Parham Taghizadegan

Silicon-containing lithium-ion cells provide higher energy density than cells with only graphite in the negative electrode. This paper investigated the impact of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on the performance of lithium-ion pouch cells with NMC-type positive electrodes (Ni83-type and Ni65-type) and 20 % chemical silicon-carbon (chemical SiC) in their negative electrode. The additive blends consisted of 2 wt. % VC with either 0, 4, or 10 wt. % FEC. Insitu volume measurements were carried out to study the impact of additives on gas volume produced during the formation step. Long-term cycling tests at 40 °C and 55 °C, and storage experiments at 60 °C, were conducted to investigate the effect of the additives on cell performance. Post-cycle analyses were conducted on cells using impedance spectroscopy (EIS) and gas chromatography-mass spectrometry (GC-MS). The results demonstrate that 2 wt % VC is an effective additive to improve cell performance in both Ni83-type and Ni65-type pouch cells with 20 % chemical SiC. The additive blend, 2 wt % with 4 wt % FEC, shows a positive impact, however, addition of 10 wt % FEC produces minimal additional impact.

Numerous efforts have been made to utilize silicon-based negative electrodes in lithium-ion batteries (LiBs). The main challenge with Si-based anodes is their significant volume expansion and contraction during cycling, which degrades LiB performance . Silicon-carbon composites are known to address many of the challenges of large-volume expansion and can deliver high specific capacity alongside acceptable cycle life. In this study, various chemically synthesized Si–C composites with different porous hard carbon precursors were investigated to examine the evolution of the filling of the carbon precursor pores with Si during impregnation via chemical vapor deposition using SiH ₄ gas. These samples were later etched with a high-pH solution to remove the Si in several steps, returning the structure to that of the carbon precursor to backtrack the changes in the carbon precursor. The as-received carbon precursor and the etched samples were compared using small-angle X-ray scattering and X-ray diffraction. These techniques, combined with high-resolution transmission electron microscopy, compression testing of the samples, and electrochemical performance measurements, provide a broad view of the evolution of the carbon precursor structure during Si deposition and during the charge/discharge processes in the LiB negative electrode, providing insight into the structure-performance-properties correlation of CVD-based Si–C composites.

Electrolyte motion–induced salt inhomogeneity (EMSI) is a critical degradation mechanism that can limit lifetime of large-format lithium-ion cells. Here, ex-situ scanning micro–X-ray fluorescence (µ-XRF) spectroscopy is employed to map electrolyte salt concentration across entire electrodes harvested from 18 650 cells. Lithium hexafluoroarsenate (LiAsF ₆ ) is used as a spectroscopically favorable salt, enabling arsenic fluorescence as an indicator of local salt concentration. Transport properties of LiAsF ₆ are shown, via Advanced Electrolyte Model calculations, to closely match those of LiPF ₆ in relevant carbonate solvents, validating its use as a surrogate salt for this technique. We confirm that the measured As distributions reflect true electrolyte salt concentration by tracking their growth and relaxation over cycling and rest, and by showing consistent patterns across the separator–anode–cathode stack, indicating a cell-level redistribution of salt. The maps reveal that the axial salt gradient along the entire length of the anode is strongly non-uniform. We observe pronounced tab-correlated banding and localized salt-depleted regions beneath the tab, which we interpret as signatures of pressure-dependent electrolyte pumping and tab-driven modulation of EMSI. This work establishes µ-XRF mapping of LiAsF ₆ as a powerful new technique to quantify electrolyte salt distributions with high spatial resolution.

Improving energy density and cycle life of lithium-ion batteries (LiBs), while maintaining or improving safety is important. Introducing silicon-containing materials to graphite negative electrodes is a promising way to increase LiB energy density. This study focuses on the impact of varying concentrations (0–10 wt%) of fluoroethylene carbonate (FEC), on the performance of Li-ion pouch cells that include 20 wt% chemical silicon-carbon (chemical SiC) in their negative electrodes. Long-term cycling tests at 40 °C and 55 °C along with long-term storage experiments at 60 °C were carried out to evaluate the effect of FEC on cell lifetime. Upon reaching 90% capacity retention, post-mortem analyses were performed using electrochemical impedance spectroscopy (EIS), while gas chromatography-mass spectrometry (GC-MS) was performed to investigate electrolyte degradation products. Scanning electron microscopy (SEM) was utilized to examine changes in the negative electrode surface after cycling at 40 °C until 90% capacity. The results demonstrate that employing FEC as an additive, in the range of 0 to 4%, has little effect on capacity retention and voltage polarization during long-term cycling or storage for cells that contain chemical SiC (at least for the sample tested here) in the negative electrode, while cells with 8 and 10% FEC performed worse.