Boosting the Potassium-Ion Storage Performance in Soft Carbon Anodes by the Synergistic Effect of Optimized Molten Salt Medium and N/S Dual-Doping

Enhancing potassium ions adsorption on mesoporous carbon spheres with abundant internal surface via engineering sulfur doping sites towards superior rate capability

Mesoporous carbon spheres (MCSs) show great potential for using as high-performance anodes in potassium-ion batteries (PIBs). Design and synthesis of MCSs with suitable multiscale structures and heteroatom doping or co-doping in MCSs are successfully employed to optimize the ion and electron transportation, however, it is still a challenge to explore MCS-based anodes with satisfactory potassium storage performance. In this work, we report novel S-doped MCS samples with abundant internal surfaces for potassium storage. The S doping sites are controlled during the synthesis, and the effect of different doping sites on the potassium storage is systematically studied. It is found that S doping between the carbon layers enlarges interlayer spacing and facilitates potassium ion adsorption. Consequently, the optimized sample shows an excellent rate capability of 144 mAh/g at 5.0 A/g, and a high reversible specific capacity of 325 mAh/g after 100 cycles at 0.1 A/g with a capacity retention of 91.2%. The important role of element doping sites on ion adsorption and ion storage performance is confirmed by theoretical investigations. Controlling the doping sites in MCSs provides a new approach to designing high-performance electrodes for energy storage and conversion applications.

Layer-Structured Multitransition-Metal Oxide Cathode Materials for Potassium-Ion Batteries with Long Cycling Lifespan and Superior Rate Capability

Manganese (Mn)-based layer-structured transition metal oxides are considered as excellent cathode materials for potassium ion batteries (KIBs) owing to their low theoretical cost and high voltage plateau. The energy density and cycling lifetime, however, cannot simultaneously satisfy the basic requirements of the market for energy storage systems. One of the primary causes results from the complex structural transformation and transition metal migration during the ion intercalation and deintercalation process. The orbital and electronic structure of the octahedral center metal element plays an important role for maintaining the octahedral structural integrity and improving the K+ diffusivity by the introduced heterogeneous [Me-O] chemical bonding. A multitransition metal oxide, P3-type K0.5Mn0.85Co0.05Fe0.05Al0.05O2 (KMCFAO), was synthesized and employed as a cathode material for KIBs. Beneficial from the larger layer spacing for K+ to better accommodate and effectively preventing the irreversible structural transformation in the insertion/extraction process, it can reach a superior capacity retention up to 96.8% after 300 cycles at a current density of 500 mA g-1. The full cell of KMCFAO//hard carbon exhibits an encouraging promising energy density of 113.8 W h kg-1 at 100 mA g-1 and a capacity retention of 72.6% for 500 cycles.

Dual-Salt-Induced Hierarchical Porous Structure in a Carbon Sheet for High Performance Supercapacitors

Porous carbon, one of the characteristic materials for electrochemical energy storage devices, has been paid wide-ranging attention. However, balancing the reconcilable mesopore volume with a large specific surface area (SSA) was still a challenge. Herein, a dual-salt-induced activation strategy was developed to obtain a porous carbon sheet with ultrahigh SSA (3082 m² g^-1), desirable mesopore volume (0.66 cm³ g^-1), nanosheet morphology, and high surface O (7.87%) and S (4.0%) content. Hence, as a supercapacitor electrode, the optimal sample possessed a high specific capacitance (351 F g^-1 at 1 A g^-1) and excellent rate performance (holding capacitance up to 72.2% at 50 A g^-1). Furthermore, the assembled zinc-ion hybrid supercapacitor also exhibited superior reversible capacity (142.7 mAh g^-1 at 0.2 A g^-1) and highly stable cycling (71.2 mAh g^-1 at 5 A g^-1 after 10,000 cycles with retention of 98.9%). This work was delivered a new possibility for the development of coal resources for the preparation of high performance porous carbon materials.

