Statistics show that global demand for lithium-ion batteries has reached 1.3 billion units, and this figure is increasing year by year as application areas continue to expand. Because of this, with the rapid surge in the use of lithium-ion batteries across various industries, battery safety performance has become increasingly important. Not only are excellent charge and discharge performance required for lithium-ion batteries, but also higher safety performance is demanded. So why do lithium batteries catch fire or even explode, and what measures can be taken to prevent and eliminate these incidents?
Laptop battery explosions are related not only to the manufacturing process of the lithium battery cells used, but also to the battery protection board encapsulated within the battery, the laptop's charge and discharge management circuitry, and the laptop's heat dissipation design. Inadequate heat dissipation design and charge/discharge management in laptops can cause the battery cells to overheat, significantly increasing cell activity and thus the likelihood of explosion and combustion.
Analysis of the Composition and Performance of Lithium-ion Battery Materials
First, let's understand the material composition of lithium batteries. The performance of lithium-ion batteries mainly depends on the structure and performance of the internal materials used. These internal materials include negative electrode materials, electrolytes, separators, and positive electrode materials. Among them, the selection and quality of positive and negative electrode materials directly determine the performance and price of lithium-ion batteries. Therefore, the research on inexpensive, high-performance positive and negative electrode materials has always been a key focus of the lithium-ion battery industry.
Carbon materials are generally used for anodes, and their development is relatively mature. However, the development of cathode materials has become a crucial factor restricting further improvements in lithium-ion battery performance and reductions in price. In currently commercially produced lithium-ion batteries, the cost of cathode materials accounts for approximately 40% of the total battery cost; therefore, a reduction in the price of cathode materials directly determines a reduction in the price of lithium-ion batteries. This is especially true for lithium-ion power batteries. For example, a small lithium-ion battery used in a mobile phone requires only about 5 grams of cathode material, while a lithium-ion power battery powering a bus may require as much as 500 kilograms of cathode material.
Although theoretically many types of materials can be used as positive electrodes in lithium-ion batteries, the most common positive electrode material is LiCoO2. During charging, the potential applied to the battery electrodes forces the compound at the positive electrode to release lithium ions, which then embed into the layered carbon structure at the negative electrode. During discharging, lithium ions are released from the layered carbon structure and recombine with the compound at the positive electrode. This movement of lithium ions generates an electric current. This is the working principle of a lithium battery.
Lithium-ion battery charge and discharge management design
During charging, the potential applied to the battery's electrodes forces the compound at the positive electrode to release lithium ions, which then embed into the layered carbon structure of the negative electrode. During discharging, lithium ions are released from the layered carbon structure and recombine with the compound at the positive electrode. This movement of lithium ions generates an electric current. While the principle is simple, in actual industrial production, many more practical issues need to be considered: the positive electrode material requires additives to maintain its activity after multiple charge-discharge cycles; the negative electrode material needs to be designed at the molecular level to accommodate more lithium ions; and the electrolyte filling the space between the positive and negative electrodes, besides maintaining stability, also needs to have good conductivity to reduce the battery's internal resistance.
While lithium-ion batteries offer the advantages mentioned above, they place high demands on protection circuits. Overcharging and over-discharging should be strictly avoided during use, and the discharge current should not be too high; generally, the discharge rate should not exceed 0.2C. The charging process of a lithium battery is shown in the figure. Before charging begins within a charging cycle, the battery voltage and temperature need to be checked to determine if charging is possible. If the battery voltage or temperature exceeds the manufacturer's allowable range, charging is prohibited. The permissible charging voltage range is 2.5V to 4.2V per cell.
When the battery is deeply discharged, the charger must have a pre-charge process to prepare the battery for fast charging. Then, according to the fast charging speed recommended by the battery manufacturer, typically 1C, the charger charges the battery with a constant current, and the battery voltage rises slowly. Once the battery voltage reaches the set termination voltage (typically 4.1V or 4.2V), constant current charging terminates, the charging current rapidly decreases, and charging enters the full charge process. During full charge, the charging current gradually decreases until the charging rate drops below C/10 or the full charge time is exceeded, at which point it enters the top-off charging phase. During top-off charging, the charger replenishes the battery with a very small charging current. After a period of top-off charging, charging is turned off.
