Ionic Transportation Characteristics in All Solid-State Battery Electrode with Sulfide and Halide Solid Electrolytes

Abstract

Bulk-type all solid-state lithium-ion battery electrodes are manufactured by compressing composite powder of active material and solid electrolyte powder. This compression increases contact area between the powder particles and reduces ion transportation resistance in the electrode. In many cases, mechanical characteristics of the solid electrolyte and active material are different, and complex stress distribution is generated in the compressed composite powder. In authors previous studies, ion transportation characteristics of electrode with soft active material and hard active material are studied experimentally and numerically. The electrode with soft active material has high ionic conductivity due to high stress is applied to the solid electrolyte in the electrode and high contact area between solid electrolyte powder is achieved. As showed from this study, Young’s modulus of active material is an important factor of ionic transportation in electrode of all solid-state battery. Based on this result, solid electrolyte the Young's modulus of the solid electrolyte is also an important factor of ionic transportation in electrode. Therefore, in this study, we conducted experiments and numerical simulation of ionic conductivity of the electrode with changing the Young’s modulus of the solid electrolyte to elucidate the influence of Young’s modulus of the solid electrolyte on ionic transportation characteristics of the electrode.Two solid electrolytes are employed in this study. One is sulfide solid electrolyte of Li5.5PS4.5Br1.5. This solid electrolyte is made by mechanical milling with vibration mill and heat treatment. The other is halide solid electrolyte of Li3InCl6. This halide solid electrolyte is made with wet process from LiCl and InCl3. These solid electrolytes are mixed with dummy active material of ZrO2 and nylon. The dummy active materials used to simulate only ionic transportation by a solid electrolyte and mechanical conditions. Young’s modulus of ZrO2 is about 200 GPa and is almost same as Young’s modulus of hard active materials such as LCO and NCM. Young’s modulus of nylon is about 6 GPa and is almost same as Young’s modulus of soft active materials such as graphite. From these two solid electrolytes and two dummy active materials, four dummy electrodes.Fig.1 illustrates effective Young’s modulus of Li5.5PS4.5Br1.5 powder and Li3InCl6 powder measured with changing the applied pressure from 0MPa to 100MPa. From this figure, the Young’s modulus of Li5.5PS4.5Br1.5 is from 100MPa to 700MPa and that of Li3InCl6 is about half of Li5.5PS4.5Br1.5. Therefore, it can be said that, compared to sulfide solid electrolyte, halide solid electrolyte is relatively soft solid electrolyte.Fig.2 illustrates relative ionic conductivity as a function of the volumetric fraction of dummy active material. Here, the ionic conductivity is normalized by the ionic conductivity of pure solid electrolyte (= 0% volumetric fraction of dummy active material). As shown in fig.2 (a), the relative ionic conductivity of sulfide solid electrolyte and halide solid electrolyte are almost same when the dummy active material is soft (nylon). However, in the case with hard dummy active material (ZrO2), as shown in fig.2 (b), the relative ionic conductivity with halide solid electrolyte is lower than that with sulfide solid electrolyte at high dummy active material volumetric fraction. With hard active material, mechanical pillar is generated by active material in the electrode and strain of the solid electrolyte is low. In the case with sulfide solid electrolyte, relatively high stress can be generated with the low strain, but in the case with halide solid electrolyte, stress is relatively low with the low strain. This is because, as shown in fig.1, Young’s modulus of sulfide solid electrolyte is higher than that of the halide solid electrolyte, and high stress for high ionic conductivity can be generated even with low strain in the case with sulfide solid electrolyteThis Study is supported by JKA foundation (2021M-188) Figure 1