Concentrated LiBF4/PC Electrolyte Solutions for 5 V LiNi0.5Mn1.5O4 Positive Electrode in Lithium-Ion Batteries

Abstract

Introduction Lithium-ion batteries (LIB) are used for a wide range of purposes, such as portable devices and electric vehicles. The demand for high-energy-density LIB is ever-increasing. To improve energy density of LIB, positive electrode materials working at high potentials have been widely investigated. Spinel LiNi0.5Mn1.5O4 is one of the most promising active materials for high-energy-density LIB because it works at extremely high potentials (ca. 4.7 V vs. Li/Li+). However, almost all electrolyte solutions are oxidatively decomposed at such high potentials, which results in a loss of reversible capacity and lowers the Coulombic efficiency. Recently, highly concentrated electrolyte solutions have been reported to offer unique features, such as a wide potential window and high thermal stability [1]. We have already reported that concentrated LiPF6/PC effectively suppressed the decomposition of solvent molecules, while the polarization in charge/discharge reactions much increased [2]. This issue remained to be solved before practical application. In this work, highly concentrated LiBF4/PC electrolyte solutions are used and charge and discharge properties of Li|LiNi0.5Mn1.5O4cells were investigated. Experimental The active material of LiNi0.5Mn1.5O4 was mixed with Ketjenblack (10 mass%) as a conductive additive and poly(vinylidene fluoride) (10 mass%) as a binder as a binder using 1-methyl-2-pyrrolidone to form a slurry. The slurry was spread out onto an aluminum current collector, and then dried at 80oC for 18 h under vacuum. The electrode sheets were punched out into disks of 13 mm in diameter. Charge-discharge tests were performed with constant current mode (30 μA cm-2, C/10 rate) using two-electrode coin-type cells at 30oC. The test cells were assembled in a glove box filled with Ar. Li foil served as a counter electrode. The electrolyte solutions were used 0.83 and 7.25 mol kg-1 LiBF4 dissolved in propylene carbonate (PC), together with 0.83 and 4.27 mol kg-1 LiPF6 / PC for comparison. Results and discussion Figure 1 shows initial charge-discharge curves of Li|LiNi0.5Mn1.5O4 cells in concentrated electrolyte solutions. Two plateaus appeared at around 4.7 V during charging and discharging, which is very similar to those obtained for conventional electrolyte solutions with 0.83 mol kg-1 (ca. 1 M) LiPF6. When the electrolyte concentration was 0.83 mol kg-1, initial charge capacities far exceeded the theoretical capacity of 148 mAh g-1, as summarized in Table 1, which resulted in a poor Coulombic efficiency. 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 initial charge capacities were reasonably low in 4.27 mol kg-1 LiPF6/PC and 7.25 mol kg-1 LiBF4/PC, and as a result high Coulombic efficiencies were achieved. These results clearly indicate that the oxidative decomposition of electrolyte solution was effectively suppressed by using concentrated electrolyte solutions, particularly for LiBF4/PC. In addition, the polarization in charge/discharge reactions, which was evaluated from the difference between the voltages at 50% state of charge (SOC) and 50% depth of discharge (DOD), was 33 mV in 7.25 mol kg-1 LiBF4/PC, which is much lower than that of 4.27 mol kg-1 LiPF6/PC (131 mV). In general, the large polarization occurs due to the poor ionic conductivity of the concentrated electrolyte solution with a high viscosity. However, the ionic conductivity of 7.25 mol kg-1 LiBF4/PC (0.14 mS cm-1) was lower than that of 4.27 mol kg-1 LiPF6/PC (0.40 mS cm-1). We will discuss it in our presentation. Acknowledgement This research is supported by the Super Cluster Program from MEXT and JST. Reference [1] S. Jeong et al., Electrochem. Solid-State Lett., 6, A13-A15 (2003).[2] R. Masuhara et al., 228thECS Meeting, #406, Phoenix, USA (2015). Figure 1