Doping Regulation in Polyanionic Compounds for Advanced Sodium-Ion Batteries.

It has long been the goal to develop rechargeable batteries with low cost and long cycling life. Polyanionic compounds offer attractive advantages of robust frameworks, long-term stability, and cost-effectiveness, making them ideal candidates as electrode materials for grid-scale energy storage systems. In the past few years, various polyanionic electrodes have been synthesized and developed for sodium storage. Specifically, doping regulation including cation and anion doping has shown a great effect in tailoring the structures of polyanionic electrodes to achieve extraordinary electrochemical performance. In this review, recent progress in doping regulation in polyanionic compounds as electrode materials for sodium-ion batteries (SIBs) is summarized, and their underlying mechanisms in improving electrochemical properties are discussed. Moreover, challenges and prospects for the design of advanced polyanionic compounds for SIBs are put forward. It is anticipated that further versatile strategies in developing high-performance electrode materials for advanced energy storage devices can be inspired.

A Bifunctional Fluorophosphate Electrolyte for Safer Sodium-Ion Batteries.

Most of the currently developed sodium-ion batteries (SIBs) have potential safety hazards due to the use of highly volatile and flammable alkyl carbonate electrolytes. To overcome this challenge, we report an electrochemically compatible and nonflammable electrolyte, tris(2,2,2-trifluoroethyl) phosphate (TFEP) with low-concentration sodium bis(fluorosulfonyl)imide (0.9 M), which is designed not only to match perfectly with the hard carbon (HC) anode but also to enhance the thermal stability of SIBs. Experimental results and theoretical calculations reveal that TFEP molecules have a significantly low barrier to decompose before Na+ inserts into HC, forming a stable inorganic solid-electrolyte interface layer, thus improving the electrochemical and structural stabilities of HC anodes. An HC/Na3V2(PO4)3 full cell using TFEP electrolyte shows a high capacity retention of 89.2% after 300 cycles and a dramatically reduced exothermic heat at elevated temperature, implying its potential application for safe and low-cost larger-scale energy storage.

Recent Progress in Iron-Based Electrode Materials for Grid-Scale Sodium-Ion Batteries.

Grid-scale energy storage batteries with electrode materials made from low-cost, earth-abundant elements are needed to meet the requirements of sustainable energy systems. Sodium-ion batteries (SIBs) with iron-based electrodes offer an attractive combination of low cost, plentiful structural diversity and high stability, making them ideal candidates for grid-scale energy storage systems. Although various iron-based cathode and anode materials have been synthesized and evaluated for sodium storage, further improvements are still required in terms of energy/power density and long cyclic stability for commercialization. In this Review, progress in iron-based electrode materials for SIBs, including oxides, polyanions, ferrocyanides, and sulfides, is briefly summarized. In addition, the reaction mechanisms, electrochemical performance enhancements, structure-composition-performance relationships, merits and drawbacks of iron-based electrode materials for SIBs are discussed. Such iron-based electrode materials will be competitive and attractive electrodes for next-generation energy storage devices.

An All-Phosphate and Zero-Strain Sodium-Ion Battery Based on Na3V2(PO4)3 Cathode, NaTi2(PO4)3 Anode, and Trimethyl Phosphate Electrolyte with Intrinsic Safety and Long Lifespan.

Development of intrinsically safe and long lifespan sodium-ion batteries (SIBs) is urgently needed for large-scale energy storage applications. However, most of the currently developed SIBs suffer from insufficient cycle life and potential unsafety. Herein, we construct an all-phosphate sodium-ion battery (AP-SIB) using a Na3V2(PO4)3 cathode, NaTi2(PO4)3 anode, and nonflammable trimethyl phosphate (TMP) electrolyte. The AP-SIB exhibits not only high safety, high rate performance, and ultralong cycle life but also zero-strain characteristics due to the inverse volume change of the phosphate cathode and anode during charge and discharge cycles, offering a safer and cycle-stable Na-ion technology for electric storage applications.

Phosphate Framework Electrode Materials for Sodium Ion Batteries.

Sodium ion batteries (SIBs) have been considered as a promising alternative for the next generation of electric storage systems due to their similar electrochemistry to Li-ion batteries and the low cost of sodium resources. Exploring appropriate electrode materials with decent electrochemical performance is the key issue for development of sodium ion batteries. Due to the high structural stability, facile reaction mechanism and rich structural diversity, phosphate framework materials have attracted increasing attention as promising electrode materials for sodium ion batteries. Herein, we review the latest advances and progresses in the exploration of phosphate framework materials especially related to single-phosphates, pyrophosphates and mixed-phosphates. We provide the detailed and comprehensive understanding of structure-composition-performance relationship of materials and try to show the advantages and disadvantages of the materials for use in SIBs. In addition, some new perspectives about phosphate framework materials for SIBs are also discussed. Phosphate framework materials will be a competitive and attractive choice for use as electrodes in the next-generation of energy storage devices.

