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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]

In this study, polyacrylonitrile and boric acid are used as the nitrogen source and boron source, respectively. The Na3V2(PO4)2F3 cathode material with a carbon layer coated by nitrogen and boron was successfully synthesized by using a mechanical ball milling assisted sol–gel method (represented as NVPF@NBC). Simultaneously, Na3V2(PO4)2F3 materials with single-atom-doped carbon have been successfully prepared in this study by using the same method. Among the above materials, NVPF@NBC exhibits appreciable electrochemical performance because of the synergistic effect of diatoms in carbon. NVPF@NBC shows an outstanding specific discharge capacity of 127.2 mA h g−1 at 0.2C, and the capacity retention after 100 cycles is 97.6%. In particular, NVPF@NBC exhibits a discharge specific capacity of 106.3 mA h g−1 at 5C, and the capacity retention after 500 cycles is 90.59%. The X-ray diffraction analysis of the synthesized materials shows that the NVPF@NBC material has good crystallinity, indicating that the diatoms of N and B do not change the crystal structure of the material. TEM shows that the NVPF@NBC material exhibits uniform distribution of particles and a uniform carbon coating of about 4 nm. Nitrogen–boron co-doping does not influence the valence state of V3+ in the material. The results indicate that the co-doping of nitrogen and boron reinforces the crystallinity of the material and strengthens the sodium storage performance of the material, thereby greatly ameliorating the electrochemical performance of the material. [ABSTRACT FROM AUTHOR]

The heterostructures, xNa0.7MnO2–yLi4Mn5O12(0 < x < 1, 0 < y ≤ 1), were synthesized by a solid-phase method. X-Ray diffraction (XRD) analyses revealed that the as-prepared samples were heterostructures Na0.7MnO2 and Li4Mn5O12. Electron microscopy was used to study the lattice fingerprint area of 0.6Na0.7MnO2–0.4Li4Mn5O12 that showed (111) crystal surface of Li4Mn5O12 and (002) crystal surface of Na0.7MnO2. With different molar ratios of lithium and sodium, Li4Mn5O12 and Na0.7MnO2 in the formed heterostructure will change accordingly, during the synthesis process. The XPS data also showed that Mn3+/Mn4+ in the materials presented different ratios. The heterostructure, 0.6Na0.7MnO2–0.4Li4Mn5O12, exhibits superior high-rate capability and excellent cycle performance. The rate discharge capacity of 0.6Na0.7MnO2–0.4Li4Mn5O12 was maintained at 163.25 mA h g−1 at 500 mA g−1. When cycled at 100 mA g−1, the discharge capacity was maintained at 189.80 mA h g−1 after 50 cycles. The construction of heterostructure can take the advantage of the high capacity of the layered structure of Na0.7MnO2 and the stable spinel structure of Li4Mn5O12, which can form a synergistic effect and improve the electrochemical performance of cathode materials. [ABSTRACT FROM AUTHOR]

• Na 3 V 2 O 2 (PO 4) 2 F/C materials doped with Mg2+ were prepared successfully. • Mg2+ doping can boost cycling stability and rate performance of Na 3 V 2 O 2 (PO 4) 2 F/C. • The diffusion coefficients of Na+ in composites were enhanced by Mg2+ doping. Na 3 V 2 O 2 (PO 4) 2 F, a significant member of the polyanionic sodium superionic conductor family, exhibits high theoretical specific capacity and operating voltage. However, its application potential is hindered by its limited intrinsic conductivity. In this study, a series of Na 3 V 2–x Mg x O 2 (PO 4) 2 F/C cathode materials (x = 0, 0.05, 0.1, and 0.15) were synthesized using sol–gel and high-temperature solid-state methods. The impact of Mg2+ doping on the structural and electrochemical properties of Na 3 V 2 O 2 (PO 4) 2 F was examined. Na 3 V 1.9 Mg 0.1 O 2 (PO 4) 2 F/C demonstrated remarkable electrochemical performance, with a first cycle discharge specific capacity of 114.2 mAh/g and a capacity retention of 96.2 % after 500 cycles at 5C. The Na+ diffusion coefficient was 3.4 × 10–13 cm2 s−1. The full cell assembled with hard carbon as the counter electrode has a first cycle discharge specific capacity of 111.16 mAh/g at 0.1C and a capacity retention of 91.89 % after 50 cycles. This superior performance is attributed to the Mg2+ doping, which enhances the structural stability and accelerates sodium-ion diffusion. [ABSTRACT FROM AUTHOR]

Summary: Herein, a series of lithium‐sodium hybrid ion Na3−xLixV2(PO4)3/C (x = 2.5, 2.0, 1.5, 1.0, 0.5) as cathode materials are prepared by sol‐gel method. The lithium storage properties of Na3−xLixV2(PO4)3/C reveal that the lithium sodium ratio has a strong influence on material structure and the electrochemical properties. The multiple voltage platforms are observed at charge and discharge curve for the Na0.5Li2.5V2(PO4)3 material, as it contains rhombohedral and monoclinic Li3V2(PO4)3 phases, whereas a single voltage platform can be observed when x is less than 2, which are mixtures of Na3V2(PO4)3 with rhombohedral structure and Li3V2(PO4)3 with rhombohedral structure. Notably, NaLi2V2(PO4)3/C prepared by the calcination at 700°C for 8 hours contains a single rhombohedral phase. And when the charging and discharging voltage at the range 2.5 to 4.5 V and current density at 0.5C, NaLi2V2(PO4)3/C and Na2.5Li0.5V2(PO4)3/C are prepared by the pre‐calcination at 400°C for 4 hours and subsequent calcination at 750°C for 8 hours exhibit high first‐cycle specific discharge capacities of 124.8 and 130.0 mAh·g−1. The capacities are 118.4 and 115.1 mAh·g−1 after 50 cycles, indicating excellent capacity retention. [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]