Developing a Hybrid Solid Electrolyte (Interphase) and Unraveling Its Mechanism Towards Long-Life Sodium Metal and Sodium Solid-State Batteries

In response to escalating global energy demands and environmental concerns, the focus has shifted from lithium-metal batteries to sodium-metal batteries due to lithium's scarcity and rising costs. Sodium-metal batteries emerge as a promising alternative, providing a high theoretical specific capacity (∼1166 mAh/g) and a low anode potential (-2.714 V vs. standard hydrogen electrode), all while being more abundant and cost-effective. However, their widespread adoption is impeded by the unstable and fragile solid electrolyte interphase (SEI) formed from spontaneous reactions between metallic sodium and liquid electrolytes. This instability leads to dendrite formation, which can cause short-circuiting and degrade battery performance. Addressing these challenges is crucial for unlocking the full potential of sodium-metal batteries as a viable and sustainable energy storage solution. This study introduces innovative mechano-electrochemical protective strategies to enhance the stability of sodium metal anodes, offering significant advancements toward long-life sodium-metal batteries and sodium solid-state batteries.
Our study focused on the use of tin fluoride (SnF₂) and silicon nitride (Si₃N₄) nano particles, both proven to form durable artificial SEI layers on the surface of sodium metal. These protective layers—enriched with NaF and Na₃N compounds—significantly enhanced the cycling performance of the batteries. They effectively suppressed dendrite formation, improving the cycling stability of the sodium anodes by approximately 5.5 times (1100) hours compared to uncoated sodium anodes.
Building on these foundational insights, we developed a novel hybrid protective strategy by integrating polyethylene oxide (PEO) with SnF₂ as an additive. This combination was carefully selected to provide chemical stability, morphological adaptability, and mechanical flexibility for addressing the challenges encountered during sodiation and desodiation.
The synergy between PEO and SnF₂ maximized the strengths of both materials, yielding a composite artificial SEI with outstanding cycling performance. Under a 0.25 mA/cm² current density in a carbonate-based electrolyte (1M NaPF₆ in 1:1 EC/DMC), the hybrid system achieved 10 times better performance compared to bare sodium, maintaining stability for up to approximately 2000 hours. Even under a more challenging 0.5 mA/cm² current density, it continued cycling for 800 hours, significantly outperforming untreated sodium metal. Furthermore, at 0.5 mA/cm² in ether-based electrolyte (1M NaPF₆ in tetraglyme), the composite SEI extended cycling to an impressive 3000 hours. This hybrid system efficiently minimized electrolyte decomposition, prevented dendrite formation, and stabilized the plating and stripping processes, underscoring its potential to advance sodium metal battery technology.
To further leverage these advancements, an innovative polymer electrolyte directly on the sodium surface was developed by hybrid solution containing NaPF₆ onto the sodium anode. This unique approach enables the formation of a solid polymer electrolyte layer directly on the sodium surface, enhancing ion conduction and acting as both a solid polymer electrolyte and a protective SEI. This configuration offers a new pathway for polymer electrolytes in sodium batteries. We also extended this concept by developing a standalone polymer electrolyte system composed of PEO, SnF₂, and NaPF₆. This polymer electrolyte demonstrated promising performance, showing stable cycling behavior and improved ion transport.
The integration of these hybrid polymer-based solutions underscores a significant leap toward the development of reliable and high-performance sodium-metal batteries. These strategies not only stabilize the sodium anode but also enhance the longevity and safety of the batteries, presenting an important step in overcoming the challenges of dendrite growth, SEI instability, and electrolyte degradation.