Coaxial Three-Layered Carbon/Sulfur/Polymer Nanofibers with High Sulfur Content and High Utilization for Lithium-Sulfur Batteries.

Great progress has been made on the cyclability and material utilization in recent development of lithium-sulfur (Li-S) batteries; however, most of the sulfur electrodes reported so far have a considerable low loading of sulfur (60%), which causes a substantial decrease in energy density and is therefore difficult for application in batteries. To deal with this issue, we fabricate a novel sulfur composite with a coaxial three-layered structure, in which sulfur is deposited on carbon fibers and coated with poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), thus enabling a high sulfur loading of 70.8 wt % without the expense of its electrochemical performance. Benefiting from the rigid conductive framework of carbon fibers and flexible buffering matrix of the polymer for blocking the diffusion loss of discharge intermediates, the as-fabricated composite electrode exhibits a high initial reversible capacity of 1272 mA h g-1 (based on the total mass of the composite), a stable cyclability with a retained capacity of 807 mA h g-1 after 200 cycles, and a high Coulombic efficiency of ∼99% upon extended cycling, offering a new selection for practical application in Li-S batteries.

Graphene-Scaffolded Na3V2(PO4)3 Microsphere Cathode with High Rate Capability and Cycling Stability for Sodium Ion Batteries.

High voltage, high rate, and cycling-stable cathodes are urgently needed for development of commercially viable sodium ion batteries (SIBs). Herein, we report a facile spray-drying method to synthesize graphene-scaffolded Na3V2(PO4)3 microspheres (NVP@rGO), in which nanocrystalline Na3V2(PO4)3 is embedded in graphene sheets to form porous microspheres. Benefiting from the highly conductive graphene framework and porous structure, the NVP@rGO material exhibits a high reversible capacity (115 mAh g-1 at 0.2 C), long-term cycle life (81% of capacity retention up to 3000 cycles at 5 C), and excellent rate performance (44 mAh g-1 at 50 C). The electrochemical properties of a full Na-ion cell with the NVP@rGO cathode and Sb/C anode are also investigated. The present results suggest promising applications of the NVP@rGO material as a high performance cathode for sodium ion batteries.

Cyclic Multiblock Copolymers via Combination of Iterative Cu(0)-Mediated Radical Polymerization and Cu(I)-Catalyzed Azide-Alkyne Cycloaddition Reaction.

Cyclic multiblock polymers with high-order blocks are synthesized via the combination of single-electron transfer living radical polymerization (SET-LRP) and copper-catalyzed azide-alkyne cycloaddition (CuAAC). The linear α,ω-telechelic multiblock copolymer is prepared via SET-LRP by sequential addition of different monomers. The SET-LRP approach allows well control of the block length and sequence as A-B-C-D-E, etc. The CuAAC is then performed to intramolecularly couple the azide and alkyne end groups of the linear copolymer and produce the corresponding cyclic copolymer. The block sequence and the cyclic topology of the resultant cyclic copolymer are confirmed by the characterization of 1 H nuclear magnetic resonance spectroscopy, gel permeation chromatography, Fourier transform infrared spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Yolk-Shell TiO2@C Nanocomposite as High-Performance Anode Material for Sodium-Ion Batteries.

Yolk-shell TiO2@C nanocomposites have been synthesized successfully through a simple self-catalyzing solvothermal method. The structural and morphological characterizations reveal that TiO2@C nanocomposite has a yolk-shell microsphere morphology with diameters of 1-2 µm, and both yolk and shell are composed of TiO2 nanoparticles (∼10 nm). The as-prepared yolk-shell TiO2@C composites exhibit superior sodium storage properties, with a specific capacity of 210 mAh g-1, an outstanding cycle life of 85% capacity retention of 2000 cycles and extraordinary rate performance at 40 C rate. All the results indicate that the yolk-shell TiO2@C nanocomposite can be suggested as a promising anode material for high-performance sodium-ion batteries.

Understanding Voltage Decay in Lithium-Rich Manganese-Based Layered Cathode Materials by Limiting Cutoff Voltage.