MOF-derived nitrogen-doped porous carbon nanofibers with interconnected channels for high-stability Li+/Na+ battery anodes

Heteroatom-doped porous carbon materials have been widely used as anode materials for Li-ion and Na-ion batteries, however, improving the specific capacity and long-term cycling stability of ion batteries remains a major challenge. Here, we report a facile based metal-organic framework (MOFs) strategy to synthesize nitrogen-doped porous carbon nanofibers (NCNFs) with a large number of interconnected channels that can increase the contact area between the material and the electrolyte, shorten the diffusion distance between Li⁺/Na⁺ and the electrolyte, and relieve the volume expansion of the electrode material during cycling; the doping of nitrogen atoms can improve the conductivity and increase the active sites of the carbon material, can also affect the microstructure and electron distribution of the electrode material, thereby improving the electrochemical performance of the material. As expected, the obtained NCNFs-800 exhibited excellent electrochemical performance with high reversible capacity (for Li⁺ battery anodes: 1237 mA h g^-1 at 100 mA g^-1 after 200 cycles, for Na⁺ battery anodes: 323 mA h g^-1 at 100 mA g^-1 after 150 cycles) and long-term cycling stability (for Li⁺ battery anodes: 635 mA h g^-1 at 2 A g^-1 after 5000 cycles, for Na⁺ battery anodes: 194 mA h g^-1 at 2 A g^-1 after 5000 cycles).

Synthesis Strategies and Applications for Pitch-Based Anode: From Industrial By-Products to Power Sources

It is significant for saving energy to manufacture superb-property batteries. Carbon is one of the most competitive anode materials in batteries, but it is hard for commercial graphite anodes to meet the increasingly higher energy-storage requirements. Moreover, the price of other better-performing carbon materials (such as graphene) is much higher than graphite, which is not conducive to massive production. Pitch, the cheap by-product in the petroleum and coal industries, has high carbon content and yield, making it possible for commercialization. Developing pitch-based anodes can not only lower raw material costs but also realize the pitch's high value-added utilization. We comprehensively reviewed the latest synthesis strategies of pitch-derived materials and then introduced their application and research progress in lithium, sodium, and potassium ion batteries (LIBs, SIBs, and PIBs). Finally, we summarize and suggest the pitch's development trend for anodes and in other fields.

Integrated Design from Microstructural Engineering to Binder Optimization Enabling a Practical Carbon Anode with Ultrahigh ICE and Efficient Potassium Storage

Potassium-ion batteries (PIBs) are emerging as a powerful alternative to lithium-ion battery systems in large-scale energy storage owing to plentiful resources. Nevertheless, pursuing high-yield anode materials with high initial Coulombic efficiency (ICE) and superior rate capability is still one of the most critical challenges in practical application. Herein, an integrated electrode (PC-x) derived from a petroleum coke precursor (carbon residue rate as high as 89%) is regulated from microstructural engineering to binder optimization devoting to high ICE and efficient potassium storage. Excitingly, with a strong assist from a sodium carboxymethyl cellulose (CMC) binder, the PC-900 anode displays an ultrahigh ICE of 80.5%, one of the highest values reported for PIB carbon anodes. Simultaneously, the PC-900 anode submits a high capacity (304.3 mAh g^-1), superb rate (138.2 mAh g^-1 at 10C), and excellent stability. Furthermore, the full cell exhibits an outstanding rate and cycling performance (210.7 mAh g^-1 at 0.5C), confirming its large-scale application prospects. The ultrahigh ICE and excellent performance are mainly attributable to the beneficial microstructures (low surface area, functional group content, and larger interlayer spacing) created by microstructural engineering. Meanwhile, binder optimization also plays a crucial role in reducing the irreversible capacity and interface impedance, further improving the ICE and rate capability. Importantly, mechanism analysis confirms two-stage K⁺ storage behavior: reversible adsorption at edges and defects (>0.25 V) and intercalation into crystalline layers (<0.25 V). This work provides an efficient and easily scalable electrode design strategy for future practical applications of PIBs.

