Identification of Redox Shuttle Generated in LFP/Graphite and NMC811/Graphite Cells

Abstract

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.

Potential and Limitations of Research Battery Cell Types for Electrochemical Data Acquisition

“…Additionally, even within the class of glass fiber separators, there are differences in the SiO 2 content so that two given glass fiber separators might not provide directly comparable cell test results. Recently, it has been found that PET containing tape (here we uses a PET‐based separator) within cells can form redox shuttle molecules, namely DMT, which cause self‐discharge of the cell [35] . This effect, however, can be negated using VC as an additive, which is the case in the present study.…”

“…Recently, it has been found that PET containing tape (here we uses a PETbased separator) within cells can form redox shuttle molecules, namely DMT, which cause self-discharge of the cell. [35] This effect, however, can be negated using VC as an additive, which is the case in the present study. The thickness of the separator and also its porosity and compressibility will directly influence the stack height and thus the cell pressure.…”

Reversible Self-discharge of LFP/Graphite and NMC811/Graphite Cells Originating from Redox Shuttle Generation

“…The initial discharge capacities of 2VC cells are mostly unaffected by the formation temperature, due to the better passivation of the negative electrode with VC (⩾165 and ⩾200 mAh for 4.06 and 4.2 V, respectively). 13,15 The reversible losses of the NMC811/AG CTRL cells in Fig. 4 are smaller than those of the LFP/AG cells in Fig.…”

“…This is consistent with Buechele et al, who saw lower FCE and thus lower discharge capacity when increasing the formation temperature beyond 40 °C. 15 The initial discharge capacities of 2VC cells also decrease with increasing formation temperature, but not as much as for CTRL cells, since VC contributes to the buildup of a well-passivating SEI, making the influence of formation temperature less apparent. 13 An additional 4.2 V hold after formation does not have a strong influence on the initial capacities (see Fig.…”

Mechanistic Understanding of Additive Reductive Degradation and SEI Formation in High‐Voltage NMC811||SiOx‐Containing Cells via Operando ATR‐FTIR Spectroscopy

“…The influence and activity of LiDFP and 2‐SBA as redox shuttle, as reported by Metzger et al., are discussed in the supporting information (Figure S8, Supporting Information). [ 36–38 ] The electrochemical performance of cells containing BE and different concentrations of additives are shown in Figures S3–S6 (Supporting Information).…”