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

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