Safety Enhanced, Block Copolymer Derived Nanoporous Battery Separators
Authors: Xing Weibing 
Published: February 2020

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 and power with engineered functionality for safety, without adversely affecting cost.

Here we present ADA efforts, in collaboration with university researchers, to develop a nanoporous battery separator that is highly tunable in safety characteristics (e.g., thermal shutdown temperature) and is derived from functionalized block copolymers (polyolefins) with low cost precursors [3, 4, 5]. In particular, we will 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 the synthesized nanoporous polyethylene (NPE) membranes, an average 25 mm thickness, revealing the desired bicontinuous cubic morphology and percolating pore architecture. Our membranes possess a narrower pore size distribution and a high porosity, as intended based on our chemistry design selection, compared with the commercial separator.

Figure 2 shows separator impedance as a function of temperature, for NEP Gen1, NPE Gen2, and a commercial separator. The onset of the impedance rise corresponded to 80°C, 115°C and 130°C for NPE Gen2, NPE Gen1, and the commercial separator, respectively. 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. 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 Li-ion batteries.

Figure 3 shows charge/discharge voltage vs. specific capacity for a representative graphite/LiCoO2 (LCO) full cell using a NPE-based hybrid separator. The full cell displayed a good discharge specific capacity of ~ 120 mAh/g (LCO active mass).

Figure 4 shows a rate capability test. The NPE-based hybrid separator demonstrated superior rate capability than the commercial separator at C rates ≥ 5C. At 20C rate, the NPE hybrid separator based Li-ion cells demonstrated an impressive rate capability of > 73% capacity retention, 2.5 times that of the commercial counterpart (29%).

Figure 5 shows discharge capacity retention vs. cycle number of the Li-ion full cells using NPE-based hybrid separator and a commercial separator (control). The NPE hybrid separator based Li-ion full cells showed excellent cycle life performance with capacity retention of 88% after 300 cycles. It is noticed that the NPE-based hybrid separator demonstrated very compatible cycle life performance as the commercial separator. This result illustrates an excellent chemical and electrochemical compatibility of the NPE-based hybrid separator.

Our study shows that the strategy of preparing block copolymer derived nanoporous separators provides a very promising and powerful tool to allow exquisite tuning of performance and safety features (e.g., thermal shutdown temperature) of the nanostructured separator. The NPE-based hybrid separator technology will afford an unprecedented performance and safety characteristics for the next generation Li-ion batteries.

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Figure 1