Increased Cycle Stability of Dual Carbon Batteries By Using LiPF6 – LiTFSI Mixed Salt Electrolyte

Abstract

Rechargeable battery with large energy capacity as well as superior rate property is strongly required for electric vehicle and hybrid vehicle. Dual carbon battery which is expected as a post Li ion battery, is based on intercalation of anion and cation into graphitic carbon and advantages on cycle stability and high discharge potential (> 4 V). However, low coulomb efficiency by gas formation is the most serious issue for this battery. In this study, effects of mixing LiPF6-LiTFSI in dimethyl carbonate (DMC) for electrolyte of DCB were studied. For electrochemical measurements, graphitic carbon of KS6 (TIMCAL) and MAGD (Hitachi Kase) were used for the positive and negative electrode, respectively. For the charge/discharge investigations, a constant current rate of 0.5 mA cm−2 was applied for both charge and discharge steps, and the potential windows of 3.0 V to 5.2 V was used. LiPF6, LiTFSI and DMC of lithium battery grade (Kishida Chemical Co., Ltd., Japan) were used. The LiPF6-LiTFSI/DMC electrolyte was prepared by dissolving LiPF6, LiTFSI (battery grade) in DMC in a dry argon-filled glove box without further purification. Coin type cell was assembled in a glove box filled with a dry argon atmosphere. It was found that mixing LiPF6 and lithium bis(trifluoro methanesulfonyl)imide (LiTFSI) at 1:1 for a support salt was highly effective for increasing coulomb efficiency and cycle stability. When LiPF6-LiTFSI in DMC was used, the discharge capacity of 90 mAh/g-cathode was sustained over 300 cycles without almost no degradation. Coulomb efficiency higher than 98% was achieved after 10 cycles suggesting a decreased gas formation, and it was also found that corrosion of Al current corrector was prevented by using LiPF6-LiTFSI mixed salt. DFT calculation and NMR measurement suggests that the complex anion of (PF5-TFSI)- and F- was formed resulting in the increased stability of intercalated anion and preventing Al current collector corrosion. It was also suggested that PF5-TFSI anion shows fast diffusivity in graphitic carbon which is in good agreement with superior rate property.