π–π interactions in a phenothiazine-based organic redox polymer lead to an ultra-high cycling stability of a lithium–organic battery.

Organic cathode materials are a sustainable alternative to transition metal oxide‐based compounds in high voltage rechargeable batteries due to their low toxicity and availability from less‐limited resources. Important criteria in their design are a high specific capacity, cycling stability, and rate capability. Furthermore, the cathode should contain a high mass loading of active material and be compatible with different anode materials, allowing for its use in a variety of cell designs. Here, cross‐linked poly(3‐vinyl‐N‐methylphenothiazine) as cathode‐active material is presented, which shows a remarkable rate capability (up to 10C) and cycling stability at a high and stable potential of 3.55 V versus Li/Li+ and a specific capacity of 112 mAh g−1. Its use in full cells with a high mass loading of 70 wt% is demonstrated against lithium titanate as intercalation material as well as lithium metal, which both show excellent performance. Through comparison with poly(3‐vinyl‐N‐methylphenothiazine) the study shows that changing the structure of the redox‐active polymer through cross‐linking can lead to a change in charge/discharge mechanism and cycling behavior of the composite electrode. Poly(3‐vinyl‐N‐methylphenothiazine) in its cross‐ and non‐cross‐linked form both show excellent results as cathode‐active materials with variable specifications regarding specific capacity, cycling stability, and rate capability.

Organic electrode materials are among the promising next generation compounds for battery energy storage as a greener and cheaper alternative to transition-metal-based electrodes. A prominent class among them are redox polymers, which can reversibly store energy and can be capable of fast redox processes. Nevertheless, drawbacks are their often low specific energy and lifetime. A main challenge is their solubility in battery electrolytes, which is detrimental to cell performance. Herein, we discuss the solubility properties of a polyvinyl-based redox polymer with a methylphenothiazine side group (PVMPT) in organic-solvent-based battery electrolytes and generate new insights into the mechanism of the redeposition process of dissolved active material. We addressed the mechanistic studies of this “polymer–electrolyte cross-talk” with microscopic and spectroscopic methods. These findings are important for the molecular design of new organic electrode materials, since the redeposited polymer showed improved cycling performance and outstanding cycling stabilities. We herein aim to draw a bigger picture of the solubility of redox polymers and its consequences and motivate the scientific community to reconsider the common conception of the deteriorating nature of the solubility of organic battery electrode materials.

Organic cathode materials are attractive for a new generation of more sustainable batteries due to their comparably low environmental footprint and toxicity. There is a continued quest for new compounds that meet the requirements of a competitive potential and a good cycling performance. We herein present phenoxazine-based polymers as cathode materials with good cycling stability, excellent rate performance, and a high discharge potential of 3.52 V vs Li|Li+ in composite electrodes. At the ultra-fast rate of 100C, a cross-linked phenoxazine poly­(vinylene) showed only slow capacity decay over 10 000 cycles with a capacity retention of 74% in cycle 10 000. Mechanistic investigations using UV/vis/near-infrared (NIR) spectroscopy and density functional theory (DFT) calculations unveiled that unlike in the homologous phenothiazine polymers, π-interactions played a minor role in phenoxazine-based polymers. Our study is the first to present phenoxazine as a redox-active unit for cathode materials and shows that an elemental change of one atom (S vs O compared to known phenothiazine-based polymers) can have a profound effect on electrochemical performance.

Organic materials are promising candidates for next-generation battery systems. However, many organic battery materials suffer from high solubility in common battery electrolytes. Such solubility can be overcome by introducing tailored high-molecular-weight polymer structures, for example, by crosslinking, requiring enhanced synthetic efforts. We herein propose a different strategy by optimizing the battery electrolyte to obtain insolubility of non-cross-linked poly(3-vinyl-N-methylphenothiazine) (PVMPT). Successive investigation and theoretical insights into carbonate-based electrolytes and their interplay with PVMPT led to a strong decrease in the solubility of the redox polymer in ethylene carbonate/ethyl methyl carbonate (3:7) with 1 M LiPF 6 . This allowed accessing its full theoretical specific capacity by changing the charge/discharge mechanism compared to previous reports. Through electrochemical, spectroscopic, and theoretical investigations, we show that changing the constituents of the electrolyte significantly influences the interactions between the electrolyte molecules and the redox polymer PVMPT. Our study demonstrates that choosing the ideal electrolyte composition without chemical modification of the active material is a successful strategy to enhance the performance of organic polymer-based batteries.

Organicc athode materials are handled as promising candidates for new energy-storage solutions based on their transition-metal-free composition. Phenothiazine-based polymers are attractive owing to their redox potentialo f3 .5 Vv s. Li/Li + and high cycling stabilities. Herein,t hree types of poly(norbornene)s were investigated, functionalized with phenothiazine units through either ad irect connection or ester linkages, as well as their crosslinked derivatives. The directly linked poly(3norbornylphenothiazine)s demonstrated excellent rate capability and cycling stabilityw ith ac apacity retention of 73 %a fter 10 000 cycles at aC -rate of 100 Cf or the crosslinked polymer. The polymer network structure of the crosslinked poly(3-norbornylphenothiazine) was beneficial for its rate performance.

Organic redox polymers are considered a “greener” alternative as battery electrode materials compared to transition metal oxides. Among these, phenothiazine-based polymers have attracted significant attention due to their high redox potential of 3.5 V vs Li/Li+ and reversible electrochemistry. In addition, phenothiazine units can exhibit mutual π-interactions, which stabilize their oxidized states. In poly­(3-vinyl-N-methylphenothiazine) (PVMPT), such π-interactions led to a unique charge/discharge mechanism, involving the dissolution and redeposition of the polymer during cycling, and resulted in an ultrahigh cycling stability. Herein, we investigate these π-interactions in more detail and what effect their suppression by molecular design has on battery performance. Our study includes a dimeric reference compound for PVMPT, polymers with bulky tolyl or mesityl substituents on the phenothiazine units to inhibit π-interactions and alternating copolymers with maleimide groups to increase spatial distancing between phenothiazine groups. UV/vis- and electron paramagnetic resonance (EPR)-spectroscopic as well as electrochemical measurements in composite electrodes demonstrate how the unique structure of PVMPT is instrumental in obtaining a high cycling stability in poly­(vinylene) derivatives of phenothiazine.

Organic redox polymers have received increasing attention as battery electrode materials due to their low toxicity and the possibility to produce them from renewable resources or petroleum. Phenothiazine is a redox-active group with highly reversible redox chemistry. Polymers based on phenothiazine have shown impressive performance as battery cathode materials regarding cycling stability and rate performance. In this chapter, the progress in this field is summarized, specific properties of phenothiazine-based polymers as cathode-active materials are highlighted and future challenges identified.