Chemical stability and electrochemical stability window

Chemical stability and electrochemical stability window

Generally, the organic electrolyte in the battery is highly polar and corrosive. Therefore, the diaphragm material must have good chemical resistance in the electrolyte and be stable for a long time. Generally, the chemical stability of the diaphragm is evaluated by measuring the corrosion resistance of the electrolyte and the rate of expansion and contraction. In the process of assembling the battery, the separator should remain flat without curling or shrinking after being dropped into the electrolyte.

In the process of charging and discharging, oxidation-reduction reactions occur inside the lithium-ion battery, and the diaphragm should have a certain resistance to electrochemical corrosion (anti-oxidation and reduction reactions) when working within a certain voltage range to ensure that the mechanical properties will not change for a long time. The electrochemical stability window of the diaphragm and electrolyte system is usually measured to determine the electrochemical stability of the diaphragm. The electrochemical workstation is used to characterize by linear scanning voltammetry (LSV) to determine whether the diaphragm material undergoes an oxidation-reduction reaction with the pole piece or electrolyte within the operating voltage range of the battery system. Generally, it is required to be greater than 4.5V (Vs.Li/Li+). This method scans the electrode at a constant speed within a certain potential interval, records the current-potential change curve, and obtains relevant electrochemical information such as peak potential, peak current, kinetic parameters, and so on. The specific test steps are:In the glove box filled with argon, take stainless steel sheet as working electrode and lithium sheet as counter electrode and reference electrode respectively, and clamp the diaphragm between them, add an appropriate amount of electrolyte to assemble the button battery, and then conduct the linear scanning test, set the corresponding scanning rate and scanning voltage range, observe the variation curve of the current obtained from the test with the potential, and obtain the range of the electrochemical stability window of the diaphragm material. As shown in Figure 1, it can be seen that the electrochemical stability window of the tested membrane can be as high as 5.0V (vs.Li/Li+), indicating that the membrane is suitable for the application of high potential electrode materials. However, this method can only obtain a transient test method. Qingdao Energy Storage Industry Technology Research Institute proposes to use in-situ differential electrochemical mass spectrometry to test the electrochemical stability window.

Chemical stability and electrochemical stability window
Figure – 1 (a) LSV graph of a diaphragm measured at a scan rate of 0.5mV/s at room temperature; (b) Relationship between voltage current and voltage CO2 and H2 production of non discharged Li-O2 battery of DME electrolyte linearly scanned from open circuit voltage to 5V at a scanning rate of 0.5mV/s

Compared with the traditional method of applying linear sweep voltammetry to determine electrolyte stability and transient properties, the article on this site proposes to use in-situ differential electrochemical mass spectrometry to test electrolyte stability. This method is dynamic, more accurate and intuitive. Compared with the traditional method of applying linear sweep voltammetry to determine electrolyte stability and transient properties, this site proposes to use in-situ differential electrochemical mass spectrometry to test electrolyte stability. This method is dynamic, more accurate and intuitive. This method uses a special Swagelok battery that provides a relatively sealed system with only channels for carrier gas to enter and exit the battery. The carrier gas is usually pure Ar, and the Li-O2 battery is a mixture of Ar-O2, which does not affect the normal operation of the battery. The battery is connected to the gas-phase mass spectrometer, and the change of the internal atmosphere of the battery carried by the carrier gas is monitored in real time during the electrochemical process. Generally, the battery electrolyte is organic, and it is generally believed that it will decompose to produce a large amount of CO2, H2, CH, C2H and other gases under high voltage, of which CO2 is the most important.

The decomposition voltage of the electrolyte can be obtained by monitoring the corresponding voltage value and the corresponding gas generation, and the stability of the electrolyte can be determined. For example, in the Li-O2 battery shown in Figure 1(b), an electrochemical mass spectrometry test is performed on a battery that has not been discharged (excluding the influence of Li2O2) and uses ethylene glycol dimethyl ether (DME) as the electrolyte. It can be seen that with the increase of the charging voltage, the battery begins to have a charging current at about 4.5V, and when the voltage further increases to 4.75V, H2 and CO2 begin to be produced, corresponding to the oxidative decomposition of the electrolyte. In this way, the electrochemical stability window of the electrolyte is obtained.