Sulfur-Doped Carbon for Potassium-Ion Battery Anode: Insight into the Doping and Potassium Storage Mechanism of Sulfur

Recent advances in rational design for high-performance potassium-ion batteries

The growing global energy demand necessitates the development of renewable energy solutions to mitigate greenhouse gas emissions and air pollution. To efficiently utilize renewable yet intermittent energy sources such as solar and wind power, there is a critical need for large-scale energy storage systems (EES) with high electrochemical performance. While lithium-ion batteries (LIBs) have been successfully used for EES, the surging demand and price, coupled with limited supply of crucial metals like lithium and cobalt, raised concerns about future sustainability. In this context, potassium-ion batteries (PIBs) have emerged as promising alternatives to commercial LIBs. Leveraging the low cost of potassium resources, abundant natural reserves, and the similar chemical properties of lithium and potassium, PIBs exhibit excellent potassium ion transport kinetics in electrolytes. This review starts from the fundamental principles and structural regulation of PIBs, offering a comprehensive overview of their current research status. It covers cathode materials, anode materials, electrolytes, binders, and separators, combining insights from full battery performance, degradation mechanisms, in situ/ex situ characterization, and theoretical calculations. We anticipate that this review will inspire greater interest in the development of high-efficiency PIBs and pave the way for their future commercial applications.

Strategies for developing layered oxide cathodes, carbon-based anodes, and electrolytes for potassium ion batteries

Lithium-ion batteries (LIBs) have become the most popular portable secondary energy storage facilities. However, the limited lithium resource results in possible unsustainable development. Potassium-ion batteries (PIBs) are considered promising alternatives to LIBs because of their high resource availability, low cost, and environmentally friendly features. In this field, high energy density layered cathodes and carbon-based anodes are also the main research objectives. However, compared to the most appealing alternative sodium-ion batteries (SIBs), despite having various theoretical advantages, PIBs exhibit poorer electrochemical performance in practice. Their poor capacity retention and narrow working voltage range seriously limit their applications. The performance of the electrodes is usually considered an important factor for battery performance, life, and safety. To solve these problems, many significant research studies have been carried out in the last decade, achieving numerous breakthroughs. Nevertheless, there are still many drawbacks and unclear mechanisms. In this comprehensive review, we examine the current state of high-performance layered oxide cathodes, electrolytes, and carbon-based anodes, to identify potential candidates for PIBs. Our focus lies on their structural characteristics, interface properties, underlying mechanisms, and modification techniques. The viewpoints of these advanced strategies are integrated, and concise development suggestions and strategies are subsequently proposed.

Biomass-derived carbon-sulfur hybrids boosting electrochemical kinetics to achieve high potassium storage performance

Potassium-ion batteries (PIBs) as an emerging battery technology have garnered significant research interest. However, the development of high-performance PIBs critically hinges on reliable anode materials with comprehensive electrochemical performance and low cost. Herein, low-cost N-doped biomass-derived carbon-sulfur hybrids (NBCSHs) were prepared through a simple co-carbonization of the mixture of a biomass precursor (coffee grounds) and sulfur powder. The sulfur in NBCSHs predominantly exists in the form of single-atomic sulfur bonded with carbon atoms (CSC), functioning as main active redox sites to achieve high reversible capacity. Electrochemical evaluations reveal that the NBCSH 1-3 with moderate sulfur content shows significantly improved potassium storage performance, such as a high reversible capacity of 484.7 mAh g-1 and rate performance of 119.4 mAh g-1 at 5 A g-1, 4.5 and 14.7 times higher than that of S-free biomass-derived carbon, respectively. Furthermore, NBCSH 1-3 exhibits stable cyclability (no obvious capacity fading even after 1000 cycles at 0.5 A g-1) and excellent electrochemical kinetics (low overpotentials and apparent diffusion coefficients). The improved performance of NBCSHs is primarily attributed to pseudocapacitance-dominated behavior with fast charge transfer capability. Density functional theory calculations also reveal that co-doping with S, N favors for achieving a stronger potassium adsorbing capability. Assemble K-ion capacitors with NBCS 1-3 as anodes demonstrate stable cyclability and commendable rate performance. Our research envisions the potential of NBCSHs as efficient and sustainable materials for advanced potassium-ion energy storage systems.

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.

