New energy vehicles refer to vehicles that use unconventional vehicle fuels
as their power source. Currently, the most common ones on the market are pure
electric vehicles and hybrid vehicles. Just like ordinary cars need internal
combustion engines, electric cars need power batteries to provide forward
energy. Currently, the most important type of power batteries is lithium-ion
batteries.
"Follow scientific laws, don't be blind" and "develop safe and reliable
technology first", at the "China Yichun 2018 Global Lithium Battery Industry
Chain Summit Forum" held in Beijing from October 17th to 18th, people from
academia and industry , have made such calls on the issue of the safety of
lithium battery development.
Lithium-ion battery (Li-ion Battery) is developed from lithium battery
(Lithium Battery). Lithium batteries have been in people's lives for a long
time. For example, button batteries are lithium batteries. The positive
electrode material of lithium batteries is manganese dioxide or thionyl
chloride, and the negative electrode is metallic lithium. After the battery
assembly is completed, the battery has voltage and will no longer be charged.
Charging of lithium batteries is generally prohibited because lithium dendrites
are easily formed during the charging and discharging process, causing an
internal short circuit in the battery.
In 1992, Japan's Sony Corporation invented a battery using carbon material
as the negative electrode and lithium-containing compounds as the positive
electrode. During the charge and discharge process, only lithium ions and no
metallic lithium exist, which is the current lithium-ion battery. Since then,
Japan's Sony Energy Development Company and Canada's Moli Energy Company have
successfully developed new lithium-ion batteries (hereinafter referred to as
"lithium batteries"). Currently, lithium batteries have been widely used in
various handheld electronic products and electric vehicles.
For lithium batteries, the performance is reflected in two indicators: one
is the charge-discharge rate, which represents the charging speed of the
battery; the other is energy density, which determines how many kilometers a car
can last. However, the blind pursuit of these two indicators largely sacrifices
the safety factor.
“Fast charging technology has no way out yet”
"At least 60% of these fire incidents occurred during charging or just
after charging, indicating that there was a big problem with charging." said
Wang Zidong, director of the National 863 Electric Vehicle Major Special Power
Battery Testing Center.
During the charging process of lithium batteries, lithium ions are inserted
and deintercalated back and forth between the two electrodes, and no oxidation
reaction occurs in the positive and negative electrodes. However, Wang Zidong
pointed out that the current charging method and usage process are all based on
redox reactions, which is not the charging method that lithium batteries should
have according to their own rules. Previous experimental results by Wang
Zidong's team showed that using the current charging method can reduce battery
life by about 30%. Therefore, under such circumstances, Wang Zidong believes
that charging with high current should not be considered.
The charge and discharge rate of a lithium battery refers to the charge and
discharge current within the unit rated capacity. For example, when a battery
with a rated capacity of 100Ah is charged and discharged at 20A, the charge and
discharge rate is 0.2C. Generally, the charging current of lithium batteries is
set between 0.2C and 1C. The larger the current, the faster the charging, but at
the same time, the battery heating phenomenon will be more serious. At present,
the charging capacity of pure electric vehicles is slow charging, basically
between 0.3C and 0.5C. On the other hand, charging with too much current will
result in insufficient capacity because the electrochemical reaction inside the
battery takes time. Just like pouring beer, pouring it too quickly will produce
foam and result in a full pour.
Qi Lu, director of the New Energy Materials and Technology Laboratory of
Peking University, said that today's multi-element positive metal composite
oxide batteries need to be charged in 8 minutes. In theory, it requires a rate
of 10C to achieve it. "This energy is unimaginable."
These technical bottlenecks are actually not new problems. Qilu is one of
the pioneers in the field of lithium battery research in China. He served as the
chief scientist of the clean energy electric vehicle power battery project for
the 2008 Olympic Games. As early as the Beijing Olympics, they targeted Various
experiments have been conducted on various problems. At that time, ternary
material batteries could already be charged within 5 minutes. In the experiment,
the heat of the ternary lithium battery cannot be released quickly during the
rapid charging process, which greatly increases the possibility of explosion.
Considering safety issues, Qilu said that this technology cannot be used in pure
electric vehicles and can only be used in battery hybrid vehicles.
In addition, fast charging of power batteries also faces a very real
problem-the city's power infrastructure cannot meet individual needs. Assuming
that a bus uses a 150kWh battery and takes 5 minutes to charge, a bus requires
100kW power supply capacity. If there are hundreds or thousands of buses
charging at the same time, it will have a great impact on the power grid. .
"Today's urban power grid simply cannot do this." Qilu said.
Currently, Wang Zidong's team is studying how to adjust the charging method
according to the battery characteristics during the charging process. After
changing different charging methods, the battery life of 500 charges with the
ordinary standard charging method can be achieved 1,000 times under the new
method, effectively slowing down the battery life. battery degradation.
Therefore, Wang Zidong said that even if there are many bottlenecks, lithium
batteries will definitely have a charging method that is particularly suitable
for them.
Qilu believes that the most appropriate method at this stage is to charge
by wiring in parking spaces, which can take two to three hours, five to six
hours, or even one night. This is fully possible with charging technology. By
first developing safe and reliable charging methods, we can promote the steady,
safe and healthy development of electric vehicles.
