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Lithium-ion battery safety issues

Author: Source: Datetime: 2016-10-26 16:44:28

In recent years, mobile phone and laptop battery explosion has long been unable to attract the eye, electric vehicle deflagration and lithium power plant fire is news. The recent occurrence of Samsung Galaxy Note 7 large-scale battery fire explosion, once again the safety of lithium-ion batteries pushed to the cusp.


In addition to the use of the external factors, the safety of lithium-ion battery depends mainly on the basic electrochemical system and the electrode / cell structure, design and production processes and other internal factors, and the electrochemical system used in batteries is The most fundamental factor in determining the safety of the battery. The safety is most important when use in devices, like solar powered portable generator. So here from several different angles to analyze the safety of lithium-ion battery problems.

 Lithium - ion battery

Thermodynamic point of view: research has confirmed that not only in the cathode, the cathode material surface is also covered with a thin layer of passive film, covering the positive and negative surface of the passive film on the performance of lithium-ion batteries will have a very important , And this particular interfacial problem only exists in non-aqueous organic electrolyte systems. The author here to emphasize that, from the Fermi level of view, the existing lithium-ion battery system is thermodynamically unstable, it has been able to work stably because the cathode and anode surface passive film in the power The isolation of positive and negative electrodes and the electrolyte further reaction.

 

Therefore, the safety of lithium and positive and negative surface of the passivation film integrity and compactness directly related to the understanding of this issue to understand the safety of lithium electricity will be critical.

 

Heat transfer angle: the lithium-ion battery unsafe behavior (including the battery in the overcharge and over discharge, rapid charge and discharge, short circuit, mechanical abuse conditions and high temperature thermal shock, etc.) easily trigger the battery side of the dangerous side reaction heat, And a passive film that destroys the surface of the negative electrode and the positive electrode.

 

When the temperature of the core rises to 130 ℃, the SEI film on the negative electrode is decomposed, resulting in the intense redox reaction of the highly active lithium carbon anode exposed to the electrolyte, resulting in heat to the battery into high-risk state. When the internal temperature of the battery is increased to above 200 ℃, the cathode of the positive electrode is decomposed to oxygen, and the reaction of the positive electrode with the electrolyte produces a lot of heat and high internal pressure. When the battery temperature reaches 240 ℃ or more, but also accompanied by lithium carbon anode with the exothermic reaction of the binder.

 

It can be seen that the damage of the SEI film on the negative electrode surface leads to the exothermic reaction of the highly active lithium intercalation negative electrode and the electrolyte, which is the direct cause of the battery temperature rise and the thermal runaway of the battery. The decomposition of the cathode material is only one part of the thermal runaway reaction, not even the most important factor.

 

Lithium iron phosphate (LFP) structure is very stable Under normal state does not occur thermal decomposition, but other hazardous side reactions still exist in the LFP battery, so the LiFePO4 battery "safety" is only relative sense. From the above analysis we can see that the temperature control of lithium safety significance. Relative to the 3C small battery, the large power battery structure due to batteries, work and environmental factors such as heat dissipation is more difficult, so large power battery system thermal management design is essential.

 

Flammability of electrode materials: organic solvents used in lithium are flammable and flash point is too low, unsafe behavior caused by thermal runaway is easy to ignite low flammable liquid components of the flash point of the battery combustion. Lithium negative carbon materials, separators and positive conductive carbon are also flammable.

 

Lithium battery burning occurs more likely than the battery explosion, but the battery explosion must be accompanied by combustion. So we should pay more attention when our batteries use in portable solar power generator. In addition, when the battery is cracked and the ambient air humidity is high, the moisture and oxygen in the air easily reacts strongly with the lithium-inserted carbon negative electrode to generate a lot of heat and cause the battery to burn. The flammability of the electrode material is a major difference between the lithium ion battery and the aqueous secondary battery.

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