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Lead-free perovskite CaCu 3 Ti 4 O 12 (CCTO) dielectrics are extremely important candidates for capacitor−varistor dual-function materials. However, their overall success in applications is somewhat controlled by the longstanding issues such as relatively large dielectric loss and insufficiently high electric breakdown field. Herein, we report the success in the preparation of an optimized lead-free (1−x)CaCu 3 Ti 4 O 12 −xSrTiO 3 (CCTO− STO) composite system with improved dielectric and nonlinear properties via interface engineering. Interestingly, looking closer at the grain boundaries using transmission electron microscopy, it is found that an obvious interface region with a transition layer of a wrinkled structure is formed between the CCTO matrix phase and STO dopant phase. Significantly, all the composite ceramic samples present high permittivity in the order of about 10 3 to 10 4 , and the 0.9CCTO−0.1STO composite ceramic sample exhibits a lower dielectric loss of about 0.068 at room temperature and at 1 kHz. Excitingly, the optimized 0.9CCTO−0.1STO composite ceramic sample also exhibits a remarkably elevated breakdown field strength of about 14.03 kV/cm and a large nonlinear coefficient of about 16.11. The improvement in nonlinear properties with a high breakdown field strength and large nonlinear coefficient could be attributed to the interfacial effect in the composite structure, originating from the formation of the transition layer with a wrinkle structure at the interface between CCTO and STO phases. Such effects can result in great electrical heterogeneity caused by the higher resistance of the grain boundary and the enhanced potential barrier at the interface region. The new insights on the formation of the interfacial wrinkle structure near the phase boundaries of the CCTO−STO composite system and their effects on improvement of electrical properties can stimulate future research on lead-free CCTO−STO-based systems toward capacitor−varistor dual-function applications and may offer an effective way to design other lead-free dielectric materials as well.

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