Influences of Anions' Solvation on Charge Storage Performance and Mechanism of Poly(phenothiazine) Cathode

Abstract

Lithium-ion batteries (LIBs) are widely used energy storage devices in portable electronics and electric vehicles because of their high specific energy and high efficiency. However, constituent metals of LIBs such as lithium, nickel, manganese, and cobalt are limited and localized resources that pose fragile, undesirable supply bottlenecks in manufacturing. Organic batteries are promising alternatives to LIBs to overcome potential shortages of Lithium and other strategic metals and alleviate the need for cross-border LIBs production. Additionally, polymeric materials have high designability that provides abundant space for performance improvement.

Organic electrode materials (OEMs) and organic electrolytes are two important components of organic batteries. To date, most research has focused on improving the energy performance of organic electrode materials for higher capacity and cycle stability to exceed that of LIBs. However, the mechanisms of charge storage are not well understood, leaving researchers without guiding principles in their materials searches. To better understand the charge storage processes of organic batteries, an experimental study is proposed to (1) quantify the influence of electrolytes, and specifically anion solvation structure on charge storage performance of and (2) elucidate the redox and kinetics mechanism of a high performance polymer cathode material. Specifically, we use a poly(3-vinyl-N-methylphenothiazine) (PVMPT) electrode and explore its electrochemical behavior in three organic electrolytes where electrolyte salts are tetrabutylammonium trifluoromethanesulfonate (TBAOTf), tetrabutylammonium hexafluorophosphate (TBAPF₆), tetrabutylammonium perchlorate (TBAClO₄), respectively and solvent is ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3:7 wt%. Cyclic voltammetry (CV) and chronoamperometry (CA) are used to calculate the kinetics parameters, in situ quartz crystal microbalance with dissipation monitoring (EQCM-D) is used to quantify ion fluxes, swelling ratio, mass transfer and charge transfer, galvanostatic charge discharge (GCD) is used to characterize rate capacity and cyclic stability. We discuss and propose the major factors of triflate anion (OTf⁻), hexafluorophosphate anion (PF₆⁻), and perchlorate anion (ClO₄⁻) effect on electrochemistry behavior of PVMPT electrode, also clarify the redox and kinetics mechanism for this system.