What about the safety of lithium batteries?

In recent years, mobile phone and laptop batteries burning explosion have already can't attract eyeball, the electric car deflagration and lithium electricity fire is the news of the factory. And recent SamsungGalaxyNote7 large-scale battery fire explosion, will once again lithium ion battery security problem into the spotlight as never before.

In addition to the use of external factors, lithium ion power battery depends mainly on the basic of electrochemical system and safety of the structure of electrode/batteries, internal factors such as design and production process, the batteries used in electrochemical system is the most fundamental factor of battery safety. The author here will be from several different angles to analyze the safety of the lithium ion battery.

Thermodynamic point of view: studies have confirmed that is not just in the anode, the surface of the anode materials are also covered a layer of thin, passivation membrane covering the positive anode passivation membrane on the surface of the aspects of lithium ion battery performance will be a very important impact, and the special interface problems exist only in nonaqueous organic electrolytic fluid system. I want to emphasize here is, from the Angle of the Fermi level in terms of the existing system of lithium ion battery is unstable in thermodynamics, it is able to work stable because of the positive and negative surface passivation film on the kinetics of isolated from the anode and electrolyte are further reaction.

Therefore, the safety of the lithium electricity and is the cathode surface passivation membrane integrity and is directly related to the density degree and know the problem in understanding the security problems of lithium electricity will be crucial.

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Heat transfer point of view: the unsafe behavior of the lithium ion battery (including battery overcharge the discharge, fast charge and discharge, short circuit, mechanical conditions and high temperature thermal shock, and so on and so forth) easy to trigger the battery internal risk of side effects and generate heat, direct destruction of the cathode and anode surface passivation membrane.

When cell temperature rise to 130 ℃, the SEI film on the surface of the anode decomposition, lithium lead to highly active carbon negative exposed to severe oxidation reduction reaction in the electrolyte, the heat generated by the high-risk battery into the state. When the battery internal local temperature to 200 ℃ above, the decomposition of the anode surface passivation membrane oxygen evolution anode, and continue with the electrolyte violent reaction to produce a lot of heat and form a high internal pressure. When the battery temperature is above 240 ℃, also with lithium carbon anode with intense exothermic reaction of binder.

Visible, the breakage of the SEI film cathode surface leading to highly active intercalated-li anode and electrolyte highly exothermic reaction, battery temperature is the direct reason for causing the battery thermal runaway. And decomposition of the anode material of exothermic just thermal runaway reaction one of the links, is not even the most main factor.

lithium iron phosphate (LFP) structure is very stable thermal decomposition conditions not usually happen, but the other dangerous side effects still exist in the LFP battery, therefore the LFP batteries in the "security" is a relative sense. We can see from the above analysis, the significance of temperature control on the lithium electricity safety. Relative to the 3 c small battery, large power battery because the cell structure, work mode and environment various factors lead to more difficulty heat dissipation, so large power battery system is very important for the thermal management design.

Electrode materials flammability: lithium electricity using organic solvents are flammable and flash point is too low, lead to unsafe behavior of thermal runaway is easy to ignite low flash point of flammable liquid components and cause battery burning. Li-ion battery anode carbon materials, diaphragm, and the anode conductive carbon also has the flammability.

Lithium electricity the chance of combustion is higher than the risk of battery explosion, but the battery must be accompanied by burning explosion. In addition, when the battery cracking and the external environment of high humidity of the air, water and oxygen in the air is extremely easily with the embedded carbon negative electrode of lithium severe chemical reactions release large amounts of heat and cause burning of the battery. Flammability of electrode material is lithium ion battery relative to one of the big difference between drainage secondary battery.

Overcharge lithium metal related problem: any kind of commercialization of secondary battery, need effective preventing overcharge measures to ensure the battery to full charge state, and avoid improper overcharge safety problem. Lithium electricity charge will result in many serious consequences, such as the anode material of crystal structure damage and deterioration life cycles, aggravate the electrolyte on the surface of the anode oxidation and thermal runaway, lithium and negative analysis and cause security problems such as short circuit/thermal runaway.

So, to prevent overcharge is extremely important to the safety of the lithium electricity use. With drainage secondary battery, control the charging voltage is lithium ion battery only preventing overcharge protection measures. Mainly comes from the anode materials in li-ion battery charging voltage change completely off lithium state near the end of the cause, and it is difficult to detect the complete degree of graphite anode charging process (because of its built-in lithium potential very close to the metal lithium), in order to circumvent the cathode voltage monitoring difficulty, is commonly used lithium ion battery limit capacity design.

, of course, is the limit of another main role is to ensure that the cathode has enough extra capacity and prevent negative lithium. However, there are three things will change negative excess capacity:

Graphite anode attenuation rate is higher than the capacity of the anode material, it has been in almost all the anode material combination was confirmed on the system.

