Reversible anion intercalation into graphite from aluminum perchlorate “water‐in‐salt” electrolyte

Abstract

Anion intercalated graphite has achieved tremendous attention for the application as cathode materials in dual-ion batteries due to their high working potentials, reversibility, and low cost. However, the process of anion intercalation into graphite requires high oxidative stability of the electrolytes and high reaction potentials above 1.4 V vs. Ag/AgCl (4.5 V vs. Li+). Reaching such high potentials is difficult with conventional aqueous electrolytes due to their limited electrochemical window. Here, we demonstrate a highly concentrated “water-in-salt” electrolyte of aluminum perchlorate that demonstrates a wide electrochemical stability window of 4.0 V. The “water-in-salt” electrolyte suppresses the dissociation of water at high potentials, facilitating a stable and reversible perchlorate (ClO4−) anion intercalation into the graphite with a Coulombic efficiency of over 95% for more than 2000 cycles. The structural and chemical changes of the anion-intercalated graphite are studied by Operando Raman, Operando X-ray diffraction, and X-ray photoelectron spectroscopy. Our study provides an insight into ClO4− anion intercalation into graphite in the supersaturated aluminum perchlorate “water-in-salt” electrolyte, demonstrating a suitable platform for future high-voltage aqueous energy storage systems.