**4. Conclusions**

In this work, a high *W*rec of ~62 J cm−<sup>3</sup> and an *η* of ~74% are achieved in the BST/0.4BFO-0.6STO film. The film shows superior thermal stability (from −50 to 200 ◦C), frequency reliability (from 500 Hz to 20 kHz) and fatigue endurance (108 cycles). Most importantly, prominent mechanical stability can also be obtained. The energy storage behaviors show no obvious deterioration after undergoing different bending radii (from 12 to 2 mm), and even after 10<sup>4</sup> bending cycles. All of these outstanding performances demonstrate that the designed flexible BST/0.4BFO-0.6STO thin film is expected to pave the way for its application in flexible energy storage electronic devices.

**Author Contributions:** Conceptualization, C.Y., L.L. and Z.C.; methodology, C.Y.; investigation, W.W., J.Q. and C.G.; resources, C.Y.; data curation, W.W., J.Q. and M.F.; writing—original draft preparation, W.W. and J.Q.; writing—review and editing, C.Y. and Z.C.; supervision, C.Y., L.L. and Z.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Grant No. 51972144), Shandong Provincial Natural Science Foundation (Grant No. ZR2020KA003), the Project of "20 Items of University" of Jinan (Grant No. 2020GXRC051) and the Australian Research Council (DP190100150).

**Conflicts of Interest:** The authors declare no conflict of interest.
