Lithium Battery Safety Issues Caused by Samsung's Note 7 Battery Accident |

Since the public sale in foreign countries on August 19th and since the public release in China on September 1st, a series of battery accidents involving Samsung Note 7 have occurred successively at home and abroad.

For the first issue of the Note 7, Samsung stated that it was due to the poor quality of the battery, so it implemented a wide range of recalls and replacements. However, recently, several domestic incidents that were not in the scope of the recall have caused Samsung to believe that the State Bank version of the Note 7 battery without quality problems has cast a shadow over this incident.

Although Samsung and National Bank Battery supplier ATL issued a statement on September 19, the domestic Note 7 battery accident was caused not by the battery quality but by external heating. However, this conclusion has not been approved by the purchasing users. They have stated that they did not externally heat Note 7, and they use the official standard charger.

As the sale continues, we can't help asking whether the National Bank version of Note 7 is a problem with the battery. What is the working principle of the lithium battery? How will this battery accident affect the entire battery and mobile phone industry? ?

Focusing on relevant issues, Lei Fengwang (searching for the "Lei Feng Net" public concern) invited two big players in the industry on the afternoon of the 28th to conduct a thorough answer and science popularization for the small partners of the Kontron Open Class.

Â Guest introduction

Liu Jianwen, postdoctoral fellow at Huazhong University of Science and Technology, an expert in electrochemical engineering.

Wu Bo, a postdoctoral fellow at the University of Leeds, is an expert in data science.

Â Lithium battery operation principle introduction

Lithium-ion batteries were introduced in China in the late 1990s. After more than 20 years of development, lithium batteries are still small compared to lead-acid batteries and nickel-metal-nickel-metal hydride batteries in the secondary battery (non-disposable battery) market. Public groups. According to the survey, the current market share is about 20%-30%. From this lower market share, it can actually reflect that lithium-ion batteries do have some fatal flaws.

Nevertheless, we cannot deny the various advantages of lithium-ion batteries. For example, the working voltage of lithium-ion battery can generally reach 3.5v-3.8V, and the lithium-ion battery developed by the laboratory can reach 5V. In addition, lithium-ion battery also has better capacity and energy density. The above two characteristics It is usually more than twice as high as traditional nickel-chromium nickel-metal hydride batteries. More importantly, lithium-ion batteries are also more environmentally friendly than traditional heavy-metal batteries.

The general lithium ion battery is mainly composed of three parts: a positive electrode, a negative electrode, and an electrolyte.

Positive electrode materials are relatively early in our country, and industrial development is relatively early one. General commercial applications include: layered lithium manganese oxide, manganese materials of spinel and olivine structure, and lithium iron phosphate. The only commercially available material for negative electrodes is graphite. Unlike traditional secondary batteries, electrolytes for lithium-ion batteries are organic electrolytes. There is a major safety risk here. This organic electrolyte is flammable compared to aqueous electrolytes.

It should be pointed out that the common commercial electrolytic salt is a substance called lithium hexafluorophosphate. One of the characteristics of this material is that decomposition occurs when the ambient temperature reaches 60 degrees Celsius, which is very easy to achieve in the daily use of our mobile phones. The decomposition of lithium hexafluorophosphate also releases gaseous substances, which creates a great safety hazard.

In addition to the above three blocks, another important component of lithium-ion batteries is the diaphragm. The diaphragm is usually made of a plastic film of PP or PE material. Its main function is to isolate the positive and negative electrodes of the battery and prevent short circuits. It should be noted that, in order to ensure the smooth passage of lithium ions during charge and discharge, the patency of the diaphragm requires that the arrangement of the aperture structure be absolutely uniform. Therefore, the manufacturing process requires high standards. In the early years, the domestic production capacity of the diaphragm was not available. Here, the technological level of the separator also plays an important role in the safety of the battery.

When it comes to the working principle of lithium-ion batteries, the industry generally also refers to lithium-ion batteries as "rocking chair batteries." As shown in the figure, the positive electrode of a lithium-ion battery is usually selected from materials whose lithium atoms are relatively easy to be depleted. When charging, lithium atoms are separated from the positive electrode to become lithium ions, and then transported through the electrolyte through the pore size structure in the middle of the separator. Reach negative and store in graphite. During discharge, lithium ions are separated from the negative electrode, transported through the electrolyte, pass through the separator, and return to the positive electrode material. This is the charge-discharge process of the entire lithium-ion battery. Lithium ions oscillate back and forth like a person in a rocking chair.

