Advancements in n-type π-conjugated polymeric materials for enhanced battery applications

Abstract

The evolution of energy storage technologies has led to significant advancements in battery chemistry, crucial for renewable energy storage and electric vehicles. This review explores the application of polymeric materials for battery applications due to their structural tunability, and molecular-level design flexibility. Focusing on BIAN based n-type π-conjugated polymers, this article presents breakthroughs in polymer binders and anodic active materials that enhance the efficiency, capacity, and rate capability of LIBs, electro polymerizable additives that increase the operating potential range of batteries and electro-catalysts that are affordable, easy to synthesize and efficient.

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Introduction

From the rudimentary voltaic piles of the 19th century to the cutting-edge metal-ion and air batteries of today, energy storage technology has undergone a profound evolution, shaping the very foundation of modern society. The global battery market, valued at over $120 billion in 2023, is projected to exceed $400 billion by 2035.¹ This growth is driven by the relentless demand for higher energy density, longer lifespan, and faster charging capabilities. Electric vehicles, which accounted for nearly 14 million sales in 2023 alone, rely on continual advancements in battery chemistry to push the boundaries of efficiency and sustainability. Meanwhile, the transition to renewable energy sources has made large-scale battery storage indispensable for stabilizing power grids and ensuring energy security.

As the world moves toward electrification, the expectations placed on battery systems have never been greater. This demand necessitates the development of better electrodes and electrolytes, along with a fundamental rethinking of every component. Researchers across various scientific domains are now challenged to develop novel materials that can overcome persistent issues related to cycle life, mechanical degradation, and electrode stability in today's commercial battery systems.

To meet these evolving requirements, the focus has gradually shifted beyond traditional material improvements towards a more holistic approach to material design. The challenges of capacity fading, mechanical degradation, and interfacial instability have underscored the need for innovative materials that can enhance overall system performance. Among these advancements, polymeric materials have emerged as promising candidates, inspiring a paradigm shift in battery component design beyond conventional approaches.

Polymeric materials have garnered increasing attention in recent years due to their unique combination of properties, making them ideal candidates for battery applications. Since the early 2000s, interest in organic electrode materials has re-emerged, leading to the synthesis and study of various redox-active monomers and polymers in laboratory settings. Unlike traditional inorganic materials, polymers offer significant advantages, including structural tunability, lightweight nature, and the ability to be tailored at the molecular level for specific functions. This versatility allows for the design of materials that improve mechanical integrity, enhance ionic conductivity, and optimize electrochemical performance while maintaining stability under extreme cycling conditions.

While the progress in carbon-based materials has tried to address some of the pertaining challenges associated with the traditional energy storage and conversion materials, the lack of sustainable, cost effective and scalable procedures pose a major hurdle in their wide-scale and industrial adoption.^2,3 Moreover, the widespread availability of organic monomers further supports the development of polymers for battery and energy related applications. Structural tunability, lower carbon footprint, and sustainability of organic monomers make them an excellent choice for high-energy battery systems.⁴ Functional groups within these polymers not only contribute to sustainability, but also enable tailor-made applications, enhancing their adaptability for various energy storage needs.⁵

As an example in the recent developments in the progress of n-type polymers, Gong et al.⁶ demonstrated the creation of air-stable n-type graphene using ammonia annealing, achieving high electron mobility in OFET devices.⁷ Building on this, Lee et al.⁷ developed a green synthesis route using ionic liquids to nitrogen-dope graphene oxide, enabling p–n switching behavior depending on the operating environment. Takeuchi's⁸ fluorinated hexa-peri-hexabenzocoronene (HBC) derivatives further pushed the boundaries of chemical bandgap tuning by lowering HOMO and LUMO levels through fluorination. Hatakeyama et al.⁷ innovated by encapsulating azafullerene into carbon nanotubes, creating the first n-type carbon peapods via charge transfer, while Dai et al.⁹ demonstrated that polyethylene imine–functionalized carbon nanotubes could serve as air-stable n-type conductors with electron mobilities as high as 8000 cm² V⁻¹ s⁻¹. These studies collectively addressed key limitations in electron transport, air sensitivity, and scalability, and offer valuable design strategies for next-generation binders and redox-active components in lithium-ion batteries. Some of the other applications have also focused on development and design of high-performance n-type (electron-transporting or n-channel) polymers for various organic optoelectronic devices and complementary circuits.¹⁰ Recent advancements have significantly improved the performance of organic thin-film transistors, all-polymer solar cells, and organic thermoelectrics, among others.^11–13

It needs to be mentioned that in electrochemical systems, materials that react with cations are classified as n-type, while those that react with anions are referred to as p-type. Typically, n-type materials exhibit lower average voltage and higher specific capacity compared to p-type materials making them suitable for anode applications, while p-type redox-active organic polymers have been engineered as cathodes predominantly.^10,14 Mechanistically, to achieve efficient electron transport and hopping within the polymer backbone (which results in faster charge carrier diffusion and interface kinetics), the lowest unoccupied molecular orbital (LUMO) energy levels of the polymer should be sufficiently low to enable efficient and stable charge transport. For p-type polymers, which function as electron donors, a higher highest occupied molecular orbital (HOMO) level facilitates oxidation, enhancing their effectiveness as cathode materials. In contrast, in n-type electrodes, when the polymer's LUMO is significantly lower than that of the electrolyte, it efficiently accepts electrons, thereby preventing excessive reduction of the electrolyte (schematically shown in Fig. 1). This electron acceptance promotes the formation of a thinner and more stable solid electrolyte interphase (SEI), which is beneficial for competitive battery performance, which in turn promotes faster interfacial kinetics.¹⁵

Fig. 1 Schematic representation of n-doping and p-doping of conjugated polymeric binders.

Though most n-type materials studied in the literature involve the reversible reduction of the oxygen atom in a carbonyl group, a diverse range of nitrogen-containing molecules has also been explored, including azo, imine, sulfonamide, and nitrile redox centers.¹⁵ Most reports focusing on the design of an optimal n-type polymer involve functionalizing fused (hetero)arenes with strong electron-withdrawing groups in a strategically conducive manner.¹³ This yields highly electron-deficient building blocks, which are then typically combined with conventional (hetero)arene and/or π-bridge blocks to form π-conjugated polymers with suitable electronic structures.^15–17

However, despite this extensive research on n-type redox materials and polymers for energy storage devices, no publications to date have specifically explored and compiled the works regarding the potential of n-type polymers in energy storage applications, to the best of our knowledge. While traditional anode materials such as graphite have failed in terms of delivering high capacity, and silicon suffers from high interfacial diffusion kinetics and volume expansion, n-type conjugated polymers have catered to the needs, with the provision of high gravimetric capacity, faster diffusion kinetics, and lower structural changes during the charge–discharge process.^18,19 Furthermore, to add to their diverse applications, the n-type polymers have also addressed the issues of traditional binders such as PVDF, PAA, CMC and SBR, by facilitating modulating the solid electrolyte interphase, providing a conductive network, while performing the known function of maintaining the electrode integrity.^20,21 While most reviews focus on conjugated polymers while discussing their applications in energy storage devices, this feature article is an attempt towards exploring and compiling different n-type polymers, especially with the acenaphthoquinone moiety in the backbone as an organic electrode precursor for various applications such as anode material, binders, ORR catalysis and carbon precursor material. Acenaphthoquinone features a fused ring system with carbonyl groups, facilitating the formation of extended π-conjugated systems, which are essential for efficient electron transport in n-type materials.¹⁵ The electron-deficient nature of the quinone moiety improves the polymer's ability to accept electrons, thereby boosting its n-type properties. Furthermore, the rigid planar structure of acenaphthoquinone contributes to the formation of polymers with well-ordered morphologies, which is beneficial for charge transport.²² Considering the imminent advantages of this moiety, in the evolving landscape of materials science, bis-imino acenaphthoquinone (BIAN) based polymers stand out by uniquely integrating multiple functionalities serving not only as n-type conductors but also as binders, electrolyte additives, and electrocatalysts. This multifunctionality positions them as a versatile framework aligned with the broader direction of advanced energy materials research. As the demand for high-performance batteries continues to rise, particularly those requiring fast charging, high energy density, and long cycle life there is an increasing need for binder systems that can actively contribute to electrochemical performance rather than serve as passive components. In response, conjugated polymer-based binders have emerged as promising alternatives, offering not only mechanical resilience but also enhanced electronic and ionic conductivity. These properties enable improved charge transport, better electrode integrity, and more stable solid-electrolyte interphases (SEIs).

