Polyimides: Promising Energy-Storage Materials

Redox-Active High-Performance Polyimides as Versatile Electrode Materials for Organic Lithium- and Sodium-Ion Batteries

Organic electrode materials for rechargeable batteries show great promise for improving the storage capacity, reducing production costs, and minimizing environmental impact toward sustainability. In this study, we report a series of newly synthesized arylamine-based polyimides, TPPA-PIs, with three different bridge functionalizations on the imide rings and isomeric constituents that can work as versatile battery electrodes. As a lithium-ion battery cathode, a maximum energy density of 248 Wh kg-1 with high voltage operation up to 4.0 V can be achieved. As a lithium-ion battery anode, the TPPA-PIs showed a reversible storage capacity of 806 mA h g-1 at 100 mA g-1 current density with good rate capability up to a current density of 2000 mA g-1. Moreover, when applied as sodium-ion battery anodes, TPPA-PIs delivered an optimum specific capacity of up to 218 mA h g-1 after 50 cycles at a 50 mA g-1 current density and revealed a long cycling stability up to 1000 cycles under a high current density of 1000 mA g-1. More importantly, these electrochemical performances of TPPA-PIs are among the best compared with other reported polymer-based electrodes. The mechanistic studies show that both bridge functionalization on the imide units and isomerism impact the electrochemical performance by regulating their intrinsic properties such as charge storage behavior, ion diffusivity, and activation energy. We believe that such a detailed study of the structural design to electrochemical performance of these polymeric electrodes will offer insights into materials development and optimization for next-generation multifunctional energy storage devices in a wide range of applications.

Flexible Backbone Effects on the Redox Properties of Perylenediimide-Based Polymers

Organic electrode materials are appealing candidates for a wide range of applications, including heterogeneous electrocatalysis and electrochemical energy storage. However, a narrow understanding of the structure-property relationships in these materials hinders the full realization of their potential. Herein, we investigate a family of insoluble perylenediimide (PDI) polymers to interrogate how backbone flexibility affects their thermodynamic and kinetic redox properties. We verify that the polymers generally access the highest percentage of redox-active groups with K+ ions (vs Na+ and Li+) due to its small solvation shell/energy and favorable soft-soft interactions with reduced PDI species. Through cyclic voltammetry, we show that increasing the polymer flexibility does not minimize barriers to ion-insertion processes but rather increases the level of diffusion-limited processes. Further, we propose that the condensation of imides to iminoimides can truncate the imide polymer chain growth for certain diamine monomers, leading to greater polymer solubilization and reduced cycling stability. Together, our results provide insight into how polymer flexibility, ion-electrode interactions, and polymerization side reactions dictate the redox properties of PDI polymers, paving the way for the development of next-generation organic electrode materials.

Amino acid-appended pyromellitic diimide liquid materials, their photoluminescence, and the thermal response that turns the photoluminescence off

We report a liquid material based on an L-valine-appended pyromellitic diimide framework. This liquid adopts a room-temperature liquid with Tg at -50 °C and can dissolve naphthalene derivatives to show various photoluminescent colors. Furthermore, the on/off photoluminescence of these solutions can be controlled by heating.

Design Principles of Quinone Redox Systems for Advanced Sulfide Solid-State Organic Lithium Metal Batteries

The emergence of solid-state battery technology presents a potential solution to the dissolution challenges of high-capacity small molecule quinone redox systems. Nonetheless, the successful integration of argyrodite-type Li6PS5Cl, the most promising solid-state electrolyte system, and quinone redox systems remains elusive due to their inherent reactivity. Here, a library of quinone derivatives is selected as model electrode materials to ascertain the critical descriptors governing the (electro)chemical compatibility and subsequently the performances of Li6PS5Cl-based solid-state organic lithium metal batteries (LMBs). Compatibility is attained if the lowest unoccupied molecular orbital level of the quinone derivative is sufficiently higher than the highest occupied molecular orbital level of Li6PS5Cl. The energy difference is demonstrated to be critical in ensuring chemical compatibility during composite electrode preparation and enable high-efficiency operation of solid-state organic LMBs. Considering these findings, a general principle is proposed for the selection of quinone derivatives to be integrated with Li6PS5Cl, and two solid-state organic LMBs, based on 2,5-diamino-1,4-benzoquinone and 2,3,5,6-tetraamino-1,4-benzoquinone, are successfully developed and tested for the first time. Validating critical factors for the design of organic battery electrode materials is expected to pave the way for advancing the development of high-efficiency and long cycle life solid-state organic batteries based on sulfides electrolytes.

Carbonyl Chemistry for Advanced Electrochemical Energy Storage Systems

On the basis of the sustainable concept, organic compounds and carbon materials both mainly composed of light C element have been regarded as powerful candidates for advanced electrochemical energy storage (EES) systems, due to theie merits of low cost, eco-friendliness, renewability, and structural versatility. It is investigated that the carbonyl functionality as the most common constituent part serves a crucial role, which manifests respective different mechanisms in the various aspects of EES systems. Notably, a systematical review about the concept and progress for carbonyl chemistry is beneficial for ensuring in-depth comprehending of carbonyl functionality. Hence, a comprehensive review about carbonyl chemistry has been summarized based on state-of-the-art developments. Moreover, the working principles and fundamental properties of the carbonyl unit have been discussed, which has been generalized in three aspects, including redox activity, the interaction effect, and compensation characteristic. Meanwhile, the pivotal characterization technologies have also been illustrated for purposefully studying the related structure, redox mechanism, and electrochemical performance to profitably understand the carbonyl chemistry. Finally, the current challenges and promising directions are concluded, aiming to afford significant guidance for the optimal utilization of carbonyl moiety and propel practicality in EES systems.

