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• Realizing excellent fusion of optical fiber and lithium-ion battery. • Implementation of internal temperature monitor by implanting FBG. • FBG sensor displays better signal-to-noise ratio than thermocouple. • The internal temperature is 3.83 °C higher than the external temperature at 2C. • Revealing relationship between charge/discharge rates and temperature gradient. Current cell performance monitoring, which relies on measurements of sporadic surface temperature through the battery management system (BMS), does not provide a reliable information on the true internal battery state. Here we proposed and demonstrated in-operation temperature monitoring of lithium-ion batteries using an implanted femtosecond-laser-inscribed fiber Bragg grating (FBG) sensor. For comparison, both FBG and thermocouple are simultaneously implanted in an 18650 cell. Over repeat cycles with different conditions (0.5C, 1C and 2C), we experimentally demonstrated that implanted FBG and thermocouple sensors show very similar temperature response curves but the FBG sensor provide a much better signal-to-noise ratio. The monitored internal temperature by FBG is 3.71 °C higher than the external temperature during 2C cycling, demonstrating the necessity to record internal temperature especially at long cycles and higher rates within BMS. This operando FBG sensor is a powerful tool in current battery monitoring and its state-of-health evolutions. [ABSTRACT FROM AUTHOR]