Synthesis, characterizations, and utilization of oxygen-deficient metal oxides for lithium/sodium-ion batteries and supercapacitors

Research Progress on the Preparation of Manganese Dioxide Nanomaterials and Their Electrochemical Applications

Manganese dioxide (MnO2) nanomaterials have shown excellent performance in catalytic degradation and other fields because of their low density and great specific surface area, as well as their tunable chemical characteristics. However, the methods used to synthesize MnO2 nanomaterials greatly affect their structures and properties. Therefore, the present work systematically illustrates common synthetic routes and their advantages and disadvantages, as well as examining research progress relating to electrochemical applications. In contrast to previous reviews, this review summarizes approaches for preparing MnO2 nanoparticles and describes their respective merits, demerits, and limitations. The aim is to help readers better select appropriate preparation methods for MnO2 nanomaterials and translate research results into practical applications. Finally, we also point out that despite the significant progress that has been made in the development of MnO2 nanomaterials for electrochemical applications, the related research remains in the early stages, and the focus of future research should be placed on the development of green synthesis methods, as well as the composition and modification of MnO2 nanoparticles with other materials.

Trifunctional L-Cysteine Assisted Construction of MoO2/MoS2/C Nanoarchitecture Toward High-Rate Sodium Storage

The volume collapse and slow kinetics reaction of anode materials are two key issues for sodium ion batteries (SIBs). Herein, an "embryo" strategy is proposed for synthesis of nanorod-embedded MoO2/MoS2/C network nanoarchitecture as anode for SIBs with high-rate performance. Interestingly, L-cysteine which plays triple roles including sulfur source, reductant, and carbon source can be utilized to produce the sulfur vacancy-enriched heterostructure. Specifically, L-cysteine can combine with metastable monoclinic MoO3 nanorods at room temperature to encapsulate the "nutrient" of MoOx analogues (MoO2.5(OH)0.5 and MoO3·0.5H2O) and hydrogen-deficient L-cysteine in the "embryo" precursor affording for subsequent in situ multistep heating treatment. The resultant MoO2/MoS2/C presents a high-rate capability of 875 and 420 mAh g-1 at 0.5 and 10 A g-1, respectively, which are much better than the MoS2-based anode materials reported by far. Finite element simulation and analysis results verify that the volume expansion can be reduced to 42.8% from 88.8% when building nanorod-embedded porous network structure. Theoretical calculations reveal that the sulfur vacancies and heterointerface engineering can promote the adsorption and migration of Na+ leading to highly enhanced thermodynamic and kinetic reaction. The work provides an efficient approach to develop advanced electrode materials for energy storage.

Using algae in Li-ion batteries: A sustainable pathway toward greener energy storage

This paper reviews and analyzes the innovations and advances in using algae and their derivatives in different parts of Li-ion batteries. Applications in Li-ion battery anodes, electrolytes, binders, and separators were discussed. Algae provides a sustainable feedstock for different materials that can be used in Li-ion batteries, such as carbonaceous material, biosilica, biopolymers, and other materials that have unique micro- and nano-structures that act as biotemplates for composites structure design. Natural materials and biotemplates provided by algae have various advantages, such as electrochemical and thermal stability, porosity that allows higher storage capacity, nontoxicity, and other properties discussed in the paper. Results reveal that despite algae and its derivatives being a promising renewable feedstock for different applications in Li-ion batteries, more research is yet to be performed to evaluate its feasibility of being used in the industry.

Vanadium-Based Cathodic Materials of Aqueous Zn-Ion Battery for Superior-Performance with Prolonged-Life Cycle

Aqueous Zn-ion battery systems (AZIBs) have emerged as the most dependable solution, as demonstrated by successful systematic growth over the past few years. Cost effectivity, high performance and power density with prolonged life cycle are some major reason of the recent progress in AZIBs. Development of vanadium-based cathodic materials for AZIBs has appeared widely. This review contains a brief display of the basic facts and history of AZIBs. An insight section on zinc storage mechanism ramifications is given. A detailed discussion is conducted on features of high-performance and long life-time cathodes. Such features include design, modifications, electrochemical and cyclic performance, along with stability and zinc storage pathway of vanadium based cathodes from 2018 to 2022. Finally, this review outlines obstacles and opportunities with encouragement for gathering a strong conviction for future advancement in vanadium-based cathodes for AZIBs.

