About the Compatibility between High Voltage Spinel Cathode Materials and Solid Oxide Electrolytes as a Function of Temperature

The reactivity of mixtures of high voltage spinel cathode materials Li₂NiMn₃O₈, Li₂FeMn₃O₈, and LiCoMnO₄ cosintered with Li_1.5Al_0.5Ti_1.5(PO₄)₃ and Li_6.6La₃Zr_1.6Ta_0.4O₁₂ electrolytes is studied by thermal analysis using X-ray-diffraction and differential thermoanalysis and thermogravimetry coupled with mass spectrometry. The results are compared with predicted decomposition reactions from first-principles calculations. Decomposition of the mixtures begins at 600 °C, significantly lower than the decomposition temperature of any component, especially the electrolytes. For the cathode + Li_6.6La₃Zr_1.6Ta_0.4O₁₂ mixtures, lithium and oxygen from the electrolyte react with the cathodes to form highly stable Li₂MnO₃ and then decompose to form stable and often insulating phases such as La₂Zr₂O₇, La₂O₃, La₃TaO₇, TiO₂, and LaMnO₃ which are likely to increase the interfacial impedance of a cathode composite. The decomposition reactions are identified with high fidelity by first-principles calculations. For the cathode + Li_1.5Al_0.5Ti_1.5(PO₄)₃ mixtures, the Mn tends to oxidize to MnO₂ or Mn₂O₃, supplying lithium to the electrolyte for the formation of Li₃PO₄ and metal phosphates such as AlPO₄ and LiMPO₄ (M = Mn, Ni). The results indicate that high temperature cosintering to form dense cathode composites between spinel cathodes and oxide electrolytes will produce high impedance interfacial products, complicating solid state battery manufacturing.