How should we maintain the lithium battery of our favorite machine
correctly? This problem has been bothering many loyal mobile phone users,
including me. After consulting some information, I recently had the opportunity
to consult with a PhD student majoring in electrochemistry and the deputy
director of a well-known domestic battery research institute. Now I will write
down some relevant knowledge and experiences I have recently acquired for the
benefit of all readers.
The positive electrode material of lithium-ion batteries usually consists
of active compounds of lithium, and the negative electrode is carbon with a
special molecular structure. The main component of the common cathode material
is LiCoO2. When charging, the potential applied to the two poles of the battery
forces the compound of the cathode to release lithium ions and embed them in the
carbon in which the molecules of the cathode are arranged in a lamellar
structure. During discharge, lithium ions are precipitated from the carbon in
the lamellar structure and recombine with the compound of the positive
electrode. The movement of lithium ions creates an electric current.
Although the principle of chemical reaction is very simple, in actual
industrial production, there are many more practical issues that need to be
considered: the material of the positive electrode needs additives to maintain
the activity of multiple charges and discharges, and the material of the
negative electrode needs to be designed at the molecular structure level to
Accommodate more lithium ions; the electrolyte filled between the positive and
negative electrodes, in addition to maintaining stability, also needs to have
good conductivity and reduce the internal resistance of the battery.
Although lithium-ion batteries rarely have the memory effect of
nickel-cadmium batteries, the principle of the memory effect is crystallization,
and this reaction almost never occurs in lithium batteries. However, the
capacity of lithium-ion batteries will still decrease after repeated charging
and discharging. The reasons are complex and diverse. Mainly due to changes in
the positive and negative electrode materials themselves. From a molecular
level, the hole structure that accommodates lithium ions on the positive and
negative electrodes will gradually collapse and become blocked. From a chemical
point of view, it is the active passivation of the positive and negative
electrode materials, causing side reactions to generate stable lithium ions.
Other compounds. Physically, the positive electrode material gradually peels
off, which ultimately reduces the number of lithium ions in the battery that can
move freely during charging and discharging.
Overcharging and over-discharging will cause permanent damage to the
positive and negative electrodes of lithium-ion batteries. From a molecular
level, it can be intuitively understood that over-discharging will cause the
negative electrode carbon to excessively release lithium ions and cause its
lamellar structure to collapse. Overcharging will force too many lithium ions
into the negative carbon structure, making it impossible for some of the lithium
ions to be released. This is why lithium-ion batteries are usually equipped with
charge and discharge control circuits.
Unsuitable temperatures will trigger other chemical reactions inside the
lithium-ion battery to produce compounds that we do not want to see. Therefore,
many lithium-ion batteries are equipped with protective temperature-control
separators or electrolyte additives between the positive and negative
electrodes. When the battery heats up to a certain level, the pores of the
composite membrane close or the electrolyte denatures, and the internal
resistance of the battery increases until the circuit is broken, and the battery
no longer heats up, ensuring that the battery charging temperature is
normal.
Can deep charging and discharging increase the actual capacity of
lithium-ion batteries? Experts told me unequivocally that this makes no sense.
They even said that the so-called "activation" of the first three full charges
and discharges, based on the knowledge of their two doctors, could not figure
out why this is necessary. However, why do many people change the capacity
marked in the Battery Information after deep charge and discharge? What follows?
will be mentioned.
Lithium-ion batteries generally have management chips and charge control
chips. There are a series of registers in the management chip, which store
values such as capacity, temperature, ID, charging status, and number of
discharges. These values will gradually change during use. I personally think
that the main function of the instruction in the instruction manual, "It should
be fully charged and discharged once a month or so after use, is to correct the
improper values in these registers so that the battery's charge control and
nominal capacity match the actual situation of the battery."
The charging control chip mainly controls the charging process of the
battery. The charging process of lithium-ion batteries is divided into two
stages, the constant current fast charge stage (when the battery indicator light
is yellow) and the constant voltage current decreasing stage (the battery
indicator light flashes green). In the constant current fast charge stage, the
battery voltage gradually increases reaches the standard voltage of the battery,
and then switches to the constant voltage stage under the control chip. The
voltage no longer rises to ensure that it will not be overcharged. The current
gradually weakens to 0 as the battery power increases, and charging is finally
completed.
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