Electrical abuse is one of the common safety hazards in the use of lithium-ion batteries, and the main factors causing electrical abuse are overcharging and over-discharging of the battery.
I. Overfilling Overcharging triggers an irreversible thermal runaway chain reaction by destroying the electrode structure and generating lithium dendrites to puncture the diaphragm and form an internal short circuit. This process not only stems from the imbalance of electrochemical reactions, but is also closely related to material thermal stability and battery design (e.g., diaphragm performance, heat dissipation capability). Therefore, the overcharge protection function of the battery management system (BMS) and the optimization of the thermal stability of the electrode materials are the key to preventing this type of accident. During normal charging, lithium ions move in an orderly manner between the positive and negative electrodes. However, when overcharging, the positive electrode material (such as lithium cobaltate) will be excessive “loss” of lithium ions and structural collapse, the release of oxygen; the negative electrode will receive enough lithium ions, excess lithium will be on the surface of the growth of twigs, the formation of sharp “lithium dendrites”. The diaphragm in the middle of the battery is like a layer of paper, used to separate the positive and negative electrodes. Lithium dendrites grow sharper and sharper, will poke through the diaphragm, so that the positive and negative electrodes in direct contact, the formation of “internal short circuit”. At this time, the current inside the battery suddenly becomes large, and the local temperature rises instantly. The high temperature generated by the internal short circuit will ignite a series of dangerous reactions.
When overcharging occurs in lithium batteries, the lattice structure of the positive electrode material is distorted, and the transition metal oxides, which are in a thermodynamically unstable state, begin to decompose, releasing highly reactive oxygen. These oxygen quickly diffuse into the electrolyte, and the electrolyte containing carbonate organic solvents (such as vinyl carbonate, dimethyl carbonate, etc.) undergoes intense redox reaction. Driven by the free radical chain reaction, the reaction rate grows exponentially, instantly releasing a large amount of reaction heat, causing the local temperature to rise sharply, providing an initial heat source for the occurrence of thermal runaway.
II. Over-discharge
When over-discharge occurs in lithium batteries, the lattice structure of the positive electrode material will be irreversibly damaged due to electrochemical stress, and the material particles will be broken to form sharp active material fragments. After these fragments penetrate the diaphragm, a conductive path will be established between the positive and negative electrodes, triggering an internal short-circuit fault. The Joule heat generated by the short-circuit current will trigger a series of exothermic side reactions such as thermal decomposition, SEI membrane decomposition and electrolyte oxidation within the battery, which will lead to thermal runaway of the battery when the rate of heat generation exceeds the heat dissipation capacity.
Under normal discharge conditions, lithium ions are dislodged from the negative electrode and embedded in the positive electrode through the transport channel composed of electrolyte and diaphragm. When the over-discharge process occurs, the lithiation degree of the positive electrode material breaks through the thermodynamic stability interval, and the lithium ions in its layered crystal structure are excessively de-embedded, triggering lattice distortion and structural collapse, and forming active material fragments with sharp edges; at the same time, the negative electrode material is unbalanced due to the sustained de-embedding of lithium ions leading to the imbalance of inter-layers' stress, and the layered structure is twisted and broken, which triggers the irreversible precipitation of lithium metal. Although the lithium deposits formed by this process lack regularity in morphology compared to the lithium dendrites formed by overcharging, the irregular lithium particles generated still pose a significant threat to the internal safety of the battery.
III. Protection
To avoid overcharge and overdischarge triggering short circuit in Li-ion battery, we need to start from multi-dimensional aspects such as technical protection, usage standardization, material optimization, etc. The core is to control the voltage boundary and temperature threshold, i.e., “voltage-temperature dual control”, while the BMS and the charger (inverter) construct the hardware defense, the user's standardization of usage is the basis, and the material upgrading (e.g., solid-state batteries) and structural optimization (thermal isolation) are the key elements from the perspective of the safety of Li-ion batteries. Material upgrades (e.g. solid-state batteries) and structural optimization (thermal isolation) essentially enhance safety. Just like when driving a car, it is important to observe the speed limit (voltage boundary) and check the brakes (BMS protection) in order to minimize the risk of accidents.