*Article* **Flexible Lead-Free Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 Dielectric Film Capacitor with High Energy Storage Performance**

**Wenwen Wang 1,†, Jin Qian 1,†, Chaohui Geng 1, Mengjia Fan 1, Changhong Yang 1,\*, Lingchao Lu <sup>1</sup> and Zhenxiang Cheng <sup>2</sup>**


University of Wollongong, Innovation Campus, North Wollongong, NSW 2500, Australia; cheng@uow.edu.au **\*** Correspondence: mse\_yangch@ujn.edu.cn

† These authors contributed equally to this work.

**Abstract:** Ferroelectric thin film capacitors have triggered great interest in pulsed power systems because of their high-power density and ultrafast charge–discharge speed, but less attention has been paid to the realization of flexible capacitors for wearable electronics and power systems. In this work, a flexible Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 thin film capacitor is synthesized on mica substrate. It possesses an energy storage density of *W*rec ~ 62 J cm<sup>−</sup>3, combined with an efficiency of *η* ~ 74% due to the moderate breakdown strength (3000 kV cm−1) and the strong relaxor behavior. The energy storage performances for the film capacitor are also very stable over a broad temperature range (−50–200 ◦C) and frequency range (500 Hz–20 kHz). Moreover, the *W*rec and *η* are stabilized after 10<sup>8</sup> fatigue cycles. Additionally, the superior energy storage capability can be well maintained under a small bending radius (*r* = 2 mm), or after 10<sup>4</sup> mechanical bending cycles. These results reveal that the Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 film capacitors in this work have great potential for use in flexible microenergy storage systems.

**Keywords:** flexible; film capacitor; Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3; energy storage properties
