New choice for lithium-ion battery packs: Electrochemical equilibrium reduces the problem of poor consistency

Brand AVX TPSE226M035R0125 Low impedance tantalum capacitor AVX 22

Electronic scale crystal oscillator 3.2*2.5mm 3225 16M (16.000MHZ) 12PF 10PPM 20PPM 30PPM

L0504-Murata muRata common mode inductor 90Ω 150mA

The long-term consistency of lithium-ion battery cells is a problem that plagues the design of lithium-ion battery packs. The consistency we refer to here is not only the parameters such as capacity and voltage in the traditional sense, but also the capacity decline of single-cell batteries. Factors such as speed, internal resistance decay rate, and temperature distribution of the battery pack.

Ideally, the same batch of lithium-ion batteries should have the same electrochemical performance, but in fact there will be inconsistencies between the lithium-ion single cells due to errors in the manufacturing process. Battery packs are often made up of hundreds or even thousands of single cells connected in series and in parallel, so the capacity of the battery pack is greatly affected by the inconsistency of the battery cells (the inconsistencies that have the greatest impact on battery pack performance include Coulomb Inconsistent efficiency, inconsistent self-discharge rate, inconsistent increase in internal resistance, etc., studies have shown that even if the cycle life of a single cell reaches 1000 or more, the battery life of the battery pack may be less than 200 times after the battery pack is formed [1].

Therefore, for a battery pack consisting of a large number of single cells, equalization equipment is necessary. Currently, the common balancing method on the market is mainly to realize the voltage balance between the single cells by means of electronic devices, so the technology is also Greatly different. Recently, Alexander U. Schmid and others at the University of Stuttgart in Germany used Ni metal hydride batteries (NiMH) and Ni-Zn batteries to achieve electrochemical equilibrium of the battery pack, which provided a new idea for the balance of the battery pack.

Due to the limitation of the working principle of the lithium ion battery, its ability to resist overcharging is very weak, and in the case of overcharging, problems such as electrolyte decomposition and lithium deposition may occur. In the case of overcharge of NiMH battery, H2O in the electrolyte will decompose O2 and H2 in the positive and negative electrodes, and O2 and H2 can recombine the generated water under the action of the catalyst to form a complete cycle. At a small rate of C/3-C/10, the rate of gas generation is almost the same as the rate of recombination, so the overcharge resistance of the NiMH battery is very good. Based on the above principles, Alexander U. Schmid used a NiMH battery and a similar Ni-Zn battery to equalize the lithium-ion battery. When using this electrochemical equalization method, the conventional voltage monitoring and electronic equalization unit can be omitted, which effectively reduces the complexity of the battery pack management and improves the reliability of the battery pack.

Alexander U. Schmid selected LiFePO4 and Li4Ti5O12 materials as experimental objects because the two materials have a certain tolerance to overcharge and the voltage rises rapidly after complete delithiation. At this time, NiMH and Ni-Zn batteries Taking on the role of the current bypass, excess current will flow into the NiMH and Ni-Zn batteries, thus avoiding overcharging of the lithium ion battery.

The working principle is as shown in the figure below. The NiMH battery or Ni-Zn battery used for equalization is connected to the lithium ion battery in parallel. When a group of low-capacity batteries in the battery pack are fully charged, the voltage reaches the threshold. At this time, the NiMH battery connected in parallel assumes the function of shunting, and all the current flows through the NiMH battery substantially, and no longer flows through the lithium ion battery, thereby avoiding overcharging of the lithium ion battery. The change in voltage and current of the Li-ion battery and NiMH during this process is shown in Figure b. In the case of a perfect match, the lithium-ion battery current is shown as a red curve.

The following table shows the information of the batteries used in the experiment. The experiments mainly used LFP/graphite, LMO/LTO, LFP/LTO, Ni-Zn and NiMH batteries.

