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Lithium-ion batteries have a higher energy density, especially the battery module, in the event of an accident, conventional means can not effectively control the rapid spread of heat or prevent the continuous explosion of the battery, which may have serious consequences. Especially in the case of lithium batteries burning or exploding during air transportation, the consequences to passengers and aircraft would be very serious or even catastrophic.

So how exactly did the lithium battery accident happen, let’s look at the characteristics of the lithium battery itself.

Lithium battery characteristics

1 High Energy
Lithium battery is a high-energy item, if it catches fire and explodes, the danger is enormous. Take the 300Wh/kg square lithium ion battery for example, 1kg square lithium battery contains 1.08 × 10 6J of energy; 1g TNT can release 4184J of energy, the conversion shows that the energy contained in 1kg square lithium battery is equivalent to about 258g TNT, the energy contained in a 100kWh pack of square lithium battery about This is equivalent to 86 kg of TNT. In the small space of a transport vehicle, this energy release is enough to cause serious damage.

Lithium batteries consist of a large number of high energy square lithium batteries tightly stacked; when transported, mostly a large number of lithium batteries are tightly stacked in the small space of the transport vehicle. From the safety management point of view, it is a typical high-risk point.

2 Large volume
Reducing the mass of individual packages, using safe packaging and decentralized storage are effective measures to reduce transportation risks. The mass of the entire package of lithium batteries for general electric vehicles, often with hundreds of kilograms, the energy reaches tens to hundreds of kilowatt-hours. The volume becomes larger and the mass heavier, so that reducing the mass of individual packages can not be achieved. We have done the following experiments: put 12pcs 506328 type polymer ternary cathode material lithium ion battery (Note: the anode material is graphite, single cell rated capacity 20Ah, charging cut-off voltage is 4. 2V) into a lithium battery module (about 4V, 220Ah), put it into a metal box made of 5. 0mm thick steel plate, and then pass the heating rod in a closed condition. The 5.0mm thick steel plate of the metal box is partially deformed after full combustion, which causes thermal runaway.

If a safety package is to be used to stop the heat and flame spread of a lithium battery fire, it needs to be flame retardant, insulated and explosion proof. Currently, there is no packaging that can limit the release of such a large amount of energy, and the cost of special packaging with certain efficacy is too high to be used for large quantities of power batteries. The increasing mass and size also make the handling and location of the battery more difficult, especially in transportation, where decentralized storage becomes impractical.

3 Cell Consistency
Differences between the individual cells in a lithium battery system are known as uniformity issues. This difference can result in inefficient use of energy and can easily cause thermal runaway of the lithium battery.

Currently, there are two ways to solve the consistency problem:
1) Study the lithium battery system and control the quality of the manufacturing process to resolve the differences from the source. Mature industrial production process has reduced the failure rate of single lithium battery to the level of one in ten million, but because the lithium battery system consists of thousands of single cells, greatly increasing the probability of accidents;
The battery equalization technology is used to force the difference between the individual batteries to become smaller.

After repeated use of the lithium battery, the differences may be further amplified, therefore, the battery management system (BMS) is particularly important, and the evaluation of the BMS is also critical.

4 High Voltage
The voltage of lithium batteries reaches hundreds of volts, far beyond the safe voltage for humans.
The dangers of high voltage leakage are:
(1) Causing electric shock to the handling personnel;
Damage to the battery protection circuit, resulting in the failure of the protective function of the battery system;
(3) may spontaneously discharge, igniting other flammable materials.

Lithium batteries are generally transported with a charge, and the charge level is directly related to safety. There is no safety test for high voltage in the UN38.3 series of tests, nor is there a risk assessment method for high voltage in the transportation regulations and standards. The transportation risks associated with high voltage in lithium batteries have been overlooked.

Our current transportation problems

1 Intrinsic Security Unresolved
The chemical composition of lithium batteries determines their flammability. Although lithium battery safety standards and measures have been advancing continuously, the factors that cause thermal runaway of lithium batteries are still difficult to avoid completely.

There are still some contradictions between the high energy density battery system and safety, which have not yet been resolved. Different modes of transportation have different levels of acceptance of safety. The causes of thermal runaway of lithium batteries are complex, and the existing countermeasures are inadequate. The unpredictable risk poses a great safety threat to air transportation.

2 Less research on combustion mechanism and hazards

At present, less research on the fire hazard of large-size lithium batteries, the lack of awareness of fire performance and hazards, related research is mostly concentrated in the small-scale, or even a few lithium battery fire studies, not representative.

Lithium-ion battery combustion will produce toxic gases and dust, but research is mostly concentrated in the analysis of the gas components after direct combustion, less research on the components of the toxic gases produced by combustion, the hazards of toxic gases and dust, toxic gas diffusion mechanism, few studies reported.

Take two fully charged G00236-20Ah type 20Ah polymer ternary cathode material lithium ion cell (cathode material is graphite, charging cut-off voltage is 4.2V), one bare, one in the homemade flame retardant package, energize the heating rod, heating at the speed of 5 ? / min, causing thermal runaway.

The experimental results show that the combustion behavior of the exposed lithium battery is very different from that of the lithium battery in the flame-retardant package, the exposed lithium battery caused thermal runaway, full combustion, high flame, less smoke; the same specification of lithium battery in the flame-retardant package, also thermal runaway, but no flame escape, producing a lot of white irritating gas.

