Tailoring hollow microflower-shaped CoSe2 anodes in sodium ion batteries with high cycling stability

Stable C-Se-Co interface of CoSe2@N-doped carbon aerogels for efficient sodium storage

The storage characteristics of sodium ions in CoSe2 are intricately linked to the doping species and concentrations of heteroatoms within the carbon matrix. However, a systematic study of the impact of heteroatom doping on the interfacial forces between the carbon matrix and CoSe2 has not been systematically investigated. In this work, CoSe2 nanoparticles coated with different heteroatom doping (N/S) carbon aerogels derived from sodium alginate (SA) were constructed to investigate the influence of dopant atoms on the interfacial forces at the C matrix and CoSe2 interface. The confinement effect of Co-SA-NH2 junction zones facilitates the formation of stable C-Se-Co interface. The higher pyridinic nitrogen can promote the reinforced interfacial connection of CSe bond, further decreasing the interface distance between CoSe2 and N-doped carbon aerogels (NCA) to 3.00 Å, alleviating the interfacial volume expansion to 15 %, thus increasing the sodium ion migration rate and cycling stability. However, the addition of sulfur inhibits the transformation of other nitrogen species into pyridinic nitrogen. Furthermore, sulfur shares the same valence electron configuration as selenium, it replaces selenium in the CSe bond position, thereby reducing the conductivity and stability of the interface. As a consequence, CoSe2@N-doped carbon aerogels (CoSe2@NCA) exhibits the lowest sodium diffusion barrier (1.10 eV) and the highest sodium ion negative adsorption energy (-2.21 eV). As expected, CoSe2@NCA delivers superior long-term cycling performance (519 mAh g-1 at 1.0 A g-1 after 800 cycles) and excellent reversible capacity at high current density (474 mAh g-1 at 5.0 A g-1).

MOFs Containing Solid-State Electrolytes for Batteries

The use of metal-organic frameworks (MOFs) in solid-state electrolytes (SSEs) has been a very attractive research area that has received widespread attention in the modern world. SSEs can be divided into different types, some of which can be combined with MOFs to improve the electrochemical performance of the batteries by taking advantage of the high surface area and high porosity of MOFs. However, it also faces many serious problems and challenges. In this review, different types of SSEs are classified and the changes in these electrolytes after the addition of MOFs are described. Afterward, these SSEs with MOFs attached are introduced for different types of battery applications and the effects of these SSEs combined with MOFs on the electrochemical performance of the cells are described. Finally, some challenges faced by MOFs materials in batteries applications are presented, then some solutions to the problems and development expectations of MOFs are given.

Ether-based electrolytes for sodium ion batteries

Sodium-ion batteries (SIBs) are considered to be strong candidates for large-scale energy storage with the benefits of cost-effectiveness and sodium abundance. Reliable electrolytes, as ionic conductors that regulate the electrochemical reaction behavior and the nature of the interface and electrode, are indispensable in the development of advanced SIBs with high Coulombic efficiency, stable cycling performance and high rate capability. Conventional carbonate-based electrolytes encounter numerous obstacles for their wide application in SIBs due to the formation of a dissolvable, continuous-thickening solid electrolyte interface (SEI) layer and inferior stability with electrodes. Comparatively, ether-based electrolytes (EBEs) are emerging in the secondary battery field with fascinating properties to improve the performance of batteries, especially SIBs. Their stable solvation structure enables highly reversible solvent-co-intercalation reactions and the formation of a thin and stable SEI. However, although EBEs can provide more stable cycling and rapid sodiation kinetics in electrodes, benefitting from their favorable electrolyte/electrode interactions such as chemical compatibility and good wettability, their special chemistry is still being investigated and puzzling. In this review, we provide a thorough and comprehensive overview on the developmental history, fundamental characteristics, superiorities and mechanisms of EBEs, together with their advances in other battery systems. Notably, the relation among electrolyte science, interfacial chemistry and electrochemical performance is highlighted, which is of great significance for the in-depth understanding of battery chemistry. Finally, future perspectives and potential directions are proposed to navigate the design and optimization of electrolytes and electrolyte/electrode interfaces for advanced batteries.

