Vincent L. Chevrier

Sodium-ion batteries (SIBs) with Earth-abundant elements are promising for global electrification, but electrolyte stability impacts electrochemical performance and safety. This study compares non-fluorinated 1,2-diethoxyethane (DEE) and fluorinated 1,2-bis(2,2-difluoroethoxy)ethane (F4DEE) as electrolyte solvents in Na _0.97 Ca _0.03 [Mn _0.39 Fe _0.31 Ni _0.22 Zn _0.08 ]O ₂ (NCMFNZO)/hard carbon (HC) pouch cells up to 4.0 V. Fluorination slightly reduces ionic conductivity and increases viscosity but significantly enhances electrochemical stability and safety. Cells with F4DEE exhibit lower impedance, reduced gas evolution, and less voltage decay during high-voltage storage at 40 °C. Long-term cycling shows ∼85% capacity retention after 500 cycles at 25 °C and ∼80% at 40 °C with less transition metal dissolution, outperforming DEE-based cells. Isothermal microcalorimetry reveals lower parasitic heat generation with F4DEE, while soft X-ray absorption spectroscopy confirms stabilized Ni and Mn oxidation states, indicating suppressed electrolyte oxidation. Accelerating rate calorimetry reveals improved thermal stability with F4DEE. These findings highlight fluorinated ether solvents as a promising approach to enhance SIB lifespan and safety, with ongoing challenges requiring further solvent and additive optimization.

Prelithiation promises higher energy density and better cycle performance by providing additional lithium into a Li-ion battery and is therefore gaining increasing attention in industry. However, to achieve extra lithium introduction, additional processes or species need to be introduced, leading to potentially new parasitic reactions. In this work, in operando isothermal microcalorimetry is used to evaluate both the prelithiation process and the associated parasitic reactions. We show with the case of cathode prelithiation using Li₂O₂ as the prelithiation additive, that while the prelithiation event completed on the first charge, an elevated heat signal extended beyond the first cycle, indicating continued presence of additional parasitic reactions even after the major prelithiation event. The heat signals from such additional parasitic reactions decreased over cycling. After three cycles, similar heat signals from parasitic events were observed for the prelithiation cells and the control cells, indicating stabilization and possibility of no long-term side effects from prelithiation. This first demonstration of in operando isothermal microcalorimetry method for prelithiation application provides valuable insights into heat production in prelithiation cells and offers a first look into prelithiation-associated parasitic reactions and their evolution.

The structural evolution of Si during lithiation and delithiation is uniquely dependent on the cycling conditions and can show either reversible or path dependent behavior. In this paper, metallurgical Si (large crystalline particles of pure Si) is cycled to exhibit both reversible and path dependent cycling while in-operando calorimetry is performed with a high precision isothermal calorimeter. The enthalpy potential and waste heat are studied in both the reversible and path dependent regimes. The enthalpy of crystallization of Li₁₅Si₄ is quantified to be 21 kJ mol_Si ⁻¹. The parasitics (heat of side reactions) of pure Si are studied, showing a stronger current than time dependence. The voltage dependence of the parasitic power is quantified, showing increasing parasitics below 170 mV. The results of this in depth calorimetric study of metallurgical Si provides valuable insights into the heat production and energy efficiency of Si as a negative electrode material in Li-ion batteries.

IEC 61850 has become the reference standard for Substation Automation Systems (SAS) in smart power grids. Switched Ethernet is used for machine to machine communication within SAS. In order to meet stringent real-time constraints, the IEC 61850 application layer protocols can be mapped into different IEEE802.1Q priorities according to their real-time constraints and application criticality. However, the delay evaluation to guarantee real-time requirements can be difficult to perform, especially for lower priority but still real-time constrained traffic. In fact, most existing end-to-end worst-case delay analyses provide upper-bounds, leading to some pessimism and consequently network resource over-provision. In this paper, we present a new method for determining a tight worst-case delay. This method is based on the study of flow characteristics from a given network path. As a flow is interfered by other concurrent flows on its path, their relative offsets with the considered flow greatly impact on its delay. Studying all combinations to find the actual worst-case delay results in high complexity. We show that this complexity can be reduced by only analysing local worst-case delay at each switch in stead of the whole path where the change at each switch would need re-analysing the already analysed switches. An algorithm is also proposed to perform the analysis. An illustrating example shows that our method can reduce the pessimism as it provides the tight worst-case delay instead of the upper-bound of the worst-case delay.

