A donor-acceptor (D-A) conjugated polymer for fast storage of anions

Arrangement of Ordered D-A Components in a Metal-Organic Framework for Cocatalyst-Free Photocatalytic Hydrogen Evolution with Efficient Proton Conduction

The arrangement of donor-acceptor (D-A) components in order at a molecular level provides a means to achieve efficient electron-hole separation for promoting the activity of photocatalysts. Herein, we report the coordination assembly of D-A molecules with desired staggered energy levels in two isostructural metal-organic frameworks (MOFs) 1 and 2, which exhibit high photocatalytic hydrogen evolution activity without using any cocatalysts and photosensitizers. The modulation of active metal sites of the D-A MOFs leads to an increase in photocatalytic hydrogen evolution rates from 1260 to 3218 µmol h-1 g-1. A detailed mechanism study revealed that the energy bond defined by the D-A components assisted with metal centers is the key to efficiently generating photogenerated charge carriers, and 2 has an appropriate affinity to proton to reduce the energy barrier for hydrogen evolution. Besides, the enhanced proton transport kinetics based on the arrayed free carboxyl groups in the hydrogen-bonded network endows 2 with higher proton conductivity than 1, thus promoting the usage rate of active metal sites in 2 for enhanced hydrogen evolution reaction kinetics.

Striving to Disclose the Electrochemical Processes of Organic Batteries

ConspectusOrganic/polymeric materials are promising as electrode materials for batteries because of their advantages of flexibility, high specific capacity due to the possible multielectron transfer, low cost from green natural resources, and weak intermolecular interactions that enable the storage of low-cost large-sized or multivalent metal ions. However, the development of organic electrode materials (OEMs) and organic batteries and the understanding of the electrochemical process face great challenges in the characterization of polymers and the charge storage mechanisms: (1) the charged and/or discharged states of OEMs are often air unstable, which makes the ex situ characterizations susceptible to the interference of air. (2) OEMs, particularly polymeric materials, are designed to be insoluble to deliver high cyclability, which makes it difficult for them to be separated from the electrode. (3) Possible multielectron transfer makes it difficult to determine whether the proposed charge storage mechanism or the experiment results are wrong when the actual capacity mismatches with the theoretical capacity based on the proposed mechanisms. (4) It is difficult to achieve single crystals of polymers, and hence, it seems impossible to know the actual locations of the stored ions in the polymers. (5) The typical methods for characterization of insoluble polymers are only qualitative, and it is challenging to quantify the amount of stored ions. (6) Even for most in situ characterizations, they can only give the tendency of qualitative structural evolution.In this Account, we give an overview of the significance of organic batteries and the challenges related to the characterization and charge storage mechanisms of organic electrode materials. Then, we summarize our efforts in recent years to reveal the charge storage mechanisms and insights into the electrochemical process. Focusing on the complexity of polymer materials, we proposed a strategy to control the reaction kinetics in order to obtain high-quality single crystals or microcrystals of polymers. The chemical structure and reaction mechanism of polymers could be successfully revealed by single crystal structure analysis. To avoid the inconvenient characterizations brought by the insolubility of polymers, soluble monomers or oligomers were studied under the same conditions to simulate and analyze the electrochemical process of polymers. We also proposed the synthesis of isomers for a deep understanding of the structure-property relationships of OEMs. On the other hand, traditional qualitative characterization instruments or techniques were reconsidered to give more information or even quantitative results via insightful analysis of the data or smart design of experiments. In addition, by introducing internal standard substances, it was also possible to realize quantitative characterizations. Strategies to convert the black box of different charged/discharged states into detectable materials or signals were also developed. This Account provides a summary of our recent progress in understanding the electrochemical process of OEMs and prospects of future development of rechargeable organic batteries.

A Metal-Organic Framework with Mixed Electron Donor TTF and Electron Acceptor NDI Ligands for High-Performance Hybrid Lithium-Ion Capacitors

Integrating mixed electron donor (D) and electron acceptor (A) ligands into metal-organic frameworks (MOFs) is an effective yet relatively unexplored approach for improving the anode performance of hybrid lithium-ion capacitors (HLICs). In this study, using an electron donor 2,6-bis(4'-pyridyl)tetrathiafulvalene and an electron acceptor N,N'-bis(5-isophthalic acid) naphthalene diimide as ligands, a new Zn-TTF/NDI MOF (1) is constructed as a pseudocapacitive anode of HLICs. Crystallographic characterization revealed that MOF 1 adopts a two-dimensional (2D) coordination network. A three-dimensional (3D) supramolecular framework is formed through face-to-face TTF packing of the adjacent 2D layers. As a result, the 2D MOF 1 with both electron-donating TTF and electron-accepting NDI units not only has rich active sites and excellent charge conductivity for reversible Li+ storage but also, owing to its 3D-supramolecular architecture, provides open channels for ion transport, leading to the merits of enhanced capacity utilization and high power density. The MOF 1||activated carbon HLIC exhibited maximum specific energy (133.7 Wh kg-1) and high specific power (12.9 kW kg-1) with stable cycling performance. The remarkable performance originates from the synergistic effect of the mixed electron-donating TTF and electron-withdrawing NDI ligands, interligand charge transfer, and structural stability.

