Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes: Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy

The applications of solid-state NMR and MRI techniques in the study of rechargeable sodium-ion batteries

In order to develop new electrode and electrolyte materials for advanced sodium-ion batteries (SIBs), it is crucial to understand a number of fundamental issues. These include the compositions of the bulk and interface, the structures of the materials used, and the electrochemical reactions in the batteries. Solid-state NMR (SS-NMR) has unique advantages in characterizing the local or microstructure of solid electrode/electrolyte materials and their interfaces-one such advantage is that these are determined in a noninvasive and nondestructive manner at the atomic level. In this review, we provide a survey of the recent advances in the understanding of the fundamental issues of SIBs using advanced NMR techniques. First, we summarize the applications of SS-NMR in characterizing electrode material structures and solid electrolyte interfaces (SEI). In particular, we elucidate the key role of in-situ NMR/MRI in revealing the complex reactions and degradation mechanisms of SIBs. Next, the characteristics and shortcomings of SS-NMR and MRI techniques in SIBs are also discussed in comparison to similar Li-ion batteries. Finally, an overview of SS-NMR and MRI techniques for sodium batteries are briefly discussed and presented.

Yolk-Shell Sb@Void@Graphdiyne Nanoboxes for High-Rate and Long Cycle Life Sodium-Ion Batteries

Antimony (Sb) has been pursued as a promising anode material for sodium-ion batteries (SIBs). However, it suffers from severe volume expansion during the sodiation-desodiation process. Encapsulating Sb into a carbon matrix can effectively buffer the volume change of Sb. However, the sluggish Na⁺ diffusion kinetics in traditional carbon shells is still a bottleneck for achieving high-rate performance in Sb/C composite materials. Here we design and synthesize a yolk-shell Sb@Void@graphdiyne (GDY) nanobox (Sb@Void@GDY NB) anode for high-rate and long cycle life SIBs. The intrinsic in-plane cavities in GDY shells offer three-dimensional Na⁺ transporting channels, enabling fast Na⁺ diffusion through the GDY shells. Electrochemical kinetics analyses show that the Sb@Void@GDY NBs exhibit faster Na⁺ transport kinetics than traditional Sb@C NBs. In situ transmission electron microscopy analysis reveals that the hollow structure and the void space between Sb and GDY successfully accommodate the volume change of Sb during cycling, and the plastic GDY shell maintains the structural integrity of NBs. Benefiting from the above structural merits, the Sb@Void@GDY NBs exhibit excellent rate capability and extraordinary cycling stability.

Interface Engineering Enables High-Performance Sb Anode for Sodium Storage

Heterointerface engineering has been verified to be an effective approach to enhance the energy density of alkali-ion batteries by resolving inherent shortcomings of single materials. However, the rational construction of heterogeneous composite with abundant heterogeneous interfaces for sodium-ion batteries (SIBs) is still a significant challenge. Herein, inspired by density functional theory calculations, interface engineering can greatly decrease the energy bandgap and migration barrier of Na ions in Sb and Na₃Sb phases, as well as enhance the mechanical properties. A porous heterointerface MOFC-Sb is fabricated by utilizing MOF-C as a support and buffer, exhibiting excellent electrochemical performances for sodium storage. The MOF-C-Sb anode with its rich heterointerface presents an improved electrochemical performance of 540.5 mAh g^-1 after 100 cycles at 0.1 A g^-1, and 515.9 mAh g^-1 at 1.6 A g^-1 in term of sodium storage, efficiently resolving the serious volume expansion issues of metal Sb. These results indicate the structural superiority of heterointerface-engineered structure and afford valuable information for the rational design and construction of Sb-based anode materials for high-performance electrochemical energy storage.

5D total scattering computed tomography reveals the full reaction mechanism of a bismuth vanadate lithium ion battery anode

We have used operando 5D synchrotron total scattering computed tomography (TSCT) to understand the cycling and possible long term deactivation mechanisms of the lithium-ion battery anode bismuth vanadate. This anode material functions via a combined conversion/alloying mechanism in which nanocrystals of lithium-bismuth alloy are protected by an amorphous matrix of lithium vanadate. This composite is formed in situ during the first lithiation of the anode. The operando TSCT data were analyzed and mapped using both pair distribution function and Rietveld methods. We can follow the lithium-bismuth alloying reaction at all stages, gaining real structural insight including variations in nanoparticle sizes, lattice parameters and bond lengths, even when the material is completely amorphous. We also observe for the first time structural changes related to the cycling of lithium ions in the lithium vanadate matrix, which displays no interactions beyond the first shell of V-O bonds. The first 3D operando mapping of the distribution of different materials in an amorphous anode reveals a decline in coverage caused by either agglomeration or partial dissolution of the active material, hinting at the mechanism of long term deactivation. The observations from the operando experiment are backed up by post mortem transmission electron microscope (TEM) studies and theoretical calculations to provide a complete picture of an exceptionally complex cycling mechanism across a range of length scales.

