A High-Voltage Potassium-Ion Battery with a Potassium Manganese Hexacyanoferrate Cathode

Abstract

Potassium-ion batteries (KIBs) are a promising complementary technology to lithium-ion batteries that have the potential to be more sustainable and lower in cost. In KIBs lithium is replaced with potassium, the copper current collector is replaced with aluminium, and cathodes can be cobalt and nickel free. KIBs have the crucial advantage over sodium-ion batteries that potassium can reversibly intercalate into graphite whilst sodium cannot. Graphite offers a low operating voltage, high capacity, and good rate performance as a potassium-ion anode material.There remain significant scientific challenges in the development of commercial potassium-ion batteries, notably developing high-capacity high-voltage cathode materials and electrolytes that are compatible with high-voltage cathodes and graphite anodes. In a recent perspective we identified potassium manganese hexacyanoferrate (KMF) as the most promising cathode material as it offers high energy density whilst also being low in cost [1]. However, the cyclability and coulombic efficiency of KMF are typically poor owing to parasitic reactions from water remaining in its structure and aluminium current collector corrosion among other mechanisms.Here we present recent work to improve the performance of the KMF//graphite electrochemical system. Firstly, the citrate-assisted synthesis of KMF is optimised using high throughput synthesis and characterisation techniques to reduce the defect and water content whilst tailoring the particle size [2]. Secondly, an ionic-liquid based electrolyte, 1 M KFSI in Pyr1,3FSI (potassium bis(fluorosulfonyl)imide in N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide)), is employed. This electrolyte successfully forms a passivating layer on the KMF cathode and surpresses aluminium corrosion which enables highly efficient cycling. Additionally, graphite operates exceptionally well with this same ionic-liquid based electrolyte.We will end by offering our perspective on the current state of this research area and promising research directions.