Nontopotactic Reaction in Highly Reversible Sodium Storage of Ultrathin Co9 Se8 /rGO Hybrid Nanosheets

Thermal Stability and Sublimation of Two-Dimensional Co9Se8 Nanosheets for Ultrathin and Flexible Nanoelectronic Devices

An understanding of the structural and compositional stability of nanomaterials is significant from both fundamental and technological points of view. Here, we investigate the thermal stability of half-unit-cell thick two-dimensional (2D) Co₉Se₈ nanosheets that are exceptionally interesting because of their half-metallic ferromagnetic properties. By employing in situ heating in the transmission electron microscope (TEM), we find that the nanosheets show good structural and chemical stability without changes to the cubic crystal structure until sublimation of the nanosheets starts at temperatures between 460 and 520 °C. The real-time observations of the sublimation process show preferential removal at {110} type crystal facets. From an analysis of sublimation rates at various temperatures, we find that the sublimation occurs through noncontinuous and punctuated mass loss at lower temperatures while the sublimation is continuous and uniform at higher temperatures. Our findings provide an understanding of the nanoscale structural and compositional stability of 2D Co₉Se₈ nanosheets, which is of importance for their reliable application and sustained performance as ultrathin and flexible nanoelectronic devices.

Metal Selenides Anode Materials for Sodium Ion Batteries: Synthesis, Modification, and Application

The powerful and rapid development of lithium-ion batteries (LIBs) in secondary batteries field makes lithium resources in short supply, leading to rising battery costs. Under the circumstances, sodium-ion batteries (SIBs) with low cost, inexhaustible sodium reserves, and analogous work principle to LIBs, have evolved as one of the most anticipated candidates for large-scale energy storage devices. Thereinto, the applicable electrode is a core element for the smooth development of SIBs. Among various anode materials, metal selenides (MSe_x ) with relatively high theoretical capacity and unique structures have aroused extensive interest. Regrettably, MSe_x suffers from large volume expansion and unwished side reactions, which result in poor electrochemistry performance. Thus, strategies such as carbon modification, structural design, voltage control as well as electrolyte and binder optimization are adopted to alleviate these issues. In this review, the synthesis methods and main reaction mechanisms of MSe_x are systematically summarized. Meanwhile, the major challenges of MSe_x and the corresponding available strategies are proposed. Furthermore, the recent research progress on layered and nonlayered MSe_x for application in SIBs is presented and discussed in detail. Finally, the future development focuses of MSe_x in the field of rechargeable ion batteries are highlighted.

Insights into the Potassium Ion Storage Behavior and Phase Evolution of a Tailored Yolk-Shell SnSe@C Anode

Tin chalcogenides are regarded as promising anode materials for potassium ion batteries (PIBs) due to their considerable specific capacity. However, the severe volume effect, limited electronic conductivity, and the shuttle effect of the potassiation product restrict the application prospect. Herein, based on the metal evaporation reaction, a facile structural engineering strategy for yolk-shell SnSe encapsulated in carbon shell (SnSe@C) is proposed. The internal void can accommodate the volume change of the SnSe core and the carbon shell can enhance the electronic conductivity. Combining qualitative and quantitative electrochemical analyses, the distinguished electrochemical performance of SnSe@C anode is attributed to the contribution of enhanced capacitive behavior. Additionally, first-principles calculations elucidate that the heteroatomic doped carbon exhibits a preferable affinity toward potassium ions and the potassiation product K₂ Se, boosting the rate performance and capacity retention consequently. Furthermore, the phase evolution of SnSe@C electrode during the potassiation/depotassiation process is clarified by in situ X-ray diffraction characterization, and the crystal transition from the SnSe Pnma(62) to Cmcm(63) point group is discovered unpredictably. This work demonstrates a pragmatic avenue to tailor the SnSe@C anode via a facile structural engineering strategy and chemical regulation, providing substantial clarification for the phase evolution mechanism of SnSe-based anode for PIBs.

