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With their advantages of abundant raw material reserves, safety, and low toxicity and cost, sodium-ion batteries (SIBs) have gained increasing attention in recent years. Thanks to a high theoretical specific capacity (124 mAh g−1), a high operating voltage (about 3.2 V), and a very stable three-dimensional layered structure, sodium ferric fluorophosphate (Na2FePO4F, NFPF) has emerged as a strong candidate to be used as a cathode material for SIBs. However, applications are currently limited due to the low electronic conductivity and slow ion diffusion rate of NFPF, which result in a low actual specific capacity and a high rate performance. In this study, the authors used a high-temperature solid-phase technique to produce Na2−xLixFePO4F/C (0 ≤ x ≤ 2) and evaluated the impact on electrode performance of materials with different Na+ and Li+ contents (values of x). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were also used to analyze the material's crystal structure and nanostructure. The results show that the material had the best room-temperature performance when x = 0.5. At a charge–discharge rate of 0.1 C, the first discharge-specific capacity of the resulting Na1.5Li0.5FePO4F/C cathode material was 122.9 mAh g−1 (the theoretical capacity was 124 mAh g−1), and after 100 cycles, it remained at 118 mAh g−1, representing a capacity retention rate of 96.2% and a Coulomb efficiency of 98%. The findings of this study demonstrate that combining lithium and sodium ions improves the electrochemical performance of electrode materials. [ABSTRACT FROM AUTHOR]

Sodium‐ion batteries (SIBs) work similarly to lithium‐ion batteries, but they cost less and are safer. However, the battery has some shortcomings, such as low energy density and poor stability, which hinder its development and application. The development of electrode materials with low cost and high‐performance characteristics is a hot and difficult point in the research of SIBs. In this paper, the existing sodium ion cathode and anode materials are systematically reviewed. Firstly, the electrochemical structures and properties of various cathode and anode materials were introduced, and the limitations of cathode and anode materials of SIBs were analyzed. Based this, the current modification methods were summarized, such as surface coating of materials, doping of heteroatoms and metal elements to improve the electrochemical performance. In this paper, the research progress and modification measures of SIBs are summarized and prospected. It is helpful to further improve its comprehensive performance and lay a foundation for the future application of cathode and anode materials for SIBs. [ABSTRACT FROM AUTHOR]

The cathode material Na 3 V 2 (PO 4) 2 F 3 of sodium-ion battery is well-known for its large number of ion migration channels and high working voltage. However, the electrochemical performance of Na 3 V 2 (PO 4) 2 F 3 is not very outstanding. Thus, in the present study, Na 3 V 2 (PO 4) 2 F 3 cathode materials were successfully synthesized by using the sol-gel method and mechanical milling method to enhance the electrochemical performance. The physicochemical properties of synthesized Na 3 V 2 (PO 4) 2 F 3 were investigated by using X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transition electron microscopy. X-ray diffraction spectroscopy indicates that the doping of nitrogen and sulfur did not alter the crystal form of Na 3 V 2 (PO 4) 2 F 3. Transition electron microscopy image shows that Na 3 V 2 (PO 4) 2 F 3 has a thin carbon layer, and x-ray photoelectron spectroscopy illustrates the successful doping of nitrogen and sulfur into the carbon layer. The cyclic voltammetry curves show that the nitrogen and sulfur co-doped Na 3 V 2 (PO 4) 2 F 3 samples have good reversibility and low polarization. Materials with 15% thiourea has a high discharge specific capacity (126.9 mA h g−1 at 0.2 C) at the first cycle and excellent cycle stability (126.3 mA h g−1 after 100 cycles, a capacity retention of 99.5%) among the synthesized cathode materials. In the present study, the electrochemical performance of the Na 3 V 2 (PO 4) 2 F 3 cathode material was enhanced by regulation of co-doping of nitrogen and sulfur atoms. [ABSTRACT FROM AUTHOR]

Lithium-ion batteries (LIBs) present the advantages of long cycle life, high voltage, and energy density and are widely made in the field of energy storage. LiVOPO 4 (LVOP), a cathode material used in LIBs, has a high conceptual capacity of 159 mAh g−1 and high operating voltage of 3.9 V. However, its low electrical conductivity and cycle performance limit its commercial applications. According to the X-ray diffraction results, orthogonal crystal LVOP and monoclinic crystal Li 3 V 2 (PO 4) 3 (LVP) coexisted in the synthesised composite material. The transmission electron microscopy results also indicated that the LVOP and LVP phases coexist, which were coated by carbon layer of about 2.5 nm. The discharge of LVOP–LVP composite material initially was 143.2 mAh g−1, and that after 120 cycles was 132.2 mAh g−1 (at 0.1 C and 3–4.5 V). Thus, the electronic conductivity and first discharge specific capacity of the material enhanced due to the introduction of fast ion conductor LVP into LVOP. Electrochemical performance improved because the introduction of LVP led to an increase in Li+ pervasion channels in the original material and the acceleration of the Li+ transmission speed. [ABSTRACT FROM AUTHOR]

The C@Fe 1−x S/FeS 2 heterostructure was designed using a facile hydrothermal and vulcanization method. The as-prepared sample exhibits significant electrical conductivity, excellent electrochemical performance. The heterostructure not only makes the crystalline framework of the material more stable, but also accelerates ionic diffusion kinetics. [Display omitted] • Synthesized C@Fe 1−x S/FeS 2 heterostructure by hydrothermal and vulcanization method. • Heterostructure provides stable framework, it accelerates ionic diffusion kinetics. • As one of cathode, the C@Fe 1−x S/FeS 2 shows excellent cycle performances. The high theoretical capacity of iron-based sulfide makes it a promising cathode material for sodium-ion batteries. However, the extensive application of Na+ is limited by numerous issues, such as poor electrical conductivity stemming from slow ionic conduction kinetics. In this study, a diphase heterostructure nanocomposite (C@Fe 1−x S/FeS 2) was designed by combining heterostructure engineering with carbonaceous materials. According to X-ray diffraction (XRD) analyses, the as-prepared sample exhibited a diphase structure. Microscopic examinations revealed a dragon fruit-like morphology formed by carbon and Fe 1−x S/FeS 2 , where carbon is wrapped by Fe 1−x S/FeS 2. The C@Fe 1−x S/FeS 2 composite exhibited remarkable electrochemical performance as cathode materials for sodium-ion batteries. Notably, the discharge capacity maintained a value of 370.4 mA h g−1 after 500 cycles at a current density of 500 mA g−1 within the voltage range of 0.8–3.0 V. Remarkably, the discharge capacity remained at 253.6 mA h g−1 after 2000 cycles, even when cycled at a current density of 1000 mA g−1. Additionally, the diphase heterostructure C@Fe 1−x S/FeS 2 improves the conductivity of the material and buffers capacity loss. These advantageous features are attributed to the dragon fruit-like structure constructed from carbon and Fe 1−x S/FeS 2. [ABSTRACT FROM AUTHOR]