Fig. 1

History of battery sensing[12-21]"

Fig. 2

Schematic diagram of acoustic emission (a) and ultrasonic detection (b)"

Table 1

Comparison of acoustic emission and ultrasonic detection"

Fig. 3

Sensing principle of acoustic emission and typical waveform signals: (a) Acoustic emission technology sensing device and the obtained acoustic signal[22, 24]; (b)—(d) Typical waveforms and power spectra of acoustic signals[22][These waveform signals can represent particle cracking (b) and gas formation (c) during cycling]"

Fig. 4

Ultrasonic detection technology is employed to explore the state of batteries: (a) The distribution of the electrolyte and the gas production of the battery were imaged by the different transmittance of ultrasonic signal in the battery[19]; (b) The time of flight and the amplitude of acoustic signals are used as the indicators of the state of batteries[30]; (c) The evolution of acoustic wave signals during charge[31]; (d) Schematic illustration of the feedforward neural network[31]"

Fig. 5

Optical sensing methods: (a) There is no direct interaction between light and the environment (such as fiber Bragg grating, Fabry-Perot sensor and so on); (b) Light interacts directly with the environment (such as titled fiber Bragg grating, fiber-optics evanescent wave sensor, long period fiber grating and so on)"

Fig. 6

Optical sensing methods: there is no direct interaction between light and the environment: (a) Schematic of three commonly used optical fiber, including fiber Bragg grating (left) and Fabry-Perot sensor (middle) based on the principle of reflection and scattering fiber based on scattering (right); (b) Place an optical fiber sensor inside a 18650 battery or pouch cell to monitor the internal temperature of the battery[21, 32]; (c) Temperature distribution map of a 18650 battery during cycling[33]; (d) Heat variation during battery formation process related to electrochemical reactions[21]; (e) Implanting a optical fiber sensor into or placing it on the surface of the electrode to monitor the evolution of stress during cycling; Stress evolution of graphite anode[35] (f) and InLi anode[38] (g) during cycling"

Fig. 7

Optical sensing methods: light interacts directly with the environment: (a) Schematic of three commonly used optical fiber, including long period fiber grating (left), titled fiber Bragg grating (middle) and fiber-optics evanescent wave sensor (right); (b) Place an optical fiber sensor on the surface of graphite anode in Swagelok cell to monitor its spectroscopy[41]; (c) Reflectance spectroscopy measurements of graphite anode[41]; (d) Placing a fiber optic sensor inside the battery and in contact with the electrolyte to monitor the working state of the electrolyte[44, 46, 47]; (e) Stable and reproducible correlation between the voltage and the optical power level of the implanted fiber sensor during cycling[46]; (f) Temperature and infrared absorption spectrum of electrolyte during the charging process of a 18650 battery[44]"