Lithium-ion batteries, as a hot and hot new energy source, have been widely used in all walks of life. With the gradual expansion of applications, there are higher requirements for the safety of batteries. During the charging and discharging process of lithium-ion batteries, there will be different degrees of expansion. On the one hand, it will affect the deformation of the battery assembly space; At the single cell level, there are many characterization methods for the characterization of cell expansion. For example, by applying a certain pressure to the surface of the cell, the expansion thickness of the cell can be tested, or the thickness of the cell can be fixed to test the cell. The amount of expansion obtained by this method is the overall expansion of the cell, including multi-layer positive and negative pole pieces, diaphragms, aluminum plastic films or aluminum shells, etc. It is impossible to accurately locate the source of expansion and quantify a certain The expansion ratio of the material, so there are still certain limitations for lithium battery researchers. Button battery is a kind of battery most used by lithium battery researchers. It is composed of a single layer of positive and negative electrodes and a separator. However, due to the constraints of the positive and negative steel shells, the expansion of the pole pieces cannot be measured. If the influence of the positive and negative steel shells is excluded, the model button battery is used, and only the expansion behavior of the single-layer battery is explored, the expansion performance of the active material can be directly analyzed, which is helpful to evaluate the feasibility of material modification and process formulation optimization. Button battery is a kind of battery most used by lithium battery researchers. It is composed of a single layer of positive and negative electrodes and a separator. However, due to the constraints of the positive and negative steel shells, the expansion of the pole pieces cannot be measured. If the influence of the positive and negative steel shells is excluded, the model button battery is used, and only the expansion behavior of the single-layer battery is explored, the expansion performance of the active material can be directly analyzed, which is helpful to evaluate the feasibility of material modification and process formulation optimization. 

Figure 1. Schematic of a model coin cell battery

1. Test equipment

Main features:

1. The size of the equipment is small (length*width*height 120*150*280mm), which can be placed in the glove box;

2. Model button battery fixture, which can be used to assemble various types of full batteries;

3. Good sealing to ensure long-term test stability and obtain more reliable test results;

4. High-precision thickness measurement system, thickness measurement resolution 0.1µm, accuracy ±1µm;

5. In-situ test full battery expansion thickness curve;

6. It can test the ionic conductivity of solid electrolyte;

7. The software automatically merges the model battery thickness change data and charge and discharge data (compatible with some charge and discharge equipment), and outputs a test data report.

Figure 2. Schematic diagram of the model battery test system equipment

2. Application direction

2.1 Li-Li symmetrical battery

Assemble Li-Li symmetrical batteries, and test the thickness change during the mutual deposition of lithium; test parameters: current density is 0.5mA/cm2, charge and discharge for 2h each, and set aside for 5min; during the lithium deposition process, the total thickness of the battery gradually increases, and each deposition is 2mAh of lithium, the total battery thickness increases by 2 μm, and the corresponding volume expansion is about 0.76 mm3/mAh.

Figure 3. Li-Li symmetrical battery test results

2.2 NCM-Li half-cell

Assemble the NCM-Li battery, and test the thickness change during charging and discharging; test parameters: the current density is 0.6mA/cm2, 3~4.3V; during the charging process, the thickness of the battery expands by about 4μm/mAh, and the volume expansion is about (0.6mm3 /mAh), mainly due to the deposition of lithium in the ternary to the surface of the lithium sheet. During discharge, the thickness shrinks by about 3μm/mAh, and the volume shrinkage is about (0.5mm3/mAh), which is mainly due to the reduction of the thickness of the lithium sheet due to the continuous extraction of lithium from the lithium sheet and the intercalation of the ternary.

Figure 4. NCM-Li half-cell test results

2.3 Ternary-graphite battery

The thickness of the battery decreased in the initial shelving stage, mainly due to the fact that in the shelving stage after assembly, under certain pressure conditions, the interface contact between the positive and negative electrodes will gradually become closer; the thickness of the first charge expanded by about 2.5 μm, The discharge thickness shrinks by 1 μm, which is related to the low first efficiency of the battery in the first cycle, mainly due to the influence of SEI film formation; in the subsequent cycles, the thickness changes of charging and discharging are about 1.33 μm/mAh, and the corresponding volume changes are 0.2mm3/mAh, which is mainly caused by the lithium deintercalation of graphite; the thickness of the graphite coating is about 100 μm, if the expansion of the positive electrode is ignored, the thickness expansion percentage of the graphite is 2%.

Figure 5. NCM-Gr battery test results

2.4 NCM-Si/C battery

When testing the thickness expansion of the model battery, ignoring the expansion of the positive NCM pole piece, the measured total expansion is mainly the expansion of the negative electrode SiC, and then deducting the thickness of the copper foil, the expansion percentage can be calculated. Comparing the two materials, the expansion ratio of material A is generally greater than that of material B, and when the first cycle is fully charged, the difference between the two is small, and when the subsequent cycle continues, the maximum expansion thickness of B will decrease compared with the first cycle, and the subsequent cycle will decrease. Slowly increasing, for material A, the maximum expansion thickness of each circle has been increasing, which may be related to the different modification methods of the two materials.

Figure 6. NCM-Si/C cell test results

3. Summary

In this paper, the model battery coin-type in-situ swelling test system MCS1000 is used to carry out the swelling test of the model coin-type battery. Only the swelling behavior of the single-layer battery is explored, and the swelling performance of the active material can be directly analyzed, which is helpful to evaluate the material modification and process. Feasibility of formulation optimization.

4. References

[1] Bernhard Bitzer, Andreas Gruhle. A new method for detecting lithium plating by measuring the cell thickness. Journal of Power Sources 262 (2014) 297~302.

[2] Electrochemical Dilatometry of Si-bearing Electrodes: Dimensional Changes and Experiment Design. Journal of The Electrochemical Society A, 102843.R1