Carboxymethyl Cellulose Binder Greatly Stabilizes Porous Hollow Carbon Submicrospheres in Capacitive K-Ion Storage

Sulfur-Decorated Ti3 C2 TX MXene for High-Performance Sodium/Potassium-Ion Batteries

As post-lithium ion batteries, both sodium-ion batteries (SIBs) and potassium ion batteries (PIBs) possess great potential for large scale energy storage. However, the application of both SIBs and PIBs are hindered by the lack of suitable electrode materials. Here, we synthesized the sulfur decorated Ti3 C2 Tx (S-T3 C2 Tx ) MXene as electrode material for SIBs and PIBs. Thanks to the sulfur functional group and the formation of Ti-S bond, which facilitates the sodium in-/desertion and strengthens the potassium ion adsorption ability, as well as enhances ion reaction kinetics and improved structure stability, the S-T3 C2 Tx exhibit excellent sodium/potassium storage performance, high reversible capacities of 151 and 101 mAh g-1 at 0.1 mA g-1 were achieved for SIBs and PIBs, respectively. Moreover, the S-T3 C2 Tx exhibits remarkable long-term capacity stability at a high density of 500 mA g-1 , providing an impressive storage of 88 mAh g-1 for SIBs and 41 mAh g-1 for PIBs even after 2000 cycles. This work could give a deep comprehension of the heteroatom modification influence on the MXene-based framework and promote the application of MXene electrodes.

Ultrastable Graphite-Potassium Anode through Binder Chemistry

Graphite with abundant reserves has attracted enormous research interest as an anode of potassium-ion batteries (PIBs) owing to its high plateau capacity of 279 mAh g-1 at ≈0.2 V in conventional carbonate electrolytes. Unfortunately, it suffers from fast capacity decay during K+ storage. Herein, an ultrastable graphite-potassium anode is developed through binder chemistry. Polyvinyl alcohol (PVA) is utilized as a water-soluble binder to generate a uniform and robust KF-rich SEI film on the graphite surface, which can not only inhibit the electrolyte decomposition, but also withstand large volume expansion during K+ -insertion. Compared to the PVDF as binder, PVA-based graphite anode can operate for over 2000 cycles (running time of 406 days at C/3) with 97% capacity retention in KPF6 -based electrolytes. The initial Coulombic efficiency (ICE) of graphite anode is as high as 81.6% using PVA as the binder, higher than that of PVDF (40.1%). Benefiting from the strong adhesion ability of PVA, a graphite||fluorophosphate K-ion full battery is further built through 3D printing, which achieves a record-high areal energy of 8.9 mWh cm-2 at a total mass loading of 38 mg cm-2 . These results demonstrate the important role of binder in developing high-performance PIBs.

Organic-Inorganic Hybrid Interfaces Enable the Preparation of Nitrogen-Doped Hollow Carbon Nanospheres as High-Performance Anodes for Lithium and Potassium-Ion Batteries

An abundant hollow nanostructure is crucial for fast Li⁺ and K⁺ diffusion paths and sufficient electrolyte penetration, which creates a highly conductive network for ionic and electronic transport. In this study, we successfully developed a molecular-bridge-linked, organic-inorganic hybrid interface that enables the preparation of in situ nitrogen-doped hollow carbon nanospheres. Moreover, the prepared HCNSs, with high nitrogen content of up to 10.4%, feature homogeneous and regular morphologies. The resulting HCNSs exhibit excellent lithium and potassium storage properties when used as electrode materials. Specifically, the HCNS-800 electrode demonstrates a stable reversible discharge capacity of 642 mA h g^-1 at 1000 mA g^-1 after 500 cycles for LIBs. Similarly, the electrode maintains a discharge capacity of 205 mA h g^-1 at 100 mA g^-1 after 500 cycles for KIBs. Moreover, when coupled with a high-mass-loading LiFePO₄ cathode to design full cells, the HCNS-800‖LiFePO₄ cells provide a specific discharge capacity of 139 mA h g^-1 at 0.1 C. These results indicate that the HCNS electrode has promising potential for use in high-energy and environmentally sustainable lithium-based and potassium-based batteries.

