Molecular Designs to Achieve High-Rate and Ultra-Stable Organic Electrode Materials for Future Sustainable Batteries

Abstract

Organic electrode materials (OEMs) are recently drawing much attention as a promising alternative to conventional transition metal oxide electrodes due to their many advantages, including abundance, sustainability, bio-compatibility, low cost, and easy tunability.[1] Most importantly, the flexible nature of organic materials due to loosely packed intermolecular structures allows fast diffusion for charge-carrying ions, which is essential for the high-rate performance of electrode materials. However, in practice, most OEMs still suffer from slow rate capability and low capacity utilization due to their electrically insulating nature. In a typical battery, the rate capability of an electrode is determined by the kinetic factors for the following three steps during the charge/discharge processes: i) redox reaction of active materials, ii) electron conduction through the active materials and the conductive carbon additives to the current collector, iii) and ion diffusion inside the active materials.[2] Previously, most studies have focused on improving the electrical conductivity of OEMs to achieve high-rate capability; however, such attempts typically sacrificed their specific capacity and/or cycle stability. Here, we present novel molecular design strategies to achieve high-rate capability of OEMs without deterioration of their specific capacity and cycle stability. First, we recently proposed that a small structural reorganization of the redox center during the redox reaction would be a key to achieving fast rate capability in OEMs for the first time.[3] We revealed that a novel p-type redox center phenoxazine (PXZ) had faster redox kinetics than its widely used analogue phenothiazine (PTZ) due to negligibly smaller structural changes, which was evidenced by theoretical calculations using the density functional theory (DFT) method and experimental measurements. In practical coin-cells, such low reorganization of the PXZ center led to high-rate performance (73% capacity retention at 20C) of a PXZ trimer electrode material (3PXZ) with a narrow voltage plateau at 3.7 V vs. Li/Li+. Then, in this presentation, we introduce our new OEMs bearing n-type redox centers with low reorganization energy, including tetrazine (Tz)[4-6] and thieno-isoindigo (TIIG).[7] The new OEMs showed superior rate capability (> 50% capacity retention at 50C) and high cycle stability (> 80% capacity retention after 500 cycles). Finally, we present novel OEMs spontaneously forming various nanostructures via self-assembly after fabricating electrodes.[8] The high surface area of the nanostructured OEMs provided fast electron transfer from the insulating active materials to the conductive carbons and easy access of the charge-carrying ions to the active materials, leading to ultrafast rate performance (> 50% capacity retention at 100C) even with high active content more than 70 wt% in the electrode.