3.2v 25ah lifepo4 battery cell.Explain the safety issues of lithium-ion power batteries as vehicle p

Time:2024.12.23Browse:0

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  Power battery safety is one of the focus issues that all parties pay most attention to during the large-scale promotion and application of new energy vehicles. This article explains the safety issues and effective solutions of lithium-ion power batteries as vehicle power systems. The safety issues of power battery systems are mainly divided into three levels, namely "evolution", "triggering" and "expansion". "Evolution" means that before a power battery safety incident occurs, the fault may have gone through a long-term evolution process; "trigger" is the turning point of the "evolution" process, or it may be an unexpected situation that destroys the power battery system and causes safety hazards. trouble. Regarding the "triggering" of power battery safety accidents, the core issue is the thermal runaway of lithium-ion power batteries. After the "triggered" onset of thermal runaway, "expanded" episodes of thermal runaway should be avoided.

  Below, we first start with some lithium battery safety stories in recent years and introduce the concepts of "evolution", "trigger" and "expansion". After that, these three levels will be explained one by one.

  1. Review of safety incidents of lithium power batteries

  It can be seen that lithium-ion power battery accidents mainly manifest themselves as fire caused by thermal runaway. Fire is a worrying safety issue, but in fact, the damage caused by the current accident is limited. In addition to the burning of the battery pack, damage to the vehicle body, and ignition of surrounding vehicles, casualties are relatively rare. In most accidents, personnel can receive timely danger warnings and safely evacuate the scene of the accident. The casualties in Incident 4 (BYD taxi collision and fire accident) were mainly caused by the high-speed collision causing the occupants of the car to faint and unable to escape. The injuries in Incident 5 (Boeing 787 battery fire accident) were caused by the evacuation process after the emergency landing of the aircraft. A moderate fall injury.

  1) The "evolution" of battery system safety. There are two situations before a safety incident occurs. One is the decrease in reliability caused by the long-term aging of the battery system, also called safety "evolution", such as incidents 1, 2, 3, 5, and 7 in Table 1; the other is the battery failure caused by emergencies The system is damaged and causes battery thermal runaway and fire, which is also called a safety "sudden change", such as incidents 4 and 6 in Table 1. The "evolution" and "mutation" of security are collectively referred to as "evolution".

  2) "Trigger" of battery system safety accidents. Long-term aging of the battery system and battery system damage caused by emergencies may further "evolve" into thermal runaway and fire of lithium-ion power batteries. The turning point of a lithium-ion power battery from normal operation to thermal runaway and fire is called "trigger".

  3) The “expansion” of battery system safety incidents. After a single battery or some batteries in a battery pack are "triggered" by thermal runaway, the thermal runaway and combustion will release a lot of heat in a short period of time. This heat is transferred to surrounding batteries and battery system accessories, which will cause corresponding secondary damage, such as thermal runaway and combustion of surrounding batteries in sequence, or flame propagation and ignition of the wiring harness and interior decoration in the car. This phenomenon of thermal runaway and fire spreading to the surrounding areas is called the "expansion" of the accident.

  2. The “evolution” of lithium power battery safety

  There are two situations before a security incident occurs. One is the decrease in reliability caused by the long-term aging of the battery system, also called safety "evolution", such as accidents 1, 2, 3, 5, and 7 in Table 1; the other is the battery failure caused by emergencies The system is damaged and causes battery thermal runaway and fire, which is also called a safety "sudden change", such as incidents 4 and 6 in Table 1 (Tesla high-speed collision and fire accident). From a time scale perspective, safety evolution takes a long time, while safety mutation takes a short time. For example, Incident 1 (Prius battery smoke and fire incident) is classified as a safety evolution. The battery withstood a long period of on-board vibration before the joints became loose due to assembly problems; Incident 5 (Boeing 787 battery fire incident) is classified as a safety evolution. From the design flaw to the internal short circuit triggering, the battery needs to go through a long internal short circuit "growth and incubation period" and a long period of unreasonable use; and Incident 6 was a safety mutation, and the power battery pack was mechanically deformed after an instant impact. The battery is not only squeezed but also moved, causing short circuits and thermal runaway.

  Comparatively speaking, safety mutations are difficult to predict, but the design of the battery system can be improved through existing accident patterns; safety evolution takes a long time and is accompanied by the aging of the battery system. The battery system can be evaluated by detecting the aging degree of the battery system. Security changes.

  The aging of any component of the battery system may trigger safety incidents. In Incident 1 (Prius battery smoke and fire accident), incorrect assembly procedures caused the battery cable connectors to loosen under long-term vehicle vibration conditions, which in turn led to an increase in resistance at the connectors. During the driving of a hybrid electric vehicle, the charging and discharging current of the battery generates a lot of heat at the loose joints, heating part of the battery, and eventually leads to the occurrence of battery thermal runaway accidents. In Incident 7, the failure of the battery management system caused the battery pack to continue to be overcharged for a long time, eventually leading to a thermal runaway accident.

  In addition to the aging of other components of the battery system, the safety evolution of the battery itself is mainly reflected in the development of internal short circuits. Internal short circuit is considered to be the main cause of Series 5 Incidents. An internal short circuit will go through a fairly long "growth and gestation period" before it finally occurs. There are many reasons for internal short circuit in lithium-ion power batteries, among which the growth of metal dendrites inside the battery is one of the main reasons for internal short circuit. Metal dendrite growth can come from the dissolution and re-growth of transition metals (copper, iron, etc.) in the battery positive electrode, or from the precipitation and growth of lithium metal. Defects in the battery design and production process will be conducive to the growth of metal dendrites, such as impurities mixed into the battery during the manufacturing process, or wrinkles in the battery pole plates due to assembly stress. Metal dendrites are more likely to grow near impurities and wrinkles. . The precipitation and growth of lithium metal are also related to the charging rate and charging temperature. High-rate charging or low-temperature charging may increase the possibility of lithium metal precipitation.

  The long-term growth of metal dendrites may squeeze into the pores of the diaphragm and eventually pierce the diaphragm, causing internal short circuit or even thermal runaway accidents. It should be noted that before the growth of lithium dendrites pierces the barrier and causes thermal runaway, the safety of aging batteries has changed compared with fresh batteries: on the one hand, due to the decrease in energy density, the damage caused by thermal runaway of batteries may decrease. ; But on the other hand, due to the existence of internal metal dendrites, aging batteries may be more susceptible to thermal runaway.


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