Brief introduction of lithium polymer battery

Table of Contents


What is a lithium polymer battery

The so-called lithium polymer battery is a battery that uses a carbon material that can absorb and desorb lithium ions as the negative electrode active material, and the lithium ion symbol is Li-ion. As we all know, batteries are generally composed of basic elements such as positive electrodes, negative electrodes, separators, and electrolytes, so these materials used in lithium polymer batteries are generally the following substances:

Positive electrode: lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), etc.;

Negative electrode: artificial graphite series, natural graphite series, coke series, etc.;

Diaphragm: single-layer or multi-layer microporous film composed of polyethylene (PE), polypropylene (PP), etc.;

Electrolyte: monobasic, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), etc. binary or ternary mixture

Most of the lithium polymer batteries on the market use lithium cobalt oxide as the positive electrode and graphite series as the negative electrode.

The working mechanism of the lithium polymer battery

When the battery is charged, the lithium ions in the positive electrode material are dissolved out and embedded in the modified graphite layer of the negative electrode; when the battery is discharged, the lithium ions are deintercalated from the graphite layer and backfilled into the layered structure of the positive electrode cobalt lithium oxide through the separator.

With the charging and discharging, lithium ions are continuously inserted and extracted from the positive and negative electrodes. The rated voltage of the lithium polymer battery is 3.7V, the charging limit voltage is 4.2V, and the discharge limit voltage is 3.0V.

The charging process of lithium polymer battery is divided into two steps:

The first is constant current charging, the current is constant, and the voltage is constantly rising. When the voltage is charged to 4.2V, it is automatically converted to constant voltage charging. During constant voltage charging, the voltage is constant, and the current is getting smaller and smaller until the charging current is less than the preset value. until the value is fixed.

When charging the battery of the mobile phone with direct charging, it clearly shows that the battery is full, but it still shows that it is charging. In fact, the voltage at this time has reached 4.2V, so the battery is displayed as full, and it is in the process of constant voltage charging. .

Then some people may ask, why do constant voltage charging, just use constant current to charge 4.2V directly.

In fact, it is easy to explain, because each battery has a certain internal resistance. When charging with constant current to 4.2V, this 4.2V is not the actual voltage of the battery, but the voltage of the battery plus the internal resistance of the battery.

The sum of the voltage consumed, if the current is large, the voltage consumed on the internal resistance is also large, so the actual battery voltage may be much lower than 4.2V, so use the constant voltage charging process to slow the charging current. Slow down, so that the actual voltage of the battery is very close to 4.2V.

In the process of using lithium-ion batteries, you don’t need to know so much. You only need to know how to charge, how to discharge, and how to use them to make the battery play its best role and maintain good cycle performance.

Let me share some of my views on this aspect:

It is best to use the original charger for lithium-ion batteries, otherwise it will damage the battery to a certain extent and affect its service life;

Regarding the issue of direct charging and cradle charging, I think it is best to use cradle charging if time permits, which can not only prolong the service life of the battery but also increase the single charge capacity;

If the battery is not used for a long time, it is best to charge it with about 40% of the power and store it at a temperature of 10 to 30 ℃, and replenish it every six months or so;

For a new battery that has just been bought, some people say that the first three charges must be charged for more than ten hours in order to fully activate the battery. I don’t think this is necessary. Of course, for a new battery, the capacity of the first few times may be less than that in the future. That is caused by the long-term storage of the battery and the passivation of the surface of the active material.

I think one or two hours after the first few chargings with the cradle until the green light is on is enough. Of course, the charging time will depend on the charging current of your charger and your battery. The capacity of the battery will vary. In short, it is not that the longer the charging time, the better. This will not only activate the battery, but will affect the life of the battery. In severe cases (if your charger voltage accuracy is not controlled enough), it will explode.

In the long-term use of batteries, people often have a misunderstanding in understanding. I need to tell you that people often think that the number of times the battery is used is certain, such as 500 times, so it must be fully utilized every time. Therefore, it is required that each charge must be fully charged, and each discharge must be cleaned as much as possible.

It seems that this is the most effective use. In fact, the service life of the battery has a certain relationship with your usage. If the cycle life of your battery is 500 times, if you use it properly, then maybe it can be used 700 to 800 times, so the correct way to use it is to not charge too much, about 90% is enough, during use When you see that the battery is low, you can charge it immediately, instead of waiting for it to automatically shut down or not to charge it after it automatically shuts down.

We have done such an experiment in the laboratory: the battery life of 80% DOD is longer than that of 100% DOD The battery life is about 30% longer;Another important point is that lithium-ion batteries have very strict requirements on chargers, and the restrictions on charging voltage are very strict. You must see whether its requirements are 4.1V or 4.2V, because if the negative electrode is used graphite series, then the limit The voltage is 4.2V. If the negative electrode is a coke series, then the limit voltage is 4.1V. This is generally stated on the battery. In short, the charger and the battery must match. You cannot use a 4.2V charger to charge 4.1V. Battery;

Another point must be emphasized: Never use a Ni-MH battery or Ni-Cr battery charger to charge a lithium-ion battery, as this will not only damage the battery but may also explode;Li-ion batteries have no memory function, so each charge does not need to be discharged like nickel-hydrogen batteries and nickel-chromium batteries, and it can be charged and discharged anytime, anywhere.


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