Interfacial Stability of Phosphate-NASICON Solid Electrolytes in Ni-Rich NCM Cathode-Based Solid-State Batteries

A nondegrading, low-impedance interface between a solid electrolyte and cathode active materials remains a key challenge for the development of functional all-solid-state batteries (ASSBs). The widely employed thiophosphate-based solid electrolytes are not stable toward oxidation and suffer from growing interface resistance and thus rapid fading of capacity in a solid-state battery. In contrast, NASICON-type phosphates such as Li1+xAlxTi2–x(PO4)3 and Li1+xAlxGe2–x(PO4)3 are stable at high potentials, but their mechanical rigidity and high grain boundary resistance are thought to impede their application in bulk-type solid-state batteries. In this work, we present a comparative study of a LiNi0.8Co0.1Mn0.1O2 (NCM-811) cathode composite employing either β-Li3PS4 (LPS) or Li1.5Al0.5Ti1.5(PO4)3 (LATP) as a solid electrolyte. LPS is employed as a separator in both cases to assemble a functional ASSB. To avoid high-temperature processing of LATP, along with subsequent detrimental interfacial reactions with NCM materials, the ASSBs are constructed and operated in a hot-press setup at 150 °C. The cathode interfaces are investigated using in situ electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy, which reveals that the interface resistance is strongly suppressed and the chemical state of the composite is unchanged during cycling when employed with LATP. The cell using LATP is reversibly charged and discharged for multiple cycles and outperforms a comparable cell using a thiophosphate composite electrode. The results indicate that LATP in the cathode composite represents an excellent candidate to overcome interfacial challenges in bulk-type solid-state batteries.