Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania Coated by Carbon for Excellent Sodium-Ion Battery Anodes

Plasma Technology for Advanced Electrochemical Energy Storage

"Carbon Peak and Carbon Neutrality" is an important strategic goal for the sustainable development of human society. Typically, a key means to achieve these goals is through electrochemical energy storage technologies and materials. In this context, the rational synthesis and modification of battery materials through new technologies play critical roles. Plasma technology, based on the principles of free radical chemistry, is considered a promising alternative for the construction of advanced battery materials due to its inherent advantages such as superior versatility, high reactivity, excellent conformal properties, low consumption and environmental friendliness. In this perspective paper, we discuss the working principle of plasma and its applied research on battery materials based on plasma conversion, deposition, etching, doping, etc. Furthermore, the new application directions of multiphase plasma associated with solid, liquid and gas sources are proposed and their application examples for batteries (e. g. lithium-ion batteries, lithium-sulfur batteries, zinc-air batteries) are given. Finally, the current challenges and future development trends of plasma technology are briefly summarized to provide guidance for the next generation of energy technologies.

Significantly Improving the Initial Coulombic Efficiency of TiO2 Anode for Sodium-Ion Batteries

Titanium dioxide (TiO2) can serve as a candidate anode material for sodium-ion batteries (SIBs) with the merits of their low cost, abundance, and environment friendliness. However, its low initial Coulombic efficiency (ICE) and sluggish sodium-ion diffusion greatly limit its further practical applications. Herein, we report a one-step prepotassiation strategy to modify commercial TiO2 by a spontaneous chemical reaction using potassium naphthalene (K-Nt). Prepotassiation effectively compensates for the irreversible Na loss and induces a homogeneous, dense, and robust artificial solid electrolyte interphase (SEI) on its surface. The well-distributed artificial SEI suppresses the excessive electrolyte decomposition, contributing to rapid interfacial kinetics and stable Na+ insertion/extraction. Therefore, such modified commercial TiO2 anodes demonstrate significantly improved ICE (72.4%) and outstanding rate performance (176.4 mAh g-1 at 5 A g-1). This simple and efficient method for promoting ICEs and interfacial chemistry also demonstrates universality and practical value for other anodes in SIBs.

Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries

Solid-state electrolytes (SEs) have attracted overwhelming attention as a promising alternative to traditional organic liquid electrolytes (OLEs) for high-energy-density sodium-metal batteries (SMBs), owing to their intrinsic incombustibility, wider electrochemical stability window (ESW), and better thermal stability. Among various kinds of SEs, inorganic solid-state electrolytes (ISEs) stand out because of their high ionic conductivity, excellent oxidative stability, and good mechanical strength, rendering potential utilization in safe and dendrite-free SMBs at room temperature. However, the development of Na-ion ISEs still remains challenging, that a perfect solution has yet to be achieved. Herein, we provide a comprehensive and in-depth inspection of the state-of-the-art ISEs, aiming at revealing the underlying Na⁺ conduction mechanisms at different length scales, and interpreting their compatibility with the Na metal anode from multiple aspects. A thorough material screening will include nearly all ISEs developed to date, i.e., oxides, chalcogenides, halides, antiperovskites, and borohydrides, followed by an overview of the modification strategies for enhancing their ionic conductivity and interfacial compatibility with Na metal, including synthesis, doping and interfacial engineering. By discussing the remaining challenges in ISE research, we propose rational and strategic perspectives that can serve as guidelines for future development of desirable ISEs and practical implementation of high-performance SMBs.

Three-in-one oxygen-deficient titanium dioxide in a pomegranate-inspired design for improved lithium storage

Defects engineering has played an ever-increasing important role in electrochemistry, especially secondary lithium batteries. TiO₂ is regarded as a promising anode due to its attractive cycling stability, low volume strain and great abundance, while challenges of intrinsic poor electrical and ionic conductivity remain to be addressed. Herein, we report a three-in-one oxygen vacancy (V_O)-involved pomegranate design for TiO_2-x/C composite anode, which provides highly improved electrical conduction, shortened Li⁺ pathway and promoted Li⁺ redox. N-doped mesoporous carbon acts as a robust scaffold to support the whole structure, electron highway and efficient reductant to generate V_O on TiO₂ nanoparticles during crystallization. Theoretical calculations reveal the crucial role of surface V_O on TiO₂ in Li electrochemistry. Resultantly, the optimal TiO_2-x/C anode achieves significantly enhanced cycling performance (203 mAh/g retained after 2000 cycles at 1 A/g). Postmortem analysis reveals the robustness of V_O and reasonable structure stability upon cycles for improved battery performance.