Hence, in the first section we discuss the potential of polymers with a specific focus on hetero-atom doped polymers in their applications as anodes. Following this, this article aims to shed light on the application of BIAN based binders in lithium batteries and BIAN based electrolyte additive as next-generation advanced additive systems. Towards the end we share our perspectives on the catalytic properties of such polymers.

Anode active material

The anode plays a vital role in determining the performance of a battery, and an ideal anode material should offer high reversible capacity, improved gravimetric and volumetric energy density, cost-effectiveness, environmental compatibility, enhanced energy and power density, and a low anodic potential (Li⁺/Li). Based on the lithium storage mechanism (lithiation and delithiation), anode materials can be divided into three categories: (1) intercalation-based anodes (LiMX₂), which include carbonaceous materials and titanium oxides; (2) alloying-based anodes (Li_xM), where M represents elements such as Si, Ge, Sn, their oxides; and (3) conversion mechanism-based anodes (Li₂X + M), where M includes transition metals like Fe, Co, and Ni, while X represents elements such as O, P, and S.²³ Among these, graphite-based anodes are widely used in commercial batteries due to their favorable working potential, cost-effectiveness, and safety. However, challenges such as anode polarization, limited energy density, high energy demands, and slow charge transfer kinetics have spurred the development of alternative materials.²⁴ Moreover, issues like pulverization, delamination, and an unstable solid electrolyte interphase (SEI) have hindered the commercialization of silicon-based anodes.^25,26 In this context, polymer-based electrode materials are gaining increasing attention due to their environmentally friendly nature, abundant resources, ease of preparation and structural diversity. Among the various types of polymer-based electrodes, conducting polymers, carbonyl polymers, radical polymers, sulfide polymers, and imine-functionalized polymers have been extensively studied.²⁷ Conductive polymers, particularly those with n-type characteristics, have garnered significant attention as potential anode materials due to their unique structural and electronic properties. These polymers feature overlapping p_z orbitals from adjacent carbon atoms, which form a conjugated π-electron system along the polymer backbone. This conjugation enables the movement of electrons across the polymer, imparting electrical conductivity to the material. Moreover, these polymers can be engineered to exhibit specific electrode morphologies that improve performance metrics such as rate capability, energy density, and cycling stability. Unlike traditional materials, conductive polymers offer unparalleled versatility in design and functionalization, enabling the creation of highly efficient, and cost-effective anode materials with enhanced capacity and faster charge–discharge cycles.²⁸ Their ability to combine flexibility, tunable properties, and high conductivity makes them highly promising for next-generation energy storage applications. By carefully controlling the doping of these polymers, their electronic structure can be significantly modified.²⁹ This doping process adds charge carriers, facilitating more efficient electron transfer during charge and discharge cycles, thereby improving overall electrochemical performance. Building on this idea, our group developed an innovative approach by introducing a donor–acceptor (D–A) pair into the polymer chain. This D–A interaction facilitates the transfer of electron density between the donor and acceptor moieties, leading to a decrease in the polymer's band gap.³⁰ As a result, electron movement becomes easier, enhancing charge carrier mobility and improving electron transport along the polymer chain. This mechanism boosts the polymer's electrical conductivity, which is critical for optimizing performance in LIBs. Our group synthesized a POP, specifically POL202, using 2,2′-bithiophene-5,5′-dicarboxaldehyde and 1,2,4,5-benzene tetraamine tetrahydrochloride moieties.³¹ POL202-based anodic half-cells exhibited exceptional performance in LIBs. At a current density of 1000 mA g⁻¹, the cells achieved a high reversible capacity of 515 mAh g⁻¹. At higher current densities, the material retained 205 mAh g⁻¹ at 5000 mA g⁻¹ and 270 mAh g⁻¹ at 2000 mA g⁻¹, demonstrating excellent rate capability. Additionally, POL202's porous structure limited volume expansion of the electrode to only 47.6% when cycled at 2000 mA g⁻¹, further emphasizing its stability and resilience under high-rate conditions.³¹ Another study, we focused on nitrogen-rich, n-type porous organic polymers, which offer excellent electrochemical performance. The morphology of the polymer is affected by the choice of reactants and spatial conformation of the reactants. As the orientation of chain growth is a consequence of covalent bond formation and angle of the bonds, reactant geometry is crucial in designing the POPs in terms of porosity and thereby the surface area of the POP. A polymer containing BIAN and melamine moieties (PBM), was synthesized with a hexagonal pore structure and a pore diameter of 1.5 nm. This nitrogen-rich porous organic n-type conjugated polymer was synthesized via polycondensation between acenaphthoquinone and melamine, where each moiety was designed to perform specific functions, for maximizing specific capacity and lithium-ion kinetics (Fig. 2).³²Fig. 2(d) shows the TEM image of PBM. The electrodes were fabricated using PBM as the active material, polyvinylidene fluoride (PVDF) as the binder, and acetylene black as the conductive additive, combined in a weight ratio of 3 : 1 : 1. The average mass loading of the electrode was maintained between 0.65 and 0.70 mg cm⁻², with a BIAN content of 0.75 mg per electrode to ensure consistent active site availability in the case of a 0.70 mg cm⁻² areal loading. The PBM-based anode demonstrated remarkable electrochemical performance, especially in terms of rate capability, cycling stability, and specific capacity. It delivered 850 mAh g⁻¹ at 400 mA g⁻¹ over 3000 cycles and retained a high reversible capacity of 300 mAh g⁻¹ at 1000 mA g⁻¹ for 1100 cycles (Fig. 2b).³² The reaction mechanism of PBM was investigated in detail to understand its role as an active anode component in lithium-ion batteries. The dQ/dV vs. voltage profiles showed a clear increase in peak intensity during the charge–discharge process, with the lithiation peak gradually shifting. This shift indicated a reduction in the delithiation overpotential, suggesting improved reaction kinetics over extended cycling. The continuous increase in capacity, along with the changes observed in the differential capacity plots, suggested that lithium intercalation became progressively easier with cycling. This behavior was attributed to the expansion of interlayer spacing within the PBM structure as lithium ions penetrated between the layers.³² Furthermore, the double-layer region in the charge–discharge plot between 1 and 3 V was observed to broaden with continued cycling, indicating increased surface charge storage, likely due to an enhanced surface area resulting from electrode expansion during repeated charge–discharge cycles. Density functional theory (DFT) studies were conducted to elucidate the lithium storage mechanism in PBM, revealing a two-step lithiation process involving distinct redox-active sites within each monomer unit. The first lithiation stage occurs at the imine (C=N) groups, where significant charge transfer between lithium ions and nitrogen atoms facilitates strong Li–N coordination, enabling one lithium ion to be stored per C=N unit. In the second stage, lithium ions interact with the conjugated C=C bonds in the naphthalene backbone, allowing additional lithium storage through reduction of the aromatic system, again accommodating one lithium ion per C=C unit. This dual-site mechanism was validated by voltage profiles showing a distinct change in the slope at ∼0.9 V, corresponding to the transition between the two lithiation steps. Galvanostatic charge–discharge data indicated the gradual increase in capacity contribution from imine group and naphthalene moiety from cycles 1 to 2000. These results highlight the progressive activation of lithium storage sites and the highly reversible nature of both lithiation processes, which together underpin the high capacity and long-term cycling stability of PBM as an anode material.³² This was evident from post-mortem studies of the anode as well. Fig. 2(e) and (f) indicate the FE-SEM images of PBM based anode before and after cycling. The images clearly indicated the pristine electrode exhibited a thickness of 11.6 μm, which increased to 18.3 μm after cycling, indicating structural changes during prolonged operation. In the pristine state (Fig. 2e), well-defined crystalline particles with distinct boundaries were clearly visible. However, after 2000 cycles (Fig. 2f), the particle boundaries became indistinct, suggesting morphological changes attributed to the formation of SEI. The expansion of the PBM electrode also resulted in a noticeable change in particle morphology evidenced by a 6.7 μm increase in electrode thickness, corresponding to a 157.8% increase in volume, as shown in the FE-SEM cross-sectional images. The observed volume expansion indicates a morphological transformation of the PBM electrode, likely exposing new active sites for lithium storage. This structural evolution contributed to the gradual increase in specific capacity. The maximum theoretical capacity of the polymer, based on the complete utilization of all active sites, was calculated to be 1068.5 mAh g⁻¹.³³ In yet one more study, we synthesized an imine and azo-functionalized POP using BIAN and Bismarck Brown moieties (Fig. 3a). When used as an anode material in LIBs, this polymer exhibited a high capacity of 500 mAh g⁻¹ at 0.4 A g⁻¹ (Fig. 3b and c). Interestingly, the initial discharge capacity was relatively low at 130 mAh g⁻¹, but steadily increased over successive cycles, ultimately reaching a peak value of 500 mAh g⁻¹. This gradual rise in capacity is attributed to the progressive electrochemical activation of the redox-active functional groups present in the polymer framework—namely the C=C, C=N, and N=N bonds, which become increasingly accessible and participate more effectively in lithium storage as cycling proceeds. The electrodes were fabricated using BBP as the active material, polyvinylidene fluoride (PVDF) as the binder, and acetylene black as the conductive additive, combined in a weight ratio of 8 : 1 : 1 and maintain an areal loading of 1.0 mg cm⁻². The high capacity exhibited by these BIAN-based active materials is highly commendable; however, there remains room for improvement in their cycling stability. One promising direction for enhancing long-term performance is the exploration of alternative binder systems. As previously discussed, various n-type conductive polymers, including BIAN-based binders mentioned in previous section, could serve as promising alternatives to conventional binder systems for BIAN-based electrodes. These materials not only provide essential mechanical integrity but also enhance electronic conductivity and ensure better chemical compatibility with the active material.

