(637i) Elucidating the Molecular Geometry and Ionic Diffusivity of Intercalant AlCl4- Anions for Rechargeable Aluminum-Graphite Batteries

Rechargeable aluminum-graphite batteries using chloroaluminate-containing ionic liquid electrolytes store charge when molecular AlCl₄^- anions intercalate into graphite^1,2. Such batteries are a promising alternative to lithium-ion because they utilize electrodes that are globally abundant, low-cost, and inherently safe, while exhibiting ultra-fast rate capabilities, high cycle life and discharge voltages of ca. 2.0 V. Recently, we elucidated the relationship between graphite structure and macroscopic electrochemical properties such as high-rate performance and the maximum intercalation composition.³ To better understand what enables such rapid molecular-level ionic diffusion and consequently high-rate cell-level cycling, we performed solid-state ²⁷Al magic-angle-spinning (MAS) NMR, density functional theory (DFT), and electrochemical methods to understand the molecular-level environments and ion transport properties of intercalated AlCl₄^- anions. First, to directly observe local environments of chloroaluminate species intercalated within the graphite electrodes, solid-state ²⁷Al MAS NMR measurements were acquired on intercalated graphite electrodes at various states-of-charge. DFT calculations were performed to simulate different molecular geometries of intercalated AlCl₄^- anions, yielding quantitatively the NMR shift contributions from chemical shift, aromatic ring-current, and electric quadrupolar coupling interactions. Correlation of experimental NMR and DFT results established that the high extents of local disorder observed in the experimental ²⁷Al NMR spectra can be attributed to distributions of intercalated chloroaluminate anions in different molecular configurations. These results also suggest that chloroaluminate anions can intercalate even before the graphite layers expand to form ordered stages by adapting distorted molecular geometries.

With this new understanding of the molecular-level environments of sterically bulky chloroaluminates, we then performed electrochemical measurements on engineered graphite electrodes to understand and control anion transport properties. Liquid-phase exfoliation and centrifugation were employed to modify the c-axis thickness of highly ordered pristine graphites to generate few-layered graphene intercalation structures. In full-cell electrochemical tests of the exfoliated-graphite electrodes, variable-rate galvanostatic cycling revealed increased capacity retention (75% vs. 43% for non-exfoliated synthetic graphite @ current density of 960 mA/g). Furthermore, variable-rate cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) experiments revealed modified effective ionic diffusivities and ohmic resistances. Collectively, this multi-scale study of chloroaluminate intercalation into graphitic electrodes has revealed new insights that are key contributing factors to their high-rate capability, which can be adapted to engineer Al-graphite batteries with enhanced performance.

(1) Lin, M.-C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.-Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B.-J.; Dai, H. An Ultrafast Rechargeable Aluminium-Ion Battery. Nature 2015, 520, 324–328.

(2) Sun, H.; Wang, W.; Yu, Z.; Yuan, Y.; Wang, S.; Jiao, S. A New Aluminium-Ion Battery with High Voltage, High Safety and Low Cost. Chem. Commun. 2015, 51, 11892–11895.

(3) Xu, J. H.; Turney, D. E.; Jadhav, A. L.; Messinger, R. J. Effects of Graphite Structure and Ion Transport on the Electrochemical Properties of Rechargeable Aluminum-Graphite Batteries. ACS Appl. Energy Mater. 2019, 2, 7799–7810.