Highly Concentrated Electrolytes for 5 V-Class Positive Electrodes in Lithium-Ion Batteries

Introduction

There are great needs for lithium-ion batteries with higher energy density for realizing electric vehicles and hybrid electric vehicles. The energy density of lithium-ion batteries can be increased by using positive electrode materials with high capacities and/or high working potentials. Several kinds of active materials with working potentials higher than that of LiCoO₂ have been reported. Among them, spinel LiN_0.5Mn_1.5O₄ exhibits extremely positive potentials at ca. 4.7 V vs. Li/Li⁺, and is a promising positive electrode material.  However, almost all electrolyte solutions oxidatively decompose at such high electrode potentials, which results in a loss of reversible capacity and lowers the Coulombic efficiency. A number of studies have been so far done on the surface coating of LiN_0.5Mn_1.5O₄ with metal oxides.  Li-rich solid-solution layered oxides, such as Li₂MnO₃-LiMO₂ (LMNC, M=Ni, Co, Mn), are also promising active materials with high working potentials (ca. 4.8-2.0 V) and high specific capacity (ca. 320 mAh g^-1). However, LMNC is known to deteriorate upon charge/discharge cycles due to the layered-to-spinel phase transformation, in addition to the irreversible electrolyte decomposition.

In this work, highly concentrated electrolytes [1] were used for LiNi_0.5Mn_1.5O₄ and LMNC positive-electrodes and the effects of lithium-salt concentration on the charge-discharge performance were studied. Experimental

 The active material of LiNi_0.5Mn_1.5O₄ or LMNC (80 wt%) was mixed with acetylene black (10 wt%) as a conductive additive and poly(vinylidene fluoride) (10 wt%) as a binder as a binder using 1-methy-2-pyrrolidone to form a slurry. The slurry was spread out onto Al foil, and then dried at 80^oC for 18 h under vacuum. The electrode sheets were punched out into disks of 13 mm in diameter. Charge-discharge tests were performed at a constant current mode (30 mA cm^-2, C/10 rate) using two-electrode coin-type cells at 30 ^oC. The test cells were assembled in a glove box filled with Ar. Li foil served as a counter electrode. The electrolyte solutions used were 0.833 and 4.45 mol kg^-1 LiPF₆ dissolved in propylene carbonate (PC). Results and discussion

Fig. 1 shows the initial charge-discharge curves for Li | LiNi_0.5Mn_1.5O₄ cell with 0.833 mol kg^-1 LiPF₆/PC. The charge capacity (164 mAh g^-1) was higher than the theoretical capacity (148 mAh g^-1), and a high irreversible capacity (46 mAh g^-1) was observed. The irreversible capacity was mainly due to the oxidative decomposition of the electrolyte solution.

On the other hand, the initial charge capacity was 144 mAh g^-1 when highly concentrated (4.45 mol kg^-1) LiPF₆/PC was used as an electrolyte (Fig. 2), which is close to theoretical capacity. The discharge capacity in the 1st cycle was 130 mAh g^-1, and hence the irreversible capacity was 14 mAh g^-1. The discharge capacity increased by 12 mAh g^-1 higher, while the irreversible capacity decreased by 32 mAh g^-1 by using the concentrated (4.45 mol kg^-1) LiPF₆/PC system. These results suggest that oxidative decomposition of the electrolyte solution was suppressed by increasing the concentration of Li salts. Similar suppression of the electrolyte decomposition was also observed for LMNC positive-electrodes. Therefore, the use of highly concentrated electrolytes is effective for the high-potential positive-electrodes in lithium ion batteries.

However, the overpotential in charge/discharge reactions, which was estimated from the difference between the potentials at 50% state of charge (SOC) and 50% depth of discharge (DOD), was as high as 0.205 V for the concentrated electrolyte of 4.45 mol kg^-1 LiPF₆/PC, as shown in Fig.1. This value was much higher than that for 0.833 mol kg^-1 LiPF₆/PC (0.030 V). The large overpotential is likely due to a poor ionic conductivity of the concentrated electrolyte solution with a high viscosity. This issue remains to be solved before practical application, and is currently under investigation. Reference

1) S. Jeong et al., Electrochem. Solid-State Lett., 6(1) A13-A15  (2003).