Shanika Abeysooriya

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.

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.

Dicationic organic salts are an interesting class of solid‐state electrolyte materials due to their unique structure. Here we present, for the first time, the synthesis and characterization of three dicationic‐FSI salts, 1,2‐bis(N‐methylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C₂‐Pyrr1][FSI]₂), 1,2‐bis(N‐ethylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C₂‐Pyrr2][FSI]₂) and 1,2‐bis(N‐n‐propylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C₂‐Pyrr3][FSI]₂). The structure and dynamics of the organic salts were probed using variable temperature solid‐state NMR and were compared with the thermal and transport properties. The investigation revealed that [C₂‐Pyrr1][FSI]₂, with shorter alkyl‐side chains on the dication, displayed increased transport properties compared to [C₂‐Pyrr2][FSI]₂ and [C₂‐Pyrr3][FSI]₂. To determine the proficiency of these dicationic‐FSI salts as electrolyte materials for battery applications, 10 mol% and 50 mol% lithium bis(fluorosulfonyl)imide (LiFSI) was mixed with [C₂‐Pyrr1][FSI]₂ and [C₂‐Pyrr2][FSI]₂. Increased transport properties were observed for [C₂‐Pyrr1][FSI]₂/10 mol % LiFSI in comparison to [C₂‐Pyrr2][FSI]₂/10 % LiFSI, while pulse field gradient NMR analysis revealed the highest Li⁺ self‐diffusion ratio for [C₂‐Pyrr1][FSI]₂/50 % LiFSI out of the four Li‐salt‐containing mixtures.