A LiNi_0.8Co_0.1Mn_0.1O₂ positive-electrode exhibits a high initial discharge capacity of ca. 200 mAh g^-1 by charging up to 4.3 V vs. Li/Li⁺ [1]. However, the cycle performance is poor in conventional electrolyte solutions due to the oxidative decomposition of the electrolyte at such a high potential. In our previous work, the charge/discharge cycle performance of LiNi_0.8Co_0.1Mn_0.1O₂ was greatly improved in the nearly saturated 8.67 mol kg^-1 LiBF₄/dimethyl carbonate (DMC) electrolyte solution, which has low DMC/Li molar ratio of 1.28. Unfortunately, the high viscosity and low ionic conductivity restrict its practical use. In this work, we applied fluorinated ester, i.e. methyl 3,3,3,-trifluoropropionate (FMP), as a diluent to decrease the viscosity. Then, we replaced a part of DMC in LiBF₄/DMC+FMP with propylene carbonate (PC) to improve its stability against oxidation. LiNi_0.8Co_0.1Mn_0.1O₂ showed excellent cycle durability in the resultant ternary system of LiBF₄/PC+DMC+FMP electrolyte solution.

The slurry consisted of 80 wt% LiNi_0.8Co_0.1Mn_0.1O₂ powder, 10 wt % carbon conductive agents (Ketjenblack) as a conductive additive, and 10 wt % polyvinylidene fluoride (PVdF) as a binder in an N-methyl-2-pyrrolidone (NMP) was coated onto Al foil. The sheet was dried at 80 ℃ for 18 h under vacuum, and punched into 13 mm diameter discs. Then, the disc electrodes were used to assemble two-electrode coin-type cells with Li foil as a counter electrode. The electrolyte solutions used in this study were 2.5 mol kg^-1 LiBF₄/DMC+FMP (1:2 by volume, DMC/Li=1.28, 11.4 mPa s) and 2.5 mol kg^-1 LiBF₄/PC+DMC+FMP (1:3:8 by volume, PC+DMC/Li=1.28, 14.5 mPa s). Li|LiNi_0.8Co_0.1Mn_0.1O₂ half-cells were prepared by using these electrolyte solutions and cycled galvanostatically at 0.1 C (1 C=275 mAh g^-1) between 3.0 and 4.3 V at 30^oC.

In 2.5 mol kg^-1 LiBF₄/DMC+FMP, the discharge capacity decreased to 88.0 % of the initial one in 50 charge/discharge cycles. On the other hand, 96.5% of the initial discharge capacity (195 mAh g^-1) was maintained even at the 50th cycle in 2.5 mol kg^-1 LiBF₄/PC+DMC+FMP. The average coulombic efficiency (99.6 %) in the 50 cycles was higher than that obtained for 2.5 mol kg^-1 LiBF₄/DMC+FMP (99.5%). The Raman spectra suggests that adding FMP to dilute the saturated LiBF₄/DMC has damage to the solvate structure and leads to the increase of free solvate. However, the PC+DMC+FMP-based electrolyte solution is stabilized against oxidation by forming aggregates (AGGs) [2], which should result in improvement of charge/discharge cycle performance of LiNi_0.8Co_0.1Mn_0.1O₂ positive-electrode.

References:

[1] Y. Bi et al., J. Power Sources, 283 (2015) 211–218.

[2] T. Doi et al., Sustainable Energy Fuels 2 (2018) 1197–1205.