Defect Engineering in Titanium-Based Oxides for Electrochemical Energy Storage Devices

Bi-C monolayer as a promising 2D anode material for Li, Na, and K-ion batteries

Electrode materials with high electrochemical efficiency are required for battery technology that can be used to store renewable energy. Bismuth (Bi) has shown great potential as an electrode material for metal ion batteries due to its large volumetric capacity and reasonable operating potential. However, the cycling performance deteriorates due to the drastic volume changes that occur during alloying and dealloying. Herein, we design a 2D Bi-C metal sheet using density functional theory and investigate the feasibility of this nanosheet for alkali metal ion batteries. The predicted metallic Bi-C monolayer (ML) are highly stable and show sound electrode performance. Moreover, alkali metal atoms exhibit high diffusivities on both sides (Bi and C sides) with low energy barriers of 0.252/0.201, 0.217/0.169, and 0.179/0.136 eV for Li, Na, and K ions, respectively. Furthermore, the Bi-C ML shows high theoretical storage capacities of (485 mA h g^-1) for Li and Na and (364 mA h g^-1) for K and low open-circuit voltage of 0.12, 0.24, and 0.32 V for Li, Na, and K ions, respectively. These exciting findings show that the predicted Bi-C ML can be used as an anode material for Li-, Na- and K-ion batteries.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

Single-Crystal Nickel-Based Cathodes: Fundamentals and Recent Advances

Lithium-ion batteries (LIBs) represent the most promising choice for meeting the ever-growing demand of society for various electric applications, such as electric transportation, portable electronics, and grid storage. Nickel-rich layered oxides have largely replaced LiCoO2 in commercial batteries because of their low cost, high energy density, and good reliability. Traditional nickel-based oxide particles, usually called polycrystal materials, are composed of microsized primary particles. However, polycrystal particles tend to suffer from pulverization and severe side reactions along grain boundaries during cycling. These phenomena accelerate cell degradation. Single-crystal materials, which exhibit robust mechanical strength and a high surface area, have great potential to address the challenges that hinder their polycrystal counterparts. A comprehensive understanding of the growing body of research related to single-crystal materials is imperative to improve the performance of cathodes in LIBs. This review highlights origins, recent developments, challenges, and opportunities for single-crystal layered oxide cathodes. The synthesis science behind single-crystal materials and comparative studies between single-crystal and polycrystal materials are discussed in detail. Industrial techniques and facilities are also reviewed in combination with our group’s experiences in single-crystal research. Future development should focus on facile production with strong control of the particle size and distribution, structural defects, and impurities to fully reap the benefits of single-crystal materials.

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A Novel Hybrid Point Defect of Oxygen Vacancy and Phosphorus Doping in TiO2 Anode for High-Performance Sodium Ion Capacitor

Although sodium ion capacitors (SICs) are considered as one of the most promising electrochemical energy storage devices (organic electrolyte batteries, aqueous batteries and supercapacitor, etc.) due to the combined merits of battery and capacitor, the slow reaction kinetics and low specific capacity of anode materials are the main challenges. Point defects including vacancies and heteroatoms doping have been widely used to improve the kinetics behavior and capacity of anode materials. However, the interaction between vacancies and heteroatoms doping have been seldomly investigated. In this study, a hybrid point defects (HPD) engineering has been proposed to synthesize TiO₂ with both oxygen vacancies (OVs) and P-dopants (TiO₂/C-HPD). In comparison with sole OVs or P-doping treatments, the synergistic effects of HPD on its electrical conductivity and sodium storage performance have been clarified through the density functional theory calculation and sodium storage characterization. As expected, the kinetics and electronic conductivity of TiO₂/C-HPD3 are significantly improved, resulting in excellent rate performance and outstanding cycle stability. Moreover, the SICs assembled from TiO₂/C-HPD3 anode and nitrogen-doped porous carbon cathode show outstanding power/energy density, ultra-long life with good capacity retention. This work provides a novel point defect engineering perspective for the development of high-performance SICs electrode materials.

Ultra-thin graphene cube framework@TiO2 heterojunction as high-performance anode materials for lithium ion batteries

Here, we proposed a new strategy to build the integrated graphene cube (Gr) framework@TiO₂ composite to improve the ion transport kinetics and electrical conductivity of TiO₂ as a long-life and high-capacity anode for lithium ion batteries. Combined with the salt template method for ultra-thin framework, the distinct structure of Gr@TiO₂ shows an excellent electrochemical performance, e.g., initial coulombic efficiency (ICE), rate performance and specific capacity, due to the increased kinetics of lithium ions. Through this method, the integrity is dramatically improved and the pulverization and agglomeration of the anode after long-term cycles are restrained. The optimized Gr@TiO₂ displays a high stable reversible capacity of 179.5 mAh g^-1 after 4000 cycles at 1 A g^-1, excellent rate performance (125.5 mAh g^-1 at 5 A g^-1). Kinetic studies through Electrochemical Impedance Spectra, Galvanostatic Intermittent Titration Technique and Linear Sweep Voltammetry confirm that the electrical conductivity and ion transport kinetics are dramatically improved through the ultra-thin graphene cube framework as a heterojunction structure of Gr@TiO₂.