Hollow CoFe-based hybrid composites derived from unique S-modulated coordinated transition bimetal complexes for efficient oxygen evolution from water splitting under alkaline conditions

The oxygen evolution reaction (OER) is an important reaction in water splitting. However, the high cost and slow-rate catalysts hinder commercial applications. Although an important process for manufacturing of hollow structures, it is difficult to construct complicated hollow structures with an excellent and regulable shape for multi-component materials. In this study, we demonstrate that sulfur-Co,Fe bimetallic nitrogen carbon hollow composite hybrids (x-S-CoFe@NC) can be synthesized by regulating the amount of sulfur and using the hydrothermal method. For OER, 32-S-CoFe@NC exhibits excellent electrocatalytic activity with a low overpotential of 232 mV, which is higher than those of 0-S-CoFe@NC (270 mV), 23-S-CoFe@NC (247 mV), and RuO₂ (243 mV) catalysts at 10 mA cm^-2. In addition, with air as the cathode, a rechargeable Zn-air battery with outstanding long-life cycling stability for 80 hours based on 32-CoFe@NC + Pt/C is proposed. The advanced technique described here supplies a new route for preparing hollow transition bimetal carbon hybrids with an adjustable composite arrangement for electrocatalysis and water splitting.

Bitumen-Derived Onion-Like Soft Carbon as High-Performance Potassium-Ion Battery Anode

Potassium-ion batteries (PIBs) have been regarded as a competitive alternative for lithium-ion batteries, owing to the natural abundance, low cost, and similar rocking-chair working mechanism of potassium element. However, it is challenging to simultaneously prepare suitable potassium ion anode materials of low voltage plateau, high capacity, and long cycle life. In this work, onion-like soft carbon (OLSC) of high heteroatom content is prepared by using solvent-sensitive self-assembly properties of asphaltene molecules. The OLSC electrode exhibits a low voltage plateau because of a high degree of graphitization. Meanwhile, it possesses excellent cycling stability and rate capability due to the high stability of the onion-like structure and fast transport of potassium ions, the latter of which is caused by heteroatom-induced expanded interlayers as found by first-principle calculations. Compared with existing carbon materials, the OLSC synthesized in this study exhibits a high reversible capacity of 466 mAh g^-1 at 20 mA g^-1 , a reversible capacity of 222 mAh g^-1 and capacity retention of 95% after 1600 cycles at 1 A g^-1 . This work connects the nanostructure of carbon materials and electrochemical performance and provides new insights in improving carbon-based anodes for PIBs.

ZnS modified N, S dual-doped interconnected porous carbon derived from dye sludge waste as high-efficient ORR/OER catalyst for rechargeable zinc-air battery

Facile and rational design of high-efficient oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) bifunctional electrocatalysts is significant for rechargeable Zinc-air batteries. In this study, ZnS modified N, S dual-doped interconnected porous carbon (ZnS/NSC) derived from the dye sludge waste is successfully fabricated via a facile ZnCl₂-assisted pyrolysis process. The effect of ZnCl₂ and carbonization temperature on the microstructure and electrocatalytic performance is systematically investigated. By virtue of the synergistic effect between ZnS nanoparticles and N, S dual-doped porous carbon network, the obtained catalyst ZnS/NSC calcined at 1000 °C exhibits a decent bifunctional electrocatalytic performance with potential gap (ΔE=E_OER,10-E_ORR,1/2) of 0.76 V comparable with commercial electrocatalysts (Pt/C and RuO₂). In addition, a rechargeable zinc-air battery employed ZnS/NSC-1000 as the air cathode also displays the favorable electrochemical performance, in which the power density is 125 mW cm^-2, the specific capacity is 763.27 mAh g^-1 and the cycling stability at 10 mA cm^-2 is more than 85 h, indicating a promising application prospect in rechargeable Zinc-air batteries.

