Operando monitoring of state of health for lithium battery via fiber optic ultrasound imaging system

With the rapid development of lithium batteries, it’s of great significance to ensure the safe use of it. An ultrasound imaging system based on fiber optic ultrasound sensor has been developed to monitor the internal changes of lithium batteries. Based on Fabry-Perot interferometer (FPI) structure which is made of a glass plate and an optical fiber pigtail, the ultrasound imaging system possesses a high sensitivity of 558 mV/kPa at 500 kHz with the noise equivalent pressure (NEP) of only 63.5 mPa. For the frequency response, the ultrasound sensitivity is higher than 13.1 mV/kPa within the frequency range from 50 kHz to 1 MHz. Meanwhile, the battery imaging system based on the proposed sensor has a superior resolution as high as 0.5 mm. The performance of battery safety monitoring is verified, in which three commercial lithium-ion ferrous phosphate/graphite (LFP||Gr) batteries are imaged and the state of health (SOH) for different batteries is obtained. Besides, the wetting process of an anode-free lithium metal batteries (AFLMB) is clearly observed via the proposed system, in which the formation process of the pouch cell is analyzed and the gas-related "unwetting" condition is discovered, representing a significant advancement in battery health monitoring field. In the future, the commercial usage can be realized when sensor array and artificial intelligence technology are adopted.