All-solid-state lithium-ion electrolytes offer substantial safety benefits compared to flammable liquid organic electrolytes. However, a great challenge in solid electrolyte batteries is forming a stable and ion conducting interface between the electrolyte and active material. This study investigates and characterizes a possible solid-state electrode-electrolyte pair for the high voltage active cathode material LiMn1.5Ni0.5O4 (LMNO) and electrolyte Li1+xAlxGe2-x(PO4)(3) (LAGP). In situ X-ray diffraction measurements were taken on pressed pellets comprised of a blend of LMNO and LAGP during exposure to elevated temperatures to determine the product materials that form at the interface of LMNO and LAGP and the temperatures at which they form. In particular, above 600 degrees C a material consistent with LiMnPO4 was formed. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to image the morphology and elemental compositions of product materials at the interface, and electrochemical characterization was performed on LMNO-coated LAGP electrolyte pellet half cells. Although the voltage of Li/LAGP/LMNO assembled batteries was promising, thick interfacial phases resulted in high electrochemical resistance, demonstrating the need for further understanding and control over material processing in the LAGP/LMNO system to reduce interfacial resistance and improve electrochemical performance.