Lithium-ion conducting Li7La3Zr2O12 (LLZO) garnets are being explored as a replacement for the flammable organic electrolytes used in batteries. However, LLZO garnets require high temperature sintering to densify the structure, but that microstructure and electrochemical properties can vary with lithium content as the lithium volatizes during sintering. The effects of sintering the LLZO garnet requires a detailed examination and study to determine how lithium content can affect physical properties, phase purity and density, as well as performance through ionic conductivity. Studying these parameters produced ionic conductivities above 10-4 S cm-1 in samples that had increased density by enabling liquid phase sintering through the eutectic between Al2O3 and Li2O. Despite this high conductivity, the movement of Li+ through a solid electrolyte encounters even slower kinetics through the rigid electrolyte-cathode interface to the active cathode material. A cathode for LLZO garnets requires a new design with both ionic conduction and electronic conduction pathways while reducing interfacial resistance when co-sintered. Excess lithium within LLZO garnet reduced formation of nonconductive LaCoO3 when co-sintered with the active material, LiCoO2 (LCO), which enables a new completely solid-state cathode for lithium metal batteries to be designed and interfacial resistance to be minimized. LCO, however, is limited to 4.2 V to ensure long life cycle without lattice deformation. Unlocking the potential 5 V cycling with of LLZO garnet necessitated the development of a higher voltage cathode. Chlorinating the oxygen site of lithium spinel, LiMn2O4, using a citric acid method stabilizes the 2 V plateau, which increases the capacity to 180 mAhr g-1, and triple doping with Co, Fe, and Ni enables customization of the properties while shifting the voltage to 5 V. The high voltage spinel and LLZO garnet enables high voltage cycling with increased safety potential enabling a pathway to a safe 400 Wh kg-1 cell, 150 Wh kg-1 higher than the current state of the art.