Anion-repulsive polyoxometalate@MOF-modified separators for dendrite-free and high-rate lithium batteries

Abstract

Commercial polyolefin separators in lithium batteries encounter issues of uncontrolled lithium-dendrite growth and safety incidents due to their low Li⁺ transference numbers ($t_{{\mathrm{Li}}^{+}}$) and low melting points. To address these challenges, this study proposes an innovative approach by upgrading conventional separators through the incorporation of metal-organic framework (MOF)-confined polyoxometalate (POM). The presence of POM restricts anion diffusion through electrostatic repulsion while facilitating Li⁺ transport within MOF nanochannels through their affinity for lithium ions. Moreover, MOF confinement effectively mitigates the acidification of electrolytes induced by POM. As a proof-of-concept, the polypropylene separators decorated with phosphotungstic acid@UIO66 (denoted as PW₁₂@UIO66-PP) exhibit remarkable lithium-ion conductivity of 0.78 mS cm⁻¹ with a high $t_{{\mathrm{Li}}^{+}}$ of 0.75 at room temperature. The modified separators also display excellent thermal stability, preventing significant shrinkage even at 150°C. Furthermore, Li symmetric cells employing PW₁₂@UIO66-PP separators exhibit stable cycling for 1000 h, benefiting from rapid Li-ion transport and uniform deposition. Additionally, the modified separator shows promising adaptability to industrial manufacturing of lithium-ion batteries, as evidenced by the assembly of a 4 Ah NCM811/graphite pouch cell that retains 97% capacity after 350 cycles at C/3, thus highlighting its potential for practical applications.