In recent years, the explosion of mobile phones and laptop batteries has long been unable to attract attention. The explosion of electric vehicles and the fire of lithium-ion factories are considered news. The recent explosion of the large-scale battery of the Samsung Galaxy Note 7 has once again pushed the safety of lithium-ion batteries to the forefront.
In addition to external factors in terms of use, the safety of a lithium-ion battery depends mainly on the underlying factors of the basic electrochemical system and the structure, design and production process of the electrode/cell, while the electrochemical system used in the cell is The most fundamental factor in determining battery safety. Here I will analyze the safety of lithium-ion batteries from several different perspectives.
Thermodynamics: Research has confirmed that not only the negative electrode, but also the surface of the positive electrode material is covered with a very thin passivation film. The passivation film covering the positive and negative electrodes is very important for all aspects of lithium ion battery performance. The effect, and this particular interface problem only exists in non-aqueous organic electrolyte systems. What I want to emphasize here is that from the perspective of Fermi level, the existing lithium-ion battery system is thermodynamically unstable, and it can work stably because the passivation film on the positive and negative surfaces is dynamic. The further reaction between the positive and negative electrodes and the electrolyte is theoretically isolated.
Therefore, the safety of lithium battery is directly related to the integrity and compactness of the passivation film on the positive and negative electrodes. Understanding this problem will be crucial to understanding the safety of lithium batteries.
Heat transfer angle: Unsafe behavior of lithium-ion batteries (including battery over-charge and over-discharge, rapid charge and discharge, short circuit, mechanical abuse conditions and high-temperature thermal shock) easily trigger dangerous side reactions inside the battery to generate heat, directly The passivation film on the surface of the negative electrode and the positive electrode was destroyed.
When the cell temperature rises to 130 ° C, the SEI film on the surface of the negative electrode decomposes, causing a high redox lithium carbon negative electrode to be exposed to the electrolyte to undergo a vigorous redox reaction, and the generated heat causes the battery to enter a high-risk state. When the internal temperature of the battery rises above 200 ° C, the positive electrode surface passivation film decomposes the positive electrode to generate oxygen, and continues to react violently with the electrolyte to generate a large amount of heat and form a high internal pressure. When the battery temperature reaches 240 ° C or higher, it is accompanied by a violent exothermic reaction of the lithium carbon negative electrode with the binder.
It can be seen that the damage of the SEI film on the surface of the negative electrode leads to a violent exothermic reaction between the high-activity lithium-ion negative electrode and the electrolyte, which is a direct cause of the battery temperature rise and the thermal runaway of the battery. The liberation of the positive electrode material is only one part of the thermal runaway reaction, and it is not even the most important factor.
Lithium iron phosphate (LFP) structure is very stable. Normally, thermal decomposition does not occur, but other dangerous side reactions still exist in LFP batteries, so the "safety" of LFP batteries is only relative. From the above analysis, we can see the importance of temperature control for the safety of lithium batteries. Compared with the 3C small battery, the large power battery is more difficult to dissipate heat due to various factors such as the structure of the battery, the working mode and the environment. Therefore, the thermal management design of the large power battery system is very important.
Flammability of electrode materials: The organic solvents used in lithium batteries are flammable and the flash point is too low. The thermal runaway caused by unsafe behavior easily ignites the low flash point flammable liquid component and causes the battery to burn. The lithium negative electrode carbon material, the separator and the positive electrode conductive carbon are also flammable.
The probability of lithium burning is higher than the probability of battery explosion, but the battery explosion must be accompanied by burning. In addition, when the battery is cracked and the air humidity in the external environment is high, the moisture and oxygen in the air are easily subjected to a violent chemical reaction with the lithium-incorporated carbon negative electrode to release a large amount of heat to cause the battery to burn. The flammability of the electrode material is a major difference between a lithium ion battery and a water based secondary battery.
Overcharge and metal lithium related issues: Any commercial secondary battery requires effective overcharge prevention measures to ensure that the battery is fully charged and avoids the safety problems caused by improper overcharging. Lithium battery overcharging will lead to many serious consequences, such as damage to the crystal structure of the cathode material, deterioration of cycle life, increase of oxidation of the electrolyte on the surface of the cathode and thermal runaway, and lithium deposition of the anode to cause short circuit/thermal runaway. Sexual problems.
Therefore, preventing overcharge is extremely important for the safe use of lithium batteries. Unlike the water-based secondary battery, controlling the charging voltage is the only over-charge protection measure for lithium-ion batteries. The change of lithium battery charging voltage is mainly caused by the positive electrode material being close to the complete delithiation state, and it is difficult to detect the completion degree of the graphite negative electrode charging process (because its lithium insertion potential is very close to metallic lithium), in order to bypass the difficulty of monitoring the negative electrode voltage, lithium Ion batteries generally use a positive limit capacity design.
Of course, another major role of positive limit capacitance is to ensure that the negative electrode has sufficient extra capacity to prevent lithium from being deposited on the negative electrode. However, there are three situations that can change the excess capacity of the negative:
The capacity of the graphite negative electrode is attenuated faster than that of the positive electrode material, which has been confirmed in almost all cathode material matching systems.
