Interfaces between electrodes and the electrolyte in solid state lithium batteries frequently involve disparate materials in contact with one another under the effect of a large electrochemical potential—rather extreme conditions that can often lead to interfacial reaction and degradation, ultimately resulting in cell failure. Here, methodologies and analyses for probing the fundamental behavior at solid-solid interfaces for solid state lithium batteries are developed and explored, specifically investigating cathode interfaces with the solid electrolyte Li₇La₃Zr₂O₁₂ (LLZO).

To investigate of cathode compatibility with LLZO from a primarily electrochemical perspective, LiMn₂O₄ (LMO) was chosen as a model cathode due to its wide operable voltage window and the extensive body of literature in other electrolyte systems available for comparison. Thus, LMO|LLZO|Li full cells are constructed and galvanostatic cycling combined with electrochemical impedance spectroscopy (EIS) is used to assess changes in cell capacity and resistances as a function of cycle number. A series of symmetric cells isolating individual electrode- electrolyte interfaces aids in pinpointing the specific sources of cell capacity fade. Combined with X-ray photoelectron spectroscopy (XPS) depth-profiling, these analyses indicate a region of interdiffusion occurs at the LMO-LLZO interface, thereby suggesting their incompatibility as a cathode-electrolyte pairing.

To further explore behavior at the buried solid-solid interfaces, a novel electrochemical mass spectrometry (EC-MS) system is developed and implemented to probe gas-evolution from these interfaces under electrochemical load. It is commonly accepted, that despite extensive mitigation procedures, Li₂CO₃ is able to re-form on the surface of LLZO after its removal. Li₂CO₃ can therefore become a contaminant present at cell interfaces during synthesis, Interfacial Li₂CO₃ oxidation occurs in LMO|LLZO|Li cells starting at charging potentials relevant for the operation of common cathode materials. Evolution of CO₂ and O₂ from Li₂CO₃ decomposition is detected by EC-MS, and further confirmed via its detection under the same conditions in blocking Au|LLZO|Li cells. The decomposition of interfacial Li₂CO₃ is shown to cause large increases in cell impedance alongside significant capacity loss.

The techniques developed in the proceeding studies are then applied to a solid state Li-O₂ system, where patterned gold electrodes deposited onto LLZO act as cathodes, providing a conductive-network for the nucleation of the discharge product. The use of a carbon-free cathode and the absence of liquid electrolytes—two cell common cell components that result in parasitic side reactions—facilitates direct assessment of the Li-O₂ chemistry in a solid state system. It is found that by cycling these cells at high temperatures, which is permitted through the use of solid electrolytes, the thermally-enhanced electronic conductivity of the discharge products permits much thicker discharge product film growth than in traditional aprotic electrolyte Li-O₂ cells.