Recent Advances in Polymers for Potassium Ion Batteries

Potassium-ion batteries (KIBs) are considered to be an effective alternative to lithium-ion batteries (LIBs) due to their abundant resources, low cost, and similar electrochemical properties of K⁺ to Li⁺, and they have a good application prospect in the field of large-scale energy storage batteries. Polymer materials play a very important role in the battery field, such as polymer electrode materials, polymer binders, and polymer electrolytes. Here in this review, we focus on the research progress of polymers in KIBs and systematically summarize the research status and achievements of polymer electrode materials, electrolytes, and binders in potassium ion batteries in recent years. Finally, based on the latest representative research of polymers in KIBs, some suggestions and prospects are put forward, which provide possible directions for future research.

Fundamental Understanding and Research Progress on the Interfacial Behaviors for Potassium-Ion Battery Anode

Potassium-ion batteries (PIBs) exhibit a considerable application prospect for energy storage systems due to their low cost, high operating voltage, and superior ionic conductivity. As a vital configuration in PIBs, the two-phase interface, which refers to K-ion diffusion from the electrolyte to the electrode surface (solid-liquid interface) and K-ion migration between different particles (solid-solid interface), deeply determines the diffusion/reaction kinetics and structural stability, thus significantly affecting the rate performance and cyclability. However, researches on two-phase interface are still in its infancy and need further attentions. This review first starts from the fundamental understanding of solid-liquid and solid-solid interfaces to in-depth analyzing the effect mechanism of different improvement strategies on them, such as optimization of electrolyte and binders, heterostructure design, modulation of interlayer spacing, etc. Afterward, the research progress of these improvement strategies is summarized comprehensively. Finally, the major challenges are proposed, and the corresponding solving strategies are presented. This review is expected to give an insight into the importance of two-phase interface on diffusion/reaction kinetics, and provides a guidance for developing other advanced anodes in PIBs.

Durable silicon-carbon composites self-assembled from double-protected heterostructure for lithium-ion batteries

HYPOTHESIS

Silicon-carbon composites have been faced with the contact issues between silicon and carbon in the form of material aggregation and inferior dispersion, leading to electrode cracking or kinetic degradation during cycling. In addition to dispersion improvement from interfacial linkage between self-assembled Si nanoparticles (SiNPs) and carbon fibers (CNFs), the positive influences of high-content carboxymethyl cellulose(CMC) (25 wt%) and amorphous carbon are also expected, respectively after the second-step self-assembly and subsequently sintering.

EXPERIMENTS

A novel composite (i.e. Si-CNF@C) with the decoration of entire SiNPs in the framework of both CNFs and amorphous carbon was prepared via two-step electrostatic self-assembly followed by sintering. Such a composite with heterogeneous nanostructure was used as a lithium-ion battery anode without additional binders or conductive agents.

FINDINGS

SiNPs can be well protected with CNFs and amorphous carbon against the dispersion and contact problems under both effects of electrostatic attraction and chemical bonding. With the double-protected heterostructure, such a novel Si-CNF@C electrode exhibits highly reversible capacities of 1200 mAh g^-1, 982 mAh g^-1, and 849 mAh g^-1 after 100, 500, and 1000 cycles at 0.5 A g^-1, respectively. The long-term cycling stability with a capacity loss of 0.036% per cycle over 1000 cycles is comparable.

Oxygen Loss in Layered Oxide Cathodes for Li-Ion Batteries: Mechanisms, Effects, and Mitigation

Layered lithium transition metal oxides derived from LiMO₂ (M = Co, Ni, Mn, etc.) have been widely adopted as the cathodes of Li-ion batteries for portable electronics, electric vehicles, and energy storage. Oxygen loss in the layered oxides is one of the major factors leading to cycling-induced structural degradation and its associated fade in electrochemical performance. Herein, we review recent progress in understanding the phenomena of oxygen loss and the resulting structural degradation in layered oxide cathodes. We first present the major driving forces leading to the oxygen loss and then describe the associated structural degradation resulting from the oxygen loss. We follow this analysis with a discussion of the kinetic pathways that enable oxygen loss, and then we address the resulting electrochemical fade. Finally, we review the possible approaches toward mitigating oxygen loss and the associated electrochemical fade as well as detail novel analytical methods for probing the oxygen loss.

