Encapsulation of MnS Nanocrystals into N, S-Co-doped Carbon as Anode Material for Full Cell Sodium-Ion Capacitors

Engineering Heterostructured Fe-Co-P Arrays for Robust Sodium Storage

Transition metal phosphides attract extensive concerns thanks to their high theoretical capacity in sodium ion batteries (SIBs). Nevertheless, the substantial volume fluctuation of metal phosphides during cycling leads to severe capacity decay, which largely hinders their large-scale deployment. In this regard, heterostructured Fe-Co-P (FeP/Co2P) arrays are firstly constructed in this work for SIBs. The novel self-supported construction without insulated binders favors fast charge migration and Na+ ion diffusion. In addition, the special heterostructure with abundant heterointerfaces could considerably mitigate the volume change during (de)sodiation and provide increased active sites for Na+ ions. Density functional theoretical (DFT) calculations confirm the built-in electric field in the heterointerfaces, which greatly hastens charge transfer and Na+ ion transportation, thereafter bringing about enhanced electrochemical performance. Most importantly, the FeP/Co2P heterostructure discloses higher electrical conductivity than that of bare FeP and Co2P based on the theoretical calculations. As anticipated, the heterostructured Fe-Co-P arrays demonstrate superior performance to that of Fe-P or Co-P anode, delivering high reversible capacities of 634 mAh g-1 at 0.2 A g-1 and 239 mAh g-1 at 1 A g-1 after 300 cycles.

Structure and Defect Engineering of V3 S4-x Sex Quantum Dots Confined in a Nitrogen-Doped Carbon Framework for High-Performance Sodium-Ion Storage

Constructing quantum dot-scale metal sulfides with defects and strongly coupled with carbon is significant for advanced sodium-ion batteries (SIBs). Herein, Se substituted V3 S4 quantum dots with anionic defects confined in nitrogen-doped carbon matrix (V3 S4-x Sex /NC) are fabricated. Introducing element Se into V3 S4 crystal expands the interlayer distance of V3 S4 , and triggers anionic defects, which can facilitate Na+ diffusions and act as active sites for Na+ storage. Meanwhile, the quantum dots tightly encapsulated by conductive carbon framework improve the stability and conductivity of the electrode. Theoretical calculations also unveil that the presence of Se enhances the conductivity and Na+ adsorption ability of V3 S4-x Sex . These properties contribute to the V3 S4-x Sex /NC with high specific capacity of 447 mAh g-1 at 0.2 A g-1 , and prominent rate and cyclic performance with 504 mAh g-1 after 1000 cycles at 10 A g-1 . The sodium-ion hybrid capacitors (SIHCs) with V3 S4-x Sex /NC anode and activated carbon cathode can achieve high energy/power density (maximum 144 Wh kg-1 /5960 W kg-1 ), capacity retention ratio of 71% after 4000 cycles at 2 A g-1 . This work not only synthesizes V3 S4-x Sex /NC, but also provides a promising opportunity for designing quantum dots and utilizing defects to improve the electrochemical properties.

2D heterostructural Mn2O3 quantum dots embedded N-doped carbon nanosheets with strongly stable interface enabling high-performance sodium-ion hybrid capacitors

The ingenious architectural structural engineering is extensively identified as a cogent means for facilitating the electrochemical properties of conversion-type anode materials for sodium-ion storage. Herein, a delicate, scalable and controllable solvent-free strategy is proposed to synthesize ultrafine Mn2O3 quantum dots embedded into N-doped carbon to generate two-dimensional (2D) composites (MNC) with robust interfacial heterostructural interactions for high sodium ion storage and fast reaction kinetics, which averts the use of solvents and environmental pollution, greatly reduces time and production costs. The introduction of metallic Mn species simultaneously achieves the construction of ultrafine Mn2O3 quantum dots and strong interfacial heterostructural COMn bonds between metal species and 2D N-doped carbon matrix. The synergistic effect of the formation of oxide quantum dots, the combination of 2D N-doped carbon and the construction of robust interfacial interactions provides the stable electrode structure, fast reaction kinetics and high electrochemical storage capability of anode materials. Hence, MNC composites in SIBs convey remarkable reversible rate capability. Its superior capacity reaches 215 mAh g-1 for 50 cycles at 0.2 A g-1 and 155 mAh g-1 for 1000 cycles at a high current density of 5 A g-1, which shows good long-term stability. The assembled sodium-ion hybrid capacitors (SIHCs) device delivers outstanding energy density of 138 Wh kg-1 at a power density of 126 W kg-1 and 98% capacity retention after 2000 cycles at 2 A g-1, and tremendous capability for practical applications (69 LEDs can be easily lighted). This work not merely offers guidance for the rational interfacial engineering design of high-capacity Mn-based electrode materials in a feasible and scalable solvent-free tactics for Na+ storage, but also broadens the routes for projecting a better electrode material for other battery systems.

