Studies of the Effect of Triphenyl Phosphate on Positive Electrode Symmetric Li-Ion Cells

The effect of triphenyl phosphate (TPP) as a flame retardant additive on the cycling behavior and AC impedance of Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 /Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 (NCA/NCA) and Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 /Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 (NMC/NMC) positive symmetric cells was studied. Symmetric cells with two identical electrodes (one was charged first and the other was fresh) were used to determine the impact of the TPP additive on the positive electrode, without the confusing effect of a second, different, electrode. The capacity retention of NCA symmetric cells with TPP was almost as good as cells without TPP when the TPP concentration was no more than 20%. At higher TPP content, impedance increase became problematic. NMC/NMC cells showed significantly worse capacity retention than NCA/NCA cells even without TPP for reasons that are, as yet, unclear. TPP additions above 20% to NMC/NMC cells again lead to dramatic increases in impedance and very poor capacity retention. The addition of 2 wt% vinylene carbonate (VC) decreased the impedance and improved the capacity retention of the NCA/NCA cells with high TPP content. Therefore, electrolytes with up to 40% TPP are compatible with NCA in the presence of 2% VC.