A Lithiophilic Donor-Acceptor Polymer Modified Separator for High-Performance Lithium Metal Batteries

As traditional lithium-ion batteries near their theoretical limits, the advancement of lithium-metal batteries (LMBs) becomes crucial for achieving higher energy densities. However, uncontrolled ion transport and unstable solid electrolyte interface (SEI) layer are key factors inducing lithium dendrite growth, hindering the development of LMBs. Separator modification is an effective strategy to address the challenges of LMBs. To tackle the issues, a donor-acceptor polymer (ArMT) consisting of benzene rings and triazine was successfully synthesized and modified onto commercial polypropylene (ArMT@PP) as separators for LMBs. Benefitting from the highly lithiophilic triazine organic units, this ArMT exhibits affinity towards Li+ and simplifies the solvation structure of Li+ during the diffusion process, thus decreasing the ion diffusion activation energy, thereby accelerating the migration of Li+. Furthermore, triazine organic units with appropriate pore size regulate the plating/stripping behavior of lithium metal anodes, thereby facilitating the formation of a stable solid electrolyte interface (SEI) layer. As a result, the assembled Li|ArMT@PP|Li symmetric cells exhibit stable plating/stripping over 800 h. Moreover, the LiFePO4|ArMT@PP|Li cells achieved excellent cycling stability with 127.3 mAh g-1 after 1200 cycles at 1 C and a high capacity retention of 90.58 %. This design strategy ensures a durable and dendrite-free anode and paves the way for the development of high-energy-density LMBs.

Insight into Robust Anion Coordination Behavior of Organic Cathode with Dual Elongated π-Conjugated Motifs

Organic electrode materials offer multi-electron reactivity, flexible structures, and redox reversibility, but encounter poor conductivity and durability in electrolytes. To overcome above barriers, we propose a dual elongation strategy of π-conjugated motifs with active sites, involving the extended carbazole and electropolymerized polymer, which enhances electronic conductivity by the electronic delocalization of electron-withdrawing conjugated groups, boosts theoretical capacity by increasing redox-active site density, and endows robust electrochemical stability attributed to the nanonetwork feature of polymer structures. As a proof-of-concept, 5,11-dihydridoindolo[3,2-b]carbazole (DHIC) is selected as the model cathode material for a dual-ion battery, with elongated carbazole groups functioning both as redox-active centers and polymerization anchors. Electrochemical comparisons and theoretical simulations validate the excellent specific capacity, accelerated reaction kinetics, and enhanced anion storage stability imparted by the dual elongated π-conjugated system containing both carbazole motif and electropolymerized DHIC (pDHIC). Simultaneously, the coordination interaction between pDHIC and anions is innovatively evidenced through operando electron paramagnetic resonance spectra. As anticipated, pDHIC cathode delivers an unprecedentedly high specific capacity of 197 mAh/g at 50 mA/g, far outperforming graphite cathodes, and maintains excellent cycling stability with a capacity retention of 86.1 % over 500 cycles. This synergetic strategy sheds light on the performance revolution of organic electrode materials.

Organic molecular design for high-power density sodium-ion batteries

Organic materials, with abundant resources, low cost, high flexibility, tunable structures, lightweight nature, and wide operating temperature range, are regarded as promising candidates for sodium-ion batteries (SIBs). Unfortunately, their poor electronic and ionic conductivity remain significant challenges, hindering the achievement of high power density for sodium storage. Power density, a critical factor in battery performance evaluation, is essential for assessing fast charging capabilities. Therefore, it is essential to summarize strategies for high-power density SIBs in further development. To address these limitations and guide future development, this highlight summarizes key advancements in SIB research over the past decade. We outline the effective molecular design strategies for improving high-power-density sodium storage, with a focus on structural optimizations ranging from the backbone to the side chains. Additionally, we propose future perspectives on electrodes, electrolytes, and potential applications to enhance the power density of organic sodium-ion batteries. This review is intended to give a comprehensive guideline on the future design of organic materials for fast-charge ability and overall performance.

