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
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.
RESUMEN
Sodium-ion batteries (SIBs) are the appropriate alternatives to lithium-ion batteries (LIBs) for the large-scale energy storage applications because of the abundant resources and wide distribution of sodium on earth. O3-NaCrO2 is a promising cathode material for SIBs due to its stable structure and low-cost raw materials. In this paper, we design and synthesize a powder consisting of submicrometer-sized O3-NaCrO2 spheres (s-NaCrO2) self-assembled with nanoflakes, which exhibits faster ion migration ability and strong structure robustness. The galvanostatic intermittent titration technique test reveals the higher apparent Na+ diffusion coefficient of s-NaCrO2 when compared with a normal NaCrO2 powder with an irregular particle morphology. The s-NaCrO2 shows impressive electrochemical properties with a capacity of 90 mAh g-1 at 50 C. In addition, outstanding cycling stability is shown when tested at 20 C, where a capacity of 90 mAh g-1 is maintained with a retention of 87% after 1500 cycles. Also, s-NaCrO2 is advantageous at high (50 °C) and low (-10 °C) temperatures. The full cells assembled employing Sb/C as the anode exhibit good rate capability with 85 mAh g-1 obtained at 50 C.