Degradation at the Na3SbS4/Anode Interface in an Operating All-Solid-State Sodium Battery

All-solid-state sodium batteries utilize earth-abundant elements and are sustainable systems for large-scale energy storage and electric transportation. Replacing flammable carbonate-based electrolytes with solid-state ionic conductors promotes battery safety. Using solid-state electrolytes (SEs) also eliminates the need for packing when fabricating tandem cells, potentially enabling further enhanced energy density. Na₃SbS₄, a Na⁺ conductor, remains stable in dry air and shows high Na⁺ conductivity (σ ≈ 1.0 × 10^-3 S/cm) and is thus a promising SE for applications in sodium batteries. However, upon repeated electrochemical cycling, Na₃SbS₄-containing Na batteries exhibit decaying capacity and limited cycle life, which is likely associated with the decomposition of Na₃SbS₄ at the electrode/electrolyte interface. This work presents an in-depth analysis of the decomposition chemistry occurring at the Na₃SbS₄/anode interface using combined in situ Raman and post-mortem characterization. The results indicate that the SbS₄^3- counterion is electrochemically reduced when experiencing Na⁺ reduction potentials, and this reduction chemistry likely follows multiple pathways. The observed reduction products include SbS₃^3-, the Sb₂S₇^4- dimer, the NaSb binary phase, and Na₂S. We also observed the irreversibility of the decomposition and, as a consequence, the accumulation of the degradation products over cycles. Also notable is the heterogeneity of this degradation chemistry across the interface. Through the spectroelectrochemical characterizations, we reveal the possible mechanisms of the Na₃SbS₄ decomposition at the solid electrolyte/anode interface in an operating device.

Faceted Antimony Particles with Interiors Reinforced with Reduced Graphene Oxide as High-Performance Anode Material for Sodium-Ion Batteries

The attainment of "true reinforcement" in a composite and harnessing of the associated beneficial effects have been demonstrated here through the development of faceted crystalline Sb particles having the interiors reinforced with reduced graphene oxide (rGO). Such a unique and "near-ideal" micro/nanocomposite architecture has been achieved via a facile/cost-effective route by facilitating heterogeneous nucleation/growth of Sb-oxide particles on/around dispersed rGO sheets upon incorporation of the same directly into the precursor suspension, followed by the reduction of Sb-oxide to Sb, in intimate contact with the rGO, during the subsequent single heat-treatment step. As a potential anode material for Na-ion batteries, the as-developed Sb/rGO composite exhibits a reversible Na-storage capacity of ∼550 mAh/g (@ 0.2 A/g) and a fairly high first cycle Coulombic efficiency (CE) of ∼79%, with the good reversibility being attributed to the coarse particle size of Sb and encompassing of rGO sheets inside the Sb particles. Furthermore, despite the coarse particle size, the Sb/rGO-based electrode exhibits outstanding cyclic stability, with negligible capacity fade up to 150 cycles (viz., ∼97% capacity retention), and rate capability, with >86% capacity being obtained upon raising the current density from 0.1 to 2 A/g, resulting in a capacity of ∼490 mAh/g, even at 2 A/g.

Entropy and crystal-facet modulation of P2-type layered cathodes for long-lasting sodium-based batteries

P2-type sodium manganese-rich layered oxides are promising cathode candidates for sodium-based batteries because of their appealing cost-effective and capacity features. However, the structural distortion and cationic rearrangement induced by irreversible phase transition and anionic redox reaction at high cell voltage (i.e., >4.0 V) cause sluggish Na-ion kinetics and severe capacity decay. To circumvent these issues, here, we report a strategy to develop P2-type layered cathodes via configurational entropy and ion-diffusion structural tuning. In situ synchrotron X-ray diffraction combined with electrochemical kinetic tests and microstructural characterizations reveal that the entropy-tuned Na_0.62Mn_0.67Ni_0.23Cu_0.05Mg_0.07Ti_0.01O₂ (CuMgTi-571) cathode possesses more {010} active facet, improved structural and thermal stability and faster anionic redox kinetics compared to Na_0.62Mn_0.67Ni_0.37O₂. When tested in combination with a Na metal anode and a non-aqueous NaClO₄-based electrolyte solution in coin cell configuration, the CuMgTi-571-based positive electrode enables an 87% capacity retention after 500 cycles at 120 mA g⁻¹ and about 75% capacity retention after 2000 cycles at 1.2 A g⁻¹.

The use of Mn-rich layered cathodes in Na-based batteries is hindered by inadequate cycling reversibility and sluggish anionic redox kinetics. Here, the authors report a strategy to stabilize the structure and promote anionic redox via configurational entropy and ion-diffusion structural tuning.

Spatial and Temporal Analysis of Sodium-Ion Batteries

As a promising alternative to the market-leading lithium-ion batteries, low-cost sodium-ion batteries (SIBs) are attractive for applications such as large-scale electrical energy storage systems. The energy density, cycling life, and rate performance of SIBs are fundamentally dependent on dynamic physiochemical reactions, structural change, and morphological evolution. Therefore, it is essential to holistically understand SIBs reaction processes, degradation mechanisms, and thermal/mechanical behaviors in complex working environments. The recent developments of advanced in situ and operando characterization enable the establishment of the structure-processing-property-performance relationship in SIBs under operating conditions. This Review summarizes significant recent progress in SIBs exploiting in situ and operando techniques based on X-ray and electron analyses at different time and length scales. Through the combination of spectroscopy, imaging, and diffraction, local and global changes in SIBs can be elucidated for improving materials design. The fundamental principles and state-of-the-art capabilities of different techniques are presented, followed by elaborative discussions of major challenges and perspectives.

