Lithium Batteries and Electron Paramagnetic Resonance As a New Research Tool

Lithium-ion batteries are ubiquitous in today's technology and a great deal of research is currently aimed at improving their performance (lightness and energy density) particularly by determining which materials will form the ‘perfect’ electrode materials.¹

Figure 1. Lithium-ion batteries. Image Credit: Vitaliy Hrabar/Shutterstock.com

Current commercial lithium batteries generally have an anode made from graphitic carbon because it is a light and low cost material and provides an acceptable specific capacity (SC) of about 370 mAh/g (milliamp hours per gram). ^1,2

The search for the most efficient anode goes on, with suggestions ranging from anodes made of carbon nanotubes that could triple current performance by achieving an SC of 1,000 mAh/g, to silicon with an energy density of 4,200 mAh/g, and lithium metal that has recently increased energy density of lithium batteries by 400%.^3,4,5

Many of these anode materials have difficulties that need to be addressed, for example silicon reacts with the lithium electrolyte and could cause instability in a battery structure. Also, metal anodes such as lithium can develop structural irregularities called dendrites, which are crystallized metal structures that grow during prolonged battery use and disrupt the anode.

However in 2014 researchers at Stanford University⁶ produced a pure-lithium anode using protective layers of interconnected carbon domes (nanospheres) that did not expand during charging and form dendrites.

Lithium Oxygen Battery

One of the most exciting areas of battery research involves the lithium oxygen battery, which moves away from conventional ‘lithium methodology’ and uses oxidation of lithium at the anode and reduction of oxygen at the cathode under control of metal catalysts such as ruthenium and manganese as the battery storage process.⁷

Lithium-air batteries can theoretically achieve energy densities of 3,840 mAh/g (similar to conventional petrol) and have major potential in automotive batteries.⁸ However, because of the complex oxidative electrochemistry involved in developing these batteries, new analytical methods such as Electron Paramagnetic Resonance (EPR) have had to be adopted to help understand the oxidation processes.

Electron Paramagnetic Resonance Spectroscopy

EPR or Electron Spin Resonance (ESR) spectroscopy⁹ is a useful analytical technique for studying materials with unpaired electrons. Similar in principle to Nuclear Magnetic Resonance (NMR), EPR allows the study of the energy levels of unpaired electron spins that are excited by microwaves in a powerful magnetic field.

EPR has shown a particular effectiveness in the study of metal complexes or organic radicals where unpaired electrons are present. Battery research is a relatively new area for EPR but already its reliability has been proven in the study of lithium dendrites formed in battery systems during charge and discharge cycle (in operando) experiments.¹⁰

EPR provides an efficient way to locate and analyze ‘electron’-related phenomena on the anode and cathode surface during operation and in conjunction with Scanning Electron Microscopy (SEM) could actually see redox processes occurring in real-time.

EPR Imaging and Instrumentation

A research group based in France using Bruker ELEXSYS EPR instrumentation has recently taken EPR a stage further for battery research¹¹ and actually developed an EPR imaging process. This means that the oxidative processes occurring on the surfaces of electrodes involving superoxo and peroxo species (0₂^-n) could be visualized for the first time during analysis in a similar fashion to Magnetic Resonance Imaging (MRI).

The group used a Bruker ELEXSYS E580 spectrometer, with a specially designed microwave compatible electrochemical cell and Li₂Ru_0.75Sn_0.25O₃ electrodes, to monitor the formation and disappearance of radical oxygen species during redox cycling.

Currently in situ or in operando EPRI is still in the early stages of development and could benefit from higher resolution. The improvement of EPR imaging resolution is currently being approached by an increase in the gradient strength up to 1Tcm^-1 and also by the use of an EPR micro-resonator with a higher B1 field to increase both sensitivity and resolution.

With these improvements in operando EPRI is set to become a powerful analytical tool for battery electrode characterization providing unique information about redox processes in the anion - as well as the cation - network.

EPRI can now provide the means to investigate the kinetics of the redox species in a range of systems such as Li-rich Nickel Molybdenum Cobalt (NMC) electrodes, Li– air, Li–S and Li–organic batteries with peroxides/superoxides, polysulfides or radical anions.

Bruker Instrumentation

The ELEXSYS E 580 is a highly sensitive EPR instrument that allows Fourier Transform Pulse-EPR data acquisition that can be used successfully for imaging the redox processes of paramagnetic species at micrometric resolution.

Figure 2. PatternJet-II Channel. Image Credit: Bruker Corporation

References

1. Wood D.L., Li J., Daniel C., Prospects for reducing the processing cost of lithium ion batteries, Journal of Power Sources, Volume 275, 1 February 2015, Pages 234–242.
2. Liu, T et. al., ‘Cycling Li-O2 Batteries via LiOH Formation and Decomposition.’ Science (2015). DOI: 10.1126/science.aac7730.
3. Harks P.P.R.M.L., Mulder F.M., Notten P.H.L., In situ methods for Li-ion battery research: A review of recent developments, Journal of Power Sources, Volume 288, 15 August 2015, Pages 92–105.
4. Scrosati B., Garche J., Lithium batteries: Status, prospects and future, Journal of Power Sources, Volume 195, Issue 9, 1 May 2010, Pages 2419–2430.
5. Goripartia S., Mielea E., De Angelisa F., Di Fabrizioc E., Zaccariaa R.P., Capigliaa C., Review on recent progress of nanostructured anode materials for Li-ion batteries, Journal of Power Sources, Volume 257, 1 July 2014, Pages 421–443.
6. Zheng G., Lee S.W., Liang Z., Lee H-W., Yan K., Yao H., Wang H., Li W., Chu S., Cui Y., Interconnected hollow carbon nanospheres for stable lithium metal anodes, Nature Nanotechnology 2014, 9, 618–623 doi:10.1038/nnano.2014.152
7. Badwal S. P. S., Giddey S.S., Munnings C., Bhatt A. I., Hollenkamp A.F., (24 September 2014). Emerging electrochemical energy conversion and storage technologies. Frontiers in Chemistry 2. doi:10.3389/fchem.2014.00079.
8. Liu T, Leskes, Michal Y., Wanjing M. A. J., Zhou L, Bayley P. M., Kim G., Grey C., (2015-10-30). ‘Cycling Li-O2 batteries via LiOH formation and decomposition’. Science 350 (6260): 530–533. doi:10.1126/science.aac7730. ISSN 0036-8075. PMID 26516278.
9. Lund, A., Shiotani, M., Shimida, S., Principles and Applications of ESR spectroscopy, 2011,Springer, New York.
10. Wandt J., Marino C., Gasteiger H.A., et al., Operando electron paramagnetic resonance spectroscopy – formation of mossy lithium on lithium anodes during charge–discharge cycling, 2015. Energy Environ. Sci., 8, 1358.
11. Sathiya M., Leriche J.-B., Salager E., Gourier D., Tarascon J.-M., Vezin H., Electron paramagnetic resonance imaging for real-time monitoring of Li-ion batteries, 2015. Nature Comm., 6:6276 [DOI: 10.1038/ncomms7276]

This information has been sourced, reviewed and adapted from materials provided by Bruker BioSpin - NMR, EPR and Imaging.

For more information on this source, please visit Bruker BioSpin - NMR, EPR and Imaging.