Phenothiazine-Based Redox Polymers As Cathode-Active Materials in Li/Organic Batteries

Abstract

The rising demand for energy storage is increasing the research on so-called next generation or beyond lithium ion battery technologies.[1] The most promising candidates are lithium metal-based batteries (LMBs) with the Li/S battery as the most advanced example.[2] Even though LMBs show a high energy storage ability, the sluggish kinetics combined with safety concerns keeps the field open for alternative battery principles. One of these are organic-based materials which offer a great opportunity for the next generation of energy storage.[3] Their energy densities are mostly lower compared to their inorganic counterparts, but the ability of fast electron transfer and therefore high kinetics leads to an increased power density (e.g. 33.6 kW kg−1[4] vs. common battery materials, e.g. 4.75 kW kg−1[5]). Redox polymers are one of the most versatile organic-based materials for energy storage as they show decreased solubility and offer countless possibilities for modifications of the polymeric backbone and the side chains, while the polymer itself can be conjugated or non-conjugated.[6] Herein, we present a non-conjugated redox polymer with a substituted poly(vinylene) backbone.[7] The redox active side chain consists of a methyl phenothiazine moiety and undergoes an one-electron transfer process during oxidation and forms a radical cation. This oxidation is accompanied by an anion insertion into the polymeric structure. Due to a π-π-interaction between the side chains this polymer is able to enable long-term cycling with increased C-rates resulting in an ultra high cycling stability of 10,000 cycles with a capacity retention of 93%. In this work, we will describe these interactions and verify them with spectroscopic methods like EPR- and X-ray photoelectron spectroscopy (XPS). Additionally, density functional theory calculations (DFT) are shown which support the strong stabilizing effect of the π-π-interactions in this organic material.[7] Beside the mechanistic studies on molecular scale, we will comment further on the mechanism on the macroscopic electrode level. Here, we performed microscopic (SEM, LSM) and spectroscopic experiments (XPS, UV/Vis) to uncover the unique cycling behavior of PVMPT.[8] Finally, we will present an approach to increase the cycling capacity of the material and discuss the influence on the stabilizing interactions.[9]