Structural and Electrochemical Properties of Type VIII Ba8Ga16-δSn30+δ Clathrate (δ ≈ 1) during Lithiation

Clathrates of the tetrel (Tt = Si, Ge, Sn) elements are host-guest structures that can undergo Li alloying reactions with high capacities. However, little is known about how the cage structure affects the phase transformations that take place during lithiation. To further this understanding, the structural changes of the type VIII clathrate Ba₈Ga_16-δSn_30+δ (δ ≈ 1) during lithiation are investigated and compared to those in β-Sn with ex situ X-ray total scattering measurements and pair distribution function (PDF) analysis. The results show that the type VIII clathrate undergoes an alloying reaction to form Li-rich amorphous phases (Li_xBa_0.17Ga_0.33Sn_0.67, x = 2-3) with local structures similar to those in the crystalline binary Li-Sn phases that form during the lithiation of β-Sn. As a result of the amorphous phase transition, the type VIII clathrate reacts at a lower voltage (0.25 V vs Li/Li⁺) compared to β-Sn (0.45 V) and goes through a solid-solution reaction after the initial conversion of the crystalline clathrate phase. Cycling experiments suggest that the amorphous phase persists after the first lithiation and results in considerably better cycling than in β-Sn. Density functional theory (DFT) calculations suggest that topotactic Li insertion into the clathrate lattice is not favorable due to the high energy of the Li sites, which is consistent with the experimentally observed amorphous phase transformation. The local structure in the clathrate featuring Ba atoms surrounded by a cage of Ga and Sn atoms is hypothesized to kinetically circumvent the formation of Li-Sn or Li-Ga crystalline phases, which results in better cycling and a lower reaction voltage. Based on the improved electrochemical performance, clathrates could act as tunable precursors to form amorphous Li alloying phases with novel electrochemical properties.

Solid-state nuclear magnetic resonance of spin-9/2 nuclei 115 In and 209 Bi in functional inorganic complex oxides

Indium and bismuth are technologically important elements, in particular as oxides for optoelectronic applications. ¹¹⁵ In and ²⁰⁹ Bi are both I = 9/2 nuclei with high natural abundances and moderately high frequencies but large nuclear electric quadrupole moments. Leveraging the quadrupolar interaction as a measure of local symmetry and polyhedral distortions for these nuclei could provide powerful insights on a range of applied materials. However, the absence of reported nuclear magnetic resonance (NMR) parameters on these nuclei, particularly in oxides, hinders their use by the broader materials community. In this contribution, solid-state ¹¹⁵ In and ²⁰⁹ Bi NMR of three recently discovered quaternary bismuth or indium oxides are reported, supported by density functional theory calculations, numerical simulations, diffraction and additional multinuclear (²⁷ Al, ^69,71 Ga, and ¹²¹ Sb) solid-state NMR measurements. The compounds LiIn₂ SbO₆ , BiAlTeO₆ , and BiGaTeO₆ are measured without special equipment at 9.4 T, demonstrating that wideline techniques such as the QCPMG pulse sequence and frequency-stepped acquisition can enable straightforward extraction of quadrupolar tensor information in I = 9/2 ¹¹⁵ In and ²⁰⁹ Bi even in sites with large quadrupolar coupling constants. Relationships are described between the NMR observables and local site symmetry. These are amongst the first reports of the NMR parameters of ¹¹⁵ In, ¹²¹ Sb, and ²⁰⁹ Bi in oxides.

Electrochemically Engineering Antimony Interspersed on Graphene toward Advanced Sodium-Storage Anodes

Nanoengineering of metal anode materials shows great potential for energy storage with high capacity. Zero-dimensional nanoparticles are conducive to acquire remarkable electrochemical properties in sodium-ion batteries (SIBs) because of their enlarged surface active sites. However, it is still difficult to fulfill the requirements of practical applications in batteries owing to the deficiency of efficient and scalable preparation approaches of high-performance metal electrode materials. Herein, an electrochemical cathodic corrosion method is proposed for the tunable preparation of nanostructured antimony (Sb) by the introduction of a surfactant, which can efficiently avoid the agglomeration of Sb atom clusters generated from the Zintl compound and further stacking into bulk during the electrochemical process. Subsequently, graphene as the support and conductive matrix is uniformly interspersed by generating Sb nanoparticles (Sb/Gr). Moreover, the reversible crystalline-phase evolution of Sb ⇋ NaSb ⇋Na₃Sb for Sb/Gr was studied by in situ X-ray diffraction (XRD). Benefiting from the interconnection of the conductive network, Sb/Gr anodes deliver a high capacity of 635.34 mAh g^-1, a retained capacity of 507.2 mAh g^-1 after 150 cycles at 0.1 C (1 C = 660 mAh g^-1), and excellent rate performance with the capacities of 473.41 and 405.09 mAh g^-1 at 2 and 5 C, respectively. The superior cycle stability with a capacity of 346.26 mAh g^-1 is achieved after 500 cycles at 2 C. This electrochemical approach offers a new route toward developing metal anodes with designed nanostructures for high-performance SIBs.

