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

Partial boronization of transition metal within double heterostructure nanosheet achieves high-rate and long-cycling lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) have emerged as highly promising next-generation chemical power sources, characterized by their elevated specific energy. Their exceptionally high theoretical energy density has attracted considerable attention in both academic research and industrial application. Nonetheless, the shuttle effect and sluggish redox kinetics significantly hinder their advancement, resulting in the rapid deterioration of battery capacity and substantially limiting their practical application. To address this issue, this work reports a molten salt-assisted strategy. Using ZnCo-ZIF as a precursor, a novel dual heterostructure nanosheet (Co/CoB/NBC) composed of nitrogen and boron co-doped carbon (NBC), transition metal boride (CoB), and metallic element (Co) is successfully constructed and applied to modify the polypropylene (PP) separator in LSBs. The lithium-sulfur (Li-S) battery utilizing the Co/CoB/NBC-PP separator demonstrates remarkable cycle stability, successfully completing 800 cycles at a high rate of 8 C, with an initial discharge capacity reaching to 757 mAh g-1 and a capacity retention of 80%. At 2 C, the initial discharge capacity is recorded at 1013 mAh g-1, and following 1000 cycles, the average capacity degradation and capacity retention rate are measured at 0.025% and 75%, respectively. Furthermore, when subjected to cycling at a low current density of 0.5 C over the course of 100 cycles, the capacity retention rate is 81.3%. It is worth noting that when the sulfur loading of the electrode is as high as 5.2 mg cm-2 and electrolyte-to-sulfur (E/S) ratio is 2.88 μL mg-1, the first discharge capacity at 0.2 C is 825 mAh g-1. This study provides a novel and highly valuable method for designing dual heterostructure nanosheet among metallic element, metal boride and carbon, aimed at enhancing the energy density and longevity of LSBs. This work also establishes a robust foundation for the continued advancement and practical implementation of LSBs technology.

Synchronous embedded growth of Mo2C nanodisk arrays immobilized on porous carbon nanosheets for ultra-stable sodium storage

Sodium ion capacitors (SICs) that combine the merits of both rechargeable batteries and supercapacitors have gained widespread recognition for their high energy density and extended cycle life as new energy storage devices. However, the purposeful design of advanced battery-type anodes has become an urgent need to remedy the dynamics mismatch with the capacitive cathode. Herein, we propose a simple but efficient bottom-up approach to build three-dimensional Mo2C/C hybrid architectures in situ as anodes for SICs. By finely regulating the ratio of carbon and molybdenum sources, the optimized Mo2C/C, where even thinner subunit assembled Mo2C nanodisk (∼47.1 nm in thickness) arrays are immobilized on carbon nanosheet substrate via the synchronous embedded growth, rapid electron and ion diffusion/transport expressways, abundant active sites and robust structural stability were achieved for efficient sodium storage. Benefiting from the synergistic contributions of the components, the optimum Mo2C/C anode displays an outstanding rate and long-cycle properties as a competitive anode. Moreover, the constructed Mo2C/C-based SICs exhibited an energy density of ∼16.7 W h kg-1 at 10 kW h kg-1, along with ∼22.5% capacitance degradation over 4000 cycles at 1 A g-1. This contribution will guide the precise synthesis of other versatile Mo2C-based hybrids towards energy-related applications and beyond.

Pseudocapacitive Potassium-Ion Intercalation Enabled by Topologically Defective Soft Carbon toward High-Rate, Large-Areal-Capacity, and Low-Temperature Potassium-Ion Batteries

Carbonaceous materials are widely investigated as anodes for potassium-ion batteries (PIBs). However, the inferior rate capability, low areal capacity, and limited working temperature caused by sluggish K-ions diffusion kinetics are still primary challenges for carbon-based anodes. Herein, a simple temperature-programmed co-pyrolysis strategy is proposed for the efficient synthesis of topologically defective soft carbon (TDSC) based on inexpensive pitch and melamine. The skeletons of TDSC are optimized with shortened graphite-like microcrystals, enlarged interlayer spacing, and abundant topological defects (e.g., pentagons, heptagons, and octagons), which endow TDSC with fast pseudocapacitive K-ion intercalation behavior. Meanwhile, micrometer-sized structure can reduce the electrolyte degradation over particle surface and avoid unnecessary voids, ensuring a high initial Coulombic efficiency as well as high energy density. These synergistic structural advantages contribute to excellent rate capability (116 mA h g-1 at 20 C), impressive areal capacity (1.83 mA h cm-2 with a mass loading of 8.32 mg cm-2 ), long-term cycling stability (capacity retention of 91.8% after 1200 h cycling), and low working temperature (-10 °C) of TDSC anodes, demonstrating great potential for the practical application of PIBs.

