Microscopy: Science, Technology, Applications and Education
A.
Méndez-Vilas and J. Díaz (Eds.)
______________________________________________
Analysis of Li-Ion Battery Materials by Electron Energy Loss
Spectroscopy
F. Cosandey
Department of Materials Science and Engineering, Rutgers University, Piscataway, NJ 08854, USA
[email protected]
Electron Energy Loss Spectroscopy (EELS) is a powerful technique for studying Li-Ion battery materials as Li distribution
in the electrode material can be mapped with nanometer scale resolution. In addition, valence state of the transition metal
in the positive electrode and charge transfer during lithiation and delithiation processes can be analyzed by measuring the
relative intensity of the transition metal L3 and L2 lines. The valence state measurement of transition elements by EELS
technique is reviewed in this paper and recent results obtained on LiMn1.5Ni0.5O4 and Li-FeOF/C nanocomposite
electrodes are presented to illustrate the potential benefits of EELS technique in studies of Li-Ion battery materials.
Keywords Li-Ion battery material; Electron Energy Loss Spectroscopy (EELS); LiMn1.5Ni 0.5O4, FeOF/C
nanocomposites
1. Introduction
Li-Ion batteries are composed of a negative electrode (C,Sn...) and a positive electrode (LiCoO2, LiFePO4, LiMn1.5Ni
0.5O4...) separated by an ion conducting electrolyte. When a battery is fully charged, the Li is stored in the negative
electrode with the positive electrode containing a low Li concentration and with the transition element (Fe, Mn, Co,
Ni...) at a high valence state. During discharge (which corresponds to a lithiation process of the positive electrode), Li
diffusees through the electrolyte into the positive electrode with a concomitant decrease in the valence state of the
transition element [1]. For instance, in the LiCoO2 positive electrode material, the reaction at the cathode during Li
intercalation (battery discharge) leads to a decrease in Co valence state from Co4+ to Co3+ as shown in the following
reaction (1):
Co4+O2 + (Li+ + e- ) LiCo3+O2
(1)
In multi-component oxides charge transfer can occur in prinicple on all or only on one of the transition elements.
For instance in the LiMn1.5Ni 0.5O4 electrode, charge transfer upon lithation occurs on the Ni ion with a reduction from
Ni4+ to Ni2+ as decribed by reaction (2) while the valence state of Mn remains unchanged at Mn4+ .
Mn4+1.5Ni4+0.5O4 + (Li++e-) LiMn4+1.5Ni2+0.5O4
(2)
Therefore, in order to understand the behavior of battery materials, it is necessary to be able to monitor the valence
state of all transition elements and to determine the local Li concentration. Amongst all the experimental techniques to
study battery materials, Electron Energy Loss Spectroscopy (EELS) play a unique role as both the Li distribution and
valence state of the transition element can be mapped with sub-nanometer spatial resolution [2]. In addition, in view of
the high energy resolution of EELS spectrometer bonding information and site occupancy can be determined also from
analysis of near edge structue. In this paper, the basic principle of EELS will not be discussed but excellent EELS
reviews can be found in the following papers [2,3].
Two main features associated with battery charge and discharge processes will be discussed here which are (1)
valence state measurement of the transition element by EELS and (b) analysis of the Li-K edge to determine phase
structure and site occupancy. Recent results obtained on two positive electrode systems used in Li-Ion batteries namely
the intercalation oxide LiMn1.5Ni0.5O4 [4] and conversion FeOF/C nanocomposite systems are presented [5,6]
2. Experimental Procedure
Electron energy loss spectroscopy (EELS) spectra were collected with a Gatan GIF-200 spectrometer attached to a
JEOL 2010F field emission microscope operating at 197 KeV. The EELS spectra were obtained in STEM mode with a
total beam current of 0.2 nA for a probe size of about 0.5 nm. For the FeOF/C system, the samples were cooled to LN2
temperature and imaged with a total electron dose limited to 104 C/cm2 in order to minimize electron beam damage and
elemental loss. Further experimental details can be found in the following papers [4,7]. In order to limit exposure to
air, the TEM samples were prepared in a He filled glove box and sealed before transfer to the TEM.
