[Abstract] This article briefly introduces the basic failure model of lithium batteries and the rate capacity effect of lithium batteries. The internal self-discharge mechanism of button-type lithium batteries is discussed, and the possible causes of internal failure are analyzed from the perspective of product manufacturing process. [Keywords] ; rate capacity effect; self-discharge; diaphragm puncture; spot weldingu003cpu003eu003c/pu003e The main failure modes of lithium-ion batteries are: capacity attenuation, gas or liquid leakage, collector corrosion, thermal runaway, etc. . Among them, capacity decay is the most common failure mode, and there are many factors leading to this failure: in terms of electrodes, repeated charge and discharge reduce the active surface area of u200bu200bthe electrode, increase current density, and increase polarization; in terms of electrolyte solution, electrolyte or conductivity Decomposition of the salt leads to a decrease in its electrical conductivity, and the decomposition products cause interface passivation. In addition, the diaphragm is blocked or damaged, and the internal short circuit of the battery will shorten the life of the battery. 1. failure model The current research believes that temperature and operating current are two important stresses that accelerate the capacity degradation of lithium-ion batteries, and tests have verified the failure model. Under the same temperature conditions, the relationship between the life of a lithium battery pack
and the discharge current basically follows the acceleration model with electric stress as the accelerating variable: (1) formula (1) is the basic failure model of lithium-ion batteries, in the double logarithmic coordinate system Above, there is also a linear relationship between lithium battery pack failure and discharge current. 2. Rate capacity effect of lithium battery pack has rate capacity effect, which means that the actual capacity of the battery will vary with the load. The larger the load, the smaller the battery capacity. The principle is that the battery life depends largely on the state of the reactive area on the negative electrode. In the case of stable discharge of small current, the reaction area is evenly distributed on the negative electrode, which can be fully used; but when the battery is discharged with large current, the reaction area on the negative electrode surface is unevenly covered, resulting in some internal The active sites of the layer cannot participate in the reaction, which will cause the battery to quickly lose power at a high discharge rate. 3. Self-discharge of lithium battery pack The self-discharge inside the lithium battery will also cause irreversible loss of battery capacity. The reason may be that a series of irreversible reactions have occurred inside the battery. The irreversible reaction between the anode and the electrolyte (relatively mainly occurs in two materials prone to structural defects, lithium manganate and lithium nickelate, such as the reaction between lithium manganate anode and lithium ions in the electrolyte). The irreversible reaction between the cathode material and the electrolyte (the SEI film formed during formation is to protect the negative electrode from the corrosion of the electrolyte). The irreversible reaction caused by impurities in the electrolyte itself and the reaction of O2 in the solvent. A similar reaction irreversibly consumes lithium ions in the electrolyte, thereby losing battery capacity. Among them, the irreversible reaction of impurities in the electrolyte is particularly common. In addition, in the manufacturing process of the battery, the micro short circuit caused by the mixing of impurities can also cause an irreversible reaction. This phenomenon is the main reason for the excessive self-discharge of individual batteries. Dust in the air or metal powder on the production of pole pieces and diaphragms can cause a short circuit inside the battery. Since absolute dust-free production is impossible, this possibility is bound to exist. When the dust is not enough to penetrate the separator and short-circuit the positive and negative electrodes, the impact on the battery is not great; but when the dust is severe enough to penetrate the separator, the impact on the battery will be very obvious. Whether to pierce the diaphragm or not, so when testing the self-discharge rate of a large number of batteries, it is often found that the self-discharge rate of most batteries is concentrated in a small range, while the self-discharge of only a small part of the battery is obviously biased. High and discrete, these are the batteries with pierced diaphragms. In general digital products, the button battery used as a power source needs to be soldered on the circuit board (PCBA), so there must be solderable tabs (connecting tabs) on the surface of the button battery; but the surface of the button battery is usually stainless steel, and it cannot be soldered directly. Yes, the manufacturer will spot weld the nickel-plated or zinc-plated solder tabs on the button battery to facilitate the soldering of the factory using the button battery. It is more difficult to control the spot welding of the button battery tabs. The main reason is that the button battery is too thin and the negative lithium tab is directly connected to the battery negative shell. Spot welding is a typical process with large instantaneous current and heat. Therefore, it is easy to cause the internal diaphragm of the battery to burn through, causing the internal positive and negative plates to short-circuit, and the battery is discharged in a very short time.
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