The current production of lithium-ion batteries lacks rapid online non-destructive testing methods for observing the quality of degassing and electrolyte injection. This study presents an ultrasonic phased array C-scan imaging method, combined with linear straight scanning, for rapid detection of gases and electrolyte infiltration within batteries. This method can visualize the location of gases and the distribution of electrolyte infiltration. We have developed a system for detecting internal defects in lithium-ion batteries based on ultrasonic phased-array C-scan imaging technology. The finite element simulations are performed to validate the feasibility of the proposed imaging method. Finally, the different sizes of simulated gases and electrolyte infiltration in battery samples are visualized and characterized by developed system. Results confirmed its effectiveness in detecting defects. This study promotes and expands the application of this technology in online detection of lithium-ion batteries.