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Density functional theory (DFT) was used to investigate the effect of electrolyte additives such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl ethylene sulfite (VES), and ethylene sulfite (ES) in propylene carbonate (PC)-based Li-ion battery electrolytes on SEI formation at graphitic anodes. The higher desolvation energy of PC limits Li+ intercalation into graphite compared to solvated Li+ in EC. Li+(PC)3 clusters are found to be unstable with graphite intercalation compounds and become structurally deformed, preventing decomposition mechanisms and associated SEI formation in favor of co-intercalation that leads to exfoliation. DFT calculations demonstrate that the reduction decomposition of PC and electrolyte additives is such that the first electron reduction energies scale as ES > VES > VEC >PC. The second electron reduction follows ES > VES > VEC > VC > PC. The reactivity of the additives under consideration follows ES > VES > VEC > VC. The data demonstrate the supportive role of certain additives, particularly sulfites, in PC-based electrolytes for SEI film formation and stable cycling at graphitic carbon-based Li-ion battery anodes without exfoliation or degradation of the anode structure.

Current lithium-ion battery anode research involves significant investigations of semiconducting materials, particularly Si as its theoretical specific capacity is >4000 mAh/g1. Previous theoretical studies showed that porous Si with a large pore size and high porosity can maintain its structure after Li ion induced alloying and swelling. Metal-assisted chemical (MAC) etching is shown here to form internally mesoporous nanowires in the form of a layer, etched from highly doped Si2-4. Some porous materials are well known to exhibit pseudocapacitive behaviour in aqueous electrolytes5,6. Maintaining the structure without stress-induced cracked caused by volumetric changes in material is crucial in achieving a high capacity and long cycle retention. Almost all investigations of nanoscale Si involve their deposition onto a metallic current collector electrode within the battery cell. Here, we demonstrate that pseudocapacitive behaviour can be harnessed when Si nanowires are etched to maximum mesoporosity, forming an electrically dead layer on silicon current collector electrodes. This limits insertion or alloying processes to form Li-Si phases7and charge is stored within the electric double layer, even in Li-ion containing electrolytes. Measurements using cyclic and linear voltammetry supported by Raman scattering spectroscopy and electron microscopy confirm surface charge storage mechanisms; pseudocapacitance is not observed when the same nanowires are used on stainless steel current collectors. In such cases, the rate of lithiation is shown to be related to the degree of porosity and the net surface electronic density of the porous silicon in accumulation mode during charging. References C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat. Nanotechnol. 3, 31 (2008). W. McSweeney, O. Lotty, N. Mogili, C. Glynn, H. Geaney, D. Tanner, J. Holmes and C. O'Dwyer, J. Appl. Phys. 114,034309 (2013). A. I. Hochbaum, D. Gargas, Y. J. Hwang, P. Yang, Nano Lett. 9, 3550 (2009). W. McSweeney, O. Lotty, J. D. Holmes and C. O'Dwyer, ECS Trans., 35, 25 (2011). P. Simon and Y. Gogotsi, Nat. Mater. 7, 845 (2008). T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nat. Mater. 9, 146 (2010). W. McSweeney, O. Lotty, C. Glynn, H. Geaney, J. D. Holmes and C. O'Dwyer, Electrochim. Acta, 135, 356 (2014).