Emerging organic electrode materials for sustainable batteries

The search for appropriate electrode materials to fulfill the demands of fast-expanding consumer electronics, electric vehicles, and grid integration of renewable energy markets has attracted considerable interest in organic materials over the past few years¹. Compared to traditional transition metal-based inorganic electrodes, organic electrodes have several advantages, including resource sustainability, low cost, outstanding stability, high flexibility, and a low carbon footprint². These materials possess the potential to be used in revolutionary technologies³. Organic electrode materials can be traced back to their invention in the 1960s⁴. In the 1970s, conducting polymers were subsequently investigated by numerous chemists as lithium-ion battery (LIB) electrode materials⁵. The highly explored conducting polymers include polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(p-phenylene)⁶. Coin-type batteries with polyaniline cathodes and Li-Al alloy anodes were introduced in the late 1980s, but they swiftly vanished from the market because they never demonstrated optimal performance. Low doping level restrictions, usually below 50%, indicate that fewer than half of the redox-active groups participate in the charging/discharging process, which prevents the storage capacity from exceeding 150 mA h g⁻¹. This has prompted researchers to look for alternate cathode materials⁷. Inorganic electrodes have been conventionally used as standard electrodes in batteries for a long time⁸. Electrode materials such as LiFeO₂, LiMnO₂, and LiCoO₂ have exhibited high efficiencies in lithium-ion batteries (LIBs), resulting in high energy storage and mobile energy density⁹. However, the hindered ionic transfer due to its rigid crystalline structure causes a slow rate capability, and the scarcity of elements such as lithium and cobalt makes it challenging to keep up with the increased demands¹⁰. The potential environmental pollution that arises due to the use of these toxic materials poses a major threat¹¹. As an alternative, several innovations, such as developing polymer-based electrodes, have attracted increased flexibility, high theoretical capacitance, and controlled conductivity. Some notable polymer-based electrode materials developed in recent years include polyanilines and polythiophene¹². Although the use of polymers as electrodes for LIBs seems to be a suitable alternative, their poor cycling ability and low specific capacitance are considerable drawbacks¹². Several metal-organic framework (MOF)-based electrodes, notably several pristine MOFs, such as Fe-MIL-53¹³, Co-BDC¹⁴, and MIL-47¹⁵, and MOF-derived metal oxides, such as Fe₃O₄/C, SnO₂@N-RGO, and TiO₂/C, have been developed as electrode materials for LIBs^16,17,18. These MOF materials have shown appreciable performance, such as high capacitance retention, columbic efficiency, and reversible capacitance¹⁹. However, the poor cycling stability exhibited by MOFs remains a concern and is an area that is still being researched²⁰. The introduction of organic electrodes has had a considerable impact on electrode development for LIBs²¹. Some of the organic electrodes that have recently gained increased attention include viologens²² and aromatic-imides²³. The high theoretical capacitance, functional stability, structural tenability, and eco-friendliness of these materials have led to the development of versatile and efficient electrodes for secondary ion batteries²⁴. The first reported OEM was dichloroisocyanuric acid, developed by D. L. Williams et al. in 1969⁴. The novel OEM showed a cathode efficiency of 57%, with an average current density of 5.8 mAcm⁻². Later, in 1981, David et al.²⁵ developed polyacetylene, which can undergo reversible n-type and p-type doping. The newly developed conducting polymer resulted in 20 charge/discharge cycles within the range of 0.1 to 1 mA along with Voc = 3.6 V, 3.3 V, and 1.3 V and Isc = 4 mA, 2 mA, and 0.3 mA at periods of 0.5 min, 1 min, and 3.5 min, respectively. In 2002, Nakahara et al.²⁶ developed novel organic radicals for rechargeable batteries. The compound they synthesized for this purpose was 2,2,6,6-tetramethylpiperidinyloxy methacrylate (PTMA). The electrochemical studies of PTMA indicated that it retained a discharge capacity of 91% at a current density of 1 mA cm⁻², with both the cathodic and anodic capacities recorded to be 1.06 C and 1.07 C at 3 V and 4 V, respectively. Furthermore, in 2008, Chen and coworkers developed a low-cost method for preparing dilithium rhodizonate (Li₂C₆O₆), a renewable organic electrode for sustainable LIBs²⁷. Li₂C₆O₆ was synthesized from commercially available rhodizonic acid and lithium carbonate. Electrochemical studies indicated that this electrode has a reversible capacity of 580 mAhg⁻¹ at a specific energy density of approximately 1300 Whkg⁻¹. In 2011, Takayuki Matsunaga and coworkers constructed a coin cell battery with triquinoxalinylene as the cathode and Li as the anode²⁸. After investigating up to 110 charge/discharge cycles, the electrochemical studies revealed that triquinoxalinylene has a total capacity of 258 Ahkg⁻¹. In 2017, Haiping Wu et al. developed methyl viologen hexafluorophosphate coated over Cu as the anode material in a lithium coin battery²⁹. Electrochemical studies indicated that the battery had a retention of up to 94.7% after 200 cycles and maintained a columbic efficiency of 91% after 306 cycles. In 2020, Zhao et al. investigated the sodium ion storage mechanism in triquinoxalinylene (TQA) organic electrodes³⁰. The electrode exhibited a retention capacity ranging from 35% to 81%, suggesting that the poor cycling stability of triquinoxalinylene can be improved by providing more anchoring sites with the help of sodium sulfonates and a sodiated Nefion layer at the electrode‒electrolyte interface, which facilitates sodium ion transfer. Despite the progress seen in several years for viologens and aromatic imide-based organic batteries, further research is required to make their efficiencies equivalent with those of conventional LIBs. Moreover, reducing production costs needs to be considered for commercialization^22,23. Hence, carbonyl-based electrode materials have recently emerged as promising materials for use in batteries. The high specific capacitance, rate performance, and cyclic stability of carbonyl-based electrodes enhance their power density and energy density, thus facilitating enhanced energy storage and reduced recharging time¹⁰. Inspired by these advantages, scientists have found that carbonyl-based OEMs have more potential as commercial electrodes for energy storage. Another area being more frequently explored is the development of carbonyl-based polymers, as they bring about certain improvements, such as low solubility, light weight, multiple active sites, and easier chemical modifications¹⁰. In pursuing high-capacity cathode materials, organo-carbonyl compounds where C=O bond formation or cleavage occurs during the charge/discharge process have received considerable interest. The carbonyl group exhibits exceptional oxidative capability³¹. It also undergoes reversible one-electron reduction. Additional carbonyl groups conjugated with each other produce multivalent anions, thus increasing the electron storage capacity³¹. Functional moieties that stabilize negative charges are essential in electrical energy storage applications. Although organic electrode materials for energy storage based on carbonyls have recently advanced, several challenges, such as high solubility in electrolytes, low intrinsic electronic conductivity, large volume changes, and low tap density, need to be addressed before they can be commercialized³².