Ga2Te3-Based Composite Anodes for High-Performance Sodium-Ion Batteries

Recently, metal chalcogenides have received considerable attention as prospective anode materials for sodium-ion batteries (SIBs) because of their high theoretical capacities based on their alloying or conversion reactions. Herein, we demonstrate a gallium(III) telluride (Ga₂Te₃)-based ternary composite (Ga₂Te₃-TiO₂-C) synthesized via a simple high-energy ball mill as a great candidate SIB anode material for the first time. The electrochemical performance, as well as the phase transition mechanism of Ga₂Te₃ during sodiation/desodiation, is investigated. Furthermore, the effect of C content on the performance of Ga₂Te₃-TiO₂-C is studied using various electrochemical analyses. As a result, Ga₂Te₃-TiO₂-C with an optimum carbon content of 10% (Ga₂Te₃-TiO₂-C(10%)) exhibited a specific capacity of 437 mAh·g^-1 after 300 cycles at 100 mA·g^-1 and a high-rate capability (capacity retention of 96% at 10 A·g^-1 relative to 0.1 A·g^-1). The good electrochemical properties of Ga₂Te₃-TiO₂-C(10%) benefited from the presence of the TiO₂-C hybrid buffering matrix, which improved the mechanical integrity and electrical conductivity of the electrode. This research opens a new direction for the improvement of high-performance advanced SIB anodes with a simple synthesis process.

Enhanced electrochemical performance of the cobalt chloride carbonate hydroxide hydrate via micromorphology and phase transformation

Micromorphology and conductivity are two vital factors for the practical capacitance of the electrode materials for supercapacitors. In this work, a novel two-step electrochemical activation method involving a cyclic voltammetry (CV) treatment within 0-0.7 V followed by a CV treatment within -1.2-0 V is explored to induce the micromorphology and phase transformation of the cobalt chloride carbonate hydroxide hydrate (CCCH) nanoneedle arrays. The first-step activation transforms the CCCH to Co(OH)₂ and then the reversible transformation between Co(OH)₂ and CoOOH generates plenty of pores in the sample, thereby increasing the specific capacitance from 0.54 to 1.74 F cm^-2 at the current density of 10 mA cm^-2. The second-step activation inducing the reversible transformation between Co(OH)₂ and Co not only endows the final sample with a nanosheets-assembled fasciculate structure but also decreases the internal resistance via generating Co⁰ in the final sample (CCCH-P75N50). Consequently, the CCCH-P75N50 shows a high specific capacitance of 3.83 F cm^-2 at the current density of 10 mA cm^-2. Besides, the aqueous asymmetric supercapacitor assembled with CCCH-P75N50 and commercial conductive carbon cloth (CC) delivers a high energy density of 2.75 mWh cm^-3 at a power density of 37.5 mW cm^-3. This work provides a novel, facile and promising method to optimize the micromorphology and conductivity of Co-based electrodes.

Overcoming Ion Transport Barrier by Plasma Heterointerface Engineering: Epitaxial Titanium Carbonitride on Nitrogen-Doped TiO2 for High-Performance Sodium-Ion Batteries

Anatase TiO₂ is a promising anode material for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to its high specific capacity, low cost, and excellent cycle stability. However, low electrical conductivity and poor Na⁺ ion transport in TiO₂ limit its practical applications. Here, substantially boosted Na⁺ ion transport and charge transfer kinetics are demonstrated by constructing a near-ideal non-rectifying titanium carbonitride/nitrogen-doped TiO₂ (TiC_x N_{1- x} /N-TiO₂ ) heterostructure. Owing to the fast plasma effects and metastable hybrid phases, the TiC_x N_{1- x} is epitaxially grown on TiO₂ . Energy band engineering at the interface induces high electron densities and a strong built-in electric field, which lowers the Na⁺ diffusion barrier by a factor of 1.7. As a result, the TiC_x N_{1- x} /N-TiO₂ electrode exhibits excellent electrochemical performance. The reversible specific capacities at rates of 0.1 and 10 C reach 312.3 and 173.7 mAh g^-1 , respectively. After 600 cycles of charge and discharge at 10 C, the capacity retention rate is 98.7%. This work discovers an effective non-equilibrium plasma-enabled process to construct heterointerfaces that can enhance Na⁺ ion transport and provides generic guidelines for the design of heterostructures for a broader range of energy storage, separation, and other devices that rely on controlled ionic transport.

Tin-Based Anode Materials for Stable Sodium Storage: Progress and Perspective

Because of concerns regarding shortages of lithium resources and the urgent need to develop low-cost and high-efficiency energy-storage systems, research and applications of sodium-ion batteries (SIBs) have re-emerged in recent years. Herein, recent advances in high-capacity Sn-based anode materials for stable SIBs are highlighted, including tin (Sn) alloys, Sn oxides, Sn sulfides, Sn selenides, Sn phosphides, and their composites. The reaction mechanisms between Sn-based materials and sodium are clarified. Multiphase and multiscale structural optimizations of Sn-based materials to achieve good sodium-storage performance are emphasized. Full-cell designs using Sn-based materials as anodes and further development of Sn-based materials are discussed from a commercialization perspective. Insights into the preparation of future high-performance Sn-based anode materials and the construction of sodium-ion full batteries with a high energy density and long service life are provided.