Electron-Beam- and Thermal-Annealing-Induced Structural Transformations in Few-Layer MnPS3

Quasi-two-dimensional (2D) manganese phosphorus trisulfide, MnPS₃, which exhibits antiferromagnetic ordering, is a particularly interesting material in the context of magnetism in a system with reduced dimensionality and its potential technological applications. Here, we present an experimental and theoretical study on modifying the properties of freestanding MnPS₃ by local structural transformations via electron irradiation in a transmission electron microscope and by thermal annealing under vacuum. In both cases we find that MnS_1-xP_x phases (0 ≤ x < 1) form in a crystal structure different from that of the host material, namely that of the α- or γ-MnS type. These phase transformations can both be locally controlled by the size of the electron beam as well as by the total applied electron dose and simultaneously imaged at the atomic scale. For the MnS structures generated in this process, our ab initio calculations indicate that their electronic and magnetic properties strongly depend on both in-plane crystallite orientation and thickness. Moreover, the electronic properties of the MnS phases can be further tuned by alloying with phosphorus. Therefore, our results show that electron beam irradiation and thermal annealing can be utilized to grow phases with distinct properties starting from freestanding quasi-2D MnPS₃.

Boosting the Initial Coulomb Efficiency of Sisal Fiber-Derived Carbon Anode for Sodium Ion Batteries by Microstructure Controlling

As anode material for sodium ion batteries (SIBs), biomass-derived hard carbon has attracted a great deal of attention from researchers because of its renewable nature and low cost. However, its application is greatly limited due to its low initial Coulomb efficiency (ICE). In this work, we employed a simple two-step method to prepare three different structures of hard carbon materials from sisal fibers and explored the structural effects on the ICE. It was determined that the obtained carbon material, with hollow and tubular structure (TSFC), exhibits the best electrochemical performance, with a high ICE of 76.7%, possessing a large layer spacing, a moderate specific surface area, and a hierarchical porous structure. In order to better understand the sodium storage behavior in this special structural material, exhaustive testing was performed. Combining the experimental and theoretical results, an "adsorption-intercalation" model for the sodium storage mechanism of the TSFC is proposed.

Proficient One-Step Heat-Up Synthesis of Manganese Sulfide Quantum Dots for Solar Cell Applications

The necessity to develop renewable energy resources that are highly durable and flexible with superior energy density and capacitance ability has attracted considerable interest in the field of solar cell research. Semiconducting compound materials that are easily available, hazard-free and cost-effective are emerging as potential solutions to tackle this challenge. Herein, we present multiple molecular precursors used to grow manganese sulfide nanoparticles through a proficient one-step heat-up approach. For all of the tested samples, the X-ray diffraction peaks correspond to a γ-MnS hexagonal wurtzite structure. UV-Vis spectroscopy yielded absorption wavelengths of 359-420 nm and band-gap energies of 3.78-4.0 eV. Photoluminescence analysis shows characteristics of red and blue shift from 451-602 nm. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) reveal a narrow size distribution with nanosticks and large contact areas, which are critical for improved catalytic performance. The current study provides an improved pathway to a well-grown and uniform nanocrystal structure for applications in energy devices.

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

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

Stretchable sodium-ion capacitors based on coaxial CNT supported Na2Ti3O7 with high capacitance contribution

Stretchable sodium-ion capacitors (SSICs) are promising energy storage devices for wearable electronic devices, and the development bottleneck is the realization of stretchable battery-type electrodes with desirable electrochemical properties during dynamic deformation. Herein, we find that electrostatic modification of acidified carbon nanotubes with polyamines can introduce active sites and modulate the surface pH microenvironment, thereby developing a route to realize the in situ coaxial nanometerization of sodium titanate (nCNT@NTO). The nCNT@NTO anode material has a fast Na⁺ transport and the high capacitive contribution, which can deliver a high specific capacity (206.5 mA h g^-1 at 0.1 A g^-1) and high rate performance (maintain 51% capacity at 10 A g^-1), and the ideal cycle stability (∼93% capacity retention after 1000 cycles at 5 A g^-1). In addition, acrylate-rubber with high stickiness and stretchability are served as the elastic matrix both of the stretchable electrodes and quasi-solid-state electrolytes, which endows strong adhesion between electrodes and electrolytes. Thus, the accordingly assembled SSIC delivers high energy density of 8.8 mW h cm^-3 (at a power density of 0.024 W cm^-3), and excellent deformation stability (89% capacitance retention after 500 stretching cycles under 100% strain).

