When the weather gets cold, the capacity of lithium batteries that are
originally full of energy will be reduced. Lithium batteries seem to have
entered a state of hibernation, which brings a lot of trouble to users of new
energy vehicles and digital products. The topic of today's article is the impact
of low temperature on lithium batteries and the industry's research and
development progress.
Is lithium battery most afraid of low temperature?
In tests conducted by AAA, an electric car had a range of 105 miles (169
kilometers) at 75 degrees Fahrenheit (about 24 degrees Celsius), which dropped
to 20 degrees Fahrenheit (about 7 degrees Celsius). The drop is as high as 60%
to 43 miles (approximately 69 kilometers). Batteries are somewhat similar to
people. They become less active when the climate turns colder. Lead-acid
batteries, lithium batteries and fuel cells will all be affected by low
temperatures, but to varying degrees.
Take lithium iron phosphate batteries, which are most commonly used in
electric buses, as an example. This type of battery is highly safe and has a
long cell life, but its low-temperature performance is slightly worse than
batteries of other technical systems. Low temperature has an impact on the
positive and negative electrodes, electrolytes and adhesives of lithium iron
phosphate. For example, the lithium iron phosphate cathode itself has poor
electronic conductivity and is prone to polarization in low-temperature
environments, thereby reducing battery capacity. Affected by low temperature,
the graphite lithium insertion speed is reduced, and metallic lithium is easily
precipitated on the surface of the negative electrode. If the battery is left
alone for insufficient time after charging, When it is put into use, all the
metallic lithium cannot be embedded inside the graphite again. Some metallic
lithium continues to exist on the surface of the negative electrode, which is
very likely to form lithium dendrites and affect the safety of the battery. At
low temperatures, the viscosity of the electrolyte will increase, and the
lithium ion migration resistance will also increase. In addition, in the
production process of lithium iron phosphate, the adhesive is also a very
critical factor, and low temperature will also have a great impact on the
performance of the adhesive.
It is also a lithium battery, and lithium titanate battery has excellent
low temperature resistance. The lithium insertion potential of the
spinel-structured lithium titanate negative electrode material is about 1.5V,
and lithium dendrites will not be formed. The volume strain during the charge
and discharge process is less than 1%. Nanosized lithium titanate batteries can
charge and discharge at high currents, achieving low-temperature fast charging
while ensuring battery durability and safety. For example, Yinlong New Energy,
which specializes in lithium titanate batteries, has normal charge and discharge
capabilities at -50-60°C.
Although lithium-ion batteries with graphite as the negative electrode can
be discharged at -40°C, it is more difficult to achieve conventional current
charging at -20°C and lower temperatures. This is also an area that the industry
is actively exploring.
The industry’s exploration of low-temperature resistant lithium
batteries
Enterprises and scientific research institutions in the industry are
exploring and researching the low-temperature resistance of batteries, mostly
focusing on improving the process of existing positive and negative electrode
materials, and creating conditions for the battery to work at low temperatures
by increasing the local ambient temperature of the battery.
Today's battery materials are moving toward nanometerization. The particle
size, resistance, and AB plane axis length of the material will affect the
low-temperature characteristics of the battery. Waterma prepared lithium iron
phosphate materials through three processes, using different processes to
nanometerize and coat them. The results show that the increase in the AB plane
axis length makes the lithium ion migration channel larger, which is beneficial
to increasing the battery rate. Performance; Judging from the materials produced
by the three processes, granular graphite with large interlayer spacing has
relatively small body resistance and ion migration resistance; in terms of
electrolyte, Waterma uses low-temperature additives based on a fixed solvent
system and lithium salt to Discharge capacity increased from 85% to 90%. It is
understood that as early as the end of 2016, Waterma has achieved a 0.5C
charging constant current ratio of 62.9% in an environment of -20, -30, and
-40°C, and a discharge of 94% at -20°C. At present, Waterma's low-temperature
batteries have been widely promoted in Inner Mongolia, three northeastern
provinces and other regions.
In addition, Penghui Energy’s power batteries can be used in an environment
of -20~60℃ and do not require heating and cooling systems. The low-temperature
resistance of Thornton New Energy's ternary batteries has been greatly improved.
The batteries can be discharged normally in a -20°C environment, which can meet
the needs of many vehicle companies.
Why does charging require more temperature than discharging?
Careful readers may find that many companies' battery products can achieve
normal discharge at low temperatures, but at the same temperature, it is more
difficult or even impossible to charge normally. Why?
According to industry insiders, when Li+ is embedded in graphite material,
it must first be solvated. This process consumes a certain amount of energy and
prevents Li+ from diffusing into the graphite. On the contrary, when Li+ comes
out of the graphite material and enters the solution, there will be a solvation
process. process, and solvation does not consume energy, Li+ can quickly escape
from graphite. Therefore, the charge acceptance ability of graphite materials is
significantly inferior to the discharge acceptance ability.
There are certain risks in battery charging in low temperature
environments. Because as the temperature decreases, the dynamic characteristics
of the graphite negative electrode worsen. During the charging process, the
electrochemical polarization of the negative electrode is significantly
intensified. The precipitated metallic lithium easily forms lithium dendrites,
which penetrates the separator and causes short circuit of the positive and
negative electrodes. .
Therefore, industry insiders recommend that you try to avoid charging
lithium-ion batteries at low temperatures. When the battery must be charged at
low temperatures, it is necessary to charge the lithium-ion battery with a small
current (i.e. slow charge) as much as possible, and fully shelve the lithium-ion
battery after charging to ensure that the metallic lithium precipitated from the
negative electrode can react with the graphite. , re-embedded inside the
graphite negative electrode.
Of course, lithium titanate batteries have material advantages. They can
still achieve fast charging at low temperatures. This kind of willfulness is
difficult to learn from batteries made of other materials.
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