Exploring the threshold of Hf in ZrO2 for garnet-type Li7La3Zr2O12 solid electrolytes

Garnet-type cubic Li7La3Zr2O12 (LLZO) is widely considered as one of the most promising solid electrolyte candidates for the next-generation solid-state lithium batteries. However, high cost also limits the wide application of LLZO electrolyte. Allowing for the existence of trace hafnium (Hf) element in zirconium dioxide (ZrO2) raw material and thus determining the threshold can significantly reduce the production cost of LLZO. Herein, the garnet-type xHf-LLZO solid electrolytes were synthesized at 1200 °C for 6 h using ZrO2 raw material with different x mol% Hf (x = 0, 1.0, 1.8 and 2.7), in which 10 % sulfobetaine-modified polyvinylidene fluoride (SB-PVDF) was added for the composite electrolyte membranes in the solid state lithium cells. Furthermore, the effect of Hf element on the structure and electrochemical performances of LLZO solid electrolyte and the threshold of Hf in ZrO2 were explored. The results showed that the tetragonal phase began to appear in LLZO when the molar ratio of Hf was 1.8 mol%, while the relative density and the room temperature ionic conductivity could remain 91.71 % and 1.08 × 10−4 S/cm, respectively. The corresponding solid-state lithium cell delivered a specific discharge capacity of 130.2 mAh/g at 0.5C, and the capacity retention rate were as high as 90.7 % after 200 cycles at 1.0C. Seemingly, the threshold of Hf in ZrO2 to prepare LLZO was about 1.8 mol%, thus reducing the cost for large-scale production of high-performance LLZO. •The effect of Hf on the structure and performances of LLZO solid electrolyte was studied.•The threshold of Hf in LLZO for solid state lithium battery was determined for the first time.•Determining the threshold of Hf decreased the mass production cost of LLZO electrolyte.•This work may promote the commercialization of the solid state lithium battery.