Lithium Difluorophosphate and Fluoroethylene Carbonate Containing Electrolytes: How Instrumental Analytics Helps to Understand Synergistic Effects on Performance Improvement in High‐Voltage Li Battery Applications

Sequential improvement of lithium-ion batteries is an ongoing process that addresses operational factors such as high voltage, safety, and/or lifetime. Tailored electrolytes serve a critical role, as they are heavily involved in the formation of the solid electrolyte interphase (SEI) and the cathode electrolyte interphase (CEI). For high-voltage applications, a properly formed CEI is essential to prevent continuous decomposition of active material or electrolyte. Preventing transition metal deposition on the negative electrode is important, as this may trigger the rollover effect, which results in a drastic loss of capacity. Specific additives employed in the electrolyte can help prevent these side reactions. This study investigates different electrolyte formulations combining various concentration ratios of fluoroethylene carbonate (FEC) and lithium difluorophosphate (DFP). Long-term cycling at increased upper cut-off voltages revealed that not only can the rollover effect occurring for FEC and EC as cosolvents be prevented, but also increased capacity retention by the addition of DFP. Transition metal deposition is drastically reduced for the employment of both DFP and FEC as cosolvents, revealing synergistic effects. Additionally, degradation products of in situ formed VC and DFP, which counteract the negative effect of FEC and EC, are revealed via instrumental analytics.