Pyrene-TEMPO-Modified Carbon Electrodes for Organic Radical Lithium-Ion Batteries

Abstract

Organic radical batteries (ORBs) are a promising alternative to traditional lithium-ion batteries (LIBs) because of their environmentally friendly materials and potential to be low-cost, light weight, and mechanically flexible. Among ORB materials, stable organic radical polymers have great potential as high energy density and high power density cathode materials due to their small molecular weight and fast, reversible redox reactions. A stable radical polymer for LIB applications contains an organic radical unit with redox potentials suitable for the LIB anode or cathode that is immobilized on an electrode via a main chain polymer. Conductive and light-weight polymer main chains are necessary to ensure fast electron transfer to the current collector, without adding significant weight to the cell. Despite extensive research, organic radical materials present some key challenges which limit their present use in electrochemical cells. Many polymer backbones are non-conductive, leading to increased electrode resistance and poor cell performance at high charge/discharge rates. Many organic radical materials also have relatively low thermal stability and low packing density. Moreover, many organic radicals are soluble in typical organic battery electrolytes, resulting in capacity fading.In this research, we apply pyrene as a polymer backbone to 2,2,6,6-tetramethyl-piperidine-1-oxyl radical (TEMPO) for the first time in LIB applications to reduce the solubility of the active organic radical and enable π−π stacking interactions with aromatic carbons. We prepared a surface-immobilized p–TEMPO on fullerene C60 and graphene foam (GF) electrodes via π−π stacking interactions to increase electrode conductivity. Pyrene is used to modify certain carbon allotropes via π−π stacking interactions. It was hypothesized that this approach would enable strong bond formation of the TEMPO radical with the electrode, while providing a non-destructive and simple modification approach of conductive carbon surfaces. The successful conjugation of pyrene and TEMPO molecules was established by NMR spectroscopy. The redox activity of p-TEMPO in LIB electrolytes was validated in solution. The non-covalent attachment of p-TEMPO to fullerene C60 and GF electrodes was established by XPS. The mechanisms and reversibility of the TEMPO redox reaction was then explored for p-TEMPO immobilized on the carbon electrodes. While both electrodes showed diffusion-limited current, and quasi-reversible reactions, C60-p-TEMPO has better reversibility and less resistance than GF-p-TEMPO. C60-p-TEMPO electrodes exhibit exceptional cycling stability over 600 cycles. These results are outstanding as the C60 has a high tendency to dissolve in LIB electrolytes. LIB half-cell testing identified challenges and opportunities for the p-TEMPO-modified electrodes. C60-p-TEMPO discharge capacity at ~5C rate is comparable to PTMA-based literature, however no distinct TEMPO response was observed in the charge or discharge curves. GF-p-TEMPO electrodes showed a TEMPO response in cycle 1 only, suggesting possible p-TEMPO dissolution in the electrolyte. Overall, the results show that the use of a noncovalent immobilization method allows facile preparation of p-TEMPO functionalized electrodes and demonstrates the potential of p-TEMPO noncovalent bonding in organic LIB applications. Figure 1