Two-Dimensional Metal Oxide Nanomaterials for Next-Generation Rechargeable Batteries

Interface chemistry of two-dimensional heterostructures - fundamentals to applications

Two-dimensional heterostructures (2D HSs) have emerged as a new class of materials where dissimilar 2D materials are combined to synergise their advantages and alleviate shortcomings. Such a combination of dissimilar components into 2D HSs offers fascinating properties and intriguing functionalities attributed to the newly formed heterointerface of constituent components. Understanding the nature of the surface and the complex heterointerface of HSs at the atomic level is crucial for realising the desired properties, designing innovative 2D HSs, and ultimately unlocking their full potential for practical applications. Therefore, this review provides the recent progress in the field of 2D HSs with a focus on the discussion of the fundamentals and the chemistry of heterointerfaces based on van der Waals (vdW) and covalent interactions. It also explains the challenges associated with the scalable synthesis and introduces possible methodologies to produce large quantities with good control over the heterointerface. Subsequently, it highlights the specialised characterisation techniques to reveal the heterointerface formation, chemistry and nature. Afterwards, we give an overview of the role of 2D HSs in various emerging applications, particularly in high-power batteries, bifunctional catalysts, electronics, and sensors. In the end, we present conclusions with the possible solutions to the associated challenges with the heterointerfaces and potential opportunities that can be adopted for innovative applications.

Microbe-Assisted Assembly of Ti3C2Tx MXene on Fungi-Derived Nanoribbon Heterostructures for Ultrastable Sodium and Potassium Ion Storage

As a typical family of two-dimensional (2D) materials, MXenes present physiochemical properties and potential for use in energy storage applications. However, MXenes suffer some of the inherent disadvantages of 2D materials, such as severe restacking during processing and service and low capacity of energy storage. Herein, a MXene@N-doped carbonaceous nanofiber structure is designed as the anode for high-performance sodium- and potassium-ion batteries through an in situ bioadsorption strategy; that is, Ti₃C₂T_x nanosheets are assembled onto Aspergillus niger biofungal nanoribbons and converted into a 2D/1D heterostructure. This microorganism-derived 2D MXene-1D N-doped carbonaceous nanofiber structure with fully opened pores and transport channels delivers high reversible capacity and long-term stability to store both Na⁺ (349.2 mAh g^-1 at 0.1A g^-1 for 1000 cycles) and K⁺ (201.5 mAh g^-1 at 1.0 A g^-1 for 1000 cycles). Ion-diffusion kinetics analysis and density functional theory calculations reveal that this porous hybrid structure promotes the conduction and transport of Na and K ions and fully utilizes the inherent advantages of the 2D material. Therefore, this work expands the potential of MXene materials and provides a good strategy to address the challenges of 2D energy storage materials.

Nano Polymorphism-Enabled Redox Electrodes for Rechargeable Batteries

Nano polymorphism (NPM), as an emerging research area in the field of energy storage, and rechargeable batteries, have attracted much attention recently. In this review, the recent progress on the composition and formation of polymorphs, and the evolution processes of different redox electrodes in rechargeable metal-ion, metal-air, and metal-sulfur batteries are highlighted. First, NPM and its significance for rechargeable batteries are discussed. Subsequently, the current NPM modulation strategies of different types of representative electrodes for their corresponding rechargeable battery applications are summarized. The goal is to demonstrate how NPM could tune the intrinsic material properties, and hence, improve their electrochemical activities for each battery type. It is expected that the analysis of polymorphism and electrochemical properties of materials could help identify some "processing-structure-properties" relationships for material design and performance enhancement. Lastly, the current research challenges and potential research directions are discussed to offer guidance and perspectives for future research on NPM engineering.

Understanding the High-Performance Anode Material of CoC2 O4 ⋅2 H2 O Microrods Wrapped by Reduced Graphene Oxide for Lithium-Ion and Sodium-Ion Batteries

Metal oxalate has become a most promising candidate as an anode material for lithium-ion and sodium-ion batteries. However, capacity decrease owing to the volume expansion of the active material during cycling is a problem. Herein, a rod-like CoC₂ O₄ ⋅2 H₂ O/rGO hybrid is fabricated through a novel multistep solvo/hydrothermal strategy. The structural characteristics of the CoC₂ O₄ ⋅2 H₂ O microrod wrapped using rGO sheets not only inhibit the volume variation of the hybrid electrode during cycling, but also accelerate the transfer of electrons and ions in the 3 D graphene network, thereby improving the electrochemical properties of CoC₂ O₄ ⋅2 H₂ O. The CoC₂ O₄ ⋅2 H₂ O/rGO electrode delivers a specific capacity of 1011.5 mA h g^-1 at 0.2 A g^-1 after 200 cycles for lithium storage, and a high capacity of 221.1 mA h g^-1 at 0.2 A g^-1 after 100 cycles for sodium storage. Moreover, the full cell CoC₂ O₄ ⋅2 H₂ O/rGO//LiCoO₂ consisting of the CoC₂ O₄ ⋅2 H₂ O/rGO anode and LiCoO₂ cathode maintains 138.1 mA h g^-1 after 200 cycles at 0.2 A g^-1 and has superior long-cycle stability. In addition, in situ Raman spectroscopy and in situ and ex situ X-ray diffraction techniques provide a unique opportunity to understand fully the reaction mechanism of CoC₂ O₄ ⋅2 H₂ O/rGO. This work also gives a new perspective and solid research basis for the application of metal oxalate materials in high-performance lithium-ion and sodium-ion batteries.

