Comparative study of electrolyte additives using electrochemical impedance spectroscopy on symmetric cells

摘要

The effect of various electrolyte additives and additive combinations added to a 1 M LiPF_6 EC:EMC electrolyte on the positive and negative electrodes surface of 1 year old wound LiCoO_2/graphite cells and Li[Ni_(0.4)Mn_(0.4)Co_(0.2)])O_2/graphite cells was studied using electrochemical impedance spectroscopy (EIS) on symmetric cells. The additives tested were: vinylene carbonate (VC), trimethoxyboroxine (TMOBX), fluoroethylene carbonate (FEC), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and H_2O alone or in combination. In general, compared to control electrolyte, the additives tested reduced the impedance of the positive electrode and increased the impedance of the negative electrode with the exception of LiTFSI in LiINi_(0.4)Mn_(0.4)Co_(0.2)]O_2/graphite wound cells. Higher charge voltage led to higher positive electrode impedance, with the exception of 2%VC + 2% FEC, and 2% LiTFSI. In some cases, some additives when mixed with another controlled the formation of the SEI at one electrode, and shared the formation of the SE1 at one electrode when mixed with a different additive.