Phenoxazine Polymer-based p-type Positive Electrode for Aluminum-ion Batteries with Ultra-long Cycle Life

d-Orbital Induced Electronic Structure Reconfiguration toward Manipulating Electron Transfer Pathways of Metallo-Porphyrin for Enhanced AlCl2 + Storage

The positive electrodes of non-aqueous aluminum ion batteries (AIBs) frequently encounter significant issues, for instance, low capacity in graphite (mechanism: anion de/intercalation and large electrode deformation induced) and poor stability in inorganic positive electrodes (mechanism: multi-electron redox reaction and dissolution of active materials induced). Here, metallo-porphyrin compounds (employed Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ as the ion centers) are introduced to effectively enhance both the cycling stability and reversible capacity due to the formation of stable conjugated metal-organic coordination and presence of axially coordinated active sites, respectively. With the regulation of electronic energy levels, the d-orbitals in the redox reactions and electron transfer pathways can be rearranged. The 5,10,15,20-tetraphenyl-21H,23H-porphine nickle(II) (NiTPP) presents the highest specific capacity (177.1 mAh g-1), with an increment of 32.1% and 77.1% in comparison with the capacities of H2TPP and graphite, respectively, which offers a new route for developing high-capacity positive electrodes for stable AIBs.

Regulating the Double-Way Traffic of Cations and Anions in Ambipolar Polymer Cathodes for High-Performing Aluminum Dual-Ion Batteries

The strong Coulombic interactions between Al3+ and traditional inorganic crystalline cathodes present a significant obstacle in developing high-performance rechargeable aluminum batteries (RABs) that hold promise for safe and sustainable stationary energy storage. While accommodating chloroaluminate ions (AlCl4 -, AlCl2+, etc.) in redox-active organic compounds offers a promising solution for RABs, the issues of dissolution and low ionic/electronic conductivities plague the development of organic cathodes. Herein, electron donors are synthetically connected with acceptors to create crosslinked, bipolar-conjugated polymer cathodes. These cathodes exhibit overlapped redox potential ranges for both donors and acceptors in highly concentrated AlCl3-based ionic liquid electrolytes. This approach strategically enables on-site doping of the polymer backbones during redox reactions involving both donor and acceptor units, thereby enhancing the electron/ion transfer kinetics within the resultant polymer cathodes. Based on the optimal donor/acceptor combination, the bipolar polymer cathodes can deliver a high specific capacity of 205 mAh g-1 by leveraging the co-storage of AlCl4 - and AlCl2+. The electrodes exhibit excellent rate performance, a stable cycle life of 60 000 cycles, and function efficiently at high mass loadings, i.e., 100 mg cm-2, and at low temperatures, i.e., -30 °C. The findings exemplify the exploration of high-performing conjugated polymer cathodes for RABs through rational structural design.

Intermolecular Hydrogen Bonding Networks Stabilized Organic Supramolecular Cathode for Ultra-High Capacity and Ultra-Long Cycle Life Rechargeable Aluminum Batteries

Rechargeable aluminum batteries (RABs) are a promising candidate for large-scale energy storage, attributing to the abundant reserves, low cost, intrinsic safety, and high theoretical capacity of Al. However, the cathode materials reported thus far still face challenges such as limited capacity, sluggish kinetics, and undesirable cycle life. Herein, we propose an organic cathode benzo[i] benzo[6,7] quinoxalino [2,3-a] benzo [6,7] quinoxalino [2,3-c] phenazine-5,8,13,16,21,24-hexaone (BQQPH) for RABs. The six C=O and six C=N redox active sites in each molecule enable BQQPH to deliver a record ultra-high capacity of 413 mAh g-1 at 0.2 A g-1. Encouragingly, the intermolecular hydrogen bonding network and π-π stacking interactions endow BQQPH with robust structural stability and minimal solubility, enabling an ultra-long lifetime of 100,000 cycles. Moreover, the electron-withdrawing carbonyl group induces a reduction in the energy level of the lowest unoccupied molecular orbital and expands the π-conjugated system, which considerably enhances both the discharge voltage and redox kinetics of BQQPH. In situ and ex situ characterizations combined with theoretical calculations unveil that the charge storage mechanism is reversible coordination/dissociation of AlCl2 + with the N and O sites in BQQPH accompanied by 12-electron transfer. This work provides valuable insights into the design of high-performance organic cathode materials for RABs.