KOH activated nitrogen and oxygen co-doped tubular carbon clusters as anode material for boosted potassium-ion storage capability

Carbon nanomaterials have become a promising anode material for potassium-ion batteries (KIBs) due to their abundant resources, low cost, and excellent conductivity. However, among carbon materials, the sluggish reaction kinetics and inferior cycle life severely restrict their commercial development as KIBs anodes. It is still a huge challenge to develop carbon materials with various structural advantages and ideal electrochemical properties. Therefore, it is imperative to find a carbon material with heteroatom doping and suitable nanostructure to achieve excellent electrochemical performance. Benefiting from a Na₂SO₄template-assisted method and KOH activation process, the KOH activated nitrogen and oxygen co-doped tubular carbon (KNOCTC) material with a porous structure exhibits an impressive reversible capacity of 343 mAh g^-1at 50 mA g^-1and an improved cyclability of 137 mAh g^-1at 2 A g^-1after 3000 cycles with almost no capacity decay. The kinetic analysis indicates that the storage mechanism in KNOCTC is attributed to the pseudocapacitive process during cycling. Furthermore, the new synthesis route of KNOCTC provides a new opportunity to explore carbon-based potassium storage anode materials with high capacity and cycling performance.

Microstructure regulation of pitch-based soft carbon anodes by iodine treatment towards high-performance potassium-ion batteries

Soft carbon has been regarded as one of the most promising anode materials for potassium-ion batteries. However, the rearrangement of planar aromatics at high carbonization temperature usually yields a highly graphitized structure, which generally leads to inferior rate and cycle performance. In addition, the role of intrinsic carbon defects on potassium storage has not been well reported yet. In this work, crosslinked pitch-based soft carbon nanosheets have been synthesized through the iodination/dehydroiodination process at low temperature and carbonization with NaCl template. The iodine-treatment efficiently crosslinks the planar aromatics to three-dimensional framework by alkyl-bridged linkages, and reduces the strong π-π interaction during carbonization. This unique microstructure yields an ordered-in-disordered carbon microstructure, enlarged interlayer spacing, and abundant intrinsic defect sites. Benefited from these merits, the optimal sample displays 140% increase of reversible capacity to the pristine pitch-based carbon at 5 A g^-1. Particularly, it also presents 87.4% capacity retention after 1000 cycles at 1 A g^-1. This facile but simple strategy is expected to expand to other high-performance carbon materials and further understand the effect of intrinsic defects for potassium storage and beyond.

N-doped pinecone-based carbon with a hierarchical porous pie-like structure: a long-cycle-life anode material for potassium-ion batteries

Pinecone-based biomass carbon (PC) is a potential anode material for potassium-ion batteries because it is abundant, cheap, renewable, and easy to obtain. However, because of inferior kinetics and the effects of volume expansion due to the large radius of the K⁺ ion, it does not meet commercial performance requirements. In this study, nitrogen-doped PC (NPC) was prepared by carbonization in molten ZnCl₂ with urea as a nitrogen source. A strategy based on synergistic effects between N doping and ZnCl₂ molten salt was used to produce a hierarchically porous pie-like NPC with abundant defects and active sites and an enlarged interlayer distance—properties that enhance K⁺ adsorption, promote K⁺ intercalation/diffusion, and reduce the effects of volume expansion. This NPC exhibited a high reversible capacity (283 mA h g⁻¹ at 50 mA g⁻¹) and superior rate performance and cyclic stability (110 mA h g⁻¹ after 1000 cycles at 5 A g⁻¹), demonstrating its potential for use in potassium-ion batteries.

Pinecone-based biomass carbon (PC) is a potential anode material for potassium-ion batteries because it is abundant, cheap, renewable, and easy to obtain.

Prospects of Electrode Materials and Electrolytes for Practical Potassium-Based Batteries

Potassium-ion batteries (PIBs) have attracted tremendous attention because of their high energy density and low-cost. As such, much effort has focused on developing electrode materials and electrolytes for PIBs at the material levels. This review begins with an overview of the high-performance electrode materials and electrolytes, and then evaluates their prospects and challenges for practical PIBs to penetrate the market. The current status of PIBs for safe operation, energy density, power density, cyclability, and sustainability is discussed and future studies for electrode materials, electrolytes, and electrode-electrolyte interfaces are identified. It is anticipated that this review will motivate research and development to fill existing gaps for practical potassium-based full batteries so that they may be commercialized in the near future.