The following figure shows the capacity-voltage curves of several batteries used in the experiment. 2'NiZn means that two Ni-Zn batteries are connected in series. It can be seen that the maximum voltage of two series-connected Ni-Zn batteries is 3.95V. (I=150mA), can be used on the LFP/C battery to avoid overcharging. A Ni-Zn battery can be connected in parallel with the LFP/LTO battery to avoid overcharging of the battery, or two NiMH batteries connected in series with the LMO/LTO in series. The maximum voltage will reach 3V or more, and the maximum voltage of the LMO/LTO battery is 2.8. V or so, but as long as the LMO/LTO battery voltage does not exceed 3.2V is acceptable, and the LMO/LTO battery increases from 2.8-3.2V with a capacity of only 0.65Ah, which is about 6.5% of the normal temperature capacity, so the performance of the battery Has little effect.

The figure below shows how the LMO/LTO battery works with two NiMH batteries connected in series. It can be seen that during the charging process of the battery pack, the LMO/LTO battery is first filled. When a certain point is reached, the current begins to change. The current flowing through the LMO/LTO battery begins to decrease, the current flowing through the NiMH battery increases, and finally the current flowing through the LMO/LTO battery drops to zero, and all current flows through the NiMH battery, so the battery pack at this time The voltage no longer increases. During the discharge process, the two batteries start to discharge at the same time. Since the capacity of the NiMH battery is small, the current drops to zero quickly, and the discharge is mainly completed by the LMO/LTO battery.

The figure below shows the operation of the LFP/C-2NiZn battery module. It can be seen that almost all current will enter the LFP/C battery when charging starts, and only about 80mA will pass through the NiZn battery. Then at t=1.2h, the current flow direction has completely changed, and the current begins to flow mainly through the NiZn battery. Therefore, in order to avoid overheating of the NiZn battery, the charging current of the module is divided into several steps, firstly 1.1A, then 0.75. A, then 0.3A, then 0.15A. At the beginning of the discharge process, the NiZn battery provides the maximum current, and then its current begins to decrease, and the current of the LFP/C battery begins to increase gradually.

The following table summarizes the effects of several batteries in parallel with NiZN and NiMH batteries. From the first column, it can be seen that several parallel modes can make the maximum voltage of the battery pack smaller than the maximum limit voltage of the lithium ion battery, avoiding lithium. The ion battery has been overcharged. As can be seen from the second column, in addition to the LFP/LTO-NiZn battery can not fully utilize the capacity of the lithium-ion battery, the other two parallel methods can fully utilize the capacity of the lithium-ion battery, so that the battery pack can be balanced. (third column). It can be seen from the fourth column that the maximum discharge current of the battery pack is less than the maximum current of the lithium ion battery due to the influence of the parallel NiZn and NiMH batteries. Therefore, in actual use, high-power NiZn and NiMH batteries are required to Ensure that the performance of the battery pack is not degraded.

The following figure shows the charging and discharging operation of two LFP/C-2NiZn batteries connected in series. The initial capacity difference between two series LFP/C batteries is 200mAh. After the following one charge and discharge, the difference between the two battery packs The reduction is 100 mAh, which means that 8% of the capacity of the two series of battery packs in one cycle achieves equalization.

The work of Alexander U. Schmid provides a new idea for battery pack equalization. Due to the design characteristics of NiMH and NiZn batteries, when overcharge occurs, the water in the electrolyte will decompose in the positive and negative electrodes respectively, producing O2 and H2. Under the action of the catalyst in the battery, O2 will combine with H2 to produce water, complete a cycle, so NiMH and NiZn have very good anti-overcharge performance, we can just take advantage of this, through single or several series of NiMH, The NiZn battery is connected in parallel with the lithium ion battery. When the charging voltage reaches the upper limit, almost all of the current flows through the NiMH and NiZn batteries, thereby avoiding overcharging of the lithium ion battery. We can also use this to achieve a balanced lithium-ion battery pack. As long as we continue to charge the battery pack, we can ensure that all the batteries can be fully charged, without worrying about overcharging of the battery, thus improving the battery pack. The consistency of the amount of content, the experiment also confirmed that a charge and discharge cycle can achieve 8% capacity balance (LFP / C-2NiZn). The biggest advantage of this method is that it does not need to monitor the voltage of the single cells in the battery pack during the whole process, which is completely automatic, which greatly simplifies the structure of the battery pack and improves the reliability of the battery pack.