Redox Shuttle Additives for Lithium-Ion Battery

Redox Shuttle Additives for Lithium-Ion Battery
Overcharge of lithium-ion batteries can be dangerous. Overcharge generally occurs when a
current is forced through a cell, and the charge delivered exceeds its charge-storing
capability.1-3 Overcharge of lithium-ion batteries can lead to the chemical and electrochemical
reaction of batteries components, rapid temperature elevation, self-accelerating reactions, and
even explosion. Redox shuttle additives have been proposed for overcharge protection of secondary lithium-ion batteries for decades.4-8 Generally, the redox shuttle molecule can be reversibly oxidized
and reduced at a defined potential slightly higher than the end-of-charge potential of the
cathode. This mechanism can protect the cell from overcharge by locking the potential of the
cathode at the oxidation potential of the shuttle molecules. The detailed mechanism is
shown in Figure 1. On the overcharged cathode surface, the redox shuttle molecule (S) is
oxidized to its (radical) cation form (S+), which, via diffusion across the cell electrolyte,
would be reduced back to its original or reduced state on the surface of the anode. The
reduced form would then diffuse back to the cathode and oxidize again. The “oxidationdiffusion-
reduction-diffusion” cycle can be repeated continuously due to the reversible nature of the redox shuttle to shunt the overcharge current. The redox shuttling mechanism at overcharge can be regarded as a controlled internal short, and the net result of the shuttling is to convert the overcharge electricity power into heat, which avoids the reactions that occur between the electrodes and electrolyte at high voltage. Redox shuttles can also be used for automatic capacity balancing during battery manufacturing and repair.