Tuning Ion Diffusivity and High-Rate Cycling Performance Via Graphite Exfoliation for Rechargeable Aluminum-Graphite Batteries

Abstract

Rechargeable aluminum-graphite batteries using chloroaluminate-containing ionic liquid electrolytes store charge when chloroaluminate anions (e.g. AlCl4 -) intercalate into graphite1,2. Such batteries utilize electrodes that are globally abundant, low-cost, and safe, while exhibiting ultra-fast rate capabilities, high cycle life and discharge voltages of ca. 2 V. Recently, we elucidated the relationship between graphite structure and macroscopic electrochemical properties such as high-rate performance and the maximum intercalation composition.3 Variable-rate cyclic voltammetry (CV) revealed potential-dependent transport regimes that yield new insights into their ultra-fast rate capabilities. To directly observe the chloroaluminate species intercalated within the graphite electrodes, we performed solid-state 27Al MAS NMR measurements, which revealed their various local environments and populations. To aid the interpretation of the experimental results, DFT calculations were performed to compute the magnitude of 27Al NMR shift contributions due to varying graphite d-spacing, aromatic ring-current shielding, and quadrupolar interactions.With this increased understanding of the elusive transport mechanism of sterically-bulky chloroaluminates, the current technological objective is to improve the high-rate capacity retention of synthetic graphite electrodes. To tune the ionic diffusivity within the graphite framework, we employed liquid-phase exfoliation and centrifugation to modify the c-axis thickness of the highly-ordered pristine graphites, generating few-layered graphene structures that we characterized using Atomic Force Microscopy (AFM), Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). In full-cell electrochemical tests of the exfoliated-graphite electrodes, variable-rate galvanostatic cycling revealed increased capacity retention (~75% @ current density of 960 mA/g, compared to ~43% for non-exfoliated synthetic graphite). Furthermore, galvanostatic intermittent titration technique (GITT) experiments revealed modified ionic diffusivity and ohmic resistances. These results are expected to aid the rational design of graphite electrodes for enhanced performance in aluminum-graphite batteries.(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.