A Highly Soluble Redox Shuttle with Superior Rate Performance in Overcharge Protection
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A Highly Soluble Redox Shuttle with Superior Rate Performance in Overcharge Protection Susan A. Odom1, Aman Kaur1, Selin Ergun1, Corrine F. Elliott1, and Matthew D. Casselman1. 1
Department of Chemistry, University of Kentucky, Lexington, KY 40506, U.S.A.
ABSTRACT The demand for a stable and compatible redox shuttles for use in lithium-ion batteries has prompted us to explore strategies to tune and improve the properties of redox shuttles. We have studied over 50 new diarylamine derivatives synthesized in our laboratory including one compound in which we introduced trifluoromethyl groups (–CF3) at the positions para to the nitrogen atom in N-ethylphenothiazine (EPT). The high electronegativity of the CF3 group raises the oxidation potential, and its incorporation also significantly increases solubility in battery electrolyte. Here we report 3,7-bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) as a new redox shuttle, which we have observed to have the highest reported solubility in battery electrolyte of all redox shuttles that maintain extended overcharge performance. We have compared its performance with 1,3-di-tert-butyl-2,5-dimethoxybenzene (DBB), EPT, and other robust redox shuttles. In our hands, overcharge cycling of BCF3EPT far surpasses any reported redox shuttle, and – because it can be dissolved at higher concentrations – it tolerates faster charging rates than both DBB and EPT. INTRODUCTION Redox shuttles can be used to prevent lithium-ion batteries (LIBs) from entering overcharge, a condition in which the voltage of a battery rises past the end-of-charge potential of the cathode. In this condition, irreversible overdelithiation, oxidation of the electrolyte, and the generation of gases can all lead to accelerated aging, lowered capacities, and safety issues.1 Redox shuttles act as an internal shunt, transporting current between battery electrodes rather than allowing the redox reactions at the electrode/electrolyte interface, which may degrade the battery materials. While hundreds of compounds have been tested as redox shuttle electrolyte additives, only a few derivatives have been reported to last over 100 cycles of 200% charge or its equivalent. One of the classes of compounds we have been studying for overcharge protection is phenothiazine,2-6 represented with generic substituents at the N-position (R) and 3- and 7positions (X) in Figure 1.
Figure 1. A representation of the chemical structure of phenothiazine redox shuttles containing substituents at the N position (R) and 3 and 7 positions (X).
The majority of redox shuttles – including the widely-studied, robust redox shuttle 1,4-ditert-butyl-2,5-dimethoxybenzene7-10 (DBB, Figure 2) – are limited in solubility to ca. 0.1 M in common battery electrolytes for LIBs. In an effort to increase the intramolecular dipole moments and thus facilitate the dissolution process, Zhang and Amine developed asymmetric derivatives of dialkoxybenzenes (Figure 2) that exhibit higher solubility but unfortunately also decreased stability,11-13 which can be explai
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