KTi₂(PO₄)₃ Electrode with a Long Cycling Stability for Potassium-Ion Batteries

In this work, rhombohedral KTi₂(PO₄)₃ is introduced to investigate the related theoretical, structural, and electrochemical properties in K cells. The suggested KTi₂(PO₄)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 KTi₂(PO₄)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 K₃Ti₂(PO₄)₃ phase, these new findings provide insight into the reaction mechanism in Na superionic conductor (NASICON)-type KTi₂(PO₄)₃. The present approach can also be extended to the investigation of other NASICON-type materials for potassium-ion batteries.