Interfacial Design on Graphene-Hematite Heterostructures for Enhancing Adsorption and Diffusion towards Superior Lithium Storage

Hematite (α-Fe₂O₃) is a promising electrode material for cost-effective lithium-ion batteries (LIBs), and the coupling with graphene to form Gr/α-Fe₂O₃ heterostructures can make full use of the merits of each individual component, thus promoting the lithium storage properties. However, the influences of the termination of α-Fe₂O₃ on the interfacial structure and electrochemical performance have rarely studied. In this work, three typical Gr/α-Fe₂O₃ interfacial systems, namely, single Fe-terminated (Fe-O₃-Fe-R), double Fe-terminated (Fe-Fe-O₃-R), and O-terminated (O₃-Fe-Fe-R) structures, were fully investigated through first-principle calculation. The results demonstrated that the Gr/Fe-O₃-Fe-R system possessed good structural stability, high adsorption ability, low volume expansion, as well as a minor diffusion barrier along the interface. Meanwhile, investigations on active heteroatoms (e.g., B, N, O, S, and P) used to modify Gr were further conducted to critically analyze interfacial structure and Li storage behavior. It was demonstrated that structural stability and interfacial capability were promoted. Furthermore, N-doped Gr/Fe-O₃-Fe-R changed the diffusion pathway and made it easy to achieve free diffusion for the Li atom and to shorten the diffusion pathway.

Robust hollow TiO2 spheres for lithium/sodium ion batteries with excellent cycling stability and rate capability

We report a facile solvothermal synthesis of hollow TiO2 nanospheres using phenolic resin nanospheres as templates under magnetic stirring condition, followed by annealing, which demonstrate excellent lithium/sodium storage performance.

Hierarchical NiCo2Se4 nanoneedles/nanosheets with N-doped 3D porous graphene architecture as free-standing anode for superior sodium ion batteries

In order to cope with the problem of insufficient lithium metal reserves, sodium ion batteries (SIBs) are proposed and extensively studied for the next-generation batteries. In our work, hierarchical NiCo₂Se₄ nanoneedles/nanosheets are deposited on the skeleton of N-doped three dimensional porous graphene (NPG) by a convenient solvothermal method and subsequent gas-phase selenization process. Compared with NiCo₂Se₄ powder, the optimized NiCo₂Se₄/N-doped porous graphene composite (denoted as NCS@NPG) as self-supporting anode exhibits the excellent electrode activity for SIBs, with a specific capacity of 500 mAh/g and 257 mAh/g at a current density of 0.2 A/g and 6.4 A/g, respectively. The high specific capacity as well as rate capacity can be attributed to the three-dimensional graphene skeleton with high electrical conductivity and pore structure, which provides convenient ion and electron transmission channels.

A high-throughput assessment of the adsorption capacity and Li-ion diffusion dynamics in Mo-based ordered double-transition-metal MXenes as anode materials for fast-charging LIBs

Utilizing the latest SCAN-rVV10 density functional, we thoroughly assess the electrochemical properties of 35 Mo-based ordered double transition metal MXenes, including clean Mo2MC2 (M = Sc, Ti, V, Zr, Nb, Hf, Ta) and surface functionalized structures Mo2MC2T2 (T = H, O, F and OH), for the potential use as anode materials in lithium ion batteries (LIBs). The first principles molecular dynamics simulations in combination with the calculations of the site adsorption preferences for Li atoms on all investigated MXenes reveal that both Li-saturated adsorption structures and theoretical capacities of Mo-based MXenes are fundamentally influenced by the surface terminations. We find that the adsorption of Li atoms on either -OH or -F functionalized MXenes is chemically unstable. In particular, the F-groups prefer to form a separate fluoride layer with Li atoms, detaching from the Mo2MC2 substrates. The Li atoms could form a stable single adsorption layer on the -H, -O and intrinsic MXenes surface, exhibiting theoretical capacities in the range from 121 mA h g-1 to 195 mA h g-1. Besides -F and -OH terminations, the remaining Mo-based MXenes also possess superior flat open circuit voltage (OCV) profiles with the most reversible storage capacity below 1.0 V during the charging/discharging cycles. We further predict the low barrier heights of Li-ion diffusion, at a range of 0.03-0.06 eV for most Mo-based MXenes except -O and -H terminations, exceeding that of graphene or Ti3C2. Furthermore, combining the Vineyard transition state theory (TST) with the phonon spectra obtained from density functional perturbation theory (DFPT), the mean planar diffusion coefficient is calculated to be 2 × 10-8 m2 s-1 at 300 K for intrinsic Mo2MC2 monolayers. Although the overall specific capacity is fundamentally restricted with the relatively heavy molecular mass of MXenes, we conclude that Mo-based structures, especially the intrinsic Mo2MC2 (M = Sc, Ti, V) monolayers, might be promising anode materials from the aspect of fast charging/discharging application for LIBs.

Advanced pillared designs for two-dimensional materials in electrochemical energy storage

Two-dimensional (2D) materials have attracted increased attention as advanced electrodes in electrochemical energy storage owing to their thin nature and large specific surface area. However, limited interlayer spacing confines the mass and ion transport within the layers, resulting in poor rate performance. Considerable efforts have been made to deal with this intrinsic problem of pristine 2D materials. Among them, interlayer engineering through pillared designs offers abundant electrochemical active sites and promotes ion diffusion. Synergetic effects between incorporated species and 2D hosts offer much better conductivity and surface modification. As a result, 2D materials with advanced pillared designs demonstrate great enhancement of specific capacity/capacitance and rate performance. Herein, we summarize the recent progress of pillared 2D materials in relation to the intercalated species. Moreover, we highlight their typical applications in lithium-ion storage and beyond to provide some insights on future trends towards this research area.

