Electron energy-loss spectroscopy, due to its spatial resolution (of the order of nanometers) and sensitivity to light elements has been very useful in the study of materials for lithium batteries. By coupling simulations using ab initio codes with experimental spectra, we were able to interpret the evolution of compositions in silicon (negative electrode), in LiFePO4 and NaFePO4 (positive electrodes) and the surface reactivity of nickel/manganese oxides (formation of the SEI with the electrolyte).
Searching for a way to measure locally the composition of LixSi alloys formed during cycling of a battery with a silicon negative electrode, we have developed an effective and original method. [Danet 2010] Engineered in the PhD thesis of Julien Danet , the synthesis of standards and the measurement of the positions of the associated plasmon has demonstrated the formation of a Li2.9Si phase during the first discharge. In the thesis of Magali Gautier , the first charge of the Si battery has been studied and, in addition to heterogeneity harmful to the proper functioning of the battery, the composition Li2Si of the amorphous phase was confirmed [Gauthier 2013].
Figure : (a) TEM image obtained on the nanometric Si-based electrode x = 3.3 along with the nano-diffraction pattern corresponding to area labelled 1 (inset). (b) Low-loss spectra of the areas labelled 1 (dashed line) and 2 (full line) in the TEM image (a). From Ref [Gauthier 2013].
For the rapid and simultaneous study of several crystals of LiFePO4 and FePO4 , we have developed a method based on the EFTEM technique in the region of plasmons [Moreau 2009] . This method, now widely used by other groups, required a thorough study of the electronic structure of these compounds [Kinyanjui 2010], and helped to highlight an original reactivity of these compounds when grafted using diazonium chemistry [Madec 2013]. This has also led us to synthesize and study the NaFePO4 compound (in batteries and with EELS) [Moreau 2010, Moreau 2012], a compound which is now the subject of intense research around the world.
Note that in our group, the EELS studies are almost always coupled with electronic structure calculations by ab initio methods (DFT, WIEN2k code, VASP) to more accurately interpret the spectroscopic results [Moreau 2012, Kinyanjui 2010, Moreau 2009, Mauchamp 2008a, Mauchamp 2008b].
Figure: EFTEM EELS mapping of LFPC-NO2-10L. Purple crystals correspond to a phase of composition close to LiFePO4 while orange ones correspond to a phase of composition close to FePO4. From ref [Madec 2013].
If the materials are important, the reactivity of the surface with respect to the electrolyte is also crucial for te long time performance of these systems. For oxides of manganese and nickel such as LiMn1/2Ni1/2O2 and LiNi0.4Mn1.6O4 [Wang 2012, Cuisinier 2012], we were able to demonstrate varying degrees of dissolution of manganese in the observed deposits, as well as an heterogeneous composition for these deposits around these particles. Measurements at the temperature of liquid nitrogen were often necessary to avoid radiation damage due to the electron beam.
Figure : EELS line scans of interphasial layer of LMN-25 and LMN-55. Spectra (e) and related quantification diagrams (f) refer to the points highlighted on the images (d). From Ref [Cuisinier 2012]
[Gauthier 2013] "Nanoscale compositional changes during first delithiation of Si negative electrodes"
M. Gauthier, J. Danet, B. Lestriez, L. Roué, D. Guyomard, and P. Moreau, Journal of Power Sources 227 (2013) 237-242. doi : 10.1016/j.jpowsour.2012.11.047
[Madec 2013] "Synergistic Effect in Carbon Coated LiFePO4 for High Yield Spontaneous Grafting of Diazonium Salt. Structural Examination at the Grain Agglomerate Scale"
L. Madec, D. Robert, P. Moreau, P. Bayle-Guillemaud, D. Guyomard, and J. Gaubicher, J. Am. Chem. Soc. 135 (2013) 11614−11622. doi : 10.1021/ja405087x
[Cuisinier 2012] "Quantitative MAS NMR characterization of the LiMn1/2Ni1/2O2 electrode/electrolyte interphase"
M. Cuisinier, J.-F. Martin, P. Moreau, T. Epicier, R. Kanno, D. Guyomard, and N. Dupre, Solid State Nuclear Magnetic Resonance 42 (2012) 51–61. doi :10.1016/j.ssnmr.2011.09.001
[Moreau 2012] "Revisiting lithium K and iron M2,3 edge superimposition: The case of lithium battery material LiFePO4"
P. Moreau, F. Boucher, Micron 43 (2012) 16–21. doi :10.1016/j.micron.2011.05.008
[Wang 2012] "Effect of glutaric anhydride additive on the LiNi0.4Mn1.6O4 electrode/electrolyte interface evolution: A MAS NMR and TEM/EELS study"
Z. Wang, Nicolas Dupré, L. Lajaunie, P. Moreau, J.-F. Martin, L. Boutafa, S. Patoux, and D. Guyomard, Journal of Power Sources 215 (2012) 170-178. doi : 10.1016/j.jpowsour.2012.05.027
[Moreau 2010] "Structure and Stability of Sodium Intercalated Phases in Olivine FePO4"
P. Moreau, D. Guyomard, J. Gaubicher, and F. Boucher, Chem. Mater. 22 (2010) 4126–4128. doi : 10.1021/cm101377h
[Kinyanjui 2010] "Origin of valence and core excitations in LiFePO4 and FePO4"
M. K. Kinyanjui, P. Axmann, M. Wohlfahrt-Mehrens, P. Moreau, F. Boucher, and U. Kaiser, J. Phys.: Condens. Matter 22 (2010) 275501. doi : 10.1088/0953-8984/22/27/275501
[Danet 2010] "Valence electron energy-loss spectroscopy of silicon negative electrodes for lithium batteries"
J. Danet, T. Brousse, K. Rasim, D. Guyomard, and P. Moreau, Phys. Chem. Chem. Phys. 12 (2010) 220–226. doi : 10.1039/b915245h
[Moreau 2009] "Fast determination of phases in LixFePO4 using low losses in electron energy-loss spectroscopy"
P. Moreau, V. Mauchamp, F. Pailloux, and F. Boucher, Applied Physics Letters 94 (2009) 123111. doi : 10.1063/1.3109777
[Mauchamp 2008a] "Electron energy-loss spectroscopy in the low-loss region as a characterization tool of electrode materials"
V. Mauchamp, F. Boucher, and P. Moreau, Ionics 14 (2008) 191-195. doi : 10.1007/s11581-008-0208-1
[Mauchamp 2008b] "Local field effects at LiK edges in electron energy-loss spectra of Li, Li2O and LiF"
V. Mauchamp, P. Moreau, G. Ouvrard, and F. Boucher, Phys. Rev. B: Condens. Matter Mater. Phys. 77 (2008) 045117. doi: 10.1103/PhysRevB.77.045117