August 2017 - Recent work shows promise for LiBF4 as an electrolyte salt for high voltage lithium-ion batteries. LiBF4 is more hydrolytically stable than LiPF6, 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 LiBF4 and LiPF6 have better performance in certain high voltage (4.35 V) lithium-ion cells than electrolytes with LiPF6 or LiBF4 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 LiPF6:LiBF4 and no other additives performed as well as electrolyte with LiPF6 and state-of-the-art additive blends in these cells. The synergism between LiPF6 and LiBF4 was explored using XPS, symmetric cells and EIS analysis. These show that LiBF4 reduces impedance growth at the positive electrode, while LiPF6 improves passivation at the negative electrode.