Dilution Effects of Highly Concentrated Dimethyl Carbonate-Based Electrolyte Solutions for LiNi0.8Co0.1Mn0.1O2 Positive Electrode

Abstract

Introduction Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) 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 severe crack formation owing to repeated shrinkage/expansion during cycling. In our previous study, we found that crack formation in NCM811 particles is remarkably suppressed in highly concentrated LiBF4/DMC electrolyte solution, and good cycle performance is obtained with low irreversible capacities up to the 50th cycle [2]. However, high concentration caused a high viscosity, low ionic conductivity and high cost, which limit the use in practical cells. To reduce the high viscosity, we chose 1,1,2,2–tetrafluoroethyl 2,2,3,3–tetrafluoropropyl ether (HFE) to dilute the highly concentrated electrolyte solution. We also discussed the effects of lithium salts by comparing the results of diluted LiBF4/DMC and LiPF6/DMC. Experimental A slurry consisting of 80 wt% LiNi0.8Co0.1Mn0.1O2 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°C for 18 h under vacuum, and punched into 13 mm-diameter discs. 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 8.67 mol kg-1 LiBF4/DMC (226 mPa s, molar ratio of DMC/Li = 1.28 ) and 0.96 mol kg-1 LiBF4/DMC+HFE (1:2 by volume, 1.46 mPa s, DMC/Li = 3.00), 5.05 mol kg-1 LiPF6/DMC (27.8 mPa s, DMC/Li = 2.2) and 0.96 mol kg-1 LiPF6/DMC+HFE (1:2 by volume, 1.72 mPa s, DMC/Li = 3.00). Li| NCM811 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°C. Results and Discussion Raman spectra (900~970 cm-1) of these electrolyte solutions are shown in Fig. 1. When the 8.67 mol kg-1 LiBF4/DMC was diluted with HFE (Fig. 1(a)), the content of free DMC molecules remarkably increased in 0.96 mol kg-1 LiBF4/DMC+HFE (DMC/Li = 3.00). Because a low miscibility between HFE and concentrated LiBF4/DMC, more diluted solutions with lower molar ratio of DMC/Li could not be obtained. The content of free DMC molecules in 0.96 mol kg-1 LiBF4/DMC+HFE was higher than that in 3.70 mol kg-1 LiBF4/DMC though the DMC/Li ratios were the same (3.00) for both solutions. This fact suggested that HFE has negative influence on the solvation structure in LiBF4/DMC electrolyte solution. On the other hand, when 5.05 mol kg-1 LiPF6/DMC was diluted with HFE, the content of free DMC molecules only slightly increased in 0.96 mol kg-1 LiPF6/DMC+HFE (DMC/Li = 3.00). The content of free DMC rarely changed when compared with 3.70 mol kg-1 LiPF6/DMC (DMC/Li = 3.00) in Fig. 1(b). That is because LiPF6 salt has a highly dissociative, and DMC molecules are more tightly coordinated with Li+ in LiPF6/DMC. Hence the addition of HFE has little influence on the solvated structure. Figure 2 shows the variations of the discharge capacity and coulombic efficiency with cycle number in these electrolyte solutions. In 8.67 mol kg-1 LiBF4/DMC, 93.3 % of the initial discharge capacity was maintained at the 50th cycle. However, the capacity decreased to 76.9 % at the 50th cycle in 0.96 mol kg-1 LiBF4/DMC+HFE. The average coulombic efficiency (99.5 %) for the 50 cycles was lower than that obtained in 8.67 mol kg-1 LiBF4/DMC (99.7 %). These results suggest that more free DMC was oxidized in LiBF4/DMC diluted with HFE. On the other hand, Li|NCM811 cells using 0.96 mol kg-1 LiPF6/DMC+HFE exhibited a good cycling performance. Compared with the data in 5.05 mol kg-1 LiPF6/DMC, the discharge capacity increased and the average coulombic efficiency was also higher. This is due to the solvation structure in the concentrated solution was maintained after diluted with HFE, by which the high stability against oxidative decomposition was not affected. Further, the overpotential of electrode was suppressed in diluted LiPF6/DMC+HFE. Figure 3 shows cross-sectional SEM images of NCM811 particles after cycled. The crack formation was more serious in 0.96 mol kg-1 LiBF4/DMC+HFE (a) than 8.67 mol kg-1 LiBF4/DMC (b). In contrast, particle fracture was inhibited in 0.96 mol kg-1 LiPF6/DMC+HFE (c) and the crack propagation within the particle was similar with 5.05 mol kg-1 LiPF6/DMC (d). References : [1] Y. Bi et al., Power Sourc es, 283 (2015) 211–218. [2] Y. Cao et al., J. Electrochem Soc., 166 (2019) 82-88. Figure 1