Dual-Doped Cubic Garnet Solid Electrolyte with Superior Air Stability

Li7La3Zr2O12 (LLZO) garnet is one kind of solid electrolytes drawing extensive attention due to their good ionic conductivity, safety and stability towards lithium metal anode. However, the stability problem during synthesis and storage results in high interfacial resistance and prevents them from practical applications. We synthesized air-stable dual-doped Li5.6La2.9Ga0.26Zr1.87Nb0.05O12 ((Ga, Nb)-LLZO) cubic phase garnet with ionic conductivity of 9.28  10-3 S cm-1. The impurity phase species formation on the garnet pellets after air exposure were investigated. LiOH and Li2CO3 can be observed on the garnet pellets by Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), once the garnets are exposed to humid air or in contact with water. The (Ga, Nb)-LLZO garnet is found to form less LiOH and Li2CO3 which can be further reduced or removed after drying treatment. To confirm the stability of the garnet, electrochemical test of Li//Li symmetric cell was also performed in comparison with previously reported garnet (Li6.9La2.6Ca0.22Zr1.82Nb0.25O12, (Ca, Nb)-LLZO). The dual-doped (Ga, Nb)-LLZO shows less polarized and stable plating/striping behavior than (Ca, Nb)-LLZO. Through Rietveld refinement of XRD patterns of prepared materials, dopant Ga is found to preferably occupy the Li site and Nb takes the Zr site, while dopant Ca mainly substitute La in the reference sample. The inherited properties of the dopants in (Ga, Nb)-LLZO and their structural synergy explain the greatly improved air stability and reduced interfacial resistance. This may open a new direction to realize garnet based solid electrolytes with lower interfacial resistance and superior air stability.