Understanding the stability of aromatic redox shuttles for overcharge protection of lithium-ion cells

Molecular orbital theory is employed to improve understanding of the stability of aromatic compounds as redox shuttles for overcharge protection of lithium-ion cells and batteries. The theoretical calculations clearly demonstrate that the pi-pi interaction between the aromatic ring and the substitution groups is critical to maximize the stability of radical cations, which are generated when the aromatic redox shuttles are oxidized at the positive electrode. It is also demonstrated that the unsubstituted hydrogen atoms on the aromatic ring also contribute significantly to the instability of the radical cations. The model suggests that full substitution on the aromatic ring or protection of the H atoms with bulky groups is highly desirable for the stability of the redox shuttles. Using the calculation results as a guide, a novel stable redox shuttle with a redox potential of 4.46 V vs Li+/Li was developed. The preliminary experimental results show that lithium-ion cells incorporating this redox shuttle are robust to overcharge even at elevated temperatures. (c) 2006 The Electrochemical Society.