Architectural Engineering Achieves High-Performance Alloying Anodes for Lithium and Sodium Ion Batteries

Tremendous efforts have been dedicated to the development of high-performance electrochemical energy storage devices. The development of lithium- and sodium-ion batteries (LIBs and SIBs) with high energy densities is urgently needed to meet the growing demands for portable electronic devices, electric vehicles, and large-scale smart grids. Anode materials with high theoretical capacities that are based on alloying storage mechanisms are at the forefront of research geared towards high-energy-density LIBs or SIBs. However, they often suffer from severe pulverization and rapid capacity decay due to their huge volume change upon cycling. So far, a wide variety of advanced materials and electrode structures are developed to improve the long-term cyclability of alloying-type materials. This review provides fundamentals of anti-pulverization and cutting-edge concepts that aim to achieve high-performance alloying anodes for LIBs/SIBs from the viewpoint of architectural engineering. The recent progress on the effective strategies of nanostructuring, incorporation of carbon, intermetallics design, and binder engineering is systematically summarized. After that, the relationship between architectural design and electrochemical performance as well as the related charge-storage mechanisms is discussed. Finally, challenges and perspectives of alloying-type anode materials for further development in LIB/SIB applications are proposed.

Bridging Structural Inhomogeneity to Functionality: Pair Distribution Function Methods for Functional Materials Development

The correlation between structure and function lies at the heart of materials science and engineering. Especially, modern functional materials usually contain inhomogeneities at an atomic level, endowing them with interesting properties regarding electrons, phonons, and magnetic moments. Over the past few decades, many of the key developments in functional materials have been driven by the rapid advances in short-range crystallographic techniques. Among them, pair distribution function (PDF) technique, capable of utilizing the entire Bragg and diffuse scattering signals, stands out as a powerful tool for detecting local structure away from average. With the advent of synchrotron X-rays, spallation neutrons, and advanced computing power, the PDF can quantitatively encode a local structure and in turn guide atomic-scale engineering in the functional materials. Here, the PDF investigations in a range of functional materials are reviewed, including ferroelectrics/thermoelectrics, colossal magnetoresistance (CMR) magnets, high-temperature superconductors (HTSC), quantum dots (QDs), nano-catalysts, and energy storage materials, where the links between functions and structural inhomogeneities are prominent. For each application, a brief description of the structure-function coupling will be given, followed by selected cases of PDF investigations. Before that, an overview of the theory, methodology, and unique power of the PDF method will be also presented.

Rational Design of Sb@C@TiO2 Triple-Shell Nanoboxes for High-Performance Sodium-Ion Batteries

Antimony is an attractive anode material for sodium-ion batteries (SIBs) owing to its high theoretical capacity and appropriate sodiation potential. However, its practical application is severely impeded by its poor cycling stability caused by dramatic volumetric variations during sodium uptake and release processes. Here, to circumvent this obstacle, Sb@C@TiO₂ triple-shell nanoboxes (TSNBs) are synthesized through a template-engaged galvanic replacement approach. The TSNB structure consists of an inner Sb hollow nanobox protected by a conductive carbon middle shell and a TiO₂ -nanosheet-constructed outer shell. This structure offers dual protection to the inner Sb and enough room to accommodate volume expansion, thus promoting the structural integrity of the electrode and the formation of a stable solid-electrolyte interface film. Benefiting from the rational structural design and synergistic effects of Sb, carbon, and TiO₂ , the Sb@C@TiO₂ electrode exhibits superior rate performance (212 mAh g^-1 at 10 A g^-1 ) and outstanding long-term cycling stability (193 mAh g^-1 at 1 A g^-1 after 4000 cycles). Moreover, a full cell assembled with a configuration of Sb@C@TiO₂ //Na₃ (VOPO₄ )₂ F displays a high output voltage of 2.8 V and a high energy density of 179 Wh kg^-1 , revealing the great promise of Sb@C@TiO₂ TSNBs as the electrode in SIBs.

Controllable Synthesis of Peapod-like Sb@C and Corn-like C@Sb Nanotubes for Sodium Storage

Antimony (Sb) is regarded as an attractive anode material for sodium-ion batteries (SIBs) due to its high theoretical capacity of 660 mAh g^-1. Combining Sb with carbonaceous materials has been considered as an effective way to resolve the serious volume expansion issues. Sb/C composites mainly consist of two types, that is, Sb confined inside a carbon matrix and Sb deposited on the surface of a carbon matrix, and both have shown superior sodium storage performance. However, which structure is more beneficial for achieving high electrochemical performance is still unclear. In this work, peapod-like Sb@C and corn-like C@Sb nanotubes are synthesized via a nanoconfined galvanic replacement reaction and used as model materials for sodium storage to explore the above issue. When evaluated as anode materials for SIBs, the peapod-like Sb@C shows a higher rate capability and a significantly better long-term cycling stability compared to those of the corn-like C@Sb. Electrochemical analysis reveals that the peapod-like Sb@C exhibits faster Na⁺ and electron transport kinetics and higher proportions of surface capacitive contributions. These results demonstrate the structural superiority of the nanoconfined structure and provide valuable information for the rational design and construction of Sb-based anode materials for high-performance electrochemical energy storage.