Reviving Cost-Effective Organic Cathodes in Halide-Based All-Solid-State Lithium Batteries

The evolution of inorganic solid electrolytes has revolutionized the field of sustainable organic cathode materials, particularly by addressing the dissolution problems in traditional liquid electrolytes. However, current sulfide-based all-solid-state lithium-organic batteries still face challenges such as high working temperatures, high costs, and low voltages. Here, we design an all-solid-state lithium battery based on a cost-effective organic cathode material phenanthrenequinone (PQ) and a halide solid electrolyte Li2ZrCl6. Thanks to the good compatibility between PQ and Li2ZrCl6, the PQ cathode achieved a high specific capacity of 248 mAh g-1 (96 % of the theoretical capacity), a high average discharge voltage of 2.74 V (vs. Li+/Li), and a good capacity retention of 95 % after 100 cycles at room temperature (25 °C). Furthermore, the interactions between the high-voltage carbonyl PQ cathode and both sulfide and halide solid electrolytes, as well as the redox mechanism of the PQ cathode in all-solid-state batteries, were carefully studied by a variety of advanced characterizations. We believe such a design and the corresponding investigations into the underlying chemistry give insights for the further development of practical all-solid-state lithium-organic batteries.

A photocatalytic redox cycle over a polyimide catalyst drives efficient solar-to-H2O2 conversion

Circumventing the conventional two-electron oxygen reduction pathway remains a great problem in enhancing the efficiency of H2O2 photosynthesis. A promising approach to achieve outstanding photocatalytic activity involves the utilization of redox intermediates. Here, we engineer a polyimide aerogel photocatalyst with photoreductive carbonyl groups for non-sacrificial H2O2 production. Under photoexcitation, carbonyl groups on the photocatalyst surface are reduced, forming an anion radical intermediate. The produced intermediate is oxidized by O2 to produce H2O2 and subsequently restores the carbonyl group. The high catalytic efficiency is ascribed to a photocatalytic redox cycle mediated by the radical anion, which not only promotes oxygen adsorption but also lowers the energy barrier of O2 reduction reaction for H2O2 generation. An apparent quantum yield of 14.28% at 420 ± 10 nm with a solar-to-chemical conversion efficiency of 0.92% is achieved. Moreover, we demonstrate that a mere 0.5 m2 self-supported polyimide aerogel exposed to natural sunlight for 6 h yields significant H2O2 production of 34.3 mmol m-2.

Supramolecular Engineering of Ti3C2Tx MXene -Perylene Diimide Hybrid Electrodes for the Pseudocapacitive Electrochemical Storage of Calcium Ions

The rare combination of metallic conductivity and surface redox activity enables 2D MXenes as versatile charge storage hosts for the design of high-rate electrochemical energy storage devices. However, high charge density metal ions including but not limited to Ca+2 and Mg+2 pose challenges such as sluggish solid-state diffusion and also inhibiting the charge transfer across electrode-electrolyte interfaces. In this work, free-standing hybrid electrode architectures based on 2D titanium carbide-cationic perylene diimide (Ti3C2Tx@cPDI) via supramolecular self-assembly are developed. Secondary bonding interactions such as dipole-dipole and hydrogen bonding between Ti3C2Tx and cPDI are investigated by zeta potential and Fourier-transformed infrared (FTIR) spectroscopy . Ti3C2Tx@cPDI free-standing electrodes show typical volumetric capacitance up to 260 F cm-3 in Mg2+ and Ca2+ aqueous electrolytes at charging times scales from 3 minutes to a few seconds. Three-dimensional (3D) Bode maps are constructed to understand the charge storage dynamics of Ti3C2Tx@cPDI hybrid electrode in an aqueous Ca-ion electrolyte. ,Pseudocapacitance is solely contributed by the nanoscale distribution of redox-active cPDI supramolecular polymers across 2D Ti3C2Tx. This study opens avenues for the design of a wide variety of MXene@redox active organic charge hosts for high-rate pseudocapacitive energy storage devices.

Ferrocene Appended Porphyrin-Based Bipolar Electrode Material for High-Performance Energy Storage

The versatile properties of bipolar organic electrode materials have attracted considerable attention in the field of electrochemical energy storage (EES). However, their practical application is hindered by their inherent limitations including low intrinsic electrical conductivity, low specific capacity, and high solubility. Herein, a bipolar organic molecule combining both porphyrin and ferrocene moieties (CuDEFcP) [5,15-bis(ethynyl)-10,20-di ferrocenyl porphinato]copper(II)) has been developed. It is proposed as a new organic electrode material with multifunctional application for rechargeable organic lithium-based batteries (ROLBs) and dual-ion organic symmetric batteries (SDIBs). Superior performance was delivered as cathode material in lithium based dual-ion batteries (LDIBs), with a high initial discharge capacity of 300 mAh. g-1 at 0.2 A. g-1 and a reversible capacity of 58 mAh. g-1 after 5000 cycles at 1 A. g-1. However, employing it as an anode material in lithium-ion batteries (LIBs), a reversible capacity of 295 mAh. g-1 at 0.2 A. g-1 was delivered. In SDIBs, in which CuDEFcP is used as both anode and cathode, an average discharge voltage of 2.4 V and an energy density of 261 Wh.kg-1 were achieved.

Towards Durable and High-Rate Rechargeable Aluminum Dual-ion Batteries via a Crosslinked Diphenylphenazine-based Conjugated Polymer Cathode

Rechargeable aluminum battery (RAB) is expected to be a promising energy storage technique for grid-scale energy storage. However, the development of RABs is seriously plagued by the lack of suitable cathode materials. Herein, we report two p-type conjugated polymers of L-PBPz and C-PBPz with the same building blocks of diphenylphenazine but different linkage patterns of linear and crosslinked structures as the cathode materials for Al dual-ion batteries. Compared to the linear polymer skeleton in L-PBPz, the crosslinked structure endows C-PBPz with amorphous nature and low dihedral angles of the polymer chains, which severally contribute to the fast diffusion of AlCl4 - with large size and the electron transfer during the redox reaction of diphenylphenazine. As a result, C-PBPz delivers a much better rate performance than L-PBPz. The crosslinked structure also leads to a stable cyclability with over 80000 cycles for C-PBPz. Benefiting from the fast kinetics, meanwhile, the C-PBPz cathode could realize a high redox activity of 117 mAh g-1, corresponding to an areal capacity of 2.30 mAh cm-2, even under a high mass loading of 19.7 mg cm-2 and a low content of 10 wt% conductive agent. These results might boost the development of polymer cathodes for RABs.