Facile hydrothermal synthesis and enhanced electrochemical properties of a layered NiSiO/RGO nanocomposite with an interesting dandelion-like structure

Materials with unique structures can exhibit different properties and are widely studied in the preparation of new materials. Herein we reported a hydrothermal method to fabricate a layered nickel silicate/reduced graphene oxide (NiSiO/RGO) nanocomposite with an interesting dandelion-like structure. The morphology, composition, and electrochemical performance of RGO, NiSiO, and NiSiO/RGO were comparatively investigated in the current work. The results showed that the NiSiO/RGO nanocomposite has a dandelion-like hollow core-shell structure and shows good electrochemical performance. Compared with NiSiO, the original discharge capacity of NiSiO/RGO increased from 1291.6 mA h g^-1 to 1653.9 mA h g^-1; meanwhile, the reversible specific capacity of NiSiO/RGO increased from 649.6 mA h g^-1 to 691.4 mA h g^-1 after testing at a current density of 100 mA g^-1 for 100 cycles. Moreover, the prepared NiSiO/RGO maintained a coulombic efficiency of about 97% after the initial charging and discharging cycle. This unique hollow dandelion-like structure enhanced the electrical conductivity and further resulted in lower diffusion resistance and higher reversible capacity. This work demonstrated that the layered NiSiO/RGO with an interesting dandelion-like structure can act as an alternative anode material for lithium-ion batteries.

Defect engineering and characterization of active sites for efficient electrocatalysis

Electrocatalysis plays a decisive role in various energy-related applications. Engineering the active sites of electrocatalysts is an important aspect to promote their catalytic performance. In particular, defect engineering provides a feasible and efficient approach to improve the intrinsic activities and increase the number of active sites in electrocatalysts. In this review, recent investigations on defect engineering of a wide range of electrocatalysts such as metal-free carbon materials, transition metal oxides, transition metal dichalcogenides and metal-organic frameworks (MOFs) will be summarized. Different defect creation strategies will be outlined, for example, heteroatom doping and removal, plasma irradiation, hydrogenation, amorphization, phase transition and reduction treatment. In addition, we will overview the commonly used advanced characterization techniques that could confirm the existence and identify the detailed structures, types and concentration of defects in electrocatalysts. The defect characterization tools are beneficial for gaining an in-depth understanding of defects on electrocatalysis and thus could reveal the structure-performance relationship. Finally, the major challenges and future development directions on defect engineering of electrocatalysts will be discussed.

Application of Scanning Tunneling Microscopy in Electrocatalysis and Electrochemistry

Abstract

Scanning tunneling microscopy (STM) has gained increasing attention in the field of electrocatalysis due to its ability to reveal electrocatalyst surface structures down to the atomic level in either ultra-high-vacuum (UHV) or harsh electrochemical conditions. The detailed knowledge of surface structures, surface electronic structures, surface active sites as well as the interaction between surface adsorbates and electrocatalysts is highly beneficial in the study of electrocatalytic mechanisms and for the rational design of electrocatalysts. Based on this, this review will discuss the application of STM in the characterization of electrocatalyst surfaces and the investigation of electrochemical interfaces between electrocatalyst surfaces and reactants. Based on different operating conditions, UHV-STM and STM in electrochemical environments (EC-STM) are discussed separately. This review will also present emerging techniques including high-speed EC-STM, scanning noise microscopy and tip-enhanced Raman spectroscopy.

Graphic Abstract

Cu3(PO4)2: Novel Anion Convertor for Aqueous Dual-Ion Battery

A novel anion electrode Cu₃(PO₄)₂ is proposed at the first time. The reaction mechanism of Cu₃(PO₄)₂ electrode is investigated. The dual-ion cell is constructed by using pretreated Cu₃(PO₄)₂ and Na_0.44MnO₂.

SUPPLEMENTARY INFORMATION

The online version of this article (10.1007/s40820-020-00576-1) contains supplementary material, which is available to authorized users.

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.

Function and Application of Defect Chemistry in High-Capacity Electrode Materials for Li-Based Batteries

Current commercial Li-based batteries are approaching their energy density limitation, yet still cannot satisfy the energy density demand of the high-end devices. Hence, it is critical to developing advanced electrode materials with high specific capacity. However, these electrode materials are facing challenges of severe structural degradation and fast capacity fading. Among various strategies, constructing defects in electrode materials holds great promise in addressing these issues. Herein, we summarize a series of significant defect engineering in the high-capacity electrode materials for Li-based batteries. The detailed retrospective on defects specification, function mechanism, and corresponding application achievements on these electrodes are discussed from the view of point, line, planar, volume defects. Defect engineering can not only stabilize the structure and enhance electric/ionic conductivity, but also act as active sites to improve the ionic storage and bonding ability of electrode materials to Li metal. We hope this review can spark more perspectives on evaluating high-energy-density Li-based batteries.

