601525 polymer battery.Briefly describe the scheme of lithium battery protection circuit

　　Among the various types of batteries currently in use, lithium batteries (also known as lithium-ion secondary batteries or lithium-ion batteries) are a new type of power source that have only been developed in the past decade. Lithium batteries are different from general chemical power sources. The charging and discharging process is achieved through the insertion and deintercalation of lithium ions in the positive and negative electrodes of the battery. The negative electrode of a lithium battery is a carbon material, such as graphite; the positive electrode is a lithium-containing transition metal oxide, such as lithium cobalt oxide (LiC002). Moreover, both the positive and negative electrode materials of lithium batteries use lithium ion intercalation compounds with a layered structure in which lithium ions can be freely inserted and detached. The lithium ions between the layers will undergo an electrochemical reaction in an appropriate electrolyte. During charging, lithium ions are extracted from the positive electrode lattice driven by the external electric field, pass through the electrolyte, and are embedded in the negative electrode lattice. The process during discharge is exactly the opposite. Lithium ions return to the positive electrode, and electrons reach the positive electrode through the external circuit to recombine with the lithium ions. Compared with commonly used nickel-cadmium and nickel-hydrogen batteries, lithium batteries have many superior characteristics, mainly in the following aspects: (1) Lithium batteries do not contain any toxic elements such as mercury and cadmium, and are truly green and environmentally friendly batteries. (2) The working environment temperature range is wide, and it can generally work between -30℃ and 0℃, and has excellent high and low temperature discharge performance. (3) High specific energy, that is, lithium batteries of the same weight provide higher energy than other batteries. The specific energy of lithium batteries is generally 2 to 3 times that of nickel-cadmium batteries and nickel-metal hydride batteries. Therefore, it is beneficial to reduce the size and weight of portable electronic devices. (4) The power supply voltage of lithium batteries is high, generally 3.6V, which is about three times the voltage of nickel-cadmium batteries and nickel-metal hydride batteries. For electronic equipment with higher power supply voltage requirements, the number of series cells required in the battery pack can also be greatly reduced. Therefore, the lithium battery used in combination can easily obtain higher voltage. (5) No memory effect. Nickel-cadmium batteries and nickel-metal hydride batteries have memory effects and must be discharged regularly, otherwise the batteries will fail due to the memory effect. Lithium batteries have no memory effect and can be charged directly regardless of the remaining power. This allows the lithium battery performance to be fully utilized. (6) Long service life. Lithium batteries use carbon negative electrodes. The carbon negative electrodes do not generate metallic lithium during the charge and discharge process, thus preventing the battery from being damaged by a short circuit of internal metallic lithium. At present, the cycle life of lithium batteries can reach more than 5,000 times, which is much higher than other types of batteries. (7) The self-discharge rate is low. Self-discharge rate, also known as charge retention rate, refers to how much the battery automatically discharges when it is not in use. The self-discharge rate of lithium batteries is 2% to 5%, that of nickel-cadmium batteries is between 25% and 30%, and that of nickel-metal hydride batteries is between 30% and 35%. Therefore, lithium batteries retain charge for the longest time under the same environment. Based on the above advantages, lithium batteries are widely used in portable electronic devices. On the other hand, lithium batteries have high energy density, making it difficult to ensure battery safety. Specifically, in the overcharge state, the electrolyte will be decomposed, causing the temperature and pressure inside the battery to rise; in the overdischarge state, the electrolytic material in the negative electrode, copper, will melt and cause an internal short circuit, causing the temperature to rise: When the external circuit is short-circuited or the discharge current is too large, due to the characteristics of high internal resistance, the internal power consumption of the battery will increase and the temperature will also rise, which may cause oxidation or decomposition of the electrolyte, resulting in shortened lithium battery life. In addition, if the lithium battery is excessively discharged, the electrolyte in the battery will change, and the number of recharge cycles will be reduced, thereby affecting the service life of the lithium battery.

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