All-Electrochemical Nanofabrication of Stacked Ternary Metal Sulfide/Graphene Electrodes for High-Performance Alkaline Batteries

Energy-storage materials can be assembled directly on the electrodes of a battery using electrochemical methods, this allowing sequential deposition, high structural control, and low cost. Here, a two-step approach combining electrophoretic deposition (EPD) and cathodic electrodeposition (CED) is demonstrated to fabricate multilayer hierarchical electrodes of reduced graphene oxide (rGO) and mixed transition metal sulfides (NiCoMnS_x ). The process is performed directly on conductive electrodes applying a small electric bias to electro-deposit rGO and NiCoMnS_x in alternated cycles, yielding an ideal porous network and a continuous path for transport of ions and electrons. A fully rechargeable alkaline battery (RAB) assembled with such electrodes gives maximum energy density of 97.2 Wh kg^-1 and maximum power density of 3.1 kW kg^-1 , calculated on the total mass of active materials, and outstanding cycling stability (retention 72% after 7000 charge/discharge cycles at 10 A g^-1 ). When the total electrode mass of the cell is considered, the authors achieve an unprecedented gravimetric energy density of 68.5 Wh kg^-1 , sevenfold higher than that of typical commercial supercapacitors, higher than that of Ni/Cd or lead-acid Batteries and similar to Ni-MH Batteries. The approach can be used to assemble multilayer composite structures on arbitrary electrode shapes.

MnS@N,S Co-Doped Carbon Core/Shell Nanocubes: Sulfur-Bridged Bonds Enhanced Na-Storage Properties Revealed by In Situ Raman Spectroscopy and Transmission Electron Microscopy

Rational structure and morphology design are of great significance to realize excellent Na storage for advanced electrode materials in sodium-ion batteries (SIBs). Herein, a cube-like core/shell composite of single MnS nanocubes (≈50 nm) encapsulated in N, S co-doped carbon (MnS@NSC) with strong CSMn bond interactions is successfully prepared as outstanding anode material for SIBs. The carbon shell significantly restricts the expansion of the MnS volume in successive sodiation/desodiation processes, as demonstrated by in situ transmission electron microscopy (TEM) of one single MnS@NSC nanocube. Moreover, the in situ generated CSMn bonds between the MnS core and carbon shell play a significant role in improving the Na-storage stability and reversibility of MnS@NSC, as revealed by in situ Raman and TEM. As a result, MnS@NSC exhibits a high reversible specific capacity of 594.2 mAh g^-1 at a current density of 100 mA g^-1 and an excellent rate performance. It also achieves a remarkable cycling stability of 329.1 mAh g^-1 after 3000 charge/discharge cycles at 1 A g^-1 corresponding to a low capacity attenuation rate of 0.0068% per cycle, which is superior to that of pristine MnS and most of the reported Mn-based anode materials in SIBs.

Electrolyte Technologies for High Performance Sodium-Ion Capacitors

Bridging the energy gap between batteries and capacitors, while in principle delivering a supercapacitor-like high power density and long lifespan, sodium-ion capacitors (SIC) have been considered promising energy storage devices that could be commercialized in the near future due to the natural abundance of sodium sources and the performance superiority of SIC devices. Therefore, in the past decade, substantial research efforts have been devoted to their structure and property improvements. With regard to the electrochemical performance of an ion capacitor, except for the electrode, the composition and structure of the electrolytes are also of great importance. Thus, in this mini review, we present a brief summary of the electrolytes developed recently for high performance SIC, including aqueous, organic, and ionic liquid based electrolytes. The influence factors such as ionic conductivities, electrolyte concentrations, electrochemical stable windows, as well as the cost and safety issues are discussed. Furthermore, the future perspectives and challenges in the science and engineering of new electrolytes are also considered. We hope that this review may be helpful to the energy storage community regarding the electrolytes of advanced SIC systems.