Free-Standing Carbon Nanofiber Composite Networks Derived from Bacterial Cellulose and Polypyrrole for Ultrastable Potassium-Ion Batteries

Carbon materials derived from bacterial cellulose have been studied in lithium-ion batteries due to their low cost and flexible characteristics. However, they still face many intractable problems such as low specific capacity and poor electrical conductivity. Herein, bacterial cellulose is used as the carrier and skeleton to creatively realize the composite of polypyrrole on its nanofiber surface. After carbonization treatment, three-dimensional carbon network composites with a porous structure and short-range ordered carbon are obtained for potassium-ion batteries. The introduction of nitrogen doping from polypyrrole can increase the electrical conductivity of carbon composites and provide abundant active sites, improving the comprehensive performance of anode materials. The carbonized bacterial cellulose@polypyrrole (C-BC@PPy) anode exhibits a high capacity of 248 mA h g^-1 after 100 cycles at 50 mA g^-1 and a capacity retention of 176 mA h g^-1 even over 2000 cycles at 500 mA g^-1. Combined with density functional theory calculations, these results indicate that the capacity of C-BC@PPy is contributed by N-doped and defect carbon composite materials as well as pseudocapacitance. This study provides a guideline for the development of novel bacterial cellulose composites in the energy storage field.

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.

Ultra-small-sized multi-element metal oxide nanofibers: an efficient electrocatalyst for hydrogen evolution

Compared to noble metals, transition metal oxides (TMOs) have positive development prospects in the field of electrocatalysis, and the synergy between the elements in multi-element TMO-based materials can improve their catalytic activity. However, it is still a challenge to synthesize multi-component TMO-based catalysts and deeply understand the effects of components on the catalytic performance of the catalysts. Here, we demonstrate multi-element ultra-small-sized nanofibers for efficient hydrogen production. The ternary NiFeCoO nanofiber-based electrode reached an overpotential of 82 mV at the current density of 10 mA cm^-2 with a Tafel slope of 56 mV dec^-1 in 1 M KOH, which are close to those of Pt plate (66 mV at 10 mA cm^-2; the Tafel slope is 32 mV dec^-1). In addition, the current density maintained 97% of its initial value after 10 h operation. We used the ternary NiFeCoO nanofiber-based electrode as an efficient counter electrode in photoelectrochemical hydrogen production to demonstrate the versatility of these nanofibers.

Design of Flexible Films Based on Kinked Carbon Nanofibers for High Rate and Stable Potassium-Ion Storage

With the emergence of wearable electronics, flexible energy storage materials have been extensively studied in recent years. However, most studies focus on improving the electrochemical properties, ignoring the flexible mechanism and structure design for flexible electrode materials with high rate capacities and long-time stability. In this study, porous, kinked, and entangled network structures are designed for highly flexible fiber films. Based on theoretical analysis and finite element simulation, the bending degree of the porous structure (30% porosity) increased by 192% at the micro-level. An appropriate increase in kinking degree at the meso-level and contact points in entanglement network at the macro-level are beneficial for the flexibility of fiber films. Therefore, a porous and entangled network of sulfur-/nitrogen-co-doped kinked carbon nanofibers (S/N-KCNFs) is synthesized. The nanofiber films synthesized from melamine as nitrogen sources and segmented vulcanization exhibited a porous, kinked, and entangled network structure, and the stretching degree increased several times. The flexible S/N-KCNFs anode delivered a higher rate performance of 270 mAh g^-1 at a current density of 2000 mA g^-1 and a higher capacity retention rate of 93.3% after 2000 cycles. Moreover, the foldable pouch cell assembled by potassium-ion hybrid supercapacitor operated safely at large-angle bending and showed long-time stability of 88% capacity retention after 4000 cycles. This study provides a new idea and strategy for the flexible structure design of high-performance potassium-ion storage materials.

