Energetic and durable all-polymer aqueous battery for sustainable, flexible power

The study addresses the long-standing challenge of achieving high-performance, safe, and sustainable flexible batteries by developing all-polymer aqueous sodium-ion batteries (ASIBs). Conventional Li-ion systems rely on scarce minerals and flammable organic electrolytes, raising safety and sustainability concerns. Aqueous Na-ion batteries offer intrinsic safety and abundant sodium resources but suffer from limited suitable electrode materials, low energy density, and poor cycling stability. Organic electrodes provide tunable structures and sustainability, yet few function as anodes at low potentials and aqueous cathode options are typically limited and underperform. Polyaniline (PANI), a well-known conducting polymer with multiple redox states, in principle could serve as both cathode and anode via p- (anion) and n- (cation) doping. However, in aqueous media, highly oxidized pernigraniline salt (PNS) undergoes deprotonation and the fully reduced metal pernigranilate (MPN) is unstable due to protonation of nitrenes, preventing stable redox cycling; prior symmetric PANI aqueous devices thus behaved like supercapacitors without distinct redox. The research goal is to design an electrolyte that stabilizes PANI’s redox products, enabling true battery behavior with symmetric PANI electrodes, high voltage stability, and durable cycling suitable for flexible, sustainable power.

The work builds on: (1) growing interest in flexible and safe energy storage for wearables; (2) limitations of traditional Li-ion batteries (flammable organic electrolytes, sustainability issues); (3) progress and remaining challenges in aqueous Na-ion batteries, where energy density and cycle life are constrained by electrode materials; (4) promise of organic/polymer electrodes for aqueous batteries due to molecular tunability and sustainability, though few organic anodes operate effectively in aqueous Na systems and cathodes often remain inorganic or low-capacity; (5) extensive prior studies of PANI as a cathode in various systems, but in aqueous electrolytes highly oxidized PANI tends to deprotonate and reduced PANI is prone to protonation, undermining stability; and (6) electrolyte-structure strategies such as water-in-salt and molecular crowding to expand aqueous electrochemical stability windows. Previous symmetric PANI aqueous devices primarily exhibited capacitive behavior without robust redox peaks. This study leverages polymer–aqueous cosolvents to modulate water activity and interfacial chemistry to overcome these limitations.

• Electrolyte design: Prepared a polymer–aqueous electrolyte (PAE) consisting of PEGDME–H2O cosolvent with 2 m NaTFSI. PEGDME molecular weight and salt concentration were screened; 2 m NaTFSI–PEGDME(450)–H2O was selected for optimal balance of electrochemical stability window (ESW) and ionic conductivity. Comparative formulations (including NaClO4, NaOTf, NaFSI in PAE) were also tested.
• Solvation and water-structure characterization: Employed 1H NMR, FTIR, and Raman spectroscopy to analyze hydrogen-bond networks and O–H vibrational modes, quantifying fractions of symmetric/asymmetric H-bonded and non-H-bonded water. 23Na NMR and Raman of C–O–C stretching quantified PEGDME’s participation in Na+ solvation layers and the fraction of solvated ether groups.
• Electrode material and cell assembly: Synthesized PANI (emeraldine base) and a carbonyl-polymer PNFE (for comparison). Fabricated symmetric PANI coin cells (CR2032) using Ti mesh current collectors, KB conductive agent, and PTFE binder (7:2:1). Separator: Whatman GF/D glass fiber; electrolyte: 2 m NaTFSI–PAE. Flexible film and fiber batteries were fabricated via roll-to-roll coating on flexible meshes and CNT fibers, encapsulated in SEBS pouches and parylene, with electrolyte infiltration.
• Electrochemical testing: Three-electrode setup to determine ESW and conductivity; CV (0.1–2.2 V, 0.1 mV/s), GCD cycling at 1C (147 mA/g) between 0.1–2.2 V, EIS (10^-2–10^5 Hz). LSV of PAE to probe SEI reduction onset. Kinetic analysis using i = a v^b and I(v) = k1 v + k2 v^0.5 to separate capacitive and diffusion contributions.
• Mechanistic and interfacial studies: DFT (VASP, PBE-GGA, DFT-D2) to compute Gibbs free energy changes for dual-ion doping (Na+ n-doping at anode, TFSI− p-doping at cathode) and theoretical capacity. Operando-inspired ex situ FTIR, Raman, and XPS (including sputter-depth profiling) on electrodes at defined states to track redox states, anion/cation doping, and SEI composition. SEM to examine electrode surface morphology evolution.
• Flexibility and recyclability: Demonstrated charge–discharge under bending (90°) for film and fiber batteries; cutting tests for safety. Recycled PANI from spent cells via solvent washing (water/ethanol, IPA) and dissolution in NMP, followed by reuse in new cells tested by GCD.

