CR2016 battery.Technical analysis of structural transformation and lithium ion occupation change during lithium titanate electrochemical lithium insertion process

　　Since lithium titanate (Li4/3Ti5/3O4) has "zero strain" characteristics, that is, the crystal structure only experiences less than 2% volume change during the battery reaction, this excellent characteristic will greatly extend the battery life. Cycle life, and can ensure extraordinary safety and rate performance. Therefore, in order to better develop and design lithium titanate batteries, more and more research groups have done a lot of in-depth research on the structural changes and electrochemical behavior of Li4/3Ti5/3O4 during the lithium ion deintercalation process from the microscopic mechanism. Research. So far, it has been found that both Li4/3Ti5/3O4 and the final Li7/3Ti5/3O4 structure after lithium insertion have poor lithium ion transport rates, and it has been believed that the structural transformation process of Li4/3Ti5/3O4 can be a two-phase phase transition. The model explains that the system only consists of Li4/3Ti5/3O4 and Li7/3Ti5/3O4. Unexpectedly, the lithium insertion process will greatly increase the transport rate of lithium ions inside the mesophase Li4/3+xTi5/3O4, resulting in good rate performance. It is obvious that this phenomenon cannot be explained by the existing two-phase model. In this regard, people have put forward various hypotheses, thinking that it may be due to the formation of a uniform solid solution, or the formation of a pseudo-solid solution, and this pseudo-solid solution is only composed of two phases of nanoscale Li4/3Ti5/3O4 and Li7/3Ti5/3O4. composed of particles. However, precisely because of the "zero strain" characteristics of this material, this directly results in the inability to accurately determine the tiny structural changes that occur during the electrochemical reaction using conventional detection methods.

　　【Achievements Introduction】

　　Recently, researchers Zhang Wei (first author), Mehmet Topsakal (co-first author), Amy C. Marschilok (corresponding author), Deyu Lu (corresponding author) and Wang Feng (corresponding author) of Brookhaven National Laboratory used in-situ X-ray Absorption spectrum measurements combined with the latest first-principles calculations analyzed the local structural transformation experienced by lithium titanate during the electrochemical lithium insertion process and the corresponding changes in lithium ion occupancy, and were reported in J.Am.Chem.Soc. Published a research paper titled "Multi-Stage StructuralTransformations in Zero-StrainLithiumTitanateUnveiledbyinSituX-rayAbsorptionFingerprints".

　　[Highlights of this article]

　　This study designed a feasible in-situ electrochemical cell to ensure that TiK-edgeX light absorption spectra with sufficiently strong signals can be obtained in fluorescence mode. Various characteristic changes in the absorption spectrum reveal that the lithium insertion process of Li4/3Ti5/3O4 will lead to multi-stage structural transformations at both the sub-cell and particle scales, which has been experimentally confirmed within a wide range of lithium ion concentrations. The solid solution phase Li4/3+xTi5/3O4 exists, and this solid solution phase will mix with the two phases (Li4/3Ti5/3O4 and Li7/3Ti5/3O4). From the sub-unit cell scale, first-principles calculations reveal that although Li4/3Ti5/3O4 is a zero-strain material, there are four different TiO6 octahedrons in a single unit cell, which will change with the different occupation of lithium ions. It experiences obvious distortion and Ti-O bond length changes, and these changes can correspond to the X-ray absorption spectrum characteristics one-to-one. Furthermore, experimental and computational results also explain why Li4/3Ti5/3O4 has unique "zero strain" properties.

　　[Picture and text introduction]

　　Figure 1. In-situ X-ray absorption spectroscopy is used to track the structural transformation of Li4/3Ti5/3O4 during the electrochemical lithium insertion process.

　　(a) Schematic diagram of the in-situ electrochemical cell and in-situ X-ray absorption spectrometry experimental device;

　　(b) is a series of in-situ X-ray absorption spectra and the corresponding constant current discharge curve;

　　(c) Several X-ray absorption spectra are placed side by side to show various isosbestic points, which are marked by circles in the figure. After zooming in on a certain isabsorption point, it can be seen that this is not a single isabsorption point, which is different from the single isabsorption point produced by the traditional two-phase reaction;

　　(d) is the change curve of the integrated intensity of Pre-peakB and the peak position of the main peak D with the lithium ion concentration; although the causes of these two parameters are different, they both show a consistent change pattern.

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