Phenolic compounds degradation: Insight into the role and evidence of oxygen vacancy defects engineering on nanomaterials

Oxygen vacancy as a typical point defect has incited substantial interest in photocatalysis due to its profound impact on optical absorption response and facile isolation of photocarriers. The presence of oxygen vacancy can introduce the midgap defect states, which promote extended absorption in the visible region. The redistribution of electron density at the surface can stimulate the adsorption and activation kinetics of adsorbates, manifesting optimal photocatalytic performance. Despite such alluring outcomes, the ambiguity in understanding the precise location, appropriate concentration, and oxygen vacancy role is still a long-standing task. The present review article comprehensively outlines the identification of oxygen vacancy defects at bulk or on the surface and its ultimate effect on the photocatalytic degradation of phenolic compounds. Particular emphasis has been drawn to summarize the critical influence of oxygen vacancy on different factors such as crystal structure, bandgap energy, electronic structure, and charge carrier mobility by integrating experimental results and theoretical calculations. We have also explored the reaction pathways and the intermediate chemistry of phenol photodegradation by analyzing the molecular activation (O₂, H₂O, and sulphate activation) through oxygen vacancy defects. Finally, the review concludes with the various challenges and future perspectives, aiming to provide a firm base for further progressions towards photocatalysis.

Recent Progress on Intercalation-Based Anode Materials for Low-Cost Sodium-Ion Batteries

Intercalation-based anode materials can be considered as the most promising anode candidates for large-scale sodium-ion batteries (SIBs), owing to their long-term cycling stability and environmental friendliness, as well as their natural abundance. Nevertheless, their low energy density, low initial coulombic efficiency, and poor cycling lifespan, as well as sluggish sodium diffusion dynamics are still the main issues for the application of intercalation-based anode materials in SIBs in terms of meeting the benchmark requirements for commercialization. Over the past few years, tremendous efforts have been devoted to improving the performance of SIBs. In this Review, recent progress in the development of intercalation-based anode materials, including TiO₂ , Li₄ Ti₅ O₁₂ , Na₂ Ti₃ O₇ , and NaTi₂ (PO₄ )₃ , is summarized in terms of their sodium storage performance, critical issues, sodiation/desodiation behavior, and effective strategies to enhance their electrochemical performance. Additionally, challenges and perspectives are provided to further understand these intercalation-based anode materials.

"Self-doping" defect engineering in SnP3@gamma-irradiated hard carbon anode for rechargeable sodium storage

Reasonable design of defect engineering in the electrode materials for sodium-ion batteries (SIBs) can significantly optimize battery performance. Here, compared with the traditional "foreign-doping" defects method, we report an innovative gamma-irradiation technique to introduce the "self-doping" defects in the popcorn hard carbon (HC). Considering the advantages of adsorption-intercalation-alloying sodium storage mechanism, the defect-rich HC-coated alloy structure (SnP₃@HC-γ) was integrated. Due to the high energy and strong penetrability of γ-rays, the constructed "self-doping" defect engineering effectively expands the interlayer structure of HC and forms the irregular ring structure. Simultaneously, the exposed large number of coordination unsaturated sites can accelerate the reaction kinetics on the surface. Based on the synergistic effect of the SnP₃@HC-γ, the composites exhibit an excellent reversible capacity of 668 mAh g^-1 at 0.1 A g^-1 in SIBs. Even, after 400 cycles at 1.0 A g^-1, an exceptional cyclability with 88% capacity retention (430 mAh g^-1) can be maintained. We envision that the γ-irradiation technology used in this research not only overturns the general perception that "self-doping" defects will reduce performance, but also provides reliable technical support for large-scale construction of high-defect, high-capacity and stable sodium-ion anode materials.

Rock-Salt MnS0.5Se0.5 Nanocubes Assembled on N-Doped Graphene Forming van der Waals Heterostructured Hybrids as High-Performance Anode for Lithium- and Sodium-Ion Batteries

Manganese-based chalcogenides would be of latent capacity in serving as anodes for assembling lithium- and/or sodium-ion batteries (LIBs/SIBs) due to their large theoretical capacity, low price, and low-toxicity functionality, while the low electroconductivity and easy agglomeration behavior may impede their technical applications. Here, a solid-state solution of MnS_0.5Se_0.5 nanocubes in rock-salt phase has been synthesized for the first time at a relatively low temperature from the precursors of Mn(II) acetylacetonate with dibenzyl dichalcogens in oleylamine with octadecene, and the MnS_0.5Se_0.5 nanocubes have been assembled with N-doped graphene to form a new kind of heterostructured nanohybrids (shortened as MnS_0.5Se_0.5/N-G hybrids), which are very potential for the fabrication of metal-ion batteries including LIBs and/or SIBs. Investigations revealed that there have been dense vacancies generated and active sites increased via nonequilibrium alloying of MnS and MnSe into the solid-solution MnS_0.5Se_0.5 nanocubes with segregation and defects achieved in the low-temperature solution synthetic route. Meanwhile, the introduction of N-doped graphene forming heterojunction interfaces between MnS_0.5Se_0.5 and N-doped graphene would efficiently enhance their electroconductivity and avoid agglomeration of the active MnS_0.5Se_0.5 nanocubes with considerably improved electrochemical properties. As a result, the MnS_0.5Se_0.5/N-G hybrids delivered superior Li/Na storage capacities with outstanding rate performance as well as satisfactorily lasting stability (1039/457 mA h g^-1 at 0.1 A g^-1 for LIBs/SIBs). Additionally, full-cell LIBs of the anodic MnS_0.5Se_0.5/N-G constructed with cathodic LiFePO₄ (LFP) further confirmed the promising future for their practical application.

