Improvements in the energy density of lithium-ion batteries (LIBs), through the adoption of novel high capacity anodes as alternatives to graphite, are being widely pursued. Silicon, with a theoretical capacity of 3579 mAh g^-1, lithiation voltage comparable to that of graphite, and high abundance, is considered the most appropriate alternative. The advantages of high energy density are countered by severe capacity fade, limiting the cycle life of Si-containing cells. Fundamental mechanistic understanding of lithiation and de-lithiation processes in Si-electrodes is essential to identify causes of capacity loss. Several in-situ and ex-situ microscopy and spectroscopy techniques have provided information on structural, chemical, and morphological changes of electrodes at specific states of charge and during calendar/cycle life ageing.

We adopted Energy Dispersive X-ray diffraction (EDXRD) to track structural changes in all components of pure Graphite (Gr) and Silicon/Graphite (Si/Gr = 15/73 w/w) based lithium-ion cells. Beamline 6 A-B at the Argonne Photon Source (APS), equipped with a fixed 3º angle single element Ge detector and incident white X-ray beam was used to obtain in operando EDXRD spectra from CR2032-type coin cells. The coin cells were assembled with a Li metal counter electrode, and 1.2 M LiPF₆ in EC/EMC (3:7 w/w) + 10 wt% FEC as the electrolyte, and underwent electrochemical cycling at a ~C/8 rate. Lattice parameters corresponding to the spectral peaks were derived from the energy of the diffracted X-rays. Variations in the average Gr layer spacing during lithiation of the pure Gr half-cell was used to calibrate an “average layer spacing vs. state of lithiation” in the Gr-component. The lattice parameters of different phases formed upon lithiation and delithiation of Si and graphite were estimated which can, thus, be used to obtain the extent of lithiation in Si and in graphite quantitatively at every state-of-charge. The consequences of this lithiation-delithiation behavior will be highlighted during our presentation.

Acknowledgement:

The authors acknowledge colleagues at Argonne, especially S. Trask, B. Polzin, A. Jansen, and D. Dees. The electrodes and electrolytes are from Argonne’s Cell Analysis, Modeling and Prototyping (CAMP) Facility, which is supported within the core funding of the Applied Battery Research (ABR) for Transportation Program. Support from the U.S. Department of Energy’s Vehicle Technologies Program (DOE-VTP), specifically from Peter Faguy, is gratefully acknowledged. This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.