This contribution presents a safe way of discharging batteries at a cell level using water, aqueous salt solutions, or organic solutions of redox-shuttle additives as discharge agents. The discharge solutions can be added to the cell from the outside when discharge is required, or, in case they are already contained in the cell, they can be activated by adequate technical trigger mechanisms.
A number of examples of compounds is shown and compared for 20 Ah prismatic Li-ion cells. Typical discharge times are in the range of 30 to 60 hours (shorter times are expected for higher concentrations of the discharge agents than tested here). The end-of-discharge voltage is typically between 0 and 1 V, depending on the respective discharge agent. Discharge with water and aqueous solutions is the cheapest way of discharge, but since it is accompanied by gas evolution, it may cause secondary risks via the evolved gas. The organic solutions of redox-shuttle additives enable a complete discharge without any gas evolution, and are thus the safer option (cf. Figure: Evolution of cell voltage after addition of redox-shuttle additives). During discharge the chemical energy contained in the charged cell is converted into thermal energy. For the timescale of the given experiment, however, no noticeable heating of the cell was observed (with the temperature changes remaining < 3 °C, which was within the natural variation of the ambient temperature).
The underlying discharge reaction mechanisms are explained and requirements and limitations for the discharge agents are discussed.
Acknowledgement: Financial support of parts of this work from the German Association of the Automotive Industry (VDA) is gratefully acknowledged.