Highly compacted TiO2/C micospheres via in-situ surface-confined intergrowth with ultra-long life for reversible Na-ion storage

TiO₂ as the promising anode material candidate of sodium-ion battery suffers from poor conductivity and slow ion diffusion rate, which severely hampers its development. Highly compacted TiO₂/C microspheres without inner pores/tunnels are synthesized by a very facile one-pot rapid processing method based on novel in-situ surface-confined inter-growth mechanism. This highly compacted TiO₂/C microspheres exhibit an excellent electrochemical performance of reversible Na⁺ storage despite with relatively large particle/aggregation size from submicrometer to micrometer. An outstanding cycling stability extending to 10,000 cycles is gained with a high retention capacity of 140.5 mAh g^-1 at a current rate of 2 A g^-1. An ultra-high reversible capacity of 362 mAh g^-1 close to its theoretic specific capacity is obtained at a current rate of 0.05 A g^-1. The successful combination of highly compacted structure with large particle size, excellent electrochemical performance as well as rapid cost-effective preparing process might provide a potential industrial approach for efficiently synthesizing electrode materials for Na ion batteries.

Oxygen Vacancy Engineering in Titanium Dioxide for Sodium Storage

Titanium dioxide (TiO₂ ) is a promising anode material for sodium-ion batteries (SIBs) due to its low cost, natural abundance, nontoxicity, and excellent electrochemical stability. Oxygen vacancies, the most common point defects in TiO₂ , can dramatically influence the physical and chemical properties of TiO₂ , including band structure, crystal structure and adsorption properties. Recent studies have demonstrated that oxygen-deficient TiO₂ can significantly enhance sodium storage performance. Considering the importance of oxygen vacancies in modifying the properties of TiO₂ , the structural properties, common synthesis strategies, characterization techniques, as well as the contribution of oxygen-deficient TiO₂ on initial Coulombic efficiency, cyclic stability, rate performance for sodium storage are comprehensively described in this review. Finally, some perspectives on the challenge and future opportunities for the development of oxygen-deficient TiO₂ are proposed.

The oxygen vacancy in Li-ion battery cathode materials

The substantial capacity gap between available anode and cathode materials for commercial Li-ion batteries (LiBs) remains, as of today, an unsolved problem. Oxygen vacancies (OVs) can promote Li-ion diffusion, reduce the charge transfer resistance, and improve the capacity and rate performance of LiBs. However, OVs can also lead to accelerated degradation of the cathode material structure, and from there, of the battery performance. Understanding the role of OVs for the performance of layered lithium transition metal oxides holds great promise and potential for the development of next generation cathode materials. This review summarises some of the most recent and exciting progress made on the understanding and control of OVs in cathode materials for Li-ion battery, focusing primarily on Li-rich layered oxides. Recent successes and residual unsolved challenges are presented and discussed to stimulate further interest and research in harnessing OVs towards next generation oxide-based cathode materials.

Novel NiCl2 Nanosheets Synthesized via Chemical Vapor Deposition with High Specific Energy for Thermal Battery

Two-dimensional (2D) nanomaterials possessing a unique sheet structure, compared to correlative bulk materials, exhibit excellent properties, especially in the energy storage and energy conversion field. In this case, NiCl₂ nanosheets with thicknesses of 2-8 nm are first prepared by a simple chemical vapor deposition method. For the Li-B/LiF-LiCl-LiBr/NiCl₂ thermal battery, the specific energy of NiCl₂ nanosheets increases from 510 W h kg^-1 (NiCl₂ rods) to 616 W h kg^-1 at an operation temperature of 500 °C and a current density of 0.2 A cm^-2. The 2D morphology and large numbers of defects not only improve the redox reaction rates and the lithium storage capacity, but also enhance the adsorption capacity with the flake-like binder MgO, which prolong the discharge time by suppressing the discharge product diffusion to the electrolyte. These results indicate that NiCl₂ nanosheets have a great possibility to become a desirable candidate of cathode materials for assisting in the development of high energy output and provide a new way to restrain the immersion between the electrode and electrolyte.

Construction of tetrahedral CoO4 vacancies for activating the high oxygen evolution activity of Co3-xO4-δ porous nanosheet arrays

This study presents low-crystalline and non-stoichiometric cobalt oxide (Co3-xO4-δ) porous nanosheet arrays (PNAs) grown on carbon fiber cloth (CFC) (Co3-xO4-δ PNAs/CFC) by a facile in situ anodic oxidation strategy. We firstly verified that the above prepared low crystalline cobalt oxide contained tetrahedral CoO4 vacancies, resulting in the creation of O vacancies at adjacent octahedral CoO6 sites, allowing the generation of tetragonal-pyramidal CoO5 sites which were regarded as active sites and being accessible for the oxygen evolution reaction (OER) with different reaction mechanisms compared to that of traditional CoO6 sites in high-crystalline and stoichiometric Co3O4, thus endowing Co3-xO4-δ PNAs/CFC with significantly improved OER activity and superior stability compared to their crystalline counterparts (Co3O4 PNAs/CFC), as illustrated by experiments and density functional theory (DFT) calculations. This study will open up a new approach for the synthesis of defect-rich materials and provide new insight into the structure-property relationship of OER catalysts.

