In electrical performance testing, high-temperature testing is an essential part, including high-temperature storage, high-temperature cycling, and thermal chamber testing. Although we will not discuss extreme testing conditions such as thermal shock for now, thermal runaway caused by thermal chamber testing is a destructive test. Under normal high temperature conditions, the battery cells stored at high temperatures may experience expansion, impedance increase, and material softening, while high temperature cycling can accelerate the performance degradation of the battery cells. So, what is the fundamental reason for the failure of lithium-ion batteries at high temperatures? Today we will briefly discuss this issue.
1.The Effect of High Temperature on Positive Electrode Materials
Under the same negative electrode and electrolyte conditions, the performance of different positive electrode materials at high temperatures varies. Generally speaking, the high-temperature performance order of positive electrode materials is: lithium iron phosphate>lithium cobalt oxide>medium low nickel ternary>high nickel ternary ≈ lithium manganese oxide. This indicates that the more stable the crystal structure of the positive electrode material, the better its performance 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 high nickel ternary materials, and the increase in rock salt phase leads to an increase in battery impedance.
Transition metal element deposition: The dissolved transition metal elements will deposit on the negative electrode graphite, damaging the SEI film on the negative electrode surface and accelerating the consumption of active lithium.Surface coating and bulk doping: Effective surface coating or bulk doping is a key measure to improve the high-temperature storage performance of high nickel ternary materials.
By comparing the XRD patterns before and after storage, it can be seen that the bulk structure of the positive electrode material did not show significant changes during storage. However, there was a significant change in the roughness of the material surface, indicating that side reactions occurred on the material surface after high-temperature storage.
Further analysis using high-resolution transmission electron microscopy (HRTEM) revealed the presence of non electrochemically active rock salt phases with increased thickness on the surface of the stored positive electrode material, as well as spinel phases in localized areas within the grains. These changes can lead to a decrease in the reversible capacity of high nickel ternary materials.
XPS analysis showed that electrolyte side reaction products such as LixPOyFz and LiF appeared on the surface of the stored positive electrode material, indicating that even after a simple electrochemical process, LiPF6 in the electrolyte would decompose and produce side reaction products that deposited on the surface of the positive electrode material. In this case, the F element mainly exists in PVDF, and the degree of electrolyte side reactions is relatively low. However, after high-temperature storage, the proportion of PVDF adhesive detected decreased, while the proportion of by-products such as LixPOyFz, LixPFy, and LiF increased. This indicates that during high-temperature storage, electrolyte side reactions continue to occur, leading to a gradual increase in side reaction products on the positive electrode surface.
The presence of these by-products significantly reduces the conductivity of the battery, as their ionic or electronic conductivity is low. The accumulation of a large amount of by-products increases the internal impedance of the battery, thereby reducing its storage performance.
To improve the storage performance of high nickel ternary material batteries at high temperatures, surface coating treatment, such as using solid electrolytes, can be considered. This treatment helps to reduce side reactions at the interface between the electrode and electrolyte, stabilize the surface structure of the material, and suppress the leaching of transition metal elements, thereby reducing the damage of transition metal deposition and reduction on the negative electrode SEI film, and reducing the consumption of active lithium ions in the positive electrode during SEI film repair process.
Another method is to stabilize the layered structure of high nickel cathode materials through doping, in order to suppress their transformation into spinel and rock salt phases during storage, thereby reducing the generation of non electrochemically active components in the material during storage. In the process of industrialization, the doping of aluminum elements (usually several hundred to several thousand ppm) is a common solution, but this doping may affect the capacity of the material.
In contrast, lithium iron phosphate exhibits superior high-temperature performance due to its relatively stable structure.
2.The impact of high temperature on negative electrode materials
The impact of high temperature on the negative electrode is mainly reflected in the damage of SEI film, including the corrosion of SEI film by transition metals, the generation of HF, and poor film formation caused by electrolyte problems. Under high temperature conditions, the fragmentation and reconstruction of SEI film consume a large amount of active lithium ions and increase the impedance of the battery. In contrast, the damage to the negative electrode material itself has a relatively small impact on the overall performance. This highlights the importance of SEI film in negative electrode performance.
