PNNL

Abstract

Graphite is regarded as a promising positive electrode (=cathode) for anion intercalation for decades due to its high operating potential (> 4.50 V vs. Li|Li+; > 2.22 V vs. Zn|Zn2+; > 1.46 V vs. standard hydrogen electrode; 1.26 V vs. Ag|AgCl), high abundance, low cost, low toxicity without use of any transition metals, and high overall safety. Nevertheless, due to its high operation potential, it is challenging to find suitable electrolytes, particularly in any aqueous system due to parasitic side reactions such as oxygen evolution reactions at the cathode. In this work, natural graphite is enabled as a cathode material using an aqueous system in a new “water-in-bisalt” electrolyte, consisting of 20 m NaFSI and 0.5 m Zn(TFSI)2. We present a graphite || zinc (Zn) metal dual-ion battery that operates at a record high voltage of ˜2.3 – 2.5 V in a single electrolyte Zn system. Anions intercalate into a natural graphite cathode at an onset potential of ˜1.5 V vs. Ag|AgCl (˜2.46 V vs. Zn|Zn2+), delivering a reversible capacity of ˜110 mAh g–1 with high rate capability reaching a capacity of nearly 60 mAh g–1 at a charge/discharge current of 5000 mA g–1 (equals 50C assuming 100 mAh g–1 theoretical capacity). Over 200 cycles are achieved with over 80% capacity retention in a graphite || Zn metal full cell, showing the stability of the graphite cathode and the Zn metal negative electrode (=anode) in the electrolyte. Electrochemical, chemical, and structural characterization, along with density functional theory calculations, collectively prove anion intercalation takes place and provide insights into the mechanism of anion intercalation into natural graphite.