Fig. 2 (a) Synthetic scheme of PBM, (b) schematic representation of battery with PBM anode, (c) comparison of reversible capacity delivered by commercial anodic active materials with PBM at different current densities, (d) TEM image of PBM, (e) cross-sectional FE-SEM.

Fig. 3 (a) Synthetic scheme of BBP, (b) charge–discharge studies of anodic half-cells with BBP active material, (c) rate studies of anodic half-cells with BBP active material. Reproduced with permission. Copyright 2023, Elsevier.

Hence, overall, these studies highlighted the potential of functionalized organic polymers in improving the performance of LIB anodes.³⁴Table 1 shows a brief comparison of different polymer-based anodic active materials. Apart from this, hard carbons (HCs), also referred to as non-graphitizable carbons, are materials that possess a unique structure characterized by numerous short-range disordered carbon layers. These carbon layers are spaced at an expanded interlayer distance of ≥3.5 Å. HCs resist graphitization even under extreme temperatures up to 3000 °C, unlike graphite, which consists of long-range ordered graphitic planes. The expanded interlayer spacing in HCs results from the disordered nature of the material, which allows for better accommodation of Li-ions during cycling in LIBs. This unique structure, with its high surface area,³⁵ numerous defect sites, and nanopores, is highly advantageous for ion storage and diffusion, making HCs suitable for a wide range of energy storage applications.³⁶ The nanovoids or pores in HCs further contribute to fast ion transport, enhancing the rate capability and cycling stability of LIBs. Apart from structural modifications, elemental doping is also beneficial as it can enable introduction of various heteroatoms such as nitrogen, sulphur, boron, phosphorous etc.³⁰ Nitrogen doping is highly effective due to the minimal size difference between carbon and nitrogen atoms. This doping not only actively participates in lithium-ion storage but also improves electronic conductivity and introduces defect sites that promote the perpendicular movement of lithium ions. These attributes are particularly beneficial for fast-charging applications, as they improve both ion diffusion and charge transfer efficiency. Based on these concepts of N-doping advantages, our group utilized BIAN-melamine based porous organic polymer (PBM) as a single source of carbon and nitrogen for synthesizing N-doped HCs.³⁷ The synthesis temperature significantly influenced the morphology of the active material, which in turn affected the battery performance. The PBM polymer was pyrolyzed at three different temperatures, yielding materials labeled PyPBM600, PyPBM700, and PyPBM800. Morphological studies revealed that PyPBM800 exhibited 61.6% crystalline nature with an average crystallite size of 3.2 nm, while PyPBM600 showed a more amorphous structure with 49% crystalline nature and an average crystallite size of 1.7 nm.³⁷ The increased crystallinity at higher pyrolysis temperatures indicated enhanced graphitization (C–C bond formation), while lower-temperature pyrolysis retained more C–N bonds. The amorphous structure of PyPBM600 resulted in a decrease in charge transfer resistance and a high exchange current density (j₀) of 8.8 × 10⁻⁴ mA cm⁻², which enhanced its rate capability. Despite this, PyPBM600 being more amorphous than PyPBM800, showed higher initial capacity but poor capacity retention due to parasitic side reactions. Specifically, the PyPBM800-based anode exhibited a reversible capacity of 86 mAh g⁻¹ at 4000 mA g⁻¹ and maintained 99% capacity retention after 1500 cycles. In contrast, the PyPBM600-based anode delivered a higher initial capacity of 120 mAh g⁻¹ at the same current density but had a 79% capacity retention after 1000 cycles. Hence, the PyPBM600 anode demonstrated superior initial capacity but exhibited lower long-term cycling stability due to parasitic side reactions, while PyPBM800 showed better capacity retention and stable performance over extended cycles. Furthermore, full cells were assembled with LiNCAO as the cathode and PyPBM800 based anode mass loading of 8.0 mg cm⁻² which showed a reversible capacity of 1.2 mAh with 99.8% coulombic efficiency. When charged at a 15-minute rate with a current density of 2.4 mA cm⁻², the full cell achieved an energy efficiency of 90% and a specific energy of 325 Wh kg⁻¹. Following this, in another study, the impact of nitrogen doping and optimal synthesis temperature on maximizing capacity was investigated by copolymerizing BIAN with Bismarck Brown and pyrolyzing at different temperatures. The PyBBP800-based anode exhibited a reversible capacity of 420 mAh g⁻¹ at a low current density of 50 mA g⁻¹. The PyBBP600-based anode delivered 440 mAh g⁻¹ under the same conditions.³³ However, at higher current densities of 2 A g⁻¹ and 4 A g⁻¹, the PyBBP600 anode maintained 150 mAh g⁻¹ and 113 mAh g⁻¹ discharge capacities, with 76.7% and 86.0% capacity retention after 5000 cycles, respectively whereas the PyBBP800 anode provided 200 mAh g⁻¹ and 145 mAh g⁻¹ at the same current densities, with 81.0% and 86.2% capacity retention after 5000 cycles.³³ Both anodes exhibited 100% coulombic efficiency throughout the testing. These findings emphasized the significant role of nitrogen-doped carbon materials and synthesis temperature in improving battery performance, positioning BIAN-based polymers and their derived carbon materials as promising candidates. While selecting different active materials can enhance capacity, long-term cycling stability remains a challenge due to issues such as electrode expansion, particle detachment, and interfacial degradation. To overcome these challenges, the choice of binder becomes essential, as it not only ensures mechanical integrity but also plays a key role in ion transport and interphase formation.

