According to data from the State Administration for Market Regulation, at
least 40 fire accidents involving new energy vehicles occurred in China in 2018.
Since April this year, electric vehicle fire and smoke accidents have occurred
one after another. Power battery safety is a sensitive topic, and It is a topic
that cannot be avoided.
Recently, Wang Zidong, deputy secretary-general of the China Power Battery
Innovation Alliance and deputy secretary-general of the China Electric Vehicle
Charging Infrastructure Charging Alliance, conducted a multi-dimensional
analysis of electric vehicle safety issues at the first China International
Electric Vehicle Safety Technology Innovation Conference. He believes that
before there is an obvious technological breakthrough in power battery
materials, it will be difficult to make further breakthroughs after the specific
energy develops to a certain level. At the same time, the negative impact on
security is growing. Before mastering the fire rules of lithium batteries,
controlling the balance between energy density, safety and long life is an issue
that cannot be ignored.
The battle over power battery technology routes
This problem has been around for a long time, and it is also the game
between energy density and safety.
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Wang Zidong said, "We must admit that the battery pack is a component
containing high-energy substances and is inherently dangerous. Moreover, as the
specific energy and specific power of the battery increase, the risk of
accidents will increase."
Among the many technical routes for lithium batteries, the duel between
lithium iron phosphate and ternary is the most stalemate.
Lithium iron phosphate has high safety and long life, but its energy
density is not as high as that of ternary, but this can be compensated by
increasing the battery capacity. Its low-temperature performance is poor, mainly
because of its low low-temperature performance and poor material consistency in
small-capacity batteries.
Ternary batteries have high energy density, good consistency, and good
low-temperature performance, but are slightly less safe and have far less cycle
life than lithium iron phosphate batteries.
"Currently, China has the most mature industrial chain for lithium iron
phosphate batteries, and we have mastered many core technologies in related
fields, while ternary batteries, represented by Japan and South Korea, are
relatively more mature." Wang Zidong believes that this This kind of duel
between technical lines is more like China vs. Japan and South Korea.
If evaluated solely from the performance of the power battery itself, he
listed 10 dimensions: 1. Safety of the battery pack, 2. Energy density of the
battery pack (not a single cell), 3. Cycle life of the battery pack, 4. The cost
of the battery pack, 5. Charging rate, 6. Battery cell consistency, 7. Low
temperature performance, 8. Group utilization, 9. Convenience of recycling and
reuse, 10. The positive and negative electrode materials can be recycled,
repaired and reused. .
As a technical route that can lead the trend, it cannot have too obvious
shortcomings in any of the above aspects. It needs to achieve a balance in all
aspects to be a feasible route, rather than a single performance index being
high. For example, energy density.
Therefore, from the analysis and comparison of the above 10 aspects, in
this duel, Sanyuan and lithium iron phosphate fought fiercely and painfully.
There were wins and losses, and there were also ties. After these 10 duels, Wang
Zidong’s personal referee gave a Simple final conclusion: where safety is a
concern and energy density requirements are not very high, lithium iron
phosphate batteries are the first choice.
Where is the ceiling of power batteries?
In Wang Zidong’s view, it is necessary to have a clear understanding of
improving the energy density of power batteries: to improve the performance of
power batteries for electric vehicles that can be industrialized, it is not only
necessary to significantly improve the performance of positive and negative
electrode materials, but also needs to be improved in many aspects. Only with
relatively large breakthroughs can it be possible to achieve real improvements
in power batteries.
So from the definition of industrialized power batteries, the goal proposed
by the country is: by 2020, electric vehicles can run 400km on a single charge,
and the specific energy of a single battery reaches: 300Wh/kg (350), 600Wh/L
(700), 0.6 yuan/Wh, the battery system reaches: 220Wh/kg (260), 300Wh/L (380),
1.0 yuan/Wh, cycle life 1500 times (80% DOD).
Wang Zidong said that judging from the indicator data, it is still
relatively difficult to achieve these indicators.
The current product overview of domestic power battery companies is: in
terms of lithium iron phosphate, the energy density of large-scale production of
energy-type lithium iron phosphate power batteries is roughly between
140-180Wh/kg. In terms of three raw materials, the energy density of large-scale
production of ternary cathode lithium-ion power batteries for pure electric
drive is roughly between 180-260Wh/kg.