DFT-Guided Design and Fabrication of Carbon-Nitride-Based Materials for Energy Storage Devices: A Review

Carbon nitrides (including CN, C₂N, C₃N, C₃N₄, C₄N, and C₅N) are a unique family of nitrogen-rich carbon materials with multiple beneficial properties in crystalline structures, morphologies, and electronic configurations. In this review, we provide a comprehensive review on these materials properties, theoretical advantages, the synthesis and modification strategies of different carbon nitride-based materials (CNBMs) and their application in existing and emerging rechargeable battery systems, such as lithium-ion batteries, sodium and potassium-ion batteries, lithium sulfur batteries, lithium oxygen batteries, lithium metal batteries, zinc-ion batteries, and solid-state batteries. The central theme of this review is to apply the theoretical and computational design to guide the experimental synthesis of CNBMs for energy storage, i.e., facilitate the application of first-principle studies and density functional theory for electrode material design, synthesis, and characterization of different CNBMs for the aforementioned rechargeable batteries. At last, we conclude with the challenges, and prospects of CNBMs, and propose future perspectives and strategies for further advancement of CNBMs for rechargeable batteries.

Understanding the Structure-Performance Relationship of Lithium-Rich Cathode Materials from an Oxygen-Vacancy Perspective

Li-rich layered oxide cathode materials are regarded as an attractive candidate of next-generation Li-ion batteries (LIBs) to realize an energy density of >300 Wh kg^-1. However, challenges such as capacity fade, cycle life, oxygen release, and structural transformation still restrain its practical application. Micro/nanotechnology is one of the effective strategies to enhance its structural stability and electrochemical performance. An in-depth understanding of the relationship between micro/nanostructures and the electrochemical performance of Li-rich layered oxides is undoubtedly important for developing high-performance cathode materials. Herein, Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ with different micro/nanostructures including irregular particles, microspheres, microrods, and orthogonal particles are synthesized. Starting from the amount of surface oxygen vacancies in the different structures, the influence of oxygen vacancies on every step during the charge-discharge processes is analyzed by experimental characterizations and theoretical calculations. It is indicated that intrinsic oxygen vacancies can enhance the electrical conductivity and decrease the energy barrier for ion migration, which exerts a significant influence on promoting the kinetics and capacity. Among the different micro/nanostructures, microrods with abundant oxygen vacancies can not only promote lithium ion transport but also stabilize a cathode electrolyte interface (CEI) film by adjusting the distribution of surface elements with lower nickel content. The microrods deliver an initial discharge capacity of up to 306.1 mAh g^-1 at 0.1C rate and a superior cycle performance with a capacity retention of 91.0% after 200 cycles at 0.2C rate.

Formation and stability of small polarons at the lithium-terminated Li4Ti5O12 (LTO) (111) surface

Zero strain insertion, high cycling stability, and a stable charge/discharge plateau are promising properties rendering Lithium Titanium Oxide (LTO) a possible candidate for an anode material in solid state Li ion batteries. However, the use of pristine LTO in batteries is rather limited due to its electronically insulating nature. In contrast, reduced LTO shows an electronic conductivity several orders of magnitude higher. Studying bulk reduced LTO, we could show recently that the formation of polaronic states can play a major role in explaining this improved conductivity. In this work, we extend our study toward the lithium-terminated LTO (111) surface. We investigate the formation of polarons by applying Hubbard-corrected density functional theory. Analyzing their relative stabilities reveals that positions with Li ions close by have the highest stability among the different localization patterns.

Recent Advances of Bimetallic Sulfide Anodes for Sodium Ion Batteries

The high usage for new energy has been promoting the next-generation energy storage systems (ESS). As promising alternatives to lithium ion batteries (LIBs), sodium ion batteries (SIBs) have caused extensive research interest owing to the high natural Na abundance of 2.4 wt.% (vs. 0.0017 wt.% for Li) in the earth's crust and the low cost of it. The development of high-performance electrode materials has been challenging due to the increase in the feasibility of SIBs technology. In the past years, bimetallic sulfides (BMSs) with high theoretical capacity and outstanding redox reversibility have shown great promise as high performance anode materials for SIBs. Herein, the recent advancements of BMSs as anode for SIBs are reported, and the electrochemical mechanism of these electrodes are systematically investigated. In addition, the current issues, challenges, and perspectives are highlighted to address the extensive understanding of the associated electrochemical process, aiming to provide an insightful outlook for possible directions of anode materials for SIBs.