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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