A recent article in Nature Communications presented a polymer-aqueous electrolyte for stabilizing polymer electrode redox products by modulating the solvation layers and developing a solid-electrolyte interphase. Dual-functional polyaniline (PANI) was used as the anode to improve the high-voltage stability of the polyaniline cathode in a polymer-aqueous electrolyte (PAE).
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Background
Lithium-ion batteries are widely used in portable electronics, but their reliance on mineral resources and the use of toxic and flammable organic electrolytes present significant challenges in terms of safety and sustainability.
Aqueous sodium-ion batteries (ASIBs) have emerged as a more sustainable alternative due to the abundance of sodium sources and the intrinsic safety of aqueous electrolytes. However, ASIBs lack suitable electrode materials, restricting their practical applications due to limited energy density and poor cycling stability.
Organic electrode materials are a promising alternative in ASIBs because of their high capacity, sustainability, and design flexibility. Given the need for flexibility and processability in batteries for flexible electronics, all-polymer ASIBs using polymer electrodes show potential.
This study presents an all-polymer ASIB with symmetric PANI electrodes and a PAE. PANI is a cost-effective, easily synthesized polymer electrode material with multiple redox states.
Methods
PANI was synthesized through a two-step process: dissolving aniline monomer in HCl solution, followed by the slow addition of ammonium persulfate (APS) at 0–5 °C. The PAE was then prepared using sodium bis(fluorosulfonyl)imide (NaTFSI) and poly(ethylene glycol) dimethyl ether (PEGDME).
Symmetric PANI coin batteries were assembled with PANI as the active material, ketjen black as the conductive agent, and polytetrafluoroethylene (PTFE) as the binder. An asymmetric PANI coin battery was assembled using two electrode sheets, a piece of Whatman glass fiber membrane, and NaTFSI-PAE.
Flexible film batteries were prepared using a similar procedure, with electrodes deposited on flexible stainless-steel mesh. A styrene-ethylene-butylene-styrene pouch was used for encapsulation. Finally, fiber batteries were fabricated using carbon nanotubes, synthesized via continuous chemical vapor deposition, as the current collectors.
The electrochemical stability windows of the electrolytes were evaluated using a three-electrode system with acetylene black as the working electrode, active carbon as the counter electrode, and an Ag/AgCl reference electrode. The electrochemical performance of the all-polymer batteries was assessed using coin cells through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) tests.
The electrode materials were characterized using nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FTIR) spectrometry, and scanning electron microscopy (SEM). Additionally, spin-polarized density functional theory (DFT) calculations were conducted to investigate the electrochemical mechanisms.
Results and Discussion
The synthesized all-polymer battery delivered a specific capacity of 139 mAh/g and an energy density of 153 Wh/kg at a 1 C rate. It maintained 92.0 % of its capacity after 4800 cycles (381 days), with an average coulombic efficiency (CE) of 99.5 %, demonstrating greater stability than most ASIBs and aqueous Li-ion batteries. The proposed strategy using PAE effectively stabilized the highly oxidized/reduced PANI.
FTIR was used to monitor the structural changes of PANI cathodes during the charging process in two different electrolytes: NaTFSI-PAE and NaTFSI-H2O. The benzenoid-quinonoid transformation in PANI cathodes was highly reversible after discharge in NaTFSI-PAE. In contrast, the quinonoid structure of the PANI cathode in NaTFSI-H2O became significantly broader and stronger than that of pristine PANI, even after full discharge.
The enhanced stability of the highly oxidized PANI cathode was attributed to the lower activity of H2O in PAE. Additionally, the anion species played a key role in the stability of the PANI cathode. This was confirmed by testing different electrolytes (with smaller anions than NaTFSI) in symmetric PANI batteries, all of which showed lower ionic conductivities and reduced cycling stability compared to NaTFSI-PAE.
Regarding electrode material recyclability, PANI was easier to recycle than other inorganic electrode materials. PANI could be separated from the electrode materials by washing with methylpyrrolidone after long-term use in the ASIB. FTIR and GCD analysis showed that recycled PANI could be reused as an active electrode material.
Conclusion
The researchers successfully fabricated all-polymer ASIBs using intrinsic PANI as symmetric electrodes. The instability issues of PANI electrodes, which arise from fully oxidized pernigraniline and fully reduced metal pernigranilate in neutral aqueous electrolytes, were addressed by using PAE with a wide electrochemical stability window and a dense solid-electrolyte interphase.
Systematic characterizations, including FTIR and NMR, revealed the structural variations of Na-ion solvation layers and H2O in PAE, which decreased the H2O activity. DFT simulations further confirmed that PANI could function as symmetric electrodes via a dual-ion doping mechanism in PAE.
The fabricated flexible battery showed a higher capacity than most advanced film/pouch Li/Na-ion aqueous batteries. Additionally, batteries reassembled with recycled PANI demonstrated ideal performance. These findings contribute to the development of low-cost, high-energy organic electrodes and aqueous electrolytes for sustainable and flexible energy storage.
Journal Reference
Hong, Y., et al. (2024). Energetic and durable all-polymer aqueous battery for sustainable, flexible power. Nature Communications. DOI: 10.1038/s41467-024-53804-2, https://www.nature.com/articles/s41467-024-53804-2
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