Journal Article triboelectric nanogenerator triboelectric nanogenerators air breakdown charge density charge excitation charge hopping charge lock‐free strategy charge loss mechanisms charge self‐injection checkered board durability electricity supply energy density energy harvesting friction force and heat suppression high‐output performance interfacial electrical energy extraction liquid suspension non-contact output improvement quasi‐dipole potential relative permittivity secondary self‐charge excitation self-polarization self‐layered method solid-liquid triboelectrification stacked design triboelectric nanogenerators (TENGs) triboelectric series