Rational nanostructured FeSe2 wrapped in nitrogen-doped carbon shell for high-rate capability and long cycling sodium-ion storage

Transition metal selenides (TMSs) have drawn substantial attention as promising anode materials for sodium-ion batteries (SIBs) on account oftheir rapid reaction kinetics and high reversible capacity. However, the undesirable capacity decay and inferior rate performance still hamper their large-scale application. Herein, an anode material comprising combination of olivary nanostructure FeSe₂ core and nitrogen-doped carbon shell (designated as FeSe₂@NC) is well designed by in-situ polymerization and selenization method. The well-designed nitrogen-doped carbon shell can not only alleviate the volume variation during the electrode cycling but also provide an optimized ion/electron transport pathway. The resulting FeSe₂@NC electrodes exhibit a superior rate capability of 228.4 mA h g^-1 at 10 A g^-1 and a long cycling performance of 246.5 mA h g^-1 at 5 A g^-1 after 1000 cycles, which can be assigned to the enhanced structural integrity and improved electrical conductivity. The strategy would present a promising thought for structure design of TMSs as anode materials, which could enhance high-rate and long-lasting cycle performances for SIBs.

Structural engineering of bimetallic selenides for high-energy density sodium-ion half/full batteries

The bimetallic selenide ZnSe/MoSe2@NC fabricated by in situ selenation of a Zn/Mo MOF shows potential for application in high-energy density sodium ion batteries.

Highly dispersive CoSe2 nanoparticles encapsulated in carbon nanotube-grafted multichannel carbon fibers as advanced anodes for sodium-ion half/full batteries

Highly dispersive CoSe2 nanoparticles encapsulated in carbon nanotube-grafted multichannel carbon fibers are synthesized through a confined-regulated interfacial engineering strategy, which delivers excellent electrochemical performance.

Constructing electronic interconnected bimetallic selenide-filled porous carbon nanosheets for stable and highly efficient sodium-ion half/full batteries

Owing to their large theoretical capacity and relatively high electronic conductivity, transition metal selenides have been investigated as potential anodes for energy storage applications. On the other hand, the quick capacity decline induced by volume expansion during cycling and unconnected conducting network of the transition metal selenide-based electrode severely limit their employment in sodium-ion batteries (SIBs). Herein, a simple solvent ultrasonic technique and pyrolysis selenation process were used to make a porous N-doped carbon nanosheet-supported FeSe₂/CoSe₂ electrode. The electrochemical kinetics could be improved, and the stress generated by volume expansion could be efficiently adjusted by exquisitely constructed boundary of the FeSe₂/CoSe₂-CN electrode. As expected, the FeSe₂/CoSe₂-CN porous nanosheets exhibited a high Na⁺ storage capacity of 350 mA h g^-1 (10 A g^-1, 1000 cycles). Kinetic studies were conducted to explore the Na⁺ storage mechanism of FeSe₂/CoSe₂-CN. The as-constructed full sodium-ion batteries, when combined with Na₃V₂(PO₄)₂O₂F, have a phenomenal energy density (109 W h kg^-1), encouraging the exploration of energy-related components with the high-energy density properties.

Pancake-Like MOF Solid-State Electrolytes with Fast Ion Migration for High-Performance Sodium Battery

Solid-state electrolyte (SSE) of the sodium-ion battery have attracted tremendous attention in the next generation energy storage materials on account of their wide electrochemical window and thermal stability. However, the high interfacial impedance, low ion transference number and complex preparation process restrict the application of SSE. Herein, inspired by the excellent sieving function and high specific surface area of red blood cells, we obtained a solid-like electrolyte (SLE) based on the combination of the pancake-like metal-organic framework (MOF) with liquid electrolyte, possessing a high ionic conductivity of 6.60 × 10^-4 S cm^-1, and excellent sodium metal compatibility. In addition, we investigated the ion restriction effect of MOF's apertures size and special functional groups, and the ion transference number increased from 0.16 to 0.33. Finally, the assembled Na_0.44MnO₂//SLE//Na full batteries showed no obvious capacity decrease after 160 cycles. This material design of SLE in our work is an important key to obtain fast ion migration SLE for high-performance sodium-ion batteries.

