(A) Li(Ni1-x-yCoxAly)O2 (NCA)/SiO-graphite(Supplier A), full charge to 4.2V corresponds to 260 mAh capacity；
(B)LiCoO2 (LCO)/Si Alloy-graphite(Supplier B), when 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 corresponds to a capacity of 165 mAh；
2. Test equipment and process：in-situ XRD test, in-situ volume expansion test, in-situ stress expansion test, in-situ thickness expansion test. The stress and thickness test setup is shown in the figure below.
Figure 1. Expansion Force and Expansion Thickness Test Equipment
Figure 3 shows the volume, stress and thickness expansion test curves of three types of batteries during charging and discharging. From the results, the volume expansion and stress expansion of batteries A and B are similar, and larger than that of battery C, and the expansion curves of batteries A and C have similar plateau regions in the high voltage range, while battery B has a similar expansion curve in the high voltage range. The expansion curve of the high voltage region is a steep increase or decrease. Since the result of this curve is caused by the common expansion of the positive and negative electrodes, when analyzing the contribution of the individual negative electrodes, it is necessary to know the expansion amount of the corresponding individual material.
Figure 3. Volume, stress and thickness expansion test curves of three types of batteries during charging and discharging
The curves (a) and (b) of Figure 4 are the volume expansion ratios of pure Si and pure graphite obtained in other related literatures during the charging and discharging process, and Figure (c) is the NCA material obtained by in-situ XRD in this paper. Expansion ratio. It can be seen from the results that the volume expansion of silicon and graphite will be 280% and 10% respectively during the charging process, and the expansion curve of silicon shows a linear increase trend with the increase of SOC, while the expansion curve of graphite is in the order of 2L→2. There is a step in the phase transition process, and there is no significant volume expansion at this stage. The expansion trend of NCA during the charging and discharging process is opposite to that of silicon and graphite. The volume shrinkage of 4.5% occurs during the entire charging process, and the most important shrinkage occurs in the high SOC range.
Figure 4. The volume change ratio curves of the three pure electrode materials during the charging and discharging process
Through the dV/dQ curve fitting, the influence of each component on the total voltage-capacity curve of the electrode when Si and Gr are combined is obtained, as shown in Figure 5. FIG. 6 is an exploded view of the volume expansion curve of each component corresponding to the full cell of the SiO/Gr composite electrode and the NCA electrode. From the results, the reason why the volume expansion curve of cell A has a plateau region in the high voltage segment is that the shrinkage of NCA offsets the expansion of SiO, so the expansion curve of the full cell shows a plateau region.
Figure 5. Composite voltage-capacity curve fitting of Si and Gr
Figure 6. Decomposition of the volume expansion curve of each component corresponding to the full cell of SiO/Gr composite electrode and NCA electrode
Figure 7 shows the expansion force and capacity change curves of cells B and C during long-term cycling. Comparing the cycling and expansion performance of the two cells, the irreversible expansion force and capacity decay rate of the LCO/Si alloy carbon-doped cell are both higher than those of NCA. /Si-C cells.
Figure 7. Expansion force and capacity change curves of cells B and C during long-term cycling
In this paper, the author uses in-situ characterization methods to test the volume, stress and thickness changes of the electrodes, and quantitatively analyzes the volume expansion ratio of each component of the silicon composite electrode by combining the calculation method, so as to lay a foundation for in-depth understanding of the expansion mechanism of silicon-based materials.
IEST Recommended test equipment
In-Situ Gassing Volume Analyzer：GVM2200（IEST），Has the following characteristics：
1.Lidian co-core test system: long-term in-situ online monitoring, and high resolution 1μL;
2.Realize different temperature test environments：20~85℃；
3.Special test software: real-time acquisition and display of mechanical test system data, automatic drawing of volume change curve and electrical performance curve;
In-Situ Cell Swelling Analyzer（IEST）：Using a highly stable and reliable automation platform, equipped with a high-precision thickness measurement sensor, it can measure the thickness change and change rate of the entire charging and discharging process of the battery cell, and can realize the following functions:
1.Constant pressure condition test battery expansion thickness curve;
2.Test the battery expansion force curve under the condition of constant gap;
3.Battery compression performance test: stress-strain curve-compression modulus;
4.Step-by-step test of battery expansion force;
5.Different temperature control：-20~80℃。
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).