The development history of lithium-ion batteries - brief introduction of overcharging

The probability of accident (LIB) of the lithium battery is because of its own defect in extremely low. In March 2009, The battery International society "26 international battery seminar & Exhibit" held in Florida, on the Sandia national laboratories (SNL) announced the battery itself cause accident probability below 1 PPM .While engaged in battery safety countermeasures of American TIAXLLC said the accident probability between 0.2 ~ 0.5 parts per million, more than the quality control method for management of six sigma (six sigma) limit.

1 PPM of probability is extremely low, can be called a rare phenomenon.1 year in the United States died in the probability of lightning is about 1 PPM. The probability of first prize in Japan to buy * * * 0.1 PPM. That is to say, the LIB due to its own defects in the probability of accident slightly higher than the first prize, and lightning die basic same. This is because the LIB took multiple safety countermeasures.

LIB, however, the actual accident probability is much higher than in the order of PPM.This is because most of the faults caused as a result of the accident is not the battery itself, but the external factors. In the previously mentioned to SNL said the vast majority of the cause of the accident are extrusion, heating, charging and other external factors.

In fact, few people know the situation. Until now, many view is also the main cause of the accident as the battery itself is flawed. Japanese battery industry will recently proposed test sheet metal into the battery will threaten the safety of test method. Specific methods is after winding battery components insert sheet metal, by loading pressure, force of metal lead to internal short circuit. Investigation in this process, the battery whether the phenomenon such as fever, smoke and fire. This is because there are critics "manufacturing process with LIB pieces of metal will be the internal short circuit fuse, eventually cause accidents".

Most battery accident due to improper use

Most manufacturers have implemented the basic same test. Its representative is in charge, with nails through the nail test of the battery. On this basis, the impurities such as if the battery in sheet metal, triggering a small internal short circuit, through the aging test should be screen out. This means that the defective batteries as low possibility of goods into the market. Battery industry will test is actually implemented in repeating the same test, its necessity suspect.

Of course, there is no internal short circuit, even when they were built in use process, the battery is also entirely possible internal short circuit. Assumptions when charging, electrode generated within the dendrite shape metal, causing the internal short circuit. There are two main reasons that cause this failure. One is charging and discharging, electrode doped metal dissolution to the electrolyte, the extreme precipitation.

The other one is inside the battery contains hundreds of PPM level (relative to the proportion of the total) electrolyte amounts of water. After water into the battery, can react with supporting electrolyte LiPF6, produce hydrofluoric acid (HF).HF acid, dissolve the positive active material and the collection of electricity, when charging and discharging, make these material in extreme precipitation in metal dendrite form.LIB just appeared, also made in the drying chamber, dry winter, compared with high wet summer products of winter products in all aspects of the performance is better, therefore, LIB is sensitive to production environment humidity.

Now, the battery manufacturing process to take the measures to completely remove moisture. For example, to dry various materials. Is the most important step, for easy to moisture absorption of carbon negative electrode used for drying, the electrolyte after dehydration tower, removal of trace moisture. And manufacturing to set the dew point between 40 ℃ to 60 ~ - in the drying chamber. To be on the safe side, after the battery components, but also use vacuum dryer to dry element again.

Comprehensive strategy means that in the manufacturing process, even the slightest mistake, may result in cells appear to itself to causes of major accidents.LIB manufacturers, therefore, must remember, in the battery manufacturing process will be a little slack.

However, no matter how battery manufacturers in the manufacturing process is careful, enhance security, users are not used correctly, can prevent the happening of the accident. Like SNL and TIAX, points out, compared with the battery itself defects, external factors is much more likely to cause an accident.

Charging is an enemy of secondary battery

What is the proper use of batteries? The most important is to avoid overcharge. Trace of charging is also likely to lead to smoke and fire accidents. FUJITSUTECHNORESEARCH has, for example, 4.25 V and 4.30 V voltage charging LIB respectively under the extrusion test and the wear nail test (figure 1).As a result, the LIB of 4.25 V batteries without problems, and 4.30 V charging LIB appeared smoke and fire. The difference of just 50 MV, it may lead to a major accident.

Figure 1: the battery burning one big reason is that a charge

Charging voltage (just 50 MV, battery combustion occurs (a, b).(photo: FUJITSUTECHNORESEARCH)

Charging lead to the risk of accidents not only exists in the LIB, also exist in all of the secondary battery. Such as security is much higher than the LIB of Ni - Cd (nickel cadmium) secondary battery. Using aqueous electrolyte, the positive and negative respectively using nickel (Ni) and cadmium (Cd) compounds. There is no fire element. In spite of this, the battery will still be at risk for charging status.

Ni - Cd after secondary battery is full, the anode will produce oxygen, the cathode will produce hydrogen. Once the hybrid form of oxygen and hydrogen gas volume ratio of 1:2 "detonating gas", to meet a certain incentives, explosion easily.

