Hard X-ray Photoelectron Spectroscopy (HAXPES) Investigation of the Silicon Solid Electrolyte Interphase (SEI) in Lithium-Ion Batteries

Abstract

Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonate (EC), fluoroethylene carbonate (FEC), or EC with 15% FEC (EC:FEC), extracted from cells and analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES). All of the electrolytes generate an SEI which is integrated with Si containing species. The EC and EC:FEC electrolytes result in the generation of LixSiOy after the first cycle while LixSiOy is only observed after five cycles for the FEC electrolyte. The SEI initially generated from the EC electrolyte is primarily composed of lithium ethylene dicarbonate (LEDC) and LiF. However, after five cycles, the composition changes, especially near the surface of silicon because of decomposition of the LEDC. The SEI generated from the EC:FEC electrolytes contains LEDC, LiF, and poly(FEC) and small changes are observed upon additional cycling. The SEI generated with the FEC electrolyte contains LiF and poly(FEC) and small changes are observed upon additional cycling. The stability of the SEI correlates with the observed capacity retention of the cells.

Determination of the Solid Electrolyte Interphase Structure Grown on a Silicon Electrode Using a Fluoroethylene Carbonate Additive

“…The C1s data shows the reemergence of a C-O species at 285.3 eV originating from solvent decomposition along with the same previously observed C-C, C-H, -CO 3 species described previously. This is consistent with the growth of C-O species reported previously and due to decomposition of the solvent 27, 33 . Smaller changes are observed for the P2p data which shows the growth of P-O type species and the loss of P-F components.…”

“…studied the SEI over binder-free electrodes using synchrotron based XPS measurements of delithiated electrodes after 1 or 5 cycles in LiPF 6 EC, EC/FEC, and FEC electrolytes 33 . They reported EC based electrolyte resulted in an SEI that was LiF poor on the surface but more LiF like closer to the electrode surface 33 . In contrast the LiF was most concentrated on the surface for the EC/FEC electrolyte while the core contained less LiF 33 .…”

“…They reported EC based electrolyte resulted in an SEI that was LiF poor on the surface but more LiF like closer to the electrode surface 33 . In contrast the LiF was most concentrated on the surface for the EC/FEC electrolyte while the core contained less LiF 33 . Furthermore the EC/FEC electrolyte contained more polymeric-FEC species and was thinner than the pure EC SEI (estimated to be ~10 nm thick) based on Si signals emanating from under the SEI layer and the EC/FEC SEI contained about half the total F content as the EC samples 33 .…”

Mechanical Damage of Surface Films and Failure of Nano-Sized Silicon Electrodes in Lithium Ion Batteries

“…A relatively stable passivating ability of the surface film has been assumed. 21,22,28,29 This assumption can be validated from the results shown in Figs. 4d and 4e.…”

“…To accomplish these goals, the performance of Si electrode in a standard electrolyte solution was compared with that in an experimental electrolyte containing fluoroethylene carbonate (FEC) as a co-solvent. 10,[19][20][21][22] The failure of Si electrodes is less severe (vide infra) upon the addition of FEC into the standard electrolyte. Hence, comparison of the performance of Si electrodes in the two different electrolyte solutions provides an insight into the failure modes of Si electrodes.…”