Tailored synthesis of NdMnxFe1-xO3 perovskite nanoparticles with oxygen-vacancy defects for lithium-ion battery anodes

In this study, we synthesize nanostructured NdMnxFe1-xO3 perovskites using a facile method to produce materials for the high-working-efficiency anodes of Li-ion batteries. A series of characterization assessments (e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron microscopy) were conducted, and the results confirmed the efficacious partial replacement of Fe ions with Mn ions in the NdFeO3 perovskite structure, occurrence of both amorphous and crystalline structures, presence of oxygen vacancies (VO), and interconnection between nanoparticles. The possibility of Mn ion replacement significantly affects the size, amount of VO, and ratio of amorphous phase in NdMnxFe1-xO3 perovskites. The NdMnxFe1-xO3 perovskite with x = 0.3 presents a notable electrochemical performance, including low charge transfer resistance, durable Coulombic efficiency, first-rate capacity reservation, high pseudo-behavior, and elongated 150-cycle service life, whereas no discernible capacity deterioration is observed. The reversible capacity of the anode after the 150th-cylcle was 713 mAh g-1, which represents a high-capacity value. The outstanding electrochemical efficiency resulted from the optimum presence of VO, interconnection between the nanoparticles, and distinctive properties of the NdFeO3 perovskite. The interconnection between nanoparticles was advantageous for forming a large electrolyte-electrode contact area, improving Li-ion diffusion rates, and enhancing pseudocapacitive effect. The attributes of perovskite crystals, coexistence of Mn and Fe throughout the charge/discharge process, and optimum VO precluded the electrode devastation that caused the Li2O-phase decomposition catalysis, enabling favorable reversible Li storage.

Combining Structural Modification and Electrolyte Regulation to Enable Long-Term Cyclic Stability of MoO3-x @TiO2 as Cathode for Aqueous Zn-Ion Batteries

Orthorhombic MoO3 (α-MoO3 ) with multivalent redox couple of Mo6+ /Mo4+ and layered structure is a promising cathode for rechargeable aqueous Zn-ion batteries (AZIBs). However, pure α-MoO3 suffers rapid capacity decay due to the serious dissolution and structural collapse. Meanwhile, the growth of byproduct and dendrite on the anode also lead to the deterioration of cyclic stability. This article establishes the mechanism of proton intercalation into MoO3 and proposes a joint strategy combining structural modification with electrolyte regulation to enhance the cyclic stability of MoO3 without sacrificing the capacity. In ZnSO4 electrolyte with Al2 (SO4 )3 additive, TiO2 coated oxygen-deficient α-MoO3 (MoO3-x @TiO2 ) delivers a reversible capacity of 93.2 mA h g-1 at 30 A g-1 after 5000 cycles. The TiO2 coating together with the oxygen deficiency avoids structural damage while facilitating proton diffusion. Besides, the additive of Al2 (SO4 )3 , acting as a pump, continuously supplements protons through dynamic hydrolysis, avoiding the formation of Zn4 SO4 (OH)6 ·xH2 O byproducts at both MoO3-x @TiO2 and Zn anode. In addition, Al2 (SO4 )3 additive facilitates uniform deposition of Zn owing to the tip-blocking effect of Al3+ ion. The study demonstrates that the joint strategy is beneficial for both cathode and anode, which may shed some light on the development of AZIBs.

Vacancy designed 2D materials for electrodes in energy storage devices

Vacancies are ubiquitous in nature, usually playing an important role in determining how a material behaves, both physically and chemically. As a consequence, researchers have introduced oxygen, sulphur and other vacancies into bi-dimensional (2D) materials, with the aim of achieving high performance electrodes for electrochemical energy storage. In this article, we focused on the recent advances in vacancy engineering of 2D materials for energy storage applications (supercapacitors and secondary batteries). Vacancy defects can effectively modify the electronic characteristics of 2D materials, enhancing the charge-transfer processes/reactions. These atomic-scale defects can also serve as extra host sites for inserted protons or small cations, allowing easier ion diffusion during their operation as electrodes in supercapacitors and secondary batteries. From the viewpoint of materials science, this article summarises recent developments in the exploitation of vacancies (which are surface defects, for these materials), including various defect creation approaches and cutting-edge techniques for detection of vacancies. The crucial role of defects for improvement in the energy storage performance of 2D electrode materials in electrochemical devices has also been highlighted.

