EQCM (Electrochemical Quartz Crystal Microbalance) analysis is an in-situ monitoring of the mass change of the lithium iron phosphate battery electrode during the electrochemical reaction process. The principle of this method is that the accumulated mass changes proportionally with the resonant frequency [13, 14]. The following is a diagram of the EQCM device.

Using EQCM data to obtain the equivalent of adsorbed material, we can estimate the type of material. The redox reaction on the electrode will lead to material dissolution, electrolyte decomposition and surface facial mask formation. Because the quality of electrode surface will change during these reactions, EQCM is a suitable analytical method.

The piezoelectric quartz crystal used in EQCM will induce elastic strain and shear strain by changing the dipole moment when subjected to mechanical pressure or tension. The perturbation potential generates vibrations parallel to the surface. The transverse sound generated by these vibrations can penetrate the crystal in the thickness direction. Wavelength （λ） It is obtained from equation (1.1).

A new facial mask with the thickness of TR is formed on the crystal by electrochemical reaction. Wavelength is shown in Figure 2.

The vibration frequency depends on the wavelength and is directly related to the quality change of the surface, so the formula (2.1), namely Sauerbrey equation.

Where f is the vibration frequency of the crystal; F0 is the resonant frequency;Is a change in quality; A is the piezoelectric active area; ρ Q is the density of quartz; μ Q is the shear modulus;Becauseit can be expressed as surface density

The above equation can be simplified into equation (2.2).

Where C is the quartz crystal constant [15]. The mass unit equivalent (mpe) is the mass per mole of electron transfer, as shown in Formula (2.3).

Using Eq. (2.2) and Eq. (2.3), we can get the instantaneous mpe as shown in Eq. (2.4).

Where W ‘is the instantaneous molar mass; Z is the atomic number; F is Faraday constant; Is a change in quality;

Is the number of charges transferred to the electrode；I is the current through the given voltage range；

Is the ratio of mass to voltage change;

Is the scanning rate of cyclic voltammetry [16].

If the charge transfer of the electrochemical reaction is caused by the increase of the electrode mass, the mpe value becomes the mass of the material adsorbed on the electrode surface. By deriving mpe from the function of E or t, we can determine the products and properties of each stage of the electrochemical reaction.

This analytical method has been widely used in the study of electrode electrolyte interaction in lithium batteries. For example, the research includes the mass change during the reaction of lithium negative electrode and electrolyte to form a surface facial mask [15].

The corrosion of aluminum collector was also studied. The corrosion reaction will cause the electrode material to be separated from the electrolyte or the reaction product to be adsorbed on the electrode surface. EQCM can be used to study these mass changes [16].

As shown in Figure. 3, EQCM is used to detect the change of mpe of LiMn2O4 electrode plate with time. It can be seen that some Mn will dissolve out during the cycle. At the same time, OCV open circuit voltage detection can identify different chemical reactions. The dissolution of MnO2 will cause the mass reduction [17].