Necessary protective measures for lithium-ion batteries
Lithium is the lightest of the alkali metals, with an atomic number of 3 and an atomic weight of 6.941. To improve safety and voltage, scientists have developed materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms tiny nanoscale storage lattices that can be used to store lithium atoms. This way, even if the battery casing breaks and oxygen enters, oxygen molecules are too large to fit into the tiny battery, thus preventing lithium atoms from contacting oxygen and exploding.
When the charging voltage exceeds 4.2V, lithium-ion batteries begin to exhibit side effects. The higher the pressure, the greater the risk. When the voltage of a lithium-ion battery exceeds 4.2V, less than half of the lithium atoms remain in the positive electrode material, often leading to battery failure and a permanent decrease in battery capacity. If the battery is overcharged, subsequent lithium metal accumulates on the surface of the material because the negative electrode is already filled with lithium atoms. These lithium atoms grow into dendritic crystals from the cathode surface towards the lithium ions. These lithium crystals can short-circuit the anode and cathode through the separator paper. Sometimes the battery explodes before a short circuit occurs. This is because during overcharging, substances such as the electrolyte decompose to produce gas, causing the battery casing or pressure valve to expand and rupture, allowing oxygen to enter and react with the lithium atoms accumulated on the negative electrode surface, leading to an explosion of the negative electrode.
Therefore, a voltage upper limit must be set when charging lithium-ion batteries to balance battery life, capacity, and safety. The optimal upper limit for charging voltage is 4.2V. Lithium-ion batteries also require a low voltage limit for discharging. When the battery voltage drops below 2.4V, some materials begin to deteriorate. Furthermore, because batteries self-discharge, the voltage decreases over time; therefore, it's best not to stop discharging at 2.4V. During discharge between 3.0V and 2.4V, lithium-ion batteries only release about 3% of their capacity. Therefore, 3.0V is the ideal discharge cutoff voltage. In addition to voltage limits, current limits are also necessary during charging and discharging. When the current is too high, lithium ions do not have time to enter the storage cell and will accumulate on the material surface.
When these ions gain electrons, they crystallize lithium atoms on the surface of the material, which is as dangerous as overcharging. If the battery compartment fails, it will explode. Therefore, lithium-ion battery protection must include at least three parts: an upper limit for charging voltage, a lower limit for discharging voltage, and an upper limit for current. A typical lithium-ion battery pack, in addition to the lithium-ion batteries, also has a protection board, which primarily provides these three protections. However, the protection provided by this three-layer protection board is clearly insufficient, and lithium-ion battery explosions continue to occur frequently worldwide. To ensure the safety of battery systems, the causes of battery explosions must be analyzed more carefully.
