Science and technology
The influence of cyclic aging and storage aging on battery safety performance
With the development of global diversification, our lives are constantly changing, including the various electronic products we come into contact with, so you must not understand some of the components of these products, such as batteries. Under different aging methods, the aging attenuation mechanism and external characteristics of the battery are not the same, and the safety performance changes caused are also different. The aging decay path can be divided into two types: cycle aging and storage aging.
The influence of cyclic aging on battery safety performance
Under normal temperature/high temperature cycles, the battery's resistance to electrical abuse such as overcharging and short circuits will become poor. The important performance is that the aging battery catches fire and explodes under the test of overcharge and short circuit, and fails the test, and the new battery All can successfully pass the above test. An important reason for the decrease in the resistance of the battery to electric abuse is the increase in internal resistance, which leads to an increase in the Joule heat of the battery in the case of electric abuse, and thermal runaway is more likely to occur. Studies have shown that the safety performance of the battery under mechanical abuse such as acupuncture and extrusion does not change much before and after the cycle aging attenuation, which indicates that the mechanical performance of the battery basically does not change with the cycle aging.
The battery's thermal stability changes under normal temperature/high temperature cycles and old chemical conditions are related to the material system. Studies have shown that after room temperature/high temperature cyclic aging, the battery's self-generated heat starting temperature Tonset and thermal runaway temperature TTR under the adiabatic thermal runaway test are reduced to a certain extent, and the self-generated heat rate is also slightly increased, indicating cyclic aging Batteries are more prone to self-heating and thermal runaway under abnormal temperature shocks; and some studies have shown that after a normal temperature/high temperature cycle, the self-heating rate of the battery is reduced, and the thermal stability of the battery is improved. The important reason for this difference is the change of SEI film during the cycle.
During the cycle, the unstable components of the SEI film on the negative electrode surface of some batteries gradually turn into steady-state components, and the SEI film gradually becomes stable, which can better protect the graphite negative electrode and improve heat dissipation. The stability of the battery; during the cycle, the SEI film on the surface will continue to break, resulting in a new unstable SEI film, and the protective purpose of the graphite anode is gradually weakened, causing the graphite anode to start to react with the battery. Electrolyte at lower temperature. Thermal stability is reduced. Under high-speed charging, some batteries will precipitate lithium in the negative electrode, which will reduce the thermal stability of the battery.
Under low-temperature cycle aging, the safety performance of the battery will change significantly. Studies have shown that after low-temperature cycle aging, the battery's spontaneous heat start temperature Tonset will be significantly reduced under the adiabatic runaway test, and spontaneous heat may occur within the normal use range (50°C). And there will be batteries. The heating rate will be greatly increased, and the thermal stability of the battery will drop sharply. After low-temperature cycle aging, an important reason for the deterioration of the thermal stability of the battery is the precipitation of lithium on the surface of the negative electrode. The precipitated lithium metal is very active and can react with the electrolyte at a lower temperature, resulting in a decrease in the battery's self-heating starting temperature Tonset and a rapid increase in the self-heating rate, which seriously endangers the safety of the battery.
The influence of storage aging on battery safety performance
Regarding the storage and aging of the battery at room temperature/high temperature, research shows that in the adiabatic runaway test, the self-heating start temperature Tonset is newly added to the aging and aging battery, and the self-heating rate is reduced to a certain degree, and self-heating occurs. With the increase of the storage time, the increase of the initial temperature Tonset and the decrease of the self-heating rate become more obvious, indicating that the storage aging of the battery after use has been improved.
The improvement of the thermal stability of the battery after storage aging is mainly attributed to the gradual stability of the SEI film on the surface of the negative electrode. Under storage conditions, the SEI film of the negative electrode will not be broken and regenerated, and the unstable components will be stored for a long time. The medium gradually transforms into a steady-state component, and the stability of the SEI film is improved, which can better protect the graphite negative electrode and improve the thermal stability of the battery. However, gas may be generated during the storage and aging of the battery, which may cause the battery to swell and affect the safety of the battery. In the case of electric abuse such as overcharging and short circuit of cyclic aging, due to the increase of internal resistance, the Joule heat of the battery will increase, resulting in the decrease of the battery's resistance to electric abuse after storage and aging.