High Pseudocapacitance Boosts Ultrafast, High-Capacity Sodium Storage of 3D Graphene Foam-Encapsulated TiO2 Architecture

Anatase TiO₂ is an attractive anode for Li-ion batteries and Na-ion batteries because of its structural stability. However, the electrochemical capability of anatase TiO₂ is unsatisfactory due to its intrinsically low electrical conductivity and poor ion diffusivity at the electrode/electrolyte interface. We prepared 3D lightweight graphene aerogel-encapsulated anatase TiO₂, which exhibits a high reversible capacity (390 mA h g^-1 at 50 mA g^-1), a superior rate performance (164.9 mA h g^-1 at 5 A g^-1), and a long-term cycling capability (capacity retention of 86.8% after 7800 cycles). The major energy-storage mechanism is surface capacitance dominated, which favors a high capacity and fast Na⁺ uptake. The inherent features of 3D porous aerogels provide additional active reaction sites and facilitate fast charge diffusion and easy ion access. This will enable the development of 3D interconnected, graphene-based, high-capacity active materials for the development of next-generation energy-storage applications.

Selenium vacancy-rich and carbon-free VSe2 nanosheets for high-performance lithium storage

VSe2 is a typical transition metal dichalcogenide with metallic conductivity, which makes it a potentially promising electrode material for lithium-ion batteries (LIBs). However, further research into the VSe2 nanomaterial for electrochemical applications has been seriously impeded by the practical difficulty of synthesizing phase-pure VSe2. In this work, Se vacancy-rich VSe2 nanosheets were synthesized by a one-step solvothermal method with suitable reactants. Benefiting from the strong reduction ability of hydrazine hydrate, V4+ was partly reduced into V3+, resulting in abundant Se vacancies being generated in situ in the as-obtained VSe2 nanosheets. Positron annihilation lifetime spectroscopy, X-ray absorption spectroscopy and photoluminescence spectroscopy all confirmed the existence of Se vacancies. When applied as the anode material for LIBs, the VSe2 nanosheets can deliver a remarkable reversible capacity of 1020 mA h g-1 at 0.1 A g-1 after 100 cycles, and even at 2 A g-1 a high specific capacity of 430 mA h g-1 is reached. Electrochemical characterizations further reveal that the Se vacancies in the VSe2 nanosheets can significantly enhance lithium-ion diffusion kinetics and increase the number of electrochemical active sites, which are responsible for the good lithium-storage performance. This work may provide an alternative approach for rational design of other high-performance electrode materials for LIBs to satisfy demand for future sustainable development.

Thermodynamic analysis and kinetic optimization of high-energy batteries based on multi-electron reactions

Multi-electron reaction can be regarded as an effective way of building high-energy systems (>500 W h kg⁻¹). However, some confusions hinder the development of multi-electron mechanisms, such as clear concept, complex reaction, material design and electrolyte optimization and full-cell fabrication. Therefore, this review discusses the basic theories and application bottlenecks of multi-electron mechanisms from the view of thermodynamic and dynamic principles. In future, high-energy batteries, metal anodes and multi-electron cathodes are promising electrode materials with high theoretical capacity and high output voltage. While the primary issue for the multi-electron transfer process is sluggish kinetics, which may be caused by multiple ionic migration, large ionic radius, high reaction energy barrier, low electron conductivity, poor structural stability, etc., it is urgent that feasible and versatile modification methods are summarized and new inspiration proposed in order to break through kinetic constraints. Finally, the remaining challenges and future research directions are revealed in detail, involving the search for high-energy systems, compatibility of full cells, cost control, etc.

Two-dimensional metal-organic frameworks and their derivatives for electrochemical energy storage and electrocatalysis

Two-dimensional (2D) metal-organic frameworks (MOFs) and their derivatives with excellent dimension-related properties, e.g. high surface areas, abundantly accessible metal nodes, and tailorable structures, have attracted intensive attention as energy storage materials and electrocatalysts. A major challenge on the road toward the commercialization of 2D MOFs and their derivatives is to achieve the facile and controllable synthesis of 2D MOFs with high quality and at low cost. Significant developments have been made in the synthesis and applications of 2D MOFs and their derivatives in recent years. In this review, we first discuss the state-of-the-art synthetic strategies (including both top-down and bottom-up approaches) for 2D MOFs. Subsequently, we review the most recent application progress of 2D MOFs and their derivatives in the fields of electrochemical energy storage (e.g., batteries and supercapacitors) and electrocatalysis (of classical reactions such as the HER, OER, ORR, and CO2RR). Finally, the challenges and promising strategies for the synthesis and applications of 2D MOFs and their derivatives are addressed for future development.

