6 Sep 2016

Optimizing next-generation lithium ion batteries with infrared spectroscopy

Lithium batteries have become the mainstay of modern portable electronics, providing a reliable power source to run mobile phones, laptop computers and electric vehicles. Lithium technology is now even being developed on a large scale to provide electricity storage to supplement the National Grid during peak demand [1].

ATR-FTIR spectroscopy accessories allow users to make useful measurements in real-time.

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The issue with lithium batteries remains the same – energy density – and prompts the question how can more power be packed into a smaller and lighter space?

In pursuit of a radically better lithium-ion battery, using a ‘lithium-rich’ cathode has shown the potential to have around a 50% higher energy density than conventional lithium cells [2]. Studying the chemical processes in lithium batteries, especially the role of oxygen in cathode oxidation processes, has been made easier by the use of methods based on IR and Raman spectroscopy.

Understanding Electrochemical Processes using Infrared

To produce a truly efficient and safe lithium battery an excellent understanding of the internal processes that occur as part of electrochemical cycling is essential.

The electrode/electrolyte interface process is particularly important as this controls the stability and kinetics of the cell affecting power, cycling stability and safety [3]. There is still not a great understanding of the interfacial reactions of lithium-ion batteries and this means that surface characterization methods such as IR spectroscopy are valuable.

In situ IR, where the electrode/electrolyte interface is studied during operation, is thought to be best as it provides continuous data during the electrochemical cycle meaning erroneous data from relaxation and contamination is avoided. In situ FT-IR can be carried out in the lab using an ATR accessory equipped with a reaction cell, such as the Golden Gate ATR spectrometer accessory from Specac.

Vibrational spectroscopy (Raman and IR) is a powerful analytical tool for the in situ investigation of surface processes occurring in lithium-ion batteries. The use of Raman and IR spectroscopy as complementary techniques means that Raman can be used to examine structural changes in the electrode material and IR to probe the interface between the lithium and the organic electrolyte.

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Source: Nature Comms | Ya-Qing Bae

Observing Dendrite Formation and Electrode Degradation 

When a lithium battery goes through a number of charge/discharge cycles, especially at a fast rate, the surface of the lithium electrode undergoes degradation and small fibres of lithium can be formed called dendrites resulting in a lower battery capacity.

Dendrite formation occurs over the service life of the battery when lithium fibres sprout from the surface of the lithium electrode and proliferate across the electrolyte until they touch the other electrode. This can result in the battery short-circuiting, causing it to overheat and, in worse case scenarios, catch fire.

Current research believes that dendrite formation is due to seed crystal contaminants in the electrolyte [4] providing a focus for the formation of electrode subsurface structures leading to dendrites. If the dendrite issue can be resolved then lighter, more energy dense lithium batteries that can also use lithium anodes will result.

An equally important issue is the solid–electrolyte interphase and its degradation following constant cycling and reformation that can lead to poor battery performance.

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Golden Gate ATR with the Reaction Cell accessory

Using Infrared to Study Electrodes in Practice 

Surface sensitive FTIR spectroscopy has been used very successfully as a tool to identify surface species on lithium and carbon electrodes.

The strength of this technique comes from its ability to provide specific information about chemical bonds and functional groups meaning it can be used to identify transient lithium species. In addition, this methodology is non-destructive and can be used for in situ electrode studies provided it is supported by a comprehensive library of IR spectra for common lithium species.

A recent study [5] examined the process of dendrite formation using FTIR and provided a solution to dendrite formation and unstable electrolyte issues. This research used an ionic liquid (N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide) containing a specific mixture of lithium salts to treat the electrodes prior to charging and cycling.

This process provided ‘a durable and lithium ion-permeable solid–electrolyte interphase (SEI)’ [5] that lasted for over 1000 cycles with a Coulombic efficiency of more than 99.5%.

The use of IR to study this method showed low dendrite formation and a low rate of change in the electrolyte during the period of the experiment. IR is ideal to examine in situ the formation and subsequent evolution of the SEI layer in lithium batteries but as IR and Raman are based upon reflectance measurements, polished electrode surfaces are more suitable in test cells [7].

IR spectroscopy in electrochemical cells requires constant temperature and a stable uncontaminated environment and this is usually provided by specially designed ATR (attenuated total reflectance) cells in which the electrochemical experiment is located.

Infrared Accessories to Assist in your Research 

The Golden Gate ATR accessory is a high performance single reflection monolithic diamond stage from Specac.

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The Specac Golden Gate ATR accessory

This ATR accessory is the gold standard for use in spectroscopic electrochemical experiments as it is available with a range of sampling options for material analysis, allowing for analysis at a range of temperatures in a reaction cell.

The Golden Gate ATR accessory requires minimal sample preparation and is ideal for high throughput qualitative and quantitative analysis of electrolyte liquids, gels and solids using transmission spectroscopy. A reaction cell version is also available for in situ electrode IR measurements at pressures of up to 3000 psi and if required temperatures approaching 200°C.

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The Specac Quest ATR accessory

The ‘Quest’, also from Specac, is an ATR accessory for more routine high throughput testing over an extended wavelength. With four interchangeable crystal pucks, all reflective gold-coated optics, and Synopti-Focal Array technology it is perfect for sample analysis in the mid- and far-infrared.

Check out #SpectroscopySolutions for more.

References

  1. https://www.sheffield.ac.uk/news/nr/willenhall-battery-energy-storage-1.558968
  2. Dong-Hwa Seo et al. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials, Nature Chemistry (2016) DOI: 10.1038/nchem.2524
  3. P. Verma, P. Maire, P. Novák, Electrochimica Acta 55 (2010) 6332
  4. K. J. Harry, D. T. Hallinan, D. Y. Parkinson, A. A. MacDowell, N.P. Balsara, Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes, Nature Materials 13, 69–73 (2014) DOI:10.1038/nmat3793
  5. A. Basile, A. I. Bhatt, and A. P. O'Mullaneb, Stabilizing lithium metal using ionic liquids for long-lived batteries, Nat Commun. 2016; 7: ncomms11794. DOI:  10.1038/ncomms11794
  6. M. Matsui, K. Dokko and K. Kanamura, In Operando FTIR Spectroscopy for Lithium-Ion Batteries, Journal of The Electrochemical Society, 157 (2010) A121-A129
  7. Rechargeable Lithium Batteries: From Fundamentals to Applications, ed. Alejandro Franco, WoodHead Publishing series in energy no 81, Elsevier, 2015