The interfacial origin of performance improvement and fade of high-voltage cathodes of LiNi 0.5 Co 0.2 Mn 0.3 O 2 for high-energy lithium-ion batteries has been investigated. Performance improvement was achieved through interfacial stabilization using 5 wt % methyl (2,2,2-trifluoroethyl) carbonate (FEMC) of fluorinated linear carbonate as a new electrolyte additive. Cycling with the FEMC additive at 3.0−4.6 V versus Li/Li + results in the formation of a stable solid electrolyte interface (SEI) layer and effective passivation of cathode surface, leading to improved cycling performance delivering enhanced discharge capacities to 205−182 mAhg −1 and capacity retention of 84% over 50 cycles. The SEI layer notably includes plenty of metal fluorides and −CF-containing species formed by additive decomposition. On the contrary, the origin of performance fade in electrolyte only was ineffective surface passivation and dissolution of metal elements, which leads to oxygen loss, surface structural degradation and crack formation at the LiNi 0.5 Co 0.2 Mn 0.3 O 2 particles. The data provide a basic understanding of the interfacial stabilization mechanism on high-voltage layered oxide cathodes.

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High-capacity Li-rich layered composite oxide, xLi 2 MnO 3 • (1-x)LiMO 2 (M = Mn, Ni, Co), is a promising candidate cathode material for high-energy electrochemical energy storage. Enabling the high-performance of high-voltage cathode relies on an electrolyte breakthrough and the solid electrolyte interface (SEI) stabilization. In this study, the 0.6Li 2 MnO 3 • 0.4LiNi 0.45 Co 0.25 Mn 0.3 O 2 (Li 1.2 Mn 0.525 Ni 0.175 Co 0.1 O 2 , LMNC) cathode is operated at 2.5-4.8 V with 5 wt% fluorinated linear carbonate, di-(2,2,2 trifluoroethyl)carbonate (DFDEC), as a high-voltage electrolyte additive, for the first time and applied to a high-energy lithium-ion battery. The cathode with DFDEC outperforms that in electrolyte only, delivering a high capacity of 250 mAhg −1 with an excellent chargedischarge cycling stability at the rate of 0.2C. Upon the use of DFDEC, the cathode surface is effectively passivated by a stable SEI composed of DFDEC decomposition products, which inhibit a detrimental metal dissolution and structural cathode degradation. A full-cell based on the SEI-stabilized LMNC cathode and graphite anode successfully demonstrates doubled energy density (∼278 Whkg −1 ) compared to ∼136 Whkg −1 of a commercialized cell of graphite//LiCoO 2 and an excellent cycling stability.

The ammonia-free coprecipitation process using citric acid as a new chelating agent successfully produced a micro-sized spherical Ni0.5Co0.2Mn0.3(OH)2 precursor and a well-performed cathode active material LiNi0.5Co0.2Mn0.3O2 for Li-ion batteries.

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Increasing demand of high-energy lithium-ion batteries for being adopted in electric vehicles and energy storage systems drives the development of high-voltage and high-capacity cathode together with functional electrolyte with high anodic and thermal stabilities. Lithium-rich layered material of xLi2MnO3-(1-x)Li(Ni, Co, Mn)O2 is an attractive cathode material because of their higher capacity than 200 mAh/g. Its performance however is often difficult to be achieved, in particular, at high voltage operation (> 4.3 V vs. Li/Li+), due to severe oxidative decomposition of conventional electrolyte consisting of LiPF6 salt and non-aqueous carbonate-based organic solvents. Operation of the cathode at higher voltage than 4.6 V can lead to a further increase in the capacity. Extensive research efforts have been made to develop appropriate electrolyte components with high anodic stability but yet to be established. We have been screening and evaluating a number of fluorinated carbonates as electrolyte additives for high-performance operation of 4.8 V Li1.2Mn0.525Ni0.175Co0.1O2 cathode. The use of a low fraction additive comprises low cost, compared to solvent. Here we present the first report on a new fluorinated carbonate as a high-performance electrolyte additive for 4.8 V Li1.2Mn0.525Ni0.175Co0.1O2 cathode operated at a wide temperature range. The SEI formation mechanism, composition and stability, and their relation to high-voltage cycling performance, and cycling performance of full-cells are discussed. The Li1.2Mn0.525Ni0.175Co0.1O2 cathode material was synthesized at 900 oC in air using the carbonate coprecipitate precursor. The crystal structure of coprecipitate precursor and cathode material were identified by X-ray diffraction analysis, measured in the 2θ range of 10 - 80o with the scan rate of 2o/min. Lithium coin cells, consisted of Li1.2Mn0.525Ni0.175Co0.1O2 as a working electrode, a lithium foil as counter electrode and the electrolyte of 1M LiPF6/EC:EMC (3:7 volume ratio) with 5 wt% additive of fluorinated carbonate was assembled in the Ar-filled glove box. The 2016 coin half- and full-cells were evaluated for their cycling ability at C/5 rate between 2.5 and 4.8 V. AC impedance spectra were also collected during cycling. For characterization of solid electrolyte interface (SEI) composition, attenuated total reflection FTIR combined with X-ray photoelectron spectroscopic (XPS) analyses were conducted. Figure 1a compares the cycling ability of Li1.2Mn0.525Ni0.175Co0.1O2 cathode without and with additive. With additive, the initial charge and discharge capacities are 350 and 256 mAh/g, respectively, with initial coulombic efficiency of 73%. The cathode delivers the capacity retention of 89% with the discharge capacity of 227 mAh/g at the 50th cycle. On the contrary, without additive, inferior capacities of 222 – 156 mAh/g and capacity retention of 70% over 50 cycles are observed. The use of additive is found to be very effective in enabling high-voltage cycling performance of Li1.2Mn0.525Ni0.175Co0.1O2 cathode. Our spectroscopic surface chemistry studies suggest that with additive, the cathode surface is effectively passivated with a stable SEI layer with maintained surface cathode structure (Figure 1b-iii), leading to a suppressed change in charge transfer resistance with cycling. On the contrary, the occurrence of surface structural degradation by the formation of dissolvable Mn2+proably followed by oxygen loss is observed when cycled without additive (Figure 1b-ii). Further discussion of the SEI formation mechanism and stability, their correlation to interfacial resistance and cycling performance, and the cycling performance of full-cells would be presented in the meeting. Acknowledgements: This research was financially supported by the Korean Ministry of Education and National Research Foundation (2012026203) and by the Ministry of Trade, Industry & Energy (A0022-00725).