Selenizing CoMoO4 nanoparticles within electrospun carbon nanofibers towards enhanced sodium storage performance

Transition metal oxides/selenides as anodes for sodium-ion batteries (SIBs) suffer from the insufficient conductivity and large volumetric expansion, which leads to the poor electrochemical performance. To address these issues, we herein demonstrate a facile selenization method to enhance the sodium storage capability of CoMoO₄ nanoparticles which are encapsulated into the electrospun carbon nanofibers (CMO@carbon for short). The partially and fully selenized CoMoO₄ within carbon nanofibers (denote as CMOS@carbon and CMS@carbon, respectively) can be readily obtained by controlling the annealing temperature (at 400 and 600 °C, correspondingly). When examined as anode materials for SIBs, the CMOS@carbon nanofibers display an outstanding electrochemical performance with a higher reversible capacity of 396 mA h g^-1 after 200 cycles at 0.2 A g^-1 and a high-rate capacity of 365 mA h g^-1 at 2 A g^-1, as compared with the CMO@carbon and CMS@carbon counterparts. The enhanced sodium storage performance of the CMOS@carbon can be owing to the partial selenization of the CoMoO₄ nanoparticles which are rooted into the porous electrospun carbon nanofibers, thus endowing them with superior ionic/electronic charge transfer efficiencies and a cushion against the electrode pulverization during cycling. Moreover, this work proposed a useful strategy to enhance the sodium storage performance of metal oxides via controlled selenization, which is promising for exploiting the advanced anode materials for SIBs.

Core-Shell FeSe2 /C Nanostructures Embedded in a Carbon Framework as a Free Standing Anode for a Sodium Ion Battery

Embedding the functional nanostructures into a lightweight nanocarbon framework is very promising for developing high performance advanced electrodes for rechargeable batteries. Here, to realize workable capacity, core-shell (FeSe₂ /C) nanostructures are embedded into carbon nanotube (CNT) framework via a facile wet-chemistry approach accompanied by thermally induced selenization. The CNT framework offers 3D continuous routes for electronic/ionic transfer, while macropores provide adequate space for high mass loading of FeSe₂ /C. However, the carbon shell not only creates a solid electronic link among CNTs and FeSe₂ but also improves the diffusivity of sodium ions into FeSe₂ , as well as acts as a buffer cushion to accommodate the volume variations. These unique structural features of CNT/FeSe₂ /C make it an excellent host for sodium storage with a capacity retention of 546 mAh g^-1 even after 100 cycles at 100 mA g^-1 . Moreover, areal and volumetric capacities of 5.06 mAh cm^-2 and 158 mAh cm^-3 are also achieved at high mass loading 16.9 mg cm^-2 , respectively. The high performance of multi-benefited engineered structure makes it a potential candidate for secondary ion batteries, while its easy synthesis makes it extendable to further complex structures with other morphologies (such as nanorods, nanowires, etc.) to meet the high energy demands.

MOF-derived ultrasmall CoSe2 nanoparticles encapsulated by an N-doped carbon matrix and their superior lithium/sodium storage properties

Ultrasmall CoSe₂ nanoparticles encapsulated by an N-doped carbon matrix were prepared by selenizing a novel Co-metal organic framework precursor. The excellent electrochemical performance may be due to the synergistic effect of the N-doped carbon matrix and the ultrasmall CoSe₂.

