Experimental Bayesian optimization for Li ion conductivity in NASICON-type LiZr₂(PO₄)₃.

Zeolite-geopolymer hybrid materials have been formed when kaolin was used as a starting material. Their characteristics are of interest because they can have a wide pore size distribution with micro- and meso-pores due to the zeolite and geopolymer, respectively. In this study, Zeolite-geopolymer hybrid bulk materials were fabricated using four kinds of kaolinitic clays (a halloysite and three kinds of kaolinite). The kaolinitic clays were first calcined at 700 °C for 3 h to transform into the amorphous aluminosilicate phases. Alkali-activation treatment of the metakaolin yielded bulk materials with different amounts and types of zeolite and different compressive strength. This study investigated the effects of the initial kaolinitic clays on the amount and types of zeolite in the resultant geopolymers as well as the strength of the bulk materials. The kaolinitic clays and their metakaolin were characterized by XRD analysis, chemical composition, crystallite size, ²⁹Si and ²⁷Al MAS NMR analysis, and specific surface area measurements. The correlation between the amount of zeolite formed and the compressive strength of the resultant hybrid bulk materials, previously reported by other researchers was not positively observed. In the studied systems, the effects of Si/Al and crystalline size were observed. When the atomic ratio of Si/Al in the starting kaolinitic clays increased, the compressive strength of the hybrid bulk materials increased. The crystallite size of the zeolite in the hybrid bulk materials increased with decreasing compressive strength of the hybrid bulk materials.

Materials with high formability and Li‐ion diffusivity are desired to realize safe bulk‐type all‐solid‐state Li‐ion batteries with high energy density. Spinel‐type Li₂FeCl₄, which is expected as a Li‐ion‐conductive electrode, adopts the high‐temperature cubic phase (space group: Fd‐3m) by a mechanochemical synthesis method. The powder‐compressed pellet shows a high relative density of 92% and large neck formation between the particles. An analysis of the distribution of relaxation times from the AC impedance results indicates that the contribution of the grain boundary impedance is almost negligible (≈3%) in the powder‐compressed pellet. Even the Li diffusion coefficient, which is underestimated by the Nernst–Einstein equation, is several orders of magnitude higher than those of conventional oxide and sulfide electrode materials. Analyzing the diffusion pathways using a classical force field calculation suggests that stabilizing the high‐temperature phase at room temperature delocalizes Li ions in the diffusion pathway, thus realizing high Li‐ion diffusivity. The Li₂FeCl₄ green compact containing 10 wt% carbon as an electron‐conductive additive shows a one‐electron charge reaction with a capacity of 126 mAh g⁻¹, with no large overpotential at a high operating voltage of 3.6 V versus Li/Li⁺ at 30 °C.

The optimal sintering conditions for LiZr₂(PO₄)₃, a NASICON-type solid electrolyte with high conductivity, were explored. It was also found that these optimum sintering conditions could be efficiently discovered by using Bayesian optimisation.

All-solid-state Li-ion batteries are expected to be the next generation of batteries with a high energy density and safety. However, for Li-ion batteries to endure high-voltage operations, the decomposition of solid electrolytes must be suppressed first. A high potential at the cathode tends to promote battery degradation because of the oxidation of the cathode electrolyte. This study aims to achieve the high-potential operation of all-solid-state batteries using LiAlCl₄ as a chloride electrolyte with a high oxidation resistance. However, batteries with commonly used oxide electrodes (e.g., LiFePO₄) exhibit low capacity (∼0.5 mA h g⁻¹), despite having working potentials less than the oxidation potential of LiAlCl₄. First-principles calculations and ²⁷Al MAS-NMR measurements suggest that acid–base reactions based on the hard and soft acid–base (HSAB) rule occur between the electrode and the electrolyte. In contrast, a high voltage of ∼3.65 V (vs. Li⁺/Li) and high-capacity utilisation (reversible capacity ∼100 mA h g⁻¹) are observed at room temperature by combining the same chloride electrode (Li₂FeCl₄) without side reactions between these chlorides. These results indicate that material design based on the HSAB rule is also instructive when considering electrode/electrolyte material combinations, which realizes a 3.5 V-class all-solid-state Li-ion battery.

Currently, NASICON-type LiZr₂(PO₄)₃ (LZP)-related materials are attracting attention as solid electrolytes. There are experimental reports that Li-ion conductivity can be improved by doping a small amount of Ca or Y into stoichiometric LZP. In previous studies, doping with only one element having a narrow search space has been attempted, and thus, further improvement of the Li-ion conductivity is conceivable by using multi-element doping. When multi-element doping is attempted, because the search space becomes enormous, it is necessary to evaluate the Li-ion conductivity using a low-cost method. Here, force-field molecular dynamics using a bond valence force field (BVFF) approach was performed to evaluate the Li-ion conductivity. We confirmed that the Li-ion conductivity of stoichiometric LZP derived from BVFF (6.2 × 10⁻⁶ S/cm) has good agreement with the first principle calculation result (5.0 × 10⁻⁶ S/cm). Our results suggest that the Li-ion conductivity can be further improved by simultaneously doping LZP with Ca and Y [6.1 × 10⁻⁵ S/cm, Li_35/32Ca_1/32Y_1/32Zr_31/16(PO₄)₃]. In addition, Bayesian optimization, which is an informatics approach, was performed using exhaustively computed conduction property datasets in order to validate efficient materials search. The averages for Bayesian optimization over 1000 trials show that the optimal composition can be found about seven times faster than by random search.