From this process, we can see that lithium ions reciprocate in the electrolyte. First, the viscosity of the electrolyte must not be too large. If it is too large, it will form a transmission resistance and cause local overheating. In addition, the diaphragm must also be sufficiently smooth, and requires a certain degree of strength. The diaphragm must not hinder the passage of lithium ions, nor should it break or increase its pore size after several charge and discharge processes. This is an important guarantee for the safety of lithium-ion batteries. condition.

Â What caused the Note 7 battery accident?

From the working principle of the lithium ion battery introduced above, we can analyze some of the causes of the battery accident.

Analyzed from the perspective of the electrolyte. Since lithium hexafluorophosphate, which is an electrolytic salt, decomposes at a relatively low temperature and generates gas, it may itself pose a danger to the enclosed environment of the battery. For example, bulging may occur on some soft pack batteries. In addition, since the lithium-ion battery electrolyte uses an organic electrolyte instead of an aqueous electrolyte, the organic material is easily burned, and thus poses a safety hazard.

The other hand is a diaphragm. As described above, there are generally two requirements for separators in the industry: one is to require a uniform distribution of the pore size, and the other is to ensure the strength of the separator material. If the strength of the aperture is not enough, perforation occurs after several charge and discharge, it will cause the battery anode and cathode short circuit, resulting in a lot of heat, which is another important reason for the explosion of lithium-ion batteries.

The third is the material of the electrode. Now some positive electrode materials may undergo deformation or collapse after repeated charge and discharge processes, which will directly lead to the formation of a physical structure on the electrode surface of the battery. This bulge is likely to pierce the diaphragm, causing a short circuit between the positive and negative electrodes, which can cause an explosion.

The last is the problem of overcharging and overdischarging of the battery. Usually we use commercial lithium-ion batteries, depending on the positive electrode material, the operating voltage is generally between 3.5v and 3.8v, and the discharge voltage is generally between 2.0v and 2.5v, and some may be lower down to 1.5v, which is based on Different materials to set. In other words, when we charge the battery, the voltage should not exceed 3.8v or 4v, and the voltage at the point of discharge should not be lower than 1.5v. Otherwise, it will be overcharged or overdischarged, which may cause potential safety problems for the battery.

However, in response to this problem, most of the industry's manufacturing processes have introduced mechanisms to protect circuits. This has largely prevented battery safety accidents caused by overcharge and overdischarge problems.

The above analysis is based on the battery structure and working principle of possible battery explosion accident possible reasons, but to find the specific cause of the Samsung Note 7 battery accident, you must get the accident battery for actual analysis.

As a party to the 4th bombing incident at Note 7, the old friends of the group also participated in this seminar. Laohui asked questions: The batteries of both ATL and SDI suppliers have experienced problems, and the cell phone technology in the cell phone is also very mature. This time, the Note 7 cell accident is not caused by the quality of the cell itself, but What other reasons, such as overcharge caused?

Liu Jianwen said: The current battery production and manufacturing process is a very systematic project. Anything that goes wrong with a small link can affect the performance of the entire battery. Therefore, the reasons for this are very complicated. a lot of. For example, when a problem occurs in the detection of the cell molding, there may be a â€œfish that escapes the netâ€. Even the sales and transportation of the battery may affect the final performance of the battery performance. For example, when the transportation environment is overheated, initial decomposition of the electrolytic salt may occur.

Â What is the safety inspection before the battery is shipped out?

To explain the problem of detection of the battery before shipment, we need to briefly introduce the production process of lithium-ion battery.

1, first need to prepare the electrode material, the positive and negative electrode materials in the reaction kettle sizing, that is, in the reaction vessel to make the positive and negative electrode materials.

2, then the smear, the so-called smear is to make a good slurry evenly coated on the electrode carrier fluid above. The following is a baking sheet that volatilizes the moisture and other volatile organic compounds in the slurry. After baking, the tablet is platted. The platter is to arrange the battery pole pieces according to the requirements of the production process. This is followed by further adjustments to the pole piece, such as brushing some of the naked-eye powder on the pole piece, or pressing the pole piece equally with hydraulic pressure and rolling.

3. After the above series of steps, the electrode will be coiled below and then wrapped to form a cell. At this time, although the electrolyte has not yet been added, the battery is still not working, but the voltage and resistance can already be measured. Therefore, a series of tests on the battery will begin at this production node, for example, detecting whether the battery is short-circuited or open circuited, so that a portion of defective products can be eliminated.

4, after the completion of the production of batteries will usually be loaded into the steel shell, then basically has the battery's appearance. The next important step is to load the electrolyte and then go through the welding and sealing operations. After strict air tightness testing, a lithium battery for ordinary mobile phones is made.