Advanced portable electronics, electric vehicles and energy storage systems need more than doubled energy density lithium-ion battery than current one consisting of graphite anode and LiCoO2 cathode. High-capacity silicon-based anode materials and high-voltage three-components layered oxide cathode materials have been actively researched for higher energy density batteries. Various methods for improving the cycling performance of silicon-based anode have been developed but mostly evaluated using a half-cell configuration.1,2 Just few reports on the performance of high-energy full-cells with silicon-based anode together with high-voltage cathode have been reported.3–5 This is associated with poor cycling ability of silicon-based anodes and continuous oxidative electrolyte decomposition above 4.3 V vs. Li/Li+. Nonetheless, interfacial reaction mechanisms for performance fade or enhancement of full-cells have not been studied in depth. Interfacial control with electrolyte components and surface structural stabilization of both anode and cathode are promising approaches for enhancement of full-cell performance.6,7 In this presentation, we report the enhanced cycling performance of high-energy full-cell with silicon-carbon composite anode and high-voltage layered LiNi0.5Co0.2Mn0.3O2 cathode (NCM) by interfacial control and the role of electrolyte component in enhancing the performance. The 2016 coin full-cells of NCM//silicon-carbon composite were assembled with different electrolyte compositions. Cycling performance of full-cells was tested between 3.0 and 4.55 V at the rate of C/3 (330 mAg-1) for 100 cycles. Impedance spectroscopy, Raman spectroscopy, FT infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscope have been used to analyze electrochemical behaviors depending on electrolyte composition and surface chemistry of both cathode and anode. Figure 1a-b compare the cycling performance of NCM//silicon-carbon composite full-cells in the conventional electrolyte of 1M LiPF6/EC:EMC (3:7) and a new electrolyte consisting of new solvents and additives. With the conventional electrolyte, the full-cell shows a rapid capacity fade and a low coulombic efficiency over 100 cycles. In contrary, the full-cell with a new electrolyte exhibits a significant performance enhancement; capacity retention is 71 % at the 100th cycle, and coulombic efficiency is maintained as higher than 99 %. Surface analyses of cathode and anode reveal that with new electrolyte a stable solid electrolyte interphase (SEI) layer forms at both surfaces of cathode and anode and a metal dissolution event from cathode is inhibited. Further discussion on interfacial control-performance relationship would be presented in the meeting. Acknowledgements This research was supported partly by the Korean Ministry of Education (2012026203) and partly by the Korean Ministry of Trade, Industry & Energy (A0022-00725 & 10049609). References 1. N. Dimov, S. Kugino, and M. Yoshio, J. Power Sources, 136, 108 (2004). 2. M. Gauthier, D. Mazouzi, D. Reyter, B. Lestriez, P. Moreau, D. Guyomard, and L. Roué, Energy Environ. Sci., 6, 2145 (2013). 3. S. J. Lee, J.G. Han, Y. Lee, M.H. Jeong, W. C. Shin, M. Ue, and N.S. Choi, Electrochim. Acta, 137, 1 (2014). 4. X. Zhao, M. Li, K.H. Chang, and Y.M. Lin, Nano Res., 7, 1429 (2014). 5. L. Ji, H. Zheng, A. Ismach, Z. Tan, S. Xun, E. Lin, V. Battaglia, V. Srinivasan, and Y. Zhang, Nano Energy, 1, 164 (2012). 6. J.S. Kim, C.C. Nguyen, H.J. Kim, and S.W. Song, RSC Adv., 4, 12878 (2014). 7. Y.M. Lee, K. M. Nam, E.H. Hwang, Y. K. Kwon, D. H. Kang, S.S. Kim, and S.W. Song, J. Phys. Chem. C, 118, 10631(2014). Figure 1

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