Thermal runaway behavior of lithium batteries -Lithium - Ion Battery Equipment

Thermal runaway behavior and mechanism of lithium batteries -Lithium - Ion Battery Equipment

The traditional view is that the thermal runaway of lithium batteries is caused by the internal short circuit caused by the shrinkage or even melting of the polyolefin separator caused by the heating inside the battery [1]. The latest research shows that the mechanism of thermal runaway of the battery is different for different battery systems and abuse conditions. Factors such as cross-reaction between positive and negative electrodes in high-specific energy batteries, lithium precipitation on the negative side caused by overcharge or low-temperature cycling, etc., may cause thermal runaway of the battery. In this regard, the team of Academician Ouyang Minggao has made systematic and in-depth results.

1.1Joule: Thermal runaway of lithium battery without internal short circuit[2]

Under normal conditions, it is believed that the thermal runaway of lithium batteries is caused by internal short circuits under abuse conditions, or at least the thermal runaway process will be accompanied by internal short circuits caused by problems such as diaphragm shrinkage. However, a recent Joule article by Academician Ouyang Minggao reported for the first time that the battery can still experience severe heat release without an internal short circuit. This is caused by the chemical crossover of the positive and negative electrodes: during the charging process of the ternary positive electrode, a phase change occurs to release oxygen, and the highly oxidizing gas reacts with the highly reducing lithiated negative electrode, resulting in a violent heat release of the battery and thermal runaway.(Lithium - Ion Battery Equipment)

The team of Academician Ouyang specially selected polyimide (PET)/non-woven separator with excellent thermal stability to eliminate the influence of short circuit in the battery. They tested the thermal effect of a 25Ah NCM523/graphite full cell using an electric vehicle acceleration calorimeter. The results show that the temperature at which thermal runaway occurs in the full cell is significantly lower than the melting temperature of the separator, indicating that the thermal runaway of the battery occurs before the battery is short-circuited. Subsequently, they carried out TG-DSC, high-temperature XRD and mass spectrometry detection of each component of the battery to confirm its heat generation mechanism and runaway behavior. During the charging process, the single NCM523 cathode undergoes a phase transition from layered to spinel and generates a small amount of heat up to 276 °C. However, when the positive and negative materials were tested together, the corresponding heat production in this process increased by 7 times and directly caused the thermal runaway of the battery. The strong thermal effect caused by the intersection of positive and negative electrodes provides theoretical guidance for the development of all-solid-state lithium batteries with high specific energy.

1.2 ACSAppliedMaterialsandInterfaces: Thermal runaway of battery caused by lithium deposition in negative electrode after fast charge[3]

With the expansion of lithium battery application scenarios, low temperature charging, fast charging and even overcharging will occur from time to time. Low temperature, high rate and overcharge often lead to the precipitation of metal lithium plating on the negative electrode side. So, what is the impact of the negative electrode lithium deposition on the thermal runaway behavior of lithium batteries in these cases? In response to the above problems, the team of Academician Ouyang Minggao recently used traditional ARC and DSC techniques combined with NMR methods to study the thermal runaway behavior and corresponding mechanisms of lithium batteries at different charging rates. They first tested the thermal runaway temperature at three rates of 0.33C/0.15C/0.3C, in which the thermal runaway temperature T2 of the battery at a high charge rate of 3C was reduced to 103.5°C (the corresponding T2 of 0.33C was 215.5°C) . According to the relationship between the full-cell voltage and temperature, they found that the thermal runaway temperature occurred before the shrinkage and melting of the separator, so the mechanism of thermal runaway in this process has little to do with the short circuit in the battery. The researchers combined 7Li-NMR spectroscopy and SEM to confirm the existence of metallic lithium coating on the surface of the negative electrode. A new high-intensity exothermic peak appears at 146.7 °C in the DSC curve of the mixture of anode powder and electrolyte (containing lithium metal coating), which confirms that the advance of thermal runaway under fast charge is caused by the reaction between lithium metal and electrolyte. 



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