Polydopamine Doping and Pyrolysis of Cellulose Nanofiber Paper for Fabrication of Three-Dimensional Nanocarbon with Improved Yield and Capacitive Performances

Biomass-derived three-dimensional (3D) porous nanocarbons have attracted much attention due to their high surface area, permeability, electrical conductivity, and renewability, which are beneficial for various electronic applications, including energy storage. Cellulose, the most abundant and renewable carbohydrate polymer on earth, is a promising precursor to fabricate 3D porous nanocarbons by pyrolysis. However, the pyrolysis of cellulosic materials inevitably causes drastic carbon loss and volume shrinkage. Thus, polydopamine doping prior to the pyrolysis of cellulose nanofiber paper is proposed to fabricate the 3D porous nanocarbons with improved yield and volume retention. Our results show that a small amount of polydopamine (4.3 wt%) improves carbon yield and volume retention after pyrolysis at 700 °C from 16.8 to 26.4% and 15.0 to 19.6%, respectively. The pyrolyzed polydopamine-doped cellulose nanofiber paper has a larger specific surface area and electrical conductivity than cellulose nanofiber paper that without polydopamine. Owing to these features, it also affords a good specific capacitance up to 200 F g⁻¹ as a supercapacitor electrode, which is higher than the recently reported cellulose-derived nanocarbons. This method provides a pathway for the effective fabrication of high-performance cellulose-derived 3D porous nanocarbons.

In Situ Monitoring the Potassium-Ion Storage Enhancement in Iron Selenide with Ether-Based Electrolyte

As one of the promising anode materials, iron selenide has received much attention for potassium-ion batteries (KIBs). Nevertheless, volume expansion and sluggish kinetics of iron selenide result in the poor reversibility and stability during potassiation-depotassiation process. In this work, we develop iron selenide composite matching ether-based electrolyte for KIBs, which presents a reversible specific capacity of 356 mAh g^-1 at 200 mA g^-1 after 75 cycles. According to the measurement of mechanical properties, it is found that iron selenide composite also exhibits robust and elastic solid electrolyte interphase layer in ether-based electrolyte, contributing to the improvement in reversibility and stability for KIBs. To further investigate the electrochemical enhancement mechanism of ether-based electrolyte in KIBs, we also utilize in situ visualization technique to monitor the potassiation-depotassiation process. For comparison, iron selenide composite matching carbonate-based electrolyte presents vast morphology change during potassiation-depotassiation process. When changing to ether-based electrolyte, a few minor morphology changes can be observed. This phenomenon indicates an occurrence of homogeneous electrochemical reaction in ether-based electrolyte, which results in a stable performance for potassium-ion (K-ion) storage. We believe that our work will provide a new perspective to visually monitor the potassium-ion storage process and guide the improvement in electrode material performance.

Controllable Preparation of Core-Shell Composites and Their Templated Hollow Carbons Based on a Well-Orchestrated Molecular Bridge-Linked Organic-Inorganic Hybrid Interface

Controlling the interfacial effect is facing challenges because of the weak interactions between the inorganic and the organic materials. We found that the silane coupling agents with -NH₂ groups (e.g., KH550) play a key role as a molecular bridge that links an inorganic silica template with an organic precursor (i.e., pyrrole) in the process of constructing a spherical silica core-polypyrrole shell structure. The molecular bridge is also suitable for inorganic core templates with cube or rod shapes for the construction of different core-shell structures. These template core-polymeric shell structures can be transformed into well-defined hollow carbons after carbonization and template removal. The outer diameter, hollow-core size, and carbon shell thickness of hollow carbon materials (e.g., hollow carbon spheres) could be facilely controlled by changing the template size or the pyrrole amount. We believe that our work will provide a guideline for the preparation of well-orchestrated carbon-based composites and their templated hollow carbons.

