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Na 2 Fe 0. 6Mn 0.4 PO 4 F/C composite materials are synthesized with various carbon sources via a simple spray-drying method in this study, and the effect of carbon sources on structure, morphology, and electrochemical properties of Na 2 Fe 0.6 Mn 0.4 PO 4 F/C materials are investigated in detail. XRD and SEM results indicate that the reduction ability of carbon sources has a key impact on the structure and morphology of Na 2 Fe 0.6 Mn 0.4 PO 4 F/C composite materials. Among these Na 2 Fe 0.6 Mn 0.4 PO 4 F/C materials, the sample prepared with ascorbic acid presents a uniform hollow spherical architecture. Electrochemical analysis demonstrates that the Na 2 Fe 0.6 Mn 0.4 PO 4 F/C sample prepared with ascorbic acid has optimal electrochemical performance. The sample shows high discharge capacities of 95.1 and 48.1 mAh g -1 at 0.05C and 1C rates, respectively, and it exhibits an improved cycle stability (91.7% retention after 100 cycles at 0.5C), which are superior to Na 2 Fe 0.6 Mn 0.4 PO 4 F/C materials prepared with other carbon sources. This study demonstrates that the reduction ability of carbon sources significantly influences the electrochemical properties of fluorophosphate/C composite materials. This work also provides a promising strategy to obtain high performance cathode materials for sodium-ion batteries., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Tang, Zhang, Sui, Wang, Li and Wu.)

Cathode materials made of x LiVPO 4 F· y Li 3 V 2 (PO 4 ) 3 /C ( x:y = 1:0, 2:1, 0:1) are synthesized via a feasible sol-gel method for high-performance lithium-ion batteries. The structures, morphology, and electrochemical properties of the composites are thoroughly investigated. The results show that LiVPO 4 F/C, Li 3 V 2 (PO 4 ) 3 /C, and 2LiVPO 4 F·Li 3 V 2 (PO 4 ) 3 /C can be synthesized under 750°C without the formation of impurities. Meanwhile, the unique morphology of the 2LiVPO 4 F·Li 3 V 2 (PO 4 ) 3 /C composite, which is porous, with nanoflakes adhering to the surface, is revealed. This composite integrates the advantages of LiVPO 4 F and Li 3 V 2 (PO 4 ) 3 . There are four discharge plateaus near 4.2, 4.1, 3.7, and 3.6 V, and the cathode material delivers high capacities of 143.4, 141.6, 133.2, 124.1, and 117.6 mAh g -1 at rates of 0.1, 0.2, 0.5, 1, and 2 C, respectively. More importantly, the discharge capacity can be almost fully recovered when the discharge rate returns to 0.1 C. The study is highly promising for the development of cathode material for LIBs., (Copyright © 2020 Zhong, Zhang, Liu and Sui.)

In this paper, we report a facile one-step hydrothermal method to synthesize tetragonal Na 3 V 2 (PO 4 ) 2 F 3 @C particles which are connected by carbon nanotubes (CNTs) networks, using water as hydrothermal solvents. In this strategy, the reduction and crystallization of materials are carried out in the hydrothermal process (180°C, 12 h), no additional heat treatment is required. The well-crystallized Na 3 V 2 (PO 4 ) 2 F 3 tetragonal grains (5-10 μm) are coated with amorphous nano-carbon and connected by highly conductive CNTs. The addition of CNTs can not only improve the conductivity of materials but also effectively inhibit the Na 3 V 2 (PO 4 ) 2 F 3 grains over growth. The Na 3 V 2 (PO 4 ) 2 F 3 @C/CNTs composite possesses very flat charge/discharge platforms of 3.6 and 4.1 V. The sample exhibits an initial discharge specific capacity of 120.2 and 74.3 mAh g -1 at 0.1 and 10 C rate, respectively, and shows excellent cyclical stability. The composite owns excellent electrochemical performances owing to the three-dimensional highly conductive network which is co-constructed by the CNTs and nano-carbon coating layer., (Copyright © 2019 Guo, Hu, Zhang, Zhang, Hou, Sui and Wu.)

LiFeBO 3 /C cathode material with hollow sphere architecture is successfully synthesized by a spray-drying method. SEM and TEM results demonstrate that the micro-sized LiFeBO 3 /C hollow spheres consist of LiFeBO 3 @C particles and the average size of LiFeBO 3 @C particles is around 50-100 nm. The thickness of the amorphous carbon layer which is coated on the surface of LiFeBO 3 nanoparticles is about 2.5 nm. LiFeBO 3 @C particles are connected by carbon layers and formed conductive network in the LiFeBO 3 /C hollow spheres, leading to improved electrical conductivity. Meanwhile, the hollow structure boosts the Li + diffusion and the carbon layers of LiFeBO 3 @C particles protect LiFeBO 3 from moisture corrosion. Consequently, synthesized LiFeBO 3 /C sample exhibits good electrochemical properties and storage performance.

Na 3 V 2 (PO 4 ) 3 /C nanofibers are prepared by a pre-reduction assisted electrospinning method. In order to maintain the perfect fibrous architecture of the Na 3 V 2 (PO 4 ) 3 /C samples after calcining, a series of heat treatment parameters are studied in detail. It is found that the heat treatment process shows important influence on the morphology and electrochemical performance of Na 3 V 2 (PO 4 ) 3 /C composite nanofibers. Under the calcining conditions of 800°C for 10 h with a heating rate of 2.5°C min -1 , the well-crystallized uniform Na 3 V 2 (PO 4 ) 3 /C nanofibers with excellent electrochemical performances are successfully obtained. The initial discharge specific capacities of the nanofibers at 0.05, 1, and 10C are 114.0, 106.0, and 77.9 mAh g -1 , respectively. The capacity retention still remains 97.0% after 100 cycles at 0.05C. This smooth, uniform, and continuous Na 3 V 2 (PO 4 ) 3 /C composite nanofibers prepared by simple electrospinning method, is expected to be a superior cathode material for sodium-ion batteries.