Erlendur Jónsson

Modern battery electrolytes: ion-ion interactions in Li+/Na+ conductors from DFT calculations.

Sodium-ion batteries, the sodium counterpart of the ubiquitous lithium-ion batteries, are currently being developed as a complementary technology to assure resource availability. As battery electrolytes tend to be one of the more limiting parts of any battery for both performance and life-length, chemical and physical data on sodium-ion battery electrolytes are important for rational development. Here the cation-anion interaction, a key property of any salt used in an electrolyte, of a number of salts is probed using numerous DFT methods via the ion-pair dissociation reaction: AlkAn ⇌ Alk(+) + An(-), where An(-) is any anion and Alk(+) is Na(+) or Li(+), the latter used here for a straight-forward literature and methodology comparison. Furthermore, the applicability of different DFT functionals for these types of calculations is benchmarked vs. a robust higher accuracy method (G4MP2).

Novel pseudo-delocalized anions for lithium battery electrolytes

A novel anion concept of pseudo-delocalized anions, anions with distinct positive and negative charge regions, has been studied by a computer aided synthesis using DFT calculations. With the aim to find safer and better performing lithium salts for lithium battery electrolytes two factors have been evaluated: the cation-anion interaction strength via the dissociation reaction LiAn ⇌ Li(+) + An(-) and the anion oxidative stability via a vertical ionisation from anion to radical. Based on our computational results some of these anions have shown promise to perform well as lithium salts for modern lithium batteries and should be interesting synthetic targets for future research.

Electrochemical oxidation stability of anions for modern battery electrolytes: a CBS and DFT study

The electrochemical stability vs. oxidation is a crucial property of anions in order to be suitable as components in lithium-ion batteries. Here the applicability of a number of computational approaches and methods to assess this property, employing a wide selection of DFT functionals, has been studied using the CCSD(T)/CBS method as the reference. In all, the vertical anion oxidation potential, ΔEv, is a fair way to calculate the stability vs. oxidation, however, a functional of at least hybrid quality is recommended. In addition, the chemical hardness, η, is identified as a novel approach to calculate the stability vs. oxidation.

Molecular simulation study of CO2 and N2 absorption in a phosphonium based organic ionic plastic crystal

An organic ionic plastic crystal (OIPC), methyl(diethyl)isobutylphosphonium hexafluorophosphate [P122i4][PF6], was investigated for CO2 and N2 absorption using molecular simulations. Ab initio calculations showed that both the cation and anion exhibit larger binding energy for CO2 compared with N2. The CO2 absorption, as calculated from classical molecular dynamics simulations, increased by a factor of 7.5 from 275 K to 325 K, while that of N2 showed low absorption at both temperatures. The simulations suggest that the significant increase in CO2 absorption at 325 K is attributed to a higher degree of disorder and increase in the free volume due to the gas/solid interfaces. While the ab initio calculations were helpful in identifying specific interaction sites on the constituent ions, the classical MD simulations elucidated the importance of interfaces in gas absorption studies in this material. The results show that the OIPC can be a promising material for CO2 separations from CO2/N2 mixture.

Understanding Fluoroethylene Carbonate and Vinylene Carbonate Based Electrolytes for Si Anodes in Lithium Ion Batteries with NMR Spectroscopy

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF6 by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by 13C-13C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using 29Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.