A “Two-In-One” Strategy in Sodium Metal Batteries: A Cost-Effective Co/Mo Metal–Organic Framework-Modified Filter Paper Separator with Ion-Sieving and Desolvation Effects in Confined Channels

In this work, a Co/Mo metal–organic framework (MOF)-modified filter paper (Co–Mo-MOF@FP) was designated as the separator of sodium metal batteries (SMBs). The Co–Mo-MOF, formulated as [Co(4,4′-bpy)0.5MoO4] (4,4′-bpy = 4,4′-bipyridine), features a three-dimensional (3D) porous architecture with two-dimensional (2D) inorganic bimetallic Co–O–Mo layers bridged by the 4,4’-bpy pillars, leading to a large interlayer spacing of 13.1 Å with a confined pore window of ca. 3.3 Å × 8.1 Å, which provides channels only for the Na+ transport. Therefore, the electrolyte-impregnated Co–Mo-MOF@FP behaves like a single-ion conductor with an ion transference number of 0.76 and a diffusion barrier of 0.27 eV. Meanwhile, the Mo centers with multiple valence states and the rich Co–O–Mo linkages can tolerate electron attack and display excellent electrochemical durability. As a result, the Na |Co–Mo-MOF@FP| Na symmetric cell exhibits a considerable cycling life for over 500 h at 0.5 mA cm–2, and the Na |Co–Mo-MOF@FP| Na3V2(PO4)3 full cells present an initial capacity of ∼100 mAh g–1 with a capacity retention of 81.8% over 1000 cycles at 0.1 A g–1, which are much superior to those with the blank filter paper (FP) separator. This work proposes a strategy of “two-in-one”, in which the confined channel of the Co–Mo-MOF coating on a cheap FP separator and the resultant ion-sieving effect can accelerate uniform Na plating, suppress Na dendritic growth, induce the formation of a stable solid electrolyte interphase (SEI), and promote the desolvation of Na+, thus boosting diffusion kinetics.