Charge Transport in TEMPO/Ionic Liquid Redox Polymers
Authors: Karlsson Christoffer , Suga Takeo , Nishide Hiroyuki 
Published: February 2020

In the emerging field of organic energy storage, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) redox polymers are among the most promising candidate compounds for the active charge storage component in the positive electrode of next generation batteries.1–3 They are a more sustainable alternative to the metal oxide based cathodes used today, which are currently the main limiting factor in battery devices, both in terms of cost and performance, and they are the main contributor to the cost and high carbon footprint of lithium ion batteries.4,5 Organic batteries will likely serve as a compliment to lithium ion batteries in the future, among many alternative technologies currently being developed, to satisfy the vast needs of energy storage with different requirements depending on the application.3,4

TEMPO typically operates around 3.5 V vs Li and can reach specific capacities of 150 mAh/g, yielding specific energies of up to 500 mWh/g, which is competitive with inorganic materials.3,6 Enabling thin and flexible battery devices that are more environmentally friendly, organic compounds nevertheless suffer from issues such as poor stability and low power.4 The stability limitations originate from unwanted side reactions of the charged species, as well as dissolution of the active material in the electrolyte. Cross-linked TEMPO redox polymers however avoid capacity fading due to dissolution, but still have problems with unwanted reactions with the solvent. Ionic liquids offer many advantages compared to conventional electrolytes, such as chemical and electrochemical stability, low flammability, high ionic conductivity, and negligible vapor pressure.7The polar solvents of conventional electrolytes are often responsible for degrading side reactions, and the stability can therefore be improved by using ionic liquids. Furthermore, the high ionic strength enables efficient shielding of the charged species, and ionic liquids have thus demonstrated improved stability of battery devices. The low flammability and absence of vapor pressure also gives large safety advantages.
The successful application of new organic compounds in battery devices relies, among other things, on detailed knowledge of the reactions leading to self-discharge and degradation of the material, as well as understanding the charge transport properties that are crucial for providing conductivity through the material and enabling high power. Therefore, the charge transport properties of TEMPO redox polymers with imidazolium based ionic liquid were studied using interdigitated array electrodes. This setup enables investigation of the redox hopping charge transport through the polymer, under steady state conditions, as well as under mass transport or kinetically limited conditions, at varying potential with respect to a reference electrode (E in Figure 1).8The mixed valence redox conduction apparent close to the TEMPO formal potential was compared in the presence of conventional electrolytes and in the presence of ionic liquid in different configurations. The implications on the viability of TEMPO/ionic liquid materials for organic energy storage devices is discussed.

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Figure 1. a) In situ conductance of TEMPO redox polymer conforms to the mixed valence conduction model. b) Schematic of the charge propagation mechanism through the polymer film under a potential bias (Ebias) between two gold electrodes (WE1, WE2), while the potential (E) with respect to a reference electrode (RE) is controlled.

Figure 1