The effect of the cutoff voltages on the working voltage decay and cyclability of the lithium-rich manganese-based layered cathode (LRMO) was investigated by electrochemical measurements, electrochemical impedance spectroscopy, ex situ X-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy line scan technologies. It was found that both lower (2.0 V) and upper (4.8 V) cutoff voltages cause severe voltage decay with cycling due to formation of the spinel phase and migration of the transition metals inside the particles. Appropriate cutoff voltage between 2.8 and 4.4 V can effectively inhibit structural variation as the electrode demonstrates 92% capacity retention and indiscernible working voltage decay over 430 cycles. The results also show that phase transformation not only on high charge voltage but also on low discharge voltage should be addressed to obtain highly stable LRMO materials.

Emulsion Templating Cyclic Polymers as Microscopic Particles with Tunable Porous Morphology.

Cyclic polymers are a particular class of macromolecules without terminal groups. Most studies has involved their physical properties and polymer composition, while attention has rarely been paid to their emulsification in an oil-water system. Herein we synthesized a cyclic polymer with polystyrene side chains via ring-expansion metathesis polymerization and click-chemistry. This cyclic polymer was compared with linear polystyrene in order to investigate the effect of cyclic topology on preparing porous particles by emulsion templating methods. The contribution of cyclic topology to emulsification originates from the formation of hollow microspheres with the use of cyclic polymer while linear polymer only afforded solid microspheres. With addition of hexadecane as soft template, both cyclic polymer and linear polymer emulsions were successfully converted into porous particles. Superior to linear polymer, cyclic polymer enables the stabilization of emulsion droplets and the tuning of porous morphology. It is revealed that cyclic polymer with nanoring shape tends to assemble at the interfacial area, leading to the Pickering effect that decelerates the macrophase separation. Furthermore, the unique porous feature of polymer particles affords a convenient application for the detection of trace explosive.

Antimony Nanocrystals Encapsulated in Carbon Microspheres Synthesized by a Facile Self-Catalyzing Solvothermal Method for High-Performance Sodium-Ion Battery Anodes.

Antimony/carbon (Sb@C) microspheres are initially synthesized via a facile self-catalyzing solvothermal method, and their applicability as anode materials for sodium-ion batteries is investigated. The structural and morphological characterizations reveal that Sb@C microspheres are composed of Sb nanoparticles (∼20 nm) homogeneously encapsulated in the C matrix. The self-catalyzing solvothermal mechanism is verified through comparative experiments by using different raw materials. The as-prepared Sb@C microspheres exhibit superior sodium storage properties, demonstrating a reversible capacity of 640 mAh g(-1), excellent rate performance, and an extended cycling stability of 92.3% capacity retention over 300 cycles, making them promising anode materials for sodium-ion batteries.

Hierarchical carbon framework wrapped Na3V2(PO4)3 as a superior high-rate and extended lifespan cathode for sodium-ion batteries.

Hierarchical carbon framework wrapped Na3 V2 (PO4 )3 (HCF-NVP) is successfully synthesized through chemical vapor deposition on pure Na3 V2 (PO4 )3 particles. Electrochemical experiments show that the HCF-NVP electrode can deliver a large reversible capacity (115 mA h g(-1) at 0.2 C), superior high-rate rate capability (38 mA h g(-1) at 500 C), and ultra-long cycling stability (54% capacity retention after 20 000 cycles).

High-Performance Olivine NaFePO4 Microsphere Cathode Synthesized by Aqueous Electrochemical Displacement Method for Sodium Ion Batteries.

Olivine NaFePO4/C microsphere cathode is prepared by a facile aqueous electrochemical displacement method from LiFePO4/C precursor. The NaFePO4/C cathode shows a high discharge capacity of 111 mAh g(-1), excellent cycling stability with 90% capacity retention over 240 cycles at 0.1 C, and high rate capacity (46 mAh g(-1) at 2 C). The excellent electrochemical performance demonstrates that the aqueous electrochemical displacement method is an effective and promising way to prepare NaFePO4/C material for Na-based energy storage applications. Moreover, the Na2/3FePO4 intermediate is observed for the first time during the Na intercalation process through conventional electrochemical techniques, corroborating an identical two-step phase transition reaction both upon Na intercalation and deintercalation processes. The clarification of the electrochemical reaction mechanism of olivine NaFePO4 could inspire more attention on the investigation of this material for Na ion batteries.

Mesoporous amorphous FePO4 nanospheres as high-performance cathode material for sodium-ion batteries.

