A solid electrolyte is an important component of all-solid-state batteries. It can not only be used as a lithium ionic conductor, substituting for a liquid organic electrolyte, but also can be used to block direct contact between the positive and negative electrodes, like a separator [11]. Solid electrolytes generally contain Li3N, LiPON, perovskite, LISICON, NASICON, garnet, etc. [12,13,14,15,16,17]. Some of these solid electrolytes have high ionic conductivity (~10−3 S·cm−1). However, some issues still exist, such as instability in an ambient atmosphere (Li10GeP2S12, LGPS) and the metal cation being easily reduced by lithium (such as Ti4+ in LixLa2/3 -x/3TiO3, LLTO) [18,19]. The cubic garnet LLZO was discovered by Murugan et al. [17] in 2007 and attracted world-wide attention for its advantages, e.g., the simple preparation process, high ionic conductivity (~10−3 S·cm−1) at room temperature, high electrochemical window (0~6 V vs. Li/Li+), and electrochemical stability of lithium metal. On the other hand, LLZO also has some defects, such as an unstable cubic phase and a low density of ceramics [20,21]. Moreover, a mass of LLZO mother powder is needed to compensate for lithium loss when sintering at high temperatures [21,22]. Many solutions have been adopted to solve the above issues. For example, Al, Ga, Fe, Ta, Nb, W, Y, and Sb doping were used to stabilize the cubic phase [23,24,25,26,27,28,29,30]; hot pressing sintering, plasma sintering, and microwave sintering were adopted to improve the relative density and sintering additives [31,32,33]; and Y2O3, Al2O3, B2O3, CaO, MgO, Li3PO4, and Li4SiO4 were investigated to reduce the grain-boundary resistance [34,35,36,37,38,39,40]. Usually, in order to evaluate the electrochemical performance, LLZO was used as solid electrolyte in all-solid-state batteries [41,42,43].