Part 3: Nanostructured Cathode Materials for Lithium Ion Batteries 79
6.3 Results
6.3.1 Materials Characterization
Table 6.1: Particle size as a function of mill time for carbon-FeF3composite milled under argon at 200 RPM for the times indicated.
Mill time with carbon crystallite size (nm)
as-received 69
6 h 41
12 h 26
24 h 21
26 h 18
X-ray diffraction (XRD) patterns were collected using Cu Kα radiation. In situ XRD mea- surements were performed with the pouch cell oriented for reflection. Rietveld analysis was used to determine lattice parameters and crystal size. CrystalMakerR and CrystalDiffractR software packages were used to design unit cells and simulate XRD patterns. A Tecnai TF30 field-emission transmission electron microscope (TEM) was used for imaging the cathode active material nanocom- posite at 300 kV bias. The sample was dispersed in ethanol and prepared on a C-FlatTMholey carbon grid. Bright-field and dark-field images were acquired using a 10µm objective aperture. The electron diffraction patterns were acquired using a 10µm selected area diffraction (SAD) aperture.
M¨ossbauer spectrometry was performed with a conventional constant acceleration system with a radiation source of57Co in a Rh matrix. Velocity and isomer shift calibrations were performed with reference to a room-temperatureα-Fe spectrum.
Two-Theta Angle (°)
Figure 6.3: XRD patterns (a and b) and M¨ossbauer spectra (c and d) from FeF3 as obtained (a and c), and the cathode material comprising carbon-FeF3 prepared by ball-milling (b and d).
material is attracted to a permanent magnet. Ball-milling the pristine material with carbon creates a carbon-FeF3nanocomposite that changes the magnetic environment of the Fe atoms. As mill time is increased, the original magnetic sextet gives way to an increasing fraction of a central peak from superparamagnetic relaxation [113,114], with an isomer shift of 0.5 mm/s (Fig. 6.1). After 36 hours of ball-milling, 75% of the ‘ferromagnetic Fe’ transformed to ‘superparamagnetic Fe’.
After significant reduction in particle size, the majority of iron has become paramagnetic, re- sulting in the doublets in the center of the spectra. The transition into the paramagnetic state indicates that the iron domains in the material are below the threshold size for domain magnetism [113,114], which is considerably reduced with particle size and carbon-coating. Continued milling for 48h causes the material to decompose from FeF3 into rutile-structured FeF2, evidenced by the new peak emerging in the M¨ossbauer spectrum around 2.5 mm/s, and confirmed with XRD.
XRD patterns of the ball-milled carbon-FeF3 nanocomposite corresponding to the M¨ossbauer
(c)
Figure 6.4: Images of the ball-milled carbon-FeF3 composite: (a) bright-field TEM image, (b) dark-field TEM image taken from the FeF3(100) diffraction ring, and (c) electron diffraction pattern acquired from the same area of (a) and (b).
spectra in Fig. 6.1are shown in Fig. 6.2. The additional peak, the (002) peak of graphite, is visible.
These results are consistent with previous studies of FeF3 with similar materials preparation[98,99, 104]. FeF3 is commonly indexed to a R3c rhombohedral crystal structure [98, 99, 104]. However, the unit cell is only slightly distorted from the cubic ReO3 structure, having a decrease inα-angle from 90◦to 88.23◦. This shearing of the unit cell causes the diffractions at 33◦, 40◦, and 54◦to split into two peaks. When particle sizes are small, however, there is considerable peak overlap, so for simplicity, we use the ReO3 cubic indices to help interpret changes in the diffraction patterns with lithiation.
Increasing mill time results in continuously broader peaks. The crystallite particle size as a function of mill time is given in Table6.1, as determined from Rietveld analysis. The reduction in
that while further reduction in particle size is not occurring as rapidly, the material continues to be better mixed with the carbon matrix, which influences the environment of the Fe atoms.
The XRD patterns and M¨ossbauer spectra of the the ball-milled carbon-FeF3 nanocomposite used in this study are shown in Fig. 6.3(b) and (d) in comparison with pristine FeF3 (a) and (c).
The material milled with carbon for 36 h was selected because the fraction of superparamagnetic iron in the material is dominant over the antiferromagnetic without indication of any additional phase formation in the XRD. The average crystallite size of 18 nm is small enough to benefit from reduced particle size effects commonly observed in nanocomposite cathode materials [99,98].
Bright-field and dark-field TEM was performed on the ball-milled carbon-FeF3 nanocomposite.
Images acquired from the same region of the sample are shown in Fig. 6.4. The bright-field image in Fig. 6.4(a) shows the aggregated FeF3 nanoparticles surrounded by amorphous carbon. The dark-field image in Fig. 6.4(b) was taken using the FeF3 (100) diffraction ring (d = 3.60 ˚A). An average particle size of 15±8 nm was determined by examining several regions of the sample, which is consistent with the value of 18 nm obtained from XRD. In the electron diffraction pattern of the carbon=FeF3 nanocomposite shown in Fig. 6.4(c), the innermost fine ring corresponds to the 10µm SAD aperture used to obtain the diffraction pattern. The remaining diffraction rings visible in Fig. 6.4(d) match well with indexes of FeF3 as a ReO3 cubic structure, consistent with the XRD pattern of Fig. 6.3(b). A number of electron diffraction patterns were taken and impurities from ball-milling such as Fe or Mn metals were not found, nor were any additional elements visible by energy-dispersive X-ray spectroscopy (EDS) analysis of the milled materials.