Anion Vacancies Regulating Endows MoSSe with Fast and Stable Potassium Ion Storage

Vacancy engineering is a promising approach for optimizing the energy storage performance of transition metal dichalcogenides (TMDs) due to the unique properties of vacancies in manipulating the electronic structure and active sites. Nevertheless, achieving effective introduction of anion vacancies with adjustable vacancy concentration on a large scale is still a big challenge. Herein, MoS_2(1-x)Se_2x alloys with anion vacancies introduced in situ have been achieved by a simple alloying reaction, and the vacancy concentration has been optimized through adjusting the chemical composition. Experimental and density functional theory calculation results suggest that the anion vacancies in MoS_2(1-x)Se_2x alloys could enhance the electronic conductivity, induce more active sites, and alleviate structural variation in the alloys during the potassium storage process. When applied as potassium ion battery anodes, the most optimized vacancy-rich MoSSe alloy delivered high reversible capacities of 517.4 and 362.4 mAh g^-1 at 100 and 1000 mA g^-1, respectively. Moreover, a reversible capacity of 220.5 mAh g^-1 could be maintained at 2000 mA g^-1 after 1000 cycles. This work demonstrates a practical approach to modifying the electronic and defect properties of TMDs, providing an effective strategy for constructing advanced electrode materials for battery systems.

A Review on the Promising Plasma-Assisted Preparation of Electrocatalysts

Electrocatalysts are becoming increasingly important for both energy conversion and environmental catalysis. Plasma technology can realize surface etching and heteroatom doping, and generate highly dispersed components and redox species to increase the exposure of the active edge sites so as to improve the surface utilization and catalytic activity. This review summarizes the recent plasma-assisted preparation methods of noble metal catalysts, non-noble metal catalysts, non-metal catalysts, and other electrochemical catalysts, with emphasis on the characteristics of plasma-assisted methods. The influence of the morphology, structure, defect, dopant, and other factors on the catalytic performance of electrocatalysts is discussed.

Core-shell anatase anode materials for sodium-ion batteries: the impact of oxygen vacancies and nitrogen-doped carbon coating

In this work, the impact of oxygen vacancies and nitrogen-doped carbon coating on the sodium-ion storage properties of anatase TiO2 has been demonstrated. Oxygen vacancies and nitrogen-doped carbon coating were introduced simultaneously by the calcination of core-shell structured TiO2 spheres in a reducing atmosphere. Compared to the anatase TiO2 with and without oxygen vacancies, TiO2-x@NC exhibits much better electrochemical performance in the storage of sodium ions. A high reversible capacity of 245.6 mA h g-1 is maintained at 0.1 A g-1 after 200 cycles, and a high specific capacity of 155.6 mA h g-1 is achieved at a high rate of 5.0 A g-1. The significantly improved electrochemical performance of the core-shell structured anatase TiO2 spheres is attributed to the synergistic effect of the oxygen vacancies in the anatase lattice and surface nitrogen-doped carbon coating. This work provides an efficient strategy for improving the electrochemical performance of metal-oxide-based electrode materials for sodium-ion batteries.

Ultrafast Zinc-Ion Diffusion Ability Observed in 6.0-Nanometer Spinel Nanodots

Rechargeable aqueous Zn-ion batteries (ZIBs) have recently attracted much attention due to their low cost and superior safety. Unfortunately, their low capacity and poor cycle life still hinder their practical application. Here, we have developed a general synthesis strategy for ultrasmall spinel oxide nanodots (Mn₃O₄, CoMn₂O₄, MnCo₂O_4.5, Co₃O₄, and ZnMn₂O₄) with abundant oxygen vacancies and highly active surface. Among them, 6.0-nanometer-sized Mn₃O₄ nanodots deliver the best Zn-ion storage ability with a high reversible capacity of 386.7 mA h g^-1 at 0.1 A g^-1, excellent rate performance, and a long-term stability of 500 cycles at 0.5 A g^-1. Taking advantage of the highly activated surficial atoms, shortened transfer pathway, and introduction of numerous oxygen vacancies, an ultrahigh Zn²⁺ diffusion coefficient of 2.4 × 10^-10 cm² s^-1 has been detected during the discharge process. This value is more than 2 orders of magnitude higher than that of other spinel oxide nanostructures in previous reports and also the highest one in all of the as-reported ZIB cathode materials to date. Our finding offers promising opportunities for the development of ZIB cathode materials with high energy density, long-term cycling stability, excellent flexibility, and wearability.

Defect-Assisted Selective Surface Phosphorus Doping to Enhance Rate Capability of Titanium Dioxide for Sodium Ion Batteries

Phosphorus doping is an effective strategy to simultaneously improve the electronic conductivity and regulate the ionic diffusion kinetics of TiO₂ being considered as anode materials for sodium ion batteries. However, efficient phosphorus doping at high concentration in well-crystallized TiO₂ nanoparticles is still a big challenge. Herein, we propose a defect-assisted phosphorus doping strategy to selectively engineer the surface structure of TiO₂ nanoparticles. The reduced TiO_2-x shell layer that is rich in oxygen defects and Ti³⁺ species precisely triggered a high concentration of phosphorus doping (∼7.8 at. %), and consequently a TiO₂@TiO_2-x-P core@shell architecture was produced. Comprehensive characterizations and first-principle calculations proved that the surface-functionalized TiO_2-x-P thin layer endowed the TiO₂@TiO_2-x-P with substantially enhanced electronic conductivity and accelerated Na ion transportation, resulting in great rate capability (167 mA h g^-1 at 10 000 mA g^-1) and stable cycling (99% after 5000 cycles at 10 A g^-1). Combining in/ex situ X-ray diffraction with ex situ electron spin resonance clearly demonstrated the high reversibility and robust mechanical behavior of TiO₂@TiO_2-x-P upon long-term cycling. This work provides an interesting and effective strategy for precise heteroatoms doping to improve the electrochemical performance of nanoparticles.

