Solid-state batteries offer a promising alternative to conventional lithium-ion batteries, addressing key safety and performance limitations. This study introduces a novel almost-solid-state battery system, implementing a thin 55 nm lithium phosphorous oxynitride (LiPON) layer on slurry-based graphite electrodes. By leveraging 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 electrode impedance of a few 10 Ω cm2, efficient lithium-ion transport and stable cycling behavior at room temperature. Furthermore, X-ray photoelectron spectroscopy and scanning transmission electron microscopy confirm the structural integrity of LiPON and the formation of a solid-liquid electrolyte interface, which enhances charge transfer and long-term stability. These findings highlight the potential of almost-solid-state batteries for safer, more compact and cost-effective energy storage solutions.