Processing and Implementation of Substituted Li7La3Zr2O12 Solid Electrolytes in Solid-State Lithium Batteries

<p>Modern electronic devices and electrified vehicles rely heavily on the use of Li-ion batteries. A battery with a solid electrolyte would be a safer alternative to current Li-ion technologies utilizing organic liquid electrolytes. The stuffed lithium garnet with the nominal composition of Li7La3Zr2O12 (LLZO) is of great interest in as a solid electrolyte owing to its stability against metallic lithium and lithium ionic conductivity of over 5x10-4 S/cm.</p><p>An approach for tape casting and sintering of substituted Li7La3Zr1.75Nb.25Al.1O12 (LLZNbO) sheets suitable for use in solid state battery development is described. The use of Li3BO3 as a sintering aid in both pellet and cast tape samples is examined. 150-175 micron thick LLZNbO +0.5% Li3BO3 tapes sintered at 1000oC for 6 hours exhibited ionic conductivity values of 2-3x10-4 S/cm.</p><p>Sintering of tape cast LLZNbO sheets at 1050oC without mother powder covering at low levels of lithium loss is performed. The addition of 3%wt. of nano-sized MgO is found to increase sintered tape sheet and pellet density while restricting grain growth and promoting a more homogeneous microstructure. Pellets of LLZNbO with 3%wt. MgO demonstrate density of up to 94% of theoretical and ionic conductivity of 3.6x10-4 S/cm.</p><p>The addition of an interfacial layer of zinc was found to promote wetting of the LLZNbO surface but its impact on a Li/LLZNbO/Li cell’s interfacial resistance and critical current density was indeterminate. The quality of the Li/LLZNbO interface is found to be the most important factor in determining the critical current density (CCD) of a symmetric cell in pellet studies with higher interfacial resistivity predicting low CCD.</p><p>Interaction between LLZNbO and several cathode materials, including LiCoO2, is observed at temperatures of >900oC required for co-sintering of a composite cathode. A process for attaching a LiCoO2 cathode to a sintered LLZNbO sheet through the melting of Li3BO3 is proposed. LiCoO2 is, however, found to undergo reduction during annealing in an argon atmosphere at the required temperatures.</p><p>Finally, several recommendations are made for implementing a solid-state battery with an LLZNbO electrolyte along with avenues for improving the electrode/electrolyte interfaces and the properties of the electrolyte itself.</p>