RESUMEN
Layered V10O24·nH2O with a large interlayer spacing of 14 Å is hydrothermally synthesized and used as a cathode for potassium-ion batteries. It exhibits a capacity of 110 mA h g-1 with a capacity retention of 99.2% over 700 cycles. Its storage mechanism is identified as pseudo-capacitive intercalation.
RESUMEN
Among various cathode materials for sodium-ion batteries, Na3V2(PO4)3 has attracted much attention due to its outstanding electrochemical performance. However, the toxicity and expensive price of vanadium limit its practical application. Therefore, the substitution of vanadium with nontoxic and inexpensive transition metal elements is significant. We select the earth-abundant iron element to partially replace the vanadium element, and successfully synthesize Na3.36FeV(PO4)3 with a Na superionic conductor structure. Furthermore, a Na3.36FeV(PO4)3 cathode with an optimal carbon content can deliver an initial capacity of 97.6 mA h g-1 at 0.5C with a high capacity retention of 96.4% after 200 cycles. In addition, it also delivers an initial capacity of 90.4 mA h g-1 at 10C, and a capacity retention of 80% can be obtained after 5000 cycles. We also found that the lack of sodium in the material can be compensated by an electrochemical reaction. Furthermore, the in situ X-ray diffraction analysis reveals that the sodium storage process follows a pseudo-solid-solution reaction mechanism and the volume change ratio is less than 3% during charging/discharging. In order to study the practical application capability of Na3.36FeV(PO4)3, we assemble the pre-activated cathode and a hard carbon anode into a full cell, which exhibits high initial discharge capacities of 103 and 91.3 mA h g-1 at 0.5C and 10C, respectively. This work will provide new insights into the structural engineering of low-toxicity and ultralong-life NASICON-type cathode materials for SIBs.
RESUMEN
Potassium-ion batteries (PIBs) are promising for large-scale energy storage due to the abundant reserves of the element potassium yet few satisfactory cathode materials have been developed due to the limitation of the large ionic radius of the potassium ion. Cubic perovskite fluorides have three-dimensional diffusion channels and a robust structure, which are favorable for ion transfer, but their poor electronic conductivity needs to be compensated. Here, we synthesized cubic KFeF3 powder by a solvothermal procedure. After the combination with reduced graphene oxide (rGO) and carbon coating, its electronic conductivity is greatly improved. In the optimal sample KFeF3/rGO-PVA-500, KFeF3 nano-particles (smaller than 50 nm) distribute on the rGO surface evenly. Owing to the special structure, KFeF3/rGO-PVA-500 provides an excellent rate performance and cycling stability. In particular, a high capacity retention of 94% is obtained after 1000 cycles at 200 mA g-1. In addition, a hybrid reaction mechanism combining mainly solid solution and partly conversion processes is revealed by employing in situ and ex situ characterization.
RESUMEN
Iron-based mixed-polyanionic cathode Na4Fe3(PO4)2(P2O7) (NFPP) has advantages of environmental benignity, easy synthesis, high theoretical capacity, and remarkable stability. From NFPP, a novel Li-replaced material NaLi3Fe3(PO4)2(P2O7) (NLFPP) is synthesized through active Na-site structural engineering by an electrochemical ion exchange approach. The NLFPP cathode can show high reversible capacities of 103.2 and 90.3 mA h g-1 at 0.5 and 5C, respectively. It also displays an impressive discharge capacity of 81.5 mA h g-1 at an ultrahigh rate of 30C. Density functional theory (DFT) calculation demonstrates that the formation energy of NLFPP is the lowest among NLFPP, NFPP, and NaFe3(PO4)2(P2O7), indicating that NLFPP is the easiest to form and the conversion from NFPP to NLFPP is thermodynamically favorable. The Li substitution for Na in the NFPP lattice causes an increase in the unit cell parameter c and decreases in a, b, and V, which are revealed by both DFT calculations and in situ X-ray powder diffraction (XRD) analysis. With hard carbon (HC) as the anode, the NLFPP//HC full cell shows a high reversible capacity of 91.1 mA h g-1 at 2C and retains 82.4% after 200 cycles. The proposed active-site-specific structural tailoring via electrochemical ion exchange will give new insights into the design of high-performance cathodes for lithium-ion batteries.
RESUMEN
The mixed polyanionic material Na4Fe3(PO4)2(P2O7) combines the advantages of NaFePO4 and Na2FeP2O7 in capacity, stability, and cost. Herein, we synthesized carbon-coated hollow-sphere-structured Na4Fe3(PO4)2(P2O7) powders by a scalable spray drying route. The optimal sample can deliver a high discharge capacity of 107.7 mA h g-1 at 0.2C. It also delivers a capacity of 88 mA h g-1 at 10C and a capacity of retention of 92% after 1500 cycles. Ex situ X-ray diffraction analysis indicates a slight volume change (less than 3%) in the Na4Fe3(PO4)2(P2O7) lattice cell. Therefore, such a spraying-derived carbon-coated Na4Fe3(PO4)2(P2O7) powder is a very attractive cathode electrode for sodium-ion batteries.
