Analyze overcharge standards for lithium batteries at home and abroad

by:CTECHi     2021-08-16

(1) The current international standards corresponding to ordinary lithium batteries: Take the IEC62133 standard and the current domestic industry standards for ordinary lithium batteries as an example. The terms related to overcharging are as follows: Section 4.3.9 ofIEC62133:2002 Test process: ThecellisdischargedasdescribedinIEC61960, thenchargedfromapowersupplyof≥10V,atthechargingcurrentIrec,recommendedbythemanufacturer,for2,5C5/Irech. Test criterion: Nofire, noexplosion. (2) The current domestic industry standard corresponding to ordinary lithium batteries: Test process: The battery is fully charged, and then charged with 3C5A until the battery voltage For 4.6V/4.8V5V, the current drops to close to 0A. End the test until the battery catches fire, explodes, or the surface temperature of the battery drops to 10°C lower than the peak value. Test criterion: No fire, no explosion (3) Standard analysis: For the end user, since all the battery packs are equipped with protection IC, there will be overcharge protection under normal conditions to prevent the battery from overcharging. However, if the protection IC fails abnormally, it is important for the battery cell to withstand a certain amount of overcharge. At present, defective chargers and defective BatteryPacks are flooding the market, which also increases the possibility of overcharged batteries for consumers. And danger. At this stage, for ordinary electronic product lithium batteries, the most common overcharge standard is 3C/4.6V or 3C/4.8V, but there are also more stringent requirements such as 3C/5.0V, 1C/10V, 1C/12V, 3C /10V, etc., these strict requirements need to be achieved by optimizing the design or changing the material. (4) Analysis of the cause of failure: Through anatomical observation of the battery cell that did not catch fire and exploded after being overcharged, we can usually find the following facts: ①There is a small amount of gas generated inside; ②The negative electrode is golden yellow to reddish, and then changes rapidly. white. Put it in water, there will be a very violent reaction; ③The positive electrode is gray; ④The current collector Al foil and Cu foil have no obvious changes; ⑤By means of DSC and other means, it can be found that the diaphragm has not changed significantly; through the charging and discharging of the lithium battery Analysis of the mechanism and actual overcharge phenomenon. We know that during overcharge, excessive lithium ions are extracted from the positive electrode, inserted or deposited on the negative electrode of the battery, which makes the thermal stability of the two electrodes worse, and the positive electrode tends to decompose and release chemical energy. At the same time, a lot of heat is generated, and the release of oxygen can catalyze the decomposition of the electrolyte. When the temperature is high enough, it will cause a chemical reaction of the negative electrode. The active metal lithium deposited on the negative electrode reacts with the solvent and then releases heat, which converts the chemical energy into heat energy. The temperature of the battery will rise rapidly from this, which will eventually lead to thermal runaway and danger. accident. The current rate in the overcharged state of lithium batteries is also an important factor that affects the overcharge performance of the battery, especially for high-capacity batteries. This is mainly because the lithium in the battery and the graphite carbon in the negative electrode of the battery form a LiC6n compound, and its reaction rate is constant. When charging at a low current, lithium atoms will not accumulate, so it is safer. The formation of lithium atoms at high currents is faster than the formation of LiC6n. Therefore, in this case, lithium atoms will accumulate. The battery is prone to negative reactions or formation of lithium dendrites, which will cause a large amount of heat to be released and cause danger. The size of the battery capacity will affect the heat generation and heat dissipation rate of the battery, and it is also an important factor affecting the battery overcharge performance. Under the same chemical system, the overcharge performance of low-capacity batteries is better than that of high-capacity batteries, which is one of the reasons why high-capacity lithium batteries are relatively unsafe. (5) Suggested solutions: According to the above failure analysis, we can use materials with better thermal stability (such as electrolyte with overcharged additives, which polymerizes during overcharge to increase the internal resistance of the battery to reduce Calorific value) to increase the anti-overcharge performance, reducing the body density can also optimize the overcharge performance to a certain extent. For the designers and users of terminal electronic products, they should try to avoid charging the battery with high current.

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