Bismuth oxychloride anchoring on graphene nanosheets as anode with a high relative energy density for potassium ion battery

In this work, we developed bismuth oxychloride anchoring on graphene nanosheets (BiOCl/G) composite via a simple one-step hydrothermal process for KIBs' anode, which delivers a high reversible specific capacity of 251 mAh g^-1 at 50 mA g^-1 after 50 cycles. Meanwhile, our BiOCl/G composite also exhibits a low voltage plateau during potassiation-depotassiation process, and such low voltage plateau in anode is helpful to improve the energy density of the full battery. In addition, we also provide the energy changes for migration of K-ion of our composite according to the density functional theory calculation and the result shows that the introduction of graphene in BiOCl can reduce the adsorption energy variation, which is in favor of K-ion intercalation process. In consideration of low potential plateau of our composite, we also introduce a new evaluation method, relative energy density (E_R), which not only includes the specific capacity, but also combines the potential plateau of the anode materials during potassiation-depotassiation process. According to the calculation, our BiOCl/G composite obtain an ultra-high E_R of 541 Wh kg^-1 at 50 mA g^-1 after 50 cycles with a relative energy conversion efficiency of 81%.

Ultra-Stable Potassium Ion Storage of Nitrogen-Doped Carbon Nanofiber Derived from Bacterial Cellulose

As a promising energy storage system, potassium (K) ion batteries (KIBs) have received extensive attention due to the abundance of potassium resource in the Earth's crust and the similar properties of K to Li. However, the electrode always presents poor stability for K-ion storage due to the large radius of K-ions. In our work, we develop a nitrogen-doped carbon nanofiber (N-CNF) derived from bacterial cellulose by a simple pyrolysis process, which allows ultra-stable K-ion storage. Even at a large current density of 1 A g-1, our electrode exhibits a reversible specific capacity of 81 mAh g-1 after 3000 cycles for KIBs, with a capacity retention ratio of 71%. To investigate the electrochemical enhancement performance of our N-CNF, we provide the calculation results according to density functional theory, demonstrating that nitrogen doping in carbon is in favor of the K-ion adsorption during the potassiation process. This behavior will contribute to the enhancement of electrochemical performance for KIBs. In addition, our electrode exhibits a low voltage plateau during the potassiation-depotassiation process. To further evaluate this performance, we calculate the "relative energy density" for comparison. The results illustrate that our electrode presents a high "relative energy density", indicating that our N-CNF is a promising anode material for KIBs.

Nitrogen and sulfur co-doped vanadium carbide MXene for highly reversible lithium-ion storage

As an emerging group of two-dimensional (2D) layered material, MXenes have received significant attention in the direction of energy storage. However, the restacking of MXene flakes severely hinders the ion transport within electrodes, which limits their application for lithium-ion batteries (LIBs). To address this issue, herein, we rationally designed and optimized the structure of N, S co-doped V₂CT_x MXene, which exhibits excellent electrochemical performance with a high reversible capacity of 590 mAh g^-1 after 100 cycles at 0.1 A g^-1 when used as anode of LIBs. Even at a high current density of 2 A g^-1, a reversible capacity of 298 mAh g^-1 is obtained after 300 cycles, which outperforms most of the V₂CT_x-based anode materials reported so far. The lithium-ion storage mechanism of N, S co-doped V₂CT_x MXene was studied by a series of characterizations. The results show that the significant improvement of electrochemical performance should be attributed to the facilitated charge transfer after N and S co-doping in V₂CT_x MXene, which can effectively improve the ion transfer kinetics during the lithiation-delithiation process. Furthermore, the expanded interlayer spacing of N, S co-doped V₂CT_x provides more active sites for the adsorption of lithium ions, promoting the insertion capacity of lithium ions. This work indicates that the N, S co-doped 2D V₂CT_x MXene should be a promising anode material for high-performance LIBs.

