With the rapid development of the Internet of Things (IoT), piezoelectric energy harvesting has emerged as a highly promising power solution for autonomous IoT devices. To increase the extracted energy from the harvesters, various rectifiers have been developed. Bias-flip rectifiers, one of the most widely used rectifiers, utilize extra components to periodically flip the voltage across the harvester to achieve higher output power. This work proposes a bias-flip rectifier with an energy investment technique to boost the total harvesting power. By optimizing the energy invested from the load to the harvester and the loading conditions, the circuit can achieve higher FoM compared to conventional SSHI rectifiers under the same configurations. The proposed circuit was designed in a 180-nm BCD process, the simulation results show a 5.97 X energy enhancement compared to a full bridge rectifier (FBR) and a 1.2 X enhancement compared to conventional synchronized switch harvesting on inductor (SSHI) rectifiers.