Zachary NealeMSE 599 Term Paper

Characterization of Lithium Ion Battery Cathodes using FTIR

Introduction: Li-ion Batteries

The increasing energy demand for consumer portable electronics has placed an importance in higher energy density lithium ion batteries. For a long time the market has been dominated by LiCoO2 cathode batteries for their high energy and reliability. However, the cost to manufacture these batteries are much higher than conventional nickel-cadmium batteries. In addition, LiCoO2 cathode batteries are susceptible to thermal runaway which can degrade the material and lead to an exothermic reaction with the electrolyte.

With the drive towards renewable energy sources there has been an increasing interest in large scale energy storage. Although lithium ion batteries have the energy capacity suitable for energy storage, current cathode materials make the battery economically unviable. From this more development has gone into cathode materials that are cheaper and have higher thermal stability than LiCoO2. One such cathode material currently being researched is lithium iron orthosilicate (Li2FeSiO4). Since Li2FeSiO4 is made from constituent chemicals that are earth abundant, the cost of producing these cathodes are theoretically significantly lower than LiCoO2. In addition, the atomic structure of Li2FeSiO4 comprises of alternating lithium, iron, and silicon tetrahedral sites shown in figure 1 [1]. The strong bond between the Si-O bonds significantly increase the stability of the cathode during delithiation.

Figure 1. Crystal structure of Li2FeSiO4 with the Pmn21 orthorhombic space group.

As a cathode, the chemical composition will change during charging and discharging of the battery. Li2FeSiO4 will become Li2-xFeSiO4 when recharging the battery. The stability of delithiated Li2-xFeSiO4 is of great concern to the performance of the battery. Since there is a possibility of extracting two lithium ions per formula unit, the theoretical capacity of a Li2FeSiO4 cathode is 330 mAh/g [2]. However, the complete delithiation to FeSiO4 can leave the structure instable and make the charge/discharge cycle irreversible.

A challenge regarding Li2FeSiO4 cathodes are their inherent low electrical conductivity. This has restricted extraction of a second lithium ion from cathode during charging [3]. A common method to increase the electrical conductivity of the cathode is to add a carbon coating as well as decrease particle size.

In many studies that characterized Li2FeSiO4, Fourier transformed infrared spectroscopy (FTIR) is used. FTIR can check for the phase purity of Li2FeSiO4 by searching for Si-O bonded signals and the absence of lithium metasilicate signals, a common byproduct of improper synthesis. In addition, FTIR is able to determine the carbon groups present in the material if the cathode is surface coated with carbon.

Fourier Transformed Infrared Spectroscopy

FTIR is an optical technique utilizing infrared light for chemical analysis. It falls under a large group of spectroscopy techniques simply known as IR spectroscopy. The basic principle behind IR spectroscopy is that diatomic molecules will absorb certain wavelengths of infrared light. The IR spectrometer will measure the irradiance of light emitted and compare it to the irradiance of light transmitted or reflected at specific wavelengths. Based on this information chemical composition can be identified.

The infrared light absorbed by the material increases the vibrational energy of molecules. Like light and electrons, vibrational energy of a diatomic system is quantized shown in figure 2. From this there are two rules in order for absorption of infrared light to occur. First, the molecule absorbing the light must have a net dipole moment. For example, diatomic molecules such as N2 with no dipole moment will not absorb infrared. This is one of the reasons why IR

Figure 2. Potential energy of a harmonic oscillator (curve 1) and an anharmonic oscillator (curve 2)

spectroscopy can be performed in an open air environment since the two most abundant atmospheric gases are nonpolar. The second rule is that the energy of infrared light must be high enough to promote the molecule to a higher vibrational state. This is analogous to the energy requirement of electrons to increase in energy level. Therefore different molecules will exhibit different amount of energies required to promote to higher vibrational states. This allows for a chemical fingerprint of various diatomic molecules in the infrared region of light.

Older versions of IR spectroscopy use a dispersive method for separating frequencies of light. In the dispersive method the light transmitted through a sample is diffracted by a prism or a diffraction grating shown in figure 3. A photodetector then has to collect intensity data for each individual wavelength. This method can be very time consuming taking up to several minutes for each sample.

To improve the measuring speed a method of measuring all wavelengths of infrared light simultaneously is needed. This is achieved by using a Michelson interferometer which is the method used by FTIR shown in figure 4. The interferometer is a beam splitting device that is used to produce a single signal of light that contains all the frequencies. Since only one signal is produced the amount of time required to measure it is greatly reduced to seconds compared to minutes using the dispersive method.

The interferometer works by using a beam splitting mirror to direct two rays of light towards two mirrors. One mirror is stationary and will reflect the incident light back toward the beam splitter. The first beam of light has to travel a total distance of 2L to return back to its original position. The second beam of light is directed towards a second mirror which is adjustable in position. This makes the second beam have to travel a total distance of 2L + 2δ where δ is the displacement of the mirror from L. When the two beams return to the beam splitter they will interfere with each other based on the optical path difference between the two, producing an interference pattern which is used to distinguish the individual wavelengths.

Figure 3. Dispersive IR using a grating to separate IR wavelengths.

If only one wavelength of light is considered, constructive interference will occur when the

adjustable mirror is at positions n2λ away from center. Upon these positions the second beam

will have a phase difference equal to nλ when it returns to the beam splitter, thus there is complete constructive interference with the first beam. The interference pattern detected for a

single frequency will simply be a cosine wave with maximum intensities at values of δ=n2λ.

For multiple frequencies each wavelength will produce its own sinusoidal interference pattern

based on constructive interference at δ=n2λ. However, since multiple wavelengths are

considered, the position for maximum constructive interference will be different for each wavelength. This will result in a combination of many different sinusoidal interferences that will produce an overall additive interference pattern called an interferogram shown in figure 5. This result is similar to the concept of a wave packet produced from two slightly different wavelength waves traveling through an optical fiber.

