Interphasial and Thermal Reactivity Investigation of Silicon-Graphite Composite Anode Electrodes

Summary:

Nowadays, lithium-ion batteries (LIBs) represent the most widely used secondary battery. They stand out because of their energy and/or power density and play an important role in the further development of electric mobility and mobile applications (e.g. laptops, cellphones, power tools, etc). To fulfill the needs for the next-generation high-capacity LIBs, Silicon (Si) and its derivatives (e.g. Si/C, Si-rich SiOx, x ≤ 1) have gained huge attention as anode materials.1,2 However, despite the beneficial features such as low cost and high capacity, Si shows limited cycle life, huge volume changes during (de-) alloying, low Coulomic efficiency, unstable SEI, etc.
One strategy to overcome these problems is to modify, with the help of small doses of electrolyte additives, the chemical nature, and features of the solid electrolyte interface (SEI) layer. Because the SEI layer is directly linked to the cycle life, obtainable capacity, Coulomic in/efficiency, thermal reactivity, etc. of a LIB system, it is an effective, economical, and scalable approach to large scale commercialization of Si anode based LIBs.3
In this context, we report an in-depth investigation of several electrolyte additives, namely fluoroethylene carbonate (FEC), vinylene carbonate (VC), tetraethyl orthosilicate (TEOS), and (2-Cyanoethyl)triethoxysilane (TEOSCN), and a blend of FEC, VC, and TEOSCN with the help of electrochemical analysis, X-ray photoelectron spectroscopy (XPS), density functional theory (DFT) calculations, and differential scanning calorimetry (DSC). The surface characterization revealed the composition, relative thickness of the SEI layer harvested from the different additives. It has also shown that the blend of FEC, VC, and TEOSCN resulted in a thinner, inorganic rich, and high thermal stable SEI layer. This could be explained by the polymerization reaction of −C≡N to C=N double and C−N single bonds, yielding a more chemically stable and highly ionically conductive SEI layer. Furthermore, the ternary additive resulted in a higher amount of LiF, which can be explained by the possible attack of the C≡N reduction strong nucleophiles on FEC and/or LiPF6 to form LiF as the XPS analysis is showing.

References:
(1) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for Li-Ion Batteries. Chem. Rev. 2014, 114 (23), 11444–11502. https://doi.org/10.1021/cr500207g.
(2) Eshetu, G. G.; Figgemeier, E. Confronting the Challenges of Next-Generation Silicon Anode-Based Lithium-Ion Batteries: Role of Designer Electrolyte Additives and Polymeric Binders. ChemSusChem. Wiley-VCH Verlag 2019. https://doi.org/10.1002/cssc.201900209.
(3) Aupperle, F.; von Aspern, N.; Berghus, D.; Weber, F.; Eshetu, G. G.; Winter, M.; Figgemeier, E. The Role of Electrolyte Additives on the Interfacial Chemistry and Thermal Reactivity of Si-Anode-Based Li-Ion Battery. ACS Appl. Energy Mater. 2019, 2 (9), 6513–6527. https://doi.org/10.1021/acsaem.9b01094.