21700 battery

　　Focus on the phenomenon of lithium precipitation and comprehensively summarize the fast charging technology of 21700 battery for electric vehicles

　　When it comes to electric vehicles, the most concerning issue is battery life.

　　The current general range is about 400km, which is really enough for most travel scenarios. However, why do most people (including me) always feel that this mileage is far from enough and hope to add another three to five hundred kilometers to feel at ease?

　　The reason is that charging is far less convenient than refueling.

　　If a fuel-powered vehicle is out of gas, you can turn on your mobile phone to navigate and drive for three to five kilometers and spend three to five minutes to refuel. Charging is different. First of all, you may not be able to find a charging station. Secondly, even if you find a charging station, you may not be able to queue up. Finally, even if you are queued up, it will take at least an hour to charge.

　　What restricts the convenience of charging is mainly that the charging speed is too slow. It has two restricting effects:

　　Slow charging speed means that electric vehicle owners will waste valuable time, which is included in the direct cost.

　　Slow charging speed means that the service efficiency of renting a site to open a charging station is low, and the input and output are not cost-effective, which indirectly restricts the popularity of charging stations.

　　Two perspectives on automotive lithium battery fast charging technology

　　We often compare 21700 battery to a "water tank model": while most batteries use a chemical reaction of conversion, accompanied by a significant material transformation process, lithium-ion batteries use a very unique lithium intercalation ( Intercalation) chemical reaction, lithium ions are indeed poured back and forth between the positive and negative electrodes like water.

　　Since the lithium-ion battery is compared to a water tank model, what is the difference between water and oil? Why is it so fast to pour oil into a fuel car and so slow to "pour electricity" into an electric car?

　　This is the so-called lithium battery fast charging technical issue, which should be understood from two perspectives:

　　From the perspective of the charging device: Do the charging pile and the vehicle-mounted high-voltage system have high power output capabilities?

　　From the perspective of 21700 battery: On the premise of ensuring safety and longevity, do lithium-ion batteries have the ability to withstand high power input?

　　Recently, Porsche released the Taycan, a luxury electric car. The most eye-catching feature is the on-board 800V high-voltage system, which can support 350kW of ultra-fast charging power [3]. What is the concept of 350kW? The air conditioners equivalent to half of the residential area are stopped, and the saved electricity is charged into a small electric car at the same time.

　　(Porsche Taycan)

　　Porsche's move is mainly to break through technology from the perspective of charging devices, which is also an area that automobile OEMs and parts factories are focusing on. In fact, what is more difficult and perhaps more important to study is another angle: Do lithium-ion batteries have the ability to withstand high power input?

　　The paper shared today discusses this topic: , translated into Chinese: "A review of lithium-ion battery fast charging issues."

　　What risks does fast charging bring?

　　In short, fast charging brings three effects: Thermal effect, Liplating and Mechanical effect.

　　The thermal effect is easy to understand. According to Joule's law, the heat generation is related to the square of the current: J=I^2R

　　Considering P=UI, from the perspective of the charging device, when increasing the charging power, the current does not increase, but the voltage can only be increased. This is why the vehicle-mounted 800V high-voltage system is so important for ultra-fast charging.

　　The voltage of the vehicle's high-voltage system is increased, which only reduces the heat generated in the charging cable. It is impossible to significantly increase the voltage of a single cell of a lithium-ion battery. They must endure the heat generated by large currents in two aspects:

　　Total heat generation: Both the heat dissipation performance of the battery core itself and the overall heat dissipation performance of the battery pack need to be enhanced.

　　Non-uniformity: If the thermal management of the car is good, the temperature difference between different cells can be ±2°C, and if it is poor, it can be ±5°C. However, this is only the temperature on the surface of the battery core. What happens inside during fast charging? The following two figures show that during fast charging, the maximum temperature difference inside the battery cell is as high as more than 10°C, with the positive electrode having the highest temperature.

　　If the battery core is given, no matter how much work the OEM does on thermal management, it will be difficult to fundamentally improve the internal temperature inconsistency of the battery core caused by fast charging. In order to improve this performance, battery cell manufacturers need to specifically improve electrode materials and battery cell design.

