(714i) Interfacial Engineering of Lithium Batteries with Conformal Polymer Thin Films

Coupled chemical and mechanical phenomena at electrode interfaces dictate the long term stability of lithium ion and lithium metal batteries. In lithium ion batteries, electrically insulating, ionically conductive solid electrolyte interphases (SEI) that form on graphite anodes passivate against electrochemical reduction of the electrolyte and other side reactions. Unfortunately, this passivation does not occur in next-generation Si and Li metal anodes. High voltage cathodes are also prone to detrimental interfacial processes, resulting in electrolyte oxidation, gas formation, and transition metal dissolution. In response, a myriad of coating technologies has been developed to mitigate these interfacial processes. The coatings should conformally and completely coat the electrochemically active surfaces and accommodate volume dilation associated with lithium insertion/extraction.

This presentation will describe the development of conformal, compliant polymer thin film coatings for lithium battery electrodes. The polymer thin films are prepared by initiated chemical vapor deposition (iCVD), which provides exquisite control over film composition and morphology by facilitating heterogeneous free-radical polymerization of vinyl monomers with thickness precision on the order of 1 nm. Poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (pV4D4) was deposited onto silicon thin film electrodes using iCVD. 25 nm-thick pV4D4 films on Si electrodes improved initial coulombic efficiency by 12.9% and capacity retention over 100 cycles by 64.9% relative to untreated electrodes. In another example, thin conformal coatings of perfluorinated polymer were deposited onto battery-grade Cu current collectors as artificial solid electrolyte interphases. Galvanostatic cycling of Li in an asymmetric Li||Cu cell showed roughly a 1% improvement in coulombic efficiency over bare, untreated Cu. The polymer coating also resulted in 24% denser electrodeposited Li (lower porosity) and stabilized the interfacial impedance of the Li anode over 250 cycles. In the bare Cu, the impedance increased by roughly a factor of 10 over the same number of cycles. The mechanism by which the polymer coating enhances coulombic efficiency and suppresses side reactions will be discussed, along with the performance in practical anode-free lithium metal batteries. Our additional strategies to utilize thin film coatings in lithium batteries will be briefly introduced.