Versatile Nitrogen-Centered Organic Redox-Active Materials for Alkali Metal-Ion Batteries

Versatile nitrogen-centered organic redox-active molecules have gained significant attention in alkali metal-ion batteries (AMIBs) due to their low cost, low toxicity, and ease of preparation. Specially, their multiple reaction categories (anion/cation insertion types of reaction) and higher operating voltage, when compared to traditional conjugated carbonyl materials, underscore their promising prospects. However, the high solubility of nitrogen-centered redox active materials in organic electrolyte and their low electronic conductivity contribute to inferior cycling performance, sluggish reaction kinetics, and limited rate capability. This review provides a detailed overview of nitrogen-centered redox-active materials, encompassing their redox chemistry, solutions to overcome shortcomings, characterization of charge storage mechanisms, and recent progress. Additionally, prospects and directions are proposed for future investigations. It is anticipated that this review will stimulate further exploration of underlying mechanisms and interface chemistry through in situ characterization techniques, thereby promoting the practical application of nitrogen-centered redox-active materials in AMIBs.

Recent Progress in Covalent Organic Frameworks for Cathode Materials

Covalent organic frameworks (COFs) are constructed from small organic molecules through reversible covalent bonds, and are therefore considered a special type of polymer. Small organic molecules are divided into nodes and connectors based on their roles in the COF's structure. The connector generally forms reversible covalent bonds with the node through two reactive end groups. The adjustment of the length of the connector facilitates the adjustment of pore size. Due to the diversity of organic small molecules and reversible covalent bonds, COFs have formed a large family since their synthesis in 2005. Among them, a type of COF containing redox active groups such as -C=O-, -C=N-, and -N=N- has received widespread attention in the field of energy storage. The ordered crystal structure of COFs ensures the ordered arrangement and consistent size of pores, which is conducive to the formation of unobstructed ion channels, giving these COFs a high-rate performance and a long cycle life. The voltage and specific capacity jointly determine the energy density of cathode materials. For the COFs' cathode materials, the voltage plateau of their active sites' VS metallic lithium is mostly between 2 and 3 V, which has great room for improvement. However, there is currently no feasible strategy for this. Therefore, previous studies mainly improved the theoretical specific capacity of the COFs' cathode materials by increasing the number of active sites. We have summarized the progress in the research on these types of COFs in recent years and found that the redox active functional groups of these COFs can be divided into six subcategories. According to the different active functional groups, these COFs are also divided into six subcategories. Here, we summarize the structure, synthesis unit, specific surface area, specific capacity, and voltage range of these cathode COFs.

Polyimide Compounds For Post-Lithium Energy Storage Applications

The exploration of cathode and anode materials that enable reversible storage of mono and multivalent cations has driven extensive research on organic compounds. In this regard, polyimide (PI)-based electrodes have emerged as a promising avenue for the development of post-lithium energy storage systems. This review article provides a comprehensive summary of the syntheses, characterizations, and applications of PI compounds as electrode materials capable of hosting a wide range of cations. Furthermore, the review also delves into the advancements in PI based solid state batteries, PI-based separators, current collectors, and their effectiveness as polymeric binders. By highlighting the key findings in these areas, this review aims at contributing to the understanding and advancement of PI-based structures paving the way for the next generation of energy storage systems.

4-Electron redox enabled by a perylene diimide containing side-chain amines for efficient organic cathode development

A perylene diimide containing side-chain amines (PDIN) was studied as an organic cathode for application in lithium batteries, showing a high capacity of 174 mA h g-1. The chemical structures, experimental results, and calculation analyses verify that PDIN performed a 4-electron redox reaction jointly involving its CO and side-chain amine groups. This study promotes the development of organic cathodes with multi-electron redox reactions.

Long-Life, High-Rate Rechargeable Lithium Batteries Based on Soluble Bis(2-pyrimidyl) Disulfide Cathode

Organosulfides are promising candidates as cathode materials for the development of electric vehicles and energy storage systems due to their low-cost and high capacity properties. However, they generally suffer from slow kinetics because of the large rearrangement of S-S bonds and structural degradation upon cycling in batteries. In this paper, we reveal that soluble bis(2-pyrimidyl) disulfide (Pym2 S2 ) can be a high-rate cathode material for rechargeable lithium batteries. Benefiting from the superdelocalization of pyrimidyl group, the extra electrons prefer to be localized on the π* (pyrimidyl group) than σ* (S-S bond) molecular orbitals initially, generating the anion-like intermedia of [Pym2 S2 ]2- and thus decreasing the dissociation energy of the S-S bond. It makes the intrinsic energy barrier of dissociative electron transfer depleted, therefore the lithium half cell exhibits 2000 cycles at 5 C. This study provides a distinct pathway for the design of high-rate, long-cycle-life organic cathode materials.

Small-Molecule Organic Cathodes with Carbon Coating for Highly Efficient Potassium-ion Batteries

Small-molecule organic cathodes face dissolution in potassium-ion batteries (PIBs). For the first time, an interesting and effective strategy is unveiled to resolve this issue by designing a new soluble small-molecule organic compound namely [N,N'-bis(2-anthraquinone)]-1,4,5,8-naphthalenetetracarboxdiimide (NTCDI-DAQ, 237 mAh g-1 ): Through the precise manipulation of carbonization temperature and time, the molecules on the surface of NTCDI-DAQ particles can be transformed into amorphous carbon with controllable thickness. This strategy called surface self-carbonization can form a carbon protective layer on organic cathodes and significantly increase their insolubility against liquid electrolytes without affecting the electrochemical behavior of bulk particles. As a result, the as-obtained NTCDI-DAQ@C sample displays significantly improved cathode performance in PIBs. In half cells, NTCDI-DAQ@C shows superior capacity stability of 84 % compared to 35 % of NTCDI-DAQ during 30 cycles under the same conditions. In full cells with a KC8 anode, NTCDI-DAQ@C delivers a peak discharge capacity of 236 mAh g-1 cathode and a high energy density of 255 Wh kg-1 cathode in 0.1-2.8 V, with 40 % capacity retention during 3000 cycles at 1 A g-1 . To the best of our knowledge, the integrated performance of NTCDI-DAQ@C is among the best of soluble organic cathodes reported in PIBs.