Table 1 Comparison of the BIAN based active material with n-type polymers and commercial anodes

Material	Current density (mA g^−1)	Capacity (mAh g^−1) (efficiency %)	Cycles
Tp-Ta-COF^38	200	418 (100)	800
E-TFPB-COF^39	100	968 (100)	300
Poly-diaminophenylsulfone−triazine^40	100	565 (99)	100
Polythiophene^35	500	100 (100)	1000
Poly(4,7-dicarbazyl-[2,1,3]-benzothiadiazole)^41	200	312 (99)	400
Poly-thieno[3,2-b] thiophene-carbon composite^42	500	424 (100)	700
Graphite^34	372	200 (99)	1000
BBP ^33	400	550 (99)	1500
PBM ^43	750	740 (99)	2000

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BIAN based materials as binders

The evolution of binder materials for lithium-ion batteries (LIBs) has progressed significantly, shifting from passive structural components to highly functional materials that actively contribute to battery performance enhancement. Conventional binders such as poly(vinylidene fluoride) (PVDF) were primarily employed to maintain electrode cohesion and mechanical stability. In recent years, several pioneering research efforts have advanced the field of n-type π-conjugated materials for energy storage, particularly by addressing the historical challenges of low charge mobility, poor ambient stability, and limited multifunctionality. Jiang et al.⁴⁴ developed sulfonated poly(oxadiazole) (POD) as a water-processable, electron-deficient binder for silicon anodes, offering both ionic and electronic conductivity along with strong mechanical integrity thereby replacing traditional insulating binders like PVDF and CMC. Nishide and coworkers⁴⁴ introduced poly(nitroxylstyrene) radical polymers with tunable redox properties, where strategic substituent modifications enabled switching from p-type to n-type behavior. This achievement marked one of the first demonstrations of redox-switchable, all-organic battery materials with rapid redox kinetics and environmental sustainability.⁴⁵ Meanwhile, Baumgartner and others⁶ have guided the design of conjugated small molecules and polymers such as perylene diimides (PDI), naphthalene diimides (NDI), and azaacenes, offering high electron mobility and chemical tunability through backbone engineering and electron-withdrawing substitutions. Additionally, Yu et al. developed a series of novel n-type conductive polymer binders (CPBs) for SiMP anodes in lithium-ion batteries which showed better cycling performance in comparison to conventional PAA and CMC binders.⁴⁴ In addition to their applications in anodes, conducting polymers such as PEDOT:PSS, PPy, and PANI have been investigated as cathodic binders in cathodes for the Li ion, Li–S, and LMBs.^46,47 In a recent study, Olmo et al. reported a mixed ionic–electronic conducting binders composed of PEDOT:PSS and an organic ionic plastic crystal enabled the development of a solid-state battery comprising of an LFP cathode without the use of any carbon additive.⁴⁸ The mixed conducting binder effectively replaced both the conventional binder and carbon additives in the solid-state Li/LiFePO₄ cell, resulting in an improved discharge capacity (157 mAh g⁻¹ at C/10) and enhanced rate capability compared to a solid-state cathode formulation employing an ion conductive polymeric binder along with a carbon additive.⁴⁸ Also, Zhu et al. demonstrated a poly(bithiophene)–carbon (PBT/C) composite with n-type redox properties, high reversible capacity, and cycling stability, synthesized by ball-milling chemically polymerized poly(bithiophene) with carbon nanofibers, that exhibited a two-electron redox capacity of ∼850 mAh g⁻¹, with half of the capacity delivered at a low potential plateau of 1.25 V in Li⁺ electrolyte.⁴⁹

Bis-imino acenaphthene (BIAN)-based n-type conjugated polymers such as bis-imino-acenaphthenequinone–paraphenylene (BP) copolymer and BIAN–fluorene copolymer have demonstrated strong potential in their applications in their applications as anodic binder materials. Owing to their low-lying LUMO energy levels, these polymers can undergo stable n-doping, enabling them to accept electrons and maintain conductivity under the reducing conditions present at the lithium-ion battery anode. This section explores how such conjugated architectures influence key performance metrics and provide a foundation for multifunctional material design in next-generation batteries. At the molecular level, the lowest unoccupied molecular orbital (LUMO) of the binder plays a crucial role in determining its electron affinity and its reductive stability at the anode–electrolyte interface. In typical lithium-ion battery systems, the graphite anode operates at potentials close to 0–0.2 V vs. Li/Li⁺, where many electrolyte solvents (e.g., EC and DMC) are thermodynamically unstable and undergo reduction to form the SEI (solid electrolyte interphase). However, if the binder itself has a LUMO energy level lower (more positive in reduction potential) than that of the solvent molecules, it can participate in the initial reduction reaction, thus controlling the formation of the SEI in a more predictable and uniform manner. Unlike traditional PVDF binders, which are non-conductive and degrade over time, BIAN based polymers were observed to enhance electronic conductivity, improve solid electrolyte interface (SEI) formation, and provide better mechanical stability. Their ability to undergo n-doping before electrolyte decomposition leads to the formation of a thin, stable SEI layer, reducing interfacial resistance and enabling faster lithium-ion diffusion with diffusion coefficients significantly higher than PVDF.

BIAN-based binders for the graphite anode

The widespread use of graphite as an anode material in lithium-ion batteries (LIBs) is hindered by several performance-limiting factors, particularly under high-rate cycling conditions. Key challenges include unstable solid electrolyte interphase (SEI) formation, weak adhesion to the current collector, and limited electronic conductivity. Conventional binders such as poly(vinylidene fluoride) (PVDF) offer inadequate solutions to these issues due to their weak interactions with both the graphite framework and the current collector, leading to suboptimal electrochemical performance. Additionally at high C-rates, PVDF is associated with safety concerns due to its non-conducting nature.

In recent years, various binder modifications have been explored to enhance the electrochemical performance of graphite anodes beyond traditional PVDF-based systems. Park et al. introduced a glycerol-modified SBR/CMC binder, which improved electrochemical impedance and facilitated Li⁺ migration by increasing the free volume within the polymer matrix. However, excessive glycerol content could lead to binder plasticization, potentially compromising mechanical integrity. To address the growing need for binders compatible with solid electrolytes, D. O. Shin et al.⁵⁰ developed Li⁺-CMC, a conductive variant of carboxymethyl cellulose that significantly reduced internal resistance and enhanced ionic conductivity. While this approach improved high-temperature performance, its effectiveness remained dependent on lithium salt concentration.

The self-supporting electrode structure eliminated the need for additional processing solvents, though PTFE decomposition during the initial cycling posed a challenge. Seeking alternatives to SBR, Sandaruwan et al.⁵¹ employed white latex (WL) as a binder, demonstrating superior adhesion and lower electrolyte swelling compared to PVDF. While WL was easier to process and offered improved initial coulombic efficiency, its long-term cycling stability required further validation.

To improve electronic conductivity, D. A. Gribble et al.⁵² explored PEDOT:PSS, a conductive polymer binder that enhanced charge transport and reduced SEI formation, lowering the risk of thermal runaway. However, PEDOT:PSS exhibited low wettability, potentially hindering electrolyte penetration.⁵³ In another study, Francon et al.⁵⁴ investigated cellulose-rich fibers modified with functional groups, which provided high adhesion strength and environmental sustainability.

To address many of these limitations, advanced binder systems have been developed, with bis-imino-acenaphthenequinone (BIAN)-based polymers emerging as a promising class of materials. Our group reported a BIAN–fluorene copolymer,⁵⁵ which exploits the synergistic properties of its molecular components to enhance anode performance. As shown in Fig. 4a, the planar naphthalene core of the BIAN unit facilitates π–π stacking interactions with graphite, reinforcing the anode's structural integrity. Additionally, the polymer backbone preserves electronic conductivity, further supporting efficient lithium-ion diffusion during cycling.