In-Situ Fabrication of Bone-Like CoSe2 Nano-Thorn Loaded on Porous Carbon Cloth as a Flexible Electrode for Na-Ion Storage

Sodium-ion batteries (SIBs) based on flexible electrode materials are being investigated recently for improving sluggish kinetics and developing energy density. Transition metal selenides present excellent conductivity and high capacity; nevertheless, their low conductivity and serious volume expansion raise challenging issues of inferior lifespan and capacity fading. Herein, an in-situ construction method through carbonization and selenide synergistic effect is skillfully designed to synthesize a flexible electrode of bone-like CoSe₂ nano-thorn coated on porous carbon cloth. The designed flexible CoSe₂ electrode with stable structural feature displays enhanced Na-ion storage capabilities with good rate performance and outstanding cycling stability. As expected, the designed SIBs with flexible BL-CoSe₂ /PCC electrode display excellent reversible capacity with 360.7 mAh g^-1 after 180 cycles at a current density of 0.1 A g^-1 .

A Dual Protection System for Heterostructured 3D CNT/CoSe2/C as High Areal Capacity Anode for Sodium Storage

3D electrode design is normally opted for multiple advantages, however, instability/detachment of active material causes the pulverization and degradation of the structure, and ultimately poor cyclic stability. Here, a dually protected, highly compressible, and freestanding anode is presented for sodium-ion batteries, where 3D carbon nanotube (CNT) sponge is decorated with homogeneously dispersed CoSe2 nanoparticles (NPs) which are protected under carbon overcoat (CNT/CoSe2/C). The 3D CNT sponge delivers enough space for high mass loading while providing high mechanical strength and faster conduction pathway among the NPs. The outer amorphous carbon overcoat controls the formation of solid electrolyte interphase film by avoiding direct contact of CoSe2 with electrolyte, accommodates large volume changes, and ultimately enhances the overall conductivity of cell and assists in transmitting electron to an external circuit. Moreover, the hybrid can be densified up to 11-fold without affecting its microstructure that results in ultrahigh areal mass loading of 17.4 mg cm-2 and an areal capacity of 7.03 mAh cm-2 along with a high gravimetric capacity of 531 mAh g-1 at 100 mA g-1. Thus, compact and smart devices can be realized by this new electrode design for heavy-duty commercial applications.

Designed Formation of Hybrid Nanobox Composed of Carbon Sheathed CoSe2 Anchored on Nitrogen-Doped Carbon Skeleton as Ultrastable Anode for Sodium-Ion Batteries

Research on sodium-ion batteries (SIBs) has recently been revitalized due to the unique features of much lower costs and comparable energy/power density to lithium-ion batteries (LIBs), which holds great potential for grid-level energy storage systems. Transition metal dichalcogenides (TMDCs) are considered as promising anode candidates for SIBs with high theoretical capacity, while their intrinsic low electrical conductivity and large volume expansion upon Na⁺ intercalation raise the challenging issues of poor cycle stability and inferior rate performance. Herein, the designed formation of hybrid nanoboxes composed of carbon-protected CoSe₂ nanoparticles anchored on nitrogen-doped carbon hollow skeletons (denoted as CoSe₂ @C∩NC) via a template-assisted refluxing process followed by conventional selenization treatment is reported, which exhibits tremendously enhanced electrochemical performance when applied as the anode for SIBs. Specifically, it can deliver a high reversible specific capacity of 324 mAh g^-1 at current density of 0.1 A g^-1 after 200 cycles and exhibit outstanding high rate cycling stability at the rate of 5 A g^-1 over 2000 cycles. This work provides a rational strategy for the design of advanced hybrid nanostructures as anode candidates for SIBs, which could push forward the development of high energy and low cost energy storage devices.

NiSe2 nanooctahedra as anodes for high-performance sodium-ion batteries

Octahedral NiSe₂ was fabricated by a facile hydrothermal method. It showed excellent Na ion storage performance when used as an anode material for sodium-ion batteries.