Carbonyl compounds from organic molecular systems were first explored for energy storage applications⁴. Extensive research over ten years has been carried out to determine the structure-activity correlation of such molecular systems. As a result, better insight into the various synthetic methodologies for the diverse functionalization of carbonyl compounds, their high oxidation stability and reversible reduction capacity, etc., has been obtained. Various organic moieties containing carbonyl groups were explored, the majority of which were quinones, carboxylic acids, and their derivatives (Fig. 1)^33,34,35.

Carbonyl-containing compounds suitable for use as organic electrode materials.

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Quinones are highly exploited as cathode materials due to their quick reversible electrochemical behavior and high storage capacity³⁶. For example, 1,4-benzoquinone can attain a theoretical capacity of 496 mA h g⁻¹ and a discharge voltage of 2.7 V, higher than that of the traditional LiCoO₂ cathode (155 mA h g⁻¹)³⁷. Nevertheless, these substances frequently experience low voltage limits, poor electronic conductivity, and severe electrolyte dissolution.

Carboxylate-containing molecules are primarily regarded as anode materials³⁴. Terephthalates are frequently utilized as anodes in metal-ion organic batteries³⁸. These compounds increase the capacity and cycling stability of electrode materials. Anhydrides are fabricated as cathode materials³⁵ that allow the reversible percolation of alkali ions onto conjugated carbonyl structures. The very low solubility, low cost, and ease of functionalization of diimides make them very appealing. Low voltage, theoretical capacity limitations, and decomposition are great challenges that need to be overcome in the case of diimides³⁹.

Organic carbonyl compounds are capable of undergoing reversible redox reactions. This electrochemical property thus enables them to be used as suitable electrode materials for energy storage applications. Organic electrodes can be categorized into three types based on their redox mechanisms: n-type, p-type, and bipolar electrode materials⁴⁰. Due to the high theoretical capacity exhibited by organic carbonyls, they are exclusively categorized as n-type electrodes and are mostly used as cathode materials^41,42. Enolate formation of carbonyls imparts a negative charge within the molecular substrate. Based on the stabilization of the anionic charge, organic carbonyl electrodes can be classified into three groups: Group I, Group II, and Group III^43,44]. Group I carbonyl electrodes consist of vicinal carbonyls (i.e., in the ortho and para positions) within the whole conjugated system. The enolate formation of one carbonyl group is facilitated by the adjacent/neighboring carbonyl group to form stable enolates. Group II mainly involves aromatic carboxylic acids and aromatic imide derivatives, where the carbonyls are attached to an aromatic core. This aromatic core helps in dispersing the induced anionic charge due to enolate formation. Group III mainly comprises qunoline derivatives. Although they share some common characteristics with Groups I and II, their main anionic stabilization occurs via the extra aromatic systems formed upon reduction^45,46.

In addition, organic carbonyl salts of lithium and sodium form another class of electrodes that exhibit high theoretical capabilities and fast kinetics. They are broadly classified into three groups (see Table 1; additional examples are included in the Supplementary Information). Group I consists of aromatic carboxylate salts of lithium ions. Group II comprises quinone/oxocarbon salts of lithium ions. Dilithium diimide-based systems that undergo tautomerization to insert Li⁺ ions upon reversible redox reactions are categorized as Group III⁴⁷.

The charging/discharging mechanism of OEMs is entirely a redox reaction on the surface of the electrode⁴⁸. The electrodes are generally placed in an electrolyte that conducts ions. An ion-permeable septum is used to separate the two electrodes. During charging, the electrolyte ions compensate for the electrode’s charge. In contrast, in the discharging cycle, the counterions migrate into the electrolyte from the surface of the electrode (Fig. 2a).

a Charging-discharging cycle of a rechargeable battery. b Li-ion storage mechanism in carbonyl-based n-type systems.

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Unlike inorganic electrodes, organic electrodes do not undergo a significant structural change during the charge/discharge cycle³⁶. The main advantage of organic electrodes is the practical use of the same electrode for both aqueous and nonaqueous metal ion batteries simultaneously. In one study, perylene dianhydride was used as an electrode for Mg²⁺, Ca ²⁺, Li⁺, Na⁺, and K⁺ counterions with various charges and radii^49,50,51. Redox reactions, typically referred to as the n-type mechanism, occur between the neutral and negatively charged states in carbonyl-based organic electrode materials⁵². N-type OEMs undergo reduction and combine with cations such as K⁺, Li⁺, Na⁺, Ca²⁺, Mg²⁺, Zn²⁺, and Al³⁺⁵³. While charging, these electrode materials are oxidized to a neutral state, and the counter cation undergoes decoupling. N-type organic electrode materials have a redox potential of less than 3 V, which helps them to be used as anodes or cathodes in metal ion batteries⁵². Electroactive carbonyl groups change to enolate monoanions (-C-O-) during the reduction process, and reverse enol-keto tautomerism occurs during oxidation². The nature of the radical intermediates produced during the redox process affects the performance of OEMs. Carbonyl electrode materials have two charge storage mechanisms: the conventional diffusion-controlled process and the surface-controlled pseudocapacitive process. Figure 2b illustrates the lithium-ion storage mechanism exhibited by various carbonyl-containing n-type systems⁵⁴. It can be seen that quinoline undergoes enol formation upon reduction, causing it to bind with lithium ions. Rylene dye (Fig. 2b) is also known to behave in a similar way. Thus, carbonyl-containing groups store ions via a simple redox mechanism that is simple and efficient⁵⁴.

