Since several decades, many efforts have been made all over the world to achieve the highly efficient conversion from low-grade thermal energy into electrical energy and the conversion efficiency is usually described by Thermoelectric Figure of Merit, i.e., ZT coefficient. However, the highest ZT coefficient have not reached 4 in spite of worldwide scientific researches for many years. In this work, a method for reaching an unprecedented ZT coefficient was proposed by utilizing phase-transition induced sudden change of electric potential for the oxidation-reduction couple at an electrode material in a thermogalvanic cell (TGCs). It was found that such TGCs could exhibit ZT coefficient of the order 100 and even higher, and this new type of thermogalvanic cell was named phase-transition thermogalvanic cell (PT-TGCs). By optimizing the experimental design, the PT-TGCs may convert waste heat into electrical power more effectively and would be applied in various areas in the future.