The increasing demand for wearable technology necessitates the development of flexible and safe batteries. While lithium-ion batteries dominate portable electronics, their reliance on mineral resources and flammable/toxic electrolytes raises safety and sustainability concerns. Aqueous sodium-ion batteries (ASIBs) offer a more sustainable alternative due to the abundance of sodium and their intrinsic safety. However, the limited energy density and poor cycling stability of current ASIBs hinder their practical applications. Organic electrode materials are promising candidates for ASIBs due to their structural designability, high capacity, and sustainability. Despite this potential, the low redox potential of many organic materials limits their use, with only a few carbonyl derivatives successfully employed as anodes, and cathodes primarily consisting of activated carbon and other inorganic materials with inadequate capacity and cycling stability. Current flexible aqueous alkali-metal-ion batteries also mostly rely on inorganic materials, predominantly in Li-ion systems, furthering sustainability concerns. The use of polymer electrode materials in all-polymer ASIBs is crucial for achieving the required flexibility and processability for flexible battery applications. This research addresses these challenges by introducing an energetic and flexible all-polymer ASIB using polyaniline (PANI) as symmetric electrodes with a polymer-aqueous electrolyte (PAE). PANI is a cost-effective, easily synthesized polymer with multiple redox states, making it a suitable candidate for symmetric electrodes. However, the conventional understanding of PANI's instability in protic conditions as a cation-doped anode has limited its application in aqueous electrolytes. This study overcomes this limitation through innovative electrolyte design, aiming to create a high-performance all-polymer ASIB with PANI as symmetric electrodes.

Extensive research has been dedicated to improving the performance and sustainability of batteries for wearable electronics. Aqueous batteries, particularly ASIBs, have emerged as a potential solution due to their inherent safety and the abundance of sodium resources. Several studies have explored the use of organic electrode materials to improve capacity and sustainability, but challenges persist regarding their low redox potentials and cycling stability. Many works have focused on carbonyl derivatives for anodes and activated carbon-based materials for cathodes, but these combinations often fall short in terms of energy density and long-term performance. The field of flexible batteries has also seen significant advances, with researchers exploring various materials and designs to achieve flexibility and good mechanical properties. However, most flexible aqueous batteries still utilize inorganic electrode materials, which limits their sustainability. This study builds upon the existing body of research by focusing on the potential of all-polymer batteries, particularly using polyaniline as a sustainable and readily processible electrode material.

The study began with the design and synthesis of a polymer-aqueous electrolyte (PAE) to enhance the stability of polyaniline (PANI) electrodes. Polyethylene glycol dimethyl ether (PEGDME) was chosen as the polymer component due to its low cost, low density, low volatility, and high biocompatibility. The effects of PEGDME's molecular weight and concentration on the electrochemical stability window (ESW) and ionic conductivity of the PAE were investigated. Spectroscopic techniques, including proton nuclear magnetic resonance (¹H-NMR), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy, were employed to characterize the solvation structures of water and sodium ions in the PAE. The aim was to understand how PEGDME modifies the hydration layers and interacts with water molecules to stabilize the redox products of PANI. First-principles density functional theory (DFT) simulations were conducted to investigate the dual-ion doping mechanism of PANI, supporting experimental observations. Three-electrode measurements were performed to determine the practical capacities of PANI as both cathode and anode. Electrochemical techniques such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were utilized to assess the performance of the all-polymer aqueous sodium-ion batteries (ASIBs) assembled using PANI as symmetric electrodes and the optimized PAE. The kinetics of the electrode reactions were analyzed using the empirical equations to discern the contributions of ion diffusion and surface absorption to the charge storage process. FTIR, Raman, and X-ray photoelectron spectroscopy (XPS) were used to track the redox state of both PANI electrodes during charge-discharge cycles. To characterize the solid-electrolyte interphase (SEI), scanning electron microscopy (SEM) and XPS analysis with sputtering were performed. Large-scale fabrication of flexible all-polymer film and fiber batteries was achieved using roll-to-roll and dip-coating techniques, respectively. The mechanical flexibility and performance of these flexible batteries were assessed under bending conditions. Finally, a recycling procedure for the PANI electrodes was developed and evaluated, assessing its sustainability and reusability.

The research yielded several key findings: 1. A novel polymer-aqueous electrolyte (PAE) was developed, utilizing polyethylene glycol dimethyl ether (PEGDME) to modulate the solvation layers and form a stable solid-electrolyte interphase (SEI). This PAE expanded the electrochemical stability window to 3.2 V. Spectroscopic analysis (¹H-NMR, FTIR, Raman) confirmed the disruption of the hydrogen bond network in water and the formation of stronger H-bonds between water and PEGDME, leading to reduced water activity. ²³Na NMR and Raman spectroscopy studies showed that PEGDME was the dominant component in the Na⁺ solvation layer. 2. DFT calculations and experimental results demonstrated that polyaniline (PANI) can function as both cathode and anode via a dual-ion doping mechanism in the PAE electrolyte. This is remarkable, as the n-doping of PANI in aqueous electrolytes was previously deemed impossible. 3. The all-polymer ASIBs assembled with symmetric PANI electrodes and the optimized PAE exhibited exceptional electrochemical performance: a specific capacity of 139 mAh/g, an energy density of 153 Wh/kg, and an impressive capacity retention of 92% after 4800 cycles (381 days). This outperforms most state-of-the-art aqueous sodium-ion batteries. 4. XPS and SEM analysis confirmed the formation of a SEI layer on the PANI anode, crucial for enabling n-doping in the aqueous environment. The SEI was found to be primarily composed of sodium alkoxides (RCH₂CH₂ONa). 5. The all-polymer ASIBs demonstrated excellent processability, flexibility, and recyclability. Large-scale flexible film and fiber batteries were successfully fabricated. These batteries retained significant capacity even under bending, and the PANI electrodes could be easily recycled, demonstrating the sustainability of this approach.

This study successfully addresses the long-standing challenge of developing high-performance all-polymer aqueous batteries. The use of a novel polymer-aqueous electrolyte (PAE) is critical for overcoming the instability issues associated with polyaniline (PANI) electrodes in protic solvents. The PAE's ability to modulate the solvation structure of water and sodium ions, along with the formation of a stable SEI layer, enables the reversible dual-ion doping mechanism of PANI, achieving both n-doping and p-doping in an aqueous environment. The exceptional electrochemical performance of the resulting all-polymer ASIBs clearly demonstrates the effectiveness of this approach. The demonstration of large-scale fabrication and the successful recycling of the PANI electrodes showcase the sustainability and scalability of this technology. The results have significant implications for the development of next-generation flexible and sustainable energy storage devices.

This research successfully demonstrates a high-performance, sustainable, and flexible all-polymer aqueous sodium-ion battery. The key innovation lies in the design of a polymer-aqueous electrolyte enabling stable cycling of polyaniline electrodes through dual-ion doping. This work achieves high energy density, excellent cycling stability, and facile recyclability. Future research could explore different polymer electrolytes to further enhance performance and explore other conductive polymers suitable for this type of battery design.

While the all-polymer ASIBs demonstrated exceptional performance, some limitations exist. The long-term stability of the PAE under extreme conditions (e.g., very high or low temperatures) still needs further investigation. The scalability of the roll-to-roll fabrication process may need optimization for mass production. The precise composition and formation mechanism of the SEI layer warrant further detailed study. The current study focused on PANI; future work could explore the applicability of this electrolyte and SEI formation strategy to other conducting polymers.