Facile Synthesis of Battery-Type CuMn2O4 Nanosheet Arrays on Ni Foam as an Efficient Binder-Free Electrode Material for High-Rate Supercapacitors

The development of battery-type electrode materials with hierarchical nanostructures has recently gained considerable attention in high-rate hybrid supercapacitors. For the first time, in the present study novel hierarchical CuMn₂O₄ nanosheet arrays (NSAs) nanostructures are developed using a one-step hydrothermal route on a nickel foam substrate and utilized as an enhanced battery-type electrode material for supercapacitors without the need of binders or conducting polymer additives. X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques are used to study the phase, structural, and morphological characteristics of the CuMn₂O₄ electrode. SEM and TEM studies show that CuMn₂O₄ exhibits a nanosheet array morphology. According to the electrochemical data, CuMn₂O₄ NSAs give a Faradic battery-type redox activity that differs from the behavior of carbon-related materials (such as activated carbon, reduced graphene oxide, graphene, etc.). The battery-type CuMn₂O₄ NSAs electrode showed an excellent specific capacity of 125.56 mA h g^-1 at 1 A g^-1 with a remarkable rate capability of 84.1%, superb cycling stability of 92.15% over 5000 cycles, good mechanical stability and flexibility, and low internal resistance at the interface of electrode and electrolyte. Due to their excellent electrochemical properties, high-performance CuMn₂O₄ NSAs-like structures are prospective battery-type electrodes for high-rate supercapacitors.

Oxygen vacancies engineering in electrocatalysts nitrogen reduction reaction

Ammonia is important, both as a fertilizer and as a carrier of clean energy, mainly produced by the Haber-Bosch process, which consumes hydrogen and emits large amounts of carbon dioxide. The ENRR (Electronchemical Nitrogen Reduction Reaction) is considered a promising method for nitrogen fixation owing to their low energy consumption, green and mild. However, the ammonia yield and Faraday efficiency of the ENRR catalysts are low due to the competitive reaction between HER and NRR, the weak adsorption of N2 andthe strong N≡N triple bond. Oxygen vacancy engineering is the most important method to improve NRR performance, not only for fast electron transport but also for effective breaking of the N≡N bond by capturing metastable electrons in the antibonding orbitals of nitrogen molecules. In this review, the recent progress of OVs (oxygen vacancies) in ENRR has been summarized. First, the mechanism of NRR is briefly introduced, and then the generation methods of OVs and their applicationin NRR are discussed, including vacuum annealing, hydrothermal method, hydrogen reduction, wet chemical reduction, plasma treatment and heterogeneous ion doping. Finally, the development and challenges of OVs in the field of electrochemical nitrogen fixation are presented. This review shows the important areas of development of catalysts to achieve industrially viable NRR.

Multifunctional Surface Construction for Long-Term Cycling Stability of Li-Rich Mn-Based Layered Oxide Cathode for Li-Ion Batteries

Li-rich Mn-based layered oxides (LMLOs) are promising cathode material candidate for the next-generation Li-ion batteries (LIBs) of high energy density. However, the fast capacity fading and voltage decay as well as low Coulombic efficiency caused by irreversible oxygen release and phase transition during the electrochemical process hinder their practical application. To solve these problems, in the present study, a multifunctional surface construction involving a coating layer, spinel-layered heterostructure, and rich-in oxygen vacancies is successfully conducted by a facile thermal reduction of the LMLO particles with potassium borohydride (KBH₄ ) as the reducing agent. The multifunctional surface structure plays synergistic effects on suppressing the interface side reaction, reducing the dissolution of transition metal, increasing electron conductivity and lithium diffusion rate. As a result, electrochemical performances of the LMLO cathode are effectively enhanced. With optimization of the addition of KBH₄ , the electrode delivers a reversible capacity of 280 mAh g^-1 at 0.1 C, which maintains after 100 cycles. The capacity retention with respect to the initial capacity is as high as 98% at 1 C after 400 cycles. The present work provides insights into designing a highly effective functional surface structure of LMLO cathode materials for high-performance LIBs.

High Pressure Rapid Synthesis of LiCrTiO4 with Oxygen Vacancy for High Rate Lithium-Ion Battery Anodes

Lithium-ion battery based on LiCrTiO₄ (LCTO) is considered to be a promising anode material, as they provide higher safety and durability beyond than that of graphite electrode. However, the applications of this transformative technology demand improved inherent electrical conductivity of LCTO as well as a simple and rapid synthetic route. Here, LCTO with oxygen vacancies (OVs) is fabricated using high-pressure synthesis technology in only 40 min. The optimal synthesis pressure is 0.8 GPa (LCTO-0.8). The reversible capacity of LCTO-0.8 at 1C is 131 mA h g^-1 after 1000 cycles and the capacity retention is nearly 97%, and the reversible capacity of LCTO synthesized at atmospheric pressure (LCTO-P) is 85 mA h g^-1 under the same circumstances. Even at 5C, the reversible capacity is 110 mA h g^-1 , which is 77% higher than LCTO-P. Furthermore, it is confirmed by theoretical calculations that the introduction of OVs has the occupation of electronic states at the Fermi level, which greatly enhances the intrinsic conductivity of LCTO. Specifically, the electronic conductivity has increased by two orders of magnitude compared with LCTO-P. Therefore, high-pressure synthesis technology endows LCTO with superior characteristics, providing a new avenue for industrialization.

