Toward Practical Quasi-Solid-State Batteries: Thin Lithium Phosphorous Oxynitride Layer on Slurry-Based Graphite Electrodes

This study introduces a novel quasi-solid-state battery system as a proof of concept. A 55-nm solid-state electrolyte layer of lithium phosphorous oxynitride (LiPON) is deposited on slurry-based graphite electrodes and assembled against lithium metal to evaluate interfacial compatibility and electrochemical performance under controlled conditions. In contrast to thin-film quasi-solid-state batteries, this approach leverages a realistic electrode architecture, where LiPON adjusts to the rough surface of the slurry-cast graphite. By utilizing LiPON's dual functionality as both a solid-state electrolyte and a separator, the system eliminates the need for a conventional separator, while requiring only 5–10% of the liquid electrolyte used in equivalent systems. This design significantly reduces internal resistance and prevents contact loss during cyclic volume changes. Electrochemical analyses, including cyclic voltammetry, galvanostatic cycling, and impedance spectroscopy, demonstrate lithium intercalation stages consistent with those in liquid electrolyte-based systems, stable cycling behavior at room temperature and reduced electrode impedance of a few 10 Ω cm². Furthermore, X-ray photoelectron spectroscopy and scanning transmission electron microscopy confirm the formation of a solid–liquid electrolyte interface and the structural integrity of LiPON, which enhances charge transfer and long-term stability. These findings highlight the potential of quasi-solid-state batteries for safer, more compact, and cost-effective energy storage solutions.