How does lithium-ion battery work?

The cathode material of a lithium ion battery usually consists of an active compound of lithium, and the anode is a carbon of a special molecular structure. The main component of the common positive electrode material is LiCoO2. When charging, the potential applied to the two poles of the battery forces the compound of the positive electrode to release lithium ions and is embedded in the carbon in which the negative electrode molecules are arranged in a sheet structure. During discharge, lithium ions are precipitated from the carbon of the sheet structure and recombined with the compound of the positive electrode. The movement of lithium ions produces a current.

Although the principle of a chemical reaction is very simple, in actual industrial production, there are many practical problems to be considered: the material of the positive electrode needs additives to maintain the activity of multiple charge and discharge, and the material of the negative electrode needs to be designed at the molecular structure level. Contains 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 memory effect is crystallized, and the reaction is almost nonexistent in lithium-ion batteries. However, the capacity of lithium-ion batteries will still decline after repeated charging and discharging, and the reasons are complex and varied. From the perspective of the molecular level, the structure containing lithium ions on the anode and cathode will gradually collapse and become blocked. From the chemical point of view, is the positive and negative materials active passivation, side reactions occur to generate stable other compounds. Physical conditions such as the gradual peeling of the anode material eventually reduce 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 perspective, it can be intuitively understood that excessive discharge will cause the anode carbon to release lithium ions excessively, causing the sheet structure to collapse. Overcharging will hardly plug too much lithium into the negative carbon structure, and some of the lithium ions will no longer be released. This is why lithium-ion batteries are usually equipped with a charge-discharge control circuit.

The unsuitable temperature will cause other chemical reactions inside the lithium-ion battery to form compounds that we do not want to see, so there are protective temperature-controlled separators or electrolyte additives between the positive and negative electrodes of many lithium-ion batteries. When the temperature of the battery is raised to a certain extent, the composite membrane is closed or the electrolyte is denatured, and the internal resistance of the battery is increased until the circuit is broken, and the battery is no longer heated to ensure that the battery charging temperature is normal.

Can deep charge and discharge increase the actual capacity of lithium-ion batteries? The expert told me clearly that this is meaningless. They even said that the so-called "activation" of the first three full charges and discharge is also unnecessary. However, why so many people change the capacity indicated in Battery Information after deep charging? It will be mentioned later.

Lithium-ion batteries generally have a management chip and a charge control chip. The management chip has a series of registers, which contain values such as capacity, temperature, ID, state of charge, and a number of discharges. These values will gradually change during use. I personally think that the main function of the "use should be fully charged and discharged once a month" in the instructions for use should be to correct the improper values in these registers, so that the battery 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 battery is divided into two stages, the constant current fast charging phase (when the battery indicator is yellow) and the constant voltage current decreasing phase (the battery indicator light flashes green. During the constant current fast charging phase, the battery voltage is gradually increased. The standard voltage to the battery is then transferred to the constant voltage stage under the control chip, the voltage is no longer raised to ensure that it will not overcharge, and the current gradually decreases to zero as the battery power rises, and the charging is finally completed.

The electricity statistic chip can calculate the battery's power by recording the discharge curve (voltage, current, time), which is the wh. the value we read in Battery Information. The lithium-ion battery will change its discharge curve after repeated use. If the chip has no chance to read the complete discharge curve again, the calculated power is not accurate. So we need to deep charge the chip to calibrate the battery.

The page contains the contents of the machine translation.