High-energy performance and interfacial phenomena of 55 ^○C full-cells with Li-rich layered oxide cathode and graphite anode under high charge cut-off voltage of 4.7V are reported. Anodic and thermal instabilities of conventional electrolyte are the critical issues to limit high-voltage and high-temperature cell operation. Our recent research results have shown that the use of functional additives enables the operation of Li-ion cells in such harsh condition. Surface analysis data indicated that structural degradation of both cathode and anode materials and instability of surface film at cathode were main reasons for the rise of interfacial resistance in the conventional electrolyte. Nonetheless, those problems can be resolved through interfacial stabilization using appropriate electrolyte additives. The surface film/ solid electrolyte interphase (SEI) layer formation and composition, and their correlation to high-voltage cycling performance are discussed.

       Cathode active material of Li_1.13Mn_0.463Ni_0.203Co_0.203O₂ (LMNC) was synthesized by solid state method at 900 ^○C using the precursor of (Mn_0.463Ni_0.203Co_0.203)CO₃ coprecipitate and LiOH.H₂O in the air. Lithium 2016 coin full-cells, consisting of Li_1.13Mn_0.463Ni_0.203Co_0.203O₂ cathode, graphite anode, the electrolyte of 1M LiPF₆/EC:EMC as the base electrolyte and with blended additives of 5 wt% di-(2,2,2 trifluoroethyl)carbonate (DFDEC) and 3 wt% vinylene carbonate (VC) were assembled in an argon-filled glove box. The full-cells were tested between 2.5 and 4.7 V (4.75 V vs. Li/Li⁺) at the rate of C/5 at 55 ^○C in the constant temperature chamber. For the characterization of surface/SEI composition, attenuated total reflection FTIR combined with X-ray photoelectron spectroscopic (XPS) analyses were conducted.

       Fig. 1 compares the cycling ability of Li_1.13Mn_0.463Ni_0.203Co_0.203O₂//graphite full-cells at 55 ^○C in the base electrolyte only and with blended additives of DFDEC-VC between 2.5 and 4.7 V at the rate of C/5. In the base electrolyte, the full-cell exhibits initial charge and discharge capacities of 320 and 214 mAhg⁻¹, respectively, with initial coulombic efficiency of 67 %. Discharge capacity decreases to 80 mAhg⁻¹ after 50 cycles, yielding very low capacity retention of 37 % and poor coulombic efficiency. On the contrary, with blended additives, significantly improved cycling performance is achieved; initial charge and discharge capacities are 323 and 227 mAhg⁻¹, respectively, corresponding to initial coulombic efficiency of 71 %, and improved capacity retention of 77 % at the 50th cycle, delivering discharge capacities of 227−174 mAhg⁻¹. The detailed studies of surface composition and formation mechanism, and their relation to high-voltage and high-temperature cycling performance would be presented in the meeting.

Acknowledgements

This research was supported by the Korean Ministry of Trade, Industry & Energy (A0022-00725 & R0004645), Chungnam National University, and Nano Material Technology Development Program through the National Research Foundation funded by the Ministry of Science, ICT and Future Planning (2009-0082580).