To prevent this phenomenon occurs, through the improvement, Ni - Cd secondary battery in the cathode adopted more than necessary quantity of cadmium compounds. Thus, in the end of charging, the anode will be full of advance and produce oxygen, and negative there are not part of the charge, will continue to charge. The positive produce oxygen moves to negative, react with negative charge to complete part of the metal cadmium. After reaction of metal cadmium return not charging status. This makes negative there is always not charging part, can keep the positive conveying to the oxygen consumption.

This method appears to be on the safe side, in fact, there is also a weakness. Under low temperature, the reaction speed slow, oxygen cadmium negative oxygen reaction will be reined in. Oxygen and absorb the balance once broken, the cathode will reach full of state and generate hydrogen gas, thus forming the detonating gas. This kind of situation easily in the fast charging, such as occurs when using large current charging.

Charging is why dangerous?

For the LIB, the effects of charging more serious. The reason is that of charging can cause two big problems. One is more than negative lithium capacity (theory capacity, graphite of 372 MAH/g) of the lithium will translate into metallic lithium, precipitation on dendrite. This is likely to cause internal short circuit.

The other one is the positive produces high reactive oxygen. If the oxygen and the electrolyte in the LIB, lithium and carbon compounds, namely oxidation reaction, may lead to a fever and catch fire.

Here in the representation of the anode material of cobalt acid lithium (LiCoO2) as an example, to explain it in detail.LiCoO2 structure is inserted between the lithium cobalt oxide (CoO2) layer. Charging, lithium from the anode to cathode. After lithium from "empty house" (gap) will increase. Cobalt (Co) will enter, into the "empty house" lodged. As the charge, the more "empty house", or the higher the temperature, cobalt is easy to move .Lithium cobalt moved once site, before and cobalt CoO2 oxygen will lose partner, also from the anode.

The oxygen is atomic state exist alone "nascent oxygen", reactivity is very strong, react with any material is easy to note 1).LIB contains easy oxidation of electrolyte, lithium and carbon compounds, these substances will combine with oxygen, causes fever and catch fire.

Note 1) the oxygen in air is formed by the combination of oxygen atoms (02) (keys) is full, the reactivity is not strong. If strong reactivity, creature in breathing, lung can react with oxygen and burns, unable to survive. We must thank oxygen do not have strong reactivity.

"The nascent oxygen" under the condition of how to produce? We Li1 - xCoO2 x as parameters, quality by using thermal analysis method to detect the quality of change when the temperature changes, studied the related conditions (figure 2).Is, according to the results of x, the greater the charge schedule, the greater the quality began to decrease temperature is lower, the greater the reduction. The performance of the quality to reduce oxygen is stripping. And we also found that the greater the charge schedule (x), the more oxygen stripping, the lower the temperature of oxygen stripping. That is to say, to suppress the nascent state oxygen ", not in excess of the prescribed under the voltage of charging (avoid charging), battery does not heat up 2) is the necessary condition.T o satisfy these conditions, the assembly enterprises LIB users must be careful.

Note 2) in the synthesis of LiCoO2 adding aluminum (Al) and magnesium (Mg) is also an effective means to suppress nascent oxygen. These substances are added oxygen stripping temperature will rise, to a certain extent can contain stripping reaction.

Figure 2: the greater the charge schedule, the smaller the anode quality

Through the Li1 - xCoO2 x difference said the anode quality change. The bigger the x said charge schedule. As oxygen stripping, the anode quality.

Quick charge is the cause of charging

Easily happened in the charging situation there are two main types: fast charging, battery unit are unbalanced. First to see quick charge.

LIB is recommended using constant current/constant voltage charging (CC/CV: constant current/constan tvoltage).This is the first charging down at constant electric charge (CC), after reaching the prescribed voltage, switch to the constant voltage charging charging (CV) method (figure 3).Usually the LIB, at the beginning of the charging charging voltage of 4.2 V, current for 1 c note 3).Because the current is 1 c, so in theory, after an hour, terminal voltage will reach 4.2 V.But the charging voltage between terminal voltage and is called "overpressure" * deviation. Therefore, when the charging voltage is 4.2 V, the actual voltage below 4.2 V.

Note 3) charge at 1 c refers to the current theory can be 1 hour full of charge. Battery capacity is 2000 MAH, 1 c to 2000 ma.5 c charge is 5 times 1 c, is the charging current of 10000 MA.

* overvoltage = electrode reaction when an electrical current, electrode potential will deviate from the equilibrium potential. This offset is called overvoltage. For example, in the electrolytic, load voltage is not higher than theory, electrolytic will happen. For the battery, the actual battery voltage is lower than is decided by the combination of the anode, electrolyte and cathode of the theory of voltage.

Figure 3: constant current/constant voltage charging (CC/CV)

LIB we recommend starting with constant current charging, after reach regulation voltage, switch to the constant voltage charging of CC/CV.

