Phenothiazine Polymers: The Versatile Building Blocks for Next-Generation Battery Technology
In the race to develop sustainable energy storage solutions that move beyond traditional lithium-ion technology, organic battery materials have emerged as promising candidates. A recent scientific Account published in the journal Accounts of Materials Research by Esser and colleagues highlights the remarkable versatility of phenothiazine-based polymers as electrode materials for next-generation batteries [1].
Why Organic Battery Materials Matter
As we transition toward alternative energy sources, the demand for efficient, sustainable, and environmentally friendly energy storage solutions continues to grow. Organic redox polymers offer several advantages over conventional inorganic materials:
Lower environmental impact and toxicity
Potential for production from renewable resources
Flexibility in molecular design and tuning
Compatibility with various battery chemistries beyond lithium
Among these organic materials, phenothiazine (PT) stands out as a particularly promising building block due to its highly reversible redox chemistry and relatively high redox potentials.
The Power of Phenothiazine: Two Electrons Are Better Than One
Phenothiazine is classified as a p-type redox-active group, meaning it can be oxidized to cationic forms in a reversible fashion. What makes PT particularly valuable is its capacity for two successive one-electron oxidations:
First oxidation: neutral PT → radical cation PT•+ (≈3.5V vs. Li/Li+)
Second oxidation: radical cation PT•+ → dication PT2+ (≈4.1V vs. Li/Li+)
These high redox potentials make PT-based materials excellent candidates for positive electrode materials. However, the second oxidation process typically suffers from limited reversibility in battery applications due to the highly electrophilic nature of the dicationic form, which tends to undergo irreversible side reactions.
The Fascinating World of π-Interactions
One of the most intriguing aspects of PT-based materials is their tendency to form π-interactions (π−π-interactions) between the redox-active units. These interactions stabilize the radical cationic form, creating an intermediate oxidation state that can strongly influence battery performance.
In poly(3-vinyl-N-methylphenothiazine) (PVMPT), these π-interactions manifest as:
"Pimers" - interactions between a neutral PT unit and a PT radical cation
"π-dimers" - interactions between two PT radical cations
These interactions lead to supramolecular hole transport and can significantly enhance the polymer's electrical conductivity—reaching the semiconductor range despite having an aliphatic backbone. This phenomenon results in remarkable stability during cycling, with PVMPT demonstrating capacity retention over 10,000 cycles at high charge/discharge rates.
Engineering Solutions to Maximize Performance
The research team applied various strategies to control these π-interactions and optimize battery performance:
Electrolyte engineering: By changing the electrolyte composition from EC/DMC (1:1) to EC/EMC (3:7), they reduced the solubility of oxidized PVMPT, allowing access to its full theoretical capacity.
Carbon architecture: Using highly porous conductive carbon (Ketjenblack 600) encapsulated PVMPT within pores, preventing dissolution even in its oxidized form.
Cross-linking: Cross-linked PVMPT (X-PVMPT) showed reduced solubility and enabled stable cycling with nearly full theoretical capacity.
Molecular design: Changing the PT structure by replacing sulfur with oxygen, altering the polymer backbone, or introducing electron-donating substituents allowed fine-tuning of the π-interactions.
Applications in Full Cell Configurations
The versatility of PT-based polymers becomes even more evident in their application to various full cell configurations:
Dual-ion batteries: X-PVMPT with lithium titanate delivered stable cycling at 1.86V with minimal capacity loss.
All-organic batteries: A naphthalene diimide-PT copolymer used as both positive and negative electrode demonstrated remarkable stability over 1,000 cycles.
Anionic batteries: X-PVMPT paired with a viologen-functionalized polystyrene showed good performance at 0.9V.
Symmetric cells: By introducing electron-donating methoxy groups to stabilize the dicationic form, researchers created a polymer (X-PSDMPT) that could serve as both positive and negative electrode.
Aluminum batteries: X-PVMPT showed exceptional performance in an aluminum battery configuration, with specific capacities surpassing graphite electrodes and retention of 88% capacity after 5,000 cycles.
Conjugated Systems: Enhancing Conductivity
The research team also explored conjugated PT polymers, which offer intrinsic semiconductivity through π-conjugation along the polymer chain. This approach enabled:
Fabrication of electrodes with reduced conductive carbon additives
Ultrahigh rate capabilities (up to 100C)
High mass loading electrodes (90% active material)
Development of donor-acceptor type polymers with visible-light absorption
These conjugated systems even found application in photobatteries, where PT-bithiophene copolymer (P(PT-T2)) was paired with a five-junction organic solar cell to create a device that could be charged by light in less than 15 minutes.
The Road Ahead: Challenges and Possibilities
While phenothiazine polymers have demonstrated impressive performance in various battery configurations, challenges remain for their practical implementation. Future research will need to focus on:
Scaling electrode thickness and active material mass loading
Designing materials to enable efficient counteranion diffusion
Identifying optimal battery configurations for specific applications
Advancing from laboratory demonstrations to commercial viability
Conclusion
The research by Esser and colleagues showcases the remarkable versatility of phenothiazine-based polymers as electrode materials for next-generation batteries. By understanding and controlling π-interactions, engineers can design materials with exceptional stability, rate capability, and compatibility with various battery chemistries.
As we continue to seek sustainable alternatives to conventional battery technologies, phenothiazine polymers represent a promising avenue for developing organic electrode materials that are environmentally friendly, highly functional, and adaptable to diverse energy storage needs.
This article is a summary of the original research published by Esser et al. in Accounts of Materials Research [1].
References
[1] Esser, B., Morhenn, I. H., & Keis, M. (2025). Phenothiazine Polymers as Versatile Electrode Materials for Next-Generation Batteries. Accounts of Materials Research. DOI: 10.1021/accountsmr.5c00053
[2] Billon, J. P. (1961). Proprietés Électrochimiques de La Phenothiazine. Étude de Son Oxydation a Une Électrode de Platine Dans l'acetonitrile. Bull. Soc. Chim. Fr., 1923-1929.
[3] Kolek, M., Otteny, F., Schmidt, P., Mück-Lichtenfeld, C., Einholz, C., Becking, J., Schleicher, E., Winter, M., Bieker, P. M., & Esser, B. (2017). Ultra-High Cycling Stability of Poly(Vinylphenothiazine) as a Battery Cathode Material Resulting from π−π Interactions. Energy Environ. Sci., 10(11), 2334-2341.
[4] Otteny, F., Kolek, M., Becking, J., Winter, M., Bieker, P., & Esser, B. (2018). Unlocking Full Discharge Capacities of Poly(Vinylphenothiazine) as Battery Cathode Material by Decreasing Polymer Mobility Through Cross-Linking. Adv. Energy Mater., 8(33), 1802151.
[5] Acker, P., Rzesny, L., Marchiori, C. F. N., Araujo, C. M., & Esser, B. (2019). Π-Conjugation Enables Ultra-High Rate Capabilities and Cycling Stabilities in Phenothiazine Copolymers as Cathode-Active Battery Materials. Adv. Funct. Mater., 29(45), 1906436.
Mentions
Prof. Birgit Esser - Institute of Organic Chemistry II and Advanced Materials, Ulm University
Isabel H. Morhenn - Institute of Organic Chemistry II and Advanced Materials, Ulm University
Michael Keis - Institute of Organic Chemistry II and Advanced Materials, Ulm University
