Synergistic Effect of LiPF₆and LiBF₄as Electrolyte Salts in Lithium-Ion Cells

Recent work shows promise for LiBF 4 as an electrolyte salt for high voltage lithium-ion batteries. LiBF 4 is more hydrolytically stable than LiPF 6 , and in some cases has proved to be more stable at high voltage. This work shows that 1.0 M electrolytes consisting of equal parts LiBF 4 and LiPF 6 have better performance in certain high voltage (4.35 V) lithium-ion cells than electrolytes with LiPF 6 or LiBF 4 alone. This is shown using long-term cycling and ultra-high precision coulometry on lithium-ion cells, with and without electrolyte additives. Ultra-high precision coulometry showed that electrolyte with 1:1 LiPF 6 :LiBF 4 and no other additives performed as well as electrolyte with LiPF 6 and state-of-the-art additive blends in these cells. The synergism between LiPF 6 and LiBF 4 was explored using XPS, symmetric cells and EIS analysis. These show that LiBF 4 reduces impedance growth at the positive electrode, while LiPF 6 improves passivation at the negative electrode.