A Multiscale Investigation of a Cation Water Exchange Mechanism for Reversible K+ Insertion into Prussian Blue Analogs

The growing interest in potassium ion battery systems is predominantly motivated by the natural abundance of potassium as well as its promising high energy densities ascribed from a low K+/K redox potential. Among the many intercalation hosts, Prussian blue (PB) and its analogues (PBAs) are the most favorable and intriguing earth-abundant hosts for potassium ions. Nickel hexacyanoferrate (NiHCF) is particularly promising due to its low strain and high cycling stability in potassium systems. The exact insertion and de-insertion mechanism of K+ ions into PBA has not yet been fully identified, thus the charge storage mechanism remains poorly understood. This work uses a multiscale investigation strategy highlighted by an in-situ multi-harmonics electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) to uncover the dynamic, reversible K+ insertion into NiHCF. Findings reveal an electrolyte concentration dependent stoichiometry for a water-participated K+ insertion and de-insertion. The NiHCF material is subject to a dramatic restructuring merely at high potassium salt concentrations, while a high charge retention is observed through various K salt concentrations. A complicated and dynamic cation-water exchange mechanism is observed pointing to a salt concentration governed microstructural evolution of NiHCF.