Crown Ether Electrolyte Additive Enables High-Rate and Stable Polyviologen Cathode Material for Chloride Ion Batteries

A variety of inorganic and inorganic cathode materials for chloride ion storage are reported. However, their application in chloride ion batteries (CIB) is hindered by poor rate capability and cycling stability. Herein, an organic poly(butyl viologen dichloride) (PBVCl2) cathode material with significantly enhanced rate and cycling performance in the CIB is achieved using a crown ether (18-Crown-6) additive in the tributylmethylammonium chloride-based electrolyte. The as-prepared PBVCl2 cathodes exhibit impressive capacity increases from 149.4 to 179.1 mAh g-1 at 0.1 C and from 57.8 to 111.9 mAh g-1 at 10 C, demonstrating the best rate performance with the highest energy density among those of various reported cathodes for CIBs. This impressive performance improvement is a result of the great boosts in charge transfer, ion transport, and interface stability of the battery by the use of 18-Crown-6, which also contributes to a more than twofold increase in capacity retention after 120 cycles. The electrode reaction mechanism of the CIB based on highly reversible chloride ion transfer is revealed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.

Electropolymerized Bipolar Poly(2,3-diaminophenazine) Cathode for High-Performance Aqueous Al-Ion Batteries with An Extended Temperature Range of -20 to 45 °C

Achieving reversible insertion/extraction in most cathodes for aqueous aluminum ion batteries (AAIBs) is a significant challenge due to the high charge density of Al3+ and strong electrostatic interactions. Organic materials facilitate the hosting of multivalent carriers and rapid ions diffusion through the rearrangement of chemical bonds. Here, a bipolar conjugated poly(2,3-diaminophenazine) (PDAP) on carbon substrates prepared via a straightforward electropolymerization method is introduced as cathode for AAIBs. The integration of n-type and p-type active units endow PDAP with an increased number of sites for ions interaction. The long-range conjugated skeleton enhances electron delocalization and collaborates with carbon to ensure high conductivity. Moreover, the strong intermolecular interactions including π-π interaction and hydrogen bonding significantly enhance its stability. Consequently, the Al//PDAP battery exhibits a large capacity of 338 mAh g-1 with long lifespan and high-rate capability. It consistently demonstrates exceptional electrochemical performances even under extreme conditions with capacities of 155 and 348 mAh g-1 at -20 and 45 °C, respectively. In/ex situ spectroscopy comprehensively elucidates its cation/anion (Al3+/H3O+ and ClO4 -) storage with 3-electron transfer in dual electroactive centers (C═N and -NH-). This study presents a promising strategy for constructing high-performance organic cathode for AAIBs over a wide temperature range.

Poly(3-Methylthiophene)/Graphene Composite Cathode for Rechargeable Aluminum-Ion Batteries

To reduce the dependence on traditional fossil energy, developing efficient energy storage systems is urgent. The reserves of aluminum resources in the earth's crust are extremely rich, which makes aluminum-ion batteries a promising competitor of new energy storage devices. Here, we report a poly(3-methylthiophene)/graphene (P3TH/Graphene) composite as the cathode of an aluminum-ion battery. The adjustment of polymer chain spacing by the methyl side chain provides a channel conducive to the transport of large-size AlCl4- complexes. The addition of electron donor groups also changes the electron delocalization characteristics of polymers and improves the specific capacity of the material. At the same time, the in situ composite of graphene can enhance the Π-Π interaction to form a favorable electronic transmission channel. At a current density of 200 mA g-1, the P3TH/Graphene composite showed a specific capacity of ∼150 mA g-1. The flexible structure of the polymer also guarantees the excellent rate capability of the composite.

The Reverse of Electrostatic Interaction Force for Ultrahigh-Energy Al-Ion batteries

The two-dimensional (2D) MXenes with sufficient interlayer spacing are promising cathode materials for aluminum-ion batteries (AIBs), yet the electrostatic repulsion effect between the surface negative charges and the active anions (AlCl4 - ) hinders the intercalation of AlCl4 - and is usually ignored. Here, we propose a charge regulation strategy for MXene cathodes to overcome this challenge. By doping N and Co, the zeta potential is gradually transformed from negative (Ti3 C2 Tx ) to near-neutral (Ti3 CNTx ), and finally positive (Ti3 CNTx @Co). Therefore, the electrostatic repulsion force can be greatly weakened between Ti3 CNTx and AlCl4 - , or even formed a strong electrostatic attraction between Ti3 CNTx @Co and AlCl4 - , which can not only accommodate more AlCl4 - ions in the Ti3 CNTx @Co interlayers to increase the capacity, but also solve the stacking and expansion problems. As a result, the optimized Al-MXene battery exhibits an ultrahigh capacity of up to 240 mAh g-1 (2-4 times the capacity of graphite cathode, 60-120 mAh g-1 ) and a potential ultrahigh energy density (432 Wh kg-1 , 2-4 times the value of graphite, 110-220 Wh kg-1 ) based on the mass of cathode materials, comparable to LiFePO4 -based lithium-ion batteries (350-450 Wh kg-1 , based on the mass of LiFePO4 ).

