Molecular Designs to Achieve Fast-Rechargeable and Highly Stable Organic Electrode—Low Reorganization and In Situ Self-Nanostructuring

Organic electrode materials (OEMs) are drawing significant attention as promising alternatives to conventional transition metal oxide electrodes for secondary batteries. Their appeal stems from numerous benefits, which include abundance, sustainability, biocompatibility, cost-effectiveness, and ease of chemical tunability. Notably, their flexible nature with large free volume, owing to loosely packed intermolecular structures, is expected to facilitate fast diffusion of charge-carrying ions — a key attribute for enhancing the high-rate performance of electrode materials. Yet, the practical application of OEMs is hindered by their slow rate capability with limited capacity utilization, a consequence of their intrinsically electronic insulating properties. Furthermore, high solubility of OEMs in organic electrolytes have brought rapid capacity fading within a few initial cycles, compelling one to prepare polymers via complicated synthesis and/or to fabricate composites with expensive nanocarbons for securing both cycle stability and rate capability.<br/>In a typical metal-ion 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 reactions of active materials, ii) conduction of the generated/inserted electrons through the active materials and the conductive carbon additives to/from the current collector, iii) and diffusion of ions inside the active materials to/from the electrolyte. Previously, most studies to achieve high-rate capability of OEMs have focused on improving their electrical conductivity; however, such attempts were unsatisfactory except for a few while sacrificing their specific capacity.<br/>Herein, we present novel molecular design strategies to simultaneously achieve high-rate capability and cycle stability of OEMs without deterioration of their specific capacity. First, we recently proposed that a small structural reorganization of the redox center during the redox reactions would be a key to achieving fast rate capability in OEMs. 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 various experimental measurements. Such low reorganization of the PXZ center led to a narrow voltage plateau at 3.7 V vs. Li/Li+ and high-rate performance (73% capacity retention at 20C) of a PXZ trimer (3PXZ) electrode than 3PTZ, in a Li-organic coin cell.<br/>However, the structural reorganization corresponds to a property that determines the kinetics of redox reactions at the molecular scale, while the other two factors influencing the rate capability of an electrode (i.e., electron conduction and ion diffusion) occur at a bulk scale beyond the molecules. To facilitate the ion diffusion and electron conduction, it is essential to fabricate nanostructures with a high surface area within an electrode. However, mass production of nanostructured inorganic materials on a large scale is challenging due to their reliance on top-down preparation methods.<br/>Finally, in this talk, we present a novel small molecule OEM, V3PXZ, which spontaneously forms nanostructures via in-situ electrochemical post-crosslinking during the cell operation after electrode fabrication from a soluble active material. It should be noted that the in-situ electrochemical coupling produces insoluble crosslinked polymers of V3PXZ accompanying nanostructure formation without generating any by-products. Notably, the self-nanostructured V3PXZ electrode demonstrates remarkably high long-term stability with a capacity loss per cycle as low as 0.0048% over 10,000 cycles. Furthermore, the aqueous-processed V3PXZ electrode retains more than 50% specific capacity at exceptionally high rate of 100C even with 70wt% of active content in the electrode.