Synthesis and Dual-Mode Electrochromism of Anisotropic Monoclinic Nb12O29 Colloidal Nanoplatelets

Transition metal oxide nanocrystals with dual-mode electrochromism hold promise for smart windows enabling spectrally selective solar modulation. We have developed the colloidal synthesis of anisotropic monoclinic Nb₁₂O₂₉ nanoplatelets (NPLs) to investigate the dual-mode electrochromism of niobium oxide nanocrystals. The precursor for synthesizing NPLs was prepared by mixing NbCl₅ and oleic acid to form a complex that was subsequently heated to form an oxide-like structure capped by oleic acid, denoted as niobium oxo cluster. By initiating the synthesis using niobium oxo clusters, preferred growth of NPLs over other polymorphs was observed. The structure of the synthesized NPLs was examined by X-ray diffraction in conjunction with simulations, revealing that the NPLs are monolayer monoclinic Nb₁₂O₂₉, thin in the [100] direction and extended along the b and c directions. Besides having monolayer thickness, NPLs show decreased intensity of Raman signal from Nb-O bonds with higher bond order when compared to bulk monoclinic Nb₁₂O₂₉, as interpreted by calculations. Progressive electrochemical reduction of NPL films led to absorbance in the near-infrared region (stage 1) followed by absorbance in both the visible and near-infrared regions (stage 2), thus exhibiting dual-mode electrochromism. The mechanisms underlying these two processes were distinguished electrochemically by cyclic voltammetry to determine the extent to which ion intercalation limits the kinetics, and by verifying the presence of localized electrons following ion intercalation using X-ray photoelectron spectroscopy. Both results support that the near-infrared absorption results from capacitive charging, and the onset of visible absorption in the second stage is caused by ion intercalation.

Strongly interfacial-coupled 2D-2D TiO2/g-C3N4 heterostructure for enhanced visible-light induced synthesis and conversion

Two-dimensional (2D) nanosheet-based nanocomposites have attracted intensive interest owing to the unique electronic and optical properties from their constituent phases and the synergistic effect from the heterojunctions. In this study, an interfacial coupled TiO₂/g-C₃N₄ 2D-2D heterostructure has been prepared via in situ growth of ultrathin 2D-TiO₂ on dispersed g-C₃N₄ nanosheets. This strongly coupled 2D-2D TiO₂/g-C₃N₄, different from the weakly bonded 2D-TiO₂/g-C₃N₄ heterostructures produced by mechanical mixing, has unique electronic structures and chemical states due to strong interlayer charge transfer, confirmed by both experimental and theoretical analyses. Significantly enhanced visible-light responses have been observed, indicating a great potential for visible-light induced photosynthesis and photocatalysis. For benzylamine coupling reactions under visible-light irradiation, 80 % yield rate has been achieved, superior to ∼30 % yield rate when adopting either 2D-TiO₂ or g-C₃N₄ structure. The enhanced photocatalytic activity can be attributed to the adequate separation of photo-generated electrons at the strongly coupled 2D-2D heterojunction interfaces.

Novel Two-Dimensional Porous Materials for Electrochemical Energy Storage: A Minireview

Two dimensional (2D) porous materials have great potential in electrochemical energy conversion and storage. Over the past five years, our research group has focused on Simple, Mass, Homogeneous and Repeatable Synthesis of various 2D porous materials and their applications for electrochemical energy storage especially for supercapacitors (SCs). During the experimental process, through precisely controlling the experimental parameters, such as reaction species, molar ratio of different ions, concentration, pH value of reaction solution, heating temperature, and reaction time, we have successfully achieved the control of crystal structure, composition, crystallinity, morphology, and size of these 2D porous materials including transition metal oxides (TMOs), transition metal hydroxides (TMHOs), transition metal oxalates (TMOXs), transition metal coordination complexes (TMCCs) and carbon materials, as well as their derivatives and composites. We have also named some of them with CQU-Chen (CQU is the initialism of Chongqing University, Chen is the last name of Lingyun Chen), such as CQU-Chen-Co-O-1, CQU-Chen-Ni-O-H-1, CQU-Chen-Zn-Co-O-1, CQU-Chen-Zn-Co-O-2, CQU-Chen-OA-Co-2-1, CQU-Chen-Co-OA-1, CQU-Chen-Ni-OA-1, CQU-Chen-Gly-Co-3-1, CQU-Chen-Gly-Ni-2-1, CQU-Chen-Gly-Co-Ni-1, etc. The introduction of 2D porous materials as electrode materials for SCs improves the energy storage performances. These materials provide a large number of active sites for ion adsorption, supply plentiful channels for fast ion transport and boost electrical conductivity and facilitate electron transportation and ion penetration. The unique 2D porous structures review is mainly devoted to the introduction of our contribution in the 2D porous nanostructured materials for SC. Finally, the further directions about the preparation of 2D porous materials and electrochemical energy conversion and storage applications are also included.

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.

Carbon-Phosphorus Bonds-Enriched 3D Graphene by Self-Sacrificing Black Phosphorus Nanosheets for Elevating Capacitive Lithium Storage

Heteroatom-doping engineering has been verified as an effective strategy to tailor the electronic and chemical properties of materials. The high amount doping of nonmetal atoms to achieve desired performance, however, is always a grand challenge. Herein, a new strategy to achieve ultrahigh-level doping of phosphorus in a 3D graphene skeleton is proposed by sacrificing heterostructured two-dimensional black phosphorus on graphene. Via this approach, the phosphorus-loading in graphene hydrogel reached a record of 4.84 at. %, together with the formation of tunable pores of size 1.7-17.5 nm in graphene. During reaction kinetic analysis, the highly phosphorus-doped 3D graphene hydrogel anode exhibited more favorable capacitive-controlled ion storage behaviors, leading to a specific capacity as high as 1000 mA h g^-1 after 1700 cycles, which is superior to the pristine graphene hydrogel electrode. This simple but effective phosphorization offers an effective doping strategy for producing ultrahigh-level phosphorous doping but avoids the usual use of toxic phosphorous precursors. Furthermore, the modulation on the activation process over cycling investigated in this work gives us a new insight into designing stable anodes for carbonaceous electrode materials.