Different methodologies have been applied for the synthesis of carbonyl-based OEMs. The oxidation of hydroquinone is one of the methods used to synthesize quinone, which is widely used in OEMs. This can be achieved by the reaction of hydroquinone with permanganate (KMnO₄) or hydrogen peroxide (H₂O₂)⁵⁵. Polycondensation is used if the electrode consists of a conjugated carbonyl-polymer. Friedel–Crafts acylation is employed for such polymerization reactions. Condensation reactions are used for synthesizing diketones and α-dicarbonyl compounds (Claisen condensation of ester), which are used for carbonyl-based OEMs. The reaction conditions, such as redox activity, stability, and compatibility, are crucial for synthesizing OEMs. Researchers often alter these techniques and develop new synthetic strategies to improve the performance of carbonyl-based organic electrode materials in various applications, such as batteries, supercapacitors, and electrochemical devices⁵⁵. Pavel A. Troshin et al. synthesized a quinone-based polymer electrode material. The synthesis involves the reaction of hydroquinone with paraformaldehyde in the presence of acetic acid and sulfuric acid to form the polymeric intermediate, which, upon reflux with sulfur, produces polyhydroquinone-methylene. The same group also synthesized two analogs of such electrodes based on quinone, as shown in Scheme 1a⁵⁶. Dylan Wilkinson and coworkers developed a quinone-based cathode material specially designed for lithium and sodium organic batteries (Scheme 1b). It involves the reaction between 9,10-anthraquinone-2-carbaldehyde and (S)-indacene1,3,5,7(2H,6H)-tetraone in a catalytic amount of glacial acetic acid⁵⁷. However, another strategy involves the use of hexaketocyclohexane (HKH) octahydrate developed by Tiantian Gu et al. (Scheme 1c). It involves the ethanolic reflux of 1,2-diaminoanthraquinone with HKH in the presence of a trace amount of glacial acetic acid⁵⁸.

Synthesis of quinone-based OEMs: a polymeric OEMs, b an anthraquinone-based OEM, and c a hexaketocyclohexane-based OEM.

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The electrochemical properties of organic electrode materials determine various parameters, such as cycle life⁵⁹, recyclability²¹, power density⁶⁰, electronic conductivity⁵², energy efficiency⁶¹, thermal stability⁶², chemical stability⁶³, and energy density^31,64.

Energy density

Energy density, one of the most important factors affecting the electrochemical properties of energy storage devices, can be calculated using Eq. 1⁶⁵. Commercial lithium-ion batteries contain graphite, Li₄Ti₅O₁₂, and other materials as anodes⁶⁶. Lithium-ion batteries have an energy density of less than 260 W hkg⁻¹, and many countries and organizations have proposed a target of 500 W hkg^[−1 67. Modifications should be made to the battery systems and electrode materials to achieve this target⁶⁴. Lithium metal is the most widely used anode because of its low redox potential and high capacity. Li- or Ni-rich layered oxides and sulfur are used as cathodes. Organic and inorganic electrodes were obtained using half-cells with lithium metal as the reference and counter electrode. Azo and carbonyl compounds (Fig. 2b), such as carboxylate, are best suited for organic electrode materials because they have high capacity, redox potential, and overpotential⁶⁵.

The nominal voltage (V) is the electrical voltage present during a battery’s normal operation.

The battery capacity (Ah) is the total amount of electricity stored by the battery.

The battery weight (kg) is the total mass of the given battery.

C_s is the specific capacitance of the electrode, and ΔV is the operational voltage.

Compared with traditional inorganic cathode materials, organic electrode materials with carbonyl and organosulfur compounds have promising gravimetric energy densities, chemical spaces, and working voltages⁶⁷. Many strategies to improve energy density include increasing the specific area, number of active sites, carbon content, molecular weight reduction, and electrolyte optimization⁵². (Table 2).

Power density

The power density measures the number of charging and discharging cycles in a battery⁶⁸. The discharge energy shows the actual value of energy stored during the fast-charging process⁶⁹. Conducting polymers and radicals have high power density but require high concentrations of carbon additives to ensure that the electrodes are conductive⁷⁰. Carbonyl-based polymers give an output of 81% of the total energy produced at low current density due to the rapid redox kinetics of C = O groups⁷¹ (Table 3). Increasing specific areas and extending conjugation are some strategies for improving the number of charging and discharging cycles⁵².

• V₀ = Maximum voltage

• I₀ = Maximum current

• kg = Mass of the given battery

I is the applied constant current, ΔV is the operational potential and m is the total mass of the supercapacitor in grams.

Cycle life is an essential factor in evaluating the performance of batteries for practical applications⁷². Solubility, structural changes, and volume variations are factors that affect cycling performance³. Small carbonyl compounds (Fig. 3) exhibit good cyclic stability, retaining 80% of their capacity for over 1000 cycles. This can be improved by optimizing electrolytes, separators, or binders by combining them with insoluble substrates⁷³.

Common carbonyl compounds with excellent cyclic stability and improved conductivity.

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Electrical conductivity

The electrical conductivity is yet another factor that affects the electrochemical properties of organic electrode materials⁷⁴. Electrically conductive carbon additives can be used to improve the conductivity, which affects the power and energy density of batteries⁷⁵. In general, carbonyl and imine compounds are poor conductors, but some carbonyl compounds (Fig. 3) that undergo extensive lithiation generally exhibit high electrical conductivity. Introducing delocalized π-electron systems or polymers into the structure can improve the conductivity^31,73.

