The Role of Additives in Electrode Stability for Improving Capacity Retention in High Energy Density Lithium Metal Batteries
Authors:   , Yim Taber Siu-Keen , Kalra Vibha 
Published: February 2025

The growth of sustainable energy is crucial to reduce carbon emissions and our impact on the environment. To match the increasing sustainable energy production, energy storage solutions must also be developed. Furthermore, the electrification of society will require an array of energy solutions. Lithium-ion batteries have spurred the electrification transition, but they are reaching their theoretical limits. Therefore, next generation batteries with greater energy density are needed. Lithium-sulfur (Li-S) batteries have emerged as an attractive alternative due to the abundance and low cost of sulfur, low environmental impact, and high gravimetric energy density. However, a limiting challenge that prevents its commercialization thus far is the polysulfide shuttling effect. This occurs during the sulfur reduction process which creates intermediate polysulfides that can shuttle from the cathode to anode, resulting in capacity fade. The lithium metal at the anode can also degrade the electrolyte and form dendrites, causing a short circuit. Another battery chemistry that does not rely on sulfur is the anode free lithium metal battery (AFLMB). This battery configuration provides maximum energy density since the anode is simply a bare current collector. However, lithium metal in this system poses similar challenges. In this thesis I aim study the role of additives on electrode stability in these high energy density battery systems to improve their capacity retention. In Li-S batteries, the primary challenge is polysulfide shuttling. To address this issue, studies have altered the electrolyte composition, confined sulfur, or used cathode additives. In this thesis, we study caffeine as an additive to create a more stable cathode. For the first time, we observe the interaction of polysulfides with the carbonyl group of the caffeine cathode additive using in-operando Fourier transform infrared (FTIR) spectroscopy. By coupling FTIR spectra and electrochemical response in the battery, we could propose a polysulfide interaction mechanism. The lithium metal also presents challenges in a Li-S battery. To create a more stable lithium anode, we study an interlayer containing a lithium nitrate (LiNO₃) salt additive in a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer matrix host. Using X-ray photoelectron microscopy (XPS), we observe a synergistic effect between the salt and polymer that creates a robust solid electrolyte interface (SEI) with a greater proportion of lithium fluoride, a beneficial SEI species. The improved cathode and anode stability both improved the capacity retention of the Li-S battery. In the pursuit of higher energy density, we study a gel polymer electrolyte (GPE) in a carbonate electrolyte based AFLMB. By using a GPE, we can avoid a cell flooded with liquid electrolyte and eliminate the conventional Celgard separator. We expand on the PVDF-HFP and LiNO₃ interlayer concept by incorporating acryloxypropyl polyhedral oligomeric silsesquioxane (POSS). POSS improves the salt dissolution and ionic conductivity, while LiNO₃ creates a spherical lithium deposition morphology on copper, which we observe using scanning electron microscopy (SEM). Finally, this thesis will discuss the technique development and challenges of in-operando SEM to further study lithium deposition morphology on bare copper electrodes.