Multifunctional Separator for Li-Ion Batteries Enabling Acid Scavenging and Long Cyclability

Abstract

The membrane separator is indispensable in lithium-ion batteries (LIBs), electrically isolating the electrodes while hosting liquid electrolyte for facile ion transport. Microporous polyolefin materials, consisting of polyethylene (PE) and/or polypropylene (PP), represent the state-of-the art in separator technology. They offer sufficient mechanical properties, chemical inertness, effective pore sizes <100 nm, porosity ranging from 30-78%, and relatively low costs. However, safety issues associated with thermal shrinkage above 70℃ and poor electrolyte wettability are significant limitations. Replacing these conventional polyolefin separators with multifunctional microporous membranes provides opportunities to address these limitations and other challenges in lithium ion batteries. LiMn2O4 cathodes, for example, attract widespread interest due to the abundance and low cost of Mn, especially in comparison to Co, and reasonable energy densities. However, these cathodes do not provide the calendar and cycle life of the layered oxide cathodes, especially at elevated temperatures (>50°C), due to dissolution of Mn and structural instabilities.1 One of the underlying reasons is the presence of HF in commercial LiPF6-carbonate electrolytes; HF will attack and dissolve the Mn, leading to severe capacity decay, as shown in Figure 1.2 Chemically active multifunctional separator can scavenge HF from the electrolyte, improving the stability of LiMn2O4, but they must be fabricated by low cost, scalable processes without compromising other separator properties.Herein, a multifunctional separator will be described, wherein poly(4-vinylpyridine) (P4VP) is incorporated into separators prepared by polymerization induced phase separation (PIPS). In PIPS, a homogenous solution of difunctional monomers and porogen is polymerized; the difunctional monomers polymerize and crosslink while the porogen phase separates into nanoscale domains. Our previous study employed PIPS to prepare highly porous and thermally stable separators, exhibiting comparable ion transport to conventional Celgard 2500 Polypropylene (PP) separators and negligible thermal shrinkage at 150 °C (digital images in Figure 1).3 Moreover, when incorporated into LIB half cells, these separators enable better electrochemical performance than Celgard 2500 separators due to their reduced thicknesses and superior electrolyte uptake. In the study, P4VP is integrated with the PIPS process to scavenge both HF and PF5 (precursor to HF) through acid-base interactions. A thin layer of P4VP coating was fabricated via initiated chemical vapor deposition (iCVD) process and makes full use of high porosity of separators due to the conformal nature of the coating, increasing the surface area to scavenge the acids. The P4VP-coated PIPS separator enables better capacity retention in Mn-based cathode materials, relative to cells using inert polyolefin separators. The scavenged concentration of HF and transition metal will be compared to validate the HF scavenging property. Post-mortem analysis of the Li anode also confirms the suppression of Mn dissolution and deposition on the anode at elevated temperatures (50°C).