Synergetic Coupling of Redox-Active Sites on Organic Electrode Material for Robust and High-Performance Sodium-Ion Storage

Organic electrode materials (OEMs), valued for their sustainability and structural tunability, have been attracting increasing attention for wide application in sodium-ion batteries (SIBs) and other rechargeable batteries. However, most OEMs are plagued with insufficient specific capacity or poor cycling stability. Therefore, it′s imperative to enhance their specific capacity and cycling stability through molecular design. Herein, we designed and synthesized a heteroaromatic molecule 2,3,8,9,14,15-hexanol hexaazatrinaphthalene (HATN-6OH) by the synergetic coupling of catechol (the precursor of ortho-quinone)/ortho-quinone functional groups and HATN conjugated core structures. The abundance of catechol/ortho-quinone and imine redox-active moieties delivers a high specific capacity of nine-electron transfer for SIBs. Most notably, the π–π interactions and intermolecular hydrogen bond forces among HATN-6OH molecules secure the stable long-term cycling performance of SIBs. Consequently, the as-prepared HATN-6OH electrode exhibited a high specific capacity (554 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate capability (202 mAh g⁻¹ at 10 A g⁻¹), and stable long-term cycling performance (73 % after 3000 cycles at 10 A g⁻¹) in SIBs. Additionally, the nine-electron transfer mechanism is confirmed by systematic density functional theory (DFT) calculation, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman analysis. The achievement of the synergetic coupling of the redox-active sites on OEMs could be an important key to the enhancement of SIBs and other metal-ion batteries.     

A redox-active conjugated microporous polymer cathode for highperformance lithium/potassium-organic batteries

Organic redox-active materials have emerged as a class of electrode materials for rechargeable batteries due to their high redox activity,low cost,structure diversity and flexibility.However,the high solubility of organic small molecules in organic electrolytes commonly leads to the fast capacity decay with cycling.Herein,we report a redox-active conjugated microporous polymer of poly(pyrene-co-anthraquinone)(Py Aq)cathode material consisting of pyrene and anthraquinone units.Benefiting from the highly cross-linked polymer structure with insoluble nature in organic electrolytes,the high surface area and the plentiful redox-active carbonyl groups,the Py Aq cathode demonstrates outstanding electrochemical performances for both lithium-ion batteries(LIBs)and potassium-ion batteries(KIBs).Specifically,the Py Aq cathode for LIBs delivers a high reversible capacity of 169 m Ah g^-1 at the current density of 20 m A g^-1,a high rate capability(142 m Ah g^-1 at 1000 m A g^-1)and an excellent cycling stability for 4000 cycles.Additionally,the Py Aq cathode for KIBs also exhibits a high reversible capacity of143 m Ah g^-1 with a long cycling life over 800 cycles.The excellent electrochemical performance demonstrates that the newly developed Py Aq could be an attractive cathode material for the advanced energy storage technologies.     

Cyclotetrabenzil Derivatives for Electrochemical Lithium-Ion Storage

Organic electrode materials could revolutionize batteries because of their high energy densities, the use of Earth-abundant elements, and structural diversity which allows fine-tuning of electrochemical properties. However, small organic molecules and intermediates formed during their redox cycling in lithium-ion batteries (LIBs) have high solubility in organic electrolytes, leading to rapid decay of cycling performance. We report the use of three cyclotetrabenzil octaketone macrocycles as cathode materials for LIBs. The rigid and insoluble naphthalene-based cyclotetrabenzil reversibly accepts eight electrons in a two-step process with a specific capacity of 279 mAh g⁻¹ and a stable cycling performance with ≈65 % capacity retention after 135 cycles. DFT calculations indicate that its reduction increases both ring strain and ring rigidity, as demonstrated by computed high distortion energies, repulsive regions in NCI plots, and close [C⋅⋅⋅C] contacts between the naphthalenes. This work highlights the importance of shape-persistency and ring strain in the design of redox-active macrocycles that maintain very low solubility in various redox states.     

