Molecularly Engineered Nanoporous Membranes for Safety Enhanced Battery Separators

Abstract

A battery separator physically and electronically separates the anode and cathode but permits Li+ transport between them. Battery separators play a critical role in battery safety by providing, e.g., thermal shutdown functionality in abusive events, or, by preventing lithium metal dendrites or other defects in the cell from penetrating the membrane which can cause internal short circuit leading to uncontrolled thermal runaway, as in the case of the Boeing 787 battery fire incident [1]. One of the challenges with designing safe battery separators is the trade-off between mechanical robustness and porosity/transport properties. For example, high separator tortuosity is good for dendrite resistance but can lead to higher separator resistance [2]. Separator design is further complicated by additional constrains including tolerance to abuse conditions, stable at high voltages (e.g., > 4V), chemically inert to other cell materials, and low cost to meet the performance and cost targets. There is a need to tailor design battery separators that enable high energy/power density with engineered functionality for safety, without adversely affecting performance. Here we present our efforts to develop a nanoporous battery separator that is highly tunable in physical/mechanical properties (e.g., porosity, elastic modulus, toughness) and safety characteristics (e.g., thermal shutdown temperature). Our nanoporous separators are derived from functionalized block copolymers (polyolefins) with low cost precursors [3, 4, 5]. We will also present a hybrid separator concept where the rapid thermal shutdown-capable nanoporous separator developed is hybridized with a highly porous, mechanically and thermally robust separator. This approach renders a hybrid separator technology that affords high energy density, high rate capability, long cycle life and unprecedented safety characteristics for the next generation Li-ion batteries. Figure 1 shows scanning electron microscopy (SEM) images of a representative, synthesized nanoporous polyethylene (NPE) membrane samples, ~ 25 mm in thickness, revealing the desired bicontinuous cubic morphology and percolating pore architecture. Figure 2 shows separator impedance as a function of temperature, for NEP1-3 membrane samples and a commercial separator. An environment (EV) chamber was programmed to increase the temperature at a very low rate of 0.5°C/min for accurate temperature shutdown measurements. The onset of the impedance rise corresponded to ~ 120°C for the NPE separators and ~ 130°C for the commercial separator. The increase of the separator impedance is a result of the separator thermal shutdown, i.e., micropore closing due to the melting/shrinking of the separator, which impedes the ionic motion. It is interesting to note that the separator impedance increased by one to two orders of magnitude at the peak of the thermal shutdown comparing with the initial impedance value. The drastic separator impedance increase associated with the separator thermal shutdown is responsible for providing the safety feature to the otherwise normal operation of Li-ion batteries. The enhanced thermal shutdown features seen in the NPE separators will render earlier, much more rapid and effective thermal shutdown at abusive events (e.g., short circuit, overcharge, etc.), which leads to much improved safety charasteristics for batteries. Our study shows that the molecularly engineered nanoporous membranes are very promising to allow exquisite tuning of mechanical properties (e.g., porosity, elastic modulus, toughness) and safety features (e.g., thermal shutdown temperature). The NPE-based hybrid separator approach will afford an unprecedented performance and safety characteristics for the next generation batteries. Figure 1