Thermal Stability of C6O6
2--Based Organic Cathodes in Li-Ion and Na-Ion Batteries
Authors: Zhao Liwei , Chihara Kuniko , Kitajou Ayuko , Kobayashi Eiji , Okada Shigeto 
Published: February 2020

Li-ion batteries have long been used as efficient power sources for a wide variety of applications due to their high energy and power densities. Most state-of-the-art cathodes are based on electroactive inorganic materials that contain transition metals as redox elements. However, these cathode materials are synthesized from limited mineral resources and far from renewable. Therefore, to meet the rapid increasing demand for Li-ion batteries with higher energy density and larger scale, low-cost cathode active materials based on abundant minor-metal free elements have become increasingly desirable. As a new approach, many organic-based active materials have been reported. Among them, dilithium rhodizonate (Li2C6O6) is known to be an attractive candidate due to its relatively higher electrical conductivity [1]. Li2C6O6 cathodesshowed a stable reversible capacity around 300 mAh/g, and larger gravimetric and volumetric capacities than many commonly used inorganic lithium-inserted materials.
Besides of high energy density Li-ion batteries, recently, ambient temperature Na-ion batteries have drawn interest as a power source for large-scale grid energy storage due to the low cost and abundant resources of sodium. Na is located below Li in the periodic table and they share similar chemical properties in many aspects. The fundamental principles of Na-ion batteries and Li-ion batteries are identical. Therefore, much of new components for Na-ion batteries could be developed according to their counterparts for Li-ion batteries. In our previous work, a sodium counterpart of Li2C6O6, disodium rhodizonate (Na2C6O6) was applied as cathode materials for Na-ion batteries [2]. Na2C6O6 has a layer structure and ideal conjugate property for the large Na+ ion insertion/extraction. Na2C6O6 cathodes exhibited excellent electrochemical properties, including large initial reversible capacity of 270 mAh/g, corresponding to more than 2 Na insertion per molecule. Moreover, the application of Na2C6O6cathodes in Na-ion batteries with Na-predoped hard carbon anodes was also confirmed.
Safety is one of the most important issues for the practical application and development of batteries. As Li2C6O6 and Na2C6O6 are organic materials, the thermal and chemical stabilities in the organic electrolyte are inherently doubtful. In the present study, thermal properties of Na2C6O6 cathodes in Na-ion battery with 1 M NaClO4/PC electrolyte was studied in detail by DSC and TG-MS to evaluate the thermal risk. Moreover, it was compared with the Li counterpart, Li2C6O6 cathodes in Li-ion battery with 1 M LiClO4/PC electrolyte.
In this study, Li2C6O6 powder, which was lab-synthesized according to ref. 3, was used for Li-ion batteries, while commercial Na2C6O6 powder was used for Na-ion batteries. A 2032 coin-type cell was used for electrochemical pretreatment to get charge or discharged cathode. The cathode consisted of 70 wt.% active material, 25wt.% acetylene black, and 5 wt.% PVdF binder. A Li or Na foil was used as the counter electrode, while 1 M LiClO4/PC and NaClO4/PC as the electrolyte for the Li- and Na-ion cell. Charge-discharge cycling was performed at 1/5 C rate for both Li and Na cells, but the voltage range was 2.2-3.5 V for Li cell and 1.5-2.8 V for Na cell, respectively. After electrochemical treatment, the cathode was taken out from the cell, washed by solvent, and then dried. The electrode powder then was removed from the Al current collector and sealed into a stainless steel pan together with either some amount of the electrolyte or not for DSC analysis.
DSC analysis was carried out on the mixtures of the organic cathode and the electrolyte. Before electrochemical cycling, both Li2C6O6 and Na2C6O6 cathodes showed good thermal stability in the associated electrolyte till 250oC. Neither exo- or endothermic peak was observed. It suggests a good thermal stability of the rhodizonic cathodes in the organic electrolytes. After 1stdischarge, both lithiated and sodiated rhodizonic cathodes were found to give similar, or even less heat generation in the reactions with the electrolytes, although the exothermic onset temperature decreased a little. Detailed results will be presented on the conference.

References

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[3] H. Kim, D.-H. Seo, G. Yoon, W. A. Goddard, III, Y. S. Lee, W.-S. Yoon, K. Kang, J. Phys. Chem. Lett., 5 (2014) 3086-3092.