Advanced Hybrid Polymer/Ceramic PEO-SnF2 Protective Mechanism for Sodium Metal Anodes
Authors: Damircheli Roya , Hoang Binh , Castagna Ferrari Victoria , Lin Chuan-Fu 
Published: August 2024

In the face of mounting global energy requirements and pressing environmental issues, the scientific community turned to lithium-metal batteries, celebrated for their superior capacity and efficiency. However, challenges arising from the scarcity and increasing costs of lithium have redirected research efforts towards sodium batteries. Sodium metal anodes, with their remarkable theoretical specific capacity of ~1166 mAh/g, low anode potential of -2.714 V vs standard hydrogen electrode, and cost benefits, are increasingly recognized as promising candidates for conventional energy storage systems. Notwithstanding their advantages, the unstable and fragile solid electrolyte interphase (SEI) due to the spontaneous reaction between metallic Na and a liquid electrolyte remains a significant limitation. As this SEI layer expands, it hinders ion transport, causing increased polarization and diminishing the battery's energy efficiency. Specifically, during the plating process, Na ions are deposited beneath the SEI layer, causing significant expansion. This expansion fractures the fragile SEI, allowing dendrites to grow through these defects. As these dendrites grow, they can pierce the separator, posing a risk of short-circuiting the battery. While numerous studies have proposed designs for protecting sodium metal interphase, to overcome these challenges, this study amalgamates mechano-electrochemical protective strategies to enhance sodium metal anode stability, offering groundbreaking solutions to these challenges.

Central to our exploration were tin fluoride (SnF2) and silicon nitride (Si3N4), both established as effective in generating robust artificial SEI layers on the sodium metal surface. These layers, rich in NaF and NaxN respectively, demonstrated a notable improvement in cycling performance, effectively suppressing dendrite growth and enhancing the battery's cycling stability by nearly 3.5 times compared to bare sodium anodes.

Building upon these foundational insights, a novel approach was conceptualized: a hybrid protective mechanism combining polyethylene oxide (PEO) with the presence of SnF2 as an additive, is particularly efficacious. This combination was specifically chosen to provide chemical stability, morphological conformality, and mechanical flexibility to accommodate the mechano-electrochemical instability upon sodiation/desodiation processes. Through this novel strategy, we aimed to amalgamate the strengths of both components, ensuring optimal outcomes. This composite artificial SEI under a 0.25 mA/cm2 current density, delivers a cycling performance 10 times superior to bare sodium metal and survived up to 1800 hours of cycling. Impressively, at a high current density of 0.5 mA/cm2, the hybrid system still performed much better than its untreated counterpart and continued cycling for up to 800 hours of cycling. The synergy of PEO and SnF2 remarkably minimized electrolyte decomposition, inhibited dendrite formation, and stabilized Na plating/stripping processes, all of which consolidated its promise for sodium metal battery technology.