Molecular layer deposition of polyhydroquinone thin films for Li-ion battery applications

Abstract

Many next-generation materials for Li-ion batteries are limited by material instabilities. To stabilize these materials, ultrathin, protective coatings are needed that conduct both lithium ions and electrons. Here, we demonstrate a hybrid chemistry combining molecular layer deposition (MLD) of trimethylaluminum (TMA) and p-hydroquinone (HQ) with oxidative molecular layer deposition (oMLD) of molybdenum pentachloride (MoCl₅) and HQ to enable vapor-phase molecular layer growth of poly(p-hydroquinone) (PHQ)—a mixed electron and lithium ion conducting polymer. We employ quartz crystal microbalance (QCM) studies to understand the chemical mechanism and demonstrate controlled linear growth with a 0.5 nm/cycle growth rate. Spectroscopic characterization indicates that this hybrid MLD/oMLD chemistry polymerizes surface HQ monomers from the TMA-HQ chemistry to produce PHQ. The polymerization to PHQ improves air stability over MLD TMA-HQ films without crosslinking. Electrochemical measurements on hybrid MLD/oMLD films indicate electronic conductivity of ~10⁻⁹ S/cm and a Li-ion conductivity of ~10⁻⁴ S/cm. While these coatings show promise for Li-ion battery applications, this work focuses on establishing the coating chemistry and future studies are needed to examine the stability, structure, and cycling performance of these coatings in full Li-ion cells.