Polyimides as Promising Cathodes for Metal-Organic Batteries: A Comparison between Divalent (Ca2+, Mg2+) and Monovalent (Li+, Na+) Cations

Ca- and Mg-based batteries represent a more sustainable alternative to Li-ion batteries. However, multivalent cation technologies suffer from poor cation mass transport. In addition, the development of positive electrodes enabling reversible charge storage currently represents one of the major challenges. Organic positive electrodes, in addition to being the most sustainable and potentially low-cost candidates, compared with their inorganic counterparts, currently present the best electrochemical performances in Ca and Mg cells. Unfortunately, organic positive electrodes suffer from relatively low capacity retention upon cycling, the origin of which is not yet fully understood. Here, 1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide was tested in Li, Na, Mg, and Ca cells for the sake of comparison in terms of redox potential, gravimetric capacities, capacity retention, and rate capability. The redox mechanisms were also investigated by means of operando IR experiments, and a parameter affecting most figures of merit has been identified: the presence of contact ion-pairs in the electrolyte.

Nonconjugated Redox-Active Polymers: Electron Transfer Mechanisms, Energy Storage, and Chemical Versatility

The storage of electric energy in a safe and environmentally friendly way is of ever-growing importance for a modern, technology-based society. With future pressures predicted for batteries that contain strategic metals, there is increasing interest in metal-free electrode materials. Among candidate materials, nonconjugated redox-active polymers (NC-RAPs) have advantages in terms of cost-effectiveness, good processability, unique electrochemical properties, and precise tuning for different battery chemistries. Here, we review the current state of the art regarding the mechanisms of redox kinetics, molecular design, synthesis, and application of NC-RAPs in electrochemical energy storage and conversion. Different redox chemistries are compared, including polyquinones, polyimides, polyketones, sulfur-containing polymers, radical-containing polymers, polyphenylamines, polyphenazines, polyphenothiazines, polyphenoxazines, and polyviologens. We close with cell design principles considering electrolyte optimization and cell configuration. Finally, we point to fundamental and applied areas of future promise for designer NC-RAPs.

Carbonyl-Based Redox-Active Compounds as Organic Electrodes for Batteries: Escape from Middle-High Redox Potentials and Further Improvement?

Extracting─from the vast space of organic compounds─the best electrode candidates for achieving energy material breakthrough requires the identification of the microscopic causes and origins of various macroscopic features, including notably electrochemical and conduction properties. As a first guess of their capabilities, molecular DFT calculations and quantum theory of atoms in molecules (QTAIM)-derived indicators were applied to explore the family of pyrano[3,2-b]pyran-2,6-dione (PPD, i.e., A0) compounds, expanded to A0 fused with various kinds of rings (benzene, fluorinated benzene, thiophene, and merged thiophene/benzene). A glimpse of up-to-now elusive key incidences of introducing oxygen in vicinity to the carbonyl redox center within 6MRs─as embedded in the A0 core central unit common to all A-type compounds─has been gained. Furthermore, the main driving force toward achieving modulated low redox potential/band gaps thanks to fusing the aromatic rings for the A compound series was discovered.

Polyimides as Promising Materials for Lithium-Ion Batteries: A Review

Lithium-ion batteries (LIBs) have helped revolutionize the modern world and are now advancing the alternative energy field. Several technical challenges are associated with LIBs, such as increasing their energy density, improving their safety, and prolonging their lifespan. Pressed by these issues, researchers are striving to find effective solutions and new materials for next-generation LIBs. Polymers play a more and more important role in satisfying the ever-increasing requirements for LIBs. Polyimides (PIs), a special functional polymer, possess unparalleled advantages, such as excellent mechanical strength, extremely high thermal stability, and excellent chemical inertness; they are a promising material for LIBs. Herein, we discuss the current applications of PIs in LIBs, including coatings, separators, binders, solid-state polymer electrolytes, and active storage materials, to improve high-voltage performance, safety, cyclability, flexibility, and sustainability. Existing technical challenges are described, and strategies for solving current issues are proposed. Finally, potential directions for implementing PIs in LIBs are outlined.

Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Boron-based nonmetallic materials (such as B₂O₃ and BN) emerge as promising catalysts for selective oxidation of light alkanes by O₂ to form value-added products, resulting from their unique advantage in suppressing CO₂ formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al₂O₃-supported B₂O₃ catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B₂O₃/Al₂O₃ catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH₃^•) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH₃^• and CH₃OO^• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

Dual redox-active porous polyimides as high performance and versatile electrode material for next-generation batteries

Energy storage will be a primordial actor of the ecological transition initiated in the energy and transport sectors. As such, innovative approaches to design high-performance electrode materials are crucial for the development of the next generation of batteries. Herein, a novel dual redox-active and porous polyimide network (MTA-MPT), based on mellitic trianhydride (MTA) and 3,7-diamino-N-methylphenothiazine (MPT) monomers, is proposed for applications in both high energy density lithium batteries and symmetric all-organic batteries. The MTA-MPT porous polyimide was synthesized using a novel environmentally-friendly hydrothermal polymerization method. Rooted in its dual redox proprieties, the MTA-MPT porous polyimide exhibits a high theoretical capacity making it a very attractive cathode material for high energy density battery applications. The cycling performance of this novel electrode material was assessed in both high energy density lithium batteries and light-weight symmetric all-organic batteries, displaying excellent rate capability and long-term cycling stability.

