High-temperature testing is an essential part of electrical performance testing, including high-temperature storage, high-temperature cycling, and hot-box testing. Although we will not discuss extreme test conditions such as thermal shock for the time being, thermal runaway caused by hot-box testing is a destructive test. Under normal high-temperature conditions, the cell may swell, increase impedance, and soften materials after high-temperature storage, while high-temperature cycling will accelerate the performance decline of the cell. So, what is the root cause of lithium-ion battery failure at high temperatures? Today we will briefly explore this issue.

Under the same anode and electrolyte conditions, the performance of different cathode materials at high temperatures varies. Generally speaking, the order of high-temperature performance of cathode materials is: lithium iron phosphate > lithium cobaltate > low and medium nickel ternary > high nickel ternary ≈ lithium manganate. This shows that the more stable the crystal structure, the better the performance of the cathode material at high temperatures.

Taking high-nickel ternary as an example, storage failure under high temperature conditions is mainly related to the following factors:

• Accumulation of by-products: After high-temperature storage, by-products accumulate on the surface of the high-nickel ternary material, and the increase of rock salt phase leads to the rise of battery impedance.
• Transition metal element deposition: The dissolved transition metal element will be deposited on the negative electrode graphite, destroying the SEI film on the negative electrode surface, thus accelerating the consumption of active lithium.
• Surface coating and bulk phase doping: Effective surface coating or bulk phase doping is a key measure to improve the high-temperature storage performance of high-nickel ternary materials.

By comparing XRD patterns before and after storage, it can be seen that the bulk phase structure of the cathode material did not change significantly during storage. However, the roughness of the material surface changed significantly, indicating that a side reaction occurred on the material surface after high-temperature storage.

Further analysis revealed that a non-electrochemically active rock salt phase with increased thickness appeared on the surface of the cathode material after storage, leading to a decrease in the reversible capacity of the high-nickel ternary material.

The impact of high temperature on the anode is mainly reflected in the destruction of the SEI film, which includes the corrosion of the transition metal on the SEI film, the generation of HF, and the poor formation of the film due to electrolyte problems. Under high temperature conditions, the fragmentation and reconstruction of the SEI membrane consumes a large amount of active lithium ions and increases the impedance of the battery.

In contrast, graphite typically outperforms SiO materials at high temperatures. Graphite is more stable under high temperature conditions, whereas silica-oxide materials face issues due to gas release from residual bases. Regulating the surface work function of silicon-based electrodes or optimizing the LUMO energy levels of electrolyte additives is crucial for improving the high-temperature performance of silica-oxide materials.

The performance of the electrolyte at high temperatures is critical to the overall performance of the battery. Several key factors are outlined below:

• Solvents: Solvents with low boiling points and low viscosities are prone to higher vapor pressures in high-temperature environments, leading to gas generation, which may affect the interfacial stability of the cell.
• Lithium salts: Lithium hexafluorophosphate (LiPF6) is thermally unstable at high temperatures and may release HF (hydrogen fluoride), which is harmful to the cell. Replacing it with lithium salts like FSI (fluorosulfonate) can improve stability.
• Additives: Additives play a crucial role in the high-temperature stability of electrolytes. They react with electrode materials to form protective films, which affect side reactions at high temperatures.

In liquid electrolyte systems, high temperatures exacerbate interfacial side reactions and electrolyte drying out. Solid electrolytes can significantly improve high temperature performance.

At high temperatures, performance degradation in lithium-ion batteries often manifests as yellowing and increased permeability of the PE diaphragm, decreased mechanical properties, and rising impedance. Improvements include:

• Coating aluminum oxide or boehmite to inhibit yellowing and oxidation.
• Using a high-permeability PE base film that reduces electrolyte oxidative polymerization.

The improvement of lithium-ion battery performance degradation at high temperatures requires the optimization of the four core materials: cathode, anode, electrolyte, and diaphragm. Understanding the causes of failure is key to improving the high temperature performance of lithium-ion batteries.

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