An oriented tube array porous carbon anode prepared using a self-blowing mold of salt templates for high-rate potassium storage

Porous carbon materials with oriented porosity are very useful in ion batteries, but their high cost and complex fabrication hinder their wide application. In this paper, we used cheap and water-soluble NaHCO3 grains to prepare unique porous carbon with an orderly arranged tube array via one-step carbonization. During the preparation process, a novel self-blowing mold of salt templates was discovered for the first time, and the resulting numerous high-speed gas jets can act as gas state templates to induce the formation of the oriented porous carbon into a mesoscale tube array with rich micropores. Besides, the amount of CO functional groups has been enhanced greatly by the chemical activation of H2O and CO2 derived from the decomposition of NaHCO3, which can improve the reversible specific capacity of the electrode by forming a C-O-K compound with potassium. Thanks to the coupling effect of the hierarchical porous structure with an orderly tube array and rich CO functional groups, the obtained porous carbon materials exhibited excellent kinetics and impressive rate capability as the anode of potassium-ion batteries (PIBs) with high capacities of 209 mA h g-1 at 10 A g-1 and 156 mA h g-1 at 30 A g-1. This work not only provides a facile, green, sustainable approach to fabricating novel carbon materials, but also demonstrates the promising prospect of oriented porous carbon in exploring advanced electrode materials for PIBs.

Enhanced As-COF nanochannels as a high-capacity anode for K and Ca-ion batteries

Covalent organic frameworks can be used for next-generation rechargeable metal-ion batteries due to their controllable spatial and chemical architectures and plentiful elemental reserves. In this study, the arsenic-based covalent organic framework (As-COF) is designed by employing the geometrical symmetry of a semiconducting phosphazene-based covalent organic framework that uses p-phenylenediamine as a linker and hexachorocyclotriphosphazene as an As-containing monomer in a C3-like spatial configuration. The As-COF with engineered nanochannels demonstrates exceptional anodic behavior for potassium (K) and calcium (Ca) ion batteries. It exhibits a high storage capacity of about 914(2039) mA h g-1, low diffusion barriers of 0.12(0.26) eV, low open circuit voltage of 0.23(0.18) V, and a minimal volume expansion of 2.41(2.32)% for K (Ca) ions. These attributes collectively suggest that As-COF could significantly advance high-capacity rechargeable batteries.

High-Yield Synthesis of Colloidal Carbon Rings and Their Applications in Self-Standing Electrodes of Li-O2 Batteries

The use of carbon materials in porous electrodes has impressive advantages. However, precisely tailoring the multilevel pore structure of carbon electrodes remains an unsolved challenge. Here, we report a highly efficient site-selective growth strategy to synthesize colloidal carbon rings by templating patchy droplets. Carbon rings are used for the direct fabrication of self-standing porous electrodes with hierarchical pores for lithium-oxygen batteries (LOBs). In situ atomic force microscopy reveals that during discharge the discharge products densely nucleate and grow on carbon rings, demonstrating that such rings are a very potential electrode material in LOBs. The hollow carbon ring electrode (HCRE) possesses micrometer-scale channels formed by random packing of rings and nanochannels consisting of ring-shaped hollow cavities connected by nanosized pores in the wall. Both channels contribute to ion transportation and gas diffusion, but the storage of the discharge products mainly lies in micrometer-scale channels, leading to a high discharge capacity of LOBs (20 658 mAh/g). Our work paves a new way to construct hierarchically porous electrodes for application in electrocatalysis and electrochemical energy storage.

High S-doped amorphous carbon/carbon quantum dots coupled micro-frame for highly efficient potassium storage

Anode materials with excellent rate capability, capacity, and cycle life have been a challenge in obtaining cost-effective K-ion batteries (KIBs). Based on the concept of waste recycling, we prepared the S-doped (21.5%) amorphous carbon/carbon quantum dots coupled micro-frame (SCMF) by combining chemical exfoliation and S/Se-assisted carbonization. SCMF exhibited the advantages of integrating amorphous carbon and carbon quantum dots (CQDs). The CQDs serve as fast electron channels, while amorphous carbon can accommodate more large-size K-ions and mitigate volume expansion. In KIBs, SCMF maintained a high reversible capacity (414.0 mAh g-1, after 100 cycles at 100 mA g-1), a good rate capability (224.0 mAh g-1, 2000 mA g-1), and excellent capacity retention (208.9 mAh g-1, after 2000 cycles at 1000 mA g-1). The molecular dynamic simulation revealed that CQDs provided fast electron transport channels and that C, O and S atoms had suitable interactions with K, facilitating potassium storage. Moreover, the potassium-ion capacitor (PIC) assembled from SCMF and activated carbon exhibited stable electrochemical performance, proving its potential for application. The research provided valuable insights into the reuse of biomass waste in new secondary batteries.

