Detailed explanation of 18650 lithium battery 3000mah safety technology

　　"When a battery is finished, what is its state like? It is closely related to the safety of our battery." Ma Zifeng said. At the 100-person Automobile Conference held on January 12, Ma Zifeng, a distinguished professor at Shanghai Jiao Tong University, shared his report from the following aspects: first, safety technology strategy; second, lithium battery model construction; third, state model construction . Fourth, application and evaluation of the model.

　　At present, lithium-ion batteries have developed into four generations of products, from lithium cobalt oxide for mobile phones to lithium manganate, to lithium iron phosphate, and now ternary and other materials. There are still many people pursuing these materials. A higher goal is to move toward more than 300Wh, or even 500Wh. So in terms of material systems, current research is diverse and new concepts emerge one after another, but why is there such a goal?

　　Ma Zifeng believes, "In fact, a very important point is the anxiety about cruising range. Everyone hopes that the cruising range will be higher and the energy density can be continuously improved. But everyone knows that after so many years of research and development, so many new ones have been developed There are only a few cathode materials that can actually be used industrially, namely the lithium cobalt oxide, lithium manganate, lithium iron phosphate, ternary materials, etc. mentioned just now.”

　　A very important point here is the safety of the battery, and safety first depends on the safety of its own materials. Professor Ma's team conducted analysis and research through thermal analysis and related systems. The results showed that among these materials, the thermal stability of lithium iron phosphate is the best in terms of safety. In this regard, Ma Zifeng said: "We can find that the higher the energy density of the battery, the greater the form of overheating or thermal runaway actually generated. Therefore, to increase the safety of the battery, these high-energy batteries may need Specific protection devices have been added to the system design, such as cooling systems, explosion-proof systems, etc., to reduce battery safety."

　　Therefore, battery safety and long cycle life are very important issues in the entire battery design. So, how to make a battery and battery system? Ma Zifeng believes, "Actually, we can learn from the management model of our chemical industry. Everyone knows that chemical industry is a very useful industry, but chemical industry has received a lot of dislike across the country today, and many places do not welcome chemical industry. Because I think there are safety issues and even toxicity issues in the chemical industry. But in fact, you can see that our petrochemical industry has a large number of reactors with high pressure and high temperature. How do they ensure safety? In fact A good method is the aspect of system control. In the process of the emergence of system cybernetics, chemical systems engineering actually played a very important role."

　　This requires us to evaluate whether we can introduce the principles of chemical system engineering to control batteries and battery systems. The key is that we need to analyze the safety factors and unsafe factors of power battery production. Professor Ma pointed out, "In batteries, there are many influencing factors, so the management of a system, in addition to its charging and discharging control, is also related to its thermal management and safety management. Among these, in fact, the The estimation of status is very important. The estimation of status includes the nuclear power status, which is what we can often see about a battery, how much power it has. In addition, it also includes its health status and power status. "

　　When the battery of our mobile phone is low, we will be prompted that it is time to charge. So, how to monitor the status of this battery? In fact, this state is achieved based on some system engineering methods in the chemical industry. The discharge curve of a battery is just like the changes in concentration, pressure, and temperature in the reactor in our chemical reaction engineering. These changes require a certain mathematical model to accurately describe them.

　　This is related to the material chemistry system of the battery. During the establishment of the battery model, different materials such as lithium iron phosphate, lithium cobalt oxide, and lithium manganate have different curve characteristics. The discharge curve of lithium iron phosphate is a very flat curve, while lithium manganate has two discharge platforms. How to determine its SOC status from these two discharge platforms? SOC is actually our curve. At 4.0V, we assume that SOC is 100%, but when it reaches 2.5V, the discharge is cut off, and it is really gone. In fact, when making battery management systems and models, whether it is feasible to cut the SOC to 5% or even 1% requires evaluating the remaining capacity of the battery during battery management. This requires an accurate state estimate, and in addition to the material chemical system, this estimate also includes internal resistance. Internal resistance is related to the material system and temperature. In addition, there are also interface effects and the battery management system.

　　In fact, the only things that can really be measured in battery cell testing are open circuit voltage and temperature, and the others cannot be seen. So we must first determine the open circuit voltage, and then determine the management of the open circuit voltage and this state of charge. This is Based on the important theoretical foundation in the SOC model. Therefore, it can be said that the accuracy of OCV and SOC models is the key to controlling the health of the battery.

　　Before making this model, we must first build a model of the lithium battery cell, so the lithium battery model involves all aspects. The charging and discharging process of lithium-ion batteries is actually a chemical reaction process, and this chemical reaction process is a multi-complex system. It involves electrochemical reactions, motor dynamics, and transmission processes. Charge transfer and heat transfer.

　　Therefore, different methods are needed to build a battery model. Professor Ma gave three models. The first is an electrochemical mechanism model; the second is a black box model; and the third is a model based on equivalent circuits.

　　Finally, Ma Zifeng concluded: "Through our efforts in the past few years, we have successively established some estimation models such as open circuit voltage, health status, and SOC. The selection of this model function, its form, parameter estimation methods, and calculations, We have proposed some specific methods of our own. At the same time, we also believe that the basis of battery safety system protection is the safety of the material chemical system itself. Battery protection devices, such as voltage limiting overcharge protection and BMS optimization control scheme, are an effective measure."

　　In fact, if you want to accurately control and predict, different battery material systems, different battery structures, such as cylindrical, soft-packed and square cases, or different cell cell capacities, will have different charging conditions. The discharge characteristic curve, and based on the characteristic curve, actual production data is used to correct the model. This is also the basis for the strategic cooperation between Ma Zifeng's team and different companies, because it can meet its specific needs.

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