From a technical point of view, if the specific energy of the battery pack
reaches 260Wh/kg, based on the energy consumption calculation of 10KWh/kg/100km,
the weight of the 40KWh battery pack cell of an electric vehicle traveling 400km
cannot exceed 99.5kg, and the total weight of the battery pack cannot exceed
153kg, the specific energy of the flexible packaging battery needs to exceed
402Wh/kg, and the difficulty can be imagined.
It can be deduced from this that a battery with a specific energy of
350Wh/kg (if it can be produced) needs to be made into a large-capacity (above
80Ah) aluminum alloy hard-shell power battery, which can save the weight of
40KWh after modularization. The total weight of the battery core must be
controlled within 114.3kg, which can only account for 74.7% of the weight of the
battery pack. The remaining aluminum alloy box (25kg), thermal management system
(2kg), connectors and fixings (11.7kg), etc. The weight cannot exceed 38.7kg,
the weight ratio of the battery pack must be 25.3%, the total weight of the
battery pack must not exceed 153kg, and the specific energy of the battery pack
can reach 262Wh/kg.
"Why do people think of soft-packaged batteries when they mention high
energy density? From the perspective of vehicle engineering, we look at the
energy density of the power battery system, not the energy density of individual
cells. From individual cells to modules to system integration, the middle There
are many links, including connectors between batteries, connecting cables
between modules, boxes, fixed frames, support frames, thermal conductive
structures, etc., which will add a lot of weight." Wang Zidong pointed out that
it is necessary to improve the system energy density and reliability. Optimize
performance and security.
Safety of power batteries
After ten years of accumulation, my country's power battery industry has
made great improvements, especially in terms of understanding and understanding
of power batteries. It should be said that it is competent for the current use
of electric vehicles.
In terms of current power battery materials, if there is no obvious
technological breakthrough, after the specific energy develops to a certain
level, it will be difficult to make further breakthroughs. At the same time, the
negative impact on safety is getting bigger and bigger. "Many people have asked
me that fuel vehicles often catch fire, and more so than electric vehicles. Why
are the requirements for electric vehicles so high?" Wang Zidong said, there is
a concept that needs to be clarified: there are patterns in the fires of fuel
vehicles. It is related to many known factors. The key point is that the
flammable substance in fuel vehicles is fuel, which is sealed in an environment
isolated from the outside world and separated from oxygen (combustion
accelerant) and fire source. Once this isolation condition is If it breaks (such
as aging pipelines, oil leakage and high engine temperatures), accidents will
occur.
The flammable substance in the power battery system is the electrolyte,
which is sealed in the same container environment with the combustion accelerant
oxygen (the positive electrode material will decompose to produce oxygen when it
encounters high temperatures) and the fire source (internal short circuit and
overcharge will generate high temperatures). , so its security uncertainty
becomes particularly prominent.
Wang Zidong made an analogy about this: which one caused more harm to
humans, the cold or the 2003 SARS virus? Of course it was a cold, but humans
were afraid of SARS because we had no drugs to treat the SARS virus at the time.
Therefore, before mastering the rules of lithium battery fire, controlling the
balance between energy density, safety and long life is an issue that cannot be
ignored.
1. How to understand power batteries?
Before the power battery is born, it is necessary to consider in advance:
the assembleability design, installability design, maintainability design,
adjustability design, recycling and easy disassembly design of the battery
module and battery pack (system), etc.
The design of these properties is very important. We cannot solve these
performance problems by manufacturing all batteries in disaster areas. Lithium
batteries are born with explosive tempers. Why do lithium batteries turn into
"ticking time bombs"?
Lithium-ion batteries are mainly composed of six parts, namely positive
electrode, aluminum foil, negative electrode, copper foil, separator, and
electrolyte.
The electrolyte inside the battery contains a large amount of organic
compounds, such as ethylene carbonate, diethyl carbonate, and dimethyl
phosphate. Each of these guys has explosive properties, with the words "keep
away from fire" written on their faces. In addition, once the positive and
negative electrodes of the battery are short-circuited, they will generate a lot
of heat and even produce sparks. Therefore, people will naturally think of using
something to separate the positive and negative electrodes, so a separator is
introduced.
After the battery separator is thinned, once this film is damaged, the
problem will be serious. Lithium batteries themselves also have hidden dangers
of puncturing the separator. This phenomenon is called "dendrites". This problem
is a disease written in the genes of lithium batteries. During the use of
lithium batteries, some small burrs will form on the electrode surface. These
small burrs are called "dendrites", and the dendrites will grow larger and
larger, and will eventually penetrate the diaphragm and cause a short
circuit.