Controllably Enriched Oxygen Vacancies through Polymer Assistance in Titanium Pyrophosphate as a Super Anode for Na/K-Ion Batteries

Although sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are promising prospects for next-generation energy storage devices, their low capacities and inferior kinetics hinder their further application. Among various phosphate-based polyanion materials, titanium pyrophosphate (TiP₂O₇) possesses outstanding ion transferability and electrochemical stability. However, it has rarely been adopted as an anode for SIBs/PIBs due to its poor electronic conductivity and nonreversible phase transitions. Herein, an ultrastable TiP₂O₇ with enriched oxygen vacancies is prepared as a SIB/PIB anode through P-containing polymer mediation carbonization, which avoids harsh reduction atmospheres or expensive facilities. The introduction of oxygen vacancies effectively increases the pseudocapacitance and diffusivity coefficient and lowers the Na insertion energy barrier. As a result, the TiP₂O₇ anode with enriched oxygen vacancies exhibits ultrastable Na/K ion storage and superior rate capability. The synthetic protocol proposed here may offer a simple pathway to explore advanced oxygen vacancy-type anode materials for SIBs/PIBs.

Anatase TiO2 as a Na+-Storage Anode Active Material for Dual-Ion Batteries

Anatase TiO₂ used as the sodium-storage anode is coupled with a graphite cathode to construct dual-ion batteries. The batteries display wide voltage window (1.0-4.7 V), long cycling stability (98 mAh g^-1 after 1400 cycles at 500 mA g^-1), and considerable rate performance (102 mAh g^-1 at 1500 mA g^-1). Furthermore, the kinetic behaviors of Na⁺ and PF₆^- ions are investigated through electrochemical impedance spectroscopy, the galvanostatic intermittent titration technique, and the potentiostatic intermittent titration technique. The corresponding apparent ion diffusion coefficient is calculated, and the results indicate that the transport of PF₆^- anions in the graphite cathode is swift.

Plasma-Introduced Oxygen Defects Confined in Li4Ti5O12 Nanosheets for Boosting Lithium-Ion Diffusion

Although Li₄Ti₅O₁₂ (LTO) is considered as a promising anode material for high-power Li-ion batteries with high safety, the sluggish Li-ion diffusion coefficient restricts its widespread application. In this work, oxygen vacancy was successfully incorporated into LTO by an eco-friendly and cost-effective plasma process. The deficient LTO delivers much higher capacities of 173.4 mAh g^-1 at 1C rate after 100 cycles and 140.5 mAh g^-1 at 5C after 1000 cycles than those of pristine LTO. Meanwhile, even at a high rate of 20C, it displays an ultrahigh capacity of 133.1 mAh g^-1 after 500 cycles with a Coulombic efficiency of 100%. Detailed analysis reveals that the lithium storage mechanisms in the oxygen-deficient LTO, especially at high rate, were dominated by the insertion behavior and dual-phase conversion due to the fast ion-diffusion ability, rather than the widely reported surface capacitance by other approaches. This work highlights that defect generation by plasma in nanomaterials is an effective way to promote ion mobility, especially at high rates, and thus can be extended to other electrode materials for advanced energy-storage applications.

Polyvinylpyrrolidone-Induced Uniform Surface-Conductive Polymer Coating Endows Ni-Rich LiNi0.8Co0.1Mn0.1O2 with Enhanced Cyclability for Lithium-Ion Batteries

The Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ (NCM₈₁₁) cathode has attracted great interest owing to its low cost, high capacity, and energy density. Nevertheless, rapid capacity fading is a critical problem because of direct contact of NCM₈₁₁ with electrolytes and hence restrains its wide applications. To prevent the direct contact, the surface inert layer coating becomes a feasible strategy to tackle this problem. However, to achieve a homogeneous surface coating is very challenging. Considering the bonding effect between NCM₈₁₁, polyvinylpyrrolidone (PVP), and polyaniline (PANI), in this work, we use PVP as an inductive agent to controllably coat a uniform conductive PANI layer on NCM₈₁₁ (NCM₈₁₁@PANI-PVP). The coated PANI layer not only serves as a rapid channel for electron conduction, but also prohibits direct contact of the electrode with the electrolyte to effectively hinder side reaction. NCM₈₁₁@PANI-PVP thus exhibits excellent cyclability (88.7% after 100 cycles at 200 mA g^-1) and great rate performance (152 mA h g^-1 at 1000 mA g^-1). In situ X-ray diffraction and in situ Raman are performed to investigate the charge-discharge mechanism and the cyclability of NCM₈₁₁@PANI-PVP upon electrochemical reaction. This surfactant-modulated surface uniform coating strategy offers a new modification approach to stabilize Ni-rich cathode materials for lithium-ion batteries.