3D Porous Self-Standing Sb Foam Anode with a Conformal Indium Layer for Enhanced Sodium Storage

Antimony (Sb) has been considered as a promising anode for sodium-ion batteries (SIBs) because of its high theoretical capacity and moderate working potential but suffers from the dramatic volume variations (∼250%), an unstable electrode/electrolyte interphase, active material exfoliation, and a continuously increased interphase impedance upon sodiation and desodiation processes. To address these issues, we report a unique three-dimensional (3D) porous self-standing foam electrode built from core-shelled Sb@In₂O₃ nanostructures via a continuous electrodepositing strategy coupled with surface chemical passivation. Such a hierarchical structure possesses a robust framework with rich voids and a dense protection layer (In₂O₃), which allow Sb nanoparticles to well accommodate their mechanical strain for efficiently avoiding electrode cracks and pulverization with a stable electrode/electrolyte interphase upon sodiation/desodiation processes. When evaluated as an anode for SIBs, the prepared nanoarchitectures exhibit a high first reversible capacity (641.3 mA h g^-1) and good cyclability (456.5 mA h g^-1 after 300 cycles at 300 mA g^-1), along with superior high rate capacity (348.9 mA h g^-1 even at 20 A g^-1) with a first Coulomb efficiency as high as 85.3%. This work could offer an efficient approach to improve alloying-based anode materials for promoting their practical applications.

Microplasmas for Advanced Materials and Devices

Microplasmas are low-temperature plasmas that feature microscale dimensions and a unique high-energy-density and a nonequilibrium reactive environment, which makes them promising for the fabrication of advanced nanomaterials and devices for diverse applications. Here, recent microplasma applications are examined, spanning from high-throughput, printing-technology-compatible synthesis of nanocrystalline particles of common materials types, to water purification and optoelectronic devices. Microplasmas combined with gaseous and/or liquid media at low temperatures and atmospheric pressure open new ways to form advanced functional materials and devices. Specific examples include gas-phase, substrate-free, plasma-liquid, and surface-supported synthesis of metallic, semiconducting, metal oxide, and carbon-based nanomaterials. Representative applications of microplasmas of particular importance to materials science and technology include light sources for multipurpose, efficient VUV/UV light sources for photochemical materials processing and spectroscopic materials analysis, surface disinfection, water purification, active electromagnetic devices based on artificial microplasma optical materials, and other devices and systems including the plasma transistor. The current limitations and future opportunities for microplasma applications in materials related fields are highlighted.

3D printing of cellular materials for advanced electrochemical energy storage and conversion

3D printing, an advanced layer-by-layer assembly technology, is an ideal platform for building architectures with customized geometries and controllable microstructures. Bio-inspired cellular material is one of most representative 3D-printed architectures, and attracting growing attention compared to block counterparts. The integration of 3D printing and cellular materials offer massive advantages and opens up great opportunities in diverse application fields, particularly in electrochemical energy storage and conversion (EESC). This article gives a comprehensive overview of 3D-printed cellular materials for advanced EESC. It begins with an introduction of advanced 3D printing techniques for cellular material fabrication, followed by the corresponding material design principles. Recent advances in 3D-printed cellular materials for EESC applications, including rechargeable batteries, supercapacitors and electrocatalysts are then summarized and discussed. Finally, current trends and challenges along with in-depth future perspectives are provided.

n-Butanol/Water Interface-Aided Physicochemical Tuning of Two-Dimensional Transition-Metal Oxides

Herein, we report a facile regulation of the interface of two immiscible solvents, n-butanol and water, to achieve the physicochemical tuning of the transition-metal oxide nickel cobaltite. The crystal nucleation and the growth of nickel cobaltite into distinct morphology are highly dependent on the orientation and the mass transfer of the reactive species through the reactive interface layer. A distinct two-dimensional flakelike (1 nm thickness) nickel cobaltite is formed at the interface of n-butanol/water in a 1:1 solvent ratio. Rather, one-dimensional needles and irregular interconnected networks are achieved, as aqueous and organic counterparts are, respectively, increased. The impact of the solvent ratio on doping metal ions (Co²⁺ and Ni²⁺) at the interstitial sites of fcc spinel structure is evident from the X-ray and electronic absorption investigations. It is presumed that the interface-assisted synthesis may provide a simple and novel way to develop and adopt various transition-metal oxides for wide applications.

Facile Synthesis of Flower-Like MnCo2 O4 @PANi-rGO: A High-Performance Anode Material for Lithium-Ion Batteries

Flower-like MnCo₂ O₄ was prepared in a self-assembly process and used in the formation of MnCo₂ O₄ @polyaniline (MnCo₂ O₄ @PANi) that proceeds by a simple in situ polymerization. The MnCo₂ O₄ @PANi-reduced graphite oxide (MnCo₂ O₄ @PANi-rGO) composite was then synthesized by introducing rGO into MnCo₂ O₄ @PANi. This modification improves the overall electronic conductivity of the MnCo₂ O₄ @PANi-rGO because of the dual conductive functions of rGO and PANi; it also provides a buffer for the changes in electrode volume during cycling, thus improving the lithium-storage performance of MnCo₂ O₄ @PANi-rGO. The electrochemical performance of the samples was evaluated by charge/discharge cycling testing, cyclic voltammetry, and electrochemical impedance spectroscopy. MnCo₂ O₄ @PANi-rGO delivers a discharge capacity of 745 mAh g^-1 and a Coulombic efficiency of 100 % after 1050 cycles at a current density of 500 mA g^-1 , and is a promising anode material for lithium-ion batteries.

