Tailoring Electrolytes to Enable Low-Temperature Cycling of Ni-Rich NCM Cathode Materials for Li-Ion Batteries

The Ni-rich LiNixMnyCozO2 (x + y + z = 1, x > 0.5, Ni rich NMC) materials are one of the most potential cathodes for high energy density lithium-ion batteries (LIBs) due to their high specific capacity and relatively low cost. However, performances of LIBs with the Ni-rich NCM cathode below 0 degrees C are restricted by low ion conductivity of the electrolyte and a slow ion diffusion rate at the electrode-electrolyte interphase. Here, gamma-butyrolactone (GBL) with a low melting point and high ion conductivity is used to partially replace ethylene carbonate, which is conducive to lower the freezing point and increase the low-temperature ionic conductivity of the electrolyte, and the addition of GBL improves the dissolution of lithium difluoro-(oxalato)borate (LiDFOB) in a traditional carbonate solvent. Instead of lithium hexafluorophosphate (LiPF6), LiDFOB can form a F-, B-, and 0-rich interfacial phase at the Ni-rich NCM cathode, suppressing the fatal interface reaction and reducing the interface impedance. As a result, the electrolyte using GBL as the cosolvent and LiDFOB as the lithium salt can significantly improve the specific discharge capacity and cycling stability of LiNi0.8Co0.1Mn0.1O2/Li cells at 0 degrees C and -30 degrees C. At 0 degrees C, the LiNi0.8Co0.1Mn0.1O2/Li cells have a discharge specific capacity of 160 mA h g(-1) and a capacity retention rate of 99% over 100 cycles. They deliver a decent capacity at -30 degrees C. This rational design of an electrolyte via optimizing the combination of a solvent and a lithium salt has been confirmed to be a low cost but rather an effective method to improve the low-temperature performances of LIBs.