FePO4 nanospheres are synthesized successfully through a simple chemically induced precipitation method. The nanospheres present a mesoporous amorphous structure. Electrochemical experiments show that the FePO4/C electrode demonstrates a high initial discharging capacity of 151 mAh g(-1) at 20 mA g(-1), stable cyclablilty (94% capacity retention ratio over 160 cycles), as well as high rate capability (44 mAh g(-1) at 1000 mA g(-1)) for Na-ion storage. The superior electrochemical performance of the FePO4/C nanocomposite is due to its particular mesoporous amorphous structure and close contact with the carbon framework, which significantly improve the ionic and electronic transport and intercalation kinetics of Na ions.

Sodium ion insertion in hollow carbon nanowires for battery applications.

Hollow carbon nanowires (HCNWs) were prepared through pyrolyzation of a hollow polyaniline nanowire precursor. The HCNWs used as anode material for Na-ion batteries deliver a high reversible capacity of 251 mAh g(-1) and 82.2% capacity retention over 400 charge-discharge cycles between 1.2 and 0.01 V (vs Na(+)/Na) at a constant current of 50 mA g(-1) (0.2 C). Excellent cycling stability is also observed at an even higher charge-discharge rate. A high reversible capacity of 149 mAh g(-1) also can be obtained at a current rate of 500 mA g(-1) (2C). The good Na-ion insertion property is attributed to the short diffusion distance in the HCNWs and the large interlayer distance (0.37 nm) between the graphitic sheets, which agrees with the interlayered distance predicted by theoretical calculations to enable Na-ion insertion in carbon materials.

High capacity, reversible alloying reactions in SnSb/C nanocomposites for Na-ion battery applications.

A new SnSb/C nanocomposite based on Na alloying reactions is demonstrated as anode for Na-ion battery applications. The electrode can achieve an exceptionally high capacity (544 mA h g(-1), almost double that of intercalation carbon materials), good rate capacity and cyclability (80% capacity retention over 50 cycles) for Na-ion storage.

A soft approach to encapsulate sulfur: polyaniline nanotubes for lithium-sulfur batteries with long cycle life.

A novel vulcanized polyaniline nanotube/sulfur composite was prepared successfully via an in situ vulcanization process by heating a mixture of polyaniline nanotube and sulfur at 280 °C. The electrode could retain a discharge capacity of 837 mAh g(-1) after 100 cycles at a 0.1 C rate and manifested 76% capacity retention up to 500 cycles at a 1 C rate.

Clewlike ZnV2O4 hollow spheres: nonaqueous sol-gel synthesis, formation mechanism, and lithium storage properties.

Hollow ZnV(2)O(4) microspheres with a clewlike feature were synthesized by reacting zinc nitrate hexahydrate and ammonium metavanadate in benzyl alcohol at 180 degrees C for the first time. GC-MS analysis revealed that the organic reactions that occurred in this study were rather different from those in benzyl alcohol based nonaqueous sol-gel systems with metal alkoxides, acetylacetonates, and acetates as the precursors. Time-dependent experiments revealed that the growth mechanism of the clewlike ZnV(2)O(4) hollow microspheres might involve a unique multistep pathway. First, the generation and self-assembly of ZnO nanosheets into metastable hierarchical microspheres as well as the generation of VO(2) particles took place quickly. Then, clewlike ZnV(2)O(4) hollow spheres were gradually produced by means of a repeating reaction-dissolution (RD) process. In this process, the outside ZnO nanosheets of hierarchical microspheres would first react with neighboring vanadium ions and benzyl alcohol and also serve as the secondary nucleation sites for the subsequently formed ZnV(2)O(4) nanocrystals. With the reaction proceeding, the interior ZnO would dissolve and then spontaneously diffuse outwards to nucleate as ZnO nanocrystals on the preformed ZnV(2)O(4) nanowires. These renascent ZnO nanocrystals would further react with VO(2) and benzyl alcohol, ultimately resulting in the final formation of a hollow spatial structure. The lithium storage ability of clewlike ZnV(2)O(4) hollow microspheres was studied. When cycled at 50 mA g(-1) in the voltage range of 0.01-3 V, this peculiarly structured ZnV(2)O(4) electrode delivered an initial reversible capacity of 548 mAh g(-1) and exhibited almost stable cycling performance to maintain a capacity of 524 mAh g(-1) over 50 cycles. This attractive lithium storage performance suggests that the resulting clewlike ZnV(2)O(4) hollow spheres are promising for lithium-ion batteries.