The history of lithium battery development -- SONY factory fire caused a big crisis in lithium ion business

On the morning November 4, 1995, SONY Battery, which tended to grow steadily, has met great crisis. Battery business unit that located in Fukushima Koriyama - SONY Energy Technology Co., LTD lithium ion battery (LIB) manufacturing site broke out fire in the factory.

That day, I just travel back from overseas, when the safety confirmation to his wife to learn the news. I couldn't believe my ears, immediately contact the factory, the answer is that the factory of the aging test chamber and burnt to the ground to charge and discharge chamber. Although the staff tried to put the fire out before the fire to expand, but without success.

* aging test: LIB not out immediately after the factory production, but to place 1 ~ 2 weeks after charging. This is called aging test. There are two purpose. One purpose is to find out the doping is a foreign body, the internal circuit of the battery. When the battery in place of self-discharge, battery side pressure drop. According to the nature of a place at a certain time to test the pressure drop, eliminate pressure drop in the standard of the battery. Another purpose is on the surface of the cathode carbon membrane formation of Solid electrolyte interface (Solid - Electrolyte Interface, SEI).SEI is mainly the electrolyte and supporting electrolyte decomposition products, such as carbon functional groups on the surface of the composite film, stick on the surface of the carbon in the process of aging test, have cut off electronics, through the solid electrolyte property of ion.LIB cathode using lithium compounds, already has a strong activity, severe reactions can lead to accidents. SEI can inhibit the reaction to a certain extent, to curb violent reaction, to ensure the safety of the LIB.

To avoid being listed in the "list of dangerous goods"

The next day, the author cannot consider other, went straight to the prefecture county mountain. Factory to the site, they see 3, second floor, the wall has a big gouges where whole pieces were missing. Because of the charge and discharge chamber without Windows, fire hose into don't go, so the firefighters had to excavator opened a hole in the wall. Turn to the east, the scene before ever. All the walls of the second floor was burned down, can see the floor internal become black. Looks like a layer and three layer of white walls between the black belt, as if a whammy of three white black and white flag.

Because indoor smoke has not yet dispersed, I can't enter the site in the day, the second day, that is, on November 6,special and fire brigade began exploration field. SONY is the identity of the author at the time recording media and energy battery development department minister, the energy department and factory manufacturing department, general affairs department and facilities department colleagues together to witness the exploration process.

Police search technology group is really real, equipped with the experts, precisely from the technical problems are put forward. The author theoretically explained the under the conditions of not charging or discharging accept aging test battery could not have reason to fire. Police said the final approval, it is concluded that the "cannot be considered a battery fire. It is unable to determine the reason, the conclusion of "still need further investigation.

After, I frequently travel to Tokyo fire department and county mountain fire agency, whenever firefighters to the factory, also have to explain to them the LIB is not dangerous goods. Because once they are included in the list of dangerous goods, will be bound in the future business. Is stored in the factory will register in excess of the prescribed number of dangerous goods, shipping mark in the goods even when the words "dangerous goods". However, can be put on this hat.

The problem is the development and manufacture of communication

We also investigated the cause of the fire, of course. Generally speaking, the supply of batteries including multiple security test. We put the test project of the "nail" and "squeeze" test back again, and investigated the parts whether there is any problem.

Figure 2: LIB safety test sample

Security testing including "nail", "squeeze" and so on more than 50 projects.

After aging test is to charge LIB in a tray, put a period of time. The number of test LIB usually keep in tens of thousands of pieces. Development department, therefore, from the perspective of eliminate hidden dangers, deems the tray with flame retardant. The importance of flame retardant, however, does not accurately convey to the mill site. Development department didn't know it, tray didn't adopt high flame retardant material, but the molecular weight of polypropylene is not big. Factory in order to reduce the cost.

Through repeat test the final conclusion is obtained: the low possibility of battery fire, fire ignition for some other reason fire ignited flammable tray. We found, according to burning test was carried out on the tray is really easy to fire. And the low melting point of polypropylene, once ignited will melt, pattering drip down, like wax dripping from the lighted candles. Fire location has identified is the top of the shelf. Because the properties of the tray, melting polypropylene will continuously on the shelf beneath. At this point, the combustion of water tray, can cause a fire of polypropylene droplet splash, unable to fire. If the tray has the flame retardancy, the fire will never spread.

