Ionic Liquid Electrolytes Enabling the Development of Highly Efficient High-Voltage Potassium-Ion Full Cells with a Potassium Manganese Hexacyanoferrate Cathode
Authors: Wheeler Samuel , Fiore Michele , Hurlbutt Kevin , Capone Isaac , Fawdon Jack , Ruffo Riccardo , Pasta Mauro 
Published: May 2020

Potassium-ion (K-ion) batteries are a promising complementary technology to lithium-ion in which less scarce and less expensive elements can be used; potassium instead of lithium, aluminum instead of copper, and cobalt-free cathodes. They have the advantage over sodium-ion batteries in that graphite, the standard commercial anode for lithium-ion, can be used[1] and potassium has a lower reduction potential (K+/K) than that of sodium and even lithium. Currently, the research focus is on developing energy-dense, high-voltage cathode materials. Prussian blue analogues, with their open-framework structure, are well-suited to reversibly insert the relatively large potassium ion.[2] Potassium manganese hexacyanoferrate (KMF) is of particular interest due to its high theoretical capacity (155 mAh g-1) and high voltage (around 4 V vs. K+/K) making it one of the most promising K-ion candidate cathode materials.[3] However, significant challenges remain before KMF K-ion batteries are suitable for commercialization. Current collector corrosion at high voltages and material instability lead to low coulombic efficiency on cycling and severe capacity fade.[4]
In this work we report a significant improvement in KMF electrode performance by utilizing a highly stable ionic liquid electrolyte. This electrolyte suppresses corrosion of the current collector and deleterious reactions on the cathode. Performance of the KMF material is significantly controlled by its composition and particle size, which can be controlled during synthesis. We employ high-throughput techniques to systematically study a wide synthesis parameter space in order to minimize hexacyanoferrate vacancy and water content whilst controlling for particle size. We found that changing particle size considerably impacted specific capacity and cyclability, and a compromise must be made. Our optimized KMF material had a specific capacity of 119 mA h g-1, a coulombic efficiency of 99.3%, and relatively good cyclability. Crucially, graphite also displayed excellent electrochemical performance in the same ionic liquid electrolyte (maintaining 99% of initial capacity after 400 cycles and a coulombic efficiency of more than 99.9%) and exhibited a highly reversible electrode reaction studied by in operando XRD. Optimized KMF | ionic liquid electrolyte | graphite full-cell chemistry reported here proves a viable strategy for highly efficient high-voltage potassium-ion batteries.
[1] Komaba et al., Potassium intercalation into graphite to realize high-voltage/high-power potassium-ion batteries and potassium-ion capacitors, Electrochemistry Communications (2015), 60, 172-175
[2] Hurlbutt et al., Prussian Blue Analogs as Battery Materials, Joule (2018), 2, 10, 1950-1960
[3] Bie et al., A novel K-ion battery: hexacyanoferrate(II)/graphite cell, J. Mater. Chem. A (2017), 5, 9, 4325-4330
[4] Hosaka et al. Highly concentrated electrolyte solutions for 4 V class potassium-ion batteries, Chem. Commun. (2018), 54, 8387-8390