Lithium-ion batteries use carbon materials as the negative electrode and lithium-containing compounds as the positive electrode. They contain no metallic lithium, only lithium ions. The term "lithium-ion battery" refers to any battery that uses lithium-ion intercalation compounds as the positive electrode material. The charging and discharging process of a lithium-ion battery is essentially the process of lithium-ion insertion and extraction. During this process, an equivalent number of electrons are simultaneously inserted and extracted (conventionally, the positive electrode uses insertion or extraction, while the negative electrode uses insertion or extraction). During charging and discharging, lithium ions shuttle back and forth between the positive and negative electrodes, a process figuratively called a "rocking chair battery."
When a battery is charged, lithium ions are generated at the positive electrode. These lithium ions then move through the electrolyte to the negative electrode. The carbon layer at the negative electrode has a layered structure with many micropores. The lithium ions that reach the negative electrode embed themselves into these micropores. The more lithium ions embedded, the higher the charging capacity. Similarly, when the battery is discharged (i.e., when we use the battery), the lithium ions embedded in the carbon layer at the negative electrode are released and move back to the positive electrode. The more lithium ions that return to the positive electrode, the higher the discharge capacity.
Generally, lithium battery charging current is set between 0.2C and 1C. The higher the current, the faster the charging, but the greater the battery heat generation. Moreover, charging with too high a current will not fully fill the battery, because the electrochemical reaction inside the battery requires time. It's like pouring beer; pouring too quickly will create foam and prevent the battery from filling completely.
Precautions for using (discharging) lithium-ion batteries
For batteries, normal use involves the discharge process. There are a few things to keep in mind when discharging lithium-ion batteries:
First, the discharge current should not be too high. Excessive current will cause the battery to overheat, potentially leading to permanent damage. This is not a problem for mobile phones and can be disregarded.
Secondly, over-discharge should be avoided. Lithium-ion batteries store electrical energy through a reversible electrochemical change. Excessive discharge can lead to irreversible reactions in this process. Therefore, lithium-ion batteries are highly susceptible to over-discharge; if the discharge voltage drops below 2.7V, the battery may become unusable. Fortunately, mobile phone batteries are equipped with internal protection circuits. Before the voltage drops to a level that could damage the battery, the protection circuits activate and stop the discharge.
