Physics Colloquium, Moniek Tromp, University of Groningen, Zernike Institute for Advanced Materials

Abstract:

An important element in the reduction of CO₂ is the change of vehicles with internal combustion engines to electric battery powered vehicles. The as such produced renewable energy can be used for individual mobility as well as for a temporary intermediate storage of excess energy. A viable electric mobility concept requires however stable cycle batteries with high specific energy (minimising weight, maximising driving range).

Li ion batteries are widely used in applications such as mobile phones and laptops, and will likely be key to future electromobility. The requirements for such batteries present a major challenge. Although current cathode materials can achieve promising capacities, at high voltages an accelerated capacity fading is observed, which in literature is mainly related to a metal dissolution from the cathode material and the subsequent deposition on the graphite anode. In order to suppress the problem of metal dissolution and precipitation (of all the different metals present in the material) a detailed understanding of its mechanism is inevitably needed.

There are no studies monitoring the dissolution and precipitation in operando conditions, neither can studies describing the kinetics and electrochemical dependence of the metal dissolution be found. Only post-mortem analyses were done to analyse if transition metals are dissolved or not. The quantification and spatially resolved information, in an operando manner, as a function of cell voltage, would be required in order to establish a clear correlation between transition metal dissolution into the electrolyte as well as deposition on the anode and the observed battery performance losses. Electronic as well as structural information of the different metals present is required to reveal the exact speciation and to understand the mechanisms occurring.

An alternative promising battery is the lithium sulfur battery with a potential twofold energy density increase. Its main problem is however the overall deactivation of the battery due to the diffusion of soluble polysulfides. To improve the cycle life of Li-S batteries, better insights in the reaction mechanisms occurring during subsequent discharge and charge cycles are required.

For Li ion battery systems, ex situ or post mortem characterisation has been performed, but its relevance to the mechanism is unclear, as mentioned. In-situ characterization of the speciation in LiS batteries have included XRD, UV-Vis and NMR, but none of them has been able to obtain a complete understanding of intermediates involved in the electrochemical conversion, mainly due to the fact that they detect either solid or liquid species, but not both at the same time.

X-ray absorption spectroscopy (XAS) is a characterisation technique which provides detailed electronic and structural information on the material under investigation, in a time- and spatially resolved manner. Long range ordering of the material is not required and thus it can be applied to solid, amorphous, and liquid phases.

Here, I will explain the strengths and limitations of XAS for battery research. A novel operando XAS cell design will be described [1], including the challenges to perform reliable experiments (electrochemically and spectroscopically). The cell allows time and spatial resolved XAS, providing insights in the type, location and reversibility of the intermediates formed in electrodes and electrolyte separately. Obtained insights in cycling and deactiviation mechanisms for the different battery types will be discussed [1-6] and future research directions described.

[1] Y. Gorlin, A. Siebel, M. Piana, T. Huthwelker, H. Jha, G. Monsch, F. Kraus, H.A. Gasteiger, M. Tromp, J. Electrochem. Soc. 162(7): A1146-A1155, 2015.

[2] Y. Gorlin, M. U. M. Patel, A. Freiberg, Q. He, M. Piana, M. Tromp, H. A. Gasteiger, J. Electrochem. Soc. 2016, 163(6), A930-A939.

[3] J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger, M. Tromp, J. Mater. Chem. A 2016, 4, 18300-18305.

[4] A. T. S. Freiberg, A. Siebel, A. Berger, S. M. Webb, Y. Gorlin, H. A. Gasteiger, M. Tromp, J. Phys. Chem. C 2018, 122, 10, 5303-5316.

[5] A. Berger, A. T. S. Freiberg, R. J. Thomas, M. U. M. Patel, M. Tromp, H. Gasteiger, Y. Gorlin, J. Electrochem. Soc. 2018, 165(7), A1288-A1296.

[6] R. Jung, F. Linsenmann, R. J. Thomas, J. Wandt, S. Solchenbach, F. Maglia, C. Stinner, H. A. Gasteiger, J. Electrochem. Soc. 2018, accepted for publication.