Wound pouch cells with a Li(Ni_0.6Mn_0.2Co_0.2)O₂ (NMC622) positive electrode and graphite/Si alloy negative electrode are shown to have significantly better performance than equivalent cells with a LiCoO₂ (LCO) positive electrode. Silicon containing NMC622 full cells have better performance with a 4.35 V upper cutoff voltage (UCV) than with a 4.25 V UCV. The addition of small amounts (≤8 wt%) of high surface area NMC622 to LCO-based positive electrodes in single layer pouch cells with graphite/Si alloy negative electrodes improves cycling performance and coulombic efficiency. Adding fluoroethylene carbonate (FEC) further improves the performance of cells showing the benefits of NMC622 and FEC can be combined. High precision calorimetry and cycling, Archimedes volume measurements, and cross-section scanning electron microscopy support the hypothesis of interplay between the parasitic reactions at the positive and negative electrodes. The NMC622 positive electrode is proposed to stabilize the Si alloy in the negative electrode via a CO₂ release mechanism, though further confirmation of the mechanism is needed.

Carbon dioxide is shown to be an effective additive to standard Li-ion electrolyte for extending the cycle life of full pouch cells containing an engineered silicon alloy. CO₂ was introduced to pouch cells by adding a few milligrams of dry ice to cells before sealing. The cells contained composite negative electrodes formulated with 15 to 17 wt% of an engineered silicon alloy and a LiCoO₂ positive electrode. Parasitic electrolyte reactions were measured in-situ by isothermal micro-calorimetry and high precision coulometry and compared to cells containing 1-fluoro ethylene carbonate (FEC). Extended cycling of cells containing CO₂ were compared to cells containing FEC. Cell gas generation and gas consumption were measured by applying the Archimedes principle. A new approach using small tubes in pouch cells to differentiate volume changes from gassing and solid expansion is introduced. Cells with CO₂ showed significantly lower parasitic thermal power, improved coulombic efficiency and better capacity retention compared to cells containing electrolytes with FEC. The gas generation/consumption experiments showed that Si alloy reacts with CO₂ during cycling until it is fully consumed. Combining FEC and CO₂ reduces the consumption rate of CO₂. Microscopy of cross-sectioned cycled electrodes showed a thin SEI layer and minimal silicon alloy erosion. The combined work establishes CO₂ as a powerful precursor to an effective SEI layer on silicon alloys. Finally, CO₂ is shown to be an effective SEI former for graphite in EC-free electrolytes.

Li-ion pouch cells utilizing a negative electrode formulated with 15 wt% of an engineered Si alloy in a graphite composite electrode were cycled in an isothermal heat flow calorimeter against a LiCoO₂ positive electrode. Two different electrolytes were investigated: a blend of ethylene carbonate and ethyl methyl carbonate (3EC:7EMC) and a blend of ethylene carbonate, ethyl methyl carbonate and 1-fluoro ethylene carbonate (27EC:63EMC:10FEC). Both electrolytes were 1 M in LiPF₆ salt. The parasitic thermal power and coulombic efficiency was derived from isothermal heat flow measurements and high precision current-source meters. Cells without FEC showed high parasitic thermal power which increased with cycle number indicative of a surface area increase which was confirmed by post-cycling scanning electron micrographs and surface area measurements. Cells with FEC showed relatively stable parasitic thermal power. These measurements demonstrate the surprising function of FEC in controlling or attenuating the evolution of surface area in Si alloys. Vinylene carbonate was also found to be effective at controlling the increase in alloy surface area.