RESUMO
Building 3D electron-conducting scaffolds has been proven to be an effective way to alleviate severe dendritic growth and infinite volume change of sodium (Na) metal anodes. However, the electroplated Na metal cannot completely fill these scaffolds, especially at high current densities. Herein, we revealed that the uniform Na plating on 3D scaffolds is strongly related with the surface Na+ conductivity. As a proof of concept, we synthesized NiF2 hollow nanobowls grown on nickel foam (NiF2@NF) to realize homogeneous Na plating on the 3D scaffold. The NiF2 can be electrochemically converted to a NaF-enriched SEI layer, which significantly reduces the diffusion barrier for Na+ ions. The NaF-enriched SEI layer generated along the Ni backbones creates 3D interconnected ion-conducting pathways and allows for the rapid Na+ transfer throughout the entire 3D scaffold to enable densely filled and dendrite-free Na metal anodes. As a result, symmetric cells composed of identical Na/NiF2@NF electrodes show durable cycle life with an exceedingly stable voltage profile and small hysteresis, particularly at a high current density of 10 mA cm-2 or a large areal capacity of 10 mAh cm-2. Moreover, the full cell assembled with a Na3V2(PO4)3 cathode exhibits a superior capacity retention of 97.8% at a high current of 5C after 300 cycles.
RESUMO
Iron carbodiimide (FeNCN) belongs to a type of metal compounds with a more covalent bonding structure compared to common transition metal oxides. It could provide possibilities for various structural designs with improved charge-transfer kinetics in battery systems. Moreover, these possibilities are still highly expected for promoting enhancement in rate performance of sodium (Na)-ion battery. Herein, oriented FeNCN crystallites were grown on the carbon-based substrate with exposed {010} faces along the [001] direction (O-FeNCN/S). It provides a high Na-ion storage capacity with excellent rate capability (680 mAh g-1 at 0.2 A g-1 and 360 mAh g-1 at 20 A g-1), presenting rapid charge-transfer kinetics with high contribution of pseudocapacitance during a typical conversion reaction. This high rate performance is attributed to the oriented morphology of FeNCN crystallites. Its orientation along [001] maintains preferred Na-ion diffusion along the two directions in the entire morphology of O-FeNCN/S, supporting fast Na-ion storage kinetics during the charge/discharge process. This study could provide ideas toward the understanding of the rational structural design of metal carbodiimides for attaining high electrochemical performance in future.
RESUMO
The V4+/V3+ (3.4 V) redox couple has been well-documented in cathode material Na3V2(PO4)3 for sodium-ion batteries. Recently, partial cation substitution at the vanadium site of Na3V2(PO4)3 has been actively explored to access the V5+/V4+ redox couple to achieve high energy density. However, the V5+/V4+ redox couple in partially substituted Na3V2(PO4)3 has a voltage far below its theoretical voltage in Na3V2(PO4)3, and the access of the V5+/V4+ redox reaction is very limited. In this work, we compare the extraction/insertion behavior of sodium ions from/into two isostructural compounds of Na3VGa(PO4)3 and Na3VAl(PO4)3, found that, by DFT calculations, the lower potential of the V5+/V4+ redox couple in Na3VM(PO4)3 (M = Ga or Al) than that in Na3V2(PO4)3 is because of the extraction/insertion of sodium ions through the V5+/V4+ redox reaction at different crystallographic sites, that is, sodium ions extracting from the Na(2) site in Na3VM(PO4)3 while from the Na(1) site in Na3V2(PO4)3, and further evidenced that the full access of the V5+/V4+ redox reaction is restrained by the excessive diffusion activation energy in Na3VM(PO4)3.
RESUMO
On the basis of resource abundance and low cost, high capacity layered P2-type Na2/3Fe1/2Mn1/2O2 material is regarded as a potential cathode material for sodium-ion batteries but suffers from its unstable structure during cycling. In this work, P2-type Na2/3Fe1/2Mn1/2O2 layered materials were synthesized by a chelating agent assisted sol-gel method with NH3·H2O. With the addition of NH3·H2O and the control of the synthesis conditions, highly active material with a more stable structure and better electrochemical performance was obtained. Furthermore, the influences of structure changes during different voltage ranges (1.5-4.0 V and 1.5-4.3 V vs Na(+)/Na) on the Na(+) storage behaviors were also evaluated and compared. It is confirmed that, when being charged to 4.2 V, an OP4-type phase emerges, which can reduce the damage by the gilding of the MeO2 layers but leads to an unstable crystal structure. For long-term cycling, it is preferred to cut off at 4.0 V rather than at 4.3 V. For the optimized P2-type Na2/3Fe1/2Mn1/2O2 calcined at 900 °C, a discharge capacity of 92 mAh/g remains after 40 cycles in the voltage range of 1.5-4.0 V, and the Coulombic efficiency remains 100%.