystalline structure that melts at approximately 57–60 °C may cause it to exhibit "plastic crystal" phase behavior at specific temperature and concentration ranges. This property may have the potential to form microscopic crystal clusters or phase separations within the polymer system. Such behavior, particularly under high C-rate conditions or prolonged cycling, can lead to structural degradation at the electrolyte–electrode interface, capacity loss, or reduced ion transport continuity. Therefore, while SN is an important ionic environment regulator contributing to initial performance, it is considered a component that requires careful optimization in terms of system stability, taking into account its crystallization tendency and polar character. In conclusion, this thesis studies demonstrates that the developed solvent/plasticizer systems play a decisive role not only in ionic performance but also in multi dimensional performance criteria such as printability, drying behavior, atmospheric stability, and cell stability. The PEO-STD formulation is considered a strong candidate for flexible, environmentally friendly, and printable battery technologies due to its technical properties and production process compatibility. With these characteristics, it holds significant potential for future wearable energy systems and printable microelectronic applications.