Literature Appreciation: In-situ Measurement of Volume-Stress-Thickness Variation of Different Silicon Anodes in Pouch Batteries

In 2017, the J.R.Dahn research group used the in-situ characterization method to test the volume, stress and thickness changes of the electrodes for Pouch Batteries with different silicon negative electrodes, combined with the calculation method, the volume swelling ratio of each component of the silicon composite electrode is quantitatively analyzed, so as to lay the foundation for a deep understanding of the swelling mechanism of silicon-based materials.

1. Experimental Protocol

1.1 In This Experiment, Make Three Kinds of Batteries:

(A) Li(Ni1-x-yCoxAly)O2 (NCA)/SiO-graphite (supplier A), fully charged to 4.2V corresponds to 260 mAh;

(B) LiCoO2 (LCO)/Si Alloy-graphite (supplier B), fully charged to 4.35V, the corresponding capacity is 230 mAh;

(C) Li(Ni1-x-yCoxAly)O2 (NCA)/nano Si-C (supplier C), fully charged to 4.4V, the corresponding capacity is 165 mAh;

2.Test Equipment and Process:

In-situ XRD test, in-situ volume swelling test, in-situ stress swelling test, in-situ thickness swelling. The stress and thickness testing setup is shown below.

Figure 1.Swelling force and swelling thickness test equipment

3. Result Analysis

Figure 3 is the test curves of volume, stress and thickness swelling of three types of batteries during charge and discharge. From the results, the volume swelling and stress swelling of batteries A and B are equivalent, which is larger than the swelling of battery C, and the swelling curves of batteries A and C in the high voltage range have similar plateau areas, the swelling curve of battery B in the high voltage region is steeply increasing or decreasing. Since the result of this curve is caused by the common swelling of the positive and negative electrodes, when analyzing the contribution of the individual negative electrodes, it is necessary to know the swelling of the corresponding individual materials.

Figure 3. The volume, stress and thickness swelling test curves of the three types of batteries during charge and discharge

The curves (a) and (b) of Figure 4 are the volume swelling ratios of pure Si and pure graphite in the charging and discharging process obtained in other related literatures, Figure (c) is the swelling ratio of the NCA material obtained by in-situ XRD in this article. It can be seen from the results that silicon and graphite will produce volume swelling of 280% and 10% respectively during the charging process, and the swelling curve of silicon shows a linear increase trend with the increase of SOC. However, the swelling curve of graphite will have a step in the phase transition process of 2L→2 order, and there is no obvious volume swelling at this stage. The swelling trend of NCA during charge and discharge is opposite to that of silicon and graphite. The volume shrinkage of 4.5% will occur during the entire charging process, and the most important shrinkage occurs in the high SOC interval.

Figure 4. Volume change ratio curves of three pure electrode materials during charge and discharge process

Through dV/dQ curve fitting, the influence of each component on the total voltage capacity curve of the electrode is obtained when Si and Gr are combined, as shown in Figure 5. Figure 6 is an exploded diagram of volume swelling curves of each component corresponding to a full battery of SiO/Gr composite electrodes and NCA electrodes. From the results, the reason for the volume swelling curve of cell A to appear in the high-voltage section is that the shrinkage of NCA counteracts the swelling of SiO, so the swelling curve of the full cell shows a plateau.

Figure 5. Composite voltage capacity curve fitting of Si and Gr

Figure 6. Decomposition of the volume swelling curves of the components corresponding to the full battery of SiO/Gr composite electrodes and NCA electrodes

Figure 7.Shows the swelling force and capacity change curves of cells B and C during the long cycle, comparing the cycle and swelling performance of the two cells, the irreversible swelling force and capacity decay rate of the LCO/Si alloy carbon-doped cells are greater than those of the NCA/Si-C cells.

Figure 7. The long cycle expansion force and capacity curves of cells B and C

4. Summarize

In this paper, the author used the in-situ characterization method to test the volume, stress and thickness changes of the electrode, combined with the calculation method, quantitatively analyzed the volume swelling ratio of each component of the silicon composite electrode, to lay the foundation for an in-depth understanding of the swelling mechanism of silicon-based materials.

5. Document Original Text

A.J. Louli, Jing Li, S. Trussler, Christopher R. Fell, and J. R. Dahn. Volume, Pressure and Thickness Evolution of Li-Ion Pouch Cells with Silicon-Composite Negative Electrodes. Journal of The Electrochemical Society, 164 (12) A2689-A2696 (2017).

IEST Related Test Equipment Recommendation

SWE Series In-situ Swelling Analysis System (IEST): Using a highly stable and reliable automation platform, equipped with a high-precision thickness sensor, it can measure the thickness change and change rate of the battery throughout the charging and discharging process, and can realize the following functions:

1.The battery swelling thickness curve is tested under constant pressure conditions.

2.Test battery swelling force curve under constant gap condition.

3.Battery compression performance test: stress-strain curve-compression modulus.

4.Step-by-step test of battery swelling force.

5.Different temperature control: -20 ~ 80 ℃.