Sodium cluster-driven safety concerns of sodium-ion batteries†

Abstract

Sodium-ion batteries (SIBs) present a resource-sustainable and cost-efficient paradigm poised to overcome the limitation of relying solely on lithium-ion technologies for emerging large-scale energy storage. Yet, the path of SIBs to full commercialization is hindered by unresolved uncertainties regarding thermal safety and lingering debates over the origin of thermal runaway. Herein, through multiscale equivalent analysis from Ah-grade cells to microstructures of battery components, we probe that the difference in the chemical environment for cation storage in anodes is the mechanistic origin underlying the inferior thermal safety of SIBs compared to lithium-ion batteries (LIBs). Bearing a quasi-metallic nature, sodium clusters that form in hard carbon (HC) anodes during routine sodiation predominantly initiate cell self-exothermic reactions, significantly earlier than the decomposition of the solid–electrolyte interphase (SEI) typically observed in LIBs. Solid-state NMR measurements elucidate that clustered sodium in HC exhibits electronic properties more akin to metallic states than lithium in graphite, with even higher electron state densities at the Fermi level than bulk sodium. This heightened reactivity triggers the decomposition of linear carbonates, ultimately culminating in a thermal runaway event almost on par with scenarios involving sodium plating. Our work challenges the prevailing brief that the thermal safety insights between LIBs and SIBs are interchangeable and highlights the necessity of stabilizing deeply sodiated HC for practically safe sodium-based battery chemistries.