A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries

Abstract

Ga-doped Li 7 La 3 Zr 2 O 12 garnet structures are among the most promising electrolytes for all solid state Li-ion-batteries. The synthesis and processing of garnet-type fast Li-ion conductors depend on conventional sol-gel and solid state syntheses and sintering that are usually done at temperatures above 1050 °C to reach the high Li-conducting cubic phase. This process results in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode-electrolyte assembly. Here, we tackle this problem and report a novel low temperature synthesis-processing route to stabilize the cubic phase of Li 7 La 3 Zr 2 O 12 , while keeping the nanocrystallites at ~ 200-300 nm. Li 7 La 3 Zr 2 O 12 phases are obtained at temperatures of as low as 600 o C by a modified sol-gel combustion method utilizing mainly nitrate precursors, and the sintering temperature is lowered by ~ 200 °C 2 compared to the state-of-art. Through a new model experiment, we also shed light on the conditions influencing the tetragonal to cubic phase transformation via homogeneous Gadiffusion and incorporation occurring at a surprisingly low temperature of ~ 100 o C for a postannealing step. The sintered pellets of newly obtained Li 6.4 Ga 0.2 La 3 Zr 2 O 12 deliver high bulk Li-ion conductivities in the range of ~ 4.0 x 10 -4 S/cm at 20 °C, and a wide thermal operation window is accessible through its characteristic activation energy of ~ 0.32 eV. We report that there is an optimum in sintering-processing conditions for the cubic c-Li 6.4 Ga 0.2 La 3 Zr 2 O 12 solid state electrolytes and their Li-ionic conductivity and the (Raman) near order characteristics that can be tracked through changes in Li-O vibrational modes. Based on this alternative route, low-temperature synthesized powders can be sintered to relatively dense pellets at around only 950 °C. For higher sintering temperatures (e.g. 1100 o C), Li-loss progresses as confirmed by structural studies and a reduction of both, ceramic pellet density and ionic conductivity, as well as distortions in the Li-sublattice are found. Through this work, an alternative low temperature processing route for Ga-doped Li 7 La 3 Zr 2 O 12 garnet type electrolytes for all solid state batteries is suggested. The new synthesis method and the use of c-Li 6.4 Ga 0.2 La 3 Zr 2 O 12 nanoparticles could open pathways in terms of preventing Li-loss during the process and advancing future solid electrolyte-electrode assembly options for all solid state Li-ion batteries.Figure 2. a)In-situ heated XRD powder patterns to study in a model experiment the incorporation of post-added Ga 2 O 3 addition to originally t-Li 7 La 3 Zr 2 O 12 and phase change with respect to temperature (5 °C/min heating rate and annealing 15 minutes at each temperature). Desired fast conducting Li-cubic phase evolves already at temperature as low as 85°C for the post-Ga 2 O 3 addition route; indicating the fast and low-temperature diffusion kinetics of gallium ions to the garnet structure. b) Differential dilatometry...

Interface‐Engineered All‐Solid‐State Li‐Ion Batteries Based on Garnet‐Type Fast Li+ Conductors

“…[16][17][18][19] One of the most commonly used dopants is Al 3+ , which stabilizes the cubic phase by substituting for three Li + , thereby creating two Li vacancies and resulting in the stoichiometry of Li 7−3 x Al x La 3 Zr 2 O 12 for the cubic phase. [ 20 ] Despite the promises, all-solid-state Li-ion batteries based on cubic LLZO solid electrolytes have still challenges in realizing high practical performances due to their high electrode-electrolyte interfacial resistances.…”

“…Using doping strategies, i.e., by introducing Al 3+ , Ga 3+ , Ta 5+ , Nb 5+ , etc., the high conductive cubic phase can be stabilized at room temperature by introducing vacancies into the Li sublattice of the structures. [16][17][18][19] One of the most commonly used dopants is Al 3+ , which stabilizes the cubic phase by substituting for three Li + , thereby creating two Li vacancies and resulting in the stoichiometry of Li 7−3 x Al x La 3 Zr 2 O 12 for the cubic phase.…”

“…Using doping strategies, i.e., by introducing Al 3+ , Ga 3+ , Ta 5+ , Nb 5+ , etc., the high conductive cubic phase can be stabilized at room temperature by introducing vacancies into the Li sublattice of the structures. [16][17][18][19] One of the most commonly used dopants is Al 3+ , which stabilizes the cubic phase by substituting for three Li + , thereby creating two Li vacancies and resulting in the stoichiometry of Li 7−3 x Al x La 3 Zr 2 O 12 for the cubic phase.[ 20 ] Despite the promises, all-solid-state Li-ion batteries based on cubic LLZO solid electrolytes have still challenges in realizing high practical performances due to their high electrode-electrolyte interfacial resistances. For the investigations on all-solid-state batteries based on garnet-type cubic LLZO, most of the current research efforts are concentrated on the cathode material, most prominently on LiCoO 2 .…”

Flexible, Scalable, and Highly Conductive Garnet‐Polymer Solid Electrolyte Templated by Bacterial Cellulose

“…The inorganic LLZO ceramic itself exhibits excellent bulk ionic conductivity, which can reach about 10 −3 S cm −1 at room temperature through doping various elements. [49][50][51] The continuous ion conducting network offers fast Li-ion conductive pathways inside the fibers and acts as a scaffold to offer mechanical support for the solid polymer electrolyte. It is also reported that the interface interaction between the polymer and the inorganic ceramic can effectively increase the ion transport ability along the fiber surface.…”

High lithium ionic conductivity in the garnet‐type oxide Li7−2xLa3Zr2−xMoxO12 (x=0‐0.3) ceramics by sol‐gel method

“…The sintering time of highly conductive Al‐doped Li 7 La 3 Zr 2 O 12 has also been greatly reduced by a polymerized complex method . Recently, tetragonal phase LLZO has been synthesized at 600°C by a modified sol‐gel combustion method utilizing mainly nitrate precursors . Combined with post‐Ga 2 O 3 addition process, cubic phase modified LLZO also can be obtained at 650°C.…”