Energy efficiency

Energy efficiency depends on the coulombic efficiency and voltage efficiency⁷⁶. The voltage efficiency depends on the charge and discharge processes. Organic electrode materials have low voltage efficiency compared with inorganic electrode materials⁷⁷. Introducing conductive carbons, optimizing the electrolyte, and reducing the particle size are methods for improving the voltage efficiency⁷⁸. The coulombic efficiency of an electrode greatly depends on the energy density of the battery, and it should be above 90% for efficient operation in practical applications. Some carbonyl-containing materials have unusually high efficiencies close to 100%. The high efficiency of carbonyls as cathodes does not significantly affect the cathode-to-anode mass ratio, but when used as an anode, it further leads to low utilization of active materials and in turn reduces the energy efficiency^31,73.

Thermal and chemical stability

Thermal and chemical stability affect the efficiency and performance of organic electrode materials⁷⁹. The thermal stability of organic materials with low molecular weights is generally poorer than that of inorganic materials¹⁰. When organic materials are chemically unstable, the difficulty in developing electrode materials and the cost of manufacturing increase⁸⁰. An OEM must exhibit a fair amount of thermal stability for its long run and be able to withstand high-temperature conditions. Good chemical stability is essential for preventing decomposition during long-term usage and easy electrode synthesis^31,73.

Cost effectiveness

Cost effectiveness is one of the important factors for the development and commercialization of organic electrode materials^81,82. Estimating the costs of organic electrode materials is challenging due to the limited availability of commercially viable options, particularly those exhibiting redox activity in batteries. Many organic electrode materials can be derived from biomass and common industrial chemicals. Consequently, the future cost of organic electrode materials is expected to align with the advancement of the biorefinery industry. These materials primarily consist of carbon, hydrogen, oxygen, and sometimes nitrogen and sulfur—elements that are abundant in the biosphere. Numerous naturally occurring compounds serve as valuable electrode precursors, indicating the potential for organic electrodes to offer renewable and environmentally friendly solutions. Therefore, resource availability is unlikely to pose a constraint for the large-scale application of organic electrode materials in the future^31,73.

Overall, we have discussed the basic aspects of OEMs including their synthesis, storage mechanisms and several electrochemical parameters and the challenges associated with manufacturing costs. In the upcoming sections, we discuss the recent advances and developments in carbonyl-based OEMs.

Carbonyl compounds have been widely explored in terms of their redox capabilities, which have been applied in batteries and supercapacitors. In this review, we have summarized a few significant studies on the development of organic electrode materials for secondary rechargeable batteries.

In 2021, Jun Li et al. conducted a theoretical study on a set of carbonyl compounds regarding HOMO-LUMO energy levels for charge and discharge. They selected a few carbonyl compounds, such as para-carbonyl-, meta-dicarbonyl-, and ortho-dicarbonyl-based systems, and compared their LUMO levels. The LUMO levels of the meta-dicarbonyl-forming dyes sharply decreased, which resulted in a decrease in their specific capacity. Steady and smooth curves were obtained from the discharge curves of pentacene-5,7,12,14-tetraone (PT) with para-dicarbonyl sites and pyrene-4,5,9,10-tetraone (PTO) with orthodicarbonyl sites. The best electrochemical performance is exhibited by multiple carbonyls with ortho-dicarbonyl groups due to the simultaneous high specific capacity and high discharge voltage. The discharge patterns of TPHA (triphenylene-2,3,6,7,10,11-hexaone), TTOA (tribenzotetraphen-2,3,6,7,11,12,15,16-octaone), and CDDA indicate that the discharge potential decreases with increasing specific capacity. From an overall comparison, it can be seen that CDDA has a better discharge profile than TPHA and TTOA⁸³.

Munseok S. Chae and coworkers synthesized 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as a cathode material for calcium-ion batteries. The electrochemical studies showed that the reversible capacity was 158 mAhg⁻¹ at a current density of 10 mAg⁻¹ in aqueous solution. XPS analysis indicated that Ca²⁺ ions were accommodated by PTCDA between two carbonyl groups. The π-electrons present enhance Ca²⁺ ion storage. Figure 4a and b show the results of cyclovoltammetry studies performed for PTCDA. CV and discharge studies were conducted at various specific capacities of 159 mAhg⁻¹, 158 mAhg⁻¹, 154 mAhg⁻¹, 140 mAhg⁻¹, and 122 mAhg⁻¹ at current densities of 62 mAg⁻¹, 125 mAg⁻¹, 250 mAg⁻¹, 500 mAg⁻¹, and 1000 mAg⁻¹, respectively. Overall, the results indicated that PTCDA is an efficient cathode for Ca²⁺ ion storage⁸⁴

a Cyclic voltammetry studies of PTCDA electrodes at a scan rate of 0.1 mV/s. b Galvanostatic charge/discharge curves of PTCDA at a current density of 10 mAg⁻¹. c Cyclic voltammetry studies of Li//PTTA cells at a scanning rate of 0.1 mV/s. d Galvanostatic curves of Li/PTTA cell half cells with 1) 1 M LiPF6 EC/DMC and 2) 1 M LiPF6 EC/DMC + benzo-15-crown-5 at current rates C/3 and C/4 (where C = 344 mAg⁻¹), respectively⁵⁵.

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Zi Feng Chen and coworkers developed an indeno[3,2-b]fluorene-6,12-dione (IFDO) electrode as a cathode material in lithium primary batteries. The cathode exhibited very good energy density. IFDO exhibited a four-electron reduction upon the addition of fluoroethylene carbonyl (FEC). The results of electrochemical studies indicated that IFDO has a high specific capacity of 652 mAh g⁻¹. The cyclic voltammetric studies were performed under two different conditions, i.e., in DME + 10% FEC and with DME alone. It was observed that the presence of FEC makes the electrochemical process of IFDO purely irreversible and vice versa. This variation is thus attributed to the poor solubility and four-electron reduction shown by IFDO. At room temperature, the IFDO electrode demonstrated an ultrahigh energy density of 1392 W h kg^−1 85.