1662
©FORMATEX 2010
Microscopy: Science, Technology, Applications and Education
A. Méndez-Vilas and J. Díaz (Eds.)
______________________________________________
3. Results and Discussion
3.1
EELS determination of transition element valence state
The EELS edge of transition elements is characterized by two strong L3 and L2 lines (historically called white lines
because of their dominant high intensities) resulting from the transition of electrons from the spin-orbit split levels 2p3/2
and 2p1/2 to unoccupied 3d states. With the exception of metallic Cu with the d-band fully occupied, all the other
transition elements from Ti to Ni have these two L3 and L2 lines. The relative intensity and energy position of these L3
and L2 lines are strongly dependent on the d-band occupancy and therefore on valence state of the transition element
[8,9]. A typical EELS spectrum taken from the LiMn1.5Ni 0.5O4 electrode is shown in Figure 1.
In this spectrum, the O-K, Mn-L and Ni-L edges are observed with the Mn and Ni edges possessing two sharp L3 and L2
peaks. There are also peaks associated with the O-K edge whose relative intensities are dependent on crystal symmetry
and structure.
In order to determine the valence state of Mn or Ni, the intensity of the L3 and L2 peaks must be extracted. First the
background under the edge is removed. For extracting the L3 and L2 line intensities, various methods have been
proposed [10,11,12] which include (1) taking the positive part of the second derivative of the spectra under each peak,
(2) removing the continuous L edge contribution and taking the L-line intensity ratio from curve fitting or within an
given energy width and (3) by taking the maximum peak intensity but without removing the continuous L edge
contribution. One advantage of the second derivative method is that the measurements do not depend on background
removal, plural scattering and continuous L edge contribution and provide a good relative measurement of line
intensity. However, the measurement cannot be related directly to d band occupancy and for this measurement, method
(2) is preferred. In general, care must be taken to perform the analysis on a thin part (<30nm) of the sample.
Otherwise, plural scattering should be removed by first by taking the low loss region that includes the zero and plasmon
losses and perform a plural scattering removal analysis
[2,3].
A summary of published studies performed on various
Mn- L3 L2
manganese oxides with a range in valence state is shown
in Figures 2a and 2b [12-15]. It can be seen that for Mn,
O-K
the L3/L2 intensity ratio increases as the Mn valence state
decreases (c.f. Figure 2a). All four studies show the
same trend but some variation between investigators
exists due to differences in energy resolution of the
detector and on the analysis method used to extract the
Ni- L3 L2
L3/L2 line intensity ratio.
Figure 1 EELS spectrum of LiMn1.5Ni
showing O-K , Mn-L and Ni-L edges.
a
0.5O4
electrode material
b
Figure 2 Summary of (a) Mn L3/L2 intesity ratio and (b) Mn L3 peak energy as a function of Mn valence state taken from
various studies [12-15]
©FORMATEX 2010
1663
Microscopy: Science, Technology, Applications and Education
A.
Méndez-Vilas and J. Díaz (Eds.)
______________________________________________
Therefore for quantitative analysis, it is imperative that a master calibration curve is performed for each instrument
and for each transition element to be studied. The L3 peak energy is also dependent on valence state and decrease with
decreasing valence state as shown in Figure 2b. Here also, there are some variations between the various studies but the
systematic decrease in energy is the same for all data. Despite the care taken by the investigators, these results
underline the difficulties in achieving precise quantitative energy measurements. Energy calibration can be done with
respect to a know element whose energy remains unchanged upon valence state changes. Such element could be for
instance carbon with a strong π* pre-peak located at 284 eV. Despite these variations in absolute energy measurements,
the magnitude of the energy decreases is identical in all three studies presented here.
3.2
EELS of surface modified LiMn1.5Ni 0.5O4
Recently, we have studied the effects of surface acidic treatment and of rate capability of LiMn1.5Ni 0.5O4 at high
operating temperatures [4]. After phosphoric or HF surface treatment, the rate capability of LiMn1.5Ni 0.5O4 at a high
temperature of 60oC improved significantly. A HRTEM and EELS study was subsequently conducted in order to
understand the effects of the acid treatment on particle surface chemistry and valence state for both Mn and Ni. The Ni
a
b
Figure 3 (a) Ni L3 and L2 and (b) Mn L3 and L2 lines taken from surface and bulk of the nanoparticle.