Due to the unreasonable design of the electrode structure, or under improper use conditions (such as high magnification, low temperature and overcharge, etc.), the anode is partially decomposed.
The side reaction of the electrolyte and the impurities causes the degree of charge of the anode to increase and gradually loses the extra lithium storage capacity.
The occurrence of any of the above conditions will lead to insufficient lithium storage capacity of the negative electrode to precipitate lithium, which is the culprit causing lithium battery safety problems. These problems are even more serious in large-capacity power batteries, and even with BMS, these problems cannot be fundamentally solved.
What I want to emphasize here is that the above three factors will become more prominent with the use of batteries, which means that the safety of old batteries will be more serious than the new ones, and this problem has not received enough attention.
A topic that has been discussed very hot in the past two years is the “gradient development†of power batteries, which reuses the power battery (the theoretical 70% of the remaining capacity) that has reached the end of its useful life for energy storage. The starting point of this idea is good, but considering the safety hazards of the old battery and the current poor quality of the power battery of most manufacturers in China, I personally do not think that the development of the power battery gradient has practical operability in the short term.
In fact, we can also compare the safety of water-based secondary batteries and lithium batteries from another angle. All secondary batteries, whether water-based or organic secondary batteries, are based on the basic principle of positive limiting capacity (capacity of excess negative electrode).
If this premise disappears, the consequence of overcharging is that the secondary battery of the water system produces hydrogen, and for the lithium ion battery, it is the lithium of the negative electrode. However, the aqueous electrolyte used in various water-based secondary batteries has a unique property that water can be decomposed into hydrogen and oxygen upon overcharging, and hydrogen and oxygen can be combined on the electrode or on the surface of the composite catalyst. Water, then we can easily understand that the secondary battery of the water system generally adopts the principle of “oxygen cycle†to achieve overcharge protection.
In a lithium ion battery, once the high-active metal lithium is precipitated in the negative electrode, the metal lithium cannot be eliminated inside the battery, which inevitably leads to safety problems. Although water-based secondary batteries limit the further increase in energy density due to the decomposition voltage of water, it is not to be forgotten that water also provides a near-perfect and irreplaceable anti-overcharge solution for water-based secondary batteries.
From this point of view, in comparison with lithium ion batteries and water-based secondary batteries, the organic electrolyte used in lithium batteries does not have the characteristics of reversible decomposition and recovery, and high-active metal lithium cannot be eliminated once it is formed. So in a sense, lithium-ion batteries have no solution to safety issues!
Comprehensive application of some technical measures, such as thermal control technology (PTC electrode), positive and negative surface ceramic coating, overcharge protection additive, voltage sensitive diaphragm and flame retardant electrolyte can effectively improve the safety of lithium battery, but These measures are impossible to fundamentally solve the safety problem of lithium batteries, because lithium batteries are thermodynamically unstable systems. On the other hand, these measures not only increase the cost, but also reduce the energy density of the battery.
If we consider the above factors comprehensively, we will understand that the "safety" of lithium batteries is only relative. Some readers may have noticed that in general batteries such as alkaline manganese, lead acid and nickel metal hydride batteries, consumers can buy bare core directly in the store, and lithium-ion batteries are an exception.
According to the regulations of the lithium battery industry, battery manufacturers will only sell their batteries to authorized Pack companies. Then Pack will pack the batteries and protection boards into battery packs for sale to electrical manufacturers instead of consumers, and battery packs. It must be used in strict accordance with the prescribed method with a dedicated charger. The logic behind this particular business model is based primarily on the safety considerations of lithium batteries.
The shocking Boeing 787 "Dream" passenger aircraft lithium battery fire incident, as well as the recent Samsung Galaxy Note 7 large-scale battery fire and explosion incident, once again sounded the alarm for the safety of lithium-ion batteries.
Compared to Samsung, Apple has been relatively conservative and stable in terms of battery, battery capacity and charging upper limit voltage are lower than Samsung. Unlike the 4.4V high-voltage LCO on the Galaxy Note 7, Apple still uses the same 4.35V LCO cathode material as the i-Phone 6 series on the recently released i-Phone 7.
Apple's conservative and robust strategy on the battery, I personally believe that mainly based on security considerations, Apple would rather sacrifice battery capacity and energy density to ensure security. According to media reports, the direct economic loss of Samsung due to the large recall of Galaxy Note 7 may be as high as 2 billion US dollars, and the loss of indirect brand value will be immeasurable.
What I need to emphasize here is that BMS does not solve the safety problem of lithium-ion battery, which is determined by the basic working principle of BMS. The safety of a power battery system is fundamentally dependent on the individual cells, and the safety issues of the large power battery after being grouped will be magnified and thus more prominent. In recent years, the domestic lithium battery industry has been filled with the argument that lithium-ion batteries will dominate the rivers and lakes and replace other secondary batteries. From the perspective of safety, this argument is undoubtedly ridiculous.
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