The cause of the thermal runaway problem in lithium-ion batteries problem is still unclear. This bottle neck has prevented increases in the energy density of lithium-ion batteries, of which the technology may stagnate for many years. The diversity of cell chemistries makes this problem more difficult to analyze. To solve this hard problem, Tsinghua University and its collaborators have established a thermal analysis database, correlated work is published in Applied Energy, 2019, 246: 53-64. The database contains comparable data for different kinds of cells using accelerating rate calorimetry and differential scanning calorimetry. Three characteristic temperatures are summarized based on the common features of the cells in the database. In attempting to explain the mechanisms that are responsible for the characteristic temperature phenomena, we have gained new insight into the thermal runaway mechanisms of lithium-ion batteries. The results of specially designed tests show that the major heat source during thermal runaway for cells with Li(NixCoyMnz)O2 cathode and carbon-based anode is the redox reaction between the cathode and anode at high temperature. In contrast to what is commonly thought, internal short circuits are responsible for very little of the total heat generated during thermal runaway, although they contribute to triggering the redox reactions after the separator collapses. The characteristic temperatures provide comparable parameters that are useful in judging the safety of a newly designed battery cell. Moreover, the novel interpretation of the thermal runaway mechanism provide guidance for the safety modelling and design of lithium-ion batteries.