Sodium metal anodes with multiphasic interphases for room temperature sodium–sulfur pouch cells†

Abstract

The SEI is pivotal for the reversibility of RT-Na/S batteries. The performance of RT-Na/S batteries critically depends on the solid electrolyte interphase's (SEI) mechanical and ionic transport properties. An ideal SEI is sufficiently stiff yet ductile and supports ionic diffusion to minimize the dendrite formation. In the present study, we report the design of a multiphasic SEI comprising NaF, NaI, and NaNH₂ to suppress the dendrite growth. An optimized amount of the novel organic additive, i.e., CH₃NH₃I, helps facilitate the formation of the desired SEI. Density Functional Theory (DFT) calculations reveal that the chemical composition effectively balances the electro-mechanical attributes of the SEI, i.e., a high stiffness (≈66 GPa) while maintaining a desired critical strain (≈40%). Besides this, the major chemical constituents of the SEI facilitate fast ion kinetics due to the low energy barriers associated with NaNH₂ and NaI (≈0.26 eV and ≈0.5 eV, respectively). This multiphasic SEI enables reversible sodium plating and stripping for an unprecedented time of over 3200 hours. The uniqueness of the multiphasic SEI becomes apparent when demonstrated in the coin and pouch format of the RT-Na/S battery, maintaining a stable operation for over 500 cycles with an initial discharge capacity of ∼700 mA h g⁻¹. These findings underscore the potential of the multicomponent SEI to enhance the stability and reversibility of the RT-Na/S batteries.