Batteries are electrochemical devices that store electrical energy in the form of chemical energy. Among known batteries, Li ion batteries (LiBs) provide the highest gravimetric and volumetric energy densities, making them ideal candidates for use in portable electronics and plug-in hybrid and electric vehicles. Conventional LiBs use an organic polymer electrolyte, which exhibits several safety issues including leakage, poor chemical stability and flammability. The use of a solid-state (ceramic) electrolyte to produce all-solid-state LiBs can overcome all of the above issues. Also, solid-state Li batteries can operate at high voltage, thus, producing high power density. Various types of solid Li-ion electrolytes have been reported; this review is focused on the most promising solid Li-ion electrolytes based on garnet-type metal oxides. The first studied Li-stuffed garnet-type compounds are Li5La3M2O12 (M = Nb, Ta), which show a Li-ion conductivity of ∼10(-6) at 25 °C. La and M sites can be substituted by various metal ions leading to Li-rich garnet-type electrolytes, such as Li6ALa2M2O12, (A = Mg, Ca, Sr, Ba, Sr0.5Ba0.5) and Li7La3C2O12 (C = Zr, Sn). Among the known Li-stuffed garnets, Li6.4La3Zr1.4Ta0.6O12 exhibits the highest bulk Li-ion conductivity of 10(-3) S cm(-1) at 25 °C with an activation energy of 0.35 eV, which is an order of magnitude lower than that of the currently used polymer, but is chemically stable at higher temperatures and voltages compared to polymer electrolytes. Here, we discuss the chemical composition-structure-ionic conductivity relationship of the Li-stuffed garnet-type oxides, as well as the Li ion conduction mechanism.

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Lithium ion batteries are the most promising energy storage system on the market today; however, safety issues associated with the use of flammable organic polymer-based electrolytes with poor electrochemical and chemical stabilities prevent this technology from reaching maturity. Solid lithium ion electrolytes (SLIEs) are being considered as potential replacements for the organic electrolytes to develop all-solid-state Li ion batteries. Out of the recently discovered SLIEs, the garnet-related structured Li-stuffed metal oxides are the most promising electrolytes due to their high total (bulk + grain boundary) Li ion conductivity, high electrochemical stability window (∼6 V versus Li(+)/Li at room temperature), and chemical stability against reaction with an elemental Li anode and high-voltage metal oxide Li cathodes. This Perspective discusses the structural-chemical composition-ionic conductivity relationship of Li-stuffed garnets, followed by a discussion on the Li ion conduction mechanism, as well as the electrochemical and chemical stability of these materials. The performance of a number of all-solid-state batteries employing garnet-type Li ion electrolytes is also discussed.

The fundamental electrical transport properties including ionic conductivity, dielectric constants, loss tangent, and relaxation time constants of Li-excess garnet-type cubic (space group Ia3̄d) Li5+2xLa3Ta2-xYxO12 (x = 0.25, 0.5 and 0.75) have been studied in the temperature range of -50 to 50 °C using electrochemical AC impedance spectroscopy. A correlation has been established between the excess Li content and the Li(+) ion migration pathways. The loss tangent (tan δ) for all samples exhibits a relaxation peak corresponding to the dielectric loss because of dipolar rotations due to Li(+) migration. Comparing the modulus analysis of Li-excess garnets with fluorite-type oxygen ion conductors, we propose the local migration of Li(+) ions between octahedral sites around the "immobile" Li(+) ions in tetrahedral (24d) sites. In the samples with x = 0.25 and 0.5, Li(+) ions seem to jump from one octahedral (96h) site to another bypassing the tetrahedral (24d) site between them (path A), both in local and long-range order migration processes, with activation energies of ∼0.69 and 0.54 eV, respectively. For the x = 0.75 member, Li(+) ions exhibit mainly long-range order migration, with an activation energy of 0.34 eV, where the Li hopping between two octahedral sites occurs through the edge which is shared between the two LiO6 octahedra and a LiO4 tetrahedron (path B). The present AC impedance analysis is consistent with the ab initio theoretical analysis of Li-excess garnets that showed two conduction paths (A and B) for Li ion conduction with different activation energies.

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Novel Li-stuffed garnet-like "Li 5+2x La 3 Nb 2−x Y x O 12 " (0.05 ≤ x ≤ 0.75) was prepared via solid-state reaction in air and characterized using ex situ and in situ powder X-ray diffraction (PXRD), 7 Li and 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA), and AC impedance spectroscopy. Rietveld refinement with the PXRD data confirmed the formation of a cubic, garnet-like Ia-3d structure. 7 Li MAS NMR showed a single sharp peak close to 0 ppm as the usual trend for fast Li ion conducting garnets. Among the materials studied, "Li 6.5 La 3 Nb 1.25 Y 0.75 O 12 " showed a very high bulk ionic conductivity of 2.7 × 10 −4 S cm −1 at 25 °C, which is the highest value found in garnet-type compounds, and is only reported for Li 7 La 3 Zr 2 O 12 . The in situ PXRD measurements revealed structural stability up to 400−600 °C after water treatment as well as chemical compatibility with high voltage Li cathodes Li 2 MMn 3 O 8 (M = Co, Fe). The current work demonstrates that slight Al contamination from the Al 2 O 3 crucible, which is commonly observed in this class of materials and was detected by 27 Al MAS NMR, does not affect the Li ionic conductivity and chemical stability of "Li 5+2x La 3 Nb 2−x Y x O 12 " garnets. It also shows that Li stuffing is critical to improve further the ionic conductivity of parent compound Li 5 La 3 Nb 2 O 12 . Y 3+ seems to be a very efficient dopant to improve the ionic conductivity of garnets compared to other investigated dopants that include M 2+ (M = alkaline earth metals), In 3+ , and Zr 4+ .

The garnet-type ''Li 6.5 La 2.5 Ba 0.5 ZrTaO 12 '', crystallizing with cubic symmetry was prepared according to a conventional solid state synthesis method using metal oxides and salt precursors of high purity. The formation of the ''single-phase'' garnet-type structure was studied by powder X-ray diffraction (PXRD). Electron microprobe analysis (EMPA) coupled with a wavelength-dispersive spectrometer (WDS) showed a rather homogeneous distribution of Ta ions and Zr ions compared to that of Ba ions and La ions in ''Li 6.5 La 2.5 Ba 0.5 ZrTaO 12 ''. Li ion dynamics were complementarily studied using variable-temperature AC-impedance spectroscopy and 7 Li NMR measurements. The bulk (ion) conductivities probed are in very good agreement with results reported earlier, illustrating the excellent reproducibility of the Li transport properties of ''Li 6.5 La 2.5 Ba 0.5 ZrTaO 12 ''. In particular, AC impedance and NMR results indicate that the Li transport process studied is of long-range nature. Finally, the chemical compatibility of the electrolyte ''Li 6.5 La 2.5 Ba 0.5 ZrTaO 12 '' was tested with Li 2 FeMn 3 O 8 , being a high-voltage cathode material. As shown by variable-temperature PXRD measurements, the garnet-type structure (bulk) was found to be stable up to 673 K.

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