Microstructural Insights into Performance Loss of High-Voltage Spinel Cathodes for Lithium-ion Batteries.

Spinel-structured LiNix Mn2-x O4 (LNMO), with low-cost earth-abundant constituents, is a promising high-voltage cathode material for lithium-ion batteries. Even though extensive electrochemical investigations have been conducted on these materials, few studies have explored correlations between their loss in performance and associated changes in microstructure. Here, down to the atomic scale, the structural evolution of these materials is investigated upon the progressive cycling of lithium-ion cells. Transgranular cracking is revealed to be a key feature during cycling; this cracking is initiated at the particle surface and leads to the penetration of electrolytes along the crack path, thereby increasing particle exposure to the electrolyte. The lattice structure on the crack surface shows spatial variances, featuring a top layer of rock-salt, a sublayer of a Mn3 O4 -like arrangement, and then a mixed-cation region adjacent to the bulk lattice. The transgranular cracking, along with the emergence of local lattice distortion, becomes more evident with extended cycling. Further, phase transformation at primary particle surfaces and void formation through vacancy condensation is found in the cycled samples. All these features collectively contribute to the performance degradation of the battery cells during electrochemical cycling.

Wet-Environment-Induced Structural Alterations in Single- and Polycrystalline LLZTO Solid Electrolytes Studied by Diffraction Techniques.

Li7La3Zr2O12 (LLZO) is one of the potential candidates for Li metal-based solid-state batteries owing to its high Li+ conductivity (≈10-3 S cm-1) at room temperature and large electrochemical stability window. However, LLZO undergoes protonation under the influence of moisture-forming Li2CO3 layers, thereby affecting its structural and transport properties. Therefore, a detailed understanding on the impact of the exchange of H+ on Li+ sites on structural alteration and kinetics under the influence of wet environments is of great importance. The present study focuses on the Li+/H+ exchange in single-crystal and polycrystal Li6La3ZrTaO12 (LLZTO) garnets prepared using the Czochralski method and solid-state reactions subjected to weathering in air, aqueous solutions at room temperature, and in aqueous solution at 363 K using X-ray diffraction (XRD) and neutron diffraction (ND) techniques. Based on 36 single-crystal diffraction and 88 powder diffraction measurements, we found that LLZTO crystallizes with space group (SG) Ia3̅d with Li located in 96h (Li(2)) and 24d (Li(1)) sites, whereas the latter one is displaced toward the general position 96h forming shorter Li(1)-Li(2) jump distances. The degradation in air, wet air, water, and acetic acid leads to a Li+/H+ exchange that preferably takes place at the 24d site, which is in contrast to previous reports. Higher Li+/H+ was observed for LLZTO aged in water at 363 K that reduced the symmetry to SG I4̅3d from SG Ia3̅d. This symmetry reduction was found to be related to the site occupation behavior of Li at the tetrahedral 12a site in SG I4̅3d. Moreover, Li+ is exchanged by H+ preferably at the 48e site (equivalent to 96h site). We also found that the equilibrium H+ concentrations in all media tested remains very similar, which is related to the H+ diffusion in the LLZTO-controlled exchange process. Only the increase in temperature led to a significant increase in the exchange capacity as well as in the Li+/H+ exchange rate. Overall, we found that the exchange rate, exchange capacity, site occupation behavior of Li+ and H+, as well as the structural stability of LLZTO, strongly depend on the composition. These findings suggest that measurements on a single LLZTO variant sample do not lead to a general conclusion for all garnets to guide the field toward better materials. In contrast, each composition has to be analyzed exclusively to understand the interplay of composition, structure, and exchange kinetic properties.

Highly Conductive Garnet-Type Electrolytes: Access to Li6.5La3Zr1.5Ta0.5O12 Prepared by Molten Salt and Solid-State Methods.

Tantalum-doped garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTO) is a promising candidate to act as a solid electrolyte in all-solid-state batteries owing to both its high Li+ conductivity and its relatively high robustness against the Li metal. Synthesizing LLZTO using conventional solid-state reaction (SSR) requires, however, high calcination temperature (>1000 °C) and long milling steps, thereby increasing the processing time. Here, we report on a facile synthesis route to prepare LLZTO using a molten salt method (MSS) at lower reaction temperatures and shorter durations (900 °C, 5 h). Additionally, a thorough analysis on the properties, i.e., morphology, phase purity, and particle size distribution of the LLZTO powders, is presented. LLZTO pellets, either prepared by the MSS or the SSR method, that were sintered in a Pt crucible showed Li+ ion conductivities of up to 0.6 and 0.5 mS cm-1, respectively. The corresponding activation energy values are 0.37 and 0.38 eV, respectively. The relative densities of the samples reached values of approximately 96%. For comparison, LLZTO pellets sintered in alumina crucibles or with γ-Al2O3 as sintering aid revealed lower ionic conductivities and relative densities with abnormal grain growth. We attribute these observations to the formation of Al-rich phases near the grain boundary regions and to a lower Li content in the final garnet phase. The MSS method seems to be a highly attractive and an alternative synthetic approach to SSR route for the preparation of highly conducting LLZTO-type ceramics.