Building Fast Ion-Conducting Pathways on 3D Metallic Scaffolds for High-Performance Sodium Metal Anodes

Abstract

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.

3D mixed ion/electron-conducting scaffolds for stable sodium metal anodes

“…In a recent development, Li et al 34 addressed the intrinsic sodiophobic characteristics and poor ionic conductivity of commercial nickel foam (NF) through a chemical fluorination modification strategy (Fig. 10a).…”

“…Currently, researchers are engaged in exploring three main strategies to solve the aforementioned issues: (I) the optimization of electrolyte composition and additives; 23–25 (II) the development of artificial SEIs; 26–30 (III) the utilization of three-dimensional (3D) conductive scaffolds for fabricating composite Na–scaffold anodes (CSSAs). 31–36 However, the first two strategies can only sustain the prolonged cycling life of Na metal anodes at low current densities and low areal capacities, but are unable to withstand the extreme volume change and mechanical deformation induced by the Na metal during long-term cycling at high current densities required for commercial applications, which leads to the repeated rupture of the SEI film and uncontrolled dendrite growth, posing a severe threat to the reversibility and safety of Na metal anodes. 37 In contrast, the third strategy exhibits significant improvements in enhancing the cycling stability of Na anodes at high current densities.…”

“…Simultaneously, the excessive accumulation of the Na metal can block the ion transport pathways inside the scaffold, further preventing ions from being transferred and deposited within the scaffold, which causes the interior of the scaffold to completely lose its buffering effect on the volume expansion of the Na metal. 34,44,45 Therefore, the construction of fast ion transport pathways on the EC scaffold to enhance and balance the rapid transfer of both ions and electrons has emerged as a potential choice for realizing high-performance Na metal anodes (Fig. 1d).…”

Spatially Confined in Situ Formed Sodiophilic-Conductive Network for High-Performance Sodium Metal Batteries

“…The huge volume expansion results in rupture of the SEI film, leading to disrupted Na + flux in the vicinity and consequently causing uneven Na deposition and dendritic Na growth. In contrast, 3D EC scaffolds with abundant pore structure have sufficient internal space and can serve as a host to buffer the volume deformation of deposited Na during cycling. ,, Regrettably, the intrinsic “sodiophobicity” of the scaffolds results in poor Na affinity, typically manifesting a high Na nucleation barrier, which adversely affects uniform Na nucleation and deposition. − Currently, the incorporation of Na-alloying metals like Sn, Sb, and Zn into the scaffolds has been reported to significantly enhance their affinity for Na (“sodiophilicity”). ,,− Nevertheless, the composite Na/scaffold anode still demonstrates an unsatisfactory cycle life owing to the blocked ion diffusion pathways within the EC scaffolds. , Sluggish ion diffusion kinetics may lead to the preferential Na + deposition on the top surface of the scaffolds rather than being densely filled, which undermines the structural benefits of the 3D scaffolds in regulating Na nucleation and spatial confinement, a situation exacerbated at high current densities. Therefore, constructing an ideal 3D scaffold with both Na affinity and robust ion transport capabilities is crucial for achieving a highly reversible dendrite-free Na metal anode.…”

“…DFT calculations were further performed to analyze the Na + diffusion barrier through the Na 2 S component in the artificial SEI layer and NaF/Na 2 O components in the original SEI layer. , Figure h illustrates the Na + diffusion paths and associated energies in Na 2 S (220), Na 2 S (111), NaF (200), and Na 2 O (220). NaF, known for its high ionic conductivity, is generally acknowledged to effectively promote Na + diffusion, − as reflected in DFT calculations revealing an extremely low diffusion barrier of 0.133 eV.…”