MALDI-MS was used to study the formation of polymeric surface films on both model anode and cathode materials, namely thin films of Au and LiMn2O4, respectively. Thin films are useful model electrodes for MALDI-MS experiments to prevent the signal from polymers utilized as binders in commercial battery electrodes from obscuring signal from the SEI. Control experiments consisting of uncycled cells containing PF6- electrolytes detected the presence of high mass oligomers associated1 with the formation of oligomeric structures, which means that LiPF6 electrolytes are unsuitable for MALDI-MS. Thus, the electrolyte systems chosen for MALDI-TOF experiments consist of propylene carbonate (PC) and ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 mixtures with LiClO4 salts.
The mass spectra obtained from both the Au anodes and the LiMn2O4 cathodes showed remarkable similarity, as seen in panel c. The composition of the electrolyte, however, has a large effect on the SEI composition. An average peak spacing of ~175 m/z was observed for the EC/DMC system (panel b), which corresponds with a polycarbonate structure formed by the co-polymerization of EC and DMC. In the case of the PC based electrolyte, a different polycarbonate with an average peak spacing of 167 m/z was observed in the mass spectra (panel a). Both polymers have been reported in the past and DFT calculations suggest a formation mechanism in which carbonate rings open at low voltages forming a reactive radical that induces polymerization of the other solvent molecules.1Lastly, different electrodes materials were studied at cathode potentials (Au, Pt, LMO and Au coated LMO), but no effect was observed on the final structure of the SEI.
In order to further study the organic SEI formed on cathode/electrolyte systems which more closely resemble commercial lithium-ion battery systems, DESI-MS was introduced as a powerful alternative to MALDI-MS, since it does not present the limitations discussed in the previous part of this work. It was found that it is possible to obtain high quality, high resolution polymeric mass spectra directly from the surface of slurry electrodes cycled in EC\DMC\LiPF6. The presence of poly(ethylene glycol) dimethyl ether (PEG) as a product of the copolymerization of EC and DMC was observed using this method. The presence of PEG in the SEI was confirmed for both anodic and cathodic half-cells, similarly to the results obtained by MALDI-MS.
The interesting similarity in the composition of the SEI at completely different environments and potential ranges (cathode and anode) suggests that the oligomerization of the organic electrolyte is not sensitive to the electric potential applied to the electrode surface.
We also carried out investigations of polymerization mechanisms of the solvent molecules using density functional methods. All calculation of energies, forces and force constants were performed using the B3LYP hybrid-density functional method with the cc-pVDZ basis set as implemented in the Gaussian 09 code. Equilibrium and transition state structures were determined for formation of dimers and trimers resulting from reaction of the oxidized EC molecule and solvent molecules. The reaction energies are consistent with the observation of polymeric species.
1. Tavassol H., Butchker J.W., Ferguson G.A., Curtiss L.A., Gewirth A.A., Solvent oligomerization during SEI formation in Li-ion battery anodes, J. Electrochem. Soc. 159(6) (2012) A730-A738.