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.

Facile Tailoring of Multidimensional Nanostructured Sb for Sodium Storage Applications

Nanoengineering of metal electrodes are of great importance for improving the energy density of alkali-ion batteries, which have been deemed one of most effective tools for addressing the poor cycle stability of metallic anodes. However, the practical application of nanostructured electrodes in batteries is still challenged by a lack of efficient, low-cost, and scalable preparation methods. Herein, we propose a facile chemical dealloying approach to the tunable preparation of multidimensional Sb nanostructures. Depending on dealloying reaction kinetics regulated by different solvents, zero-dimensional Sb nanoparticles (Sb-NP), two-dimensional Sb nanosheets (Sb-NS), and three-dimensional nanoporous Sb are controllably prepared via etching Li-Sb alloys in H₂O, H₂O-EtOH, and EtOH, respectively. Morphological evolution mechanisms of the various Sb nanostructures are analyzed by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction measurements. When applied as anodes for sodium ion batteries (SIBs), the as-prepared Sb-NS electrodes without any chemical modifications exhibit high reversible capacity of 620 mAh g^-1 and retain 90.2% of capacity after 100 cycles at 100 mA g^-1. The excellent Na⁺ storage performance observed is attributable to the two-dimensional nanostructure, which ensures high degrees of Na⁺ accessibility, robust structural integrity, and rapid electrode transport. This facile and tunable approach can broaden ways of developing high performance metal electrodes with designed nanostructures for electrochemical energy storage and conversion applications.

Electrode Materials for High-Performance Sodium-Ion Batteries

Sodium ion batteries (SIBs) are being billed as an economical and environmental alternative to lithium ion batteries (LIBs), especially for medium and large-scale stationery and grid storage. However, SIBs suffer from lower capacities, energy density and cycle life performance. Therefore, in order to be more efficient and feasible, novel high-performance electrodes for SIBs need to be developed and researched. This review aims to provide an exhaustive discussion about the state-of-the-art in novel high-performance anodes and cathodes being currently analyzed, and the variety of advantages they demonstrate in various critically important parameters, such as electronic conductivity, structural stability, cycle life, and reversibility.

Galvanic Replacement Synthesis of Highly Uniform Sb Nanotubes: Reaction Mechanism and Enhanced Sodium Storage Performance

One-dimensional nanotubes are very useful for achieving excellent performance in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to the fact that tubular structures can effectively alleviate the structural strain and shorten the ion diffusion length during repeated cycling. In this work, we report a Cu₂Sb-mediated growth strategy to controllably fabricate highly uniform Sb nanotubes (NTs), as well as Cu@Cu₂Sb and Cu₂Sb@Sb composite NTs, via a facile galvanic replacement reaction using Cu nanowires (NWs) as sacrificial templates. Benefiting from their structural merits, the Sb NTs manifest excellent sodium storage performance with superior rate performance (286 mAh g^-1 at 10 A g^-1) and extraordinary cycling stability (342 mAh g^-1 after 6000 cycles at 1 A g^-1). Furthermore, a full cell with Sb NTs as anode and Na₃(VOPO₄)₂F as cathode exhibits a high energy density (252 Wh kg^-1) and high output voltage (2.7 V), revealing their significant application promise in the next-generation SIBs.

Electrochemical exfoliation of graphene-like two-dimensional nanomaterials

Unlike zero-dimensional quantum dots, one-dimensional nanowires/nanorods, and three-dimensional networks or even their bulk counterparts, the charge carriers in two-dimensional (2D) materials are confined along the thickness while being allowed to move along the plane. They have distinct characteristics like strong quantum confinement, tunable thickness, and high specific surface area, which makes them a promising candidate in a wide range of applications such as electronics, topological spintronic devices, energy storage, energy conversion, sensors, biomedicine, catalysis, and so on. After the discovery of the extraordinary properties of graphene, other graphene-like 2D materials have attracted a great deal of attention. Like graphene, to realize their potential applications, high efficiency and low cost industrial scale methods should be developed to produce high-quality 2D materials. The electrochemical methods usually performed under mild conditions are convenient, controllable, and suitable for mass production. In this review, we introduce the latest and most representative investigations on the fabrication of 2D monoelemental Xenes, 2D transition-metal dichalcogenides, and other important emerging 2D materials such as organic framework (MOF) nanosheets and MXenes through electrochemical exfoliation. The electrochemical exfoliation conditions of the bulk layered materials are discussed. The numerous factors which will affect the quality of the exfoliated 2D materials, the possible exfoliating mechanism and potential applications are summarized and discussed in detail. A summary of the discussion together with perspectives and challenges for the future of this emerging field is also provided in the last section.