An Aqueous Conducting Redox-Polymer-Based Proton Battery that Can Withstand Rapid Constant-Voltage Charging and Sub-Zero Temperatures

Electrodes based on organic matter operating in aqueous electrolytes enable new approaches and technologies for assembling and utilizing batteries that are difficult to achieve with traditional electrode materials. Here, we report how thiophene-based trimeric structures with naphthoquinone or hydroquinone redox-active pendent groups can be processed in solution, deposited, dried and subsequently polymerized in solid state to form conductive (redox) polymer layers without any additives. Such post-deposition polymerization offers efficient use of material, high mass loading (up to 10 mg cm⁻²) and good flexibility in the choice of substrate and coating method. By employing these materials as anode and cathode in an acidic aqueous electrolyte a rocking-chair proton battery is built. The battery shows good cycling stability (85 % after 500 cycles), withstands rapid charging, with full capacity (60 mAh g⁻¹) reached within 100 seconds, allows for direct integration with photovoltaics, and retains its favorable characteristics even at −24 °C.     

A High‐Energy‐Density Potassium Battery with a Polymer‐Gel Electrolyte and a Polyaniline Cathode

A safe, rechargeable potassium battery of high energy density and excellent cycling stability has been developed. The anion component of the electrolyte salt is inserted into a polyaniline cathode upon charging and extracted from it during discharging while the K⁺ ion of the KPF₆ salt is plated/stripped on the potassium‐metal anode. The use of a p‐type polymer cathode increases the cell voltage. By replacing the organic‐liquid electrolyte in a glass‐fiber separator with a polymer‐gel electrolyte of cross‐linked poly(methyl methacrylate), a dendrite‐free potassium anode can be plated/stripped, and the electrode/electrolyte interface is stabilized. The potassium anode wets the polymer, and the cross‐linked architecture provides small pores of adjustable sizes to stabilize a solid‐electrolyte interphase formed at the anode/electrolyte interface. This alternative electrolyte/cathode strategy offers a promising new approach to low‐cost potassium batteries for the stationary storage of electric power.     

Preferential Pyrolysis Construction of Carbon Anodes with 8400 h Lifespan for High-Energy-Density K-ion Batteries

Carbonaceous materials are promising anodes for practical potassium-ion batteries, but fail to meet the requirements for durability and high capacities at low potentials. Herein, we constructed a durable carbon anode for high-energy-density K-ion full cells by a preferential pyrolysis strategy. Utilizing S and N volatilization from a π–π stacked supermolecule, the preferential pyrolysis process introduces low-potential active sites of sp² hybridized carbon and carbon vacancies, endowing a low-potential “vacancy-adsorption/intercalation” mechanism. The as-prepared carbon anode exhibits a high capacity of 384.2 mAh g⁻¹ (90 % capacity locates below 1 V vs. K/K⁺), which contributes to a high energy density of 163 Wh kg⁻¹ of K-ion full battery. Moreover, abundant vacancies of carbon alleviate volume variation, boosting the cycling stability over 14 000 cycles (8400 h). Our work provides a new synthesis approach for durable carbon anodes of K-ion full cells with high energy densities.     

Realizing Low-Temperature Graphite-based Rechargeable Potassium-Ion Full Battery

Graphite (Gr) has been considered as the most promising anode material for potassium-ion batteries (PIBs) commercialization due to its high theoretical specific capacity and low cost. However, Gr-based PIBs remain unfeasible at low temperature (LT), suffering from either poor kinetics based on conventional carbonate electrolytes or K⁺-solvent co-intercalation issue based on typical ether electrolytes. Herein, a high-performance Gr-based LT rechargeable PIB is realized for the first time by electrolyte chemistry. Applying unidentate-ether-based molecule as the solvent dramatically weakens the K⁺-solvent interactions and lowers corresponding K⁺ de-solvation kinetic barrier. Meanwhile, introduction of steric hindrance suppresses co-intercalation of K⁺-solvent into Gr, greatly elevating operating voltage and cyclability of the full battery. Consequently, the as-prepared Gr||prepotassiated 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (KPTCDA) full PIB can reversibly charge/discharge between −30 and 45 °C with a considerable energy density up to 197 Wh kg_cathode⁻¹ at −20 °C, hopefully facilitating the development of LT PIBs.     