Ultra-High Sulfur-Doped Hierarchical Porous Hollow Carbon Sphere Anodes Enabling Unprecedented Durable Potassium-Ion Hybrid Capacitors

Sulfur doping is a promising path to ameliorate the kinetics of carbon-based anodes. However, the similar electronegativity of sulfur and carbon and the poor thermal stability of sulfur severely restrict the development of high-sulfur-content carbon-based anodes. In this work, ultra-high sulfur-doped hierarchical porous hollow carbon spheres (SHCS) with a sulfur content of 6.8 at % are synthesized via a direct high-temperature sulfur-doping strategy. An SHCS has sulfur bonded to the carbon framework including C-S-C and C-SO_x-C, which enlarges its interlayer distance (0.411 nm). In the K half-cell, benefiting from the considerable content and the reasonable architecture of sulfur, the SHCS exhibits significantly improved reversible specific capacity, initial Coulombic efficiency, and cyclability than hierarchical porous hollow carbon spheres without sulfur. Remarkably, the potassium ion hybrid capacitor device fabricated with the SHCS anode achieves excellent energy/power density (135.6 W h kg^-1/17.7 kW kg^-1) and unprecedented durability over 26,000 cycles at 2 A g^-1. This research provides a superior strategy to design high-sulfur-content carbon-based anodes with excellent potassium storage performance.

Local Structures of Soft Carbon and Electrochemical Performance of Potassium-Ion Batteries

Due to climate variation and global warming, utilization of renewable energy becomes increasingly imperative. Rechargeable potassium-ion batteries (PIBs) have lately attracted much attention due to their earth-abundance and cost-effectiveness. Because soft carbon materials are cheap, abundant, and safe, extensive feasible research studies have indicated that they could become promising anode materials for PIBs. In spite of gaining achievements, fundamental questions regarding effects of the basic structure unit inside soft carbon on potassium storage potential have not been sufficiently addressed yet. Here, a series of soft carbon pyrolyzed from 900 to 2900 °C were systematically and quantitatively characterized by combining Raman spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray pair distribution function analysis, and advanced evaluation of wide-angle X-ray scattering data. All these characterizations reveal structural details of soft carbon with increasing pyrolysis temperature. Our results show that the potassium storage behavior, especially the potential plateau is closely correlated to non-uniformity in interlayer distance and defect concentration in soft carbon, which is further confirmed by reverse Monte Carlo (RMC) modeling and density functional theory calculation. On the basis of these results, optimizing strategies are discussed to design an advanced soft carbon anode. This work provides significant insights into the structure engineering of soft carbon for high-performance rechargeable PIBs.

An AlCl3 coordinating interlayer spacing in microcrystalline graphite facilitates ultra-stable and high-performance sodium storage

Metal chloride-intercalated graphite intercalation compounds (MC-GICs) show a perfect sandwich structure with high electronic conductivity and chemical stability, but there are few applications for MC-GICs in anode materials of sodium ion batteries (SIBs). Herein, we selected a splendid host microcrystalline graphite (MG) to synthesize an AlCl3 intercalated MG intercalation compound (AlCl3-MGIC) anode material and demonstrated that it is suitable for SIBs via electrolyte optimization. The AlCl3-MGIC electrode is primarily compared in four electrolytes. Sodium storage is proposed for co-intercalation and conversion reactions by simultaneously selecting a compatible NaPF6/diethylene glycol dimethyl ether (DEGDME) electrolyte. As a result, the AlCl3-MGIC anode delivers a specific capacity of 202 mA h g-1 at a current density of 0.2 A g-1 after 100 cycles and still exhibits a satisfactory capacity of 198 mA h g-1 after 900 cycles. Density functional theory calculations further illustrate that DEGDME solvent molecules offer moderate adsorption energy to sodium ions that guarantees structure stabilization of GICs during repeated cycling. This work provides a theoretical basis for designing sodium ion storage with a graphite layered structure and unveiling the prospects of MC-GIC materials as high-performance anodes.

