The main test method of polymer electrolyte

The test of polymer electrolyte is mainly divided into the basic performance test of the material and the electrochemical test. Among them, the basic performance tests of materials mainly include: scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy, differential scanning calorimetry, thermogravimetric analysis, mechanical tensile testing, etc.; electrochemical tests mainly include: cyclic voltammetry, electrochemical stability window, ion conductivity, ion migration number, rate charge and discharge, long cycle performance, etc. This part focuses on the electrochemical stability window, the determination of key parameters such as ion conductivity, ion mobility, and the application of solid-state nuclear magnetism.

1. Electrochemical stability window

The electrochemical stability window is the voltage range within which the polymer electrolyte can work stably. The operating voltage range of lithium batteries is generally 3~4.5V (vs.Li/Li+), which requires the polymer electrolyte to ensure normal and stable operation of the battery within this electrochemical stability window. The test polymer electrolyte has been electrochemically tested at room temperature or operating temperature. Generally, the metal flip sheet is used as the negative electrode and the reference electrode, and the stainless steel sheet is used as the positive electrode. The polymer electrolyte is sandwiched between a lithium sheet and a steel sheet to form a three-electrode system. An electrochemical workstation is used to perform linear scanning voltammetry or cyclic voltammetry. Usually the test range is 0~6v. Set an appropriate voltage sweep speed to test the three-electrode system.

2. Ionic conductivity

Ionic conductivity is usually tested by electrochemical impedance spectroscopy (EIS). The electrochemical impedance spectroscopy method is an electrical measurement method that uses a small amplitude sine wave potential as a disturbance signal. Due to the small amplitude electrical signal perturbing the system, on the one hand, it can avoid having a large impact on the system, and on the other hand, it also makes the relationship between the disturbance and the response of the system approximately linear. This makes the mathematical processing of the measurement results simple. Obtain the impedance of the polymer electrolyte through the EIS test, and then calculate the ionic conductivity of the polymer electrolyte according to the following formula:

              σ=L/RS              (1-1)

In the formula: σ—ionic conductivity (S/cm); L—electrolyte thickness (cm); R—electrolyte impedance (Ω); S—electrolyte area (cm2).

3. Number of ion mobility

The measurement method of polymer electrolyte lithium ion migration number is to sandwich the polymer electrolyte between two lithium sheets, assemble the battery, first test the initial impedance value with EIS, and then apply a DC voltage to it to obtain an initial current and a steady-state current. Then test its steady-state impedance value, and the data obtained calculate the lithium ion migration number according to the following formula.

The main test method of polymer electrolyte
(1-2)

Where: t+—the number of ion migration; ΔV—the voltage applied to both ends of the battery (V); R0el—initial impedance value (Ω); Rsel—steady-state impedance value (Ω); I0—initial current ( A); Is—steady-state current (A).

4. Solid state NMR

Solid-state high resolution nuclear magnetic resonance (solid-state high resolution nuclear magnetic resonance solid-state NMR) technology is an important structural analysis method. It studies the different local environments around various nuclei, that is, short-to-medium-range interactions. It is very suitable for studying the microstructure of solid materials and can provide very rich and detailed structural information. It can be used to analyze the structure of solid materials with high crystallinity, and can also be used for the structure analysis of solid materials and amorphous materials with low crystallinity. Solid-state NMR and ray diffraction, neutron diffraction, electron diffraction and other methods to study the long-range overall structure of solids are complementary to each other. Especially when studying non-crystals, solid-state NMR methods are even more important because of the absence of long-range order. Now solid-state NMR has been widely used to study the microstructure of inorganic materials (such as molecular sieves, catalysts, ceramics, glass, etc.) and organic materials (such as high molecular polymers, membrane proteins, etc.).

In solid-state batteries, the electrode-solid electrolyte interface has a greater contact resistance, so the interface compatibility problem mainly affects the electrochemical performance of the battery. For the characterization of the solid-solid interface in current solid-state batteries, there are not many effective methods. Solid-state NMR is a non-destructive and highly selective testing method for materials. It mainly uses the chemical shift changes in the solid-state nuclear magnetic resonance spectrum to investigate the interaction between the nucleus and the local microenvironment of each child, so as to effectively detect the body phase information in the battery materials (electrode materials and solid electrolytes). Solid-state NMR can detect the spontaneous lithium ion exchange in the lithium-containing multiphase battery material system (such as between multiple lithium-containing electrode materials or between lithium-containing electrode materials and lithium-containing electrolytes), so as to obtain the selective information of the charge transfer in the multiphase interface.