Highly conductive Co3Se4 embedded in N-doped 3D interconnected carbonaceous network for enhanced lithium and sodium storage

Traditional cobalt selenides as active materials in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) would suffer from drastic volume expansions and large stacking effects, leading to a low cycling stability. In this work, we utilized a facile template method for preparing Co₃Se₄@N-CN (CSNC) that encapsulated Co₃Se₄ nanoparticles into 3D interconnected nitrogen-doped carbon network (N-CN). Satisfactorily, it possesses excellent cycling stability with enhanced lithium and sodium energy storage capacity. As an anode material in LIBs, CSNC exhibited a prominent reversible discharge performance of 1313.5 mAh g^-1 after 100 cycles at 0.1 A g^-1 and 835.6 mAh g^-1 after 500 cycles at 1.0 A g^-1. Interestingly, according to the analysis from cyclic voltammetry, the in-situ generated Se might provide extra capacity that leaded to a rising trend of capacity. When utilized as an anode in SIBs, CSNC delivered an outstanding capacity of 448.7 mAh g^-1 after 100 cycles at 0.1 A g^-1 and could retain 328.9 mAh g^-1 (77.2% of that of 0.1 A g^-1) even at a high current density of 5.0 A g^-1. The results demonstrate that CSNC is a superior anode material in LIBs and SIBs with great promise. More importantly, this strategy opens up an effective avenue for the design of transition metal selenide/carbonaceous composites for advanced battery storage systems.

Highly Efficient Sodium-Ion Storage Enabled by an rGO-Wrapped FeSe2 Composite

Exploitation of superior anode materials is a key step to realize the pursuit of high-performance sodium-ion batteries. In this work, a reduced graphene oxide-wrapped FeSe₂ (FeSe₂ @rGO) composite derived from a metal-organic framework (MOF) was synthesized to act as the anode material of sodium-ion batteries. The MOF-derived carbon framework with high specific surface area could relieve the large volumetric change during cycling and ensure the structural stability of electrode materials. Besides, the rGO conductive network allowed to promote the electron transfer and accelerate reaction kinetics as well as to provide a protection role for the internal FeSe₂ . As a result, the FeSe₂ @rGO composite exhibited a high capacity of 350 mAh g^-1 after 600 cycles at 5 A g^-1 . Moreover, in situ XRD was conducted to explore the reaction mechanism of the FeSe₂ @rGO composite upon sodiation/de-sodiation. Importantly, the presented method for the synthesis of MOF-derived materials wrapped by rGO could not only be used for FeSe₂ @rGO-based sodium-ion batteries but also for the different transition metal-based composite materials for electrochemical devices, such as water splitting and sensors.

Urchin-Like Fe3 Se4 Hierarchitectures: A Novel Pseudocapacitive Sodium-Ion Storage Anode with Prominent Rate and Cycling Properties

Transition metal chalcogenides have received great attention as promising anode candidates for sodium-ion batteries (SIBs). However, the undesirable cyclic life and inferior rate capability still restrict their practical applications. The design of micro-nano hierarchitectures is considered as a possible strategy to facilitate the electrochemical reaction kinetics and strengthen the electrode structure stability upon repeated Na⁺ insertion/extraction. Herein, urchin-like Fe₃ Se₄ hierarchitectures are successfully prepared and developed as a novel anode material for SIBs. Impressively, the as-prepared urchin-like Fe₃ Se₄ can present an ultrahigh rate capacity of 200.2 mAh g^-1 at 30 A g^-1 and a prominent capacity retention of 99.9% over 1000 cycles at 1 A g^-1 , meanwhile, a respectable initial coulombic efficiency of ≈100% is achieved. Through the conjunct study of in situ X-ray diffraction, ex situ X-ray absorption near-edge structure spectroscopy, as well as cyclic voltammetry curves, it is intriguing to reveal that the phase transformation from monoclinic to amorphous structure accompanied by the pseudocapacitive Na⁺ storage behavior accounts for the superior electrochemical performance. When paired with the Na₃ V₂ (PO₄ )₃ cathode materials, the assembled full cell enables high energy density and decent cyclic stability, demonstrating potential practical feasibility of the present urchin-like Fe₃ Se₄ anode.

