Tetragonal vs. cubic phase stability in Al – free Ta doped Li7La3Zr2O_(12) Garnet Li Ion Solid Electrolyte

摘要

Solid Li-ion electrolytes have been attracting increasing interest as a means to address the safety issues associated with flammable liquid electrolytes and enable the next generation of battery technology. In order to be viable, significantly high ionic conductivity must be exhibited. The Li rich garnet crystal structure based on Zr and discovered by Weppner1 et. al. in 2007 (Li7La3Zr2O12, LLZO) has been identified as a solid electrolyte with conductivities of ～ 1 mS/cm when stabilized as the cubic polymorph. However, a tetragonal polymorph of LLZO is also possible and exhibits ～2 orders of magnitude lower ionic conductivity. The cubic phase of LLZO can be stabilized by supervalent charge doping where the excess charge is balance by Li vacancies . It has been predicted computationally by Bernstein et. al. that the critical Li vacancy concentration to stabilize the cubic phase is 0.4 - 0.5 mols per formula unit . This critical doping level has been confirmed experimentally for Al~(3+) doping on the Li~(1+) site. However, it is advantageous to dope on the primary lattice, which acts as a skeletal framework, instead of on the Li sublattice, where the diffusion of the charge carrying ions takes place. Ta~(5+) substitution for Zr~(4+) has been demonstrated in the literature as a way to stabilize the cubic polymorph of LLZO. However, compositions which were below the critical doping level (subcritically doped) have been reported as cubic. Exploring the phase stability of LLZO is complicated by the fact that Al ions can be incorporated into the garnet from Al2O3 calcination media at high temperature and act as a dopant. Furthermore, Al has a disproportionately larger effect as a dopant compared to Ta because it creates twice the Li vacancies (Kroger-Vink notation: Li_(Li)~x + Al~x→ Al_(Li)~~(··)+ 2V'_(Li) vs. Zr°_(Zr) + M° → Ta·_(Zr) + V'_(Li)). Therefore, a small amount of Al can make a large difference on the Li vacancy concentration, and thus the resulting phase. Because of this, compositions which should have an insufficient number of Li vacancies from the purposefully added dopant to stabilize the cubic phase have been reported as cubic.