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.
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