The thinner the diaphragm, the flammable electrolyte, the undercurrent of
dendrites that will grow on their own, and the decomposition of materials at
high temperatures that will automatically separate out oxygen, the entire
lithium battery is like a powder barrel, combustion accelerant, and lighter in a
small room inside, and then separated by a layer of plastic wrap, which would
make anyone shudder to think about it. Now, the most important thing is to
control the "lighter".
2. How to ensure the safety of the power battery system?
The safety of the battery system must be solved by the battery core. To
ensure the safety of the battery core, it must use more stable materials and
safer designs.
Wang Zidong said that now we are deliberately lowering the safety
requirements of battery cells to reduce costs and increase energy density. It is
difficult to pass the thermal runaway propagation experiment. The safety of the
entire vehicle should still be evaluated based on the basic safety requirements
at the source. Make sure to use a thicker separator to ensure the safety of the
battery core. The design of the battery core to increase energy density should
not be achieved by thinning the separator thickness.
Safe management when charging the battery pack is key! Since the most
critical and core issues in the use of power lithium batteries in groups are:
first, safety, and second, lifespan. Especially during fast charging, the
difference between batteries in the battery pack increases. How to solve the
service life of the battery pack faces huge challenges. .
Factors that affect the safe use and cycle life of batteries, in addition
to the battery's own craftsmanship and product quality, are also a crucial
issue: safety control and thermal management technology when charging batteries
in groups. Without perfect battery group safety control and thermal management
technology, the safety and long life cycle of the battery cannot be guaranteed.
Therefore, the power battery charging management system is as important as the
safety of the battery itself.
3. Fast charging technology has high requirements on power batteries
Regarding the issue of charging speed, everyone hopes to achieve fast
charging. The current charging speed of high-energy power batteries can
replenish 80% of the energy in about 40-60 minutes. For urban commuting
transportation, it does not constitute the real use of electric vehicles. There
are barriers to use, but there may be some problems for those who hope to use
electric vehicles to solve transportation problems between cities.
From fast charging to ultra-fast charging (200-400kW), 90% of energy can be
replenished within 10 minutes, which will effectively shorten the gap between
electric vehicles and internal combustion engine vehicles.
Wang Zidong said that the current design plan is to reduce the thickness of
the electrodes, change the battery structure, and select materials more suitable
for fast charging. These will increase the production cost of power batteries,
reduce their energy density, and also reduce the power battery system. The
service life needs to be optimized from an overall consideration.
On the other hand, how to reduce the differences between single cells
during the fast charging process of the battery pack requires reasonable thermal
management system design to increase the service life of the power battery.
4. How to solve the safety problem of power battery?
Wang Zidong believes that new energy vehicle safety accidents are mainly
caused by thermal runaway of power batteries. Thermal runaway is not only the
result, but also the causes are complicated. The source of the accident is
difficult to identify, and safety issues should be highly valued.
The industry continues to reflect on safety issues, and the blind pursuit
of high energy density has become the focus. Professionals point out that
theoretically, battery energy density is inversely proportional to safety. When
companies pursue high energy density, safety issues are exposed. Although it is
not clear that fire incidents have occurred How relevant it is to the pursuit of
energy density, but as high-nickel ternary batteries enter the market, new
energy vehicles are facing higher safety technical requirements.
How to strike a balance between high energy density and improved safety has
become a major problem that needs to be solved in the current industry. Various
companies are improving overall safety from multiple levels of single cell,
module design and battery pack structural design.
Improving the safety of power batteries is mainly done at three levels,
including single cell, module design and battery pack structural design to
improve overall safety. In terms of single cells, additives can be added to the
electrolyte. It can be improved by reducing its flammability, improving the
temperature resistance of the separator, or improving the stability of the
cathode material. In terms of module design, safety can be improved by
strengthening temperature control design, BMS charging management, or changing
the single connection method. At the vehicle level, The battery can be
positioned to better dissipate heat, improve charging methods, and reduce safety
hazards caused by improper charging.
Since the battery pack is a component containing high-energy substances, it
is inherently dangerous. Moreover, as the specific energy and specific power of
the battery increase, the risk of accidents will increase. Therefore, it is
necessary to study the contradictory balance between energy density and safety,
including the balance of material performance, the balance of battery module
structure, the balance of the battery pack system level, the balance of cost
acceptability, and the balance in the multi-level utilization process. , balance
in the recycling process of power battery materials.
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