Antisite disorder and bond valence compensation in Li2FePO4F cathode for Li-ion batteries
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The formation of Li-Fe antisite defects was observed in Li2FePO4 cathode material upon electrochemical cycling. It was revealed, that oxygen atoms, which are coordinated by electrochemically active Li atoms, induce the formation of defect. This work was published in Chem. Mater., 2016, 28 (21), pp 7578–7581 DOI: 10.1021/acs.chemmater.6b03746
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Antisite disorder and bond valence compensation in Li2FePO4F cathode for Li-ion batteries
- 1. Antisite disorder and bond valence compensation in Li2FePO4F cathode for Li-ion batteries Olesia M. Karakulina, Nellie R. Khasanova, Oleg A. Drozhzhin, Alexander A. Tsirlin, Joke Hadermann, Evgeny V. Antipov, Artem M. Abakumov. Chemistry of Materials, 28(21):7578–7581, 2016.
- 2. 2 1. Introduction Li2FePO4F. Polymorphs 2. Synthesis Electrochemical exchange 3. Method Electron diffraction tomography 4. Results and discussion 4.1 Li2FePO4F - 75oC 4.2 Li/Fe antisite disorder. Reason 4.3 LiFePO4 vs Li2FePO4F 5. Conclusions 6. Acknowledgements Outline
- 3. 3 layered tavorite 3D structure The radii of alkali metal and transition metal define the structure. Li2FePO4F 1. Li2FePO4F. Polymorphs LiFePO4 Li2FePO4FF- higher electrode potential + electronegative anion was not synthesized
- 4. 4 Li - + e- V 2. Li2FePO4F. Electrochemical exchange N.R. Khasanova, et al. Chem. Mater. 2012, 24, 4271−4273 Na+ LiNaFePO4F 4.5 V LiFePO4F Li2FePO4F 2.5 V discharge +Li+ charge Na+ 75oC charge -Li+ x10 1. Deintercalation of Na upon charge 2. Intercalation of Li upon discharge
- 5. 5 Li - + e- V 2. Li2FePO4F. Electrochemical exchange N.R. Khasanova, et al. Chem. Mater. 2012, 24, 4271−4273 Li+ Energy (keV)Counts LiNaFePO4F 4.5 V LiFePO4F Li2FePO4F 2.5 V discharge +Li+ charge Na+ 75oC charge -Li+ x10 EDX spectrum no Na in Li2FePO4F
- 6. 6 Li2FePO4F • Preserves 3D framework • Relative intensities changed 2. Li2FePO4F. Electrochemical exchange LiNaFePO4F 4.5 V LiFePO4F Li2FePO4F 2.5 V discharge +Li+ charge Na+ 75oC charge -Li+ x10 N.R. Khasanova, et al. Chem. Mater. 2012, 24, 4271−4273 Structure was not determined from XRD We applied electron diffraction tomography (EDT) X-day diffraction patterns
- 7. 7 Source 3. Electron diffraction tomography (EDT) Rotation axis We can • detect positions of light elements We can • decrease dynamical effect • consider intensities as proportional to structure factor • reconstruct reciprocal space in 3D • perform ab-initio structure determination electron beam Data off zone diffraction patterns Object We can • analyze multiphase samples • use a little amount of sample ~100-200 nm single crystal Titling range 100-120o Process: tilt a crystal with 1o step and acquire diffraction patterns
- 8. 8 4.1. Li2FePO4F - 75oC Ordered model 90o Projection along b Cross section for x ≈ 0 Cross section for x =0 Difference Fourier map Firstly, structure was refined using ordered model.
- 9. 9 4.1. Li2FePO4F - 75oC Experimental Fourier map Calculated Fourier map Difference Fourier map Fourier maps show electrostatic potential in the cell. Li positions: excess of potential Fe positions: lack of potential Difference Fourier map Ordered model Ordered model did not fit well
- 10. 10 4.1. Li2FePO4F - 75oC Disordered model Li1.96(6)Fe1.04(6)PO4F Li positions: excess of potential Fe positions: lack of potential Difference Fourier map Ordered model No significant positive or negative peaks
- 11. 11 4.1. Li2FePO4F - 75oC Ordered model Disordered model R=0.264 R=0.219 Li1.96(6)Fe1.04(6)PO4F Conclusion Li2FePO4F-75oC has a disordered structure
- 12. 12 4.2. Li/Fe antisite disorder. Reason Hypothesis 1: Thermal effect Li2FePO4F prepared at 25oC EDT: Severe antisite disorder LiFePO4 cycled at 100oC EDT: No antisite disorder Conclusion Reason elevated temperature crystal structure features? Li Na PO4FFe 1.84 0.16
- 13. 13 4.2. Li/Fe antisite disorder. Reason Fe+2 Li+ Fe+3 Conclusion x=0.97, 1.13, 1.68 Synchrotron powder XRD: no anti-site disorder Chemical exchange Ant-site disorder occurs only upon electrochemical charge Na2FePO4F LixNa2-xFePO4FLiBr+ acetonitrile
- 14. 14 4.3. LiFePO4 vs Li2FePO4F 3 oxygen of PO4 tetrahedron are shared with FeO4F2 octahedra Li2FePO4F 4 oxygens of PO4 tetrahedron are shared with FeO6 octahedra LiFePO4
- 15. 15 4.3. LiFePO4 vs Li2FePO4F Li1 leaves structure upon charge Problem: O is not well coordinated LiFePO4F O3 and O5 are coordinated by electrochemically active Li 1
- 16. 16 4.3. LiFePO4 vs Li2FePO4F Problem: O is not well coordinated Li2FePO4F Solution: Fe moves into Li positions Conclusion Underbonded O atoms induce Fe migration LiFePO4F
- 17. 17 Conclusions 1. Underbonded O atoms forced Fe ones to migrate to Li positions. 2. The chemical exchange does not result in the same phase as electrochemical exchange. 3. Elevated temperature does not lead to degradation of crystal structure of LiFePO4. Reference: Olesia M. Karakulina, Nellie R. Khasanova, Oleg A. Drozhzhin, Alexander A. Tsirlin, Joke Hadermann, Evgeny V. Antipov, and Artem M. Abakumov. Chemistry of Materials, 28(21):7578–7581, 2016.
- 18. 18 Acknowledgement Financial support: FWO grant G040116N Research Fund - Flanders