SnS2 Nanosheets Anchored on Nitrogen and Sulfur Co-Doped MXene Sheets for High-Performance Potassium-Ion Batteries

Potassium-ion batteries (KIBs) are emerging as the prospective alternatives to lithium-ion batteries in energy storage systems owing to the sufficient resources and relatively low cost of K-related materials. However, serious volume expansion and low specific capacity are found in most materials systems resulting from the large intrinsic radius of K⁺. Herein, SnS₂ nanosheets anchored on nitrogen and sulfur co-doped MXene (SnS₂ NSs/MXene) are creatively designed as advanced anode materials for KIBs. SnS₂ NSs/MXene with a unique hierarchical structure can not only provide fast transmission channels for K⁺ but also avoid the accumulation of K⁺ and volume expansion. These novel features make SnS₂ NSs/MXene electrodes exhibit a superior reversible specific capacity of 342.4 mA h g^-1 under 50 mA g^-1. Also, they maintain 206.1 mA h g^-1 at an even higher current density of 0.5 A g^-1 over 800 cycles almost without capacity decay. Moreover, the multistep alloying reaction mechanism of SnS₂ NSs/MXene composites and K⁺ is revealed by the ex situ X-ray diffraction measurement. In addition, the density functional theory calculations confirm the existence of Ti-S bonds between SnS₂ nanosheets and MXene, which significantly enhance the structural stability and cycling electrochemical performance of SnS₂ NSs/MXene composites.

A Superior Potassium-Ion Anode Material from Pitch-based Activated Carbon Fibers with Hierarchical Pore Structure Prepared by Metal Catalytic Activation

The pitch-based activated carbon fibers with nickel sulfide nanoparticles (ACF/NiS) were designed by in situ polymerization of ethylene tar with the addition of nickel nitrate followed by melt spinning, stabilization, carbonization, steam activation, and vulcanization processes. The ACF/NiS with hierarchical pore structure and abundant active sites was used as an anode material to improve Coulombic efficiency and increase capacity of potassium-ion batteries (PIBs). The results showed the obtained ACF/NiS with excellent specific surface area of 1552 m² g^-1 and high mesopore volume contribution of 38%, which delivered a high initial Coulombic efficiency of 84.22%, a high capacity of 292.5 mAh g^-1, and retained 95.7% capacity retention after 200 cycles at 0.5 A g^-1 current density. The NiS provided abundant active sites for the adsorption of potassium-ion, and the rich hierarchical structure shortened the electrolyte penetration path and expanded the storage space of potassium-ion, making it easier to store potassium-ion inside the ACF/NiS anode to obtain a better performance. This work presented one strategy for designing the hierarchical pore structure of pitch-based ACF to boost the capacity storage of PIBs and revealed that ACF-based carbon materials served as potential anodes for high-performance PIBs.

Free-Standing N-Doped Carbon Nanotube Films with Tunable Defects as a High Capacity Anode for Potassium-Ion Batteries

Potassium-ion batteries (KIBs) have aroused enormous interest for future energy storage technology. However, the current anodes for KIBs greatly suffer from the rapid capacity fading and inferior rate capability. Herein, a free-standing flexible anode, that is, nitrogen-doped carbon nanotube paper (NCTP), which is derived from the pyrolysis of organic polypyrrole materials, is demonstrated for high-performance potassium storage. The correlations between the material structure and electrochemical properties have been investigated by a series of material analysis and characterizations, as well as electrochemical tests. The research results show that the annealing temperature dramatically affects the N-doping content, the carbon defects, and the graphitization degree. Electrochemical tests indicate that the NCTP annealed at 700 °C displays the best performances with a high reversible capacity of 250.1 mA h g^-1 at 100 mA g^-1 and superior rate capability retaining 133 mA h g^-1 at 5 A g^-1. The excellent electrochemical properties are derived from a synergic contribution from the moderate N-doping, carbon defect, and high electronic conductivity of the materials. The facile pyrolysis strategy and the appealing performances involved in this work could provide some hints to manipulate high-performance anode materials of KIBs.