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Highly Concentrated Electrolyte for 5 V Systems

Monday, 20 June 2016: 15:40
Grand Ballroom (Hyatt Regency)
M. Inaba, R. Masuhara, Y. Shimizu, M. Hashinokuchi, T. Doi (Doshisha University), H. Inoue, K. Takahashi, H. Nakagawa, and T. Inamasu (GS Yuasa International Ltd.)
Highly concentrated electrolytes have many unique properties,1-4 such as Li+-intercalation into graphite anode without EC,1,3 suppression of S82- ion dissolution in Li-S batteries, etc., and is attracting much attention of many researchers.  In highly concentrated electrolytes, all solvent molecules are strongly coordinated with Li+ ions, and hence the stability of the electrolytes against oxidation is improved significantly.  Yoshida et al. reported that LiCoO2, which is a 4 V-class cathode, can be charged and discharged in a LiTFSI/triglyme (1:1) electrolyte with good cycleability,3 though triglyme is an ether compound and usually cannot be used as solvent. 

5 V-calss cathodes, e.g. LiNi0.5Mn1.5O4 and LiCoPO4, are promising for the next-generation LIBs with high energy densities.  Unfortunately no electrolyte systems that tolerate the highly oxidative 5 V-class cathode have been reported so far.  In the present study, we investigated the effect of concentration on the stability of highly concentrated electrolytes, LiPF6/PC and LiBF4/PC, against a 5 V-class cathode, LiNi0.5Mn1.5O4 to realize 5 V-class LIBs with high energy densities.

Figure 1 compares charge/discharge curves of a 5 V class spinel LiNi0.5Mn1.5O4 in standard (0.83 mol kg-1, Li/PC = 1/11.8) and highly concentrated (4.9 mol kg-1, Li/PC = 1/2) LiBF4/PC at 30°C.  The charge/discharge rate of C/10 was employed to emphasize electrolyte decomposition. Thought the LiNi0.5Mn1.5O4 can be charged and discharged in both electrolytes, the irreversible capacity (Qirr) in 1 M LiBF4/PC was high (76 mAh g-1) because of vigorous electrolyte decomposition.  In contrast, Qirr was significantly reduced in the concentrated electrolyte, which indicated that stability against oxidation was improved in the highly concentrated electrolyte.  Similar tendency was also observed in highly concentrated LiPF6/PC electrolytes, though polarization on charging and discharging were much higher.

A fresh half-cell was fully charged to 5.0 V, and kept at 60°C for 3 days in LiPF6/PC electrolytes.  The amount of Mn deposited on lithium counter electrode was evaluated by ICP as a measure of dissolved Mn ions. The amount of dissolved Mn ions decreased with increasing Li/PC ratio, which indicated that the use of highly concentrated electrolytes is also effective for suppressing Mn ion dissolution.

This work was supported by a Kyoto Area Super Cluster Program, Japan Science and Technology Agent (JST), Japan.

References:[1] S.-K. Jeong et al., J. Power Sources, 175, 540 (2008).; [2] K. Dokko, et al., J. Electrochem. Soc., 160, A1304 (2013).; [3] K. Yoshida, et al., J. Am. Chem. Soc., 133,13121 (2011).; [4] Y. Yamada, et al., ACS Appl. Mater. Interfaces, 6, 10892 (2014).