5, then need to be in a special workshop on the production of good batteries for "chemical" operation. The so-called achievement is the process of activating the positive and negative electrode materials of the battery, and the formed battery will form a passivation film on the surfaces of the positive and negative electrodes. This step is very important. Before the battery is shipped from the factory, it is usually necessary to convert the battery to activate the positive and negative electrodes so that the battery can form such a film. In addition, the requirements for chemical conversion equipment are also very high in this step. It is required that the current signal for activating the battery be as accurate and stable as possible, and it must not be subject to much fluctuation.

In addition, some batteries are tested before they leave the factory. For example, let a battery do several dozen charge/discharge cycles and test the performance of the battery.

In general, from the point of view of the production process, the detection of the battery before leaving the factory mainly includes the detection of the battery core, the detection of the airtightness, the forming process after the molding, and the endurance test.

In response to the above process, Wu Bo supplemented the data related to the testing process. He said: Before the battery is molded, many tests on the chemical properties must be done regardless of the electrical characteristics of the battery. Once the cell is molded, the next step is to perform some electrical tests related to the electronic properties of the cell. In addition, many compliance and certifications are required after the battery is manufactured. There are more than a dozen common compliance and certifications.

As shown in the figure, the battery should meet the national standard of 2014 or the national standard of 2013 after the production of the battery. The international standard of the United States also includes the UL standard, and the transportation also includes the United Nations UN 38.3 standard.

The UN 38.3 standard of the United Nations stipulates seventy-eight tests for battery safety. Including the detection of low pressure, the detection of high and low temperature, the detection of physical impact, the detection of overcharge and overdischarge, the detection of external accidental short circuit, and so on. All test items need to complete two or thirty samples. However, due to the different detection items of the national standard, it may require 20 or 30 samples. In addition, the time spent on each test item is not the same. For example, the charge and discharge test requires more than ten hours.

After these standards are tested, each standard has twenty or thirty samples, and each standard contains multiple test items. These results form a complex data collection for general battery manufacturers or mobile phone manufacturers. How to effectively manage and use this data also needs attention and attention.

This is followed by the battery manufacturing process, such as the above-mentioned chemical testing prior to cell molding, and the series of test procedures mentioned by Dr. Liu, such as chemical conversion and repeated charge and discharge. These test results can also form big data. It is also a process of collecting and managing big data, so it also needs special attention.

Before the news showed that Samsung broke out after the Note 7 battery accident, the South Korean government asked Samsung to carry out X-ray scanning and detection of each cell phone battery. Then, after the inspection will form a large number of X-ray photos, how to manage and identify these photos, how to deal with these X-ray photos, but also need to rely on the relevant theoretical knowledge of big data.

Also, Dr. Liu mentioned just now that batteries generally have a protective circuit. In a market with more than 1,000 million lithium batteries, there are a group of companies that specialize in protecting circuits. Their output value has reached more than 10 billion yuan, accounting for 10% to 15% of the entire market.

In addition, as a Note 7 battery supplier ATL company, there is news recently that they intend to add a remote management module inside the battery, so that the battery data can be remotely reported, in the cloud on the operation of the battery in real-time monitoring And management.

(The protection circuit in the figure can regulate the user's charge and discharge process of the battery, as well as protect the battery from overcharge and overdischarge, as well as accidental short-circuit and other common problems.)

In short, I think that once the entire battery is in contact with data acquisition and management, it is already approaching the task of big data. From the chemical detection before the cell molding to the airtightness detection, formation process and endurance test before shipment of the cell, and various certification tests after leaving the factory, such a large amount of test data has caused some confusion to the cell manufacturers. They need to cross-border to find resources for big data.

Â A battery incident occurred in both suppliers. Is there a problem with the battery design of Note 7?

From the point of view of the accident, this conclusion cannot be made. Since the first commercially available lithium-ion battery was introduced by Sony Corporation of Japan in 1990, they used exactly the same materials and structures as those used in current mobile phone batteries, such as positive and negative materials, electrolytes, and separators. As of now, twenty or thirty years have passed. There has been no fundamental change in this formula. Therefore, it cannot be said that there is a problem in the design process of the battery because if there is a problem, all the batteries have problems.

However, the occurrence of the Note 7 battery accident did remind us that we need to make some improvements in the design of the battery. For example, just as Dr. Wu also said, in the battery shipment inspection process, if it can be combined with big data, software and hardware, or full inspection and spot inspection, it will be able to find more defective products. This is also a need for us. The direction of improvement.

