How to solve the electrochemical equilibrium of lithium ion batteries?

Batteries are generally composed of hundreds or thousands of cell monomers, so the battery capacity is also affected by individual cells. Studies have shown that even if a single cell has a cycle life of more than 1,000 times, it forms a battery. After the battery, the battery life may be less than 200 times. This shows that the balance of the battery pack is very important.

For a long time, the low consistency of lithium-ion cell monomers is a difficult problem for the design of lithium-ion batteries. Here, the consistency refers not only to the traditional parameters such as capacity and voltage. It also includes factors such as the rate of capacity decline, the rate of internal resistance decline, and the temperature distribution of the battery.

Ideally, the same batch of lithium-ion batteries should have the same electrochemical properties, but in fact, due to errors in the manufacturing process, there will be inconsistencies between lithium ion monomer batteries. Batteries are often made up of hundreds, even thousands of individual cells connected in series. Therefore, the capacity of the battery pack is greatly affected by the inconsistency of the individual cell(the inconsistencies that have the greatest impact on the performance of the battery pack include the inconsistency of the Coulomb efficiency, the inconsistency of the self-discharge rate, the inconsistency of the internal resistance increase speed, etc.), Studies have shown that even if the cycle life of a single cell reaches more than 1,000 times, the battery life after the formation of a battery pack may be less than 200 times.

Therefore, for a battery pack consisting of a large number of monomer cells, equilibrium equipment is necessary. The common equilibrium method on the listing surface is mainly to achieve voltage equilibrium between monomer cells with the help of electronic equipment, so the technology is also very similar. Alexander U. Schmid of the University of Stuttgart in Germany recently used Ni metal hydride cells(NiMH) and Ni-Zn cells to achieve electrochemical equilibrium of batteries, providing a new idea for battery balance.

Due to the limitation of the working principle of lithium-ion batteries, their ability to resist overcharging is very weak. In the case of overcharging, electrolyte decomposition and lithium analysis may occur. In the case of overcharging of the NiMH battery, H2O in the electrolyte will decompose the O2 and H2 produced by the positive and negative poles, and O2 and H2 can be recombined under the action of a catalyst to form water to form a complete cycle. At the low magnification of C/3-C / 10, the gas generation rate is almost the same as its recombination rate, so the NiMH battery has a very good overcharge resistance. Based on the above principles, AlexanderU. Schmid used NiMH cells and similar Ni-Zn cells to equalize lithium-ion batteries. When using this electrochemical equilibrium method, traditional voltage monitoring, and electronic equilibrium units can be omitted, which effectively reduces the complexity of battery management and improves the reliability of the battery pack.

AlexanderU.Schmid selected LiFePO4 and Li4TI5O12 materials as experimental objects because both materials have a certain tolerance to overcharge, and the voltage rises rapidly after completely delithium. At this point, the NiMH and Ni-Zn batteries assume the role of current Byass, and the excess current will flow into the NiMH and Ni-Zn batteries, thereby avoiding overcharging of lithium-ion batteries.

The working principle is as shown in the figure below. The balanced NiMH cell or Ni-Zn cell is connected to a lithium-ion cell in parallel. When a group of series low-capacity cells in the battery pack are fully charged, the voltage reaches the threshold. At this time, the NiMH battery in parallel with it assumed the role of a shunt. All currents basically flowed through the NiMH battery and no longer flowed through the lithium-ion battery, thus avoiding overcharging of the lithium-ion battery. In this process, the changes in the voltage and current of the lithium-ion battery and the NiMH are shown in figure B below. In the case of a perfect match, the current of the lithium-ion battery is shown as a red curve.

AlexanderU.Schmid's work provides a new idea for battery balance. Because of the design characteristics of the NiMH and NiZn cells, when overcharging occurs, the water in the electrolyte will decompose at the positive and negative poles, respectively, producing O2 and H2. Under the action of the catalyst in the battery, O2 will combine with H2 to produce water and complete a cycle, so NiMH and NiZn have very good anti-overcharging properties, and we can just take advantage of this. With a single or several tandem NiMH and NiZn batteries in parallel with lithium-ion batteries, the current will flow almost completely through the NiMH and NiZn batteries when the charging voltage reaches the upper limit, thus avoiding over-charging of lithium-ion batteries. We can also use this to equalize the lithium-ion batteries. We can ensure that all cells can be fully charged as long as we continue to charge the batteries, without worrying that some cells will overcharge, thus improving the consistency of the battery's internal capacity. The experiment also confirmed that a charge and discharge cycle can achieve 8 % capacity equilibrium(LFP/C -2 NiZn). The biggest advantage of this method is that the voltage monitoring of the individual cells in the battery pack is not required in the entire process. It is completely automatic, so it greatly simplifies the structure of the battery pack and improves the reliability of the battery pack.

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