High-nickel NCA material is a newly emerging high-capacity cathode material in recent years. With high-capacity characteristics, it has gained a foothold in the field of high specific energy lithium-ion batteries and currently competes with only high-nickel NCM811 material. In the design of high specific energy cells, we are very concerned about how one can ensure that the battery than the energy to meet the demand, but also to ensure that the battery has a good cycle performance, this depends on our lithium-ion battery designer at Battery formulation and system matching to do the meticulous work, more importantly, we need to make appropriate adjustments from the material structure point of view to enhance the material in the long-term cycle of structural stability.This requires us to use in the high-nickel NCA And the NCM material attenuation mechanism, we have done a lot of reports on the mechanism of the decay of high-nickel NCA materials. For example, we have reported the research of Nagoya University in Japan The results ("Nagoya University: high-temperature nickel NCA high-temperature decay mechanism") and the United States of New York State University Research into ("Recent advances in the decay of the anti-nuclear shell structure - the decay mechanism of high-nickel NCA materials"), the current research basically shows that the high-nickel NCA material can cause structural instability after removing Li in the recycling process, Resulting in the loss of O, making the transition metal transition occurred in the material structure, causing the material structure from the layered structure to the spinel structure, and eventually transformed into rock salt structure.We are going to introduce you today from the German KIT research The Karin Kleiner et al's research results.
In the experiment, Karin Kleiner used a 7Ah cylindrical cell. The cell was divided into two batches, one of which was cycled for 34 weeks (50 ° C, 40-80% SoC) at a rate of 8C (55 A) and the other batches were the same The time of storage, and then two batches of batteries were disassembled and analyzed, respectively, of which the material from the recycled battery was called fatigued LNCAO and the material from storage was called pristine LNCAO. The following table shows the basic physical properties of the two materials Indicators.From the data in the table can be seen, compared to the stored battery, the battery capacity after the cycle decline to about 26% ± 9%.
In order to study the cause of the LNCAO material's decay during cycling, Karin Kleiner analyzed the structure and found that there are three phases in LNCAO during charge and discharge, one of which is the basic phase in LNCAO rh1, a phase rh2 with more Li + intercalation and higher degree of reduction, and a phase rh3 with less Li + intercalation and higher degree of oxidation.
The study of these three phases found that in the non-circulating pristine LNCAO material, the rh2 phase mainly existed between 3.6-3.8 V (at the time of charging) and 3.8-3.1 V (except for the base phase rh1) Discharge), as shown in Figure A below, and no rh3 phase was observed in the pristine LNCAO material.
In the cycled fatigue LNCAO material, we observed the presence of the rh2 phase from 3.6V to 4.1V, and at 4.1V, the phase fraction of rh2 was about 27%, with the capacity loss of LNCA in the circulation of 26 % Rh3 phase is observed at lower potential during discharge, but disappeared before reaching the discharge cut-off voltage.It is not difficult to find that rh2 phase and LNCAO material are in the process of cycling The capacity decline has a close relationship.
The picture below shows a small fragment in the crystal structure of LNCAO material, surrounded by Ni atoms. The author has drawn three circles, 1, 2 and 3, respectively. According to the results of Raman spectroscopy, O, the second is made up of Ni, Co and Al, and the third is composed of Li.Now we can calculate the distance between the three circles and the center of the circle by taking the Ni element in the middle of the three circles as the center, O distance, Ni-metal distance and Ni-Li distance).
The graph below shows the Ni-metal distance and the Ni-O distance at different voltages for pristine LNCAO without cycling and after cycling fatigue LNCAO (using EXFAS and Powder diffraction methods, respectively, with some differences between the two methods, but with The final trend is the same.) As can be seen from the curve, pristine LNCAO Ni-metal changes in the discharge process value of about 0.4A, but after cycling fatigue LNCAO material Ni-metal change value of only 0.3A The difference between the Ni-metal values of the two materials at higher potentials is larger, which indicates that the LNCAO material can not be fully delithiated after charging, which explains why the rh2 phase exists.
By Karin Kleiner's study we know that in addition to the main rh1 phase in LNCAO material, there are two other phases rh2 and rh3, the crystal structure of the two phases did not change much during charging and discharging, indicating that they Lack of electrochemical activity, especially in the fully charged state of LNCAO material rh2 phase ratio of 27%, which is LHCAO material itself, about 26% of the capacity decline is very close, indicating that rh2 phase and LNCAO material There is a close relationship between the two kinds of inactive phase (rh2 and rh3) formation mechanism, at present two theories, one is the 'isolated particle' theory, this theory that in LNCAO particles, part of Of the small particles lose the connection with the host, resulting in this part of the particles can not participate in the charge-discharge reaction, resulting in the material appeared in the new phase.Another theory that the two inactive phase Is mainly related to the diffusion of Li +. For example, the Li2 + phase is not sufficiently removed due to Li + diffusion in the interior of the particle. However, excessive removal of Li + occurs on the surface of the particles .
Overall, Xiao Bian argues that the second theory is more accurate, so in order to reduce the appearance of the rh2 phase, we need to shrink the LNCAO particles and reduce the distance Li + diffuses, but this poses another problem - too large specific surface area , Resulting in increased side effects, which requires us to trade-off, to find a suitable particle size to enhance the LNCAO material performance.