Guzaliya et al. synthesized a novel OEM, poly(triquinoyl 1,2,4,5-tetraaminobenzene) (PTTA), for lithium-ion storage. This OEM was synthesized via the condensation of triquinoyl and 1,2,4,5-tetraaminobenzene. Cyclic voltammetry studies were conducted on PTTA at a scanning rate of 0.1 mV s⁻¹, and the charge‒discharge profile of PTTA was studied with lithium as the half cell and 1 M LiPF₆ EC/DMC and 1 M LiPF₆ EC/DMC + benzo-15-crown-5 as the counter electrodes (Fig. 4c, d). Electrochemical studies concluded that PTTA has a capacity greater than 350 mAh g⁻¹. This is a result of three-electron reduction, and the average cell discharge potential of PTTA was found to be 0.88 V⁵⁵

Micheal Ruby Raj and coworkers developed a perylene-based multicarbonyl polyimide electrode for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). Cyclovoltammetry studies of the polyimide electrode against Li-cells and Na-cells were carried out at different scanning rates ranging from 0.1 to 1 mV s⁻¹. Electrochemical studies thus indicated that the electrode undergoes a three-electron transfer redox process. The charge-discharge was 209 mA h g⁻¹ for both the LIB and SIB. The electrode showed reversible capacities of 58 mA h g⁻¹ for the LIB and 77 mA h g⁻¹ for the SIB at 50 mA g⁻¹ for 20 cycles. Lifelongity stability was observed for the electrode with a current density of 15 mA h g⁻¹ for LIBs and 78 mA h g⁻¹ for SIBs over 1000 cycles⁸⁶.

Kristin B et al. synthesized two polyimide-dicyanotriphenylamine derivatives labeled triphenylamine-naphthalenetetracarboxylic polyimide (TPA-NTCPI) and triphenylamine pyromellitic polyimide (TPA-PMPI) as organic electrode materials for lithium-ion batteries. Figure 5a illustrates the cathode performance of TPA-NTCPI in LIBs. The charge/discharge profiles of TPA-NTCPI at a scanning rate of 0.1 Ag⁻¹ are depicted here. The charge/discharge profile confirmed the occurrence of Li-enolate and dianion species through electron transfer. This inference is confirmed by the presence of two plateaus at 2.52 V and 2.3 V (as shown in Fig. 5b). From the current density profiles of lithium cobalt oxide (LCO) and TPA-NTCPI, it can be inferred that TPA-NTCPI exhibits a capacity of 150 mAhg^-1, which is superior to that of LCO (101 mAh/g). Moreover, TPA-NTCPI exhibited impressive cycling stability, exhibiting a capacitance retention of 73.1% after 1000 cycles (as shown in Fig. 5c). Electrochemical studies showed that the electrodes can attain a high specific capacity of 150 mAhg⁻¹ at a current density of 0.1 Ag⁻¹. The extended conjugation in TPA-NTCPI hence contributed to its long cyclic stability. TPA-PMPI, on the other hand, showed a capacity of 1600 mAhg⁻¹ at a current density of 0.1 Ag⁻¹ after 100 cycles⁸⁷.

a Galvanostatic charge/discharge curves of TPA-NTCPI at a current density of 0.1 Ag⁻¹ as the cathode material in LIBs. b Rate capability profiles of TPA-NTCPI and LCO at different current densities. c Cyclic stability profiles of TPA-NTCPI at a current density of 0.5 Ag⁻¹ for 1000 cycles⁸⁷.

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Kang Li et al. developed a thioether-connected carbonyl-rich linear polymer for use as an anode for sodium- and lithium-ion batteries. Structurally, carbonyl groups are responsible for intercalating Li⁺/Na⁺ ions, and the presence of sulfur atoms enhances the conductivity. The synthesized electrode labeled poly(pyrene tetralone sulfide) (PPTS) was subjected to electrochemical studies. Cyclic voltammetry studies were conducted to determine the performance of the electrodes in the LIBs at a scan rate of 1 mV s⁻¹. The electrodes exhibited a high discharge specific capacity of 588.8 mAh g⁻¹ and a reversible charge specific capacity of 205.2 mAh g⁻¹. The results obtained indicate that PPTS can be used for future applications in LIBs and SIBs⁸⁸. Chao et al. developed a novel method for synthesizing ultrathin quiononine poly-dopamine (PDA) coatings on 3D porous carbon materials. The synthetic method adopted was a heterogeneous nucleation process. Figure 6 illustrates the cyclic voltammetric studies of the electrode performed at a scanning rate of 1 mV s⁻¹ and rate capability studies of PDA, PC-PDA-O₂ (oxygen-induced poly carbon-poly dopamin), and PC-PDA-APS (poly carbon-poly dopamine-ammonium persulfate). Figure 6a compares PC-PDA-O2 and PC-PDA-APS via cyclic voltammetry. The cyclic voltammograms indicate that PC-PDA-APS has a better current response than does PC-PDA-O2 at a scanning rate of 1 mV/s. Moreover, PC-PDA-APS exhibited better cycling stability, with an initial discharge capacity of 228 mAhg⁻¹, a capacitance of 141 mAh/g for 100 cycles, and a retention of 62%, as illustrated in Fig. 6b. The novel 3D polymer composite was subjected to electrochemical studies, where PC-PDAAPS showed the highest specific capacity of 322 mAhg⁻¹ at a current density of 0.1 A g⁻¹ and good rate performance of 102 mA h g⁻¹ at a current density of 10 A g⁻¹ and ultralong cycling stability for up to 500 cycles⁸⁹.

a Cyclic voltammetry profiles of PC-PDA-O₂ and PC-PDA-APS at a 0.1 mVs⁻¹ scan rate. b Coulombic efficiency and cycling stability of PDA, PC-PDA-O₂, and PC-PDA-APS at a current density of 0.25 Ag^[−1 89.