Energy density and safety are a contradiction
In February 2018, the Ministry of Finance, the Ministry of Industry and
Information Technology and other four ministries and commissions jointly issued
the "Notice on Adjusting and Improving Fiscal Subsidy Policies for the Promotion
and Application of New Energy Vehicles", canceling subsidies for pure electric
vehicles with a driving range of less than 150 kilometers, and canceling
subsidies for pure electric vehicles with a driving range of less than 150
kilometers. The subsidy for pure electric vehicles with a driving range of 300
kilometers has been increased to 34,000 yuan, and the subsidy for models with a
driving distance of more than 400 kilometers has been increased to 50,000
yuan.
Wang Yunshi, director of the China Transportation Energy Center at the
University of California, Davis, analyzed that this means that after pure
electric vehicles reach a driving range similar to that of gasoline vehicles,
the longer the driving range, the better. The new policy may hope to promote the
development of power batteries through requirements on the energy density of
power battery systems.
The energy density of lithium batteries (Wh/kg) refers to the amount of
energy that the battery can store per unit weight, which is mainly determined by
the material characteristics of the battery. Calculating that the energy density
of ordinary lead-acid batteries is about 40Wh/kg, if lead-acid batteries are
used to drive a family car for more than 200 kilometers, nearly 1 ton of
batteries will be needed. Therefore, under the premise of controlling the
battery weight within a certain range, the greater the energy density of the
battery, the longer the cruising range of the car.
The higher the energy density value, the better. However, batteries are
small devices with highly concentrated energy. When more energy is collected in
a smaller volume, if it is used improperly, such as when the temperature rises
or a sudden violent collision occurs, the consequences will be. It can even be
compared to a bomb.
According to the latest data released by the New Energy Research
Institute’s True Lithium Research, after June 2018, the installed capacity of
120Wh/kg battery packs accounted for approximately 95%, while the proportion in
June a year ago was only 7.3%. In other words, the progress in energy density of
domestic battery systems far exceeds overseas at an "astonishing speed".
2017-2018.8 Installed proportion of battery packs with different energy
densities in China’s electric vehicle passenger car production Source: True
Lithium Research
Although the number of installed high-energy-density battery packs has
greatly increased, the issue of how to balance energy density and safety has not
yet been resolved.
"Today, energy density is undoubtedly inversely proportional to battery
safety, and we have not yet solved this problem." Qilu said.
At present, the lithium batteries mass-produced in China have a theoretical
cell energy density of 300 to 400Wh. If there is no way to break through this
upper limit, the material space can be expanded by using anti-friction
diaphragms to increase energy density. The function of the separator is to
separate the positive and negative electrodes of the battery, prevent the two
electrodes from contacting and causing a short circuit, and allow electrolyte
ions to pass through.
"This is the simplest and most dangerous method." Wang Zidong said.
Wang Zidong said that his research team had previously learned about the
Samsung Note7 fire incident. The inspection found that the separator used in the
Samsung Note7 battery was about 45 microns to 46 microns thick. When using
homogeneous materials, some power battery manufacturers He is even considering
using a 10-micron and 8-micron thick separator. In his opinion, such an idea is
"very bold."
Since particles cannot be avoided during the battery manufacturing process,
the actual separator will have some minor "work injuries". As long as there is
no breakthrough in the material, the ultra-thin separator, flammable
electrolyte, and undercurrent of dendrites will If you take risks at this stage,
there will be an explosion hazard.
"Before we master the ignition laws of lithium batteries, controlling the
balance between energy density, safety and lifespan is an issue we cannot
ignore." Wang Zidong said.
In fact, the "fish and bear" problem of energy density and safety not only
troubles the development of lithium battery technology in China, but also
troubles South Korea and Japan, which are the leaders in the industry. Seunghoon
Han, an analyst from Deutsche Securities Korea, said that currently no company
dares to say that its technical route is completely certain, but every company
believes that their batteries are the safest. In view of the fact that many
other industries have solved many safety problems through standardization and
standardization, the safety problems faced by the development of the lithium
battery industry today may also need standardization to solve. These development
bottleneck issues do not affect fast charging technology and increasing energy
density as the future direction of technological development.
"Only after the safety indicators are standardized and standardized,
technological development will make it easier to determine which ones are safer
and which ones are unsafe." Han said.
On the other hand, high energy density means the need for high-density
materials. High-density materials will determine the amount of stored electrical
energy. When the thickness that a material can achieve reaches the safe limit
but is still lower than people's expectations, many people turn their attention
to seeking new materials. Wang Zidong believes that if there is no breakthrough
in materials and breakthroughs in high energy density, the bottleneck period
will be stagnant for a long time, possibly 10 or 50 years.
However, Qilu is not optimistic about the recently popular graphene and
nanomaterials. He said that these materials, including the previously used
lithium ferrous sulfate, are actually low-density materials, while the densities
of ternary materials and multi-element materials are much higher, and the
density can even be made higher in the future.
"From a material perspective, graphene has good electrical conductivity,
but can the concept be the same when it is deduced from the material to
batteries and then to electric vehicles?" Qilu said, "Nanomaterials will not
have any specific applications in this field."
Whether it is fast charging or high energy density, Qilu believes that we
must guard against arrogance and impetuosity and remain vigilant, especially for
technologies such as solid-state lithium batteries that can ensure energy
density and are safe to use, without electrolyte materials with good
conductivity. Before it appears, one should not have too many expectations for
the industrialization of this type of battery.
"We still have to go all out to develop our feasible technology. I think
the most important thing is our 'tomorrow's technology.'" Qilu said.
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