The Influence of Polymerization Temperature on the Volume Expansion of Battery Cells Analysis

Preface

Formation is a critical step in the production process of lithium-ion batteries. The primary purpose of formation is to generate a solid electrolyte interphase (SEI) on the surface of the anode, which serves to isolate electrons while allowing ion conduction. The quality of the SEI formation directly affects the subsequent cycling performance of the battery. Therefore, controlling appropriate formation conditions (such as formation temperature, charging rate, applied pressure, etc.) is a crucial production step. The SEI formation process is accompanied by an increase in the volume of the battery. This increase is attributable to two main factors: the gaseous byproducts generated during the film formation reaction and the expansion of the anode structure as lithium ions are extracted from the cathode and intercalate into the anode.

 

In this study, in-situ volume monitoring instruments (GVM) are employed to conduct in-situ volume testing of NCM523/graphite cells (theoretical capacity of 2400 mAh) at different formation temperatures, thereby analyzing the impact of formation temperature.

1. Experimental Equipment and Testing Methods

1.1 Experimental Equipment

Model GVM2200 (IEST Yuan Energy Technology) was used for the testing, with a temperature range of 20°C to 85°C. It supports dual-channel (2 cells) simultaneous testing. The appearance of the equipment is shown in Figure 2

1.2 Testing Information

NCM523/Graphite cell system, charged at 0.5C constant current (CC) to 4.2V, with a theoretical capacity of 2400 mAh.

 1.3 Testing Method

Initially, weigh the battery cell (m₀) and place the cell to be tested into the corresponding channel of the equipment. Launch the MISG software and set the corresponding cell identification numbers and sampling frequency parameters for each channel. The software will automatically read the volume change, testing temperature, current, voltage, capacity, and other data.

 2. In-Situ Volume Expansion Analysis of Battery Cells

Five parallel samples were subjected to formation at temperatures of 25°C, 45°C, 55°C, 65°C, and 85°C according to the process shown in Figure 4(a). The resulting volume expansion curves and differential capacity curves are presented in Figures 4(b) and (c). As the formation temperature increases, the corresponding gas generation rate also gradually rises. When the battery is charged to around 3.7V, the volume curve of the battery reaches a relatively stable maximum value, followed by a slight volume contraction during the constant voltage phase. From the enlarged volume expansion curves and differential capacity curves, it can be observed that the increase in formation temperature causes the volume expansion to occur earlier, with the peak positions of each phase transition shifting to the left. This indicates that the polarization of the battery is continuously decreasing. However, when the temperature exceeds 55°C, the first phase transition reaction peak becomes sharper, which may be related to the intensified SEI formation reactions at elevated temperatures.

During formation, a solid electrolyte interphase (SEI) forms on the surface of the graphite electrode to prevent solvent co-intercalation. The physical and chemical properties of the interface significantly affect the polarization potential and lifespan of lithium-ion batteries. An ideal SEI layer requires high ionic conductivity, good electronic insulation, and excellent thermal and electrochemical stability to ensure rapid lithium ion transport while effectively isolating electrons to reduce side effects. The main components of the SEI include electrolyte salts, LiF, Li₂CO₃, RCO₂Li, carbonates, etc. Only with successful formation of a stable SEI can lithium ions intercalate and de-intercalate stably with graphite. The capacity retention and storage lifespan of lithium-ion batteries are also directly dependent on the stability of the SEI.

The formation of the SEI involves two reverse processes: growth (increase) and dissolution (decrease) of the SEI. Research indicates that SEI growth is associated with the electrochemical reduction process of the electrolyte solvent and is not overly sensitive to temperature. In contrast, an increase in temperature significantly accelerates the dissolution of the initially formed SEI into the electrolyte. Therefore, the SEI interfaces formed at different temperatures exhibit distinct characteristics.

At high temperatures, both the solvent molecules and the electrode's activity are relatively high, making the electrochemical performance at the electrode/electrolyte interface more complex. The organic components of the SEI dissolve more easily in organic electrolytes than inorganic components, leading to collapse of the SEI film. Consequently, at high temperatures, inorganic components become the primary constituents of the SEI film, significantly reducing the electrode's capacity to withstand volume changes. High temperatures can also trigger severe side reactions and produce more gas; moreover, the increased speed of lithium ion transport at elevated temperatures generates greater electrochemical stress at the interface, contributing to instability.

At low temperatures, the formed SEI tends to be denser, resulting in lower ionic conductivity and restricting rapid lithium transport. Furthermore, excessively low temperatures, due to high polarization, can lead to direct deposition of metallic lithium. Hence, only within an optimal temperature range can the formed interfacial film possess the best ionic conductivity and stability.

In conclusion, the formation temperature affects the viscosity and conductivity of the electrolyte as well as the ionic diffusion rate of the electrode materials, thereby influencing the formation results. Generally, higher formation temperatures result in lower electrolyte viscosity, higher electrolyte conductivity, and faster ionic diffusion rates in the electrode materials. Consequently, higher temperatures lead to smaller battery polarization and better formation effects. However, excessively high formation temperatures can damage the already formed SEI structure, increase side reactions, and accelerate the volatilization of low-boiling components in the electrolyte, which is detrimental to the formation effects. Therefore, the commonly selected formation temperature in the industry is typically between 45°C and 70°C.

3.Conclusion

This study employs a controllable dual-channel in-situ gas volume monitoring instrument to conduct in-situ volume expansion tests under different formation temperature conditions. It was found that the higher the temperature, the earlier and larger the volume expansion of the battery cells. By quantitatively characterizing the volume of the battery cells, this approach can assist battery researchers in determining optimal formation conditions.

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