A Novel Electrolyte Additive for High-Voltage LiNi0.5Mn1.5O4 Positive Electrode

  The Ni-doped manganese spinel (LiNi_0.5Mn_1.5O₄, LNMO) has been projected as a high-voltage positive electrode for lithium-ion batteries (LIBs). The high working voltage (> 4.6 V vs. Li/Li⁺) with an outstanding structural stability must be beneficial with respect to energy density and cycle life, but this advantage is offset by the instability of cell constituents. For instance, the commonly used electrolytes are decomposed and surface films deposit.¹ Once surface films deposit, polarization increases to eventually cause a capacity fading.^2,3

  In order to mitigate the electrolyte decomposition/film growth, LNMO electrode should be passivated. A simple approach is the addition of film-forming agents, which decompose to form a protective layer to suppress additional electrolyte decomposition.

  As a film-forming agent for LNMO electrodes, tris(pentafluorophenyl)silane (TPFPS) was tested in this work. Addition of TPFPS (0.1 wt. %) into the conventional electrolyte (1.3 M LiPF₆in EC:EMC:DEC = 3:2:5 in vol. ratio) improves both the cycle performance and coulombic efficiency at 25°C and 60°C (Fig. 1a and 1b). TPFPS is oxidatively decomposed (< 4.7 V) prior to the electrolyte (Fig. 1c). The surface film derived from the additive shows a good passivating ability. The film growth and polarization increase are not serious. In contrast, the surface film derived from the additive-free electrolyte is poorly passivating. Surface film steadily grows due to continued electrolyte decomposition, which eventually leads to capacity fading due to ever-increasing electrode polarization (Fig. 1d). 

References

1. J. B. Goodenough and Y. Kim, Journal of Power Sources, 196, 6688 (2011).

2. D. Aurbach, B. Markovsky, Y. Talyossef, G. Salitra, H.-J. Kim and S. Choi, Journal of Power Sources, 162, 780 (2006).

3. T. Yoon, S. Park, J. Mun, J. H. Ryu, W. Choi, Y.-S. Kang, J.-H. Park and S. M. Oh, Journal of Power Sources, 215, 312 (2012).

Fig. 1. Cycle performance and coulombic efficiency at 25°C (a) and 60°C (b). The differential capacity plot in the 1^stcharging (c) and electrochemical impedance data obtained after 200 cycles (d).