An ultrasonic imaging technique has been developed to investigate the internal changes of pouch cells nondestructively. The local ultrasonic transmittance of pouch cells has been measured and used for imaging with a new ultrasonic scanning machine designed and built in-house. The wetting process of the cells is clearly observed via such ultrasonic imaging techniques. Furthermore, ultrasonic transmission images of fresh cells and aged cells with different electrolytes and cycling conditions exhibit very different ultrasonic transmittance, which can be caused by electrolyte dry-out or ''unwetting'' due to cell swelling. The ultrasonic imaging technique is a very sensitive method to probe failure mechanisms in Li-ion pouch cells.

LiFePO4 (LFP) is an appealing cathode material for Li-ion batteries. Its superior safety and lack of expensive transition metals make LFP attractive even with the commercialization of higher specific capacity materials. In this work the performance of LFP/graphite cells is tested at various temperatures and cycling protocols. The amount of water contamination is controlled to study the impact of water on capacity fade in LFP. Further, several additive systems that have been effective in NMC/graphite chemistries are tested in LFP/graphite cells. The presence of excess water impacts cell performance severely when no electrolyte additives are used, or when the electrodes are poorly passivated. When effective additive systems are used, the existence of up to 500 ppm excess water in the cell is does not strongly affect cycle life and storage performance. Fe dissolution is studied in LFP with micro X-ray fluorescence spectroscopy (μXRF), and most electrolyte additives virtually eliminate Fe dissolution, even at high temperature and with water contamination. Removing excess water contamination suppresses Fe dissolution in cells without electrolyte additives. Finally, the capacity retention of LFP/graphite cells at high temperature is compared with long lifetime NMC/graphite cells, demonstrating challenges for LFP/graphite cells.

Lithium-ion cells testing under different state of charge ranges, C-rates and cycling temperature have different degrees of lithium inventory loss, impedance growth and active mass loss. Here, a large matrix of polycrystalline NMC622/natural graphite Li-ion pouch cells were tested with seven different state of charge ranges (0-25, 0-50, 0-75, 0-100, 75-100, 50-100 and 25-100%), three different C-rates and at two temperatures. First, capacity fade was compared to a model developed by Deshpande and Bernardi. Second, after 2.5 years of cycling, detailed analysis by dV/dQ analysis, lithium-ion differential thermal analysis, volume expansion by Archimedes’ principle, electrode stack growth, ultrasonic transmissivity and x-ray computed tomography were undertaken. These measurements enabled us to develop a complete picture of cell aging for these cells. This then led to an empirical predictive model for cell capacity loss versus SOC range and calendar age. Although these particular cells exhibited substantial positive electrode active mass loss, this did not play a role in capacity retention because the cells were anode limited during full discharge under all the tests carried out here. However, the positive electrode mass loss was strongly coupled to positive electrode swelling and electrolyte “unwetting” that would eventually cause dramatic failure.

In an effort to better understand capacity loss mechanisms in LiFePO4 (LFP)/graphite cells, this work considers carbon-coated LFP materials with different surface area and particle size. Cycling tests at room temperature (20°C) and elevated temperatures show more severe capacity fade in cells with lower surface area LFP material. Measurements of Fe deposition on the negative electrode using micro X-ray fluorescence (µXRF) spectroscopy reveal more Fe on the graphite electrode from cells with low surface area. Measurements of parasitic heat flow using isothermal microcalorimetry show marginally higher parasitic heat flow in cells with low surface area. Cross-sectional scanning electron microscopy images of aged LFP electrodes show micro-fracture generation in large LFP particles, which are more prevalent in the low surface area material. Further, studies on the impact of vacuum drying procedures show that while Fe deposition can be inhibited by removing excess water contamination, the direct impact of Fe deposition on capacity fade is small. Particle fracture leads to the exposure of LFP fresh surface to the electrolyte, leading to more parasitic reactions and Li inventory loss.

Summary Waterflooding of heavy oil reservoirs is commonly used to enhance their productivity. However, preferential pathways are quickly developed in the reservoir because of the significant difference in viscosity between water and heavy oil and, hence, the oil is trapped. Here, we propose a platform for designing ultralow interfacial tension (IFT) solutions for reducing the capillary pressure and mobilizing the heavy oil. In this study, we formulated mixtures of organic acids and bases. We tested three different formulations: an ionic liquid (IL) formulation in which the bulk acid [4-dodecylbenzene sulfonic acid (DBSA)] and base [tetra-N-butylammonium hydroxide (N4444OH)] were mixed using general protocols for IL synthesis; an acid/base solution (ABS) in which the acid (DBSA) and base (N4444OH) were mixed in low weight fractions directly in water; and an acid salt/base solution (ASBS) in which the acid salt [sodium dodecylbenzene sulfonate (SDBS)] was used instead of the acid. All the formulations have a 1:1 stoichiometric ratio of acid and base. Salinity scans were conducted to determine the optimum salinity that gives the lowest IFT for each formulation. Corefloods were conducted in hydrophilic and hydrophobic sandpacks to evaluate the three formulations at their optimum salinities for post-waterflood heavy oil recovery. The IL and ABS formulation are acidic solutions with a pH of approximately 3. The ASBS formulation is highly basic with a pH of approximately 12. None of the formulations salted out below 14 wt% of sodium chloride (NaCl), whereas the conventional surfactant, SDBS, precipitated at a salt concentration of less than 2 wt% of NaCl. The formulation solutions (1 wt%) have different optimum salinities: 2.5 wt% NaCl for ASBS and 3 wt% NaCl for IL and ABS. Although the IL and ABS have the same composition and molar ratio of the components, their performances are completely different, indicating different intermolecular interactions in both formulations. Corefloods were conducted using sandpack saturated with Luseland heavy oil (∼15,000 cp) and a fixed Darcy velocity of 12 ft/D. A slug of 1 pore volume (PV) of each formulation was injected after waterflooding for 5 PV followed by 5 PV post-waterflooding. In the hydrophilic sandpacks, IL and ABS formulation produced an oil bank consisting mainly of a water-in-oil (W/O) emulsion, with oil recovery that was 1.7 times what was recovered by 11 PV of waterflooding solely. The majority of the oil was recovered in the 2 PV of waterflood after the IL slug. ASBS formulations produced oil-in-water (O/W) emulsions with prolonged recovery over 5 PV waterflooding after the ASBS slug. The recovery factor for ASBS was 1.6 times that recovered for 11 PV of waterflooding only. In the hydrophobic sandpacks, the ASBS formulation slightly increased the recovery factor compared with only waterflooding, whereas for IL and ABS formulations, the recovery factor decreased. In this work, we present a novel platform for tuning the recovery factor and the timescale of the recovery of heavy oil with a variable emulsion type from O/W to W/O depending on the intermolecular interactions in the system. The results demonstrate that the designed low IFT solutions can effectively reduce the capillary force and are attractive for field applications.