CO_2 and O_2 Evolution at High Voltage Cathode Materials of Li-Ion Batteries: A Differential Electrochemical Mass Spectrometry Study

摘要

A three-electrode differential electrochemical mass spectrometry (DEMS) cell has been developed to study the oxidative decomposition of electrolytes at high voltage cathode materials of Li-ion batteries. In this DEMS cell, the working electrode used was the same as the cathode electrode in real Li-ion batteries, i.e., a lithium metal oxide deposited on a porous aluminum foil current collector. A charged LiCoO_2 or LiMn_2O_4 was used as the reference electrode, because of their insensitivity to air, when compared to lithium. A lithium sheet was used as the counter electrode. This DEMS cell closely approaches real Li-ion battery conditions, and thus the results obtained can be readily correlated with reactions occurring in real Li-ion batteries. Using DEMS, the oxidative stability of three electrolytes (1 M LiPF_6 in EC/DEC, EC/DMC, and PC) at three cathode materials including LiCoO_2, LiMn_2O_4, and LiNi_(0.5)Mn_(1.5)O_4 were studied. We found that 1 M LiPF_6 + EC/DMC electrolyte is quite stable up to 5.0 V, when LiNi_(0.5)Mn_(1.5)O_4 is used as the cathode material. The EC/DMC solvent mixture was found to be the most stable for the three cathode materials, while EC/DEC was the least stable. The oxidative decomposition of the EC/DEC mixture solvent could be readily observed under operating conditions in our cell even at potentials as low as 4.4 V in 1 M LiPF_6 + EC/DEC electrolyte on a LiCoO_2 cathode, as indicated by CO_2 and O_2 evolution. The features of this DEMS cell to unveil solvent and electrolyte decomposition pathways are also described.