Scalable Fabrication of Core-Shell Sb@Co(OH)2 Nanosheet Anodes for Advanced Sodium-Ion Batteries via Magnetron Sputtering

Antimony (Sb) has captured extensive attention as a promising anode for sodium-ion batteries (SIBs) due to its high theoretical capacity and moderate sodiated potential but is held back from practical applications owing to its pulverization induced by dramatic volumetric variations during the (de)sodiation process. Herein, we report a core-shell Sb@Co(OH)₂ nanosheet anode fabricated via magnetron sputtering Sb onto the mass-productive Co(OH)₂ substrate anchored on stainless-steel mesh, which is scalable and suitable for flow-line production. In SIBs, the Sb@Co(OH)₂ anode displays superior rate performance (383.5 mAh/g at 30 A/g), high discharge capacity, and excellent stability. Compared with the sputtered Sb film electrode, the improved performance of the core-shell Sb@Co(OH)₂ nanosheet anode can be attributed to the open framework of the Co(OH)₂ substrate, not only accelerating the ion and electron transfer but also serving as the buffer for alleviating the volumetric variation and the supporting scaffold for prohibiting the aggregation. More importantly, the (de)sodiation mechanism of the Sb@Co(OH)₂ anode was explored by operando ( in situ) X-ray diffraction, and the similar alloying-dealloying processes (Sb ↔ Na _xSb ↔ Na₃Sb) for the 1st, 13th, and 30th cycles illustrate the excellent stability of the electrode.

Nanostructured Functional Hydrogels as an Emerging Platform for Advanced Energy Technologies

Nanostructured materials are critically important in many areas of technology because of their unusual physical/chemical properties due to confined dimensions. Owing to their intrinsic hierarchical micro-/nanostructures, unique chemical/physical properties, and tailorable functionalities, hydrogels and their derivatives have emerged as an important class of functional materials and receive increasing interest from the scientific community. Bottom-up synthetic strategies to rationally design and modify their molecular architectures enable nanostructured functional hydrogels to address several critical challenges in advanced energy technologies. Integrating the intrinsic or extrinsic properties of various functional materials, nanostructured functional hydrogels hold the promise to break the limitations of current materials, improving the device performance of energy storage and conversion. Here, the focus is on the fundamentals and applications of nanostructured functional hydrogels in energy conversion and storage. Specifically, the recent advances in rational synthesis and modification of various hydrogel-derived functional nanomaterials as core components in batteries, supercapacitors, and catalysts are summarized, and the perspective directions of this emerging class of materials are also discussed.

Electrochemical Alloying of Lead in Potassium-Ion Batteries

The electrochemical alloying of lead-based electrodes with potassium was investigated by galvanostatic measurements as well as by ex situ and operando X-ray diffraction. The electrochemical reduction must be activated by an initial high current pulse which prevents the passivation of the lead electrode. The alloying process leads to the formation of crystalline KPb. During the discharge, two intermediate phases are observed, K₁₀Pb₄₈ and K₄Pb₉, whereas only K₄Pb₉ seems to form during the charge. High capacity retention is observed, with, however, a limited specific capacity value because of high weight of lead.

A Dealloying Synthetic Strategy for Nanoporous Bismuth-Antimony Anodes for Sodium Ion Batteries

Metal-based anodes have recently aroused much attention in sodium ion batteries (SIBs) owing to their high theoretical capacities and low sodiation potentials. However, their progresses are prevented by the inferior cycling performance caused by severe volumetric change and pulverization during the (de)sodiation process. To address this issue, herein an alloying strategy was proposed and nanoporous bismuth (Bi)-antimony (Sb) alloys were fabricated by dealloying of ternary Mg-based precursors. As an anode for SIBs, the nanoporous Bi₂Sb₆ alloy exhibits an ultralong cycling performance (10 000 cycles) at 1 A/g corresponding to a capacity decay of merely 0.0072% per cycle, due to the porous structure, alloying effect and proper Bi/Sb atomic ratio. More importantly, a (de)sodiation mechanism ((Bi,Sb) ↔ Na(Bi,Sb) ↔ Na₃(Bi,Sb)) is identified for the discharge/charge processes of Bi-Sb alloys by using operando X-ray diffraction and density functional theory calculations.

Three-dimensional carbon network confined antimony nanoparticle anodes for high-capacity K-ion batteries

Antimony (Sb) represents a promising anode for K-ion batteries (KIBs) due to its high theoretical capacity and suitable working voltage. However, the large volume change that occurs in the potassiation/depotassiation process can lead to severe capacity fading. Herein, we report a high-capacity anode material by in situ confining Sb nanoparticles in a three-dimensional carbon framework (3D SbNPs@C) via a template-assisted freeze-drying treatment and subsequent carbothermic reduction. The as-prepared 3D SbNPs@C hybrid material delivers high reversible capacity and good cycling stability when used as the anode for KIBs. Furthermore, cyclic voltammetry and in situ X-ray diffraction analysis were performed to reveal the intrinsic mechanism of a K-Sb alloying reaction. Therefore, this work is of great importance to understand the electrochemical process of the Sb-based alloying reaction and will pave the way for the exploration of high performance KIB anode materials.