That is to say, when the charging voltage is 4.2 V, the battery is not full of, charging amount is only 70 ~ 80%.If you want to continue with, want to switch to CV charging, at the same time of maintaining charging voltage of 4.2 V, slowly add the remaining 20 ~ 30%.In CV when charging, however, current will be narrowed, if to shorten the charging time, expand the CC of charging current value, the over voltage deviation will increase, make the charging voltage of 4.2 V in advance. May take longer to full of battery.

If only need to fill the 70 ~ 80% of the battery, through large current in CC when charging, quick charging, 20 ~ 30 minutes to complete. However, to take advantage of the CC into close to 100% of the electricity, is impractical. Because the need for the purpose of this voltage is higher than the prescribed voltage of 4.2 V, will form the "quick charge = charge" equation.

Set the protection circuit response unit imbalance

Recently there is a kind of thinking: even if the charging voltage in excess of the prescribed voltage, as long as with the method of pulse charging, by means of the current interrupt, will not cause a charge. But the pause time of pulse charge means that the pulse current than continuous charging current. Although fast charging, but must be loaded above specified value of voltage, still can cause of charging. You don't get me wrong.

Another factor of the charging is imbalance between the battery cell unit performance. The following simple battery pack with two cells in series and become, for example.

Assuming that a serious deterioration of the two units, capacity imbalances If in this state for two charging unit, the unit of capacity degradation will achieve full of state ahead of time. Suppose a voltage of 4.2 V, the other party is 3.8 V. At this time, 2 times the charging voltage of 4.2 V, 8.4 V, according to the simple calculation, at the end of the charging, a unit of voltage of 4.4 V, the other one is 4.0 V.4.4 V unit is obvious in the charging state.

Avoid imbalances of charging method, in external protective circuit is set in the unit stops charging voltage of 4.2 V. However, if the unit quantity, the way that you need to set up a lot of FET, used to monitor the voltage, the cost is not cost-effective.

Count of oxidation reduction on the material

Devices such as do not rely on protection circuit, make the battery itself can prevent charging material has not stopped the development steps. Although has not been put into practical, but, as the representative of this kind of material, oxidation reduction for the prospect of the material was favored. The mechanism of action of the material is not difficult to explain, have got the Ni - Cd secondary battery and nickel metal hydride secondary battery.

About Ni - Cd secondary battery charging problem, it has already been said, when fully charged, the anode will produce oxygen, producing oxygen will be negative. In other words, it is positive to produce (oxidation) of oxygen in anode consumption (reduction), the process takes long. When charging, the battery in the following loop condition.

That is to say, as long as the formation of such circulation, can produce won't appear of charging the battery. This is called REDOX reaction. Redox (Redox) including VAT (Reduction) and oxygen (Oxidation) two steps.

LIB cannot like Ni - Cd secondary battery, gives its REDOX function directly. This is because if you don't control voltage when charging, the positive of Li + (lithium-ion) will continually be shipped to the cathode.LIB REDOX reaction must be equipped with the function of the material. This has become a subject faced by the battery developers for years.

Adopted by the REDOX of functional units of the battery pack can solve the problem of unbalanced cells (figure 4).When a particular unit with early, REDOX will start to function, the unit stop recharge electrode. But not full of the other units of REDOX function is not start, to continue their charge.

Figure 4: using REDOX uneven on the material of voltage can be solved

The past in unbalanced state of unit stop charging the battery pack (a), using REDOX battery pack can make all elements of materials to achieve the same voltage (b).According to the 3 m paper making pictures.

The author's laboratory in about 25 years ago trying to develop peroxide reduction on the material. We were trying to material only in very small electrical flow REDOX reaction, can't cope with the usual charging current.

In recent years, along with the progress of the simulation technology, the people for what would happen for the structure of the compound REDOX reaction has had certain understanding. In 2008, the United States 3 m company developed under the voltage of 3.9 V work of oxidation reduction on the material, make practical took a real color (figure 5).Now, work under 4.2 V material also in the study, if effective, should be applied to occupy the mainstream LIB LiCoO2 batteries.

Figure 5: work under 3.9 V 3 m company's oxidation reduction on the material

Through repeated prevent overcharge REDOX REDOX reaction of materials is a new generation of battery development a big focus.3 m company developed work under 3.9 V. According to the 3 m paper making pictures.

In addition, the development of flame retardant electrolyte is in progress. Adding flame retardants will seriously affect the battery performance, as a result, many ideas that flame retardant electrolyte is difficult to put into practical. Recently, the temperature of molten salt electrolyte (ionic liquid) successful development. The electrolyte has the characteristics of low steam, non-flammable. Although have argued that the imperfect ion conductivity and other properties, ionic liquids can not avoid burning, at present has not yet reached practical level, but for the future development, the author is full of expectation.

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