Wessel Willem Andries van Ekeren: Pioneering Triethyl Phosphate-Based Liquid Electrolytes for Safe Lithium- and Sodium-Ion Batteries: From Fundamental Insights to Practical Applications

Abstract

State-of-the-art liquid electrolytes, composed of flammable organic solvents, pose serious safety risks in lithium- and sodium-ion batteries. Non-flammable liquid electrolytes, particularly those based on phosphate solvents, offer a promising solution. However, they still lack compatibility with the most widely accepted carbonaceous anodes, due to continuous electrolyte decomposition or solvent co-intercalation. This thesis investigates triethyl phosphate (TEP) as a non-flammable electrolyte solvent for both lithium- and sodium-ion batteries. 

For lithium-ion batteries, an LHCE composed of 1.5 M LiFSI in TEP + diluent was examined. The effect of two diluents, bis(2,2,2-trifluoroethyl) ether (BTFE) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), on solvation structure and electrochemical performance was investigated. It was shown by Raman and Nuclear Magnetic Resonance (NMR) spectroscopy that addition of both diluents in LHCEs resulted in minor changes in interactions between Li+, TEP and the diluents, while maintaining a similar solvation structure as in high concentration electrolytes. In a lithium-ion full-cell, based on LiNi0.6Mn0.2Co0.2O2 (NMC622) | graphite, LHCEs prevented continuous TEP decomposition and TEP co-intercalation.

For sodium-ion batteries, the effects of ethers as co-solvents in TEP, various sodium salts, and several additives were investigated. Co-solvation of TEP with diglyme and tetraglyme using sodium hexafluorophosphate (NaPF6) and sodium tetrafluoroborate (NaBF4) salts revealed a significant trend between ionic conductivity and viscosity. NaPF6-based electrolytes showed higher ionic conductivities than NaBF4-based electrolytes, despite higher viscosity. NMR and computational studies revealed stronger interaction of the BF4 anion with the first solvation shell indicating more ion-ion interactions, leading to lower ionic conductivity. Despite promising fundamental insights in physicochemical properties, glymes were unable to prevent TEP decomposition. The analysis of five sodium salts in TEP-based electrolytes - NaBF4, NaClO4, NaDFOB, NaFSI, and NaPF6 - identified NaDFOB and NaFSI as particularly effective, especially when combined with 1 wt.% VC. Notably, NaFSI in TEP with 1 wt.% VC demonstrated a capacity retention of 77% after 600 cycles in high mass loading Prussian white (PW) | Hard carbon (HC) full-cells. XPS measurements revealed that, with this electrolyte, an effective passivation layer could be formed, which was poor in TEP decomposition products. The use of various additives was investigated in a fluorine-free NaBOB TEP electrolyte and it was found that they form effective passivation layers in high-mass loading PW | HC full-cells, even outperforming conventional carbonate-based electrolytes.

This thesis presents several strategies to effectively minimize TEP decomposition, advancing the development of safe liquid electrolytes for lithium- and sodium-ion batteries.