Understanding High-Rate K+-Solvent Co-Intercalation in Natural Graphite for Potassium-Ion Batteries

Graphite shows great potential as an anode material for rechargeable metal-ion batteries because of its high abundance and low cost. However, the electrochemical performance of graphite anode materials for rechargeable potassium-ion batteries needs to be further improved. Reported herein is a natural graphite with superior rate performance and cycling stability obtained through a unique K⁺-solvent co-intercalation mechanism in a 1 m KCF₃SO₃ diethylene glycol dimethyl ether electrolyte. The co-intercalation mechanism was demonstrated by ex situ Fourier transform infrared spectroscopy and in situ X-ray diffraction. Moreover, the structure of the [K-solvent]⁺ complexes intercalated with the graphite and the conditions for reversible K⁺-solvent co-intercalation into graphite are proposed based on the experimental results and first-principles calculations. This work provides important insights into the design of natural graphite for high-performance rechargeable potassium-ion batteries.     

In Situ Electropolymerization Enables Ultrafast Long Cycle Life and High-Voltage Organic Cathodes for Lithium Batteries

Organic cathode materials have attracted extensive attention because of their diverse structures, facile synthesis, and environmental friendliness. However, they often suffer from insufficient cycling stability caused by the dissolution problem, poor rate performance, and low voltages. An in situ electropolymerization method was developed to stabilize and enhance organic cathodes for lithium batteries. 4,4′,4′′-Tris(carbazol-9-yl)-triphenylamine (TCTA) was employed because carbazole groups can be polymerized under an electric field and they may serve as high-voltage redox-active centers. The electropolymerized TCTA electrodes demonstrated excellent electrochemical performance with a high discharge voltage of 3.95 V, ultrafast rate capability of 20 A g⁻¹, and a long cycle life of 5000 cycles. Our findings provide a new strategy to address the dissolution issue and they explore the molecular design of organic electrode materials for use in rechargeable batteries.     

Lithium Ferrocyanide Catholyte for High-Energy and Low-cost Aqueous Redox Flow Batteries**

Aqueous redox flow batteries (ARFBs) are a promising technology for grid-scale energy storage, however, their commercial success relies on redox-active materials (RAM) with high electron storage capacity and cost competitiveness. Herein, a redox-active material lithium ferrocyanide (Li₄[Fe(CN)₆]) is designed. Li⁺ ions not only greatly boost the solubility of [Fe(CN)₆]⁴⁻ to 2.32 M at room temperature due to weak intermolecular interactions, but also improves the electrochemical performance of [Fe(CN)₆]^4−/3−. By coupling with Zn, ZIRFBs were built, and the capacity of the batteries was as high as 61.64 Ah L⁻¹ (pH-neutral) and 56.28 Ah L⁻¹ (alkaline) at a [Fe(CN)₆]⁴⁻ concentration of 2.30 M and 2.10 M. These represent unprecedentedly high [Fe(CN)₆]⁴⁻ concentrations and battery energy densities reported to date. Moreover, benefiting from the low cost of Li₄[Fe(CN)₆], the overall chemical cost of alkaline ZIRFB is as low as $11 per kWh, which is one-twentieth that of the state-of-the-art VFB ($211.54 per kWh). This work breaks through the limitations of traditional electrolyte composition optimization and will strongly promote the development of economical [Fe(CN)₆]^4−/3−-based RFBs in the future.     

