All-organic Na-ion battery based on conjugated carbonyl compounds

In the last decade, lithium (Li) -ion batteries have been the technology of choice for the majority of battery powered devices, starting from small-scale electronics to power- demanding hybrid electric vehicles. (Armand and Tarascon, Nature 2008; Van Noorden, Advanced Science 2014) In pursuit of higher capacities, energy storage by batteries shall move from the current intercalation process to conversion reactions. Very attractive are organic compounds with low molecular weight that are capable of multiple electron transfer. Their inherent soft nature allows an exchange of Li through much more abundant sodium (Na).(Häupler, Wild, and Schubert, Adv. Energy Mater. 2015) Although Na-ion and Li-ion batteries were both already investigated in the 1970s and 1980s, due to the success of the Li-ion battery, research on Na-based batteries was largely abandoned, with the exception of the high temperature systems. Research on Na-ion batteries has only recently been revived, largely motivated by the high natural abundance of Na and novel electrode materials.(Slater et al., Adv. Funct. Mater. 2013) Na in the earths crust and in water is over 1000 times more abundant than Li. Furthermore, the number of known Na compounds is much higher than that of Li compounds. Therefore the amount of possible combinations of electrode materials that enable the development of batteries solely based on low cost elements and that will provide specific advantages that complement current state of the art Li-ion technology are expected. This project entitled All-organic Na-ion battery based on conjugated carbonyl compounds is an ambitious program that aims at understanding Na-ion kinetics in organic compounds as electrode materials in an all-organic sodium (Na) -ion battery that possesses high theoretical capacity, can be cheaply produced, is non-toxic and bio compatible. An important focus is on the grand challenge to exploit organic Na-ion battery electrode materials that can be synthesized by green chemistry from biomass, with the big advantage of being easily recyclable.

The completed project entitled "All-organic Na-ion battery based on conjugated carbonyl compounds" investigated organic electrode materials for their potential use in an all-organic sodium-ion battery. The projects overall scientific goal focused specifically on exploiting n-type organic electrode materials to develop an all-organic sodium-ion battery that is, in the ideal case, biocompatible and biodegradable. The most innovative aspect thereby was the idea to use n-type organic materials as both, the anode and cathode material in the battery. While suitable cathode materials were known, finding a comparable anode material was most challenging and required a fundamental understanding of the most important material properties. This included, but is not limited to, the redox reactions and potentials that take place upon sodium-ion storage, their reaction pathways and side reactions and their reaction kinetics. Therefore, detailed electrochemical and spectroscopy studies have been carried out. In the course of this project, n-type organic materials were identified which demonstrated good sodium-ion storage kinetics and good sodium-ion storage capacities. The most important findings for these materials, with a special focus on their sodium-ion storage performance, have been published open access, in international accessible, peer-reviewed, scientific journals. Furthermore, at the final stages of this project, and by successful collaboration with partners from theoretical and synthetic chemistry, it became possible to specifically modify the most promising materials found. In a combinatorial approach, using theoretical modelling, synthetic chemistry and battery testing, n-type organic materials where found, which showed a strong alteration in their sodium-ion storage potential upon addition of functional side groups. Some of these materials, with modified side groups, showed such a strong alteration of their sodium-ion storage potential, that it became possible to use them either as anode or cathode material in a sodium-ion battery. Thereby, this project added a substantial, scientific contribution to the potential realization of an all-organic sodium-ion battery.