Overcharge and Lithium Metal Related Issues
Author: Source: Datetime: 2016-10-27 11:05:03
Any commercial secondary battery, need to be effective anti-overcharge measures to ensure the battery to fully charged state, and to avoid inappropriate overcharge caused by security issues. Lithium charge overcharge will lead to many serious consequences, such as the crystal structure of the cathode material is damaged and deterioration of the cycle life, exacerbating the oxidation of the electrolyte surface in the positive pole caused by thermal runaway, and the negative pole of lithium caused by short circuit / thermal runaway safety Sexual problems. The lithium batteries can be used in solar powered portable generator.
Therefore, to prevent overcharge the safety of the use of lithium is extremely important. With the secondary battery is different from the water, the control charge voltage is the only lithium-ion battery protection against overcharge protection measures. Lithium charge voltage changes mainly from the cathode material near the fully de-lithium state caused, and difficult to detect the completion of the graphite anode charging process (because of its very close to the lithium potential lithium lithium), in order to bypass the negative voltage monitoring difficulties, lithium Ion battery is generally used positive tolerance design.
Of course, the positive limit capacity of another major role is to ensure that the negative electrode has enough additional capacity to prevent the negative pole of lithium. However, there are three cases where the capacity of the negative electrode is changed:
The capacity decay rate of graphite negative electrode is higher than that of positive electrode material, which has been verified on almost all cathode materials collocation system.
As the electrode structure design is unreasonable, or inappropriate use conditions (such as high magnification, low temperature and overcharge, etc.) caused by local negative lithium analysis.
Electrolyte and impurities caused by the side effects of increased negative charge level and the gradual loss of additional lithium storage capacity.
The occurrence of any of the above will lead to the lack of lithium capacity of the negative electrode lithium and lithium, and lithium is the cause of the lithium safety of the culprit. These problems in the high-capacity battery will be more serious, even if the use of BMS can not fundamentally solve these problems.
I here to emphasize that the above three factors will become more prominent with the use of the battery, which means that the old battery safety problems will be more serious than the new battery, and this issue has not attracted enough attention.
A hot topic in the last two years is the "gradient development" of power cells, which reuses battery cells that have reached the end of their useful life (theoretically 70% of capacity remaining) for energy storage purposes. This idea is the starting point is good, but taking into account the safety of the old battery hidden dangers, as well as most domestic manufacturers generally poor quality battery power status, the author does not believe that the battery gradient development in the short term with practical operability.
In fact, we can also from another point of view to compare the secondary battery and lithium battery security issues. All of the secondary batteries, whether water or organic secondary batteries, the charge safety is built on the positive limit capacity (negative capacity surplus) on the basis of the basic principles. The lithium battery could be used in portable solar power generator.
If this premise disappears, the consequences of overcharge is secondary battery hydrogen production, lithium-ion battery is negative for lithium analysis. However, the aqueous electrolytes used in various aqueous secondary batteries have a unique property that water can be decomposed into hydrogen and oxygen at the time of overcharge, and hydrogen and oxygen can be recombined on the electrode or on the surface of the composite catalyst Water, then we are not difficult to understand the water secondary batteries commonly used "oxygen cycle" principle to achieve overcharge protection.
In the lithium-ion battery, the cathode once the precipitation of highly active metal lithium, because the metal lithium can not be eliminated within the battery will inevitably lead to security issues. Although water secondary batteries limit their energy density further due to the decomposition voltage of water, do not forget that water also provides a near-perfect and irreplaceable anti-overcharge solution for water-based secondary batteries.
From this point of view, compared with lithium-ion batteries and water rechargeable batteries, lithium batteries used in organic electrolytes do not have the characteristics of reversible decomposition and recovery, and high activity of lithium once generated can not be eliminated. So in a sense, the lithium-ion batteries in the security issue is no solution!
Through a number of technical measures of the integrated application, such as thermal control technology (PTC electrode), positive and negative surface ceramic coating, overcharge protection additives, voltage sensitive diaphragms and flame retardant electrolyte can effectively improve the safety of lithium, but These measures can not fundamentally solve the safety of lithium, because lithium is thermodynamically unstable system. On the other hand, these measures not only increase the cost, but also reduce the battery energy density.
If we take into account the above factors will understand, lithium "safety" is only relative sense. Some readers may note that the general battery such as alkaline manganese, lead acid and nickel-metal hydride batteries, consumers can buy in the store directly to the bare core, and only lithium-ion battery is an exception.
In accordance with the provisions of the lithium battery industry, the battery manufacturer will only be licensed to the Pack company sells its own batteries, and then by the Pack will be batteries and protective plate packaged into battery packs sold to electrical manufacturers rather than consumers, and battery pack must be used with a dedicated charger in strict accordance with the provisions of the method used. Like our lifepo4 battery pack with BMS together. This particular business model behind the logic, mainly based on the safety considerations of lithium.
The lithium-ion battery fire that stunned the industry, and the recent explosion of the Samsung Galaxy Note 7, sparked another alarm for the safety of lithium-ion batteries.
Compared to Samsung, Apple has been relatively conservative in the battery, the battery capacity and charging voltage is lower than the Samsung ceiling. Unlike Galaxy Note 7, which uses a 4.4V high-voltage LCO, Apple's recently released i-Phone 7 still features the same 4.35V LCO cathode material as the i-Phone 6 Series.
Apple is conservative in the battery to adopt a conservative strategy, I personally think that mainly based on security considerations, Apple would rather sacrifice a little battery capacity and energy density to ensure safety. According to media reports, the Samsung Galaxy Note 7 because of the large-scale recall of direct economic losses may be as high as 20 billion US dollars, indirect brand value loss will be immeasurable.
I need to emphasize here is that, BMS does not solve the safety of lithium-ion battery, which is determined by the basic working principle of BMS. The safety of the power battery system is fundamentally determined by the single cell, and large power batteries in the group after the security issues will be amplified and thus more prominent. In recent years, the domestic lithium-ion battery industry has been filled with lithium-ion battery will dominate the arena and replace the other secondary battery argument, just from the security point of view, this argument is undoubtedly ridiculous.
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