(Invited) Highly Concentrated LiBF4-Based Electrolyte Solutions for LiNi0.5Mn1.5O4 Positive Electrodes in Lithium Ion Batteries

Abstract

Spinel LiNi0.5Mn1.5O4 is a promising active material for a positive electrode of lithium ion batteries because it shows extremely high positive working potentials at about 4.7 V vs. Li/Li+. However, conventional electrolyte solutions are thermodynamically unstable at such high potentials. Highly concentrated electrolyte solutions, as well as solvate ionic liquids, offer unique features, including a wide potential window owing to enhanced reductive and oxidative stability. The enhancement of oxidative stability is particularly noted for ether-based concentrated electrolyte solutions. However, ethereal solvents usually cannot tolerate such high potentials. We previously reported that carbonate ester-based concentrated electrolyte solutions showed high oxidative stability at LiNi0.5Mn1.5O4; irreversible capacities due to the oxidative decomposition of PC-based electrolyte solution decreased with increasing the concentration of LiPF6, while the discharge capacities increased. In the present work, we used the highly concentrated LiBF4-based electrolyte solutions to further improve the charge/discharge performance of LiNi0.5Mn1.5O4positive electrodes. When 0.83 mol kg-1 (ca. 1 M) LiPF6 /PC was used, the initial charge capacity (164 mAh g-1) far exceeded the theoretical capacity and the irreversible capacity was very high. As a result, the Coulombic efficiency was very low (72.1 %). These results suggest that irreversible reactions, such as the oxidative decomposition of the electrolyte solution, occurred at high potentials during charging. On the other hand, the use of concentrated 4.45 mol kg-1 electrolyte solution resulted in a decrease in the initial charge capacity to 144 mAh g-1 and the Coulombic efficiency increased to 90.4 % (Fig. 1a). Thus, the initial charge/discharge performance of a LiNi0.5Mn1.5O4 positive electrode was remarkably improved by using highly concentrated electrolyte solutions. However, the polarization in charge/discharge reactions, which was evaluated from the difference between the potentials during charging and discharging, significantly increased with electrolyte concentration, as seen in Fig. 1a. In addition, the discharge capacity rapidly decreased to 112 mAh g-1 at the 50th cycle; the capacity retention (85.6%) was not high. On the other hand, the concentration of electrolyte solution could be further increased with use of LiBF4 as an electrolyte. In the almost saturated 7.25 mol kg-1 LiBF4/PC, charge/discharge capacities in the 1st cycle were close to those obtained for 4.45 mol kg-1 LiPF6/PC, while the polarization in charge/discharge reactions decreased significantly, as shown in Fig. 1b. However, the capacity retention in the first 50 cycles was still low, as is the case in 4.45 mol kg-1 LiPF6/PC. The capacity fading was likely due to the deterioration of the positive composite electrode. To improve the cycle performance, we investigated the effects of conductive additives and a binder, which will be reported in our presentation. Acknowledgement This research is supported by the Super Cluster Program from MEXT and JST. Figure 1