Kinetically Accelerated Lithium Storage in High-Entropy (LiMgCoNiCuZn)O Enabled By Oxygen Vacancies

High-entropy oxides (HEOs) are gradually becoming a new focus for lithium-ion battery (LIB) anodes due to their vast element space/adjustable electrochemical properties and unique single-phase retention ability. However, the sluggish kinetics upon long cycling limits their further generalization. Here, oxygen vacancies with targeted functionality are introduced into rock salt-type (MgCoNiCuZn)O through a wet-chemical molten salt strategy to accelerate the ion/electron transmission. Both experimental results and theoretical calculations reveal the potential improvement of lithium storage, charge transfer, and diffusion kinetics from HEO surface defects, which ultimately leads to enhanced electrochemical properties. The currently raised strategy offers a modular approach and enlightening insights for defect-induced HEO-based anodes.

Perspective on Micro-Supercapacitors

The rapid development of portable, wearable, and implantable electronic devices greatly stimulated the urgent demand for modern society for multifunctional and miniaturized electrochemical energy storage devices and their integrated microsystems. This article reviews material design and manufacturing technology in different micro-supercapacitors (MSCs) along with devices integrate to achieve the targets of their various applications in recent years. Finally, We also critically prospect the future development directions and challenges of MSCs.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

Recent progress in electrochemical performance of carbon-based anodes for potassium-ion batteries based on first principles calculations

Carbonaceous materials and the composite materials of transition metals compounds in carbon matrix were widely used as anode for potassium-ion batteries (PIBs). During the research of these anode materials, first-principles calculations based on adsorption energy, density of states (DOSs) as well as diffusion energy barriers was regarded as an effectively approach to investigate their potassium storage mechanism. The underlying reasons for the improvement of electrochemical performance could be well illustrated via the corresponding calculations. Moreover, first-principles calculations also played a vital role to predict the material properties of electrodes before conducting experimental analysis. Hence, this review is to analyze in-depth the effect mechanism of K-adsorption energy, DOSs as well as diffusion energy barrier and so on for electrochemical performance of carbon-based anode materials. We summarized the corresponding research progress, the challenges of first principles calculations in PIBs, and proposed the corresponding strategies along with future perspectives for further development of carbon-based anode materials. This work not only can provide theoretical guidance for the development of anode materials with excellent physical and chemical properties, but also have reference significance for other energy storage systems.

Exploring the electrochemical performance of copper-doped cobalt-manganese phosphates for potential supercapattery applications

The significant electrochemical performance in terms of both specific energy and power delivered via hybrid energy storage devices (supercapattery) has raised their versatile worth but electrodes with flashing electrochemical conduct are still craved for better performance. In this work, binary and ternary metal phosphates based on copper, cobalt, and manganese were synthesized by a sonochemical method. Then, the compositions of copper and cobalt were optimized in ternary metal phosphates. The structural studies and morphological aspects of synthesized materials were scrutinized by X-ray diffraction and scanning electron microscopy. Furthermore, the electrochemical characterizations were performed in three- and two-cell configurations. The sample with equal compositions of copper and cobalt (50/50) demonstrates the highest specific capacity of 340 C g⁻¹ at a current density of 0.5 A g⁻¹ among all. This optimized composition was utilized as a positive electrode material in a supercapattery device that reveals a high specific capacity of 247 C g⁻¹. The real device exhibits an excellent energy density of 55 W h kg⁻¹ while delivering a power density of 800 W kg⁻¹. Furthermore, the device was able to provide an outstanding specific power of 6400 W kg⁻¹ while still exhibiting a specific energy of 19 W h kg⁻¹. The stability potential of the device was tested for 2500 continuous charge and discharge cycles at 8 A g⁻¹. Excellent capacitive retention of 90% was obtained, which expresses outstanding cyclic stability of the real device. A theoretical study was performed to investigate the capacitance and diffusion-controlled contribution in the device performance using Dunn's model. The maximum diffusion-controlled contribution of 85% was found at 3 mV s⁻¹ scan rate. The study demonstrates the utilization of ternary metal phosphates as self-supported electrode materials for potential energy storage applications.

The optimized copper-doped cobalt–manganese phosphate was utilized as a positive electrode in an asymmetric architecture (supercapattery device), which yields enhanced specific energy and power.

