Simultaneously Improved Cubic Phase Stability and Li-Ion Conductivity in Garnet-Type Solid Electrolytes Enabled by Controlling the Al Occupation Sites

Abstract

Here, we, for the first time, report on the simultaneous enhancement in cubic phase stability and Li-ion conductivity of garnettype solid electrolytes (SEs) by adding excess Li/Al. The excess Al/Li creates very large grains of up to 170 μm via the segregation of Al at the grain boundaries and enables preferential Al occupation at 96h sites over 24d sites, a behavior contrary to previous observations. The resulting SE shows improved Li-ion conductivity due to the large grain size and less blocking Li pathway caused by different preferential Al occupation. Surprisingly, it is observed that the cubic phase of the garnet-type SE is transformed to the tetragonal phase on the surface and in the bulk under the applied voltage, and the preferential Al occupation enables its cubic phase stability. Under battery operating conditions, the LLZO SE with excess Li/Al can maintain high ionic conductivity due to the cubic phase stability and large grain size. We clearly demonstrate that the cubic phase stability and ionic conductivity of LLZO can be simultaneously improved by excess Li/Al without any post-treatments. The findings and understanding will provide new insights into practical use of the garnet-type SEs for advanced all solid-state batteries.

Review of the Developments and Difficulties in Inorganic Solid-State Electrolytes

“…However, it is believed that Al 3+ occupying the 24d sites will obstruct the migration of Li + in its path [ 30 ]. Because the introduction of surplus Al into the bulk alters the preference of Al occupation at 96h sites over 24d sites, Kim et al improved the phase stability of the LLZO, and ionic conductivity reached 3.84 × 10 −4 S·cm −1 [ 31 ]. Ga doping exhibits a site preference similar to that of Al in the LLZO lattice, and the overall ionic conductivity of Ga-LLZO can be an order of magnitude higher than Al-LLZO [ 32 ].…”

Defect Strategy in Solid‐State Lithium Batteries

“…This alteration can enhance ionic conductivity and the stability of the cubic phase. [118] LISICON Type and Lithium Argyrodite Electrolytes: The LISI-CON SSEs present ultrahigh ionic conductivity (≈10 −3 -10 −2 S cm −1 ) comparable to the current organic liquid electrolytes, good machinability, and interface contact with electrodes due to the inherently soft property. [26] It has a 𝛾-Li 3 PO 4 type frame structure, and the general structure can be expressed as Li x A 1−y B y S 4 (where A = Si and Ge, B = P, Al, Zn, Ga, and Sb).…”

A data-mining approach to understanding the impact of multi-doping on the ionic transport mechanism of solid electrolytes materials: the case of dual-doped Ga0.15/Scy Li7La3Zr2O12

“…8,10 This suggests that the blocking of T d junctions signicantly impedes the connectivity of the Li + ion sublattice. [10][11][12] In recent years, research has focused on various substitution strategies to stabilize the cubic phase of LLZO at RT. Notable examples include ion substitutions such as Li + by Al 3+ , 12,13 or Ga 3+ ; 10,14,15 Zr 4+ by Ta 5+ , 16 or Bi 5+ ; 17 as well as La 3+ by Ca 2+ , Sr 2+ or Ba 2+ .…”

“…[10][11][12] In recent years, research has focused on various substitution strategies to stabilize the cubic phase of LLZO at RT. Notable examples include ion substitutions such as Li + by Al 3+ , 12,13 or Ga 3+ ; 10,14,15 Zr 4+ by Ta 5+ , 16 or Bi 5+ ; 17 as well as La 3+ by Ca 2+ , Sr 2+ or Ba 2+ . 18 Moreover, multi-ion substitutions have been explored, aiming to stabilize the cubic phase with one dopant, while utilizing others to ne-tune the Li-molar content, crystal lattice size, and pellet densication.…”