Fig. 4 (a) Structure–activity relation of BIAN–fluorene based copolymer (BF) (b) rate studies of graphite based anodic half-cells with BF as binder (c) structure–activity relation of BIAN-paraphenylene based copolymer (BP) (d) charge–discharge studies of graphite based anodic half-cells with BP as binder.

Electrochemical studies have demonstrated that graphite electrodes incorporating the BIAN–fluorene binder significantly outperform their PVDF-based counterparts (Fig. 4b), exhibiting specific capacities exceeding 250 mAh g⁻¹ at a 1C rate after 100 cycles, compared to the 165 mAh g⁻¹ achieved with PVDF. Moreover, impedance analysis confirms the formation of a more stable and conductive electrode–electrolyte interface, which is crucial for long-term cycle stability and high-rate operation. Despite its superior performance, the synthesis and purification of the BIAN–fluorene binder involves complex and time-intensive procedures, making large-scale implementation economically challenging (Fig. 5a). To overcome this issue, a more efficient synthetic strategy was developed via acid-catalyzed Schiff base condensation, significantly simplifying the production process while retaining the structural and electrochemical benefits of the original BIAN-based polymer (Fig. 5b).

Fig. 5 (a) Synthesis of BF (b) synthesis of BP.

The original synthesis route for the BIAN-based polymer monomer, although providing a high overall yield (∼92%), involves multiple steps that include the use of air- and moisture-sensitive reagents such as tetrakis(triphenylphosphine)palladium(0) and bis(triphenylphosphine)palladium(II) dichloride as catalysts, as well as anhydrous toluene and triethylamine under reflux for extended durations (up to 48 hours). Additionally, column chromatography purification is required at two intermediate stages, which adds significantly to the material and solvent consumption, labor, and waste generation—factors that are typically discouraged in large-scale or industrial synthesis due to high cost, environmental burden, and low throughput.

In contrast, the simplified synthesis route we developed proceeds in fewer steps, does not require metal catalysts, and replaces organic bases and dehydrated solvents with acetic acid as a benign and inexpensive reagent. Although the isolated yield in this route is moderately lower (∼72%), it eliminates the need for column chromatography, relying instead on simple filtration and precipitation techniques. This greatly reduces solvent usage, processing time, and equipment requirements.

From an industrial standpoint, this simplified method is significantly more appealing for scale-up. It aligns better with green chemistry principles and process intensification standards commonly followed in industry, where reaction robustness, environmental safety, cost-effectiveness, and ease of purification are prioritized over marginal gains in yield. The reduction in hazardous reagent use, waste generation, and operational complexity makes this pathway much more sustainable and commercially viable for large-scale production of BIAN-type conjugated materials.

The inherently low-lying LUMO energy level of BIAN contributes to reduced SEI formation, minimizing irreversible electrolyte decomposition and enhancing battery longevity. density functional theory (DFT) calculations further corroborate these findings, demonstrating that the polymer's low LUMO energy (−3.17 eV) enables n-doping before electrolyte reduction, promoting the formation of a thin, conductive SEI that mitigates side reactions during cycling.

The electrochemical superiority of BIAN-based binders extends across multiple performance parameters, including enhanced mechanical properties, improved lithium-ion transport, and reduced interfacial resistance. Graphite electrodes incorporating a P-BIAN copolymer⁵⁶ binder have exhibited exceptional electrochemical stability, delivering a high reversible capacity of 260 mAh g⁻¹, 95% capacity retention over 1735 cycles, and significantly lower interfacial resistance (Fig. 4d). These attributes highlight P-BIAN as a promising candidate for next-generation LIB binders, capable of addressing the limitations of conventional binder systems (Fig. 4c).

Building on this foundation, the development of composite binder systems has further expanded the capabilities of BIAN-based materials. A particularly notable example is the BIAN–LiPAA composite binder,⁵⁷ which integrates the advantages of BIAN-based conjugated polymers with poly(lithium acrylate) (LiPAA).

As shown in Fig. 6, this novel composite binder system introduces an intrinsic lithium-ion reservoir, significantly enhancing initial coulombic efficiency and facilitating efficient lithium-ion diffusion, desolvation, and Li⁺ uptake within the SEI. As a result, the BIAN–LiPAA binder promotes the formation of a stable, highly conductive SEI, thereby reducing impedance and improving charge transfer kinetics. Electrochemical testing has demonstrated its outstanding performance, delivering discharge capacities of 276, 114.5, and 62.1 mAh g⁻¹ at 1C, 5C, and 10C rates, respectively, while maintaining an impressive 94.2% capacity retention after 2000 cycles at 10C. These results establish the BIAN–LiPAA system as a superior alternative to conventional binders, offering a robust solution for fast-charging, high-performance LIBs.

Fig. 6 (a) Structural features of p-BIAN (BP)–LiPAA based composite binder system, (b) charge–discharge studies of graphite based anodic half-cells with the p-BIAN (BP)–LiPAA based binder. (c) Rate studies of graphite based anodic half-cells with the p-BIAN (BP)–LiPAA based binder.

A key limitation of non-conjugated polymer binders lies in their high LUMO energy levels and intrinsic electrical insulating properties. This restricts charge transport within the anode, increasing internal resistance and diminishing overall electrochemical efficiency. In contrast, conjugated polymer binders inherently possess electronic conductivity, reducing internal resistance and promoting more efficient charge transfer at the interphase. Furthermore, the n-doping capability of conjugated binders further decreases interfacial resistance, enhancing overall electrode performance.

Through strategic molecular design and a deep understanding of the structure–activity relationship, BIAN-based conjugated polymers have evolved into multifunctional binder systems capable of addressing critical challenges in LIB technology. Their ability to simultaneously enhance mechanical stability, interfacial adhesion, SEI formation, and electrochemical performance positions them as a transformative solution for the next generation of high-performance lithium-ion batteries.

BIAN based binders for silicon (Si) anodes

Silicon (Si) is one of the most promising anode materials for lithium-ion batteries due to its high theoretical capacity, natural abundance, and low toxicity. However, its commercialization is hindered by large volume expansion during cycling, leading to particle pulverization, electrical contact loss, electrode delamination, and the formation of a thick and resistive solid–electrolyte interphase (SEI), all of which degrade battery performance over time.

Conventional binders like PVDF rely on weak van der Waals forces, which are insufficient to maintain the structural integrity of Si electrodes during repeated expansion and contraction. Alternative binders, including CMC, PAA, and alginate, have been explored for their superior mechanical properties. Alginate, a natural polysaccharide, has demonstrated excellent performance by providing high stiffness and strong adhesion through its carboxylic groups, resulting in a stable reversible capacity over prolonged cycling. However, these linear polymers rely solely on covalent bonding, which once broken, cannot recover, leading to electrode degradation over time.

Crosslinked polymer binders have been introduced to address these issues, as they allow for enhanced mechanical properties by integrating multiple functionalities. Recent advancements have included self-healing polymers that incorporate hydrogen bonding or dynamic covalent linkages to improve electrode stability. For example, hyperbranched polymer systems with hydroxyl-functionalized networks have shown promise in stabilizing Si anodes by forming strong noncovalent interactions with the active material, leading to enhanced mechanical robustness and improved electrochemical performance.

Poly(vinyl alcohol) (PVA)-based binders have been widely studied due to their strong hydrogen bonding with Si and water-soluble, non-toxic nature. Cao et al.⁵⁸ developed a cross-linked c-PVA-g-PAAm binder, where poly(acrylamide) (PAAm) branches improved flexibility and Li⁺ conductivity, resulting in enhanced mechanical stability. However, like PAA, linear PVA suffers from limited elasticity, making it unsuitable for electrodes with extreme volume fluctuations. Taskin et al.⁵⁹ introduced a conjugated cross-linked PVA system (PF-co-PP(Car)-PVA), which combined strong covalent crosslinking with an electrically conductive backbone, improving adhesion and electronic conductivity.