Reversible Metal and Ligand Redox Chemistry in Two-Dimensional Iron-Organic Framework for Sustainable Lithium-Ion Batteries

Metal-organic frameworks (MOFs) are emerging as attractive electrode materials for lithium-ion batteries, owing to their fascinating features of sustainable resources, tunable chemical components, flexible molecular skeletons, and renewability. However, they are faced with a limited number of redox-active sites and unstable molecular frameworks during electrochemical processes. Herein, we design a novel two-dimensional (2D) iron(III)-tetraamino-benzoquinone (Fe-TABQ) with dual redox centers of Fe cations and TABQ ligands for high-capacity and stable lithium storage. It is constructed of square-planar Fe-N₂O₂ linkages and phenylenediamine building blocks, between which the Fe-TABQ chains are connected by multiple hydrogen bonds, and then featured as an extended π-d-conjugated 2D structure. The redox chemistry of both Fe³⁺ cations and TABQ anions is revealed to render its remarkable specific capacity of 251.1 mAh g^-1. Benefiting from the intrinsic robust Fe-N(O) bonds and reinforced Li-N(O) bonds during cycling, Fe-TABQ delivers high capacity retentions over 95% after 200 cycles at various current densities. This work will enlighten more investigations for the molecular designs of advanced MOF-based electrode materials.

Anion Storage Chemistry of Organic Cathodes for High-Energy and High-Power Density Divalent Metal Batteries

Multivalent batteries show promising prospects for next-generation sustainable energy storage applications. Herein, we report a polytriphenylamine (PTPAn) composite cathode capable of highly reversible storage of tetrakis(hexafluoroisopropyloxy) borate [B(hfip)4 ] anions in both Magnesium (Mg) and calcium (Ca) battery systems. Spectroscopic and computational studies reveal the redox reaction mechanism of the PTPAn cathode material. The Mg and Ca cells exhibit a cell voltage >3 V, a high-power density of ∼∼3000 W kg-1 and a high-energy density of ∼∼300 Wh kg-1 , respectively. Moreover, the combination of the PTPAn cathode with a calcium-tin (Ca-Sn) alloy anode could enable a long battery-life of 3000 cycles with a capacity retention of 60 %. The anion storage chemistry associated with dual-ion electrochemical concept demonstrates a new feasible pathway towards high-performance divalent ion batteries.

Redox-active polyimides for energy conversion and storage: from synthesis to application

As the demand for next-generation electronics is increasing, organic and polymer-based semiconductors are in the spotlight as suitable materials owing to their tailorable structures along with flexible properties. Especially, polyimide (PI) has been widely utilised in electronics because of its outstanding mechanical and thermal properties and chemical resistance originating from its crystallinity, conjugated structure and π-π interactions. PI has recently been receiving more attention in the energy storage and conversion fields due to its unique redox activity and charge transfer complex structure. In this review, we focus on the design of PI structures with improved electrochemical and photocatalytic activities for use as redox-active materials in photo- and electrocatalysts, batteries and supercapacitors. We anticipate that this review will offer insight into the utilisation of redox-active PI-based polymeric materials for the development of future electronics.

Squarephaneic Tetraanhydride: A Conjugated Square-Shaped Cyclophane for the Synthesis of Porous Organic Materials

Aromatic carboxylic anhydrides are ubiquitous building blocks in organic materials chemistry and have received considerable attention in the synthesis of organic semiconductors, pigments, and battery electrode materials. Here we extend the family of aromatic carboxylic anhydrides with a unique new member, a conjugated cyclophane with four anhydride groups. The cyclophane is obtained in a three-step synthesis and can be functionalised efficiently, as shown by the conversion into tetraimides and an octacarboxylate. Crystal structures reveal the high degree of porosity achievable with the new building block. Excellent electrochemical properties and reversible reduction to the tetraanions are shown for the imides; NMR and EPR measurements confirm the global aromaticity of the dianions and evidence the global Baird aromaticity of the tetraanions. Considering the short synthesis and unique properties, we expect widespread use of the new building block in the development of organic materials.

Squarephaneic Tetraanhydride: A Conjugated Square-Shaped Cyclophane for the Synthesis of Porous Organic Materials

Aromatic carboxylic anhydrides are ubiquitous building blocks in organic materials chemistry and have received considerable attention in the synthesis of organic semiconductors, pigments, and battery electrode materials. Here we extend the family of aromatic carboxylic anhydrides with a unique new member, a conjugated cyclophane with four anhydride groups. The cyclophane is obtained in a three-step synthesis and can be functionalised efficiently, as shown by the conversion into tetraimides and an octacarboxylate. Crystal structures reveal the high degree of porosity achievable with the new building block. Excellent electrochemical properties and reversible reduction to the tetraanions are shown for the imides; NMR and EPR measurements confirm the global aromaticity of the dianions and evidence the global Baird aromaticity of the tetraanions. Considering the short synthesis and unique properties, we expect widespread use of the new building block in the development of organic materials.

Molecular and Morphological Engineering of Organic Electrode Materials for Electrochemical Energy Storage

Organic electrode materials (OEMs) can deliver remarkable battery performance for metal-ion batteries (MIBs) due to their unique molecular versatility, high flexibility, versatile structures, sustainable organic resources, and low environmental costs. Therefore, OEMs are promising, green alternatives to the traditional inorganic electrode materials used in state-of-the-art lithium-ion batteries. Before OEMs can be widely applied, some inherent issues, such as their low intrinsic electronic conductivity, significant solubility in electrolytes, and large volume change, must be addressed. In this review, the potential roles, energy storage mechanisms, existing challenges, and possible solutions to address these challenges by using molecular and morphological engineering are thoroughly summarized and discussed. Molecular engineering, such as grafting electron-withdrawing or electron-donating functional groups, increasing various redox-active sites, extending conductive networks, and increasing the degree of polymerization, can enhance the electrochemical performance, including its specific capacity (such as the voltage output and the charge transfer number), rate capability, and cycling stability. Morphological engineering facilitates the preparation of different dimensional OEMs (including 0D, 1D, 2D, and 3D OEMs) via bottom-up and top-down methods to enhance their electron/ion diffusion kinetics and stabilize their electrode structure. In summary, molecular and morphological engineering can offer practical paths for developing advanced OEMs that can be applied in next-generation rechargeable MIBs.

