Structural and Computational Assessment of the Influence of Wet-Chemical Post-Processing of the Al-Substituted Cubic Li₇La₃Zr₂O₁₂

Abstract

Li7La3Zr2O12 (LLZO) and related compounds are considered as promising candidates for future all-solid-state Li-ion battery applications. Still, the processing of those materials into thin membranes with the right stoichiometry and crystal structure is difficult and laborious. The sensitivity of the Li-ion conductive garnets against moisture and the associated Li+/H+ cation exchange makes their processing even more difficult. Formulation of suitable polymer/ceramic hybrid solid state electrolytes could be a prosperous way to reach the future large scale production of solid state Li-ion batteries. In fact, solvent mediated and/or slurry based wet-processing of the LLZO, e.g., tape-casting, could result in irreversible Li-ion loss of the pristine material due to Li+/H+ cation exchange. The concomitant structural changes and loss in functionality in terms of Li-ion conductivity are the results of the above process. Therefore, in the present work a systematic study on the chemical stability and structural retention of Al-substituted LLZO in different solvents is reported. It was found that Li+/H+ exchange in LLZO occurs upon solvent immersion, and its magnitude is dependent on the availability of −OH functional groups of the solvent molecules. As a result, a larger degree of Li+/H+ exchange causes higher increase of the lattice parameter of the LLZO, determined by synchrotron diffraction analyses. The expansion of the cubic unit cell was ascertained, when Li+ was replaced by H+ in the host lattice, by ab initio computational studies. The application of the most common solvent as dispersion medium, i.e., high purity water, causes the most significant Li+/H+ exchange and, therefore, structural change, while acetonitrile was proven to be the best suitable solvent for wet postprocessing of LLZO. Finally, computational calculations suggested that the Li+/H+ exchange could result in diminished ionic, i.e., mixed Li+–H+, conductivity due to the insertion of protons with lower mobility than that of Li-ions.

Controlling the lithium proton exchange of LLZO to enable reproducible processing and performance optimization

“…All starting powders (milled (blue), air0 (green) and argon0 (red)) showing shear-thinning behaviour. When getting into contact with the solvent during preparation of the slurry, fast protonation of the surface is expected, as LiOH is formed and consecutively dissolved in the surrounding solvent as proposed by R. Kun et al 20 In our work, the effect of the excess LiOH and Li 2 CO 3 on the particle surface (as determined by Raman and XPS) on the protonation in the solvent and the resulting rheological behaviour needs to be considered. On the one hand, the excess LiOH dissolves very well in ethanol, on the other hand, Li 2 CO 3 has a very low solubility, which means it will remain in its original state and place upon contact with the solvent.…”

“…This process takes place when LLZO is exposed to the humidity in air [16][17][18][19] and to all solvents commonly used for wet-processing of ceramic components. 20 Although this Li + /H +exchange on the LLZO surface during storage in air (or any atmosphere with traces of humidity) or exposure to solvents is practically unavoidable, little is known about the inuence of this exchange on the sintering behaviour and processability of LLZO powders via wet-processing routes and on the resulting component properties.…”

Polyethylene oxide‐Li6.5La3Zr1.5Ta0.5O12 hybrid electrolytes: Lithium salt concentration and biopolymer blending

“…Li [22] Hybrid as well as polymer electrolyte films were prepared by solution casting method in a glovebox under argon atmosphere (H 2 O < 0.1 ppm; O 2 < 0.1 ppm). All components were dried at 60 • C under vacuum, and anhydrous solvents were stored over molecular sieve 3 Å before use.…”

“…A water free processing was necessary for the biopolymer hybrid electrolytes, as LLZO reacts with water. [22] Since typical chitosan salts such as chitosan chlorides or chitosan acetate are almost exclusively soluble in aqueous solutions, chitosan mesylate salt was prepared to achieve solubility in dimethyl sulfoxide. [24][25] For the preparation of chitosan mesylate, 1 g chitosan ("Chitosan 134″ prepared from squid pen β-chitin by four sequential heterogeneous alkaline deacetylation steps, Mahtani Chitosan, Gujarat, India), which we determined to have a fraction of acetylation F A below 0.01, a weight average Mw of 242 000 g/mol, and a Mw dispersity Đ of 1.4 using state-of-the-art analytical methods ( 1 H-NMR for F A [26][27] ; HPSEC-RID-MALLS for Mw and Đ [28] ), was dissolved in 250 mL water and 250 μL methanesulfonic acid.…”

“…As solvent for remilling, dry IPA was chosen, as it shows low Li + /H + exchange in LLZO materials. [ 22 ]…”