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.
"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.
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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. the battery’s 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".
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