To investigate the tolerance of the Li plating–stripping cycles of the Li+/Na+ mixed solid electrolyte, Li/Li symmetric cells were fabricated using NaI-NaBH4-LiI solid solutions, and the critical current densities (CCDs) at which the cell underwent a short circuit were determined. After short-circuiting, the microstructure of the solid electrolyte fracture was observed using scanning electron microscopy (SEM), and the Li precipitation morphologies were investigated. For comparison, the same set of measurements was performed for the cells with LiI and 2LiI·LiBH4. The CCDs for 9(15NaI·NaBH4)·LiI and 7(15NaI·NaBH4)·LiI were 0.38 and 0.51 mA/cm2, respectively. The CCD of the 2LiI·LiBH4 cell was 0.64 mA/cm2, which was better than that of the Li+/Na+ mixed electrolyte. However, the LiI cell showed poor tolerance to Li plating–stripping cycles; the cell underwent a short circuit at 0.09 mA/cm2. In the SEM images, Li convex outshoots were commonly observed for the three electrolytes, indicating Li penetration through the pellets. For the fracture of 2LiI·LiBH4, B-rich regions were preferentially observed at the edges of the steps. These regions are speculated to be due to the reduction of 2LiI·LiBH4 (LiBH4 + 4Li+ + 4e− → 4LiH + LiB). The decomposition products may block the further propagation of Li, resulting in a better tolerance of the BH4−-included electrolyte. The in-plane propagation of Li was a characteristic for NaI-NaBH4-LiI and was not observed for LiI and 2LiI·LiBH4 under the same current conditions. The Li propagation along the lateral side of the 9(15NaI·NaBH4)·LiI pellet was confirmed via in situ observations using optical microscopy. The present results indicate that the penetration of Li through the electrolyte as well as the in-plane precipitation and lateral propagation of Li should be considered and avoided for the practical application of the NaI-NaBH4-LiI electrolyte.