ã€introduction】
Since lithium titanate (Li 4/3 Ti 5/3 O 4 ) has a "zero strain" characteristic, that is, the crystal structure undergoes less than 2% volume change during the battery reaction, this excellent characteristic will Greatly extend battery cycle life and ensure exceptional safety and rate performance. Therefore, in order to better develop and design lithium titanate batteries , more and more research groups have done a lot and deeper on the structural changes and electrochemical behaviors of Li4/3Ti5/3O4 during lithium ion deintercalation. Research. So far, it has been found that Li 4/3 Ti 5/3 O 4 and the final Li 7/3 Ti 5/3 O 4 structure after lithium intercalation have poor lithium ion transport rates, and Li 4/3 has always been considered The structural transformation process of Ti 5/3 O 4 can be explained by a two-phase phase transition model, ie the system consists only of Li 4/3 Ti 5/3 O 4 and Li 7/3 Ti 5/3 O 4 . Unexpectedly, the lithium intercalation process greatly increases the transfer rate of lithium ions inside the mesophase Li 4/3+x Ti 5/3 O 4 , which leads to good rate performance. Obviously, this phenomenon cannot be explained by the existing two-phase model. In this regard, various hypotheses have been proposed, which may be due to the formation of a uniform solid solution or the formation of a turmeric solution, which is only composed of nanoscale Li 4/3 Ti 5/3 O 4 and Li 7 . /3 Ti 5/3 O 4 two-phase particles combined. However, due to the "zero strain" nature of this material, this directly leads to the inability to accurately determine the subtle structural changes that occur during the electrochemical reaction using conventional detection methods.
[Introduction]
Recently, Zhang Haijun (first author), Mehmet Topsakal (co-first author), Amy C. Marschilok (corresponding author), Deyu Lu (corresponding author) and Wang Feng researcher (communication author) of Brookhaven National Laboratory used the original The X-ray absorption spectrum measurement combined with the latest first principle calculations to resolve the local structural transition experienced by lithium titanate during electrochemical lithium intercalation and the corresponding lithium ion occupancy change, and in J. Am. Chem. A research paper entitled "Multi-Stage Structural Transformations in Zero-Strain Lithium Titanate Unveiled by Situ X-ray Absorption Fingerprints" was published at Soc.
[This article highlights]
In this study, a viable in-situ electrochemical cell was designed to ensure a sufficiently strong Ti K-edge X-ray absorption spectrum in fluorescence mode. The various characteristic changes in the absorption spectrum reveal that the lithium intercalation process of Li 4/3 Ti 5/3 O 4 leads to multi-stage structural transformation at the sub-cell and particle scale, which is experimentally confirmed to be large. The presence of solid solution phase Li 4/3+x Ti 5/3 O 4 in the range of lithium ion concentration, and this solid solution phase and two phases (Li 4/3 Ti 5/3 O 4 and Li 7/3 Ti 5/3 O 4 ) Mix with each other. From the sub-cell scale, the first principle calculation reveals that although Li 4/3 Ti 5/3 O 4 is a zero strain material, there are four different TiO 6 octahedrons in a single unit cell, which will follow lithium. Different ions occupy different positions and undergo significant distortion and Ti-O bond length changes, and these changes can be in one-to-one correspondence with X-ray absorption spectrum characteristics. Furthermore, the experimental and calculation results also explain why Li 4/3 Ti 5/3 O 4 has a unique "zero strain" property.
[Graphic introduction]
Figure 1. Structural transformation of Li 4/3 Ti 5/3 O 4 during electrochemical lithium intercalation using in situ X-ray absorption spectroscopy
(a) is a schematic diagram of an in situ electrochemical cell and an in situ X-ray absorption spectrum experimental device;
(b) a series of in situ X-ray absorption spectra and corresponding constant current discharge curves;
(c) To display several different isosbestic points for juxtaposing several X-ray absorption spectra, they are indicated by circles in the figure. After amplifying a certain absorption point, it can be seen that this is not a single equal absorption point, which is different from the single equal absorption point produced by the conventional two-phase reaction;
(d) is the curve of the integrated intensity of Pre-peak B and the peak position of the main peak D with the concentration of lithium ions; although the two parameters correspond to different genesis, they all show a consistent change law.
Figure 2. The variation of fingerprint characteristics of multiple maps with the concentration of lithium ions
If the system undergoes a pure two-phase phase transition, the mass percentage (α(x)) of Li 4/3 Ti 5/3 O 4 should vary linearly with lithium ion concentration, as indicated by the black dashed line in the figure. However, all normalized fingerprint fingerprint characteristic curves, such as the integrated intensity of pre-peak B? B(x), the relative peak position of the main peak D??D(x), and the α(x) obtained by the map fitting. ), both deviate from the two-phase change path.
Figure 3. Structural model corresponding to different lithium ion concentrations and corresponding bond length information
(a) is a structural model of Li 4/3 Ti 5/3 O 4 , Li 4.5/3 Ti 5/3 O 4 and Li 7/3 Ti 5/3 O 4 . Four different TiO6 octahedrons of unit cells are marked by different colors;
(b) Ti-O, Ti-Ti and Ti-Li bond length information corresponding to the structure of Fig. a. The red dotted line in the figure indicates the average bond length. After lithium intercalation, only the Ti-O and Ti-Li bond lengths changed significantly, while the Ti-Ti average bond length remained essentially unchanged.
Figure 4. Calculated X-ray absorption spectrum and projected state density at different lithium ion concentrations
(a) Ti K-edge X-ray absorption spectra for different lithium-intercal states simulated and experimentally simulated;
(b) is the change in the projected state density of Ti, Li and O in Li 4/3 Ti 5/3 O 4 and Li 7/3 Ti 5/3 O 4 .
Figure 5. Correlation between pre-peak B intensity and local distortion obtained by analyzing simulated X-ray absorption spectra
(a) pre-peaks A, B, C corresponding to the respective different Ti ions;
(b) showing the offset ΔCM of the Ti ion from the centroid position for the structure diagram;
(c) A linear relationship between the offset ΔCM and the pre-peak B integrated intensity of each different Ti ion.
Figure 6. Schematic diagram of multi-stage structural transition from sub-cell to particle size
(a) Multi-stage structural changes experienced by Li 4/3 Ti 5/3 O 4 on a particle scale;
(b) Local structural changes and atomic rearrangements experienced by Li 4/3 Ti 5/3 O 4 on the unit cell scale
[Outlook]
This work utilizes in-situ X-ray absorption spectroscopy to detect the local environment around the atom and the structural changes of the ligand. Combined with the first principle calculation, Li 4/3 Ti 5/3 O 4 is in the process of lithium insertion. The subtle structural changes experienced have been studied in detail. The distortion of TiO 6 octahedron, the reduction of Ti ions and the occupancy of lithium ions are the physical reasons that directly lead to a series of changes in spectral characteristics. Quantitative analysis of the map reveals the multi-stage structural changes experienced by Li 4/3 Ti 5/3 O 4 during lithium insertion, which is different from the phase change model derived from off-site studies or theoretical predictions. This mechanism provides a new way to understand the dynamic reaction process of lithium batteries in a non-equilibrium environment. The analytical tools and map fingerprint features thus established will help to study other "zero strain" electrode materials. In addition, through the understanding of the "zero strain" characteristics, this research results will help to design and improve the new "zero strain" electrode material, so it has guiding significance for improving the cycle life and safety of the battery.
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