Hierarchically Structured Potassium Titanium Phosphates as Reference and Diagnostic Electrodes in Potassium‐Ion Batteries

The high reactivity and unstable potential of metallic potassium limits its application as a stable reference or counter electrode for a precise and reliable electrochemical characterization of electrode active materials for potassium-ion batteries (PIBs). In this study, we investigate the application of hierarchically structured KTi$_2$(PO$_4$)$_3$/C (KTP/C) and KTiOPO$_4$/C (KTPO/C) composites as a reference (RE) or diagnostic electrode (DE) respectively. KTP/C is a promising RE due to its stable potential at 1.88 V versus K+/K on a long-term scale for more than 250 h in a three-electrode configuration with a KTP/C working electrode (WE) and RE against a metallic potassium counter electrode (CE). Instead, KTPO/C exhibits a low kinetic limitation, making this material ideally suited as a DE for PIBs. To prove the suitability of KTPO/C as a DE, a full-cell setup of K$_3$V$_2$(PO$_4$)$_3$/C (KVP/C) as WE against a KTPO/C DE was developed. The full-cell setup with a KTPO/C DE revealed a significant underestimation of the properties of KVP/C compared to the conventional half-cell setups with a metallic potassium CE. The developed RE and DE for PIBs facilitate an accurate and reliable electrochemical characterization of electrode active materials for and will accelerate the future development of PIBs.