Electrochemical Assessment of Indigo Carmine Dye in Lithium Metal Polymer Technology

Lithium metal batteries are inspiring renewed interest in the battery community because the most advanced designs of Li-ion batteries could be on the verge of reaching their theoretical specific energy density values. Among the investigated alternative technologies for electrochemical storage, the all-solid-state Li battery concept based on the implementation of dry solid polymer electrolytes appears as a mature technology not only to power full electric vehicles but also to provide solutions for stationary storage applications. With an effective marketing started in 2011, BlueSolutions keeps developing further the so-called lithium metal polymer batteries based on this technology. The present study reports the electrochemical performance of such Li metal batteries involving indigo carmine, a cheap and renewable electroactive non-soluble organic salt, at the positive electrode. Our results demonstrate that this active material was able to reversibly insert two Li at an average potential of ≈2.4 V vs. Li⁺/Li with however, a relatively poor stability upon cycling. Post-mortem analyses revealed the poisoning of the Li electrode by Na upon ion exchange reaction between the Na countercations of indigo carmine and the conducting salt. The use of thinner positive electrodes led to much better capacity retention while enabling the identification of two successive one-electron plateaus.     

Understanding High‐Rate K+‐Solvent Co‐Intercalation in Natural Graphite for Potassium‐Ion Batteries

Graphite shows great potential as an anode material for rechargeable metal‐ion batteries because of its high abundance and low cost. However, the electrochemical performance of graphite anode materials for rechargeable potassium‐ion batteries needs to be further improved. Reported herein is a natural graphite with superior rate performance and cycling stability obtained through a unique K⁺‐solvent co‐intercalation mechanism in a 1 m KCF₃SO₃ diethylene glycol dimethyl ether electrolyte. The co‐intercalation mechanism was demonstrated by ex situ Fourier transform infrared spectroscopy and in situ X‐ray diffraction. Moreover, the structure of the [K‐solvent]⁺ complexes intercalated with the graphite and the conditions for reversible K⁺‐solvent co‐intercalation into graphite are proposed based on the experimental results and first‐principles calculations. This work provides important insights into the design of natural graphite for high‐performance rechargeable potassium‐ion batteries.     

Progress of Organic Electrodes in Aqueous Electrolyte for Energy Storage and Conversion

Aqueous batteries using inorganic compounds as electrode materials are considered a promising solution for grid-scale energy storage, while wide application is limited by the short life and/or high cost of electrodes. Organics with carbonyl groups are being investigated as the alternative to inorganic electrode materials because they offer the advantages of tunable structures, renewability, and they are environmentally benign. Furthermore, the wide internal space of such organic materials enables flexible storage of various charged ions (for example, H⁺, Li⁺, Na⁺, K⁺, Zn²⁺, Mg²⁺, and Ca²⁺, and so on). We offer a comprehensive overview of the progress of organics containing carbonyls for energy storage and conversion in aqueous electrolytes, including applications in aqueous batteries as solid-state electrodes, in flow batteries as soluble redox species, and in water electrolysis as redox buffer electrodes. The advantages of organic electrodes are summarized, with a discussion of the challenges remaining for their practical application.     

Polyolefin-Based Separator with Interfacial Chemistry Regulation for Robust Potassium Metal Batteries

Potassium metal batteries (KMBs) are ideal choices for high energy density storage system owing to the low electrochemical potential and low cost of K. However, the practical KMB applications suffer from intrinsically active K anode, which would bring serious safety concerns due to easier generation of dendrites. Herein, to explore a facile approach to tackle this issue, we propose to regulate K plating/stripping via interfacial chemistry engineering of commercial polyolefin-based separator using multiple functional units integrated in tailored metal organic framework. As a case study, the functional units of MIL-101(Cr) offer high elastic modulus, facilitate the dissociation of potassium salt, improve the K⁺ transfer number and homogenize the K⁺ flux at the electrode/electrolyte interface. Benefiting from these favorable features, uniform and stable K plating/stripping is realized with the regulated separator. Full battery assembled with the regulated separator showed ∼19.9 % higher discharge capacity than that with glass fiber separator at 20 mA g⁻¹ and much better cycling stability at high rates. The generality of our approach is validated with KMBs using different cathodes and electrolytes. We envision that the strategy to suppress dendrite formation by commercial separator surface engineering using tailor-designed functional units can be extended to other metal/metal ion batteries.     

