Solid State NMR Spectroscopic Study Of Na₂MPO₄F （M=Fe,Mn） As Cathode Materials For Li/Na Ion Batteries

Polyanion-type fluorophosphates Na2MPO4F (M=Fe, Mn) are promising cathode materials for Lithium/Sodium ion batteries. To optimize the synthetic method and improve the electrochemical performance, studying the structure of materials and the charge-discharge mechanism is quite essential. Solid state NMR can probe the local chemical environment of the nucleus in solid materials, being a useful characterization tool for studying the local structure informations. In this thesis, solid state NMR technique and XRD pattern are applied to study the charge-discharge mechanism of Na2MPO4F (M=Fe, Mn).We employed the ex situ x-ray diffraction and solid state NMR techniques to study the charge and discharge process of Na2MnPO4F, including the crystal structure and sodium sites changes. Ex situ x-ray diffraction pattern shows that two new diffraction peaks are observed at31°and36°indicating that there’s intermediate phases formed with the extraction of Na+. Three peaks-209ppm,-258ppm and-295ppm are observed in23Na MAS NMR spectra, which can be assigned to Nal+Na2, Na3and Na4site in the crystal structrue, respectively. The ex situ solid state NMR study demonstrates that the Na+in Nal and Na2sites deintercalated first compared to Na3and Na4sites. The signal peaks of intermediate phase appeared at-132ppm and-330ppm when charged to4.2V. The opposite phenomenon occurred during the discharge process.The crystallographic structure and sodium sites of Na2FePO4F were also studied by23Na MAS NMR spectra. The isotropic peaks appeared at-188ppm and443ppm in the23Na MAS NMR spectra can be assigned to Nal and Na2site respectively. The sodium content of intermediate phase Na1.5FePO4F and NaFePO4F were quantified by23Na MAS NMR spectra. The results show that the Na+in Na2site deintercalated first during the early stage of charge process. Then the Na+content in Nal site decreased while the Na2site had smaller change. Comparing the chemical and electrochemical desodiation process, peaks for electrochemical desodiation phase were more complex. During the electrochemical desodiation process, the local chemical environment of Na were influenced by the viriation of Fe valance and the lattice parameters. In contrast, the peaks for chemical desodiation phase were simple for the thermodynamic stabilized state.Li/Na exchange occurred in hybrid lithium ion battery system. From the23Na、7Li and19F MAS NMR spectra of the sample after soaked in LD120electrolyte, we confirmed the number of Na exchanged in electrolyte and detected the generation of LiF. Li1.2Na0.8FePO4F material was obtained from exchanging Na2FePO4F with LiBr, and also this material exchanged alkali metal ions with LD120electrolyte. The T1measurement revealed that there are chemical exchange or cross relaxation in Li1.2Na0.8FeP04F.7Li MAS NMR spectra of different charge state during cycling suggests that Li+would (de)intercalate in a more stable position with the increase of charge-discharge cycles.