Understanding Interfacial Chemistry and Stability for Performance Enhancement of 4.55V Li-Ion Battery of LiNi0.5Co0.2Mn0.3O2//Silicon-Graphite Using FEC-Based Electrolyte
Authors: Nguyen Dan Thien , Kang Joon Sup , Nam Kyoung-Mo , Song Seung-Wan 
Published: February 2020

With increasing demand for electric and hybrid electric vehicles and energy storage systems, Li-ion batteries as their power sources are being required to store more than doubled energy density than currently used ones. Enabling the high-energy density Li-ion battery relies on high-voltage cathodes (e.g., multi-components layered oxides), high-capacity anodes (e.g., silicon (Si)-based materials) and advanced electrolyte.1 However, challenges of structural instability of cathode material, disastrous volume change and particle cracking and the instability of solid electrolyte interphase (SEI) layer of silicon anode material, and anodic instability of electrolyte at high voltage region remain. Surface coating and/or voltage control has been applied to high-voltage layered oxide cathode and silicon-based anode materials, respectively, to achieve their high-performance. Interfacial chemistry control using appropriate electrolyte composition and SEI stabilization for both cathode and anode are simpler, high impact and promising approaches for performance enhancement of a full-cell. We demonstrated that blended additives of methyl (2,2,2-trifluoroethyl)carbonate (FEMC) 2 and vinylene carbonate (VC) are effective in achieving the high-energy performance of LiNi0.5Co0.2Mn0.3O2 (NCM523)//silicon-graphite full-cells in fluoroethylene carbonate (FEC)-based electrolyte.3 Here we report the correlation of surface/SEI composition and stability to cycling performance of full-cell. The roles of solvent and additives in forming and stabilizing the surface film/SEI at cathode and anode are systematically studied using ex situ surface analysis techniques. The reasons for performance enhancement or fade are proposed in the aspects of interfacial chemistry and stability between cathode/anode and electrolyte, and interaction(s) between cathode and anode.
Lithium 2016 coin full-cells, consisting of NCM523 cathode and silicon-graphite (30:70 wt%) anode, separator, were assembled in an argon-filled glove-box. The electrolyte was 1M LiPF6/FEC:DEC (50:50, vol%) with and without FEMC and VC additives. After 100 cycles between 3.0 and 4.55V at C/3 at room temperature, the SEI layer of both cathode and anode electrode were studied using ex situ Raman, FTIR and X-ray photoelectron spectroscopy.
Figure 1 compares the cycling performance of NCM523//silicon-graphite full-cells in different electrolyte composition. With blended additives of FEMC and VC in FEC-based electrolyte, the full-cell shows a remarkable performance enhancement from EC-based one; the first discharge capacity is 168 mAh/g and initial coulombic efficiency of 86%, and capacity retention after 100 cycles is 71%. In contrast, the full-cell without additives exhibits inferior performance. Surface analyses of cathode and anode reveal that the use of blended additives induces to form a stable surface protective film/SEI at both cathode and anode, significantly reduces metal-dissolution phenomena and finally inhibits structural degradation. Further discussion would be presented in the meeting.

Acknowledgements

This research was supported by the Korean Ministry of Trade, Industry & Energy (A0022-00725 & 10049609), National Research Foundation (2012026203), Chungnam National University, and Nano-Material Technology Development Program by the Ministry of Science, ICT and Future Planning (2009-0082580).

References

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303, 150, (2016).

Figure 1