Revealing the Dominance of the Dissolution-Deposition Mechanism in Aqueous Zn−MnO2 Batteries

Zn−MnO₂ batteries have attracted extensive attention for grid-scale energy storage applications, however, the energy storage chemistry of MnO₂ in mild acidic aqueous electrolytes remains elusive and controversial. Using α-MnO₂ as a case study, we developed a methodology by coupling conventional coin batteries with customized beaker batteries to pinpoint the operating mechanism of Zn−MnO₂ batteries. This approach visually simulates the operating state of batteries in different scenarios and allows for a comprehensive study of the operating mechanism of aqueous Zn−MnO₂ batteries under mild acidic conditions. It is validated that the electrochemical performance can be modulated by controlling the addition of Mn²⁺ to the electrolyte. The method is utilized to systematically eliminate the possibility of Zn²⁺ and/or H⁺ intercalation/de-intercalation reactions, thereby confirming the dominance of the MnO₂/Mn²⁺ dissolution-deposition mechanism. By combining a series of phase and spectroscopic characterizations, the compositional, morphological and structural evolution of electrodes and electrolytes during battery cycling is probed, elucidating the intrinsic battery chemistry of MnO₂ in mild acid electrolytes. Such a methodology developed can be extended to other energy storage systems, providing a universal approach to accurately identify the reaction mechanism of aqueous aluminum-ion batteries as well.     

Electrolyte Chemistry Enables Simultaneous Stabilization of Potassium Metal and Alloying Anode for Potassium‐Ion Batteries

Alloying anodes are promising high‐capacity electrode materials for K‐ion batteries (KIBs). However, KIBs based on alloying anodes suffer from rapid capacity decay due to the instability of K metal and large volume expansion of alloying anodes. Herein, the effects of salts and solvents on the cycling stability of KIBs based on a typical alloying anode such as amorphous red phosphorus (RP) are investigated, and the potassium bis(fluorosulfonyl)imide (KFSI) salt‐based carbonate electrolyte is versatile to achieve simultaneous stabilization of K metal and RP electrodes for highly stable KIBs. This salt‐solvent complex with a moderate solvation energy can alleviate side reactions between K metal and the electrolyte and facilitate K⁺ ion diffusion/desolvation. Moreover, robust SEI layers that form on K metal and RP electrodes can suppress K dendrite growth and resist RP volume change. This strategy of electrolyte regulation can be applicable to other alloying anodes for high‐performance KIBs.     

Combining Deep Eutectic Solvents with TEMPO-based Polymer Electrodes: Influence of Molar Ratio on Electrode Performance**

For sustainable energy storage, all-organic batteries based on redox-active polymers promise to become an alternative to lithium ion batteries. Yet, polymers contribute to the goal of an all-organic cell as electrodes or as solid electrolytes. Here, we replace the electrolyte with a deep eutectic solvent (DES) composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and N-methylacetamide (NMA), while using poly(2,2,6,6-tetramethylpiperidin-1-yl-oxyl methacrylate) (PTMA) as cathode. The successful combination of a DES with a polymer electrode is reported here for the first time. The electrochemical stability of PTMA electrodes in the DES at the eutectic molar ratio of 1 : 6 is comparable to conventional battery electrolytes. More viscous electrolytes with higher salt concentration can hinder cycling at high rates. Lower salt concentration leads to decreasing capacities and faster decomposition. The eutectic mixture of 1 : 6 is best suited uniting high stability and moderate viscosity.     

