The growing demand for renewable energy systems and high-energy-density applications has intensified the need for advanced and more efficient energy storage technologies. Lithium-ion batteries, though historically valued for their versatility, efficiency and energy density, are approaching their theoretical limits. Their reliance on liquid electrolytes continues to raise concerns regarding interfacial stability, longevity, and safety. Solid-state electrolytes, particularly solid polymer electrolytes (SPEs), have emerged as safer and more stable alternatives, compatible with alkali metal anodes. Simultaneously, embedded sensors enable in situ operation monitoring and improve battery safety and performance. This dissertation, developed within an FCT-funded project on smart battery components, focused on synthesising SPEs based on sodium carboxymethyl cellulose (NaCMC), doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and plasticised with glycerol. A systematic set of compositions was prepared by varying dopant concentrations to identify optimal formulations for solid-state batteries. Membranes were characterised for optical, chemical, structural, thermal, mechanical, electrochemical, and surface properties using techniques such as ultraviolet-visible spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy, electrochemical impedance spectroscopy (EIS), Xray diffraction, polarised optical microscopy, scanning electron microscopy, thermogravimetric analysis, tensile testing, and contact angle measurements. The highest ionic conductivity (4.40 × 10-4 S/cm) was achieved for the NaCMC-based membrane with 150% LiTFSI and a 1:2 NaCMC-to-glycerol ratio, although mechanical fragility limited applicability. The 1:1 formulation with 50% LiTFSI offered a better compromise, with conductivity of 2.62 × 10-5 S/cm, Fmax = 3.0 ± 1.4 N, and εb = 58 ± 15%. All membranes exhibited thermal stability of at least 160 °C. Additionally, exploratory experiments assessed membrane interactions with loop antenna-based sensors. Electrical characterisation revealed signal shifts upon membrane application, supporting sensor sensitivity and SPE integration. Preliminary lithium-metal halfcells were also assembled and characterised via cyclic voltammetry, galvanostatic charge and discharge, and EIS, establishing protocols for future integration of the synthesised SPEs. Overall, this study provides a comprehensive assessment of biopolymer-based SPEs as multifunctional battery components, co

The main goal of this PhD project was to develop fiber-based energy storage devices, commonly also known as textile-based electrochemical energy storage devices (TEESDs), namely supercapacitors (SCs). By incorporating sustainable processes and materials, two main architectures of fiber-based 1D and 2D, were explored and integrated into textiles. The developed 1D fiber-shaped supercapacitors (FSCs) combine stretch-broken carbon fiber yarns (SBCFYs) as both current collector and active material, paired with an in-situ regenerated cellu-lose-based ionic hydrogel (RCIHs) as electrolyte. The SBCFYs and cellulose can be recovered for reuse, allowing the fabrication of new 1D FSCs without significant loss in electrochemical performance, even after 2 years and 5 months, owing to the hydrophilic nature of cellulose and stability of the SBCFYs. The hybridization of SBCFYs was further explored with V2O5, MnO2 and MoS2 as active mate-rials with a particular focus on MoS2. Hydrothermal synthesis using conventional- and microwave-as-sisted heating (CAH, MAH) was conducted. MAH hybridized SBCFYs demonstrate 95.7 % capacitance retention after 3000 cyclic voltammetry (CV) cycles. Moreover, both hybridized SBCFYs displayed capacitive behavior even after 60 days of being synthesized through both approaches. The 2D architecture was explored by utilizing screen-printing with commercially available inks, including silver, carbon, and PEDOT:PSS, as current collectors and active materials. An impregnated cotton fabric with cornstarch-based electrolyte (CotCSBE) was applied, providing an adhesive and ro-bust interface between the symmetric parts of the device. The influence of the number of PEDOT:PSS printed layers on devices performance was investigated, achieving capacitances of 5.51±0.44 mF·cm-2 at 5 mV·s-1, with 81.93 % retention after 10,000 galvanostatic charge-discharge (GCD) cycles for four layers. The potential of using reduced graphene oxide (rGO) and laser induced graphene (LIG) as both current collectors and active materials in the 2D architecture was also considered. The fabricated 1D and 2D SCs exhibited versatility on different prototypes. Six woven FSCs connected in series light up three red LEDs. Additionally, five FSCs, hybridized via MAH, connected in series and hand-stitched onto a cotton fabric, demonstrated their ability to power a humidity and temperature sensor for up to 6 minutes. Six printed and textile-based 2D SCs, also connected in series, continually powered a watch for 1 hour.

Printed and flexible electronics have gained considerable scientific attention in recent years, driving the demand for low-energy production techniques, eco-friendly materials and flexible substrates. However, effective encapsulation is essential to protect these devices in harsh environmental conditions. Thus, sustainable encapsulant materials are critical for advancing flexible electronics. In this work, we studied three encapsulant materials—commercial plastic, polyvinyl alcohol and ethyl cellulose—applied to thermoelectric touch sensors printed on paper and fabric substrates. Ethyl cellulose demonstrated promising properties in terms of flexibility, water resistance and transparency, along with a low carbon footprint. Encapsulated substrates with ethyl cellulose exhibited high contact angles (121° on fabric and 116° on paper), indicating robust water repellency. Thermal stability tests showed minimal mass loss (10%) at 315 °C, confirming its temperature resilience. Furthermore, sensors encapsulated with ethyl cellulose retained their electric performance after water submersion for 1 min and withstood 100 bending cycles, maintaining response times below 1 s and signal output around 100 µV. These findings highlight ethyl cellulose as a viable green encapsulant material compatible with large-scale sustainable electronics manufacturing.