Solid state batteries offer many potential advantages over their liquid electrolyte counterparts including improved safety, wider electrochemical voltage ranges, better chemical compatibility, and the possibility for structural batteries. Among the solid Li-ion electrolyte materials, Li₇La₃Zr₂O₁₂ (LLZO), a garnet structured compound with a relatively high room temperature ionic conductivity (~ 2 x 10^-4 S cm^-1), has attracted much research interest.

The solid state method is the most common route to synthesize LLZO. Long duration sintering at temperatures of 1200 °C or greater are used to obtain dense bodies of the highly conductive cubic phase of LLZO rather than the lower conductivity tetragonal structure.

The long time and high temperature sintering can also lead to Li loss requiring the LLZO part be covered in sacrificial mother powder. Stabilization of the cubic phase can be achieved through the addition of one or more dopants including Al, Ta and Nb.

LLZO electrolytes are thus energy intensive and materially inefficient to manufacture. Production scalability of LLZO batteries depends on developing methods to reduce sintering temperatures while maintaining desirable properties. High energy ball milling (HEBM) offers a relatively simple means to reduce particle size, improve microstructural homogeneity, and reduce sintering temperatures.

Here we report the effect of HEBM on phase stability in undoped LLZO. It is observed that HEBM is capable of causing undoped LLZO to transform from the tetragonal phase to the cubic phase at low temperatures ( <40^oC). It is seen that the HEBM cubic undoped LLZO transforms back into tetragonal LLZO with heating. This transformation is not observed in HEBM (Al,Nb or Ta)-doped LLZO.

HEBM of (Al,Nb or Ta)-doped LLZO with Li₃BO₃ is also reported. Li₃BO₃ is added to promote low temperature sintering (< 900C) which greatly reduces Li losses. Li₃BO₃ is found to be compatible with LLZO. We observe that dopants may also be incorporated into LLZO through Li₃BO₃ sintering. This is demonstrated by heat treatment of HEBM undoped LLZO with Li₃BO₃ and Al₂O₃ additives. LLZO treated this way retains the cubic structure, rather than reverting to the tetragonal phase, indicating that Al has been incorporated into the lattice.