EELS spectra taken from the edge and bulk of a 30 nm particle are shown in Figure 3a. For, Ni, the two EELS spectra
are identical in terms of relative ratio and energy indicating that the valence state of Ni has not changed. However, for
Mn, a shift of the L3 line to lower energy by 1.3 eV is clearly visible in Figure 3b for the EELS spectra taken from the
particle edge with also a reduction in L2 line intensity. Based on these results, Mn on the surface has a lower valence
state than Mn in the bulk. A quantitative analysis based on standard curves reveals a decrease from Mn4+ to Mn3+. In
addition to single spectra, we have also collected 2D maps of Mn, O, and Ni to reveal the changes is elemental
concentration and the results are shown in Figure 4. The temperature colour scale from blue to red reflects increases in
elemental concentration. By inspection of Figure 4, it can be seen that the Mn concentration increases at the particle
surface with a corresponding decrease in O content. Also mapped in Figure 4 is the Mn L3/L2 ratio. As expected, the
ratio increases at the particle surface reflecting the change in valence state from Mn4+ to Mn3+. These maximum surface
changes occur within 3-5 nm from the particle edge.
Figure 4 Mn-L, O-K and Ni-L elemental maps with Mn valence map
expressed as Mn L3/L2 intensity ratio
1664
©FORMATEX 2010
Microscopy: Science, Technology, Applications and Education
A. Méndez-Vilas and J. Díaz (Eds.)
______________________________________________
3.3
EELS of Lithiated FeOF/C nanocomposites.
A new type of positive electrode materials has been developed recently based on transition metal fluorides/C
nanocomposites (FeF2/C, FeOF/C...) [16,17]. In these materials, Li does not intercalate into the structure of the
materials such as in oxides but react with the materials and converts the initial materials into new phases as shown in
reaction (3).
Fe2+F2 + 2(Li+ +e-) 2LiF + Feo(3)
This reaction involves two electrons transfer and therefore results in a high electrode capacity. For FeOF/C a more
complex conversion process occurs with the formation of a (LiFeO) intermediate phase [7],
Fe3+OF + x(Li++e-) LiF + Feo + x(LiFeO)(4)
Upon further lithiation, the (LiFeO) phase decomposes into Li2O and Fe. In order to determine structure, chemistry and
valence state of Fe in this (LiFeO) intermediate phase we have performed STEM-EELS analysis of this materials after
partial lithiation (discharge) to 1.5 V. It is now well documented that the near edge structure of the Li-K edge is
dependent on crystal structure and bonding (Li2O, LiF, Li) and for intercalation compounds, on the site occupancy
(tetrahedral versus octahedral) [19]. In order to determine the crystal structure and chemistry of this (LiFeO) phase we
analyzed the Li-K edge and compared its characteristic features with known standards. The low loss EELS signal of
lithiated FeOF to 1.5V is shown in Figure 5. Also depicted in this Figure 5 are spectra for LiF, Li2O, and Li2CO3 and
recharged FeOF (delithiated) to 4.5V. As expected, the recharged (delithiated) electrode does not contain any Li and
the low loss signal is composed of only the Fe-M edge. For the discharge electrode, the low loss signal is composed of
the superposition of the Li-K and Fe-M edges. An enlarged view of the low loss edges for the lithiated FeOF/C
electrode (discharged to 1.5V) is shown in Figure 5b.
a
b
Fe-M and Li-K
Fe-M
Extracted Li-K
Figure 5 (a) Li-K edge taken from various Li componds (LiF,
Li2CO3 and Li2O) with low loss EELS signal from lithiated
(discharge) and de-lithiated (re-charged) FeOF (b) enlarged view
of the low loss region of discharged FeOF showing overlap
between Fe-M edge and Li-K edge with the extracted Li-K edge
[5]
O/F
a
Fe L3/L2
b
Figure 6 (a) O-K/F-KEELS signal intensity ratio map and (b)
corresponding Fe valence map form Fe-L3/L2 line intensity ratio (Median
filtered)
©FORMATEX 2010
1665
Microscopy: Science, Technology, Applications and Education
A.
Méndez-Vilas and J. Díaz (Eds.)
______________________________________________
In addition to the two prominent Li peaks separated by ∆E=6.6 eV corresponding to LiF, there is a third peak located
at a distance of 4.2 eV from the first peak. The existence of this additional peak could be from a new intermediate O
rich LixFe2O3 phase. Electron diffraction data indeed confirm the presence of a cubic type structure.
The spatial distribution of this O-rich phase has been determined by ELLS and is shown in Fig. 6a as represented by
the O-K/F-K EELS signal intensity ratio map. The corresponding Fe valence state map measured from the Fe L3/L2
intensity ratio in shown in Fig. 6b. It can be seen that a higher valence state is associated with the O rich phase
(LixFe2O3) with the lower valence state associated with the F rich phase consisting of a mixture of metallic Feo and LiF.