However, since the information regarding frequencies and intensity is compressed into one interferogram, post-processing must be done to transform this information into a readable plot. To achieve this a Fourier transform is done on the combined sinusoidal wave to convert it into a plot of intensity as a function of wavelength. The interferogram is really a function of time because the optical path difference for a continuous scan is related to time by δ=2ut where u is the velocity of the moving mirror. Therefore from the interferogram a function f (t) can be produced that gives the times when the signal is non-zero. The Fourier transform shown in equation 1 is the integral of f (t) times [cos (ωt )+i sin (ωt )] taken over all time where ω is 2π

P(δ)

δ

Interference pattern

−δ +δ0

Adjustable Mirror

Fixed Mirror

Beam Splitter50% transmission

IR Light Source

Figure 4. Michelson Interferometer

times the optical frequency. The equivalent function F (ω) is the frequencies where the signal is non-zero.

F (ω )=∫−∞

∞

f ( t ) eiωt dt (1)

To gain any useful information about the sample, the combined beam which contains all wavelengths from the IR source is transmitted through the sample. Any absorption of IR frequencies within the sample will be identified by the difference between the measured signal after the sample and signal without the sample. Since FTIR only measures one signal per test, a blank run must be run without the sample in order to receive a baseline value. In addition, the photodetector compares the measured signal to the signal of a HeNe laser in the system. The

Figure 5. Interferograms of a) a single wavelength, b) two wavelengths A and B, and c) multiple wavelengths

c) Multiple Frequencies

b) Two Frequenciesa) Single Frequency

Figure 6. Setup of FTIR system.

purpose of the HeNe laser is to calibrate the device so that it can assign the correct frequencies to its measured signal.

FTIR can use different types of transducers for measuring light intensity. The two most common categories of detectors are photon detectors and thermal detectors. The different types of

photon detectors include photovoltaic, photoconductive, and photoelectromagnetic which all utilize the production of electron-hole pairs in a semiconductor material. Photovoltaics will produce an electric signal proportional to input intensity. Photoconductive sensors will change their resistance based on the intensity of light received. Photoelectromagnetic sensors take advantage of the Hall Effect in a semiconductor. The semiconducting materials used for photodetectors depends on the bandgap energy of the material and the range of wavelengths needed to be collected. In addition, many semiconducting photodetectors require thermoelectric cooling or liquid nitrogen cooling to remain functional due to the high absorption of light. On the other hand, thermal detection devices use pyroelectric materials such as LiTaO3. Although semiconducting photodetectors are more sensitive, they are more limited to the range of frequencies they can measure because of energy cut offs due to their band gaps shown in figure 7.

FTIR used for Li2FeSiO4 Characterization

FTIR is used in many studies to identify the phase purity of a cathode material. An example of a FTIR spectra done on Li2FeSiO4 is shown in figure 8 [4]. Key peaks to look for are the bending vibrations of SiO4 tetrahedra at 595 cm-1 and 526 cm-1 [5]. In addition is the stretching vibration peak of Si-O bonds at 906 cm-1 [6]. If the sample has been prepared with a carbon coating then peaks at near-IR can be observed. For example, figure 8 shows the vibrational peaks attributed to carboxylic groups at 1506 cm-1 and 1448 cm-1. Additionally, for good phase purity the characteristic peaks for Si-O-Si in lithium metasilicate at 1100 and 780 cm-1 should be absent.

Figure 7. Response versus wavelength of photodetectors and thermal detectors.

Conclusion

FTIR has many advantages for material characterization over other optical spectroscopy methods. The use of a Michelson interferometer allows for all wavelengths of the IR light source to be measured in one signal. This greatly reduces the time take measure a sample compared to dispersive techniques. The use of a single moving mirror makes the system mechanically simple with little possibility for breakdown. In addition, the FTIR system is internally calibrated using a HeNe laser which reduces the need for user calibration. Through Fourier transform of the measured interferogram, an intensity spectrum can be created that holds chemical information of the measured sample useful for characterization.

Figure 8. Example of FTIR spectra for phase pure Li2FeSiO4

References

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[2] P. Zhang, C. H. Hu, S. Q. Wu, Z. Z. Zhu and Y. Yang, "Structural properties and energetics of Li2FeSiO4 polymorphs and their delithiated products from first-principles," Physical Chemistry Chemical Physics, no. 14, pp. 7346-7351, 2012.

[3] M. Devaraju, T. Tomai and I. Honma, "Supercritical hydrothermal synthesis of rod like Li2FeSiO4 particles for cathode application in lithium ion batteries," Electrochimica Acta, vol. 109, pp. 75-81, 2013.

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[5] M. Karakassides, D. Gournis and D. Petridis, "An infrared reflectance study of Si-O vibrations in thermally treated alkali-saturated montmorillonites," Clay Minerals, vol. 34, p. 429, 1999.

[6] M. Ganesan, "Li1-xSm1+xSiO4 as solid electrolyte for high temperature solid-state lithium batteries," Ionics, vol. 13, p. 379, 2007.

[7] C. Eames, A. R. Armstrong, P. G. Bruce and M. S. Islam, "Insights into changes in voltage and structure of Li2FeSiO4 polymorphs for lithium-ion batteries," Chemistry of Materials, vol. 24, no. 11, pp. 2155-2161, 2012.

[8] Thermo Nicolet Corporation, "Introduction to Fourier Transform Infrared Spectroscopy," 2001.

[9] P. A. Tanner, "IR instrumentation," University of Trento.

[10] Thermo Nicolet Corporation, "FT-IR vs. Dispersive Infrared," 2002.