Black SnO2-x based nanotheranostic for imaging-guided photodynamic/photothermal synergistic therapy in the second near-infrared window

The shallow penetration depth of photothermal agents in the first near-infrared (NIR-I) window significantly limits their therapeutic efficiency. Multifunctional nanotheranostic agents in the second near-infrared (NIR-II) window have drawn extensive attention for their combined treatment of tumors. Here, for the first time, we created oxygen-deficient black SnO_2-x with strong NIR (700-1200 nm) light absorption with NaBH₄ reduction from white SnO₂. Hyaluronic acid (HA) could selectively target cancer cells overexpressed CD44 protein. After modification with HA, the obtained nanotheranostic SnO_2-x@SiO₂-HA showed high dispersity in aqueous solution and good biocompatibility. SnO_2-x@SiO₂-HA was confirmed to simultaneously generate enough hyperthermia and reactive oxygen species with single NIR-II (1064 nm) light irradiation. Because HA is highly affined to CD44 protein, SnO_2-x@SiO₂-HA has specific uptake by overexpressed CD44 cells and can be accurately transferred to the tumor site. Furthermore, tumor growth was significantly inhibited following synergistic photodynamic therapy (PDT) and photothermal therapy (PTT) with targeted specificity under the guidance of photoacoustic (PA) imaging using 1064 nm laser irradiation in vivo. Moreover, SnO_2-x@SiO₂-HA accelerated wound healing. This work prominently extends the therapeutic utilization of semiconductor nanomaterials by changing their nanostructures and demonstrates for the first time that SnO_2-x based therapeutic agents can accelerate wound healing. STATEMENT OF SIGNIFICANCE: The phototherapeutic efficacy of nanotheranostics by NIR-I lightirradiation was restricted owing to the limitation of tissue penetration and maximum permissible exposure. To overcome these limitations, we hereby fabricated a NIR-IIlight-mediated multifunctional nanotheranostic based on SnO₂-x. The introduction of oxygen vacancy strategy was employed to construct full spectrum responsive oxygen-deficient SnO2-x, endowing outstanding photothermal conversion, and remarkable production activity of reactive oxygen species under NIR-II light activation. Tumor growth was significantly inhibited following synergistic PDT/PTT with targeted specificity under the guidance of photoacoustic imaging using 1064 nm laser irradiation in vivo. Our strategy not only expands the biomedical application of SnO₂, but also providea method to develop other inorganic metal oxide-based nanosystems for NIR-II light-activated phototheranostic of cancers.

Sulfur vacancies enriched Nickel-Cobalt sulfides hollow spheres with high performance for All-Solid-State hybrid supercapacitor

To pursue excellent performance of supercapacitor, an electrode material with designed morphology and tailored intrinsic properties is indeed desired. Herein, nickel-cobalt sulfides hollow spheres decorated with rich sulfur vacancies r-NiCo₂S₄ HSs) are prepared via an anion exchange of Ni-Co coordination polymer spheres, combined with wet chemical reduction. The r-NiCo₂S₄ HSs sample delivers excellent performance as an electrode: it possesses a high specific capacity (763.5C g^-1 at 1 A/g), favorable cyclability (91.40% after 5000 cycles at 10 A/g) and rate capacity (522.68C g^-1 at 15 A/g). Additionally, an all-solid-state hybrid supercapacitor device, assembled with r-NiCo₂S₄ HSs as the positive electrode and N/S co-doped activated carbon nanosheets as the negative electrode, presents an excellent energy density of 50.76 Wh kg^-1 under 800 W kg^-1 and feasible stability. Thus, combining hollow structure with sulfur vacancies could not only increase more active sites and ensure sufficient redox reactions, but also enhance electronic conductivity, facilitate ions / electrons transport and shorten diffusion path, which could be regarded as a promising approach to develop electrode materials with outstanding performance.

Lattice Engineering to Simultaneously Control the Defect/Stacking Structures of Layered Double Hydroxide Nanosheets to Optimize Their Energy Functionalities

An effective lattice engineering method to simultaneously control the defect structure and the porosity of layered double hydroxides (LDHs) was developed by adjusting the elastic deformation and chemical interactions of the nanosheets during the restacking process. The enlargement of the intercalant size and the lowering of the charge density were effective in increasing the content of oxygen vacancies and enhancing the porosity of the stacked nanosheets via layer thinning. The defect-rich Co-Al-LDH-NO₃^- nanohybrid with a small stacking number exhibited excellent performance as an oxygen evolution electrocatalyst and supercapacitor electrode with a large specific capacitance of ∼2230 F g^-1 at 1 A g^-1, which is the largest capacitance of carbon-free LDH-based electrodes reported to date. Combined with the results of density functional theory calculations, the observed excellent correlations between the overpotential/capacitance and the defect content/stacking number highlight the importance of defect/stacking structures in optimizing the energy functionalities. This was attributed to enhanced orbital interactions with water/hydroxide at an increased number of defect sites. The present cost-effective lattice engineering process can therefore provide an economically feasible methodology to explore high-performance electrocatalyst/electrode materials.

