Operando monitoring Lithium-ion battery temperature via implanting femtosecond-laser-inscribed optical fiber sensors

•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.