Garnet And NASICON Electrolytes For Next-Generation Lithium Batteries

The Li⁺solid electrolyte, which is an important component of next-generationLi-ion batteries （Li-water, Li-air, and solid-state battery）, mainly plays the followingroles:（1） to enable use of Li metal anode to increase cell voltage and capacity;（2） toeliminate irreversible Li loss from cathode side during the first charge;（3） to enable useof liquid cathodes with higher capacity than insertion hosts. Li-rich oxides withgarnet and NASICON structure have been extensively studied because of theirhigh Li⁺-ion conductivity at room temperature and electrochemically stability.The room-temperature Li⁺-conductivity of garnet Li₇La₃Zr₂O₁₂is3.1×10^-4S cm^-1,and small amount of Al³⁺contaminates the sample to enhance the electricalproperties in garnet by:（1） introducing more Li⁺vacancies by the substituted Alions on Li sites in the garnet framework;（2） acting as a sintering aid by reactingwith Li2O in the grain boundaries to form an amorphous Li2O-Al2O3phase;（3）blocking excess Li loss at high temperature. The physically reasonable lithiumdistribution is confirmed by neutron-diffraction method:（1） the octahedron48goccupied requiring two empty neighboring tetrahedral24d sites;（2） a single tetrahedron24d is occupied with displacement of octahedral Li octahedron96h away from theshared face;（3） both tetrahedra24d are occupied and the octahedron48g is vacant. Theneutron-diffraction results at high temperature showed that the lithium ions on both48gand96h sites played a major role in the Li-ion mobile process. The theoretical limit ofLi content in a [B3C2O₁₂] garnet framework is7.5Li per formula unit with the24d sites half-empty and long-range ordered by refining the neutron-diffractionresults in Al-free Li₇La₃Zr₂O₁₂samples.We prepare garnet Li_7-xLa₃Zr_2-xTa_xO₁₂samples by substitution of Zr⁴⁺by aliovalentcations, and the room-temperature Li⁺-ion conductivities are above8.0×10^-4S cm^-1forsamples with x=0.4-0.6; the Li⁺-conductivity reaches the ma_ximum value when theoctahedral sites （48g and96h） are78.2%occupied. There is no other reaction up to5Vvs. Li⁺/Li, indicating that the electrolyte is stable with lithium metal. The Li⁺/H⁺exchange in Li_7-xLa₃Zr_2-xTa_xO₁₂indicate that the electrolyte can only be used in alkalinesolution. For LiZr₂（PO₄）₃in NASICON structure, the rhombohedral NASICONframework is only stable above50oC. We find that the substitution of Ca²⁺or Y³⁺for Zr⁴⁺in LiZr₂（PO₄）₃can transform the structure to rhombohedral NASICONat room temperature and increase the ionic conductivity significantly.The bulkand total Li-ion conductivities of Li_1.15Y_0.15Zr_1.85（PO₄）₃sintered by SPS at25oC are1.4×10^-4and0.71×10^-4S cm^-1, respectively; The dopant Y³⁺reduces the size of theinterstitial space in the M1cavity, and the size change in M1and M2’ cavity facilitatedthe Li⁺transport within the NASICON framework.