AUTHOR=Kim Yunsung , Yoo Aeri , Schmidt Robert , Sharafi Asma , Lee Heechul , Wolfenstine Jeff , Sakamoto Jeff 

TITLE=Electrochemical Stability of Li6.5La3Zr1.5M0.5O12 (M = Nb or Ta) against Metallic Lithium

JOURNAL=Frontiers in Energy Research

VOLUME=4

YEAR=2016

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2016.00020

DOI=10.3389/fenrg.2016.00020

ISSN=2296-598X

ABSTRACT=<p>The electrochemical stability of Li<sub>6.5</sub>La<sub>3</sub>Zr<sub>1.5</sub>Nb<sub>0.5</sub>O<sub>12</sub> (LLZNO) and Li<sub>6.5</sub>La<sub>3</sub>Zr<sub>1.5</sub>Ta<sub>0.5</sub>O<sub>12</sub> (LLZTO) against metallic Li was studied using direct current (DC) and electrochemical impedance spectroscopy (EIS). Dense polycrystalline LLZNO (ρ = 97%) and LLZTO (ρ = 92%) were made using sol–gel synthesis and rapid induction hot-pressing at 1100°C and 15.8 MPa. During DC cycling tests at room temperature (± 0.01 mA/cm<sup>2</sup> for 36 cycles), LLZNO exhibited an increase in Li–LLZNO interface resistance and eventually short-circuiting while the LLZTO was stable. After DC cycling, LLZNO appeared severely discolored while the LLZTO did not change in appearance. We believe the increase in Li–LLZNO interfacial resistance and discoloration are due to reduction of Nb<sup>5+</sup> to Nb<sup>4+</sup>. The negligible change in interfacial resistance and no color change in LLZTO suggest that Ta<sup>5+</sup> may be more stable against reduction than Nb<sup>5+</sup> in cubic garnet versus Li during cycling.</p>