　　What are the dangers of thermal effects?

　　Two aspects: Aging and Safety

　　Regarding lifespan (Aging), what will happen if the temperature is high? We can refer to a line from teacher Zhao Zhongxiang, "Spring is here, everything revives, and the prairie has come to the season of animals again." The side-reaction of lithium-ion battery life decay is similar to that of animals on the savanna, and is strongly related to temperature.

　　What specific side effects are so disturbing? The most mentioned is the growth of negative SEI film (Solidelectrolyteinterphase).

　　About safety. Regarding the Tesla and NIO self-ignition incidents in the first half of this year, I heard an intuitive and simple way of understanding from the melon-eaters, "The weather is already hot, and charging is even hotter. When the battery temperature gradually rises to a critical point, it's like It started to burn like a pile of weeds." Is this understanding correct?

　　This understanding has a correct side: there is indeed a temperature critical point in the chain reaction of battery thermal runaway (Thermal Runaway).

　　As shown in the figure below, the spread of thermal runaway is divided into three stages. The ordinate is the logarithmic heat generation rate: at any stage, as long as the heat dissipation rate is higher than the heat generation rate, thermal runaway will not continue to spread. At the same time, we can see that the heat production rate in stage II increases significantly (note that this is a logarithmic coordinate). The starting point of temperature T2 in this stage corresponds to the "critical temperature" in the mouth of melon eaters.

　　So here’s the problem, T2 is over 100 degrees, which is not very easy to achieve. When we pass current through it, it is a charging action with an efficiency of over 95% (the proportion of heat generated is very small), and it is not heating the resistance wire. After all, the battery pack is a big thing that weighs half a ton. Even if it is given to you for free, it will be very difficult to heat it to more than 100 degrees!

　　Therefore, the thermal effect alone cannot reach the temporary temperature T2, and the battery pack is not as dangerous as a pile of weeds. So what is the force that makes the critical temperature T2 come unexpectedly?

　　This is about discussing the lithium plating effect caused by fast charging - it is like a devil and can significantly reduce the critical temperature T2.

　　Lithium-ion batteries are designed based on the lithium intercalation reaction. However, when the negative electrode current is too large or the temperature is too low, and the negative electrode potential is lower than the potential of the Li/Li+ reference electrode, lithium conversion unique to lithium metal batteries will occur ( Conversion) reaction to produce metallic lithium, which is also called Liplating.

　　The reaction of lithium conversion (Conversion) is very terrible. The safety accidents it caused once caused the bankruptcy of MoliEnergy, the promising world's first lithium battery company.

　　After the lithium evolution reaction continues, a tree-like structure will grow, which is called lithium dendrites. Let's see what it looks like:

　　The early simple understanding is that lithium dendrites are continuously produced, and eventually pierce the separator between the positive and negative electrodes, causing an internal short circuit (Internal Short Circuit). This understanding makes sense intuitively. After all, lithium dendrites are metal, and they pierce Isn’t it easy to create a non-metallic film?

　　In recent years, another explanation has gradually gained ground: lithium metal is extremely soft, and the lithium dendrites produced are not cast or forged, and are in a microscopic form that is too soft to stand up. How could it possibly pierce the diaphragm? Woolen cloth?

　　Therefore, it is not the internal short-circuit thermal runaway caused by the lithium dendrites puncturing the separator, but the dendritic structure of the lithium dendrites that greatly reduces the critical temperature T2 due to certain mechanisms, making thermal runaway more likely to occur!

　　In other words, the thermal effect during fast charging increases the battery temperature, and the lithium precipitation effect lowers the critical temperature. The two effects combine internally and externally, which together lead to the occurrence of thermal runaway.

　　In addition to the impact on safety, the reduction in the number of lithium ions during the fast charge lithium elution process also leads to capacity attenuation and has an impact on battery life. This paper also points out that the lithium evolution process seems to be partially reversible. After fast charging, as long as the battery is given a rest, the lithium metal will turn into lithium ions again (the part that cannot be recovered is called DeadLithium), critical temperature T2 will also return to its normal higher value.

