Electrochemical Discharge of Spent Lithium-Ion Batteries in a K3[Fe(CN)6]/K4[Fe(CN)6] Redox Electrolyte

Abstract

Discharging of spent lithium-ion batteries (LIBs) is one of the key stages of their recycling. The main purpose of discharging spent LIBs is to prevent ignition and to ensure safety during the mechanical shredding performed at subsequent recycling stages. In the present work, the electrochemical discharge of cylindrical LIBs in a K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox electrolyte was investigated.
Using response surface methodology (central composite rotatable design), the discharge efficiency was shown to be governed by the combined effect of electrolyte concentration and temperature. The discharge efficiency (D_e) exhibited a non-monotonic (V-shaped) dependence on temperature: the highest efficiency was achieved at 20 °C, decreased at 50 °C, and was partially recovered at 71-80 °C. The non-linearity of the temperature effect is attributed to the competition between two processes – the increase in the ionic conductivity of the solution with rising temperature and the accelerated formation of a passivating Prussian blue (Fe₄[Fe(CN)₆]₃) layer on the surface of the positive terminal.
Under the optimal discharge conditions (electrolyte concentration 25 wt.%, temperature 20 °C, ultrasonic power 120 W), a discharge efficiency of 18.3% was reached within 4 h. The application of ultrasound provided an additional gain of 4.2 percentage points, is likely attributed to mechanical disruption of the forming passivating layer and to enhanced mass transport. The results demonstrate the potential of combining a K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox electrolyte with ultrasonic treatment for the safe discharge of spent lithium-ion batteries and provide an explanation for the observed temperature dependence of the discharge efficiency.