(Invited) Investigations into the Chemical Role of Additives in Li-Ion Cells

Abstract

INTRODUCTION For over a decade, the most common electrolyte solution in commercially available lithium-ion cells has remained LiPF6 dissolved in some blend of organic carbonate solvents.1 Rather than change the salt or the solvent, many industrial production lines have adopted the use of electrolyte additives to improve cycling performance, extend calendar lifetime, decrease detrimental gas formation and improve lithium-ion cell safety. The practical advantage of this move to electrolyte additives is that performance improvements can be achieved with minimal changes to existing supply chains for electrolyte salts and solvents. However, the optimization of lithium-ion cells for various applications (automotive, grid storage, etc.) would be greatly enhanced by a more detailed understanding of the cell chemistry. In particular, it is desirable to characterize the chemical and electrochemical reactions that occur during solid-electrolyte interphase (SEI) formation for the various additives in use. This presentation will demonstrate how our group has used computational and experimental methods, together, to study SEI formation for two additives, prop-1-ene-1,3-sultone (PES)2 and pyridine boron trifluoride (PBF).3 EXPERIMENTAL Density functional theory (DFT) calculations were performed with the Gaussian 09 (G09.D01) software package using the B3LYP and M06-2X hybrid functionals. The IEFPCM-UFF solvation model and its parameterization will be discussed.4 Several experimental methods will be discussed, including coulometry, in situ volumetric measurements using the Measuring Archimedes’ Gas Evolution (MAGE) instrument, gas-chromatography coupled with mass spectrometry (GC-MS) and thermal conductivity detection (GC-TCD), X-ray photoelectron spectroscopy (XPS), and isothermal microcalorimetry. Experimental details have been described previously.5–9 RESULTS AND DISCUSSION During the initial formation cycle (i.e., the first charge step), PES forms a passive SEI at the negative electrode surface via a two-electron electrochemical reduction, which produces Li2PES (Figure 1). The decomposition of this compound and its various reactions with the solvent (EC and EMC) and with other PES molecules will be discussed. These reactions are spontaneous and result in the formation of Li2SO3 and organic sulfate species (RSO3Li) at the anode. This is a good match to the S2ppeaks observed in the XPS spectrum of the anode after formation. The predicted gas-phase products, including several hydrocarbons at the anode, are also consistent with those observed by GC-MS. PBF similarly forms a passive SEI at the graphite surface by electrochemical reduction. The reduced species, LiPBF, forms a bipyridine boron trifluoride adduct, which is accompanied by the reduction of the solvent component, ethylene carbonate (EC). This reaction produces lithium ethyl carbonate, a soluble lithium semicarbonate. This reaction pathway does not produce an appreciable amount of any gas-phase species, as demonstrated by MAGE, GC-MS, and GC-TCD results. The predicted PBF-derived dimer is consistent with the C1s and N1s peaks observed in the XPS spectrum of the anode surface after formation. In summary, carefully developed theoretical methods coupled with experimental data reveal several spontaneous pathways for the reductive decomposition of two additives, PES and PBF. It is hoped that these results will prove useful for developing new and improved electrolyte additives. Moreover, these results provide new insight into the role of the solvent molecules during SEI formation that may have significance for research into new solvents and solvent blends. REFERENCES 1. K. Xu, Chem. Rev., 114, 11503–11618 (2014). 2. B. Li, M. Xu, T. Li, W. Li, and S. Hu, Electrochem. Commun., 17, 92–95 (2012). 3. M. Nie, J. Xia, and J. R. Dahn, J. Electrochem. Soc., 162, A1186–A1195 (2015). 4. D. S. Hall, J. Self, and J. R. Dahn, J. Phys. Chem. C, 119, 22322–22330 (2015). 5. C. P. Aiken et al., J. Electrochem. Soc., 161, A1548–A1554 (2014). 6. V. L. Chevrier et al., J. Electrochem. Soc., 161, A783–A791 (2014). 7. L. Madec et al., J. Phys. Chem. C, 118, 29608–29622 (2014). 8. R. Petibon, L. M. Rotermund, and J. R. Dahn, J. Power Sources, 287, 184–195 (2015). 9. J. Self, D. S. Hall, L. Madec, and J. R. Dahn, J. Power Sources, 298, 369–378 (2015). Figure 1