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.
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