Exploring the sodium ion storage mechanism of gallium sulfide (Ga2S3): a combined experimental and theoretical approach

Developing sodium ion battery (SIB) anode materials of a low-cost and high-capacity nature for future large-scale applications still involves challenges. Herein, we have reported gallium sulfide (Ga2S3) as a novel SIB anode material for the first time. Ga2S3 nanorods have been synthesized via the facile hydrothermal preparation of a GaOOH precursor with subsequent H2S annealing. Mixed with graphene upon electrode preparation, this Ga2S3 electrode maintains a reversible specific capacity of 476 mA h g-1 after 100 cycles at a current density of 0.4 A g-1, with a coulombic efficiency of over 99%. Ex situ XRD analysis and theoretical calculations are employed to comprehensively elucidate the detailed sodium ion storage mechanism of Ga2S3, which is composed of initial Na+ intercalation, a subsequent multi-step conversion reaction between S and Na+, and an eventual alloying reaction between Ga and Na+ with the end product of Na7Ga13. Further kinetics analysis has demonstrated that the conversion reaction is the rate-limiting step due to a multi-step reaction with the intermediate phase of GaS. Moreover, the appearance of liquid metal Ga, as confirmed via TEM observations and theoretical calculations, can serve as a self-healing agent that repairs cracks in the electrode. Our findings shed light on the further design of Ga-based materials, and they also can be extended to solid-state-battery systems.

Sub-20 nm Carbon Nanoparticles with Expanded Interlayer Spacing for High-Performance Potassium Storage

Carbon materials are most promising candidates for potassium-ion battery (PIB) anodes because of their high electrical conductivities, rational potassium storage capabilities, and low costs. However, the large volume change during the K-ion insertion/extraction and the sluggish kinetics of K-ion diffusion inhibit the development of carbon-based materials for PIBs. Here, under the guidance of density functional theory, N/P-codoped ultrafine (≤20 nm) carbon nanoparticles (NP-CNPs) with an expanded interlayer distance, improved electrical conductivity, shortened diffusion distance of K ions, and promoted adsorption capability toward K ions are synthesized through a facile solvent-free method as a high-performance anode material for PIBs. The NP-CNPs show a high capacity of 270 mA h g^-1 at 0.2 A g^-1, a remarkable rate capability of 157 mA h g^-1 at an extremely high rate of 5.0 A g^-1, and an ultralong cycle life with a high capacity of 190 mA h g^-1 and a retention of 86.4% at 1.0 A g^-1 after 4000 cycles. The potassium storage mechanism and low volume expansion for NP-CNPs are revealed through cyclic voltammetry, in situ Raman, and ex situ XRD. This work paves a new way to design and fabricate carbon-based nanostructures with high reversible capacity, great rate capability, and stable long-term performance.

Ultraviolet Irradiation Treatment for Enhanced Sodium Storage Performance Based on Wide-Interlayer-Spacing Hollow C@MoS2@CN Nanospheres

The photochemistry and sodium storage process have been generally considered as two separated approaches without strong connection. Here, ultraviolet (UV) irradiation was applied to sodium-ion batteries to improve the electrochemical performance of MoS₂-based composites. C@MoS₂@CN nanospheres consist of double protective structures, including inner hollow carbon spheres with a thin wall (C) and outer N-doping carbon nanosheets (CNs) derived from polydopamine. The special nanostructure possesses the virtues such as wide-interlayer spacing, flexible feature with great structure integrity, and rich active sites, which endow the fast electron transfer and shorten the ion diffusion pathways. Under the excitation of UV-light, intense electrons and holes are accumulated within MoS₂-based composites. The excited electrons can promote the preinsertion of Na⁺. More importantly, dense electrons promote the electrolyte to decompose and hence form a stable solid electrolyte interphase in advance. After UV-light irradiation treatment in the electrolyte, the initial Coulombic efficiency of C@MoS₂@CN electrodes increased from 48.2 to 79.6%, and benefiting from the fine nanostructure, the C@MoS₂@CN electrode with UV irradiation treatment delivered a great rate performance 116  mAh g^-1 in 20 s and  super cycling stability that 87.6% capacity was retained after 500 cycles at 500 mA g^-1. When employed as anode for sodium-ion hybrid capacitors, it delivered a maximum power density of 6.84 kW kg^-1 (with 114.07  Wh kg^-1 energy density) and a maximum energy density of 244.15 Wh g^-1 (with 152.59 W kg^-1 power density). This work sheds new viewpoints into the applications of photochemistry in the development of energy storage devices.

Flexible Freestanding Carbon Nanofiber-Embedded TiO2 Nanoparticles as Anode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs), owning to the low cost, abundant resources, and similar physicochemical properties with lithium-ion batteries (LIBs), have earned much attention for large-scale energy storage systems. In this article, we successfully synthesize flexible freestanding carbon nanofiber-embedded TiO₂ nanoparticles (CNF-TiO₂) and then apply it directly as anode in SIBs without binder or current collector. Taking the advantage of flexible CNF and high structural stability, this anode exhibits high reversible capacity of 614 mAh·g^-1 (0.27 mAh·cm^-2) after almost 400 cycles and excellent capacity retention ability of ~100.