The use of ethyl acetate and methyl propanoate in combination with vinylene carbonate as ethylene carbonate-free solvent blends for electrolytes in Li-ion batteries

Li[Ni0.33Mn0.33Co0.33]O2/graphite and LiCoO2/graphite pouch cells filled with a combination of ethyl acetate (EA) and vinylene carbonate (VC), or methyl propanoate (MP) and VC with 1 M LiPF6 function very well, even though these cells contain no ethylene carbonate, thought to be essential for the negative electrode solid electrolyte interphase (SEI). Cells filled with 1 M LiPF6 in MP:VC (95:5 w:w) showed similar capacity retention and impedance after 1700 h of cycling at 40 °C as cells filled with 1 M LiPF6 in EC:ethyl methyl carbonate (EMC) + 2% VC. Cells filled with these ester containing electrolytes also showed better rate performance when cycled at -14 °C, delivering 40% of the discharge energy at a current equivalent to 4 C, while cells filled with EC:EMC + 2% VC failed to deliver any capacity at a current of 3 C. High precision coulometry measurements also showed that cells filled with 1 M LiPF6 in MP:VC (95:5 w:w) had similar coulombic efficiency and charge endpoint capacity slippage as cells filled with 1 M LiPF6 EC:EMC + 2% VC which indicates ester based electrolytes can have long lifetime.