[Technology] Detailed explanation of the advantages and disadvantages of
lithium iron phosphate power batteries
Lithium iron phosphate battery refers to a lithium-ion battery using
lithium iron phosphate as the positive electrode material. The cathode materials
of lithium-ion batteries mainly include lithium cobalt oxide, lithium manganate,
lithium nickel oxide, ternary materials, lithium iron phosphate, etc. Among
them, lithium cobalt oxide is the cathode material currently used in most
lithium-ion batteries.
working principle
The full name of lithium iron phosphate battery is lithium iron phosphate
lithium-ion battery. This name is too long, so it is simply called lithium iron
phosphate battery. Because its performance is particularly suitable for power
applications, the word "power" is added to the name, that is, lithium iron
phosphate power battery. Some people also call it "lithium iron (LiFe) power
battery."
significance
In the metal trading market, cobalt (Co) is the most expensive and has
limited storage, nickel (Ni) and manganese (Mn) are relatively cheap, while iron
(Fe) is the cheapest. The price of cathode materials is also in line with the
price trends of these metals. Therefore, lithium-ion batteries made of LiFePO4
cathode material should be the cheapest. Another feature of it is that it does
not pollute the environment.
The requirements for a rechargeable battery are: high capacity, high output
voltage, good charge and discharge cycle performance, stable output voltage,
capable of large current charge and discharge, electrochemical stability, and
safety during use (not due to overcharge, overdischarge, or short circuit) etc.
may cause combustion or explosion due to improper operation), wide operating
temperature range, non-toxic or less toxic, and no pollution to the environment.
Lithium iron phosphate batteries using LiFePO4 as the positive electrode are
good in these performance requirements, especially in terms of large discharge
rate discharge (5~10C discharge), stable discharge voltage, safety (no burning,
no explosion), and lifespan (number of cycles). ), no pollution to the
environment, it is the best, and it is currently the best high-current output
power battery.
Structure and working principle
LiFePO4 serves as the positive electrode of the battery. It is connected to
the positive electrode of the battery by aluminum foil. There is a polymer
separator in the middle, which separates the positive electrode from the
negative electrode. However, lithium ions Li can pass but electrons e- cannot
pass. The right side is composed of carbon (graphite) The negative terminal of
the battery is connected to the negative terminal of the battery by copper foil.
Between the upper and lower ends of the battery is the electrolyte of the
battery, and the battery is sealed by a metal casing.
When the LiFePO4 battery is charged, the lithium ions Li in the positive
electrode migrate to the negative electrode through the polymer separator;
during the discharge process, the lithium ions Li in the negative electrode
migrate to the positive electrode through the separator. Lithium-ion batteries
are named because lithium ions migrate back and forth during charging and
discharging.
Main performance
The nominal voltage of LiFePO4 battery is 3.2V, the termination voltage is
3.6V, and the termination voltage is 2.0V. Due to the different quality and
process of the positive and negative electrode materials and electrolyte
materials used by various manufacturers, there will be some differences in their
performance. For example, the same model (standard battery in the same package)
has a large difference in battery capacity (10% to 20%).
What should be noted here is that lithium iron phosphate power batteries
produced by different factories will have some differences in various
performance parameters; in addition, some battery properties are not included,
such as battery internal resistance, self-discharge rate, charge and discharge
temperature, etc.
The capacity of lithium iron phosphate power batteries varies greatly and
can be divided into three categories: small ones ranging from a few tenths to
several milliamp hours, medium-sized ones tens of milliamp hours, and large ones
hundreds of milliamp hours. There are also some differences in the same
parameters of different types of batteries.
Over-discharge to zero voltage test
The lithium iron phosphate power battery using STL18650 (1100mAh) has been
discharged to zero voltage test. Test conditions: Fully charge the 1100mAh
STL18650 battery with a 0.5C charge rate, and then discharge it with a 1.0C
discharge rate until the battery voltage is 0C. Then divide the batteries placed
at 0V into two groups: one group is stored for 7 days, and the other group is
stored for 30 days; after the storage expires, it is fully charged with a 0.5C
charging rate, and then discharged with a 1.0C rate. Finally, compare the
differences between the two zero-voltage storage periods.
The result of the test is that after 7 days of zero-voltage storage, the
battery has no leakage, good performance, and the capacity is 100%; after 30
days of storage, the battery has no leakage, good performance, and the capacity
is 98%; after 30 days of storage, the battery is subjected to 3 charge-discharge
cycles. The capacity is restored to 100%.
This test shows that even if the battery is over-discharged (even to 0V)
and stored for a certain period of time, the battery will not leak or be
damaged. This is a feature that other types of lithium-ion batteries do not
have.
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