12 Jun 2019

Testing nano-materials for rechargeable batteries | ATR-FTIR

Lithium Iron Phosphates are already widely used as storage cathode materials for rechargeable Li batteries. The advantages include high energy density, wide operating temperature, thermal stability, long cycle and service life and good environmental and safety credentials. 

Building upon this success a new study is focusing on the utility of hybrid lithium/iron butyl phosphates in the manufacture of electrochemical devices. The synthesis of lithium iron alkyl phosphate crystals is achieved relatively easily by mixing inexpensive alkyl phosphates with lithium. 

These hybrid lithium iron salts are characterized by the presence of elongated hexagonal crystals, which are stable up to 315°C. The insertion of lithium into the crystals gives new nanostructured materials that can be electrochemically-cycled with Li ions acting as charge carriers.

The lithium ions are able to find facilitated pathways along the crystals for travel and redox reactions. Although these hybrid lithium /iron alkyl salts are highly suitable for this application they needed to be studied and characterized. This was accomplished using a variety of analytical techniques to examine their structure and behaviour under different conditions.  One of the major techniques used was Attenuated total reflection (ATR) Fourier transform Infra-Red (FT-IR) spectroscopy using a Specac Golden Gate Attenuated Total Reflectance (ATR) accessory. 

Structure of the crystals

The Alkyl lithium iron compound was produced in the form of crystals around 10 µm long. The structure of the crystal was found to be quite independent of the R (molar ratio) of reactants used in synthesis, as shown by the FT-IR results. 
Bands for n-tributyl phosphate (P = O) could be seen at 1271 cm-1. Also there were bands for TBP/Fe (III) and TBP/Fe(II) appearing at 1160, 1101 and 1062 cm-1suggesting P = O groups interacting with Fe(III) and Fe(II). Further bands (at 563, 514, 467, 413 cm-1for Fe-O vibrations) confirmed the phosphate-iron interaction. 

The crystal structure was found to be the same with or without lithium according to the FTIR. This indicates that lithium is located far apart from the P = O groups and that the structure at the nano-scale is quite insensitive to the presence of lithium. 

Analysis of FTIR spectra showed there to be an irregular structure of [(BuO)2POO]3FeLi, which gives rise to very regular crystals. This enhanced self-assembly of the compound into its regular crystal structure gives rise to a facilitated lithium ion transport and its intercalation and de-intercalation during eventual charge-discharge processes in battery applications.

The ATR FTIR data combined with x-ray diffraction, scanning electron microscopy and cyclic voltammetry shows a stoichiometric nanostructure material with excellent electrical properties. 

ATR-FTIR technique

ATR FT-IR is an ideal method for the initial characterisation of these new materials, because it can examine the nature/strength of the bonding between the different components in the crystals even if there is weaker bonding. 

ATR requires little or no sample preparation unlike transmission IR. In addition, the pathlength is much shorter with ATR and this avoids strong attenuation of the IR signal. In the case of ATR the IR beam undergoes multiple internal reflections in the cell crystal of high refractive index (in this case zinc selenide). 

The IR beam has to enter the ATR ZnSe lens at a higher angle than the critical angle so that total internal reflection takes place. This then allows the accumulation of radiation repeatedly reflected from the interface between the sample and the ATR cell. 

The evanescent wave formed penetrates from the cell into the sample at a set depth. The depth of sample penetration is controlled by the wavelength of the IR. This Multiple ATR technique avoids the problems arising from saturation effects in transmission mode, and provides a high signal intensity and signal-to-noise ratio for superior IR absorption spectra.

The Specac Golden Gate ATR accessory is sensitive, robust, and versatile. This accessory is invaluable as it is able to provide a range of lens materials and has the ability to undertake IR measurements at elevated temperature or pressure if required. The Golden Gate accessory is ideal for solids or liquids, the pressure cell can withstand 3000 PSI and heated reactions can be monitored from 0-300°C. 

Conclusion

The development of lithium ion batteries is moving forward at a remarkable pace and the drive is towards ‘smaller and lighter’, especially for vehicles. New batteries will be expected to pack ever more power into a smaller space. 

This means the development of new lithium bearing materials for electrodes and electrolytes without the existing disadvantages. During cycling the impedance of the Li+transfer across the electrode/electrolyte interface is crucial as it can lower the cycle life of a battery. 

The formation of passivation layers on the anode is also important as it sequestrates Li from the cathode irreversibly. The development of new modifiable lithium iron alkyl hybrids has the potential to address these issues in new batteries. It has been seen that lithium insertion to form these alkyl lithium iron compounds can provide interesting nanostructured materials that could be tailored for the realization of new generation lithium ion battery technology.

References

  1. Valeria La Parola, Vincenzo Turco Liveri, et al., Iron and lithium-iron alkyl phosphates as nanostructured material for rechargeable batteries, Materials Letters 220 (2018) 58–61
  2. D. A. Skoog, F. J. Holler, T. A. Nieman, Principles of Instrumental Analysis, 5th edition, Thomson Learning, Brook, 1998
  3. J.B. Goodenough, K.-S. Park, The Li-ion rechargeable battery: a perspective, J.Am. Chem. Soc. 135 (2013) 1167–1176
  4. Peter R. Griffiths; James A. De Haseth (2007). Fourier Transform Infrared Spectrometry (2nd ed.). John Wiley & Sons

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