LiFePO4 Prismatic Cells’ In-situ Swelling and Temperature Change Analysis

In this paper, the in-situ swelling analyzer (SWE) is used to test the swelling force and the temperature change of the LiFePO4 prismatic cells under different initial preload (60kg/90 kg/120 kg) and different charge and discharge rates of the LiFePO4 prismatic cells and analyze the cell swelling and temperature rise behavior. During the charging and discharging process of lithium-ion batteries, with the continuous intercalation and extraction of lithium-ion, the stress inside the battery will increase and decrease. If the irreversible stress accumulates to a certain extent, it will lead to particle breakage or lithium precipitation, reducing the usable capacity and life of the battery^1-3. When the battery cell is packaged into an electric vehicle or 3C electronic product, it is squeezed to varying degrees due to the space constraints of the casing or other components, which affects the performance change of the battery cell during subsequent use.

Figure 1. Schematic diagram of the LiFePO4 system battery cell

1. Test Information

1.1 Test Equipment

In-situ swelling analyzer, model SWE2110 (IEST), which can exert a pressure range of 5~1000kg, and the appearance of the equipment is shown in Figure 2.

Figure 2: Appearance of SWE2110 Equipment

2. Test Parameters

2.1 Prismatic Cells Information

LiFePO4/graphite prismatic cells: 40Ah

2.2 Test Plan

(1) Swelling force and temperature change under the same magnification with different preloads (60kg/90kg/120kg);

(2) The swelling force and temperature changes of the same preload (60kg) under different magnifications (1C, 1.5C, 2.5C);

The voltage and current changes of the battery charge and discharge cycle are shown in Figure 3.

Figure 3. Schematic diagram of battery charging and discharging

2.2 Cell Thickness Swelling Test

Put the cell to be tested into the corresponding channel of the device, open the MISS software, set the corresponding cell number, sampling frequency, test pressure and other parameters for each channel, and the software will automatically read data such as cell thickness, thickness variation, temperature, current, voltage, and capacity.

3. Result Analysis

3.1 Prismatic Cell Swelling and Temperature Variation Curves under Different Initial Preload Conditions

The prismatic cells were subjected to 1C charge and discharge tests under three different initial preload conditions. The in-situ measured swelling curves, temperature changes and differential capacity curves are shown in Figure 4. It can be seen from Figure 4(a) that with the increase of the initial pre-tightening force, the variation of the maximum swelling force of the cell during charging and discharging also gradually increases, mainly because the larger the pre-tightening force, the smaller the initial gap of the cell. The smaller the value, the more the structural swelling of the cell is restricted during the process of lithium intercalation and deintercalation.

In Figure 4(b), the temperature of the cell surface increases under the three conditions during the charging and discharging process, indicating that there is a current passing through the inside of the cell, and the temperature on the cell surface will be about 3°C. When the current is removed during the rest period, the surface temperature slowly decreases. It can be seen from the peak position change of the differential capacity curve of the charging and discharging process in Figure 4(c), that as the initial preload increases from 60kg to 90kg and 120kg, the peak position first shifts to the left and then remains unchanged, indicating a certain degree of The preload is beneficial to reduce the polarization of the cell.

Moderate pressure can improve the electrical contact between particles, prevent the delamination of the electrode layer, and discharge the gas from the electrode layer, but when the pressure is too high, the compression will hinder the ion transmission, thereby increasing the ion resistance; and the uneven pressure distribution is also It will cause the pores of the diaphragm to close, and the current distribution will be uneven, resulting in local lithium precipitation. In addition, the different initial pressure will also lead to different stress evolution characteristics of the battery during long-term cycling: higher confinement pressure will cause the battery cycle to deteriorate rapidly and the capacity to decay rapidly. However, compared with the case of no pressure, proper pressure can improve the cycle stability and capacity retention of the battery.

Figure 4. Cell under 3 kinds of preload conditions (a) swelling force curve; (b) temperature change curve; (c) differential capacity curve.

3.2 LiFePO4 Prismatic Cells Swelling and Temperature Variation Curves under Different Rate Conditions

The swelling force, temperature and differential capacity curves of the battery cell under three different charge and discharge rate conditions are shown in Figure 5. The battery is charged and discharged at different rates. With the increase of the charge rate, the maximum swelling force of the cell increases from about 144kg to 164kg, and the increase of the surface temperature of the cell is more obvious.

When the rate is 2.5C, the maximum temperature rise is about 15°C, and the peak position of the differential capacity curve during the charging process of the prismatic cell is gradually shifting to the right. The above information shows that the charge and discharge rate have different effects on the swelling force, temperature and polarization of the prismatic cell.

Figure 5. (a) Swelling force change; (b) Temperature change; (c) Differential capacity curve under different charge and discharge rates of the cell.

4. Summary

The influence of charge and discharge rate on the swelling force of the LiFePO4 prismatic cells are considered from two aspects: reversible swelling and irreversible swelling. Reversible swelling refers to the volume change of the electrode material lattice due to the phenomenon of delithiation and lithium intercalation of the electrode.

The irreversible swelling includes the growth of SEI film, the formation of interfacial dendrites, gas production, and particle fragmentation and delamination caused by irreversible damage. The increase of the charge-discharge rate leads to the accumulation of a large amount of lithium-ion on the surface of the active particles, resulting in an increasing concentration difference between the inside and the surface of the particles, which leads to the formation of lithium dendrites on the surface on the one hand. On the other hand, the stress on the particles will increase with the increase of the concentration difference. The greater the stress, the easier it is for the particles to crack or even break, resulting in irreversible structural damage and swelling of the lithium battery.

5. References

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[2] Thomas M. M. Heenan, Paul R. Shearing*, Identifying the Origins of Microstructural Defects Such as Cracking within Ni-Rich NMC811 Cathode Particles for Lithium-Ion Batteries. Adv. Energy Mater. 2020, 2002655.

[3] Dai H, Yu C, Wei X, Sun Z, State of charge estimation for lithium-ion pouch batteries based on stress measurement, Energy (2017).

[4] M Lewerenz,A Marongiu,A Warnecke,DU Sauer. Differential voltage analysis as a tool for analyzing inhomogeneous aging: A case study for LiFePO4|Graphite cylindrical cells. Journal of Power Sources 368 (2017) 57~67