Articles:
iopscience.iop...
iopscience.iop...
Abstracts:
Unwanted self-discharge of #LFP/AG and #NMC811/AG cells can be caused by in situ generation of a #redox shuttle molecule after formation at elevated temperature with common alkyl carbonate electrolyte. This study investigates the redox shuttle generation for several electrolyte additives, e.g., vinylene carbonate and lithium difluorophosphate, by measuring the additive reduction onset potential, first cycle inefficiency and gas evolution during formation at temperatures between 25 and 70 °C. After formation, electrolyte is extracted from pouch cells for visual inspection and quantification of redox shuttle activity in coin cells by cyclic voltammetry. The redox shuttle molecule is identified by GC-MS and NMR as dimethyl terephthalate. It is generated in the absence of an effective SEI-forming additive, according to a proposed formation mechanism that requires residual water in the electrolyte, catalytic quantities of lithium methoxide generated at the negative electrode and, surprisingly, polyethylene terephthalate tape within the cell.
Unwanted parasitic reactions in lithium-ion cells lead to self-discharge and inefficiency, especially at high temperatures. To understand the nature of those reactions this study investigates the open circuit storage losses of LFP/graphite and NMC811/graphite pouch cells with common alkyl carbonate electrolytes. The cells perform a storage test at 40 °C with a 500 h open circuit period after formation at temperatures between 40 °C and 70 °C. Cells formed at elevated temperature showed a high reversible storage loss that could be assigned to a redox shuttle generated in the electrolyte during formation. A voltage hold after formation can reduce the shuttle-induced self-discharge as indicated by significantly lower reversible storage losses, the absence of shuttling currents in cyclic voltammetry and improved metrics in ultra-high precision cycling. The addition of two weight percent vinylene carbonate can prevent redox shuttle generation and leads to almost zero reversible self-discharge.
Sebastian Büchele is a research associate at the Institute for Applied Materials at KIT and previously conducted research at Dalhousie University in Canada
25 авг 2024