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High Discharge Capacity and Ultra-Fast-Charging Sodium Dual-Ion Battery Based on Insoluble Organic Polymer Anode and Concentrated Electrolyte

Sodium-based dual-ion batteries have shown great promise for large-scale energy storage applications due to their wide operating voltages, environmental friendliness, abundant sodium resources, and low cost, which are widely investigated by researchers. However, the development of high-performance anode materials is a key requirement for the realization of such electrochemical energy storage systems at the practical application level. Carbonaceous anode materials based on intercalation/deintercalation mechanisms typically exhibit low discharge capacities, while metal-based materials based on conversion or alloying reactions show unsatisfactory stability in performance. On the contrary, organic materials display high theoretical capacities due to their flexible molecular structure designability and stable cyclic performance with fast reaction kinetics based on the unique enolization reaction. Herein, we report an organic polymer anode material of polyimide (PNTO), combined with a high-concentration electrolyte; the sodium-based dual-ion battery system constructed exhibits outstanding electrochemical performance. The full battery shows an ultra-high specific discharge capacity of 293.2 mAh g-1 and can be cycled stably for 3200/5600/4100 cycles at ultra-high rates of 60/120/150 C without degradation. Furthermore, the dual-ion battery system demonstrates an extremely low self-discharge rate of 0.03% h-1 and superior fast-charging-slow-discharging performance. It is one of the best performances reported up to now for a dual-ion full battery based on an organic polymer anode. This novel battery system design strategy will facilitate the advancement of high-performance organic-based dual-ion batteries and is expected to be a promising candidate for large-scale energy storage applications.

Molecular structure design of planar zwitterionic polymer electrode materials for all-organic symmetric batteries

All-organic symmetric lithium-ion batteries (LIBs) show promising prospects in sustainable energy storage systems, due to their environmental friendliness, structural diversity and low cost. Nevertheless, it remains a great challenge to explore suitable electrode materials and achieve excellent battery performance for all-organic symmetric LIBs. Herein, a squaraine-anthraquinone polymer (PSQ) electrode material was designed through rational molecular engineering. The well-designed extended π-conjugated system, donor-acceptor structure, abundant redox-active sites and rational manipulation of weak inter-/intramolecular interactions endow the PSQ electrode with outstanding electrochemical performance. The capacity of the PSQ cathode can be optimized to 311.5 mA h g-1 by in situ carbon-template polymerization. Impressively, PSQ-based all-organic symmetric LIBs displayed high reversible capacity (170.8 mA h g-1 at 50 mA g-1), excellent rate performance (64.9% capacity retention at 4000 mA g-1 vs. 50 mA g-1), ultralong cycle life up to 30 000 cycles at 2000 mA g-1 and 97% capacity retention after 2500 cycles at 500 mA g-1, which is one of the best comprehensive battery performances among the all-organic LIBs reported thus far.

Solution-Processable Redox-Active Polymers of Intrinsic Microporosity for Electrochemical Energy Storage

Redox-active organic materials have emerged as promising alternatives to conventional inorganic electrode materials in electrochemical devices for energy storage. However, the deployment of redox-active organic materials in practical lithium-ion battery devices is hindered by their undesired solubility in electrolyte solvents, sluggish charge transfer and mass transport, as well as processing complexity. Here, we report a new molecular engineering approach to prepare redox-active polymers of intrinsic microporosity (PIMs) that possess an open network of subnanometer pores and abundant accessible carbonyl-based redox sites for fast lithium-ion transport and storage. Redox-active PIMs can be solution-processed into thin films and polymer-carbon composites with a homogeneously dispersed microstructure while remaining insoluble in electrolyte solvents. Solution-processed redox-active PIM electrodes demonstrate improved cycling performance in lithium-ion batteries with no apparent capacity decay. Redox-active PIMs with combined properties of intrinsic microporosity, reversible redox activity, and solution processability may have broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

Investigation of ion-electrode interactions of linear polyimides and alkali metal ions for next generation alternative-ion batteries

Organic electrode materials offer unique opportunities to utilize ion-electrode interactions to develop diverse, versatile, and high-performing secondary batteries, particularly for applications requiring high power densities. However, a lack of well-defined structure-property relationships for redox-active organic materials restricts the advancement of the field. Herein, we investigate a family of diimide-based polymer materials with several charge-compensating ions (Li+, Na+, K+) in order to systematically probe how redox-active moiety, ion, and polymer flexibility dictate their thermodynamic and kinetic properties. When favorable ion-electrode interactions are employed (e.g., soft K+ anions with soft perylenediimide dianions), the resulting batteries demonstrate increased working potentials and improved cycling stabilities. Further, for all polymers examined herein, we demonstrate that K+ accesses the highest percentage of redox-active groups due to its small solvation shell/energy. Through crown ether experiments, cyclic voltammetry, and activation energy measurements, we provide insights into the charge compensation mechanisms of three different polymer structures and rationalize these findings in terms of the differing degrees of improvements observed when cycling with K+. Critically, we find that the most flexible polymer enables access to the highest fraction of active sites due to the small activation energy barrier during charge/discharge. These results suggest that improved capacities may be accessible by employing more flexible structures. Overall, our in-depth structure-activity investigation demonstrates how variables such as polymer structure and cation can be used to optimize battery performance and enable the realization of novel battery chemistries.

