In order to investigate whether and

In order to investigate whether and how well LLZO can be densified through the dissolution-precipitation mechanism, CSP experiments were performed. As shown in Fig. 1a, when deionized water was used to wet the powder, the combination of appropriate pressure and temperature immediately led to increased densities. Under 350 MPa, a low CSP temperature of 150 °C can produce a relative density of 81.5%, while the dry pressed pellet without any densification only showed a density of 76.6%. With increasing temperature, the density continuously improved and reached 83.6% at 350 °C. Although apparatus of CSP [25], further densification was still possible by adjusting other parameters. In addition to temperature, the CSP efficiency was known to depend heavily on the aqueous solution; the solution that can more effectively dissolve the particles would typically facilitate the dissolution-precipitation process and lead to higher density [23, 28]. Therefore, we replaced water with nitric acids of different concentrations during CSP. Indeed, further improvement was observed, and stronger acidity appeared to yield better densification. As shown in Fig. 1b, when the HNO3 concentration changed from 0 (pure water) to 2 M, the density increased from 83.6% to 87.7%. Besides CSP temperature and acidity of aqueous solutions, calcination temperature of the LLZO powder was also found to influence the density. In many cases, lower calcination temperature would yield smaller particle sizes [7, 29], and smaller particles sizes were often reported to result in higher density after CSP [23, 24, 26]. Nevertheless, the trend for LLZO was quite the opposite . Fig. 1c shows the variation of densities with calcination temperatures. The powder calcined at 1200 °C exhibited the highest density after CSP, while decreasing the calcination temperature only led to lower densities. Such an unexpected behavior entailed that the CSP mechanism of LLZO could be different from most reported ceramics.