Effect of Electrolyte Compositions on Cycling Performance of Li-Ion Full Cells with Si-Graphite Composite Electrodes

Abstract

The demand for high energy densities in Li-ion batteries (LIBs) necessitates exploring bottlenecks of cell performance. Layered oxide positive electrodes, such as LiNixCoyMn1-x-yO2 (NCM), and silicon-graphite composite (Si-Gr) negative electrodes in a LIB undergo a series of complex structural and compositional changes during electrochemical cycling. The induced changes have a direct influence on the energy and power output from a battery and a profound dependence on the cycling or aging conditions. The promise of multifold enhancement in energy densities of LIBs using Si based negative electrodes is accompanied by the limited stability of the electrode-electrolyte interface. In commercial LIBs, the solid electrolyte interphase (SEI) layer on the graphite negative electrode facilitates Li+ ion conduction and minimizes reduction of the electrolyte. In cells with Si- containing negative electrodes, the large volume changes of Si during cycling results in continuous degradation of the SEI layer. The formation of a new SEI on the newly-exposed Si surfaces during subsequent cycling causes further trapping of Li+ ions, contributing to performance loss. In the present study, full cells with NCM 523 cathode, Si-Gr (containing 15 wt% Si) anode, and a baseline electrolyte (1.2 M LiPF6 in Ethylene Carbonate/Ethyl Methyl Carbonate, 3:7 w/w) were tested under calendar life ageing and cycle life ageing protocols between 3 and 4.1 V. The cycle-life cells showed ~92% capacity loss after 100 cycles, while the calendar-life cells showed ~13% capacity loss; the reasons for this difference will be highlighted during the presentation.1 In order to further improve capacity retention we studied the effect of various additives to the baseline electrolyte, and also examined EC-free electrolyte systems. Advanced diagnostic techniques, including X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) were used to examine the effect of these additives on the SEI layer.2,3 Insights from electrochemical response in conjunction with post-mortem characterization studies have fostered deeper understanding of the charge transfer mechanisms and the properties of the interfacial layers in the blended negative electrodes. Fundamental knowledge of the nature of this dynamic SEI layer is essential to design better surface films and achieve stable electrochemical cycling. Support from David Howell, Brian Cunningham, and Peter Faguy of the U.S. Department of Energy’s Office of Vehicle Technologies is gratefully acknowledged. This work was performed under the auspices of the US Department of Energy, Office of Vehicle Technologies, Hybrid and Electric Systems, under Contract No. DE-AC02-06CH11357 K. Kalaga et al., manuscript in preparation (2017). K. Kalaga, I.A. Shkrob, R.T. Haasch, C. Peebles, J. Bareño, D.P. Abraham, “Auger Electrons as Probes for Composite Micro and Nanostructured Materials: Application to Solid Electrolyte Interphases in Graphite and Silicon-Graphite Electrodes”, J. Phys. Chem. C 2017, 121, 23333−23346. J. Bareño, I.A. Shkrob, J.A. Gilbert, M. Klett, D.P. Abraham, “Capacity Fade and Its Mitigation in Li-ion Cells with Silicon-Graphite Electrodes”, J. Phys. Chem. C 121 (2017), pp 20640-20649.