All-solid-state Li batteries exhibit significant potential for use as next-generation batteries. Sulfide solid electrolytes possess high Li-ion conductivity, making them suitable for all-solid-state Li batteries with a high-power density. For using the batteries in high-power applications, it is crucial to achieve a low interface resistance between sulfide solid electrolytes and positive electrodes. Thus, elucidating the origin of the interfacial resistance quantitatively to reduce the interface resistance is critical for the practical application of all-solid-state Li batteries.

There are two hypotheses regarding the origin of the interface resistance. First, an interphase layer is formed chemically or electrochemically. [1,2] Second, a space charge layer is formed at the interface. [3] The origin of the reduction in interfacial resistance remains debated despite intensive studies. It is well known that introducing a thin oxide layer, hereafter referred to as a “buffer layer,” reduces the interface resistance. Thus, studies on introducing a buffer layer, such as Li₃PO₄, LiNbO₃, etc. are underway to reduce this interface resistance. However, the mechanism and the role of the buffer layers still remain unclear.

Accordingly, we utilized a thin-film model system with a clean interface, which can facilitate the measurement of the interface resistance and the quantitative analysis of the interface structure. [4,5] We previously developed a controlled interface using an amorphous Li₃PO₄ oxide solid electrolyte and LiCoO₂ epitaxial thin-film positive electrode for quantitative analysis; the atomically well-ordered interface exhibits an extremely low resistance (~10 Ohm cm²). [6,7]

 In this study, we quantitatively investigated the interface between an amorphous Li₃PS₄ sulfide solid electrolyte and a LiCoO₂(001) epitaxial thin-film positive electrode and the effects of a Li₃PO₄ buffer layer on the interface resistance. When Li₃PS₄ contacts directly upon LiCoO₂(001), the interface resistance exhibits a significantly high value (3.5 x 10⁴ Ohm cm²), resulting in no battery operation. Scanning transmission electron microscopy (STEM) observation and X-ray energy dispersive spectroscopy (EDX) analysis revealed that a chemical reaction layer is formed as S diffuses from Li₃PS₄ to LiCoO₂ before battery operation. In contrast, introducing a Li₃PO₄ buffer layer with 10 nm thickness reduces the interface resistance dramatically by ~1/2,800, leading to normal battery operation. STEM-EDX revealed that the Li₃PO₄ buffer layer suppresses the S diffusion from Li₃PS₄ into LiCoO₂. Consequently, an abruptly sharp interface structure remains at Li₃PO₄ buffer layer-LiCoO₂, exhibiting extremely low interface resistance. These results unveiled that the significantly high interface resistance between Li₃PS₄ and LiCoO₂ originates from the formation of the chemical reaction layer, and the Li₃PO₄ buffer layer suppresses the S diffusion causing a chemical reaction. [8] This study paves the way for the strategically designed interfacial modification using the buffer layer between sulfide solid electrolytes and positive electrodes.