A metal-organic framework with mixed electron donor and electron acceptor ligands for efficient lithium-ion storage

Electron donor tetrathiafulvalene (TTF) and electron acceptor naphthalene diimide (NDI) derivatives were used to synthesize a 3D Zn-TTF/NDI-MOF. Multiple redox active sites and charge transfer endow the pristine MOF anode with excellent rate behavior and long term cycling performance (with an average specific capacity of 956 mA h g-1 at 1 A g-1 over 600 cycles). This study highlights the great potential of elaborately-designed MOFs for developing efficient anode materials.

Redox-Bipolar Covalent Organic Framework Cathode for Advanced Sodium-Organic Batteries

Redox-active covalent organic frameworks (COFs) are promising candidates for sodium-ion batteries (SIBs). However, the construction of redox-bipolar COFs with the anions and cations co-storage feature for SIBs is rarely reported. Herein, redox-bipolar COF constructed from aniline-fused quinonoid units (TPAD-COF) is developed as the cathode material in SIBs for the first time. The unique integration of conductive aniline skeletons and quinone redox centers endows TPAD-COF with high ionic/electrical conductivity, abundant redox-active sites, and fascinating bipolar features. Consequently, the elaborately tailored TPAD-COF cathode exhibits higher specific capacity (186.4 mAh g-1 at 0.05 A g-1) and superior cycling performance (over 2000 cycles at 1.0 A g-1 with 0.015% decay rate per cycle). Impressively, TPAD-COF also displays a high specific capacity of 101 mAh g-1 even at -20 °C. As a proof of concept, all-organic SIBs (AOSIBs) are assembled using TPAD-COF cathode and disodium terephthalate anode, which also show impressive electrochemical properties, indicating the potential application of TPAD-COF cathode in AOSIBs. The work will pave the avenue toward advanced COFs cathode for rechargeable batteries through rational molecular design.

Tunable Interfacial Electric Field-Mediated Cobalt-Doped FeSe/Fe3Se4 Heterostructure for High-Efficiency Potassium Storage

The interfacial electric field (IEF) in the heterostructure can accelerate electron transport and ion migration, thereby enhancing the electrochemical performance of potassium-ion batteries (PIBs). Nevertheless, the quantification and modulation of the IEF for high-efficiency PIB anodes currently remains a blank slate. Herein, we achieve for the first time the quantification and tuning of IEF via amorphous carbon-coated undifferentiated cobalt-doped FeSe/Fe3Se4 heterostructure (denoted UN-CoFe4Se5/C) for efficient potassium storage. Co doping can increase the IEF in FeSe/Fe3Se4, thereby improving the electron transport, promoting the potassium adsorption capacity, and lowering the diffusion barrier. As expected, the IEF magnitude in UN-CoFe4Se5/C is experimentally quantified as 62.84 mV, which is 3.65 times larger than that of amorphous carbon-coated FeSe/Fe3Se4 heterostructure (Fe4Se5/C). Benefiting from the strong IEF, UN-CoFe4Se5/C as a PIB anode exhibits superior rate capability (145.8 mAh g-1 at 10.0 A g-1) and long cycle lifespan (capacity retention of 95.1 % over 3000 cycles at 1.0 A g-1). Furthermore, this undifferentiated doping strategy can universally regulate the IEF magnitude in CoSe2/Co9Se8 and FeS2/Fe7S8 heterostructures. This work can provide fundamental insights into the design of advanced PIB electrodes.

Electropolymerization of Donor-Acceptor Conjugated Polymer for Efficient Dual-Ion Storage

Rationally designed organic redox-active materials have attracted numerous interests due to their excellent electrochemical performance and reasonable sustainability. However, they often suffer from poor cycling stability, intrinsic low operating potential, and poor rate performance. Herein, a novel Donor-Acceptor (D-A) bipolar polymer with n-type pyrene-4,5,9,10-tetraone unit storing Li cations and p-type carbazole unit which attracts anions and provides polymerization sites is employed as a cathode for lithium-ion batteries through in situ electropolymerization. The multiple redox reactions and boosted kinetics by the D-A structure lead to excellent electrochemical performance of a high discharge capacity of 202 mA h g-1 at 200 mA g-1, impressive working potential (2.87 and 4.15 V), an outstanding rate capability of 119 mA h g-1 at 10 A g-1 and a noteworthy energy density up to 554 Wh kg-1. This strategy has significant implications for the molecule design of bipolar organic cathode for high cycling stability and high energy density.