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

Abstract

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 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.

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

“…2 The sensitivity of reversible self-discharge to cell voltage correlates with the observation that low voltage LFP/AG cells without electrolyte additives showed significantly more reversible self-discharge than higher voltage NMC811/AG cells without electrolyte additives. 2 The large currents needed to explain the rapid self-discharge at elevated temperature pointed towards a reversible shuttle that is generated in the battery cells. 2 Other groups also ascribed reversible selfdischarge to a shuttle reaction.…”

“…2 Buechele et al also showed that a voltage hold at 4.2 V after formation is effective at reducing reversible self-discharge in LFP/AG cells without electrolyte additives. 2 The sensitivity of reversible self-discharge to cell voltage correlates with the observation that low voltage LFP/AG cells without electrolyte additives showed significantly more reversible self-discharge than higher voltage NMC811/AG cells without electrolyte additives. 2 The large currents needed to explain the rapid self-discharge at elevated temperature pointed towards a reversible shuttle that is generated in the battery cells.…”

“…1 Buechele et al then performed systematic open circuit storage experiments with LFP/AG and NMC811/AG cells and assigned the capacity loss during storage to irreversible and reversible self-discharge. 2 They found that reversible self-discharge accounts for the majority of capacity loss in cells without electrolyte additives, that it correlates strongly with formation temperature, is lower with lithium bis(fluorosulfonyl)imide (LiFSI) conducting salt, and is suppressed if vinylene carbonate (VC) is used as electrolyte additive. 2 Buechele et al also showed that a voltage hold at 4.2 V after formation is effective at reducing reversible self-discharge in LFP/AG cells without electrolyte additives.…”

“…2 They found that reversible self-discharge accounts for the majority of capacity loss in cells without electrolyte additives, that it correlates strongly with formation temperature, is lower with lithium bis(fluorosulfonyl)imide (LiFSI) conducting salt, and is suppressed if vinylene carbonate (VC) is used as electrolyte additive. 2 Buechele et al also showed that a voltage hold at 4.2 V after formation is effective at reducing reversible self-discharge in LFP/AG cells without electrolyte additives. 2 The sensitivity of reversible self-discharge to cell voltage correlates with the observation that low voltage LFP/AG cells without electrolyte additives showed significantly more reversible self-discharge than higher voltage NMC811/AG cells without electrolyte additives.…”

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][37][38] The electrochemical performance of cells containing BE and different concentrations of additives are shown in Figures S3-S6 (Supporting Information).…”