Poly(acrylamide) (PAM)-based binders have also been extensively investigated due to their amide (–CONH₂) functional groups, which interact strongly with the Si surface. Han et al.⁶⁰ developed a charge-regulated polymer binder (PN), where a combination of positively and negatively charged polymer segments enhanced mechanical stability, self-healing ability, and ionic conductivity. This innovative approach led to a high initial discharge capacity (191.4 mAh g⁻¹), ICE (98.7%), and good cycle retention (85.4% after 200 cycles at 0.5C). Woo et al.⁶¹ further optimized cross-linked PAM binders by varying the crosslinker concentration, achieving 60% capacity retention after 500 cycles at 0.2C with 0.75 mol% crosslinker concentration.

As a widely used commercial binder, carboxymethyl cellulose (CMC) has been modified to further improve adhesion and mechanical properties. Wang et al.⁶² introduced dopamine-functionalized CMC (CMC-DOP), where catechol groups enhanced electrode integrity and adhesion, leading to a high initial specific capacity of 3418.2 mAh g⁻¹ and 1650.6 mAh g⁻¹ retention after 200 cycles. Kim et al.⁶³ developed a Zn²⁺-imidazole cross-linked CMC binder, which exhibited self-healing ability and superior elasticity due to poly(ethylene glycol) (PEG) chains.

Natural polymer-based binders, such as sodium alginate (SA), starch, and chitosan, have also been explored as eco-friendly alternatives. Yao et al.⁶⁴ designed a sodium alginate–borate hybrid (SA-SMH) binder with a 3D interconnected network, providing strong adhesion and self-healing properties, thereby improving SiO electrode integrity. Feng et al.⁶⁵ constructed a potassium tripolyphosphate (PTP)–alginate binder, where ion–dipole interactions enhanced the flexibility and cycle life of micron-Si anodes, retaining 1599.9 mAh g⁻¹ after 100 cycles at 3000 mA g⁻¹. Similarly, Hapuarachchi et al.⁶⁶ modified tapioca starch with PEG, which improved adhesion and ionic conductivity, leading to reduced electrode cracking. Rajeev et al.⁶⁷ synthesized a chitosan–polyaniline (CS-g-PANI) hybrid binder, which improved mechanical stability and electronic conductivity but had slightly weaker adhesion than pure chitosan.

Other advanced polymer binders have also been developed to introduce self-healing, adaptive, and conductive properties. Liu et al.⁶⁸ reported cross-linked polyurethane oligomer (PUO) with PAA, forming a hydrogen-bonding network that improved flexibility and adhesion to Si particles. Lee et al.⁶⁹ introduced the HA-GA binder, which undergoes adaptive repositioning and crosslinking during initial cycles, forming a stable Si micro-environment (Si-μ-env) that helped retain 1153 mAh g⁻¹ even after 600 cycles at 1C.

Among the latest developments, BIAN-based polymer binders have emerged as a highly effective solution for stabilizing Si anodes. As shown in Fig. 7a, the P-BIAN/PAA composite binder, which combines P-BIAN (poly(bisiminoacenaphthenequinone)), an n-type conducting polymer, with poly(acrylic acid) (PAA), has demonstrated significant advantages. The P-BIAN component enhances electron transport while facilitating the formation of a thin, conductive SEI that suppresses electrolyte decomposition. The carboxylate groups in PAA provide strong adhesion and mechanical flexibility to accommodate Si's volume expansion. Electrostatic hydrogen bonding between P-BIAN and PAA further enhances the self-healing properties of the binder, ensuring long-term electrode stability (Fig. 7b). Compared to conventional binders, the P-BIAN/PAA composite⁷⁰ offers superior performance by simultaneously addressing mechanical degradation, lithium-ion diffusion, and SEI stability. While conducting polymers such as polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT)⁷¹ improve conductivity, they lack the mechanical strength necessary to accommodate Si's expansion. Conversely, self-healing polymer binders, while mitigating cracking, do not adequately control SEI growth. The P-BIAN/PAA system overcomes these limitations by providing a balance of conductivity, mechanical resilience, and interfacial stability, leading to high-capacity retention and extended cycle life.

Fig. 7 (a) Structural features of BP–PAA composite binder. (b) Long cycling charge discharge studies of anodic half-cell with the BP–PAA binder. Reproduced with permission. Copyright 2018 RSC. Reproduced with permission. Copyright 2022, ACS. (c) Structural features of the crosslinked P-BIAN binder. (d) Long cycling charge discharge studies of anodic half-cell with the crosslinked P-BIAN binder Reproduced with permission. Copyright 2024, Wiley.

Further enhancements in BIAN-based materials have been made through covalently crosslinked P-BIAN binders which has shown higher mechanical strength than the polymers crosslinked through H-bonding. This strategy results in a 3D network structure with superior mechanical properties and ionic conductivity. The crosslinked P-BIAN binder⁷² effectively prevents Si pulverization, maintains electrical conductivity, and facilitates the formation of a stable SEI, significantly improving the cycling stability of Si anodes (Fig. 7c and d). Electrochemical testing has demonstrated that Si anodes with the crosslinked P-BIAN binder achieve exceptional capacity retention over extended cycling, making them highly viable for practical battery applications.

In conclusion, BIAN-based binder systems represent a major advancement in the development of high-performance binders for Si anodes, effectively addressing challenges related to mechanical integrity, electrical conductivity, and SEI stabilization. These materials pave the way for the commercialization of Si anodes in next-generation lithium-ion batteries and open new avenues for the design and optimization of conjugated polymer binders. While an effective binder enhances electrode stability and lithium-ion transport, it alone cannot fully prevent interfacial degradation and electrolyte decomposition during cycling. To further stabilize the electrode–electrolyte interface and extend battery lifespan, electrolyte additives play a crucial role by forming protective interphases and regulating side reactions. Tables 2 and 3 show the performance of various binders in graphite and silicon-based anodes respectively.

Table 2 Comparison of the performances of the BIAN-based binder and other reported binders for the graphite anode⁵⁶

Binders for graphite	Peak discharge capacity (mAh g^−1)	Current rate
PVDF	∼200	1C
	∼56	5C
	∼12	10C
CMC-Na	∼200	1C
Acryl S020	∼166	1C
AMAC	∼210	1C
LiPAA	102	1C
XG	∼350	1C
SBR	∼340	1C
SBR-PVDF	∼260	1C
BIAN–fluorene polymer	∼270	1C
Allylimidazolium-based poly(ionic liquid)	∼210	1C
P-BIAN	∼260	1C
BIAN–LiPAA	∼276	1C
	∼114	5C
	∼62	10C

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Table 3 Comparison of the performances of the BIAN-based binder and other reported binders for the silicon anode⁷⁰

Binders for silicon	Peak discharge capacity (mAh g^−1)	Current rate/current density
Organic PVA + BA 3D-framework^73	1800	0.5C
PFPQ-COONa	1100	0.5C
Polyisoindigo derivative^74	1475	0.2C
Conducting glue (D-sorbitol + VAA + PEDOT:PSS)^75	1800	0.5C
PEO/PEDOT:PSS/PEI^76	2027	1.0 A g^−1
PVA–PEI^77	1200	1.0 A g^−1
PI^78	1000	1C
PAA–PVA^79	1800	1C
CS–CG^+–GA^1	2200	1C
PAA–CMC	1600	0.5C
PAA-grafted-CMC	1950	0.1C
PAA–BP	1450	200 mA g^−1
P-BIAN/PAA ^72	2100	500 mA g ^−1

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Polymerizable conjugated molecules as electrolyte additives for high-voltage LIBs

As mentioned in previous sections, high power and energy are the requirements of any battery system. Energy density (E) of a battery is given by the following equation:¹⁵

Since a certain amount of internal resistance is inevitable in any battery system, cell voltage can be computed using the following formula:

Voltage = V_OC − IR_b (2)

where V_OC is the open circuit potential, I is the current and R_b is the internal resistance. Furthermore, V_OC is the potential difference between oxidant (cathode) and reductant (anode). V_OC is given by the following equation:where μ_A is the chemical potential of the anode, μ_C is the chemical potential of the cathode, n is the number of electrons transferred and F is Faraday's constant.