Boosting High-Rate Zinc-Storage Performance by the Rational Design of Mn2O3 Nanoporous Architecture Cathode

Manganese oxides are regarded as one of the most promising cathode materials in rechargeable aqueous Zn-ion batteries (ZIBs) because of the low price and high security. However, the practical application of Mn2O3 in ZIBs is still plagued by the low specific capacity and poor rate capability. Herein, highly crystalline Mn2O3 materials with interconnected mesostructures and controllable pore sizes are obtained via a ligand-assisted self-assembly process and used as high-performance electrode materials for reversible aqueous ZIBs. The coordination degree between Mn2+ and citric acid ligand plays a crucial role in the formation of the mesostructure, and the pore sizes can be easily tuned from 3.2 to 7.3 nm. Ascribed to the unique feature of nanoporous architectures, excellent zinc-storage performance can be achieved in ZIBs during charge/discharge processes. The Mn2O3 electrode exhibits high reversible capacity (233 mAh g-1 at 0.3 A g-1), superior rate capability (162 mAh g-1 retains at 3.08 A g-1) and remarkable cycling durability over 3000 cycles at a high current rate of 3.08 A g-1. Moreover, the corresponding electrode reaction mechanism is studied in depth according to a series of analytical methods. These results suggest that rational design of the nanoporous architecture for electrode materials can effectively improve the battery performance.

The Road Towards Planar Microbatteries and Micro-Supercapacitors: From 2D to 3D Device Geometries

The rapid development and further modularization of miniaturized and self-powered electronic systems have substantially stimulated the urgent demand for microscale electrochemical energy storage devices, e.g., microbatteries (MBs) and micro-supercapacitors (MSCs). Recently, planar MBs and MSCs, composed of isolated thin-film microelectrodes with extremely short ionic diffusion path and free of separator on a single substrate, have become particularly attractive because they can be directly integrated with microelectronic devices on the same side of one single substrate to act as a standalone microsized power source or complement miniaturized energy-harvesting units. The development of and recent advances in planar MBs and MSCs from the fundamentals and design principle to the fabrication methods of 2D and 3D planar microdevices in both in-plane and stacked geometries are highlighted. Additonally, a comprehensive analysis of the primary aspects that eventually affect the performance metrics of microscale energy storage devices, such as electrode materials, electrolyte, device architecture, and microfabrication techniques are presented. The technical challenges and prospective solutions for high-energy-density planar MBs and MSCs with multifunctionalities are proposed.

Rapid and Low-Temperature Salt-Templated Production of 2D Metal Oxide/Oxychloride/Hydroxide

2D materials have played an important role in electronics, sensors, optics, electrocatalysis, and energy storage. Many methods for the preparation of 2D materials have been explored. It is crucial to develop a high-yield, rapid, and low-temperature method to synthesize 2D materials. A general, fast (5 min), and low-temperature (≈100 °C) salt (CoCl₂ ·6H₂ O)-templated method is proposed to prepare series of 2D metal oxides/oxychlorides/hydroxides in large scale, such as MoO₃ , SnO₂ , SiO₂ , BiOCl, Sb₄ O₅ Cl₂ , Zn₂ Co₃ (OH)₁₀ 2H₂ O, and ZnCo₂ O₄ . The as-synthesized 2D materials possess an ultrathin feature (2-7 nm) and large aspect ratios. Additionally, these 2D metal oxides/oxychlorides/hydroxides exhibit good electrochemical properties in energy storage (lithium/sodium-ion batteries) and electrocatalysis (hydrogen/oxygen evolution reaction).

Biomass-Derived Carbon Paper to Sandwich Magnetite Anode for Long-Life Li-Ion Battery

Metal oxides can deliver high capacity to Li-ion batteries, surpassing conventional graphite, but they suffer from a huge volume change during charging-discharging and poor cycle life. Herein, we merge the dual strategies of 3D-network support and sandwiching design to tackle such issue. We develop a skillful O₂-NH₃ reactive pyrolysis of cellulose, where the preoxidation and the aminolysis result in the spatially separated charring of cellulose chains. A cellulose fiber is wonderfully converted into several ultrathin twisted graphenic sheets instead of a dense carbon fiber, and consequently, a cellulose paper is directly transformed into a porous flexible carbon paper with high surface area and conductivity (denoted as CP). CP is further fabricated as a 3D-network support into the hybrid CP@Fe₃O₄@RGO, where RGO is reduced graphene oxide added for sandwiching Fe₃O₄ particles. As a binder-free free-standing anode, CP@Fe₃O₄@RGO effectively fastens Fe₃O₄ and buffers the volume changes on cycling, which stabilizes the passivating layer and lifts the Coulombic efficiency. The anode thus presents an ultralong cycle life of >2000 running at a high capacity level of 1160 mAh g^-1. It additionally facilitates electron and ion transports, boosting the rate capability. CP and CP@Fe₃O₄@RGO represent a technological leap underpinning next-generation long-life high-capacity high-power batteries.

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.

Critical thickness of a surface-functionalized coating for enhanced lithium storage: a case study of nanoscale polypyrrole-coated FeS2 as a cathode for Li-ion batteries

The encapsulation or coating of conductive materials is an effective strategy to increase the electrochemical ion-storage performance of some promising electrode materials such as transition metal oxides and sulfides, which are low-cost and have high capacity, but their practical applications are hindered by their intrinsically low conductivity and large volume changes during cycling; however, to date, the effect of the thickness of conductive layers on the ion-storage performance has been rarely studied. In this study, taking nanoscale polypyrrole (PPY)-coated FeS₂ as an example, the effect of the critical thickness of the conductive PPY coating on the lithium-ion storage performance of (PPY)-coated FeS₂ as a cathode of rechargeable lithium-ion batteries (LIBs) was investigated. Via a facile vapor-phase polymerization method, uniform PPY coatings with the thickness of 1-18 nm on microsized FeS₂ particles were prepared. It was found that the critical thickness of PPY was 5 nm, at which the PPY-coated FeS₂ cathode exhibited remarkablely superior high-rate capability (808, 583, 543, 511, and 489 mA h g^-1 at 0.1, 1, 2, 5 and 10 A g^-1, respectively) and long-term stability (504 mA h g^-1 at 1.0 A g^-1 after 500 cycles) as compared to those with other coating thicknesses owing to the acheivement of optimal electrical conductivity and ion diffusion efficiency. Thus, this study provides an insight into the critical thickess of a surface-functionalized coating of active materials and opens a new avenue for the futher enhancement of the performance of energy storage deivces.

