Contact-electrification (CE) mechanism between insulators is the last puzzle to be solved between solid and solid, and it was ultimately proven to be dominated by electron transfer. The introduction of the energy band model incorporating surface states provides a possibility for the aforementioned electron transfer-dominated mechanism. In 2016, Wang's team made a groundbreaking discovery: the transfer of electrons occurs exclusively when the distance between the atoms of two substances is less than the standard bonding length, approximately 0.2 nm, which is within the range of repulsive forces. This finding laid the groundwork for the electron cloud model that was later proposed. By 2018, Wang's team, utilizing a newly engineered high-temperature resistant TENG, conducted research on charge transfer at elevated temperatures. Through these studies, they identified the dominant mechanisms of electron transfer and, in a ground-breaking move, proposed the innovative electron cloud model. This model can be broadened to encompass not only solid–solid interactions but also liquid–solid, liquid–gas, and even liquid–liquid scenarios, solving a scientific problem that has puzzled humankind for 2600 years. Interface electron transfer-transition induced photon emission was predicted by the electron cloud model and observed by Wang's team, achieving a unified explanation for the triboluminescence phenomenon previously discovered at the atomic and electronic levels. There are also self-discharging processes in the CE process between insulators, including thermal ion emission and photon excitation. Through researches on these two processes, the electron transfer-dominated CE mechanism was experimentally confirmed. In addition, there are some factors that also affect CE between insulators, including curvature effect, atomic electronegativity, material transfer and heterolytic bond, dipole polarization, and deep trapping. Through in-depth research on these influencing factors, CE processes can be further regulated, thereby enhancing the power generation and energy collection efficiency of the prepared TENG.