Due to the electrode structure design is unreasonable, or under the condition of the improper use, such as high ratio, low temperature and overcharge, etc.) caused by negative lithium local analysis.

The electrolyte and the side effects caused by negative charge degree of impurity increases gradually lose extra lithium storage capacity.

The occurrence of any of these conditions will lead to the cathode and analysis the deficiency of the lithium, lithium storage capacity and metallic lithium is the main culprit of a lithium electricity safety issues. These problems on the large capacity power battery are more serious, even the BMS cannot fundamentally solve these problems.

The author here is to emphasize that the above three factors will become more prominent with battery, that is, the security problems old batteries will be more serious than a new battery, and this problem has not caused enough attention.

Nearly two years to discuss a very hot topic is power battery "gradient development", will reach the service life of the power battery (in theory but also the remaining 70% of capacity) for reuse and used for energy storage purposes. The starting point of the idea is good, but given the security hidden danger old batteries, as well as the present domestic most of the status quo of the generally poor quality power battery manufacturer, I personally don't think power battery gradient development has practical operability in a short time.

In fact, we can also from another Angle to compare the drainage secondary battery and lithium electricity safety issues. All of the secondary battery, both water and organic system of secondary battery, charging its security is based on is the limit capacity (negative excess capacity) based on the basic principles.

Disappear if this premise, the consequence of overcharge is drainage secondary battery production, is the cathode for lithium ion battery lithium. However, various kinds of drainage secondary battery using aqueous electrolyte has a unique nature, that is water can be decomposed into hydrogen and oxygen during overcharge, and hydrogen and oxygen can be generated or composite catalyst on the surface of composite on electrode water, so it is not hard to understand we drainage secondary battery widely used the principle of "oxygen cycle" to implement the overcharge protection.

In the lithium ion battery, anode once precipitates highly reactive metal lithium, with lithium metal not eliminate and will cause security problems inside the battery. Although due to the breakdown voltage of water drainage secondary battery limits further improve the energy density, but don't forget, also for drainage water secondary battery provides a nearly perfect and irreplaceable preventing overcharge solution.

Compared to lithium ion batteries and drainage from this Angle secondary battery, lithium electricity using organic electrolyte does not have the characteristics of reversible decomposition and reconstruction, and once a highly reactive metal lithium production will not be able to eliminate. So in a sense, the lithium ion battery on security issues is no solution!

By some technical measures of comprehensive application, such as thermal control technology (PTC), is the cathode surface ceramic coating and overcharge protection additives, voltage sensitive diaphragm and flame retardant electrolyte can effectively improve the safety of the lithium electricity, but these measures can't fundamentally solve the problem of the safety of the lithium electricity, because lithium electricity on the thermodynamic system is unstable. 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 will understand, lithium electricity "security" is a relative sense. Have readers might notice that common battery such as alkaline manganese, lead and nickel metal hydride batteries, consumers can buy directly in the shop to the bare core, with lithium ion battery is an exception.

According to the regulations of the lithium electricity industry, the battery manufacturer to authorized Pack company will only sell their batteries, again by Pack the batteries and battery protection board encapsulated into package sold to electrical appliance producers rather than consumers, and the battery Pack must match with dedicated charger used in strict accordance with the provisions of the method. The logic behind this particular business model, is mainly based on lithium electricity safety considerations.

Before a Boeing 787 "dream" shocked the industry lithium battery fires, as well as the recent SamsungGalaxyNote7 a wide range of battery fire explosion, is for the safety of the lithium ion battery problem once again sounded the alarm.

Relative to the Samsung, Apple has been relatively conservative robust at battery, battery capacity and maximum charge voltage is lower than the Samsung. And in the light of the high voltage of 4.4 V GalaxyNote7 LCO, Apple in the recently released a new generation of I - phone 7 still USES is with I - 4.35 VLCO Phone6 series of the same positive electrode material.

Apple is on the battery to take partial conservative robust strategy, I personally believe is mainly based on security considerations, Apple would rather sacrifice a little battery capacity and energy density to ensure safety. Because, according to media reports, the Samsung GalaxyNote7 massive recalls a direct economic loss could be as high as $2 billion, indirect brand value loss will be immeasurable.

The author need to emphasize here is that BMS can not solve the problem of the safety of the lithium ion power battery, which is determined by the basic working principle of the BMS. The security of the power battery system fundamentally depends on the monomer batteries, and large power battery after group security problems will be magnified and more outstanding. In recent years, the domestic lithium electricity industry has been filled with lithium ion battery will unify river's lake and replace other secondary battery, just from a security perspective, this argument is absurd.

The page contains the contents of the machine translation.