This work examined the impact of depth of discharge (DOD), C-rate, upper cut-off voltage (UCV), and temperature on the lifetime of single-crystal NMC811/Artificial Graphite (AG) cells. Cells were cycled at C/50, C/10, C/5, or C/3, and 25, 50, 75, or 100% DOD at room temperature (RT, 20. ± 2°C) or 40.0 ± 0.1°C. The UCVs were 4.06 or 4.20 V. After 12000 hours of cycling, experiments such as electrochemical impedance spectroscopy, Li-ion differential thermal analysis (DTA), ultrasonic mapping, X-ray fluorescence, differential capacity analysis, synchrotron computed tomography (CT) scans, and cross-section scanning electron microscopy (SEM) were carried out. We showed that capacity loss increased slightly with DOD and C-rate, and that cells with 4.06 V UCV have superior capacity retention and impedance control compared to 4.20 V. SEM, CT scans, and differential capacity analysis show that microcracking and positive electrode mass loss did not occur regardless of DOD, C-rate, or UCV. DTA and ultrasonic mapping showed no C-rate or DOD dependency for electrolyte changes or “unwetting.” A simple square-root time model was used to model SEI growth in 4.06 V UCV cells, and a cell design with impressive performance is demonstrated.

Graphite is the most used lithium intercalation host for the negative electrode of the lithium-ion battery. Extensive research has been carried out to achieve high coulombic efficiency (CE) and long cycle life for the graphite anode. Here, LFP/graphite (graphite from Vendor 1) cells that undergo formation at 40°C with either 1.2 M LiPF6 dissolved in ethylene carbonate:dimethyl carbonate (EC:DMC), or ethylene carbonate:ethylmethyl carbonate (EC:EMC) have excellent first cycle efficiency (FCE). However, when the formation is done at 20°C, EC:EMC and ethylene carbonate:diethyl carbonate (EC:DEC) cells show much reduced FCE while EC:DMC cells retain high FCE. We prove by a variety of experiments that the reduced FCE is caused by solvent co-intercalation. We explore the impact of temperature, different graphites, electrolyte additives, and varied salt content on this effect. We show that basic additives, such as vinylene carbonate, are sufficient to eliminate the co-intercalation. With a well-designed electrolyte system containing additives, graphites that show co-intercalation in the absence of additives perform equivalently or better to graphites that do not show co-intercalation in the absence of additives.

SiOx is a silicon-based anode material for Li-ion batteries that has a high specific capacity and good cycle life. Lithiation of SiOx results in the formation of active Si cores surrounded by an inactive stabilizing matrix of irreversible lithium silicates and lithium oxides. SiOx with adjustable values of x can be synthesized by roller milling under oxygen flow, with the oxygen content being controlled by the amount of time spent milling in oxygen vs milling under argon. A fast and inexpensive oxygen content determination method was needed for determination of the oxygen content in SiOx as the roller milling process proceeded. A simple 4-step procedure to determine the oxygen content in SiOx is presented using a KOH solution, aluminized pouch bags, and Archimedes gas quantification measurements. This method can serve as a quick and inexpensive substitute to traditional oxygen content determination measurements such as gas fusion analysis.

SiOx is an attractive anode material given its high specific capacity and its increased lifetime due to its supporting matrix of lithium silicates irreversibly formed during its first lithiation. While SiOx is normally created by simultaneous evaporation and vapor deposition of Si and SiO2 powders, this can be very difficult and energy consuming method. It is shown here that SiOx with controlled oxygen content can be made by ball milling crystalline silicon powder in an oxidizing medium using two different milling techniques. To characterize the SiOx powders, oxygen content is quantified using a KOH-based method and Brunnauer-Emmett-Teller surface area is measured. Electrochemical testing using coin cells is completed and the results are compared to commercially available SiO samples. The results show that SiOx with competitive properties can be made by ball milling. Further work is required to reduce the specific surface area of the material made by ball milling.