TEMPO-Modified Linear Polyethylenimine (LPEI) As Organic Radical Battery Electrode
Authors: Underwood Robert , Klunder Kevin Jay , Yuan Mengwei , Hickey David P , Minteer Shelley , Warren Roseanne 
Published: February 2020

With increasing demand for electronic devices and electric vehicles, there is a need for good performance of electrochemical energy storage materials while considering cost, safety, and environmental impact [1][2]. Organic electrodes based on stable organic radicals such as (2,2,6,6,-tetramethylpiperidin-1-yl)oxy (TEMPO) are a promising alternative to conventional inorganic (i.e. metal, metalloid, and metal oxide) battery materials due to their highly reversible, stable electron transfer reaction. Organic radical electrodes have the potential to achieve high power density, low cost, and improved environmental sustainability compared to inorganic electrodes [2][1]. Despite these advantages, organic radical-based electrodes face problems including dissolution of the active material in common battery solvents (thus requiring radical immobilization on polymer supports) and low capacity due to poor electronic conductivity and/or low fractions of active radical material [2][3]. Previous work suggests the support polymer can affect the performance of organic radicals as electrocatalysts, mediators, or energy storage materials. Hickey et al. demonstrated enhanced electrocatalytic activity of TEMPO when immobilized on linear polyethyleneimine (LPEI), a cationic polymer with amine functionality [4] [5]. In this work we explore for the first time LPEI-TEMPO as a cathode material in sodium-ion and lithium-ion organic radical batteries.
Figure 1a demonstrates the structure and stable electron transfer mechanism of LPEI-TEMPO. LPEI-TEMPO electrode is prepared by dissolving TEMPO-modified LPEI(prepared according to [3]) in 0.1 M HCl (10 mg/mL) with 2.5:1 wt% carboxylated carbon nanotubes, 2.5 M glutaraldehyde crosslinker (0.25 L in 80 L of solution) and polytetrafluoroethylene (PTFE) binder (5 wt%). This solution is deposited on Toray carbon paper (10L) and a steel current collector (20 L) for cyclic voltammetry and battery testing, respectively. Cyclic voltammetry measurements of LPEI-TEMPO (Figure 1b) confirm the reversible electron transfer of the TEMPO radical when immobilized on LPEI (electrolyte: 0.1 M TBAPF6 in acetonitrile; counter electrode: Pt; reference electrode: Ag/Ag+; scan rate: 100 mV/s). This work will present a detailed characterization of LPEI-TEMPO electrodes in sodium- and lithium-ion battery systems, including cyclic voltammetry, charge-discharge, rate capacity, and electrochemical impedance measurements. A preliminary sodium-ion half-cell charge-discharge test (Figure 1c) indicates a capacity of 30 mAh/g of LPEI-TEMPO (electrolyte: 1 M NaClO4 in 1:1 etheylene carbonate: tetrahydrofuran; Celgard 2340 separator; sodium metal anode). We will explore performance optimization of LPEI-TEMPO organic radical batteries, including a study of LPEI-TEMPO electrode-electrolyte interactions.
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Figure 1