Improvement in the Thermal Safety of Rechargeable Lithium Batteries: Endothermic Behavior of Organic Positive-Electrode Materials
Authors: Yao Masaru , Ando Hisanori , Kataoka Riki , Kiyobayashi Tetsu , Takeichi Nobuhiko 
Published: February 2020

The current rechargeable lithium batteries have potential risks related to the thermal safety. These batteries sometimes suffers from an unexpected rapid temperature increase. This phenomenon, called thermal runaway, is triggered by some abuse conditions such as overcharge and internal short-circuits, which could lead to explosions or fire. Considering large scale battery applications including electric vehicles and stationary system, extreme caution for this safety control is particularly important. To ensure the battery safety, many approaches have been suggested. The use of a particular kind of separator, the combination with a solid electrolyte, and coating of electrode materials are common strategies. The introduction of an external battery management system is also applied. Furthermore, it is inevitable to find out a thermally safe electrode material.
We have studied a series of organic active materials to reduce the amount of minor metal-based materials in the positive electrodes from the view point of the resource concern and have demonstrated that some low-molecular-weight quinone compounds show relatively large capacities based on their multi-electron transfer-type redox reaction [1‒3]. There are many other reports describing the charge/discharge performance of such organic active materials; however, the thermal behavior of these organic ones has been rarely discussed. Here, we compares the thermal behavior of a conventional lithium cobalt oxide (LiCoO2) and an organic active material using an example of 2,5-dimethoxy-1,4-benzoquinone (DMBQ) (Fig. 1a). This organic active material shows a large initial capacity of more than 300 mAh/g and relatively good cycle-life stability.
Figure 1b shows the differential scanning calorimetry profiles of the charged (delithiated) LiCoO2-electrode and the DMBQ-electrode under the coexistence with an electrolyte solution. The LiCoO2-electrode showed a sharp exothermic peak at around 240oC. Similar exothermic behaviors are often observed for a lot of minor-metal oxides, which leads to the thermal runaway. As for the mechanism, it is commonly understood that charged minor-metal oxides often release oxygen at elevated temperatures, which then induces the combustion reaction of electrolyte solutions. On the other hand, the DMBQ-electrode showed an opposite behavior: some endothermic peaks were observed during the measurement. In general, many organic compounds have a melting point or decomposition point, and these reactions are endothermic. The endothermic behavior observed for the DMBQ-electrode in this study is also considered to reflect such melting and/or thermal decomposition of the DMBQ particles in the electrode.
Since many organic materials do not contain any minor metals, the use of these materials can alleviate resource constraint issues. Furthermore, their endothermic behavior can be a promising characteristic, which would contribute to the development of a thermally safe battery system.
References:
[1] M. Yao, et al., J. Power Sources, 195, 8336 (2010).
[2] M. Yao, et al., Mat. Sci. Eng. B, 177, 483 (2012).
[3] M. Yao, et al., Energy Procedia, 56, 228 (2014).

Figure 1