Revisiting the Structure and Electrochemical Performance of Poly(o-phenylenediamine) as an Organic Cathode Material

Poly(o-phenylenediamine) (PoPDA) has been recognized as a low-cost electroactive organic material and studied as a cathode for aqueous zinc batteries or as an anode for nonaqueous lithium batteries. However, there remains a lot of confusion about its synthesis, structure, and electrochemical application. Especially, the previously studied PoPDA samples were mostly synthesized at room temperature, which were proved by us to be just a dimer, that is, 2,3-diaminophenazine (DAPZ). By various characterization methods including elemental analysis and mass spectrometry, we verified that the product synthesized at high temperature, PoPDA-H, was a polymer based on DAPZ as the structural repeat unit and with some imperfect substitutes (OH and NH₃⁺CH₃COO^-). Based on the reversible redox reaction of phenazine units and the stable polymer structure within 1.3-3.8 V vs Li⁺/Li, PoPDA-H was more appropriate to be applied as a cathode rather than as an anode for lithium batteries. It achieved a high energy density of 490 Wh kg^-1 (2.12 V × 231 mAh g^-1) at 50 mA g^-1 and a high cycling stability (79%@1000th cycle) at 500 mA g^-1, both of which were comparable to previously reported expensive pyrazine- and carbonyl-based polymers. This work clarifies many misunderstandings of PoPDA, which is important to its further development toward practical application in energy-storage devices.

Benzoquinone-Pyrrole Polymers as Cost-Effective Cathodes toward Practical Organic Batteries

Organic cathode materials (OCMs) for rechargeable Li and Na batteries show great advantages in resource sustainability and huge potential in electrochemical performance but suffer from dissolution problems and costly synthesis. Herein, for the first time, we investigated the copolymer of benzoquinone (BQ) and pyrrole (Py), namely, poly(benzoquinone-pyrrole) (PBQPy), as an OCM for Li batteries. The low-cost raw materials and solvent-free synthesis provide PBQPy much brighter prospects in large-scale production compared to other carbonyl-based polymer cathode materials. Nevertheless, PBQPy showed one of the best electrochemical performances among all OCMs, including excellent energy density (2.32 V × 255 mAh g^-1 = 592 Wh kg^-1), rate capability (79%@2000 mA g^-1), and cycling stability (81%@1000th cycle). By introducing poly(benzoquinone-methyl pyrrole) for comparison, as well as employing density functional theory calculations and various characterizations for in-depth understanding, the synthesis mechanism, polymer structure, electrochemical behavior, and redox mechanism were clearly clarified. It is believed that this work will encourage more efforts to develop cost-effective OCMs toward practical organic batteries.

Conjugated Polyelectrolytes: Underexplored Materials for Pseudocapacitive Energy Storage

Conjugated polyelectrolytes (CPEs) are characterized by an electronically delocalized backbone bearing ionic functionalities. These features lead to properties relevant for use in energy-storing pseudocapacitor devices, including ionic conductivity, water processability, gel-formation, and formation of polaronic species stabilized by electrostatic interactions. In this Perspective, the basis for evaluating the figures of merit for pseudocapacitors is provided, together with the techniques used for their evaluation. The general utility and challenges encountered with neutral conjugated polymers are then discussed. Finally, recent advances on the use of CPEs in pseudocapacitor devices are reviewed. The article is concluded by discussing how their miscibility in aqueous media permits the incorporation of CPEs in living materials that are capable of switching function from extraction of energy from bacterial metabolic pathways to pseudocapacitor energy storage.

Polyimide Copolymers and Nanocomposites: A Review of the Synergistic Effects of the Constituents on the Fire-Retardancy Behavior

Carbon-based polymer can catch fire when used as cathode material in batteries and supercapacitors, due to short circuiting. Polyimide is known to exhibit flame retardancy by forming char layer in condensed phase. The high char yield of polyimide is attributed to its aromatic nature and the existence of a donor–acceptor complex in its backbone. Fabrication of hybrid polyimide material can provide better protection against fire based on multiple fire-retardancy mechanisms. Nanocomposites generally show a significant enhancement in mechanical, electrical, and thermal properties. Nanoparticles, such as graphene and carbon nanotubes, can enhance flame retardancy in condensed phase by forming a dense char layer. Silicone-based materials can also provide fire retardancy in condensed phase by a similar mechanism as polyimide. However, some inorganic fire retardants, such as phosphazene, can enhance flame retardancy in gaseous phase by releasing flame inhibiting radicals. The flame inhibiting radicals generated by phosphazene are released into the gaseous phase during combustion. A hybrid system constituted of polyimide, silicone-based additives, and phosphazene would provide significant improvement in flame retardancy in both the condensed phase and gas phase. In this review, several flame-retardant polyimide-based systems are described. This review which focuses on the various combinations of polyimide and other candidate fire-retardant materials would shed light on the nature of an effective multifunctional flame-retardant hybrid materials.

Calcium Based All-Organic Dual-Ion Batteries with Stable Low Temperature Operability

In response to the application requirements of secondary batteries at low temperature, an all-organic dual-ion battery with calcium perchlorate contained acetonitrile as the electrolyte (CAN-ODIB) is fabricated in this work. The electrochemical energy is stored in CAN-ODIB via the association and disassociation of calcium and perchlorate ions in perylene diimide-ethylene diamine/carbon black composite based anode and polytriphenylamine based cathode with highly reversible redox states. Benefiting from the energy storage mechanism, CAN-ODIB exhibits excellent electrochemical performances in tests with the temperature ranging from 25 to -50 °C. Especially, CAN-ODIB at -50 °C reserves ≈61% of the capacity at 25 °C (83.4 mA h g^-1 ) with the current density of 0.2 A g^-1 . CAN-ODIB also shows excellent cycling stability at low temperature by retaining 90.3% of the initial capacity at 1.0 A g^-1 after 450 charge-discharge cycles at -30 °C. The impedance analysis of CAN-ODIB at different temperatures indicates that the low temperature performance of CAN-ODIB depends more on the electrode materials than the electrolyte, which provides the important guidance for the further design of secondary batteries operable at low temperatures.

Storing Mg Ions in Polymers: A Perspective

The electrochemical performance of rechargeable Mg batteries (RMBs) is primarily determined by the cathodes. However, the strong interaction between highly polarized Mg²⁺ and the host lattice is a big challenge for inorganic cathode materials. While endowed with weak interaction with Mg²⁺ , organic polymers are capable of fast reaction kinetics. Besides, with the advantages of light weight, abundance, low cost, and recyclability, polymers are deemed as ideal cathode materials for RMBs. Although polymer cathodes have remarkably progressed in recent years, there are still significant challenges to overcome before reaching practical application. In this perspective, the challenges faced by polymer cathodes are critically focused, followed by the retrospection of efforts devoted to design polymers. Some feasible strategies are proposed to explore new structures and chemistries for the practical application of polymer cathodes in RMBs.

