Constructing Chelation for Boosting Storage of Large-Sized or Multivalent Ions

Rechargeable lithium-ion batteries (LIBs) are currently the most popular energy storage devices. However, the essential elements for commercial LIBs, i.e., lithium, cobalt, and nickel, are scarce, leading to an increase in cost, which together with the environmental concerns results in concern for future energy storage and calls for large-scale post-LIBs, including Na-, K-, Ca-, Mg-, Zn-, Al-, and dual-ion batteries. However, these post-LIBs are facing challenges in storage of either large-sized (Na-, K-, Ca-ions, anions) or high-charge-density multivalent (Ca-, Mg-, Zn-, Al-) metal ions. The large ionic sizes will inevitably result in the sluggish ionic diffusion, difficulty in storage, enormous volume variation, pulverization, low capacity, and poor cyclability. While the high charge/radius ratio of multivalent ions leads to strong electrostatic interactions with solvent molecules and electrode lattice, strong solvation, high desolvation energy, possible storage of complex ions, sluggish ionic diffusion and reaction kinetics, low actual capacity, and poor rate capability and cyclability. From this point of view, redox-active organic electrode materials (OEMs) are promising for these post-LIBs. OEMs can be easily synthesized from natural resources, which could meet the low-cost requirements for large-scale applications. Another advantage of OEMs is that the electrochemical performance could be facilely tuned through molecular design. High specific capacity could be expected, surpassing inorganic materials with intercalation chemistry. More importantly, organic materials have the merits of flexibility, which makes it intriguing for storage of large-sized ions with fast kinetics, and relatively small volume variation compared with inorganic electrode materials. In addition, organic/polymeric materials are composed by molecules via weak intermolecular interactions and hence should be suitable for storage of multivalent metal ions with reduced electrostatic interactions. In the past two decades, various OEMs have been reported with decent electrochemical performance for both LIBs and post-LIBs. However, they are still facing a lot of challenges, and effective strategies to enhance the performance of organic batteries are scarce. In the past few years, it has also been found that the storage of ions may lead to chelation with the OEMs when the OEMs have multiple active sites for improving the theoretical capacity or substituents for enhancing the intermolecular interactions. Later, we found that the intentional design of adjacent functional groups for chelation could enhance the storage of ions. In this Account, we first give an overview of challenges faced by post-LIBs and then propose a strategy to boost the storage of large-sized or multivalent metal ions by promoting chelation with stored ions, based on the recent works from our group and others. The findings on chelation with both Li ions and other monovalent and multivalent metal ions are summarized. We hope this Account could stimulate further research on molecular design through chelation to enhance the performance of organic batteries.