Synergistic Effect of LiPF 6 and LiBF 4 as Electrolyte Salts in Lithium-Ion Cells

Abstract

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. Recent work has shown that LiBF 4 may be a promising alter-native salt, or co-salt, for high voltage lithium ion cells. 1–9 LiBF 4 has a similar chemical structure to LiPF 6 , which is the best-known and most widely used electrolyte salt for lithium-ion batteries. Both these salts have different advantages and disadvantages in lithium-ion cells. LiPF 6 has high solution conductivity in carbonate solvents, and helps passivate the negative electrode SEI. However, LiPF 6 is known to have poor thermal and hydrolytic stability. 10,11 Decompo-sition of LiPF 6 results in the formation of reactive species, notably HF and PF 5 , which are thought to cause a cascade of undesirable side-reactions inside the cell. 10–12 LiBF 4 has often been studied as a substitute for LiPF 6 in lithium-ion cells because of its improved thermal and hydrolytic stability. 3,12–17 LiBF 4 has demonstrated sev-eral other important advantages over LiPF 6 : improved performance at sub-zero temperatures, improved passivation of the Al current col-lector, and improved performance at the positive electrode.