In addition, battery design solutions also need improvement. For example, from the electrolyte point of view, we can completely add some flame retardants in the electrolyte to prevent the electrolyte from burning and prevent over-discharge of additives. In the diaphragm section, the use of a stronger, more uniform material. In battery protection circuits, strive to do more scientific and more advanced and so on.

I am currently engaged in research work on electrolytes, which is how to effectively prevent the explosion of the electrolyte. Some of the research results have been applied in the enterprise. In general, on the one hand, it is not possible to arbitrarily assume that there are problems with the design of the battery, and on the other hand, it is necessary to continuously improve the process.

Â Note 7 Is the battery's energy density too high, does this have an impact on battery safety?

As mentioned earlier, a great advantage of lithium-ion batteries over conventional secondary batteries is their high energy density. The theoretical energy density of mobile phone lithium-ion batteries can reach 500Wh/kg. The lead-acid battery is only 50Wh/kg or 60Wh/kg. Lithium-ion batteries have grown almost tenfold compared to the two, which should be a huge advantage for lithium-ion electronics.

The lithium-ion batteries we now use in electric vehicles also require higher energy densities. This makes the battery output more power, which is a necessary condition for electric vehicles, but also a huge advantage. At present, we have not seen any reports of safety accidents in electric vehicles caused by high energy density.

If the energy density and safety are to be related, it can only be analyzed from this point of view: high energy density allows high-current discharge, such as in the electric car requires a large-scale discharge of the battery, the industry known as 10C or 20C discharge, this lithium battery can do, but the lead-acid battery can not do. This large-scale discharge will cause the current to be too large, resulting in local overheating and security risks. So if you analyze this, there is a certain relationship there. But under normal circumstances, we will not look at the safety and energy density of the battery.

Â Note 7 What impact will battery accidents have on the entire battery and mobile phone industry?

Through these several incidents, everyone should pay more attention to the safety of lithium-ion batteries. The summary from the previous answer is:

1. Whether it is possible to add substances that prevent combustion and explosion in the electrolyte during the production process;

2, in the diaphragm, can not be more reliable, more evenly arranged aperture.

3, in the entire process of battery testing, how can we do more reliable detection and effective data records for subsequent calls at any time.

4. How to ensure the safety of these non-production processes in the final transportation and packaging of the battery.

In addition, Dr. Wu mentioned previously that after the Note 7 battery accident, Samsung began to use X-ray inspection on a large scale, that is, the X-rays were emitted before the battery was shipped. This method can bring a lot of convenience, such as the shell, after the laser inspection can confirm whether the shell is damaged. If it is broken, electrolyte leakage is a big risk. For another example, whether the diaphragm inside the battery is perforated or not, whether the protection circuit has fallen off, or the like. The introduction of X-ray inspection is a very convenient and important technological innovation.

In addition to the above thematic sharing section, Liu Jianwen also gave his opinions on several issues raised by the group of friends.

1. How long does a mobile phone lithium battery last when it self-ignites?

The autoignition time generally does not exceed one minute, because the auto-ignition is mainly the combustion of the electrolyte, of course, the diaphragm itself can also be burned, but there is no severe combustion of the electrolyte. In this part, I have done experiments and put our traditional electrolytes in the membrane for burning. It is about 40-50 seconds, and it will not take more than one minute. When the back septum is burned, no open flame can be seen.

2. What is the difference between a power battery and a lithium battery?

The problem should be to ask the difference between a small lithium battery and a power lithium battery. In the case of positive and negative electrode materials and electrolytes, the power battery material has a higher energy density, and at the same time, it must be able to withstand larger-scale discharges. In addition, higher requirements are imposed on the electrolyte, for example, the electrolyte will be added on an ordinary basis to prevent overcharge and flame retardant additives. And power batteries generally have to do more with more batteries. In contrast, small batteries or mobile lithium batteries usually have only one or two batteries in parallel.

3. Is battery material guaranteed for 8 years? Do you have such a request?

I think this requirement is not necessary. The current lithium battery can theoretically charge and discharge 2000 times. This requirement is to ensure that the capacity is more than 80%. Of course, when we actually use it, it is generally more than 2,000 times. In some cases, when people see the battery capacity is very low, everyone still uses it. Therefore, this cannot be guaranteed for 8 years.

4. Does the battery explode or ignite spontaneously?

It should be said that any explosion or self-ignition accident still has certain harbingers. For example, the explosion will cause the battery to begin to bulge. This is the gas produced by the decomposition of the electrolyte. In addition, smoke will be emitted during spontaneous combustion.

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