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Sudhir D. Jagdale and coworkers designed pseudo capacitors based on azo-functionalized anthraquinone (AZOAQ). The integration of graphene foil (GF) with AZOAQ helped to produce a suitable electrode material, which was then subjected to electrochemical studies to determine its cyclic stability. Cyclovoltammetry studies of the AZOAQ/GF electrode were conducted at a scanning rate of 5 mVs⁻¹. The studies concluded that the electrode possesses a specific capacitance of 173.32 Fg⁻¹ at a scan rate of 5 mVs⁻¹ and 270 Fg⁻¹ at a current density of 0.5 Ag⁻¹. The same electrode, AZOAQ/GF, was then used to make a symmetric two-electrode supercapacitor that exhibited a high specific capacitance of 159.12 F g⁻¹ at a current density of 0.5 Ag⁻¹. The electrode exhibited a capacitance retention of 93.22% after 10,000 GCD (galvanostatic charge/discharge) cycles with an energy density and power density of 28.64 Wh kg⁻¹ and 1080.02 W kg⁻¹, respectively. (Fig. 7a–c)⁹⁰.

a Cyclic voltammetry profile of AZOAQ in DMF solution. b Galvanostatic charge/discharge profiles of AZOAQ/GF at various current densities. c Cyclic voltammetry profile of AZOAQ/GF at various scanning rates. d Cyclic voltammetry studies of PHATN-CMP at a 0.1 mVs⁻¹ scanning rate. e Galvanostatic charge/discharge profiles of PHATN-CMP at a current density of 50 mAg⁻¹. f Galvanostatic charge/discharge curves of symmetric all-organic batteries based on PHATN-CMP at 50 mAg^[−190,92.

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Liang Huang et al. developed an organic electrode based on the trilithium salt of tetrahydroxyanthraquinone (Li₃THAQ) for lithium-ion and Li-metal batteries. The novel electrode was subjected to various characterization methods to confirm that its chemical formula was Li₃C₁₄H₄O₆. Upon conducting several electrochemical studies, it was interesting to see that the Li₃THAQ electrode has a sustained reversibility between Li₂THAQ and Li₄THAQ, indicating that the electrode has a large interlayer spacing, thus making its intercalation property unique. Cyclic voltammetry studies of Li₃THAQ were performed at different scanning rates of 0.1, 0.2, 0.5, and 1.0 mV s⁻¹. The electrode exhibited a high reversible capacity of 192 mAh g⁻¹, with a high discharge potential of 2.93 V against Li/Li⁺ batteries. The long-term cycling performance of Li₃THAQ showed that the novel electrode has a capacity retention of 95% after 500 cycles⁹¹.

Suriguga Li et al. synthesized two novel cathode and anode organic polymers based on hexaazatrinaphthalene conjugated microporous polymers (PHATN-CMP) and hexaazatrinaphthalene-hexaketocyclohexane octahydrate-conjugated microporous polymers (HAHK-CMP) for organo-lithium batteries. The key feature of these two electrodes lies in their multiple redox sites, which enables the electrodes to potentially have high charge/discharge values. The cyclic voltammetry studies of these electrodes were conducted at a scanning rate of 0.1 mVs⁻¹. The PHATN-CMP and HAHK-CMP electrodes exhibited galvanostatic charge/discharge values of 169.6 mAhg⁻¹ and 130.2 mAhg^−1, respectively, at a current density of 50 mA/g. The PHATN-CMP electrode was then incorporated into LIBs, resulting in a symmetric organic LIB. These newly fabricated batteries exhibited charge/discharge values of 82.2 mAhg⁻¹ and 72.6 mAhg^−1, respectively. Figure 7d shows the cyclic voltammetry studies of PHATN-CMP and HAHK-CMP at a scanning rate of 0.1 mVs⁻¹. The voltammogram shows intense redox peaks, which indicate increased polarization. The galvanostatic charge/discharge curves of PHATN-CMP at a scanning rate of 0.05 Ag⁻¹ are shown in Fig. 7e. The curves indicate that Li⁺ ion storage occurs via both controlled diffusion and surface reactions. Another set of galvanostatic charge-discharge curves of PHATN-CMP at a scanning rate of 0.05 A/g, as shown in Fig. 7f, implied that PHATN-CMP has initial charge/discharge capacities of 169.6 mAhg⁻¹ and 130.2 mAhg⁻¹, whereas HAHK-CMP has lower charge/discharge capacities of 148.8 mAhg⁻¹ and 93.1 mAhg^[−1 92.

An organic electrode material based on 1,4,5,8-naphthalene tetracarboxylic acid dianhydride-2,3-diaminophenothiazine (NTDP) was developed by Jiali Wang et al.⁶³ for aqueous zinc-ion batteries. The electrode contains more than 6 cyano and 2 carbonyl groups, which were characterized with the help of IR, Raman, and XRD spectroscopy, thus making it an electrode with multiple active sites along with a large π-conjugated backbone and aromaticity. Cyclic voltammetry studies of the NTDP were conducted at a scanning rate of 0.4 mVs⁻¹ (Fig. 8). The electrode showed a high specific capacitance of 205.1 mAhg⁻¹ at a current density of 0.05 Ag⁻¹. The high specific capacitance of the electrode at a current density of 20 A/g was recorded to be 194.9 mAhg^-1. NTDP showed good stability, with a capacity retention of 91.2% after 9000 cycles and a high specific capacitance of 190 mAhg⁻¹. The NTDP/Zn battery, which was also constructed, showed good flexibility. Electrochemical studies of NTDP/Zn were conducted at different bends of 45⁰, 90⁰_, 135⁰, and 180⁰. The battery exhibited a high specific capacitance of 112.6 mAhg⁻¹ at all bends and exhibited a high capacitance of 205.1 mAhg⁻¹ at a current density of 0.05 Ag⁻¹. This flexible nature of the NTDP/Zn battery and its overall electrochemical studies indicate that this novel electrode can be incorporated and used in wearable devices in the future⁹³.

a Cyclic voltammetry profiles of the NTDP at a 0.4 mVs⁻¹ scanning rate. b Different cyclic performance states of flexible NTDP//Zn cells. c Cyclic stability profile of the NTDP cell at a current density of 15 Ag^[−193.

