Date: 2025-06-20 hits: 104
A. Overcharge
Overcharge destroys the electrode structure, generates lithium dendrites that pierce the diaphragm, forms an internal short circuit, and then triggers an irreversible thermal runaway chain reaction. This process is not only due to the imbalance of electrochemical reactions, but is also closely related to the thermal stability of materials and battery design (such as diaphragm performance and heat dissipation capacity). Therefore, the overcharge protection function of the battery management system (BMS) and the thermal stability optimization of electrode materials are the key to preventing such accidents.
During normal charging, lithium ions move in an orderly manner between the positive and negative electrodes. However, during overcharging, the positive electrode material (such as lithium cobalt oxide) will collapse due to excessive "throwing" of lithium ions and release oxygen; after the negative electrode receives enough lithium ions, the excess lithium will grow on the surface like branches, forming sharp "lithium dendrites". The diaphragm in the middle of the battery is like a layer of paper, which is used to separate the positive and negative electrodes. The longer and sharper the lithium dendrites are, the more they will pierce the diaphragm, allowing the positive and negative electrodes to directly contact, forming an "internal short circuit". At this time, the current inside the battery suddenly increases, and the local temperature rises instantly. The high temperature generated by the internal short circuit will ignite a series of dangerous reactions.
When a lithium battery is overcharged, the lattice structure of the positive electrode material is distorted, and the transition metal oxide in a thermodynamically unstable state begins to decompose, releasing highly active oxygen. This oxygen quickly diffuses into the electrolyte and undergoes a violent redox reaction with the electrolyte containing carbonate organic solvents (such as ethylene carbonate, dimethyl carbonate, etc.). Driven by the free radical chain reaction, the reaction rate increases exponentially, and a large amount of reaction heat is released instantly, causing the local temperature to rise sharply, providing the initial heat source for the occurrence of thermal runaway.
B. Overdischarge
When a lithium battery is overdischarged, the lattice structure of its positive electrode material will be irreversibly damaged due to electrochemical stress, and the material particles will be broken into sharp active material fragments. After these fragments penetrate the diaphragm, they will establish a conductive path between the positive and negative electrodes, causing an internal short circuit failure. The Joule heat generated by the short-circuit current will trigger a series of exothermic side reactions such as thermal decomposition, SE! membrane decomposition and electrolyte oxidation inside the battery. When the heat generation rate exceeds the heat dissipation capacity, it will cause thermal runaway of the battery.
Under normal discharge conditions, lithium ions are deintercalated from the negative electrode and embedded in the positive electrode through the transmission channel composed of the electrolyte and the separator. When the over-discharge process occurs, the degree of lithiation of the positive electrode material exceeds the thermodynamic stability range, and the lithium ions in its layered crystal structure are over-intercalated, causing lattice distortion and structural collapse, forming active material fragments with sharp edges. At the same time, the negative electrode material causes interlayer stress imbalance due to continuous lithium ion deintercalation, and the layered structure is distorted and broken, which in turn causes irreversible precipitation of metallic lithium. Although the lithium deposits formed in this process lack regularity in morphology compared to the lithium dendrites formed by overcharging, the irregular lithium particles produced by it still pose a significant threat to the internal safety of the battery.
C. Protection
To avoid short circuits in lithium batteries caused by overcharging and overdischarging, we need to start from multiple dimensions such as technical protection, usage specifications, and material optimization. The core is to control the voltage boundary and temperature threshold, that is, "voltage and temperature dual control." BMS and chargers build a hardware defense line. User specification use is the basis, while material upgrades (such as solid-state batteries) and structural optimization (thermal isolation) essentially improve safety. Just like when driving, you must comply with the speed limit (voltage boundary) and check the brakes (BMS protection) to minimize the risk of accidents.