As observed from eqn (1)–(3), energy density of battery can be increased by increasing the operating potential of the battery within the theoretical limits. One important constraint towards this end is the stability of the electrolyte and thereby the stability of the electrode–electrolyte interface at high voltages.

In this context, many strategies such as using high concentration electrolytes, polymer–gel electrolytes, solid electrolytes, etc., were used. Another strategy is to prepare a protective coating on the cathode to prevent undesirable oxidation of electrolyte at higher voltages and enhances the mechanical stability of the electrode.⁸⁰ Protective coatings using materials such as Al₂O₃, MnO₂, AlF₃etc., were used.²² Apart from such inorganic coatings, it is known that materials consisting of amines, aromatic heterocyclic rings can exhibit oxidative polymerization.⁷³ Such properties can be exploited to create an in situ polymeric coating on the cathode. Using such a unique chemistry of amines, our group demonstrated the application of BIAN based free amine terminal molecule for high voltage cathodes. Fig. 8 shows the synthesis of BIANODA.²² BIANODA was used as an electrolyte additive to conventional 1.0 M LiPF₆ in ethylene carbonate, diethyl carbonate-based electrolyte. Under oxidative conditions, terminal amines lead to polymerization and subsequent coating on the cathode surface. Such intricate design of an electrolyte additive led to higher capacity retention, lower charge-transfer resistance and led to stable electrolyte–electrode interface in LiMn_xNi_yCo_zO₂ based cathodic half-cells. Cathodic half-cells with BIANODA based electrolyte additive exhibited higher reversible capacity and retention in the potential range of 3.0–4.5 V (vs. Li/Li⁺). Furthermore, in the higher potential range of 3.0–4.8 V, cathodic half-cells with BIANODA based cathodic half-cells exhibited remarkable stability, which in reflected as stable coulombic efficiency. In contrast, cathodic half-cells without BIANODA exhibited inconsistent coulombic efficiency indicating severe side reactions at higher voltage ranges. This study exhibits the intriguing benefits of BIAN based polymerizable additives for enabling high voltage operation in lithium-ion batteries. Table 4 presents the comparison of various additives used in batteries.

Fig. 8 (a) Synthesis of BIANODA (b) structural features of BIANODA.

Table 4 Comparison of performance of various additives

Additive	Capacity retention (%)	Cycles	Current	Upper voltage (V)	Configuration	Ref.
Li[N(SiMe3)(SO2CF3)]	90.1	55	0.5C	4.2	LiNi1/3Mn1/3 Co1/3O2||graphite	81
Li[N(SiMe3)(SO2C4F9)]	90.7	55	0.5C	4.2	LiNi1/3Mn1/3 Co1/3O2||graphite	81
Lithium difluoro(oxalato) borate (LiDFOB)	84	100	1 mA cm^−2	4.3	LiNi1/3Mn1/3 Co1/3O2||Li	82
Lithium difluorophosphate (LiDFP)	78.2	200	1C	4.5	LiNi1/3Mn1/3 Co1/3O2||graphite	83
DMBAP	59	100	1C	4.5	LiNi1/3Mn1/3 Co1/3O2||Li	84
BIANODA	71	100	1C	4.5	LiNi1/3Mn1/3 Co1/3O2||Li	22

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Oxygen reduction reaction

Electrochemical energy conversion and storage devices are popular research topics among researchers and industrialists due to their high energy efficiency, environmental friendliness, and versatility. Especially, fuel cells provide high energy conversion efficiency and low emissions. Metal–air batteries boast high energy density, lightweight design, and cost-effectiveness. These technologies play crucial roles in renewable energy systems, energy management, and greenhouse gas reduction, making them attractive for various applications. The oxygen reduction reaction (ORR) process in these devices is crucial in determining performance efficiency. However, the ORR process suffers from sluggish kinetics due to the high activation energy required for oxygen reduction. This kinetic limitation results in energy losses and necessitates a high overpotential to drive the reaction. Incorporating an electrocatalyst addresses these challenges by lowering the activation energy, accelerating the reaction kinetics, and reducing the overpotential, thereby improving the capacity and rate performance of the device. For optimal performance, an electrocatalyst should possess (1) a high reactive surface area, (2) reliable catalytic performance with minimized side reactions and by-products, (3) increased electrochemical reversibility with efficient oxygen mass transport and a less complex reaction mechanism, (4) optimization for performance-sensitive parameters such as pH, temperature, and device architecture, and (5) non-toxic, sustainable, and economically feasible materials.

In this context, metal-based catalysts, particularly platinum (Pt)-based carbon composites and noble metal oxides such as RuO₂ and IrO₂, have been commercialized due to their high catalytic conversion efficiency and superior electrochemical performance.⁸⁵ However, these nano catalysts have intrinsic drawbacks, such as high cost—accounting for 35% to 50% of the device cost— limited natural abundance, poor durability, and a few other performances related problems like slow kinetics, instability, and susceptibility to species crossovers. Therefore, alternative materials have been explored to develop cost-efficient, stable, and effective ORR electrocatalysts.⁸⁶ This has led to increased interest in metal-free carbon based electrocatalysts, which offer tunable and controllable morphology and properties such as surface area, induced catalytic sites, conductivity, and economic viability. Since pristine carbon does not facilitate the adsorption or activation of ORR intermediates, chemical doping (inducing basal plane defects) or structural defects (armchair and zigzag edges) are introduced to enhance catalytic activity.

Doping with single or multiple heteroatoms of varying electronegativity modifies the π-electron distribution (delocalization) adjacent to carbon atoms, thereby influencing the material's physical and chemical properties.^87,88 Among heteroatoms, nitrogen doping has received the most attention over alternatives such as boron, sulfur, and phosphorus due to (1) higher positive charge density resulting from significant dipole–dipole interactions, (2) nitrogen hydrogenation effects where at a particular potential the nitrogen will destabilize the surrounding atoms, resulting in generation of electrons, and (3) modification of surface physicochemical properties, including hydrophilicity, alkalinity, defect density, and electron conductivity. Liu et al. ranked nitrogen catalytic activity in the following order:⁸⁸ pyridinic N > pyrrolic N > graphitic N > oxidised N > carbon skeleton using both experimental studies as well as DFT methods. Induced heteroatom doping and structural modifications in n-type carbonaceous materials improve performance, but understanding and controlling the amount and position of active sites during synthesis remains challenging. Consequently, researchers have turned to polymer compounds containing inherent heteroatom moieties such as nitrogen and sulfur. These compounds offer controlled active site distribution, porous structures, and desirable molecular weights, contributing to superior electrocatalytic properties. Recently, polymers such as aromatic BIAN, polyaniline, polypyrene, and poly-o-toluidine have been employed as ligands for transition metals (Fe, Cu, and Pt), exhibiting enhanced redox chemistry. For instance, polyacrylate-based hydrogel exhibited OER activity due to its enhanced open porous network and active high surface area for the reaction to occur. Palkovits et al. explored covalent triazine micro/mesoporous ordered frameworks as catalysts in alkaline media which demonstrated synergy between heteroatoms and porous morphological structures leading to improved OER activity.⁸⁹ These frameworks showed higher thermal stability, a good electroactive surface area, and higher current density than platinum-based catalysts.

