Potassium Salts of 9-Oxo-9 H -fluoren-4-carboxylic Acid: Phase Formation, Crystal Structure, and Electrochemical Behavior in Potassium-Ion Batteries

Abstract Potassium-ion batteries (KIBs) are attracting increasing interest as alternatives to lithium-based systems owing to the natural abundance and low cost of potassium. Organic carbonyl compounds have emerged as promising electroactive materials, as their redox properties can be tuned through molecular design and crystal structure. Here, we examine potassium derivatives of 9-oxo-9H-fluoren-4-carboxylic acid (HFl). Depending on reaction conditions, two distinct crystalline phases are obtained from the parent acid: the fully deprotonated potassium salt (KFl) and a hydrogen potassium carboxylate (HKFl2). While acid salts of monoprotic carboxylic acids are well-known, their formation and structural implications in fluorenone-based systems remain largely unexplored. Single-crystal X-ray diffraction reveals that both structures comprise extended K–O coordination networks built from distorted KO6 environments, while HKFl2 additionally features strong O–H···O– hydrogen bonds linking neutral and deprotonated molecules. KFl electrodes show reversible K-ion insertion at ca. 1.5 V vs K+/K, despite pronounced first-cycle irreversibility and capacity fading, which may be related to electrolyte-dependent side processes and active-material dissolution. Ex situ diffraction shows that KFl is the electrochemically active phase, whereas HKFl2 converts into KFl upon electrolyte contact. These results demonstrate that subtle variations in protonation state lead to distinct crystalline phases that directly govern electrochemical behavior.