Superflexible and Lead-Free Piezoelectric Nanogenerator as a Highly Sensitive Self-Powered Sensor for Human Motion Monitoring

Highlights Continuous piezoelectric BaTi0.88Sn0.12O3 (BTS) films were deposited on glass fiber fabrics successfully. Superflexible, highly sensitive self-powered piezoelectric sensors were fabricated based on polarization-free BTS. Low curie temperature would be a benefit for flexible piezoelectric sensors because small alterations of force will trigger large changes in polarization. The superflexible sensors are highly desirable for wearable devices to detect human motion. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00649-9.


S2 Flexibility, Thickness and Weight of Samples
The flexibility of BTS-GFF/PVDF composite film is shown in Fig. S1. The film can be bent at will and can be restored to its original state under any folding. The thickness of the prepared sample was measured by spiral micrometer. The thickness of BTS-GFF/PVDF composite film is about 22 μm. After plating a layer of 2 cm × 3 cm silver electrode on both sides, the thickness increases to ~33 μm. Finally packaged with BOPP tape, the thickness of BTS-GFF/PVDF sensor is ~110 μm. The sensor is very light, with a weight of only 0.5002 g.   Table S1. The measuring range of the current testing software is 200 nA and 2 μA. When the external force is small (1N~4N), we choose 200 nA as the measuring range; When the external force increases (≥ 5N), the output current increases, so we choose 2 μA as the measuring range, the corresponding base noise becomes larger (Fig. S3a). There is obvious base noise in the current figures, so we calculate the limit of detection according to the current curve. The limit of detection is three times the height of base noise peak. When the measuring range is 200 nA, the average peak value of base noise is about ±0.0223 μA, and the limit of detection is ±0.066 μA. When the measuring range is 2 μA, the average peak value of base noise is ±0.044 μA, and the limit of detection is ±0.132 μA. A response time of 14 ms and a recovery time of 62 ms have been calculated from the enlarged current versus time curve under the force of 1 N, 0.5 Hz. We also illustrate the changes of polar nano regions in BTS under different force according to the working mechanism to explain that the sensitivity will become small when the force reaches to a critical value.  BaTi0.88Sn0.12O3 nano powders were prepared by sol-gel method. DSC test was reported to determine the characteristic temperatures of phase transition processes. As shown in Fig. S4, two obvious steps can be seen in the DSC curve, the ending of which is ~41.8 ℃, indicating the completement of ferroelectric-paraelectric phase transition, and could be regarded as Tc of BTS nano powders.

S5 Voltage Signal of GFF/PVDF Sensors
In order to prove that piezoelectric effect plays a dominant role in the progress of droplets fall and spread, we tested the output performance of GFF/PVDF sensors under the same experimental conditions. The result is presented in Fig. S5. There is almost no signal generated by the sensor and the output of BTS-GFF/PVDF sensor is mainly due to the piezoelectric effect of BTS.

S6 Dielectric Properties of Composite Films
The dielectric constant (εr) of BTS-GFF/PVDF composite (εr≈56.8) is twice that of GFF/PVDF (εr≈27.4). The increase of dielectric constant is beneficial to the increase of charge density, which will enhance the output of devices.

Supplementary Movies
Movie S1 Open-circuit voltage (Voc) generation under finger bending.
Movie S2 Open-circuit voltage (Voc) generation under balloon pressing.