Lithium cycling efficiency of ternary solvent electrolytes with ethylene carbonate—dimethyl carbonate mixture

Abstract

Lithium cycling efficiency for ternary solvent (mol ratio 1:1:1) electrolytes of different molar conductivities containing LiPF 6 and LiClO 4 with ethylene carbonate (EC)—dimethyl carbonate (DMC) binary mixture of constant mixed ratio (mol ratio 1:1) was investigated by galvanostatic experiments at 25 °C. The solvents applied to the EC-DMC mixture are 1,2-dimethoxyethane (DME), 2-methyltetrahydrofuran (2-MeTHF) and ethylmethyl carbonate (EMC). The molar conductivity of the EC-DMC-DME ternary solvent electrolytes gradually increased with the addition of DME. However, the molar conductivities of the EC-DMC-2-MeTHF and EC-DMC-EMC ternary solvent electrolytes gradually decreased with the addition of 2-MeTHF and EMC. The decrease of the molar conductivity for these solutions is attributed to a decrease of the dissociation degree for electrolytes versus a decrease of the dielectric constant rather than that of the viscosity of the EC-DMC-2-MeTHF and EC-DMC-EMC ternary solvent mixtures. The lithium cycling efficiency of every ternary electrolyte containing LiPF 6 was larger than those of the EC-DME, EC-EMC and EC-DMC binary electrolytes containing LiPF 6 at about 20 cycles. Especially, the efficiency of LiPF 6/EC-DMC-DME electrolyte became about 80% at 40 cycles. The nickel (working) electrode surface in binary and ternary electrolytes after dissolution by cyclic voltammetry was observed by atomic force microscopy. The formation of lithium dendrite was already observed during the first cycle in the LiPF 6/EC-DMC electrolyte. However, it was found that the addition of ethers such as DME and 2-MeTHF to the LiPF 6/EC-DMC electrolyte was helpful to suppress the formation of lithium dendrite on the nickel electrode.