Abstract
Abstract In this work, rhombohedral KTi 2 (PO 4 ) 3 is introduced to investigate the related theoretical, structural, and electrochemical properties in K cells. The suggested KTi 2 (PO 4 ) 3 modified by electro‐conducting carbon brings about a flat voltage profile at ≈1.6 V, providing a large capacity of 126 mAh (g‐phosphate) −1 , corresponding to 98.5% of the theoretical capacity, with 89% capacity retention for 500 cycles. Structural analyses using electrochemical performance measurements, first‐principles calculations, ex situ X‐ray absorption spectroscopy, and operando X‐ray diffraction provide new insights into the reaction mechanism controlling the (de)intercalation of potassium ions into the host KTi 2 (PO 4 ) 3 structure. It is observed that a biphasic redox process by Ti 4+/3+ occurs upon discharge, whereas a single‐phase reaction followed by a biphasic process occurs upon charge. Along with the structural refinement of the electrochemically reduced K 3 Ti 2 (PO 4 ) 3 phase, these new findings provide insight into the reaction mechanism in Na superionic conductor (NASICON)‐type KTi 2 (PO 4 ) 3 . The present approach can also be extended to the investigation of other NASICON‐type materials for potassium‐ion batteries.