A composite of Fe3O4@C and multilevel porous carbon as high-rate and long-life anode materials for lithium ion batteries

The iron oxide-based anode materials are widely studied and reported due to their abundance, low cost, high energy density and environmental friendliness for lithium ion batteries (LIBs). However, the application of LIBs is always limited by the poor rate capability and stability. In order to tackle these issues, a novel material with carbon-encapsulated Fe₃O₄ nanorods stuck together by multilevel porous carbon (Fe₃O₄@C/PC) is prepared through directly carbonizing the Fe-based metal-organic framework under a nitrogen atmosphere. This novel material shows a high specific capacity and rate performance. The initial specific capacity can reach 1789 mAh g^-1 at a current density of 0.1 A g^-1, and the specific capacity still remains 1105.3 mAh g^-1 and 783.5 mAh g^-1 after 150 cycles at the current densities of 0.1 A g^-1 and 1 A g^-1, respectively. Even under a current density as high as 12 A g^-1, the specific capacity can achieve 309 mAh g^-1 after 2000 cycles with an average attenuation rate of 0.019% per cycle. Overall, the simple strategy, low cost and high capacity can make the practical application possible.

Facile Bottom-up Preparation of WS2-Based Water-Soluble Quantum Dots as Luminescent Probes for Hydrogen Peroxide and Glucose

Photoluminescent zero-dimensional (0D) quantum dots (QDs) derived from transition metal dichalcogenides, particularly molybdenum disulfide, are presently in the spotlight for their advantageous characteristics for optoelectronics, imaging, and sensors. Nevertheless, up to now, little work has been done to synthesize and explore photoluminescent 0D WS2 QDs, especially by a bottom-up strategy without using usual toxic organic solvents. In this work, we report a facile bottom-up strategy to synthesize high-quality water-soluble tungsten disulfide (WS2) QDs through hydrothermal reaction by using sodium tungstate dihydrate and L-cysteine as W and S sources. Besides, hybrid carbon quantum dots/WS2 QDs were further prepared based on this method. Physicochemical and structural analysis of QD hybrid indicated that the graphitic carbon quantum dots with diameters about 5 nm were held onto WS2 QDs via electrostatic attraction forces. The resultant QDs show good water solubility and stable photoluminescence (PL). The excitation-dependent PL can be attributed to the polydispersity of the synthesized QDs. We found that the PL was stable under continuous irradiation of UV light but can be quenched in the presence of hydrogen peroxide (H2O2). The obtained WS2-based QDs were thus adopted as an electrodeless luminescent probe for H2O2 and for enzymatic sensing of glucose. The hybrid QDs were shown to have a more sensitive LOD in the case of glucose sensing. The Raman study implied that H2O2 causes the partial oxidation of QDs, which may lead to oxidation-induced quenching. Overall, the presented strategy provides a general guideline for facile and low-cost synthesis of other water-soluble layered material QDs and relevant hybrids in large quantity. These WS2-based high-quality water-soluble QDs should be promising for a wide range of applications in optoelectronics, environmental monitoring, medical imaging, and photocatalysis.

Two-Dimensional Bismuth Oxide Heterostructured Nanosheets for Lithium- and Sodium-Ion Storages

Two-dimensional (2D) bismuth oxide (Bi₂O₃) heterostructured nanosheets (BOHNs) were first fabricated by a solution-based molecular self-assembly approach. The synthesized BOHNs nanosheets feature mixed α- and β-phases and rich surface/edge-active sites. When utilized as anode materials for rechargeable batteries, dual-phase BOHNs deliver an initial discharge capacity as high as 647.6 mAh g^-1 and an increased capacity of over 200 mAh g^-1 remained after 260 cycles for lithium-ion batteries (LIBs), and a stable cycling capacity at ∼50 mAh g^-1 after 500 cycles for sodium-ion batteries (SIBs). A novel flexible 2D/1D/2D structure is further developed by implanting 2D BOHNs into conductive 1D carbon nanotubes and 2D graphene to form composite (BOHNCG) paper as free-standing anodes for both LIBs and SIBs. The capacity of 2D/1D/2D BOHNCG as a LIB anode reaches 823.5 mAh g^-1, corresponding to an enhancement of ∼27%, and remains at >110 mAh g^-1 after 80 cycles as a SIB anode with greatly improved cycling stability. This work verifies the promising potential of 2D BOHNs for practical energy-related devices and enriches the current research on emerging 2D nanomaterials.

MnO2@Corncob Carbon Composite Electrode and All-Solid-State Supercapacitor with Improved Electrochemical Performance

Two corncob-derived carbon electrode materials mainly composed of micropores (activated carbon, AC) and mesopores/macropores (corncob carbon, CC) were prepared and studied after the anodic electrodeposition of MnO₂. The capacity of the MnO₂/activated carbon composite (MnO₂@AC) electrode did not noticeably increase after MnO₂ electrodeposition, while that of the MnO₂/corncob carbon composite (MnO₂@CC) electrode increased up to 9 times reaching 4475 mF cm⁻². An asymmetric all-solid-state supercapacitor (ASC) was fabricated using AC as the anode, MnO₂@CC as the cathode, and polyvinyl alcohol (PVA)/LiCl gel as the electrolyte. An ultrahigh specific capacitance of 3455.6 mF cm⁻² at 1 mA cm⁻², a maximum energy density of 1.56 mW h cm⁻², and a long lifetime of 10,000 cycles can be achieved. This work provides insights in understanding the function of MnO₂ in biomass-derived electrode materials, and a green path to prepare an ASC from waste biomass with excellent electrochemical performance.