It is a basic principle of safety management "heinrich rule". This is the insurance company as a technician in the United States has heinrich through investigation, analysis of the law of more than 5000 industrial accident found. Behind a major accident, there will be 29 secondary accident. Behind in 29 secondary accident, and 300 class-action accident seems to be not worth mentioning. The discovery, he is also known as "1:29:30 0 law." Before the fire, should be too much a small accident, but no attention. Open Gao Jian's novel "the last supper," the word, "the flame of the fire must be than cigarettes cause fire flame strong". The result is true.

5 months of discussions come to the conclusion

After the discussion of fire department continues. The fire broke out after 3 months of February 9, 1996, we came to Tokyo fire department prevention department, in a table near the window, and dangerous goods class inspections and class staff discussed is the cause of the accident. Desk in the room, dense, heaped on the desk and bookcase with file. Although the author heart whisper "so many files, a fire broke out in one thousand to do", but not show on the face a trace. Out of the window to see the emperor in hand city, quiet scenery. But, in our mind is full of worries about the day's discussion results with hesitation.

We are careful, patiently explained the principle and structure of the LIB to Tokyo fire department, multiple security agencies, and why we by recreating the experiment's vision of a fire. Discuss the result is: without changing the operation of the LIB regulation, can maintain the same. At the moment, there has never been easy. On April 4, 1996, after that, we take the opinions of the Tokyo fire department, and mountain county fire department meeting for 4 hours, finally gets the conclusion of the LIB does not belong to dangerous goods.

In this way, the cognizance of the fire accident. Next to develop countermeasure, to eliminate a recurrence of such problems. The fire occurred in SONY own factory, we from recognize unlucky, but must not be the same accident in the factories, and end of the personal computer, mobile phone enterprise users there.

To prevent over charging and discharging

The following will introduce after the accident, we have the security technology in the LIB.LIB using organic solvent as electrolyte, cathode equipped with lithium and carbon compounds, completely is the fuel assembly. Moreover, huge energy is enclosed in a narrow space. Battery when use, energy is released a little, if at all put out, will cause an accident. Therefore, in order to prevent this happening, LIB multiple security measures must be taken. Countermeasure principle is to prevent over charge and discharge.

Cylinder type of structure is the flake of the positive and the diaphragm, the negative overlap and roll into tubular, immersion in the electrolyte as a whole.(click to enlarge)

The first response is mechanical connection (mechanicallink, also called current cut-off device) (figure 3).This is when the battery internal pressure rise due to abnormal, by cutting off the positive lead made the current disconnect the device. The anode lead bonding at the top of the protruding wire circuit breaker

Cut-out operation with the aid of the positive electrode active material LiCoO2 adding a small amount of lithium carbonate (Li2CO3) way. When the battery after charging, voltage of about 4.3 V, lithium carbonate decomposition produces carbon dioxide gas. Gas pressure makes the pressure inside the battery, circuit breaker were pushed up, lead to cut off the welding wire.

If you want to make the positive active material containing a small amount of lithium carbonate, you just need to slightly adjust when generating LiCoO2.LiCoO2 synthesis by means of Co3O4 and lithium carbonate mixed calcination. Synthesis, the dosage of lithium carbonate is greater than demand. As a result, a small amount of excess lithium carbonate will not react, residues in the anode.

When internal short circuit result in higher temperature and internal pressure, lead circuit breaker were pushed up, cut off the positive lead of the welding.(click to enlarge)

Using natural diaphragm to cut off the circuit

Through SEM observation, many microporous membrane surface distribution. With electrolyte microporous memory formation ion conductive channel between electrodes.(click to enlarge)

Diaphragm also plays an important role in the safety countermeasures of LIB. Diaphragm usually use polyethylene (PE) microporous membrane. From SEM photos on the surface of the PE membrane, diameter of sub-micron grades of PE fibrils (thinner than a fiber is called "fibril") formed a thin pad, similar to the non-woven it also has many sub-micron grades of microporous (figure 5).

