Striving to Disclose the Electrochemical Processes of Organic Batteries

Organic/polymeric materials are promising as electrode materials for batteries because of their advantages of flexibility, high specific capacity due to the possible multielectron transfer, low cost from green natural resources, and weak intermolecular interactions that enable the storage of low-cost large-sized or multivalent metal ions. However, the development of organic electrode materials (OEMs) and organic batteries and the understanding of the electrochemical process face great challenges in the characterization of polymers and the charge storage mechanisms: (1) the charged and/or discharged states of OEMs are often air unstable, which makes the ex situ characterizations susceptible to the interference of air. (2) OEMs, particularly polymeric materials, are designed to be insoluble to deliver high cyclability, which makes it difficult for them to be separated from the electrode. (3) Possible multielectron transfer makes it difficult to determine whether the proposed charge storage mechanism or the experiment results are wrong when the actual capacity mismatches with the theoretical capacity based on the proposed mechanisms. (4) It is difficult to achieve single crystals of polymers, and hence, it seems impossible to know the actual locations of the stored ions in the polymers. (5) The typical methods for characterization of insoluble polymers are only qualitative, and it is challenging to quantify the amount of stored ions. (6) Even for most in situ characterizations, they can only give the tendency of qualitative structural evolution.

In this Account, we give an overview of the significance of organic batteries and the challenges related to the characterization and charge storage mechanisms of organic electrode materials. Then, we summarize our efforts in recent years to reveal the charge storage mechanisms and insights into the electrochemical process. Focusing on the complexity of polymer materials, we proposed a strategy to control the reaction kinetics in order to obtain high-quality single crystals or microcrystals of polymers. The chemical structure and reaction mechanism of polymers could be successfully revealed by single crystal structure analysis. To avoid the inconvenient characterizations brought by the insolubility of polymers, soluble monomers or oligomers were studied under the same conditions to simulate and analyze the electrochemical process of polymers. We also proposed the synthesis of isomers for a deep understanding of the structure–property relationships of OEMs. On the other hand, traditional qualitative characterization instruments or techniques were reconsidered to give more information or even quantitative results via insightful analysis of the data or smart design of experiments. In addition, by introducing internal standard substances, it was also possible to realize quantitative characterizations. Strategies to convert the black box of different charged/discharged states into detectable materials or signals were also developed. This Account provides a summary of our recent progress in understanding the electrochemical process of OEMs and prospects of future development of rechargeable organic batteries.