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 and industrial partners, to develop a highly tunable, nanoporous battery separator derived from functionalized block copolymers (polyolefins) with low cost precursors.

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 compares the electrolyte wicking property of a commercial and a nanoporous separator. The electrolyte is a Li-ion battery electrolyte consisting of 1M LiPF₆ in a carbonate based solvent mixture. The nanoporous separator clearly shows much better wicking (absorbing) of the electrolyte, compared with the commercial counterpart. The electrolyte contact angles were estimated to be 31° for NPE and 69°for the commercial samples, respectively, consistent with the electrolyte wettability observation.

Figure 3 shows charge/discharge voltage vs. specific capacity during formation for a commercial and NPE separator based Li/LiCoO₂ (LCO) half cells, displaying characteristic LCO voltage profiles vs. Li/Li⁰. The NPE separator based cells delivered an equivalent (or slightly higher) specific capacity, compared to the control cells. This result is very encouraging and represents the first demonstration of this class of block polymer derived, high-density linear polyethylene based nanoporous separators in a real lithium battery chemistry.

Figure 4 shows discharge capacity vs. cycle number for the commercial and NPE separator based graphite/LCO full cells. These cells were cycled at £ 1C rates. Excellent cycle life stability was observed for the NPE separator based full cells, overlapping closely with the commercial separator based full cells.

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 of the nanostructured separator for the next generation Li-ion batteries.

References

1. National Transportation Safety Board Incident Report, NTSB/AIR-14/01, PB2014-108867, Jan 7, 2013.

2. Pankaj Arora and Zhengming (John) Zhang, Chem. Rev. 104, 4419-4462, (2004).