On the Stability and Reactivity of Redox Shuttles in Their Neutral and Radical Cation Forms
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On the Stability and Reactivity of Redox Shuttles in Their Neutral and Radical Cation Forms Susan A. Odom1, Matthew Casselman1, Aman Preet Kaur1, Selin Ergun1, and Naijao Zhang1 1
Department of Chemistry, University of Kentucky, Lexington, KY 40506, U.S.A.
ABSTRACT The performance of aromatic compounds as redox shuttles for overcharge protection in lithiumion batteries is quite variable and is often difficult to predict. Redox shuttles may decompose in battery electrolyte in their neutral and radical cation forms, both of which are present during overcharge protection. While hundreds of compounds have been evaluated as redox shuttle candidates and a few have stood out as top performers, the reasons for increased stability over similar candidates with slightly different structures is often unclear, and the exploration of decomposition of redox shuttles has been severely limited, restricting our ability to design improved versions of redox shuttles that do not suffer from the same reactions in lithium-ion batteries. To better understand the stability and reactivity of redox shuttles (also relevant to the improvement of positive electrode materials in non-aqueous redox flow batteries) our research has focused on measuring the stability of neutral and oxidized forms of redox shuttle candidates as well as using a variety of spectroscopic methods to analyze the byproducts of decomposition, both from radical cations generated in model solvents and electrolytes from postmortem analysis of failed batteries. INTRODUCTION Electrolyte additives called redox shuttles have been studied in lithium-ion batteries (LIBs) as a means of preventing overcharge, a condition in which the voltage of a battery rises above the end-ofcharge potential of the cathode. While hundreds of compounds have been tested as redox shuttles, only a few show extended performance in overcharge including 1,4-dialkyl-2,5-dialkoxybenzenes,1-4 Nalkylphenothiazines,5-8 and TEMPO derivatives (Figure 1).9 The performance of aromatic compounds as redox shuttles for overcharge protection in LIBs is quite variable and is difficult to predict. Redox shuttles may decompose in battery electrolyte in their neutral and radical cation forms, both of which are present during overcharge, which can lead to an inability to protect from overcharge. The reasons for increased stability for some compounds compared to similar candidates with slightly different structures is often unclear, and the decomposition of redox shuttles has not been significantly explored in the scientific literature, which restricts our ability to design improved versions of redox shuttles that do not decompose by similar mechanisms in LIBs.
Figure 1. Representations of the structures of 1,4-dialkyl-2,5-dialkoxybenzenes (left), Nalkylphenothiazines (middle), and TEMPO derivatives (right). To better understand the stability and reactivity of redox shuttles, our research has focused on measuring the stability of neutral and oxidized forms of redox shuttle candidates as well as using a variety of spectrosco
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