Stabilization of the solid electrolyte interphase (SEI) layer of the silicon (Si)−graphite composite anode for Li-ion batteries leads to an improvement of cycling performance. The SEI stabilization is achieved by utilizing a binary additive of fluoroethylene carbonate (FEC) and a fluorinated linear carbonate, di(2,2,2-trifluoroethyl)carbonate (DFDEC). The SEI composition analyses using attenuated total reflectance Fourier transform infrared (ATR FTIR) and X-ray photoelectron spectroscopy reveal that FEC alone plays a role in producing relatively more and various organic compounds including anhydride but lower concentration of inorganic salts, lowering interfacial resistances than those of conventional electrolyte and other additives. The SEI composition of silicon−graphite composite anode with FEC additive is distinguished from those reported for Si only and graphite only. The role of DFDEC alone, which possessed six fluorine atoms, is found to be the production of plenty of inorganic compounds such as Li 2 CO 3 , LiF, and OPF 3−y (OR) y /Li x PF y O z compounds, thickening the SEI layer. Blending of FEC and DFDEC results in the SEI thickening with the formation of mixtures of organic and inorganic compounds, which permits effective surface passivation of the anode, SEI robustness, and structural robustness of the silicon− graphite anode material. The anode with the binary additive of 10 wt % FEC and 1 wt % DFDEC outperforms the one in the conventional electrolyte and with FEC alone, delivering improved initial Coulombic efficiency of 84%, high discharge capacity of 742−601 mAh g −1 of a whole active material, and 81% capacity retention at the 50th cycle.

Utilization of high-voltage electrolyte additive(s) at a small fraction is a cost-effective strategy for a good solid electrolyte interphase (SEI) formation and performance improvement of a lithium-rich layered oxide-based high-energy lithium-ion cell by avoiding the occurrence of metal-dissolution that is one of the failure modes. To mitigate metal-dissolution, we explored fluorinated dual-additives of fluoroethylene carbonate (FEC) and di(2,2,2-trifluoroethyl)carbonate (DFDEC) for building-up of a good SEI in a 4.7 V full-cell that consists of high-capacity silicon-graphite composite (15 wt% Si/C/CF/ C-graphite) anode and Li 1.13 Mn 0.463 Ni 0.203 Co 0.203 O 2 (LMNC) cathode. The full-cell including optimum fractions of dualadditives shows increased capacity to 228 mAhg −1 at 0.2C and improved performance from the one in the base electrolyte. Surface analysis results find that the SEI stabilization of LMNC cathode induced by dual-additives leads to a suppression of soluble Mn 2+ -O formation at cathode surface, mitigating metal-dissolution event and crack formation as well as structural degradation. The SEI and structure of Si/C/CF/C-graphite anode is also stabilized by the effects of dual-additives, contributing to performance improvement. The data give insight into a basic understanding of cathode-electrolyte and anodeelectrolyte interfacial processes and cathode-anode interaction that are critical factors affecting full-cell performance.

Silicon-graphite composites have drawn significant attention as promising anode materials for higher energy density Li-ion batteries due to larger theoretical specific capacity of silicon than currently commercialized graphite.1 They, however, suffer from a large volume change during lithiation and delithiation, causing the particle cracking, structural degradation of the electrode, an instability of the solid electrolyte interphase (SEI) and finally leading to poor cycle life. In order to address the problems, several strategies in the aspects of improved electrical conductivity and buffering effect of volume change have been developed but the fabrication of silicon composites with carbon materials combined with the use of functional binder and electrolyte additives 2- 4 are recognized as a promising approach. In this presentation, we report the control of electrode-electrolyte interfacial reaction and the formation of a stable SEI layer using various additives, and SEI characterization using ATR FTIR spectroscopy combined with X-ray photoelectron spectroscopy. In the conventional electrolyte of 1M LiPF6/EC:EMC (3:7 volume ratio), the half-cell with Si-graphite composite anode, which is cycled between 0.01 and 1.5 V at the rate of 0.2C, shows a rapid capacity fade and a low capacity retention of 68 % at the 50th cycle. On the contrary, the cell with the designed blended additives yields improved capacity retention to 81 %, with well-maintained coulombic efficiency of higher than 99 %. Further studies of additive-dependent SEI composition and stability and their correlation to cycling performance would be discussed in the meeting. Acknowledgements This research was supported by the Korean Ministry of Trade, Industry & Energy (10049609), and Nano-Material Technology Development Program by the Ministry of Science, ICT and Future Planning (2009-0082580).   References 1. Y. Zhang, X.G. Zhang, H.L. Zhang, Z.G. Zhao, F. Li, C. Liu, and H.M. Cheng, Electrochim. Acta, 51, 4994 (2006). 2. S.-W. Song. and S.-W. Baek, Electrochem. Solid-State Lett., 12, A23 (2009). 3. J.-S. Kim, C. C. Nguyen, H.-J. Kim, and S.-W. Song, RSC Adv., 4, 12878 (2014). 4. D. T. Nguyen, J. Kang, K. M. Nam, Y. Paik, and S. W. Song, J. Power Sources, 303, 150 (2016)