Dual carbon-confined Sb2Se3 nanoparticles with pseudocapacitive properties for high-performance lithium-ion half/full batteries

Transition metal selenides have attracted enormous research attention as anodes for lithium-ion batteries (LIBs) due to their high theoretical specific capacities. Nevertheless, the low electronic conductivity and dramatic volume variation in electrochemical reaction processes result in rapid capacity fading and poor rate capability. Herein, a metal-organic framework is used as a template to in situ synthesize Sb₂Se₃ nanoparticles encapsulated in N-doped carbon nanotubes (N-CNTs) grafted on reduced graphene oxide (rGO) nanosheets. The synergistic effects of N-doped carbon nanotubes and reduced graphene oxide nanosheets are beneficial for providing good electrical conductivity and maintaining the structural stability of electrode materials, leading to stable cycling performance and superior rate performance. Kinetic analysis suggests that the electrochemical reaction kinetics is dominated by pseudocapacitive contribution. Notably, a high discharge capacity of 451.1 mA h g^-1 at a current density of 2.0 A g^-1 is delivered after 450 cycles. Even at a high current density of 10.0 A g^-1, a discharge capacity of 192.6 mA h g^-1 is maintained after 10 000 cycles. When coupled with a commercial LiFePO₄ cathode, the full batteries show an excellent discharge specific capacity of 534.5 mA h g^-1 at 0.2 A g^-1. This work provides an effective strategy for constructing high-performance anodes for Li⁺ storage.

Demystifying the Lattice Oxygen Redox in Layered Oxide Cathode Materials of Lithium-Ion Batteries

The practical application of lithium-ion batteries suffers from low energy density and the struggle to satisfy the ever-growing requirements of the energy-storage Internet. Therefore, developing next-generation electrode materials with high energy density is of the utmost significance. There are high expectations with respect to the development of lattice oxygen redox (LOR)-a promising strategy for developing cathode materials as it renders nearly a doubling of the specific capacity. However, challenges have been put forward toward the deep-seated origins of the LOR reaction and if its whole potential could be effectively realized in practical application. In the following Review, the intrinsic science that induces the LOR activity and crystal structure evolution are extensively discussed. Moreover, a variety of characterization techniques for investigating these behaviors are presented. Furthermore, we have highlighted the practical restrictions and outlined the probable approaches of Li-based layered oxide cathodes for improving such materials to meet the practical applications.

Roadmap on Ionic Liquid Electrolytes for Energy Storage Devices

Ionic liquids are considered to be promising electrolyte solvents or additives for rechargeable batteries (i. e., lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, aluminum-ion batteries, etc.) and supercapacitors. This is related with the superior physical and electrochemical properties of ionic liquids, which can influence the performance of rechargeable batteries. Therefore, it is necessary to write a roadmap on ionic liquids for rechargeable batteries. In this roadmap, some progress, critical techniques, opportunities and challenges of ionic liquid electrolytes for various batteries and supercapacitors are pointed out. Especially, properties and roles of ionic liquids should be considered in energy storage. Ionic liquids can be used as electrolyte salts, electrolyte additives, and solvents. For optimizing ionic liquid-based electrolytes for energy storage, their applications in various energy storage devices should be considered by combing native chemical/physical properties and their roles. We expect that this roadmap will give a useful guidance in directing future research in ionic liquid electrolytes for rechargeable batteries and supercapacitors.

Adjusting the Valence State of Vanadium in VO2 (B) by Extracting Oxygen Anions for High-Performance Aqueous Zinc-Ion Batteries

VO₂ generally has a higher theoretical capacity and layered structure suitable for the intercalation/extraction of zinc ions. However, Zn²⁺ ions with high charge density interact with the crystal lattice and limit further improvement in electrochemical performance. Defect engineering is a potential modification method with very promising application prospects, but the established procedures for preparing defects are complicated. In this study, VO_2-x (B) with oxygen deficiency is prepared by a simple solution reaction with NaBH₄ . The presence of oxygen deficiencies is confirmed by positron annihilation lifetime spectroscopy, UV/Vis absorbance spectroscopy and others. Owing to the presence of oxygen defects, the aqueous Zn/VO_2-x (B) battery exhibits improved specific capacity, excellent reversibility, and structural stability. Ex situ characterization techniques are employed to demonstrate the reversible insertion-extraction mechanism of Zn²⁺ ions from and into the host material. In addition, the Zn/VO_2-x (B) batteries still exhibit considerable electrochemical performance, even with high-loading electrodes (about 4 mg cm^-2 ).