Anode-Free Lithium Metal Batteries Based on an Ultrathin and Respirable Interphase Layer

Anode-free lithium (Li) metal batteries are desirable candidates in pursuit of high-energy-density batteries. However, their poor cycling performances originated from the unsatisfactory reversibility of Li plating/stripping remains a grand challenge. Here we show a facile and scalable approach to produce high-performing anode-free Li metal batteries using a bioinspired and ultrathin (250 nm) interphase layer comprised of triethylamine germanate. The derived tertiary amine and Li_xGe alloy showed enhanced adsorption energy that significantly promoted Li-ion adsorption, nucleation and deposition, contributing to a reversible expansion/shrinkage process upon Li plating/stripping. Impressive Li plating/stripping Coulombic efficiencies (CEs) of ≈99.3 % were achieved for 250 cycles in Li/Cu cells. In addition, the anode-free LiFePO₄ full batteries demonstrated maximal energy and power densities of 527 Wh kg⁻¹ and 1554 W kg⁻¹, respectively, and remarkable cycling stability (over 250 cycles with an average CE of 99.4 %) at a practical areal capacity of ≈3 mAh cm⁻², the highest among state-of-the-art anode-free LiFePO₄ batteries. Our ultrathin and respirable interphase layer presents a promising way to fully unlock large-scale production of anode-free batteries.     

Armor-like Inorganic-rich Cathode Electrolyte Interphase Enabled by the Pentafluorophenylboronic Acid Additive for High-voltage Li||NCM622 Batteries

Lithium metal batteries (LMBs) comprising Li metal anode and high-voltage nickel-rich cathode could potentially realize high capacity and power density. However, suitable electrolytes to tolerate the oxidation on the cathode at high cut-off voltage are urgently needed. Herein, we present an armor-like inorganic-rich cathode electrolyte interphase (CEI) strategy for exploring oxidation-resistant electrolytes for sustaining 4.8 V Li||LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) batteries with pentafluorophenylboronic acid (PFPBA) as the additive. In such CEI, the armored lithium borate surrounded by CEI up-layer represses the dissolution of inner CEI moieties and also improves the Li⁺ conductivity of CEI while abundant LiF is distributed over whole CEI to enhance the mechanical stability and Li⁺ conductivity compared with polymer moieties. With such robust Li⁺ conductive CEI, the Li||NCM622 battery delivered excellent stability at 4.6 V cut-off voltage with 91.2 % capacity retention after 400 cycles. The excellent cycling performance was also obtained even at 4.8 V cut-off voltage.     

A bi-component polyoxometalate-derivative cathode material showed impressive electrochemical performance for the aqueous zinc-ion batteries

Rechargeable aqueous zinc-ion batteries are recently gaining incremental attention because of low cost and material abundance, but their development is plagued by limited choices of cathode materials with satisfactory cycling performance. The polyoxometalates perform formidable redox stability and able to participate in multi-electron transfer, which was well-suited for energy storage. Herein, a bi-component polyoxometalate-derivative KNiVO (K₂[Ni(H₂O)₆]₂[V₁₀O₂₈]·4H₂O polyoxometalates after annealing) is firstly demonstrated as a cathode material for aqueous ZIBs. The layered KV₃O₈ (KVO) In the bi-component material constitutes Zn²⁺ migration and storage channels (K⁺ were substantially replaced by Zn²⁺ in the activation phase), and the three-dimensional NiV₃O₈ (NiVO) part acts as skeleton to stabilize the ion channels, which assist the cell to demonstrate a high-rate capacity and specific energy of 229.4 mAh/g and satisfactory cyclability (capacity retention of 99.1% after 4500 cycles at a current density of 4 A/g). These results prove the feasibility of POM as cathode materials precursor and put forward a novel pattern of the Zn²⁺ storage mechanism in the activated-KNiVO clusters, which also provide a new route for selecting or designing high-performance cathode for aqueous ZIBs and other advanced battery systems.