Homologous Strategy to Construct High-Performance Coupling Electrodes for Advanced Potassium-Ion Hybrid Capacitors

Anion-Regulated Sulfur Conversion in High-Content Carbon Layer Confined Sulfur Cathode Maximizes Voltage and Rate Capability of K-S Batteries

Potassium-sulfur (K-S) batteries have attracted attention in large-scale energy storage systems. Small-molecule/covalent sulfur (SMCS) can help to avoid the shuttle effect of polysulfide ions via solid-solid sulfur conversion. However, the content of SMCS is relatively low (≤40%), and solid-solid reactions cause sluggish kinetics and low discharge potentials. Herein, SMCS is confined in turbo carbon layers with a content of ≈74.1 wt% via a C/S co-deposition process. In the K-S battery assembled by using as-fabricated SMCS@C as cathode and KFSI-EC/DEC as an electrolyte, anion-regulated two-plateau solid-state S conversion chemistry and a novel high discharge potential plateau at 2.5-2.0 V with a remarkable reversible capacity of 384 mAh g-1 at 3 A g-1 after 1000 cycles are found. The SMCS@C||K full cell showed energy and power density of 72.8 Wh kg-1 and 873.2 W kg-1, respectively, at 3 A g-1. Mechanism studies reveal that the enlarged carbon layer space enables the diffusion of K+-FSI- ion pairs, and the coulombic attraction between them accelerates their diffusion in SMCS@C. In addition, FSI- regulates sulfur conversion in situ inside the carbon layers along a two-plateau solid-state reaction pathway, which lowers the free energy and weakens the S─S bond of intermediates, leading to faster and more efficient S conversion.

Fundamental Understanding and Optimization Strategies for Dual-Ion Batteries: A Review

There has been increasing demand for high-energy density and long-cycle life rechargeable batteries to satisfy the ever-growing requirements for next-generation energy storage systems. Among all available candidates, dual-ion batteries (DIBs) have drawn tremendous attention in the past few years from both academic and industrial battery communities because of their fascinating advantages of high working voltage, excellent safety, and environmental friendliness. However, the dynamic imbalance between the electrodes and the mismatch of traditional electrolyte systems remain elusive. To fully employ the advantages of DIBs, the overall optimization of anode materials, cathode materials, and compatible electrolyte systems is urgently needed. Here, we review the development history and the reaction mechanisms involved in DIBs. Afterward, the optimization strategies toward DIB materials and electrolytes are highlighted. In addition, their energy-related applications are also provided. Lastly, the research challenges and possible development directions of DIBs are outlined.

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.

Wrinkled Flower-Like rGO intercalated with Ni(OH)2 and MnO2 as High-Performing Supercapacitor Electrode

This study reports a simple one-step hydrothermal method for the preparation of a Ni(OH)₂ and MnO₂ intercalated rGO nanostructure as a potential supercapacitor electrode material. Having highly amorphous rGO layers with turbostratic and integrated wrinkled flower-like morphology, the as-prepared electrode material showed a high specific capacitance of 420 F g^-1 and an energy density of 14.58 Wh kg^-1 with 0.5 M Na₂SO₄ as the electrolyte in a symmetric two-electrode. With the successful intercalation of the γ-MnO₂ and α-Ni(OH)₂ in between the surface of the as-prepared rGO layers, the interlayer distance of the rGO nanosheets expanded to 0.87 nm. The synergistic effect of γ-MnO₂, α-Ni(OH)₂, and rGO exhibited the satisfying high cyclic stability with a capacitance retention of 82% even after 10 000 cycles. Thus, the as-prepared Ni(OH)₂ and MnO₂ intercalated rGO ternary hybrid is expected to contribute to the fabrication of a real-time high-performing supercapacitor device.

Homologous Nitrogen-Doped Hierarchical Carbon Architectures Enabling Compatible Anode and Cathode for Potassium-Ion Hybrid Capacitors

Potassium-ion hybrid capacitors (PIHCs) have been considered as an emerging device to render grid-scale energy storage. Nevertheless, the sluggish kinetics at the anode side and limited capacity output at the cathode side remain daunting challenges for the overall performances of PIHCs. Herein, an exquisite "homologous strategy" to devise multi-dimensional N-doped carbon nanopolyhedron@nanosheet anode and activated N-doped hierarchical carbon cathode targeting high-performance PIHCs is reported. The anode material harnessing a dual-carbon structure and the cathode candidate affording a high specific surface area (2651 m² g^-1 ) act in concert with a concentrated ether-based electrolyte, resulting in an excellent half cell performance. The related storage mechanism is systematically revealed by in situ electrokinetic characterizations. More encouragingly, the thus-derived PIHC full cell demonstrates a favorable energy output (157 Wh kg^-1 ), showing distinct advantages over the state-of-the-art PIHC counterparts.

Oxygen-Containing Functional Groups Regulating the Carbon/Electrolyte Interfacial Properties Toward Enhanced K+ Storage

Oxygen-containing functional groups were found to effectively boost the K⁺ storage performance of carbonaceous materials, however, the mechanism behind the performance enhancement remains unclear. Herein, we report higher rate capability and better long-term cycle performance employing oxygen-doped graphite oxide (GO) as the anode material for potassium ion batteries (PIBs), compared to the raw graphite. The in situ Raman spectroscopy elucidates the adsorption-intercalation hybrid K⁺ storage mechanism, assigning the capacity enhancement to be mainly correlated with reversible K⁺ adsorption/desorption at the newly introduced oxygen sites. It is unraveled that the C=O and COOH rather than C-O-C and OH groups contribute to the capacity enhancement. Based on in situ Fourier transform infrared (FT-IR) spectra and in situ electrochemical impedance spectroscopy (EIS), it is found that the oxygen-containing functional groups regulate the components of solid electrolyte interphase (SEI), leading to the formation of highly conductive, intact and robust SEI. Through the systematic investigations, we hereby uncover the K⁺ storage mechanism of GO-based PIB, and establish a clear relationship between the types/contents of oxygen functional groups and the regulated composition of SEI.

Fly Ash Carbon Anodes for Alkali Metal-Ion Batteries

Graphite has become a critical material because of its essential role in the lithium-ion battery (LIB) industry. However, the synthesis of graphite requires an energy-intensive thermal treatment. Also, when used in sodium-ion and potassium-ion batteries (SIBs and PIBs), the graphite anode shows poor capacities and cycling stability, which hinders the development of next-generation battery technologies. Finding suitable anode materials for commercial alkali metal-ion batteries is not only urgent for the energy storage industry, but is also important for economic and sustainable development. In this work, we use fly ash carbon (FAC), a residue of crude oil combustion, as an anode material for alkali metal-ion batteries. The FAC anodes show relatively high capacities and excellent cycling stability. The charge storage mechanism of FAC anode is shown to be intercalation coupled with redox reactions of oxygen functional groups. This work shows that FAC is a promising scalable anode material for alkali metal-ion batteries.