Binary Solvent Induced Stable Interphase Layer for Ultra-Long Life Sodium Metal Batteries
Authors: Vaidyula Rinish Reddy , Nguyen Mai H , Weeks Jason Alexander , Wang Yixian , Wang Ziqing , Kawashima Kenta , Paul-Orecchio Austin G , Celio Hugo , Dolocan Andrei , Henkelman Graeme , Mullins Charles Buddie 
Published: December 2024

Lithium-ion batteries (LIBs) face concerns about production capacity and critical material shortages in the pursuit of reliable energy storage for intermittent renewable sources.1,2 As an alternative, sodium-ion batteries (SIBs) offer a comparable storage mechanism using abundant and cost-effective sodium as the energy host.3 With sodium's abundance surpassing lithium by ≈1000 times, SIBs present a compelling solution for future energy storage needs.1 Sodium metal boasts a high theoretical capacity of 1166 and 1131 mA h cm−3, coupled with a low redox potential of −2.71 V versus standard hydrogen electrode (SHE).4 These attributes pave the way for exploring the potential applications of metal anode-based batteries in energy density-intensive electric vehicles and more efficient stationary storage systems for renewable energy sources.5 However, addressing the challenges associated with parasitic reactions involving alkali metal anodes is essential. These reactions can lead to the formation of unstable solid electrolyte interphase (SEI) and dendrites, which not only diminish battery longevity but also pose safety hazards.6

The past decade has seen extensive research efforts aimed at achieving stable cycling of sodium metal batteries (SMBs), addressing challenges like parasitic reactions and dendrite formation.1,7,8 Various strategies, including the use of additives,4 artificial SEI formation,9 and directed sodium deposition,10 have been explored to enhance SMB lifespan. Notably, modifying the electrolyte composition, particularly the choice of solvent, stands out as a straightforward yet practical approach.8 Solvent selection significantly influences the physical and chemical properties of the electrolyte, impacting salt solvation and SEI layer formation. Recent studies have shown promising results with ether-based solvents such as diglyme (Dig) and tetrahydrofuran (THF), promoting stable SEI layer formation due to the presence of aggregates (AGGs) solvation structure.7,11 However, the sustainability of high salt concentrations remains a concern due to cost implications and limited ionic conductivity.8,12 Here, solvent engineering offers a solution by adjusting the mixture ratio of solvents to create localized high-concentration electrolytes, enhancing SMB stability.8,13 Furthermore, environmentally friendly solvents like 2-methyl tetrahydrofuran (MTHF) present a promising co-solvent option, offering weaker Na⁺ ion solvation characteristics and lower viscosity, potentially improving SMB performance, especially at lower salt concentrations.14,15 Hence, introducing MTHF as a co-solvent to Dig holds the potential to enhance SMB rate capability and stability, even at reduced salt concentrations.
In this study, a stable sodium metal battery (SMB) is achieved by tuning the electrolyte solvation structure through the addition of co-solvent 2-methyl tetrahydrofuran (MTHF) to diglyme (Dig). The introduction of cyclic ether-based MTHF results in increased anion incorporation in the solvation structure, even at lower salt concentrations. Specifically, the anion stabilization capabilities of the environmentally sustainable MTHF co-solvent lead to a contact-ion pair-based solvation structure. Time-of-flight mass spectroscopy analysis reveals that a shift toward an anion-dominated solvation structure promotes the formation of a thin and uniform SEI layer. Consequently, employing a NaPF6-based electrolyte with a Dig:MTHF ratio of 50% (v/v) binary solvent yields an average Coulombic efficiency of 99.72% for 300 cycles in Cu||Na cell cycling. Remarkably, at a C/2 cycling rate, Na||Na symmetric cell cycling demonstrates ultra-long-term stability exceeding 7000 h, and full cells with Na0.44MnO2 as a cathode retain 80% of their capacity after 500 cycles. This study systematically examines solvation structure, SEI layer composition, and electrochemical cycling, emphasizing the significance of MTHF-based binary solvent mixtures for high-performance SMBs.

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