Volumetric energy density is a universal and the most important standardized figure-of-merit to quantify energy technologies, regardless of the operating mechanism, form of energy, energy scale, etc. In terms of triboelectric nanogenerators (TENGs), considering their cycle-by-cycle operation mechanism, output energy density has been demonstrated as the key factor limiting the output performance. However, the state-of-the-art output energy density of TENGs in the ambient environment is only <104 J m−3, as limited by two crucial factors: charge density and breakdown threshold voltage. In this work, we proposed a comprehensive strategy including charge excitation, interface insulation, and material screening to boost the maximized output energy density of the TENGs. In particular, charge excitation was effectively realized by an external high-voltage (HV) source, while interface insulation was achieved through liquid oils to alleviate the electric field and suppress the breakdown effect. Moreover, a set of comprehensive parameters involving triboelectrification, dielectric properties, leakage properties, etc. are fully investigated for screening triboelectric materials. With that, we experimentally achieved a record-high charge density of 2.3 mC m−2 among SFT-TENGs and yielded the maximum output energy density of 4.3 × 105 J m−3, as a new record in the field. A theoretical maximum energy density of 1.1 × 106 J m−3 is predicted as simulated by the finite-element method. This work demonstrates the boosted capability of TENGs' maximized output energy density from both theoretical and experimental perspectives, which will be an important step towards designing high-performance TENGs for efficient energy harvesting with broad application scope.