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Abstract Interphase layers that form at contact points between the solid electrolyte (SE) and cathode active material in solid‐state lithium‐ion batteries (SS‐LIBs) increase cell impedance, but the mechanisms for this interphase formation are poorly understood. Here, we demonstrate a simple workflow to study cathode–electrolyte interphase (CEI) formation using 4D‐scanning transmission electron microscopy (4D‐STEM) that does not require SS‐LIB assembly. We show benefits of MoCl₅:EtOH as a chemical delithiating agent, and prepare chemically delithiated cathode LiNi_0.6Co_0.2Mn_0.2O₂(NMC) powder in contact with Li₁₀GeP₂S₁₂(LGPS) SE powder as a SS‐LIB CEI surrogate. We map the composition and structure of the CEI layers using 4D‐STEM, energy dispersive X‐ray spectroscopy (EDS), and electron pair distribution function analysis (ePDF). EDS indicates O migration from NMC into LGPS. ePDF analysis indicates sulfate and phosphate formation localized on the surface of LGPS, as well as Li₂O formation within the LGPS phase, and self‐decomposition of NMC. These results are consistent with an electrochemical self‐discharge mechanism for interphase formation arising from coupled redox reactions of sulfur oxidation in LGPS and transition metal reduction in NMC. This suggests that coatings which stop anion transport but allow Li⁺and e⁻transport may prevent interphase formation and reduce impedance in SS‐LIBs. 

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.