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.

Regulation of nitrogen configurations and content in 3D porous carbons for improved lithium storage

The incorporation of active nitrogen species in carbon materials has been widely demonstrated as a viable means to produce superior lithium storage materials, while the precise regulation of nitrogen configurations as well as their content still remains a formidable challenge. Herein, nitrogen-free porous carbon frameworks were synthesized by a self-templating strategy from disodium citrate, and post-annealing yielded 10.4 at% N that was primarily pyrrolic-N and pyridinic-N with an atomic ratio of about 3 : 1, with negligible inactive graphitic-N. A gravimetric capacity of 570 mA h g^-1 at a current density of 4 A g^-1 was measured for a Li half-cell based on the as-prepared N-doped 3D carbon materials. Lithium-ion capacitors with this N-doped carbon as the anode and commercial AC as the cathode yielded energy densities of 58.9 and 142.6 W h kg^-1 with the corresponding power densities of 7400 and 185 W kg^-1, respectively. We suggest that the carbon materials with high content of pyrrolic-N and pyridinic-N especially pyrrolic-N have improved lithium storage.

Fe3O4 nanoplates anchored on Ti3C2Tx MXene with enhanced pseudocapacitive and electrocatalytic properties

Ti₃C₂T_x, as novel members of the two-dimensional material family, hold great promise for electrochemical energy storage and catalysis, however, the electrochemical performance of Ti₃C₂T_x is largely limited by the self-restacking of their layers due to van der Waals forces. In this study, we report a high-performance electrode material, Ti₃C₂T_x supported Fe₃O₄ nanoplates (denoted as MXene-Fe), synthesized by a simple in situ wet chemistry method in a solvothermal system. The mesoporous MXene-Fe material as a supercapacitor electrode exhibits a high specific capacitance of 368.0 F g^-1 at 1.0 A g^-1 and long cycling stability with about 81% capacitance retention after 10 000 cycles at 10.0 A g^-1. Moreover, the optimized MXene-Fe also displays high electrocatalytic activity and stability toward the oxygen evolution reaction in alkaline solution (1.0 M KOH) with a low overpotential of 290 mV at 10 mA cm^-2 and a small Tafel slope of 65.1 mV dec^-1. This work provides an effective strategy for developing novel Ti₃C₂T_x-based functional materials with outstanding electrochemical performance for supercapacitors and electrocatalysis.

Recent progress in electrochemical performance of binder-free anodes for potassium-ion batteries

Potassium ion batteries (PIBs) are regarded as one of the most promising candidates for large-scale stationary energy storage beyond lithium-ion batteries (LIBs), owing to the abundance of potassium resources and low cost. Unfortunately, the practical application of PIBs is severely restricted by their poor rate capacity and unsatisfactory cycle performance. In traditional electrodes, a binder usually plays an important role in integrating individual active materials with conductive additives. Nevertheless, binders are not only generally electrochemically inactive but also insulating, which is unfavorable for improving overall energy density and cycling stability. To this end, in terms of both improved electronic conductivity and electrochemical reaction reversibility, binder-free electrodes offer great potential for high-performance PIBs. Moreover, the anode is a crucial configuration to determine full cell electrochemical performance. Therefore, this review analyzes in detail the electrochemical properties of the different type binder-free anodes, including carbon-based substrates (graphene, carbon nanotubes, carbon nanofibers, and so on), MXene-based substrates and metal-based substrates (Cu and Ni). More importantly, the recent progress, critical issues, challenges, and perspectives in binder-free electrodes for PIBs are further discussed. This review will provide theoretical guidance for the synthesis of high-performance anode materials and promote the further development of PIBs.

Tuning the electronic conductivity of porous nitrogen-doped carbon nanofibers with graphene for high-performance potassium-ion storage

In the designed graphene/porous nitrogen-doped carbon nanofibers, graphene can improve the electronic conductivity of the composite materials, and a large amount of mesopores provided much more exposed N-doped active sites for adsorbing K⁺.

Waste cigarette butt-derived nitrogen-doped porous carbon as a non-mercury catalyst for acetylene hydrochlorination

The development of advanced carbon materials as metal-free catalysts holds great importance for mercury catalyst replacement in acetylene hydrochlorination.