The influence of the transformation temperature plays a crucial role in the production and manufacturing of lithium-ion batteries. Its main purpose is to generate a solid electrolyte interphase (SEI) on the negative electrode surface, which isolates electrons and facilitates ion conduction. The quality of the SEI film directly impacts the battery’s cycling performance. Therefore, controlling the appropriate transformation conditions such as temperature, charging rate, pressure, etc., is a vital production step. During the SEI film formation process, the battery volume increases. This is attributed to the gaseous products generated during the film formation reaction and the expansion of the negative electrode structure after lithium ions detach from the positive electrode and embed into the negative electrode.
Experimental equipment and test methods:
Experimental equipment: We used Model GVM2200 (IEST Energy Technology), which has a test temperature range of 20°C to 85°C and supports dual-channel (2 cells) synchronous testing.
Test information: We employed an NCM523/Graphite system cell, applying a constant current (CC) of 0.5C to reach 4.2V, with a theoretical capacity of 2400mAh.
Test method: Initially, the cell was weighed (m0), then placed into the corresponding channel of the equipment. The MISG software was activated, and the appropriate cell number and sampling frequency parameters for each channel were set. The software automatically recorded volume changes, test temperature, current, voltage, capacity, and other data.
In-situ volume expansion analysis of cells:
As the influence of the temperature is increased, the volume of gas produced also gradually increased. When the battery was charged to approximately 3.7V, the volume curve reached a relatively stable maximum, with a slight shrinkage in the constant pressure stage. Analyzing the amplified volume expansion curve and differential capacity curve, it became evident that higher temperatures caused volume expansion to occur earlier, and the peak position of each phase change shifted to the left. This indicates a reduction in battery polarization. However, at temperatures above 55°C, the first phase transition reaction summit became sharper, which could potentially lead to a more intense SEI film formation reaction at high temperatures.
During the transformation process, a solid electrolyte interface (SEI) forms on the surface of the graphite electrode to prevent solvent co-embedding. The physical and chemical properties of this interface significantly impact the polarization potential and lifespan of lithium-ion batteries. An ideal SEI layer requires high ion conductivity, excellent electronic insulation, and good thermal and electrochemical stability. These characteristics ensure the rapid transmission of lithium ions while effectively isolating electrons. The main components of the SEI layer include electrolyte salts, LiF, Li2CO3, RCO2Li, carbonate, and others. Only when a stable SEI layer is successfully formed can lithium ions be stably inserted and extracted from the graphite electrode. The capacity retention and storage lifespan of lithium-ion batteries directly depends on the stability of the SEI layer.
The formation of the SEI layer involves two competing processes: growth and dissolution. Research indicates that the growth of SEI is associated with the electrochemical reduction process of electrolytic solvents and is not very sensitive to temperature. In contrast, the higher influence of the temperature significantly accelerates the dissolution of the initial SEI into the electrolyte. Consequently, SEI interfaces formed at different temperatures exhibit distinct characteristics. At high temperatures, solvent molecules, and electrodes exhibit greater activity, resulting in more complex electrochemical properties at the electrode/electrolyte interface. Organic components of the SEI layer dissolve more easily in organic electrolytes compared to inorganic components, leading to the collapse of the SEI membrane. Thus, at high temperatures, inorganic components become the primary constituents of the SEI film, reducing the electrode’s ability to withstand volume deformation. Moreover, high temperatures promote extensive side reactions and increased gas production. The faster lithium ion transmission at high temperatures generates greater electrochemical interface stress, contributing to interface instability.
At low temperatures, the formed SEI layer tends to be denser, resulting in lower ion conductivity, which restricts the rapid transmission of Li ions. Additionally, low temperatures can cause direct lithium metal deposition due to high polarization. Therefore, the optimal interface membrane, with the best ion conductivity and stability, is formed within a specific temperature range.
In summary, the transformation temperature affects the viscosity and conductivity of the electrolyte, as well as the ion diffusion rate of the electrode material, thereby influencing the transformation efficiency. Generally, higher conversion temperatures correspond to lower electrolyte viscosity, higher electrolyte conductivity, and faster ion diffusion in the electrode material. Consequently, a higher influence of the temperature lead to reduced battery polarization and improved conversion efficiency. However, excessively high conversion temperatures can damage the structure of the formed SEI layer, increase side reactions, and accelerate the volatilization of low-boiling-point components in the electrolyte, negatively impacting the transformation efficiency [2]. As a result, the temperature range commonly selected in the industry is 45-70°C.
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