Acknowledgements This research is supported in part by NECCES a DOE-BES-EFRC funded center. Award # DE-SC0001294.
Special thanks also to IAMDN for the use of electron microscopy facility. Discussions with G.G. Amatucci and N. Pereira are
gratefully acknowledge.
References
[1] Whittingham MS, Lithium batteries and cathode materials, Chem. Rev. 2004, 104; 4271-4301
[2] Keast VJ, Scott AJ, Brydson R, Williams DB, Bruley J, Electron energy-loss near edge structure – a tool for the investigation of
electronic structure on the nanometer scale, J. of Microscopy, 2001; 203 (2): 135-175.
[3] Egerton RF, Malac M, EELS in the TEM, J. of Electron Spectroscopy and Related Phenomena, 2005, 143: 43-50.
[4] Hagh NM, Rangan S, Cosandey F, Bartynski R, Amatucci GG, Electrochemical performance of surface treated nanostructured
LiMn1.5Ni0.5O4-δ spinel at elevated temperature, J. Electrochemical Society, 2010; 157 (3) : A305-A319.
[5]
Cosandey F, Rao KS, Pereira N, Amatucci GG, Electron Energy Loss Spectroscopy Study of Lithiation in FeOF/C
Nanocomposite Battery Material, IMC 17 Conference, 2010 (To be published)
[6] Pereira N, Badway F, Wartelsky M, Gunn S, Amatucci GG, Iron oxyfluorides as high capacity cathode materials for lithium
batteries, J. Electrochemical Society, 2009 ; 156 (6) :A407-A416.
[7] Cosandey F, Al-Sharab J, Badway F, Amatucci GG, Stadelmann P, EELS spectroscopy of FeFx/C nanocomposite electrodes
c 2007; 13: 1-9.
used in Li-Ion batteries, Microscopy and Microanalysis,
[8] Leapman RD, Grunes La, Fejes PL, Study of the L23 edges in the 3d transition metals and their oxides by electron-energy-loss
spectroscopy with comparisons to theory, Physical Review B, 1982; 26 (1): 614-635
[9] Botton GA, Appel CC, Horsewell A, Stobbs WM, Quantification of the EELS near-edge structure to study Mn doping in oxides,
J. of Microscopy, 1995; 180: 211-216.
[10] Riedl T, Gemming T, Wetzig K, Extraction of EELS white-line intesities of manganese compounds: Methods, accuracy and
valence sensitivity, Ultramicroscopy, 2006; 106: 284-291.
[11] Kuruta H, Lefevre E, Colliex C, Brydson, Electron-energy-loss near-edge structure in the K-edge spectra of transition-metal
oxides, Physical Review B, 1993; 47 (20): 13763-13768.
[12] Kurata H, Colliex C, Electron-energy-loss core-edge structure in manganese oxide, Physical Review B, 1993; 48 (4): 21022108.
[13] Rask JH, Miner BA, Determination of manganese oxidation states in solids by electron energy-loss spectroscopy,
Ultramicroscopy, 1987; 21: 321-326
[14] Wang Zl, Bentley J, Evans ND, Mapping the valence state of transition-metal elements using energy-filtered transmission
electron microscopy, J. Phys. Chem. 1999; 103: 751-753.
[15] Paterson JH, Krivanek OL, ELNES of 3d transition-metal oxides II. Variations with oxidation state and crystal structure,
Ultramicrosocpy, 1990; 32: 319-325
[16] Badway F, Cosandey F, Pereira N, Amatucci GG, Carbon metal fluoride nanocoposites; High-capacity reversible metal fluoride
converssion materials as rechargeable positive electrodes for Li batteries, J. of the Electrochemical Society, 2003; 150 (10):
A1318-1327
[17] Amatucci GG, Pereira N, Fluoride based electrode materials for advanced energy storage devices, J. Of Fluorine Chemistry,
2007; 128; 243-262
[18] Hightower A, Ahn CC, Fultz B, Rez P, Electron energy-loss spectroscopy on lithiated graphite, Applied Physics Letters, 2000 ;
77(2) : 238-240.
[19] Mauchamp V, Boucher F, Moreau P, Electron energy-loss spectroscopy in the low-loss region as a characterization tool of
electrode materials, Ionics, 2008 ; 14 : 191-195.
1666
©FORMATEX 2010