Molecular-Level to Cell-Level Understanding of the High-Rate Cycling Capability of Rechargeable Aluminum-Graphite Batteries

Abstract

Rechargeable aluminum-graphite batteries using chloroaluminate-containing ionic liquid electrolytes are a promising beyond-lithium technology because they utilize electrodes that are globally abundant, low-cost, and inherently safe.1,2 Notably, these batteries have demonstrated ultrafast charge/discharge rates, which are critical for advancing high-power and rapid-charging applications such as electric transportation. Since Lin et al.’s pioneering report in 2015, several groups have engineered graphene-like materials to further improve the high-rate capability and capacity retention to as high as 120 mAh/g @ 400 A/g, on the cell-level.3 Recently, we elucidated the relationship between mesoscale graphite structure and macroscopic electrochemical properties such as high-rate performance.4 However, molecular-level understanding of the electrochemical charge storage mechanism upon cycling, which involves the intercalation of sterically bulky, molecular AlCl4 - anions into graphite, remains incomplete. To better understand what enables such rapid molecular-level ionic diffusion and consequently high-rate cell-level cycling, we combined electrochemical studies with solid-state 27Al magic-angle-spinning (MAS) NMR and density functional theory (DFT) 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. Correlation of experimental NMR and DFT results established that the high extents of local disorder observed in the experimental solid-state 27Al NMR spectra can be attributed to distributions of intercalated chloroaluminate anions in different molecular configurations. Second, to address the technological objective of further improving the high-rate capacity retention of graphite electrodes, we employed liquid-phase exfoliation and centrifugation to modify the c-axis thickness of the highly ordered pristine graphites. In full-cell electrochemical tests of the exfoliated-graphite electrodes, variable-rate galvanostatic cycling revealed increased capacity retention, while 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, Pages: 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) Chen, H.; Xu, H.; Wang, S.; Huang, T.; Xi, J.; Cai, S.; Guo, F.; Xu, Z.; Gao, W.; Gao, C. Ultrafast All-Climate Aluminum-Graphene Battery with Quarter-Million Cycle Life. Sci. Adv. 2017, 3.(4) 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.