Cyanogel-Enabled Homogeneous Sb-Ni-C Ternary Framework Electrodes for Enhanced Sodium Storage

Antimony (Sb) represents an important high-capacity anode material for advanced sodium ion batteries, but its practical utilization has been primarily hampered by huge volume expansion-induced poor cycling life. The co-incorporation of transition-metal (M = Ni, Cu, Fe, etc.) and carbon components can synergistically buffer the volume change of the Sb component; however, these Sb-M-C ternary anodes often suffer from uneven distribution of Sb, M, and C components. Herein, we propose a general nanostructured gel-enabled methodology to synthesize homogeneous Sb-M-C ternary anodes for fully realizing the synergestic effects from M/C dual matrices. A cyano-bridged Sb(III)-Ni(II) coordination polymer gel (Sb-Ni cyanogel) has been synthesized and directly reduced to an Sb-Ni alloy framework (Sb-Ni framework). Moreover, graphene oxide (GO) can be in situ immobilized within the cyanogel framework, and after reduction, reduced graphene oxide (rGO) is uniformly distributed within the alloy framework, yielding a homogeneous rGO@Sb-Ni ternary framework. The rGO@Sb-Ni framework with optimal rGO content manifests a high reversible capacity of ∼468 mA h g^-1 at 1 A g^-1 and stable cycle life at 5 A g^-1 (∼210 mA h g^-1 after 500 cycles). The proposed cyanogel-enabled methodology may be extended to synthesize other homogeneous ternary framework materials for efficient energy storage and electrocatalysis.

Nanostructured Na2Ti9O19 for Hybrid Sodium-Ion Capacitors with Excellent Rate Capability

Herein, we report a new Na-insertion electrode material, Na₂Ti₉O₁₉, as a potential candidate for Na-ion hybrid capacitors. We study the structural properties of nanostructured Na₂Ti₉O₁₉, synthesized by a hydrothermal technique, upon electrochemical cycling vs Na. Average and local structures of Na₂Ti₉O₁₉ are elucidated from neutron Rietveld refinement and pair distribution function (PDF), respectively, to investigate the initial discharge and charge events. Rietveld refinement reveals electrochemical cycling of Na₂Ti₉O₁₉ is driven by single-phase solid solution reaction during (de)sodiation without any major structural deterioration, keeping the average structure intact. Unit cell volume and lattice evolution on discharge process is inherently related to TiO₆ distortion and Na ion perturbations, while the PDF reveals the deviation in the local structure after sodiation. Raman spectroscopy and X-ray photoelectron spectroscopy studies further corroborate the average and local structural behavior derived from neutron diffraction measurements. Also, Na₂Ti₉O₁₉ shows excellent Na-ion kinetics with a capacitve nature of 86% at 1.0 mV s^-1, indicating that the material is a good anode candidate for a sodium-ion hybrid capacitor. A full cell hybrid Na-ion capacitor is fabricated by using Na₂Ti₉O₁₉ as anode and activated porous carbon as cathode, which exhibits excellent electrochemical properties, with a maximum energy density of 54 Wh kg^-1 and a maximum power density of 5 kW kg^-1. Both structural insights and electrochemical investigation suggest that Na₂Ti₉O₁₉ is a promising negative electrode for sodium-ion batteries and hybrid capacitors.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Alloy-Based Anode Materials toward Advanced Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are considered as promising alternatives to lithium-ion batteries owing to the abundant sodium resources. However, the limited energy density, moderate cycling life, and immature manufacture technology of SIBs are the major challenges hindering their practical application. Recently, numerous efforts are devoted to developing novel electrode materials with high specific capacities and long durability. In comparison with carbonaceous materials (e.g., hard carbon), partial Group IVA and VA elements, such as Sn, Sb, and P, possess high theoretical specific capacities for sodium storage based on the alloying reaction mechanism, demonstrating great potential for high-energy SIBs. In this review, the recent research progress of alloy-type anodes and their compounds for sodium storage is summarized. Specific efforts to enhance the electrochemical performance of the alloy-based anode materials are discussed, and the challenges and perspectives regarding these anode materials are proposed.

Melt-Spun Fe-Sb Intermetallic Alloy Anode for Performance Enhanced Sodium-Ion Batteries

Owing to the high theoretical sodiation capacities, intermetallic alloy anodes have attracted considerable interest as electrodes for next-generation sodium-ion batteries (SIBs). Here, we demonstrate the fabrication of intermetallic Fe-Sb alloy anode for SIBs via a high-throughput and industrially viable melt-spinning process. The earth-abundant and low-cost Fe-Sb-based alloy anode exhibits excellent cycling stability with nearly 466 mAh g^-1 sodiation capacity at a specific current of 50 mA g^-1 with 95% capacity retention after 80 cycles. Moreover, the alloy anode displayed outstanding rate performance with ∼300 mAh g^-1 sodiation capacity at 1 A g^-1. The crystalline features of the melt-spun fibers aid in the exceptional electrochemical performance of the alloy anode. Further, the feasibility of the alloy anode for real-life applications was demonstrated in a sodium-ion full-cell configuration which could deliver a sodiation capacity of over 300 mAh g^-1 (based on anode) at 50 mA g^-1 with more than 99% Coulombic efficiency. The results further exhort the prospects of melt-spun alloy anodes to realize fully functional sodium-ion batteries.

