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

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.