UC Berkeley

Li-ion batteries (LIBs) are used to power many devices, ranging from personal electronics to electric vehicles. LIB performance has been optimized for operation at moderate charging and discharging rates (>1 hour for full charge) and at temperatures near 25oC. However, at operating conditions away from these optimal conditions, LIB performance severely declines. At "extreme" fast charging (XFC) rates (<15 min. charge time), and/or low operating temperatures, LIBs suffer from the electrochemical plating of metallic Li on the graphite negative electrode. In this work, I demonstrate the first fully operando electrochemical method for the detection of Li plating during XFC using electrochemical impedance spectroscopy (EIS). Using a three-electrode cell to directly monitor the impedance response of graphite in a full cell LIB, and the distribution of relaxation times (DRT) to deconvolute the processes at the graphite electrode, I find that an increase in the graphite SEI resistance indicates the onset of Li plating. I cross-validate this finding with highly sensitive mass spectrometry titrations of the cycled graphite electrodes to measure the inactive Li on the surface of the graphite. I find that the technique has a sensitivity to Li plating of <0.6% of the graphite's capacity. I also use the EIS information and the surface carbonate MST results to offer possible explanations for the observed SEI resistance behavior.

I also apply the operando EIS technique to commercially-relevant two-electrode full cells. I find that the SEI resistance increase observed in three-electrode cells is no longer resolvable in the two-electrode EIS data. However, the charge transfer resistance reliably increases 20-30% SOC prior to the onset of Li plating. I demonstrate the reliability of this analysis for rates ranging from 2C to 6C, confirmed with MSTs. I also confirm that the technique is viable over at least 100 cycles of 4C charging. However, the signal is not resolvable for cycling performed at elevated temperatures (45oC).

Even at lower current rates, LIB discharge performance is severely limited at temperatures below 0oC. Electrolyte engineering is one promising pathway to improve LIB C/3 discharge at -20oC. I use operando EIS to confirm that graphite charge transfer is the limiting process for LIB discharge with LP57 baseline electrolyte. Characteristic time calculations confirm that this charge transfer resistance is limited by the electrochemical reaction kinetics, not mass transport, in the LIB. gamma-butyrolactone (GBL) and ethyl acetate (EA) have been proposed as possible replacements for ethylene carbonate (EC), a common electrolyte component that undergoes a large viscosity increase at low temperatures, which may be the cause of poor LIB performance at low temperatures. Upon initial cycling experiments, neither chemical was found to be suitably compatible with the graphite electrodes.

Silica nanoparticles (SiNPs), however, improve LIB low temperature discharge capacity retention. Operando EIS experiments show that the addition of SiNPs to the baseline LP57 decreases the graphite charge transfer resistance during discharge at -20oC. Coupled ion-electron transfer (CIET) kinetic theory is implemented to measure the exchange current density, i_0, of the graphite and NMC622 electrodes at 30oC and -20oC with this electrolyte, as well as other variations of LP57. The SiNP electrolyte improves the graphite i_0 by more than an order of magnitude, supporting the finding of a reduced charge transfer resistance at the graphite. This is the first implementation of CIET to low temperature LIB studies and to electrodes with commercially-relevant loadings. Furthermore, this combination of operando EIS and CIET theory can serve as a quantitative way to drive electrolyte engineering for LIBs and other intercalation-based electrochemical systems.

I also discuss possible future research plans for the continuation of both the XFC and low temperature studies. Lastly, I look to critically analyze the environmental and ethical concerns associated with LIB manufacturing and widespread EV adoption, and I explore possible pathways to ensure a just and equitable clean energy future.