Advanced Nanostructured Anode Materials for Sodium-Ion Batteries

Sodium-ion batteries (NIBs), due to the advantages of low cost and relatively high safety, have attracted widespread attention all over the world, making them a promising candidate for large-scale energy storage systems. However, the inherent lower energy density to lithium-ion batteries is the issue that should be further investigated and optimized. Toward the grid-level energy storage applications, designing and discovering appropriate anode materials for NIBs are of great concern. Although many efforts on the improvements and innovations are achieved, several challenges still limit the current requirements of the large-scale application, including low energy/power densities, moderate cycle performance, and the low initial Coulombic efficiency. Advanced nanostructured strategies for anode materials can significantly improve ion or electron transport kinetic performance enhancing the electrochemical properties of battery systems. Herein, this Review intends to provide a comprehensive summary on the progress of nanostructured anode materials for NIBs, where representative examples and corresponding storage mechanisms are discussed. Meanwhile, the potential directions to obtain high-performance anode materials of NIBs are also proposed, which provide references for the further development of advanced anode materials for NIBs.

Real-time powder diffraction studies of energy materials under non-equilibrium conditions

Energy materials form the central part of energy devices. An essential part of their function is the ability to reversibly host charge or energy carriers, and analysis of their phase composition and structure in real time under non-equilibrium conditions is mandatory for a full understanding of their atomic-scale functional mechanism. Real-time powder diffraction is increasingly being applied for this purpose, forming a critical step in the strategic chemical engineering of materials with improved behaviour. This topical review gives examples of real-time analysis using powder diffraction of rechargeable battery electrodes and porous sorbent materials used for the separation and storage of energy-relevant gases to demonstrate advances in the insights which can be gained into their atomic-scale function.

Improvement of Na Ion Electrode Activity of Metal Oxide via Composite Formation with Metal Sulfide

The composite formation with a conductive metal sulfide domain can provide an effective methodology to improve the Na-ion electrode functionality of metal oxide. The heat treatment of TiO₂(B) under CS₂ flow yields an intimately coupled TiO₂(B)-TiS₂ nanocomposite with intervened TiS₂ domain, since the reaction between metal oxide and CS₂ leads to the formation of metal sulfide and CO₂. The negligible change in lattice parameters and significant enhancement of visible light absorption upon the reaction with CS₂ underscore the formation of conductive metal sulfide domains. The resulting TiO₂(B)-TiS₂ nanocomposites deliver greater discharge capacities with better rate characteristics for electrochemical sodiation-desodiation process than does the pristine TiO₂(B). The ²³Na magic angle spinning nuclear magnetic resonance analysis clearly demonstrates that the electrode activities of the present nanocomposites rely on the capacitive storage of Na⁺ ions, and the TiS₂ domains in TiO₂(B)-TiS₂ nanocomposites play a role as mediators for Na⁺ ions to and from TiO₂(B) domains. According to the electrochemical impedance spectroscopy, the reaction with CS₂ leads to the significant enhancement of charge transfer kinetics, which is responsible for the accompanying improvement in electrode performance. The present study provides clear evidence for the usefulness in composite formation between the semiconducting metal oxide and metal sulfide in exploring new efficient NIB electrode materials.

The chemistry of ZnWO4 nanoparticle formation

The need for a new approach to describing nanoparticle nucleation and growth different from the classical models is highlighted. In and ex situ total scattering experiments combined with additional characterization techniques are used to unravel the chemistry dictating ZnWO4 formation.

The need for a change away from classical nucleation and growth models for the description of nanoparticle formation is highlighted. By the use of in situ total X-ray scattering experiments the transformation of an aqueous polyoxometalate precursor mixture to crystalline ZnWO₄ nanoparticles under hydrothermal conditions was followed. The precursor solution is shown to consist of specific Tourné-type sandwich complexes. The formation of pristine ZnWO₄ within seconds is understood on the basis of local restructuring and three-dimensional reordering preceding the emergence of long range order in ZnWO₄ nanoparticles. An observed temperature dependent trend in defect concentration can be rationalized based on the proposed formation mechanism. Following nucleation the individual crystallites were found to grow into prolate morphology with elongation along the unit cell c-direction. Extensive electron microscopy characterization provided evidence for particle growth by oriented attachment; a notion supported by sudden particle size increases observed in the in situ total scattering experiments. A simple continuous hydrothermal flow method was devised to synthesize highly crystalline monoclinic zinc tungstate (ZnWO₄) nanoparticles in large scale in less than one minute. The present results highlight the profound influence of structural similarities in local structure between reactants and final materials in determining the specific nucleation of nanostructures and thus explains the potential success of a given synthesis procedure in producing nanocrystals. It demonstrates the need for abolishing outdated nucleation models, which ignore subtle yet highly important system dependent differences in the chemistry of the forming nanocrystals.