Electrochemical properties and structural stability of Ga- and Y- co-doping in Li₇La₃Zr₂O₁₂ ceramic electrolytes for lithium-ion batteries

Ga-, Y- co-doped Li₇La₃Zr₂O₁₂ ceramic electrolytes with composition of Li_7-3xGa_xLa_3-yY_yZr₂O₁₂ (x = 0.2 and y = 0, 0.125, 0.25, 0.5) were prepared via conventional solid-state reaction in air and characterized using X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM) and AC impedance spectroscopy. XRD analysis and Raman spectra showed that all samples formed a single cubic phase with garnet-type structure. Among the materials investigated, Li_6.4Ga_0.2La_2.75Y_0.25Zr₂O₁₂ sample presented the highest lithium-ion conductivity of 1.61 mS cm⁻¹ at room temperature, a minimum activation energy of 0.281 eV along with the maximum relative density of 96.6%. The stability of Li_6.4Ga_0.2La_2.75Y_0.25Zr₂O₁₂ material was also investigated in liquid electrolyte, air, LiCl solution, deionized water and LiOH solution. Although the Li⁺/H⁺ exchange was confirmed by XRD, Raman spectra, Fourier Transform infrared spectroscopy (FTIR) and thermo-gravimetric analysis (TGA), the garnet-type cubic structure was found to be stable for the sample after immersion in organic liquid electrolyte, in LiCl solution, deionized water, at room temperature for two weeks. The lithium-ion conductivity was more or less decreased in the sample treated in air, LiCl solution, deionized water and LiOH solution. However, no decrease in conductivity was observed in the sample upon organic liquid electrolyte treatment for a long time, making it a suitable alternative in fabricating hybrid lithium-ion batteries. Therefore, through construction of the symmetric Li/Garnet/Li cells, the interfacial resistance decreases from 970 to 205 Ω cm² by adding liquid electrolyte as a buffer layer. Meanwhile, the full cells consisting of hybrid electrolyte, lithium-metal anode, and LiCoO₂ cathode demonstrate a high initial discharge capacity of 140 mAh g⁻¹ and excellent cycling stability with 130 mAh g⁻¹ retained after 100 cycles at a charge/discharge rate of 15 mA g⁻¹ at room temperature.