　　Technical Difficulties in Monitoring Lithium Evolution—Nondestructive Diagnosis

　　As discussed above, the lithium precipitation effect plays an extremely critical role in the degradation of battery life and safety issues caused by fast charging.

　　The first thing to do is to rationally design the cells and battery packs, thoroughly understand the battery model, accurately estimate the current status of the battery, control the fast charging process, and try to avoid lithium precipitation during the fast charging process. Unfortunately, for mass-produced products like cars with hundreds of thousands of units, it is still very, very difficult to fully achieve this, especially when our 21700 battery are not fully understood.

　　Don't panic, from lithium precipitation to lithium dendrites and then to lithium loss of control, it is not an instant process, but a process that gradually spreads. If we can detect the lithium precipitation effect at an early stage, take preventive measures in advance or warn the car owner to repair it quickly, we can avoid the development of thermal runaway and cause personal and property losses.

　　The paper mentioned that the detection methods of lithium dendrites include: optical microscopy (optical microscoppy), scanning electron microscopy (SEM, Scanning Electron Microscopy), transmission electron microscopy (TEM, Transmission Electron Microscopy), nuclear magnetic resonance spectroscopy (NMR spectroscopy, Nuclear Magnetic Resonance spectroscopy), X-ray Diffraction technology (XRD, X-raydiffracation), etc.

　　Unfortunately, these methods are not non-destructive diagnosis (Non-destructive diagnosis), but require the battery to be disassembled and observed. This is obviously impractical for batteries that are tightly sealed in battery packs; even if these methods are used for spot inspection, due to the significant inconsistency between cells, a small number of randomly inspected cells cannot explain the entire battery pack. security status.

　　Are there any non-destructive diagnostic methods? This paper comprehensively summarizes related research. There are roughly 6 categories of methods as shown in the figure below:

　　1) Arrhenius plot (Arrheniusplot) 2) Internal resistance-capacity curve (Resistance-Capacity) 3) Nonlinear frequency response analysis (NFRA, Nonlinear Frequency Response Analysis) 4) Coulomb efficiency analysis (Coulombicefficiency) 5) Differential voltage analysis (DVA) , DifferentialVoltageAnalysis) 6) Capacity Increment Analysis (ICA, IncrementalCapacityAnalysis).

　　Since there is such a non-destructive diagnostic method, and the eight immortals cross the sea and show their magical powers, there must be one that can be effective, right? Sadly, this is not the case!

　　Although the above analysis methods are diverse, they are essentially all mathematical transformations of the same external measurement signals of current and voltage in the time dimension: transforming the coordinate axis, calculating differentials, calculating integrals, etc.

　　For example, a five-thousand-year-old tomb is unearthed, a bone is found (voltage and current signals), and a question is asked to ask you to draw the DNA of the owner of the bone (lithium dendrite state). It cannot be said that it is impossible, but at least it is very difficult; especially if you do not know the double helix structure model of DNA (the crystallization mechanism of lithium-ion batteries), it will be even more difficult.

　　There is a worse situation, the question becomes: you are given a bone and asked to infer the name of the owner of the bone. Since the bones don't contain this information at all, you can't deduce it even if you try hard! If this road doesn't work, why not try another way and look for ancient historical materials!

　　Yes, if it cannot be inferred from external voltage and current signals, can we find another way to non-destructively detect lithium dendrites?

　　Coincidentally, at the just-concluded 3rd International Battery Safety Workshop (IBSW, International Battery Safety Workshop), global lithium-ion giants gathered together, and there was a relevant report introducing a promising method: designing a lithium-ion battery on the negative electrode of the battery. The sensor with clever structure is specially designed to detect the phenomenon of lithium evolution. If this method can successfully move out of the laboratory and be industrialized, it can fundamentally solve the lifespan decay and safety issues caused by lithium precipitation.

　　After talking so much, the melon-eaters may say: We don’t care about any of this. We just want to know when we can enjoy the same super fast charging technology as the Porsche Taycan at the price of BAIC New Energy?

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