Insight into the Storage Mechanism of Sandwich-Like Molybdenum Disulphide/Carbon Nanofibers Composite in Aluminum-Ion Batteries

Aluminum-ion batteries (AIBs) have become a research hotspot in the field of energy storage due to their high energy density, safety, environmental friendliness, and low cost. However, the actual capacity of AIBs is much lower than the theoretical specific capacity, and their cycling stability is poor. The exploration of energy storage mechanisms may help in the design of stable electrode materials, thereby contributing to improving performance. In this work, molybdenum disulfide (MoS2) was selected as the host material for AIBs, and carbon nanofibers (CNFs) were used as the substrate to prepare a molybdenum disulfide/carbon nanofibers (MoS2/CNFs) electrode, exhibiting a residual reversible capacity of 53 mAh g-1 at 100 mA g-1 after 260 cycles. The energy storage mechanism was understood through a combination of electrochemical characterization and first-principles calculations. The purpose of this study is to investigate the diffusion behavior of ions in different channels in the host material and its potential energy storage mechanism. The computational analysis and experimental results indicate that the electrochemical behavior of the battery is determined by the ion transport mechanism between MoS2 layers. The insertion of ions leads to lattice distortion in the host material, significantly impacting its initial stability. CNFs, serving as a support material, not only reduce the agglomeration of MoS2 grown on its surface, but also effectively alleviate the volume expansion caused by the host material during charging and discharging cycles.

Configurational Entropy Strategy Enhanced Structure Stability Achieves Robust Cathode for Aluminum Batteries

Rechargeable aluminum batteries (RABs) are an emerging energy storage device owing to the vast Al resources, low cost, and high safety. However, the poor cyclability and inferior reversible capacity of cathode materials have limited the enhancement of RABs performance. Herein, a high configurational entropy strategy is presented to improve the electrochemical properties of RABs for the first time. The high-entropy (Fe, Mn, Ni, Zn, Mg)3 O4 cathode exhibits an ultra-stable cycling ability (109 mAh g-1 after 3000 cycles), high specific capacity (268 mAh g-1 at 0.5 A g-1 ), and rapid ion diffusion. Ex situ characterizations indicate that the operational mechanism of (Fe, Mn, Ni, Zn, Mg)3 O4 cathode is mainly based on the redox process of Fe, Mn, and Ni. Theoretical calculations demonstrate that the oxygen vacancies make a positive contribution to adjusting the distribution of electronic states, which is crucial for enhancing the reaction kinetics at the electrolyte and cathode interface. These findings not only propose a promising cathode material for RABs, but also provide the first elucidation of the operational mechanism and intrinsic information of high-entropy electrodes in multivalent ion batteries.

Intramolecular Hydrogen Bond Improved Durability and Kinetics for Zinc-Organic Batteries

Organic compounds have the advantages of green sustainability and high designability, but their high solubility leads to poor durability of zinc-organic batteries. Herein, a high-performance quinone-based polymer (H-PNADBQ) material is designed by introducing an intramolecular hydrogen bonding (HB) strategy. The intramolecular HB (C=O⋯N-H) is formed in the reaction of 1,4-benzoquinone and 1,5-naphthalene diamine, which efficiently reduces the H-PNADBQ solubility and enhances its charge transfer in theory. In situ ultraviolet-visible analysis further reveals the insolubility of H-PNADBQ during the electrochemical cycles, enabling high durability at different current densities. Specifically, the H-PNADBQ electrode with high loading (10 mg cm-2) performs a long cycling life at 125 mA g-1 (> 290 cycles). The H-PNADBQ also shows high rate capability (137.1 mAh g-1 at 25 A g-1) due to significantly improved kinetics inducted by intramolecular HB. This work provides an efficient approach toward insoluble organic electrode materials.