However, this does not mean that the negative electrode material itself is not important. For example, graphite typically performs better than silicon oxide materials at high temperatures. On the one hand, graphite has good stability under high temperature conditions, while silicon oxide materials are prone to problems due to the release of residual alkali gas at high temperatures. On the other hand, there are significant differences in chemical composition and electronic surface properties between graphite and silicon oxide electrodes, which affect their thermal stability. The edge plane work function of graphite is lower than that of silicon oxide, which makes the transfer of charge to the electrolyte smoother in the early stages of formation. Due to the high work function of silicon oxide, it hinders the transfer of charges, resulting in an increase in the solubility of SEI film on the surface of silicon oxide, leading to an increase in self discharge at high temperatures.
Therefore, adjusting the surface work function of silicon-based electrodes or optimizing the LUMO energy level of electrolyte additives is crucial for improving the high-temperature performance of silicon oxide materials.
The SEM image of the graphite electrode shows no significant degradation on the surface after storage. The SEI film of the silicon oxide electrode partially dissolves after high-temperature storage.
3.The impact of high temperature on electrolyte
The performance of the electrolyte is crucial for the overall performance of the battery under high temperature conditions. The following is an overview of several key factors:
Solvent: Low boiling point and low viscosity solvents are prone to generate high vapor pressure in high-temperature environments, leading to gas generation, which may affect the interface stability of the battery cell. For example, DMC (dimethyl carbonate) and carboxylic acid ester solvents, although having excellent kinetic properties, should be used with caution under high temperature conditions.
Lithium salt: Currently widely used lithium hexafluorophosphate (LiPF6) has poor thermal stability at high temperatures and may release harmful HF (hydrogen fluoride) to battery cells. This is also an important reason for replacing lithium salts with FSI (fluorosulfonate) and other alternatives. In addition, if the purity of lithium salt is not high, serious side reactions may occur at high temperatures.
Additives: Additives play a crucial role in the high-temperature stability of electrolytes. They preferentially react with electrode materials to form a protective film, which in turn affects the suppression effect of side reactions at high temperatures. For example, S-based additives such as PS (polysulfide), PST (polysulfide), and MMDS (1,3-dimethylthiosulfate) exhibit excellent performance in maintaining high temperature performance. However, ODFB (tetrafluoroethylene oxide) has significant benefits in high-temperature cycling, but may lead to performance degradation due to gas generation during high-temperature storage.
Interface side reactions and electrolyte drying: In liquid electrolyte systems, high temperatures can exacerbate interface side reactions and electrolyte drying, which is also an important reason why solid electrolytes can significantly improve high-temperature performance. Solid electrolytes can effectively reduce these issues and improve the stability of batteries under high temperature conditions.
Overall, improving the performance of electrolytes under high temperature conditions mainly requires two aspects: optimizing the solvent system and improving the interface control after film formation.
4.The Effect of High Temperature on Diaphragm
Under high temperature conditions, the performance degradation of lithium-ion batteries is mainly manifested by the yellowing of the surface of the positive electrode facing the PE separator, an increase in the permeability value of the separator, a decrease in mechanical properties, and an increase in impedance. Usually, these issues are attributed to the oxidation of the membrane and the deposition of electrolyte oxidation products. The following are improvement measures:
1) Coating with alumina or boehmite can suppress the yellowing and blackening of the diaphragm.
2) Different PE base films. The mechanism is that boehmite or alumina can neutralize the HF generated by the reaction between LiPF6 and H2O, suppressing pressure drop. For the positive electrode oxygen release, alumina and boehmite can eliminate singlet oxygen and reduce the oxidation reaction of the electrolyte. In addition, a dense polar ceramic coating can block the initially formed non-polar – [C-O-F-P] – on the surface of the positive electrode. The surface smoothness of PE separator will change the deposition rate of electrolyte polymer, causing black substances to deposit around the positive electrode. A PE membrane with good permeability is conducive to the migration of low polymer electrolyte through the membrane to the negative electrode, reducing its accumulation on the surface and inside of the membrane, thereby reducing the persistence of electrolyte oxidation polymerization.
In summary, the improvement plan for the performance degradation of lithium-ion batteries at high temperatures mainly involves the optimization of four core materials. Auxiliary materials (such as the specific surface area of conductive agents, the binding of various components by binders) and production processes (such as moisture control, optimization of chemical composition and volume system, mixing process, etc.) also have important impacts on high-temperature performance. Understanding the cause of failure is the key to improving the high-temperature performance of batteries.