In state-of-the-art lithium ion batteries, separators (microporous membranes) play a passive yet critical role – hosting liquid electrolyte and maintaining physical separation of the electrodes. However, as the demands on lithium ion batteries increase, with an emphasis on greater energy density, longevity (cycle/calendar life), and safety, engineering separators to take a on more active role in the cell (electro)chemistry is expected to be an important strategy. Myriad membrane materials and separator designs have been developed to impart additional functionality, for example, acid and/or transition metal scavenging, temperature responsiveness, enhanced thermal stability, increased ion dissociation, combustion suppression, and mechanical strength.

In this talk, I will preset my group’s approach to additive manufacturing of next-generation lithium ion battery separators. Our approach is based on polymerization induced phase separation (PIPS), wherein polymerizable monomers (or prepolymer resins) are mixed with porogen. Through rapid, low-cost, readily scalable photopolymerization, the monomers are converted to a crosslinked polymer network, which results in the porogen becoming immiscible and phase separating through spinodal decomposition. By tuning the thermodynamics of the polymer-porogen mixture and photopolymerization kinetics, the porosity and pore size of the resulting polymeric phase can be tuned. We have shown that ethylene carbonate (EC) mixed with common acrylate monomers, such as 1,4-butanediol diacrylate, is an effective porogen. Most importantly because EC is an indispensable component in liquid electrolytes, it does not need to be extracted from the separator prior to incorporation into the electrochemical cell.

By controlling the ratio of the 1,4-butanediol diacrylate (BDDA) monomer to EC, monolithic microporous membranes are readily prepared with 25 µm thickness and pore sizes and porosities ranging from 6.8 to 22nm and 15.4% to 38.54%, respectively. The optimal poly(1,4-butanediol diacrylate) (pBDDA) separator has a porosity of 38.5% and average pore size of 22 nm; uptakes 127% liquid electrolyte by mass, and has an ionic conductivity of 1.98 mS/cm, which is higher than that of Celgard 2500. Lithium ion battery half cells consisting of LiNi_0.5Mn_0.3Co_0.2O₂ cathodes and pBDDA separators were shown to undergo reversible charge/discharge cycling with an average discharge capacity of 142 mAh/g and a capacity retention of 98.4% over 100 cycles - comparable to cells using state-of-the-art separators. Furthermore, the pBDDA separators were shown to be thermally stable to 400°C, lack low temperature thermal transitions that can compromise cell safety, and exhibits no thermal shrinkage up to 150°C. I will also discuss my group’s efforts to engineer separators with additional functionality to improve cell performance under abuse conditions.