As we all know, the first charge/discharge of graphite cathode is only about 90% Coulomb efficiency because the electrolyte decomposes on the surface of cathode to form SEI film and consumes part of the active Li. But in fact, not only the graphite cathode has a low coulomb efficiency in the first charge/discharge process, even the cathode material, which we usually think will not form an interface film in the first charge/discharge, does not have a 100% coulomb efficiency in the first charge/discharge. The study shows that 12-30% of the first de-lithium capacity of all layered cathode materials cannot be re-embedded in the material, and the Coulomb efficiency of the electrode increases to about 100% during the subsequent cycle. So why can't part of Li dive into the cathode material again after de-lithiuming?
(Source: WeChat public number "newenergy-leader" ID: newenergy-leader Author: by the fence)
There are several explanations for the irreversible capacity of the layered cathode material during the first charge/discharge process: 1) it is believed that the additional capacity during the de-lithium process mainly comes from the electrode/electrolyte side reaction, so it is irreversible; 2) the cathode material undergoes irreversible phase change during the first de-lithium process, especially on the surface of the particles, which causes part of the capacity to be irreversible; 3) after the first de-lithium process, the material kinetic properties of the cathode material can be changed. material kinetic properties decrease during this lithium embedding, resulting in partial Li inability to re-embed into the material.
Winter et al. suggested that the above three causes may exist simultaneously for NCM materials, for example, Winter's study found that the first irreversible capacity of NCM333 material was 36.3 mAh/g, and the irreversible capacities due to the above three causes were 10.0, 4.2 and 22.1 mAh/g, respectively, while Manthiram's study showed that electrode interface side reactions Lee and Kim showed that increasing the Co content from 0 to 0.2 for LiNi1-yCoyO2 material can reduce the first irreversible capacity from 55 mAh/g to 27 mAh/g. Lee suggested that this is mainly due to the presence of Co element to reduce the Li/Ni mixing.
Although there are different opinions on the cause of the first irreversible capacity of the layered material, it is generally agreed that the change in the kinetic properties of Li embedded in the electrode is one of the important reasons for the irreversible capacity of the NCM material. Kinetic properties are an important characteristic of cathode materials, and studies have shown that after increasing the Li content from 0.6 to 0.8 in LixMO2 materials, the diffusion coefficient of Li+ decreases by two orders of magnitude because most of the Li sites are occupied, especially in high Ni materials due to the presence of large amounts of Ni, so the generation of Li2NiO2 on the material surface after the first de-lithiuming can significantly affect the Li+ embedding.
Recently, Hui Zhou (first author) and M. Stanley Whittingham (corresponding author) from Binghamton University, State University of New York, USA, investigated the origin of the irreversible capacity of NCM811, the most hotly studied material, during the first charge.
Li-ion batteries are usually discharged according to a constant current discharge regime, but previous studies have shown that as the Li content of the NCM material increases, the diffusion coefficient of Li+ decreases by up to two orders of magnitude, and the authors added a constant voltage discharge process at the end of the discharge to allow Li+ to fully embed back into the material. The following figure a shows the first charge/discharge curves of different NCM811 material electrodes, from which it can be seen that the irreversible capacity of the battery is reduced by 11.2 mAh/g by lapping the electrodes, and if combined with the constant voltage discharge, the first irreversible capacity of NCM811 can be reduced from 37.5 mAh/g to 12.1 mAh/g, and the first charge/discharge Coulomb efficiency can be increased to 94.8%.
From the above analysis, it is easy to see that the irreversible capacity of NCM811 material in the first charge/discharge process mainly comes from the slower Li+ diffusion rate. Therefore, theoretically, the irreversible capacity of the first charge/discharge can be effectively reduced by increasing the temperature. The following figure shows the first charge and discharge curves of NCM811 electrode at 21, 45 and 60°C. From the figure a, it can be seen that the kinetic properties of NCM811 material are significantly improved after the temperature is increased, and the capacity that needs to be discharged at constant pressure is significantly reduced. 96.4%, but the irreversible capacity of NCM811 material in the first charge/discharge process at 60℃ showed a slight increase, which was mainly due to the increase of side reaction between NCM811 and electrolyte caused by high temperature.
Therefore, it is easy to see from the above experimental data that the irreversible capacity of the NCM material in the first charge/discharge is mostly affected by the Li+ kinetic conditions, and other factors only account for about 5% of the first irreversible capacity of the NCM material. In order to verify the above view, the authors used the GITT method to measure the diffusion coefficient of Li+ for NCM materials with different Li contents. It is obvious that the reduction of NCM diffusion coefficient at the end of Li-embedding at room temperature is the main reason for the first irreversible capacity of NCM materials.
We know that there is almost no reversible capacity loss in the first charge/discharge process for olivine structured materials like LFP, and the Coulomb efficiency of the first charge/discharge is close to 100%, so why does the NCM material have such irreversible capacity? This starts from the crystal structure characteristics of NCM material, which is a layered structure. Studies have shown that when the Li content in the layered material exceeds 0.6, it leads to the expansion of a and contraction of c in the lattice parameter, which makes the diffusion channel of Li+ narrow, thus leading to the reduction of the diffusion coefficient of Li.
Hui Zhou's study shows that about 80% of the irreversible capacity of NCM811 material during the first charge/discharge is due to the reduction of Li+ diffusion coefficient at the end of lithium embedding, so that some Li+ cannot be re-embedded into the NCM material during constant current discharge because of the reduced kinetic properties, so the electrode thickness, conductive agent type and different Ni content have basically no effect on the first irreversible capacity. Therefore, electrode thickness, conductive agent type and different Ni content basically have no effect on the first irreversible capacity, but laminating and raising the working temperature can effectively improve the kinetic properties of NCM materials, thus reducing the irreversible capacity during the first charge/discharge.
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