Assessing the importance of cation size in the tetragonal-cubic phase transition in lithium garnet electrolytes.

Lithium garnets are promising solid-state electrolytes for next generation lithium-ion batteries. These materials have high ionic conductivity, a wide electrochemical window and stability with Li metal. However, lithium garnets have a maximum limit of 7 lithium atoms per formula unit (e.g. La3Zr2Li7O12), before the system transitions from a cubic to a tetragonal phase with poor ionic mobility. This arises from full occupation of the Li sites. Hence, the most conductive lithium garnets have Li between 6-6.55 Li per formula unit, which maintains the cubic symmetry and the disordered Li sub-lattice. The tetragonal phase, however, forms the highly conducting cubic phase at higher temperatures, thought to arise from increased cell volume and entropic stabilisation permitting Li disorder. However, little work has been undertaken in understanding the controlling factors of this phase transition, which could enable enhanced dopant strategies to maintain room temperature cubic garnet at higher Li contents. Here, a series of nine tetragonal garnets were synthesised and analysed via variable temperature XRD to understand the dependence of site substitution on the phase transition temperature. Interestingly the octahedral site cation radius was identified as the key parameter for the transition temperature with larger or smaller dopants altering the transition temperature noticeably. A site substitution was, however, found to make little difference irrespective of significant changes to cell volume.

Evaluation of Ga0.2Li6.4Nd3Zr2O12 garnets: exploiting dopant instability to create a mixed conductive interface to reduce interfacial resistance for all solid state batteries

Lithium garnets are promising solid electrolytes; however, they suffer from intrinsically high interfacial resistance. In this work we exploit Ga dopant instability to form Li/Ga eutectic mixtures that give very low resistance at the Li interface.

Facile Synthesis of Al-Stabilized Lithium Garnets by Solution-Combustion Technique for All Solid-State Batteries

Solid-state lithium batteries (SSLBs) with ceramic electrolytes are proposed to result in improved energy density and safety compared to liquid electrolyte-based Li-ion batteries. Among the various inorganic ceramic electrolytes, Li7La3Zr2O12...

On-surface lithium donor reaction enables decarbonated lithium garnets and compatible interfaces within cathodes

Lithium garnets have been widely studied as promising electrolytes that could enable the next-generation all-solid-state lithium batteries. However, upon exposure to atmospheric moisture and carbon dioxide, insulating lithium carbonate forms on the surface and deteriorates the interfaces within electrodes. Here, we report a scalable solid sintering method, defined by lithium donor reaction that allows for complete decarbonation of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) and yields an active LiCoO2 layer for each garnet particle. The obtained LiCoO2 coated garnets composite is stable against air without any Li2CO3. Once working in a solid-state lithium battery, the LiCoO2-LLZTO@LiCoO2 composite cathode maintains 81% of the initial capacity after 180 cycles at 0.1 C. Eliminating CO2 evolution above 4.0 V is confirmed experimentally after transforming Li2CO3 into LiCoO2. These results indicate that Li2CO3 is no longer an obstacle, but a trigger of the intimate solid-solid interface. This strategy has been extended to develop a series of LLZTO@active layer materials.

Low Electronic Conductivity of Li7La3Zr2O12 (LLZO) Solid Electrolytes from First Principles

Lithium-rich garnets such as Li7 La3 Zr2 O12 (LLZO) are promising solid electrolytes with potential applications in all–solid-state lithium-ion batteries. The practical use of lithium-garnet electrolytes is currently limited by pervasive lithium-dendrite growth during battery cycling, which leads to short-circuiting and cell failure. One proposed mechanism for dendrite growth is the reduction of lithium ions to lithium metal within the electrolyte. Lithium garnets have been proposed to be susceptible to this growth mechanism due to high electronic conductivities [Han et al. Nature Ener. 4 187, 2019]. The electronic conductivities of LLZO and other lithium-garnet solid electrolytes, however, are not yet well characterised. Here, we present a general scheme for calculating the intrinsic electronic conductivity of a nominally-insulating material under variable synthesis and operating conditions from first principles, and apply this to the prototypical lithium-garnet LLZO. Our model predicts that under typical battery operating conditions, electron and hole carrier-concentrations in bulk LLZO are negligible, irrespective of initial synthesis conditions, and electron and hole mobilities are low (<1 cm2 V−1 s−1 ). These results suggest that the bulk electronic conductivity of LLZO is not sufficiently high to cause bulk lithium-dendrite formation during cell operation. Any non-negligible electronic conductivity in lithium garnets is therefore likely due to extended defects or surface contributions.

Low Electronic Conductivity of Li7La3Zr2O12 (LLZO) Solid Electrolytes from First Principles

Lithium-rich garnets such as Li7 La3 Zr2 O12 (LLZO) are promising solid electrolytes with potential applications in all–solid-state lithium-ion batteries. The practical use of lithium-garnet electrolytes is currently limited by pervasive lithium-dendrite growth during battery cycling, which leads to short-circuiting and cell failure. One proposed mechanism for dendrite growth is the reduction of lithium ions to lithium metal within the electrolyte. Lithium garnets have been proposed to be susceptible to this growth mechanism due to high electronic conductivities [Han et al. Nature Ener. 4 187, 2019]. The electronic conductivities of LLZO and other lithium-garnet solid electrolytes, however, are not yet well characterised. Here, we present a general scheme for calculating the intrinsic electronic conductivity of a nominally-insulating material under variable synthesis and operating conditions from first principles, and apply this to the prototypical lithium-garnet LLZO. Our model predicts that under typical battery operating conditions, electron and hole carrier-concentrations in bulk LLZO are negligible, irrespective of initial synthesis conditions, and electron and hole mobilities are low (<1 cm2 V−1 s−1 ). These results suggest that the bulk electronic conductivity of LLZO is not sufficiently high to cause bulk lithium-dendrite formation during cell operation. Any non-negligible electronic conductivity in lithium garnets is therefore likely due to extended defects or surface contributions.

The influence of co-doping strontium and zirconium on structure and ionic conductivities of Li6+yLa3−ySryNbZrO12 solid electrolytes

To enhance the ionic conductivity of lithium garnets, a co-doping strategy was adopted with both strontium and zirconium for [Formula: see text] ([Formula: see text], 0.25, 0.5, 0.75 and 1.0) (LLSNZO). By increasing the content of Sr, lithium garnet ceramics maintain cubic structure when [Formula: see text] is in the range of 0–0.75. A secondary phase of [Formula: see text] appeared in the ceramic when [Formula: see text]. We also studied the cross-section of lithium garnets with silver electrode. Results showed that the density of LLSNZO ceramics increased continuously against the increase of Sr content, while their total ionic conductivity enhanced initially and then reduced, with the maximum reached when [Formula: see text]. It is indicated that ionic conductivity of lithium garnets is not only decided by the density but also decided by the concentration and the mobility of [Formula: see text].