Furthermore, other classes of polymers, such as π-conjugated polymers like polyaniline, polythiophene, and polypyrroles, have also been explored as catalysts. These polymers inherently contain heteroatoms such as sulfur and nitrogen, which serve as active surface moieties for ORR/OER reactions. Additionally, conjugation lowers the activation energy for oxygen splitting, decreases the reaction potential, and enhances conductivity. Our group has synthesized BIAN-based polymeric catalysts and demonstrated superior ORR catalytic activity compared to other polymer-based catalysts due to their functional advantages (Fig. 9).⁹⁰ The BIAN-para-phenylene copolymer (BP) was synthesized using polycondensation and predominantly contained 94.48% pyridinic nitrogen with a smaller fraction (5.52%) of pyrrolic nitrogen. These nitrogen moieties, associated with naphthalene units, served as active centers for the ORR. Cyclic voltammetry performed in an oxygen-saturated environment in both aqueous and non-aqueous media confirmed the presence of two peaks, indicating two active ORR sites. Density functional theory (DFT) studies further investigated the origin of these peaks and the ORR mechanism. Mülliken population analysis of the optimized energy structure revealed two distinct electropositive carbon sites adjacent to nitrogen atoms (Fig. 10). This explains the stepwise formation of peroxide ions around −0.35 V, followed by reduction or disproportionation at −0.82 V to hydroxyl ions, consistent with a two-electron reduction mechanism. The polymer demonstrated excellent stability due to its high molecular weight, large hydrodynamic radius, coiled π-conjugated segments, and significant entanglement or cross-linking, reducing electrochemical quenching phenomena.

Fig. 9 Structural features of BP for ORR catalysis.

Fig. 10 Mülliken charge analysis of BP.

To enhance durability, metal-free catalytic activity, and electrochemical performance, the BIAN para-phenylene copolymer was combined with graphene oxide (GO) sheets through annealing at 400 °C. This process preserved the original polymer framework while reducing surface oxygen from the GO sheet and increasing interaction with the polymer, leading to a higher concentration of pyrrolic and graphitic nitrogen. The composite exhibited increased surface defect concentration (higher D/G ratio in Raman spectroscopy) and reduced volumetric change during the ORR process. It also demonstrated a similar cathodic reduction peak as the copolymer but with a higher onset potential, indicating lower activation energy for reduction and improved electron transfer kinetics. These findings open new avenues for ORR catalysts beyond conventional N-doped carbon-based materials like graphene, carbon nanotubes (CNTs), carbon quantum dots, and metal-based catalysts. The n-type conjugated polymer-based carbonaceous electrocatalysts provide greater control over synthesis and design, enabling highly defined active sites while maintaining catalytic activity comparable to metal-based catalysts. This novel in situ, low-temperature annealing synthesis method offers several advantages such as a controlled environment for reaction and structure preservation over traditional techniques, such as pyrolysis, chemical vapor deposition (CVD), arc discharge methods, and ex situ techniques like plasma and hydrothermal treatments. A BIAN-based catalyst is scalable, cost-effective, and environmentally friendly, making it a promising approach for future ORR catalyst development.

The ability of a catalyst to reduce or oxidize oxygen in a non-aqueous medium is a prerequisite for its application in lithium–air batteries. The n-type BIAN-based graphene oxide composite described above exhibited bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity in an oxygen-saturated, ester-based polar aprotic solvent (0.1 M LiTFSI in TEGDME solution). This polymer composite displayed two reduction peaks, attributed to superoxide formation and its subsequent reduction to lithium oxides via a one-electron, quasi-reversible process. To further enhance the kinetics of the bifunctional electrocatalyst, overcome increased overpotential, and prevent passivation during discharge/charge cycles in the lithium–air battery, our group synthesized a BIAN-paraphenylene (BP)-based polymer-transition metal (iron) complex⁹⁰ (Fig. 11). This uninterrupted π-conjugated polymer catalyst, featuring linear or branched chains and predetermined active sites with imine-based ligands, demonstrated long-term stability over 160 cycles with 100% coulombic efficiency. The BP–Fe catalyst outperformed a conventional Ketjen black catalyst cathode in terms of capacity (500 mAh g⁻¹ at 250 mA g⁻¹), capacity retention, coulombic efficiency, and lower overpotential. Active imine nitrogen from BIAN–paraphenylene and the Fe ionic site together act as active sites for both ORR (discharge) and OER (charge). Adsorption of oxygen at these active sites is followed by the formation of lithium superoxide via single-electron reduction. This superoxide intermediate subsequently disproportionates, forming lithium peroxide. In the case of the BP–Fe catalyst, the adsorption reaction forms Fe³⁺–Li₂O₂ during discharge. It reversibly decomposes into Li⁺ and O₂ during the OER, which may explain its high durability and reversibility (Fig. 11). This type of polymer–metal complex catalyst, with inherently controlled heteroatoms as active sites, can be used as a cathode catalyst in lithium–air batteries to achieve long cell cycles with adequate capacity (Table 5).

Fig. 11 Synthesis and mechanism of catalysis of the BIAN-Fe complex.

Table 5 Comparison table of catalytic properties of various materials

Catalyst	Onset potential (V)	Number of e-transferred	Cycling stability	Current (mA g^−1)	Ref.
Pt/C	0.95 vs. RHE	4	—	—	91
Pt/C	—	—	40	1000	91
Anisotropic Pt	—	—	70	1000	92
Pt/CNT	—	—	130	2000	93
CNT	—	—	75	2000	93
Co–N–MWCNT	3.1 vs. Ag/Ag^+	2	50	400	94
PEI–AQ	1.9 vs. Li/Li^+	2	25	100	95
BP	−0.13 vs. Hg/HgO	2	—	—	96
BP–GO	−0.19 vs. Hg/HgO	2	—	—	96
BP–Fe	0.39 vs. Ag/Ag^+	2	160	250	90

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These applications demonstrate the use of n-type conjugated polymers (CPs), specifically BIAN based polymers, in various components of lithium-ion batteries, showcasing specific case studies that highlight their unique properties and impact on enhancing battery performance. It examines how BIAN based polymers contribute to improving cycle life, capacity, and voltage through diverse applications. For instance, as binders, BIAN based binders exhibit robust mechanical stability due to their conjugated structure, while the flexibility of n-type BIAN based polymer design enables lower LUMO energy levels, resulting in thinner and stronger solid electrolyte interfaces (SEIs). Carefully tailored conjugated polymers can significantly boost the cycle life and reversible capacity of batteries. In spite of this, in terms of industrial application, water solubility is a crucial factor that needs attention. A balanced design encompassing the advantages of n-type polymers along with water solubility is desirable. When utilized as active materials, BIAN based polymers can be tailored into porous structures or employed as sources of carbon and heteroatoms to create heteroatom-doped carbons, which facilitate rapid charging and enhance power density. BIAN based active materials exhibited high charge storage ability. However, in terms of future perspectives, factors such as electrode wettability, areal mass loading are to be optimized to scale up the applicability of these systems. As electrolyte additives, BIAN based additives can form in situ protective layers that act as artificial SEIs, raising voltage limits and improving energy density. The results presented could inspire the application of such additives in full-cell and pouch-cell type batteries to enhance the practical applicability. Moreover, imine functionality in BIAN based polymers enables catalytic properties in lithium–air batteries; by promoting polarity for oxygen adsorption, driving oxygen reduction and evolution reactions. These functional groups can also be arranged to function as ligands, forming complexes with non-precious metals to improve catalytic activity. The diverse applications of BIAN based polymers underscore their significant potential in advancing energy technology. In terms of future prospects, design modifications can be brought out to increase the catalytic active sites and thereby increase the catalytic activity. The versatility of n-type CPs gives an exhaustive scope for future research studies in various other fields such as other alkali-metal ion batteries, capacitors etc.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this feature review article can be found in the respective articles of the journals. The details of permissions acquired for respective figures have been clearly stated in the article. Additionally, the authors have included all required information regarding the synthesis as well as battery performance in the article. The authors have provided additional information in the figures as well offering the readers additional aid that can enhance the understanding of the scientific findings.

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