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Zheng et al. developed a π-d conjugated coordination structure formed between the cathode and electrolyte; a polyvinylidene fluoride-based Li_0.33La_0.557TiO₃ (PVDF-LLTO) polymer was used as the solid-state electrolyte, and 2,5-dihydroxy-1,4-benzoquinone (DHBQ) was used as the cathode. The C=O and O-H groups in DHBQ can be coordinated with La³⁺ in LLTO, as demonstrated by first-principle calculations and experiments. This π-d conjugate coordination structure strengthens the contact interface between the electrode material and solid electrolyte, extending the cycle life and durability of the battery. Using a PVDF-LLTO solid electrolyte and organic cathode materials, they attempted to construct π-d conjugated coordination structures that enhanced ion transport at the cathode/electrolyte interface. DHBQ is a bridge ligand and intermediate in a variety of processes because it can readily shed two protons and form a divalent anion. It serves as the cathode material in this instance. When combined with La³⁺ in LLTO, the C=O and O-H groups in DHBQ may form a π-d ligand structure. DHBQ-based all-solid-state lithium batteries (ASSLBs) have a much higher electrochemical activity than liquid batteries. The manufactured battery, which has a coulombic efficiency of more than 95%, is found to sustain a capacity of up to 184.7 mAh g⁻¹ after 500 cycles, with an initial discharge capacity of 238.4 mAh g⁻¹ at 0.1 C. The π-d conjugated coordination structure between the cathode and electrolyte preserves their interfacial contact and stabilizes the organic electrode material. Chemical or physical interactions between the electrodes and electrolytes are successfully eliminated by introducing a π-d coordination structure into the ASSLBs. The results showed a lower interfacial resistance between DHBQ and the PVDF-LLTO solid electrolyte bond. The enhanced cycle stability and rate performance of DHBQ in ASSLBs is achieved by the strong bonding of DHBQ to LLTO, which can withstand changes in the DHBQ volume while preserving the low interfacial resistance. It is possible to easily modify the functional groups of organic compounds to strengthen their bonds with inorganic solid-state batteries (SSEs) and to increase their mechanical properties. In ASSLBs, high interfacial resistance and slow reaction kinetics might be resolved concurrently with the solubility problem of organic electrodes using SSE-based organic LIBs⁹⁴.

Wang et al.⁹⁵ created a dual redox center for the storage of Na using conjugated carbonyls of metal-organic polymers (MOPs) and copper. Here, they made linear Cu-based MOPs by utilizing tetra amino benzoquinone molecules (TABQ, C₆H₈O₂N₄) as organic ligands and Cu²⁺ ions as transition metal centers. To improve Na⁺ diffusion, a coplanar delocalization skeleton is constructed in each unit of Cu-TABQ. FTIR and ex situ XPS demonstrated that active metal ions (Cu²⁺) and organic ligands (benzoquinone framework) function as dual redox centers for Na storage, resulting in three Na⁺ transfers per coordination unit. This yields a high reversible capacity of 322.9 mAh g⁻¹ at 50 mAg⁻¹ for Cu-TABQ at voltages between 1.0 and 3.0 V. The findings provide new guidance for the advancement of metal-ion battery technology and demonstrate that Cu-TABQ is a prime choice for high-performance cathodes in Na-organic battery construction. This leads to an exceptional rate performance of 198.8 mAh g⁻¹ at 4000 mA g⁻¹ and good cycling performance with nearly no deterioration within 700 cycles. An understanding of the clever design of MOP cathode materials for future batteries will be provided by examining the Na storage mechanism (Scheme 2)⁹⁵.

Schematic diagram of the sodium ion storage mechanism in Cu-TABQ⁹⁵.

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The stable coordination between Cu²⁺ and the TABQ ligand with d orbitals significantly enhances the electrical conductivity and redox activity. The sodium storage mechanism was determined using FTIR, Raman, XPS, and other characterization techniques. Cu-TABQ has two redox centers, one for Cu^2+/Cu⁺ (one electron) and one for C=O/C–O (two electrons), allowing three electron transfers for each unit and boosting the specific capacity. This demonstrates the enormous potential for creating coordination polymer cathode materials with numerous redox centers that work well, and it will make it easier to design cutting-edge cathodes for sodium-ion batteries⁹⁵.

Sugandha Singh et al.⁹⁶ explored cobalt(II) modification in a conjugated organic polymer based on phenanthroline for trifunctional electrocatalysis. Using 1,3,5-tri bromobenzene (TBB) as a donor and 3,8-diethynyl-1,10-phenanthroline (phen) as an acceptor, they designed and synthesized a novel redox-active semiconducting donor–acceptor pair conjugated organic polymer via a Pd(PPh₃)₄-catalyzed Sonogashira coupling reaction to yield TBB-phen as a COP (Scheme 3). At room temperature, TBB-phen demonstrated semiconducting behavior and a charge-separated state. This material exhibited notable hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) activity. This is the first report of a conjugated organic polymer (COP) donor‒acceptor pair as an active electrocatalyst for the HER and ORR. They used an in situ stabilizing metal to better utilize this COP, resulting in a metal-modified COP that provided an enhanced ORR and HER in addition to the OER. TBB-phen showed low overpotentials for the ORR and HER due to sufficient p-conjugation and charge separation. With regard to electrocatalytic applications, this study presents a new class of organic materials that have not been studied before. Moreover, this study revealed that phenanthroline is a potential candidate for use as a cathode catalyst in energy conversion and energy storage systems. The increased performance and functionality of a low-cost transition metal can be achieved by taking advantage of the tunability of the donor‒acceptor concept and the simplicity of metal stabilization⁹⁶.

