Redox shuttles and their use as overcharge protection in lithium-ion cells

Lithium-ion batteries are currently used to power most portable electronic devices. They also have use in electric and hybrid electric vehicles and as a storage device for clean, non-constant energy sources, such as solar and wind power. A lot of effort and money is currently being expended on lithium-ion battery research. Much of this research goes towards increasing the power or energy density of the batteries, but there is also significant interest in making these batteries safer and more user friendly. One way of doing this is to use a redox shuttle electrolyte additive to protect cells against overcharge.

As more and more cells are placed together in series strings, it becomes more and more important to ensure that the cells are properly balanced in the pack. If the capacities are not properly balanced, or if they become unbalanced during use, individual cells can experience overcharge, which can further degrade cells or pose a safety risk. Redox shuttles can be used to prevent overcharge conditions and they can also be used to periodically rebalance the individual cells in a pack, which would increase the overall lifetime of the pack.

Many molecules, as well as the properties that make some molecules better shuttles than others, are discussed in this thesis. Molecules that make good redox shuttles must have an oxidation potential that is suitable for use with the electrodes used in the cell and they must provide overcharge protection for a large number of cycles. The molecule 2,5-di-t-butyl-1,4-dimethoxybenzene, several substituted analogues of 2,2,6,6-tetramethylpiperidine-1-oxyl and several substituted phenothiazines are shown here to be excellent for use in cells that contain a LiFePO₄ positive electrode. For higher potential positive electrodes, fluorinated naphthalenes or 2,5-di- t-butyl-1,4-bis(2,2,2-trifluoroethoxy)benzene are shown to be useful shuttles.