Contribution of Raman micro-spectroscopy to the study of the electrolyte for Li-metal batteries
front of the jury composed of:
– Mr Laurent SERVANT, Professor, University of Bordeaux – ISM – Rapporteur
– Mrs Cristina IOJOIU, Research Director, CNRS, Grenoble INP – LEPMI – Rapporteur
– Mrs Sandrine LYONNARD, Engineer, CEA – SyMMES – Grenoble – Examiner
– Mr Lorenzo STIEVANO, Professor, University of Montpellier – ICGM – Examiner
– Mr François HENN, Professor, University of Montpellier – L2C – Examiner
– Mr Stefano DEABATE, Associate Professor, ENSCM – IEM – Thesis supervisor
Li-metal batteries are ideal candidates for next-generation high-energy density storage devices, thanks to the Li metal anode having ultrahigh capacity (3860 mAh g-1) and the lowest redox potential (-3.04 V vs. S.A.H.E.). However, their wide commercial deployment is currently hindered by the notorious Li dendrites generation. To overcome this main issue, the early low-concentration electrolytes (LCEs) have been replaced with high-concentration electrolytes (HCEs). The latter limit actually the generation of dendritic Li but involve new shortcomings such as poor ionic conductivity, high viscosity and increased price.
These drawbacks are solved in turn by adding a diluent to obtain localized high-concentration electrolytes (LHCEs). Nevertheless, the electrochemical performances of the Li metal anode still remain inadequate.
This work is devoted to the detailed and systematic comparison between the structure and properties of low-, high- and localized high-concentration electrolytes consisting of the same salt and solvents, the purpose implied being to propose new criteria for the development of improved electrolytes
. In order to establish more rigorous relationships between the electrolyte local solvation structure, its reactivity towards the Li metal anode leading to the formation of the solid electrolyte interface (SEI) film and, finally, the electrochemical properties, we use Raman micro spectroscopy to probe the Li+ inner solvation sheath. We show that the increase of the diluent amount in the LHCE affects the electrolyte reactivity by increasing the number of free solvent species and contact ion pairs (CIPs) to the detriment of AGG clusters. Then, due to the consequences that an excessive diluent addition has on the reactivity of the electrolyte and, ultimately, on the Li metal anode lifespan, tuning the diluent content appears for the first time as a key parameter for the designing of improved LHCEs for high-energy-density batteries.
The depletion of Li+ species at the electrode surface is usually considered the phenomenon underlying the dendrite appearance and growth. Then, we also use in
situ Raman spectroscopy to study the Li+ concentration gradients occurring across the electrolyte during the electrochemical operation and, by this way, calculate
approximate Li+ diffusion coefficients. The comparison between LCEs, HCEs and LHCEs show that the Li+ diffusion across the electrolyte decreases with the solvent addition and the increase of the electrolyte concentration. But this does not prevent the homogeneous and smooth Li deposition expected with LCHE from being obtained. Indeed, the characterization of the electrode/electrolyte interface shows that the main parameter affecting the Li deposit morphology is the Li+ mobility across the SEI film, enhanced by the presence of LiF in the case of HCEs and LHCEs.
The last part of this work is devoted to the more in-depth study of a phenomenon that we initially observed by chance during the Raman experiments : the photochemical conversion (i.e. by the laser irradiation) of the electrolyte into graphitic carbon onto a fluorine containing CuxO substrate. We show that this material is the precursor of an effective catalyst for the CO2 reduction reaction, which can represent a most interesting way to recycle exhausted Li battery electrolytes.
Keywords: Li metal battery, Raman spectroscopy, localized high-concentration electrolyte, CO2RR