Atomic-Scale Dynamics and Storage Performance of Na/K on FeF3 Nanosheet

Developing highly efficient FeF₃-based cathode materials for Na/K-ion batteries is greatly needed, which needs long cycling life and rate performance besides large voltage and capacity. Accordingly, we designed a two-dimensional (2D) FeF₃ nanosheet to obtain highly efficient Na/K-ion batteries. Moreover, first-principles calculations were implemented to discuss systematically the Na and K storage mechanism on the FeF₃(012) nanosheet. The adsorption energies of Na and K are -3.55 and -3.98 eV, respectively, which can guarantee the Na/K loading process. Interestingly, Na and K adatoms on FeF₃(012) prefer to get together in the form of the Na dimer and K tetramer, respectively. Energy barrier of the K tetramer is lower than that of the Na dimer (0.43 eV vs 0.45 eV). As a result, the K tetramer possesses a larger diffusion coefficient than the Na dimer (4.22 × 10^-10 cm²·s^-1 vs 3.32 × 10^-10 cm²·s^-1). That is to say, good Na/K-ion mobility can be achieved. Also, the FeF₃(012) nanosheet exhibits high initial discharge voltage (4.10 V for K and 3.74 V for Na). Moreover, it has a stable discharge voltage curve in Na/K-ion batteries. Besides, the FeF₃(012) nanosheet is more favorable to be fabricated as a flexible cathode material for potassium batteries. Therefore, the 2D FeF₃ nanosheet belongs to a promising cathode material in Na/K-ion batteries.

Nitrogen Boosts Defective Vanadium Oxide from Semiconducting to Metallic Merit

2D metal oxide nanosheets have attracted substantial attention for various applications owing to their appealing advantages. Yet, the exploration of effective methodology for fabrication of metallic 2D metal oxides with a high concentration of N dopants in a scalable manner remains challenging. Herein, a topochemical strategy is demonstrated on vanadium oxide nanosheets by combining 2D nanostructuring, heteroatom-doping, and defect engineering for modulating their intrinsic electronic structure and greatly enhancing their electrochemical property. O vacancies and N dopants (VON and VN bonds) are in situ formed in vanadium oxide via nitridation and lead to semiconductive-to-metallic phase transformation evidenced by experimental results and theoretical calculation. Overall, the N-VO_0.9 nanosheets exhibit a metallic electron transportation behavior and excellent electrochemical performance. These findings shed light on the rational design and electron structure tuning of 2D nanostructures for energy and electronics applications.

Hybrid CoO Nanowires Coated with Uniform Polypyrrole Nanolayers for High-Performance Energy Storage Devices

Transition metal oxides with high theoretic capacities are promising materials as battery-type electrodes for hybrid supercapacitors, but their practical applications are limited by their poor electric conductivity and unsatisfied rate capability. In this work, a hybrid structure of CoO nanowires coated with conformal polypyrrole (Ppy) nanolayer is proposed, designed and fabricated on a flexible carbon substrate through a facile two-step method. In the first step, porous CoO nanowires are fabricated on flexible carbon substrate through a hydrothermal procedure combined with an annealing process. In the second step, a uniform nanolayer of Ppy is further coated on the surfaces of the CoO nanowires, resulting in a hybrid core-shell CoO@Ppy nanoarrays. The CoO@Ppy aligned on carbon support can be directly utilized as electrode material for hybrid supercapacitors. Since the conductive Ppy coating layer provides enhanced electric conductivity, the hybrid electrode demonstrates much higher capacity and superior rate capability than pure CoO nanowires. As a further demonstration, Ppy layer can also be realized on SnO₂ nanowires. Such facile conductive-layer coating method can be also applied to other types of conducting polymers (as the shell) and metal oxide materials (as the core) for various energy-related applications.

Self-Supporting Hybrid Fiber Mats of Cu3P-Co2P/N-C Endowed with Enhanced Lithium/Sodium Ions Storage Performances

Recently, Cu₃P has been targeted as an alternative anode material for alkali-metal-ion batteries because of their safety potential and high volumetric capacity. However, designing a high-rate Cu₃P electrode with long durability is still faced with huge challenges. Here, we report a self-supporting three-dimensional (3D) composite of Cu₃P and Co₂P interconnected by N-doped C fibers (Cu₃P-Co₂P/N-C). The advanced 3D structure not only provides fast reaction kinetics but also improves the structural stability, leading to excellent rate capability and long-term cycling stability, and pseudocapacitance behavior is also beneficial to the high rate performance. Additionally, the synergistic effects between Cu₃P, Co₂P, and N-doped carbon can increase the electrical conductivity and active sites, ensuring more ion storage. The Cu₃P-Co₂P/N-C anode for lithium-ion batteries delivers high discharge capacity, superior rate performance, and ultralong lifespan over 2000 cycles accompanied by a stable capacity of around 316.9 mAh/g at 5 A/g. When the 3D structured material works in sodium-ion batteries, it also displays improved electrochemical performance. Our method provides a new insight to design advanced metal phosphides anodes for energy storage devices.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

High-Performance Flexible Freestanding Anode with Hierarchical 3D Carbon-Networks/Fe7 S8 /Graphene for Applicable Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have gained tremendous interest for grid scale energy storage system and power energy batteries. However, the current researches of anode for SIBs still face the critical issues of low areal capacity, limited cycle life, and low initial coulombic efficiency for practical application perspective. To solve this issue, a kind of hierarchical 3D carbon-networks/Fe₇ S₈ /graphene (CFG) is designed and synthesized as freestanding anode, which is constructed with Fe₇ S₈ microparticles well-welded on 3D-crosslinked carbon-networks and embedded in highly conductive graphene film, via a facile and scalable synthetic method. The as-prepared freestanding electrode CFG represents high areal capacity (2.12 mAh cm^-2 at 0.25 mA cm^-2 ) and excellent cycle stability of 5000 cycles (0.0095% capacity decay per cycle). The assembled all-flexible sodium-ion battery delivers remarkable performance (high areal capacity of 1.42 mAh cm^-2 at 0.3 mA cm^-2 and superior energy density of 144 Wh kg^-1 ), which are very close to the requirement of practical application. This work not only enlightens the material design and electrode engineering, but also provides a new kind of freestanding high energy density anode with great potential application prospective for SIBs.