Direct Growth of Oxygen Vacancy-Enriched Co3O4 Nanosheets on Carbon Nanotubes for High-Performance Supercapacitors

Ultrathin Co₃O₄ nanosheets (NSs) with abundant oxygen vacancies on conductive carbon nanotube (CNT) nanocomposites (termed as Co₃O₄-NSs/CNTs) are easily achieved by an effective NaBH₄-assisted cyanogel hydrolysis strategy under ambient conditions. The specific capacitance of Co₃O₄-NSs/CNTs with 5% CNT mass can reach 1280.4 F g^-1 at 1 A g^-1 and retain 112.5% even after 10 000 cycles, demonstrating very high electrochemical capability and stability. When assembled in the two-electrode Co₃O₄-NSs/CNTs-5%//reduced graphene oxide (rGO) system, a maximum specific energy density of 37.2 Wh kg^-1 (160.2 W kg^-1) is obtained at room temperature. Ultrathin structure of nanosheets, abundant oxygen vacancies, and the synergistic effect between Co₃O₄-NSs and CNTs are crucial factors for excellent electrochemical performance. Specifically, these characteristics favor rapid electron transfer, complete exposure of the active interface, and sufficient adsorption/desorption of electrolyte ions within the active material. This work gives insights into the efficient construction of two-dimensional hybrid electrodes with high performance for the new-generation energy storage system.

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-Deficient Cobalt-Based Oxides for Electrocatalytic Water Splitting

An apparent increased interest has been recently devoted towards the previously untrodden path for anionic point defect engineering of electrocatalytic surfaces. The role of vacancy engineering in improving photo- and electrocatalytic activities of transition metal oxides (TMOs) has been widely reported. In particular, oxygen vacancy modulation on electrocatalysts of cobalt-based TMOs has seen a fresh spike of research work due to the substantial improvements they have shown towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Oxygen vacancy engineering is an effective scheme to quintessentially tune the electronic structure and charge transport, generate secondary active surface phases, and modify the surface adsorption/desorption behavior of reaction intermediates during water splitting. Based on contemporary efforts for inducing oxygen vacancies in a variety of cobalt oxide types, this work addresses facile and environmentally benign synthesis strategies, characterization techniques, and detailed insight into the intrinsic mechanistic modulation of electrocatalysts. It is our foresight that appropriate utilization of the principles discussed herein will aid researchers in rationally designing novel materials that can outperform noble metal-based electrocatalysts. Ultimately, future electrocatalysis implementation for selective seawater splitting is believed to depend on regulating the surface chemistry of active and stable TMOs.

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.

Preparation of Y-Doped La2Ti2O7 Flexible Self-Supporting Films and Their Application in High-Performance Flexible All-Solid-State Supercapacitor Devices

Flexible all-solid-state supercapacitors have drawn more attention owing to the rapid growth of wearable electronic equipments. Herein, we have succeeded in synthesizing a series of Y-doped lanthanum titanate flexible self-supporting films (LSF-x, 0.1 ≤ x ≤ 0.5) and investigating the change of microstructures, morphological characteristics, and lattice structures of these films affected by different Y-doping contents. To further determine the optimum Y-doping content, we have explored the electrochemical properties of working electrodes prepared by LSF-x (0.1 ≤ x ≤ 0.5) samples as the main active material. As the LSF-0.2 electrode has the best areal capacitance of 1.3 F·cm-2 at 2 mA·cm-2, we use the LSF-0.2 electrodes and PVA-Na2SO4 gel to fabricate a flexible all-solid-state supercapacitor device. This device has a high areal capacitance of 255.9 mF·cm-2 at a current density of 2 mA·cm-2 with a high cell voltage of 2.1 V, while the corresponding energy density is 156.8 μWh·cm-2 with a power density of 2.1 mW·cm-2. Moreover, it also shows a long cycling life and outstanding flexibility. Therefore, the LSF-0.2 sample can be used as an excellent energy-storage material for a wearable electronic device.

Co-Construction of Sulfur Vacancies and Heterojunctions in Tungsten Disulfide to Induce Fast Electronic/Ionic Diffusion Kinetics for Sodium-Ion Batteries

Engineering novel electrode materials with unique architectures has a significant impact on tuning the structural/electrochemical properties for boosting the performance of secondary battery systems. Herein, starting from well-organized WS₂ nanorods, an ingenious design of a one-step method is proposed to prepare a bimetallic sulfide composite with a coaxial carbon coating layer, simply enabled by ZIF-8 introduction. Rich sulfur vacancies and WS₂ /ZnS heterojunctions can be simultaneously developed, that significantly improve ionic and electronic diffusion kinetics. In addition, a homogeneous carbon protective layer around the surface of the composite guarantees an outstanding structural stability, a reversible capacity of 170.8 mAh g^-1 after 5000 cycles at a high rate of 5 A g^-1 . A great potential in practical application is also exhibited, where a full cell based on the WS_{2- x} /ZnS@C anode and the P2-Na_2/3 Ni_1/3 Mn_1/3 O₂ cathode can maintain a reversible capacity of 89.4 mAh g^-1 after 500 cycles at 1 A g^-1 . Moreover, the underlying electrochemical Na storage mechanisms are illustrated in detail by theoretical calculations, electrochemical kinetic analysis, and operando X-ray diffraction characterization.