The Progress of Cobalt-Based Anode Materials for Lithium Ion Batteries and Sodium Ion Batteries

Limited by the development of energy storage technology, the utilization ratio of renewable energy is still at a low level. Lithium/sodium ion batteries (LIBs/SIBs) with high-performance electrochemical performances, such as large-scale energy storage, low costs and high security, are expected to improve the above situation. Currently, developing anode materials with better electrochemical performances is the main obstacle to the development of LIBs/SIBs. Recently, a variety of studies have focused on cobalt-based anode materials applied for LIBs/SIBs, owing to their high theoretical specific capacity. This review systematically summarizes the recent status of cobalt-based anode materials in LIBs/SIBs, including Li+/Na+ storage mechanisms, preparation methods, applications and strategies to improve the electrochemical performance of cobalt-based anode materials. Furthermore, the current challenges and prospects are also discussed in this review. Benefitting from these results, cobalt-based materials can be the next-generation anode for LIBs/SIBs.

Tuning Metallic Co0.85Se Quantum Dots/Carbon Hollow Polyhedrons with Tertiary Hierarchical Structure for High-Performance Potassium Ion Batteries

Potassium-ion batteries (KIBs) are a potential candidate to lithium-ion batteries (LIBs) but possess unsatisfactory capacity and rate properties. Herein, the metallic cobalt selenide quantum dots (Co_0.85Se-QDs) encapsulated in mesoporous carbon matrix were designed via a direct hydrothermal method. Specifically, the cobalt selenide/carbon composite (Co_0.85Se-QDs/C) possesses tertiary hierarchical structure, which is the primary quantum dots, the secondary petals flake, and the tertiary hollow micropolyhedron framework. Co_0.85Se-QDs are homogenously embedded into the carbon petals flake, which constitute the hollow polyhedral framework. This unique structure can take the advantages of both nanoscale and microscale features: Co_0.85Se-QDs can expand in a multidimensional and ductile carbon matrix and reduce the K-intercalation stress in particle dimensions; the micropetals can restrain the agglomeration of active materials and promote the transportation of potassium ion and electron. In addition, the hollow carbon framework buffers volume expansion, maintains the structural integrity, and increases the electronic conductivity. Benefiting from this tertiary hierarchical structure, outstanding K-storage performance (402 mAh g^-1 after 100 cycles at 50 mA g^-1) is obtained when Co_0.85Se-QDs/C is used as KIBs anode. More importantly, the selenization process in this work is newly reported and can be generally extended to prepare other quantum dots encapsulated in edge-limited frameworks for excellent energy storage.

Metal Organic Framework-Templated Synthesis of Bimetallic Selenides with Rich Phase Boundaries for Sodium-Ion Storage and Oxygen Evolution Reaction

Two-phase or multiphase compounds have been evidenced to exhibit good electrochemical performance for energy applications; however, the mechanism insights into these materials, especially the performance improvement by engineering the high-active phase boundaries in bimetallic compounds, remain to be seen. Here, we report a bimetallic selenide heterostructure (CoSe₂/ZnSe) and the fundamental mechanism behind their superior electrochemical performance. The charge redistribution at the phase boundaries of CoSe₂/ZnSe was experimentally and theoretically proven. Benefiting from the abundant phase boundaries, CoSe₂/ZnSe exerts low Na⁺ adsorption energy and fast diffusion kinetics for sodium-ion batteries and high activity for oxygen evolution reaction. As expected, excellent sodium storage capability, specifically a superb cyclic stability of up to 800 cycles for the Na₃V₂(PO₄)₃∥CoZn-Se full cell, and efficient water oxidation with a small overpotential of 320 mV to reach 10 mA cm^-2 were obtained. This work demonstrates the importance of phase boundaries in bimetallic compounds to boost the performance in various fields.

F-Doped NaTi2(PO4)3/C Nanocomposite as a High-Performance Anode for Sodium-Ion Batteries

We are presenting a sol-gel method for building novel nanostructures made of nanosized F-doped Na_{1-2 x}Ti₂(PO₄)_{3- x}F _x (NTP-F _x, x = 0, 0.02, 0.05, and 0.10) particles embedded in three-dimensional (3D) carbon matrices (NTP-F _x/C). This technique combines advantages of both zero-dimensional materials and 3D-carbon networks. Proper fluorine doping stabilizes the NTP structure and greatly enhances ion/electron transportation, leading to superhigh-rate electrochemical performance and ultralong cycle life. The composite electrode delivers high specific capacities of 121, 115, 112.2, 110.1, 107.7, 103.1, 85.8, and 62.5 mA h g^-1 at 0.2, 0.5, 1, 2, 5, 10, 20, and 30 C, respectively. It retains an unbelievable ∼70% capacity after a thousand cycles at a rate as high as 10 C. Electroanalytical results reveal that fluorine doping significantly enhances Na⁺ diffusion kinetics. Meanwhile, density functional theory calculations demonstrate F-doped NTPs' own outstanding electrochemical properties, which is due to the enhanced intrinsic ionic/electronic conductivity. The results show that anion doping is an efficient way to make high-performance NTP anodes for sodium-ion batteries.

