Janus-faced, multifunctional separator membranes for high-performance lithium-ion batteries

Abstract

A vigorous expansion of newly emerging application fields, including smart mobile electronics, power tools, (hybrid) electric vehicles, and grid-scale energy storage systems inspires the relentless pursuit of advanced rechargeable power sources with reliable/sustainable electrochemical performance and safety tolerance. Among the numerous power sources, undoubtedly, lithium-ion batteries (LIBs) are listed in the top tier and still garner considerable attention as an appealing electrochemical system to address the challenging issues. A battery separator membrane is supposed to be electrically inert for preventing electrical contact (resulting in safety failures) between electrodes, in addition to its another key role as an ion-conducting route. Here, as a multifunctional membrane strategy to break up the stereotypical belief about battery separators, we demonstrate a new class of Janus-faced, dual (ion/electron)-conductive/chemically-active (i.e., heavy metal ion-chelating) battery separators based on elaborately-designed heterolayered nanomat architecture. The Janus-faced, heterolayered nanomat separators (referred to as “Janus separators”) are fabricated through the in-series, concurrent electrospraying/electrospinning process. The Janus separator is composed of an ion-conductive/metal ion-chelating support layer (= a mat of densely-packed, thiol-functionalized silica particles spatially besieged by polyvinylpyrrolidone (PVP)/polyacrylonitrile (PAN) nanofibers) and a dual-conductive top layer (= a thin mat of polyetherimide (PEI) nanofibers wrapped with multi-walled carbon nanotubes (MWNTs)). The support layer acts as a chemical trap that can capture heavy metal ions dissolved in liquid electrolytes and the thin top layer serves as an upper current collector of cathodes to facilitate redox reaction kinetics. Notably, the unusual porous structure (specifically, preferential deposition of the MWNTs along the PEI nanofibers) of the top layer is theoretically elucidated using molecular dynamics simulation. As a consequence, the Janus separator enables significant improvements in the fast-rate charge/discharge reaction of lithium-ion batteries and also high-temperature cycling performance, which lie far beyond those achievable with conventional polyethylene separators. The Janus separator featuring such exceptional multifunctionality is envisioned to open a new membrane solution for high-energy/high-performance lithium-ion batteries that will power forthcoming smart ubiquitous era.