Structural and Electrochemical Consequences of Al and Ga Cosubstitution in Li₇La₃Zr₂O₁₂ Solid Electrolytes

Abstract

Several “Beyond Li-Ion Battery” concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to “non-garnet” (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10–4 S cm–1 to 1.2 × 10–3 S cm–1, which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm2, the lowest reported value for LLZO so far. These results illustrate that understanding the structure–properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

Interface Instability of Fe-Stabilized Li7La3Zr2O12 versus Li Metal

“…The resulting ASR of 1088 Ω cm 2 is significantly larger than those observed for the Al- and Ga-stabilized counterparts obtained by similar preparation routes. 4 A cyclic voltammogram of the Li|LLZO:Fe|Au cell was recorded to check the electrochemical voltage stability window of LLZO:Fe (see Figure 3). We observe small reduction and oxidation peaks at low potentials because of the alloying reaction of Au and Li and the extraction of Li from the alloy.…”

“…The interfacial impedance measurements were taken shortly after the Li|LLZO|Li cell was assembled. 4 During this time, the black coloration region may be dynamically evolving and resulted in a possible mixed conduction black layer. We were not able to separate partial electronic and ionic conduction.…”

Ion dynamics in solid electrolytes for lithium batteries

“…Regarding Li 7 La 3 Zr 2 O 12 , its tetragonal modification is a moderate Li ion conductor [119]. The replacement of La and Zr ions with isovalent or aliovalent cations greatly affects, however, (i) the defect chemistry, (ii) the crystal structure parameters and (iii) also the Li content of the garnets [40,[120][121][122]. These variations lead to a series of oxides that may largely differ in ionic conductivities [19].…”

“…The search for suitable materials that are characterized by ultra-fast ion dynamics [32]; LISICON (Li 14 Zn(GeO 4 ) 4 ) [33]; Li-β-alumina [34]; Li 10 GeP 2 S 12 [35]; s-Li 3 N (pure, sintered) [36]; d-Li 3 N (doped with H) [37]; tetragonal LLZ (Li 7 La 3 Zr 2 O 12 ) [38]; cubic-Al-LLZO (Li 7 La 3 Zr 2 O 12 with 0.9 wt% Al added) [39]; cubic-Ga-LLZO (Ga-containing garnet, bulk values) [40]; LiBSO (0.3 LiBO 2 -0.7 Li 2 SO 4 , thin film) [41]; LiPON (lithium phosphorous oxynitride, thin film) [42] [56]; LLTO (La 0. 51 Li 0.34 TiO 2.94 , bulk conductivity) [57] and negligible electronic transport has already reached an unprecedented level.…”

“…[19,22] where the relationships between chemical composition, structure and ionic conductivity of the Li-stuffed garnet-type oxide as well as relevant Li ion conduction mechanisms are discussed. Among the known Li-stuffed garnets, Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 and Al(Ga)-stabilized cubic-Li 7 La 3 Zr 2 O 12 exhibit the highest Li-ion conductivity (10 −3 S cm −1 at 295 K) [19,22,40]. While activation energies derived from impedance spectroscopy lie around 0.2 eV for the sulfides [64], for the highly conducting garnet-based oxides values of ca.…”