3 Transport test system is not perfect

UN38.3 series testing is the basic standard for lithium battery transport safety. The standard is based on the small consumer batteries, but does not provide specific requirements for high energy, large mass and multiple specifications of lithium batteries. Although gradually revised, but still lags behind the development of lithium batteries.

Currently, some of the equipment in the lithium-ion battery rated energy. Usually, the energy of a small and medium-sized lithium battery module can exceed 6200Wh, judged by 6200Wh, many of these battery modules do not need to be tested. In the process of welding and assembling, there will be false soldering, missed soldering and repeated soldering in the lugs of the individual battery, and the potential safety hazards in the battery module may be induced by external forces during transportation, which may eventually lead to short circuit. For lithium batteries with rated energy greater than 6200Wh, the regulations require the evaluation of BMS, but the evaluation methods and standards are lacking.

For the evaluation of BMS, in addition to the individual evaluation of stability under the transport environment, it is also necessary to test the whole battery system under the transport environment to reflect the control ability of BMS, otherwise there is no practical meaning. The professionalism and past experience of the evaluator may also have a direct impact on the results. The conduction effect of this regulation directly leads to the lack of consideration of transport safety in the design of lithium batteries, which is a missing link at the source.

4 Lack of transportation safety evaluation ability
There are relatively few third-party testing organizations for lithium batteries. Most of the third-party laboratories for lithium battery transportation testing do not have testing capability for lithium batteries. According to the regulations, the slightly higher energy lithium batteries can seek not to conduct UN38.3 series of tests, also led to the laboratories have not established testing capabilities. Transportation safety evaluation of lithium batteries should be based on the basis of valid test data.

The current situation is that it is difficult to find a suitable laboratory to perform the tests and obtain the corresponding data. Without valid data, the appropriate test methods and standards can not be established, not to mention the establishment of evaluation standards. In addition, with the development of related industries, a large number of tested, damaged, defective, recalled and recycled lithium batteries need to be shipped. These lithium batteries are also unsuitable for UN38.3 series testing before shipping, so reasonable safety evaluation is necessary. This vacuum has emerged and deserves attention.

5 Insufficient risk prevention and control measures Suitable packaging can effectively control the danger of dangerous goods in the package and achieve safe transportation. Due to the high specific energy and large size of lithium batteries, there is no suitable packaging material or packaging form to control the risk in the package, i.e., lithium batteries in the package, the complete combustion and does not spread to other items. Internationally, the design and small-scale use of the fire hood or fireproof package inside the compartment, similar products in China are still in the experimental stage. These packaging methods do not solve the problem of lithium battery combustion produces large amounts of toxic gases and dust. The requirements in the packaging instructions are all principle requirements, a temporary safety measures. These requirements are not specific technical indicators, lack of test verification, lack of certification by authoritative agencies.

3.6 Lack of effective fire fighting methods
The effectiveness of the lithium battery internal flame retardant materials and external packaging materials need to be further verified. Research shows that after 5min of thermal runaway of the lithium battery on the bus, the fire will spread to the interior of the bus; and the time for all personnel to escape is about 1min. It is therefore recommended that 5min be used as the refractory time of the lithium battery on the bus. If under road transport conditions, 5min is sufficient for road transport emergency evacuation, but is far from sufficient for marine, rail and air transport. The method of extinguishing lithium batteries has not yet been determined. Neither large quantities of water nor gaseous extinguishing agents are readily available in a short period of time during transportation. The heat, smoke, and toxic gases produced by lithium battery combustion can quickly cause loss of life and damage to equipment.

Conclusion
China’s lithium battery industry is booming, the mass transportation of lithium batteries is also imperative. Due to the high specific energy, dense accumulation, large volume, high quality, high consistency and high voltage characteristics of lithium batteries, the transport safety of lithium batteries is also different from consumer lithium ion batteries.

At present, the intrinsic safety of lithium battery transportation has not yet been resolved, the combustion mechanism and hazards are not well understood, the transportation test system is not complete, the transportation safety evaluation capability is lacking, the risk prevention and control measures are insufficient, and the lack of effective fire fighting methods and other issues. To solve these problems, we need the cooperation between the governmental departments and enterprises, the close cooperation between the manufacturing industry and the transportation industry, and the breakthrough of multi-faceted research.

A reasonable solution to the safety of lithium batteries to meet the transportation needs, the following elements should be considered:

Safety is relative. All devices have risks, and can cause problems in special environments.

The safety issues of lithium battery transportation should be addressed in transportation practice by continually exposing problems and seeking appropriate solutions.

Safety is a risk management issue. Different modes of transportation have different acceptable limits.

There are many measures to mitigate the risk, and applicable measures are available, such as reduced charge, special packaging, effective isolation, and quantity limits. Safety is closely related to the efficiency or performance of lithium batteries. The lithium battery industry has developed rapidly in recent years, including new systems, higher specific energy and new structures. For designers, higher specific energy cannot be pursued unilaterally. People are constantly looking for smaller, higher energy lithium batteries, will also lead to an increased probability of energy runaway danger, resulting in more serious hazards. Safety throughout the entire life cycle of lithium batteries.

The safety of lithium batteries are involved in the design, production, storage, transportation, sales, use and recycling of links, and affect each other. Transportation safety should also be considered at the forefront of product design.

All parties involved are responsible for safety. The risk of lithium batteries must be strictly controlled and the necessary precautions must be taken to ensure the safety of the entire process from production, sale, transportation, use and recycling, which should be shared by all parties involved.

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