Controllable Synthesis of Anatase TiO2 Nanosheets Grown on Amorphous TiO2/C Frameworks for Ultrafast Pseudocapacitive Sodium Storage

Pseudocapacitance has been confirmed to significantly improve the rate capability and cycling durability of electrode materials. However, rational design and controllable synthesis of intercalation pseudocapacitive materials for sodium-ion batteries (SIBs) still remain greatly challenging. Herein, a core-shell TiO₂-based anode composed of S-, Co-, and N-doped amorphous TiO₂/C framework cores and ultrathin anatase TiO₂ nanosheet shells (SCN-TC@UT) was synthesized using Ti-based metal-organic frameworks (Ti-MOFs) as self-sacrificing templates coupled with a solvothermal sulfidation process. Thanks to heteroatom doping, integration of carbon species, and 2D nanosheet coating, the kinetic properties of SCN-TC@UT have been significantly improved. As a consequence, the anode achieves ultrahigh capacitive contributions up to 90.9 and 96.3% of the total capacity at scan rates of 5 and 10 mV s^-1 and delivers unprecedented capacities of 211, 201, and 100 mA h g^-1 at 1, 5, and 30 C (1 C=335 mA g^-1) for over 800, 2000, and 18,000 cycles, respectively. Even at an ultrahigh rate of 50 C, the anode can still deliver a capacity of 108 mA h g^-1. This work demonstrates the most efficient TiO₂-based anode ever reported for SIBs and holds great potential in directing the development of amorphous materials for intercalation pseudocapacitance.

Fundamental Triangular Interaction of Electron Trajectory Deviation and P-N Junction to Promote Redox Reactions for the High-Energy-Density Electrode

Highly efficient redox reaction of active electrode materials is the guarantee for achieving high energy density for energy storage devices. Here, we design a triangle of the electrode material involving the P-N junction between NiO (p-type) and MoO₃ (n-type) and electron trajectory deviation between gold nanoparticles with NiO or MoO₃. This optimized fundamental triangle structure could facilitate the redox reaction of a metal oxide, and thus the fabricated ternary nanocomposites exhibit excellent electrochemical performance. At a lower current density (0.5 A g^-1), the mass specific capacitance of a single electrode can reach 943.3 F g^-1, while the NiO/MoO₃ tested under the same conditions only has a specific capacitance of 278.9 F g^-1. The assembled asymmetric device with activated carbon shows a higher capacitance retention rate of 98.7% after long-term cycling under different current densities, and a maximum energy density of 28.9 W h kg^-1 (power density of 400.1 W kg^-1). The crucial prerequisite of this strategy is the lower work function of gold nanoparticles compared with active materials, which significantly reduce the activation energy of NiO/MoO₃ and the formed P-N junction between p-type NiO with n-type MoO₃ in their contact interfaces. This novel design of a triangle structure could be expected to be applied in other materials to develop a kind of energy storage device with excellent electrochemical performance.

Amorphous iron oxide-selenite composite microspheres with a yolk-shell structure as highly efficient anode materials for lithium-ion batteries

Yolk-shell structured transition metal compounds have intrinsic structural advantages as anode materials and have been synthesized in a highly crystalline form. Thus, development of a synthesis process for a yolk-shell structure with an amorphous state, that displays high structural stability and fast ionic diffusion, is a notable research subject, with wide applications in fields such as energy storage. Herein, a novel approach for synthesizing amorphous materials with a yolk-shell structure using several facile phase transformation processes is presented. Crystalline iron oxide microspheres with a yolk-shell structure were formed by oxidation of the spray-dried product at 400 °C. Using the pitch/tetrahydrofuran solution infiltration method, pitch-infiltrated iron oxide was selenized at 350 °C to form a crystalline iron selenide-C composite. The following partial oxidation process at 375 °C produced a yolk-shell structured amorphous iron oxide-selenite composite. The amorphous characteristics, the yolk-shell structure, and the formation of a heterostructured interface from iron selenite during the initial cycle contributed to high electrochemical kinetic properties and excellent cycling performance of the iron oxide-selenite composite. The amorphous iron oxide-iron selenite yolk-shell microspheres exhibited enhanced reversible capacities, cycling stability, and remarkable electrochemical kinetic properties when compared to crystalline iron oxide.