In situ synthesis of tin dioxide submicrorods anchored on nickel foam as an additive-free anode for high performance sodium-ion batteries

A hybrid of tin dioxide submicrorods anchored on conductive nickel foam (SnO₂ submicrorods-Ni foam) is in-situ synthesized via a hydrothermal and a subsequent heat treatment by using stannic chloride and sodium hydroxide as the starting materials. Characterization results indicate that the synthesized SnO₂ submicrorods has a length of ∼400 nm and a diameter of ∼150 nm anchoring tightly on Ni foam. The electrochemical properties of the material as an additive-free anode for sodium-ion batteries are investigated. And a comparative research of the reversible sodium storage properties between the additive-free electrode of SnO₂ submicrorods-Ni foam and the additive electrode of SnO₂ rod-assembly microspheres is carried out. The results demonstrate that the SnO₂ submicrorods-Ni foam is a highly attractive anode for sodium ion batteries, which could exhibit much better sodium storage properties than the SnO₂ rod-assembly microspheres and other reported SnO₂-based additive electrodes. The excellent sodium storage properties of the SnO₂ submicrorods-Ni foam electrode can be attributed to its structure advantages without additive-assistant, which increase sodium storage active sites, facilitate the electronic/ionic transport and stabilize the total electrode structure during charge-discharge process.

Harnessing the unique properties of 2D materials for advanced lithium-sulfur batteries

In the past decade, lithium-sulfur batteries have attracted tremendous attention owing to their high theoretical energy densities. The electrochemical performances of lithium-sulfur batteries are strongly dependent on the electrode materials. Among all the electrode material candidates, the application of 2D materials in lithium-sulfur batteries including a sulfur cathode, a lithium anode, a separator and/or an electrolyte has gained great success in enhancing their electrochemical performance by overcoming their intrinsic obstacles. Thus, it is necessary to summarize the relationships between the unique features of 2D materials and the electrochemical performances of lithium-sulfur batteries, guiding the development of next-generation lithium-sulfur batteries. In this review, we focus on recent advances in harnessing the unique properties of 2D materials, including their high surface area, 2D feature, high mechanical strength, plentiful active sites and functional groups to improve the electrochemical properties of sulfur cathodes, lithium anodes, electrolytes and/or separators, respectively. Finally, we propose possible directions and strategies for harnessing various properties of 2D materials to promote the development and applications of lithium-sulfur batteries.

2D Oxide Nanomaterials to Address the Energy Transition and Catalysis

2D oxide nanomaterials constitute a broad range of materials, with a wide array of current and potential applications, particularly in the fields of energy storage and catalysis for sustainable energy production. Despite the many similarities in structure, composition, and synthetic methods and uses, the current literature on layered oxides is diverse and disconnected. A number of reviews can be found in the literature, but they are mostly focused on one of the particular subclasses of 2D oxides. This review attempts to bridge the knowledge gap between individual layered oxide types by summarizing recent developments in all important 2D oxide systems including supported ultrathin oxide films, layered clays and double hydroxides, layered perovskites, and novel 2D-zeolite-based materials. Particular attention is paid to the underlying similarities and differences between the various materials, and the subsequent challenges faced by each research community. The potential of layered oxides toward future applications is critically evaluated, especially in the areas of electrocatalysis and photocatalysis, biomass conversion, and fine chemical synthesis. Attention is also paid to corresponding novel 3D materials that can be obtained via sophisticated engineering of 2D oxides.

Hierarchical nanostructures derived from cellulose for lithium-ion batteries

Recent advances in natural cellulose substance derived hierarchical nanomaterials applied as anodic materials for lithium-ion batteries are summarized.

In situ synthesis of ultrasmall MnO nanoparticles encapsulated by a nitrogen-doped carbon matrix for high-performance lithium-ion batteries

Ultrasmall MnO nanoparticles encapsulated into the nitrogen-rich doped carbon matrix hybrids are synthesized by annealing Mn₂(EDTA) precursors and exhibit outstanding electrochemical performance as advanced anode material for lithium-ion batteries.

Double-Holey-Heterostructure Frameworks Enable Fast, Stable, and Simultaneous Ultrahigh Gravimetric, Areal, and Volumetric Lithium Storage

Deliberate design of advantageous nanostructures holds great promise for developing high-performance electrode materials for electrochemical energy storage. However, it remains a tremendous challenge to simultaneously gain high gravimetric, areal, and volumetric capacities as well as high rate performance and cyclability to meet practical requirements mainly due to the intractable insufficient ion diffusion and limited active sites for dense electrodes with high areal mass loadings. Herein we report a double-holey-heterostructure framework, in which holey Fe₂O₃ nanosheets (H-Fe₂O₃) are tightly and conformably grown on the holey reduced graphene oxide (H-RGO). This hierarchical nanostructure allows for rapid ion and electron transport and sufficient utilization of active sites throughout a highly compact and thick electrode. Therefore, the free-standing flexible H-Fe₂O₃/H-RGO heterostructure anode can simultaneously deliver ultrahigh gravimetric, areal, and volumetric capacities of 1524 mAh g^-1, 4.72 mAh cm^-2, and 2621 mAh cm^-3, respectively, at 0.2 A g^-1 after 120 cycles, and extraordinary rate performance with a capacity of 487 mAh g^-1 (1.51 mAh cm^-2) at a high current density of 30 A g^-1 (93 mA cm^-2) as well as excellent cycling stability with a capacity retention of 96.3% after 1600 cycles, which has rarely been achieved before.