• Electrolyte and solvation: The PAE expands the aqueous ESW to ~3.2 V without high salt concentration. Spectroscopy shows PEGDME disrupts the bulk H2O H-bond network: disappearance of strong tetrahedral H-bonded component (Raman 3249 cm^-1), blue-shifted O–H modes, and increased fractions of asymmetric H-bonded and non-H-bonded H2O. In PAE, asymmetric H-bonded H2O dominates (53–69%). 23Na NMR chemical shift at −7.51 ppm indicates PEGDME-dominant Na+ solvation with minor H2O participation; Raman confirms significant C–O–C involvement in solvation.
• Symmetric PANI mechanism: DFT indicates intrinsic PANI can lose two electrons via TFSI− p-doping (ΔG ≈ −5.88 eV, spontaneous) and gain two electrons via Na+ n-doping (energy cost 7.48 eV), enabling a two-electron-transfer overall process with theoretical capacity ~147 mAh/g. Three-electrode tests give practical capacities: cathode 135 mAh/g (0–1.0 V), anode 130 mAh/g (−1.0–0 V). CV kinetics show mixed diffusion/capacitive behavior; diffusion dominates below 1 mV/s.
• Electrochemical performance: Symmetric PANI||PANI all-polymer ASIB delivers 139 mAh/g specific capacity and 153 Wh/kg energy density at 1C, retaining 92.0% capacity after >4800 cycles (381 days) with ~99.5% average coulombic efficiency. High-voltage redox of PANI cathode (0.1–2.2 V) is reversible in PAE but not in 2 m NaTFSI–H2O.
• Interfacial chemistry (SEI): An anodic feature near −1.8 V in first CV cycles and an LSV reduction peak at −1.23 V indicate SEI formation. EIS shows emergence of RSEI and reduced Rct after the first cycle. SEM reveals increasingly smooth, dense anode surfaces with cycling. Depth-profile XPS identifies SEI dominated by sodium alkoxides/carboxylates (R–OCO2Na) attributed to PEGDME decomposition, with NaTFSI-rich outer layers and NaOH/NaF enriched inner layers; NaF may partially arise from beam/sputtering-induced salt decomposition. Na species ratio analysis corroborates a multilayer SEI architecture that stabilizes n-doped PANI anodes.
• Flexibility and recyclability: Flexible film batteries maintain ~135 mAh/g under 90° bending; fiber batteries ~122 mAh/g under bending. Devices remain operational after cutting. Series-connected film cells power LEDs and sensors. Recycled PANI electrodes deliver ~140 mAh/g initially with ~70% retention after 200 cycles, demonstrating facile materials recovery.

The central challenge—stabilizing the redox end-products of PANI in neutral aqueous media—was addressed by engineering a polymer–aqueous electrolyte that simultaneously lowers water activity via disrupted hydrogen-bond networks and supports stable interfacial chemistry. PEGDME-centered Na+ solvation layers and reduced free water reactivity widen the ESW, enabling the PANI cathode to avoid deprotonation at high states of oxidation and allowing reversible TFSI− p-doping. On the anode side, the formation of a dense, inorganic–organic SEI dominated by PEGDME-derived sodium carboxylates (with NaOH/NaF inner components) passivates the surface and facilitates Na+ n-doping in a protic medium—previously considered untenable for PANI. Together, these electrolyte and interface advancements convert symmetric PANI electrodes from capacitive behavior to true battery-type operation with diffusion-dominated kinetics at practical rates. The resulting device exhibits state-of-the-art cycling durability and competitive energy density among aqueous Na-ion systems while using metal-free organic electrodes and a mild 2 m electrolyte, aligning with sustainability goals and enabling flexible form factors. These findings are broadly relevant for designing polymer electrodes and polymer–aqueous electrolytes for high-energy, safe, and recyclable wearable power.

This work demonstrates an energetic and durable all-polymer aqueous sodium-ion battery using symmetric polyaniline electrodes enabled by a polymer–aqueous electrolyte (2 m NaTFSI in PEGDME–H2O). By modulating water structure and Na+ solvation and promoting a robust SEI, the electrolyte stabilizes both highly oxidized and reduced PANI states, unlocking a dual-ion (TFSI−/Na+) doping mechanism. The battery achieves 139 mAh/g, 153 Wh/kg, and 92% capacity retention after 4800 cycles with ~99.5% CE, and remains functional in flexible film and fiber formats. Spectroscopic analyses elucidate hydration structure, solvation, and interphase composition centered on PEGDME-derived sodium carboxylates. Recyclability of PANI is demonstrated with retained performance after reuse. Future directions include tailoring polymer backbones and side chains for targeted redox potentials, optimizing polymer–aqueous cosolvents and anion chemistry for faster kinetics and wider ESW, and integrating device-level controls (e.g., constant-voltage charging) for stable operation in diverse wearable scenarios.

• Mechanistic uncertainties remain for the initial n-doping of PANI anodes: FTIR suggests reduction may involve protons from water, and SEI composition includes NaOH; the precise sequence and contributions of Na+ versus proton-coupled processes are inferred rather than directly observed.
• SEI composition analyses rely on XPS with ion sputtering; partial salt decomposition (e.g., NaF formation) is acknowledged and can complicate quantitative depth profiling.
• While the electrolyte generality was probed with alternative anions (NaClO4, NaOTf, NaFSI), these showed lower conductivity or inferior cycling, indicating that performance may be sensitive to specific salt–polymer–water interactions.
• Conducting polymers typically lack flat voltage plateaus; practical applications may require circuit-level charge control. Rate-dependent behavior indicates mixed capacitive/diffusive storage, potentially affecting high-rate operation outside reported conditions.