Lithium ion batteries (LIB) have been adopted for the power supply of electric vehicles, and higher energy density and larger capacity cells are now in demand. Complying with these demands means that cells under abnormal conditions could become less safe. In particular, the amount of heat due to cathode decomposition could increase if overcharged. Therefore, technology is necessary to improve the safety in overcharging conditions.

A redox-shuttle compound is known as an effective protection technology for overcharging cells. A redox-shuttle compound can be reversibly oxidized and reduced at a defined potential slightly higher than the full-charge potential of the surface of cathode active materials. The oxidized redox-shuttle molecules that diffuse through the electrolytes would be reduced to their neutral state on the surface of anode active materials. Neutral molecules would then diffuse back to the cathode and be oxidized again. This shuttle reaction can convert excess energy during overcharging into heat, thereby protecting the cells by limiting the potential of the cathode to less than or equal to the oxidation potential of the shuttle molecules. However, when the overcharge current is bigger than the maximum rate of the shuttle reaction, the redox-shuttle compound cannot fully consume the current, and cathode active materials will be damaged.  

We designed and studied a new shutdown system that is a combination of a redox-shuttle compound and shutdown separator. In this system, the redox-shuttle compound will protect cathode active materials from overcharging, and at the same time, the separator will shut down because of the heat generated by the redox-shuttle reaction. First of all, we studied the rate of redox reaction and the temperature of the cell heated by the reaction of the redox-shuttle compound.

We selected 1,4-di-tert-butyl-2,5-dimethoxybenzene as the redox-shuttle compound. The reaction potential of this material is about 3.9 V vs. Li/Li⁺ (From the point of view of the principle verification, we selected the redox-shuttle compound of a lower reaction potential to reduce the extra reaction while the redox-shuttle compound was reacting). The redox-shuttle compound was dissolved up to 0.15 M with a common electrolyte, EC/DEC (3/7 by volume)/ 1.0 M LiPF₆ and used as a redox-shuttle electrolyte. Graphite and a mixture of lithium manganese oxide and lithium nickel oxide were used as the anode and cathode active material, respectively. Porous polypropylene (PP) and PP/polyethylene (PE)/PP stacked film were used as the separator. The melting points of PP and PE are 160^°C and 130^°C, respectively. Cells that varied in capacity from 3.5 to 10 Ah were prepared by changing the number of stacked layers (the size of the electrode was fixed). Each cell was charged at the rate of 0.1 to 1 C, and the cell voltage and the temperature on the surface of the cell were measured.

 As a result, as the cells were charged, a plateau was observed around 3.8 V of cell voltage, and the temperature of the cells increased at the same time. These results indicate that the redox-shuttle compound consumed current and converted it into heat. The increase in temperature of the cells heated by the redox-shuttle compound had a linear relationship with the charged current value (See figure 1). From this relational expression, when the charge current is higher than 10 A, the temperature of the cell can exceed 160^°C (higher than the melting point of PP and PE), and ion diffusion will be protected. These results and equation suggest that the new shutdown system is capable of keeping the high energy density cells safe.

On the meeting day, we will report the details of the new shutdown system and the results of the experiment using 10-Ah class cells. In particular, we will focus on the influence of the redox-shuttle reaction rate and the properties of the electrode and separator in this system.

Fig. 1 Relationship between the charging current and the highest and equilibrium temperature on the surface of the cell.