**2.3 Single-electrode mode**

As displayed in **Figure 1c**, the single-electrode mode TENG has only one bottom electrode connected to the ground [25, 26]. After contact with the top material, the two surfaces will get charged owing to the triboelectric effect. During the process of an approaching and departing of top material, the local electrical field distribution caused by charged surfaces will change. Then, there will be potential difference change between the bottom electrode and the ground, and electrons exchange between them to maintain the potential change.

## **2.4 Freestanding triboelectric-layer mode**

As for the freestanding triboelectric-layer mode, it is the only one that the motion part is a dielectric layer [10], as shown in **Figure 1d**. The dielectric layer and two electrodes are in the same order, and the gap distance between the two symmetric electrodes should much smaller than the size of dielectric layer. At the original position, the state of dielectric layer and electrode is the same as what is in the lateral-sliding mode. The dielectric layer and electrode will get oppositely charged, respectively, once the motion occurs as before mentioned. When the dielectric layer is sliding forward and backward, there will be a potential difference between the two electrodes due to the change of overlapped area, which drives the electron exchanges between them.
