Poor contact between the lithium metal and oxide SE is an issue to realize ASSLiMB with oxide SE. This poor contact causes high overvoltage at the SE-Li interface, and lithium dendrite grows into the SE, and it reaches to the cathode, then causing a short circuit at high-speed charging that exceeds critical current density (CCD). Several methods (e.g., thin-film insertion method, interface structure modification methods) are proposed to suppress the interface resistance and enhance the CCD; however, more low interface resistance and high CCD is required to achieve high-speed charging by ASSLiMB with oxide SE.

In this study, we propose a thin surface structure control method by shot peening (S.P.) to suppress the interface resistance and increase the CCD of ASSLiMB with oxide SE. S.P. is a surface treatment method by blowing abrasive grains with high-pressure air. S.P. has the effect of imparting an uneven surface and compressive residual stresses to the surface. The uneven shape is expected to increase the interfacial stress and improve the contact property as the SE bites into the Li. The compressive residual stresses suppress crack initiation on the SE surface. It has been reported that Li dendrites grow with cracks, and suppression of cracks is expected to improve CCD . The S.P. has above two effects that can improve the lithium metal anode performance, so, we investigated the influence of the S.P. on the lithium metal anode performance. We also investigated the influence of the grain size on the electrode performance because the size of the surface roughness and the compressive residual stresses can be changed by the change of the grain size used in S.P.

Pellets of oxide SE (Li6.25Ga0.25La3Zr2O12) synthesized by the solid phase reaction method were processed by S.P. with abrasive grain of Al2O3 powder. After the S.P., the surface of the pellet is coated with gold by gold sputtering. Three experiments with three abrasive gain sizes (50 µm,100 µm,300 µm) and the SE without S.P. were carried out to check the influence of the S.P.. Li metal was attached to both sides of the SE layer to construct a symmetric cell, and CCD measurements and electrochemical impedance spectroscopy measurements were performed. Surface characteristics were measured by surface three-dimensional scanning electron microscopy (surface 3DSEM) and indentation.

Fig. 1 shows the CCD and interfaces resistance without S.P. and S.P. using each abrasive grain. As shown in the figure, higher CCD and low interface resistance are achieved for the samples with S.P. with 50 µm, and 100 µm grain size than non S.P. processed samples. On the other hand, the CCD of S.P. with 300 µm grain is almost the same as the non S.P. processed samples and the interface resistance is high. These results indicate that the S.P. can improve electrode performance, and there is an optimum S.P. grain size. From indentation, the fracture toughness of S.P. processed samples were higher than a sample without S.P., and the fracture toughness increased with increasing in grain diameter. This increase in fracture toughness may have suppressed the growth of Li dendrites into the SE, leading to an increase in CCD. The surface 3DSEM showed that the S.P. creates an uneven pattern on the SE pellet and the structure size is several µm. The structure size increases with the increase in the abrasive grain size. This unevenness improves the contact between SE and Li and suppresses the interface resistance. This interface resistance suppression depends on the structure size, and the lowest interface resistance is achieved for the S.P. with the smallest grain size of 50 µm. As a result of the balance between high compressive residual stress by small size grain and the low interface resistance by large size grain, the highest CCD is obtained by middle size grain of 100 µm.

This study shows that S.P. can decrease the interfacial resistance of ASSLiMB and improve CCD. The unevenness caused by S.P. is on the order of several micrometers, and it can be said that S.P. can be applied to separators thinner than 100 micrometers.