Two-Dimensional π-Conjugated Frameworks as a Model System to Unveil a Multielectron-Transfer-Based Energy Storage Mechanism.

ConspectusAlthough electrochemical energy storage is commonplace in our society, further advancements in this technology are indispensable for the transition to a low-carbon society. Recent intensive research has expanded concepts in this field; however, finding one suitable material to obtain a high energy density accomplishing the criteria of next-generation batteries is still a conundrum. To solve this issue, material investigations based on big data combined with artificial intelligence are a present trend. On the contrary, this Account focuses on an alternative approach, i.e., fundamental research to shed light on key basic principles to design new electrode materials and new principles achieving multielectron transfer, which is a key to improve a specific capacity. In addition to the cation-redox mechanism, materials showing the multielectron-transfer mechanism based on cation-/anion-redox can enrich material choices with high theoretical energy densities. The challenge in this mechanism is that a rational design of electrode materials based on microscopic understanding of underlying electrode processes has not been fully achieved so far. This is a key bottleneck in machine-learning approaches as well because the reliability of outputs from an algorithm is dependent on the reliability of data from a corresponding microscopic electrode process. Therefore, uncovering fundamental mechanisms in electrochemical energy storage remains one of the primary goals for the present research. In our series of investigations, we developed concepts for replacing complex practical electrode materials, such as polyanion or Li-rich layered oxides, by simplified model systems based on two-dimensional (2D) π-conjugated frameworks, which are based on purely organic aromatic systems and metal-containing coordination polymers. These materials are relatively simple, but it is still possible to control their complexity of systems in order to mimic certain aspects of structure-property relations in practical electrode materials. In particular, recent studies have shown that we can tune electronic structures of 2D π-conjugated frameworks, which is a key feature to investigate electron-transfer mechanisms, along with the concept of the threefold correlation approach, i.e., the relations in chemical structures, electronic structures, and electrochemical reactions. In this Account, several model studies focusing on microscopic understandings of structure-electrochemical energy storage functions are presented in which we investigate how the structural periodicity and nature of the coordination environment affect their electronic properties and the electrochemical reactions. In particular, we investigate the effects of combinations of linkers and metal ions toward the mechanism of the electrochemical energy storage reaction. We identified few major factors determining the energy storage mechanism of 2D π-conjugated frameworks. Local configurations of coordinate covalent bonding and organic linkers interact with each other, and these effects provide unique electronic states. These electronic states are projections of intriguing electrochemical features in this materials system, such as cation/anion co-redox mechanism, anion-insertion mechanism, or inductive effect. This Account indicates that 2D π-conjugated frameworks can be applied as models to extract fundamental/microscopic principles in the complicated electrode processes, which is linked to practical electrode materials, such as oxides. Therefore, the approach shown here is a powerful tool to unveil microscopic electrochemical energy storage mechanisms, which is indispensable to advance clean energy technology and accelerate decarbonization.