Are Redox-Active Organic Small Molecules Applicable for High-Voltage (>4 V) Lithium-Ion Battery Cathodes?

While organic batteries have attracted great attention due to their high theoretical capacities, high-voltage organic active materials (> 4 V vs Li/Li+ ) remain unexplored. Here, density functional theory calculations are combined with cyclic voltammetry measurements to investigate the electrochemistry of croconic acid (CA) for use as a lithium-ion battery cathode material in both dimethyl sulfoxide and γ-butyrolactone (GBL) electrolytes. DFT calculations demonstrate that CA dilitium salt (CA-Li2 ) has two enolate groups that undergo redox reactions above 4.0 V and a material-level theoretical energy density of 1949 Wh kg-1 for storing four lithium ions in GBL-exceeding the value of both conventional inorganic and known organic cathode materials. Cyclic-voltammetry measurements reveal a highly reversible redox reaction by the enolate group at ≈4 V in both electrolytes. Battery-performance tests of CA as lithium-ion battery cathode in GBL show two discharge voltage plateaus at 3.9 and 3.1 V, and a discharge capacity of 102.2 mAh g-1 with no capacity loss after five cycles. With the higher discharge voltages compared to the known, state-of-the-art organic small molecules, CA promises to be a prime cathode-material candidate for future high-energy-density lithium-ion organic batteries.

The Emergence of Aqueous Ammonium-Ion Batteries

Aqueous ammonium-ion (NH4 + ) batteries (AAIB) are a recently emerging technology that utilize the abundant electrode resources and the fast diffusion kinetics of NH4 + to deliver an excellent rate performance at a low cost. Although significant progress has been made on AAIBs, the technology is still limited by various challenges. In this Minireview, the most recent advances are comprehensively summarized and discussed, including cathode and anode materials as well as the electrolytes. Finally, a perspective on possible solutions for the current limitations of AAIBs is provided.

Study on Chemical Graft Structure Modification and Mechanical Properties of Photocured Polyimide

The great challenge facing additive manufacturing is that the available high-performance 3D printing materials can hardly keep up with the rapid development of new additive manufacturing technology. Then, the commercial resins available in the market have some problems, such as poor thermal stability, insufficient light-curing degree, and large shrinkage after curing, which need to be solved urgently. This study reports a photocurable polyimide ink for digital light processing (DLP) 3D printing to prepare controllable 3D structures with high thermal stability, low shrinkage, and excellent comprehensive properties. In this study, pyromellitic dianhydride and diaminodiphenyl ether, the Kapton polyimide with the highest performance synthesized so far, were selected as raw materials, and 2,2'-bis(3,4-dicarboxylic acid) hexafluoropropane dianhydride containing fluorine was introduced to modify the branched-chain structure. The polyimide was prepared by one-step imidization, and then the graft with photocurable double bonds and certain functions was grafted by reaction of glycidyl methacrylate with phenolic hydroxyl groups. In this work, the solubility of the synthesized oligomer polyimide in organic solvents was greatly increased by combining three methods, thereby allowing the formation of ink for photocuring 3D printing, and the ink can be stacked to form low-shrinkage polyimide with complex controllable shape. Polyimide printed by DLP can produce complex structures with good mechanical character and thermal stability and small shrinkage. Therefore, the polyimide prepared in this study is considered to be a resin of great commercial possibility. In addition, due to its properties, it has important development potential in some fields with high demand for thermal stability, such as high-temperature cooling valves, aerospace, and other fields.

Graphene-Supported Naphthalene-Based Polyimide Composite as a High-Performance Sodium Storage Cathode

Electroactive acid anhydride with multicarbonyl is highly promising for electrochemical energy storage because of its high specific capacity and environmental benignity. Its low electrical conductivity and high dissolution in organic electrolyte, however, result in poor cycling and rate capabilities. Here, we report a naphthalene polyimide derivative (NPI) synthesized by using anhydride under condensation polymerization conditions, along with its composite with graphene (NPI-G) fabricated via in situ polymerization. The composite delivers a high reversible capacity and outstanding cycling stability and rate capability as a cathode for sodium-ion batteries (SIBs) owing to the formation of a polymer, the improvement in the electrical conductivity brought about by the highly dispersed graphene sheets, and the enhancement of structural stability resulting from the π-π stacking interaction between the phenyl groups of NPI and the six-member carbon rings of graphene. This investigation sheds light on the development, design, and screening of next-generation organic electrode materials with high performance for SIBs.

A π-Conjugated Polyimide-Based High-Performance Aqueous Potassium-Ion Asymmetric Supercapacitor

Aqueous asymmetric supercapacitor has captured widespread attention as a sustainable high-power energy resource. Organic electrode materials are appealing owing to their sustainability and high redox reactivity, but suffer from structural instability and low power density. Here the π-conjugated polyimide-based organic electrodes with different lengths of alkyl chains are explored to achieve high rate capability and long lifespan in an aqueous K⁺ -ion electrolyte. The fabricated asymmetric supercapacitor exhibits high capacities of 107 mAh g^-1 at 2 A g^-1 and 67 mAh g^-1 at 90 A g^-1 . A specific capacity of 65 mAh g^-1 which is over 70% of the initial performance is obtained after 65,000 cycles. Molecular engineering of long alkyl chains in polyimide could reduce the degree of π-conjugation and spatially block the π-conjugated imide bond with limited redox activity but improved stability against chemical degradation. Further electrochemical quartz crystal microbalance (EQCM) and ex situ FTIR and XPS characterizations reveal the pseudocapacitance behavior originated from the π-conjugated polyimide-based redox reaction with potassium ions and hydrated potassium ions. We showcase a promising polyimide-based polymer with extended π-conjugated system for high-performance asymmetric supercapacitor. This article is protected by copyright. All rights reserved.