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

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.

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.

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.

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.

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.

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.