Oxygen Vacancy-Tailored Schottky Heterojunction Activates Interface Dipole Amplification and Carrier Inversion for High-Performance Potassium-Ion Batteries

An oxygen vacancy-tailored Schottky heterostructure composed of polyvinylpyrrolidone-assisted Bi2 Sn2 O7 (PVPBSO) nanocrystals and moderate work function graphene (mWFG, WF = 4.36 eV) is designed, which in turn intensifies the built-in voltage and interface dipole across the space charge region (SCR), leading to the inversion of majority carriers for facilitating K+ transport/diffusion behaviors. Thorough band-alignment experiments and interface simulations reveal the dynamics between deficient BSO and mWFG, and how charge redistribution within the SCR leads to carrier inversion, demonstrating the impact of different defect engineering degrees on the amplification of Schottky junctions. The ordered transport of bipolar carriers can boost electrons and K ions easily passing through the inner and outer surfaces of the heterostructure. With high activity and low resistance in electrochemical reactions, the PVPBSO/mWFG exhibits an attractive capacity of 430 mA h g-1 and a rate capability exceeding 2000 mA g-1 , accompanied by minimal polarization and efficient utilization of conversion-alloying reactions. The substantial cell capacity and high-redox plateau of PVPBSO/mWFG//PB contribute to the practical feasibility of high-energy full batteries, offering long-cycle retention and high-voltage output. This study emphasizes the direct importance of interface and junction engineering in improving the efficiency of diverse electrochemical kinetic and diffusion processes for potassium-ion batteries.

Phase Engineering and Synchrotron-Based Study on Two-Dimensional Energy Nanomaterials

In recent years, there has been significant interest in the development of two-dimensional (2D) nanomaterials with unique physicochemical properties for various energy applications. These properties are often derived from the phase structures established through a range of physical and chemical design strategies. A concrete analysis of the phase structures and real reaction mechanisms of 2D energy nanomaterials requires advanced characterization methods that offer valuable information as much as possible. Here, we present a comprehensive review on the phase engineering of typical 2D nanomaterials with the focus of synchrotron radiation characterizations. In particular, the intrinsic defects, atomic doping, intercalation, and heterogeneous interfaces on 2D nanomaterials are introduced, together with their applications in energy-related fields. Among them, synchrotron-based multiple spectroscopic techniques are emphasized to reveal their intrinsic phases and structures. More importantly, various in situ methods are employed to provide deep insights into their structural evolutions under working conditions or reaction processes of 2D energy nanomaterials. Finally, conclusions and research perspectives on the future outlook for the further development of 2D energy nanomaterials and synchrotron radiation light sources and integrated techniques are discussed.

Short-Range Graphitic Nanodomains in Hypocrystalline Carbon Nanotubes Realize Fast Potassium Ion Migration and Multidirection Stress Release

Defect-rich carbon materials are considered as one of the most promising anodes for potassium-ion batteries due to their enormous adsorption sites of K+ , while the realization of both rate capability and cycling stability is still greatly limited by unstable electrochemical kinetics and inevitable structure degradation. Herein, an Fe3+ -induced hydrothermal-pyrolysis strategy is reported to construct well-tailored hybrid carbon nanotubes network architecture (PP-CNT), in which the short-range graphitic nanodomains are in-situ localized in the pea pod shape hypocrystalline carbon. The N,O codoped hypocrystalline carbon region contributes to abundant defect sites for potassium ion storage, ensuring high reversible capacity. Meanwhile, the short-range graphitic nanodomains with expanded interlayer spacing facilitate stable K+ migration and fast electron transfer. Furthermore, the finite element analysis confirms the volume expansion caused by K+ intercalation can be availably buffered due to the multidirection stress release effect of the unique porous pea pod shape, endowing carbon nanotubes with superior structural integrity. Consequently, the PP-CNT anode exhibits superior potassium-storage performance, including high reversible capacity, exceptional rate capability, and ultralong cycling stability. This work opens a new avenue for the fabrication of advanced carbon materials for achieving durable and fast potassium storage.

Densified graphene-like carbon nanosheets with enriched heteroatoms enabling superior gravimetric and volumetric potassium storage capacities

Constructing carbon electrodes with abundant heteroatoms and appropriate graphitic interlayer spacing remains a major challenge for achieving high gravimetric and volumetric potassium storage capacities with fast kinetics. Herein, we constructed 3D graphene-like N, F dual-doped carbon sheets induced by Ni template (N, F-CNS-Ni) with dense structure and rich active sites, providing a promising approach to address the facing obstacles. Highly reversible K-ion insertion/extraction is realized in the graphitic carbon structure, and K-adsorption capability is enhanced by introducing N/F heteroatoms. As a result, the N, F-CNS-Ni electrode exhibits ultrahigh gravimetric and volumetric capacities of 404.5 mA h g^-1 and 281.3 mA h cm^-3 at 0.05 A/g, respectively, and a superb capacity of 259.3 mA h g^-1 with a capacity retention ratio of 90 % even after 600 cycles at 5 A/g. This work presents a simple Ni-based template method to prepare graphene-like carbon nanosheets with high packing density and rich heteroatoms, and offers mechanism insight for achieving superior K-ion storage.