Solid-state, thin, and flexible electrochemical capacitors (ECs) are promising power sources for wearable electronics such as smart textiles and medical sensors. One of the key enablers for safe and high performance solid electrical double layer capacitors (EDLCs) is aqueous-based neutral pH polymer electrolytes (NPPEs) [1-2]. NPPEs containing chloride [3-5] or sulfate salts [6-8] as ion conductors, have been demonstrated high ionic conductivities with wide cell voltage window (>1.5 V, beyond the typical limit of aqueous-based systems). While nitrate solution can also offer similarly wide potential window (demonstrated in liquid electrolytes [9]), their polymer electrolytes may offer several additional advantages: (i) better compatibility with wide-range of polymers from its chaotropic nature, (ii) good thermal stability from deep eutectic temperature with water, and (iii) good water-retaining ability from hygroscopic nature that allows for higher retention than sulfates while maintaining better structural integrity than chlorides.

In this study, we aim to: (i) develop a class of high performance LiNO₃ based NPPEs, (ii) investigate the underlying material characteristics of the NPPE that support good electrochemical performance, and (iii) demonstrate its application in solid EDLC devices using carbon-based electrodes. The polymer electrolytes utilizing either polyacrylamide or poly(vinyl alcohol) host with various amount of LiNO₃ have been systematically studied for their ionic conductivities and performance in solid capacitive devices. While increasing the salt content can lead to higher ionic conductivity, the mechanical properties may be compromised from excessive water absorption. The optimized electrolytes exhibited a high ionic conductivity (>20 mS cm^-1) at ambient, relatively high conductivity retention at sub-zero temperatures, and long shelf-life (>30 days). These electrolytes maintained well-hydrated ions and would enable solid-state double layer capacitors, without any separator.

References:

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[6] N. Batisse and E. Raymond- Piñero, “A self-standing hydrogel neutral electrolyte for high voltage and safe flexible supercapacitors,” J. Power Sources, 348, 2017.

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[8] T. Gu and B. Wei, “High-performance all-solid-state asymmetric stretchable supercapacitors based on wrinkled MnO₂/CNT and Fe₂O₃/CNT macrofilms,” J. Mater. Chem. A, 4, 2016.

[9] K. Fic et al., “Comparative operando study of degradation mechanisms in carbon-based electrochemical capacitors with Li₂SO₄ and LiNO₃ electrolytes,” Carbon, 120, 2017.