Synthesis and structure of the Co@TBB-phen electrocatalyst⁹⁶.

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The development of nanostructured carbonyl materials as supercapacitors is a suitable alternative for energy storage purposes. Graphene oxide is mostly preferred for the manufacturing of such supercapacitors due to the presence of hydroxy and carboxy/carbonyl groups. The major advantages of such nanostructured materials are their enhanced surface area, good conductivity, and high stability, which promote large-scale energy storage⁹⁷. The nanostructured carbonyl materials used for energy storage were synthesized by chemical or electrochemical oxidation of carbon nanostructures via the introduction of oxygen functionalities into the carbon skeleton. Chemical or physical activation methods were also used for introducing oxygen functionalities onto the carbon surface⁹⁸. In 2024, Ruidas et al. developed an efficient electrode material for pseudocapacitive energy storage based on imine-linked π-conjugated covalent organic frameworks. Two novel covalent organic frameworks (COFs), i) TFPh-NDA COF from 2-hydroxy-1,3,5-benzenetricarbaldehyde (TFPh) and 5-diaminonaphthalene (NDA) and ii) TFR-NDA COF from 2,4-dihydroxybenzene-1,3,5-tricarbaldehyde (TFR) and 5-diaminonaphthalene (NDA), were synthesized. Among these, TFPh-NDA exhibited superior performance, with a specific capacitance of 583 Fg⁻¹. Moreover, an energy density of 280.58 Whkg⁻¹ at a power density of 404.06 Wkg⁻¹ and a cyclic retention of 98% after 10,000 cycles make TFPh-NDA COF a suitable electrode for energy storage applications in the future⁹⁹.

Lithium-ion batteries have demonstrated notable characteristics, including rapid response time, high open-circuit voltage, elevated specific energy, minimal self-discharge, and an absence of a memory effect. As a result, lithium-ion batteries have become commercially viable energy storage solutions. However, certain drawbacks, such as pulverization, limited capacity, structural alterations during charge‒discharge cycles, demanding manufacturing conditions, and cost considerations, remain unresolved. Hence, carbonyl-containing organic electrodes have emerged as a promising alternative to lithium-ion batteries, offering advantages such as high power density, structural versatility, flexibility, insolubility, and sustainability. Lithium-ion batteries often exhibit shortcomings such as low cycle count, rate, and lifespan due to structural changes induced by the insertion of lithium ions into the lattice. Organic electrodes address this issue by mitigating structural degradation through multiple redox reactions. A notable advantage of carbonyl-based organic electrodes is the tunable redox potential achieved through the introduction of electron-withdrawing or electron-donating groups, a property that is challenging to achieve in inorganic materials without regulating the valence of metal ions. Compared with inorganic materials, polymeric carbonyl-based organic electrodes, an extended form of carbonyl networks, offer enhanced high power density due to faster reaction kinetics. Additionally, their polymeric nature enhances flexibility, enabling their integration into wearable devices and roll-up displays, an application unattainable with rigid and fragile lithium-ion-based electrodes. From a sustainability perspective, carbonyl-based organic electrodes present a favorable option, as the materials required for their manufacturing are predominantly earth abundant, whereas lithium-ion batteries rely on limited and nonrenewable mineral sources. Carbonyl-based organic electrode materials can be developed responsibly without contributing to pollution, and they represent a promising avenue for sustainable energy storage solutions.

In the pursuit of advanced energy storage systems driven by renewable and clean energy sources, carbonyl-based organic electrodes have garnered significant attention as promising materials for future high-performance electrodes. Their appeal lies in their structural versatility, light weight, environmental friendliness, high specific capacity, power density, energy density, cyclic stability, and mechanical flexibility.

The abundant availability of biomass, high theoretical capacity, high redox stability, and structural diversity have promoted the use of organic carbonyl compounds as high-energy materials for rechargeable LIBs. The problems associated with low electronic conductivity, lack of a well-defined Li⁺-conducting channel, and interstitial sites of such materials cannot be disregarded either. Introducing multiple carbonyl groups in such cases would help to capture more lithium, resulting in multielectron reactions. Most of these organic compounds function via a redox mechanism, thus imparting an electroactive nature toward any metal. Such electrodes can function similarly to supercapacitors and may charge and discharge at temperatures as high as 120 °C, wherein most of the inorganic material electrodes fail.

Nevertheless, due to the enormous success of graphite-based and inorganic electrode materials in both research and commercialization, organic materials have received very little attention in the past several decades for the development of battery systems. Furthermore, organic chemicals necessitate high concentrations of conductive substances (carbon) of at least 50% weight percentage, which decreases the battery’s actual capacity to noncompetitive levels.

Most carbonyl organic electrodes still face challenges such as inadequate specific capacity, limited redox potential windows, and insufficient long-term stability, which hinder their practical utility. For this purpose, more research should be conducted on enhancing the conductivity and gravimetric density. Second, the tests must be carried out in environments that are similar to industrial conditions, which include high mass loadings and minimum electrolyte usage. Compared with other commercially available inorganic electrodes, carbonyl-based OEMs are highly promising for achieving sustainable reactions. Therefore, research on the development of sustainable electrodes using carbonyl-based OEMs would be very beneficial in terms of developing eco-friendly energy storage systems. Leveraging the extensive versatility of organic chemistry, these compounds offer the advantage of eco-friendly structural modifications to tailor their electrochemical properties. Despite these advantages, challenges such as susceptibility to dissolution in electrolytes, inherent low electrical conductivity, and limited volumetric energy density hinder their long-term performance and practical utility in battery systems. To address these issues, numerous molecular engineering strategies need to be explored. This progress has led to the development of highly efficient, cost-effective energy storage devices and cleaner and more environmentally friendly sustainable energy practices.