Molecular to Macroscopic Understanding of Chloroaluminate Anion Intercalation in Rechargeable Aluminum-Graphite Batteries

Author

Jeffrey Xu, CUNY City CollegeFollow

Date of Award

2021

Document Type

Dissertation

Department

Chemical Engineering

First Advisor

Robert J. Messinger

Keywords

Batteries, Energy storage, NMR, Green energy

Abstract

Today’s global energy challenges pose an urgent need to electrify transportation and better store intermittent renewable energy sources (e.g., solar and wind energy). For such large-scale battery applications, aluminum batteries are a promising “beyond lithium-ion” technology due to the high volumetric capacity, earth abundance, low-cost, and inherent safety of aluminum metal. However, there are very few compatible positive electrode materials that exhibit high energy density and cycling stability, in part due to the challenges of electrochemically intercalating highly charged Al³⁺ cations. Recently, graphite has been demonstrated as a promising positive electrode material in non-aqueous rechargeable aluminum batteries, which store charge when monovalent chloroaluminate molecular anions (e.g., AlCl₄^-) intercalate into graphite. Such aluminum-graphite batteries exhibit discharge voltages of ca. 2 V, high cycle life, and ultra-fast rate capabilities. However, it remains unclear how the electrochemical performance is linked to the fundamental processes that govern the electrochemical intercalation of AlCl₄^- anions, which occur over varying length and time scales. Notably, the relationships among graphite material and electrode structures, molecular-level AlCl₄^- environments, ion mass transport, and cell-level electrochemical properties are not well understood.

Here, the electrochemical intercalation of chloroaluminate anions into graphite is investigated from the molecular to macroscopic scales via electrochemical, spectroscopic, and theoretical methods. First, we elucidate the effects of different graphite structures on cell-level properties, such as capacity, cycle life and the extent of parasitic side reactions that contribute to capacity fade. The theoretical capacity and maximum graphite stage were estimated coulometrically via applying a hard-sphere model. Variable-rate cyclic voltammetry (CV) analyses quantify the ionic transport of AlCl₄^-, revealing potential-dependent regimes that are not strongly diffusion-limited, an unexpected result that suggests that the intercalation of the sterically bulky molecular anion into narrow graphite layers is a faster and more facile process than expected.

To probe the local environments of the intercalated AlCl₄^- anions themselves, solid-state ²⁷Al nuclear magnetic resonance (NMR) spectroscopy was used to measure the local ²⁷Al electronic and magnetic environments. The NMR results establish that the intercalated anions experience a diversity of local environments that deviate from the uniform environments suggested by ideal graphite staging models. Density Functional Theory (DFT) quantum chemical calculations were performed using a [AlCl₄^-]-coronene bilayer structure model to quantitatively interpret the experimental ²⁷Al NMR shifts. The NMR and DFT results establish that the high extents of local disorder observed in experimental ²⁷Al NMR spectra are largely due to distributions of AlCl₄^- molecular configurations that deviate from tetrahedral geometry. Notably, this includes planar-like configurations at lower cell potentials at which the graphite layers are contracted, suggesting that the anions can intercalate even before the layers fully expand, which appear to facilitate non-diffusion-limited intercalation processes.

Finally, we developed a rigorous electrochemical explanation for the (ultra)fast performance of Al-graphite batteries, in light of conflicting explanations as to how modified graphite structures are linked to enhanced high-rate capability. Using advanced electrochemical analyses, the Faradaic, pseudocapacitive, and capacitive contributions were disentangled quantitatively. Two model electrodes were studied: (i) pristine graphite and (ii) exfoliated graphite prepared by a scalable ultrasonication process. Variable-rate CVs revealed that exfoliated graphite exhibited significant pseudocapacitive contributions that accounted for ~30% higher capacity at fast cycling rates, while electrochemical impedance spectroscopy (EIS) confirmed lower charge-transfer resistances and higher effective diffusion coefficients. Reduced tortuosity and the increased accessibility of interstitial pores to AlCl₄- ions are hypothesized to alter ion mass transport within the porous electrode. Thus, mild exfoliation is shown to be simple method that enhances pseudocapacitive contributions, enhancing capacity retention at higher charge and discharge rates.

Overall, the results provide enhanced fundamental understanding of the electrochemical ion intercalation and transport processes that underlie the charge storage mechanism of graphite electrodes in rechargeable Al-graphite batteries, as well as strategies for their control. This work thus provides a scientific foundation for the continued technological development of this emerging electrochemical energy storage technology.

Recommended Citation

Xu, Jeffrey, "Molecular to Macroscopic Understanding of Chloroaluminate Anion Intercalation in Rechargeable Aluminum-Graphite Batteries" (2021). CUNY Academic Works.
https://academicworks.cuny.edu/cc_etds_theses/926