Highly concentrated electrolytes have many unique properties,^1-4 such as Li⁺-intercalation into graphite anode without EC,^1,3 suppression of S₈^2- 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. LiNi_0.5Mn_1.5O₄ 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, LiNi_0.5Mn_1.5O₄ to realize 5 V-class LIBs with high energy densities.

Figure 1 compares charge/discharge curves of a 5 V class spinel LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄ can be charged and discharged in both electrolytes, the irreversible capacity (Q_irr) in 1 M LiBF₄/PC was high (76 mAh g^-1) because of vigorous electrolyte decomposition.  In contrast, Q_irr 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).