When charging and discharging of Li + (li-ion) through the pores, the moving between the cathode. When the battery temperature anomalies due to wrong operation of rise, according to different molecular weight, the PE will melt under the temperature of 120 ~ 130 ℃, clogging pores. So that the lithium ion can't move, disconnect charge or discharge current. This phenomenon is called "diaphragm cut off".

Theory, though, but not so well. PC power supply, for example, such as the diameter of 18 mm x 65 mm of "18650" battery usable floor area of about 350 ~ 400 cm2 two pieces of the diaphragm. Area is so big the diaphragm is very difficult to whole cut off at the same time. At the same time to cut off the means to have a considerable range of softening point in PE after reaching a certain temperature, all areas and at the same time to soften.

And, if one is local softening, instead will make problems become serious. Local softening occurs, lithium ion concentration through the microporous part no closed, the formation of large current through the region. And softening of PE strength is low, it is easy to rupture (called "breakdown"), even lead to positive negative "short circuit" directly, very dangerous.

Rely on friends to solve the problem

In that case, how should solve? The answer is simple: the crystalline polymer as the diaphragm. Crystalline polymer is not ambiguous softening point, instead of melting temperature, melting point. Using special solvent molecular weight of PE (uhmw PE) made from gel, in both directions along the vertical tensile gel membrane (two-way stretch), you can get the right film (microporous membrane).This film is composed of high crystalline fibrils, have definite melting point (130 ℃, for example).Another advantage is that the melt viscosity of ultra-high molecular weight PE is great, and there was no liquidity, therefore, breakdown of temperature can be dozens of ℃ higher than the cutting temperature.

We in order to find the characteristics of the diaphragm, the variety of trial production, but has not been good results. In the end, a friend or a solved the problem. On December 25, 1991, in the research and development personnel to do, when we got the feasible diaphragm. It was Christmas presents from Santa Claus, the "Santa Claus" is half jersey into the high school classmates.

The author with a jersey in Sapporo, Hokkaido at a high school classmates for three years. In 1991, he served as manufacturing diaphragm east chemical company directors. After the progress smoothly. Half a jersey of diaphragm although there are deficiencies, but we work together, to optimize the tensile strength, thickness, pore size distribution, degree of porous, air permeability and other properties, finally reached the can be put into practical level.

PTC element is utilized to avoid the cut off

Mechanical connection and diaphragm once started, the battery will be scrapped. Therefore, we also for LIB joined a reversible safety device, is the PTC element. Is PTC (positive temperature coefficient), has the positive temperature gradient, formed from carbon particles dispersed in polymer matrix. At room temperature, carbon particles contact each other, components of low resistance. As the battery temperature rise, inflation will happen in polymer matrix, and the contact between carbon particles decreases. This will make the resistance of the element, so cut off the charging and discharging current, the temperature is called "trip". PTC element between circuit breaker and the anode lead cover. After temperature lowering, the resistance of the element restorable, therefore, can put the PTC element as the temperature of the reversible switch.

As mentioned above, the diaphragm of the cutting temperature is about 130 ℃.As long as the trip temperature of the PTC element is set to lower than the temperature, 100 ~ 110 ℃, for example, when the abnormal temperature, reversible switch PTC element will be the first to start. In this way, can avoid diaphragm cut the lead to permanent battery scrap.

But want to remind everyone here, it is believed that the temperature of the reversible switch will cause the LIB in the case of not found abnormal fever real reason is to be put into use, it is likely to cause dangerous.

A final fortress security agencies is to protect the circuit, namely BMU (battery management unit).BMU belongs to the electronic circuit, can add rich function. Its basic function is by monitoring the state of battery, prevent overcharge, over discharge, etc.

And, in addition to the security services, we also set up a strict inspection before delivery. Want to undertake a variety of tests, only qualified batch to the factory. For example, in the nail test, nails will be throughout the whole battery; Extrusion tests will use perpendicular to the battery of steel bar will be crushed to the diameter of about a quarter of a fire or smoke is unqualified. About the nail test, due to the thickness, nail insertion speed and insert the battery parts will affect the results of the test, so the test was conducted in the most demanding conditions.

To sum up, we have for the LIB adopted multiple security measures, and implementation of a number of strict test, only the supply of qualified batch. Even so, the LIB in accident.

The page contains the contents of the machine translation.