Three-dimensional hierarchical Ni3Se2 nanorod array as binder/carbon-free electrode for high-areal-capacity Na storage

A three-dimensional hierarchical Ni3Se2 nanorod array (NA) grown in situ on foam Ni is the first to act as a carbon/binder-free electrode of SIBs via a one-step reversible conversion reaction. By a special decomposition-fusion process, the morphology and composition of the NA are regulated to obtain ultrahigh areal capacity, which is three times greater than that reported for other metal selenides.

Porous cobalt sulfide hollow nanospheres with tunable optical property for magnetic resonance imaging-guided photothermal therapy

Transition-metal chalcogenides with an imaging element and tunable optical property are strongly desired as ideal high-efficiency photothermal theranostic agents to diagnose and eliminate tumors. Herein, we report on a one-pot solvothermal strategy to synthesize various porous cobalt sulfide hollow nanospheres (PCSH NSs) and elucidate the relation between PCSH NSs and their optical absorption as a guide to obtain optimal photothermal therapy (PTT) agents. After PEG modification, PEG-PCSH NSs show superexcellent photothermal conversion efficiency (∼70.1%) which is higher than that of binary transition-metal chalcogenides materials reported to date. A low dose (100 μL, 25 ppm) could completely ablate tumors under an 808 nm laser power of 0.7 W cm-2, reducing in vivo long-term residual agent content and thus lowering the possibility of side effects. Additionally, they also exhibit excellent biocompatibility, good photostability and utility for magnetic resonance imaging. Our results indicate that PCSH NSs can be considered as an outstanding PTT agent and give guidance towards the design of other photothermal theranostic agents.

Achieving Ultrafast and Stable Na-Ion Storage in FeSe2 Nanorods/Graphene Anodes by Controlling the Surface Oxide

Designing transitional metal selenides (TMSes) with superior rate and cyclic performance for sodium-ion storage remains great challenges. To achieve this task, the influence of surface oxides on Na-ion storage behavior of FeSe₂ is investigated by designing FeSe₂ with varying oxide content. It is found that surface oxide has an inhibitory effect on the activity of FeSe₂. Small-sized FeSe₂ on graphene with higher surface oxide content exhibits obviously inferior performance compared to large-sized FeSe₂ with lower oxide content. By controlling oxide content, the prepared FeSe₂ nanorods/graphene exhibits a high capacity of 459 mAh/g at 0.1 A/g and superior rate performance. Only 10% capacity decrease occurs with the increase in current density from 0.1 to 5 A/g. Even at 25 A/g (∼50 C), it delivers a capacity of 227 mAh/g with almost no decay after 800 cycles. The influence mechanism of surface oxide is investigated. The oxide can be converted to a sodiated shell with high mechanical strength and poor conductivity, which generates phase-transition resistance to suppress the sodiation of FeSe₂ core, blocks the transfer of Na-ions and electrons in subsequent sodiation processes. Understanding the effect of surface oxide on Na-ion storage will be helpful in designing TMSes and other active materials.

Formation of Hierarchical Cu-Doped CoSe2 Microboxes via Sequential Ion Exchange for High-Performance Sodium-Ion Batteries

Electrode materials based on electrochemical conversion reactions have received considerable interest for high capacity anodes of sodium-ion batteries. However, their practical application is greatly hindered by the poor rate capability and rapid capacity fading. Tuning the structure at nanoscale and increasing the conductivity of these anode materials are two effective strategies to address these issues. Herein, a two-step ion-exchange method is developed to synthesize hierarchical Cu-doped CoSe₂ microboxes assembled by ultrathin nanosheets using Co-Co Prussian blue analogue microcubes as the starting material. Benefitting from the structural and compositional advantages, these Cu-doped CoSe₂ microboxes with improved conductivity exhibit enhanced sodium storage properties in terms of good rate capability and excellent cycling performance.