In 2014, Bernhard Bitzer et al. published a method to detect changes in battery thickness in situ to monitor the lithium evolution of cells.When the battery is charged at a low temperature or at a high rate, the lithium ions extracted from the positive electrode will be deposited on the surface of the negative electrode. Compared with the lithium normally embedded in the negative electrode, the lithium precipitation will cause the thickness of the battery to increase, so the thickness can be measured to characterize the lithium precipitation.
² Making three electrodes to monitor the negative electrode potential;
² Analyze electrical signals such as capacity loss and internal resistance during battery aging
² The physical/chemical method after dismantling proves the existence of Li metal;
² Material structure expansion: graphite~10%, NMC~1%, LTO: 0.1%~0.3%;
² Volume change during lithium precipitation: According to the following calculation formula, the precipitation of 1Ah of lithium will increase the volume by 0.37cm3.
Figure 1 shows the thickness measurement experimental device, which is mainly composed of two aluminum plates and thickness sensors, with a thickness resolution of 1 µm, and is equipped with a data circulation and acquisition system BaSyTec XCPS-System.
Figure 1. The device for in-situ measurement of the thickness of the cell.
2.The thickness curve of the battery during the cycle of standard operating conditions
Figure 2 shows the open circuit thickness curve (OCT) of a 20Ah NCM/graphite cell. The charge and discharge rate of the battery is 1/4C, and each test point is left standing for 1h.The thickness change curve of the battery is divided into three stages, the slope of stage I is about 9.5 mm/Ah. The slope of stage II is about 3.0 mm/Ah, and the slope of stage III is about 6.5 mm/Ah. When the battery is fully charged to 100% SOC, the thickness increases by about 136µm.
Figure 2. The thickness change curve of the battery under steady-state conditions
3. The influence of cathode material on battery thickness
The LTO with relatively small volume expansion is used as the negative electrode, the positive electrode is NCM, and the theoretical capacity of the battery is 16Ah. The thickness change curve of the battery during charging and discharging is measured under steady-state conditions.During the charging process, the thickness of the battery is first reduced by about 3µm, and then increased to 11µm. The thickness of the battery is also reduced first and then increased slightly during discharge, but the thickness change of no more than 12µm can be compared with the change of about 140µm in Figure 2 Ignorable.
Figure 3. OCT curve of NCM/LTO battery
4.The influence of gas on battery thickness
Cycle the battery for 3 cycles under the condition of -5°C, and use two thickness sensors to measure the change in the thickness of the battery surface. One of the sensors has a spring plate underneath, and the spring force is about 2 bar.If there is gas production during the cycle, the thickness measured by the sensor with the spring should be greater than the thickness of the other sensor.But judging from the test curve in Figure 5, the thickness curves of the two are not much different, indicating that there is no obvious gas production behavior under the test conditions.
Figure 4. Dual thickness sensor device
Figure 5. Comparison of thickness curves measured by two sensors
5.The influence of gas on battery thickness
Comparing the thickness change curves of two different charging currents, when charging with a current of 3A, the battery does not show any lithium evolution.When charging with a current of 7A, it can be seen that the thickness change is greater than the thickness change at 3A, and there will be a decrease in thickness during the constant voltage stage.This is mainly because the current decreases when the voltage is constant, and the originally precipitated lithium will continue to be embedded in the graphite again. Therefore, the reversible and irreversible amounts of lithium can be calculated from the two curves.Using 7A charging and 20A discharging at a temperature of -5°C, it can be seen from the cycle curve of Fig. 7 that the irreversible thickness expansion of each circle caused by lithium evolution is continuously increasing.
Figure 6. Thickness curve of two different test conditions
Figure 7. Cycle thickness curve under the condition of lithium analysis
This paper uses an in-situ thickness expansion measuring device with a resolution of 1 µm, which can non-destructively characterize the lithium evolution of the battery cell under different charging and discharging conditions. At the same time, it analyzes the influence of various conditions on the lithium evolution, which is a fast judgment analysis. The method of lithium.
IEST Yuanneng Technology related test equipment recommendation
SWE series in-situ expansion analysis system (IEST Yuanneng Technology):
1.Integrate multiple cell in-situ characterization methods (stress & expansion thickness): simultaneously measure the expansion thickness and expansion force of the cell during charging and discharging, and quantify the change in cell expansion thickness and expansion force;
2.More precise and stable test system: using a highly stable and reliable automatic adjustment platform, equipped with a high-precision thickness measurement sensor and a pressure adjustment system, with a relative thickness measurement resolution of 0.1 µm, to achieve long-term monitoring of the long-term charging and discharging process of the battery;
3.Diversified environmental control and testing functions: SWE series equipment can adjust the temperature of the charging and discharging environment to help the study of cell expansion behavior under high and low temperature conditions;In addition to the conventional thickness and pressure tests, the expansion force, compression modulus, compression rate and other parameters of the battery can be tested.
B. Bitzer，Gruhle. A new method for detecting lithium plating by measuring the cell thickness. Journal of Power Sources, 262 (2014) 297~302.