Exploring Phosphate-Based Mixed Polyanionic Materials for Enhanced Metal-Ion Rechargeable Batteries

Affordable and high-performance energy storage solutions are essential to meet the growing energy demands of society. Rechargeable batteries employing cost-effective and readily scalable battery materials offer promising prospects for fulfilling these needs. Mixed polyanionic insertion materials offer a rich treasure house for designing robust cathode materials for rechargeable batteries, leveraging properties such as chemical/thermal stability, tunable redox voltage, and excellent electrochemical performance. 1,2 Among these, PO 4 -based materials stand out as exceptional systems, offering scalable synthesis, safe storage/handling, and high energy density. Here, we delve into three distinct classes of mixed polyanionic materials, elucidating their electrochemical behavior and mechanism. (1) Mixed Phosphates: Nanostructured carbon-coated Na 4 Co 3 (PO 4 ) 2 P 2 O 7 and Na 4 Ni 3 (PO 4 ) 2 P 2 O 7 were explored as bifunctional electrocatalysts for oxygen evolution and reduction reactions (OER and ORR). 3 Both materials showed bifunctional behavior, suggesting potential use in sodium-air batteries. For practical applications, nanoscale in-situ carbon-coated Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) material was utilized for the fabrication of sodium ion full cells using hard carbon (HC) as the anode. 4 Reduced order modeling (ROM) was employed to accurately monitor cycling-related degradation. A stable NFPP/HC full cell exhibited a robust specific capacity for over 200 cycles with minimum degradation was achieved. (2) NASICON-type Phosphate-sulfate: Mixed polyanionic NaFe 2 PO 4 (SO 4 ) 2 was explored for Li- and Na-ion batteries, leveraging the Fe +3/+2 redox couple within a voltage range of 2.0 V to 4.5 V with excellent capacity and cycling stability. Spray-drying synthesized material with uniform spherical particles was studied using powder X-ray diffraction data followed by various physical characterizations (SEM, TEM, FTIR, Raman, UV vis and calorimetry, etc.). 5,6 A single particle-based model (SPM) was employed to analyze the system for predictions of alterations associated with different geometric and physical parameters. (3) Fluorophosphates: The electrochemical properties of Na 2 FePO 4 F fluorophosphate (3 V vs Na) were also investigated using a single particle model, validated using the half-cell data and various geometric and physical parameters. The magnetic structure and properties of fluorophosphate Na 2 MnPO 4 F sodium insertion material were explo