*2.3. Preparation of BaTiO3*/*Polymer*/*Al (BPA) Films*

To determine the possibility of the as-obtained BaTiO3 nanocrystals to form a uniform film for capacitor energy-storage application, we chose sample S8 (in Table 1) as an example to prepare BaTiO3/polymer/Al films (BPA films, Figure 2) using the similar method reported in our previous work [25]. Typically, the BaTiO3 nanocrystals (S8) were mixed with a silicon-containing heat-resistant resin (CYN-01), and then some silane coupling agent (KH550) was added into the above mixture. Dimethylacetamide (DMAc, Guangzhou Jinhuada Chemical Reagent Co., Ltd., Guangzhou, China)) was used as the solvent. The mass ratio of *M*BaTiO3:*M*DMAc:*M*Polymer:*M*KH550 was kept at 100:45:25:4. The as-prepared mixture was ultrasonically treated for 30 min for a uniform slurry. The above slurry was coated on an Al foil by a bar coater (T-300CA) and a coating rod (D10-OSP010-L0400) from Shijiazhuang Ospchina Machinery Technology Co., Ltd (Shijiazhuang, China)). The as-formed films were then dried in an oven at 220 ◦C for 10 min and finally used for the test of dielectric properties.

**Figure 2.** A schematic diagram of the BaTiO3/polymer/Al film for capacitor cells.
