High-potential positive electrodes, such as LiNi_0.5Mn_1.5O₄ and LiCoPO₄, are promising to enhance the energy density of lithium-ion batteries.  However, at the high potentials of ca. 4.7 V vs. Li/Li⁺, conventional electrolyte solutions are thermodynamically unstable to be decomposed oxidatively.  Highly concentrated electrolyte solutions, including solvate ionic liquids, have been reported to offer many unique features, including a wide potential window owing to enhanced reductive and oxidative stability. ^1,2  These effects are particularly noted for ether-based concentrated electrolyte solutions, while not so remarkable for carbonate ester-based ones.³  In the present work, propylene carbonate (PC)-based concentrated electrolyte solutions were used to suppress the oxidative decomposition on LiNi_0.5Mn_1.5O₄positive electrodes.  Here PC was employed as a solvent because PC dissolves a larger amount of Li salts than EC or EC-based solvents.

When 4.45 mol kg^-1 LiPF₆ /PC (nearly saturated) was used, the initial irreversible capacity decreased to 14 mhA g^-1 and the discharge capacity increased to 130 mAh g^-1, compared to those obtained for 0.83 mol kg^-1 (ca. 1 mol dm^-3) LiPF₆ /PC.  These results clearly indicate that the oxidative decomposition of electrolyte solution was effectively suppressed, and that the initial charge/discharge performance of LiNi_0.5Mn_1.5O₄ positive electrodes was significantly improved by employing highly concentrated electrolyte solutions.  However, the discharge capacity decreased rapidly to 124 mAh g^-1 in the first 4 cycles, and then decreased gradually to 112 mAh g^-1 at the 50th cycle.  This value was higher than those obtained in 0.83 mol kg^-1 LiPF₆ /PC, but the capacity retention (85.6%) was lower.  Thus, the oxidative stability of the electrolyte solution was improved with increasing concentration, but the capacity fading was more marked than those in 0.83 mol kg^-1 LiPF₆ /PC.  We hence assumed that either the conductive additive of acetylene black or the binder is degraded.  To prove this assumption, we used a graphitized Ketjenblack conductor, which was heat-treated at 2200 ^oC and has a high oxidative stability, in place of the acetylene black conductor. As a result, the cycleability of LiNi_0.5Mn_1.5O₄ was greatly improved when Ketjenblack was used as a conductor in 4.27 mol kg^-1 LiPF₆ /PC (Fig.1a); the discharge capacity of 123 mAh g^-1 was obtained even at the 50th cycle, corresponding to 92.3 % of the initial capacity (Fig.1b).  In the concentrated electrolyte solution, the oxidative decomposition of solvent molecules was greatly suppressed, and hence the degradation of the carbon conductor became a more critical factor for cycleability.  In the present study, it was found that the decomposition of solvent molecules can be effectively suppressed by the use of concentrated electrolyte solutions without free PC molecules.  However, not only the stability of the electrolyte solution, but also the stability of the other components such as carbon conductor, binder, separator, etc. against oxidation is also important for realizing practical 5-V lithium-ion batteries.

Acknowledgments: The authors wish to thank Ms. Hidemi Inoue, Ms. Hiroe Nakagawa, and Dr. Tokuo Inamasu in GS Yuasa International Ltd. for their helpful discussions.  This research is partially supported by “the Super Cluster Program” from MEXT and JST and “Kyoto Regional Scientific Innovation Hub” from MEXT.

References: [1] T. M. Pappenfus et al., J. Electrochem. Soc., 151 (2004) A209.; [2] L. Suo et al., Science, 350 (2015) 938.; [3] D. W. McOwen et al., Energy Environ. Sci., 7 (2014) 416.