Mini-Review on the Redox Additives in Aqueous Electrolyte for High Performance Supercapacitors

Supercapacitors, also known as electrochemical capacitors, are attracting much research attention owing to their high power density, long-term cycling stability, as well as exceptional safety compared with rechargeable batteries, although the globally accepted quantitative benchmarks on the power density, cycling stability, and safety are yet to be established. However, it should be noted that the supercapacitors generally exhibit low energy density, which cannot satisfy the demands where both high energy density and power density are needed. To date, various methods have been employed to improve the electrochemical performances of supercapacitors. Among them, introducing redox additives (or redox mediators) into conventional aqueous electrolyte is regarded as one of the most promising strategies. The redox additives in aqueous electrolyte are widely demonstrated to be able to increase the charge storage capability via redox transformation and thus enhance the electrochemical performances. Herein, we present a brief review on the classification, state-of-the-art progress, challenges, and perspectives of the redox additives in aqueous electrolyte for high performance supercapacitors.

Defect Engineering on Electrode Materials for Rechargeable Batteries

The reasonable design of electrode materials for rechargeable batteries plays an important role in promoting the development of renewable energy technology. With the in-depth understanding of the mechanisms underlying electrode reactions and the rapid development of advanced technology, the performance of batteries has significantly been optimized through the introduction of defect engineering on electrode materials. A large number of coordination unsaturated sites can be exposed by defect construction in electrode materials, which play a crucial role in electrochemical reactions. Herein, recent advances regarding defect engineering in electrode materials for rechargeable batteries are systematically summarized, with a special focus on the application of metal-ion batteries, lithium-sulfur batteries, and metal-air batteries. The defects can not only effectively promote ion diffusion and charge transfer but also provide more storage/adsorption/active sites for guest ions and intermediate species, thus improving the performance of batteries. Moreover, the existing challenges and future development prospects are forecast, and the electrode materials are further optimized through defect engineering to promote the development of the battery industry.

Research Development on K-Ion Batteries

Li-ion batteries (LIBs), commercialized in 1991, have the highest energy density among practical secondary batteries and are widely utilized in electronics, electric vehicles, and even stationary energy storage systems. Along with the expansion of their demand and application, concern about the resources of Li and Co is growing. Therefore, secondary batteries composed of earth-abundant elements are desired to complement LIBs. In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to LIBs. Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Thus, besides being free from any scarce/toxic elements, the low standard electrode potentials of K/K⁺ electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K⁺ ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than that of Li⁺ ions; this is essential to realize high-power KIBs. This review comprehensively covers the studies on electrochemical materials for KIBs, including electrode and electrolyte materials and a discussion on recent achievements and remaining/emerging issues. The review also includes insights into electrode reactions and solid-state ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs compared to those of LIBs and NIBs.

Formation of needle-like porous CoNi2S4-MnOOH for high performance hybrid supercapacitors with high energy density

Seeking for suitable electrode materials and designing rational porous structures are great challenges for developing high performance supercapacitors. Herein, needle-like porous CoNi₂S₄-MnOOH (denoted as NCS-MO) were prepared via a simple two steps solvothermal method and used as battery-type electrode of supercapacitor for the first time. Owing to the multiple oxidation states of needle-like porous NCS-MO and the inherent porous structure, the electrode delivers outstanding electrochemical capacitive properties with a high gravimetric specific capacitance of 1267.7 F g^-1 at the scan rate of 1 mV s^-1. To further assess the practical electrochemical performances, we assembled a hybrid supercapacitor using the as-synthesized porous NCS-MO as cathode and active carbon as anode. The device exhibits excellent performance with a high energy density of 47.1 Wh kg^-1 at the power density of 998 W kg^-1 in an extended voltage range of 1.6 V and outstanding cycling stability. These results demonstrate that the needle-like porous NCS-MO could be promising potential electrode material for high performance supercapacitor.